TiO2 during Ethanol Oxidation: Understanding

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C-C Cleaving by Au/TiO2 during Ethanol Oxidation: Understanding Bandgap Photo-Excitation and Plasmon Mediated Charge Transfer via Quantitative In-Situ DRIFTS Tze Hao Tan, Jason Anthony Scott, Yun Hau Ng, Robert A. Taylor, Kondo-Francois Aguey-Zinsou, and Rose Amal ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01833 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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C-C Cleaving by Au/TiO2 during Ethanol Oxidation: Understanding Bandgap Photo-Excitation and Plasmon Mediated Charge Transfer via Quantitative In-Situ DRIFTS Tze Hao Tan1, Jason Scott*,1, Yun Hau Ng1, Robert A. Taylor2, Kondo-Francois Aguey-Zinsou1, Rose Amal*,1.

1.

School of Chemical Engineering, The University of New South Wales (UNSW), Kensington, New South Wales 2052, Australia

2.

School of Mechanical and Manufacturing Engineering, The University of New South Wales (UNSW), Kensington, New South Wales 2052, Australia

Email: [email protected], [email protected]

Abstract

Research on photo-enhanced heterogeneous catalysis with Au/TiO2 has gained traction in recent years owing to the potential for activity enhancement due to its localised surface Plasmon resonance effects, including oxidation reactions. While others have observed and described the effects of C-C cleaving by

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Au/TiO2, how the C-C cleaving occurs has not been reported to date. To elucidate the mechanism and to understand the fundamental impacts of visible and UV photo-excitation on the dynamics of gas phase ethanol oxidation, an in-situ, quantitative Diffuse Reflectance Infrared Fourier Transform Spectroscopy analysis on the surface of Au/TiO2 and neat TiO2 is presented. Key findings from the study include: (i) discovery of exclusive oxalate species, a critical precursor to C-C cleaving, which is also an indicator of selective ethanol adsorption at the Au-TiO2 interfacial perimeter; (ii) fortification of C-C bond cleaving by Au/TiO2 via detection of single carbon species such as formate and carbon monoxide on Au/TiO2 in the dark and under visible light illumination; (iii) validation of previous postulations regarding ethanol adsorption on TiO2 followed by oxygen activation at the Au-TiO2 interfacial perimeter; and (iv) in-situ reenactment of the different impacts by bandgap photo-excitation and plasmon mediated charge transfer, under UV and visible light illumination, respectively, on ethanol oxidation by Au/TiO2 and neat TiO2.

KEYWORDS: gold, titanium dioxide, plasmon, photo-excitation, heterogeneous catalysis, ethanol oxidation, in-situ DRIFTS

Introduction Localised surface plasmon resonance (LSPR) mediated charge transfer has been shown to have positive effects on heterogeneous Au-based plasmonic nanocatalysts. These materials enable the design of innovative hybrid photo-reactors which utilise the solar spectrum at its highest potential for both illumination and heating.1–4 In particular, Au/TiO2 has shown excellent potential through gold’s strong, localised surface plasmon resonance in the visible spectral range of solar irradiation, as reported by Tatsuma et al. and Ohtani et al.5–12 In addition, extended studies have been performed on heterogeneous catalytic oxidation reactions using Au/TiO2, including CO oxidation by Haruta et al., and


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alcohol oxidation by Sobolev et al. and Holz et al, probing the effects of catalyst preparation, particle size, and metal-support interaction on the activity of Au/TiO2.13–18 In our previous work1, we postulated that the photo-enhancement of Au/TiO2 under UV illumination was due to the synergistic roles of photo- and thermal- catalysis, while photo-enhancement under visible light illumination was solely due to plasmonic-mediated electron charge transfer from the Au deposits to the TiO2 support. Nonetheless, the work did not explicitly clarify the mechanisms behind the two different photo-excitation pathways. In the dark, Holz et al. suggested alcohol oxidation on the surface of Au/TiO2 is initiated by the adsorption of ethanol on the catalyst, followed by oxygen activation which oxidizes ethanol to intermediates such as acetaldehyde and acetate species (Equations 1-2).17,18 Camellone et al. proposed through density functional theory (DFT) calculations on methanol oxidation by Au/TiO2 that alcohol first adsorbs on the TiO2 surface together with oxygen activation at the Au-TiO2 interface, induced in the presence of an Au cluster.19 Photo-electrochemical studies by Tatsuma et al.6–8 have shown that Au nanoparticles readily inject electrons into the adjacent TiO2 substrate under visible light illumination. 2 + + 2 → 2 + 2

(1)

+ + 2 → +

(2)

While evidence has been provided in the literature on the oxygen activation mechanism, less is known regarding the mechanism associated with C-C bond cleaving by Au/TiO2. To date, no spectroscopy evidence is available on the intermediates formed due to the selective adsorption of ethanol on the perimeter of the Au-TiO2 interface as a critical precursor to C-C bond cleaving. The work of Sobolev et al. and Holz et al. touched on C-C bond cleaving in the presence of Au/TiO2 through the formation of single carbon products such as carbon dioxide and formate intermediate species.15–18


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Herein, we propose that acetate species (generated from the initial ethanol oxidation step) undergo further oxidation to form oxalate species, a process facilitated by the Au-TiO2 interface (Equation 3). The oxalate can undergo C-C cleaving on the acidic surface of TiO2 to form formate species (Equation 4), which is then converted to carbon monoxide and carbon dioxide species through further dehydration and oxidation, respectively (Equations 5-6). + 3 + 6 → + 3

[ ]

(3)

+

(4)

↔ +

(5)

+ →

(6)

Additionally, previous mechanistic studies on ethanol oxidation by Au/TiO2 have been restricted solely to qualitative analysis, which is incapable of effectively assessing the illumination effects of bandgap photo-excitation and plasmon mediated charge transfer on ethanol oxidation by Au/TiO2. To overcome this limitation, we use an in-situ quantitative DRIFTS analysis approach to probe the surface of Au/TiO2 and neat TiO2 so as to elucidate the impact of visible (LSPR effect) and UV (TiO2 bandgap excitation) photo-excitation on the dynamics of gas phase ethanol oxidation during the initial adsorption – oxygen activation step. Identifying and quantifying intermediate species formation, particularly oxalate species, has allowed us to reveal the mechanisms pertaining to oxygen activation and C-C cleaving by Au/TiO2, along with the impact of the different photo-excitation pathways on the Au-TiO2 interactions during ethanol oxidation.


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Results and Discussion Characteristics of Synthesised Au/TiO2 Au/TiO2 catalysts were synthesised and comprehensively characterised as described in our prior work.1 0.67 mol% of Au was loaded onto an Aeroxide® TiO2 P25 support (size, ~25 nm; specific surface area, ~51 m2/g; anatase to rutile ratio, 4:1) using the deposition-precipitation (DP) method. Subsequently, the Au deposits were heat treated at 400 oC in a reductive environment (20% H2 in Ar balance) to ensure the formation of metallic Au/TiO2 with the intention of maximising Au/TiO2’s plasmonic absorbance. The Au deposits were observed to be ~2 nm in diameter. The ionic state of the supported Au deposits was characterised using high angle annular dark field, high resolution scanning transmission electron microscopy (HAADF-HRSTEM) and X-ray diffraction (XRD). HAADF-HRSTEM showed the presence of metallic Au deposits with a lattice spacing of 2.35 Å, corresponding to Au(111). The Au(111) peak was not detectable in XRD due to the interference from overlapping TiO2 crystal peaks. Nonetheless, small peaks at 45°, 65°, 78° corresponding to Au(200), Au(220) and Au(331) crystal planes were observed. The diffuse reflectance ultraviolet-visible (UV-vis) spectrum of Au/TiO2 showed a distinctive LSPR peak at 550 nm. Further details can be found in Figure S1.

Impact of Au deposition on Ethanol Adsorption In-situ DRIFTS analysis of Au/TiO2 and neat TiO2 were studied in three consecutive steps: (i) in-situ pretreatment of the catalysts at 400 oC to both remove surface organics and reduce the catalysts; (ii) ethanol adsorption at 50 oC; and (iii) photo-thermal-catalytic oxidation at 100 oC under dark, UV and visible light illumination. Due to space constraints in the optical chamber, sample illumination was conducted using LED light sources with a narrow bandgap corresponding to the TiO2 bandgap and Au plasmon excitation wavelengths, 365 nm and 530 nm, respectively. Quantification of the DRIFTS results involved an initial normalising of all the measured DRIFTS spectra to the characteristic infrared


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absorbance spectra of TiO2. The normalised spectra were then smoothened and adjusted according to Kubelka-Munk theory to improve the linearity of the peak intensities in relation to the adsorbed organic concentration. Subsequently, a background correction was applied to remove the broad O-H peaks spanning the range 2400 - 3800 cm-1 which corresponds to adsorbed water and the characteristic absorbance spectra of TiO2.17,18,20 A residual peak corresponding to adsorbed water species can be observed at 3450 cm-1 in Figure 1 – 5. The adsorbed water was a by-product of heat treatment (400 oC) of the catalysts in H2 and was present for all prepared catalysts. 1.0

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Figure 1 In-situ DRIFTS spectra spanning: (A) 3800 – 3600 cm-1 and (B) 1700 – 1200 cm-1 wavenumbers for Au/TiO2 and neat TiO2 before and after ethanol adsorption at 50 oC for 20 min. Ethanol feed concentration = 1000 ppm ethanol in N2, gas flow rate = 20 mL/min, reaction performed at ambient pressure. Corrected DRIFTS spectra of Au/TiO2 and neat TiO2 following ethanol absorption are provided in (Figure 1). Examination of the fresh Au/TiO2 and neat TiO2 spectra shows OH stretching vibrations corresponding to free OH, vfree(OH), adsorbed water, vads(OH), and covalently bound OH, vbound(OH), at 3650 cm-1, 3000 – 3500 cm-1, and 1650 cm-1, respectively.17,18,20 The adsorbed water was a by-product of catalyst reduction under the hydrogen flow at 400 oC, with its OH stretch seen at 3000 – 3500 cm-1. Weak asymmetric and symmetric COO stretch intensities, v(COO), are also apparent at 1553 cm-1 and


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1443 cm-1, respectively, corresponding to residual organic contaminants on the catalysts.17,18,21 To minimise the presence of organic contaminants on the catalyst surface during in-situ DRIFTS analysis, the synthesised catalysts were pre-calcined twice (ex-situ and in-situ, see Experimental Methods, DRIFTS Analysis for further details). In-situ DRIFTS spectra of fresh neat TiO2 and Au/TiO2 showed a negligible amount of residual organic contaminants in comparison to the adsorbed ethanol species. Following ethanol adsorption, the OH peaks are completely removed suggesting that the OH was displaced by adsorbing ethanol species. Sun et al. observed a similar drop in O-H species presence during the adsorption of formaldehyde on TiO2.22 In place of the OH vibration peaks, we observed the following vibration peaks: (i) CH stretching, v(CH), at 2973 cm-1, 2929 cm-1, and 2873 cm-1; (ii) asymmetric and symmetric CH bending, δ(CH), at 1396 cm-1 and 1381 cm-1, respectively; and (iii) wagging bending, δwag(CH2), at 1356 cm-1; which all correspond to adsorbed ethanol species on the surface of both catalysts.17,18,23–25 As v(CH) (2800 – 3000 cm-1) showed the strongest DRIFTS signal the v(CH) peaks were used to quantify the adsorbed ethanol species. Quantification of the v(CH) peak areas for Au/TiO2 and neat TiO2 (Figure 1A, calculated area = 34 ± 5 and 30 ± 10, respectively) suggested that Au had minimal impact on the adsorption of ethanol on TiO2 (Scheme 1, step 1). As a control, the quantified DRIFTS spectra of Au/TiO2 and neat TiO2 were compared with that of Cu/TiO2, an alternate plasmonic metal. Interestingly, the v(CH) peak area increased in the presence of Cu (Figure S2A, calculated area = 56 ± 8) relative to neat TiO2, indicating the Cu introduced additional active sites for ethanol adsorption. The adsorption of alcohol on Cu active sites has been wellstudied by Madix et al.26–29 Thus, Cu/TiO2 represents a positive control to indicate the absence of adsorbed ethanol species on the surface of Au deposits.


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Impact of Photo-Excitation on Catalytic Oxidation of Ethanol

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Figure 2 In-situ DRIFTS spectra spanning: (A, B) 3800 – 3600 cm-1; (C, D) 2400 – 1600 cm-1; and (E, F) 1700 – 1200 cm-1 wavenumbers in ascending time steps (0 - 90 min, 15 min intervals) for Au/TiO2 (A, C, E) and neat TiO2 (B, D, F) during ethanol oxidation in air at 100 oC without illumination (i.e. in the dark). Air flow rate = 20 mL/min, reaction performed at ambient pressure.


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Corrected DRIFTS spectra of Au/TiO2 and neat TiO2 in ascending time steps (0 - 90 min, 15 min intervals) during ethanol oxidation at 100 oC in the dark (i.e. no illumination) are provided in (Figure 2). During ethanol oxidation there is a sizeable drop in the CH stretching vibration peak, v(CH), with a greater than 73% reduction 15 min after the introduction of air, suggesting rapid oxidation of the adsorbed ethanol in the presence of Au/TiO2. The CH consumption was accompanied by the appearance of intense COO stretching vibrations, v(COO), at 1553 cm-1 and 1443 cm-1 (Figure 2E), corresponding to the formation of acetate species from the ethanol. A weak acetaldehyde CO stretching vibration, v(C=O) can be seen at 1735 cm-1 (Figure 2C); the low level of adsorbed acetaldehyde is likely due to its desorption, once formed, from the catalyst surface which is then detected as an intermediate gas product as observed in our earlier study.1 In contrast, most of the v(CH) was retained on the neat TiO2 and Cu/TiO2 during oxidation in the dark condition, with a loss of 13% and 55%, respectively, during exposure to air for 90 min (Figure 2B, S3B) and no significant formation of partially oxidized intermediates (Figure 2, S3E). In the dark, the DRIFTS spectra of Cu/TiO2 was similar to that of neat TiO2, suggesting that Cu had no working role in the activation of oxygen for ethanol oxidation. Rather, the observed decrease in v(CH) may be due to the desorption of ethanol from the surface of the catalyst over time as no oxidised intermediates were observed in the effluent gas product. Kumar et al. have observed similar ethanol desorption from the surface of CuNi catalysts at temperatures as low as 50 oC.30 Combined, the DRIFTS spectra suggest that Au plays a critical role in the activation of oxygen. On the basis of DFT calculations, Camellone et al. proposed that oxygen activation occurred at the Au-TiO2 perimeter and was facilitated by electron injection from the deposited Au clusters, which was supported experimentally in photoelectrochemical studies on Au/TiO2 by Tatsuma et al.6–8,19 Holz et al. observed a similar effect whereby Au/TiO2 induced the additional oxidation of ethanol to acetaldehyde at temperatures < 125 oC.18


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In addition to oxygen activation, Au/TiO2 participates in the cleaving of the C-C bond in ethanol as is illustrated in Scheme 1. From the 2400 – 1600 cm-1 and 1700 – 1200 cm-1 regions in the Au/TiO2 DRIFTS spectra (Figure 2C, E) both; (i) CO stretching vibration, v(CO), at 2050 cm-1 corresponding to linearly adsorbed carbon monoxide,31,32 and (ii) COO stretching vibrations corresponding to formate species, v(HCOO), at 1414 cm-1 and 1340 cm-1 are apparent,33–35 providing direct evidence of C-C cleaving by Au/TiO2 (Scheme 1, step 3-4). Furthermore, the presence of adsorbed oxalate species is confirmed as indicated by its COO stretching vibration, voxa(COO), at 1320 cm-1.36–39 The revelation of oxalate formation during ethanol oxidation on Au/TiO2 is pivotal to the proposed mechanism as it verifies the selective oxidation of ethanol at the Au-TiO2 interfacial perimeter, a critical step for cleaving of the C-C bond by Au/TiO2 (Scheme 1, step 1-3). On illuminating the system with UV light (λ = 365 nm), only the bandgap of TiO2 is activated, without the LSPR effect of the Au deposits being triggered. Corrected DRIFTS spectra of Au/TiO2 and neat TiO2 in ascending time steps (0 - 90 min, 15 min intervals) during ethanol oxidation at 100 oC under UV illumination are provided in Figure 3. Quantitative comparison between the two DRIFTS spectra highlights a similar intermediate evolution throughout the analysis, suggesting that the role of Au deposits as an oxygen activator is superseded by the photo-excited TiO2 support, as is shown in Scheme 1 (step 2). Photo-excited TiO2 directly converts the ethanol to carbon dioxide, indicated by faint CO2 stretching vibrations, v(CO2), at 2361 cm-1 and 2336 cm-1; which then desorbs from the catalyst surface, as seen by its short lifetime in the DRIFTS spectra.17,18,23 The finding is consistent with the observation of CO2 within the gaseous effluent stream during ethanol oxidation by UV illuminated TiO2 at T < 150 OC in Tan et al.1 In contrast, incomplete ethanol oxidation was observed for UV illuminated Cu/TiO2, as indicated by the residual v(COO) stretch at on the surface of the catalyst (1553 cm-1 and 1443 cm-1, Figure S3E). The presence of Cu was found to be detrimental for the complete oxidation of ethanol by neat TiO2 (Figure 3).


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Figure 3 In-situ DRIFTS spectra spanning: (A, B) 3800 – 3600 cm ; (C, D) 2400 – 1600 cm-1; and (E, F) 1700 – 1200 cm-1 in ascending time steps (0 - 90 min, 15 min intervals) for Au/TiO2 (A, C, E) and neat TiO2 (B, D, F) during ethanol oxidation in air at 100 oC under UV (365 nm, 20 W/cm2) illumination. Air flow rate = 20 mL/min, reaction performed at ambient pressure.


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Differences in the neat TiO2 and Au/TiO2 spectra can be seen for v(HCOO) with Au/TiO2 exhibiting stretch intensities at 1414 cm-1 and 1340 cm-1 while neat TiO2 presents only the v(HCOO) stretch at 1340 cm-1 (Figure 3E, F). The variation suggests that the two v(HCOO) stretches correspond to different formate adsorption sites; at the Au-TiO2 interfacial perimeter (1414 cm-1) and explicitly on TiO2 (1340 cm-1). The v(HCOO) stretch at 1414 cm-1 suggests that the Au-TiO2 interfacial perimeter still actively participates in cleaving the C-C bond, supporting the previously observed photo-enhancement effect ascribed to a synergism between photocatalysis by TiO2 and thermal-catalysis by the Au deposits. Under UV illumination, the intensity of v(C=O), at 1735 cm-1, corresponding to acetaldehyde species on neat TiO2, increased by over 7.5 times compared to the dark reaction condition (Figure 3D). The accumulation of adsorbed acetaldehyde species on neat TiO2 may be associated with an undesired aldol condensation reaction, catalysed by the TiO2 surface, which promotes fouling via aromatic carbon formation, as described in the work by Falconer et al.40,41 In contrast, acetaldehyde-based species accumulation was suppressed in the instance of Au/TiO2 (Figure 3C), indicating that C-C cleaving by Au-TiO2 interfacial perimeter sites restricts catalyst fouling (Scheme 1, step 3-4). Sannino et al. photocatalytically oxidised ethanol using Au/TiO2 under UV-LED illumination and found that, with increasing Au content, fouling by carbon species was greatly reduced.42 Nonetheless, insignificant acetaldehyde formation on Cu/TiO2 (Figure S3C) indicates that the inhibition of CO2 formation by the Cu deposits could not be due to fouling of the TiO2 surface.


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The system was next illuminated with visible light (λ = 530 nm) to selectively induce plasmonic mediated charge transfer between Au and TiO2 without exciting the TiO2 support. Corrected DRIFTS spectra of Au/TiO2 and neat TiO2 in ascending time steps (0 - 90 min, 15 min intervals) during ethanol oxidation at 100 oC under the visible light illumination are provided (Figure 4). Visible light illumination is observed to have a limited impact on ethanol oxidation by neat TiO2, with only a slight increase in acetaldehyde formation (1.4 times compared to neat TiO2 in the dark, Figure 2D, 4D). Acetaldehyde production was believed to be caused by the photochemical oxidation of ethanol which was activated under visible light illumination, as observed in our earlier work. A similar photochemical ethanol oxidation effect has been reported by others.43,44 Photo-enhancement by Cu/TiO2 was insignificant and comparable to neat TiO2 (Figure 4, Figure S3B, D, F) despite absorbing visible light at λ > 400 nm (Figure S1F). As Cu/TiO2 was inactive in the dark and under visible light illumination, the findings suggest that plasmonic photo-enhancement requires direct participation of the metal deposit in the oxidation reaction. As such, plasmonic enhancement due to plasmon-mediated charge transfer was deemed to be the more favoured phenomenon for Au/TiO2 over optical near field enhancement and photothermal heating effects. DRIFTS spectra of the visible light illuminated Au/TiO2 exhibit an increase in both the v(HCOO) and v(COO) stretch intensities compared to Au/TiO2 in the dark condition (Figure 2E, 4E). The increased acetate and formate species formation suggests that plasmon-mediated charge transfer promotes both oxygen activation and C-C bond cleaving at the Au-TiO2 perimeter. It is notable that the 1400-1600 cm-1 IR region is quite congested with the stretching and bending of various species; hence quantifying the intensities of the stretch species in the region is challenging. Under visible light illumination, the peak area of v(CO) dropped by 41%, indicating a lower instance of surface adsorbed CO species which is due to the further oxidation of CO to CO2. The v(CO2) stretch was not observed in Au/TiO2 DRIFTS spectra


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ACS Catalysis

under the dark and visible light illuminated conditions as CO2 rapidly desorbs from the surface after the complete oxidation reaction, similar to the UV illumination case.

C-C Cleaving by Au/TiO2 The mechanistic steps involved in the photo-thermal oxidation of ethanol under visible light and UV illumination on the Au/TiO2 surface derived from the in-situ, quantitative DRIFTS analyses are illustrated in Scheme 1. Quantification of the adsorbed ethanol (Figure 1) indicated that that Au deposition did not affect the amount of ethanol adsorbed (Step 1). Instead, the catalytic activity exhibited by Au/TiO2 centred on Au-facilitated oxygen activation (Step 2 and 4), as was highlighted by differences in the ethanol oxidation process under the various illumination conditions (Figure 2-4). Step 3 depicts adsorption of the oxalate species on the TiO2 surface at the Au-TiO2 perimeter via bidentate bringing for both carboxylate species. Durand et al. showed that for a (TiO2) surface with oxygen vacancies, the bridging configuration predominates.45 However, other binding modes are available, all of which may lead to the cleaving of C-C bonds as well.45,46 The presence of formate species on UV-illuminated neat TiO2 (Figure 3F) indicates that C-C cleaving can occur in the absence of Au deposits, albeit at an insignificant rate without UV photo-excitation. While neat TiO2 is capable of cleaving the C-C bond of ethanol (as shown under UV illumination), this process can be enhanced in the presence of Au for both thermal- and photo- catalytic oxidation. Au facilitates oxidation of the ethyl group on TiO2 as indicated by the formation of oxalate species on the surface of Au/TiO2. The exclusive formation of oxalate species on Au/TiO2 (Step 3, Figure 2E) infers that oxygen activation occurs at Au-TiO2 interfacial perimeter sites, promoting selectivity during ethanol oxidation by the Au/TiO2 (Step 1-2). A similar oxygen activation site was proposed by both Camellone et al. and Tatsuma et al.6–8,19 In addition to oxygen activation, the oxidation of ethanol to oxalate species at the Au-TiO2 interfacial perimeter is facilitated by an electron charge transfer from the ethyl group to the Au deposits


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as shown in (step 2). The selective oxidation of ethanol to oxalate species at the Au-TiO2 interfacial perimeter is the key factor invoking the cleaving of the C-C bonds by the Au/TiO2. DRIFTS spectra of UVilluminated neat TiO2 and Au/TiO2 did not show any corresponding vibration peak at 1320 cm-1, despite proof of C-C cleaving (indicated the by formation of CO2 and formate species, Figure 3). The DRIFTS spectra suggested that the observed vibration peak corresponded to an intermediate species formed only at the Au-TiO2 interfacial perimeter. Comparison of the different DRIFTS spectra and potential intermediate species suggested that the formation of oxalate species is the only possible reaction at the Au-TiO2 interfacial perimeter.17,18,36–39,47

Scheme 1 Proposed mechanism describing the photo-thermal oxidation of ethanol by Au/TiO2 with associated electron transfer steps shown by black arrows. Plasmon mediated charge transfer and bandgap photo-excitation are indicated by red and blue highlights, respectively. Vibration peaks used to identify and quantify the intermediate species involved in each reaction step are also included.


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Under visible light illumination, plasmon-mediated charge transfer promotes the flow of electrons from Au to the TiO2, creating a region of electron deficient Au and electron rich TiO2 at the Au-TiO2 perimeter.6–8 The electron deficient Au is proposed to encourage electron transfer from the ethyl group while the electron rich TiO2 encourages electron transfer to adsorbed oxygen species. Plasmonmediated charge transfer across the Au-TiO2 interface promotes both the selective oxidation of ethanol to oxalate species (steps 2-3) and the further oxidation of carbon monoxide to carbon dioxide (step 4). In a separate study on Au/Al2O3, we observed insignificant light enhancement under both visible and UV-illuminated conditions (Figure S4). The lack of photo-enhancement was attributed to the large bandgap or high conduction band potential of gamma-Al2O3, preventing electron injection from the excited Au deposits.48,49 In addition to needing a UV excitable photocatalyst support, the study on Al2O3 implied that the transfer of plasmon-excited electrons from Au to the neighbouring support plays a critical role in photo-enhanced catalytic reactions.

Conclusion Herein, we have presented detailed in-situ DRIFTS results on ethanol oxidation by Au/TiO2 under different illumination conditions (dark, visible light and UV illumination). The study also provides quantitative comparisons of intermediate species in all three cases through post-treatment of the DRIFTS spectra to remove discrepancies arising from varying detection sensitivity, signal noise, and background interference from TiO2 and water. As a consequence, direct in-situ evidence on the critical role of Au deposits in the oxidation of ethanol by Au/TiO2 has been found. Specifically, we observed; (i) oxygen activation promotes the selective oxidation of ethanol at the Au-TiO2 interfacial perimeter as well as furthers CO oxidation to CO2, (ii) new evidence of oxalate species formation at the Au-TiO2 interfacial perimeter which enables the cleaving of C-C bonds. The DRIFTS results also indicated that C-C


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cleaving can occur in the absence of Au deposits, albeit at an insignificant rate when UV photoexcitation is not present. Additionally, the study clarified the photo-illumination effects of TiO2 bandgap photo-excitation and Au plasmon-mediated charge transfer on ethanol oxidation by Au/TiO2. Under UV illumination, the role of Au as an oxygen activator was suppressed by the photo-excited TiO2 support, although Au still actively participated in the oxidation reaction through C-C bond cleaving, as was indicated by the presence of formate intermediates. In contrast, plasmon-mediated charge transfer at the Au-TiO2 interface under visible light illumination was found to enhance the catalytic properties of Au/TiO2 by promoting the selective oxidation of ethanol to oxalate species at the Au-TiO2 interfacial perimeter as well as promote the oxidation of carbon monoxide to carbon dioxide.

Experimental Methods Materials Aeroxide® TiO2 P25 (primary particle size >25 nm, surface area ~50 m2/g, anatase to rutile ratio ~ 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), compressed ethanol (1000 ppm, nitrogen balance, Coregas®), and zero air (20% oxygen, nitrogen balance, Coregas®).

Particle Synthesis and Characterisation The deposition and precipitation method developed by Haruta et al.50 was used to load 1 atm% Au onto Aeroxide® P25 TiO2. A 300 mL solution containing 49.3 mg gold (III) chloride trihydrate was heated to 80 °C and adjusted to pH 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 continued for a further 3 hours. After cooling to room temperature, the particles were collected by centrifuging and washed with deionized water to


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remove residual chloride ions. The collected paste was then dried under vacuum at ambient temperature for 24 hours and ground into a powder in preparation for further calcination/reduction treatment. HAADF-STEM of the Au/TiO2 particles was taken on a JEOL JEM-ARM200F operating at 200 kV. Metal deposit sizing was performed using 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 analysed with a PANalytical Xpert Multi-purpose XRD system. The total metal content in the catalyst was analysed by a Perkin Elmer OPTIMA 7300 Inductively Coupled Plasma – Atomic Emission Spectrometer (ICPAES) using aqua regia as the digestive agent. Brunauer-Emmett-Teller (BET) surface area was assessed by a Micrometritics TriStar 3000 Analyser. UV-Vis spectra were measured with a Shimadzu UV-3600 UV-Vis-NIR Spectrophotometer with BaSO4 as the reference.

DRIFTS Analysis In-situ DRIFTS was performed using a Brüker VERTEX 70v FTIR spectrometer, equipped with a liquid N2 cooled MIR source, KBr optics, and a RockSolid interferometer (Figure 5). For each analysis, approximately 30 mg of catalyst was placed in a commercial in-situ DRIFTS cell [HVC-DRM-5, Harrick's Scientific, USA] possessing ZnS windows and equipped with an ohmic heating device. Due to space constraints in the optical chamber, illumination of the DRIFTS samples was conducted using Thorlabs® fibre-coupled light-emitting diode (LED) light sources (M365FP1 and M530F2), corresponding to the TiO2 bandgap and Au plasmon excitation wavelengths, 365 nm and 530 nm, respectively. A 2m custom made patch cable with Ø1000 µm, 0.50 NA, high OH, core step-index multimode fibre terminated by SMA mating sleeves was used to connect the LED source with a Ø4 mm UV-enhanced aluminium reflective collimator. The LED beam intensity was tuned to 20 W/cm2 using a Newport® 91150V calibrated reference cell and meter.


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DRIFTS samples were externally pre-calcined for 2 hrs in a tube furnace at 400 oC under a constant air flow (60 mL/min) to clean the catalyst surface. The samples were then transferred to the DRIFTS reaction cell and further treated in-situ at 400 oC under three different environments: (i) a constant air flow (40 mL/min, ramped at 10 mL/min to prevent movement of the catalysts) for 1 hr to remove any residual organics and carbon dioxide adsorbed on the catalyst surface during the transfer process; (ii) a pure Ar flow (20mL/min) for 20 min to remove oxygen from the reaction chamber; and (iii) a 20% H2 in Ar (20 mL/min) for 1 hr to reduce the catalysts. The catalysts were then cooled to 50 oC in the 20% H2 in Ar stream (20 mL/min) and purged with Ar (20 mL/min) to prepare an inert environment for subsequent analysis. DRIFTS spectra of the samples were collected at this point to determine the amount of free OH species on the catalyst surface.

Figure 5 Configuration of the in-situ DRIFTS analysis cell equipped with a fibre-coupled LED for UV (365 nm) and visible light (530 nm) illumination. LED beam intensity was tuned to 20 W/cm2 to maintain a consistent illumination effect. The catalysts were then saturated with ethanol by purging the reactor with 1000 ppm ethanol in N2 for 20 min at 50 oC. This was followed by Ar purging to stop the absorption step and remove any residual


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ethanol in the gas phase. DRIFTS spectra were taken every 5 min during the adsorption process. The temperature of the reaction chamber was then increased to 100 oC for the oxidation reaction. Different illumination conditions (dark, visible (λ = 530 nm), and UV (λ = 365 m)) were employed during the oxidation reaction. The oxidation step was performed by simultaneously introducing zero air (20% O2 in N2,

TiO2 during Ethanol Oxidation: Understanding

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