Stable Pt Single Atoms and Nanoclusters on Ultrathin CuO Film and

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Stable Pt Single Atoms and Nanoclusters on Ultrathin CuO Film and Their Performances in CO Oxidation Xiong Zhou, Wenshao Yang, Qiwei Chen, Zhenhua Geng, Xiang Shao, Jianlong Li, Yongfeng Wang, Dongxu Dai, Wei Chen, Guo Qin Xu, Xueming Yang, and Kai Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11362 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 4, 2016

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The Journal of Physical Chemistry

Stable Pt Single Atoms and Nanoclusters on Ultrathin CuO Film and Their Performances in CO Oxidation Xiong Zhou,† Wenshao Yang,‡ Qiwei Chen,† Zhenhua Geng,‡ Xiang Shao, ∥ ,* Jianlong Li,† Yongfeng Wang,⊥ Dongxu Dai,‡ Wei Chen, #,§ Guoqin Xu, # ,§ Xueming Yang,‡, * and Kai Wu†,§,* †

BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871,

China. ‡

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics,

457 Zhongshan Road, Dalian 116023, Liaoning, China. ∥

Department of Chemical Physics, School of Chemistry and Materials Science, University of

Science and Technology of China, Hefei 230026, China. ⊥

School of Electronics Engineering and Computer Science, Peking University, Beijing 100871,

China. #

Department of Chemistry, National University of Singapore, Singapore 117543, Singapore.

§

SPURc, 1 CREATE Way, #15-01, CREATE Tower, Singapore 138602, Singapore.

ABSTRACT A series of model catalysts consisting of Pt single atoms and nanoclusters supported by monolayered CuO film grown at Cu(110) were successfully prepared, which could

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be stabilized well above room temperature and exhibited a high performance in CO oxidation at temperatures as low as ~360 K. Combined scanning tunneling microscopy and temperatureprogrammed desorption measurements directly evidenced that at the initial CO oxidation stage, oxygen vacancy in the CuO lattice was generated at the nearest neighbour of the Pt nanoclusters. The experimental measurements showed that the oxidation activity was inversely proportional to the Pt nanocluster size. In contrast, the Pt single atoms possessed no surface reactivity for the CO oxidation due to the early and complete desorption of CO before its oxidation on the model catalysts commenced.


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INTRODUCTION A high performance catalyst has a prerequisite that it must possess proper interaction with reactants, i.e., neither too strong nor too weak. Oxide-supported Pt, a widely used family of catalysts in heterogeneous catalysis, has been extensively investigated in both catalysis and surface chemistry. However, the strong interaction of Pt with CO normally suppresses the catalytic performance in CO oxidation, a key catalytic process in automobile emission control and a widely employed probe reaction for scaling surface activity. The main reasons lie in that CO has a high desorption energy on Pt surface and tends to block the surface active sites for other incoming reactants such as O2.1-5 Recent studies suggested that downsizing of the Pt particle could gradually weaken the CO-Pt interaction and thus invoked a much lower CO oxidation temperature.6-9 Brune et al.6 have proved that the reactivity is Pt3 > Pt7 > Pt10 in catalyzing CO oxidation by Pt cluster on TiO2. However, when Pt downsizes to single atoms, intensive debates still exist on understanding the elementary processes involved in CO adsorption and oxidation at the atomic level on the Pt single atoms at oxide surfaces.10-18 Zhang et al.10 have reported that Pt single atoms on FeOx show the weakest CO adsorption according to Fourier-transform infrared (FTIR) spectra and have a better performance in CO oxidation than nanometer-sized Pt clusters. While Ding et al.11 have observed that due to the strong binding of CO molecules, Pt single atoms lack catalytic activity for CO oxidation at low temperature, only Pt nanoparticles show such activity. Single metal atoms at oxide surfaces have reportedly showed high activity or selectivity in several catalytic reactions. To prevent single atoms from aggregation, surface defects in the oxide supports can effectively serve as potential wells to trap and stablize these metal atoms.1012,17-20

Suitable surface structure can also provide such potential wells. For example, Diebold and


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collaborators have used reconstruction sites on Fe3O4(001) to stablize Au and Pd single atoms.21,22 Monolayerd CuO film (cf. Figure 1a) prepared by exposing a Cu(110) surface to oxygen at room temperature (RT) and subsequent annealing to 500 K in vacuum contains rectangular unit cell (0.36 nm × 0.51 nm) formed by copper and oxygen ions can also create such potential wells to trap metal atoms and clusters.23-28 Previous studies with STM24-26, XPS29 and EELS30 have shown that such a prepared film by this approach is indeed CuO film. This paper aims to address the above-mentioned debating issues, namely, stabilization of Pt single atoms and nanoclusters and understanding the reaction mechanism of CO oxidation, with combined STM and TPD techniques on prepared model catalysts of Pt supported by monolayered CuO on Cu(110) where the coexistence of both Pt single atoms and nanoclusters allows to simultaneously explore their catalytic performances in CO oxidation. Such Pt atoms and clusters supported on monolayered Cu film have not been reported in literature.

EXPEIMENTAL SECTION STM experiments. All STM experiments were performed on a Unisoku UHV LT-STM with a base pressure of 1 × 10-10 Torr. The Cu(110) single crystal was cleaned by repeated cycles of Ar+ sputtering with a kinetic energy of 1.5 keV for 20 minutes and subsequent annealing at 723 K for 15 minutes. The cleanness was confirmed by STM characterization. To prepare the monolayered CuO film, the clean Cu(110) surface was exposed to 1 × 10-7 Torr O2 (Air Products, purity > 99.99%. The purity also holds for all other gases used in our experiments) at room temperature for 100 seconds, and subsequently annealed to 500 K for 10 minutes in vacuum. An incomplete CuO film in Figure 6a was obtained at a lower O2 exposure (1 × 10-7 Torr, 80 s). A Pt


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rod (Aldrich, 99.99% purity) was employed for the Pt deposition in an e-beam evaporator which was carefully degassed prior to use. The evaporation temperature was 1913 K and the flux was controlled to be 0.01 ML/minute, as monitored by a quartz crystal microbalance. All STM images were obtained in liquid Helium temperature (4.2 K). TPD experiments. The TPD experiments were conducted with a home-made UHV TPD apparatus with a base pressure of 6 × 10−11 Torr.31 The clean single crystal, CuO monolayer and Pt nanoclusters were prepared by following the same recipe as in the STM experiments. The ordering and cleanness of the surfaces were confirmed by low energy electron diffraction and Auger electron spectroscopy (LEED/AES optics, Omicron) measurements. The inlet gasses, i.e. CO (99.9% purity), O2 (99.99% purity) and CO2 (99.9% purity), were dosed separately via high precision UHV leak valves connected to a nozzle positioned close to the sample surface which was cooled by liquid nitrogen. In this way the real gas exposure was at least two orders higher than that calculated from the gauge reading. The samples were afterwards heated at a linear rate of 2 K/s and the TPD signals were monitored with a quadrupole mass spectrometer (Extrel) which is positioned ~ 5 mm away from the surface.

RESULTS AND DISCUSSION Preparations of Pt single atoms and nanoclusters. RT deposition of 0.1 ML (1 ML ≈ 5.4 Pt atoms/nm2) Pt produces co-existent Pt single atoms and nanoclusters on the monolayered CuO film, as depicted in Figure 1b. In the high-resolution STM image (Figure 1c), the accommodated Pt single atoms sitting atop the CuO lattice can be clearly identified. As marked in Figure 1c, the small blue and yellow circles indicate the surface Cu and O ions. Each small red protrusion


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corresponds to an individual Pt atom sitting in the middle of the Cu2+ unit cell and is hence encircled by two topmost O anions and four Cu cations.25 Such a geometry was also observed for other metals such as cobalt deposited at 15-20 K on a similar CuO film.27,28 However, the simultaneously formed small Pt nanoclusters, i.e., the bright yellow protrusion in Figure 1c, can hardly be resolved at the atomic level. The detailed information can only be obtained from the line profiling analyses, as shown in inset of Figure 1b where the Pt atom displays an apparent height of 1.2 Å and an apparent diameter (FWHM, full width at half maximum) of 0.51 nm while the bilayered (and trilayered) Pt clusters possess a height of 2.2 Å (3.1 Å) and a diameter of 0.86 nm (1.27 nm), respectively. In Figure 1, all STM images are scaled by the right side color scale bar to identify the number of layers of the corresponding Pt species. The light blue, red, yellow and green color correspondingly represents the topmost Cu2+ plane in CuO, and the 1st, 2nd and 3rd layer of the Pt atoms in various prepared Pt nanoclusters. Such a color scale bar is used throughout the whole study. Obviously, the single Pt atoms (small red protrusions) are the dominating Pt species in Figure 1, with a percentage of about 68% for all the Pt atoms deposited on the CuO film.


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Figure 1. RT deposited 0.1 ML Pt on a CuO monolayer film. Scanning condition: Vbias= 0.3 V, I= 30 pA. (a) Prepared CuO monolayer film with Cu-cation lattice imaged. Unit cell (white rectangle): 0.36 nm × 0.51 nm. In the inset model, red and light blue balls represent Cu cations and O anions in the CuO film, and brown ones for Cu atoms in Cu(110) underneath the CuO monolayer. (b) Pt single atoms and nanoclusters on CuO monolayer prepared by deposition of 0.1 ML Pt at RT. Different layers of Pt species are illustrated in different colours, indicative of the apparent height by the coloured scale bar. Inset: profiles of single atom, bilayered and trilayered clusters. The arrows mark the apparent diameters of different Pt species. (c) Enlarged image of the Pt single atoms and a particular Pt cluster in the black frame in (b). Blue and yellow circles represent the Cu and O ions, respectively.

An increase of the Pt coverage can normally change the distribution of the Pt atoms and clusters. As shown by the STM images in Column a in Figure 2, an increase of the Pt coverage from 0.1 to 0.3, 0.5 and 0.7 ML leads to the decrease of the occupancy of the Pt atoms and a gradual increase in both size and quantity of the Pt nanoclusters. The corresponding statistical analyses of the Pt nanoclusters are given in Column b in Figure 2. The species with a FWHM of about 0.5 nm are due to the single Pt atoms (see the line profiling analysis in the Inset in Figure 1b), while other Pt species are ascribed to the Pt nanoclusters which contain an estimated number of Pt atoms: 0.7 nm cluster to 2 ± 1 atoms, 0.9 nm to 6 ± 3 atoms, 1.1 nm to 9 ± 4 atoms, 1.3 nm to 17 ± 7 atoms, 1.5 nm to 22 ± 8 atoms and 1.7 nm to 29 ± 11 atoms (see Supporting Information for the estimation). As such, the total number of Pt atoms in variously sized Pt nanoclusters can be estimated and directly correlated to the Pt coverage. It is thus concluded that


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the mean size of the Pt nanocluster can be scaled to the coverage: 0.95 ± 0.19 nm (0.1 ML), 0.99 ± 0.23 nm (0.3 ML), 1.06 ± 0.26 (0.5 ML) and 1.16 ± 0.28 nm (0.7 ML).

Figure 2. Morphology, size and reactivity of RT-deposited Pt species as a function of the Pt coverage. (a) STM images of the Pt species from 0.1 ML (top) to 0.7 ML (bottom): Vbias= 0.3 V, I= 30 pA. (b) Size distribution of the Pt species. Natoms is defined as the number of Pt atoms per unit area in various Pt species. (c) TPD spectra of co-adsorbed 0.03 L CO and 0.9 L O2. Black circles: raw data; black curve: CO2 data fitting; red curve: CO data fitting; orange curve: CO adsorbed on CuO; pink curve: CO physisorbed on Pt species; blue curve: CO chemisorbed on Pt atoms; green curve: CO chemisorbed on Pt nanoclusters. The dashed lines mark the peaks of the


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CO desorbed from the Pt single atoms and nanoclusters, respectively. Temperature ramping rate: 2 K/s.

Performance in CO oxidation. According to the STM characterizations, the whole series of the Pt nanoclusters in Figure 2 are very suitable for examining the size-dependent catalytic activity of the Pt/CuO model catalysts. Therefore, a series of TPD measurements were carried out to monitor the CO oxidation on various Pt-loaded samples. In the measurements, each sample was exposed to 0.03 L CO (1 L = 10-6 Torr⋅s) followed by excessive O2 (~0.9 L) at 90 K and finally ramped to 450 K at a ramping rate of 2 K/sec. The desorbed molecules were monitored by a quadrupole mass spectrometer. The spectrum for each sample is shown in Column c in Figure 2. For samples from top to bottom in Column c, the amount of the desorbed CO monotonically correlates with the Pt loading on the CuO surface. The CO desorption begins at about 90 K and completes at about 450 K, whereas product CO2 desorbs above 300 K from the Pt samples. The complex CO desorption behavior stems from various contributions by the adsorbed CO on various surface sties, i.e., the bare CuO film, the Pt atoms and various Pt nanoclusters. CO molecularly adsorbs on the CuO film at low temperature and completely desorbs below 200 K without reaction, as evidenced by our TPD and STM measurements (see Figure S1).32, 33 This is also consistent with the fact that Cu(110) can actively catalyze CO oxidation at a low temperature of < 230 K, but its catalytic activity drops to zero once a complete CuO layer is formed.34,35 Therefore, the TPD features below 200 K come from the adsorbed CO on CuO. The other features above 200 K can be assigned to the CO desorption from the Pt atoms and clusters


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(the contribution to the mass 28 signal by the cracking fragment of CO2 is negligible and the CO peak intensity hardly changes after the calibration), as indicated by the pink, blue and green curves de-convoluted from the overall CO TPD spectra. The CuO-relating spectra (in orange) have been calibrated with respect to the exposed bare CuO surface area for each sample. It is well known that CO can feasibly chemisorb on Pt surfaces, but its binding strength weakens with the Pt cluster size.6-9 This fact allows us to assign the green features in Column c in Figure 2 to the chemisorbed CO on the Pt clusters, shifting to a higher temperature with the increase of the Pt cluster size. Meanwhile, the physisorption-relating features (pink) show an opposite TPD behavior. In contrast, the blue features retain the position at about 300 K, regardless of the Pt coverage variation. The only change of the blue feature is its TPD spectral area, correlating to the amount of the desorbed CO, which decreases with the Pt coverage from 0.1 ML up to 0.7 ML. Therefore, we can unambiguously assign the CO desorption feature at 300 K to the adsorbed CO from the Pt single atoms whose specific density also decreases with the Pt coverage. Further support for the TPD assignments comes from the TPD experiments where the size distribution of the Pt nanoclusters is tuned at a fixed Pt coverage. Figures 3a-3c show the STM images of the 0.1 ML Pt sample annealed to different temperatures. Both the STM image and statistical histogram (Inset) of the Pt nanoclusters at various annealing temperatures demonstrate that the aggregation of the Pt atoms starts from about 400 K. More serious aggregation takes place at 500 K where the averaged Pt cluster size significantly increases to 1.38 nm and eventually no Pt single atoms survive such a high temperature annealing. In parallel, the TPD spectra are carried out for these annealed samples. As shown in Figure 3d, the CO desorption


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peak at about 300 K completely disappears for the sample annealed at 500 K, indicating that no Pt single atoms exist on such a sample.

Figure 3. STM images and Pt cluster size distribution (insets) of RT-deposited 0.1 ML Pt as a function of annealing temperature. Vbias= 0.3 V, I= 30 pA. (a) 360 K, (b) 400 K, (c) 500 K, and (d) corresponding TPD spectra of CO on the as-deposited and annealed Pt/CuO samples. Exposure: 0.03 L CO + 0.9 L O2. In contrast to the complex desorption behavior of the CO, the CO2 desorption turns out to be much simpler. The symmetric CO2 desorption feature positions at around 350 K and is closely related to the chemisorbed CO on the Pt nanoclusters, as shown in Column c in Figure 2. Both the desorption peak area and temperature gradually increase with the Pt coverage and reach maxima at ~380 K for the highest Pt loading (~0.7 ML) in our experiments. As a control experiment, the TPD measurement for a purely CO2-covered CuO/Cu(110) sample (Figure S2)


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reveals a much lower desorption temperature of below 200 K, verifying the weak interactions of CO2 with both CuO and Pt. The above two pieces of evidence bring us to the conclusions that the observed CO2 in Figure 2 comes from the CO oxidation process catalyzed by the Pt clusters supported on the CuO film, and the catalytic activity is inversely proportional to the Pt cluster size. According to the TPD results in Figure 2, a quantitative relationship between the desorption temperature of CO (or CO2) and the mean size of the Pt nanoclusters can be established, as shown in Figure 4. It is clear that within our experimental range, both desorption temperatures of CO and CO2 are in a linear relationship with the mean size of the Pt species on CuO. Note that this linear dependence is just valid within our particle size range since the Pt particles are quite small and cannot endlessly continue and would reach saturation when Pt cluster is large enough. The lower the CO2 desorption temperature, the higher the catalytic activity of the Pt/CuO model catalyst because all CO2 molecules can only be produced from oxidation of the adsorbed CO. Linear fittings for both data sets (black and red lines for CO and CO2, respectively) show that the lines are unparalleled, and the CO2 line has a smaller slope (117 K/nm) than CO (128 K/nm). These two lines merge at a point where the mean size is equivalent to the Pt cluster size of about 0.65 nm, echoing the fact that no CO oxidation exists for the Pt clusters smaller than 0.65 nm because CO completely desorbs prior to its oxidation. Therefore, the single Pt atoms are insufficient for the CO oxidation, in agreement with our TPD experimental result. The main reason is that CO adsorption on the Pt atom is too weak to be activated.


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Figure 4. Plots of the CO desorption and CO2 generation temperatures versus mean size of the Pt nanoclusters. Insets: schematic models for CO desorption from a Pt single atom (left) and a Pt cluster (right).

The above-described size-dependent catalytic activity of the prepared Pt model catalysts is further experimentally checked. We first induce on purpose the aggregation of the Pt atoms by annealing the as-deposited 0.3 ML Pt/CuO sample to 500 K for 10 min. In Figure 5a, the STM image shows a much narrowly dispersed Pt nanoclusters with an average size of about 1.41 nm. Subsequent TPD measurement of the sample yields only molecular CO desorption below 250 K when the temperature ramping stops at 450 K (Figure 5f). It implies that the annealing-induced Pt clusters are much larger than the as-deposited Pt ones (cf. 0.99 nm for the 0.3 ML Pt sample in Figure 2). Therefore, the absence of chemisorbed CO becomes inevitable because the 500 K annealed sample is full of much larger Pt nanoclusters on which CO should desorb at much higher temperatures outside our temperature ramping range. When additional 0.1 ML Pt is


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deposited at room temperature onto the 500 K annealed sample, both the chemisorbed CO and produced CO2 are restored below 450 K in the TPD spectra (Figure 5g). According to the size distribution (Figures 5c and 5d), more Pt nanoclusters with the size of smaller than 1.3 nm (Figure 5e which is the difference histogram of Figures 5d and 5c) are formed by the addition of the extra 0.1 ML Pt, which resumes the emergence of the Pt single atoms and small nanoclusters. It is worthy to point out that the (0.3+0.1) ML Pt sample is similar to neither the 0.4 ML nor the 0.1 ML as-deposited Pt samples shown in Figure 2. This perfectly explains why the CO2 desorption feature is smaller than that obtained with 0.4 ML as-deposited Pt sample. Hence, the difference data treatment for both the size histograms (Figure 5e) and TPD spectra in Figure 5 (not shown here) may establish a direct size-activity correlation for CO2 production where the Pt nanoclusters of 0.7 – 1.1 nm in size dominantly contribute to the CO oxidation.

Figure 5. (a) STM image of the RT-deposited 0.3 ML Pt/CuO followed by annealing at 500 K. (b) STM image of sample (a) with additionally deposited 0.1 ML Pt. (c) and (d) Pt cluster size distribution of the samples in (a) and (b), respectively. (e) Differentiated histogram of (d)-(c). (f)


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and (g) CO and CO2 TPD spectra for samples (a) and (b). Vbias= 0.3 V, I= 30 pA. Exposure: 0.03 L CO, 0.9 L O2.

Reaction mechanism and oxygen vacancy creation. After having explored the activity of the Pt single atoms and nanoclusters, we now address the reaction mechanism issue by scrutinizing the difference between the TPD spectra for the CO oxidation on the 0.3 ML Pt/CuO sample with and without co-adsorbed O2, as shown in Figures 6b and 6c (similar TPD spectra for the 0.5 ML Pt/CuO sample are shown in Figure S3). In this series of experiments, we carry out two consecutive runs of TPD measurements with the same sample: the first-run TPD experiment is performed with the sample temperature ramped up to 450 K, and the second-run experiment is carried out on the sample after the first-run experiment. It can be clearly seen that the second-run experiment results in a CO TPD spectrum nearly identical to the one achieved from the hightemperature-annealed sample (Figure 5), indicating that the first-run TPD experiment with the temperature ramped up to 450 K already causes drastic aggregation of the Pt clusters. Enhanced metal cluster ripening by reaction or adsorption has been previously reported.8,20,21,36 A noticeable difference between Figures 6b and 6c lies in that the CO TPD spectrum for the COcovered sample (Figure 6b) shows a pronounced shoulder at about 200 K that is characteristic of the defective CuO film (Figure 6a), suggesting that the CuO film has been partially reduced by the CO adsorbed on the Pt clusters.


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Figure 6. Exploration of the CO oxidation mechanism on 0.3 ML Pt/CuO. (a) CO TPD spectra on CuO, incompletely oxidized CuO and bare Cu(110) surfaces. (b) Consecutive TPD spectra of 0.03 L CO. (c) Consecutive TPD spectra of co-adsorbed 0.03 L CO and 0.9 L O2. (d) STM image of the sample treated in CO environment at 360 K. Black protrusions near the surrounding Pt nanoclusters are oxygen vacancies. Vbias= 3.0 V, I= 30 pA. Inset: Line profile of an oxygen vacancy and its neighboring Pt nanocluster. (e) Distance between the generated oxygen vacancy and its neighbouring Pt nanocluster as a function of the Pt nanocluster size.

In combination of all experimental observations in Figure 6, we therefore conclude: 1) Even the O2 exposure is 35 times higher than CO, the latter still dominates the surface adsorption on the Pt clusters; 2) The co-adsorbed O2 is involved in the CO2 production, but the major contribution comes from the depletion of the lattice oxygen in the CuO monolayer by the CO adsorbed on the Pt clusters; 3) When O2 and CO are continuously introduced into the system, the


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reduced CuO film can be completely recovered. These experimental facts exclusively point to the so-called Mars-van-Krevelen mechanism, 18, 37-40 which has been widely proposed for the oxidation reaction on the metal-loaded oxide surface: (1) CO(g) + Pt(*) → COads-Pt (2) COads-Pt + CuO(s) → CO2(g) + CuO1-x(s) + Pt(*) (3) O2(g) + CuO1-x(s) → CuO(s) where Pt(*) represents the active surface site of the Pt species. Further direct experimental evidence for the Mars-van-Krevelen mechanism can be obtained in the STM experiments. Figure 6d shows the STM image of the 0.3 ML Pt/CuO sample which is exposed to 5 L CO at 360 K. Except for the Pt single atoms and nanoclusters, new dark features are frequently observed at the peripheries of the Pt nanoclusters. This dark feature is actually a pit of 0.4 Å in depth, and can be feasibly assigned as the oxygen vacancies (OV) in the CuO film because it can be easily healed by re-exposing the defective surface to oxygen at 400 K or above. OV has been recognized to play a significant role in CO oxidation.41-45 Such a STM observation of the OV around the Pt nanoclusters provides clear and direct evidence that the lattice oxygen in the CuO film has been involved in the CO oxidation catalyzed by the Pt nanocluster sitting nearby. Similar oxygen up-taking phenomenon has been recently observed around Pt single atoms supported on Fe3O4(001) reconstructed surface under a CO atmosphere. 18 In Figure 6e, the distance between the formed OV and its nearest Pt nanocluster has been statistically analyzed and plotted as a function of the Pt nanocluster size. A striking feature is that the lateral distance between the OV and its nearest Pt nanocluster maintains at 0.34 ± 0.06 nm,


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irrelevant to the Pt cluster size. Such a distance is in surprising coincidence with the periodicity of the CuO unit cell, thus strongly suggesting that the CO oxidation takes place at the periphery of the Pt nanocluster. The short and constant distance within the experimental error implies that the nearest neighboring O (excluding those underneath the Pt atoms and nanoclusters) in the CuO lattice participates in the surface CO oxidation. In addition, the concentrated data points in the size range of 0.7 - 1.1 nm (yielding an average of 0.86 ± 0.21 nm) suggest that the most active Pt nanoclusters for the catalytic surface CO oxidation, which have the lowest CO2 generation temperature, are around 1 nm in size, in excellent agreement with the above conclusions drawn from the TPD measurements.

CONCLUSION To summarize, we have prepared and characterized a series of model catalyst that consist of Pt single atoms and nanoclusters on monolayered CuO film. The prepared Pt/CuO model catalysts have shown a very high CO oxidation activity by lowering its oxidation temperature down to as low as ~360 K. Our systematic and combined TPD and STM measurements have unambiguously demonstrated that the catalytic activity is in inverse proportion to the size of the Pt nanocluster with an optimized size in the sub-nanometer regime, i.e., 0.7 ~ 1.1 nm. Once the Pt nanocluster downsizes to the Pt single atom level, the catalytic activity for the surface CO oxidation vanishes due to the early and complete desorption of CO from the Pt single atom before the surface oxidation commences. More importantly, we have shown that the surface CO oxidation follows the Mars-van-Krevelen mechanism where lattice oxygen in the CuO monolayer is involved. Detailed and careful STM measurements show that only the nearest


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neighboring lattice oxygen at the peripheries of the Pt nanoclusters are directly involved in the surface CO oxidation. All these findings would help prepare model catalysts consisting of stabilized metallic atoms and nanoclusters supported by thin oxide films and further our understanding of surface CO oxidation.

AUTHOR INFORMATION Corresponding Author * Address correspondence to [email protected] (Kai Wu), [email protected] (Xueming Yang), [email protected] (Xiang Shao). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was jointly supported by NSFC (91527303, 21333001, 21133001, 21261130090, 21228301) and MOST (2013CB933404, 2011CB808702), China, and NRF CREATE-SPURc project (R-143-001-205-592), Singapore. ASSOCIATED CONTENT Supporting Information. Estimation of the number of Pt atoms in a Pt nanocluster, CO adsorption on bare CuO. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES


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Table of Contents

Size-dependent reactivity of CO oxidation on a whole series of prepared Pt clusters supported by monolayered CuO grown at Cu(110).


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Stable Pt Single Atoms and Nanoclusters on Ultrathin CuO Film and

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