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Hydrogenated Blue Titania for Efficient Solar-to-Chemical Conversions: Preparation, Characterization, and Reaction Mechanism of CO2 Reduction Guoheng Yin, Xieyi Huang, Tianyuan Chen, Wei Zhao, Qingyuan Bi, Jing Xu, Yi-Fan Han, and Fuqiang Huang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03473 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017
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ACS Catalysis
Hydrogenated Blue Titania for Efficient Solar-to-Chemical Conversions: Preparation, Characterization, and Reaction Mechanism of CO2 Reduction Guoheng Yin,†,|| Xieyi Huang,† Tianyuan Chen,‡ Wei Zhao,† Qingyuan Bi,*,† Jing Xu,‡ Yifan Han,*,‡ and Fuqiang Huang*,†,§ †
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China
§
Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
||
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
ABSTRACT: Here we report a facile low-temperature solvothermal method by using Li-dissolved ethanediamine to prepare uniform hydrogenated blue H−TiO2−x with wide spectrum response. H−TiO2−x possesses distinct crystalline core-amorphous shell structure (TiO2@TiO2−x) with numerous oxygen vacancies and doped H in the amorphous shell. Efficient solar-to-chemical energy conversions, likely photocatalytic reduction of CO2, degradation of contaminant, and H2 generation from water splitting, can be achieved over this blue titania. Notably, the optimized H−TiO2−x(200) shows high activity of CH4 formation at rate of 16.2 μmol g−1 h−1 and selectivity of 79% under full solar irradiation. The kinetic isotope effects measurements reveal that the cleavage of C=O bond from CO2 rather than O−H bond from H2O is the rate-determining step in CH4 formation. Meanwhile, the in situ diffuse reflectance infrared Fourier transform spectroscopy shows the existence of key intermediate of CO2− species. The formation of intermediate CO2− indicates that the defective surface of H−TiO2−x can efficiently accelerate the adsorption and chemical activation of the extremely stable CO2 molecule, which makes the single-electron reduction of CO2 to CO2− more easily. KEYWORDS: blue titania, solvothermal synthesis, solar-to-chemical conversions, CO2 reduction, hydrogen generation, degradation of methyl orange
1. INTRODUCTION Titanium dioxide (TiO2) as a promising semiconductor photocatalyst has attracted tremendous interest in the field of photocatalytic hydrogen generation, CO2 reduction and environmental pollution removal.1−3 However, its poor photocatalysis performance is severely insufficient for practical application due to the extremely low utilization ratio of solar energy caused by the large band gap, the rapid recombination of photogenerated electron-hole (e−−h+) pairs and the backward reactions.4−6 The large band gap of 3.0−3.2 eV also results in high electrical resistivity, seriously impedes the effective charge transfer in catalysis and limits its solar utilization in ultraviolet (UV) region, which only accounts for less than 5% of the full solar energy.7−9 Recently, great efforts are taken to
make TiO2 colorful for excellent visible (Vis-) or even infrared (IR-) light response.7−13 Colored TiO2, which owns narrowed band gap and efficient separation and transportation of photo-excited charges, has been widely reported to attain largely enhanced photocatalytic activity.4,14,15 Among which, hydrogenated TiO2 has been studied for a long time and the obtained gray, blue, or even black TiO2 exhibits improved Vis-light absorption, thus shows excellent photocatalytic water splitting and decontamination properties.16,17 Nevertheless, most of the hydrogenation of TiO2 was performed by H2 reduction at high annealing temperature and/or high pressure.4,7,18−20 Therefore, future work should be taken to explore more facile and effective method to synthesize hydrogenated TiO2 with excellent photocatalytic performance.
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Due to its strong polarity, low critical pressure and prominent chelating ability, ethanediamine (H2NCH2CH2NH2, EDA) has been widely used to synthesize new compounds with various morphologies and structures by solvother- mal method.21−23 Active metals like Zn can also be added to avoid the reoxidation of Ti3+ during the solvothermal process to prepare colored TiO2,24 however, there were few other studies using this facile metal-assisted solvothermal method to synthesize Ti3+ self-doped TiO2. On the one hand, alkali metals like Na and K are of high activities, which could seriously damage the crystal structure of TiO2. On the other hand, metals like Mg, Al, and Zn own low reducibility and the generated multiple valence ions are difficult to be removed from the defective TiO2 surface.25 Compared to other active metals, Li is of relatively high reducibility and can also be well dissolved in EDA to form Li(C2H8N2) or Li-(C2H8N2)y species with following equations (1~3), and the obtained solution can be applied to prepare superconducting materials:26−29 Li(s) + C2H8N2 → Li(C2H8N2) (1)
chemical conversions, including CO2 reduction, H2 generation, and pollutant degradation. Kinetic isotope effects (KIEs) measurements reveal that cleavage of C=O bond from CO2 is the rate-determining step (RDS) rather than cleavage of O−H bond from H2O in CH4 formation. Through the analysis of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), we found that H−TiO2−x can efficiently accelerate the adsorption and chemical activation of the extremely stable CO2 molecule on solar irradiation. Importantly, the formation of intermediate CO2− indicates that the defective surface of H−TiO2−x(200) can effectively facilitate electron-injection from TiO2−x to the adsorbed CO2 molecule and further accelerate the single-electron reduction process of CO2 to CO2−. Therefore, the optimized H−TiO2−x(200) achieves high performance of CH4 formation at rate of 16.2 μmol g−1 h−1, which is nine times of that of pristine TiO2. Furthermore, the enhanced light absorption can boost the photocatalytic efficiencies of H−TiO2−x on H2 generation and degradation of methyl orange (MO).
Li(s) + (x+y) C2H8N2 → Li+(C2H8N2)x + Li-(C2H8N2)y
(2)
2. RESULTS AND DISCUSSION
Li(s) + C2H8N2 → Li(C2H7N2) + [H]
(3)
X-ray diffraction (XRD) analysis in Figure 1a reveals that the rutile and anatase phases are partially reduced after solvothermal treatment and the ratio of the peak intensity of rutile (110) and anatase (101),39 IR(110)/IA(101), gradually decreases as the amount of Li increasing during the preparation process (Table S1). H−TiO2−x(0) shows no
Herein, we prepared the hydrogenated blue titania (H−TiO2−x) with uniform crystalline core-disordered shell structure (TiO2@TiO2−x) using a facile low-temperature solvothermal method with Li-dissolved EDA as solution. In the solvothermal environment, the dissolved Li species and the generated [H] can efficiently reduce TiO2 to produce numerous Vo and hydrogen can further intercalate into the matrix of TiO2 at relatively low temperature. The obtained blue H−TiO2−x(y) (y represents the amount (mg) of Li used in the preparation process) owns a characteristic core-shell structure with abundant Vo and doped H within the amorphous shell. These materials exhibit excellent solar or even Vis-light response in solar-to-
■
(a)
■ — Anatase ★— Rutile
A(101)
Intensity (a.u.)
The generated Li-relevant species and reductive hydrogen atom ([H]) with excellent reducibility and intercalating ability can reduce metal oxides easily or intercalate into them to form enormous surface defects and doped H at low temperature. Surface defects like oxygen vacancies (Vo) or disordered shell are found to efficiently enhance the catalytic performance of semiconductors.30−35 And the doped H can also result in the intermediate state (Ti−H), which can effectively change the band structure and the color of TiO2.36,37 Recently, Park et al. have successfully prepared colored titania with efficient hydrogen generation by the Li-dissolved EDA at room-temperature for 6 days, and the rutile phase was selectively disordered.38 Thus, the photocatalytic performance of TiO2, of which both phases have a disordered shell, is also worthy to be studied. Furthermore, the solar-to-chemical energy conversions, especially CO2 reduction, over the crystalline core-disordered shell titania with numerous Vo gets no deserved attention and the reaction mechanism remains to be further explored.
■ ■
■
R(110)
H-TiO2-x(200)
■ ■ ■
H-TiO2-x(100) H-TiO2-x(50)
★ 10
20
TiO2
★★
30
40
★ 50
60
70
80
2 Theta (°)
(b) H-TiO2-x(200)
Absorbance (a.u.)
1 2 3 4 5 6 7 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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H-TiO2-x(100)
TiO2 50 100 200
H-TiO2-x(50) TiO2
300
600
900
1200
1500
1800
Wavelength (nm)
Figure 1. (a) XRD and (b) UV-VIS-NIR Diffuse reflectance spectra of pristine TiO2 and H−TiO2−x samples. Inserts in (b) are photographs of pristine TiO2 and H−TiO2−x.
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phase change after solvothermal treatment (Figure S1), indicating that the changes of structure and properties are mainly due to the dissolved Li species or the intercalated H atoms. By contrast, the rutile phase is totally disordered when the dosage of Li reaches to 400 mg (only anatase phase remained, Figure S1), but this is unfavorable for the electron transfer due to the large bulk defects.40,41 Furthermore, the control experiments of single phase TiO2 (rutile or anatase) treated by the same procedure (denoted as H-R-TiO2−x or H-A-TiO2−x) demonstrate that the reduced rutile phase is disordered rather than dissolved in EDA (Figures S2−S5), which is consistent with previous study.38 Based on the analysis of XRD and X-ray photoelectron spectroscopy (XPS) Li 2s (Figure S6b), there is no existence of Li element or Li-related species in H−TiO2−x samples. Inductively coupled plasma (ICP) measurements also show that the residual Li is less than 0.001 wt%. The color of H−TiO2−x has turned from white of the pristine TiO2 to blue or dark-blue after solvothermal treatment, which suggests the significant enhancement on Vis-light absorption, as shown in Figure 1b. The photoresponse of H−TiO2−x is largely extended to visible or infrared light regions, and the absorption covers more than 60% of the whole solar energy. H−TiO2−x shows a similar morphology and particle size with that of pristine TiO2 nanoparticles (Figure S7). The pristine TiO2 and H−TiO2−x(0) are highly crystallized with a legible grain edge containing well-resolved lattice features shown in the high-resolution transmission electron microscopy (HRTEM) images (Figures 2a and S8). However, the HRTEM images of series H−TiO2−x in Figure 2 show that H−TiO2−x catalysts present a characteristic core-shell structure (TiO2@TiO2−x) that is similar to the
(a)
TiO2
(b)
5 nm
(c)
H-TiO2-x(100)
5 nm
H-TiO2-x(50)
hydrogenated titania synthesized by H2 reduction method,7,16,17 in which the core is highly crystallized while the shell is amorphous. The thickness of the disordered layer increases with the amount of Li used, as shown in Figures 2, S8, and S9, and achieves ca. 2 ~ 3 nm for H−TiO2−x(200) and 4 ~ 6 nm for H−TiO2−x(300) samples. It should be emphasized that the lattice of H−TiO2−x(300) is obviously damaged once large amount of Li was used in preparation process, and the much thicker disordered layer is unfavorable for the separation and transportation of photogenerated carriers.42,43 The disordered surface layer and Vo (or Ti3+) are also verified by Raman spectroscopy (Figure 3a). It is clear that the strongest Eg mode at 145 cm−1 attributed to the external vibration of the Ti−O bond exhibits a visible blue-shift as well as peak broadening after solvothermal treatment.18,44 Meanwhile, the extent of shift and broadening shows more distinct with the dosage of Li used rising to 200 mg, indicative of considerable number of defects (Vo or Ti3+) existed in H−TiO2−x and H−TiO2−x(200) owns the most. The slight shift to lower energies in XPS Ti 2p and the stronger peak intensity of Ti−OH in O 1s of H−TiO2−x (Figures S6a and S11) agree well with the Raman data.45−47 The localized Ti3+ spins can introduce ferromagnetism into TiO2, so the plots of magnetic field dependence of magnetization can serve as an efficacious probe to explore the reduction degree of H−TiO2−x.48,49 As shown in Figure 3b, the magnetization is more distinct after solvothermal treatment and becomes stronger with the amount of Li, indicative of the higher concentration of Ti3+ species. As a highly sensitive technique to probe paramagnetic species containing unpaired electrons, electron paramagnetic resonance (EPR) has been widely applied to verify the existence of Vo and Ti3+. The pristine TiO2 exhibits no recognizable resonance signal, suggesting that no Ti3+ in the nanoparticles, as shown in Figure 3c. In contrast, a strong
5 nm
(d)
H-TiO2-x(200)
5 nm
Figure 2. HRTEM images of (a) pristine TiO2 and (b, c, and d) H−TiO2−x samples. The amorphous shells are marked with arrows.
Figure 3. (a) Raman, (b) magnetic field dependence of magnetization, (c) EPR, and (d) 1H NMR spectra of pristine TiO2 and H−TiO2−x samples.
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EPR peak at a g-value of 2.003 ascribed to the unpaired electrons is observed in series H−TiO2−x, which indicates the existence of Vo combined with one electron induced by surface Ti3+ in H−TiO2−x samples.42,50 However, there were no obvious signal at g = 2.01 or 2.02 (surface Ti3+) observed, which may be overlapped by the strong signal of Vo.51 Moreover, the EPR intensity is enhanced with Li addition in preparation process, which is consistent well with the Raman and TEM results. The weaker photoluminescence (PL) peak of H−TiO2−x demonstrates that H−TiO2−x gives a much lower recombination rate of e− and h+ under light irradiation (Figure S12), indicating that the characteristic core-shell structure with enormous Vo can efficiently accelerate the separation and transportation of e−−h+ within the matrix.52−54 The role of H atoms in H−TiO2−x was further examined by solid 1H nuclear magnetic resonance (NMR) measurement. As shown in Figure 3d, the pristine TiO2 shows a main peak at δ = 5.61 ppm attributed to the characteristic bridging proton,55 which exhibits an obvious shift and broadening by solvothermal treatment probably due to the incorporation of H atoms at bridging sites generated in the amorphous layer during the reduction process.36 The peak at 3.65 ppm is detected in H−TiO2−x, which originates from a small amount of CH group introduced in the preparation stage.56 It is worth noting that a strong and sharp peak at 1.12 ppm corresponding to the dynamic exchange between 1H (fast motions) in different environments arises for series H−TiO2−x, which means rapid isotropic diffusion and exchange, and this can enhance the transportation of photogenerated electrons.39,57,58 The chemical environment of the H atoms in H−TiO2−x(200) seems to be more complicated as the peak at 0.74 ppm, indicative of more H species and stronger reduction.36 The generated Ti−H combining with numerous Ti3+ and the disordered shell can effectively change the band structure of H−TiO2−x and thus lead to the color change and the enhanced light absorption.36,37 Considering the ever-increasing concerns about the effective fixation and utilization of CO2 released by use of fossil fuels and human activity in chemical processing,2,59 photo-assisted reduction of CO2 was conducted to investigate the photocatalytic activity of series H−TiO2−x (Figure S13). As listed in Table 1, the blue H−TiO2−x samples exhibit largely enhanced activity and high CH4 selectivity for CO2 reduction under both solar and Vis-light irradiation. The CH4 formation rate gradually increases as the amount of Li increasing, and reaches up to 16.2 μmol g−1 h−1 for H−TiO2−x(200), which is nine times of that of pristine TiO2 (1.8 μmol g−1 h−1) under solar light and comparable with the best one using TiO2 as photocatalyst without metal loading (Table S2). Note that the CH4 selectivity also increases and shows 81% for H−TiO2−x(300) sample. The narrowed band gap caused by the Ti−H bonds and the disordered surface layer greatly extend the photoresponse of H−TiO2−x to Vis- or even IR-light
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Table 1. Photocatalytic activity of CO2 reduction over TiO2 and H−TiO2−x with different light sources.a STYb of products
Light source
Catalyst
Selectivity to CH4 (%)
CH4
H2
CO
TiO2
1.8
7.6
1.9
43
H−TiO2−x(50)
7.9
13.4
2.9
66
H−TiO2−x(100)
11.3
13.9
3.6
72
H−TiO2−x(200)
16.2
13.0
4.2
79
H−TiO2−x(300)
9.2
6.5
2.1
81
TiO2
0
0
0
—
H−TiO2−x(50)
1.0
1.9
0.5
62
H−TiO2−x(100)
1.7
2.5
0.7
68
H−TiO2−x(200)
2.7
3.1
0.9
73
H−TiO2−x(300)
2.1
2.1
0.4
77
Solarlight
Vislight
a
Reaction conditions: 50 mg cat., 2 bar CO2, 6 mL H2O, 5 h. b The unit of STY is μmol g−1 h−1. regions, which immensely improve the utilization of solar energy.36,37 Although the red shift of valence band (Figure S14) and the localized mid-gap electronic states (Ti−H) lower the kinetic energy of redox reactions, a large amount of electrons can be excited to the upper energy level in conduction band to fulfill the potential requirement for reduction. H−TiO2−x catalysts give a considerable Vis-light activity and selectivity. H−TiO2−x(200) displays a CH4 formation rate of 2.7 μmol g−1 h−1 and CH4 selectivity of 77%, while pristine TiO2 shows negligible performance under Vis-light irradiation. The enormous Vo formed in solvothermal treatment can serve as photogenerated carrier sinks in retarding charge recombination and promoting the electron transfer to the surface to participate in the reduction reaction,18,38,54 which are responsible for the enhanced photocatalytic activities. The more thicker disordered layer in H−TiO2−x(300) is unfavorable for the transportation of photogenerated carriers and thus leads to the low catalytic activity. In addition, the defective surface with numerous Vo can improve the adsorption and chemical activation of the extremely stable CO2 molecule effectively, which makes the limiting-step of single-electron reduction of CO2 more easily.60−62 The O2 generation during the photocatalytic reduction of CO2 with H2O was also observed. Take H−TiO2−x(200) under full solar irradiation for example, the space time yield (STY) of O2 is 37.9 μmol g−1 h−1, which is similar to that of theoretical rate of O2 formation ([(H2 formation rate)/2 + (CO formation rate)/2 + 2 × (CH4 formation rate)]) of ca. 41.0 μmol g−1 h−1 (Table S3).
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Table 2. Rate of methane evolution and KIEs of photocatalytic reduction of CO2 with H2O over pristine TiO2 and H−TiO2−x(200).a STYb of methane
Catalyst
Substrate
TiO2
H2O/CO2
1.8
—
TiO2
D2O/CO2
1.3
1.4±0.2
TiO2
H2O/13CO2
1.0
1.8±0.2
TiO2
D2O/13CO2
0.6
3.0±0.3
H−TiO2−x(200)
H2O/CO2
16.2
—
H−TiO2−x(200)
D2O/CO2
12.4
1.3±0.2
H−TiO2−x(200)
H2O/13CO2
10.3
1.6±0.2
H−TiO2−x(200)
D2O/13CO2
6.9
2.3±0.3
Moreover, H−TiO2−x(200) exhibits excellent stability toward the reduction of CO2 to CH4 under solar light irradiation, as shown in Figure S15. Compared with fresh H−TiO2−x(200), the characteristic core-shell structure of the used one shows no significant changes (Figures S16−S18) due to the existence of Ti−H bonds that can stabilize the surface disordered shell.63
KIEs
KIEs measurements have been widely used in investigating the catalytic process.64,65 To gain a further mechanistic understanding and the RDS of CH4 formation from photocatalytic reduction of CO2 with H2O over pristine TiO2 and blue H−TiO2−x(200), the experiments in which carbon atom in CO2 being labeled with 13C and the effect of deuterium oxide (D2O) were carried out. As listed in Table 2, D2O and 13CO2 show significant KIEs in the catalytic process over both catalysts, indicative of the cleavages of O−H bond from H2O and C=O bond from CO2 are kinetically-relevant steps in CH4 generation. The KIEs of D2O and 13CO2 decrease slightly after solvothermal treatment (1.3±0.2 vs 1.4±0.2 and 1.6±0.2 vs 1.8±0.2, respectively), which demonstrate that H−TiO2−x(200) with enormous surface defects can effectively accelerate the adsorption and chemical activation of CO2 and H2O molecules, and lower the energy barriers for CH4
a
Reaction conditions: 50 mg cat., 2 bar CO2, 6 mL H2O, solar light, 5 h. b The unit of STY is μmol g−1 h−1. TiO2
H-TiO2-x(200) 2-
H2O c-CO32- 1648 1714
0.04
(b) c-CO3 1719
HCO31430
Absorbance (a.u.)
Absorbance (a.u.)
(a)
m-CO321306
b-CO321579
b-CO321581
HCO31439 HCO3 1396
CO2- H2O 1670 1635
CO2b-CO321248 1329
0.06
Dark- 10 min
Dark- 10 min
Dark- 60 min
Dark- 60 min
1900 1800 1700 1600 1500 1400 1300 1200 1100 1900 1800 1700 1600 1500 1400 1300 1200 1100
Wavenumber (cm-1)
Wavenumber (cm-1) 2-
c-CO3 1717
0.04
TiO2
H 2O 1645
(d)
H2O 1641
-
HCO3 1431 b-CO321581
m-CO321303
Absorbance (a.u.)
(c) Absorbance (a.u.)
1 2 3 4 5 6 7 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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c-CO32- CO 2 1717 1670
2-
b-CO3 1581
H-TiO2-x(200)
HCO31439 HCO31398
0.1 m-CO3 1518
2-
b-CO321330 CO21251 HCO31222
Light-0 min
Light-0 min
Light-10 min
Light-10 min
Light-30 min
Light-30 min
1900 1800 1700 1600 1500 1400 1300 1200 1100 1900 1800 1700 1600 1500 1400 1300 1200 1100
Wavenumber (cm-1)
Wavenumber (cm-1)
Figure 4. In situ DRIFT spectra of catalytic reduction of CO2 with H2O over pristine TiO2 and H−TiO2−x(200) without (a and b) and with (c and d) solar irradiation.
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formation. Furthermore, the reaction rates of H2O/13CO2 as substrate are lower than that of D2O/CO2 and 13CO2 exhibits a stronger KIEs than D2O, indicating that the cleavage of C=O bond is the RDS in CH4 formation. And D2O/13CO2 system shows the lowest catalytic performance and strongest KIEs over pristine TiO2 and H−TiO2−x(200) due to the double isotope-labeled effect. These results further confirm that the C and H elements in CH4 generated are derived from the initial substrates of CO2 and H2O (Figures S19−S21), respectively. To further examine the adsorption and chemical activation of CO2 molecule on the defective H−TiO2−x and explore the key reaction intermediates for determining the possible reaction pathway, in situ DRIFTS of photoreduction of CO2 with moisture was carried out. For comparison, the pristine TiO2 was also investigated. As shown in Figure 4a, exposure of the pristine TiO2 to H2O/CO2 system in dark can induce the generation of strongly adsorbed H2O at 1648 cm−1, carbonate or bicarbonate species (monodentate (m-CO32−) at 1306 cm−1, bidentate (b-CO32−) at 1579 cm−1, chelating bridged carbonate (c-CO32−) at 1714 cm−1, and bicarbonate (HCO3−) at 1430 cm−1).60,66−69 There is no significant change on adsorption and activation behaviors for pristine TiO2 under solar irradiation, even extended reaction time of 30 min (Figure 4c). Interestingly, H−TiO2−x(200) exhibits different results, as shown in Figures 4b and 4d. Especially the key intermediates of CO2− species at 1248 and 1670 cm−1 are observed. The IR intensity of CO2− is enhanced largely under solar irradiation and increases obviously with the reaction time (Figure 4d). Meanwhile, the signals of primary HCO3− and CO32− species are enhanced markedly stimulated by solar light. These findings suggest that the defective surface of H−TiO2−x(200) can trap the photogenerated electrons and then effectively facilitate the process of electron injection from H-TiO2−x to the adsorbed CO2 molecule. The formation of CO2− species indicates that the single-electron reduction of CO2 to CO2− can easily occur on the defective surface of H−TiO2−x(200). Further16 -1
-1
STY of CH4 (µmol g h )
1 2 3 4 5 6 7 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
12 8 4 0 0.1
0.2
0.3
0.4
0.5
-
Integrated intensity of CO2 band (a.u.)
Figure 5. Correlation between STY of CH4 and the integrated intensity of CO2− band at 1670 cm−1.
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more, the positive correlation between STY of CH4 and the integrated intensity of CO2− band at 1670 cm−1 (Figure 5) clearly demonstrates that the key species of CO2− are responsible for the reactivity. Therefore, the appearance of intermediate CO2− can give us new insight into the reaction mechanism of photo-assisted catalytic reduction of CO2. hν − e + h+ TiO2−x →
1/2O2 + H2O + OH
OH + H+
CO2 + Ti3+
+ H+ + 2e-
(CO2 + e- → CO2-)
H2
Ti4+
CO2+ CO2-
CO32-
HCO3-
CO
2OH-
2OH-
+ e-
CO+ H+ + e-
OH+ H+
H2O
C + 4H+ + 4e-
CH4
Scheme 1. Possible reaction pathways for photoreduction of CO2 with H2O using H−TiO2−x. Based on the abovementioned catalytic performance, KIEs data, and in situ DRIFTS results, a possible reaction pathway for photoreduction of CO2 with moisture to form CH4 is proposed, as shown in Scheme 1. The process that occurs during the photocatalytic reduction of CO2 over blue titania catalyst consists of four-steps: i) The light irradiated separation and transportation of e−−h+ pair; ii) adsorption and chemical activation of CO2 and H2O; iii) the generation of CH4 accompanying with by-products of H2, CO, and O2, and iv) the desorption of the final products from catalyst surface. The solar irradiation on defective TiO2−x shell can facilitate the limiting process of single-electron reduction of CO2. Although a considerably negative electrochemical potential (−1.9 V vs NHE at pH 7) is needed for single-electron reduction of CO2 to CO2−,2 the numerous defective sites in H−TiO2−x could efficiently lower the reaction barrier and thus ensure the strong adsorption and activation of CO2 molecules.
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(a)
Solar-light
0.8
Absorbance (a.u.)
MO degradation (C/CO)
100 80 60 40 H-TiO2-x(50)
H-TiO2-x(100) 20 TiO2 0
H-TiO2-x(200) 0
2
4
6
8
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transportation of e−−h+ pairs, and thus exhibit exceptional solar-to-chemistry transformation of catalytic reduction of CO2, degradation of contaminant, and H2 generation from water splitting. H−TiO2−x(200) shows high activity of CH4 formation at rate of 16.2 μmol g−1 h−1 and selectivity of 79% to desired product, and excellent stability for CO2 reduction under full solar irradiation. The KIEs and in situ DRIFTS data reveal that the cleavage of C=O bond from CO2 is the rate-determining step and the existence of key intermediate of CO2− species in CH4 formation.
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Figure 6. (a) Solar-stimulated catalytic degradation of methyl orange. (b) Absorption spectra of methyl orange after solar-assisted degradation with H−TiO2−x(200). H2 generation from water splitting over H−TiO2−x samples under (c) solar-light and (d) Vis-light irradiation. Given the superb photocatalytic performance observed for blue titania toward CO2 reduction, we are curious to check whether these engineered catalysts would also allow for the solar-to-chemical conversions, especially the degradation of pollutants and clean H2 generation from water. As depicted in Figure 6a, H−TiO2−x samples show comparable performance to pristine TiO2 for photocatalytic degradation of MO and H−TiO2−x(200) is the best. Meanwhile, H−TiO2−x(200) can give a degradation efficiency of > 99% within a limited period of 12 min (Figure 6b). Furthermore, the photocatalytic activity of pristine TiO2 for H2 generation from water splitting is also improved by solvothermal treatment, as shown in Figure 6c, and H−TiO2−x(200) exhibits a H2 generation rate of 3.8 mmol g−1 h−1 under solar-light irradiation, which is 1.5 times higher than that of pristine TiO2. The catalytic results depicted in Figure 6d demonstrate that H−TiO2−x materials can also display considerable performance for H2 generation, while pristine TiO2 exhibits no reactivity for water splitting under Vis-light irradiation. These results show that series H−TiO2−x can achieve the degradation of MO and clean H2 generation from water splitting under the irradiation of solar or even Vis-light, and the enhancement degree of catalytic performance is positively related to the amount of Li used in solvothermal process of H−TiO2−x samples.
3. CONCLUSIONS We have successfully designed and fabricated hydrogenated blue H−TiO2−x showing unique crystalline coreamorphous shell structure (TiO2@TiO2−x) with numerous oxygen vacancies and doped H in the disordered shell by using a facile low-temperature Li-assisted solvothermal strategy. Surface electron-modified H−TiO2−x can extend the wide spectrum response, enhance the separation and
4.1. Chemicals and materials. TiO2 (P25, with the specific surface area of 45 m2 g−1, 80% anatase phase and 20% rutile phase) was obtained from Evonik. Anatase TiO2 (particle size of < 40 nm), rutile TiO2 (particle size of < 40 nm), and EDA were supplied by Aladdin. Li and HCl were supplied by Sinopharm Chemical Reagent Co., Ltd (SCRC). All the chemicals were used without further purification. 4.2. Catalyst preparation. In an Ar-filled glovebox (O2 and H2O content below 0.1 ppm), 0.5 g TiO2 was added into 50 mL Teflon-lined autoclave containing 30 mL of EDA under continuous magnetic stirring. A certain amount of Li (0, 50, 100, 200, 300, 400 mg) was added into the solution and sealed immediately. Then, the Teflon-lined autoclave was taken out of the glovebox and kept in an oven at 180 oC for 24 h. After cooling down to room temperature, the products were firstly washed with 0.2 M HCl thoroughly, then washed with deionized water and ethanol for several times (AgNO3 solution was used to examine the residual Cl−), and freeze drying overnight. H−R−TiO2−x and H−A−TiO2−x were prepared with the same process as that of H−TiO2−x(200) by adding 0.5 g TiO2(rutile) or TiO2(anatase). 4.3. Catalyst characterization. The XRD characterization of the samples was carried out on a German Bruker D8 Advance X-ray diffractometer using the Ni-filtered Cu Kα radiation at 40 kV and 40 mA. A JEOL 2011 microscope operating at 200 kV equipped with an EDX unit (Si(Li) detector) was used for the TEM investigations. The samples for electron microscopy were prepared by dispersing the powder in ethanol and applying a drop of very dilute suspension on carbon-coated grids. A JEM 2100F electron microscope operating at 200 kV equipped with an EDX unit (Si (Li) detector) was used for the HRTEM investigations. The samples for electron microscopy were prepared by dispersing the powder in ethanol and applying a drop of very dilute suspension on carbon-coated grids. UV-VIS-NIR DR spectra of the solids were recorded at room temperature on a Hitachi U4100 Spectrometer equipped with an integrating sphere and using BaSO4 as reference. The solar absorption of TiO2 samples was attained by the equation:14 A=
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Where A is the solar absorption, T is the reflectance of the sample, S is the solar spectral irradiance (W m−2 nm−1), λ is the wavelength (nm), and the (1−T)·S represents the sample absorption of solar spectral irradiance. Raman spectra were collected in a Thermal Dispersive Spectrometer using a 10 mW laser with an excitation wavelength of 532 nm. XPS data were recorded with a Perkin Elmer PHI 5000C system equipped with a hemispherical electron energy analyzer. The spectrometer was operated at 15 kV and 20 mA, and a magnesium anode (Mg Kα, hν = 1253.6 eV) was used. The C 1s line (284.6 eV) was used as the reference to calibrate the binding energies (BE). EPR spectra were investigated using a Bruker EMX-8 spectrometer at 9.44 GHz and 300 K. PL spectra were excited at 320 nm using a fluorescence spectrophotometer of MODEL Fluoro Max-3 (Horiba) at room temperature. 4.4. Catalytic activity experiment. 4.4.1. Photocatalytic CO2 reduction measurement. Photocatalytic experiments of CO2 reduction with H2O were conducted in a stainless autoclave reactor (100 mL) with a quartz window on the top (Figure S13). The full-solar light irradiation was from a 300 W Xe lamp (Aulight CEL-HX, Beijing) and the power of the light source is calibrated to AM 1.5 by a NREL-calibrated Si cell (Oriel 91150). The Vis-light was attained using a light-reflector of 400 ~ 780 nm and the reflectivity is larger than 95%. 6mL of deionized water was firstly added into the reactor. Later, 50 mg of catalyst was ultrasonically dispersed in 1 mL deionized water and drop-coated on a glass sheet (area of 4 cm2), which was placed on the catalyst holder in the upper region of the reactor. Then the autoclave was sealed, and the internal air was degassed quickly and completely using high-purity CO2 (99.99%, no other carbon-containing compounds detected) for twenty times at room temperature and charged with CO2 gas to the pressure of 2 bar. The stirrer was started (500 rpm) when the light was irradiated. After 5 h, the autoclave was placed in cool water and the gas was carefully released. The gaseous mixture was qualitatively and quantitatively analyzed using an Agilent 7820A GC equipped with a TDX-01 column connected to a TCD coupling with a HP-5 capillary column connected to a FID. The liquid product was also analyzed using the GC. The selectivity to CH4 based on electron transfer (Scheme 1) was calculated with the following equation (9):70 H2O+ 2h+ → 1/2O2 + 2H+
(5)
+
−
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+
−
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2H+ + 2e−→ 2H2
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CO2 + 8H + 8e → CH4 + 2H2O
Selectivity to CH4 (%) = 8n(CH4)/[8n(CH4) + 2n(H2) + 2n(CO)] × 100% (9) Where n(CH4), n(H2) and n(CO) are the amounts (moles) of CH4, H2, and CO generated. It should be noted that three separate experiments were performed for each test.
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The reactions did not proceed at all without the use of H−TiO2−x. And there were no C-containing compounds generated if the substance of CO2 was not charged. The stability test of the H−TiO2−x(200) was carried out on the conditions of 50 mg catalyst, 2 bar CO2, 6 mL H2O, full solar irradiation and 5 h in each run. After each photocatalytic reaction, the catalyst was just washed with deionized water several times and freeze-dried for 12 h. Kinetic isotope effects (KIEs) measurements using 13CO2 or D2O as substrates over pristine TiO2 and H−TiO2−x(200) have been performed to investigate the catalytic process. All experiments were carried out by following the same procedure as described in photocatalytic reduction of CO2 under solar light irradiation. And a gas chromatographmass spectrometry system (GC-MS, TSQ 8000 Evo, Thermo Fisher Scientific Co., USA) was used to determine the origin of the CH4 and CO products. In situ DRIFT spectra for photocatalytic reduction of CO2 were performed in the reaction cell (modified Harricks model HV-DR2) that allowed continuous gas flow through the catalyst bed (ca. 0.1 g). The reaction cell is equipped with a heater and a temperature controller. The spectra were recorded on a PerkinElmer spectrum 100 FT-IR spectrometer with a resolution of 4 cm−1. The dome of the DRIFTS cell has two KBr windows allowing IR transmission and a third (quartz) window allowing transmission of solar irradiation introduced by a 300 W Xe lamp. Prior to test, the samples were pretreated in Helium flow for 2 h at 200 oC. The temperature was then lowered to 50 oC, and the background spectrum in the presence of the sample was collected. The in situ DRIFT spectra were then conducted by introducing a CO2/H2O mixture to the IR cell at 50 oC for 60 min in the dark when the intensities of adsorption peaks reached saturation. Next, the solar light was turned on for 60 min to investigate the photocatalytic conversion and the spectra of the adsorbed species were recorded. The integrated intensity of CO2− band at 1670 cm−1 of in situ DRIFTS was recorded at the irradiation time of 0, 10, 30, 60, 90, 120, 180, 240, and 300 min. The corresponding STY of CH4 was achieved by measuring the average reaction rate in the period of 20 min. 4.4.2 Photocatalytic degradation measurement. The photocatalytic degradation was conducted by monitoring the decomposition of MO in an aqueous solution under solar-light irradiation from a 300 W Xe lamp (Aulight CEL-HX, Beijing) and the power of the light source is calibrated to AM 1.5 by a NREL-calibrated Si cell (Oriel 91150). Typically, the as-prepared sample (100 mg) was dispersed in a Pyrex glass reactor that contained MO solution (100 mL, 10 mg L−1). Prior to irradiation, the suspensions were stirred in dark for 30 min to reach the adsorption/desorption equilibrium. At given time intervals, portions of the suspension (≈ 5 mL) were taken for analysis after centrifugation. The solution was cooled by the water circulating jacket of the reactor to avoid the
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interference of the solvent evaporation. The concentration of aqueous MO was determined with a UV-Vis spectrophotometer by measuring the peak intensity at 464 nm. 4.4.3. Photocatalytic H2 generation measurement. H2 production by photocatalytic water splitting was performed by using a top-irradiation Pyrex reaction cell. Photocatalyst powder (100 mg) was dispersed by ultrasonication for 2 min into an aqueous solution (200 mL) that contained methanol (40 mL) as the sacrificial reagent. Pt (0.5 wt%) was loaded in situ by impregnation of H2PtCl6 (0.05 mL, 10 g L−1) in the suspension. Then the suspension was degassed thoroughly with pure N2. The solar light irradiation was from a 300 W Xe lamp (Aulight CEL-HX, Beijing) and the power of the light source is calibrated to AM 1.5 by a NREL-calibrated Si cell (Oriel 91150). The Vis-light was attained using a light-reflector of 400 ~ 780 nm and the reflectivity is larger than 95%. The temperature of the reaction solution was maintained at RT by a flow of water. The amount of H2 evolved was determined by an Agilent 7820A GC equipped with a TDX-01 column connected to a TCD. The photocatalytic activities were compared based on the average H2 evolution rate in the first 5 h.
AUTHOR INFORMATION Corresponding Authors *
[email protected] *
[email protected] *
[email protected] Notes The authors declare no competing financial interest.
ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publication website at DOI: xx.xxxx/acscatalxxxxxxx Full spectroscopic data and TEM images for all samples, photographs, comparison of photocatalytic activity of CO2 reduction over TiO2-based catalysts, O2 formation of CO2 photoreduction over H-TiO2-x, cycling tests, and Isotope tracer analyses.
ACKNOWLEDGMENT This work was financially supported by the National Key Research and Development Program of China (Grant no. 2016YFB0901600), the NSF of China (Grant no. 61376056, 51502331 and 51402334), and the STC of Shanghai (Grant no. 16JC1401700 and 16ZR1440400).
REFERENCES (1) Liu, L.; Chen, X. Chem. Rev. 2014, 114, 9890−9918. (2) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Angew. Chem., Int. Ed. 2013, 52, 7372−7408. (3) Liu, X.; Zhu, G.; Wang, X.; Yuan, X.; Lin, T.; Huang, F. Adv. Energy Mater. 2016, 17, 1600452. (4) Chen, X.; Liu L.; Huang, F. Chem. Soc. Rev. 2015, 44, 1861−1885.
(5) Pan, X.; Yang, M. Q.; Fu, X.; Zhang, N.; Xu, Y. J. Nanoscale 2013, 5, 3601−3614. (6) Thompson, T. L.; Yates, J. T. Chem. Rev. 2006, 106, 4428−4453. (7) Chen, X.; Liu, L.; Yu P. Y.; Mao, S. S. Science 2011, 331, 746−750. (8) Xu, Q.; Yu, J.; Zhang, J.; Zhang, J.; Liu, G. Chem. Commun. 2015, 51, 7950−7953. (9) Chang, W.; Sun, C.; Pang, X.; Sheng, H.; Li, Y.; Ji, H.; Song, W.; Chen, C.; Ma, W.; Zhao, J. Angew. Chem., Int. Ed. 2015, 54, 2052−2056. (10) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo M.; Bahnemann, D. W. Chem. Rev. 2014, 114, 9919−9986. (11) Wang, Z.; Yang, C.; Lin, T.; Yin, H.; Chen, P.; Wan, D.; Xu, F.; Huang, F.; Lin, J.; Xie, X.; Jiang, M. Energy Environ. Sci. 2013, 6, 3007−3014. (12) Dahl, M.; Liu Y.; Yin, Y. Chem. Rev. 2014, 114, 9853−9889. (13) Liu, G.; Yang, H. G.; Pan, J.; Yang, Y. Q.; Lu, G. Q.; Cheng, H.-M. Chem. Rev. 2014, 114, 9559−9612. (14) Lin, T.; Yang, C.; Wang, Z.; Yin, H.; Lü, X.; Huang, F.; Lin, J.; Xie, X.; Jiang, M. Energy Environ. Sci. 2014, 7, 967−972. (15) Yang, C.; Wang, Z.; Lin, T.; Yin, H.; Lü, X.; Wan, D.; Xu, T.; Zheng, C.; Lin, J.; Huang, F.; Xie, X.; Jiang, M. J. Am. Chem. Soc. 2013, 135, 17831−17838. (16) Yu, X.; Kim, B.; Kim, Y. K. ACS Catal. 2013, 3, 2479−2486. (17) Liu, N.; Schneider, C.; Freitag, D.; Hartmann, M.; Venkatesan, U.; Müller, J.; Spiecker, E.; Schmuki, P. Nano Lett. 2014, 14, 3309−3313. (18) Naldoni, A.; Allieta, M.; Santangelo, S.; Marelli, M.; Fabbri, F.; Cappelli, S.; Bianchi, C. L.; Psaro, R.; Santo, V. D. J. Am. Chem. Soc. 2012, 134, 7600−7603. (19) Barzan, C.; Groppo, E.; Bordiga, S.; Zecchina, A. ACS Catal. 2014, 4, 986−989. (20) Liang, Z.; Zheng, G.; Li, W.; Seh, Z. W.; Yao, H.; Yan, K.; Kong, D.; Cui, Y. ACS Nano 2014, 8, 5249−5256. (21) Zhang, P.; Wang, X.; Wang, W.; Lei, X.; Yang, H. Dalton Trans. 2016, 45, 296−299. (22) Mendil, R.; Ayadi, Z. B.; Djessas, K. J. Alloys Compd. 2016, 678, 87−92. (23) Vaquero, F.; Navarro, R. M.; Fierro, J. L. G. Int. J. Hydrogen Energy 2016, 41, 11558−11567. (24) Zhao, Z.; Tan, H.; Zhao, H.; Lv, Y.; Zhou, L.-J.; Song, Y.; Sun, Z. Chem. Commun. 2014, 50, 2755–2757. (25) Xu, J.; Tian, Z.; Yin, G.; Lin, T.; Huang, F. Dalton Trans. 2017, 46, 1047–1051. (26) Burrard-Lucas, M.; Free, D. G.; Sedlmaier, S. J.; Wright, J. D.; Cassidy, S. J.; Hara, Y.; Corkett, A. J.; Lancaster, T.; Baker, P. J.; Blundell, S. J.; Clarke, S. J. Nature Mater. 2013, 12, 158−164. (27) Hatakeda, T.; Noji, T.; Kawamata, T.; Kato, M.; Koike, Y. J. Phys. Soc. Jpn. 2013, 82, 123705. (28) Scheidt, E.-W.; Hathwar, V. R.; Schmitz, D.; Dunbar, A.; Scherer, W.; Mayr, F.; Tsurkan, V.; Deisenhofer, J.; Loidl, A. Eur. Phys. J. B 2012, 85, 279. (29) Inorganic chemistry teaching and research section of Tianjin University, Inorganic Chemistry, 4th ed.; Yang, H. X., Yan, X. R., Cui, J. Z., Wang, J. H., Wang, X. R., Qin, X., Eds.; Higher Education Press: Beijing, 2010; p 292. (30) Hu, Y.; Li, J.; Tian, J.; Xuan, Y.; Deng, B.; McNear, K. L.; Lim, D. G.; Chen, Y.; Yang, C.; Cheng, G. J. Nano Lett. 2016, 16, 7536−7544. (31) Yu, X.-Y.; Feng, Y.; Jeon, Y.; Guan, B.; Lou, X. W.; Paik, U. Adv. Mater. 2016, 28, 9006−9011.
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(32) Meng, F.; Cushing, S. K.; Li, J.; Hao, S.; Wu, N. ACS Catal. 2015, 5, 1949−1955. (33) Li, L.; Yan, J.; Wang, T.; Zhao, Z.-J.; Zhang, J.; Gong, J.; Guan, N. Nat. Commun. 2015, 6, 5881. (34) Lei, F.; Sun, Y.; Liu, K.; Gao, S.; Liang, L.; Pan, B.; Xie, Y. J. Am. Chem. Soc. 2014, 136, 6826−6829. (35) Kong, X. Y.; Choo, Y. Y.; Chai, S.-P.; Soh, A. K.; Mohamed, A. R. Chem. Commun. 2016, 52, 14242−14245. (36) Zhu, G.; Shan, Y.; Lin, T.; Zhao, W.; Xu, J.; Tian, Z.; Zhang, H.; Zheng, C.; Huang, F. Nanoscale 2016, 8, 4705−4712. (37) Wang, Z.; Yang, C.; Lin, T.; Yin, H.; Chen, P.; Wan, D.; Xu , F.; Huang, F.; Lin, J.; Xie X.; Jiang, M. Adv. Funct. Mater. 2013, 23, 5444−5450. (38) Zhang, K.; Wang, L.; Kim, J. K.; Ma, M.; Veerappan, G.; Lee, C.-L.; Kong, K.-j.; Lee, H.; Park, J. H. Energy Environ. Sci. 2016, 9, 499−503. (39) Zhu, G.; Yin, H.; Yang, C.; Cui, H.; Wang, Z.; Xu, J.; Lin, T.; Huang, F. ChemCatChem 2015, 7, 2614−2619. (40) Zhang, Z.; Lee, J.; Yates, J. T.; Bechstein, R.; Lira, E.; Hansen, J. Ø.; Wendt, S.; Besenbacher, F. J. Phys. Chem. C 2010, 114, 3059−3062. (41) Zhang, J.; Xu, Q.; Feng, Z.; Li, M.; Li, C. Angew. Chem., Int. Ed. 2008, 47, 1766−1769. (42) Zhou, X.; Häublein, V.; Liu, N.; Nguyen, N. T.; Zolnhofer, E. M.; Tsuchiya, H.; Killian, M. S.; Meyer, K.; Frey, L.; Schmuki, P. Angew. Chem., Int. Ed. 2016, 55, 3763−3767. (43) Cushing, S. K.; Meng, F.; Zhang, J.; Ding, B.; Chen, C. K.; Chen, C.-J.; Liu, R.-S.; Bristow, A. D.; Bright, J.; Zheng, P.; Wu, N. ACS Catal. 2017, 7, 1742−1748. (44) Yu, X.; Kim, B.; Kim, Y. K. ACS Catal. 2013, 3, 2479−2486. (45) Zhu, G.; Lin, T.; Lü, X.; Zhao, W.; Yang, C.; Wang, Z.; Yin, H.; Liu, Z.; Huang, F.; Lin, J. J. Mater. Chem. A 2013, 1, 9650−9653. (46) Wang, Y.; Cai, J.; Wu, M.; Zhang, H.; Meng, M.; Tian, Y.; Ding, T.; Gong, J.; Jiang, Z.; Li, X. ACS Appl. Mater. Interfaces 2016, 8, 23006−23014. (47) Eom, J.-Y.; Lim, S.-J.; Lee, S.-M.; Ryu, W.-H.; Kwon, H.-S. J. Mater. Chem. A 2015, 3, 11183–11188. (48) Hong, N. H.; Barla, A.; Sakai, J.; Huong, N. Q. Phys. Status Solidi C 2007, 4, 4461−4466. (49) Zhao, Q.; Wu, P.; Li, B. L.; Lu, Z. M.; Jiang, E. Y., J. Appl. Phys. 2008, 104, 073911. (50) Cai, J.; Wu, M.; Wang, Y.; Zhang, H.; Meng, M.; Tian, Y.; Li, X.; Zhang, J.; Zheng, L.; Gong, J. Chem 2017, 2, 877–892. (51) Zhang, H.; Cai, J.; Wang, Y.; Wu, M.; Meng, M.; Tian, Y.; Li, X.; Zhang, J.; Zheng, L.; Jiang, Z.; Gong, J. Appl. Catal. B-Environ. 2018, 220, 126–136. (52) Pallotti, D. K.; Passoni, L.; Maddalena, P.; Fonzo, F. D.; Lettieri, S. J. Phys. Chem. C 2017, 121, 9011−9021. (53) Imani, R.; Dillert, R.; Bahnemann, D. W.; Pazoki, M.; Apih, T.; Kononenko, V.; Repar, N.; Kralj-Iglič, V.; Boschloo, G.; Drobne, D.; Edvinsson, T.; Iglič, A. small 2017, 1700349. (54) Zhang, K.; Ravishankar, S.; Ma, M.; Veerappan, G.; Bisquert, J.; Fabregat-Santiago, F.; Park, J. H. Adv. Energy Mater. 2016, 1600923. (55) Chen, X.; Liu, L.; Liu, Z.; Marcus, M. A.; Wang, W. C.; Oyler, N. A.; Grass, M. E.; Mao, B.; Glans, P. A.; Peter, Y. Y. Sci. Rep. 2013, 3, 1510. (56) Nosaka, A. Y.; Fujiwara, T.; Yagi, H.; Akutsu, H.; Nosaka, Y. Langmuir 2003, 19, 1935−1937. (57) Xia, T.; Zhang, W.; Li, W. J.; Oyler, N. A.; Liu, G.; Chen, X. B. Nano Energy 2013, 2, 826−835.
Page 10 of 11
(58) Yan, Y.; Shi, W.; Yuan, Z.; He, S.; Li, D.; Meng, Q.; Ji, H.; Chen, C.; Ma, W.; Zhao, J. J. Am. Chem. Soc. 2017, 139, 2083−2089. (59) Mikkelsen, M.; Jørgensen, M.; Krebs, F. C. Energy Environ. Sci. 2010, 3, 43−81. (60) Liu, L.; Zhao, H.; Andino, J. M.; Li, Y. ACS Catal. 2012, 2, 1817−1828. (61) Olah, G. A.; Mathew, T.; Prakash, G. K. S. J. Am. Chem. Soc. 2016, 138, 6905−6911. (62) Tahir, M.; Amin, N. S. Appl. Catal. B 2013, 142-143, 512−522. (63) Zheng, Z.; Huang, B.; Lu, J.; Wang, Z.; Qin, X.; Zhang, X.; Daib, Y.; Whangbo, M.-H. Chem. Commun. 2012, 48, 5733−5735. (64) Bi, Q.-Y.; Du, X.-L.; Liu, Y.-M.; Cao, Y.; He, H.-Y.; Fan, K.-N. J. Am. Chem. Soc. 2012, 134, 8926−8933. (65) Hauwert, P.; Boerleider, R.; Warsink, S.; Weigand, J. J.; Elsevier, C. J. J. Am. Chem. Soc. 2010, 132, 16900−16910. (66) Liu, L.; Zhao, C.; Miller, J. T.; Li, Y. J. Phys. Chem. C 2017, 121, 490−499. (67) Yang, C.-C.; Yu, Y.-H.; van der Linden, B.; Wu, J. C. S.; Mul, G. J. Am. Chem. Soc. 2010, 132, 8398−8406. (68) Liu, L.; Jiang, Y.; Zhao, H.; Chen, J.; Cheng, J.; Yang, K.; Li, Y. ACS Catal. 2016, 6, 1097−1108. (69) Mino, L.; Spoto, G.; Ferrari, A. M. J. Phys. Chem. C 2014, 118, 25016−25026. (70) Zhai, Q.; Xie, S.; Fan, W.; Zhang, Q.; Wang, Y.; Deng, W.; Wang, Y. Angew. Chem., Int. Ed. 2013, 52, 5776−5779.
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Graphical Abstract: CO2 + Ti3+ VO
H+
Ti4+
H-TiO2−x
CO2H2O
Solar-to-Chemical Conversions
CO32-, HCO3-
CO, H2 VO
H2
Organic Pollution
CO2 + H2O
CH4
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