Understanding Plasmon and Band Gap Photoexcitation Effects on the

Feb 3, 2016 - To date there has been a lack of understanding on how photoexcited electron charge transfer can be beneficially combined in a hybrid pho...
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Understanding Plasmon and Band Gap Photoexcitation Effects on the Thermal-Catalytic Oxidation of Ethanol by TiO2‑Supported Gold Tze Hao Tan,† Jason Scott,*,† Yun Hau Ng,† Robert A. Taylor,‡ Kondo-Francois Aguey-Zinsou,† and Rose Amal*,† †

School of Chemical Engineering and ‡School of Mechanical and Manufacturing Engineering, The University of New South Wales (UNSW), Kensington, New South Wales 2052, Australia S Supporting Information *

ABSTRACT: To date there has been a lack of understanding on how photoexcited electron charge transfer can be beneficially combined in a hybrid photo-thermal-catalytic reaction. The effect of different excitation wavelengths on photo-thermal-catalytic oxidation by Au/TiO2 and TiO2 nanoparticles was studied via the gas-phase oxidation of ethanol over a temperature range of 100−250 °C under either visible light or UV illumination. Catalytic performance was assessed by monitoring the CO2 yield. Despite being a weak thermal catalyst (5% catalytic enhancement in comparison to neat TiO2 under thermal catalytic conditions), Au/TiO2 displayed a considerable photo-thermal synergism in the photo-thermal regime (>175 °C), with over 50% and 100% increases in catalytic performance in comparison to neat TiO2 under visible light and UV illumination, respectively. Photo-thermal-catalytic results and detailed probing of postreaction surface carbon species on Au/TiO2 indicated that photo enhancement under UV illumination was due to congruent roles of the photo and thermal catalysis, while photo enhancement under visible light illumination was due to plasmonic-mediated electron charge transfer from the Au deposits to the TiO2 support. KEYWORDS: plasmonic, photoactivity, photo-thermal catalysis, gold, titanium dioxide, ethanol oxidation

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INTRODUCTION Intense research on heterogeneous photocatalysis of titanium dioxide (TiO2) has been under way since the discovery of photo-electrochemical water splitting by Fujishima and Honda.1,2 While TiO2 exhibits great potential in solar energy applications, two key challenges remain. The first challenge is associated with utilizing the full solar spectrum. TiO2 has a band gap of approximately 3.2 eV, which requires ultraviolet (UV) irradiation with wavelengths less than 385 nm in order to excite an electron from its valence band to its conduction band. Thus, the photocatalyst utilizes less than 5% of the energy available in sunlight. The second challenge stems from deactivation of the photocatalyst due to stable byproduct formation on the photocatalyst surface, which hampers catalytic performance, particularly for the removal of volatile organic compounds (VOCs).3,4 These challenges restrain photocatalysis from being feasible for systems with a low reactant concentration. Currently, commercial heterogeneous catalytic oxidation reactions are exclusively driven by thermal energy, which has high throughput but requires a high temperature through external heating. Nevertheless, there remains a largely unexplored potential for thermal-catalytic processes to be enhanced by coupling them with photoexcitation effects or vice versa. For instance, Denny et al. observed a thermal-catalytic contribution of Pt in Pt/TiO2 during the photocatalytic degradation of selected organics, while Scott et al. found that © XXXX American Chemical Society

preilluminating Pt/TiO2 with UV light improved the catalytic oxidation of formic acid by a factor of 7.5,6 In a separate study on ethanol oxidation using Pt/TiO2, Kennedy et al.7 suggested that this synergism arose from a combination of the photocatalytic activity of TiO2 and the thermal-catalytic activity of Pt deposits instead of electronic charge interaction between Pt and TiO2. Thus, far, metal deposits have been utilized exclusively as thermal catalysts and the effect of photoexcitation on the thermal catalysis by metal deposits requires further investigation. In recent years, supported Au nanoparticles have been found (independently) to be both thermal- and photo-catalytically active. Haruta et al.8,9 suggested that the strong metal−support interaction between gold and TiO2 as well as the small size of Au/TiO2 (∼2 nm) created more active sites for CO oxidation on preparation via the deposition−precipitation method. They also studied ethanol oxidation by selected Au catalysts, finding that the products were defined by the support type and characteristics such as its acidity and basicity. Gas-phase oxidation of ethanol by Au/TiO2, performed by Holz et al.,10 demonstrated that Au/TiO2 is capable of oxidizing ethanol at a lower temperature (475 Received: December 8, 2015 Revised: January 31, 2016

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Here 200 mg of the dry catalyst was converted into a paste by mixing with 1 mL of a 70% ethanol−water solution while being sonicated in an ultrasonic bath. A 0.4 mL subsample of the paste was uniformly spread onto the aluminum support using a glass slide at a paste thickness of approximately 1.5 mm. The thin film was then dried under ambient airflow for 6 h followed by drying at 120 °C for 12 h. Thermogravimetric analysis (TGA) of the dried films showed no evidence of significant organic peaks, indicating the residual ethanol was effectively removed from the catalyst surface. Catalyst Characterization. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) of Au/TiO2 particles was carried out on a JEOL JEMARM200F instrument operating at 200 kV. Particle sizing was performed with a Phillips CM200 high resolution transmission electron microscope (HR-TEM) operated at 200 kV. Prior to the measurements, the samples were dispersed in ethanol and sonicated for 5 min with the resulting suspension loaded on a Cu grid. The crystal phase of the prepared catalysts was analyzed with a PANalytical Xpert multipurpose X-ray diffraction (XRD) System. The total metal content in the catalyst was analyzed using a PerkinElmer OPTIMA 7300 inductively coupled plasma atomic emission spectrometer (ICPAES), using aqua regia as the digestive agent. The Brunauer−Emmett−Teller (BET) surface area was assessed by a Micrometritics TriStar 3000 Analyzer. UV−vis spectra were measured with a Shimadzu UV-3600 UV−vis−NIR spectrophotometer with BaSO4 as the reference. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific ESCALAB250i X-ray photoelectron spectrometer using a monochromated Al Kα radiation source (energy ∼1.5 keV). Functional groups on the catalyst surface pre- and postreaction were identified using a PerkinElmer Spotlight 400 Fourier transform infrared (FTIR) spectrophotometer. Raman spectroscopy of the catalyst surface was performed with a Renishaw inVia Raman microscope. Thermogravimetric analysis of the catalysts was conducted with a TA Instruments Q5000 dynamic vapor sorption analyzer to quantify the total organics accumulated on the catalyst surface during the reaction. 13C solid state nuclear magnetic resonance (13C CP-MAS SSNMR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses were performed using a Brüker Biospin Avance III solids-700 MHz spectrometer and a Brüker VERTEX 70v FTIR spectrometer (with a high temperature reaction chamber housed in a modified praying mantis assembly (HVC-DRM-5, Harrick’s Scientific, USA)), respectively. Additional analysis details can be found in the Supporting Information. Reactor Configuration and Operation. Gas-phase ethanol oxidation experiments were performed using a plug flow reactor system at atmospheric pressure as shown in Scheme 1. The reactor system comprised (i) a LX300F Xe illuminator (PerkinElmer/ILC Technologies) mounted in an Eagle R300-3J lamp housing, (ii) an infrared filtering unit below the light source, and (iii) a stainless steel reactor with provision for illumination from above (annealed Kodial glass window (alkali borosilicate 7046)) and heating from below. The light source was placed 10 cm above the catalyst bed and was directed toward the reactor with a MR 60/90 light reflection unit. The infrared filter comprised a glass vessel through which 18 °C water was continually circulated to minimize heating of the catalyst film during illumination. Additional control of the spectral region of light incident upon the reactor was provided

K). Additionally, Au was used as a cocatalyst for TiO2, further enhancing the photocatalytic performance of TiO2 under UV excitation.11−14 The photocatalytic performance of Au is attributed to localized surface plasmon resonance (LSPR) effects, giving new breath to photocatalysis.15−22 LSPR effects allow the gold to absorb in the visible light region, making it a visible light active catalyst. Charge separation on Au/TiO2 was first observed by Tatsuma et al.23,24 The phenomenon was further supported by the work of Furube et al.,20,21 who suggested that charge transfer from gold would occur in the presence of a reducible oxide support, such as TiO2, which was enhanced under visible light illumination. Other work by Sakai and Tatsuma showed that the plasmonic charge transfer from gold to TiO2 is influenced by the deposit size.25 Most recently, Upadhye et al.26 found that high-temperature reversed watergas shift catalysis by Au/TiO2 was enhanced by 30−1300% under visible light illumination and proposed that the enhancement was due to either hot electron generation or absorbate polarization. Furthermore, Au/TiO2 consists of two opposing photocatalysts which can be activated either through plasmonic or band gap excitation. Silva et al.14 postulated that the photocatalytic activity of Au/TiO2 was dependent on the different excitation wavelength and that opposing charge transfer mechanisms between the semiconductor and the deposits were present when either UV or visible light were used. Gunawan et al.27 made a similar observation for TiOsupported Ag nanoparticles, which also exhibit the LSPR effect. These disparate photoexcitation pathways have led to dissimilar photo-thermal-catalytic mechanisms which are yet to be fully understood. The present study clarifies the contributions of plasmonic and band gap excitation effects in photo-thermal catalysis by probing the impact of different excitation wavelengths on the thermal-catalytic activity of Au/TiO2. The oxidation of ethanol, a known VOC, was selected, as it is a simple reaction which will assist with identifying the mechanisms. By probing the effluent gas as well as surface species on the catalysts, we assessed the effect of temperature and different illumination conditions (dark (i.e. nonilluminated), visible light, and UV) on the photothermal oxidation of ethanol by Au/TiO2.

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EXPERIMENTAL SECTION Materials. Aeroxide TiO2 P25 (primary particle size >25 nm, surface area ∼50 m2/g, anatase to rutile ratio of around 4:1) was used as the catalyst support in all experiments. Chemicals were used as supplied: gold(III) chloride trihydrate (Sigma-Aldrich), sodium hydroxide (Chem-Supply), absolute ethanol (Ajax-Finechem), compressed ethanol (500 ppm, synthetic air balance, Coregas). Particle Synthesis and Thin Film Preparation. The deposition−precipitation (DP) method of Haruta et al.28 was used to load 1 atom % Au onto Aeroxide P25 TiO2. A 300 mL solution containing 49.3 mg of gold(III) chloride trihydrate was heated to 80 °C and adjusted to a pH of 7.5 using a 0.1 M NaOH solution. The mixture was stirred at 700 rpm, whereby 1.0 g of the TiO2 was added and the stirring was continued for a further 3 h. After the mixture was cooled, the particles were collected by centrifuging and washed with deionized water to remove residual chloride ions. The collected paste was then dried under vacuum at ambient temperature for 24 h, ground into a powder, and stored ready for thin film preparation. Thin catalyst films of TiO2 and Au/TiO2 were loaded onto a 3 cm × 3 cm aluminum support using a doctor-blading method. 1871

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window provided >99% light penetration at wavelengths longer than 280 nm. Heating of the reactor was from below using an in-house-built system comprised of a temperature-controlled (Shimaden FP21) tube furnace. All catalyst films (including neat TiO2) were initially reduced in situ at 400 °C (ramp rate 5 °C/min) for 2 h under a 50 mL/ min 20% hydrogen/argon gas mix to maximize the presence of metallic Au (Figure S1 in the Supporting Information). The catalyst film was then cooled and held at a temperature of 100 °C. A 500 ppm ethanol−air gas mixture was introduced to the reactor at 100 mL/min and the system purged for 2 h. Catalyst performance under thermal, visible light/thermal and UV/ thermal conditions was assessed by measuring the activity (ethanol consumption, acetaldehyde and CO2 production) at 25 °C intervals over the range 100−250 °C. At each temperature step and prior to sampling, the temperature was held for 40 min to allow the reaction to reach a steady state. At each temperature stage, gas samples were then collected at 10 min intervals for 30 min following the 40 min equilibration period. The temperature of the catalyst film was controlled by a Shidmaden FP21 PID controller with a K-type thermocouple mounted at the base of the reactor. Samples from the reactor outlet were injected into a gas chromatograph (Shimadzu GC2010) equipped with a methanizer and flame ionization detector. Product separation was achieved using an Agilent J&W HP-PLOT Q capillary column.

Scheme 1. Configuration of Plug Flow Photoreactor Equipped with a Heat Sourcea

The incident light was passed through an infrared filter comprising a glass vessel in which water was continually circulated to minimize additional heating of the catalyst film by the light source. a

by a Schott UG-11 band-pass glass filter (UV illumination, 420 nm). The intensity of the light source was governed by the light filter. The light intensities incident upon the catalyst bed were measured to be 390 and 20 mA/cm2 under visible light illumination and UV illumination, respectively. The 3 cm × 3 cm aluminum support loaded with the thin catalyst film was mounted below the glass window within the reactor. Gas manifolds were built into the reactor to provide a well-distributed flow across the catalyst bed. The glass

Figure 1. (a) HAADF-STEM image of Au/TiO2 showing Au deposits (∼2 nm) sitting on large TiO2 supports (∼25 nm). (b) HAADF-HRSTEM image showing lattice spacing of Au deposits sitting on an anatase TiO2 support. (c) Particle size distribution of Au deposits (200 counts). (d) UV− vis spectra of neat TiO2 and Au/TiO2 highlighting the plasmonic Au deposit peak at 551 nm. (e) XRD spectra of neat TiO2 and Au/TiO2 identifying peaks corresponding to the planes of Au[200], Au[220], and Au[311] in addition to distinct TiO2 peaks. (f) XPS spectra of as-prepared Au/TiO2 showing peaks corresponding to Au 4f5 and Au 4f7, indicating that the Au exists as Au0. 1872

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RESULTS

Au/TiO2 Characteristics. The presence of supported Au deposits is confirmed by a HAADF-STEM image of Au/TiO2 (Figure 1a). The HAADF-STEM image highlighted the supported Au deposits (∼2 nm), which appeared brighter, sitting on large TiO2 supports (∼25 nm). The size distribution of Au deposits (Figure 1c) returned an average deposit size of 2.4 ± 0.1 nm, with the particle sizes ranging from 1 to 6 nm. Lattice spacing calculated from HAADF-HRTEM of Au/TiO2 (Figure 1b) was used to confirm the ionic state of Au deposits, which appeared to be in metallic form, with a lattice spacing of 2.35 Å, corresponding to the Au(111) facet. Concurrently, the lattice spacing of the TiO2 support was determined to be 3.48 Å, which corresponds to the TiO2(101) facet of anatase TiO2 crystals. From HAADF-HRSTEM, the Au deposits appeared to be spherical in shape, although we also observed other morphologies, suggesting different degrees of metal−support interactions between the Au deposits and TiO2 support. From the ICPAES analysis, the nominal Au content of the Au/TiO2 is 0.67 atom %. The UV−vis spectrum of Au/TiO2 (Figure 1d) shows a distinct absorption band at 550 nm, which is associated with the localized surface plasmon resonance of Au nanoparticles. The absorption edge of TiO2 remained unchanged at 400 nm after the deposition of Au nanoparticles and is not expected to be activated upon visible light illumination (λ >420 nm). The lack of a discernible change in the XRD peaks attributable to TiO2 anatase and rutile before and after Au deposition (Figure 1e) indicates the deposition and calcination processes did not alter the crystal structure of Aeroxide P25. Small peaks at 45, 65, and 78° corresponding to Au(200), Au(220), and Au(331) crystal planes were also observed in the XRD spectrum of Au/TiO2 (Figure 1e) with the Au peaks at 45 and 65° partially overlapping existing TiO2 crystal peaks. The primary peak of Au metal, which corresponds to Au(111) as detected by HAADF-HRSTEM, was not observable in the XRD spectrum due to interferences from existing TiO2 crystal peaks located at 38°. The specific surface areas of neat TiO2 and Au/TiO2 were determined to be 51 and 52 m2/g, respectively, indicating that there was a negligible impact of Au deposition on the TiO2 surface area. XPS analysis of the asprepared (i.e., reduced) Au/TiO2 showed that the Au deposits exist as Au0, indicating that the Au was in the metallic state prior to the oxidation reaction. The presence of Au0 is in agreement with the plasmonic peak observed in the UV−vis spectrum. Impact of Photoexcitation on Thermal-Catalytic Ethanol Oxidation. Figure 2 compares the photo-thermalcatalytic oxidation of ethanol for Au/TiO2 and TiO2 under dark, visible light and UV illuminated conditions. The figure shows the concentration of ethanol, acetaldehyde, and carbon dioxide in the effluent over the temperature range 100−250 °C. The dark condition (Figure 2a) indicates that at temperatures >125 °C, both the neat TiO2 and Au/TiO2 increasingly degrade ethanol to produce acetaldehyde as the temperature increases. Complete ethanol oxidation is achieved with CO2 appearing in the gas phase at temperatures higher than 175 °C. A comparison of the neat TiO2 and Au/TiO2 profiles indicates that the TiO2 support is the main contributor, although as the temperature increases the effect of the Au deposits escalates. Sobolev et al.29 suggested that reducing TiO2 in hydrogen at 400 °C can introduce oxygen vacancies into the material which can catalyze oxidation reactions. Control experiments demon-

Figure 2. Effect of temperature on the concentration of gases in the effluent stream during ethanol oxidation by TiO2 (open symbols) and Au/TiO2 (filled symbols): (a) in the dark; (b) under visible light illumination; (c) under UV illumination. Conditions: initial ethanol concentration 1000 ppm carbon in air, gas flow rate 100 mL/min, reactions performed at ambient pressure. Reactor gas products were taken at the steady state (at the 40 min mark for each temperature step), with an interval of 10 min between each sample.

strated that the reduction step had little influence on TiO2 support effects during the ethanol oxidation reaction within our study (Figure S2 in the Supporting Information). Under visible light illumination, catalyst proficiencies were improved while the temperature profiles of the ethanol, acetaldehyde, and CO2 were similar to those observed for the dark thermal-catalytic system (Figure 2b). For instance, under visible light illumination; (i) the ethanol concentration was lower for both neat TiO2 and Au/TiO2, which indicates a higher conversion rate, (ii) the onset of accelerated acetaldehyde consumption by Au/TiO2 occurred at a comparatively lower temperature (200 °C vs 225 °C), and (iii) CO2 generation by Au/TiO2 at temperatures >175 °C was increasingly greater as the temperature increased. Interestingly, illuminating the neat TiO2 with visible light enhanced its conversion of ethanol to acetaldehyde despite the anticipated photocatalytic inactivity. This suggests ethanol activation may be photochemical rather than photocatalytic and that ethanol and acetaldehyde oxidation follow separate reaction pathways 1873

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ACS Catalysis under illuminated conditions.30,31 Visible light illumination increased the CO2 produced by Au/TiO2 (twice as high in comparison to neat TiO2 at 250 °C), while CO2 produced by neat TiO2 did not differ significantly from that of dark catalysis. UV illumination had a distinct impact on the catalytic performance of the Au/TiO2 and TiO2 catalysts (Figure 2c). At 100 °C, the ethanol concentration was reduced from ∼960 ppm (dark conditions) to ∼265 and ∼363 ppm for Au/TiO2 and TiO2, respectively, while the carbon dioxide concentration increased from ∼0 ppm (dark conditions) to ∼302 and ∼275 ppm, respectively. As the reaction temperature was increased from 100 to 250 °C, effluent CO2 concentration dropped before increasing to ∼813 and ∼423 ppm for Au/TiO2 and neat TiO2, respectively. The parabolic profile can be separated into two distinct regimes: a photo-catalytic-driven oxidation region at temperatures 175 °C (indicated by the dotted line). As the temperature increased, (i) the effluent ethanol concentration remained approximately constant, (ii) the acetaldehyde concentration continually rose with temperature, and (iii) there was a corresponding drop in CO2 generation. The rise in acetaldehyde concentration and the drop in CO2 observed at temperatures 175 °C), ethanol oxidation over Au/TiO2 displayed a greater synergy between the photo and thermal effects. The enhancement attained from UV illumination was at least 200% higher in terms of CO2 production for Au/TiO2 in comparison to neat TiO2 (Figure 2c). Furthermore, the CO2 produced by Au/TiO2 at 250 °C under UV illumination (813 ppm) was significantly higher than the total CO2 produced from photocatalysis (265 ppm at 100 °C under UV illumination) and thermal catalysis (350 ppm at 250 °C under dark conditions) combined. In contrast, the CO2 generated by neat TiO2 under UV illumination (423 ppm at 250 °C) was only slightly higher than the yield from photocatalysis and thermal catalysis (275 ppm at 100 °C under UV illumination and 300 ppm at 250 °C under the dark conditions, respectively). Deconvoluting Thermal-Catalytic, Photocatalytic, and Plasmonic Effects of Au/TiO2. Initial analyses of ethanol conversion under different illumination types at higher temperatures >175 °C suggested that Au deposits participate in the reaction by promoting the oxidation of acetaldehyde to CO2 (differences in apparent activation energies for ethanol consumption can be found in Figure S3 in the Supporting Information, with accompanying discussion). However, comparison between the effects of different illumination sources on the catalytic properties of Au deposits was difficult, as acetaldehyde conversion rates are dependent on both the concentration of acetaldehyde and the amount of ethanol converted. To account for the difference in ethanol conversion, CO2 yield was normalized against the amount of ethanol converted, with the resulting profiles illustrated in Figure 3. The normalized CO2 yields illustrate the distinct difference in performance between the Au/TiO2 and TiO2 catalysts under the different illumination types. The area under the graphs can be separated into three regions which are attributed to thermal catalysis (dark) and photoenhancement by visible light (midtone) and UV (light tone). Similar areas for the dark catalysis region exhibited by both neat TiO2 and Au/TiO2 illustrate that Au deposits have a minimal effect on the thermal

Figure 3. Normalized CO2 yield with temperature by (a) neat TiO2 and (b) Au/TiO2 in the dark (nonilluminated) and under visible (λ >420 nm) and UV (λ 175 °C under visible light illumination (as indicated by the dark blue region). Enhancement by visible light can be attributed to the surface plasmonic resonance of the Au deposits (Figure 1d).32,33 In contrast, the extent of ethanol mineralization by neat TiO2 in the dark and with visible light illumination was similar over the entire temperature range. This is expected, as the frequency of the visible light (generated by filtering the xenon light with a GG420 long-pass filter, cutoff wavelength 420 ± 6 nm) was insufficient to photoexcite the electrons in TiO2. Comparison between the contribution by UV illumination (light tone) for both neat TiO2 and Au/TiO2 highlights two interesting features: (i) within the photocatalytic regime (temperature 175 °C), UV photoenhancement was 4 times higher for Au/TiO2 in comparison to neat TiO2. Figure 3 demonstrates a clear distinction between Au/TiO2 and neat TiO2 in the photothermal region, where there exists a synergism between photocatalysis and thermal catalysis for Au/ TiO2 which is absent in neat TiO2. Under visible light illumination, the photoenhancement was proportional to the thermal-catalytic conversion rates. Alternately, photoenhancement under UV illumination created two distinct regimes 1874

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ACS Catalysis consisting of a photocatalytic dominant regime (175 °C). The differences observed in the photoenhancement profiles may arise from two possible photoexcitation mechanisms in Au/TiO2: (i) a visible light activated surface plasmon resonance effect from the Au deposits or (ii) a band gap excitation of TiO2 under UV illumination. Unlike Pt, which is an effective thermal catalyst for ethanol oxidation,7 our Au deposits were not an efficient thermal catalyst for oxidizing ethanol to CO2 (Figure 2a). However, our experimental data indicate that the thermal catalytic properties of Au deposits can be greatly enhanced (or activated) via illumination, suggesting that heat and light can work in harmony within the Au/TiO2 system. The role of Au deposits in the photo-thermal-catalytic oxidation of ethanol was further investigated by probing the adsorbed carbon species on the surface of the spent catalysts following ethanol oxidation at 200 °C for 12 h under different illumination conditions (Figures 4−6 and Figures S4−S7 in the Supporting Information). Raman spectra of the neat TiO2 and Au/TiO2 post-reaction are provided in Figure 4. A peak at 1580 cm−1 is present for all

of strongly bound carbon species which are formed by aldol condensation of acetaldehyde on TiO2, acting to foul the TiO2 surface and lower performance. Luo and Falconer35 observed the formation of aromatic carbon species, such as toluene and alkylbenzene, during ethanol-TPD on TiO2. In contrast, the Au/TiO2 catalysts did not exhibit a “G” band following the reaction, signifying that the Au deposits restrict the aldol condensation byproducts. The observation is in agreement with NMR spectroscopy, which detected an insignificant amount of aromatic carbon on Au/TiO2. A similar effect has been observed for noble metal (Au, Pt, Pd, Rh) deposited TiO2 catalysts used for oxidizing volatile organic compounds such as ethanol, isopropyl alcohol, and toluene, in which the metal deposits enhance catalytic performance by preventing the fouling of the catalyst.7,10,13,36,37 Ex situ FTIR spectra of TiO2 and Au/TiO2 post-reaction are provided in Figure 5. Regardless of the conditions, both

Figure 5. Ex situ FTIR spectra of (a) neat TiO2 and (b) Au/TiO2 prior to and following ethanol photothermal oxidation at 200 °C under different illumination conditions for 12 h: (i) prior to reaction (no light or heating); (ii) under UV illumination; (iii) under visible light illumination; (iv) with no illumination (i.e., thermal only). Conditions: initial ethanol concentration 1000 ppm carbon in air, gas flow rate 100 mL/min, reactions performed at ambient pressure.

Figure 4. Raman spectra of neat (a) TiO2 and (b) Au/TiO2 following ethanol photo-thermal oxidation under different illumination conditions at 200 °C for 12 h: (i) under UV illumination; (ii) under visible light illumination; (iii) with no illumination (i.e., thermal only); (iv) prior to reaction (no light or heating). Conditions: initial ethanol concentration 1000 ppm carbon in air, gas flow rate 100 mL/min, reactions performed at ambient pressure.

catalysts exhibit a broad peak at 1640 cm−1 prior to reaction (i.e., without heat and light) which shifts to 1622 cm−1 following the reaction. The peak can be attributed to the OH bending vibration of molecular water, with the shift in the peak being attributed to the accumulation of water formed during ethanol oxidation.10,37,38 The band at 1412 cm−1 represents R− OH species and is assigned to adsorbed ethanol molecules.10,39 The bands at 1452 cm−1 (R−COO−) and 1715 cm−1 (RO) are attributed to symmetric COO stretching vibrations from absorbed carboxylate/carboxylic acid and CO vibrations in aldehyde, respectively.10,37,39,40 Following ethanol oxidation by the neat TiO2, ethanol (R− OH), acetaldehyde (R−COO−), and acetates/acetic acid (R

the neat TiO2 catalysts (apart from the prereaction catalyst), which corresponds to a carbon “G” band associated with ordered sp2-hybridized bonds.34 In addition, we visually observed a distinct brown coloration of the spent neat TiO2. 13 C NMR spectroscopy of the spent catalyst surface (Figure S4 in the Supporting Information) indicates that the detected sp2hybridized bonds in Raman spectroscopy correspond primarily to the accumulation of aromatic carbon on the neat TiO2 surface. Together, these observations point toward the buildup 1875

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ACS Catalysis O) are all present on the TiO2 surface to a similar extent, with the exception of the thermal-only case, where there is decreased acetaldehyde. In situ DRIFTS analysis of ethanol oxidation on neat TiO2 under thermal-only conditions was performed to gain a greater understanding of intermediate formation/ consumption during the reaction. Spectra showing the dynamics of formation, interaction with air, and temperatureprogrammed desorption (TPD) of the intermediates are provided in Figures S5−S7 of the Supporting Information. The profiles show that intermediate formation on TiO2 is a slow process and the formed intermediates accumulate over time. Air pulsing over the adsorbed species on neat TiO2 led to greater C−O intermediate formation from the oxidation of C− H without any CO2 being produced, which suggests aldol condensation may be occurring on the neat TiO2 surface. Additionally, the TPD study indicated that the species adsorbed on TiO2 desorbed directly and did not significantly convert to CO2, suggesting that neat TiO2 is a weak oxidizer. In contrast to the spectra for neat TiO2, the Au/TiO2 spectra in Figure 5 exhibit additional bands at 1560 and 1581 cm−1 which can be assigned to the presence of formates on the postreaction catalyst surface.10,41 Each of the post-reaction Au/TiO2 samples exhibits an increased acetate/acetic acid and formate/ formic acid presence on the surface at the apparent expense of adsorbed acetaldehyde. The acetate and formate peaks are especially pronounced for the visible light and nonilluminated conditions. Formate generation was further studied using in situ DRIFTS on ethanol oxidation by Au/TiO2 under visible light illumination (Figure S5 in the Supporting Information). The DRIFTS analysis showed that C−H species were readily oxidized to C−O species soon after adsorption on the Au/TiO2 surface, suggesting that the ethanol was converted either to oxalate (no C−C cleavage) or formate (C−C cleavage) species. Additionally, the presence of a CO2 peak in the Au/TiO2 DRIFTS spectra implies that formate production by the Au/ TiO2 is quite likely. Holz et al.10 utilized in situ DRIFTS to probe the gas-phase oxidation of ethanol over Au/TiO2 and observed the presence of cleaving reactions which led to the formation of formates on the TiO2 surface. Additionally, the DRIFTS findings indicated that the resulting C−O species on the Au/TiO2 surface were stable (Figure S6 in the Supporting Information). Overall, the key findings from DRIFTS and ex situ FTIR analyses are as follows: (i) the oxidation of ethanol to acetate and formate species occurs more readily on Au/TiO2 than TiO2; (ii) Au deposits catalyze C−C bond cleavage in the acetate/acetic acid to produce formates (which are then readily oxidized to CO2);42 (iii) illuminating the Au/TiO2 samples with UV substantially decreases the intensity of bands associated with the presence of acetate and formate species, as indicated by the significant reduction of bands at 1454 and 1560 cm−1; (iv) fouling of neat TiO2 is associated with aldol condensation, which promotes the accumulation of carbon species on the catalyst surface. Semiquantitative analysis of carbon on the neat TiO2 and Au/TiO2 catalysts following ethanol oxidation was conducted using TGA, and the findings are provided in Figure 6. Figure 6a indicates that the Au/TiO2 possesses a lower post-reaction carbon loading irrespective of the illumination conditions, with the differences in carbon presence being a function of the illumination type. In the case of neat TiO 2 , carbon accumulation on the visible light illuminated sample is higher than that of the thermal-only case, potentially due to the

Figure 6. Thermogravimetric analysis (TGA) of neat TiO2 and Au/ TiO2 following ethanol photothermal oxidation at 200 °C under different illumination conditions for 12 h: (a) weight percent attributed to carbon species present on the catalysts following reaction (uncertainty ±0.1%); (b) derivative weight loss of carbon-based species with temperature. Conditions: initial ethanol concentration 1000 ppm carbon in air, gas flow rate 100 mL/min, reactions performed at ambient pressure.

photochemical ethanol degradation, as previously discussed. The greater carbon accumulation for the UV illuminated sample is derived from the photocatalytic action of the TiO2, which promotes aldol condensation on the surface of TiO2. Conversely, carbon accumulation on the Au/TiO2 catalyst appears to be influenced more by radiation from a particular spectral region (e.g., visible light) than by the excitation energy invoked by illumination. Carbon accumulation was approximately 30% lower when visible light was used as the illumination source. Information on the nature of the carbon that has accumulated on the catalyst surface can be extracted from the derivative weight loss profiles in Figure 6b. Immediately apparent from Figure 6b is that the carbon accumulated on the neat TiO2 differs in comparison to the carbon on the Au/ TiO2. The carbon species present on neat TiO2 appear to be similar in nature, irrespective of the illumination conditions, and possess a higher molecular weight in comparison to those on the Au/TiO2 (as indicated by the higher oxidation peak temperature (∼390 °C)). The peaks representing carbon species on the Au/TiO2 indicate that illumination provides a dominance of easily oxidized species on the catalyst surface in 1876

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Scheme 2. Proposed Mechanism for Photoenhancement due to (A) Plasmonic-Mediated Electron Charge Transfer between Au Deposits and TiO2 and (B) UV Photoexcitation of Electrons in TiO2: (Bottom Left) Photothermal Catalytic Oxidation of Ethanol by Au/TiO2; (Bottom Right) Fouling of the TiO2 Surface by Carbon-Based Species Formed during Aldol Condensation

unsaturated Au deposit edges, which then exposes the ethyl section (C−C bonds) to activated oxygen on the TiO2 support. The cleaving of C−C bonds restricts the growth of aromatic carbon; this prevents the fouling of the catalyst and allows formaldehyde and formic acid to form, which are easier to oxidize to carbon dioxide. Sannino et al.43 photocatalytically oxidized ethanol using Au/TiO2 under UV-LED illumination and found that, with increasing Au content, fouling by carbon species was greatly reduced. Scheme 2 shows the C−C cleaving properties of Au deposits, which can be explained through the reaction mechanism at the interface of Au and TiO2. Density functional theory modeling on methanol oxidation by Camellone et al.44 described the oxidation mechanism which occurs on the surface of Au/TiO2 catalysts. The oxidation cycle begins with the adsorption and activation of oxygen molecules on the surface of the TiO2 support (step 1) which involves electron transfer from the TiO2 to the oxygen molecules, forming superoxide, O2−, species. This is supported by experimental work from Haruta et al.8,9 O−H radicals as a byproduct of superoxide formation were observed on both Au/TiO2 and neat TiO2 (Figure S8 in the Supporting Information). The formation site of the superoxide is important, as it defines the active site as well as the mechanism of ethanol oxidation. This paves the way for the adsorption of ethanol on the surface of Au/TiO2 through a dehydrogenation process (step 2). The literature suggests that dehydrogenation of the hydroxyl group of ethanol occurs on the surface of TiO2.9,10 A comparison between DRIFTS spectra of the intermediate

comparison to the case where only thermal catalysis is employed. This finding is consistent with the TPD study of surface carbon species using DRIFTS analyses (Figure S7 in the Supporting Information), which showed that the carbon species on Au/TiO2 were more easily oxidized to produce CO2 in comparison to those on the neat TiO2. From Figure 6b it also appears that visible light illumination promotes the formation of “lighter” carbon species on the Au/TiO2 surface in comparison to UV illumination. The variation in carbon species on the Au/TiO2 surface following visible light and UV illumination indicates that the two spectral regions affect the Au deposit behavior differently. The diversity likely arises from the different activation mechanisms invoked by the spectral regions (i.e., plasmonic versus photocatalytic), with the plasmonic charge seeming to facilitate greater C−C bond cleavage in this instance.

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MECHANISM Despite the lack of catalytic enhancement by Au deposits during the thermal (dark) oxidation of ethanol by TiO2, surface carbon species characterization demonstrated that Au deposits play a role in C−C bond cleavage and prevent acetaldehyde formation on the TiO2 surface (Scheme 2). The formation of formate species on the Au/TiO2 surface was detected by FTIR analysis following a 12 h oxidation reaction at 200 °C. Formate species formation was not observed for ethanol oxidation by neat TiO2, which suggests that the cleaving of C−C bonds is exclusive to Au/TiO2. Cleaving of C−C bonds is mediated by the adsorption of the carbonyl group in acetaldehyde on the 1877

DOI: 10.1021/acscatal.5b02785 ACS Catal. 2016, 6, 1870−1879

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ACS Catalysis

CO2. Photocatalytic studies by Muggli et al.46−48 showed that the oxidation of acetaldehyde to CO2 readily proceeds via acetate and formate species formation on the TiO2 surface. Similarly to the other illumination conditions, ethanol oxidation under UV illumination was inhibited by the formation of fouling carbon species. We also observed that UV illumination promoted the growth of carbon foulants on neat TiO2 (step 3b), as suggested by the increased level and complexity of carbon-based species on the TiO2 surface (Figure 6a and Figure S4 in the Supporting Information). The indiscriminate promotion of catalytic ethanol conversion to CO2 and the formation of aromatic carbon species suggest that UV illumination primarily enhances oxygen activation on the TiO2 surface (step 1), a critical step in both reactions. The presence of Au deposits provides an alternative reaction pathway which suppresses carbon fouling of the TiO2 surface (Figures S4 and S5 in the Supporting Information). Raman spectroscopy (Figure 4) suggested that the oxidation reactions on the Au/TiO2 surface occur close to the Au deposits, as no buildup of the ordered sp2 species was observed. It is possible that the photoexcited electrons on TiO2 are being drawn toward the Au deposits which act as an electron sink, thus allowing the reactions to proceed as described in Scheme 2. Photoexcited TiO2 would further oxidize the cleaved species to give carbon dioxide (step 4), regenerating the active sites for subsequent oxidation reactions, which explains the low amount of formate species observed by FTIR on the surface of UV illuminated Au/TiO 2 (Figure 5). DRIFTS analysis of intermediate formation on Au/TiO2 (Figure S5 in the Supporting Information) showed that the greater amount of accumulated intermediate species on Au/TiO2 may be readily oxidized to CO2 under illuminated conditions, which would free up the active sites to facilitate further reactions.

species on Au/TiO2 and neat TiO2 (Figure S5 in the Supporting Information) suggests that the adsorption step is accompanied by the oxidation of the methyl group. This oxidation step is unique to Au/TiO2 and occurs on the perimeter of Au/TiO2, where the dangling methyl group is more likely to interact with the Au catalyst. Cleavage of the C− C bonds occurs due to the weakening of the C−C bonds at the Au/TiO2 interface (step 3). The cleaved formate groups are further oxidized to carbon dioxide (step 4). The donated electron in the Au deposit is then transferred to the TiO2 support, replenishing the oxygen vacancies on the surface (step 5). Holz et al.10,37 showed that, in addition to the formation of oxygen vacancies, the lattice oxygen in TiO2 also participates in the oxidation process. Strunk et al.,45 in their work on methanol oxidation by Au/ZnO, demonstrated that Au deposition invoked a large amount of oxygen vacancies on the catalyst surface, mainly adjacent to the Au deposits. The mechanism detailed above consists of three interfaces at which electron charge transfer occurs (steps 1, 2, and 5). As such, there exists a possibility of enhancing ethanol oxidation by Au/TiO2 by promoting the electron charge transport across any one of the three interfaces. Plasmonic-Mediated Charge Transfer from Au to TiO2 (Scheme 2A, Step 5). Under visible light illumination, this is achieved via plasmonic mediated charge transfer from the Au deposits into the TiO2. Plasmonic-excited electrons gain approximately 2.4 eV from the absorption at 550 nm (Figure 1d). This provides sufficient energy for the electrons to overcome the Schottky barrier of TiO2, which catalyzes electron transfer from the Au deposits into the TiO2 (Scheme 2A). Tian and Tatsuma23 studied the plasmon-induced charge separation at the Au/TiO2 interface by illuminating an Audeposited TiO2 film in the presence of a hole scavenger. During visible light illumination (>420 nm), coloration of the TiO2 film (measured via absorbance at 680 nm) increased with time due to the accumulation of charge in the TiO2. In contrast, the illumination of neat TiO2 did not produce a similar coloration effect, suggesting that the observed charge did not originate from the TiO2. In a separate study on an Au/TiO2 film in a photochemical cell, Sakai et al.24 observed a reversal in the generated circuit current when the order of the Au-deposited TiO2 electrode was changed between ITO/Au/TiO2 and ITO/ TiO2/Au. The electron transfer would create a positively charged surface on the Au deposits which promoted the adsorption of electron donors such as ethanol and acetaldehyde on the Au surface. The findings imply that visible light illumination enhanced the catalytic oxidation of ethanol in our system by promoting charge transfer between the Au deposits and the TiO2 support rather than from the formation of photoexcited electrons in the Au deposits. UV Photoexcitation of Electrons in TiO2 (Scheme 2B, Step 1). Under UV illumination, the Au deposits become electron sinks and capture photoexcited electrons from TiO2.11−14 Nevertheless, Au/TiO2 did not exhibit a higher catalytic activity in comparison to neat TiO2 in the photocatalytic regime (175 °C), which increased the CO2 yield by over 200% (Figure 2c) in comparison to neat TiO2. UV illuminated neat TiO2 is capable of oxidizing ethanol to acetaldehyde and, to a certain extent,

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CONCLUSIONS Despite being a weak thermal catalyst, Au deposits were shown to increase the CO2 yield of TiO2 in the photo-thermal regime by over 1.5 times under visible light illumination. The photoenhancement of Au/TiO2 under visible light can be attributed to the LSPR effect, which mediated electron charge transfer from the Au deposits to the TiO2. The electron charge transfer promotes C−C bond cleavage by the Au deposits, which then prevents the aldol condensation byproducts. In contrast, the photoenhancement displayed upon UV illumination of Au/TiO2 was 2 times higher than that of neat TiO2 under the same conditions. The observed photoenhancement effect was attributed to a synergism between photocatalysis by TiO2 and thermal-catalysis by the Au deposits. Coupling plasmonic gold nanoparticles with active thermal catalysts may further improve the synergism between thermal and photo effects and allow for the fabrication of more efficient visible light active photo-thermal catalysts. Potential applications of such catalysts would be in solar fuel and materials systems, where heat and light can be derived from concentrated sunlight.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02785. Additional experimental information (PDF) 1878

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.S.: [email protected]. *E-mail for R.A.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Australian Research Council (ARC) under the Laureate Fellowship Scheme-FL140100081. The authors acknowledge Dr. Aditya Rawal of the NMR Facility within the UNSW Mark Wainwright Analytical Centre for NMR support. The authors also acknowledge the use of facilities within the UNSW Mark Wainwright Analytical Centre and UoW Electron Microscopy Centre (Dr. David Mitchell).

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DOI: 10.1021/acscatal.5b02785 ACS Catal. 2016, 6, 1870−1879