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摘要: 采用密度泛函理论（DFT）计算模拟Pd平板和Pd38团簇上的CO催化氧化过程，分析了CO在Pd催化剂表面上的氧化反应机理。结果表明，在Pd38团簇模型上CO催化氧化的决速步骤是O2的解离，反应能垒为0.65 eV，而在Pd平板模型上的决速步骤是CO的氧化，其反应能垒为0.87 eV。对比决速步骤的活化能发现，CO在Pd38团簇上的氧化反应更易进行，说明CO氧化更易在小颗粒催化剂表面上进行，即Pd催化剂的活性与活性组分颗粒大小相关，活性组分颗粒越小，暴露的活性位点越多，其催化活性也越高。Abstract: The catalytic oxidation of CO was comparatively investigated on the Pd slab and Pd38 cluster models by density functional theory (DFT) calculation, in order to reveal the mechanism of CO oxidation over Pd catalysts. The results show that the rate-determining step of CO oxidation on the Pd38 cluster is the dissociation of O2, with the energy barrier of 0.65 eV, whereas the oxidation of CO turns to be the rate-determining step on Pd slab, with the energy barrier of 0.87 eV. Obviously, the oxidation of CO on the Pd38 cluster is much easier than that on the Pd slab, suggesting that the activity of Pd catalysts is related to the dispersion of active Pd species; the Pd catalyst with higher Pd dispersion also exhibits higher activity in CO oxidation.
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图 1 两种Pd催化剂的模型
Figure 1 Models of Pd catalysts
(a): Pd slab model; (b): Pd38 cluster model
图 2 CO与其反应中间物在Pd38团簇模型上吸附位置示意图(图中红球为O原子，灰球为C原子，蓝球为Pd原子)
Figure 2 Adsorption configurations of all intermediates involved in CO oxidation reaction on the Pd38 cluster C, O, H and Pd atoms are shown as grey, red, white and blue balls, respectively
图 3 CO与其反应中间物在Pd平板模型上吸附位置示意图(图中红球为O原子，灰球为C原子，蓝球为Pd原子)
Figure 3 Adsorption configurations of all intermediates involved in CO oxidation reaction on the Pd slab C, O, H and Pd atoms are shown as grey, red, white and blue balls, respectively
图 4 CO在Pd38团簇模型上的催化氧化基元反应
Figure 4 Elementary steps of CO oxidation on Pd38 cluster
(a): O2 dissociation; (b): CO oxidation-1; (c): CO oxidation-2 C, O, H and Pd atoms are shown as grey, red, white and blue balls, respectively
图 5 CO在Pd平板模型上的催化氧化基元反应
Figure 5 Elementary steps of CO oxidation on the Pd slab
(a): O2 dissociation; (b): CO oxidation C, O, H and Pd atoms are shown as grey, red, white and blue balls, respectively
图 6 CO在Pd38团簇模型与Pd平板模型表面上基元反应的反应势能图
Figure 6 Potential energy diagram of CO oxidation on the Pd38 cluster and Pd slab
表 1 CO与其反应中间物在Pd模型上的吸附能与相应的结构参数
Table 1 Optimized geometric parameters and adsorption energies of possible adsorbates involved in CO oxidation on the Pd38 cluster and Pd slab
Configuration Adsorption energy E/eV Geometric parameters/nm CO-hcp/Pd38 -1.98 Pd1-C: 0.3088; Pd2-C: 0.3086; Pd3-C: 0.3084 CO-fcc/Pd38 -1.96 Pd1-C: 0.3130; Pd2-C: 0.3132; Pd3-C: 0.3130 CO-top/Pd38 -1.41 Pd-C: 0.1911 CO-edge/Pd38 -1.22 Pd-C: 0.2008 O2/Pd38 -0.89 Pd-O1: 0.2021; Pd-O2: 0.2024 O-hcp/Pd38 -3.63 Pd1-O: 0.2019; Pd2-O: 0.2020; Pd3-O: 0.2024 O-bridge/Pd38 -2.59 Pd1-O: 0.1950; Pd2-O: 0.1948 CO2-bridge/Pd38 -0.39 Pd-C: 0.2134; Pd-O: 0.2096 CO2-edge/Pd38 -0.63 Pd-C: 0.2021 CO-top/Pd-slab -1.37 Pd-C: 0.1932 O2/Pd-slab -0.72 Pd-O1: 0.2030; Pd-O2: 0.2033 O-bridge/Pd-slab -3.15 Pd1-O: 0.1981; Pd2-O: 0.1980 O-hcp/Pd-slab -3.27 Pd1-O: 0.2034; Pd2-O: 0.2033; Pd3-O: 0.2034 CO2/Pd-slab -0.27 Pd-C: 0.2167; Pd-O: 0.2122 
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表 2 CO在Pd模型上基元反应的活化能和反应能
Table 2 Activation energies and reaction energies of the elementary steps involved in CO oxidation on the Pd38 cluster and Pd slab
Configurations Elementary step Activation energy Ea/eV Reaction energy ΔE/eV O2 dissociation/Pd38 O2 → 2O 0.65 -0.48 CO oxidation-1/Pd38 CO + O → CO2 0.75 0.05 CO oxidation-2/Pd38 CO + O → CO2 0.51 -0.31 O2 dissociation/Pd-slab O2 → 2O 0.79 -0.27 CO oxidation/Pd-slab CO + O → CO2 0.87 0.24
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