Evidence of encapsulation model for strong metal-support interaction

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Evidence of encapsulation model for strong metal-support interaction under oxidized conditions: A case study on TiOx/ Pt(111) for CO oxidation by in situ wide spectral range IRAS Huan Li, Xuefei Weng, Zhenyan Tang, Hong Zhang, Ding Ding, Mingshu Chen, and Huilin Wan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02883 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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Evidence of encapsulation model for strong metal-support interaction under oxidized conditions: A case study on TiOx/Pt(111) for CO oxidation by in situ wide spectral range IRAS Huan Liǂ, Xuefei Wengǂ, Zhenyan Tang, Hong Zhang, Ding Ding, Mingshu Chen*, Huilin Wan State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols–Ethers–Esters, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, People’s Republic of CHINA. ǂ

These two authors contribute equally to this study.

*

Email for M. C.: [email protected]

ABSTRACT: Strong metal-support interaction (SMSI) has been found to significantly modify the catalytic performance in the last four decades. The origins of the promotion effects of SMSI still reamin unclear. In this work, fully covered TiOx/Pt(111) model surfaces were prepared to mimic the proposed encapsulation model. The catalytic activities for CO oxidation on such model surfaces as a function of the TiOx coverage were examined in the model catalysis reaction cell. The TiOx ordered thin flims significantly enhanced the reaction rate for CO oxidation on Pt(111). The apparent activation energy was obtained to be 53 kJ/mol for CO oxidation on the TiOx/Pt(111), which is much lower than that of 97 kJ/mol on the clean Pt(111) surface. The stability and catalytic efficiency of such SMSI model surfaces under reaction conditions were confirmed by our home-built in situ wide spectral range infrared reflection adsorption spectroscopy that is capable to measure both the surface species and the changes of a catalyst in one spectrum. The study provides an experimental evidence that a fully covered thin oxide layer can indeed enhance catalytic activities for CO oxidation on Pt

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catalysts, and confirms the complete encapsulation model. The promotion effects of SMSI may origin from the existing surface sites of the boundary of the two-dimensional thin oxide domains. Keywords: Strong metal-support interaction (SMSI), Model catalysis, In situ IRAS, Titanium oxide, Encapsulation, CO oxidation

1. Introduction Since the strong metal-support interaction (SMSI) effect was first reported to strongly modify the catalytic performance by Tauster in 19781-2, a great deal of efforts have been devoted to understanding the origins of the promotion effects3-23. Most of the SMSI effects were found on the reducible oxide supported metal catalysts after reduction treatment at high temperature. The larger metal particles were found to be partly covered by a reduced TiOx film, while the smaller metal particles preferred flatter quasi-two-dimensional

clusters1-4,6,11.

The

encapsulation

induced

chemisorption

suppression can be reversed by heating in oxygen. Many studies have addressed the evidence for the presence of oxide species on metal particles and probed the effects of such species on the catalytic and chemisorptive properties2-7,10,15, some tried to probe the effect of oxide overlayer on the supported metal particles24. Based on the large experiment facts, the origins of SMSI have been proposed to be due to electronic interactions, structure changes, or encapsulation. But it is hard to distinguish which of them is the most important since all occur together in most realistic SMSI catalysts. The presence of oxide on the metal particles has been confirmed by many surface science studies of metal nanoparticles on single crystal TiO2 surfaces25-32, and growing TiOx thin films on metal single crystal surfaces33-39. Several ultrathin metal oxides have been found to exhibit unique chemical properties and promising applications in heterogeneous catalysis5,30,40. Although these oxide layers often suppress the catalytic


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activities of the metals,2,32 the novel properties of the ultrathin oxides themselves allow new possibilities for the development of chemically active materials5,30,40. In the past several decades, the changes of catalyst properties caused by SMSI have been reported in large quantities. However, due to complexities of a realistic catalyst, there is still lack of clear understanding of the origins of the promotion effects. Especially, the encapsulation model is still not convincible evidence observed in a working catalyst. Titanium oxide film on a metal single crystal surface is certainly one of the best-characterized oxide model systems in surface science33-39. But there are only few reports regarding the ralationship between the structures and activities towards catalysis41-44. On the other hand, CO catalytic oxidation, a simple but fundemantal reaction in both academic and industrial societies, also triggers plenty of research interests. But as far as we know, different adsorption and activation properties of different types of TiOx films toward CO/O2 and the reaction details, especially from spectroscopic aspects like IR, were much rarely reported in literature, compared to the abundent morphological data. In this paper, a much reasonable procedure to measure the turnover frequency (TOF) and apparent reaction activation energy (Ea) on a single crystal surface was introduced by considering the effects from different faces, polished front face, unpolished back face and unpolished side face. Different types of TiOx/Pt(111) model surfaces were constructed to mimic the proposed SMSI surfaces and to overcome the complexity of a realistic catalyst. CO catalytic oxidation was conducted on these model surfaces to evaluate catalytic activities as a function of the TiOx coverage. The possible changes of such model surfaces under reaction conditions were monitored by home-built wide spectral range in situ IRAS. The active sites and the promotion effect of SMSI were discussed.

2. Experimental section


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The experiments were performed in an elevated pressure reactor combined with an ultrahigh vacuum (UHV) surface science setup. The detail description of this system can be found in a previous publication45,46. Briefly, the apparatus consists of an UHV section equipped with Auger spectroscopy (AES), low energy electron diffraction (LEED), Gas chromatography (GC), and an elevated pressure cell equipped with infrared reflection adsorption spectroscopy (IRAS). After preparation and characterization by AES and LEED, the model surface was transferred in situ into the elevated pressure cell. High-purity CO and O2 were introduced into the elevated pressure cell after being purified by a liquid nitrogen trap, and CO oxidation was conducted in the cell with 2 Torr CO and 2 Torr O2. During the reaction a home-built wide spectral range IRAS was used to in situ characterize the surface change47. The reaction gas mixture after kinetic test was pumped into a home-built sampling system and compressed into a GC sample loop for product analyses46. Another experimental UHV system was equipped with X-ray photoemission spectrometer (XPS) and low energy ion scattering spectrometer (LEIS) (Thermo Multilab 2000). XPS measurements were performed using Al Kα (1486.6 eV) and a hemispherical analyzer. LEIS was done using 1 keV He+ generated in a hot cathode ion gun, directed at the sample with a 45° angle of incidence. The pass energy was 50 eV. The Pt(111) surfaces (both sides) were cleaned through repeated cycles of Ar+ sputtering (2 keV, 5 µA) at 300 K, followed by oxidation (PO2 = 5×10-7 Torr) at 773 K, and finally annealing at 1100 K. The cleanness of the surface was confirmed by AES and a sharp (1×1) LEED pattern, or by XPS. The TiOx/Pt(111) model catalysts were obtained via deposition of Ti in 5×10-7 Torr O2 onto the Pt(111) surfaces at room temperature, followed by oxidization in 1×10-6 Torr O2 or annealing in UHV for 10 min respectively. The coverages of TiOx were calibrated using AES and LEED prior to kinetic measurement in the first chamber, and further confirmed by XPS and LEIS in the second chamber.


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Results and discussion Two series of TiOx/Pt(111) thin films, TiOx-Re/Pt (111) and TiOx-Ox/Pt(111), with different growth modes were prepared by controlling Ti evaporation rate and O2 pressure. Specifically, the TiOx-Re/Pt(111) was prepared by cycling deposition of Ti in 5×10-7 Torr O2 followed by annealing in UHV at 973 K for 10 min, and the TiOx-Ox/Pt(111) was prepared by cycling deposition of Ti in O2 followed by oxidizing in 1×10-6 Torr O2 at 773 K for 10 min respectively. As can be seen in Figure 1, the Ti/Pt AES ratio of the TiOx-Re/Pt(111) increases linearly then slows down with a sharp breakpoint, while that of the TiOx-Ox/Pt(111) follows similar trend to the break point and then becomes fast. Such behavior corresponds to two typical thin-film growth modes, Stranski–Krastanov (SK: layer-plus-island) and Frank–van der Merwe (FM: layer-by-layer), respectively48. Thus, the monolayer (1 ML) is defined from the break point for the TiOx-Re/Pt(111), which means the minimum amount of TiOx that is required to fully cover the substrate Pt(111) surface. A typical LEED pattern at the break point is shown for the TiOx-Re/Pt(111) in the inset in Figure 1. It is identical with that of z-TiOx reported previously 36-38, which can be regarded as a reduced type of titania film with mainly Ti3+ and a zigzag structure. This is also confirmed by XPS measurements. The AES ratio of O/Ti remains almost constant as the deposition time increases (Figure 1(A)). The fully covered TiOx-Re/Pt(111) surface is further confirmed by IRAS using CO adsorption as a probe (Figure S1), in which only very weak CO adsorption peak with IR band at 1900~2200 cm-1 is observed for the 1 ML TiOx-Re/Pt(111). Note that the film growth for the TiOx-Re/Pt(111) is self-limiting, in that different initial Ti coverages (more than 1 ML) upon annealing lead to TiOx films with an identical Ti/Pt AES and/ or XPS ratio, corresponding to 1 ML. Note also that such weak CO adsorption peak always existed with further depositing TiOx onto Pt(111) may result from the domain boundary of the zigzag structure.


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Figure 1. Ti/Pt and O/Ti AES ratios as a function of the deposition time under different preparation conditions: (A) UHV annealing at 973 K for 10min after each deposition and a typical LEED pattern at the break point is inserted, (B) oxidized at 773 K in O2 for 10min after each deposition and a typical LEED pattern at the break point is inserted. (C) XPS Ti2p spectra and (D) LEIS spectra for films prepared by oxidizing at 773 K in O2 for 10min after each deposition.

The TiOx-Ox/Pt(111) follows a multilayer growth mode. The Ti/Pt AES ratios are very close to those for the TiOx-Re/Pt(111) below 1 ML. The XPS spectra reveal that the film of the TiOx-Ox/Pt(111) possesses a Ti3+ at submonolayer, and appears Ti4+ at higher coverages, as shown in Figure 1C and Figure S2. Judged by the preparation condition and


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the similar O/Ti AES ratios (Figure 1A and B), the TiOx-Re/Pt(111) should be a reduced TiOx film as well. However, their surface structures are different, z-TiOx and w-TiOx, respectively, as shown from the LEED patterns in Figure 1A and B. According to literature reports36-38, the z-TiOx possesses a zigzag structure with a large amount of domain boundary, while the w-TiOx possesses a 2D network with a hole in the corner of a 18.2 Åx18.2 Å unit. Note that the calibrated coverage of 1 ML in XPS measurements was also confirmed from the low-energy ion scattering spectroscopy (LEIS) in which the substrate Pt signal is nearly completely suppressed by the above TiOx (Figure 1D). Furthermore, both the O/Ti AES ratio (Figure 1B, Figure S3) and O1s/Pt4p3/2 XPS ratio (Figure S4) show a low value at submonolayer, then increase fast after 1 ML, which also confirm that the part of titanium, especially from the second layer, is oxidized to be Ti4+. To better evaluate the activity for CO oxidation on the TiOx/Pt(111) model surfaces, the background contribution was carefully examined by considering the different faces of a single crystal. As shown in Figure 2A, three parts of the Pt(111) contribute to the total activity, namely the well-polished front face, the unpolished back face, and the side face. Here the side surface also includes Ta wire welded on the crystal for resistivity heating. Thick SiOx films were grown on both or either the front and/or back face to exclude their contributions in the catalytic reaction. Under CO oxidation conditions, the clean surface exposes all the three parts, the back covered one exposes the front and side faces, while the both covered case exposes only the side face. The obtained CO2 formation rates as a function of reaction temperature are shown in Figure 2B. Here the rate is the total CO2 formation amount per second. Such non-linear tendency in Arrhenius plot has been observed previously49. After examining such three cases (Figure 2A), the specific rate (TOF) for each face can be obtained, as shown in Figure 2C. Note that the TOFs for the three faces were all calculated based on the surface atom number by using 1.30×1015 atoms/cm2 for the Pt(111) surface. The reaction rates for CO oxidation on both


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the back and side faces are apparently higher than that on the well-polished front face, which may be caused from their rough surfaces (unpolished) possessing higher rate for CO oxidation45,50-52. And only the front face (polished) displays a good linear line in the Arrhenius plots, with Ea of 97 kJ/mol. The reason is that all surface Pt atoms on the polished Pt(111) face are identical with a coordination number of nine, i. e. the unique surface active site. While on the unpolished and the side faces, surface Pt atoms are not equal and with lower and different coordination numbers due to roughness. Hence there exist different surface active sites which possess higher reaction rate for CO oxidation45,50-52. Due to co-existing such different surface active sites, it is reasonable to observe non-linear Arrhenius plots for the unpolished back and side faces. The above results show that the background contribution (from back face and side face) is even higher than the value of the investigated part (the polished front face). The present procedure achieves more reliable kinetic data by using a thick SiO2 film to passivate the unpolished back face and subtracting the evaluated values from the side face.


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Figure 2. (A) Schematic for the single-surface polished Pt(111). (B) Reaction rates for CO oxidation on a Pt(111) surface with and without covering by a thick SiOx film. Note that the sample was re-cleaned by Ar+ sputtering and annealing after each reaction, and that the back face was also cleaned for the ‘No SiOx covered’ case. (C) Arrhenius plots for the three different surfaces.

Next, CO oxidation was run on both the TiOx-Re/Pt (111) and TiOx-Ox/Pt(111) model surfaces with various TiOx coverages at 473 K. The obtained reaction rates are compared in Figure 3A and B. Note that the back face of the Pt(111) was covered by a thick SiOx film, and the contribution from the side face was deducted using the value obtained in Figure 2. Note also that the TOFs for the TiOx/Pt(111) were still calculated based on the surface atom number of 1.30×1015 atoms/cm2 as the clean Pt(111) surface. It is obvious that TiOx on Pt(111) enhances the activity for CO oxidation. For the TiOx-Re/Pt(111), the TOF increases almost linearly as the TiOx coverage increases to 1 ML (Figure 3B), which value is about five times higher than that on the clean Pt surface. This reveals that the domains of the reduced two-dimensional (2D) TiOx on the Pt(111) surface serve as the active sites and are more active than the Pt(111) surface. On the other hand, a volcano curve between 0 to 1 ML with a highest value at about 0.4 ML and an almost constant rate at above 1 ML are observed for the TiOx-Ox/Pt (111) (Figure 3A and B). Such a maximum rate achieved at submonolayer suggests that the perimeter of the 2D TiOx domains on the Pt(111) surface is the active sites. Although both of the TiOx films at submonolayer were determined to be Ti3+, their local structures, including the domain boundary / surface defects, may be different due to different preparation conditions.


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Figure 3. (A) and (B) TOFs for CO oxidation on the TiOx-Re/Pt(111) and TiOx-Ox/Pt(111) films as a function of the TiOx deposition time and coverage, respectively. (C) Arrhenius plots for the clean front face, TiOx-Re/Pt(111) and TiOx-Ox/Pt(111). Reaction conditions: CO/O2 = 1/1, total pressure of 4 Torr, temperature at 473 K. Note that the model surface was freshly prepared for each reaction temperature. The TOFs for the TiOx/Pt(111) were also calculated using the value of 1.30×1015 atoms/cm2 as the clean Pt(111) surface.

For the 1 ML TiOx-Re/Pt(111) and 2 ML TiOx-Ox/Pt(111) model surfaces, CO oxidation was run at various temperatures. Note that the TiOx-Ox/Pt(111) may involve surface structuring at around 1 ML, thus the 2 ML surface with stable structure was chosen as a model surface to investigate their apparent activation energy. The obtained reaction rates as a function of the reaction temperature, i.e. Arrhenius plots, are compared in Figure 3C. The apparent activation energies (Ea) are 51 kJ/mol and 54 kJ/mol for the 1 ML TiOx-Re/Pt(111) and 2 ML TiOx-Ox/Pt(111), respectively. Both are significantly lower than that of 97 kJ/mol for the clean Pt(111), and close to that of 52.5 kJ/mol obtained from a realistic catalyst study53. The promotion effect on the TiOx-Re/Pt(111) is more distinct at low temperature range. This may be due to the much lower CO poisoning effect on the Pt surface sites adjacent to the TiOx than the bare Pt substrate, which adsorbs CO strongly at relatively low temperature. The key aspects for SMSI are the nature of the active sites and the origins of the


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promotion effects. For a realistic catalyst due to existing different sizes of supported metal nanoparticles, the encapsulation may not include all nanoparticles, as evidenced by probe molecule adsorption2,3,6,7,10,11,15,25-28. TiO2 is the most typical SMSI support. Several surface science studies have confirmed the growth and stability of an oxide thin film on a metal36-42. However, certain vital information for such SMSI model surfaces under realistic reaction conditions is lacking in reports. Here, home-built in situ wide spectral range IRAS was used to monitor the model surfaces under CO oxidation condition. As shown in Figure 4A and Figure S4, there was no obvious CO adsorption peak when the 1 ML TiOx-Re/Pt(111) surface was exposed to CO and CO/O2 mixture, indicating the Pt(111) surface is fully covered by the TiOx thin film. Then the surface was heated to the reaction temperature of 473 K. It can be seen that gas phase CO2, with IR band at 2300~2400 cm-1, emerges just reaching the reaction temperature (the second spectrum in Figure 4A) while the Ti-O related vibrational modes (1000~500 cm-1, Figure 4B) remain almost unchanged. Note that the reaction was taken in a batch reactor, the increase of the IR band intensity of gas phase CO2 can also be used to estimate the reaction rate. The evolution of IR band intensities of the gas phase CO2 and adsorbed CO are plotted as a function of the reaction time in Figure 4C. The intensity of the IR band of CO2 increases continually with reaction time, demonstrating that the model surface maintains active for CO oxidation. The decrease of the slope, e.g. the reaction rate, may be mainly resulted from the decrease of the reactant pressures of both CO and O2 and the increase of the product pressure of CO2. This is confirmed by the cycle reaction test. In which, after reaction for 10 min, the reaction mixture was analyzed to obtain an average reaction rate, and pumped out. Reactant gas of CO/O2 mixture was refilled and run the reaction again. The obtained average reaction rate for each cycle is comparable as shown in Figure 4D.


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Figure 4. (A) and (B) In-situ IRAS spectra for CO oxidation on the 1 ML (TiOx-Re) film at 473 K; (C): IR intensities for the gas phase CO2 and the adsorbed CO as a function of the reaction time under CO oxidation condition; (D) Reaction rate for five cycle CO oxidation on the same 1 ML TiOx-Re/Pt(111) surface. Note that for each cycle, reaction was taken for 10 min as shown in Figure 4C, then evacuate the reaction cell and refilled fresh CO+O2 for next repeated reaction.

Under CO oxidation condition, there appears a very weak IR band at 2084 cm-1,


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corresponding to CO adsorption on Pt. Its peak intensity almost keeps constant during the reaction. After reaction, cooling down to room temperature and pumping out, a weak IR band at 2090 cm-1 for CO adsorption is still detected. Its intensity is about one tenth of that for CO adsorption on the clean Pt(111) surface (see also Figure S1). Note that the IR band shift from 2084 cm-1 at 473 K to 2090 cm-1 at room temperature may mainly cause by the temperature differences. Moreover, the Ti-O related vibration does not change as shown in the low wavenumber region in Figure 4B, which also confirms the TiOx film being stable under CO oxidation condition. The comparable reaction rates for CO oxidation in five cycles and weak CO adsorption peak demonstrate the stability of the model SMSI surface under CO oxidation condition, and evidence that the reduced TiOx film can indeed enhance the reaction rate on the Pt(111) surface. For the 2 ML TiOx-Ox/Pt(111) surfaces, there only appears an nearly invisible IR band of CO under CO oxidation condition, as shown in Figure S4. The Ti-O related vibration bands at 1200 ~ 500 cm-1 maintain similar. These results also confirm the stability of the model SMSI surface under CO oxidation condition, and evidence that oxidized TiOx film can also enhance the reaction rate on the Pt(111) surface. A comparable system of FeOx/Pt(111) have been investigated extensively under different reaction conditions in recent years5,41-44,54-58. Freund et al. proposed that the FeO2/FeO interface in the film plays the main contribution for CO oxidation5,43,44,54,55. Bao et al. emphasized the edges/ boundary of FeO domains41,42,56. Zheng et al. found the hydroxyl group being the important species57. Substrate-induced rumpling appears to be a general phenomenon for oxide monolayers, which may be relevant for a number of catalytically relevant materials58. Regarding that iron oxide itself is also a catalyst for CO oxidation, it is reasonable that different active sites exist for the FeOx/Pt(111)59-61. Unfortunately, there is a lack of detail evaluations of the relative activities for the different active sites of FeOx/Pt(111) under comparible reaction conditions. It has been


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proposed that CO oxidation takes place under the films for the graphene/Pt(111) and BN/Pt(111)62-65. For the TiOx/Pt(111), the present results manifest that the fully covered TiOx/Pt(111) surfaces are more active than the Pt(111) surface for CO oxidation at low temperature. However, it can exclude the possible reaction path under the TiOx thin films since only very weak CO adsorption peaks were observed and the Ti-O related vibration does not change under reaction conditions. Regarding that titanium oxide itself is not an active component for CO oxidation under regular reaction conditions, it doesn’t seem possible for CO oxidation to take place on the surface of TiOx. The metal-oxide interface or boundary has been correlated well with its catalytic performance in many systems, Pd/TiO2(110)27, TiO2/Au(111)66-68, CeOx/Au(111)9,20,67,68, VOx/Rh(111)69, VOx/Pt(111)46, ZnO/Cu(111)70, Cu/Al2O3/ZnO(0001)−Zn71. Hence, we propose that the CO oxidation reaction primarily takes place at the oxide/Pt(111) interface, and is promoted by the Pt support. Although the detail structure for the TiOx/Pt(111) is not clear yet, regarding that a zigzag image was observed by STM37,38 and that the intensity of the adsorbed CO is about one tenth of the clean Pt(111) surface, the active site is most likely the boundary of the zigzag domians, where CO adsorbs on the Pt sites and O2 activates on the adjecent TiOx sites may initiate the CO oxidation with a lower activation energy and a high reaction rate. And with respect to the CO adsorbed amount, the TOF for CO oxidation on the active site would be about fifty times higher than that on the clean Pt surface. Similary, there is still a very weak CO peak for the 2 ML TiOx-Ox/Pt(111), coming from the existing domain boundary / defects. This may serve as the highly active sites for CO oxidation. In summary, two types of ordered ultrathin TiOx films, reduced (TiOx-Re) and oxidized (TiOx-Ox), were successfully grown on the Pt(111) surface. The TiOx-Re/Pt(111) can only grow up to 1 ML with mainly Ti3+, while the TiOx-Ox/Pt(111) can grow up to multilayer with the submonolayer Ti being Ti3+ and the second and above layer being Ti4+. The


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catalytic activity for CO oxidation on the TiOx-Re/Pt(111) model surfaces increases nearly linearly as increasing the TiOx coverage up to 1 ML, and is significantly higher than that on the Pt(111). The high rate is observed for the TiOx-Ox/Pt(111) at submonolayer. It then decreases to a low value at above 1 ML. The in situ wide spectral range IRAS confirms the catalytic efficiency and stability of the TiOx/Pt(111) model surfaces under the reaction condition. The studies provide experimental evidencec that a fully covered oxide thin layer can indeed enhance catalytic activities for CO oxidation on Pt catalysts, with an apparent activation energy being much lower than that on the clean Pt(111) surface. It is an ideal model surface to investigate SMSI effect for mimicing conventional metal catalysts supported on transition metal oxides. The present results verify the encapsulation SMSI model, and moreover a monolayer thickness. The promotion effects of SMSI may origin from two-dimensional monolayer oxide film that possesses active sites at the boundary of the thin oxide domains.

ASSOCIATED CONTENT Supporting Information The Supporting Information includes more information about HS-LEIS and supporting experimental data, which is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT We gratefully acknowledge the support received for this work from the National Natural Science Foundation of China (21273178, 21327901, 21573180, 91545204).

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Role of Al2O3 in the promotion of CuZnAl catalysts: a Model Study. Chem.- Eur. J. 2017, 23, 10632-10637.


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TOC


Evidence of encapsulation model for strong metal-support interaction

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