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CO Adsorption on Au/TiO Catalysts: Observations, Quantification, and Explanation of a Broad-Band Infrared Signal Camilah D Powell, Arthur W Daigh, Meagan N Pollock, Bert D Chandler, and Christopher J Pursell J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07249 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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The Journal of Physical Chemistry

CO Adsorption on Au/TiO2 Catalysts: Observations, Quantification, and Explanation of a Broad-Band Infrared Signal Camilah D. Powell, Arthur W. Daigh, Meagan N. Pollock, Bert D. Chandler, and Christopher J. Pursell* Department of Chemistry, Trinity University, One Trinity Place, San Antonio, Texas 78212 *

Corresponding Author: [email protected]

(210)999-7381

ABSTRACT The adsorption of CO on Au/TiO2 catalysts was examined at room temperature using FTIR transmission spectroscopy. Adsorption was observed as (i) a sharp peak at ~2100 cm-1 due to CO molecular vibration (the Au-CO peak), and (ii) a broad-band infrared (BB-IR) signal. The Au-CO peak and BB-IR signal are correlated and quantitatively related to the amount of CO adsorbed on the Au nanoparticles. For comparison purposes, we also examined CO adsorption on Au/Al2O3 catalysts. When supported on this non-reducible support, CO adsorption on Au showed only the Au-CO peak; the BB-IR signal was absent. This allowed us to determine that the BB-IR signal observed for CO adsorption on the Au/TiO2 catalyst is associated with the reducibility of the support. Comparison of the two catalysts also enabled us to determine that the BB-IR signal is due to a decrease in transmission through the powdered catalysts when CO adsorbs on Au/TiO2. Consistent with previously published studies, we propose that this BB-IR signal is related to the reversible, partial reduction of the TiO2 at the Au-TiO2 interface. This reduction leads to an increase in surface disorder or roughening of TiO2 particles that produces a decrease in IR transmission through the catalyst (i.e. an increase in IR scattering). These results suggest an efficient CO-Au-TiO2 adsorbate-induced electronic metal-support interaction (EMSI) that may play an important role in understanding CO reactions on Au/TiO2 catalysts.

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INTRODUCTION Understanding CO adsorption on gold nanoparticle catalysts is important to develop a complete picture of the catalytic behavior associated with CO reactions on supported gold catalysts.1 These reactions include low-temperature CO oxidation,2-6 preferential CO oxidation in the presence of excess hydrogen (PROX),7-9 and the low-temperature water-gas shift reaction (LT-WGS).10-13 While at first glance these reactions appear rather simple, the mechanisms involved have been vigorously debated in the literature over the past 30 years. Many important factors have been suggested as contributing to the catalytic mechanisms, including: the Au nanoparticle size; the charge on the Au; the contribution of low-coordinate Au active sites; the electronic interaction of the Au with the support; the importance of the Au-support perimeter interface; the role of support hydroxyls and adsorbed water. In an effort to better understand catalysis by supported gold nanoparticles, our laboratory is examining CO adsorption, CO oxidation, and PROX on gold catalysts. Previous studies involve quantifying CO adsorption on supported Au catalysts using infrared transmission spectroscopy14 and analyzing the coverage using a Temkin adsorbate interaction model.15,16 This treatment yields thermodynamic metrics (i.e. coverage-dependent enthalpies of adsorption) for characterizing CO adsorption on catalysts. We also examined the CO adsorption and CO oxidation properties of bimetallic Au-Ni17 and Pd-Au18 catalysts. To better understand the CO oxidation reaction mechanism, we developed a Michaelis-Menten approach for comparing catalysts,19 utilized intentional NaBr poisoning to estimate the number of catalytically active sites,20 and examined the role of catalyst pretreatment on deactivation and carbonate formation.21 Additionally, we recently reported on the important role of water as a co-catalyst in CO oxidation22 and how water significantly improves performance in the PROX reaction.23


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We now present our most recent results concerning CO adsorption on Au/TiO2 catalysts at room temperature using transmission FTIR spectroscopy. For comparison purposes, we also examined CO adsorption on Au/Al2O3 catalysts (i.e. a non-reducible support). While CO adsorption on both catalysts produced the sharp IR peak due to the vibration of CO adsorbed on the Au nanoparticles (Au-CO peak), we have discovered a broad-band infrared (BB-IR) signal associated with CO adsorption on the Au/TiO2 catalyst. Herein we provide a complete report of our observations, analysis and explanation of this new broad-band infrared signal.

EXPERIMENTAL All experiments were conducted using a Nicolet Magna 550 FTIR spectrometer. The sample compartment was equipped with a specially designed temperature and pressure controlled copper sample cell that has a short gas-phase optical path length (1 cm) and small volume (~1.3 cm3). KBr windows with strong infrared cut-off below 600 cm-1 were used. The gas handling system consisted of a mechanical and diffusion pump, a glass line with stainless steel transfer lines to the IR cell, and Baratron pressure gauges (P = 0-20 Torr, and 0-1000 Torr). A liquid nitrogen trap was used to purify the CO, which was a UHP Grade in an aluminum cylinder from Praxair. The entire gas handling system was rinsed with CO three times before exposing the sample to CO. The Au/TiO2 and Au/Al2O3 catalysts were purchased from Strem Chemicals (cf. 1% Au by mass; dAu ≈ 2-3 nm; TiO2 mixture of 80% anatase and 20% rutile with 45 m2/g surface area; Al2O3 with 200-260 m2/g surface area). Samples were prepared for infrared analysis by firmly pressing approximately 25 mg of the Au/TiO2 (or Au/Al2O3) catalyst into 30 x 30 mesh Ti gauze (Unique Wire Weaving Co.) with a hand press. The resulting pellet sample had a surface area of


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0.5 cm2 and thickness of 100 µm (the thickness of the Ti gauze). The IR beam is slightly larger than the pellet and therefore probes the entire catalyst sample. According to the manufacturer, the TiO2 particles have a 40 nm nominal diameter while the Au particles have a 3 nm nominal diameter (TEM provided in the Supplemental Material). Based on a simple packing calculation, our pressed powder sample of 0.5 cm2 x 100 µm with 40 nm TiO2 particles contains approximately 1014 TiO2 particles. Using bulk densities for 1% Au on TiO2, we estimate an average of 5 Au nanoparticles per TiO2 particle. The IR beam therefore samples about 5 x 1014 Au nanoparticles. The resulting gauze-supported pellet was mounted into the sample cell and evacuated to a pressure of < 1 mTorr for at least one hour. Experiments were performed at an ambient temperature of ~298 K. Transmission spectra consisted of 100 scans collected with 8 cm-1 resolution (spectral data spacing = 4 cm-1) and are presented as infrared intensity through the sample. All infrared absorbance spectra were referenced to a background spectrum of the Au/TiO2 (or Au/Al2O3) pellet under vacuum that was collected just prior to the addition of CO. Experiments were performed from both low to high pressure, and also from high to low pressure, demonstrating complete reversibility of the resulting infrared signals. After collecting a reference spectrum, the catalyst sample was exposed to a low (or high) pressure of CO and the system was allowed to equilibrate for ~ 30 minutes. An infrared spectrum was recorded and the pressure in the cell was slowly increased (or decreased) to the next pressure and allowed time to equilibrate (~10 min). After completing an experiment, the sample cell could be evacuated and the experiment repeated for a total of two or three adsorption isotherm measurements on a single sample in a single day.


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RESULTS Representative infrared transmission spectra for CO adsorption on Au/Al2O3 and Au/TiO2 catalysts are shown in Figure 1 (for PCO = 1-20 Torr at room temperature). Both catalysts show a sharp IR peak at ~ 2100 cm-1 due to the vibration of CO adsorbed on the Au nanoparticles.14 This Au-CO peak increases in intensity as the CO pressure increases, indicating that it is proportional to the amount of CO adsorbed on the Au. CO adsorption on the Au/TiO2 catalyst also produces an additional broad-band infrared (BB-IR) signal from 3000-1000 cm-1. Both the sharp Au-CO IR peak and the BB-IR signal are reversible; they increase with increasing CO pressure and decrease when the CO is removed by pumping. Multiple experiments were performed on the same catalyst sample (from high to low and from low to high CO pressure) without appreciable changes in the adsorption behavior.

Figure 1. IR transmission spectra for CO adsorption on (a) Au/Al2O3 and (b) Au/TiO2 catalysts (at room temperature for PCO = 1-20 Torr).


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In the absence of CO, the transmission of infrared light through the Au/Al2O3 and Au/TiO2 catalysts is qualitatively very similar. To more easily compare the two catalysts, their spectra (without CO) are reproduced in Figure 2. The transmission spectra for both catalysts Figure 2. IR transmission spectra for Au/TiO2 and Au/Al2O3 catalysts (in the absence of CO).

contain the following notable

characteristics: (a) a sharp cut-off below 1200 cm-1 due to strong IR absorption by the metal oxide support (TiO2 or Al2O3); (b) a decrease in light transmission between 1300-1400 cm-1 due to IR absorption by carbonates adsorbed on the supports (these are common surface impurities on these catalysts); (c) a decrease in light transmission at ~1620 and ~1550 cm-1 due to IR absorption associated with the bending vibration of strongly adsorbed water on the supports and on the supports near Au, respectively {note: assignment of the 1550 cm-1 peak to adsorbed water (and not carbonates) is based upon comparison of the IR spectra of the Au/TiO2 catalyst (which shows both the 1620 and 1550 cm-1 peaks) and the TiO2 support (which shows only the 1620 cm1

peak), along with drying and water adsorption experiments that indicate these peaks are

correlated with the amount of water adsorbed on the catalyst and support, respectively; spectra not shown}; and (d) a monotonic decrease in light transmission towards higher wavenumber due to a combination of the decreasing intensity of the IR source and wavelength dependent detector response of the spectrometer. These transmission spectra represent the overall intensity of infrared radiation as a function of IR frequency (in wavenumber units) and are affected by a number of factors. These factors include the IR source intensity profile, the IR detector


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response, all instrumental optical losses, reflection and scattering of IR radiation from the catalyst sample, absorption of IR radiation due to the Au/TiO2 and Au/Al2O3 catalyst samples, and absorption of IR radiation by any species associated with the samples (i.e. carbonate impurities and strongly bound water). The BB-IR signal in Figure 1b has the same spectral features as the transmission spectra of the Au/TiO2 catalyst in the absence of CO, which are similar to the spectral features of the Au/Al2O3 catalyst (cf. Figure 2). Thus, the BB-IR signal appears to arise from a decrease in the overall transmission of IR light through the Au/TiO2 catalyst; this decrease in light transmission occurs with increasing CO adsorption on the Au nanoparticles. Furthermore, multiple experiments were performed on numerous catalysts, including catalysts with differing amounts of adsorbed water and carbonates. The results were always similar – the infrared spectrum simply displays the differing amounts of water and carbonates, but the overall structure of the broadband IR signal is the same. Again, this is an indication that the BB-IR signal is due to a decrease in light transmission through the catalyst, which Figure 3. IR absorbance spectra for CO adsorption on (a) Au/Al2O3 and (b) Au/TiO2 catalysts (at room temperature for PCO = 1-20 Torr).

is a consequence of CO adsorption. To better understand and to


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quantify the BB-IR signal in terms of the amount of CO adsorbed, we converted the transmission spectra to absorbance spectra by referencing to a background spectrum for each of the Au/TiO2 and Au/Al2O3 catalysts that was collected immediately prior to the addition of CO (i.e. the transmission spectra displayed in Figure 2). Representative absorbance spectra are shown in Figure 3. For both catalysts, the sharp Au-CO absorption peak at ~ 2100 cm-1 is again due to the vibration of CO adsorbed on the Au nanoparticles of the catalysts. The spectrum of Au/Al2O3 is mostly flat, except for the Au-CO peak. For Au/TiO2, the BB-IR signal now appears as an increase in absorbance across the spectral range.


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We, and numerous others, have published infrared spectra for CO adsorption on Au catalysts, but have not previously reported this BB-IR signal. In our previous studies,14-16 our attention focused on the sharp, well-defined Au-CO peak centered at 2100 cm-1. We had observed an apparent baseline shift over the narrow spectral range from 2200-2000 cm-1, as demonstrated in Figure 4 (note: these are the same IR spectra as in Figure 3 simply replotted over the narrower frequency range). However, we concluded that this baseline shift was unimportant Figure 4. IR absorbance spectra for CO adsorption on (a) Au/Al2O3 and (b) Au/TiO2 catalysts (at room temperature for PCO = 1-20 Torr); same spectra as Figure 3 but plotted over a narrower frequency range.

since it did not affect the integrated area of the CO peak and therefore did not affect

our ability to use IR measurements for quantifying CO adsorption on Au/TiO2 catalysts. Upon closer inspection of the larger infrared region, we now understand that the baseline shift is actually due to the BB-IR signal that increased/decreased with increasing/decreasing CO pressure as displayed in Figure 3. We suspect that other research groups may have also observed this apparent baseline shift due to the BB-IR signal, 24 but have not appreciated its importance (as we did not recognize its importance).


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Since the BB-IR signal for the S (Broad-band intensity)

200

Au/TiO2 catalyst appears to be proportional 150

to the amount of CO adsorbed, we quantified 100

this signal (S) by integrating the absorbance

50

from 3000-1800 cm-1 and using a baseline

0 0

5

10

15

20

S (CO peak)

Figure 5. Correlation plot of the BB-IR and the Au-CO IR peak integrated absorbance signals.

from 3500-1000 cm-1. This spectral range was chosen because it avoids the carbonate and water peaks below 1800 cm-1. As shown

in Figure 5, the BB-IR integrated absorbance signal (S) is correlated with the Au-CO IR peak centered at 2100 cm-1 (determined using the integrated absorbance and baseline limits from 2200-2000 cm-1, see Figure 4).14 Since the integrated Au-CO IR peak is a quantitative measure of the amount of CO adsorbed on Au (vide infra),14-16 the BB-IR signal also appears to be a quantitative measure of the amount of CO adsorbed on Au. The BB-IR signal (S) can therefore be plotted as an isotherm for CO adsorption on the Au/TiO2 catalyst at room temperature, as shown in Figure 6. This IR spectral measurement appears to approximately follow Langmuirlike adsorption behavior, similar to the Au-CO peak.14 The adsorption isotherm associated with 150 S (BB intensity)

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the Au-CO IR peak (centered at 2100 cm-1) can 100

be analyzed using a treatment of the Temkin 50

adsorbate interaction model, as we have previously demonstrated.15,16 This model

0 0

5

10

15

Pressure CO (Torr)

Figure 6. Isotherm for CO adsorption on Au/TiO2 (at room temperature) using the BBIR integrated absorbance signal.

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extends the Langmuir model by incorporating a linear variation of the adsorption enthalpy with


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adsorbate coverage (cf. ∆Hθ = ∆H0 – θ δ∆H).

-14

The Temkin adsorbate interaction model

-16 ∆ G (kJ/mol)

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accurately describes the physicochemical

-18

behavior of CO adsorption on Au and yields

y = 7.313x - 20.776 -20

meaningful thermodynamic parameters for CO -22 0

0.2

0.4

0.6

0.8

Coverage

Figure 7. Temkin plot using the BB-IR signal. This plot give ∆H0 = -62.4 kJ/mol and δ∆H = -7.3 kJ/mol (see text for details).

1

adsorption: the adsorption enthalpy at zero coverage (∆H0), which is the intrinsic enthalpy without adsorbate-metal-adsorbate

interactions, the adsorption entropy (∆S), and the change in enthalpy from zero to full coverage (δ∆H), which is a measure of how the electronic properties of the Au-CO binding sites change with coverage. In this case, the change in enthalpy, δ∆H, is attributed to indirect, Au nanoparticle mediated, CO-CO interactions (as opposed to direct CO-CO interactions such as dipole coupling). The Temkin analysis therefore provides chemically meaningful metrics that characterize electronic changes in Au and describe differences between catalysts.15,16 We have successfully applied this analysis to several supported Au catalysts and literature data sets,16 as well as to NaBr poisoned Au catalysts20 and PdAu catalysts.18 Since the BB-IR integrated absorbance signal appears to be a quantitative measure of the amount of CO adsorbed on Au, we analyzed the BB-IR signal using the Temkin model as shown in Figure 7. The linearity is excellent and further supports the use of this adsorption model. The results from multiple experiments are given in Table 1, along with the corresponding Temkin analysis using the Au-CO IR peak centered at 2100 cm-1. The agreement is excellent, further confirming the quantitative nature of the BB-IR signal for describing CO adsorption on Au/TiO2.


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These values for the enthalpy of CO adsorption at zero coverage and the change in enthalpy with coverage also agree with our previously published values. For a large variety of catalysts, including both model and real-world catalysts, -∆H0 = 63 – 75 kJ/mol and -δ∆H = 7 – 40 kJ/mol.16 The ∆H0 enthalpy metric varies linearly with Au nanoparticle size and is related to the relative number of low-coordinate corner and edge sites. Our experimental values, -∆H0 = 61 – 62 kJ/mol, are consistent with 3 nm Au nanoparticles that have about 30% of corner and edge Au atoms.16 The δ∆H values reported in Table 1 are consistent with our previous determinations and are most likely related to support effects, preparation method of the catalysts, and the relatively narrow Au particle size distribution.

Table 1. Temkin metrics for CO adsorption on Au/TiO2 determined from CO IR peak and BB-IR signal. -∆H0 (kJ/mol)

-δ∆H (kJ/mol)

S (CO peak signal)

61.3 ± 0.4

7.3 ± 0.9

S (BB-IR signal)

62.3 ± 0.7

7.3 ± 0.9


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DISCUSSION A physicochemical explanation for the observed BB-IR signal associated with CO adsorption on Au/TiO2 catalysts must be consistent with its key features. These features include: (i) a wavelength dependent change in light transmission that mirrors the overall light transmission of the sample and (ii) the quantitative correlation with the reversible amount of CO adsorbed on Au. As detailed below, the BB-IR signal is consistent with a reversible, partial reduction of the TiO2 support and formation of localized Ti3+ centers at the Au-TiO2 interface. This leads to an increase in surface disorder and roughening of TiO2 that produces a decrease in IR transmission through the powdered catalyst sample (i.e. due to an increase in IR scattering). Support for our proposed interpretation comes from several literature reports. EPR detection of Ti3+ and reduction of titania with CO adsorption on Au/TiO2. Haruta, Conesa, and coworkers examined CO adsorption on Au/TiO2 catalysts using EPR spectroscopy.25 In one set of experiments with conditions very similar to our present study (cf. 20 Torr dose of CO at room temperature), Conesa’s group observed an EPR signal due to Ti3+ that was not associated with anion vacancy generation. They attributed these results to the partial reduction of the titania support upon CO adsorption onto the catalyst Au nanoparticles. Their interpretation involved reversible Au-TiO2 electron transfer induced by CO adsorption on Au, which appears to raise the Fermi level of the Au nanoparticle and leads to electron transfer from the Au to the TiO2 support. This indicates an efficient CO-Au-titania electronic interaction. They further suggested that the electronic transfer probably remains limited to the titania surface sites closest to the Au nanoparticles, since the process was reversible (i.e. the Ti3+ signal decreased with CO desorption from the Au/TiO2 catalyst).25 Our present study, which includes


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similar experimental conditions, is consistent with these published observations and interpretations. Surface roughening and decrease in transmission upon reduction of titania. Lu’s group used a combination of EPR, XPS, and TEM to examine the surface changes associated with the reduction of titania nanorods.26 Exposure to H2 resulted in a disordered surface layer on both the TiO2 and Au/TiO2 nanorods; the titania nanorods decorated with Au nanoparticles demonstrated a thicker disordered surface layer than the undecorated nanorods. EPR and XPS showed the formation of Ti3+ (i.e. evidence for reduction of the titania) while HRTEM showed a concomitant disordering of the TiO2 surface.26 Other groups have also reported increased surface disorder and roughening with the reduction of TiO2.27-30 For example, a very recent study by Mehta et al. examined the reduction of anatase TiO2 using a variety of techniques.27 As TiO2 nanoparticles were reduced by hydrogen, EPR and XPS data revealed an increase in localized Ti3+ sites near the surface. NMR, XRD and Raman spectroscopy demonstrated structural changes and increased disorder at the surface. Furthermore, diffuse reflectance FTIR spectroscopy showed a decrease in reflectance, indicating an increase in IR scattering loss.27 This increase in IR scattering loss with reduction of TiO2 nanoparticles is consistent with our observation of a decrease in IR transmission (i.e. the BB-IR signal). Another study of importance concerns the optical properties of amorphous thin films of TiO2 and Ti2O3 (i.e. titanium (IV) and (III) oxide, respectively).31 Abdel-Aziz et al. reported a 10% increase in the index of refraction for the Ti3+ oxide at 5000 cm-1 (the index of refraction appeared unchanged toward lower IR frequency). Since they measured %Trans. + %Reflect. = 100%, they concluded that there was no IR absorbance. Also, the 50 nm thick amorphous Ti3+


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oxide films had lower percent transmission and higher percent reflectance relative to the amorphous Ti4+ oxide films. 31 These results are consistent with our interpretation of the BB-IR signal arising from a decrease in light transmission due to surface TiO2 roughening.

CONCLUSION Using infrared transmission spectroscopy to monitor CO adsorption on Au/TiO2 catalysts, we have discovered a broad-band infrared (BB-IR) signal associated with CO adsorption. The integrated BB-IR absorbance signal is a quantitative measure of CO adsorption on the gold nanoparticles, as it is directly correlated with the IR peak due to the stretching vibration of CO on Au (Au-CO peak).14 This BB-IR signal was analyzed using the Temkin adsorbate interaction model, yielding enthalpy of adsorption values for CO on Au nanoparticles in excellent agreement with our previous studies, and those reported in the literature.15,16 We proposed a physicochemical explanation for this newly observed BB-IR signal associated with CO adsorption on Au/TiO2 catalysts that is consistent with previously published studies. We suggest that upon adsorption, CO donates electron density to the Au nanoparticles, which is then transferred to the TiO2 support. This leads to a partial reduction of the titanium centers near the Au nanoparticles, along with a concomitant disordering and roughening of the surface oxide layer. Transmission through the powdered catalyst sample therefore decreases as IR reflectance off the roughened TiO2 surfaces increases with increased CO adsorption on Au. Utilizing infrared transmission spectroscopy, this increased reflectance (and decreased transmission) appears as a BB-IR signal that contains the same spectral features as the


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transmission spectrum for the catalyst without CO adsorption. Consistent with the reversibility of CO adsorption on Au/TiO2, this associated BB-IR signal is also reversible. These results suggest an efficient CO-Au-TiO2 adsorbate-induced electronic metalsupport interaction (EMSI) that may play an important role in understanding CO reactions on Au/TiO2 catalysts. It is thought that adsorbates raise the Fermi level of the Au nanoparticles such that electrons are easily transferred from the metal to the support.25 The reversibility of this process at room temperature indicates an efficient electronic charging/discharging between the Au and TiO2, which should influence the energetics of any redox steps in catalytic processes.32 This is consistent with the observation that metal-support interactions influence the catalytic properties of Au/TiO2 catalysts and other metals on reducible supports.33-42 It is also interesting to note that, to the best of our knowledge, a broadband-infrared signal has not been reported in the literature for CO adsorption on other metal nanoparticles (e.g. Pt, Pd, Ag, Ni, Cu) supported on titania, suggesting that the Au/TiO2 metal-to-support electronic interaction is unique.

SUPPORTING INFORMATION TEM of Au/TiO2 catalyst; Reproduction of all Figures and Table.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the U.S. National Science Foundation (CBET-1160217 and CHE-1012395) and the Robert A. Welch Foundation (departmental grant W-0031). These experiments were performed by undergraduate students CDP, AWD, and MNP under the supervision of BDC and CJP at Trinity University. CDP gratefully acknowledges the financial assistance of the Ronald E. McNair Scholars Program at Trinity University, which is funded in part by a grant from the U.S. Department of Education.


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The manuscript was partially prepared by CJP during his academic sabbatical at the Okinawa Institute of Science and Technology (OIST). He gratefully acknowledges OIST and Trinity University for financial support during his sabbatical. CJP also thanks Professor Mukhles Sowwan for hosting him at OIST.


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References (1) Bond, G. C. L., C.; Thompson, D. T. Catalysis by Gold; Imperial College Press: London, 2006; Vol. 6. (2) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Lu, P.; Akita, T.; Ichikawa, S.; Haruta, M. Au/TiO2 Nanosized Samples: A Catalytic, TEM, and FTIR Study of the Effect of Calcination Temperature on the CO Oxidation. J. Catal. 2001, 202, 256–267. (3) Bond, G. C.; Thompson, D. T. Gold-Catalysed Oxidation of Carbon Monoxide. Gold Bull. (London) 2000, 33, 41–51. (4) Clark, J. C.; Dai, S.; Overbury, S. H. Operando Studies of Desorption, Reaction and Carbonate Formation During CO Oxidation by Au/TiO2 Catalysts. Catal. Today 2007, 126, 135– 142. (5) Kung, M. C.; Davis, R. J.; Kung, H. H. Understanding Au-Catalyzed Low-Temperature CO Oxidation. J. Phys. Chem. C 2007, 111, 11767–11775. (6) Widmann, D.; Leppelt, R.; Behm, R. J. Activation of a Au/CeO2 Catalyst for the CO Oxidation Reaction by Surface Oxygen Removal/Oxygen Vacancy Formation. J. Catal. 2007, 251, 437–442. (7) Deng, W.; De, J. J.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Low-Content Gold-Ceria Ccatalysts for the Water–Gas Shift and Preferential CO Oxidation Reactions. Appl. Catal., A 2005, 291, 126–135. (8) Deng, W.; Flytzani-Stephanopoulos, M. On the Issue of the Deactivation of Au–Ceria and Pt–Ceria Water–Gas Shift Catalysts in Practical Fuel-Cell Applications. Angew. Chem., Int. Ed. 2006, 45, 2285–2289.


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TOC GRAPHIC

IR

3+

Ti


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TiO2 Catalysts: Observations ... - ACS Publications

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