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Titanium Dioxide/Reduced Graphene Oxide Hybrid Photocatalysts for Efficient and Selective Partial Oxidation of Cyclohexane Yasuhiro Shiraishi, Shingo Shiota, Hiroaki Hirakawa, Shunsuke Tanaka, Satoshi Ichikawa, and Takayuki Hirai ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02611 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016
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ACS Catalysis
Titanium Dioxide/Reduced Graphene Oxide Hybrid Photocatalysts for Efficient and Selective Partial Oxidation of Cyclohexane Yasuhiro Shiraishi,*,†,‡ Shingo Shiota,† Hiroaki Hirakawa,† Shunsuke Tanaka,ǁ Satoshi Ichikawa,§ and Takayuki Hirai† †
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan ‡ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Saitama 332-0012, Japan ǁ Department of Chemical, Energy and Environmental Engineering, Kansai University, Suita 564-8680, Japan § Institute for NanoScience Design, Osaka University, Toyonaka 560-8531, Japan ABSTRACT: Partial oxidation of cyclohexane (CHA) to cyclohexanone (CHA-one) with molecular oxygen (O2) is one of the most important reactions. Photocatalytic oxidation has been studied extensively with TiO2-based catalysts. Their CHA-one selectivities are, however, insufficient because the formed CHA-one is subsequently decomposed by photocatalysis involving the reaction with superoxide anion (O2●–) produced by one-electron reduction of O2 on TiO2. Here we report that TiO2, when hybridized with reduced graphene oxide (rGO), catalyzes photooxidation of CHA to CHA-one with enhanced activity and selectivity under UV light (λ >300 nm). The TiO2/rGO hybrids produce CHA-one with twice the amount formed on bare TiO2 with much higher selectivity (>80%) than that on bare TiO2 (ca. 60%). The conduction band electrons photoformed on TiO2 are transferred to rGO, promoting efficient charge separation and enhanced photocatalytic cycles. The trapped electrons on rGO selectively promote two-electron reduction of O2 and suppresses one-electron reduction. This inhibits the formation of O2●–, which promotes photocatalytic decomposition of the CHA-one formed. These properties of rGO therefore facilitate efficient and selective formation of CHA-one on the hybrid catalyst. KEYWORDS: Photocatalysis · Titanium dioxide · Reduced graphene oxide· Cyclohexane · Partial oxidation
INTRODUCTION Selective oxidation of cyclohexane (CHA) to cyclohexanone (CHA-one) is one important transformation in industry because CHA-one is an irreplaceable intermediate in the synthesis of ε-caprolactam for nylon production.1,2 Heterogeneous catalytic systems for CHA oxidation with O2 as an oxidant has received much attention.3,4 Photocatalytic CHA oxidation has also been studied with TiO2-based catalysts5–19 because it promotes the reaction even at room temperature and atmospheric pressure. Selectivity for CHAone is, however, insufficient; therefore, its improvement is necessary. The mechanism for photocatalytic CHA oxidation on TiO2 is generally accepted as follows.9,12,15,19 Photoexcitation of TiO2 (eq. 1) produces the valence band holes (VB h+) and conduction band electrons (CB e–). The h+ oxidize CHA (C6H12) and produce cyclohexyl radical (C6H11●, eq. 2). The radicals react with O2 producing peroxy radical (C6H11OO●, eq. 3). The reaction of these radicals (eq. 4) produces cyclohexanol (C6H11OH) and CHA-one (C6H10O). TiO2 + hv → h+ + e–
C6H12 + h+ → C6H11● + H+
(2)
●
(3)
C6H11●
+ O2 → C6H11OO
C6H11OO● + C6H11OO● → C6H11OH + C6H10O + O2 (4) The cyclohexanol formed is subsequently oxidized by h+ and produces CHA-one. C6H11OH + 2h+ → C6H10O + 2H+
(5)
CHA-one is also produced via the reduction of the C6H11OO● radial by e–. C6H11OO● + e– + H+ → C6H10O + H2O
(6)
The e– on TiO2 are also consumed by one-electron reduction of O2 and produce superoxide radical (O2●–).20 O2 + e– → O2●– (–0.13 V vs NHE)
(7)
Selective CHA-one production is, however, difficult. One of the reasons is that the peroxy radical (C6H11OO●, eq. 3) is
(1)
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oxidized by h+ and is decomposed to CO2 via the cleavage of C–C bond.21 C6H11OO● + h+ → decomposition (CO2 formation) (8) The critical reason is that the formed CHA-one is subsequently decomposed to CO2 by photocatalysis. Although the detailed decomposition mechanism is still unclear, several experimental results suggest that oxidation of CHA-one by h+ and reaction with O2●– are involved in the mechanism, as follows.19,22,23
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diameter, 27 nm; BET surface area, 57 m2 g–1; anatase/rutile ratio, 83/17 w/w), supplied from the Catalyst Society of Japan. Hydrazine monohydrate was used for the reduction of GO (formation of rGO) and strong interaction between TiO2 and rGO.42 Hydrazine monohydrate was added to an NH3 solution containing TiO2 and GO, and the mixture was stirred at 333 K for 2 h. The mixture was neutralized with HCl. The solids were recovered by centrifugation, washed with water, and dried at 473 K, affording gray-black powders of TiO2/rGOx.
C6H10O + h+ + O2●–→ decomposition (CO2 formation) (9) This implies that O2●–, formed via the reduction of O2 by the CB e– photoformed on TiO2 (eq. 7), is the key species for the CHA-one decomposition. The suppression of the O2●– formation may inhibit the decomposition of CHA-one, leading to selective production of CHA-one. The present work aims at improving both activity and selectivity for CHA-one formation by simple modification of TiO2. Graphene is a 2D carbon layer prepared from cheap graphite and shows high electron mobility, high surface area, and high thermal and photostability.24–28 Several literature works prepared TiO2 hybridized with reduced graphene oxide (TiO2/rGO) and used them for photocatalytic reactions. They can be classified into four reactions such as decomposition of organic compounds,29–32 oxidation of alcohols,33,34 reduction of nitroaromatics,35,36 and hydrogen production.37–39 In these cases, the hybridized rGO enhances photocatalytic activity because it behaves as a co-catalyst that efficiently traps the CB e– of TiO2 and enhances charge separation. There is, however, no report of system that enhances the reaction selectivity. In the present work, we used TiO2/rGO hybrid catalysts for photocatalytic oxidation of CHA. The hybrid catalysts enhance the activity to about twice of that for bare TiO2. The most striking aspect of the hybrid catalysts is the enhanced CHA-one selectivity: bare TiO2 exhibits ca. 60% selectivity, while the hybrid catalyst shows more than 80% selectivity. The CB e– on the photoexcited TiO2 are smoothly transferred to rGO. This promotes separation of the h+ and e– pairs and enhances CHA oxidation. The e– trapped on rGO are consumed by selective two-electron reduction of O2, as follows: O2 + 2H+ + 2e– → H2O2 (0.68 V vs NHE)
(10)
This inhibits one-electron reduction of O2 (eq. 7) and suppresses the formation of O2●–, which promotes decomposition of the CHA-one formed (eq. 9). These properties of rGO therefore result in efficient and selective partial oxidation of CHA. RESULTS AND DISCUSSION Catalyst preparation and characterization. Graphene oxide (GO) was obtained by the conventional Hummers’ method.40,41 TiO2/rGOx catalysts [x (wt %) = GO / TiO2 × 100; x = 0.1, 0.5, 1.0, 3.0, 5.0] were synthesized with JRC-TIO-4 TiO2 (equivalent to Evonik (Degussa) P25 TiO2, average
Figure 1. (a, b) Typical TEM images of TiO2/rGO1.0. (c, d) Highresolution TEM images of the catalyst. More images are shown in Figure S1 (Supporting Information).
As shown in Figure 1 (a and b) and Figure S1 (Supporting Information), transmission electron microscopy (TEM) images of TiO2/rGO1.0 clearly show two-dimensional graphene sheet associated with TiO2 particles.42,43 In that, some TiO2 particles are not associated with the sheets, indicating that the catalyst contains TiO2 associated or unassociated with graphene sheet. As shown in Figure 1c, d, these TiO2 are a mixture of anatase and rutile particles.44 As shown in Figure S2 (Supporting Information), X-ray diffraction (XRD) of TiO2/rGO1.0 shows distinctive peaks assigned to anatase and rutile particles, and their intensity ratio is identical to those of bare TiO2. This indicates that phase transition of TiO2 particles scarcely occurs during the hybridization procedure. Figure S3 (Supporting Information) shows the diffuse-reflectance UV-vis spectra of TiO2/rGOx. The increase in the rGO amount increases the absorbance at λ >300 nm, assigned to its light absorption.45 Bandgap energies for all catalysts are 3.0–3.1 eV, similar to those of bare TiO2, indicating that hybridization of rGO scarcely affects the bandgap energy. Figure 2 shows the XPS charts for GO and TiO2/rGO1.0 at the C1s level. GO shows several surface carbon components assigned to framework (C–C, 284.6 eV; C=C, 283.7 eV), hydroxyl (C–OH, 285.8 eV), epoxy (C–O–C, 286.3 eV), carbonyl (C=O, 287.7 eV), and carboxyl groups (–COOH,
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288.9 eV), respectively.46–51 In contrast, TiO2/rGO1.0 shows a new component at 282.5 eV assigned to Ti–C group,48–51 along with a decrease in the C–O–C group. This is because, as reported for related materials,50,51 ring-cleavage of C–O–C bond on rGO by the reduction with hydrazine leads to a formation of covalent bonding with the lattice of TiO2 surface. This suggests that TiO2 strongly binds with rGO. Photoreaction. CHA (5 mL) containing catalyst (20 mg) was photoirradiated by a Xe lamp (λ >300 nm) under magnetic stirring with O2 (1 atm) at 298 K. Table 1 shows the results after 12 h photoirradiation. Note that the absence of TiO2 (entry 1) promotes almost no reaction, indicating that direct excitation by UV light scarcely promotes reaction. Bare TiO2 (entry 2) produces 22 µmol CHA-one and small amount of CHA-ol (2.5 µmol) together with a large amount of CO2 (69 µmol), resulting in low CHA-one selectivity (61%). In contrast, as shown by entries 3–5, hybridization of rGO with TiO2 enhances both activity and selectivity for the CHA-one formation. Increase in the amount of rGO produces larger amounts of CHA-one while decreasing the amounts of CO2 formed, although the amounts of CHA-ol are similar (ca. 3 µmol). Among the catalysts, TiO2/rGO1.0 (entry 5) produces the largest amount of CHA-one (40 µmol) and shows very high selectivity for CHA-one formation (83%). As shown by entries 6 and 7, a physical mixture of TiO2 with GO (1.0 wt %) or rGO (1.0 wt %) prepared by the reduction of GO with hydrazine shows almost no enhancement of the CHA-one formation and selectivity. In addition, hydrazine-treated TiO2 and its physical mixture with rGO (1.0 wt %) also scarcely enhance the activity and selectivity (entries 8 and 9). This suggests that strong interaction between TiO2 and rGO by hybridization is necessary. However, as shown by entries 10 and 11, TiO2 hybridized with larger amounts of rGO
(TiO2/rGO3.0 and TiO2/rGO5.0) produce lower amounts of CHA-one (300 nm (Figure S3, Supporting Information). This may suppress the light absorption of TiO2 and decreases activity, suggesting that TiO2 hybridized with appropriate amount of rGO (TiO2/rGO1.0) exhibits the best catalytic performance. C-C 284.6
a
C-O-C 286.3 C-OH 285.8 C=C 283.7 280
282
284
286
-COOH 288.9 C=O 287.7
288
290
Binding energy / eV C-C 284.6
b
Ti-C 282.5
280
282
C-OH 285.8 C=C 283.7 284
C-O-C 286.2 -COOH 288.7 C=O 287.7
286
288
290
Binding energy / eV
Figure 2. XPS charts (C1s level) of (a) GO and (b) TiO2/rGO1.0. Black is the obtained chart, and gray is the sum of the deconvoluted components, respectively.
Table 1. Results for photocatalytic oxidation of cyclohexane (CHA).a entry
catalyst
1 2 3 4 5
none TiO2 TiO2/rGO0.1 TiO2/rGO0.5 TiO2/rGO1.0 first reuse f second reuse f third reuse f TiO2 + GO1.0 TiO2 + rGO1.0 hydrazine-treated TiO2 hydrazine-treated TiO2 + rGO1.0 TiO2/rGO3.0 TiO2/rGO5.0 TiO2/rGO1.0 g TiO2 + p-benzoquinone h
6 7 8 9 10 11 12 13
Amounts / µmol CHA-ol b
CHA-one b
CO2 c
0.8 2.5 3.4 3.5 2.7 3.0 2.5 2.9 3.1 2.8 2.4 2.9 3.1 3.7 0.8 3.6
1.1 22.1 29.5 35.1 40.1 38.9 40.0 38.7 21.0 23.8 20.9 21.7 28.8 19.7 2.0 25.9
0.4 68.8 55.3 42.2 36.1 34.8 35.8 35.1 57.9 70.1 64.3 65.0 29.4 22.7 1.8 27.8
CHA-one selectivity / % d 61 70 77 83 82 83 82 62 62 61 61 78 73 76
H2O2 formed / µmol e ND 2.4 4.0 5.9 8.6 8.3 9.0 8.2 2.3 2.8 2.6 2.6 7.1 6.0 0.8 3.8
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a Reaction conditions: CHA (5 mL), catalyst (20 mg), O2 (1 atm), temperature (298 K), time (12 h), λ >300 nm (intensity at 300–450 nm is 27.3 W m−2). b Determined by GC-FID (Shimadzu GC-1700). c Determined by GC-BID (Shimadzu GC-2010 Plus). d = (CHA-one) / [CHA-ol + CHA-one + (1/6) CO2] ×100 (refs. 8 and 19). e Determined by redox titration with KMnO4 (detection limit, 0.2 µmol). f Reused after washing with MeOH and MeCN. g λ >420 nm (intensity at 420–500 nm is 26.9 W m−2). h p-benzoquinone (0.05 mmol) was used.
60 50 40 20 0 0
12 t/h
0 24
100
80 60 50 40 20 0 0
12 t/h
CHA−one selectivity / %
80
b TiO 2/rGO 1.0
100
Amounts / µmol
CHA−ol CHA−one CO2
100 CHA−one selectivity / %
a TiO 2
100
Amounts / µmol
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0 24
Figure 3. Time-dependent change in the product amounts during photocatalytic oxidation of CHA on (a) TiO2 and (b) TiO2/rGO1.0. Reaction conditions are identical to those in Table 1. The green circle symbols show the change in the CHA-one selectivity.
Figure 3 shows the time course for the amounts of CHA-ol, CHA-one, and CO2 during photoreactions of CHA on the respective TiO2 and TiO2/rGO1.0 catalysts. In both cases, the amounts of CHA-ol formed are negligibly small. TiO2 produces much larger amount of CO2 than CHA-one, suggesting that subsequent decomposition of CHA-one indeed produces CO2 (eq. 9).19,22,23 In contrast, TiO2/rGO1.0 produces CHA-one about twice that on TiO2, while the amount of CO2 formed on TiO2/rGO1.0 is about half of that on TiO2. As shown by green symbols, selectivities for the amount of CHA-one formed on TiO2 are ca. 60%, whereas those on TiO2/rGO1.0 are >80%. These data suggest that hybridization of rGO with TiO2 enhances photocatalytic activity and produces larger amount of CHA-one, while suppressing decomposition of the formed CHA-one, resulting in enhanced CHA-one selectivity. The apparent quantum yields (ΦAQY) for CHA-one formation were determined by the reaction with monochromated 334 nm light for 12 h, using the equation [ΦAQY (%) = ([CHA-one formed] × 2) / (photon number entered into the reaction vessel) × 100].52 The ΦAQY obtained with bare TiO2 is determined to be 2.2 %, whereas that obtained with TiO2/rGO1.0 is 4.1 %. This clearly indicates that the hybridization of rGO indeed enhances CHA-one formation. Note that, as shown by entry 5, the TiO2/rGO1.0 catalyst reused shows almost the same activity and selectivity as the virgin one, indicating that it can be reused without the loss of catalytic performance. Also note that, as shown in Figure S4 (Supporting Information), change in the amount of TiO2/rGO1.0 catalyst (10–100 mg) scarcely affect the CHA-one selectivity. In contrast, the amount of CHA-one produced increases with the catalyst amount, but 50 and 100 mg catalysts produce similar amounts of CHA-one. This indicates that the photons are efficiently absorbed by 50 mg catalyst. Several literatures revealed that carbon-doped TiO2 prepared by calcination of amorphous TiO2 with a carbon source at high temperature (>673 K) are active even under visible light due to
the formation of new energy levels.53,54 As shown in Table 1 (entry 12), TiO2/rGO1.0 used for reaction under visible light (λ >420 nm) scarcely produces CHA-one (2.0 µmol). The present catalysts are prepared with crystalline TiO2 at relatively low temperature (473 K). This may therefore scarcely promote carbon doping with TiO2 and exhibit almost no activity under visible light. Enhanced charge separation. The enhanced formation of CHA-one on TiO2/rGO is due to the transfer of photoformed CB e– on TiO2 to rGO, leading to enhanced charge separation. It is well known that P25 TiO2 is a mixture of anatase and rutile particles44 with 3.20 eV and 3.03 eV bandgap energies,55 respectively. As shown in Figure S5 (Supporting Information), electrochemical Mott-Schottky plot of bare P25 TiO2 determines the level of CB bottom to be –0.18 V (vs NHE, pH 0). As reported,55–58 the CB bottom of rutile is ca. 0.2 V more negative than that of anatase. The CB bottoms of anatase and rutile particles of P25 TiO2 can therefore roughly be determined to be –0.18 V and –0.38 V, respectively. The energy diagrams of bare TiO2 and TiO2/rGO systems can therefore be depicted as Scheme 1. The donor level of rGO (– 0.08 V vs NHE, pH 0)35,59 lies at the level more positive than the bottom of anatase CB (–0.18 V). The CB e– on rutile or anatase are therefore transferred to rGO thermodynamically favorably. Scheme 1. Energy diagrams for (a) TiO2 and (b) TiO2/rGO.
The e– transfer to rGO is supported by the photocurrent response of the catalysts measured on a fluoride tin oxide (FTO) glass under photoirradiation (λ >300 nm). As shown in Figure 4, the photocurrent density on TiO2/rGO1.0 is much larger than that on TiO2. The current density lies in the order
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of TiO2/rGO1.0 >TiO2/rGO0.1 >TiO2, which agrees with the amount of CHA-one formed by photocatalysis (Table 1). This suggests that CB e– transfer from TiO2 to rGO enhances charge separation and efficiently catalyzes photocatalytic cycles. As shown in Figure 4, a physical mixture of TiO2 and 1.0 wt % rGO exhibits almost the same photocurrent density as that of bare TiO2, which also agrees with the photoreaction data (Table 1). This suggests that strong interaction between TiO2 and rGO facilitates efficient CB e– transfer from TiO2 to rGO. The CB e– transfer is also confirmed by photoluminescence (PL) measurements. As shown in Figure S6 (Supporting Information), distinctive PL spectrum of TiO2 at 400–600 nm,60 when observed in water by photoexcitation at 340 nm, decreases with an increase in the amount of rGO hybridized. This further supports the CB e– transfer from TiO2 to rGO.
TiO 2/rGO1.0
1.2
TiO 2/rGO0.1
1
TiO 2 n = 1.65 ± 0.05 jk = 3.57 ± 0.12
20
TiO 2
−1
/ mA
TiO 2 + rGO1.0
0.8 0.6
−1
40
cm
2
60
reduction and suppresses the decomposition of CHA-one. The inhibition of one-electron reduction of O2 on TiO2/rGO is supported by electron spin resonance (ESR) measurement with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), a spin trapping reagent.63 Figure 5 shows the ESR spectra obtained at 298 K after 5 min irradiation of an O2-saturated MeCN/CHA mixture (9/1 v/v, 5 mL) with the respective catalysts and DMPO. The solution obtained by photoirradiation with TiO2 (Figure 5a) shows signals assigned to the DMPO–●OOH spin adduct (αN = 13.0 G; αHβ= 10.1 G, g = 2.0065).63 The signal intensity for the solution obtained by photoirradiation with TiO2/rGO1.0 (Figure 5b) is much weaker than that on TiO2. Figure 5c shows the ESR spectrum of the solution obtained by photoirradiation of TiO2 with p-benzoquinone, a well-known O2●– quencher.64 Its signal intensity is similar to that obtained on TiO2/rGO1.0 (Figure 5b), indicating that one-electron O2 reduction is indeed suppressed on TiO2/rGO.
j
−2
light light on off
Current density / µA cm
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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0.4 −
2e process
0 0
100
200
300
400
0.2 0
t / sec
0.02
0.04 −1/2
Figure 4. Photocurrent response of TiO2, TiO2/rGOx (x = 0.1 and 1.0), and a physical mixture of TiO2 and 1.0 wt % rGO obtained under λ >300 nm irradiation (0.5 V bias). g = 2.0065 αN = 13.0
a. TiO 2
b. TiO 2/rGO1.0 c. TiO 2 + p−benzoquinone
3420 3440 Magnetic field / G
ω
0.06
0.08
−1/2
/ rpm
Figure 6. Koutecky-Levich plots of the O2 reduction data measured by RDE analysis for TiO2 or TiO2/rGO1.0 (Figure S7, Supporting Information) in a buffered pH 7 solution at a constant potential of –0.4 V (vs Ag/AgCl).
β
αH = 10.1
3400
TiO 2/rGO1.0 n = 2.09 ± 0.07 jk = 4.30 ± 0.15
3460
Figure 5. ESR spectra (298 K) for the DMPO–●OOH spin adduct signals obtained by λ >300 nm irradiation with TiO2 or TiO2/rGO1.0 (20 mg) in an O2-saturated MeCN/CHA mixture (9/1 v/v, 5 mL) containing DMPO (0.125 mmol). p-Benzoquinone (0.05 mmol) was used for analysis.
Selective two-electron reduction of O2. The CB e– on TiO2 are mainly consumed by one-electron reduction of O2 (eq. 7) and produce O2●–,61,62 which promotes decomposition of CHAone (eq. 9). In contrast, TiO2/rGO inhibits one-electron
Photoexcited TiO2/rGO promotes two-electron reduction of O2 (eq. 10) by the e– trapped on rGO; this suppresses oneelectron reduction. The amounts of H2O2 formed during the reaction confirm this. As summarized in Table 1, the amount of H2O2 formed on TiO2/rGO1.0 by photoreaction of CHA for 12 h is 8.6 µmol, whereas TiO2 produces much smaller amount (2.4 µmol). Electrochemical rotating disk electrode (RDE) analysis further supports this. Figure S7 (Supporting Information) shows linear-sweep voltammograms of TiO2 or TiO2/rGO1.0 measured on RDE in buffered water (pH 7) under O2 at different rotating speeds. Figure 6 summarizes the Koutecky–Levich plots of the data at –0.4 V. The average number of electrons (n) involved in the overall O2 reduction can be estimated by the linear regression of the plots,65,66 using the following equations: j–1 = jk–1 + B–1ω–1/2 –1/6
B = 0.2nFν
CD
2/3
(11) (12)
j is the current density, jk is the kinetic current density, ω is the rotating speed (rpm), F is the Faraday constant (96485 C mol– 1 ), ν is the kinetic viscosity of water (0.01 cm2 s–1), C is the bulk concentration of O2 in water (1.3 × 10–6 mol cm–3), and D
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is the diffusion coefficient of O2 (2.7 × 10–5 cm2 s–1), respectively.67 The n for TiO2 is 1.65, suggesting that oneelectron O2 reduction (n = 1) indeed occurs,61,62 as supported by the ESR data (Figure 5). This indicates that, as shown in Scheme 1a, the CB e– on TiO2 mainly promotes one-electron O2 reduction and produces O2●– (eq. 7), leading to decomposition of the formed CHA-one (eq. 9). In contrast, TiO2/rGO1.0 exhibits n = 2.09, indicating that it selectively promotes two-electron reduction (n = 2). As shown in Scheme 1b, the donor level of rGO (–0.08 V)35,59 lies at the level more positive than that for one-electron reduction of O2 (–0.13 V),20 while more negative than that for two-electron reduction (0.68 V).68 This thermodynamically-favorable CB e– transfer from TiO2 to rGO therefore promotes selective two-electron reduction of O2 and suppresses one-electron reduction (O2●– formation). 60 CO2 formed / µmol
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
TiO 2
40
TiO 2/rGO1.0
20
TiO 2 + p−benzoquinone 0
0
12 t/h
24
Figure 7. Time-dependent change in the amount of CO2 formed during photooxidation of CHA-one with TiO2 or TiO2 /rGO1.0 with or without p-benzoquinone (0.05 mmol). Reaction condition: catalyst (20 mg), CHA-one (0.1 mmol), MeCN (5 mL), O2 (1 atm), light irradiation (λ >300 nm), temperature (298 K).
Suppression of CHA-one decomposition. The inhibition of one-electron reduction of O2 (inhibition of O2●– formation) on TiO2/rGO suppresses the decomposition of CHA-one. Figure 7 shows the time course of the amount of CO2 formed during photooxidation of CHA-one (0.1 mmol), when used as a starting substrate. TiO2 (black symbols) produces large amount of CO2, indicating that, as shown by eq. 9, CHA-one is indeed decomposed by photocatalysis involving h+ and O2●– .19,22,23 In contrast, TiO2/rGO1.0 (red symbols) produces much smaller amount of CO2, suggesting that it suppresses decomposition of CHA-one. As shown by blue symbols in Figure 7, TiO2, when used for the reaction together with pbenzoquinone, an O2●– quencher,63 produces smaller amount of CO2, comparable to that of the TiO2/rGO1.0 system (red). The data clearly indicates that, as shown by eq. 9, suppression of O2●– formation on the TiO2/rGO catalysts indeed inhibits the decomposition of CHA-one. This therefore results in enhanced CHA-one selectivity during the CHA photooxidation. As shown by entry 13 in Table 1, TiO2, when used for CHA photooxidation with p-benzoquinone, produces smaller amount of CO2 (28 µmol) as compared to the case with bare TiO2 (69 µmol, entry 2) because p-benzoquinone quenches the O2●– formation as is the case for the TiO2/rGO1.0 system. However, in this case, the amount of CHA-one formed (26
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µmol) is much smaller than that on the TiO2/rGO1.0 system (40 µmol). This is because, on the TiO2/rGO1.0 system, the CB e– transfer from TiO2 to rGO promotes efficient photocatalytic cycles. The above findings indicate that rGO hybridized with TiO2 acts as an efficient trapping site for the CB e– photoformed on TiO2 (activity enhancement) and an efficient active site for selective two-electron reduction of O2 (suppressing decomposition of the formed CHA-one). These two properties therefore facilitate efficient and selective photooxidation of CHA to CHA-one on the hybrid catalysts. CONCLUSION We found that the TiO2/rGO hybrid photocatalysts promote efficient and selective photooxidation of CHA to CHA-one under UV irradiation. The enhanced catalytic activity is due to the charge separation enhanced by the transfer of CB e– on the photoactivated TiO2 to rGO. The e– trapped on rGO promotes selective two-electron reduction of O2. This suppresses the formation of O2●– (one-electron reduced species), which leads to subsequent decomposition of the formed CHA-one. These properties of rGO result in successful production of CHA-one. Although several rGO-based photocatalysts have been studied, there is no report of catalyst facilitating selective photocatalytic oxidation. The selective multi-electron reduction of O2 on rGO clarified here may contribute to the creation of more efficient photocatalytic system for partial CHA oxidation and to the design of new catalytic system for selective photooxidation. EXPERIMENTAL SECTION Preparation of GO. GO was prepared as follows:40,41 Graphite powder (3.0 g) and NaNO3 (1.5 g) were added to H2SO4 (85%, 70 mL) and stirred in an ice bath. KMnO4 (9.0 g) was added slowly at 300 nm using a 2 kW Xe lamp (USHIO Inc.) with magnetic stirring. After the reaction, the gas-phase products were analyzed by GC-BID (Shimadzu; GC-2010 Plus). The catalyst was removed by centrifugation, and the resulting solution was analyzed by GC-FID (Shimadzu; GC-1700). H2O2 concentration in solution was determined by redox titration with KMnO4.69 Apparent quantum yield determination. Photoreactions were performed in CHA (5 mL) with catalyst (20 mg). After ultrasonication and O2 bubbling, the tube was photoirradiated for 12 h, where the incident light was monochromated by the 334 nm band-pass glass filter.70 The photon number entered into the reaction tube was determined with a spectroradiometer (USR-40, USHIO Inc.).71 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 10.5 mW,72 with 1,1′-diphenyl-2-picrylhydrazyl (DPPH) as a standard. Catalyst (20 mg) was added to a MeCN/CHA mixture (9/1 v/v, 5 mL) containing DMPO (0.125 mmol) within a Pyrex glass tube. After ultrasonication (5 min) and O2 bubbling (10 min), the tube was photoirradiated for 3 min. The catalyst was recovered by centrifugation, and the solution was subjected to analysis at room temperature. RDE measurements. The measurements were performed on an electrochemical system with a three-electrode cell using an Ag/AgCl electrode and a Pt wire electrode as the reference and counter electrode, respectively.73 The working electrode was prepared as follows:74 catalysts (50 mg) were dispersed in EtOH (2 mL) containing Nafion (50 mg) by ultrasonication. The slurry (20 µL) was put onto a Pt disk electrode and dried at room temperature. The linear-sweep voltammograms were obtained in an O2-saturated 0.1 M phosphate buffer solution (pH 7) with a scan rate 10 mV s–1 after O2 bubbling for 5 min. Electrochemical analysis. A three-electrode cell with an electrochemical analyzer (SI 1280B, TOYO Corp.) was used. The working electrode was prepared as follows: catalysts (50 mg) were dispersed in EtOH (3 mL) with p-ethylene glycol (60 mg) by ultrasonication (1 h). The slurry (10 µL) was put onto a FTO glass, and the glass was annealed at 623 K for 30 min.75 The working electrode was immersed in a 0.1 M Na2SO4 solution (pH 6.6) with a Pt sheet and a Ag/AgCl electrode as the counter and reference electrodes, respectively, and irradiated at λ >300 nm at a potential of 0.5 V, where the exposed area was 0.25 cm2. The perturbation signal for MottSchottky analysis was set at 10 mV. Other analysis. TEM observations were performed on an FEI Tecnai G2 20ST analytical electron microscope.76 Diffuse-reflectance (DR) UV-vis spectra were measured on an UV-vis spectrophotometer (JASCO Corp.; V-550) using BaSO4 as a reference.77 XRD patterns were measured on a
Philips X′Pert-MPD spectrometer. PL spectra were measured on a FP-6500 fluorescence spectrophotometer (JASCO).
ASSOCIATED CONTENT TEM images (Figure S1), XRD patterns (Figure S2), diffuse reflectance UV-vis spectra (Figure S3), effect of the catalyst amount on the activity and selectivity (Figure S4), electrochemical Mott-Schottky plot (Figure S5), PL spectra (Figure S6), and linear-sweep voltammograms of catalysts (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by the Grant-in-Aid for Scientific Research (No. 26289296) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), and by the Precursory Research for Embryonic Science and Technology (PRESTO) from the Japan Science and Technology Agency (JST). H.H. thanks the Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists.
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