Interaction of CO and O$ _\textbf{2} $ with Supported Pt Single-Atoms on TiO$ _\textbf{2} $(110)
doi: 10.1063/1674-0068/cjcp1911198
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摘要: Pt单原子在低温CO氧化反应中具有很高的催化活性.利用扫描隧道显微术与密度泛函理论,研究了Pt单原子在还原性TiO$ _2 $(110)表面的吸附行为及其与CO和O$ _2 $分子的相互作用.研究发现在80 K低温下,TiO$ _2 $表面的氧空位缺陷是Pt单原子的最优吸附位.将CO和O$ _2 $分子分别通入Pt单原子吸附后的TiO$ _2 $表面,研究相应的吸附构型.实验表明在低覆盖度下,单个Pt原子会俘获一个CO分子,CO分子同时与表面次近邻的五配位Ti原子(Ti$ _{\mathrm{5c}} $)成键,进而形成非对称的Pt-CO复合物构型.将样品从80 K升温到100 K后,TiO$ _2 $表面的CO分子会迁移到Pt-CO处形成Pt-(CO)$ _2 $的复合结构.对于O$ _2 $分子,单个Pt原子同样会吸附一个O$ _2 $分子,O$ _2 $分子也会与最近邻或次近邻的Ti$ _{\mathrm{5c}} $原子成键形成两种Pt-O$ _2 $构型.这些结果在单分子尺度上揭示了CO和O$ _2 $与Pt单原子的相互作用,呈现了CO与O$ _2 $反应中的初始状态.
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关键词:
- 金红石TiO$ _2 $ /
- 单原子催化 /
- CO氧化 /
- 扫描隧道显微镜
Abstract: In view of the high activity of Pt single atoms in the low-temperature oxidation of CO, we investigate the adsorption behavior of Pt single atoms on reduced rutile TiO$ _2 $(110) surface and their interaction with CO and O$ _2 $ molecules using scanning tunneling microscopy and density function theory calculations. Pt single atoms were prepared on the TiO$ _2 $(110) surface at 80 K, showing their preferred adsorption sites at the oxygen vacancies. We characterized the adsorption configurations of CO and O$ _2 $ molecules separately to the TiO$ _2 $-supported Pt single atom samples at 80 K. It is found that the Pt single atoms tend to capture one CO to form Pt-CO complexes, with the CO molecule bonding to the fivefold coordinated Ti (Ti$ _{5 \rm{c}} $) atom at the next nearest neighbor site. After annealing the sample from 80 K to 100 K, CO molecules may diffuse, forming another type of complexes, Pt-(CO)$ _2 $. For O$ _2 $ adsorption, each Pt single atom may also capture one O$ _2 $ molecule, forming Pt-O$ _2 $ complexes with O$ _2 $ molecule bonding to either the nearest or the next nearest neighboring Ti$ _{5 \rm{c}} $ sites. Our study provides the single-molecule-level knowledge of the interaction of CO and O$ _2 $ with Pt single atoms, which represent the important initial states of the reaction between CO and O$ _2 $.-
Key words:
- Rutile TiO$ _2 $ /
- Single atom catalysis /
- CO oxidation /
- Scanning tunneling microscope
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Figure 1. (a$ - $c) Sequential STM images obtained at the same area of a clean TiO$ _2 $(110) surface, after Pt SA deposition and after CO dosing. The images are obtained at 1.0 V, 10 pA, 80 K. (d) and (e) the magnified regions marked by white rectangles in (b) and (c), respectively. Yellow arrows indicate CO adsorption at Ti$ _{5 \rm{c}} $ sites. Red and white dashed crosses indicate the center of the original O$ _ \rm{V} $ positions before Pt atom and CO adsorption. (f) The difference image obtained by subtracting (d) from (e), where the contrast and curved arrows show the change after CO adsorption. (g) Line profiles obtained at the "cross" positions labeled by the number in (d) and (e), indicating the changes after Pt (orange line) and CO (blue lines) adsorption, respectively. The green lines are obtained at the same positions before Pt and CO adsorption in (a).
Figure 2. Pt-CO interaction and the configuration change at elevated temperature from 80 K to 100 K. (a1)$ - $(e1) Large scale STM image (26.5 nm$ \times $26.5 nm) obtained at different temperature (upper panels), and the corresponding magnified regions (8.7 nm$ \times $8.7 nm) marked by the yellow rectangles (lower panels). (a2)$ - $(e2) Line profiles of Pt SAs at O$ _ \rm{V} $ sites (green), Pt-CO (orange) and the Pt-(CO)$ _2 $ (blue), obtained at the corresponding position marked by the dashed color lines in the lower panels of (a1)$ - $(e1). (a3)$ - $(e3) Plot of the height distribution of the configurations of Pt SA, Pt-CO and Pt-(CO)$ _2 $ at each temperature.
Figure 3. (a$ - $d) Sequential STM images obtained at the same area of a clean TiO$ _2 $(110) surface, after Pt SAs adsorption, after exposure to 0.05 L O$ _2 $ and after exposure to 0.2 L O$ _2 $, respectively. The O$ _2 $ was dosed successively in situ with the substrate at 80 K, and the images were obtained at 1.0 V and 10 pA. The white arrows in (b)$ - $(d) indicate Pt single atom with no O$ _2 $ adsorption, and yellow arrows indicate O$ _2 $ adsorption on Pt atoms. The dashed rectangles in (c) mark the molecular O$ _2 $ at O$ _ \rm{V} $ sites and the red arrows mark the dissociated O adatoms. (e) and (f) The magnified regions as marked by dashed rectangles in (b) and (d) respectively. (g) The difference image obtained by subtracting (e) from (f). Crosses in red, yellow and black indicate the center of O$ _ \rm{V} $ sites before Pt SAs and O$ _2 $ adsorption. (h) Line profiles of Pt SAs and Pt-O$ _2 $ configurations obtained at the corresponding position as marked by the lines with the numbers in (a$ - $d).
Figure 4. Top view (left) and side view (right) of the ball-and-stick model of reduced TiO$ _2 $(110) surface with Pt, CO and O$ _2 $ adsorption. (a) Clean TiO$ _2 $(110) surface. The dashed green sphere represents O$ _ \rm{V} $. (b) Pt SA adsorption. (c) $ \alpha $-Pt-CO configuration. (d) $ \beta $-Pt-CO configuration. (e) $ \alpha $-Pt-O$ _2 $ configuration. (f) $ \beta $-Pt-O$ _2 $ configuration. The blue, yellow and gray spheres indicate the Ti, Pt, and C atoms, respectively. The red spheres represent O atoms of the substrate, and the green spheres are O atoms from CO or O$ _2 $. The black arrows mark the Ti$ _{5 \rm{c}} $-0 site and the equivalent Ti$ _{5 \rm{c}} $-1 sites.
Figure 5. The differential charge density map of (a) Pt SA, (b) $ \alpha $-Pt-CO configuration, (c) $ \beta $-Pt-CO configuration, (d) $ \alpha $-Pt-O$ _2 $ configuration, and (e) $ \beta $-Pt-O$ _2 $ configuration. Blue (red) color denotes diminishing (accumulation) of electrons. The numbers of electron are obtained from Bader analysis
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