Selectivity Enhancement in Heterogeneous Photocatalytic

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Selectivity Enhancement in Heterogeneous Photocatalytic Transformations Jiahui Kou,*,†,‡,§ Chunhua Lu,†,‡,§ Jian Wang,†,‡,§ Yukai Chen,†,‡,§ Zhongzi Xu,†,‡,§ and Rajender S. Varma*,∥ †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, ‡Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, and §Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 210009, P. R. China ∥ Regional Center of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University, Šlechtitelů 11, 783 71 Olomouc, Czech Republic ABSTRACT: Photocatalysis has been invariably considered as an unselective process (especially in water) for a fairly long period of time, and the investigation on selective photocatalysis has been largely neglected. In recent years, the field of selective photocatalysis is developing rapidly and now extended to several newer applications. This review focuses on the overall strategies which can improve the selectivity of photocatalysis encompassing a wide variety of photocatalysts, and modifications thereof, as well as the related vital processes of industrial significance such as reduction and oxidation of organics, inorganics, and CO2 transformation. Comprehensive and successful strategies for enhancing the selectivity in photocatalysis are abridged to reinvigorate and stimulate future investigations. In addition, nonsemiconductor type photocatalysts, such as Ti−Si molecular sieves and carbon quantum dots (CQDs), are also briefly appraised in view of their special role in special selective photocatalysis, namely epoxidation reactions, among others. In the end, a summary and outlook on the challenges and future directions in the research field are included in the comprehensive review.

CONTENTS 1. Introduction 2. Selective Oxidation of Organics 2.1. Morphological Control 2.1.1. Spatial Confinement Effect 2.1.2. Quantum Confined Nanoparticles 2.1.3. Formation of Special Crystal Facets 2.2. Phase 2.3. Doping 2.3.1. Doping with Metallic Ions 2.3.2. Doping with Nonmetallic Ions 2.3.3. Codoping 2.4. Cocatalysts 2.4.1. Metal Loading 2.4.2. Carbon Materials/Photocatalysts 2.5. Semiconductor Composites 2.6. Surface Treatment 2.6.1. Molecular Recognition Site (MRS) 2.6.2. Ion Modification 2.6.3. Surface Acid−Base Properties 2.6.4. Other Modifications 2.7. Effect of Inherent Properties of Chemicals 2.7.1. Photocatalyst with Selectivity 2.7.2. Substituent Effect on the Substrates 2.8. External Reaction Conditions 2.8.1. Addition of Organics 2.8.2. Effect of Solvent © 2017 American Chemical Society

2.8.3. Influence of Gas Atmosphere 2.8.4. Various Ionic Additives 2.8.5. Role of Light Sources 2.8.6. Effect of pH Value 2.8.7. Effect of Temperature 2.8.8. Influence of Reaction Time 2.8.9. Effect of Photocatalyst Amount 2.8.10. Effect of Humidity 3. Selective Reduction of Organics 3.1. Cocatalysts 3.1.1. Carbon Material/Photocatalysts 3.1.2. Metals Loading 3.1.3. Complexes 3.2. Heterostructured Materials 3.3. Surface Modification 3.4. Effect of Phase 3.5. Effect of Crystal Facets 3.6. Influence of Light Sources 3.7. Photocatalyst with Selectivity 3.8. Effect of Solvent 4. Miscellaneous 4.1. Selective Oxidation of Inorganics 4.2. Selective Reduction of Inorganics 4.2.1. CO2 Conversion

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Received: June 22, 2016 Published: January 17, 2017 1445

DOI: 10.1021/acs.chemrev.6b00396 Chem. Rev. 2017, 117, 1445−1514

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Figure 1. Strategies for enhancing the photocatalytic selectivity.

4.2.2. Conversion of Nitrates 4.2.3. NOx Conversion 4.2.4. CO Conversion 4.2.5. Reduction of Ions 4.3. Coupling Reaction 4.3.1. C−C Coupling 4.3.2. C−N Coupling 4.3.3. C−O Coupling 4.3.4. N−N Coupling 4.3.5. S−O Coupling 4.3.6. Acceptorless Dehydrogenation Coupling 4.4. Metathesis Reactions 5. Summary, Challenges, and Perspective Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments Abbreviations References

(CB), and in the meantime a hole is generated in the VB.3,9−12 If the photoinduced electrons and holes have enough activity, the active species, free radicals (•OH, O2•−, and HO2•) could be generated.3,13 Therefore, photocatalysis has been invariably considered as an unselective process (especially in water) for quite a long time, and the investigation on selective photocatalysis has been largely neglected.1−8,14 In recent years, especially during the past five years, the field of selective photocatalysis is developing rapidly15−31 and now extended to numerous latest applications namely novel selective catalytic reactions, selective CO2 transformation to fuels, and selective elimination or oxidative degradation of molecules (mixture of pollutants). The following applications are noteworthy: selective catalytic reactions in industrial chemistry;32−37 discerning conversion of CO2 to fuels is attractive for both, the environmental protection and renewal of energy;25,38−40 and selective degradation of mixed pollutants is becoming increasingly noteworthy in greener environmental remediation, especially an effective pathway to remove dilute pollutants.41−44 Hence, selective photocatalysis is becoming a highly significant research direction for advanced photocatalysis providing a broader developing space in view of emerging prospects for this field. On the other hand, these burgeoning explorations are very attractive in view of the utilization of solar energy, which is garnering immense interest because of the demonstrated harmful impact of fossil fuel consumption in the climate change debate. Compared to the traditional methods, the photocatalysis-promoted transformations are helpful to relieve the energy stress, which is one of the most crucial issues of the 21st century. The photocatalytic studies are mainly deliberated on titanium dioxide (TiO2) because of its environmental-friendly and stable

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1. INTRODUCTION Photocatalysis has splendid application prospects in various disciplines, such as greener energy sources, chemical synthesis, environmental technology, and medicine.1−8 When the energy of a photon is higher than the band gap energy (Ebg) of the photocatalyst, it can be absorbed to result in the promotion of an electron from valence band (VB) into conduction band 1446


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confinement are helpful to improve the reaction selectivity of one reactant among many without compromising the conversion.41,42,49−53 In addition, the external conditions, including aeration, the solvent usage, and deployment of additives, can be used to control the photocatalytic selectivity too.43,67−70 These investigations have brightened the prospects for photocatalysis as they enable potential applications in emerging fields such as separation processes, selective elimination or oxidative degradation of pollutant molecules from a mixture. This review focuses on the overall strategies which can improve the selectivity of photocatalysis encompassing a wide variety of photocatalysts, and modifications thereof, as well as the related vital processes of industrial significance such as reduction and oxidation of organics, inorganics, and CO2 transformation. Comprehensive and successful strategies for enhancing the selectivity in photocatalysis are summarized to reinvigorate and stimulate future investigations. In addition, nonsemiconductor type photocatalysts, such as Ti−Si molecular sieves and carbon quantum dots (CQDs), are also briefly appraised in view of their special role in special selective photocatalysis, namely epoxidation reactions, among others.

properties, which make it the excellent material for many uses in spite of the shortcoming that it cannot absorb visible light.18,19,45−47 Most of the investigations are targeted to oxidation or reduction reactions, since the photoinduced active species inherently possess reducibility or oxidizability attributes.16−19,21,41,42,48−56 Traditionally, the conversion, yield, and selectivity for the generation of target product in selective reaction are defined as follows:57 conversion(%) = [(C0 − Cr)/C0] × 100 yield(%) = Cp/C0 × 100 selectivity(%) = [Cp/(C0 − Cr)] × 100

where C0 is the initial concentration of reactant and Cr and Cp are the concentrations of reactant and target product at a certain reaction time, respectively. The enhancement of selectivity in photocatalytic processes is a formidable challenge. The strategies for enhancing the photocatalytic selectivity can be broadly divided into two categories: the modification of photocatalysts and the change of external operational conditions (Figure 1). Since the generation of •OH radicals is largely responsible for the nonselectivity of photocatalysts, many strategies for modification of photocatalysts are tailored to avoid the involvement of •OH via band gap modification, selective growth of crystal facets, and the surface treatment.58−62 Figure 2 summarizes the association

2. SELECTIVE OXIDATION OF ORGANICS Photocatalytic oxidation reactions attract burgeoning interest because they can be conducted under mild conditions using nontoxic materials and have economic and environmental advantages.15−19,48 In contrast, the conventional industrial processes are often carried out in organic solvents at high temperature and pressure, and the commonly used oxidants not only generate copious amounts of hazardous waste, but are also expensive and perilous, such as chromate and permanganate, among others.71 Photocatalytic selective oxidation is a vital and well-studied branch in photocatalysis, especially the selective generation of carbonyl compounds, e.g., ketones and aldehydes from alcohols, which are significant raw materials to synthesize vitamins, drugs, and fragrances (Table 1).67,72−83 Various reaction mechanisms for the photo-oxidation of alcohols have been proposed, which are related to the selectivity and activity.84 18O-enriched benzyl alcohol and cyclohexanol have been used as substrates to investigate the mechanism associated with TiO2. When O2 is absent, the O atom of the alcohol could remain in the product because of a two-electron transfer process (Figure 3a). First, the TiO2 surface adsorbs an alcohol molecule to obtain the structure I through a deprotonation process and h+/e− pairs are produced after TiO2 is excited by UV light (Figure 3b). In succession, the adsorbed alcohol reacts with h+ to generate a carbon radical, while TiIV is reduced by e− to form TiIII (II). There are two pathways to lead to the formation of intermediate III, an oxygen bridged structure, which is the key to the cleavage of O−O bond in O2 and the C−O bond in the alcohol. When O2 is reduced by electron/ TiIII, superoxide is formed to attack the carbon radical, which afford structure III. On the other hand, if a carbon radical reacts with dioxygen, an organic superoxyl radical can be generated which then reacts with TiIII to form intermediate III. One of the concerted bond-cleavage products of intermediate III, structure IV, can be stimulated by protons to generate H2O2, which is substantiated by iodometric titration method. The reaction mechanisms for the photooxidation of alcohol over Nb2O5 (Figure 4) and Cu/Nb2O5 with molecular oxygen are different from that over TiO2.85,86 First, Nb2O5 surface adsorb alcohol as alcoholate species, and then Nb5+ is reduced by electron

Figure 2. Potentials for various redox couples in water (pH 7) and the band-edge positions of semiconductor photocatalysts.

between band-edge positions of photocatalysts and the potentials for various redox couples in water (pH 7).63−65 Generally, the •OH radicals could not be generated on visible light responsive photocatalysts because of the inadequate VB position, while the traditional photocatalysts with wide band gap, e.g., TiO2, facilitates the generation of •OH. Though selectivity can be improved when •OH radicals are eluded, the photocatalytic efficiency usually decreases as well.61,66 As a consequence, the strategy for simultaneously enhancing selectivity and activity need to be explored. The formation of molecular recognition site (MRS) and the utilization of spatial 1447


1448

material

rutile TiO2 anatase TiO2 brookite TiO2

36 37 38

Pt-TiO2 (P25) Au-TiO2 (P25) Ag-TiO2 (P25) Cu-TiO2 (P25) Ni-TiO2 (P25) Pd-TiO2 (P25) Ir/TiO2 Rh/TiO2 Pd/TiO2 Pt/TiO2 Au/TiO2 Ag/TiO2 CQDs TiO2‑xNx (x = 0.034) Cu/Nb2O5 Nb2O5 mesoporous C3N4 mesoporous C3N4 mesoporous C3N4 mesoporous C3N4 mTiO2 1D CdS core@TiO2 shell (110) plane exposed rutile TiO2 TiO2 CdS sheet ZnS−5%GR ZnIn2S4 mpg-C3N4 photocatalyst Au/CeO2 Multi-Pd Core@CeO2 Shell Pd/CeO2 C/ZIS-0.03 (C-coated ZnIn2S4) ZnIn2S4 2.5 wt % Ag/TiO2 (Brookie)

1 2 3 4 5 6 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

entry

material

entry

atmosphere

2.1 h; 125 W medium pressure Hg lamp 4.1 h; 125 W medium pressure Hg lamp 1.8 h; 125 W medium pressure Hg lamp

irradiation condition

air air air

atmosphere

water water water

solvent

60 min; UV light emitting diode (maximum energy at λ= 366 nm) Ar 180 min; UV light emitting diode (maximum energy at λ= 366 nm) Ar 180 min; UV light emitting diode (maximum energy at λ= 366 nm) Ar 180 min; UV light emitting diode (maximum energy at λ= 366 nm) Ar 180 min; UV light emitting diode (maximum energy at λ= 366 nm) Ar 180 min; UV light emitting diode (maximum energy at λ= 366 nm) Ar 6 h; 250 W high pressure Hg lamp (315−420 nm, main wavelength at 365 nm) O2 6 h; 250 W high pressure Hg lamp (315−420 nm, main wavelength at 365 nm) O2 6 h; 250 W high pressure Hg lamp (315−420 nm, main wavelength at 365 nm) O2 6 h; 250 W high pressure Hg lamp (315−420 nm, main wavelength at 365 nm) O2 6 h; 250 W high pressure Hg lamp (315−420 nm, main wavelength at 365 nm) O2 6 h; 250 W high pressure Hg lamp (315−420 nm, main wavelength at 365 nm) O2 12 h; 450 W Xe arc lamp (λ > 700 nm) air 2 h; 300 W Xe arc lamp (λ > 420 nm) O2 48 h; 500 W ultrahigh-pressure Hg lamp (λ > 300 nm) O2 48 h; 500 W ultrahigh-pressure Hg lamp (λ > 300 nm) O2 3 h; 300 W Xe arc lamp (λ > 420 nm) O2 3 h; 300 W Xe arc lamp (λ > 420 nm) air 3 h; 300 W Xe arc lamp (λ > 420 nm) O2 3 h; 300 W Xe arc lamp (λ > 420 nm) O2 70 h; Hg lamp (λ > 400 nm) O2 8 h; 300 W Xe arc lamp (λ = 520 ± 15 nm) O2 12 h; xenon lamp (CEL-HXF 300, Techcomp) with a UV-cutoff filter (λ ≥ 420 nm). air 2 h; blue LED lamp (kmax =460 nm, ca. 10 mW/cm2). air 40 min; 300W Xe arc lamp (λ > 420 nm) O2 10 h; 300 W Xe arc lamp (λ > 420 nm) O2 3 h; 300 W Xe arc lamp (λ > 420 nm) O2 2 h; 300 W xenon lamp (λ > 420 nm); 100 °C O2 20 h ; green light from an LED O2 20 h; 300 W Xe arc lamp (λ > 420 nm) O2 20 h; 300 W Xe arc lamp (λ > 420 nm) O2 2 h; 300 W Xe arc lamp (λ > 420 nm) O2 2 h; 300 W Xe arc lamp (λ > 420 nm) O2 12 h; high-pressure xenon arc lamp N2 oxidation of 4-methoxybenzyl alcohol (4-MBA) to 4-methoxybenzaldehyde (4-MBAD)


oxidation of benzyl alcohol (BzA) to benzaldehyde (BzAD)

Table 1. Selective Photocatalytic Oxidations over Different Photocatalysts solvent

50 50 50

conv. (%)

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN solvent-free solvent-free solvent-free solvent-free solvent-free solvent-free H2O2 aqueous water solvent-free solvent-free trifluorotoluene trifluorotoluene toluene acetonitrile methyl cyanide trifluorotoluene acetonitrile acetonitrile solution trifluorotoluene trifluorotoluene trifluorotoluene pH 0 acidic water solution water trifluorotoluene trifluorotoluene trifluorotoluene trifluorotoluene acetonitrile

58 31 39

sel. (%)

99 51 69 49 99 28 7 70 45 19.6

99 40 8.1 19 39 39 8.9 8.7 4.1 3.3 6.4 2.4 92 66.3 24 19 57 44 40 70 92 34 27

conv. (%) >99 89 43 79 90 71 92 83 81 79 81 81 100 100 98 93 >99 >99 >99 68 >99 97 99 99 100 98 94 93 99 100 71 95 90 77.8

sel. (%)

75 75 75

ref

ref 72 72 72 72 72 72 91 91 91 91 91 91 92 61 93 93 73 73 73 73 90 94 95 96 57 97 15 71 98 99 99 100 100 101

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1449

material

material

1D CdS core@TiO2 shell 1D CdS core@TiO2 shell multi-Pd Core@CeO2 shell Pd/CeO2 multi-Pd Core@CeO2 shell Pd/CeO2

material

1D CdS core@TiO2 shell multi-Pd Core@CeO2 shell Pd/CeO2

57 58 59 60 61 62

entry

63 64 65

rutile TiO2 at 800 °C for 1 rutile TiO2 at 800 °C for 1 anatase TiO2 at 600 °C for anatase TiO2 at 600 °C for

entry

53 54 55 56

sintered sintered sintered sintered

48 49 50 51 52

entry

material

P25 4% Au/P25 4% Au/P25 Au@TiO2- microspheres mTiO2

entry

material

P25 4% Au/P25 4% Au/P25

45 46 47


h h 1h 1h





solvent

18.2 16.6 10.0

atmosphere

atmosphere O2 O2 O2

irradiation condition 8 h; 300 W Xe arc lamp (λ = 520 ± 15 nm) 20 h; 300 W Xe arc lamp (λ > 420 nm) 20 h; 300 W Xe arc lamp (λ > 420 nm)

solvent

trifluorotoluene trifluorotoluene trifluorotoluene

solvent

trifluorotoluene trifluorotoluene trifluorotoluene trifluorotoluene trifluorotoluene trifluorotoluene

50 50 50

conv. (%)

37 42 13

conv. (%)

39 41 10 5 12 8

conv. (%)

5.4 1.9 20 54

conv. (%)

34.0 10.6 8.2 63 23

conv. (%)

20 20 20

conv. (%)

conv. (%)

air water air 1.0 mM NaF water solution air water air 1.0 mM NaF water solution p-methoxyl benzaldehyde

atmosphere

375 W Hg lamp (λ > 320 nm) 375 W Hg lamp (λ > 320 nm) 375 W Hg lamp (λ > 320 nm) 375 W Hg lamp (λ > 320 nm) oxidation of p-methoxyl benzyl alcohol to

solvent

water water water

solvent

water water water

solvent

water water water aqueous phenol water

water water water

solvent

8 h; 300 W Xe arc lamp (λ = 520 ± 15 nm) O2 8 h; 300 W Xe arc lamp (λ = 520 ± 15 nm) O2 20 h; 300 W Xe arc lamp (λ > 420 nm) O2 20 h; 300 W Xe arc lamp (λ > 420 nm) O2 20 h; 300 W Xe arc lamp (λ > 420 nm) O2 20 h; 300 W Xe arc lamp (λ > 420 nm) O2 oxidation of p-fluoro benzyl alcohol to p-fluoro benzaldehyde

h; h; h; h;

air air CO2 air N2 (CAT)

atmosphere

air air CO2 to phenol

atmosphere

24 h; 1 solar (1000 W m−2) power light 24 h; 1 solar (1000 W m−2) power light 24 h; 1 solar (1000 W m−2) power light 3 h; 300 W Xe arc lamp (λ > 400 nm) 2 h; 300 W high-pressure Hg lamp (λ > 320 nm) oxidation of phenol to catechol 6 6 6 6

atmosphere

2 h; 300 W top-irradiated xenon lamp air 4 h; 300 W top-irradiated xenon lamp air 4 h; 300 W top-irradiated xenon lamp air oxidation of phenol to hydroquinone (HQ)

24 h; 1 solar (1000 W m−2) power light 24 h; 1 solar (1000 W m−2) power light 24 h; 1 solar (1000 W m−2) power light oxidation of benzene

material

rutile with high percentage of {110} facets anatase with {001} facets anatase with {101} facets

entry

atmosphere

oxidation of 5-(hydroxymethyl)-2-furaldehyde to 2,5-furandicarbaldehyde irradiation condition 13 h; four fluorescent black lamps (Philips, 8 W) that emit at 365 nm air 2.6 h; four fluorescent black lamps (Philips, 8 W) that emit at 365 nm air 3.95 h; four fluorescent black lamps (Philips, 8 W) that emit at 365 nm air oxidation of glycerol to hydroxyacetaldehyde (HAA)

42 43 44

rutile TiO2 anatase TiO2 brookite TiO2

39 40 41

entry

material

entry

Table 1. continued

96 74 65

sel. (%)

65 74 100 74 100 73

sel. (%)

76 100 64 75

sel. (%)

21.0 60.8 82.2 91 83

sel. (%)

7.3 8.5 33.0

sel. (%)

96 16 59

sel. (%)

25 22.5 21

sel. (%)

94 99 99

ref

94 94 99 99 99 99

ref

104 104 104 104

ref

89 89 89 103 16

ref

89 89 89

ref

102 102 102

ref

79 79 79

ref



material


entry

69 70 71

oxidation of toluene to benzalydehyde epoxidation of styrene oxidation of cyclohexane to cyclohexanone oxidation of phenoxyacetic acid to phenol oxidation of 4-chlorophenoxyacetic acid to 4-CP oxidation of 2,4-dichlorophenoxyacetic acid to 2,4-DCP epoxidation of cyclohexene

oxidation of ammonia to N2

oxidation of ammonia to N2

oxidation of ANL to quinaldine oxidation of nitrobenzene (NBz) to quinaldine oxidation of NBz to quinaldine oxidation of MPS to MPSO

73 74 75

1450

80

81

82 83

solvent

2 h; 300 W high-pressure Hg lamp (λ > 320 nm)

mTiO2

1%WO3/TiO2

oxidation of trans-ferulic acid to vanillin

oxidation of CLA to 7-KOCA

88

89

NHPI/g-C3N4

TiO2

Au/TiO2 mpg-C3N4 coupled with isobutyraldehyde mpg-C3N4 coupled with isobutyraldehyde

Au/TiO2 Au/TiO2

O2

N2 O2

1000 ppm of NH3, O2, 3% H2O and 1000 ppm of NH3, O2, 3% H2O and N2 N2

O2

N2

N2

N2

90 min; three 15 W/10 Philips fluorescent lamps (maximum energy at λ = air 365 nm) 60 min; three external Actinic BL TL MINI 15 W/10 Philips fluorescent air lamps whose main emission peak was in the near-UV region at 365 nm 8 h; 250 W W-filament bulb (λ > 420 nm) O2

50 min; 150 W Hg lamp (λ > 420 nm)

6 h; 365 nm medium-pressure Hg lamp 4 h; 150 W Hg lamp (λ > 420 nm)

4 h; 365 nm medium-pressure Hg lamp 5 h; 365 nm medium-pressure Hg lamp

flow rate: 150 mL min−1; temperature: 150 °C; UVA lamp 1.1 mW cm−2.

Flow rate: 150 mL min−1; temperature: 150 °C; UVA lamp 1.1 mW cm−2.

12 h; high-pressure Hg lamp (λ > 280 nm)


mTiO2

Ti-containing mesoporous organosilicas (T-OS) PC 500 (Crystal Global) (anatase) P25


mTiO2

O2 O2 O2

5% N2 5% N2;

38 35 15

solvent

acetone

water

water

CH3CN

ethanol CH3CN

ethanol ethanol

acetonitrile

water

water

water

87 96 58

48

4.3

15

98

99 97

99 99

60

99

11

84

90

89

66 13 9

15

conv. (%)

sel. (%)

70 100 80

sel. (%)

trifluorotoluene trifluorotoluene water

conv. (%)

30 8 4

conv. (%)

water vapor, N2, O2

atmosphere

trifluorotoluene trifluorotoluene trifluorotoluene

CdS sheet ZnS−5%GR 1% Ag- substituted TiO2


O2 O2 O2

atmosphere

solvent trifluorotoluene trifluorotoluene trifluorotoluene

40 min; seven 40 W UV fluorescent lamps (300−425 nm, maximum energy at λ = 365 nm) 40 h; 300 W Xe arc lamp (λ > 420 nm) 10 h; 300 W Xe arc lamp (λ > 420 nm) 5 h; 80 W high-pressure Hg lamp (main wavelength at 365 nm)

oxidation of 4-(nitrophenyl) methyl sulfide to 4-(nitrophenyl) methyl sulfoxide oxidation of vanillyl alcohol to vanillin

87

irradiation condition 8 h; 300 W Xe arc lamp (λ = 520 ± 15 nm) 20 h; 300 W Xe arc lamp (λ > 420 nm) 20 h; 300 W Xe arc lamp (λ > 420 nm) others material MoOx/TiO2

atmosphere

oxidation of p-nitro benzyl alcohol to p-nitro benzaldehyde irradiation condition 8 h; 300 W Xe arc lamp (λ = 520 ± 15 nm) O2 20 h; 300 W Xe arc lamp (λ > 420 nm) O2 20 h; 300 W Xe arc lamp (λ > 420 nm) O2 oxidation of p-chloro benzyl alcohol to p-chloro benzaldehyde

86

84 85

79

78

77

76

oxidation of cyclohexane to benzene

72

reaction


66 67 68

entry

material

entry

Table 1. continued

99

24.7

21

98

73 98

76 76

28

92

76

89

72

72

96 92 63

65

sel. (%)

94 99 99

ref

94 99 99

ref

ref

114

113

112

111

110 111

108 109

107

107

106

16

16

16

57 97 66

105



Chemical Reviews

Review

selectivity as well. If electron and hole are continuously generated under light irradiation, the oxidation selectivity will be unavoidably poor, owing to the overoxidation of partially oxidized product into acid and even CO2. Thus, to attain higher oxidative selectivity over photocatalysts, the control of electron and hole generation process is also of crucial importance.90 2.1. Morphological Control

Photocatalysts with different morphologies usually have a very big influence on the photocatalytic performance due to the distinct active sites, active facets, and associated adsorption− desorption ability to reactant, and so on.16−19 To simultaneously achieve sufficient conversion and selectivity, it is of great significance to control the morphology of the photocatalysts or their surface properties in compliance with different needs, such as quantum confined structure, mesoporous, core− shell, etc.16−19 2.1.1. Spatial Confinement Effect. One typical approach for the construction of selective photocatalytic systems is the synthesis of photocatalysts with defined nanostructures, such as mesoporous photocatalysts, as well as nanocrystalline photocatalyst particles or isolated photocatalysts dispersed onto inorganic supports with periodic pore systems with high surface areas.16−18,90 The pore size and crystallinity of mesoporous photocatalysts have significant effect not only on reaction activity but also on the shift to a selective pathway.115−120 The creation of nonsemiconducting micropores leads to selective decomposition of large molecules on an external semiconducting photocatalysis surface,16,121−123 although often accompanied by the decrease in the photocatalytic activity more or less. However, a core−shell composite of TiO2 nanoparticles encased in a hollow silica shows size-selective preformance for photocatalytic degradation of organics without reducing the intrinsic activity of the naked TiO2 core.18 Crystalline mesoporous TiO2 (mTiO2) materials and their application in photocatalysis have been documented.19,90 The high-yielding benzyl alcohol (BzA)-to-benzaldehyde (BzAD) conversion can be attained over mTiO2 under mild and visible light irradiation conditions in the presence/absence of O2.90 According to the density functional theory (DFT) calculations, the O 2p atomic orbitals of TiO2 can hybridize with the antibonding π molecular orbitals of BzA; thus, new energy levels appear in the band gap which can generate holes via photoexcitation. Though the oxidative product BzAD is not stable on the TiO2 surface, the controllable cease in hole formation in this type of photoreaction system termed, “selfadjustable photooxidation system”, can prevent the overoxidation of BzAD. Shiraishi et al. have reported that the adsorption degree of reactant on the mTiO2 surface can play an important role in the photocatalytic selectivity; the welladsorbed molecules will be preferentially converted.16 On the basis of adsorption measurement, the distribution ratio, D, is used to describe the degree of molecule adsorption on the photocatalyst surface. The molecules with high D are welladsorbed and have good dispersion inside the pores, which can easily react with •OH; however, the molecules with low D, on the other hand, can barely enter the pores and thus unable to react with •OH (Figure 5). For the transformation of phenoxyacetic acid to phenol, mTiO2 displays unique selectivity (72%) which is more than twice that of a conventional nonporous (nTiO2). The reason is that on the mTiO2 surface, the D value is 0 for phenol, implying the inferior adsorption ability of mTiO2 to phenol, as a result, the sequential oxidation

Figure 3. Proposed O transfer mechanism for alcoholic oxidation on TiO2 photocatalyst in BTF solvent (a) without and (b) with O2. Reprinted with permission from ref 84. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 4. Reaction pathway for photooxidation of alcohol over Nb2O5 with molecular oxygen. Reprinted with permission from ref 85. Copyright 2009 American Chemical Society.

activated alcoholate species to form Nb4+, thus leaving a hole on alcoholate. The formed Nb4+ sites can be reoxidized by O2 after carbonyl compound III, which is from the conversion of alkenyl radical II, is desorbed. Oxygen anion radical species (O2− and O3−) are formed by irradiation over TiO2 and often responsible for the deep oxidation; they do not participate in the photooxidation over Nb2O5 catalyst. This presumably explains why the carbonyl compound can be selectively obtained through the photooxidation of alcohol over Nb2O5. However, on Cu/Nb2O5, the photogenerated radical intermediate disappears, because it is immediately converted to the product in the presence of CuII, which may serve as an electron acceptor.86 Among the abundant semiconductor materials, TiO2-based entities have garnered maximum attention as active semiconductor photocatalysts.16,18,87−89 There are several conceivable strategies to improve the selectivity in photocatalytic oxidations. An effective approach would be to avoid the induction of the strong, nonselective hydroxyl radicals, •OH, including changing the band gap, phase, or crystal facets. Another promising strategy is to combine molecular selective adsorption with photocatalysis, such as surface modification. The third option is the size selective advantage, such as shape changing, using mesoporous silica (MPSi) as a support. In addition, the oxidative ability of photogenerated holes in VB of photocatalyst often affects the photocatalytic activity and 1451


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Figure 6. Molecular structures and size of various reactants. Reprinted with permission from ref 74. Copyright 2011 Elsevier.

bearing different functional groups and sizes, such as the mixture of C9P, 4-hexylphenol (C6P), and 4-butylphenol (C4P) or the mixture of 2-nitro-4-phenylphenol (2-NPP), 2NP, and 4-nitro-2,6-diphenylphenol (2-NDPP). The degradation of NP is not achieved by the naked P25, showing again the merit of nanoporous silica coating on P25 to achieve selective photocatalysis. 2-NP is more easily concentrated on P25@ CnNPS with a smaller pore diameter, because 2-NP is nonpolar, and due to the intramolecular interactions between hydroxyl and nitro groups, it is difficult to be incorporated into pores with larger amounts of free silanol groups. 2-NDPP is hardly adsorbed on any P25@CnNPS. Taking the molecular size of 2-NDPP and the pore diameter of the three core/shell particles into account, the limited diameter of the pore in P25@ CnNPS is a possible reason for the exclusion of the reactant. In addition, the molecular recognizable adsorption and photocatalytic decomposition of C6P can be achieved when the core/ shell particles are reacted with aqueous mixture comprising C4P, C6P, and C9P under dark and then UV light irradiation. On naked P25, C9P is selectively adsorbed, but C9P is selectively adsorbed on any P25@CnNPS and the adsorbed amount does not depend on the pore diameter, clearly showing a merit of CnNPS coating on the TiO2 particle. Consequently, C9P and C6P are successfully separated on P25@CnNPS, which is a noticeable advantage since the separation of C9P from C6P is difficult. Inumaru’s study also shows that the molecular selectivity could be tuned by systematically controlling the pore size of support material when well-crystallized TiO2 particles (P25, 20−30 nm in diameter) are directly embedded into surfactanttemplated MPSi.19 The mechanism of the formation of nanocomposite is schematically illustrated in Figure 7. The alkyl-grafting renders the surface of the TiO2 particles to be highly hydrophobic thus dispersing C18-TiO2 very well in the surfactant solution. Consequently, it is expected that the TiO2 particles have very high affinity toward micelles in the solution. This allows the mesoporous silica to surround the TiO2 particles very efficiently. The catalysts having diverse big pores (2.7−5.3 nm) resolve nonylphenols and heptylphenol at comparable rates due to their preferential adsorption onto the catalysts. However, heptylphenol degrades faster than nonylphenols over the catalysts possessing small pores (1.4−1.9 nm and smaller micropores) because of their different diffusion rates. The nanocomposites with pores of 2.7 nm or larger show

Figure 5. Relationship between the distribution ratio, D, and reactant conversion (0.5 h) on (A) nTiO2(x) (green open circle, x = 100; red open circle, 58; black open circle, 0) and aggTiO2(36) (blue open circle) and (B) mTiO2(x) (blue open circle, x) 65; green open circle, 61; solid green circle, 57; red open circle, 37; black open circle, 0). Reactants used are labeled as numbers in the figure: 1, phenol; 2, 2,4,6trichlorophenol; 3, chlorohydroquinone; 4, 2,4-dichlorophenol; 5, 3chlorophenol; 6, 4-chlorophenol; 7, 2-chlorophenol; 8, benzyl alcohol; 9, 2,4-dichlorophenoxyacetic acid; 10, p-cresol; 11, phenoxyacetic acid; 12, 1,2,4-trihydroxybenzene; 13, 1,3,5-trihydroxybenzene; 14, 4chlorophenoxyacetic acid; 15, 2,6-bis(hydroxymethyl)-p-cresol. Reprinted with permission from ref 16. Copyright 2005 American Chemical Society.

of phenol by •OH can be well inhibited. Furthermore, mTiO2 also shows high selectivity (>80%) for the conversion of benzene to phenol, a challenging synthetic reactions.16 The results reveal that the adsorbed molecules in an ideal selective catalytic reaction should undergo the change from a well adsorption to a poor adsorption on catalyst via a so-called, stick- and leave-alteration. Of course, mesoporous photocatalysts having their own inherent drawback of limited crystallinity and photocatalytic activity may be inferior comparable to that of commercially available TiO2, although some examples have been reported.17,73 If photocatalyst is combined with mesoporous material via simple methods such as impregnation, the sol−gel method, and surface grafting, it appears that the crystallinity of obtained photocatalyst is poor with the limited loading level (ca. 10−25 wt %) thus affecting the photocatalytic activity.115 From the viewpoint of practical utility, the usage of commercially available TiO2 with a sufficient photocatalytic performance is more advantageous than the use of mesoporous photocatalysts.115 It is believed that the coating of P25 TiO2 particles by a nanoporous silica layer (CnNPS) imparts molecular recognizable photocatalytic ability74 as summarized in Figure 6 for varied structures and size of reactants. The P25@CnNPS exhibits adsorption-relied selectivity for the decomposition of 4-nonylphenol (C9P) and 2-nitrophenol (NP) in the 4-alkylphenol mixture and NP mixtures, respectively. Absorption and photocatalytic reactions on the ensuing core/shell particles are examined by organic probes 1452


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pores to result in high conversion. In contrast, the reactant that cannot diffuse well in pores, polar molecules, displays inferior reactivity. Therefore, sequential decomposition of phenol is suppressed over TiO2@MPSi leading to high phenol selectivity. In these photocatalysts, TiO2 particles are insufficiently embedded within the MPSi phase, and parts of the TiO2 particles are exposed on its surface (Figure 8). In xTiO2@MPS

Figure 8. Schematic representation of 29TiO2@MPSi. Modified with permission from ref 124. Copyright 2009 Elsevier.

catalysts, x (wt %) = Ti/(Ti + Si) × 100; x = 0, 29, 40, 57, 76. xTiO2@MPSi catalysts with lower Ti amount (29 TiO2@MPSi and 40 TiO2@MPSi) show the highest phenol selectivity, where the selectivity declines with an enhancement in the Ti amount of the catalysts. This is because that xTiO2@MPSi with high Ti content contains TiO2 particles exposed on the external surface, and hence, allows the sequential reaction of phenol. The selective photodecomposition of charged contaminants in aqueous solution is achieved on FAU-type zeolites wrapped TiO2.125 Since FAU-type zeolites are negatively charged, they can facilitate the selectivity of charged molecule via the electrostatic interaction with cationic and anionic reactants. The selective photodecomposition of charged contaminants has been studied by using three pairs of oppositely charged reactants. On the zeolites wrapped photocatalyst, the decomposition rates for the cationic reactants are obviously faster than those for the anionic reactants. The size of cationic reactants is also important for the reaction wherein the smaller ones, e.g., tetramethylammonium, is favorably removed. Therefore, the design of photocatalyst offers a beneficial approach to selectively degrade cationic reactants in contaminants mixture by targeting the refractory substrates exclusively. A core−shell composite of TiO2 particles encapsulated in hollow SiO2 (SiO2/void/TiO2) shows the size-selective photocatalytic activity for decomposition of organics without reducing the intrinsic activity of the naked TiO2 core.18,19 Fabrication of SiO2/void/TiO2 can be conducted through consecutive coating of TiO2 with a carbon and SiO2 layer, and the carbon layer can be removed by calcination treatment, as shown schematically in Figure 9.18 These results, which contradict the above-described gas-phase and liquid-phase reactions for relatively small molecules, can be explained by the blocking effect of the hollow silica shell to prevent adsorption of such large molecules on the surface of the TiO2 core. The porous silica shell will provide an exposure channel around the encapsulated TiO2 for selective diffusion of organic substrates moving into and out of the shell depending on their molecular sizes.17,18 The SiO2/void/TiO2 composite has size selective activity in the photodegradation of organics, i.e., it exhibits photocatalytic performance for the decomposition of relatively small reactants but does not have activity for the decomposition of a large molecule. The notable feature of this photocatalytic system is that it retains the inherent performance

Figure 7. Schematic illustration of the TiO2/mesoporous silica nanocomposite formation process. (a) Hydrophobic alkyl-grafted TiO2 particles float on water; (b) surfactant adsorption enables the particles to be dispersed in water, and (c) enhances the affinity to micelles; (d and e) the TiO2 particles are embedded into mesoporous silica. Modified with permission from ref 19. Copyright 2011 The Royal Society of Chemistry.

similar decomposition rates for nonylphenol and heptylphenol; nonylphenols and heptylphenol are preferentially adsorbed on the nanocomposite, while propylphenol and phenol are not. The molecular selectivity for nanocomposite (pores of 2.7 nm) and nanocomposite (pores of 5.3 nm) corresponds well to the molecular adsorption. On the other hand, the nanocomposite catalysts with narrow pores (1.9 and 1.4 nm) decomposed heptylphenol faster than nonylphenol; heptylphenol has normal straight alkyl chains whereas nonylphenol molecules are relatively much bulkier. The results for nanocomposite (pores of 1.9 and 1.4 nm) indicate that the bulky molecule is decomposed slowly due to slow diffusion into the narrow pores. Modification of the TiO2 particles via the surface grafting of alkyl groups demonstrates a highly efficient nanocomposite formation in which the particles are completely surrounded by MPSi; nanocomposite prepared from alkyl-grafted TiO2 particles shows much higher molecular selectivity and activity than that prepared from unmodified TiO2.19 The results highlight that the direct incorporation of catalytically active nanoparticles (NPs) into mesoporous materials is a promising strategy for designing selective catalysts. The TiO2 particles embedded in MPSi (TiO2@MPSi) can selectively catalyze (selectivity >69%) the conversion of benzene (less polar substrate) to phenol (polar product), which is considered to be motivated by the molecule polarity.124 Comparatively, photoirradiation of benzene on bulk TiO2, on the other hand, shows quite low selectivity (63%), however, the yield of 2-CP is very low ( P25 > CS TiO2 > 1% Ag imp >1% Ag DP.66 The total conversion of cyclohexane over 1% Ag sub at the end of 5 h is 9%, with 63% selectivity for cyclohexanone. The substitution of Ag into the TiO2 lattice is beneficial for achieving high conversions of cyclohexane and high selectivity for cyclohexanone, while impregnation of Ag on the TiO2 surface, both on CS TiO2 and on P25, reduces the overall conversion of cyclohexane and shifts the selectivity to cyclohexanol. The reason for higher

2.3. Doping

Doping with Fe, S, N, etc. may render photocatalysts more efficient for the selective oxidation.164,165 For a semiconductor with wide band gap, active species •OH is often photogenerated, which lead to “non-selective” reaction. The band gap of semiconductors can be tuned by doping metal or nonmetal elements, especially N.60,82,166 After being doped, VB energy level is too negative to allow the oxidation of − OH by photogenerated hole into •OH. In addition, dopants may act as a recombination center that may drastically suppress O2•− formation at the CB of TiO2.60,167 2.3.1. Doping with Metallic Ions. Selective photocatalytic oxidation of cyclohexane to cyclohexanol (the maximum selectivity is ca. 82%) under mild conditions could be achieved 1459


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Figure 18. UV−vis diffuse reflectance spectra (A), the Mott−Schottky plots analyzed at a frequency of 1000 MHz (B), the estimated band-structure (C), and the EIS Nyquist plots (D) of various TiO2‑xNx samples: a, x = 0; b, x = 0.025; c, x = 0.034; d, x = 0.036; e, x = 0.101. Modified with permission from ref 61. Copyright 2013 The Royal Society of Chemistry.

Co−TiO2−SiO2 (with Ti/Si molar ratio of 0.5) possesses the highest conversion and selectivity toward benzophenone formation which is about 1.5 times higher than that of ComTiO2. It is speculated that Ti/Si materials would take advantage of both, TiO2 (an n-type semiconductor and active catalyst support) and SiO2 (high thermal stability and excellent mechanical strength). The reasonable activity of Co-TiO2− SiO2 is not only due to the presence Co ions and the interactions between Co, Ti, and silica but also its special structure. Additionally, using Co-MCM-41 as a catalyst, a byproduct 2-hydroxybenzophenone (2-HOBP) formation shows a selectivity of higher than 5%, while only a trace amount of 2-HOBP is detected with Co-TiO2−SiO2 catalyst. The addition of alkali ions to V2O5/SiO2 drastically enhances the photocatalytic activity and the selectivity to propenal formation;171 Li, Na, K, and Rb ions display the similar effect upon the selectivity to propenal. However, the photocatalytic activity of Rb-ion modified 2.5 wt %-V2O5/SiO2 is highest, ∼8 times higher than that of nonmodified 2.5 wt %-V2O5/SiO2, despite the reduced specific surface area due to the high pH value used in solution preparation of the catalyst. The input of alkali ions also improves the photocatalytic capability for propenal formation over V2O5/Al2O3 with the order, Na < K < Rb. The propenal yield over Rb-ion-modified 2.5 wt %-V2O5/ Al2O3 is 3 times higher than that of nonmodified 2.5 wt %-V2O5/Al2O3. The interaction between the surface V2O5 and the alkali ion is important for the increase of photocatalytic activity since bare alumina and Rb2O/Al2O3 are ineffective for the propenal formation. In the instance of alumina support, the specific surface area is not decreased during catalyst preparation by addition of RbOH. Pure g-C3N4 is selective for the oxidation of styrene to styrene oxide, and the selectivity is 46.6% when the conversion of styrene is 24.1% in the dark;172 visible light irradiation not only improves the conversion of styrene (37.1%) but also enhances the selectivity for the formation of styrene oxide (53%). Modification with Fe and Co ions can promote the photocatalytic selectivity and activity for the oxidation of

selectivity of P25 than 1% Ag DP can be ascribed to the existence of surface Ag centers in 1% Ag DP, which possesses low concentrations of defect sites to effectively separate the charge-carriers. In the example of photocatalytic oxidation of BzA on Rh3+/TiO2 photocatalyst, the conversion of BzA reaches >99% with high selectivity of BzAD (>97%).167 The smaller adsorption amount of BzAD than that of BzA contributes to the lower rate of BzAD oxidation (consumption) than that of BzA over Rh3+/TiO2, and thereby high BzAD selectivity in the reaction system. Ru doped TiO2 nanotubes are utilized to improve the selectivity of photocatalytic oxidation of toluene to BzAD;169 specific modification of the tubes electronic properties leads to significant change in selectivity. Ru may act as a hole transfer promoter at the VB or as a doping species in TiO2, and very importantly, it forms a recombination center that may drastically suppress O2•− generation at the CB of TiO2. The Ru3+/4+ states in TiO2 are situated 0.4 eV below the CB of anatase, that is, electrons trapped on these states have an energy not sufficient to create O2/O2•−. Excited electrons are fast trapped on the Ru levels and do not create O2•− and, consequently preventing the oxidation of toluene to benzoic acid. Pt-doped TiO2 shows higher CO2 selectivity for the C3H6 removal with increasing the reaction temperature than N-doped TiO2.170 Infrared light irradiation can greatly promote the photocatalytic activity of Pt-doped rather than N-doped TiO2. The Pt dopant is able to generate heterojunctions with TiO2, which may act as thermal catalytic sites. The infrared light can excite the intense interaction between oxygen vacancies and Pt to cause the formation of absorption peaks at 800−900 nm, and therefore photocatalytic reactivity of Pt-doped TiO2 under visible light irradiation is remarkably promoted. Co-TiO2−SiO2 shows photocatalytic selectivity for the oxidation of diphenylmethane to benzophenone,45 while TiO2−SiO2, without doping by Co, does not show any significant activity; the results are similar to those without adding catalyst, and the incorporation of Si into the TiO2 framework highly improves the performance for the conversion of diphenylmethane compared with Co-mTiO2. In particular, 1460


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styrene. On Co-modified g-C3N4 catalyst, the conversion of styrene and the styrene oxide selectivity reaches to 32.6% and 46.4% in the dark and 55.5% and 58.5% under visible light irradiation, respectively. Using Fe-modified g-C3N4 catalyst, the selectivity can reach to 66.6%, in contrast, the Cu-, Ni-, and Mn modified g-C3N4 show a poor performance in the epoxidation of styrene. The styrene conversions of Cu-g-C3N4 and Mn-gC3N4 are even much lower than that obtained in the absence of any catalyst, suggesting that the catalysis by Cu- and Mnmodified samples, in fact, inhibits the styrene oxidation. Among the metal-modified samples examined, Co-g-C3N4 catalyst exhibits the best performance in the oxidation of styrene with O2. The more positive redox potential of Co3+/Co2+ (1.808 V) than that of the other metals probably is the reason, since the stability of different valence state of the metal atoms and the quantity of the electric potential may influence the catalytic activation of different metal complexes. However, there are still some exceptional cases, for example, Li-doped ZnO displays depressed selectivity for resorcinol.173 Photoluminescence and soft X-ray absorption spectroscopy analysis suggests that the surface defect caused by Li dopant can serve as the e−−h+ combination centers, and the adsorption ability as well as surface binding are also affected, which lead to the reduction of photocatalytic activity and selectivity of Lidoped ZnO materials. 2.3.2. Doping with Nonmetallic Ions. The CB level of TiO2‑xNx is enough to lead to the reduction of oxygen by photogenerated electron, but the energy of photoinduced holes from TiO2‑xNx is inadequate to oxidize OH− into •OH. This characteristic may render the TiO2‑xNx more suitable to photocatalyze the selective oxidation of alcohols to aldehydes.61 The conversion and selectivity for the oxidation of BzA to BzAD improves quickly with the enhancement in N dopant, and TiO1.966N0.034 sample displays the greatest BA conversion (ca. 20%) and selectivity (100%), which is ∼4 times higher than that of the as-prepared TiO2. The use of acetonitrile as solvent can further improve the conversion of BzA on TiO1.966N0.034 to achieve 66% and 100% conversion of BzA with >99% selectivity of BzAD at 2 and 4 h, respectively. For the oxidation of cyclohexanol to cyclohexanone, the conversion is 24% and the selectivity is >99% at 2 h. As Figure 18 shows, all TiO2‑xNx samples are visible light responsive, and after being doped with N, the TiO2 photocatalysts transform from white color to yellow and then to dark blue (the inset in Figure 18A). Compared with undoped TiO2, no obvious difference can be discerned in the measured flat-bad potential value (Efb) after Ndoping, indicating that the CB level of TiO2‑xNx is not affected by N-dopant (Figure 18B). The VB energy level of TiO2‑xNx continuously becomes more negative with the increase in N dopant, which means a worse oxidative ability of photoinduced holes on TiO2‑xNx (Figure 18C) and hence the improvement of selectivity. Figure 18D shows the electrochemical impedance spectroscopy (EIS) Nyquist plots of the ITO/TiO2‑xNx electrode, which represent the charge migration resistance and the separate efficiency of e−-h+ pair. Under visible light irradiation, the TiO1.966N0.034 catalyst with the ideal photocatalytic performance for alcohol oxidation exhibits the best separation efficiency of e−-h+ pair. It is also reported that the selective oxidation of benzyl and cinnamyl alcohols to the corresponding aldehydes is achieved with 100% selectivity on visible-light-responsive N-doped TiO2 synthesized by a sol−gel method.166

Zhang’s work indicates that the photocatalytic selectivity for MO decomposition on the N-doped rutile TiO2 nanorods is adjustable, however, the pure rutile TiO2 has no selectivity for the degradation of mixed MB and MO solution.174 When N atoms are doped into the lattice of rutile TiO2, some surface oxygen will be replaced by N to form O−Ti−N bonding modes. After the surface saturated O3c modes on the rutile (110) are replaced by unsaturated N3c, two main changes transpire. First, compared with cationic MB, unsaturated N3c preferentially bonds with the unsaturated O1c of anionic MO in water, which enhances the adsorbent selectivity for MO. Second, which is more important, unsaturated N3c can undergo photocatalytic oxidization that greatly improves the degradation rate of MO. Meanwhile, MB cannot benefit from this improvement because of the weak adsorption on the unsaturated N3c. The results illustrate that besides enhancing visible light absorption, N-doping in rutile TiO2 can modify the surface state, improve photocatalytic activity, and realize tunable photocatalytic selectivity, which has been ignored in many previous studies. Similarly, the N-doped TiO2 shows obvious photodegradation preference on RhB in the RhBphenol mixture solution,175 which may be caused by the preferential adsorption of RhB molecules; cationic −NEt2 group of RhB has strong static electron interaction with the anionic N atom on the TiO2 surface. In addition, the utilization of solar light is more favorable than UV light to improve the selectivity toward oxidation of 4-MBA to 4-MBAD.82 The poor crystallinity is helpful to obtain highly selective N-doped catalysts, and the selectivity on the sample prepared using NH4Cl as N source at low temperature can reach 90%. The N on TiO2 surface decreases the mineralization sites and enhances the reaction speed and therefore improves the selectivity. 2.3.3. Codoping. The single S-doped and codoped S/FeTiO2 catalysts shows high selectivity (>90%) toward partial oxidation of toluene to BzAD, and S additionally modifies the selectivity.176 The yield of partial oxidation increases with the enhancement of S content in both, codoped S/Fe-TiO2 and single-doped S-TiO2. The S-containing species may affect the hydrophilic/hydrophobic character of TiO2 surface and hence the adsorption of toluene on the hydrophobic sites of TiO2. It causes the difference in catalytic selectivity through the so-call “stick-and-live” mechanism, since toluene is more hydrophobic than BzAD; decrease of photoactivity with the S content is ascribed to the elimination of •OH species in the highly active (100) anatase surface at high levels of S dopant.176 The ammonia treatment favorably alters selectivity for Ti and Mo−Ti systems, while both activity and selectivity for W−Ti;62 it modifies the M (M = Mo, W) cation surface concentration (e.g., small depletion of W and enrichment of Mo) and enhances N-concentration (both at surface and bulk) in the order TiO2-N < Mo-TiO2-N < W-TiO2−N. Codoping enhances the amount of the two species but mainly alters the chemical bond of (CN)n- species to the anatase network in the W−Ti case. The change of selectivity appears to be due to the ammonia treatment and moderately dependent on the nature of the sample, although copresence of M and N decreases CO2 yield significantly; differences among samples could be thus due to the different nature of the dominant (CN)n- species. 2.4. Cocatalysts

Cocatalysts, which usually contribute to efficient charge separation and transportation, have an important effect on improving both the reactivity and stability of semiconductor 1461


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photocatalysts.177 Though Pt is the most popular cocatalyst for various photocatalysis reactions, it usually exhibits unsatisfactory performance to enhance the selectivity.91,93,103 Because of the capability of Pt metal in trapping photoinduced electrons, loading of Pt cocatalyst often leads to the generation of more strong and nonselective •OH, which is unfavorable for selective photocatalysis. By contrast, Au and carbon materials, which are relatively mild, are a better choice for tuning the photocatalytic selectivity.89,98,103,109,110,178−180 Actually, the surface plasmon resonance excitation of Au often plays a helpful role in the photocatalytic selectivity under visible light irradiation.103,180 2.4.1. Metal Loading. Metals, especially noble metals, are the most often used cocatalysts to enhance the photocatalytic activity, and endow greater promotional properties on the photocatalytic oxidation reaction.3,181−183 As the photoinduced electrons are apt to transfer from photocatalysts to metal particles and then be trapped, the surface modification using metal particles can speed up the separation of photoinduced e−−h+ pairs to improve the photocatalytic reactivity (Figure 19). Meanwhile, the deposited metal, including Au, Pt, Ag, Ir,

Figure 20. Adsorption isotherms of HQ from water on P25 (●) and 4%Au/P25 (○). Modified with permission from ref 89. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

which suggests relatively weak adsorbent−adsorbate interactions; 4% Au/P25 can adsorb less hydroquinone from a relatively low concentration. The possible reason is that the surface Ti−OH group, which can interact with HQ, is effectively covered by Au nanoparticles and amorphous layers, and consequently there is not enough Ti−OH remaining on the P25 surface to adsorb hydroquinone. When the irradiation time is relatively long, carbon species, derived from the successive oxidation of the formed HQ, also promote HQ desorption because the yield and selectivity of the product on Au/P25 is higher than those of the product on P25. Another possible positive role of Au loading could be that Au absorbs UV light via the interband transition so effectively that photon absorption by P25 decreases to generate a smaller amount of oxidants on the TiO2 surface. Additional report documents the selective aerobic oxidation of alcohols over the surface plasmon-activated Au/TiO2;187 selectivity of photooxidation of 1-phenylethanol to acetophenone is >99% and no byproduct is detected over Au/TiO2. In contrast, only 420 nm).180 Because of the outstanding association of Au NPs with S containing compounds, the supply of sufficient amount of 2mercaptopyridine is constant, which favors this reaction and apparently follows the zero-order kinetics. In addition, no formation of overoxidation product such as sulfoxide suggests the smooth diffusion of the product from the catalyst surface. The high efficiency and selectivity of this reaction can be explained within the framework of the idea of “reasonable delivery photocatalytic reaction systems (RDPRS)”.188 The reversible formation and cleavage of the S−S bond can be photocatalytically attained by the selection of irradiation wavelength; UV light (λ > 300 nm) irradiation under anaerobic conditions causes the rereduction of 2,2′-dipyridyl disulfide to 2-mercaptopyridine.180 Among the noble-metal plasmonic photocatalysts M@TiO2 (M = Au, Pt, Ag), Au@TiO2 displays a high conversion and selectivity for the catalytic oxidation of

Figure 19. Schematic description of photocatalytic processes transpiring on metal-loaded photocatalyst.

Cu, etc., possibly promotes the adsorption and activation of oxygen, which is favored by the catalytic processes involving O2.91,184−186 Au NPs on P25 can contribute to the selective oxidative conversion of phenol to hydroquinone (HQ) under solar simulator irradiation, as well as the photocatalytic oxidation of aqueous benzene to phenol.89 The amount of the surface titanol group on P25 dramatically decreases after Au is loaded, and desorption of the generated HQ is promoted from the surface which prevents the successive oxidation of the product. The atmosphere also has important effect on the yield and selectivity of HQ. In air, the formation of HQ is difficult, barely 1% Au/P25 and 4% Au/P25 under sunlight irradiation. However, when the reaction is conducted under a CO2 atmosphere, HQ yield and selectivity on Au/P25 is enhanced simultaneously; 4% Au/P25 with 90 mg of CO2 (equivalent to 173 kPa of CO2 partial pressure) affords 3% yield and 33% selectivity, which is considerably higher than those (1% yield and 9% selectivity) obtained on 4% Au/P25 in air. On the other hand, P25 gives substantially lower yield and selectivity for HQ production even under a CO2 atmosphere. It is plausible that Au NP alters the interactions between TiO2 and HQ. A comparison of the adsorption isotherm of HQ from water on P25 and that on 4% Au/P25 is shown in Figure 20. Both the isotherms are of type S on the basis of the Giles classification, 1462


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further oxidized over Au/CeO2 in water even after 20 h and the selectivity for BzAD is extraordinary (>99%) with >99% conversion. Au/CeO2 sample is found to be recyclable without loss of its selectivity and activity; i.e., BzAD is quantitatively obtained (>99% yield) in both the second and third cycle using the same Au/CeO2 sample. In addition, the Au/CeO2 exhibits specific activities for the oxidation of a variety of aromatic alcohols as exemplified in the selective formation of aldehydes (selectivity >99%) using BzA and its derivatives substituted by electron-donating groups (o-CH3, m-CH3, and p-CH3) and electron-withdrawing groups (m-Cl and p-Cl). The photocatalytic selectivity of BzA dehydrogenation to BzAD is more than 99% when the reaction is performed in a CH3CN suspension of Pt-TiO2 under deaerated conditions.72 The loading of Pt cocatalyst is indispensable for the stoichiometric conversion of BzA to BzAD; BzAD and H2 are simultaneously formed with a molar ratio of 1:1 and BzA is converted quantitatively with high quantum efficiency of 38% at 366 nm. With O2, water is generated instead of H2, and the yield of BzAD is much lower. Moreover, solvent plays significant role in the selectivity as well. When a small amount of water (1% (v/v)) is intentionally added to the CH3CN suspension of Pt-TiO2, reaction rates of simple dehydrogenation are drastically decreased; oxygen-free and water-free conditions are important for quantitative conversion of BzA to BzAD. Since no active oxygen species are formed under the conditions, undesired oxidation of BzA and BzAD does not occur, resulting in higher selectivity of BzAD. The Pt/P25 catalyst can successfully photocatalyze the oxidation of aniline (ANL) to produce nitrosobenzene with a very high selectivity (90%);190 selectivity for nitrosobenzene formation also depends on the reaction temperature as the condensation of ANL is suppressed at low temperatures. Pt/WO3 is an efficient visible-light-responsive photocatalyst (λ > 420 nm) for the selective oxidation of cyclohexane (CHA) to cyclohexanol (CHA-ol) and cyclohexanone (CHA-one) (93% selectivity);191 Pt can promote the conversion of CHA with 0.2 wt % Pt being the optimal content for activity. In the presence of Pt particles, the photogenerated electrons easily encounter the multi-

Figure 21. Schematic illustration for the aerial oxidation of alcohol on Au/P25 photocatalyst. Adapted with permission from ref 187. Copyright 2012 American Chemical Society.

benzene to phenol under visible light irradiation in water solution.103 When the Au loading is 2 wt %, 63% yield and 91% selectivity for the oxidation of benzene to phenol are attained. However, further increasing the Au content decreases the yield and selectivity to 49% and 78%, respectively. Under the same conditions, the yield and selectivity over Pt@TiO2-microspheres are decreased to 34% and 53%, respectively. In contrast, the oxidation of benzene to phenol is negligible over Ag@TiO2-microspheres. After visible light is absorbed by the Au NPs, electrons migrate from Au NPs to TiO2 particle, and the electron depleted Au oxidizes phenoxy anions to form phenoxy radicals which can oxidize benzene to phenol. Au/CeO2 is a mild photocatalyst under visible light irradiation and is not powerful enough for subsequent oxidation of BzAD to other products, such as benzoic acid;98 BzAD is not

Figure 22. Selective oxidation of BzA to benzyl aldehyde over the core−shell Pt@CeO2, yolk−shell Pt@CeO2, supported Pt/CeO2, nanosized CeO2 powder and blank-CeO2 under visible light irradiation; (b) Plausible pathway for the photocatalytic oxidation of BzA to benzaldehyde on the core− shell Pt@CeO2. Modified with permission from ref 193. Copyright 2011 The Royal Society of Chemistry. 1463


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Figure 23. Schematic illustration for the selective photocatalytic oxidation of BzA over Ir/TiO2. Modified with permission from ref 91. Copyright 2011 The Royal Society of Chemistry.

degradation and selective photooxidation of BzA to BzAD.101 The Ag-TiO2 catalysts exhibit high selectivity for deoxygenation of epoxides to the corresponding alkenes in alcoholic suspension, i.e., (2,3-epoxypropyl)benzene (EPB) to allylbenzene (ALB), though longer reaction time is required.195 Styrene oxides are reduced to the corresponding styrenes selectively with high yields (96%) as well as the deoxygenation of EPB (99%). The Ag loading is also helpful for the selective photosynthesis of quinaldines from nitroarenes.110 The photocatalytically selective conversion of aliphatic primary and secondary alcohols except for cyclohexanol is dramatically improved by depositing a tiny quantity of Cu on Nb2O5 without reducing selectivities, however, loading of Pt, Ni, Rh, Ru, and Ag does not show promotional consequence.93 Over Cu/Nb2O5, the selectivities in the oxidation of aromatic, aliphatic and heteroatom-containing alcohols are in the range of 80−99%. In contrast to aromatic alcohols, the conversions and selectivities of aliphatic alcohols oxidation are lower, and among aliphatic alcohols, the selectivity to cyclohexanone is relatively inferior. For both, aliphatic and aromatic alcohols, primary alcohols are easily oxidized than secondary ones, which is the opposite of that observed on TiO2 film. The reason may be that the radical species with different stability are generated by the immediate electron migration to affect the reactivity of various alcohols. In another investigation, multifarious transition metals are loaded on the surface of TiO2 to enhance its photocatalytic preformance.91 Among various M/TiO2 photocatalysts studied (Ag/TiO2, Au/TiO2, Pt/TiO2, Pd/TiO2, Rh/TiO2, and Ir/ TiO2), Ir/TiO2, an unusual photocatalyst synthesized by photodeposition, shows the best activity and selectivity for BzA photocatalytic oxidation to BzAD.91 Over Ir/TiO2, the average reaction rate for the oxidation of BzA is 14538 mmol h−1 g cat−1, and for α-phenylethyl alcohol is 17289 mmol h−1 g cat−1 with >99% selectivity to acetophenone. The loading of Ir particles on TiO2 can restrain the recombination of photoinduced e− and h+ in the TiO2 semiconductor, and particularly active molecular oxygen to significantly improve its participation in photocatalytic reactions (Figure 23). Loading of alloy NPs or multiple metals may further improve the activity and selectivity of photocatalysts.196−198 Addition of Sn to the Pt catalyst promotes the selectivity of unsaturated alcohols and slows the reaction sufficiently to avoid the formation of tertiary products even at nearly complete

electron reduction of O2 to form H2O and H2O2. Thus, the O2•− formation in Pt/WO3 system is reduced, leading to the inhibition of the complete photocatalytic oxidation and the ensuing high selectivity. For gas-phase selective oxidation of IPA, the loading of Pt on the Ti- and V-containing zeolites also greatly promotes the selectivity to acetone (90−96%) and the conversion of IPA simultaneously, moreover, the activity can be kept with a time-on-stream of 5 h.192 A small presence of metal content (case of Pt with weight percentage of 0.7% and 0.1% for Pt on the Ti- and V-containing zeolites, respectively) can retard electron−hole recombination, thus improving the photoactivity. On the other hand, higher contents in metal (case of Ag with over 1.5% by weight) could be detrimental to activity as the result of the metal acting as electron−hole recombination center. The core−shell nanocomposite can effectively promote the selective photoreaction of alcohol to aldehyde as displayed by visible-light-responsive Pt@CeO2 and [email protected],193,194 The Pt core can bring about the longer lifetime of charge carriers by trapping photoinduced electrons thereby improving the photocatalytic performance. The high selectivity of benzylaldehyde (100%) can be attained over the core−shell Pt@CeO2 (Figure 22) at a short time (1, 2, or 3 h), and the yield is 9 and 27 times higher than that of nanosized CeO2 and blank-CeO2, respectively. Significantly, the yolk−shell Pt@CeO2 shows much lower activity than the core−shell structure, as that the untight interfacial contact has negative effect on the photoinduced e− capturing of Pt.193 On the core−shell Pt@CeO2, O2•− is generated instead of the nonselective •OH; however, no O2•− is detected over the yolk−shell structured Pt@CeO2 possibly due to the short charge carrier lifetime. As schematically shown in Figure 22b, the photogenerated holes oxidize the adsorbed BzA to generate the corresponding radical cation which can react with O2 or O2•− to form benzylaldehyde. On the multi-Pd core@CeO2 shell catalyst, the selectivity for photooxidation of BzA, p-methyl benzyl alcohol, p-methoxyl benzyl alcohol, and p-nitro benzyl alcohol to aldehydes is 100%.99 When the strong electron withdrawing groups (EWG) are present in the aromatic ring, such as −F and −Cl, the conversion is greatly enhanced but with a concomitant loss in selectivity. The deposition of Ag particles on the surface of TiO2 (Brookite) remarkably restrains the recombination of photoinduced h+ and e−, which is good for both photocatalytic 1464


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Figure 24. Schematic profile showing the formation of the Schottky barrier, the band bending, the electron transfer under UV irradiation (a) Au/ TiO2, (b) Ag/TiO2, and (c) AuAg/TiO2; (d) the oxygen dissociation on the metal surface near the perimeter. Reprinted with permission from ref 196. Copyright 2014 The Royal Society of Chemistry.

conversion of citral.197 With the increase of the Sn loading on the catalyst, the selectivity toward nerol and geraniol is improved; however, the catalytic activity is reduced due the decreased number of active Pt sites on the catalyst. Au−Ag alloy NPs supported on TiO2 exhibit superior CH3OH conversion and methyl formate selectivity.196 The total conversion of CH3OH on Au/TiO2 is about 50%−65% and that on Ag/TiO2 is about 35%−75%, but the selectivity toward methyl formate on Ag/TiO2 is 75%−80%, higher than that on Au/TiO2 (65%−75%). The catalyst, AuAg/(1:1)/TiO2, shows the highest CH3OH conversion up to 90% and the selectivity toward methyl formate up to 85% which is due to the special structures of the catalysts (Figure 24). Au and Ag have similar atomic configuration 5d106s1 and 4d105s1. The CB or the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of Au and Ag are formed by the hybridization of s and p orbitals. Substantial charge transfer exists from the 5s state of Ag to Au, and the charge transfer from the 6s and 5d states to the 6p state in Au and that from the 5s and 4d states to the 5p state in Ag, resulting in the spd hybridization of Au and Ag. Consequently, the electronic density in the HOMO of the alloy is higher than that in pure Au or Ag. On the one hand, the separation efficiency of the e−-h+ pairs is enhanced by transferring the photoinduced e− in time from the CB of TiO2 to the metal; on the other hand, under UV irradiation the negative charge level of the alloy surface is elevated by the spd hybridization, the formation of Schottky barriers, the e− migrates from the CB of TiO2 to metal, and the interband and intraband electron transitions. By fine-tuning the structure and composition of Au−Pd metal NPs loaded on TiO2, the selectivity of

photocatalytic oxidation of benzene to phenol can be enhanced thereby restraining the degradation of benzene and generation of phenolic compounds.198 Compared to the other cocatalyst formulations (Au NPs, Pd NPs, Au−Pd NPs, and Pdshell-Aucore), the Aushell-Pdcore NPs supported on TiO2 attain the highest selectivity of phenol by simultaneously increasing the phenol evolution rate and decreasing the HQ generation rate. 2.4.2. Carbon Materials/Photocatalysts. Carbon materials can act as an e− trap to capture the e− photoexcited from the CB of photocatalysts, and thus may help in improving the selectivity.199 Different carbon-materials (graphene (GR), carbon nanotube (CNT), and C60) loaded photocatalysts are discussed in this section.97,200,201 The deposition of GR on the surface TiO2 can effectively promote and control the photocatalytic selectivity of TiO2, possibly due to the tunable adsorptive ability of photocatalysts via changing the GR loading amount.200 The principles for the adsorption of MO and MB on TiO2-GR nanocomposites are different; MB molecules possibly adsorb onto a GR moiety via a π−π interaction. When dyes are absorbed on TiO2-GR nanocomposites, the active sites for adsorption of hole and electron scavenger (water and O2, respectively) are blocked. Moreover, the adsorption affinity of O2 on GR is low, which may be adverse to the electron migration from GR to O2. The reductive ability of the electrons stored on TiO2 with high GR loading is weak, despite the fast electron mobility on GR versus TiO2. Therefore, the higher GR loading can obviously hinder the photocatalytic activity toward MO, which primarily initiated by O2•−. On the contrary, the photocatalytic degradation of MB, which mainly relies on h+ or •OH, is seldom compromised by the electron storage in GR but promoted by the GR1465


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selective oxidation of BzA to BzAD (40% conversion with >95% selectivity) are similar to other substituted benzylic alcohols at 2, 4, 6, and 8 h. The photocatalytic mechanisms over the as-prepared TiO2-carbon photocatalysts are also analogous, which involve the O2•− as the primary active species. Different from the aforementioned photocatalysts, GR in ZnS-GR nanocomposites serves as a photosensitizer rather than an electron trap, like organic dyes.97 ZnS-GR nanocomposites can photocatalyze the selective oxidation of alcohols and epoxidation of alkenes under visible light irradiation, and 5% GR loaded catalyst displays the best photocatalytic activity toward these reactions with 88−98% selectivity to target products. The oxidation of −CH2OH groups in primary alcohols is easier than that of −CC− groups in alkenes; thus, the selective epoxidation of alkenes is comparatively more difficult than the oxidation of alcohols. Xu et al. investigated the role of defect density of GR in the photocatalytic behavior of CdS-GR nanocomposites, deploying two precursors of GR namely reduced graphene oxide (RGO) with higher defect density and solvent-exfoliated graphene (SEG) with lower defect density.203 The crystallinity and light response of CdS NPs can be enhanced by introducing both GO and SEG, since they offer an excellent two-dimensional platform for the nucleation and growth of CdS nanoparticles. The as-prepared CdS-GR (RGO, SEG) nanocomposites is selective for the oxidation of alcohols and reduction of heavy ions Cr(VI). By using the SEG with low defect density as GR precursors, the photocatalytic activity of CdS-GR is extraordinary owing to its high electron conductivity. The optimum weight addition ratio of RGO or SEG is 5%, and the extra RGO or SEG plays a negative role in the conversion of BzA and selectivity to BzAD, probably due to the difficult desorption of product BzAD on high ratio of RGO and thereby bringing about the deep oxidation of BzAD to benzoic acid. The conversion of BzA on CdS-5% SEG is 67% and the selectivity of BzAD is 71%, while the results over CdS-5% RGO are 35% and 72%, respectively. Selective oxidations of alcohols to aldehydes have been systematically compared with GR/CdS and their analog (C60 and CNT)-CdS composite photocatalysts, based on a reasonable benchmark framework; GR does not show obvious salient improvement over C60 and CNT toward the selective oxidation of alcohols.204 The roles of various carbon materials in CdS-carbon nanocomposites are analogous, mainly responsible for enhancing the visible light absorption, suppressing the recombination of electrons and holes, as well as promoting the adsorption capacity. However, excessive deposition of carbon will reduce the selectivity to the target product, owing to the excellent adsorption ability of carbon species to target product of aldehyde. The optimal weight ratio of C60, CNT, and RGO in the nanocomposites is 10%, 5%, and 5%, respectively, and the corresponding conversion at 3 h is 61%, 42%, and 40%, respectively, all along with 100% selectivity. Therefore, CdS10% C60 possesses the highest activity for the selective oxidation of alcohols, although the selectivity to the aldehydes has no apparent differences. The preparative method used for RGO−Ag3VO4 nanocomposites can affect the activity and selectivity of photohydroxylation of phenol;205,206 composite prepared from ethanol as a sacrificial agent shows complete phenol conversion with 80% CAT selectivity whereas the composite prepared from methanol shows a minor decrease in the phenol conversion with CAT selectivity of 76% along with a minor

mediated rapid charge separation and the high adsorption (Figure 25).

Figure 25. Proposed mechanism for dye adsorption and the interfacial charge separation and migration on GR/TiO2. Modified with permission from ref 200. Copyright 2012 The Royal Society of Chemistry.

For the selective oxidation of alcohols, it is critical to control the content of GR to attain the excellent photocatalytic performance.202 The 5% GR loaded TiO2 displays the best activity for the selective oxidation of BzA to BzAD that the conversion is 30% with 100% selectivity at 4 h, and similarly, 5% is also the optimum content for CNT-TiO2 nanocomposites though the conversion is only 6% with 97% selectivity under the same condition. It is worth noting that the optimal amount of carbon materials toward the oxidation of BzA is different (1% for both of GR and CNT) when P25 photocatalyst is used the conversion is 14% with 91% selectivity (over P25−1%GR) and the values being 11% conversion with 90% selectivity (for P25−1%CNT), respectively. Consequently, GR is more beneficial for enhancing the selectivity than CNT. Furthermore, GR is also helpful to improve the selectivity of oxidation 3-pyridine-methanol to nicotinic acid (vitamin B3).178 Rao et al. used the doped GR as a support to improve the photocatalytic selectivity of TiO2.147 The electronic characteristics of GR can be conveniently tuned by doping with nitrogen and boron, thereby changing the effective band gap of the composites. The interaction of TiO2 NPs with B- and N-doped GR has been studied and so is the interaction of the GR with two dye molecules possessing widely different electrondonating abilities or ionization energies. As mentioned above, the ionization energy of MB (5.3 eV) is much lower than that of RhB (6.7 eV), which makes MB to function as a better electron donor than RhB. The results show that electron-poor B-doped GR/TiO2 is selective in the decomposition of MB because of the higher electron-donating ability of this dye. Similarly, electron-rich N-doped GR/TiO2 is selective in the degradation of RhB which is not a good electron donor. The electronic properties of GR support are helpful to tune the selective reaction of an electron donor or acceptor. TiO2-GR, TiO2−CNT, and TiO2−C60 show similar visiblelight-responsive activity for the selective oxidation of BzA to BzAD;201 addition of GR, CNT, and C60 can promote the visible light absorption ability and the separation efficiency of the photogenerated e−-h+ pairs in a similar manner. The optimum content of GR, CNT, and C60 is 0.1%, 0.5%, and 1.0%, respectively, and their catalytic performance for the 1466


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leaching of V5+. As to composites prepared from IPA and EG, the phenol conversion is 40% and 20%, respectively, with higher HQ selectivity, which is due to the nonformation of composite in the case of IPA and EG. Hence, the moiety containing more of Ag3VO4 forms HQ in larger quantity compared with CAT. In contrast to anionic dyes, the degradation of cationic dyes is faster, and additionally in the mixed dye solution, the degradation of cationic dyes is twice as that of anionic dye. The mechanism of the degradation process is shown in Figure 26, with MO + rhodamine 6G (Rh6G) as a

over the 1D CdS@TiO2 CSNs is 97%, 65%, and 74%, respectively, while over CdS NWs it is only 97%, 36%, and 60%; comparable activity trend also can be observed toward other alcohols, such as 4-fluorobenzyl alcohol, 4-nitro benzyl alcohol, and 4-chloro benzyl alcohol. It is worth noting that the mechanisms for photocatalytic oxidation of alcohols on 1D CdS@TiO2 CSNs and CdS NWs are different, since the TiO2 shell can also block the photoexcited holes from the CdS core. The reaction over 1D CdS@TiO2 CSNs is mainly motivated by the photoinduced electrons and O2•−, and the longer lifetime of photoexcited e−−h+ pairs from the 1D CdS@TiO2 CSNs than that from the original 1D CdS NWs should be responsible for the enhanced photocatalytic performance. In another case, CdS/TiO2 photocatalyst displays visible-light-induced photocatalytic selectivity (>99%) for the dehydrogenation of benzyl alcohol to benzaldehyde accompanied by the generation of H2, and the activity can be improved by using Pd cocatalyst.209 MoOx/TiO2 photocatalyst is found to be selective for the oxidation of cyclohexane to benzene in the presence of gaseous oxygen at temperature of 308 K under UV illumination, and increasing molybdenum loading results in higher benzene selectivity;105 cyclohexane conversion of 15% and maximum selectivity of 65% for benzene are achieved at 308 K on 8 wt % MoOx/TiO2 under irradiation in a continuous flow reactor. Nevertheless, when MoOx is supported on α- or γ-alumina, they promote only the formation of CO2 as the main byproduct. TiO2 exhibits high activity of total oxidation to CO2 but is not selective for conversion to benzene; polymolybdate species appear to be responsible for this change in selectivity of TiO2, favoring the formation of benzene. Titanate nanotubes supported Ag/AgI photocatalysts display outstanding selectivity (>94%) for the oxidation of benzylamine to corresponding imine, and the conversion rate can reach up to 95%.210 The photocatalytic activity of VOx/TiO2 could be improved in the selective oxidation of ethanol to acetaldehyde by simultaneously using internal light source (phosphorescent particles) and external light source (UVA-LEDs); phosphors can be motivated by external UVA-LEDs and emit light close to the catalyst.211 The photon transfer is promoted by adding phosphors light source; the obvious quantum yield is improved from 2 to 30% together with a high photoreactivity, and the selectivity for acetaldehyde remains high as well, about 97%, along with ethylene and CO2 as the byproducts. The selective oxidation of ethanol to acetaldehyde is influenced by the V loading;212 discriminating sites are related to surface polymeric vanadates possessing Ti−O−V and V−O−V functionalities with 5 wt % V2O5 content catalyst displaying the highest photocatalytic selectivity. Photocatalytic decarboxylation of RhB can selectively occurs on ZrO2 -incorporated TiO 2 (ZIT);213 optimal incorporation amount of ZrO2 in ZIT for photodegradation of RhB is Zr/Ti = 15.94%. The selective photodegradation of RhB occurs via two routes, namely N-deethylation and decarboxylation in an anatase-structured TiO2 domain. Magnetic TiO2-guanidine-(Ni,Co)Fe2O4 nanocomposites have been reported with excellent selective transformation of 2-hydroxy-butandioic acid (malic acid) to C1 and C2 chemicals in aqueous solution; selectivity to HCOOH could be achieved ∼80% at less than 2 h.214 The ternary CdS−GR− TiO2 composites display higher photocatalytic activity than binary CdS−5% GR nanocatalyst for the selective oxidation of BzA to BzAD.215 The CdS−GR is the foundation of the ternary CdS−GR−TiO2 composites for the selective oxidation of alcohols, while the existence of TiO2 can tune the energy band,

Figure 26. Mechanism for degradation of dye over RGO−Ag3VO4 using a mixed dye solution (Rh6G: red and MO: pink) under visible light illumination. Modified with permission from ref 205. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

model combination; efficient removal of dyes and mixture of dyes are ascribed to the synergic control of RGO-mediated substrate adsorption and charge separation kinetics. In2S3-GR nanocomposites, with varying amounts of GR, show superior photocatalytic activity than blank In2S3 toward the selective oxidation of BzA.207 In2S3-1%GR exhibits best photocatalytic performance among all of the studied nanocomposites, the conversion of BzA being about 55% with 99% selectivity. However, both of the conversion and selectivity are decreased with 2% ratio of GR, because the excessive GR can block the light absorption of photocatalyst and retard the surfacial desportion of target product aldehyde, similar to the reasons mentioned for other carbon loaded photocatalysts. 2.5. Semiconductor Composites

Different types of composites for selective transformation are covered in this section with the associated mechanistic details. By forming composites, the lifetime of photoexcited charge carriers may be prolonged, and the photoresponsive range could be broadened by combining different electronic structures, thus culminating in the improvement of both, the conversion and yield.208 The selectivity and activity of 1D CdS nanowires (NWs) for the oxidation of alcohols to their homologous aldehydes under visible-light irradiation, can be improved by forming 1D CdS core@TiO2 shell (CdS@TiO2 CSNs) semiconductor nanocomposites.94 For example, the conversion for BzA, 4-methoxyl benzyl alcohol and 4-methyl benzyl alcohol over the 1D CdS@ TiO2 CSNs at 8 h is 34%, 39%, and 41%, respectively, which is better than the results acquired on CdS NWs (13%, 21%, and 7%, respectively). The selectivity of corresponding aldehyde 1467


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Figure 27. Proposed reaction pathways for the trans-ferulic acid photooxidation. Modified with permission from ref 113. Copyright 2015 Elsevier.

higher than 86%. Highly dispersed TiO2 NPs in SiO2 matrix (denoted as TiO2/SiO2) show high selectivity for the epoxidation of styrene.218 Compared with P25 TiO2, the further oxidation of styrene oxide into CO2 is greatly restrained on TiO2/SiO2, possibly is due to the surface features of TiO2/ SiO2 nanoparticles. A lot of oxygen vacancies are discovered on the surface of TiO2/SiO2 nanoparticles by EPR analysis. The active vacancy can capture one electron to generate an F-center, which may function as the catalytic active site and prefer the selective oxidation of styrene into styrene oxide rather than BzAD and CO2; F-center sites have a great influence on the product distribution and catalytic activity. With increasing Fcenter sites, the photocatalytic activity of TiO2/SiO2 NPs is enhanced, and more styrene oxide and less CO2 are produced, which indicates that F-centers favor selective epoxidation of alkanes rather than mineralization. The TiO2/SiO2 samples also show selectivity for the photoepoxiation of propene to propene oxide (PO);219 samples with less Ti content are more selective. When the content of Ti is 0.08%, the selectivity of PO is 60%, while it is 6% with 8.3% of Ti. However, the conversion of propene decreases with the increasing of Ti content. When the Ti content is low, the isolated TiO4 units do exist, which are responsible for propene photoepoxidation to PO. With increasing Ti content, one-dimensional polymerized TiO4 units and two-dimensional polymerized TiO5 units would be formed, followed by the formation of small particles comprising TiO6 units, resulting in the particle size enhancement; aggregated TiO2 species are responsible for propanal, ethanal, and COx production. The selective photocatalytic oxidation of benzene to phenol is attained on TiO2 combined with hydrophobic mesocellular siliceous foam (MCF).220 TiO2 NPs entrapped in MCF (TiO2@MCF) have been prepared and silylated with triethoxymethylsilane for hydrophobic modification (TiO2@MCF/CH3), and the grafted CH3 functional groups can be partly removed using UV irradiation to obtain TiO2@MCF/CH3/UV, as schematically illustrated in Figure 28. Both the adsorption of reactants (benzene) and desorption of the target products (phenol) on the surface of

promote the surface area, benefit the electron transfer, and therefore prolong the lifetime of photoexcited charge carriers. Trans-ferulic acid can be selectively oxidized to vanillin in water by using various TiO2 and WO3-loaded TiO2 samples as catalysts via parallel reaction routes, as illustrated in Figure 27.113 One route is the mineralization to CO2 and H2O, owing to the nondesorption of formed intermediates in the bulk of the solution. Other pathways involve the partial oxidation of transferulic acid to stable intermediates that can desorb in solution. Higher selectivity values are obtained by impregnation of TiO2 with H2WO4 followed by calcination; selectivity for vanillin increases with the enhancement of WO3 content, and the best selectivity, 25%, is obtained with the 1%WO3/TiO2 sample; in contrast, the selectivity for vanillin is only 10% over the commercial TiO2 (Merck). Higher WO3 loadings, however, result in decrease of the catalytic activity. The further oxidation of the aldehyde is restrained on WO3-loaded TiO2 powders because of the practically inactive WO3 on the TiO2 surface, which possibly is responsible for the increased production of vanillin. When TiO2 is supported on an inert material without photocatalytic activity, the mechanism to improve photocatalyic selectivity is different; TiO2 pillared clays shows higher selectivity to oxygenate than TiO2 (Degussa P25) with the product distribution among oxygenates being dictated by the type of the clay host, indicating that the photocatalytic activities of TiO2 depend strongly on the type of clay host. For the cyclohexane photooxidation, TiO2 pillared clay shows much higher selectivity than TiO2 alone, possibly due to the hydrophobic property of pillared clay.216 The presence of SiO2 can change the surface electric charge of TiO2, as a result, RhB adsorbs on the TiO2/SiO2 surface by the positively charged diethylamino group, whereas it adsorbs through the negatively charged carboxyl group on the TiO2 surface under the experimental conditions (pH ∼4.3).217 On TiO2/SiO2 particles, RhB initially undergoes a stepwise selective deethylation process before the chromophore structure is destroyed; average yield of the each de-ethylation step is 1468


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Figure 29. Plausible mechanism of the selective oxidation of alcohol and the reduction of NBz in N2 atmosphere. Printed with permission from ref 221. Copyright 2014 Elsevier.

Figure 28. Schematic illustration for the hydroxylation of benzene on TiO2@MCF. Modified with permission from ref 220. Copyright 2011 Elsevier.

for the RhB degradation. The cocatalyst MnOx can be used to enhance the transfer of the photoinduced h+, and the separation efficiency of the photoinduced e− and h+ of BiOI culminating in outstanding photocatalytic performance for the degration of MB. However, for Pt-BiOI, the e− transfer to Pt and reacts with O2 to produce O2•−, but O2•− cannot degrade aromatic compounds (such as RhB) efficiently. Consequently, BiOI is selective with deriving hole-type cocatalysts in enhancing the photocatalytic activity. NiO/Ag3VO4 is found to exhibit high selectivity for azodyes photooxidation under visible light and UV illumination, which is independent of the excitation wavelength.223 Oxygen plays an important role in trapping photogenerated e− in NiO/Ag3VO4 systems to afford O2−• radicals, which are the major active species causing the selective photooxidation of azodyes. With various loading amounts of V2O5 on SiO2, the selectivity for conversion of propene into PO and propanal (PA) over V2O5/SiO2 are different;224 decrease in the loading amount increases the PO selectivity, while the PA selectivity decreases. The highest PO selectivity (43%) is observed over 0.18 wt % loaded V2O5/SiO2 catalysts. A small amount of PO is formed over highly loaded V2O5/SiO2 even at 10 wt % in a flow reactor. To some extent, the transient interaction between the substrates and catalysts in the flow reactor system may prevent the PO from decomposing into aldehydes over a highly loaded catalyst. However, the sums of the selectivities into PO and PA at steady state are similar, about 35−40 C% for V2O5/SiO2 with each loading, indicating that PA is formed as a secondary product via PO. The monomeric VO4 tetrahedral species distributed on SiO2 produces PO using UV as light source, while PO is isomerizated into PA by using less dispersed V2O5 species on SiO2. For the selective photocatalytic oxidation of propylene to propenal, V2O5/Al2O3 shows high selectivity in contrast to V2O5/SiO2, indicating that the nature of Al2O3 surface influences the photocatalytic properties.171 In addition, V2O5/Al2O3 exhibits the high selectivity to partial photooxidation of cyclohexane to cyclohexanol and cyclohexanone in liquid phase;225 selectivity to the partial oxidation compounds (cyclohexanol and cyclohexanone) is 91% and the ketone/ alcohol ratio (K/A ratio) is 1.7 after being photoirradiated for 24 h. By prolonging the photoirradiation time, cyclohexyl hexanoate as a byproduct is generated and the K/A ratio decreases gradually.

photocatalyst can be accelerated via adjusting the hydrophobicity of MCF. TiO2@MCF exhibits the higher selectivity of phenol than TiO2, indicating that the hydroxylation of benzene is more favored than the further degradation of hydroxylated benzenes within the MCF cage. In another case, TiO2 within the prepared mesoporous metal organic framework (MOF) is selective for the oxidation of benzylic alcohols with moderate to high yields using sunlight as light source.177 Over 25Ti@SHK2, 50Ti@SHK2, 70Ti@SHK2, and 85Ti@SHK2, the yield of 4-methyl benzaldehyde is 21, 35, 89, and 63%, respectively, with high selectivity (>93%). The peroxide formation under photocatalytic conditions is the chief pathway in the current system that brings about the selective oxidation process. For both, the selective oxidation of BzA to BzAD and the reduction of NBz into ANL, the CdS/g-C3N4 exhibits higher conversion and yield than both blank g-C3N4 and CdS under visible light illumination.221 With the optimal amount of CdS, 10%, the conversion and selectivity for the oxidation of BzA to BzAD is about 48% and 94%, respectively, and those for the reduction of NBz into ANL are about 49% and 53%, respectively. If the use of CdS in CdS/g-C3N4 is excessive, aldehyde might be not be able to desorb from the surface of catalyst, since the excellent adsorption ability of CdS. Figure 29 displays the proposed mechanism for the photocatalytic selective oxidation of alcohols and the reduction of NBz by CdS/g-C3N4 photocatalyst in N2 atmosphere. Under the visible light irradiation, C3N4 and CdS can be motivated to create e− and h+, and e− in the CB of C3N4 can immediately transfer to the CB of CdS. Meanwhile, photogenerated h+ in the VB of CdS transfers to the VB of C3N4. In the N2 atmosphere, the oxidation of NBz is inhibited, and thereby the reduction is more likely to generate ANL, which involves a photoexcited sixelectron reduction process. Concomitantly, the existence of alcohols can cause the quench of h+ and hence the generation of aldehydes. As-prepared MnOx-BiOI exhibits higher visible-light-responsive photoactivity than BiOI and Pt-BiOI for the decomposition of RhB dye.222 The main active species of BiOI are photoinduced h+, rather than •OH and O2•−, under visible light illumination. The deriving-hole-type cocatalyst MnOx may be more effective than the deriving-electron-type cocatalyst Pt 1469


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Figure 30. Proposed adsorption state of (a) benzene, (b) chlorobenzene, (c) toluene, (d) phenol, (e) anisole, and (f) NBz on TiO2 surface. Reprinted with permission from ref 227. Copyright 2011 American Chemical Society.

2.6. Surface Treatment

in photodegradation due to the MRS. Moreover, the selectivity is expected to further enhance while using smaller domains, such as on powders, where the diffusion length is shorter. However, since the MRS is organic in nature, proper precautions have to be observed to prevent the degradation of the MRS.42 By introducing 2-hydroxyethanesulfonic acid sodium salt and 4-(tributylammonium-methyl)- benzyltributylammonium chloride to the surface of TiO2 NPs, the photocatalyst with MRS is constructed which shows the selective recognition ability for target molecules, and the hybrid photocatalysts showed selective degradation performance to saxitoxins, a class of notorious neurotoxins.228 Molecular imprinting technique has been explored as a new method for the predetermination to design specific MRS.49−53,229−231 Figure 31 displays a typical route for the

Photocatalytic reactions always take place on the surface of photocatalyst, therefore, the selective adsorption ability of photocatalyst toward reactants and products is quite crucial to the selective photoreaction, a thought that is mostly ignored for a long period. It is viable to promote the selectivity by surface adjustment, e.g., transforming the surface charge/functional group, and constructing specific structures.200 Actually, the weaker interaction between target product and photocatalyst benefits the selective reaction, because the desired products can easily desorb from the photocatalyst surface once reactant-toproduct transformation is finished, which has been proved not only by density functional theoretical calculations but also experiments.48,90,149,226 Yoshida et al. have revealed the adsorption state of benzene derivatives on TiO2 by solid-state nuclear magnetic resonance spectroscopy.227 All of the carbons of adsorbed benzene derivatives exhibit each chemical shift higher than the corresponding carbons of the molecules in the CCl4 solution, indicating that the electrons of the adsorbed molecules are withdrawn by TiO2. For the adsorbed benzene, the aromatic ring faces parallel to the TiO2 surface and is mobile, whereas in benzene derivatives they are inclined at various angles to the TiO2 surface (Figure 30). 2.6.1. Molecular Recognition Site (MRS). Recently, creation of MRS has been proposed for the enhancement of photocatalytic selectivity, due to the preferential decomposition of targeted substrate.41,42,49−53 The adsorption of reactants in the surrounding area of photocatalytic sites may increase photoefficiency. The MRS is supposed to selectively adsorb the target molecules and with their aid the photocatalysis may selectively remove highly toxic organic pollutants. Usually, surface complexes can be formed as reactants interact with the active sites and/or surface •OH groups, which has a significant effect on the photocatalytic conversion and selectivity.48,90,149 Paz et al. chose thiolated β-cyclodextrin (TCD) as the MRS, based on the affinity between TCD and hazardous contaminants, which is related to the well-defined cavity found in this cyclic glucose oligomer facilitating the hosting of molecules such as 2-methyl-1,4-naphthoquinone or p-benzoquinone.41,42 Because of the affinity between the MRS and the target molecules (e.g., 2-methyl-1,4-naphthoquinone), the target molecules adsorb on the MRS, from where they surface diffuse to the photocatalytic sites. Without TCD, the ratio between the relative degradation of 2-methyl-1,4-naphthoquinone to that of benzene is 0.75, however, in the presence of TCD, the ratio is dramatically improved to 8.11, demonstrating the enhancement

Figure 31. Typical route for preparation of MIP-coated photocatalyst and its utility in the photocatalytic decomposition; 4-CP is used as a representative of the target pollutants. Modified with permission from ref 52. Copyright 2007 The Royal Society of Chemistry.

preparation of molecular imprinted polymer (MIP)-coated photocatalyst and its photocatalytic decomposition.52 First, a precursor is prepared via a reaction of monomer in excess, e.g., o-phenylenediamine, and the target compound, e.g., 4chlorophenol (4-CP), as template. The existence of the precursor is assumed only to represent the strong interactions between the monomer building blocks and template molecules. Then, a MIP layer is coated on photocatalyst particles via an in situ polymerization and finally, the template molecules are 1470


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inant decomposition with the decrease in original concentration. Since the MIPs layer possesses a basic structure as organic polymer, the selectivity of MIP-coated TiO2 might be slowly deteriorated during a long period of UV-light illumination. To increase the stability of the nanocomposite under UV irradiation, inorganic molecularly imprinted polymers (IMIPs) coated TiO2 photocatalysts have been investigated.233 By using diethyl phthalate (DEP) as the template, the inorganic MPSi/ alumina MIPs coated TiO2 photocatalysts are prepared and their relative selectivity is evaluated by defining α as the ratio of the value of kDEP/kphenol on IMIP-P25 to that on NIP-P25; the large value of α (8.67) confirms that the inorganic molecular imprinting heightens the selectivity of marked contaminant. When the level of phenol is increased to 50 mg L−1, the relative selectivity of IMIP-P25 is further increased thus confirming that IMIP-P25 also has high selectivity in the photocatalytic removal of target contaminants at low concertration from the mixture comprising other pollutants at high concertration. Besides, the magnetic and dual conductively imprinted photocatalysts exhibit significant selectivity for the degradation of 1-methylimidazole-2-thiol.177 A Cu2+-doped 2,4-DCP molecularly imprinted TiO2−SiO2 nanocomposite catalyst not only has good capacity but also favorable selectivity toward the decomposition of the target contaminant (2,4-DCP) when admixed with phenol.234 The distinction of molecular structure between the objective and nonobjective contaminants leads to the selectivity, because of the molecularly selective pores in the Cu2+-doped 2,4-DCP imprinted catalyst. In contrast, NIP TiO2−SiO2 composite does not possess such special cavity. Compared with NIP/TiO2, RhB-MIP/TiO2 not only exhibits high adsorption capacity and excellent selectivity toward RhB but also can improve photocatalytic selectivity;235 selectivity coefficient for RhB relative to Rh6G being 2.99. MIPRhB-PPy/ TiO2 also has excellent recognition selectivity for RhB;236 apparent rate constant (k) for the photodegradation of RhB over MIPRhB-PPy/TiO2 is 3.6 times of that over NIP nanocomposites NIP-PPy/TiO2. The imprinted cavities have been created in MIPRhB-PPy/TiO2 by using template RhB in polymerization process, and the binding abilities of MIPRhBPPy/TiO2 for RhB is found to be much stronger than that for Rh6G and MB; selectivity coefficients of MIPRhB-PPy/TiO2 for RhB relative to Rh6G and MB are higher, 1.75 and 2.29, respectively, while those of NIP-PPy/TiO2 for RhB relative to Rh6G and MB are lower, 0.96 and 0.94, respectively. The selectivity can be enhanced through the functionalization of materials with organics.111,114,237−239 By coupling with aldehyde, mesoporous graphitic C3N4 (mpg-C3N4) shows excellent performance in the selective oxidation of methyl phenyl sulfide (MPS) and other substituted aromatics sulfides to sulfoxides.111 With neat mpg-C3N4, the oxidation of MPS occurs at room temperature under visible-light irradiation with a notable selectivity (99%) for methyl phenyl sulfoxide (MPSO) but with only moderate conversion (8%). The selective oxidations of various sulfides by mpg-C3N4−IBA (isobutyraldehyde) system are summarized in Table 3; in 100 mol % concentration, the conversion of MPS oxidation is 51% with MPSO as the sole product. When the use of IBA is increased 2-fold (200 mol %), the conversion and selectivity for the oxidation of MPS to MPSO at 4 h is 97% and 98%, respectively. With more IBA, 300 mol %, the main product is changed from MPSO to methyl phenyl sulfone, and the

removed from the polymer layer. Compared with the nonimprinted (NIP) photocatalyst, the molecular imprinted photocatalyst (NaCl/TiO2) exhibits higher selectivity and photodegradation capacity for ciprofloxacin; degradation ratio of ciprofloxacin could reach 70.9% in 60 min under visible irradiation.229 Coating a thin layer of MIP significantly increases the specific recognition and the photocatalytic selectivity of the targets.49−53 When a suitable functional monomer is polymerized along with TiO2 NPs and target molecules, the MIP can be coated on TiO2 photocatalyst.51 Compared with the NIP TiO2 film (NITiF), the MIP coated TiO2 film (MITiF) exhibits the improved decomposition of target contaminants due to the enhancement of adsorption ability toward the objective molecule. The value of the apparent reaction rate constant on the MITiF is obtained from the Langmuir−Hinshelwood model, which is much higher (more than 7 times) than that on the NITiF. Because of this high affinity, the MITiF exhibits special molecular recognition ability, causing the selective adsorption and photodecomposition of the aimed contaminant. By using 2-CP, 2-NP, and 4-NP as templates, photocatalysts 2CP-P25, 2-NP-P25, and 4-NP-P25 have been prepared, which show high selectivity for the photocatalytic decomposition of 2CP, 2-NP, and 4-NP, respectively.52 The rate constant on the MIP coated TiO2 is obviously higher than that on neat TiO2; for the decomposition of 2-NP and 4-NP (1.8 mg L−1), the rate constant is 10.73 × 10−3 and 7.06 × 10−3 min−1 on MIP/TiO2, but on neat TiO2 it is only 4.36 × 10−3 and 1.53 × 10−3 min−1, respectively.232 As the concentration of the objective molecules is reduced and/or as difference in chemical configuration and molecule size between the objective and other molecules becomes larger, the photocatalytic selectivity can be improved, resulting from the outstanding interaction between the objective molecules and MIP by the functional groups (−OH and -NO2). In addition, by using a suitable transition state analog (TSA) (i.e., 2-NP, 4-NP) as the template, the apparent activation energy declines over the TSA-MIP coated TiO2 photocatalysts (i.e., 2-NP-P25, 4-NP-P25), thereby enhancing the photocatalytic decomposition of the target NBz with or without the existence of nontarget pollutants. Compared with neat Degussa P25 TiO2, the obtained TSA-MIP-TiO2 not only increases the photocatalytic degradation of NBz, but also inhibits the accumulation of unwanted intermediates. As the use of nontarget pollutant bisphenol A (BPA) is 225 μmol L−1 (15 times of the addition of NBz), 2-NP-P25 and 4-NP-P25 show an individually selective factor of 5.2 and 4.4 for the photocatalytic degradation of NBz, which is about 4.4 and 3.6 times that over neat P25 TiO2, respectively, though the BPA degradation over TSA-MIP-TiO2 is slower than that over neat P25. These results confirm that the choice of an appropriate TSA of the target pollutant as the template to prepare MIP coated photocatalysts provides an alternative approach to selectively mineralize the target pollutant that cannot be directly used as template due to the low solubility of very toxic pollutants.53 MITiF exhibits high selectivity (3.24) for the degradation of salicylic acid (SA) in the mixture of SA (2 or 25 mg L−1) and phenol (25 mg L−1) and the degradation rate of SA is twice as high as that of the nontarget phenol, and the selectivity of MITiF for SA is 3.24 when the use of SA is 25 mg L−1. Moreover, the selectivity of SA can reach to 6.16 when the SA concentration is reduced to 2 mg L−1, which is another illustration for the selectivity enhancement of target contam1471


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Table 3. Oxidation of Various Sulfides by mpg-C3N4−IBA Systema

A combination of g-C3N4 and N-hydroxy compounds, namely 1-hydroxybenzotriazole (HBT), N-hydroxysuccinimide (NHSI), and N-hydroxyphthalimide (NHPI), can be used for the selective allylic oxidation of cholesteryl acetate (CLA) to 7ketocholesteryl acetate (7-KOCA), a key step in the production of vitamin D3.114 When the g-C3N4/HBT system is used under visible light irradiation, the conversion of CLA is 23% with 61% selectivity to 7-KOCA, while neat g-C3N4 or HBT is not impactful, implying the synergistic role of g-C3N4 and HBT. As HBT is replaced by NHSI, the selectivity (96%) is promoted with similar conversion (19%). The combination of g-C3N4 and NHPI delivers the best performance when 48% conversion and >99% selectivity is obtained. In the reaction process, O2 is first reduced by photogenerated e− to form active O2•−, and then the HOO− is generated after O2•− loses a hydrogen, which can react with hole to achieve the conversion of > N−OH to > N− O•. Afterward, hydrogen is abstracted from RH by the > N− O• radical to generate R• following a free radical route, which is a significant step for the whole catalytic reaction. In the presence of O2, R• is rapidly captured by O2 to form a ROOH radical, which can decompose to afford the final products on gC3N4. Furthermore, g-C3N4/NHPI system can also catalyze the oxidation of phenylethylene, α-isophorone, α-pinene, and ethylbenzene with conversion rate of 26−71% and a good selectivity to ketones (66−84%). The oxidation of βisophorone is quick and the conversion can reach to 100% with extraordinarily high selectivity (>99%) of keto-isophorone at 4.5 h. The oxidation of toluene to BzAD has >99% selectivity as well. The modification of poly vinylidene-fluoride (PVDF) changes the surface property of TiO2, and PVDF/TiO2 shows higher photocatalytic selectivity than TiO2 for the degradation of MB in the MB solution admixed with MO.237 The constants k of MB/MO is 10.5 over PVDF/TiO2 (10%), which is about 26 times higher than that of TiO2. The reason is that the adsorption ability of MB on TiO2 becomes greater after being modified by PVDF. By tuning the PVDF amount, the ratio of selectivity for MB to that for MO over all PVDF/TiO2 photocatalysts can be adjusted and is 0.4, 3.5, 5.5, 10.5, and 11.1 over TiO2, PVDF/TiO2 (3%), PVDF/TiO2 (5%), PVDF/ TiO2 (10%), and PVDF/TiO2 (15%), respectively. The surface silylation is helpful to improve the selective photooxidation rate of cyclohexane on anatase TiO2.238 Both of the active OH sites on surface and desorption rate of reactant play very important roles in the generation rate of product. When the Si amount is less than 1.0 wt %, the generation of cyclohexanone decreases, which is attributable to the varied desorption ability and the declined available OH caused by silylation. However, when the Si amount is more than 1.0 wt %, the effect of increase of desorption rate exceeds that of decrease of the OH availability. Due to the promoted desorption, the generation rate of surface deactivating carbonate and carboxylate species on TiO2 declines because of silylation. Importantly, the selectivity toward cyclohexanone formation is similar, ranging between 95% and 97%, suggesting that the silylation does not affect the reaction mechanism. The oxidative selectivities for alcoholic substrates to aldehydes are high (>96%) in the Vis/TiO2/dye-sensitized system which is not impacted by solvents.240 In the dyesensitized system, the dye, rather than the TiO2 photocatalyst, absorbs visible light and is excited. The photoexcited electron transfer from the excited dye molecule to the CB of TiO2

a

Reaction conditions: substrate (1 mmol), mpg-C3N4 (50 mg), isobutyraldehyde (2 mmol), CH3CN (3 mL), O2 (1 atm), 25 °C, light source: a mercury lamp (150 W) together with a 420 nm cutoff filter. Reprinted with permission from ref 111. Copyright 2012 The Royal Society of Chemistry

selectivity is 92%. A plausible mechanism is illustrated in Figure 32. O2 is photoreduced to generate active species O2−•, which can attack the S atom in MPS to generate intermediate persulfoxide or thiadioxirane and then form the final sulfoxide product.111

Figure 32. Plausible pathway for the selective oxidation of MPS over mpg-C3N4. Modified with permission from ref 111. Copyright 2012 The Royal Society of Chemistry. 1472


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interlayer space with hydration depending on layer charge density, and Na+ has greater effect on the degree of hydration than Li+, while the pristine K+-type titanate is barely hydrated. Thus, the composition of MxTi2‑x/3Lix/3O4 (M = K+, Li+, and Na+) is adjustable to control its protuberance in water, and therefore titanate can act as molecular sieves to bring about selectivity for photocatalytic degradation. The titanate can selectively adsorb benzene from the mixture of benzene, phenol, and C4P in water. Consequently, benzene can be selectively degraded and the efficiency depends on the degree in the enlargement of the titanate. Mul et al. have prepared Ti3+-containing blue TiO2, which comprises 85% rutile and 15% anatase with the presence of surface Ti3+ in an estimated amount of 50% of all surface sites which shows higher selectivity for the production of ketones than P25 and rutile; phenomena relates to a high concentration of surface defect sites (Ti3+) and a relatively low but still significant affinity for water.242 For the photodegradation of cationic crystal violet and anionic MO, the photocatalytic selectivity to the crystal violet can be 100% over the novel 2Dordered dome films with abundant oxygen vacancies or Ti3+ ions; dye photodegradation on the 2D-Ag-TiO2 and 2D-TiO2 films follows the photosensitization mechanism.243 The dye, instead of the TiO2 photocatalyst, is excited by light irradiation. The excited dye molecule transfers electrons into the CB of TiO2, while the dye itself is converted to its cationic radical form. The injected electrons react with O2 adsorbed on the surface of TiO2 to generate active oxygen species (e.g., O2•−), which leads to the degradation of the dye and the cationic dye radical. As mentioned earlier, the high selectivity is ascribed to the high adsorption selectivity and the less detectable •OH in the process of the photocatalytic reaction, which is most probably due to oxygen vacancies or Ti3+ ions in the 2D-AgTiO2 and 2D-TiO2 films. Li et al. have deliberated that the reaction mechanism in dry organics involves the generation and transfer of organic peroxide, which is different from the •OH mechanism in water. The O2 in dry organics is reduced by e− generated from TiO2 at the Surf-Ti3+ sites under UV irradiation and then is transformed into active oxygen species (i.e., organic peroxide). Thus, in dry organic phase, cyclohexane is oxidized into cyclohexanone and cyclohexanol at Surf-Ti3+ sites by the active oxygen species.129,130 When the TiO2 is prepared in HF solution, the surface of TiO2 is fluorinated and NaOH washing causes the conversion of abundant Ti−F species present on the facets into Ti−OH species. The surface rehydroxylation seems to promote the selective adsorption of MB and methyl violet (MV) molecules.43,44 Consequently, the selective photocatalytic degradation of MB and MV can be tuned by simply modifying the surface charge of the TiO2. The photodegradation process for MO is faster than that of MV and MB over the as-prepared TiO2 samples. This is because the negatively charged MO molecules are preferentially adsorbed on the fluorinated surface of positively charged TiO2, which favors the degradation of negatively charged contaminants. By contrast, after the TiO2 samples are modified by NaOH washing, they exhibit much better selectivity toward photodecomposition of MV and MB due to the preferential adsorption of positively charged MV and MB on the hydroxylated surface of negatively charged TiO2. When the NaOH-treated TiO2 films are treated again by NaF or HNO3 solution, resulting in the positive charge on the TiO2 surface; reversal in the photodegradation process occurs by faster decomposition of MO in comparison to MV and MB. In

generates dye radical which can selectively oxidize the alcohols to aldehydes, and itself being reduced to dye. When MB is used as a structure-targeting agent in the synthesis of ZnO, ZnO/MB hybrid thin films can be obtained.239 After extraction of MB by dipping the hybrid film in a dilute base solution, the resulting porous film exhibits satisfactory photocatalytic activity to degrade MB and eosin Y (EY); photodegradation apparent rate constant for MB is almost 5.5 times higher than that of EY, due to the synergetic effect that porous ZnO film obtained by extracting MB adsorbs MB more favorably than EY. A suitable and compact adsorption promotes the photocatalytic selectivity. Moreover, the addition of MB promotes the crystal growth of ZnO and thus affects the crystallographic orientation and the surface morphology of the hybrid film; electrodeposited hybrid thin films show porous morphology and hexagonal wurtzite crystalline structure. These facts can shed some light on the application of this technology for the selective oxidation. 2.6.2. Ion Modification. By modifying the surface with ions, including metal ions (e.g., Fe3+) and nonmetal ions (e.g., F−), selectivity can be enhanced via modulating the adsorption of the contaminants on the photocatalyst’s surface.41 Visible-light-responsive activity of the surface-modified TiO2 loaded with several metal ions such as Fe, Ag, Ce, Pt, or Cu, for the oxidation of BzA, have been studied.96 Among the metal ion-modified TiO2, the Fe (III) ion-modified TiO2 exhibits the highest photocatalytic performance with the selectivity (>99%) to BzAD. The BzA can interact with the •OH groups on the TiO2 surface to generate alcoholate species which lead to the original visible-light response of photocatalyst. The key factors for generating such a highly active photocatalytic system are considered as follows: (1) the efficient electronic transitions from the energy levels constructed by the O 2p orbitals of the alcoholate species hybridize with the occupied Fe 3d orbitals to the CB of the TiO2, (2) electron transfers to the unoccupied Fe 3d orbitals as the acceptor levels accompanied by O2 reduction with two and/or four electrons (Figure 33). Such enhancement

Figure 33. Possible electron transfers for the oxidation of benzyl alcohol on the Fe3+-modified TiO2 photocatalyst under visible-light illumination. Modified with permission from ref 96. Copyright 2014 Elsevier.

of the activity by surface modification with Fe3+ ions can, thus, contribute to the development of various selective organic synthesis systems using unlimited solar light energy. The interlayer space in two-dimensional layered titanates is adjustable, which creates them to have distinctive selectivity.241 The Li+- and Na+-exchanged titanates can enlarge their 1473


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substrates after pretreatment of the catalyst with a Brϕnsted acid, thus validating the significant promotional effect of Brϕnsted acids. Additionally, such an acceleration effect does not result in poorer selectivity, suggesting that the oxidation process on the surface of the pretreated TiO2 and Si/Ti catalysts should proceed via the proposed mechanism (oxygen atom transfer from O2 to alcohols during the TiO2 photocatalytic oxidation of alcohols in an organic solvent248 rather than the traditional photocatalytic pathways of nonselective •OH radicals and radical auto-oxidation with the existence of O2). Pt/TiO2 photocatalysts prepared with the assistance of NaOH shows higher HCHO oxidation activity than those without NaOH.249 As Figure 35 shows, O2 is first adsorbed

general, the photocatalytic selectivity of TiO2 is associated with the selective adsorption of contaminants on the surface of TiO2, which can be controlled by altering the surface charge of TiO2. Since the isoelectric point of TiO2 in water is at ∼5.8, the surface charge of TiO2 can be changed by a simple ligand interchange reaction between surface hydroxyl groups of TiO2 and H+ at acidic pH and/or OH− at alkaline pH.44 Under the acidic conditions, the surface of positively charged TiO2 promotes the selective adsorption of negatively charged contaminants, which results in their selective degradation. On the contrary, the alkaline conditions are beneficial for the decomposition of positively charged pollutants (Figure 34).44

Figure 34. Schematic illustration for the selective adsorption of charged contaminants on the surface of TiO2; A and B represent negatively and positively charged contaminants, respectively. Modified with permission from ref 44. Copyright 2011 The Royal Society of Chemistry.

Figure 35. Schematic mechanism for the enhanced oxidation of HCHO over Pt/TiO2 modified by NaOH. Adapted with permission from ref 249. Copyright 2013 American Chemical Society.

Therefore, this is not surprising that the selective photocatalytic degradation of charged contaminants can be adjusted by shifting the surface charge of TiO2 depending on pH.44 Additionally, it has been reported that RhB also preferentially anchors on an F-doping TiO2 through the cationic moiety (−NEt2 group).244,245 2.6.3. Surface Acid−Base Properties. Usually, the preferentially adsorbed reactant will react more easily with photogenerated short-lived •OH on photocatalyst surface, which can be speedily transformed to inactive surface hydroxyls.43 In some cases, the basic surface favors adsorption of some organics due to strong hydrogen bonding; hence, the photocatalytic selectivity of these organics can be improved. For TiO2, the isoelectric point of TiO2 is approximately at pH 6, therefore high pH values (>6) are beneficial to adsorbing positively charged pollutants, on the contrary, low pH values ( 4-CP > 2-CP > 4-NP > phenol > 4-chloro-2nitrophenol > 2,4,-dinitrophenol > 4-nitro-2-chlorophenol under UV irradiation. The degradation rates of NP are lower than that of only chloro substituted phenols. The complete degradation of 2,4-DCP is observed within 1 min for all the three lanthanide molybdovanadate; all monochloro substituted phenol undergo degradation within 10−15 min. The reported catalysts show significant catalytic behavior not only under

Figure 36. Selective oxidation of 4-MBA (0.8 mmol) to 4-MBAD by various catalysts (50 mg) under visible light irradiation (λ > 420 nm) at 3 h. Adapted with permission from ref 71. Copyright 2010 American Chemical Society. 1475


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The photooxidation of 4-CP and MO over Pt-CaCu3Ti4O12 occurs via a more selective mechanism than that over P25.262 CaCu3Ti4O12 has square planar Cu2+ and distinct octahedral Ti4+ sites, forming two band gaps in the visible region (2.21 and 1.39 eV). The differences in electronic structure of CaCu3Ti4O12 give the photogenerated carriers distinct chemical character from those in TiO2 and make the observed phototransformations of MO and 4-CP more selective. The enhanced localization of VB holes and Cu+ electrons, as well as the higher energy is conducive to the lower oxidizing potential of hole states at the VB edge. The reduction of the CB population (Ti3+) results from localized Cu+ serves as an electron/hole trap thereby assisting the recombination within the electronic structure of CaCu3Ti4O12. The selective photooxidation of BzA to BzAD over ZnIn2S4 has been reported originally by Chen et al.;15 ZnIn2S4, one of ternary chalcogenides, exhibits the good photocatalytic performance as well as selectivity under visible light irradiation. The selectivity to BzAD formation on ZnIn2S4 at different irradiation time stages are all between 94 and 95%; after 0.5 h irradiation, the conversion of BzA and the yield of BzAD being 47% and 45%, respectively. When the irradiation time is 3 h, the conversion of BzA reaches 69% along with the high selectivity, 94%. Li et al. have further investigated the selective photooxidation over ZnIn2S4; the best conversion of benzylamine (63%) and the highest selectivity (97%) to Nbenzylidenebenzylamine are obtained in CH2Cl2 solvent.263 The selective aerobic oxidation of all the benzylamine derivatives to their corresponding imines proceeds efficiently with medium to high conversions (74−99%) and high selectivity (94−97%). Electron-donating substituents (CH3− and CH3O−) and electron-withdrawing groups (F− and Cl−) on the phenyl ring do not have noticeable impact on the conversion rate and selectivity. Carbon coating plays a noteworthy role in improving the photocatalytic performance of ZnIn2S4.100 Carbon coated ZnIn2S4 composites prepared by varied amounts of glucose are labeled C/ZIS-0.01, C/ZIS-0.03, C/ZIS-0.05, C/ZIS-0.1, C/ZIS-0.5, and C/ZIS-0.7. Except C/ ZIS-0.7, all these composites show almost 100% selectivity and higher conversion than naked ZnIn2S4 samples (Figure 38). The best photocatalytic performance is attained on C/ZIS-0.03 sample with the conversion of BzA and the yield of BzAD being 1.6 and 1.7 times higher than that on bare ZnIn2S4 samples, respectively. The adsorptivities of ZnIn2S4 and C/ZIS-0.03 to BzA are similar, and the synergistic effect caused by ZnIn2S4

Another interesting paper reports the H2-evolution coupled selective oxidation of alcohols to aldehydes in the system comprising a cyanamide surface-functionalized melon-type C3N4 and a molecular nickel(II) bis(diphosphine) (NCNCNxNiP).260 The yield of 4-MBA to 4-MBAD in water is 66.0 ± 6.6% at normal pressure and temperature without forming byproduct carboxylic acid, and the H2 and 4-MBAD can be photogenerated in a 1:1 stoichiometry over time. Another metal-free C3N3S3 polymer has been reported as a visible-light-responsive photocatalyst for the selective oxidation of benzylic alcohols to aldehydes;261 conversion can be significantly improved by constructing the layered sandwich RGO/C3N3S3 hybrids. For the photocatalytic oxidation of BzA, 4-nitrobenzyl alcohol, 4-fluorobenzyl alcohol, and 4-methylbenzyl alcohol to corresponding aldehydes, the selectivity is 100%, but the conversion is dependent on the electron donating ability of the −R group, which declines in the order of −CH3 (54%) > −H (52%) > −NO2 (34%) > −F (25%). As Figure 37 shows, under light irradiation, the HOMO electrons

Figure 37. Schematic depiction of the photocatalytic reaction and the synergetic interaction between RGO and C3N3S3 polymer. Reprinted with permission from ref 261. Copyright 2014 American Chemical Society.

of the C3N3S3 polymer moieties jump to the LUMO orbitals, followed by fast transfer to the RGO moieties to reduce O2 molecules. The photoexcited holes react with the reactants to generate carbocations, which can be further oxidized to afford the final products by formed O2•−. The layered “sandwich” structure of the RGO/C3N3S3 hybrids can efficiently improve the separation of photogenerated carriers of the C3N3S3 polymer.

Figure 38. Selective oxidation of benzyl alcohol to benzaldehyde on prepared samples: (a) ZIS and C/ZIS-0.03 samples under visible light illumination without catalyst and in the dark with catalyst and (b) conversion and yield (C and Y) on all samples under the visible light illumination for 2 h. (a-g represent the samples of ZIS, C/ZIS-0.01, C/ZIS-0.03, C/ZIS-0.05, C/ZIS-0.1, C/ZIS-0.5, and C/ZIS-0.7, respectively.) Reprinted with permission from ref 100. Copyright 2015 Elsevier. 1476


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exhibits low selectivity (21%) for the epoxide of cyclohexene (CHE), since the formation of O2•− and •CHE radicals promotes the allylic oxidation of CHE. Acetonitrile, in view of the “shield effect”, plays a significant role in the selective epoxide formation akin to the results mentioned above. In the presence of acetonitrile the epoxide selectivity on T-S is 71% but only 11% without acetonitrile, though the CHE conversion has no obvious differences. Similarly, over T-OS-(10) and TOS-(100), the epoxide selectivity is just 67% in the presence of acetonitrile. CQDs can perform under near-infrared (NIR) light irradiation to catalyze the highly selective oxidation of BzA to BzAD (selectivity: 100%; conversion: 92%);92 •OH being the main active oxygen species. Moreover, the system also can be used for the efficient and selective conversion of other alcoholic substrates to corresponding aldehydes. The mechanism is different from that of traditional semiconductor photocatalysis, as shown in Figure 40.92 There is a π−π interaction between

and the carbon layers should be responsible for the improved photocatalytic performance. The photoexcited electron can quickly migrate to the carbon layer on ZnIn2S4 to generate O2•−, and therefore, the lifetime of charge carriers is prolonged since the combination of charge carriers is effectively restrained. Nevertheless, the excessive use of carbon in C/ZIS-0.7 decreases its photocatalytic performance in contrast to naked ZnIn2S4, because the thick carbon layer can block light absorption of ZnIn2S4 and accordingly reduce the formation of charge carriers. Thus, there is an optimal amount of carbon for preparing ZnIn2S4 with excellent photocatalytic performance. Ti molecular sieve photocatalysts show remarkable conversion and selectivity for the epoxidation of alkene by organic hydroperoxides.264 Shiraishi reports the photocatalytic epoxidation of olefins with high selectivity in acetonitrile solvent filled with O2, using a Ti-containing mesoporous silica (TMPSi) or T-OS as catalyst.106,265,266 The high selectivity for epoxide on T-MPSi is due to a “shield effect” driven by acetonitrile, as summarized in Figure 39.106 The ligand-to-metal

Figure 40. Plausible pathway for highly selective oxidation of BzA to BzAD catalyzed over CQDs under the NIR light irradiation. Reprinted with permission from ref 92. Copyright 2013 The Royal Society of Chemistry.

CQDs and BzA or BzAD which is helpful for the augmentation of BzA and BzAD on the surface of CQDs, thus promoting the catalytic process. H2O2 molecules can be adsorbed on CQDs and decomposed into active oxygen species (•OH) with strong oxidizing ability, which are prone to oxidize BzA to BzAD or benzoic acid. The CQDs’ photoexcited electron migration ability (particularly as strong electron donors) shields BzAD from deep oxidation to afford 100% selectivity to BzAD. In2S3 microsphere (Eg = 1.96 eV) presents fine conversion and selectivity for selective oxidation of aryl alcohols to corresponding aldehydes under the visible light illumination with good stability;267 conversion of BzA to BzAD in trifluorotoluene solvent is higher than that in nonfluorinated solvents, e.g., toluene, acetonitrile, and water. The selectivity is 99.8% in the trifluorotoluene solution, 66.2% in the toluene, 8.1% in the acetonitrile, and 1.1% in the aqueous solution after 4 h irradiation, which can be attributed to more dissolved oxygen in trifluorotoluene. The selectivity of the reaction is as follows: BzA = 4-MBA > 4-chlorobenzyl alcohol > 4fluorobenzyl alcohol > 4-nitrobenzyl alcohol. Photocatalyic conversion of alcohols to corresponding aldehydes on Ag3PO4 displays efficient conversion and high selectivity;268 BzA and 4-methoxy benzyl alcohol undergo >85% oxidation with selectivity of over 99%. However, in the case of cinnamyl alcohol, although the conversion is ∼90%, selectivity is partially lost as BzAD and benzene acetaldehyde

Figure 39. Acetonitrile-assisted selective photoepoxidation of olefins on T-MPSi catalyst. Adapted with permission from ref 106. Copyright 2006 American Chemical Society.

charge transfer (LMCT) occurs between Ti4+ and lattice oxygen (OL2−) when the Ti−O4 species (I) is photoexcited. Therefore, the agitated [Ti3+- OL−]* species (II) is generated, which can react with O2 to produce two kinds of oxygen radicals, O2•− and O3•− (III and IV). The epoxide of olefin can be formed after the electrophilic O3•− is directly react with olefin (route A). The vacant OL− sites on species II and IV are also electrophilic and, hence, act as positive holes. In the absence of acetonitrile, the olefin radical is formed after the proton in olefin is transferred, which then reacts with O2•− to produce byproduct, allylic oxidation products (routes B and C). However, the existence of acetonitrile, inherently a weak base, can stabilize the olefin by a hydrogen-bonding effect that the proton transfer is inhibited. The catalytic performance of T-OS and Ti-containing silica (T-S) for epoxide is similar via the acetonitrile-assisted shield effect.106 Bulk TiO2 in acetonitrile 1477


Chemical Reviews

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are also formed in addition to cinnamaldehyde (∼81%). High activity and selectivity of Ag3PO4 are attributed to the high oxidation potential, passive behavior of O2•−, and weak generation and ineffectiveness of OH• radicals. 2.7.2. Substituent Effect on the Substrates. Since TiO2 photocatalytic oxidations normally involving radicals, the selectivity of the attack to the aromatic ring in positions (ortho, meta, and para) is strongly dictated by the type of substituent. Palmisano et al. have systemically studied the oxidation of numerous aromatic organics over TiO2, and the impact of the substituent groups on the generation of monohydroxy derivatives, which is helpful to understand the reactivity of monosubstituted aromatic species.80,157,258,269 Among the studied molecules, NBz, cyanobenzene, benzoic acid (BA), and 1-phenylethanone (1-PE) contain an EWG; phenol (PH), phenylamine (PA), and N-phenylacetamide (NPA) have an electron donor group (EDG); 4-CP owns both an EWG and an EDG substituent. For the oxidation of aromatic substrates having an EDG, ortho and para monohydroxy products are usually obtained, in contrast, for those bearing an EWG, the oxidation is unselective as all the three potential isomers are attained.258 With both an EDG and an EWG, such as 4-CP and hydroxy-cyanobenzenes, the attack of the •OH radical ensues merely in the positions enthused by −OH. During the oxidation of 4-MBA and BzA, the selectivity gradually decreases during the reaction because of overoxidation, but maintains the values of ca. 38% and 60% mol, respectively, even when the conversion rate reaches 50%.78 These values are surprisingly high in aqueous solution with a TiO2-mediated photocatalytic protocol. The selectivity for 4MBA oxidation to corresponding aldehyde is higher than that for BzA, possibly due to the electron releasing methoxy group in the para position in 4-MBA. Another report shows that the oxidation of BzA and its derivatives to homologous aldehydes, including BzA, 4-MBA, 4-chlorobenzyl alcohol, 4-nitrobenzyl alcohol, 4-methylbenzyl alcohol, 4-(trifluoromethyl) benzyl alcohol, and 4-tertiary-butylbenzyl alcohol, can achieve both high conversion rate (>99%) and excellent selectivity (>99%) on TiO2 in O2 atmosphere. However, when the benzyl alcohols bearing the −OH group such as 4-hydroxybenzyl alcohol is photooxidized to 4-hydroxybenzaldehyde as well as some unidentified products, the selectivity of 4-hydroxybenzaldehyde is barely 23%. In another investigation, the effect of various substitutent groups namely, −OCH3, −CH3, −C(CH3)3, −Cl, −CF3, and −NO2 on the activity and selectivity of BzA has been compared by using TiO2 under visible light illumination.257 The activity for generating aldehyde is evaluated by a pseudo-first-order reaction, which is improved by phenyl-ring substitution with the electron-releasing groups (−OCH3, −CH3, and −C(CH3)3) and the electron-withdrawing groups (−Cl, −CF3, and −NO2). The activities can be affected by the oxidative potentials of aromatic substances, and moreover, the stability of the resonant structures of the benzylic alcohol radicals also very important, resulting in the subsequent oxidation to afford the corresponding aldehydes. The behavior of aromatic substances having an EDG is illustrated in Figure 41.269 When the hydroxyl radical goes into ortho and para positions, the highest influence to the stabilization is offered by the formula with the unpaired electron on the carbon bonded to the EDG. The aromatic ring can be protected from the oxidant attack via the presence of an

Figure 41. Resonance structures of radical intermediates formed during the oxidation of a molecule having an EDG. Modified with permission from ref 269. Copyright 2006 The Royal Society of Chemistry.

EDG, because of its inductive and delocalization impacts.78,80,157,258,269 For the oxidation of benzyl alcohols, the existence of an −OCH3 group promotes the selectivity on the rutile TiO2, while the EWG (−NO2) is unfavorable to selective reaction, because their presence usually bring about breakage of the aromatic ring to generate overoxidized products and finally CO2.157 The effect of substituent groups in different positions (BzA, 2-MBA, 3-MBA, 4-MBA, 2,4-dimethoxybenzyl alcohol, 4hydroxybenzyl alcohol, and 4-hydroxy-3-MBA) has been investigated for the partial oxidation of aromatic alcohols to the corresponding aldehydes in a photocatalytic system (labmade and commercial rutile TiO2 samples);256,257 the selectivity of compounds with substituent groups on the aromatic ring is in the order as follows: para > ortho > meta. The donor group in the meta position has no obvious impact on the oxidation of aromatic alcohol, and the ortho replaced molecule displays worse activity and selectivity than the para one, as a result of steric-hindrance effects. Though both −OH and −OCH3 are EDG, the −OH has more passive influence on reactivity and selectivity than −OCH3. The selectivity declines and activity increases when two substituent groups, e.g., −OH and −OCH3, are on the aromatic ring, owing to the activation ability of the extra EDG toward benzyl ring. When Cu(II) is added into the reaction solution (TiO2/ Cu((II)/solar hν), the substituent groups in the aromatic rings also play important role in the conversion rate and selectivities.270 Similar to the aforementioned facts, the presence of both electron donating (−OCH3, −OH) and electron withdrawing (−NO2) groups on the aromatic substrate obviously decreases the photocatalytic selectivity.270 For the oxidation of 4-(nitrophenyl) methyl sulfide on mpg-C3N4, the reaction is faster due to the strong interaction between −NO2 and the surface active site.258 The transformation of all the benzylic amines examined shows high selectivity to deliver the imine products.58 The substituents on the benzylic amines do not remarkably impact the selectivity but mildly change the conversion rates. Various substituent groups can affect the efficiency and selectivity of Au/TiO2 as well.109 The yield of quinaldine, 2,7dimethylquinoline, 2,6-dimethylquinoline, 6-methoxy-2-methylquinoline, 2,5,7-trimethylquinoline, 6-chloro-2-methylquino1478


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which are helpful to prevent the mineralization of aromatic alcohols, and therefore improve the selectivity of aldehyde. 2.8.2. Effect of Solvent. In water, the •OH is regularly formed, resulting in the low selectivity of the photocatalytic organic syntheses. Therefore, organic solvents are often used instead of water to avoid the formation of •OH and thereby enhancing the selectivity.67,68 The reactions in a dry organic phase, on the contrary, usually implicate the generation and transformation of organic peroxide. The photocatalytic results indicate that the activity and selectivity are greatly affected by the type of solvent used.218 When 1 mL H2O is added to the reaction mixture of photooxidation of styrene, the product distribution changes; conversion of styrene is enhanced from 18.1% to 24.5%; however, the selectivity for epoxidation of stytene to styrene oxide is reduced from 62% to 31%. On further increasing the amount of H2O (5 mL), the tested selectivity of styrene decreases to 15.6%. It is proposed that the decrease in selectivity for styrene oxide maybe due to different reaction mechanism. In the presence of water, the mechanism for photocatalytic oxidation on TiO2 involves unselective •OH rather than organic peroxide to cause the overoxidation and eventually mineralization of styrene. In the dry organic solvent, molecular oxygen is activated by photoexcited electron from the F-center sites of TiO2 to generate active oxygen species (i.e., organic peroxide), which subsequently epoxidizes styrene into styrene oxide and BzAD at the F-center sites. When water is replaced by CH3CN, TiO2 is a selective photocatalyst for the oxidation of aromatic alcohols to corresponding aldehydes with conversions approaching almost quantitative, relying on the inertness of the latter to the oxidation by positive holes;48,101,129,130,271 the underlying mechanisms, with and without H2O in CH3CN, are different.61,66 In the case of selective oxidation of cyclohexane to cyclohexanol and cyclohexanone over Q-TiO2, the selectivity of cyclohexanol decreases with the increase of water amount in CH3CN solvent, which is 87%, 66%, and 52% with 0, 1, and 5 mL of H2O, respectively.129 The conversion of benzyl and cinnamyl alcohols to aldehydes in dry nitrile solvents follows the order: CH3CN > CH3CH2CN > CH3(CH2)2CN and is completely restrained as the amount of water is >1% (v/v).166 The interaction between CH3CN and the surface of photocatalyst is very weak, which can be proved by quenching of its luminescence. A crucial point for obtaining aldehydes seems to be the generation of active oxygen species via oxygen reduction. 2.8.3. Influence of Gas Atmosphere. In inert gaseous atmosphere (Ar and N2), O2, an important oxidant in the photocatalytic processes, can be isolated. Consequently, the oxidizing ability of the photocatalyst is limited thus affecting the photocatalytic selectivity.16,46,72,108 Additionally, the presence of some gas, like CO2, can promote the desorption of products probably by lowering the pH value of the reaction solution, thus promoting the selectivity enhancement.89,272 The effect of O2 concentration on the selective oxidation of 4-MBA to 4-MBAD is detected, suggesting that the presence of O2 is unfavorable to the selectivity, since O2 usually functions as an electron trap to inhibit the e−-h+ recombination and therefore cause the increase of •OH radicals.273 In acetonitrile medium without O2, the generation of •OH radicals is avoided, and consequently, the thorough oxidation of BzA to CO2 can be largely avoided thus enhancing the selectivity of photocatalytic conversion of BzA to BzAD.101 The reaction between carbon radicals and hole is the chief reason for the activity of

line, and 6-fluoro-2-methylquinoline is 75, 80, 70, 60, 70, 16, and 8%, respectively. Correspondingly, the conversion of substrate NBz, 3-nitrotoluene, 4-nitrotoluene, 4-methoxynitrobenzene, 3,5-dimethylnitrobenzene, 4-chloronitrobenzene, and 4-fluoronitrobenzne, is 99, 99, 99, 80, 89, 99, and 99%, respectively. For the formation of quinaldine, −NO2 is first reducted and then undergo condensation with aldehyde as well cyclization. The yield of quinaldine is higher by using 3nitrotoluene (80% of 2,7-dimethylquinoline) than 4-nitrotoluene (70% of 2,6-dimethylquinoline), while 4-methoxynitrobenzene bearing a strong EDG at para-position can yield 60% quinaldine. The product yield (70%) for 3,5-dimethylnitrobenzene is lower than that of 3-nitrotolune (80%), as a result of inhibited cyclization by steric effect. In the case of 4-chloroand 4-fluoronitrobenzenes, the yield of quinaldines is quite poor which is ascribed to the photoexcited dehalogenation; similar results are also obtained for Ag/TiO2.110 In water, trans-ferulic acid, eugenol, isoeugenol, and vanillyl alcohol are used for the photoproduction of vanillin with a selectivity ranging from 1.4 to 21 mol %.112 The highest selectivity to vanillin (21% at a 15% conversion) is obtained when vanillyl alcohol is used. In the case of trans-ferulic acid and isoeugenol the selectivity to vanillin reaches values of ca. 12% for conversions up to ca. 50%, while for eugenol a maximum of 5% is found for conversion up to 20%. There is a double bond in the side chain of ferulic acid, isoeugenol, and eugenol but not in the vanillyl alcohol molecule, which possibly be responsible of a stronger mutual interface between the reactant and the surface of the catalyst. Consequently, the further photooxidation steps for vanillin is favored that give rise to open ring intermediates and to CO2 production. 2.8. External Reaction Conditions

The external conditions, e.g. solvent, gas atmosphere, pH value, can often affect the active species, reaction rate, and reaction pathway. Consequently, besides the properties of photocatalysts and reactant, the external conditions usually have a vital effect on the reactivity and selectivity of photocatalysis.16,43,46,69,70,72,108,218 2.8.1. Addition of Organics. The organics in photocatalytic system usually alter the involved active species; therefore, the selectivity is different after addition of organics to the reaction mixture. The frequently used organics are alcohols, such as methanol, ethanol, IPA, or tert-butanol. Furthermore, CH3CN and CH2Cl2 can also be utilized to enhance the photocatalytic selectivity.43,69,70 Product selectivity for oxidation of cyclohexane to cyclohexanol formation on TiO2 particles sharply increases with the addition of dichloromethane.69 It is inferred that cyclohexanol is generated by recombination of cyclohexylperoxy radicals and with the postulation that the Cl ions are formed by O2•− reaction with CH2Cl2. The simultaneously formation of CH2ClOO• radicals, which in turn can abstract an H atom from cyclohexane, improves the efficiency of charge utilization. The catalytic selectivity of the as-prepared fluorinated hollow TiO2 microspheres toward photodecomposition of MB in the mixture of MO and MB can be tuned by changing the portion of ethanol in the mixed solvent; enhancement occurs with the increasing fraction of ethanol in the synthesis mixture.43 Moreover, supplementing an aliphatic alcohol (methanol, ethanol, IPA, or tert-butanol) in aqueous suspensions can promote the photocatalytic selectivity toward aldehyde over TiO2.70 The aliphatic alcohols act as the strong hole-traps, 1479


Chemical Reviews

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for the photooxidation of phenol are CAT and HQ in almost all of the prepared forms of TiO2 namely rutile, anatase and P25; the photooxidation of phenol is greatly enhanced in the presence of NaF for anatase and P25 TiO2 but decreased for all the rutile TiO2 samples, though the selectivity of CAT is improved. The replacement of the surface hydroxyl groups of TiO2 by fluoride increases the production of free •OH in solution due to enhanced hole availability for water oxidation. Recent studies demonstrate that the use of Cu(II) ions as electron acceptor instead of O2 under deaerated conditions, lead to an increase of selectivity for aldehyde formation as exemplified in the selective conversion of 4-nitro-benzyl alcohol to 4-nitrobenzaldehyde; Cu(II) is completely reduced to Cu(0) by photogenerated electrons after the reaction.270,274 Cu(II) could be recovered and reused, via an aerial reoxidation of Cu(0), produced during the photolytic run in the dark.275 The oxidation rate of BzA is greatly affected by the original Cu ions concentration, light source, and temperature. With Cu(II) 0.5 mM, the BA oxidation is enhanced, while at the highest Cu(II) initial concentration (1.5 mM) the system reactivity and BA conversion decreased. In fact, since the Cu(II) species is added to the reactive solution as CuSO4, the increased Cu(II) ions initial concentration results in the enhancement of sulfate concentration as well. The selectivity for BzAD, which increases with the enhancement sulfate concentration, confirms the capability of SO42− to scavenge very reactive and unselective •OH. The adsorbed SO42− may block the TiO2 active sites (s*) thus controlling the deactivation of partial catalyst. When pH changes from 2.0 to 4.0, the oxidation of BzA and the formation of BzAD decreases. 2.8.5. Role of Light Sources. By adjusting the light irradiation, the generation of byproducts can possibly be eliminated, and therefore the selectivity of oxidations can be impacted, as the shorter wavelength with stronger energy can undertake harder reactions.48,95,276,277 The conversion of benzyl and cinnamyl alcohols to the corresponding aldehydes is achieved with 100% selectivity by visible light irradiation (400−500 nm) of nitrogen-doped TiO2.166 However, upon irradiation with UV light, the alcohol conversion is unselective toward the generation of aldehyde though conversion rate is faster. The initial rate of selective alcohol to aldehyde conversion depends linearly on light intensity below 1 mW cm−2 wherein all these reactions are carried on in nitrile solvents. The •CH2CN radicals are formed on TiO2 upon UV irradiation, while it does not form under visible light exposure, possibly because of the low steady-state concentration of radicals. In another case, if photolysis is prevented by using the proper light filters (e.g., Pyrex, λ > 275 nm), the oxidation of cyclohexane over TiO 2 yields predominantly cyclohexanone (selectivity >95%).277 In contrast, the selectivity shifts to the ketone when the reactant is exposed at λ < 275 nm. The influence of light power on oxidative selectivity of 4MBA to aldehyde has been studied;160 selectivity is 53% when the lamp power is 10 mW/cm2. Moreover, increasing the lamp power up to 17.5 mW/cm2 surprisingly promotes both conversion rate and selectivity (56%). However, excessive lamp power (40 mW/cm2) results in a decrease of selectivity to 48%, probably due to the occurrence of severe oxidation leading to faster degradation of the generated aldehyde. 2.8.6. Effect of pH Value. Sometimes pH value can influence the photocatalytic selectivity.278,279 In the mixture of benzamide and 4-hydroxybenzoic acid, for instance, low pH

photocatalytic oxidation, which can be verified through both the experimental analysis and the frontier electron density theory calculation. Ide et al. have reported a versatile method to modify the efficiency and product selectivity of heterogeneous photocatalytic oxidation by conducting the reaction under a CO2 atmosphere.89,272 The presence of CO2 can promote the hydroquione desorption on Au/P25 probably by lowering the pH value of the reaction solution, which is conducive to the enhancement of selectivity of phenol oxidation to hydroquione. Moreover, the presence of CO2 is shown to further enhance desorption of the product, which prevents the successive oxidation. The photocatalytic selectivity of P25 is 7% for the oxidation of phenol to HQ at 24 h, and it can be enhanced to 8.5% after loading 4% Au, furthermore, it can increase to 33% when CO2 is used.89 The selective photocatalytic oxidation of benzene on TiO2-supported Au NPs has been examined. When 3% Au@P25 is used in the atmospheric air, significant amounts of the successive oxidized products, such as CAT, hydroxyquinone, and 1,2,3- and 1,2,4-trihydroxybenzene, are formed, in addition to phenol;272 yield and selectivity of phenol are 8% and 62%, respectively. In contrast, when the reaction is conducted under a CO2 atmosphere (230 kPa), the formation of the successive oxidized products is largely suppressed and the phenol yield dramatically increased, resulting in 13% yield and 89% selectivity. A similar CO2 effect is observed when 4% Au@Tynoc is used as the photocatalyst; both, the yield and selectivity of phenol varied depending on the pressure of CO2, are improved when only 230 kPa of CO2 is loaded. For the photocatalytic oxidation of benzene, it is directly oxidized to phenol and the formed phenol is sequentially converted to the successive oxidized products, such as CAT, hydroxyquinone, and trihydroxybenzenes, and finally mineralized to CO2. When the starting mixture is loaded with CO2 pressure, the generation of CO2 gas from water solution, or the successive oxidation of phenol, is mechanically suppressed to improve the selectivity of phenol. If CO2 pressure is too high (>230 kPa), the oxidation of benzene is also suppressed, giving a maximum yield for phenol formation. Next, CO2 is partially dissolved in water and, accordingly, the change in the pH of the starting solution affects the electronic properties of phenol and the catalyst’s surface to possibly alter the photocatalytic performance. On the other hand, the HCO3− negatively affects the selective benzene oxidation. When the loaded amount of CO2 is smaller ( CdS NSPs/2% GR > CdS NSPs/10% GR > bare CdS NSPs.54,286 Under the irradiation of visible light, electron is photogenerated due to the excitation of CdS NSPs in the CdS NSPs/GR NCs, which can then easily move to GR to greatly enhance their lifetime. In the interim, the existence of GR also promotes the accumulation of nitro entities on the surface of CdS NSPs/GR NCs and, consequently, the adsorbed aromatic nitro organics can be successfully reduced into amino derivatives. Furthermore, the CdS NSPs/GR NCs possesses tremendous reusability, in view of the synergetic consequence of GR on the CdS NSPs surface and the use of hole sacrificial agent, ammonium formate. Another study shows that the lifetime and migration of photoexcited electrons can be enhanced after RGO nanocomposites are combined with CdS NWs.298 Additionally, the existence of RGO is beneficial to the adsorption of aromatic nitro entities on CdS NWs-RGO NCs. CdS NWs-carbon nanotubes 1D-1D nanocomposites (CdS NWs-CNT NCs) also show better performance than pure CdS NWs and even CdS NWs-RGO NCs for the selective reduction of nitro organics to amino compounds in water, which can be attributed to the reduced recombination of the charge carriers and the improved visible light response, stemming from the hybridization of CdS NWs with CNT.299 For In2S3 photocatalyst, the addition of GR likewise improves the photocatalytic performance for the selective reduction of 4-NANL to p-PhDA in water under visible light irradiation.300 The efficiency of reduction over the studied photocatalysts follow the order of In2S3-2%GR > In2S3-1%GR > In2S3-5%GR. The lifetime of photoexcited charge carries plays a primary role in the activity of In2S3-GR toward the selective reduction of nitro substances to the corresponding amino derivatives. Similar, In2S3−CNT nanocomposites display high visible-light-responsive performance for the selective hydrogenation of nitroaromatics to amines as well.292 The facile preparative process (Figure 44) entails the addition of indium chloride tetrahydrate (InCl3·4H2O) to the acid treated CNT suspension followed by thioacetamide; refluxing at 95 °C for 5 h affords the In2S3−CNT nanocomposites. The ZnIn2S4-GR nanocomposites also exhibit remarkably enhanced photocatalytic activity toward the selective reduction of nitroaromatics to amines in water compared to blank ZnIn2S4 as well.263 3.1.2. Metals Loading. Precious metals such as Pd, Pt, Au, Ag, etc. are often used to improve the photocatalytic selectivity of hydrogenation, especially Pd, which can adsorb H atoms very well (“protons”) and is thereby favorable to increase reduction reactions. Accordingly, the surface of Pd rapidly adsorbs H2 molecule as soon as it is produced from dehydrogenation, generating Pd−H reduction sites for the photocatalytic reduction reaction.20,21 CdS0.4Se0.6-Pd prefers hydrogenolysis rather than dehydrogenation (3:1) of benzaldehyde to form

aliphatic alcohols required longer reaction times over the polyoxometalate−ZrO2 nanocomposite, so that the chemoselective oxidation of a benzylic alcohol with the inclusion of a nonbenzylic ones could be accomplished by using proper reaction time.283 When a mixture of 1-phenylethanol and 2phenylethanol is irradiated in the presence of the polyoxometalate−ZrO2 nanocomposite under O2, 1-phenylethanol is oxidized to acetophenone in 86% yield, while 2-phenylethanol gets converted to 2-phenylacetaldehyde in 90%) when the atmosphere is dry nitrogen containing 10−20% of oxygen.284 However, as the applied nitrogen/oxygen gas is humidified, the selectivity of ketone decreases; the yield of cyclohexanol/cyclohexanone enhances from 0.4 to 1.0 as a result of the humidity variation from 30% to 90%. There are two possible reasons: (a) an increase in hydroxyl radical concentration and (b) prevention of deep oxidation of cyclohexanol.

3. SELECTIVE REDUCTION OF ORGANICS In comparison to the photocatalytic selective oxidation reactions, the investigations pertaining to selective photoreduction of organics are rather meagre, and most of them are focused on organic nitrocompounds only.46,54−56,285−288 The main modification strategies for promoting photoreductive selectivity are fewer as well, including production of composites, their surface modification, formation of special phase and crystal facets.46,54−56,285−288 However, the details of these strategies are very different from those of photocatalytic selective oxidation, due to the distinction of their reaction properties (Table 5). For example, Au is the most often used and effective loading metal in photocatalytic selective oxidation, while Pd is the most favorite metal in most of photocatalytic selective reduction.20,21 3.1. Cocatalysts

Cocatalyst can significantly affect the transfer and conductivity of photoexcited charge carriers in photocatalysts as they are exposed to light,54,263 and therefore, the selective reduction can be promoted by using cocatalyst, including metal, carbon materials, and complexes.20,285,289 3.1.1. Carbon Material/Photocatalysts. By using ammonium formate as a sacrificial agent for photoinduced holes, the selectivity for the reduction of 4-nitroaniline (4-NANL) to pphenylenediamine (p-PhDA) over GR@TiO2 is almost quantitative, 99% (Figure 43).285 As TiO2 is exposed to UV 1482


1483

CdS In2S3−3%CNT 1.0% Nd, Ncodoped TiO2 Pt/TiO2+Sn−Pd /Al2O3 Sn−Pd/Al2O3

reduction of styrene oxide to styrene

reduction of 4-NANL to p-PhDA

reduction of 4-NANL to p-PhDA reduction of nitrate to nitrogen

reduction of nitrate to nitrogen

reduction of nitrate to nitrogen

reduction of nitrate to ammonia synthesis of dioctylsulfide by addition of 1octanethiol on 1-octene conversion of m-nitrotoluene to mono Nalkylated product C−N coupling of benzylamine to Nbenzylidenebenzylamine C−N coupling of benzylamine to Nbenzylidenebenzylamine

16

17

18 19

20

21

22 23

26

25

24

3 wt % Au/CeO2

reduction of acetophenone to benzyl ethanol

15

TiO2

Pt(1.0%)-anatase TiO2 TiO2

Cu−Pd/TiO2 P25

3 wt % Au/CeO2

3 wt % Au/CeO2

reduction of azobenzene to hydroazobenzene

3 wt % Au/CeO2

Au-CeO2

14

deoxgenation of styrene oxide to styrene

12

Au-CeO2

reduction of NBz to azobenzene

reduction of acetophenone to benzyl ethanol

11

Au-CeO2

Au-CeO2

material

others

N2 N2 N2 N2 N2 N2 Ar N2

5 h; 100 W high-pressure Hg lamp (λ > 300 nm) 5 h; 100 W high-pressure Hg lamp (λ > 300 nm)

4 h; 8W LED (main wavelength at 380 nm) 4 h; 8W LED (main wavelength at 380 nm) 5 h; 300 W Xe arc lamp (λ > 420 nm) 300 min; 125 W medium-pressure Hg lamp (λ > 340 nm) 2 h; UV(A) irradiation (λ > 320 nm)

O2

water

acetonitrile

ethanol

Ar O2

10 vol % aqueous methanol solution dodecane

water

water

4-NANL with 40 mg of ammonium formate water with 200 mg/L HCOOH

vacuum N2

Air

air

N2 Ar

N2

Ar

16 h; 500 W halogen lamp (λ > 400 nm) 9 min; Ozone-free 300 W Xe lamp (λ > 420 nm) 1 h; 300 W Xe lamp (λ > 420 nm) 90 min; 300 W xenon lamp (λ > 420 nm)

Ar

Ar

24 h; 500 W halogen lamp (λ > 400 nm)

6 h; 500 W halogen lamp (λ > 400 nm)

solvent

90 89 69 20 86 13 84 >99

conv. (%)

30 mL of IPA as solvent, and 3 mL of 0.1 M KOH solution in IPA are mixed 30 mL of IPA as solvent, and 3 mL of 0.1 M KOH solution in IPA are mixed 30 mL IPA as solvent, and 3 mL of 0.1 M KOH solution in IPA are mixed 30 mL of IPA as solvent, and 3 mL of 0.1 M KOH solution in IPA are mixed aqueous solution of ammonium formate

IPA

Ar

IPA

IPA

IPA

Ar

solvent mixture of CH3OH and water mixture of CH3OH and water mixture of CH3OH and water mixture of CH3OH and water mixture of CH3OH and water mixture of CH3OH and water acetonitrile- H2O- methanol 2-PrOH solution

Ar

Ar

Ar

atmosphere

atmosphere

3 h; simulated sunlight (xenon lamp, 150 W) 6 h; simulated sunlight (xenon lamp, 150 W) 24 h; simulated sunlight (xenon lamp, 150 W) 20 h; simulated sunlight (xenon lamp, 150 W) 2 h; 500 W halogen lamp (λ > 400 nm)


nm) nm) nm) nm) nm) nm)

photoreduction of NBz to ANL 400 W high pressure mercury lamp (λ > 300 400 W high pressure mercury lamp (λ > 300 400 W high pressure mercury lamp (λ > 300 400 W high pressure mercury lamp (λ > 300 400 W high pressure mercury lamp (λ > 300 400 W high pressure mercury lamp (λ > 300 400 W high-pressure Hg lamp (λ > 300 nm) 2 kW Xe lamp (300−450 nm, 27.3 W m−2)


13

hydrogenation of azobene to hydroazobenzene

10

h; h; h; h; h; h; h; h;

reduction of NBz to azoxybenzene

1 1 1 1 1 1 1 4

9

reaction

asparagine- TiO2 serine-TiO2 phenylalanine-TiO2 tyrosine-TiO2 Ag- TiO2 TiO2 Ag/TiO2 rutile TiO2

1 2 3 4 5 6 7 8

entry

material

entry

Table 5. Selective Photocatalytic Reductions and Other Miscellaneous Reactions over Different Photocatalysts

100

74

76 90 of 1octene 100

69

39

58

87

80

78 96

90

90

100 85

∼98

∼100 45 98

88

99

78

96

89

94

79

95

sel. (%)

20

31

40

43.5

18

31

33

66

conv. (%)

100 97

70

97 70 95

sel. (%)

ref

ref

58

58

55

295 296

294

294

292 293

291

189

189

189

189

189

189

189

189

46 46 46 46 46 46 290 56



Review

99.5 23.5

297

99.2 29.3

ref sel. (%) conv. (%)

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toluene, however, under optimal conditions, the ratio of dehydrogenation to hydrogenolysis on CdS-Pt is 8:1.20 This aspect has been confirmed by addition of glyceraldehyde (HOCH2CH2OHCHO) to the reaction mixture with CdS-Pd wherein glyceraldehyde gets hydrogenerated to form glycerol, and the amount of produced BzAD is higher. Additionally, transfer hydrogenation reaction of glyceraldehyde does not occur in the dark, meaning that it is only reduced by photogenerated H2 or Pd-hydride equivalents on the Pdphotocatalyst surface thus validating the mechanism (Figure 45). This brings up a new strategy to develop a series of

water Ar 9 h; 300 W high-pressure Hg lamp

water Ar 9 h; 300 W high-pressure Hg lamp

Figure 44. Schematic flowchart for the preparation of In2S3−CNT nanocomposites. Reprinted with permission from ref 292. Copyright 2014 The Royal Society of Chemistry.

1%Pt/TiO2nanorods 1%Pd/TiO2nanorods

irradiation condition material

others

atmosphere

solvent

Figure 43. Schematic of the proposed mechanism for photocatalytic reduction of nitro aromatics on the GR@TiO2 nanocomposites under the UV light irradiation (λ = 365 ± 15 nm). Reprinted with permission from ref 285. Copyright 2014 American Chemical Society.

C−O coupling of ethanol to DEE

C−O coupling of ethanol to DEE

27

28

entry

Table 5. continued

reaction

Figure 45. Photoinduced transfer hydrogenation reactions. Adapted with permission from ref 20. Copyright 2012 American Chemical Society.

photocatalytic dehydrogenation reactions. Metal (M = Pt, Pd) islands on the photocatalyst surface can be used to enhance the selectivity for dehydrogenation and hydrogenolysis of BzA under sunlight irradiation.20 In the interim, the metal can improve both the activity and antiphotocorrosion ability of photocatalyst. Pd@CeO2 core−shell nanocomposite is selective for the reduction of 4-NP to 4-aminophenol (4-AP) with excessive NaBH4 under room temperature and atmospheric pressure; the reduction of 4-NP predominantly generates 4-AP with a high selectivity of ca. 99%.21 The selective reduction has also been observed for other substituted aromatic nitro compounds, such as 3-NP, 2-NP, 4-NANL, 3-NANL, 21484


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Figure 46. Schematic underlying mechanisms for the photocatalytic reductions with supported Au-NPs. Modified with permission from ref 189. Copyright 2013 The Royal Society of Chemistry.

A cyclization reaction of the imine intermediate to generate methyl quinoline has been observed when P25 is used as a photocatalyst.55 When cocatalyst Pt is used, the hydrogenation of the ensuing imine invariably afforded mono N-alkylated products in all cases. However, the selectivity toward the mono N-alkylated product is greatest using platinized anatase, which increases from 50% to 80% when the Pt loading increases from 0.3% to 1.0%. The selectivity is also influenced by the platinization method and the illumination time but not by the light intensity; Pt/TiO2 prepared via the photodeposition method has higher selectivity than that prepared by mixing of colloidal Pt with TiO2, which may be ascribed to the more exposed available surface of Pt particles in the case of the former. On the other hand, prolonged illumination time leads to the reductive alkylation of N-alkylated product by the photocatalytically generated acetaldehyde. Ag NPs (main diameter of 1.5 nm) loaded TiO2 photocatalyst, displays 100% selectivity and 84% conversion rate for the photocatalytic reduction of NBz ([NBz]0 = 1.1 mM) to ANL at 1 h with the existence of 100 mM CH3OH; even at 5.4 mM of [NBz]0, the reaction proceeds with high conversion (95%) and selectivity (81%) at 4 h.290 The product selectivity to ANL as well as the activity greatly increases with the addition of methanol, CH3OH, and a minor enhancing effect is discerned by the addition of 2-methyl-2-propanol [(CH3)3COH]. NBz is selectively adsorbed onto the Ag surfaces of Ag/TiO2 via partial electron transfer from Ag to NBz, whereas the interaction between ANL and Ag/TiO2 is weak. The kinetic analysis indicates that the recombination between the electrons flowing into the Ag NPs and the holes left in the TiO2 VB is significantly suppressed, particularly in the presence of CH3OH. The high activity and selectivity in the present Ag/TiO2-photocatalyzed reduction are due to the charge separation efficiency, the selective adsorption of reactants on the catalyst surfaces, and the restriction of product readsorption. The oxidant (NBz) and the reductant (H2O) are selectively supplied to the reduction (Ag) and oxidation (TiO2) sites, respectively, at a high concentration, which would

NANL, 4-nitrotoluene, 4-nitroanisole, 1-chloro-4-nitrobenzene, and 1-bromo-4-nitrobenzene.21 It has been proposed that S can be selectively adsorbed on the Au(111) surfaces of Au NPs.301 Consequently, disulfide can be efficiently and selectively reducted to thiol on the Au NPsloaded TiO2 by the UV light irradiation;301 6-electron reduction of NBz to ANL proceeds almost selectively and the photocatalytic activity significantly increases as a result of the Au NPs loading. Similarly, Au/CeO2 is also found to be highly selective photocatalyst for photoreduction of organics.189 The conversion rate for the reduction of NBz at 2 h, azobenzene at 6 h, acetophenone at 24 h, and styrene oxide at 16 h, is 43.5%, 40%, 31%, and 20%, respectively; the corresponding selectivity to azobenzene, hydroazobenzene, benzyl ethanol, and styrene is 96%, 78%, 99%, and 88%, respectively. IPA can act as a hydrogen donor for the formation of the transient Au−H species from Au-NPs, which can react with the NO, NN, CO double or epoxide bonds to generate the final reductive products (Figure 46).189 The excited electrons in the Au-NPs can also reduce the strength of these double or epoxide bonds, and the capacity depends on the energy of these electrons. The aromatic compounds attain higher conversions than nonaromatic ones, possibly due to the strong affinity between aromatic ring and Au-NP surface. For the reduction of styrene oxide, Au/CeO2 is more selective (88%) than Au/ZrO2 (72%), Au/TiO2 (73%), Au/Al2O3 (60%), and Au/Y (62%). Additionally, by tuning the wavelength of light source, the selectivity of products can be modified, and more challenging reductions occur at shorter wavelength. For the photocatalytic conversion of 3-nitrotoluene, selectivity toward the mono N-alkylated product is affected by the loading of Pt; best selectivity is obtained employing 0.5% Pt/anatase TiO2.55 The production of the mono-N-alkylated product is a result of the hydrogenation of imine, which is catalyzed by the Pt NPs deposited on the surface of TiO2; selectivity to mono-N-alkylated product increases from 50% to 80% when the Pt loading increases from 0.3% to 1.0% via the hydrogenation of the CN moiety. 1485


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and HCOONH4 for the reduction of nitroaromatics) can react with the photoexcited h+, while the photogenerated e− can simultaneously contribute to the reduction of nitroaromatics and CO2. The adsorption of TiO2 nanosheet is extended to the visible light range after being combined with narrow band gap CdS, and moreover, the photoexcited e− transport performance is also improved because of the interface between CdS and TiO2, thus rationalizing the photocatalytic activity of CdS-TiO2 over the bare TiO2 nanosheets (Figure 47).

contribute to the increases in not only the photocatalytic activity, but also the product selectivity. 3.1.3. Complexes. By coupling CdS NPs with iridiumbased complexes, a highly selective photocatalytic system has been developed for the visible light driven reduction of aldehyde to alcohol, possibly due to a unique conjugated structure between the iridium complexes oxyanion and the phenyl group.289 In the hybrid systems, the semiconductor harvests light energy, and complexes are activated by the photodriven electron, thus catalyzing the subsequent reactions; cyclohexane carboxaldehyde, benzylideneacetone, acetophenone, and ethylpyruvate can be quantitatively converted to the corresponding alcohols in 6 h. 3.2. Heterostructured Materials

The CdS-TiO2 nanosheet shows better photocatalytic performance than that of pure TiO2 nanosheet for the selective reduction of CO2 in gas-phase and reduction of nitroaromatics in liquid-phase;302 their activities are enhanced with the increase of photodeposition time for CdS between 15 and 60 min, the activity being highest for CdS−TiO2 nanosheet-60 among these samples (Table 6). CO2 is reduced to CO and Table 6. Photocatalytic Reduction of Different Nitroaromatics in Aqueous Medium over CdS-TiO2−NS-60a

Figure 47. Schematics of mechanism for simulated solar-light-driven (300 < λ < 800 nm) reduction of CO2 and nitroaromatics over the CdS-TiO2 nanosheets. Reprinted with permission from ref 302. Copyright 2015 American Chemical Society.

CdS modified 1D ZnO nanorods-2D graphene hybrids with ternary heteroassembly structure show excellent visible-lightresponsive photocatalytic performance for the selective reduction of 4-NANL to p-PhDA; the 5%RGO-ZnO NRsCdS displays the best photocatalytic performance as both the conversion of 4-NANL (95%) and selectivity for p-PhDA (>98%) are high at 16 min.303 In the design of RGO-ZnO NRsCdS photocatalyst, the inclusion of CdS enhances the light absorption of ZnO to the visible region, while 2D RGO sheets is beneficial to facilitate the separation and migration of photoexcited electrons from CdS. Hence, the hierarchical architecture displays obviously higher photocatalytic preformance than either the binary ZnO-CdS or bare ZnO under visible light irradiation, as a result from the effective three-level electron transfer for ternary RGO-ZnO NRs-CdS. 3.3. Surface Modification

In order to improve the selectivity, the surface of the photocatalysts has been modified via the formation of organic/inorganic hybrid catalysts;46,287,288 the modification with EDG appears to be an efficient strategy to promote the selectivity of photoreduction, which usually has a significant effect on the adsorption capability of photocatalysts as well as the reaction pathway. The modification of TiO2 with electron-donating groups (asparagine (Asp), serine (Ser), phenylalanine (Phe), and tyrosine (Tyr)), which bind to TiO2 via carboxyl group, is a useful mode to improve the photocatalytic reduction of nitroaromatic compounds;46 such modifications result in highly selective activity of NBz to ANL compared to bare TiO2, the order being the highest (>95%) on the Asp-TiO2 with high conversion (>90%) of NBz. The selectivity over different amino

Reaction time: 8 min; light source: simulated solar light (300 < λ < 800 nm); scavenger for holes in the liquid: ammonium formate (HCOONH4). Reprinted with permission from ref 302. Copyright 2015 American Chemical Society a

CH4 over the TiO2 nanosheets and CdS-TiO2 nanosheets-60 samples, and the yield over CdS-TiO2−NS-60 composite photocatalyst (CO 3.62 μmol g−1 h−1 and CH4 0.54 μmol g−1 h−1) is much higher than that over the bare TiO2 nanosheets (CO 0.99 μmol g−1 h−1 and CH4 0.09 μmol g−1 h−1). Mechanisms for the photocatalytic reduction of CO2 and nitroaromatics have been delineated. Charge carriers can be photogenerated on CdS and TiO2 under simulated solar light illumination, and then the reductants (water for CO2 reduction 1486


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plays a very important role in the CO selectivity which declines with the enhancement of calcination temperature, possibly due to the elevated crystallinity of the catalysts attained at the high temperature calcination.

acid modified TiO2 catalysts varies in short-range from 89% to 96% with asparagine/TiO2 > serine/TiO2 > phenylalanine/ TiO2. 13% of NBz is converted in the presence of TiO2 with merely any ANL, using solely Ag-TiO2 increases the NBz conversion to 86% with 70% selectivity to ANL. It is found that the adsorption capability is greatly increased for all modified TiO2; stronger adsorption capability is attributed to hydrogen bonding, n-p and p-p interaction between modified TiO2 and NBz. Only electrons, but not •CH2OH, are responsible for photoredution of NBz over TiO2. In the case of TiO2, merely any ANL is produced via the reduction of NBz by photogenerated electrons at the TiO2 surface which is kinetically hindered. Surface modification, however, brings NBz close to the TiO2 surface thus enhancing the kinetics and the yield of NBz reduction. Due to their strong electrondonating properties, Asp, Ser, and Phe act as a hole trap to prevent electron/hole recombination, providing stable surface layer with reduction pathway for NBz. Additionally, Asp, Ser, and Phe also improves the coupling between NBz and TiO2, and the migration of e− from the CB of TiO2 to NBz without significant activation energy. Surface modification of TiO2 with arginine changes the surface charge which results in the enhanced reduction of 4-NP at pH 9, where the positively charged amine group on the side chain of arginine effectively complexes with negatively charged 4-NP;287 arginine is an excellent amino acid surface modifier for the reduction of NBz288,304 presumably by bidentate binding to the TiO2 surface via the carboxyl group.288 The nanoparticulate TiO2 modified with an arginine monolayer forces the reduction of NBz to a single product, ANL, as opposed to typical oxidative pathways converting NBz to assorted NPs and dinitrobenzene. Moreover, Arg-TiO2 selectively reduces NBz, even in the presence of chemically similar molecules such as phenol. The surface modification of TiO2 NPs photocatalyst with arginine completely alters the degradation pathway from oxidation to reduction during treatment of NBz as compared to bare TiO2; by acting as a hole trap to prevent electron/hole recombination and transfers of electrons from the CB of TiO2 to NBz with small activation energy.304 However, illumination of argininemodified TiO2 has no effect on phenol in solution which can result in a selectivity factor for nitroaromatic compounds. The reason may be that (1) arginine has efficiently blocked the TiO2 surface, (2) there is no electronic coupling between Phe and arginine, and (3) while arginine acts as a hole trap, it does not transfer photogenerated holes to phenol as it does to CH3OH. The surface-modified TiO2 by VOx shows quantitative selective reduction of the nitro group in 4-nitro-benzaldehyde under milder experimental conditions252 presumably due to control of number of photogenerated electrons. The reduction of 4-nitrobenzaldehyde on glassy carbon occurs at ∼−0.65 V and its reduction by CB electrons has been observed at TiO2. In the case of (VOx)n/TiO2, charge transfer on the whole is slower and electrons can be transferred to 4-nitrobenzaldehyde through the VV/VIV surface state of the (VOx)n overlayer wherein 4-nitrobenzaldehyde can reasonably interact with this hydroxylated layer via the acidic V−OH groups. However, further investigations are needed to fully delineate these mechanistical details. For photocatalytic reforming CH3OH on Pt/TiO2−SO42−, the selectivity of H2 and CO formation can be adjusted by varying the crystallinity and surface acidity of TiO2;305 highest CO selectivity is attained on the P25 with the highest acidity. The calcination temperature of Pt/P25−1.0%SO42−-T also

3.4. Effect of Phase

For the photocatalytic transformation of various nitro-aromatic compounds, rutile is known to be more selective than other phases55,56 as shown for the facile formation of a primary amino compound exemplified by m-toluidine, while anatase catalysts afford a mixture of m-toluidine and its imine derivative (Nethylidene-3-methylaniline);55 surface properties of different phases are responsible for the selectivity. The catalytic condensation of the photocatalytically produced aldehyde and amine occurs on the Lewis acid site of anatase TiO2 to produce the imine whereas poor acidity of the rutile is the reason for its selectivity toward amine formation. On the other hand, P25 TiO2 has both Lewis and Brϕsted acid sites on its surface; Brϕsted acid sites are believed to be the reason for the cyclization reaction of the ensuing imine, which is produced on the Lewis acid sites, to finally generate the corresponding quinoline in the presence of P25 TiO2.55 Rutile TiO2 shows much superior activity and selectivity than P25 TiO2 and anatase TiO2 for the selective hydrogenation of nitroaromatics using alcohol as a hydrogen source;56 anatase TiO2 has NBz conversion of 420 nm).317,318 In earlier reported photocatalytic systems, however, H2O2 has been produced with significantly low selectivity (∼1%). 319 The high H 2 O 2 selectivity is attributed to the effective generation of 1,4endoperoxide species on the g-C3N4 surface, which inhibits the formation of one-electron reduction product, O2•−, and thereby results in the selective reduction of O2 to H2O2 by two-electron reaction. The proposed mechanism for selective H 2 O 2 generation over g-C3N4 is displayed in Figure 55.318 First, gC3N4 is photoexcited by two photons (a) to cause the separation of e−−h+ (b); h+ is localized at the N2 and N6 positions and subsequently oxidize α- and β-hydrogens of alcohol to form aldehyde (c), while e− is localized at the C1 and N4 positions of the triazine ring. O2 is reduced by the photoexcited e− to generate a superoxo radical (d), which is generally released as a •OOH radical (e) by reacting with the proton, which improves one-electron reduction of O2 (eq 3). By comparison, the superoxo radical (d) is quickly reduced by the e− photogenerated on g-C3N4, and forming a 1,4endoperoxide species (f) which up on protonation produces H2O2, and completes the photocatalytic cycle.

Figure 52. Photonic efficiency of (a) agglomerates; (b) TiO2 anatase calcined nanoplates; (c) washed nanoplates; and (d) P25 for NO oxidation and degradation of acetaldehyde. Modified with permission from ref 136. Copyright 2013 Elsevier.

Figure 53. (a and b) SEM image of the as-prepared BiOI sample; (c) TEM images of the as-prepared BiOI sample; (d) the SAED pattern of the as-prepared BiOI sample; (e) the high-resolution TEM image of the as-prepared BiOI sample. Reprinted with permission from ref 314. Copyright 2015 Elsevier.

selectivity toward N2 (SN2), possibly due to the high oxidative ability of the Pt/TiO2 photocatalytic systems; series of Pdmodified photocatalysts show a similar trend (Figure 54B). The rate of NH3 photocatalytic oxidation followed the order 1Ag > 1Pt > 1Pd > P25 > 1Au (Figure 54C), which is also paralleled by an increase of selectivity to NO3−. Ag appears to be the most active photocatalyst in terms of NH3 conversion, with a XNH3 value 1.5 times higher than that attained with P25, but it also produces huge amounts of nitrite and nitrate anions. The presence of Pt NPs leads to a consistent drop of selectivity toward N2, possibly due to the high oxidative power of the Pt/

4.2. Selective Reduction of Inorganics

Recently, many studies have been dedicated to the reduction of inorganics, especially CO2, because of the environmental problems. The efficient selective photoreduction of inorganics to useful chemicals is still a big challenge.294,320−322 4.2.1. CO2 Conversion. CO2, an inevitable product of fossil fuel combustion, contributes to possible climate change and 1490


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Figure 54. Photocatalytic ammonia conversion (XNH3) and products selectivity (SY) attained after 6 h with (a) P25, blank, and Pt-modified photocatalysts with different NPs loadings; (b) Pd-modified photocatalysts with different NPs loadings; and (c) 1.0 wt % NPs-loaded photocatalysts. Modified with permission from ref 311. Copyright 2013 Elsevier.

selectivity for CH4 is usually higher than CO, because of the reconciliation between thermodynamics and charge migration. The generation of CH4 (Eredox/SCE = −0.48 V) is more thermodynamically viable than the generation of CO (Eredox/ SCE = −0.77 V) when the supply of protons and electrons is high enough.326 Usually, alcohols are the favored products because of their ease of collection and high added value. There are two significant species, •H (hydrogen atom) and •CO2− (CO2 anion radical), involved in photocatalytic CO2 conversion, which obtain electron from the CB. Methoxyl (•OCH3) and methyl (•CH3) radicals are also reaction intermediates during the process of photocatalytic CO2 conversion.327−329 CH4 can be produced by •CH3 radical reacting with proton and electron (•CH3 + H+ + e− → CH4) and coupling of two •CH3 leads to C2H6 (•CH3 + •CH3 → C2H6).330 Paul and Hoffmann331 suggested that the dimerization of •CH3 preferably occurs via a radical-substrate reaction mechanism in a hydrogen deficient system. This section covers strategies to improve the selective formation of various products and the impacts of important factors on the reaction, such as metal loading, composite materials, special light source, and new materials.25,38,39 4.2.1.1. Cocatalysts. Both the yield and selectivity of photocatalysts, especially the selectivity for CH4 formation, are usually enhanced by loading various metals, e. g. Pt, Pd, and Cu.315,320,332−334 According to the various studies, the CO2 chemical reduction to CH4 follows two different reaction mechanisms, viz. CO2 → HCOOH → HCHO → CH3OH → CH4 and CO2 → CO → C• → CH2 → CH4.323 As metal is loaded on the surface of photocatalyst, the electrons are liable to transfer from the photocatalyst to the metal and disperse on the surface, and therefore the recombination of the photoexcited electron−hole pair is suppressed.7 The amount of metal loading has an optimum, and excessive metal loading is a futile effort, since photons cannot be absorbed because of reflection. The appropriate Pt-metal content can obviously improve the photocatalytic activity; Pt/TiO2 nanotube is more active than Pt/TiO2 NPs catalyst.320 Since Pt-metal has higher work function than TiO2, some electrons transmit via the Pt-metal and effectively avoid the recombination with holes to extend the life-span of the electron−hole pairs; 0.12 Pt/TiO2 NPs catalyst exhibits the highest efficiency toward CH4 formation (0.0565 mmol h−1 g Ti−1 after 7 h UV exposure) as the molar ratio of H2O/CO2 is 0.02. With the increase of reaction temperature, the yield of CH4 is improved on both Pt/TiO2 nanotube and Pt/TiO2 NPs catalyst, because that the higher temperature facilitates the desorption of products. In another

Figure 55. Proposed mechanism for selective H2O2 formation on the surface of g-C3N4. Adapted with permission from ref 318. Copyright 2014 American Chemical Society.

pose a severe threat to the environment. Photocatalytic transformation of CO2 into organic fuels is therefore of growing interest, since the process is simple and manageable, involving only water, CO2, photocatalyst, and light.22−29 Notwithstanding that the research on photocatalytic reduction of CO2 is still at an early stage, CO2 is a rather inert compound and its conversion to other carbon compounds is generally thermodynamically unfavorable.323 An array of products can be obtained over various photocatalysts under light illumination emanating from CO2 and H2O, namely CO, CH4, CH3OH, HCOOH, among others, involving multielectron processes.324,325 A few representative reactions with their needed electron number and corresponding reduction potential are shown in Figure 56.26 The mechanism for CO2 photoreduction is essentially complicated.315 Most scholars consider that the reaction is related to photoinduced multielectron transfer rather than single electron transfer, since the electrochemical potential for a single electron process is extremely critical (−2.14 V vs SCE).326 The formation of products can be different with various electrons and protons. CO is generated by reacting with two protons and two electrons, while the formation of CH4 requires eight electrons and eight protons. However, the 1491


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Figure 56. CO2 reduction products and corresponding reduction potential with referene to normal hydrogen electrode (NHE) at pH 7.

Figure 57. Schematic mechanism for CO2 photoreduction over Pt-TiO2 nanostructured films. Reprinted with permission from ref 315. Copyright 2012 American Chemical Society.

case, the Pt-TiO2 nanostructured films also display outstanding performance toward selective photoreduction of CO2 to CH4; the yield of CH4 is up to 1361 μmol g−1 cat h−1.315 The deposit of Pt NPs is beneficial to electron migration and thereby electron−hole separation, which is the plausible reason for improvement in the photocatalytic performance, as well as the size of the Pt NPs and the 1D structure of TiO2. CH4 can also be generated by converting CO via the six-electron reaction, which supports the selective formation of CH4 (Figure 57). Guan and co-workers have demonstrated that CH4, HCOOH, and HCHO are produced as well as H2 over Pt/K2Ti6O13, while CH3OH and C2H5OH are generated besides the aforementioned products by using a hybrid catalyst containing Fe−Cu− K/DAY (Fe-based catalyst supported on a dealuminated Y-type zeolite) and Pt/K2Ti6O13.335 The photocatalytic reduction of CO2 by Wu et al. shows that Cu loading enhances the CO2 reduction activity and the selectivity toward CH3OH.332,334 The activity of Ag/TiO2 is lower than Cu/TiO2 because of the strong affinity between Ag particles and photogenerated electrons. The distribution of Cu on/in the TiO2 particles is critical to amplify the yield of CH3OH. The maximum CH3OH yield of 1000 mmol/g cat is obtained over the 25 molar % Cu loaded TiO2, which is much higher than those of sol−gel TiO2 and Degussa P25. In

addition, the CH3OH yield is significantly increased by adding NaOH, due to the improved dissolution of CO2 and more quenched hole. The loaded Cu usually serves as an electron reservoir to improve the performance of Cu/TiO2, resulting from the Schottky barrier between Cu and TiO2 as well as the redistribution of the electric charge. Molecules of CO2 (or HCO3−) and H2O can interact with the trapped electrons on the Cu cluster, and consequently, the reduction of CO2 and the decomposition of H2O competitively occurs. OH radicals are generated via the reaction between OH− and holes on the TiO2 surface, and then these radicals possibly react with the carbon species generated from CO2 to produce CH3OH. The chemical states and the location of Cu on TiO2 play important roles in the photoreduction of CO2; isolated Cu(I) being considered as the main active site for photoreduction. A similar phenomenon is uncovered when CH4 is selectively produced with Cu species, Cu2O, deposited on TiO2;336 this boosted selectivity for CH4 production can be due to the function of Cu species as electron traps which increases the probability of multielectron reactions (e.g., eight electrons for CH4 production). The optimal Cu loading on the Cu/TiO2−SiO2 composite is found to be 0.5 wt %. Cu(I) species may be reduced to Cu(0) during the photoreduction, and the Cu(0) species can be reoxidized back to Cu(I) in aerobic atmosphere. When Cu-TiO2 is highly 1492


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The synergistic effect of the surface-Ti3+sites and GR prefers the formation of C2H6, and the yield of the C2H6 increases with the enhancement of GR use (Figure 59).25 In Gx-TiO2

dispersed on 5A molecular sieves, its selectivity for the photocatalyic conversion of CO2 to oxalic acid is enriched, and the yield of oxalic acid is up to 65.6 μg h−1 g−1 per cat.234 However, unsupported Cu-TiO2 system shows comparatively lesser formation of oxalic acid. The product formation is due to the product shape selectivity of the composite photocatalyst, as well as adsorb−desorb shuttle mechanism of CO2 on the support which has a crucial effect on selective formation of the products. Under high pressure of CO2, 2.8 MPa, the main products over Cu loaded TiO2 are methane and ethylene, rather than methyl alcohol and formaldehyde. Under optimized conditions, the yields for methane, ethylene, and ethane are 21.8, 26.2, and 2.7 μL/g, respectively.337 Pd plays a quite noteworthy role in the reduction of CO2/ HCO3− to formate, due to transfer of electrons from TiO2 to properly prepared catalytic Pd surface sites;333 specific sites on the Pd surface favor CO2/HCO3− reduction over H2 evolution (Figure 58). First, hydrogen atoms or hydrides are formed by

Figure 59. (a) Photocatalytic efficiency of Gx-TiO2 (x = 0, 1, 2, 5) and P25. The molar ratio of C2H6 to CH4 improves from 0.71 (for G0TiO2), 2.09 (G1-TiO2), 2.10 (G2-TiO2), to 3.04 (G5-TiO2). (b and c) Photocatalytic CH4 and C2H6 formation over Gx-TiO2 (x = 0, 1, 2, 5). Modified with permission from ref 25. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

nanocomposites, x represents the weight contents (wt %) of GR. With cooperation of GR, the production rate of CH4 slowly decreases but the production rate of C2H6 noticeably increases, i.e., the molar ratio of C2H6 to CH4 surges from 0.71 (for G0-TiO2), 2.09 (G1-TiO2), 2.10 (G2-TiO2), to 3.04 (G5TiO2), implying that C2H6 is easily produced with introduction of GR. Paul and Hoffmann331 have suggested that the dimerization of •CH3 preferably occurs via a radical−substrate reaction mechanism in a hydrogen deficient system. For the GxTiO2 systems, •CH3 radicals may be absorbed on the surface of GR via π-conjugation between the unpaired electron of the radical and aromatic regions of the GR.339,340 The electron-rich GR may help to stabilize the •CH3 species, which restrains the combination of •CH3 with H+ and e− into CH4. Meanwhile, subsequent growing accumulation of •CH3 on the GR raises the opportunity for the formation of C2H6 by the coupling of •CH3. 4.2.1.2. Composite Materials. In the g-C3N4/N-TiO2 composite, the ratio of g-C3N4 to N-TiO2 has a vital impact on the selectivity of CO2 reduction.341 When the mass ratio of urea to Ti(OH)4 is larger than 1:1, corresponding to g-C3N4 and N-TiO2 composites, only CO is generated as the reduced product, while CO and CH4 can be obtained on N-TiO2 samples with preparation ratios of urea to Ti(OH)4 lower than 60:40. All the prepared samples show better photocatalytic activity than P25, indicating that the formation of composites bearing g-C3N4/N-TiO2 is an effective way for the reduction of CO2. The plausible reaction mechanism for the formation of CO over composite photocatalysts under light irradiation is illustrated in Figure 60. Only two electrons are needed for each CO molecule evolution. Therefore, the photoreduction of CO2 to CO may be a dynamically favored process in the present system. Under simulated light illumination, charge carriers are formed and transferred between the interface of g-C3N4 and N-

Figure 58. Representation of the photocatalyzed reduction of HCO3− to formate by Pd(β-CD)-TiO2. Adapted with permission from ref 333. Copyright 1990 American Chemical Society.

dissociation of H2 on the Pd surface or by proton reduction on electronically charged Pd. Bicarbonate is then activated on specific Pd sites followed by its hydrogenation to formate. The photoreduction of CO2/HCO3− probably is due to electronic charging of Pd sites by conduction-band electrons of TiO2. Afterward, proton reduction occurs at the metal surface and then hydrogenation of CO2/HCO3−. It has been reported that selective formation of C1−C3 compounds occurs favorably on basic oxide (MgO) supported catalysts, whereas an acidic oxide (Al2O3, SiO2) support produces selective formation of C1 compounds.330 The products are 0.4 mmol h−1 of CH4 and 0.5 mmol h−1 of C2H6 over TiO2/Pd/Al2O3 whereas in the TiO2/Pd/SiO2 system 0.8 mmol h−1 of CH4 are formed with no C2H6. In contrast, the bulk Pd/TiO2 system exhibits a very high selectivity for the production of CH4 from the photoreduction of CO2 indicating that the nature of the support itself alters the selectivity to C1 and C2 compounds. It is believed that the lowered work function of an alkali promoted surface facilitates a high probability of electron transfer to impinge neutral CO2 molecules.331 The dimerization occurs via a radical−substrate reaction mechanism in a hydrogen deficient system. Yamashita and co-workers have confirmed the selective formation of CH3OH over a highly dispersed TiO2 species in the CO2 reduction, while the use of aggregated octahedrally coordinated TiO2 species as the catalyst shows selective formation of CH4.338 1493


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Under UV−visible-light exposure, the photoreduction of CO2 in water with BiVO4 leads selectively to ethanol, and monoclinic BiVO4 is more efficient and selective than tetragonal BiVO4.344,345 CO32− adsorbed to the Bi3+ sites on the surface via a weak Bi···O bond can effectively trap the photoformed electrons from the V 3d-block bands of BiVO4. The asymmetry of the local environment around the Bi3+ ion is more firmly in monoclinic phase than that in tetragonal phase, therefore, the Bi3+ ion in the monoclinic phase has a stronger lone pair character to form a Bi···O bond with CO32−, causing a more effective migration of photoexcited electrons from the V 3d-block bands to the anchored CO32−. The addition of NaOH in the BiVO4 system facilitates the enhancement of CH3OH yield, similar to the results mentioned above.345 The CH3OH production rate is 3.76 μmol h−1 under visible light irradiation, with ∼31% decrease as compared to that under full spectrum, indicating that UV-light also contributes to the selective CH3OH production. BiVO4 cannot produce H2 due to its energy band structures are unmatched with the water reduced potential. The photoreduction of CO2 and the decomposition of H2O proceeds competitively on the BiVO4 surfaces, and CH3OH is preferentially produced since the protons in water are difficult to capture the photogenerated electrons due to their unmatched energy band structures. However, it still is not clear whether the BiVO4 surface state can be restored during the photocatalytic CO2 reduction or not, which needs further investigation. Selective CH3OH production over NiO/InTaO4 has been observed under visible light.346 The In2Ge2O7 (EN) (EN = ethylenediamine) ultrathin nanowire can selectively reduce CO2 into CO with the existence of water vapor.39 Under the same photocatalysis conditions, Zn2GeO4 nanoribbon photocatalysts can be used to selectively produce CH4. Anpo et al. have reported that the tetrahedrally coordinated TiO2 photocatalysts, well dispersed in a silica matrix, can lead to excellent photocatalytic activity and selectivity for the generation of CH3OH via the photoreduction of CO2, while the octahedrally coordinated bulk TiO2 photocatalyst and impTi-oxide/Y-zeolite produces CH4 exclusively.192,347−349 The hydrophilic−hydrophobic performance of zeolites is the key factors in determining the reactivity and selectivity. In the case of low concentration of surface OH groups, the selectivity for the formation of CH3OH is possibly higher since the reaction for the formation of CH4 is inhibited. The plausible explanation is that the higher concentration of surface OH groups facilitates the interaction of the H2O molecules with the excited state of the tetrahedrally coordinated Ti-oxide species, possibly affecting the selectivity toward the generation of CH3OH.350,351 When Ti-β zeolites are synthesized using F− ions as anions of the structure-directing agents (SDA), the selectivity for CH3OH could be improved.348 In addition, the dispersion state of the Ti-oxide species and the pore structure also affects the formation of CH3OH.338 The ex-Ti-oxide/Yzeolite, which has the high dispersion state of the Ti-oxide species and the large three-dimensional pore channel, displays an excellent activity and selectivity for the generation of CH3OH. Due to the similar reason, Ti-MCM-48 zeolite has higher reactivity and selectivity than TS-1 or Ti-MCM-41. When Ti content is varied in Ti-containing porous silica, the selectivities for CH3OH formation are different too.347,352,353 Unlike Ti-PS (h, 50), which is tetrahedral, Ti-PS (h, 25) possessing an aggregated octahedral structure just like that of bulk TiO2 shows a higher selectivity for CH4 formation,

Figure 60. Proposed mechanism for the photoreduction of CO2 over g-C3N4 and N-TiO2 composites (a) and N-TiO2 (b). Reprinted with permission from ref 341. Copyright 2014 Elsevier.

TiO2; e− in the CB of g-C3N4 can migrate quickly to the CB of N-TiO2 for CO2 reduction to CO. In contrast, when the ratio of g-C3N4 is low, electron generated in photocatalytic reaction may be quickly consumed by CO2, thus generating CH4 and CO, due to the absence of conjugated aromatic system. 4.2.1.3. Various Materials. In general, the activity and selectivity of photocatalytic CO 2 reduction is mainly determined by the property of the photocatalyst and the reaction condition.328,342 For the CO2 photoreduction, the C2H5OH can be selectively gained over nonporous flaky C3N4 synthesized from melamine, while the mesoporous flake-like g-C3N4 prepared from urea can result in the generation of a mixture comprising CH3OH and C2H5OH.40 The amount of H+ ions usually controls the direction and the product selectivity of the CO2 reduction; differences in the crystal size, crystallinity and microstructure may be responsible for the selective generation of CH3OH and C2H5OH in the system. The helical-rod like g-C3N4 (HR-CN) is shown to have unequal quantities of left- and right-handed helical nanorods and exhibits unique optical activity to circularly polarized light at the semiconductor absorption edge;343 HR-CN gives a higher yield (9.2%) and better selectivity (97%) for CO production than bulk g-C3N4. It is assumed that the mechanism of the optical activity for HR-CN may be similar to that of chiral TiO2 nanofibers. The helical nanostructure of HR-CN leads to an asymmetric electric field, and the semiconductorbased electronic transitions from the VB to the CB, under the dissymmetric field, results in its optical activity. 1494


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Figure 61. Hypothetical reduction pathways for CO2 in different solvents. Modified with permission from ref 356. Copyright 1997 Elsevier.

photocatalytic efficiency under visible light illumination; N2 gas selectivity is approximately 85% with 1.0% Nd, N-codoped TiO2, whereas it is only around 35% with N-TiO2. Enhanced nitrate conversion and N2 selectivity can be ascribed to the synergistic impact of the optimal Nd concentration, high surface area, and appreciable visible light absorption of Nd, Ncodoped TiO2. Among the hole scavengers studied, HCOOH is found to be the most effective as the nitrate conversion is >98% with 85% N2 selectivity, while with other hole scavengers the conversion is below 40%. The amount of HCOOH also influences the reaction efficiency and product selectivity. The initial increase in HCOOH concentration leads to the increased conversion and N2 selectivity; an initial 200 mg/L concentration of HCOOH delivers the maximum conversion percentage of nitrate (98%). However, nitrate conversion and N2 selectivity gradually decreases when the amount of hole scavenger is greater than 200 mg/L. Additionally, when the initial pH varies from 3.5 to 2.8, the nitrate conversion yield only changes from 95% to 99% while N2 selectivity fluctuates from 82% to 85%. Ag/TiO2 finds useful applications in degradation of nitrite ions where its preparative method defines its efficacy.359 The conversion of nitrite ions to N2 on Ag/TiO2 (P) (the photoinduced synthesized Ag/TiO2) reaches 99.5% after 40 min of irradiation, much higher than that achieved on 1 wt % Ag/TiO2 (C) (Ag/TiO2 prepared by the conventional chemical reduction method) (55%). The different catalytic activity and selectivity on Ag/TiO2 (P) and Ag/TiO2 (C) may be attributed to their dissimilar morphology and size of deposited Ag. In contrast to the Ag/TiO2 (C) catalyst, Ag/TiO2 (P) samples shows more homogeneous Ag morphology and narrow size distribution; deposited Ag particles on the Ag/TiO2 (C) catalyst are typically aggregated and heterogeneously dispersed. The bigger metal particles not only decrease the number of active sites trapping the electrons but also enrich much more photogenerated electrons and become new recombination centers of photogenerated carriers. Better separation of photogenerated carriers occurs on Ag/TiO2 (P) catalysts, which have much smaller and homogeneous dispersion of Ag. Consequently, the photocatalytic efficiency on the Ag/TiO2 (P) catalysts is significantly superior to that on the corresponding Ag/TiO2 (C) catalyst. Cu−Pd nanoalloys deposited on TiO2 (Cu−Pd/TiO2) show high selectivity for conversion of NO3− to NH3.295 TiO2 calcination temperature, Pt/Cu ratio, metal loading amount, additions, and the pH value all play important roles in the reaction. First of all, the coexistence of metallic Cu and Pt is crucial for the selective nitrate reduction to N2. The yield for NH3 over Cu−Pd/TiO2 under UV irradiation for 5 h is found to be 76%, which corresponds to 78% of selectivity. There are two possible explanations for the high selectivity to NH3. One is that Cu and Pd atoms are homogeneously distributed in the Cu−Pd nanoalloy. Hydrogenation of NO2− is considered to be a critical step controlling selectivity toward N2 and NH3.360,361

whereas Ti-PS (c, 50) exhibits a better selectivity for CH3OH generation. The selectivity toward CO evolution during the photocatalytic conversion of CO2 can be controlled by doping.354 The addition of dopant of Zn species in Ga2O3 inhibits the H2 evolution from water splitting and, consequently, Zn-doped, Ag-modified Ga2O3 displays better selectivity (87%) for the generation of CO than bare, Ag-modified Ga2O3 (26%). 4.2.1.4. Effect of Reaction Conditions. Increase in CO2 pressure is one of the strategies for increasing the concentration of CO2 and the CO2 reduction selectivity;355 yield of HCOOH increases linearly but slightly with the pressure, while the yield of CH3OH enhances rapidly with the increase of CO2 pressure to 1.0 MPa, and then decreased sharply with its further increase. The possibly reason it that CO2 pressure increases the absorption of hydrogen and carbon species on the TiO2 surfaces, improving the reaction between the two species and then the formation of lower hydrocarbons as gaseous products. When water is replaced by the reductants, the reaction mechanism possibly changes, which can provide both high yield and great selectivity toward target products.356,357 As Figure 61 illustrates, •CO2− anion radicals in low polar solvents, such as CCl4 and CH2Cl2, tend to be adsorbed on the positively charged Cd2+ sites of CdS surface by the carbon atom of •CO2− anion radicals, resulting in the CO formation (•CO2− + •CO2−→ CO + •CO32−).356 If solvents of high dielectric constant such as water and propylene carbonate are used, the formed •CO2− anion radicals may be highly stabilized by the solvents, leading to weak interaction with photocatalyst surfaces. Then the carbon atom of •CO2− anion radicals usually react with a proton to give formate (•CO2− + 2H+ + e− → HCO2H). The production of CO is more favorable in the case of CdS particles rather than TiO2 nanocrystals embedded in SiO2 matrices (Q-TiO2/SiO2), suggesting that •CO2− anion radicals tend to be more easily adsorbed on the CdS surface rather than Q-TiO2/SiO2 surface.357 4.2.2. Conversion of Nitrates. Nitrates are highly mobile in the environment and cannot be constrained by many materials. Nitrates and their metabolites, which include nitrite, nitrosamine, and ammonium ions, are toxic to human health, particularly to children. With the development of modern industry and agriculture, the concentration of nitrite ions in drinking water is significantly rising because of excessive industrial effluents, over use of nitrogeneous fertilizers, and incomplete abiotic denitrification process in the soil. Nitrate pollution of groundwater restricts its use in a drinking water supply system unless the nitrate is removed.358 TiO2 exhibits considerable activity for the photocatalytic reduction of nitrate ions. In order to improve its selectivity for nitrogen, metaldoping and loading have been investigated.294,321,322 Both the nitrate reduction and N2 selectivity of neodymium (Nd), N-codoped TiO2 are higher than those of N-doped TiO2 alone.293 The codoped TiO2 with 1.0% Nd displays the highest surface area, most nitrate dark adsorption, and the best 1495


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reoxidized to the Ti4+ by oxygen. NO can also be adsorbed directly on the Lewis acid site which is unoccupied by NH3 and transformed to inactive nitrate species. The formation of a NH2 radical on TiO2 under photoirradiation has also been confirmed by electron paramagnetic resonance spectroscopy.369 Photogenerated holes and electrons on TiO2 react with adsorbed NH3 and Ti4+ species, respectively, to generate the NH2 radical and Ti3+ species. The half-life of the NH2 radical determined by the Arrhenius equation is 1.4 min for the photo-SCR of NO with NH3. It has been found that the photo-SCR occurs under not only UV ray (99% consumption of ANL, affording quinaldine in only ca. 50% and 2,3dimethylindole (15%) in addition to small amounts of other reduction products (5%). The authors contend that the electorns trapping ability of Au can promote the separation 1498


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Figure 64. Schematic illustration of plausible pathways for the formation of cyclized products. Modified with permission from ref 108. Copyright 2010 Elsevier.

considered to be the best medium among the solvents studied (acetonitrile, methanol, toluene, and trifluorotoluene). The radical scavenger, 2,6-ditert-butyl-4-methylphenol, cannot have an effect on the yield, eliminating the autoxidation possibility via radical chain mechanism. The couping reaction is also initiated by photoinduced e− and h+ pairs on mpg-C3N4 photocatalyst (Figure 65); first, the amine possibly loses an

photoirradiation of NBz affords only 60% yield of the product quinaldine. By comparison, 75% product quinaldine yield is obtained at 5 h over Au/TiO2. For the transform of 3nitrotoluene and 4-methoxynitrobenzene to quinaldine, the higher activity and selectivity are obtained on Au/TiO2 that the yield of quinaldine is 80% and 60%, respectively, and in contrast, blank TiO2 affords only 68% and 42% of quinaldine, respectively, indicating the significant role of Au loading in the fast and selective synthesis of quinaldine. Using supports, the selectivity can be further enhanced as exemplified for the higher selectivities of cyclohexanol and cyclohexanone formation on Au@TiO2/MCM-41 than those on Au/TiO2.374 The synthesis of quinaldines has been conducted on Ag/ TiO2 with Ag content varying from 0.5 to 2.0 wt %, and the loaded Ag with low amount is found to facilitate the electron− hole separation because that the electron migration from the TiO2 CB to Ag particles is thermodynamically conceivable due to the higher Fermi level of TiO2 than that of Ag.101 As the Ag deposition is below 1.5 wt %, the catalytic activity improves with the enhancement of Ag amount and then declines, since the excessive Ag possibly acts as electron−hole recombination centers, and accordingly, 1.5 wt % of Ag loading is the optimal amount. The impact of addition of water on the selective formation of quinaldines in ethanolic solvent containing ANL or NBz has been noticed.109,187 In both cases, the use of water significantly reduces the selectivity of quinaldine due to the unselective attack from the photoformed •OH radicals; when the amount of water increases from 0 to 4%, the yield of quinaldine from NBz and ANL decreases from 75% to 6% and 5%, respectively. Light-excited g-C3N4 can also activate oxidative coupling approach wherein the conversion of benzylamine reaches 34% and 60% at 60 and 80 °C, respectively, with high selectivity (99%) of N-benzylidene benzylamine.375 Acetonitrile is

Figure 65. Plausible pathway for aerobic oxidative coupling of amines. Modified with permission from ref 375. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

electron to generate the carbocationic-radical type intermediate, afterward, the corresponding imine is formed by reacting with the O2•−. Subsequently, the imine is coordinated to positively charged h+, which makes it more liable to confront nucleophilic attack by the amine to form the aminal group and then the final coupled product. 4.3.3. C−O Coupling. A highly selective dehydrogenative C−O coupling reaction of ethanol to 1,1-diethoxyethane 1499


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(DEE) is accomplished on Pt-loaded TiO2 nanotubes and nanorods, and the conversion can reach to 157.7 mmol g−1 h−1.297 In darkness, no reaction is observed; however, under UV irradiation, DEE is obtained with selectivity of 99.6%, and the reaction reaches an equilibrium state within a short duration of typically 40 min. Without any metal loading, TiO2-nanorods are inactive in the present reaction, so the use of metal cocatalysts (Pt, Pd, Au, and Rh) is necessary for the C−O coupling reaction. Pt and Pd are highly active cocatalysts, whereas Au and Rh show very low activity with 99%). During the reaction process, dehydrogenation of ethanol into acetaldehyde first occurs via reacting with photoinduced holes, and subsequently, acetalization between acetaldehyde and ethanol takes place facilitated by H+ ions (Figure 66).

Figure 67. Proposed pathway for photoacetalization. Reprinted with permission from ref 376. Copyright 2016 The Royal Society of Chemistry.

Figure 66. Proposed mechanism for the photocatalytic dehydrogenation coupling of ethanol into DEE over noble metal loaded TiO2. Modified with permission from ref 297. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA.

compounds, and a few unidentified products may be involved as shown by the reaction pathways depicted in Figure 68.

The photocatalytic acetalization of aldehydes/ketones over gC3N4 using molecular oxygen and visible light under ambient conditions has been first reported by Tsang et al.;376 conversion of benzaldehyde with excess methanol to the corresponding acetal is >97% with a selectivity >98% in the absence of any acid additive. When visible light is irradiated on g-C3N4, surface charge separation of N and O2 can be generated, which may help to deprotonate alcohol to form adsorbed protons on the surface initially, as summarized in Figure 67. Such transient surface charge species will also induce fast acetalization on the material surface from polar aldehydes and alcohols via an ionic mechanism. This may entail the formation of an oxonium ion (g) as reflected by the negative value of ρ = −1.9 in the Hammett study to form acetal. 4.3.4. N−N Coupling. The 2,2′-dinitrobiphenyl (DNBP) can undergo intramolecular reductive cyclization reactions on P25-TiO2 under UV light illumination due to the close spatial proximity of the interacting NO2 groups that lie in two different benzene rings, and selectively generate nonconventional production of benzo[c]cinnoline (BC).377 However, the pure rutile TiO2 phase does not have any photoreactivity for DNBP reduction under the same experimental conditions. The yield and selectivity could be simply tuned by light irradiation under ambient conditions. Under an argon atmosphere, 23.8 μmol of BC (95%) and 2,2′-biphenyldiamine (5%) are produced over 50 mg P25-TiO2 in 50% aqueous IPA after 20 h UV light irradiation, and the amount gradually increased with the irradiation time beyond 20−24 h due to further reduction of benzo[c]cinnoline. It is revealed that the rate-determining step for DNBP photocatalytic reduction using TiO2 is the selective formation of BC via some transient intermediates; nitroso and hydroxylamine derivatives benzo[c]cinnoline dioxide, azoxy

Figure 68. Possible reaction pathways of DNBP reduction by TiO2 and UV light irradiation. Modified with permission from ref 377. Copyright 2015 The Royal Society of Chemistry.

4.3.5. S−O Coupling. Atmosphere has an important impact on the selective photocatalysis.296 Without O2, dioctylsulfide is selectively produced from 1-octanethiol, whereas the high selectivity for n-dioctylsulfide simply vanishes with the existence of O2, because of concurrent oxidation reactions by oxygen; under nitrogen, the main product is the linear dioctylsulfide, with a selectivity of 96% (with respect to 1octanethiol conversion). Whereas in the presence of oxygen, the 1-octene conversion is only 57.5% after 300 min and the main products are the linear and the branched dioctyl-disulfide (selectivities equal to 18% and 6%, respectively). The competition exists between oxo- and thio-photocatalysis when the latter is performed in the presence of oxygen. TiO2’s surface can be sulfurized by S atoms from 1-octanethiol: O2−(surface) 1500


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+ C8H17SH → S2− (surface) + C8H17OH. The photoassisted self-sulfurization of TiO2 creates sulfide sites for adsorption of sulfur-containing reactants (thio-compounds). 4.3.6. Acceptorless Dehydrogenation Coupling. In organic media or neat reagents, primary alcohols (including biorenewable ethanol and butanol) can undergo the acceptorless dehydrogenation coupling (ADC) reaction over the highly active P25-TiO2 photocatalyst under UV irradiation, resulting in the selective generation of acetals.378 First, aldehydes are formed via hole-induced dehydrogenation of alcohols which then condense in situ with unreacted alcohols to selectively generate acetals (selectivity >99%). Noble metal cocatalysts such as Pt, Au, and Rh can promote the ADC reaction to different levels with no obvious loss in reaction selectivity.

5. SUMMARY, CHALLENGES, AND PERSPECTIVE Selective reactions are usually proposed as a crucial steps in many organic syntheses. Though photocatalytic reactions have been considered to be “nonselective” processes, selective reactions are undoubtedly a significant research area of photocatalysis, which has garnered much attention and interest. Attempt has been made to emphasize the accomplishments attained to date on the improvement of photocatalytic selectivity. As described in the aforementioned sections, the major efforts have focused on oxidation and reduction reactions due to the peculiarity of photocatalysts, which have been extensively investigated.46,54−56,67,72−83,285−288 Although not fully exploited with availability of meager literature reports,116,373 special photocatalysts can work for some other reactions namely coupling and addition reactions. Among the photocatalytically selective investigations, oxidation reactions have been studied the most, and the majority of examples are focused on the oxidation of organics, especially selective oxidation of hydroxyl groups to carbonyls. The key to the modification of photocatalysts for high selectivity is avoiding the formation of radicals, particularly •OH, which are widely regarded as unselective species. Some photocatalysts, e.g. C3N4, may naturally have good selectivity due to their narrow band gap, which is unsuitable to generate •OH. For those photocatalysts with wide band gap, modifications are prerequisite to obtain high selectivity, which can generally be achieved by using dopants, forming special crystal facets and phase, because of the changed band gap or performance. However, the selectivity is usually enhanced at the expense of less conversion by using these methods. In addition, the change of adsorbability is helpful to the desired improvement of photocatalytic selectivity, since reactants have to be adsorbed on the surface of catalysts before the performance of photocatalysis. These kinds of strategies include MRS, surface acid−base properties, spatially confinement effect, and so on, which possibly are conducive to simultaneously increase the selectivity and conversion. External conditions can also be used to tune the selectivity of photocatalysis by changing solvent, atmosphere, and pH value. It is worth revealing that epoxidations can be catalyzed by Ti−Si molecular sieves under light irradiation, and the underlying mechanism is different from the traditional semiconductor photocatalysis. In comparison with photocatalytic selective oxidations, the investigations pertaining to selective photoreduction of organics are scant, and most of them are restricted to nitro compounds exclusively. Forming composites, modifying surfaces, exposing particular phase and crystal facets, as well as changing external conditions are often used strategies for enhancing the selectivity in reduction. Since a variety of reaction conditions are deployed in the photocatalytic investigation, it is hard to articulate which photocatalyst is the best for selectively in photocatalytic reactions. However, according to these documented investigations, it can be safely concluded that the modified photocatalyst with wide band gap, e.g., TiO2, may be a better choice to simultaneously get high selectivity and activity for both photocatalytic oxidation and reduction. In recent years, it has become clear that the burgeoning interest in photocatalytic CO2 conversion into organic fuels is due to the pressure and demands from the energy and environmental protection. CO2 is a rather inert compound and its conversion to other carbon compounds is generally thermodynamically unfavorable, and the research on photo-

4.4. Metathesis Reactions

The occurrence of photocatalytic metathesis reactions appears to be characteristic for the silica surface. Although other reactions occur to a small extent, the distribution of products is fairly dissimilar to that observed for the metathesis reaction. Glucose, a model biomass product, can be photocatalytically reformed on the Ni/TiO2−SiO2 catalyst.379 CO selectivity, besides H2 production, is of concern because CO easily poisons the noble metal-based catalysts in fuel cells at even very low concentrations. The loaded Ni can enhance the rate of H2 formation and suppress the selectivity of CO which is strongly dependent on the preparation method, and photoassisted deposition of Ni is the optimal way for the formation of H2 along with the worst CO selectivity. When Ni metal is deposited on TiO2−SiO2, the migration of electrons from TiO2−SiO2 to the Ni metal ensues until the two Fermi levels are aligned. The Ni metal deposited on TiO2−SiO2 provides active sites for H2 production, where photoinduced electrons are migrated to protons to generate H2. Ni metal promotes the HCOOH decomposition reaction which inhibits the CO and H2O products, resulting in much less CO being produced on Ni/TiO2−SiO2 than on TiO2−SiO2. Furthermore, Ni actually modifies the photocatalytic process of the semiconductor by changing its surface properties. The migration of excited electrons from the TiO2−SiO2 to the Ni metal causes the enhancement of the hydroxyl group acidity, which would affect the adsorption of glucose on the TiO2−SiO2 surface. This, in turn, affects the photocatalytic process on the TiO2−SiO2 surface and results in depressing the yield for CO. Rutile TiO2-based photocatalyst is highly selective for the conversion of glucose to arabinose and erythrose;380 selectivity of arabinose over rutile, anatase, and P25 TiO2 is 75, 49, and 61%, while that of erythrose is 21, 9.0, and 4.5%, respectively. When cocatalyst is loaded on rutile TiO2, the conversion of glucose is significantly enhanced. Metal cocatalysts can serve as efficient electron traps preventing photogenerated electrons and holes from recombining and serve as active sites for H2 production, while they display a negligible influence on the selectivity. Rh shows a higher conversion than that of Pt, and Pd shows a comparable activity to Rh. Inexpensive and earth-abundant metals, e.g., Cu and Ni, can also be used for conversion of glucose, giving conversions of 41% and 27%, respectively. It has been demonstrated for the first time that the photodegradation of glucose initially involves C1−C2 bond cleavage (α-scission) to produce arabinose in water as a solvent.380 1501


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catalytic reduction of CO2 is still at an early stage. An array of products can be obtained from CO2 in water over various photocatalysts under light illumination, such as CO, CH4, CH3OH, and HCOOH among others. Usually, alcohols are the favored products because of ease of collection and high added value, especially methanol, a superb fuel for internal combustion engines, fuel cells, stoves, etc., which has been considered as a vital constituent of such an anthropogenic carbon cycle in the framework of a “Methanol Economy”.381 Nonetheless, CH4 is easier to form because of the low reduction potential; more investigations are focused on the selective generation of CH4. Consequently, future studies pertaining to the conversion of CO2 to methanol should be receiving more attention. In addition, the selective photocatalytic conversion of biomass, which is an abundant and carbon-neutral renewable energy resource to produce biofuels and valuable chemicals, such as the valorization of lignin,31 should garner more consideration in terms of both sustainability and environmental protection. Overall, despite the enormous achievements and attempts made in the field of selective photocatalysis, the enhancement of selectivity is usually accompanied by the reduction of conversion, and the area of application of selective photocatalysis is constrictive. The rational design and synthesis of efficient photocatalytic materials that can lead to high selectivity toward desirable reactions, such as CO2 conversion, is still a challenge. Therefore, it remains a long road ahead before photocatalysis can find widespread applications and uses for transforming various compounds on an industrial scale. This calls for more research efforts in this burgeoning and fascinating area of science and technology.

degree in Materials Science and Engineering from Institute of Coal Chemistry, Chinese Academy of Sciences, in 2013. Currently, he is a postdoctoral researcher in College of Material Science and Engineering, Nanjing Tech. University. His current research interests primarily focus on the design and synthsis of highly efficient oxides and sulfides photocatalysts for energy utilization and environmental self-cleaning under visible-light irradiation. Yukai Chen received his BEng degree in Materials Science and Engineering from NJTech University in 2015. Currently, he is a postgraduate in the State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, NJTech University. He is now researching in the fields of advanced functional materials, nanomaterials and photocatalysis with primary research focus on the seeking and synthesizing zinc oxide-based semiconductor photocatalysts with highly efficiency under visible-light irradiation. Zhongzi Xu received his BEng and Master degrees in Silicate Engineering from Nanjing Chemical Engineering College in 1982 and 1984, respectively. He obtained his Ph.D. degree in Inorganic Metallic Materials Engineering from the Department of Silicate Engineering, Nanjing Chemical Engineering College, in 1988. He became a professor and Ph.D. Tutor in 1994 and 1995, respectively. He is now the chief scientist of National Program on Key Basic Research Project (973 Program) and Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites. He has been recipient of the First Class Prize of The Jiangsu Scientific and Technological Progress Award in 2008, First Class Prize of Technological Invention of China Petroleum Chemical Industry Federation in 2010, and Second Class Prize of The State Scientific and Technological Progress Award in 2012. He has obtained 28 National Invention Patents and published more than 160 papers. An early career researcher, he is active in the areas of cement and cementitious materials. His research interests include the mechanism of alkali aggregate reaction in cement, development of high-performance cementitious materials, and inorganic nanomaterials.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]; [email protected]. Fax: +1-(513) 569-7677.

Rajender S. Varma, (H-Index 90), was born in India (Ph.D., Delhi University 1976). After postdoctoral research at Robert Robinson Laboratories, Liverpool, U.K., he was faculty member at Baylor College of Medicine and Sam Houston State University prior to joining Sustainable Technology Division at U.S. Environmental Protection Agency in 1999 with adjunct appointment at Palacky University, Olomouc, Czech Republic. He has over 40 years of research experience in management of multidisciplinary technical programs and is extensively involved in sustainable aspects of chemistry that includes photocatalysis, synthesis, environmental sciences, and development of environmentally benign synthetic methods using alternate energy input using microwaves, ultrasound, mechanochemistry, etc; efficient technologies for greener remediation of contaminants; and environmental sciences. Lately, he is focused on greener approaches to assembly of nanophotocatalysts and sustainable applications of magnetically retrievable nanophotocatalysts in benign media. He is member of the editorial advisory board of several international journals and has published over 450 scientific papers and been awarded 15 U.S. Patents.

ORCID

Rajender S. Varma: 0000-0001-9731-6228 Notes

The authors declare no competing financial interest. Biographies Jiahui Kou is an associate professor in College of Materials Science and Engineering Nanjing Tech University. She received her Ph.D. degree from Nanjing University in 2008 under the guidance of Prof. Zhigang Zou. From 2010 to 2013, she worked in Dr. Varma’s group as an ORISE (Oak Ridge Institute for Science and Education) postdoctoral fellow at the National Risk Management Research Laboratory, U.S. Environmental Protection Agency. Her current research interests are mainly focused on homogeneous and heterogeneous photocatalysis, advanced environmental materials synthesis, as well as the design and morphological control of nanomaterials. Chunhua Lu is a full professor in College of Materials Science and Engineering, Nanjing Tech University, China. He has extensive experience in photocatalysis and phototransformation with scientific focus that includes nanomaterial, semiconductor photocataylsts, photocataylst films, and luminescent materials. He is the author or coauthor of more than 150 scientific papers.

ACKNOWLEDGMENTS National Natural Science Foundation of China (No. 51303079), Natural Science Foundation of Jiangsu Province (Nos. BK20141459 and BK20150919), Project on the Integration of Industry, Education and Research of Jiangsu Province (No. BY2015005-16), Key University Science

Jian Wang received his BEng degree in Material Chemistry from Taiyuan University of Technology in 2007. He obtained his Ph.D. 1502


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4-MBA 4-MBAD MIP MITiF

Research Project of Jiangsu Province (No. 15KJB430022), Qing Lan Project, Six Talent Peaks Project in Jiangsu Province (No. XCL-029), and Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD) are gratefully acknowledged.

MO MOF mpg-C3N4 MPS MPSi MPSO MRS mTiO2 MV NANL NBz NIP NITiF NHE NHSI NHPI NOCM nTiO2 NP 2-NP NPs 2-NPP 2-NDPP NSPs NTP NWs O2•− •OH PA 1-PE PH PHA PhDA Phe PlO PO PVDF Q-TiO2 RhB Rh6G RGO Ser TCD TF T-MPSi T-OS T-S TSA Tyr VB VHSV ZIT

ABBREVIATIONS AC activated carbons ADC acceptorless dehydrogenation coupling Ag sub Ag-substituted Ag imp Ag-impregnated Ag DP Ag-impregnated Degussa P25 ANL aniline 4-AP 4-aminophenol Asp asparagine BA benzoic acid BC Benzo[c]cinnoline BP benzophenone BPA bisphenol A BzA benzyl alcohol BzAD benzaldehyde CAT catechol CB conduction band CdS@TiO2 CSNs 1D CdS core@TiO2 shell CLA cholesteryl acetate CnNPS nanoporous silica layer CNT carbon nanotube 2-CP 2-chlorophenol 4-CP 4-chlorophenol C4P 4-butylphenol C6P 4-hexylphenol C9P 4-nonylphenol CQDs carbon quantum dots CS TiO2 unsubstituted combustion-synthesized TiO2 CSNs core−shell nanocomposites 2, 4-DCP 2, 4-dichlorophenol DEE diethoxyethane DEP diethyl phthalate DFT density funcional theory DN- BODIPY 3,4-dinitrophenyl-BODIPY DNB dinitrobenzene DNBP 2,2′-dinitrobiphenyl DPM diphenylmethane EDG electron donor group EIS electrochemical impedance spectroscopy EN ethylenediamine EWG electron withdrawing group EY eosin Y GR graphene HAA hydroxyacetaldehyde HBT 1-hydroxybenzotriazole HN- BODIPY 4-hydroxyamino-3-nitrophenyl-BODIPY HOMO the highest occupied molecular orbitals HQ hydroquinone HR-CN helical-rod like g-C3N4 IBA isobutyraldehyde IMIPs inorganic molecularly imprinted polymers IPA isopropanol 7-KOCA 7-ketocholesteryl acetate LMCT ligand-to-metal charge transfer LUMO the lowest unoccupied molecular orbitals MB Methyl Blue

4-methoxybenzyl alcohol 4-methoxybenzaldehyde = p-anisaldehyde molecular imprinted polymer the molecular imprinted polymer coated TiO2 film methyl orange metal organic framework mesoporous graphitic C3N4 methyl phenyl sulfide mesoporous silica methyl phenyl sulfoxide molecular recognition site mesoporous TiO2 methyl violet nitroaniline nitrobenzene nonimprinted nonimprinted TiO2 film normal hydrogen electrode N-hydroxysuccinimide N-hydroxyphthalimide nonoxidative coupling of methane nonporous TiO2 nitrophenol 2-nitrophenol nanoparticles 2-nitro-4-phenylphenol 4-nitro-2,6-diphenylphenol nanospheres natural phosphate nanowires superoxide radical species hydroxyl radicals propanal 1-phenylethanone phenol phenylamine phenylenediamine phenylalanine propylene oxide propene oxide poly vinylidene-fluoride quantum confined TiO2 nanoparticles rhodamine B rhodamine 6G reduced graphene oxide serine thiolated β-cyclodextrin thin film Ti-containing mesoporous silica Ti-containing mesoporous organosilicas Ti-containing silica transition state analog tyrosine valence band volume hourly space velocity ZrO2-incorporated TiO2

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Selectivity Enhancement in Heterogeneous Photocatalytic

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