Visible-Light-Induced Partial Oxidation of ... - ACS Publications

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Visible-Light-Induced Partial Oxidation of Cyclohexane by Cr/Ti/Si Ternary Mixed Oxides with Molecular Oxygen Daijiro Tsukamoto, Akimitsu Shiro, Yasuhiro Shiraishi,* and Takayuki Hirai Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan ABSTRACT: Cr/Ti/Si ternary mixed oxides (CrTiSi) were prepared by a hydrolysis of tetraethyl orthosilicate (TEOS), Cr(NO3)3, and titanium tetraisopropoxide (TTIP) followed by calcination. These oxides were used as catalysts for partial oxidation of cyclohexane with molecular oxygen under irradiation of visible light (λ > 400 nm). The CrTiSi catalysts produce cyclohexanol and cyclohexanone with high selectivity (>91%) and show much higher activity than the CrSi binary oxides. Among them, the catalyst with equimolar amounts of Cr and Ti shows the highest activity. Visible-light irradiation of the CrSi catalyst promotes a reduction of tetrahedrally coordinated Cr oxide species (Td6+) and produces the excited state (Td4+*), which behaves as an active site for oxidation. In contrast, the CrTiSi catalyst contains tetrahedrally coordinated Cr and Ti oxide species that are connected through the Cr
O
Ti bond. The photoformed Td4+* species is strongly stabilized due to delocalization of excited electrons on the Cr
O
Ti species. This suppresses rapid deactivation of Td4+* and results in enhanced photocatalytic activity.

1. INTRODUCTION The partial oxidation of cyclohexane (CHA) to cyclohexanol (CHA-ol) and cyclohexanone (CHA-one) has attracted much attention because these products are the intermediates in εcaprolactam synthesis, which is used in the manufacture of nylon polymers.1 Of particular interest is the catalytic oxidation of CHA in heterogeneous systems with molecular oxygen (O2) as an oxidant.2 Photocatalytic CHA oxidation in liquid/solid heterogeneous systems with O2 has also been studied extensively with various catalysts such as TiO2,3 Fe porphyrin-modified TiO2,4 polyoxotungstate-modified SiO2,5 and V2O5-impregnated Al2O3.6 Some of these systems promote partial CHA oxidation with high selectivity (>98%).3b,f,g,4a,5 However, all of these systems require UV light for catalyst activation. Earlier, we proposed a photocatalytic CHA oxidation process with Cr/Si binary mixed oxides (CrSi), prepared by a hydrolysis of tetraethyl orthosilicate (TEOS) with Cr(NO3)3 followed by calcination.7 The CrSi catalyst containing highly dispersed chromate species promotes partial oxidation of CHA to CHA-ol and CHA-one with >97% selectivity in MeCN at room temperature under visible-light irradiation (λ > 400 nm). As shown in Scheme 1, the Cr species are dispersed well in the Si matrices, and the chromate species with a tetrahedral coordination (Td6+) acts as the active site for oxidation. Photoexcitation of Td6+ produces the excited state (Td4+*) via a photoinduced charge transfer from the terminal oxygen (OT) to the Cr center (Scheme 1c). The electrophilic OT on the Td4+* species attracts the hydrogen atom of CHA and produces a [CHA
Td5+*] complex (Scheme 1d). The reaction of the complex with O2 gives rise to the partial oxidation products. r 2011 American Chemical Society

The purpose of the present work is to improve the rate of CHA oxidation while maintaining high oxidation selectivity. Some literature reports that Ti species, when incorporated into Cr-containing materials, enhances the photocatalytic activity under visible-light irradiation, even though Ti species does not absorb visible light.8
10 Rodrigues et al.8 reported that MCM-48 containing Cr and nanosized TiO2, prepared by a hydrolysis of TEOS, Cr(NO3)3, and titanium tetraisopropoxide (TTIP), shows higher activity for degradation of acetaldehyde in the gas phase than MCM-48 containing Cr alone. Shen et al.9 reported that MCM-41 containing Cr and Ti species, prepared by a hydrolysis of TEOS, Cr(NO3)3, and tetrabutyl titanate, promotes water splitting with higher activity than MCM-41 containing Cr alone. Kamegawa et al.10 reported that MCM-41 containing Cr and Ti species, prepared by a chemical vapor deposition with TiCl4 and CrO2Cl2, promotes photopolymerization of ethylene with higher activity than MCM-41 containing Cr alone. All these reports suggest that the incorporation of Ti to Cr-containing materials probably produces an oxo-bridged Cr
O
Ti species and shows enhanced activity, although the direct evidence for the formation of Cr
O
Ti species and the mechanism for activity enhancement are not provided. In the present work, Cr/Ti/Si ternary mixed oxides (CrTiSi) were employed for CHA oxidation to clarify the effect of Ti incorporation. These oxides were prepared by a hydrolysis of TEOS, Cr(NO3)3, and TTIP followed by calcination. The CrTiSi catalysts Received: June 14, 2011 Revised: September 1, 2011 Published: September 06, 2011 19782

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Scheme 1. Mechanism of Photocatalytic Oxidation of CHA on the CrSi Catalyst7

Table 1. Properties of Catalysts Ti (mol %)

SBET (m2 g
1)a

particle size (μm)b

VCHA (mmol g
1)c

qCHA (molecules nm
2)d

QCHA (mmol g
1)e

0.043 0.043

0.023

651 647

54.2 55.4

1.57 2.21

1.45 2.06

3.89  10
2 2.86  10
2

Cr0.04Ti0.04Si

0.037

0.036

690

62.1

1.75

1.52

2.74  10
2

Cr0.04Ti0.05Si

0.036

0.049

676

62.6

2.31

2.06

3.66  10
2

Cr0.04Ti0.07Si

0.036

0.072

679

62.8

2.80

2.48

3.58  10
2

Cr0.04Ti0.22Si

0.037

0.222

642

62.9

2.02

1.90

2.77  10
2

Cr0.04Ti1.68Si

0.040

1.681

516

62.6

1.69

1.97

3.69  10
2

Cr1.44Ti0.04Si

1.444

0.035

590

55.2

2.62

2.68

3.21  10
2

0.036

741

48.0

2.43

1.97

3.44  10
2

catalyst

Cr (mol %)

Cr0.04Si Cr0.04Ti0.02Si

Ti0.04Si a

b

c

BET surface area determined by N2 adsorption/desorption analysis. Determined by laser scattering analysis. Monolayer adsorption capacity of CHA (p/p0 = 0.10
0.30), determined by vapor CHA adsorption. d Adsorption capacity of CHA per unit surface area [= (VCHA/103)  (10
18/SBET)  N (N = Avogadro’s number)]. e Adsorption capacity of CHA per unit weight of catalysts determined by liquid-phase adsorption analysis. The experiments were performed by stirring a MeCN solution (10 mL) containing CHA (100 μmol) with the respective catalyst (50 mg) at 298 K for 5 h.

show much higher activity for CHA oxidation than CrSi while maintaining high selectivity. The amount of Ti incorporated into the catalyst is crucial for catalytic activity, and the CrTiSi catalyst containing equimolar amounts of Cr and Ti shows the highest activity. Electron spin resonance (ESR) analysis revealed that the catalyst containing equimolar amounts of Cr and Ti produces a large amount of Td4+* active species. Several experimental findings suggest that the catalyst contains an oxo-bridged Cr
O
Ti species. The photoformed Td4+* is stabilized strongly by delocalization of excited electrons in the Cr
O
Ti species and hence accelerates CHA oxidation.

2. EXPERIMENTAL SECTION 2.1. Materials. All of the reagents used were supplied from Wako, Tokyo Kasei, and Sigma-Aldrich and used without further purification. Water was purified by the Milli-Q system. Japan Reference Catalyst, JRC-TIO-4 TiO2 particles (equivalent to Degussa P25), were kindly supplied from the Catalysis Society of Japan and used for reactions as a reference. 2.2. Catalyst Preparation. CrxTiySi catalysts [x (mol %) = Cr/(Cr + Ti + Si)  100; y (mol %) = Ti/(Cr + Ti + Si)  100] were prepared with different amounts of Cr and Ti sources:7 In the typical synthesis of Cr0.04Ti0.04Si, TEOS (10.4 g, 50 mmol), Cr(NO3)3 3 9H2O (5.6 mg, 0.02 mmol), and TTIP (8.0 mg, 0.02 mmol) were dissolved in ethylene glycol (8.4 g) and stirred at 363 K for 3 h under N2. Water (80 g) was added to the mixture and stirred at 363 K for 5 h. The obtained gel was dried at 373 K for 12 h and calcined at 773 K for 5 h under air, affording CrxTiySi as yellow powder. The CrSi and Ti-containing silica (TiSi) were

prepared in a similar manner. The properties of catalysts are summarized in Table 1. 2.3. Photoreaction. Each catalyst (50 mg) was added to a mixture of CHA (1 mL) and MeCN (9 mL) in a Pyrex glass tube (capacity, 20 cm3; ϕ 10 mm), and the tube was sealed with a rubber septum cap. The catalyst was dispersed thoroughly by ultrasonication for 5 min, and O2 was bubbled through the solution for 5 min. The samples were photoirradiated with magnetic stirring by a Xe lamp (2 kW; Ushio Inc.) filtered through an aqueous NaNO2 solution (3 wt %) to give light wavelength of >400 nm. The light intensity at 400
530 nm was 16.0 mW m
2, and the temperature of the solution during reaction was 313 K. After the reaction, the gas-phase product was analyzed by GCTCD (Shimadzu; GC-14B). The solution was recovered by centrifugation, and the liquid-phase products were analyzed by GC-FID (Shimadzu; GC-1700). 2.4. ESR Analysis. The spectra were recorded at the X-band using a Bruker EMX-10/12 spectrometer with a 100 kHz magnetic field modulation at a microwave power level of 4.0 mW, where microwave power saturation of the signals does not occur.7 The magnetic field was calibrated using 1,10 -diphenyl-2-picrylhydrazyl (DPPH) as standard. Catalyst (50 mg) was placed in a quartz ESR tube and treated under O2 flow at 673 K for 1 h. The tube was evacuated at 473 K for 4 h and cooled to room temperature. The required quantity of CHA or O2 was then introduced to the tube. The tube was placed on the ESR sample cavity and photoirradiated at 77 K using a Xe lamp (500 W; USHIO Inc.) at λ >240 nm (without filter) or at λ >400 nm (with filter). After photoirradiation, measurement was started with continued photoirradiation. 19783

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Table 2. Results of Photocatalytic Oxidation of CHA under Visible Lighta yield (μmol) run

catalyst

partial oxidation

CHA-ol CHA-one CO2

selectivity (%)b

TONc

1

Cr0.04Si

2.1

3.9

0.6

98.3

16.8

2

Cr0.04Ti0.02Si

3.5

6.0

0.6

99.0

26.5

3 4

Cr0.04Ti0.04Si Cr0.04Ti0.05Si

2.8 2.9

9.3 5.1

0.9 0.5

98.8 99.0

39.3 26.7

5

Cr0.04Ti0.07Si

2.5

3.8

0.8

98.0

21.0

6

Cr0.04Ti0.22Si

1.0

2.4

1.0

95.3

11.0

7

Cr0.04Ti1.68Si

0.8

1.0

1.2

91.0

5.4

8

Ti0.04Si

N.D.

N.D.

N.D.

9

Cr1.44Ti0.04Si

3.4

4.2

1.8

96.2

0.6

10

Cr2O3

N.D.

N.D.

N.D.

11d TiO2 12e Cr0.04Ti0.04Si

0.8 2.8

2.2 9.1

5.5 0.8

76.6 98.9

38.6

13f

2.8

9.2

0.9

98.8

39.0

Cr0.04Ti0.04Si

a

Reaction conditions: CHA (1 mL), MeCN (9 mL), catalyst (50 mg), O2 (1 atm), photoirradiation time (5 h), λ > 400 nm. b = [(CHA-ol + CHA-one)/(CHA-ol + CHA-one + (1/6)CO2)]  100. c = [(CHA-ol + CHA-one)/(Cr atoms on catalyst)]. d Catalyst (10 mg). e First reuse of the catalyst (run 3) after washing with MeCN. f Second reuse.

2.5. Other Analysis. The total Cr and Ti amounts of catalysts were determined by an inductively coupled plasma atomic emission spectrometer (SII Nanotechnology; SPS 7800) after dissolution in HF. BET surface area was determined by N2 adsorption/desorption analysis at 77 K using an AUTOSORB-1C/TCD analyzer (Yuasa Ionics Co., Ltd.). The monolayer adsorption capacity of CHA (VCHA) and CHA adsorption capacity per unit surface area of catalysts (qCHA) were determined by the vapor adsorption analysis at 298 K using a BELSORP 18PLUSSP analyzer (BEL Japan, Inc.). Diffuse reflectance UV
vis spectra of the catalysts were measured on a UV
vis spectrophotometer (Jasco Corp.; V-550 with Integrated Sphere Apparatus ISV-469) using BaSO4 as reference. Particle size distribution was determined using a Horiba LA-910 laser light-scattering particle size analyzer.

3. RESULTS AND DISCUSSION 3.1. Selectivity for Photocatalytic Oxidation. CrxTiySi catalysts [x (mol %) = Cr/(Cr + Ti + Si)  100; y (mol %) = Ti/ (Cr + Ti + Si)  100] were prepared by a hydrolysis of TEOS, Cr(NO3)3, and TTIP followed by calcination (Table 1). Photocatalytic oxidation of CHA was performed with respective catalysts under visible-light irradiation (λ >400 nm) using a Xe lamp. Table 2 summarizes the yields of CHA-ol, CHA-one, and CO2 obtained by photoirradiation (5 h) of a mixture of CHA (1 mL) and MeCN (9 mL) with catalyst (50 mg) in the presence of O2. Both CrSi and CrTiSi catalysts promote CHA oxidation under visible light (runs 1
7), whereas the Ti0.04Si catalyst without Cr does not (run 8). The CrTiSi catalysts promote selective CHA oxidation as well as CrSi. As shown in run 1, Cr0.04Si shows 98% selectivity for partial oxidation products (CHA-ol and CHA-one). The Cr0.04TiySi catalysts with e0.07 mol % Ti (runs 2
5) also show high selectivity (g98%), although the catalysts with g0.22 mol % Ti show decreased selectivity (runs 6 and 7; 95%

Figure 1. Time-dependent change in the amounts of CHA-ol, CHAone, and CO2 during photooxidation of CHA on (a) Cr0.04Si, (b) Cr0.04Ti0.04Si, and (c) TiO2. Reaction conditions: CHA (1 mL, 9.3 mmol), MeCN (9 mL), catalyst (Cr0.04Si and Cr0.04Ti0.04Si, 50 mg; TiO2, 10 mg), O2 (1 atm), λ > 400 nm.

and 91%). As shown in run 11, a common TiO2 particle (equivalent to Degussa P25) shows 77% selectivity, indicative of high oxidation selectivity of CrTiSi catalysts. Figure 1 shows the time-dependent change in the yields of CHA-ol, CHA-one, and CO2 during photoreaction of CHA with respective catalysts. With TiO2 (Figure 1c), the amount of CO2 is larger than those of CHA-ol and CHA-one. In contrast, Cr0.04Si and Cr0.04Ti0.04Si produce a much smaller amount of CO2 (Figure 1a, b), indicating that these catalysts selectively produce CHA-ol and CHA-one. On TiO2, these products are subsequently decomposed to CO2 by photocatalytic reactions. Figure 2 shows the time-dependent change in the CO2 formed during photoreaction of 100 μmol of CHA, CHA-ol, or CHAone as the starting material. With TiO2 (black circle), CO2 is scarcely produced from CHA (Figure 2a) but produced significantly from CHA-ol and CHA-one (Figure 2b, c). This indicates that, in the photocatalytic CHA oxidation on TiO2, the formed CHA-ol and CHA-one are subsequently decomposed and produce CO2. In contrast, on Cr0.04Si and Cr0.04Ti0.04Si catalysts (white diamond and black square), CO2 production from CHA-ol and CHA-one is much suppressed. These data clearly indicate that, in the photocatalytic CHA oxidation on the CrSi and CrTiSi catalysts, decomposition of partial oxidation products is suppressed and results in high oxidation selectivity. 3.2. Photocatalytic Activity. The CrTiSi catalysts show high catalytic activity. Table 2 (final column) summarizes the turnover number (TON) for the formation of CHA-ol and CHA-one [= (CHA-ol + CHA-one)/(Cr atoms on the catalyst)] during 5 h photoirradiation. The TON increases with an increase in the Ti amount of the catalysts, and Cr0.04Ti0.04Si containing equimolar amounts of Cr and Ti (run 3) shows the highest value (39.3), which is more than double that of Cr0.04Si (16.8). Further incorporation of Ti, however, decreases TON (runs 4
7): the TONs for Cr0.04Ti0.22Si and Cr0.04Ti1.68Si are 11.0 and 5.4, respectively. Incorporation of a large amount of Cr is also not effective: the TON of Cr1.44Ti0.04Si is only 0.6 (run 9). These suggest that the catalyst containing low and equimolar amounts of Cr and Ti shows high catalytic activity. As described previously,7c the adsorption degree of CHA onto the catalyst surface also affects the catalyst activity; the enhanced 19784

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absorption of highly dispersed titanate species,12 as observed in the spectrum of Ti0.04Si (f). These suggest that Cr0.04Ti0.04Si contains Cr6+ and Ti4+ species. In contrast, the catalyst with larger amounts of Cr (Cr1.44Ti0.04Si) shows red-shifted absorption at 580
800 nm (e), assigned to the d
d transition of octahedral Cr3+ in the Cr2O3 clusters (h),11 indicating that the catalyst contains polymerized Cr3+ species. As shown in Table 1 (run 10), Cr2O3, when used as a catalyst, does not promote oxidation, suggesting that polymerized Cr3+ is inactive. The low activity of Cr1.44Ti0.04Si (run 9) is therefore due to the formation of polymerized Cr3+. As shown in Table 1 (runs 6 and 7), the catalysts with larger amounts of Ti (Cr0.04Ti0.22Si and Cr0.04Ti1.68Si) also show decreased activity. As shown by the spectra c and d, these catalysts show increased absorption at 400 nm.

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Figure 3. Diffuse reflectance UV
vis spectra of catalysts. (a) Cr0.04Si, (b) Cr0.04Ti0.04Si, (c) Cr0.04Ti0.22Si, (d) Cr0.04Ti1.68Si, (e) Cr1.44Ti0.04Si, (f) Ti0.04Si, (g) TiO2, and (h) Cr2O3.

adsorption of CHA accelerates the oxidation. Table 1 shows the monolayer adsorption capacity of CHA per unit surface area (qCHA) determined by the vapor CHA adsorption analysis and the adsorption capacity of CHA (QCHA) determined by the liquidphase adsorption analysis. There is no clear relationship between the adsorption degree and the catalytic activity. This suggests that the adsorption effect is negligible for the enhanced activity of CrTiSi. As shown in Figure 1b, the Cr0.04Ti0.04Si catalyst maintains high activity even after 24 h photoreaction. In addition, the catalyst is reusable for further reactions: the catalyst recovered after reaction (run 3), when reused after simple washing with MeCN, shows almost the same activity and selectivity as the virgin catalyst (runs 12 and 13). 3.3. UV
vis Spectra. Figure 3 shows the diffuse reflectance UV
vis spectra of catalysts. Cr0.04Si (a) shows three distinctive bands at 245, 350, and 440 nm, assigned to the ligand-to-metal charge transfer (LMCT) absorption of highly dispersed chromate species.11 Cr0.04Ti0.04Si (b) shows three similar bands and an additional absorption at 400 nm. This suggests that the structure of Cr species on Cr0.04Ti0.22Si is different. The reduction of Cr species occurs via the electron transfer from OT to the Cr center and is suppressed by an increase in the electron density of OT. As reported,14 the electron density of the oxygen atom for carbonyl compounds, such as acetone, increases by the electrostatic interaction between the carbonyl oxygen and the Ti4+ site on TiO2. This implies that, in the Cr0.04Ti0.22Si system, the incorporation of a large amount of Ti probably leads to an electrostatic interaction between OT and the Ti4+ species. This increases the electron density of OT and suppresses the electron transfer to the Cr center. This probably suppresses the Td4+* formation. The increase in 400
460 nm absorption of Cr0.04Ti0.22Si (Figure 3c) may arise from the electrostatic interaction between OT and Ti4+ species. The above findings indicate that the Ti amount in the catalyst is crucial for catalytic activity, and the Cr0.04Ti0.04Si catalyst containing an equimolar amount of Cr and Ti shows the highest activity. These findings and the literature results8
10 suggest that Cr0.04Ti0.04Si probably contains an oxo-bridged Cr
O
Ti species, as shown in Scheme 2a, and the species demonstrates high photocatalytic activity. As shown in Table 2 (runs 3 and 4), Cr0.04Ti0.04Si shows very high catalytic activity (TON = 39.3), but the Cr0.04Ti0.05Si catalyst containing a slightly larger amount of Ti shows significantly decreased activity (TON = 26.7). This indicates that almost all of the Cr atoms in the Cr0.04Ti0.04Si catalyst exist as the Cr
O
Ti species; further addition of Ti changes the structure of the species and decreases the catalytic activity. As reported,15 the hydrolysis of Cr and Ti precursors occurs very rapidly as compared to that of TEOS. This probably allows a preferable 1:1 interaction between the Cr and Ti species and, hence, results in the formation of a stoichiometric amount of oxo-bridged Cr
O
Ti species on the Cr0.04Ti0.04Si catalyst. As shown in Figure 3a and b, the LMCT absorption bands of Cr0.04Ti0.04Si are similar to those of Cr0.04Si, indicating that the CrdO groups remain unchanged in the Cr0.04Ti0.04Si catalyst. This supports the proposed structure of the Cr
O
Ti species (Scheme 2a). 3.5. Effect of Ti on the Excited State. On the Cr0.04Si catalyst, the photoformed Td4+* is deactivated by reverse electron transfer

(Scheme 1).16 The enhanced Td4+* formation on Cr0.04Ti0.04Si (Figure 4b) is because the excited electron (e
) on Td4+* is transferred to the adjacent Ti4+ and delocalizes e
on the Cr
O
Ti species (Scheme 2). This suppresses rapid deactivation of Td4+* and shows high catalytic activity. The e
transfer from Td4+* to Ti4+ is confirmed by the ESR analysis with O2. As shown in Figure 5a, the ESR spectrum of Ti0.04Si measured with O2 (20 Torr) under UV irradiation (λ > 240 nm) shows a signal assigned to O2•
(gxx = 2.007, gyy = 2.010, gzz = 2.026).17 This is because UV irradiation promotes the LMCT transition of the titanate species (Ti4+
O2
f Ti3+
O
) and reduces O2 by Ti3+. As shown in Figure 5b, the O2•
signal is also observed on Cr0.04Ti0.04Si under visible-light irradiation (λ > 400 nm). In contrast, the signal is not observed on Cr0.04Si or Ti0.04Si under visible light (Figure 5c, d). These suggest that, on Cr0.04Ti0.04Si, the e
transfer from Td4+* to Ti4+ indeed occurs. If e
on Td4+* is fully transferred to Ti4+, Td4+* must be oxidized to Td5+. However, as shown in Figure 4b, the Td5+ signal intensity is much lower than that on Cr0.04Si, indicating that the e
on Td4+* is not fully transferred to Ti4+. This suggests that, as shown in Scheme 2c and c0 , the e
on Td4+* is delocalized over the Cr
O
Ti chain. This probably lengthens the lifetime of Td4+* and efficiently produces the [CHA
Td5+*] complex (Scheme 2d), thus resulting in an acceleration of CHA oxidation. The mechanism for delocalization of e
on the Cr
O
Ti species must be clarified. Several literatures report the electron delocalization over the oxo-bridged binuclear complexes, such as Fe
O
Ti,18a Fe
O
Mn,18b and Mn
O
Mn systems.19 In these systems, the energetic overlap of 3d orbitals of respective metals leads to an electron delocalization, along with a formation of new absorption band assigned to the metal-to-metal charge transfer (MMCT) transition. As shown in Figure 3b, the diffuse reflectance UV
vis spectrum of Cr0.04Ti0.04Si does not show MMCT transition, indicating that the overlap of Cr 3d and Ti 3d orbitals does not occur in the Cr
O
Ti species. Another possibility of the electron delocalization over the oxobridged metal species is the similar reduction potentials of respective metals; the reversible electron transfer between the two metals promotes electron delocalization. As reported,20,21 the Mo6+
O
Zn2+ system [
0.54 V (Mo6+) and
0.76 V (Zn2+) vs NHE] with a relatively small potential difference (Δ = 0.22 V) 19786

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Scheme 2. Mechanism for Photocatalytic Oxidation of CHA on the Cr0.04Ti0.04Si Catalyst

Scheme 3. Proposed Oxidation-Reduction Potential of Cr0.04Ti0.04Sia

a

Figure 5. ESR spectra (77 K) measured in the presence of O2 (20 Torr), (a) under UV irradiation (λ > 240 nm, 0.5 h) with Ti0.04Si and under visible-light irradiation (λ > 400 nm, 18 h) with (b) Cr0.04Ti0.04Si, (c) Cr0.04Si, and (d) Ti0.04Si.

shows electron delocalization, whereas the Mo6+
O
Pd2+ [+0.99 V (Pd2+) vs NHE; Δ = 1.53 V] and Mo6+
O
Cu2+ systems [+0.34 V (Cu2+) vs NHE; Δ = 0.88 V] with larger potential difference scarcely promote delocalization. As shown in Scheme 3, photoexcitation of Td species promotes a transition from the O 2p to Cr 3d orbital.11 As shown in Figure 5b, photoexcitation of Cr0.04Ti0.04Si with O2 promotes one-electron reduction of O2 (O2/O2•
;
0.13 V vs NHE),22 indicating that the reduction potential of Td4+* is more negative than that of Ti4+ (Ti4+/Ti3+;
0.04 V vs NHE).23 In contrast, as shown in Figure 5a, photoexcitation of Cr0.04Si does not promote O2 reduction. These data clearly suggest, as shown in Scheme 3, that the reduction potential of Td4+* lies between
0.04 and
0.13 V, and the reduction potential of Td4+* is close to that of Ti4+ (Δ < 0.09 V). Similar reduction potentials of Td4+* and Ti4+ therefore promote

The reduction potentials of O2, Ti4+, and Ti3+ are from refs 22 and 23.

a reversible e
transfer between Td4+* and Ti4+.21 This, thus allows delocalization of e
on the Cr
O
Ti species and, hence, lengthens the lifetime of Td4+* species.

4. CONCLUSION The CrTiSi ternary mixed oxides were used as catalysts for photocatalytic oxidation of CHA with O2 under visible light (λ > 400 nm). The CrTiSi catalyst containing equimolar amounts of Cr and Ti (0.04 mol %) shows much higher activity than Cr/Si while maintaining high selectivity for partial oxidation products (CHA-ol and CHA-one). The Cr0.04Ti0.04Si catalyst contains an oxo-bridged Cr
O
Ti species. Visible-light excitation of the chromate species leads to a formation of the Td4+* active species, which is stabilized by delocalization of excited electrons within the Cr
O
Ti species. The electron delocalization probably occurs due to the similar reduction potentials of Td4+* and Ti4+. This suppresses rapid deactivation of Td4+* and, hence, accelerates CHA oxidation. 19787

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’ AUTHOR INFORMATION Corresponding Author

*Fax: +81-6-6850-6273. Tel.: +81-6-6850-6271. E-mail: shiraish@ cheng.es.osaka-u.ac.jp.

’ ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research (No. 23656503) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). We thank the Division of Chemical Engineering for the Lend-Lease Laboratory System. D.T. thanks the Japan Society for Promotion of Science (JSPS) for Young Scientist and the Global COE Program “Global Education and Research Center for BioEnvironmental Chemistry” of Osaka University. ’ REFERENCES (1) (a) Schuchardt, U.; Cardoso, D.; Sercheli, R.; Pereira, R.; da Cruz, R. S.; Guerreiro, M. C.; Mandelli, D.; Spinace, E. V.; Pires, E. L. Appl. Catal., A 2001, 211, 1–17. (b) Bellussi, G.; Perego, C. Cattech 2000, 4, 4–16. (c) Castellan, A.; Bart, J. C. J.; Cavallaro, S. Catal. Today 1991, 9, 237–254. (2) (a) Zhao, R.; Ji, D.; Lv, G.; Qian, G.; Yan, L.; Wang, X.; Suo, J. Chem. Commun. 2004, 904–905. (b) Raja, R.; Sankar, G.; Thomas, J. M. J. Am. Chem. Soc. 1999, 121, 11926–11927. (3) (a) Giannotti, C.; Le Greneur, S.; Watts, O. Tetrahedron Lett. 1983, 24, 5071–5072. (b) Lu, G.; Gao, H.; Suo, J.; Li, S. J. Chem. Soc., Chem. Commun. 1994, 2423–2424. (c) Boarini, P.; Carassiti, V.; Maldotti, A.; Amadelli, R. Langmuir 1998, 14, 2080–2085. (d) Sclafani, A.; Herrmann, J. M. J. Phys. Chem. 1996, 100, 13655–13661. (e) Shimizu, S.-I.; Kaneko, T.; Fujishima, T.; Kodama, T.; Yoshida, H.; Kitayama, Y. Appl. Catal., A 2002, 225, 185–191. (f) Almquist, C. B.; Biswas, P. Appl. Catal., A 2001, 214, 259–271. (g) Brusa, M. A.; Grela, M. A. J. Phys. Chem. B 2005, 109, 1914–1918. (h) Gonzalez, M. A.; Howell, S. G.; Sikdar, S. K. J. Catal. 1999, 183, 159–162. (4) (a) Amadelli, R.; Bregola, M.; Polo, E.; Crassiti, V.; Maldotti, A. J. Chem. Soc., Chem. Commun. 1992, 1355–1357. (b) Molinari, A.; Amadelli, R.; Antolini, L.; Maldotti, A.; Battioni, P.; Mansuy, D. J. Mol. Catal. A 2000, 158, 521–531. (5) Maldotti, A.; Molinari, A.; Varani, G.; Lenarda, M.; Storaro, L.; Bigi, F.; Maggi, R.; Mazzacani, A.; Sartori, G. J. J. Catal. 2002, 209, 210–216. (6) (a) Teramura, K.; Tanaka, T.; Yamamoto, T.; Funabiki, T. J. Mol. Catal. A 2001, 165, 299–301. (b) Teramura, K.; Tanaka, T.; Kani, M.; Hosokawa, T.; Funabiki, T. J. Mol. Catal. A 2004, 208, 299–305. (7) (a) Shiraishi, Y.; Teshima, Y.; Hirai, T. Chem. Commun. 2005, 4569–4571. (b) Shiraishi, Y.; Teshima, Y.; Hirai, T. J. Phys. Chem. B 2006, 110, 6257–6263. (c) Shiraishi, Y.; Ohara, H.; Hirai, T. New J. Chem. 2010, 34, 2841–2846. (8) Rodrigues, S.; Ranjit, K. T.; Uma, S.; Martyanov, I. N.; Klabunde, K. J. Adv. Mater. 2005, 17, 2467–2471. (9) Shen, S.; Guo, L. Catal. Today 2007, 129, 414–420. (10) Kamegawa, T.; Shudo, T.; Yamashita, H. Top. Catal. 2010, 53, 555–559. (11) (a) Weckhuysen, B. M.; De Ridder, L. M.; Schoonheydt, R. A. J. Phys. Chem.1993, 97, 4756–4763. (b) Weckhuysen, B. M.; Verberckmoes, A. A.; Buttiens, A. L.; Schoonheydt, R. A. J. Phys. Chem. 1994, 98, 579–584. (c) Weckhuysen, B. M.; Wachs, I. E.; Schoonheydt, R. A. Chem. Rev. 1996, 96, 3327–3349. (d) Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. Chem. Rev. 2005, 105, 115–183. (12) (a) Gao, X.; Bare, S. R.; Fierro, J. L. G.; Banares, M. A.; Wachs, I. E. J. Phys. Chem. B 1998, 102, 5653–5666. (b) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina, A.; Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic, G. J. Phys. Chem. 1994, 98, 4125–4132.

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(13) Wada, K.; Yamada, H.; Watanabe, Y.; Mitsudo, T. J. Chem. Soc., Faraday Trans. 1998, 94, 1771–1778. (14) (a) Henderson, M. A. J. Phys. Chem. B 2004, 108, 18932–18941. (b) Henderson, M. A. Langmuir 2005, 21, 3443–3450. (c) Haubrich, J.; Kaxiras, E.; Friend, C. M. Chem.
Eur. J. 2011, 17, 4496–4506. (15) (a) Jehng, J.; Wachs, I. E.; Weckhuysen, B. M.; Schoonheydt, R. A. J. Chem. Soc., Faraday Trans. 1995, 91, 953–961. (b) Zhu, Z.; Hartmann, M.; Maes, E. M.; Czernuszewicz, R. S.; Kevan, L. J. Phys. Chem. B 2000, 104, 4690–4698. (16) (a) Yamashita, H.; Mori, K.; Shironita, S.; Horiuchi, Y. Catal. Surv. Asia 2008, 12, 88–100. (b) Kamegawa, T.; Morishima, J.; Matsuoka, M.; Thomas, J. M.; Anpo, M. J. Phys. Chem. C 2007, 111, 1076–1078. (17) (a) Morishita, M.; Shiraishi, Y.; Hirai, T. J. Phys. Chem. B 2006, 110, 17898–17905. (b) Murata, C.; Yoshida, H.; Kumagai, J.; Hattori, T. J. Phys. Chem. B 2003, 107, 4364–4373. (18) (a) Sherman, D. M. Phys. Chem. Miner. 1987, 14, 364–367. (b) Sherman, D. M. Am. Mineral. 1990, 75, 256–261. (19) (a) McGrady, J. E.; Stranger, R. J. Am. Chem. Soc. 1997, 119, 8512–8522. (b) Okuno, T.; Nishida, Y. Polyhedron 1996, 15, 1509–1515. (20) (a) Akimoto, M.; Shima, K.; Ikeda, H.; Echigoya, E. J. Catal. 1984, 86, 173–186. (b) Akimoto, M.; Ikeda, H.; Okabe, A.; Echigoya, E. J. Catal. 1984, 89, 196–208. (c) Hu, J.; Burns, R. C. J. Catal. 2000, 195, 360–375. (21) Ward, M. D.; Brazdil, J. F.; Grasselli, R. K. J. Phys. Chem. 1984, 88, 4210–4213. (22) Arai, T.; Yanagida, M.; Konishi, Y.; Iwasaki, Y.; Sugihara, H.; Sayama, K. J. Phys. Chem. C 2007, 111, 7574–7577. (23) Dobos, D. Electrochemical Data; Elsevier Sci. Pub. Co.: Amsterdam-Oxford-New York, 1975.

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