Reaction Mechanisms for the CO Oxidation on Au/CeO2 Catalysts: Activity of Substitutional Au3+/Au+ Cations and Deactivation of Supported Au+ Adatoms
Reaction Mechanisms for the CO Oxidation on Au/CeO2 Catalysts: Activity of Substitutional Au3+/Au+ Cations and Deactivation of Supported Au+ Adatoms
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DOI：
10.1021/ja902109k
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发表时间：
2009-08-05
影响因子：
15
通讯作者：
Fabris, Stefano
中科院分区：
文献类型：
--
作者：
Camellone, Matteo Farnesi;Fabris, Stefano
通过哈伯德项 (DFT+U) 解释现场库仑相互作用的密度泛函理论计算揭示了孤立的 Au 原子以及 Au/CeO2 催化剂中的小团簇催化 CO 氧化的机制。含有带正电的 Au 离子的二氧化铈 (111) 表面，无论是作为负载的 Au+ 吸附原子还是作为替代的 Au3+ 离子，都可以激活分子 CO 并催化其氧化为 CO2。在负载单个 Au+ 吸附原子的情况下，CO 氧化的极限速率由从吸附原子到氧化物载体的吸附物溢出决定。然后反应通过晶格氧和 O 空位形成进行 CO 氧化。这些空位很容易吸引负载的 Au+ 吸附原子，并将其转化为带负电荷的 Au δ-吸附原子，从而使催化剂失活，阻止进一步的 CO 吸附。作为替代点缺陷分散到二氧化铈晶格中的 Au3+ 离子可以维持一个完整的催化循环，该循环由三个单独的步骤组成，在反应过程中保持其活性：AuxCe1-xO2 系统中的 Au 阳离子通过形成表面 O 空位，在没有任何活化能的情况下促进 CO 的多次氧化。氧分子吸附在这些空位上并形成 O 形态，然后催化 CO 分子的氧化，关闭催化循环并恢复化学计量的 AuxCe1-xO2 系统。可逆的 Ce4+/Ce3+ 和 Au3+/Au+ 还原之间的相互作用支撑了分散的 Au 原子进入二氧化铈基底的高催化活性。结果表明，替代性 Au 离子的正氧化态在整个催化循环中得以保留，从而防止了 AuxCe1-xO2 催化剂在操作条件下失活。最后，虽然与 O 空位结合的单个 Au+ 吸附原子在 CO 氧化过程中被证明会失活，但计算预测，对于小至 Au-2 的簇尺寸，在 O 空位处成核的金纳米粒子的反应性可以恢复。
Density functional theory calculations that account for the on-site Coulomb interaction via a Hubbard term (DFT+U) reveal the mechanisms for the oxidation of CO catalyzed by isolated Au atoms as well as small clusters in Au/CeO2 catalysts. Ceria (111) surfaces containing positively charged Au ions, either as supported Au+ adatoms or as substitutional Au3+ ions, are shown to activate molecular CO and to catalyze its oxidation to CO2. In the case of supported single Au+ adatoms, the limiting rate for the CO oxidation is determined by the adsorbate spillover from the adatom to the oxide support. The reaction then proceeds with the CO oxidation via lattice oxygen and O vacancy formation. These vacancies are shown to readily attract the supported Au+ adatoms and to turn them into negatively charged Au delta- adspecies that deactivate the catalyst, preventing further CO adsorption. Au3+ ions dispersed into the ceria lattice as substitutional point defects can instead sustain a full catalytic cycle consisting of three individual steps maintaining their activity along the reaction process: Au cations in AuxCe1-xO2 systems promote multiple oxidations of CO without any activation energy via formation of surface O vacancies. Molecular oxygen adsorbs at these vacancies and forms O adspecies that then catalyze the oxidation of molecular CO, closing the catalytic cycle and recovering the stoichiometric AuxCe1-xO2 system. The interplay between the reversible Ce4+/Ce3+ and Au3+/Au+ reductions underpins the high catalytic activity of dispersed Au atoms into the ceria substrate. It is shown that the positive oxidation state of the substitutional Au ions is retained along the catalytic cycle, thus preventing the deactivation of AuxCe1-xO2 catalysts in operation conditions. Finally, although a single Au+ adatom bound to an O vacancy is shown to deactivate during CO oxidation, the calculations predict that the reactivity of gold nanoparticles nucleated at O vacancies can be recovered for cluster sizes as small as Au-2.