Research Article pubs.acs.org/acscatalysis
Partial Oxidation of Alcohols on Visible-Light-Responsive WO3 Photocatalysts Loaded with Palladium Oxide Cocatalyst Osamu Tomita,† Takahide Otsubo,† Masanobu Higashi,† Bunsho Ohtani,‡ and Ryu Abe*,† †
Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
‡
S Supporting Information *
ABSTRACT: Particles of tungsten oxide loaded with a palladium oxide cocatalyst (PdOx/WO3) exhibit higher selectivity in comparison to other photocatalysts, such as Pt/ WO3, Pd/TiO2, and Pt/TiO2, in the partial oxidation of alcohols such as 2-propanol to the corresponding aldehydes or ketones (e.g., 80% selectivity for acetone production with ca. 96% conversion of 2-propanol) in water containing molecular O2. A detailed investigation of 2-propanol oxidation as a model reaction revealed significant differences between the reactivities of the WO3 and TiO2 systems. On TiO2 photocatalysts, complete decomposition to CO2 proceeded readily, due to the occurrence of direct oxidation of 2-propanol and acetone adsorbed by holes, resulting in significantly low selectivity for partial oxidation. On the other hand, the rates of acetone peroxidation on WO3 photocatalysts were much lower than those on TiO2 due to the low affinity of the WO3 surface to the substrates, particularly acetone. The low affinity of the WO3 surface also enables preferential generation of hydroxyl radicals (•OH) from water, which react with 2-propanol much more efficiently than with acetone, further increasing selectivity for acetone in the WO3 system. Most importantly, the loading of a palladium oxide cocatalyst on WO3 drastically improved the selectivity for acetone by almost completely suppressing the peroxidation of acetone during photoirradiation. A considerable amount of hydrogen peroxide (H2O2) was confirmed to accumulate during photoirradiation on PdOx/WO3 due to the high selectivity of the PdOx cocatalyst for the two-electron reduction of O2 molecules, while such accumulation was not observed for Pt/WO3. The H2O2 in the PdOx/ WO3 system preferentially reacted with photogenerated holes when the concentrations of acetone and H2O2 increased, suppressing the peroxidation of acetone by the holes. The PdOx/WO3 photocatalyst was more selective for partial oxidation of other alcohols than other photocatalysts, while the selectivity depended on the alcohols used, suggesting the availability of the unique reactivity of the PdOx/WO3 photocatalyst for partial oxidation of various organic compounds. KEYWORDS: partial oxidation of alcohols, photocatalytic conversion, heterogeneous photocatalysis, tungsten oxide, visible light
1. INTRODUCTION Photocatalytic conversion of organic compounds by utilizing light energy is the candidate for a variety of environmentally benign synthetic processes. One of the advantages of the photocatalytic conversion system is the availability of the abundant and harmless reactants, such as molecular oxygen (O2) and water (H2O). For example, molecular O2 can be used as an efficient oxidant in the oxidation of organic compounds on TiO2 photocatalysts, upon which photoexcited electrons reduce O2 and holes oxidize organic compounds directly or indirectly through the generation of active intermediates such as hydroxyl radicals (•OH).1−10 Some synthetic reactions, such as the partial oxidation of alkyl-substituted benzyl alcohols to the corresponding aldehydes, have been demonstrated using TiO2 photocatalysts with molecular O2.11−20 In some cases, H2O can be used as an oxygen source through the generation of • OH by holes.21−23 For example, the direct hydroxylation of benzenes to phenols has been demonstrated using TiO2 photocatalysts.21 However, one of the major drawbacks of the TiO2 photocatalyst system is the facile occurrence of sequential oxidation, which lowers selectivity for the desired products.24−26 As is well known, TiO2 generally shows high photocatalytic activity toward nonselective oxidation to CO2 © 2016 American Chemical Society
and, thus, has been widely studied in photocatalysts for environmental purification.1−10 As a result, the number of reports on selective oxidation procedures utilizing TiO2 with molecular O2 have so far been limited,27−31 whereas some reports have successfully employed Pt-loaded TiO2 photocatalysts in the absence of O2, where photoexcited electrons reduce H+, or some other intermediate, instead of O2.21,32−39 Another drawback of the TiO2 photocatalyst system is the necessity of ultraviolet (UV) light with wavelength shorter than ca. 400 nm for band gap excitation, which sometimes induces undesirable photochemical reactions.40−46 Additionally, conventional UV light sources, such as high-pressure Hg lamps, are generally insufficient for total conversion efficiency of electricity to photons in the UV region, although improvements in efficiency in UV LED systems can be expected in the future. Thus, the construction of highly selective synthetic systems utilizing visible light, which can be obtained from LEDs that emit visible light with relatively high efficiency or perhaps even Received: August 20, 2015 Revised: January 5, 2016 Published: January 6, 2016 1134
DOI: 10.1021/acscatal.5b01850 ACS Catal. 2016, 6, 1134−1144
Research Article
ACS Catalysis
have diameters ranging from 50 to 200 nm with ca. 11 m2 g−1 of specific surface area. Scanning electron microscopy (SEM) images of the collected fine WO3 particles and the as-received TiO2 (P-25) particles are shown in Figure S1 in the Supporting Information. X-ray diffraction (XRD) measurements revealed that the fine particles consisted of the triclinic phase predominantly, along with a small fraction of the monoclinic phase (see Figure S2 in the Supporting Information). 2.2. Noble-Metal Loading by Photodeposition. Noblemetal species (Pd or Pt) were loaded onto the WO3 and TiO2 samples via the following photodeposition method. The photocatalyst particles were suspended in an aqueous methanol solution (10 vol %) containing the required amount of PdCl2 or H2PtCl6·6H2O as precursor (from 0.05 to 0.5 wt %, calculated relative to the amount of photocatalyst particles). For PdCl2, the necessary amount of PdCl2 powder was dissolved to give a 9.3 mmol L−1 HCl aqueous solution to, forming the PdCl42− complex, which was then added to the suspension of photocatalyst particles as a precursor for photodeposition. WO3 samples were irradiated with visible light emitted from a homemade LED irradiation system (LED 400 ± 10 nm, 54 mW × 28) for 4 h. For photodeposition on the TiO2 samples, which require UV light shorter than 400 nm for photoexcitation, a Xe lamp (LX-300F, Cermax 300 W, 300 < λ < 500 nm) was used as the light source, with photoirradiation carried out for 3 h. After three repeated washings with Milli-Q water and centrifugation, the sample was dried in air at 333 K. The prepared samples were characterized with XRD (Rigaku, MiniFlex II, Cu Kα), Brunauer−Emmett−Teller (BET) specific surface area measurements (BEL SORP-mini II), SEM (JEOL, JSM-7400F), transmission electron microscopy (TEM, JEOL, JEM-1400 or JEM-2100F), and X-ray photoelectron spectroscopy (XPS, ULVAC Phai, MT-5500, Mg Kα). The binding energy of the impurity C 1s peak was adjusted to 284.6 eV to correct the chemical shifts of other elements. 2.3. Photocatalytic Reaction. The photocatalytic oxidation of 2-propanol was conducted using a top-irradiation-type Pyrex reaction cell, in which the photocatalyst particles (0.1 g) were continuously suspended in an aerated aqueous 2-propanol solution (100 mL; initial 2-propanol amount 131 μmol) using a set of a stirrer bar and a magnetic stirrer. The head space (ca. 233 mL) of the reactor was filled with ambient air, and the reaction cell was tightly sealed with a rubber cap. Some reactions were conducted in a side-irradiation-type Pyrex reaction cell (100 mL; head space ca. 120 mL) tightly sealed with a silicon septum. The temperature of the solution was kept at 279 K during the reaction by a flow of external circulating cooling water. In most cases, photocatalytic reactions were carried out using a 300 W xenon lamp (LX-300F, Cermax 300 W) equipped with a cold mirror that afforded irradiation within a limited wavelength range of 300−500 nm, enabling the photoexcitation of both the WO3 and TiO2 photocatalysts. For the reactions under visible light irradiation, the Xe lamp was equipped with a cutoff filter (L-42, Hoya) to eliminate UV light shorter than 400 nm. Sample aliquots were withdrawn from the reactor cell after each irradiation period and then filtered through a PVDF filter (Mini-Uni Prep, 0.1 μm) to remove the photocatalyst particles. Product analysis was performed by means of gas chromatography (Shimadzu, GC-14A, FID, PEG20 M Uniport, N2 carrier) and ion chromatography (Shimadzu, CDD-10ASP, Shim-pack IC-SA2). The generation of CO2 in the gas phase was analyzed by means of gas chromatography (Shimadzu, GC-8A, TCD, New Carbon, Ar carrier) or micro
natural sunlight, is highly desired for achieving environmentally benign conversion processes based on photocatalysis. Particles of tungsten oxide loaded with nanoparticulate platinum (Pt/WO3) have recently been shown to exhibit high photocatalytic activity for the decomposition of various organic compounds under visible light.47 Although WO3 has generally been regarded as an inactive photocatalyst in the oxidation of organic compounds using O2, due to an insufficient conduction band minimum (CBM) for the reduction of O2 via oneelectron processes, loading with a nanoparticulate Pt cocatalyst significantly increases the probability of multielectron reduction of O2 by photoexcited electrons, thus enhancing the oxidation of organic substances by holes remaining in the valence band.47 We have also demonstrated that Pt/WO3 photocatalysts can produce phenol directly from benzene with fairly high selectivity (>70%) using O2 and H2O as reactants under UV or visible light.23,48 The hydroxylation of benzene using 18Olabeled O2 and H2O revealed that the holes generated on Pt/ WO3 react primarily with H2O molecules, even in the presence of benzene in aqueous solution, selectively generating •OH radicals that subsequently react with benzene to produce phenol.23 On the other hand, a considerable portion of the holes generated on TiO2 photocatalysts react directly with benzene molecules, enhancing undesirable peroxidations, including cleavage of the aromatic ring.23 These findings motivated us to apply the WO3 photocatalysts to other synthetic reactions initiated by •OH radicals, such as the oxidation of alcohols.49−52 Partial oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively, are important processes in the chemical industry, and therefore the development of efficient catalytic or photocatalytic systems has been extensively studied to achieve highly selective reactions.53−75 In the present study, a visiblelight-responsive WO3 photocatalyst was applied to the partial oxidation of alcohols in water using molecular O2 as the oxidant. The oxidation of 2-propanol was first examined in detail as a model reaction to understand the properties of WO3 photocatalysts in comparison with those of TiO2, before the oxidation of other alcohols was also examined to evaluate the feasibility and scope of this procedure.
2. EXPERIMENTAL SECTION 2.1. Preparation of Photocatalyst Samples. Commercially available WO3 powder (mixture of triclinic and monoclinic phases, 4.8 m2 g−1, Kojundo Chemical Laboratory Co., Ltd.) and TiO2 powder (mixture of anatase and rutile phases, 55 m2 g−1, Evonik P-25) were used as photocatalysts. The fine particles of WO3, which have higher activity than aggregated large particles, were collected from the commercial WO3 samples as follows.47 The as-purchased WO3 powder (6.5 g), containing both fine particles and large aggregates, was suspended in Milli-Q water (ca. 50 mL) and exposed to ultrasonic dispersion for more than 10 min. The resultant suspension was then centrifuged at 1000 rpm for 10 min. After the supernatant containing fine WO3 particles was collected in another bottle, an appropriate amount of Milli-Q water (ca. 50 mL) was added to the remaining suspension, still containing the mixture of fine and large particles. This suspension was again subjected to ultrasonic dispersion and subsequent centrifugation. After five repetitions of this process, the fine particles were finally collected from the combined supernatants by centrifugation at 10000 rpm for 15 min and subsequent drying in air at 333 K. The particles obtained were confirmed to 1135
DOI: 10.1021/acscatal.5b01850 ACS Catal. 2016, 6, 1134−1144
Research Article
ACS Catalysis gas chromatography (Agilent, 3000A Micro GC, TCD, Plot U, Ar carrier). The amount of H2O2 produced during the photocatalytic oxidation of 2-propanol was analyzed with iodometry, as follows. After filtration, 1 mL of 0.1 mol L−1 potassium hydrogen phthalate (C8H5KO4) aqueous solution and 1 mL of 0.4 mol L−1 potassium iodide (KI) aqueous solution were added to the solution, which was then kept for 4 min. The H2O2 molecules reacted with iodide anions (I−) under acidic conditions (H2O2 + 3I− + 2H+ → I3− + 2H2O) to produce triiodide anions (I3−) possessing a strong absorption at around 350 nm. The amount of I3− was determined by means of UV− vis spectroscopy (Shimadzu, UV-1800) on the basis of the absorbance at 350 nm, from which the amount of H2O2 produced during each reaction was estimated.
Figure 2. XPS of TiO2 samples loaded with (a) Pd species and (b) Pt species prepared by photodeposition under ultraviolet and visible light (amount of Pd and Pt, 0.5 wt %).
3. RESULTS AND DISCUSSION 3.1. Characterization of Noble-Metal Species Loaded on Photocatalyst Particles. Figure 1 shows the XPS spectra
One of the reasons for the presence of PdII species (i.e., PdO) in the WO3 sample was the insufficient numbers of photons produced by the LED system. Thus, photoirradiation was conducted for 3 h using the Xe lamp to reduce the Pd precursor to its metallic state on WO3. As shown in Figure S3 in the Supporting Information, the fraction of Pd0 species had indeed increased in comparison to the previous sample prepared by LED (see Figure 1a). However, the peaks attributed to the Pd II species were clearly observed. Considering the reduction potential from PdCl42− to Pd metal (+0.6 V vs standard hydrogen electrode (SHE)), the lower level of CBM in WO3 (+0.5 V vs SHE) than in TiO2 (−0.2 V vs SHE), i.e. the lower reduction potential of photoexcited electrons, might be the reason for incomplete reduction on WO3. Another likely reason would be that the Pd precursors were actually reduced to Pd0 during photoirradiation but were readily reoxidized by exposure to the O2-containing atmosphere, especially at the WO3 surface. Thus, the WO3 samples loaded with Pd species will, hereafter, be denoted as PdOx/WO3 because they contain both Pd0 and PdII (i.e., PdO) species. Other samples loaded with metallic species are denoted Pt/WO3, Pt/TiO2, and Pd/TiO2. As for the PdOx/WO3 and Pt/WO3 photocatalysts, the samples prepared via the LED setup were used as photocatalysts in subsequent experiments. Figure 3 shows TEM images of WO3 particles loaded with (a) Pt and (b) PdOx cocatalyst; the loading amount was set at
Figure 1. XPS of WO3 samples loaded with (a) Pd species and (b) Pt species prepared by photodeposition under visible light (amount of Pd and Pt, 0.5 wt %).
of (a) Pd and (b) Pt species loaded on WO3 particles via photodeposition with a homemade LED system (at 400 nm), along with those of PdO, Pd, and Pt for comparison. The WO3 sample loaded with Pd was found to contain both PdII and Pd0 species, as shown in Figure 1a. Considering the general mechanism of photodeposition on semiconductor photocatalysts by photoexcited electrons, in which the metal precursors are first reduced on the semiconductor surface and sequentially on the deposited (reduced) species, it was likely that the core part of the cocatalyst predominantly consisted of highly reduced metal species (Pd0) and the outer (shell) part consisted of oxide species (e.g., PdO) or adsorbed cations and complexes (e.g., Pd2+ or PdCl42−). Since the peak positions of the Pd 3d spectrum in Pd halides, such as PdCl2, are higher than those of PdO in the XPS database, the Pd species on WO3 were presumed to be Pd oxide, i.e., PdO, judging from good agreement with the peak position in reference PdO. On the other hand, the peak positions in the Pt 4f XPS spectrum of the Pt-loaded WO3 sample were in agreement with those of Pt foil, indicating that H2PtCl6 was almost completely reduced to metallic Pt particles under photoirradiation conditions (with the homemade LED). In contrast to results for the WO3 samples, both the Pd and Pt precursors were completely reduced to their metallic states in the TiO2 samples under the photoirradiation with a 300 W Xe lamp (see Figure 2).
Figure 3. TEM images of (a) Pt/WO3 and (b) PdOx/WO3 (amount of Pt and Pd, 0.5 wt %).
0.5 wt % of metal in each case. As seen in Figure 3a, the diameters of the loaded Pt nanoparticles were confirmed to be 3−5 nm, as demonstrated in our previous study.47 However, the particle sizes of Pd species on WO3 samples were larger than those of Pt, distributed in the range ca. 10−15 nm (see Figure 3b, for example). On the other hand, the particles of both Pt and Pd loaded on TiO2 samples were found to have 1136
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each step were greatly influenced by the type of photocatalyst. In the case of PdOx/WO3, the generation of acetone (shown by circles in Figure 4a) proceeded with a fairly steady rate and was saturated after irradiation of 4 h. Interestingly, no appreciable decrease in the amount of acetone was observed with even longer irradiation periods (up to 7 h) and the generation of CO2 was negligible (below the detection limits). As shown in Table 1 (entry 2), the selectivity for acetone, which was calculated on the basis of the amount of 2-propanol consumed, was 90% for the PdOx (0.1 wt %)/WO3 photocatalyst in the initial period (2 h), with a low conversion of 2-propanol (27%). Even at high conversion (96%) after a longer irradiation (7 h), the selectivity for acetone was still high (ca. 80%). In this reaction (entry 2), a small amount of formic acid (ca. 4 μmol) was detected by ion chromatography, indicating that C−C bond cleavage in 2-propanol and/or acetone partially occurred during the reaction. The occurrence of such cleavage reactions was certainly one of the reasons that the selectivity for acetone was a maximum of ca. 90% despite the absence of CO2 generation on PdOx/WO3. The amount of acetone produced on Pt/WO3 started to decrease relatively quickly after 2 h of irradiation. Thus, the Pt (0.1 wt %)/WO3 sample exhibited a relatively high selectivity for acetone (ca. 80%) only in the initial period (∼2 h) with middling conversions (ca. 50%, see entry 6, Table 1). The selectivity drastically decreased to ca. 8% during longer irradiation periods of 7 h due to significant peroxidation of acetone to CO2. No catalytic reaction took place on either the Pt/WO3 and PdOx/WO3 samples under dark conditions in the presence of 2-propanol or acetone (see Figures S5 and S6 in the Supporting Information for the results with Pt/WO3). Thus, both PdOx and Pt nanoparticles loaded onto WO3 samples undoubtedly worked as cocatalysts promoting the multielectron reduction of O2 by photoexcited electrons (O2 + 2e− + 2H+ → H2O2, E°(O2/H2O2) = +0.68 V; O2 + 4e− + 4H+ → 2H2O, E°(O2/
similar sizes (∼3 nm; see Figure S4 in the Supporting Information). 3.2. Photocatalytic Oxidation of 2-Propanol over WO3 and TiO2 Photocatalysts. Figure 4 shows the time course for
Figure 4. Time course of photocatalytic oxidation of 2-propanol over (a) WO3 and (b) TiO2 photocatalysts in aerated aqueous solutions of 2-propanol under ultraviolet and visible light irradiation (amount of photocatalyst, 0.1 g; amount of cocatalyst, 0.1 wt % as metal; pH before reaction, 5.0−6.0 (without adjustment); irradiation wavelength, 300 < λ < 500 nm).
photocatalytic oxidation of 2-propanol on WO3 and TiO2 samples in aerated water (100 mL) containing 2-propanol (ca. 130 μmol) under irradiation with light (300 < λ < 500 nm). With the onset of light irradiation, the amount of 2-propanol (shown as squares) decreased in all cases, with acetone (circles) generated in the aqueous solution. With the exception of the PdOx/WO3 system, the amounts of acetone started to decrease after reaching a plateau, accompanied by the generation of CO2 in the gas phase (shown as triangles). These results indicated that 2-propanol was sequentially oxidized to produce CO2 on these photocatalysts (Pt/WO3, Pd/TiO2, and Pt/TiO2) via acetone and other intermediates, whereas the reaction rates in
Table 1. Conversion and Acetone Selectivity for Oxidation of 2-Propanola entry photocatalyst
metal species
amt of Pd or Pt (wt %)
1
WO3
PdOx
0.05
2
WO3
PdOx
0.1
3 4d
WO3 WO3
PdOx PdOx
0.5 0.1
5 6
WO3 WO3
Pt Pt
0.05 0.1
7d 8 9
WO3 WO3 TiO2
Pt unmodified Pd
0.1
10
TiO2
Pt
0.1
11
TiO2
unmodified
0
0.1
conversion (%)b (irradiation time (h)) 56.7 84.0 27.4 52.2 96.0 29.3 98.0 35.9 90.1 97.3 54.0 99.3 44.9 11.3 42.6 99.2 75.3 99.4 56.1 99.2
(7) (10) (1) (2) (7) (1) (7) (9) (7) (10) (1) (7) (1.5) (7) (1) (7) (1) (7) (1) (7)
selectivity (%)c (amt of acetone (μmol)) 90.0 84.8 89.7 84.0 79.7 84.6 71.0 86.6 75.4 67.9 81.4 8.4 88.8 97.6 78.0 25.8 73.4 1.5 63.4 24.9
(52.9) (73.9) (28.2) (50.3) (87.8) (27.6) (77.3) (33.2) (96.8) (94.1) (50.9) (9.6) (45.3) (11.6) (40.9) (31.5) (60.5) (1.4) (42.6) (29.6)
amt of CO2 (μmol)
