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Precisely Applying TiO2 Overcoat on Supported Au Catalysts Using Atomic Layer Deposition for Understanding the Reaction Mechanism and Improved Activity in CO Oxidation Chunlei Wang, Hengwei Wang, Qi Yao, Huan Yan, Junjie Li, and Junling Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11047 • Publication Date (Web): 16 Dec 2015 Downloaded from http://pubs.acs.org on December 17, 2015

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The Journal of Physical Chemistry

Precisely Applying TiO2 Overcoat on Supported Au Catalysts Using Atomic Layer Deposition for Understanding the Reaction Mechanism and Improved Activity in CO Oxidation

Chunlei Wang, Hengwei Wang, Qi Yao, Huan Yan, Junjie Li, Junling Lu*

Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui 230026 (P. R. China)

* To whom correspondence should be addressed. E-mail: [email protected]

KEYWORDS: Au catalyst, CO oxidation, perimeter sites, TiO2 overcoat, atomic layer deposition

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ABSTRACT: For TiO2 supported Au catalysts, the Au particle size and the interfacial perimeter sites between Au particles and the TiO2 support both play important roles in CO oxidation reaction. However, changing the Au particle size inevitably accompanied with the change of the perimeter length makes it extremely difficult to identify their individual roles. Here we reported a new strategy to isolate them by applying TiO2 overcoat to Au/Al2O3 and Au/SiO2 catalysts using atomic layer deposition (ALD) where the new Au-TiO2 interfacial length was precisely tuned to different degrees while preserving the particle size. High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of CO chemisorption all confirmed that the TiO2 overcoat preferentially decorates the low-coordinated sites of Au nanoparticles and generates Au-TiO2 interfaces. In CO oxidation, we demonstrated a remarkable improvement of the catalytic activities of Au/Al2O3 and Au/SiO2 catalysts by the ALD TiO2 overcoat. More interestingly, the activity as a function of TiO2 ALD cycles obviously showed a volcano-like behavior, providing direct evidence that the catalytic activities of TiO2 overcoated Au catalysts strongly correlate with the total length of perimeter sites. Finally, our work suggests that this strategy might be a new method for atomic level understanding the reaction mechanism, and high performance catalyst design.


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1. INTRODUCTION Supported Au nanoparticle catalysts have shown extraordinarily high activity in CO oxidation.1-3 Intensive studies have been carried out to unravel the origin of the exceptionally high activity, while the nature of the active sites is still debated, and multiple reaction mechanisms have been proposed.4-9 For instance, in the gold-only mechanism, molecular O2 adsorbs and dissociates at the low-coordination Au sites and then directly reacts with the adjacent CO to form CO2, thus the activity of supported Au catalysts mainly depends on the total number of low-coordination Au sites.6, 10-12 While in the support-involved mechanism, molecular O2 adsorbs at the Au/reducible oxide interface (the perimeter sites)2, 5, 13 or on the support14 and reacts with CO which is delivered from Au. In this case, the total length of the perimeter sites was proposed to govern the catalytic activity. Another topic under debate is the Au particle size effect. Several explanations were suggested, ranging from size-related change in the number of low-coordination Au sites,6, 10-12 size-related change in the length of perimeter sites at the Au-TiO2 interface,2,

5, 13

quantum size effect,7 strain effect15 to cooperative effects

between metallic and cationic Au species.8, 16 One reason is likely due to that variation of particle size is inevitably accompanied with the change of the number of low-coordination sites and the length of perimeter sites, which makes it extremely difficult to link the catalytic activity to an individual factor. Inverse oxide/metal model catalyst by depositing oxide on metal single crystal in an ultrahigh vacuum system provides a direct way to illustrate the individual role of the metal-oxide interfaces.17-25 However, the well-known “material gap” and “pressure gap” sometimes might have difficulties to bridge the results obtained from the model catalyst system to the corresponding real catalytic conditions.26-29 Inspired from the inverse model catalyst, here we propose a new strategy of precisely applying oxide overcoating onto supported metal catalyst to extend the methodology of inverse model catalyst into a real catalyst system, thus the “material gap” and “pressure gap” is avoided; therein, the metal


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particle size is kept unchanged during oxide overcoating, while the length of metal-oxide interfaces is tuned by the coverage of oxide overcoat. In this case, precise control over the coverage of oxide overcoat from submonolayer to multilayers could be extremely critical. Atomic layer deposition (ALD) is a thin film growth technique through self-limiting binary reactions between gaseous precursors and the substrate,30-31 which can uniformly grow on various substrates with high surface area and high porosity. More importantly, ALD provides atomically precise control over the thickness of oxide film, since the growth rates of oxides ALD typically fall in the range of 0.1 – 1.5 Å/cycle.30, 32-33 For instance, the growth rate of TiO2 ALD by alternatively exposing to titanium isopropoxide (TTIP) and water at 150 °C is about 0.2 Å/cycle.34 Therefore, precisely applying oxide overcoat onto supported metal catalyst using ALD would allow tuning the metal-oxide interface to a large extent while maintaining the metal particle size, in a similar manor of inverse model catalyst, as shown in the schematic model in Figure 1. As a consequence, it could allow a better understanding of the individual role of the metal-oxide interface of supported metal catalyst under practical reaction conditions. Another benefit from this strategy is that the activity of metal catalyst might be improved by the oxide overcoat. In order to demonstrate our strategy, here we applied TiO2 overcoat onto supported Au catalysts using ALD to tune the Au-TiO2 interfaces while maintaining the Au particle size, so that the individual role of the Au-TiO2 interfacial perimeter sites in CO oxidation is addressed. Meanwhile, in order to avoid the possible variations induced by the different Au-TiO2 interfaces via either P25 or TiO2 ALD overcoat, Al2O3 and SiO2, two inert supports were chosen as the starting materials to obtain Au/Al2O3 and Au/SiO2 catalysts.35-36 After that, TiO2 overcoat with different cycles was deposited onto these catalysts using ALD. TiO2 nucleates on Au nanoparticle surface and creates new interfaces between Au and the TiO2 overcoat, along with a deposition of TiO2 on the support surface, as illustrated in the schematic model in Figure 1b. Next, by varying the


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number of TiO2 ALD cycles, the exposed Au and the interfacial length is precisely tuned in a broad range without changing the particle size (Figure 1c). In CO oxidation, we observed a remarkable improvement of the catalytic activity of Au/Al2O3 and Au/SiO2 catalysts by the TiO2 overcoat. The improved activity showed a volcano-like behavior as a function of TiO2 ALD cycles, providing direct evidence that the total length of perimeter sites plays the key role for the catalytic activity in CO oxidation. Our work suggests that this strategy might be a new method for atomic level understanding the reaction mechanism and high performance catalyst design.

2 EXPERIMENTAL SECTION Au catalyst synthesis. Au/Al2O3 and Au/SiO2 catalysts were prepared using the deposition–precipitation (DP) method with urea and deposition–precipitation (DP) method with ammonium hydroxide respectively.36-39 Sphere gamma-alumina (BET surface area=32-40 m2/g purity=99.5%) were used as the support. HAuCl4.4H2O (Sinopharm Chemical Reagent Co, Ltd, Au content, 47.8%) was used as the Au precursor. Typically, 6 g Al2O3 was added to 100 mL aqueous solution, which contained HAuCl4 (4.2 × 10−3 M) and urea (0.42 M) at an initial pH of 2. For preparing the Au/SiO2 catalyst, 1.0 g spherical SiO2 and 100 mL deionized water contained HAuCl4 (0.7 × 10−3 M) were co-added and ammonia was used to carefully adjust the pH value between 9 and 10. These two mixtures were both vigorously stirred at 75 °C for 12 h in the absence of light to avoid the gold precursor decomposition. Next, the suspensions were centrifuged and thoroughly washed with deionized water for at least six times to remove chlorine residual. Finally, the obtained materials were dried at 70 °C in air overnight and further calcined at 250 °C in 10% O2 in He to obtain the Au/Al2O3 and Au/SiO2 catalysts. Moreover, an Au/Al2O3 catalyst with a larger size was obtained by calcination at 450 °C in 10% O2 in He. Unsupported Au nanocrystals with a size of about 20 nm were synthesized by a


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standard sodium citrate reduction method.40 In brief, 50 ml of 0.01% HAuCl4 aqueous solution was placed in a round-bottom flask and heated to boiling under stirring. Next, 4.5 ml of a 1% solution of sodium citrate was quickly added to the boiling solution, and the mixture was kept boiling for 30 min with continuous stirring. After that, the solution was cooled to room temperature and washed with water and ethanol for several times. Finally, the unsupported Au nanocrystals were grafted onto a TEM grid for the TiO2 ALD coating at 150 °C. TiO2 ALD. ALD was carried out in a viscous flow reactor (GEMSTAR-6TM Benchtop ALD, Arradiance) with ultrahigh purity N2 (99.999%) carrier gas at a flow rate of 200 ml/min and a pressure of 1 Torr. TiO2 ALD was performed at 150 °C by alternatively exposing titanium isopropoxide (TTIP, 99.7%, Sigma-Aldrich) and Millipore water. The TTIP precursor was heated to 80 °C to get a reasonable vapor pressure, and water was kept at room temperature.34, 41-42 The inlet lines were heated to 115 °C to avoid any condensation. The timing sequence of TiO2 ALD was 10, 200, 8, and 250 sec for TTIP exposure, N2 purge, water exposure and N2 purge, respectively. Different cycles of TiO2 ALD were executed onto the Au/Al2O3 and Au/SiO2 catalysts to get different thickness of TiO2 overcoating layer, which were denoted as xc-Au/Al2O3 and xc-Au/SiO2. Characterizations. The Au contents in the Au/Al2O3 and Au/SiO2 catalysts were determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES). High-resolution transmission electron microscopy (HRTEM) measurements were performed on a JEOL-2010 instrument operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were taken on a Thermo-VG Scientific Escalab 250 spectrometer equipped with an aluminum anode (Al Kα = 1486.6 eV). The binding energies in XP spectra were referenced to the C1s binding energy at 284.8 eV. Atomic force microscopy (AFM) measurements were carried out on a Veeco DI Nano-scope Multi Mode V system, which was employed to investigate the morphology of TiO2


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overcoat on Au surface. An Au film coated quartz crystal (purchased from Inficon) was used as the starting Au surface. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of CO chemisorption were performed on a Nicolet iS10 spectrometer, equipped with an MCT detector and a low temperature reaction chamber (Praying Mantis Harrick). A cold trap at about -80 °C was used in the gas inlet line to remove any possible iron carbonyls contamination in the 10% CO in He line.43-44 Before DRIFTS measurements, the samples were calcined at 250 °C in 10% O2 in He for 30 min and followed by reduction in 10% H2 for another 30 min. After cooling the sample to room temperature under flowing ultrahigh purity He, a background spectrum was collected. Subsequently, the sample was exposed to 10 % CO in He at a flow rate of 20 ml/min for about 15 min until saturation. In order to avoid any variations caused by the decline of CO chemisorption peak during He purge,12 here we collected the DRIFT spectra of all samples during the flow of 10% CO in He. The CO chemisorption spectra of the xc-Au/Al2O3 catalysts were obtained by subtracting the CO chemisorption DRIFT spectrum of TiO2 to remove the gas phase CO. All the DRIFT spectra were collected with 256 scans at a resolution of 4 cm-1. Catalytic activity. The activities of all supported Au catalysts in CO oxidation reaction were conducted using a fixed-bed tubular quartz reactor at atmospheric pressure. 100 mg uncoated Au/Al2O3 catalyst, well-mixed with 1 g fine quartz particles (60/80 mesh) was used for the reaction test. For other xc-Au/Al2O3 and xc-Au/SiO2 catalysts, the amount of catalyst was adjusted to keep the same Au content. Prior to reaction, the catalyst was first calcined in 10% O2 in He at a flow rate of 40 ml/min at 250 °C for 1h. Next, the reaction gas consisting of 2% CO and 8% O2 balanced with He was fed to the reactor at a total flow rate of 50 ml/min. The reaction products were analyzed by an on-line gas chromatograph (Fuli, GC-9790II) with a thermal conductivity detector. The CO conversion was calculated based on the ratio of the CO consumed to the CO fed.


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3 RESULTS TiO2 ALD on Au/Al2O3 catalyst: Morphology. The Au loading of Au/Al2O3 catalyst was determined to be 1.2% using ICP-AES and the particle size was 2.9±0.45 nm, seen in Figure 2a. After applying 50 cycles of TiO2 overcoat, HRTEM clearly showed that the thickness of the TiO2 overcoat on both Au nanoparticles and Al2O3 support was about 1.5 nm (Figure 2b), indicating an average growth rate of ~0.3 Å/cycle, in a good agreement with the literature results.45 More importantly, the Au particle size did not show any visible changes after applying TiO2 overcoating, as what we expected (Figure 2c). However, whether the TiO2 overcoat is a uniform film or only decorated islands is still not clear under current resolution. In this case, it might be a big challenge for HRTEM to reveal this detailed morphology of the TiO2 overcoat on small Au nanoparticles, since TEM images are obtained by projecting the 3D features of the object into a 2D picture. In order to conquer this issue, unsupported Au nanocrystals with a size of ~ 20 nm were employed as a model Au surface to investigate the growth of TiO2 ALD using HRTEM. After applying 20 cycles of TiO2 ALD, we observed that TiO2 islands (highlighted by the black arrows) instead of a uniform film were mainly formed on the corner and defect sites of Au nanocrystals (Figure 3). Atomic Force Microcopy (AFM) measurements were further carried out on a continuous thick Au film coated on a quartz crystal (purchased from Inficon), another model Au surface, to investigate the morphology of TiO2 ALD overcoat. Compared with the pristine Au film, AFM measurements again clearly illustrated that TiO2 patches, instead of a uniform film on the entire surface were formed on the Au film after applying 50 cycles of TiO2 ALD (Figure 4). Comparing with the uncoated Au film, XPS measurements confirmed that TiO2 film was successfully deposited on the continuous Au film by the presence of Ti 2p peaks at 458.7 and 464.5 eV. The separation between the two peaks matches very well the reported values for Ti4+.46-47 DRIFTS of CO chemisorption and XPS studies. DRIFTS of CO chemisorption


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measurements were carried out to evaluate the gradual overcoating of Au nanoparticles by TiO2 overcoat with an increase of ALD cycles. As shown in Figure 5a, the uncoated Au/Al2O3 sample showed a strong peak at 2101 cm-1, assigned to linear CO on low-coordination neutral Au sites.12,

36, 48-49

After applying TiO2 ALD overcoat, the

intensity of CO peak decreased dramatically and continued decreasing as increasing the TiO2 ALD cycles. The change of the peak intensity suggested that Au particles were gradually covered by TiO2 during ALD process along with the variation of interfacial length between Au particles and TiO2 ALD overcoat. After 50 cycles of TiO2 ALD with the thickness of about 1.5 nm as shown in Figure 5a, a weak CO peak could still be observed. XPS was also carried out to understand the influence of the electronic properties of Au nanoparticles by TiO2 overcoat. As shown in Figure 5b, the Au 4f binding energy remained nearly constant at 83.4 eV after TiO2 ALD overcoat, implying that the TiO2 overcoat does not considerably change the electronic properties of Au nanoparticles.50-51 Catalytic activities. The activity of these TiO2 overcoated Au/Al2O3 catalysts in CO oxidation were evaluated in a fixed-bed reactor. As what we expected, the uncoated Au/Al2O3 catalyst showed a very low catalytic activity towards CO oxidation below 150 °C, and 100% CO conversion was obtained at about 280 °C (Figure 6a), consistent with literature.52 After applying 2-cycle TiO2 overcoat, the activity was largely increased by achieving the 100% CO conversion at 165 °C. Increasing the number of TiO2 ALD cycles (or the overcoat thickness), the activity aggressively increased and reached a maximum on 20c-Au/Al2O3, on which 100% conversion was achieved at as low as 46 °C, comparable to the activity of TiO2 P25 supported Au catalysts with a similar Au particle size reported in the literatures.53-55 After that, further increasing the number of TiO2 ALD cycles, the activity started decreasing. Obviously, the activity as a function of the number of TiO2 ALD cycles showed a volcano-like behavior, which is further highlighted by plotting 50% CO conversion with the reaction temperatures on the


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corresponding samples, as shown in Figure 6b. Figure 6c showed the Arrhenius plots of these xc-Au/Al2O3 samples. Uncoated Au/Al2O3 showed an activation barrier of 44.3 kJ/mol. On the other hand, the Au/Al2O3 catalysts with different cycles of TiO2 ALD overcoat demonstrated a similar activation barrier of near 30 kJ/mol, close to the ones on TiO2 P25 supported Au catalysts.22, 55 Stability of TiO2 coated Au/Al2O3 catalyst. The stability of 20c-Au/Al2O3 catalyst in CO oxidation was further examined. As shown in Figure 7, the initial CO conversion was 100% at 46 °C, while it gradually deactivated and became rather stable at ~80% conversion after around 3 hours. Once we regenerated the catalyst by calcination at 200 °C for 1 h, the activity was able to be fully recovered. However, the deactivation rate after the first catalyst generation seems to be slightly accelerated. TEM measurements were carried on the used catalysts after a total reaction time of 15 h along with three calcination steps. It clearly showed that the average of Au particle size remained about 3.0 ± 0.43 nm, without showing any visible changes in particle size (Figure 8).

4 DISCUSSION There are only few studies about TiO2 ALD on Au surfaces. Martinson, et al. reported that an island growth of TiO2 ALD on a bare Au surface using a sequence of tetrakis(dimethylamido)titanium (TDAMTi) and water at 150 °C, where a slower growth rate at the initial 30 ALD cycles was observed by in situ quartz crystal microbalance measurements.56 They also pointed out some unintentional surface oxygen on the nominally bare Au might contribute to the partial TiO2 nucleation. In our case, HRTEM images of Au nanocrystals with 20 cycles of TiO2 overcoat (Figure 3) and AFM measurements of a continuous Au film with 50 cycles of TiO2 overcoat (Figure 4) both provide solid evidence of an island growth of TiO2 ALD on Au surface, in line with the previous studies,56 which infers that the TiO2 overcoat on 50c-Au/Al2O3 (Figure 2b) is porous, allowing the embedded Au nanoparticle accessible for catalytic function. More


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interestingly, we noticed that the TiO2 overcoat seems preferentially decorates on the low-coordinated Au sites (Figures 3 and 4). This is not surprising since low-coordinated Au sites are more active than terrace Au sites towards the nucleation of the TTIP precursor. It is worthy to note that the average growth rate of TiO2 overcoat on Au nanoparticles was estimated to be ~0.3 Å/cycle (about 0.1 monolayer) based on the TEM measurements, satisfying the requirements of precise control over the oxide coverage on metal surfaces down to submonolayer range for inverse model catalysts, thus allowing the precise tuning the metal-oxide interfaces. Using CO as a probe molecule, DRIFTS of CO chemisorption measurements further confirmed the gradual overcoating of Au nanoparticles on the Au/Al2O3 sample as increasing the number of TiO2 ALD cycles, evidenced by the continuous decrease in the CO peak intensity. On the other hand, the presence of a weak CO peak even after 50 cycles of TiO2 ALD overcoating, implying that the embedded Au nanoparticles are still accessible for catalytic function and the TiO2 overcoat on Au particles is interconnected porous film as described in the scheme model in Figure 1. According to the literatures,2, 13 the CO peak at 2103 cm-1 is assigned to linear CO on low-coordination neutral Au sites, thus the decrease of the CO peak intensity with TiO2 overcoat also implies that low-coordination neutral Au sites are blocked by the TiO2 overcoat, consistent with the observation by HRTEM and AFM in Figures 3 and 4. Moreover, the CO peak position remained almost constant at ~2103 cm-1 with TiO2 overcoat, suggesting that the changes in electronic properties of Au nanoparticles induced by TiO2 overcoat should be minor, which is further confirmed by the XPS measurement (Figure 5b). In CO oxidation reaction, Au/Al2O3 catalysts often show a large activity variation even the Au particle size is rather similar.52,

57-59

Several factors have been identified to

contribute to the activity, such as Au particle size, oxidation state of Au species,16 water effect,60-61 alumina phase,59 preparation methods,16, 52, 57 the amount of chlorine residual,62 etc. In our case, the uncoated Au/Al2O3 catalyst showed a poor activity by achieving


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100% CO conversion at about 280 °C, which might be partially related with the relative larger Au particle size and the density of hydroxyl group on support, compared with literatures.57-59 Figure 6 clearly demonstrated that the catalytic activity of Au/Al2O3 catalyst in CO oxidation was dramatically improved after applying TiO2 overcoat (also seen the catalytic improvement on another Au/Al2O3 catalyst with a larger size of 4.2 ± 0.45 nm and an Au/SiO2 catalyst with a size of 3.0 ± 0.4 nm by TiO2 overcoat, Figures S1-S6 in the supporting information). Our observation is in line with the dramatic increase of the catalytic activity of nonporous Au by TiO2 ALD overcoat63 and Pt/SiO2 catalyst by the FeOx64 or Fe(OH)x65 overlayer in CO oxidation, where the reactions takes place at the metal-oxide interfaces. On the contrary, Dai observed a significant catalytic activity drop after applying an inert SiO2 overcoat onto an Au/TiO2 catalyst by ALD.66 Therefore, the generated Au-TiO2 interfaces plays the essential role for the remarkable activity increase on the TiO2 overcoated Au/Al2O3 and Au/SiO2 catalysts. We believe the electronic effect induced by the TiO2 overcoat on the catalytic activity might be minor, since the electronic property of Au nanoparticles did not seem to be considerably changed after applying TiO2 overcoat according to our DRIFTS of CO chemisorption and XPS results (Figure 5b). More interestingly, the activity of xc-Au/Al2O3 catalysts as a function of the number of TiO2 ALD cycles showed a volcano-like behavior (Figure 6a,b). Such observation suggests that the Au-TiO2 interfacial perimeter sites are the active sites, because the total length of the perimeter sites are expected to first increase and reach to a maximum at a certain number of TiO2 ALD cycles and then decrease by further increasing the number of ALD cycles due to the gradual encapsulation. However, the total length of the perimeter sites of TiO2 coated Au/Al2O3 catalysts was not able to be quantified here. When the TiO2 ALD overcoat was below 20 cycles, the gradual increase of the activity with TiO2 ALD cycles can be dominantly attributed to the increase of the total Au-TiO2 perimeter sites, since the Au particle size and electronic properties remained constant and


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the number of exposed low-coordination sites was reduced indicated by the decrease of CO peak intensity in the DRIFT spectra (Figure 5a). In another word, enhanced activity along with the reduction of the number of exposed low-coordination Au sites clearly suggests the rate-determining step in CO oxidation might be O2 activation at the perimeter sites, but NOT CO adsorption, consistent with literatures.67-71 When the TiO2 ALD overcoat was above 20 ALD cycles, the decease of catalytic activity might be due to two reasons: the decrease of Au-TiO2 perimeter sites by the coalescence of TiO2 islands at high coverages and the greatly diminished number of exposed low-coordinated Au sites, indicated by the vast decrease of CO peak intensity in DRFITS spectra (Figure 5a). The stability of Au/Al2O3 catalyst with 20 cycles of TiO2 overcoat was further examined. The activity were able to be fully regenerated by calcination in 10% O2 in He at 200 °C strongly indicate that the gradual catalyst deactivation is due to the accumulation of carboxyl groups on the catalyst surface, rather than the sintering Au nanoparticles.72-73 TEM measurements on the used catalyst further support our suggestions (Also seen in Figure S6, in the supporting information).

5 CONCLUSIONS We have successfully demonstrated that a new strategy of precisely applying TiO2 overcoat onto alumina and silica supported Au catalysts using ALD to tune the Au-TiO2 interface to a large extent while maintaining the Au particle size; which allows a better understanding of the individual role of the interfacial perimeter sites without introducing any contributions from the particle size effect. TEM, AFM and DRIFTS of CO chemisorption all suggest that the TiO2 overcoat preferentially decorates the low-coordinated sites of Au nanoparticles. In CO oxidation, we observed a remarkable improvement of the catalytic activity of Au/Al2O3 and Au/SiO2 catalysts by the TiO2 overcoat. More interestingly, the activity as a function of the number of TiO2 ALD cycles showed a volcano-like behavior, providing direct evidence that the total length of


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perimeter sites are the active sites in CO oxidation. The rate-determining step in CO oxidation might be O2 activation at the perimeter sites but not CO adsorption. Our work suggests that this strategy might be a general new method for atomic level understanding the reaction mechanism, and high performance catalyst design in other catalyst systems.

AUTHOR INFORMATION Corresponding Authors *[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the One Thousand Young Talents Program under the Recruitment Program of Global Experts, the NSFC (51402283 and 21473169), the Fundamental

Research

Funds

for

the

Central

Universities

(WK2060030014,

WK2060190026, and WK2060030017), and the startup funds from University of Science and Technology of China.

SUPPORTING INFORMATION Additional characterizations and activity test on Au/Al2O3 and Au/SiO2 catalysts.

REFERENCES (1) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S., Gold Catalysts Prepared by Coprecipitation for Low-Temperature Oxidation of Hydrogen and of Carbon-Monoxide. J. Catal. 1989, 115, 301-309. (2) Haruta, M., Size- and Support-Dependency in the Catalysis of Gold. Catal. Today 1997, 36, 153-166. (3) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N., Novel Gold Catalysts for the


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Reference Catalyst in CO Oxidation. J. Catal. 2006, 239, 307-312.


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Figure 1 A schematic model of precisely tuning the interfaces between Au and TiO2 ALD overcoat. a) Au/Al2O3 catalyst. b) Applying TiO2 overcoating onto the Au/Al2O3 catalyst using ALD to decorate the Au nanoparticles. c) Varying TiO2 ALD cycles to precisely tune the interfacial length of Au-TiO2 interface.


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Figure 2 TEM images of Au/Al2O3 catalysts with and without TiO2 ALD overcoat: a) Au/Al2O3, b) 50c-Au/Al2O3, c) the particle size distribution of these two catalysts.


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Figure 3 HRTEM images of unsupported Au nanocrystals with 20 cycles of TiO2 overcoat at low (a) and high (b) magnifications. Here TiO2 islands formed on the Au nanocrystals are highlighted by the black arrows.


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Figure 4 AFM measurements of a bare Au film in the height (a) and tapping amplitude (b) modes. AFM measurements of an Au film with 50 cycles of TiO2 ALD overcoating in the height (c) and tapping amplitude (d) modes. e) XP spectra of the bare Au film and the Au film with 50 cycles of TiO2 ALD overcoating in the Ti 2p region. Here the TiO2 islands formed on the Au film are highlighted by the white arrows in (c-d).


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Figure 5 a) DRIFT spectra of CO chemisorption on xc-Au/Al2O3 at the saturation coverage. b) XP spectra of the 0c-Au/Al2O3 and 20c-Au/Al2O3 sample in the Au 4f region.


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Figure 6 a) Catalytic activities of Au/Al2O3 catalysts with different cycles of TiO2 ALD overcoat (xc-Au/Al2O3) in CO oxidation reaction. b) A plot of the reaction temperatures for 50% CO conversion as a function of the number of TiO2 ALD cycles. c) Arrhenius plots of these xc-Au/Al2O3 catalysts.


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Figure 7 The stability of 20c-Au/Al2O3 catalyst in CO oxidation where the sample was regenerated through calcination at 200 °C.


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Figure 8 a) A representive TEM image of the used 20c-Au/Al2O3 catalyst after reaction test; b) the particle size distribution of this used catalyst.


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