Tibiletti D, Fonseca A Amieiro, Burch R, Chen Y, Fisher J M, Goguet A, Hardacre C, Hu P, Thompsett D
CenTACat and School of Chemistry and Chemical Engineering, Queen's University Belfast, Belfast BT9 5AG, Northern Ireland, UK.
J Phys Chem B. 2005 Dec 1;109(47):22553-9. doi: 10.1021/jp054576s.
A combined experimental and theoretical investigation of the nature of the active form of gold in oxide-supported gold catalysts for the water gas shift reaction has been performed. In situ extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) experiments have shown that in the fresh catalysts the gold is in the form of highly dispersed gold ions. However, under water gas shift reaction conditions, even at temperatures as low as 100 degrees C, the evidence from EXAFS and XANES is only consistent with rapid, and essentially complete, reduction of the gold to form metallic clusters containing about 50 atoms. The presence of Au-Ce distances in the EXAFS spectra, and the fact that about 15% of the gold atoms can be reoxidized after exposure to air at 150 degrees C, is indicative of a close interaction between a fraction (ca. 15%) of the gold atoms and the oxide support. Density functional theory (DFT) calculations are entirely consistent with this model and suggest that an important aspect of the active and stable form of gold under water gas shift reaction conditions is the location of a partially oxidized gold (Audelta+) species at a cerium cation vacancy in the surface of the oxide support. It is found that even with a low loading gold catalysts (0.2%) the fraction of ionic gold under water gas shift conditions is below the limit of detection by XANES (<5%). It is concluded that under water gas shift reaction conditions the active form of gold comprises small metallic gold clusters in intimate contact with the oxide support.
对用于水煤气变换反应的氧化物负载型金催化剂中活性金形态进行了实验与理论相结合的研究。原位扩展X射线吸收精细结构（EXAFS）和X射线吸收近边结构（XANES）实验表明，新鲜催化剂中的金呈高度分散的金离子形式。然而，在水煤气变换反应条件下，即使在低至100℃的温度下，EXAFS和XANES的证据仅表明金迅速且基本完全还原，形成含有约50个原子的金属簇。EXAFS光谱中存在Au-Ce距离，以及约15%的金原子在150℃下暴露于空气后可被再氧化，这表明一部分（约15%）金原子与氧化物载体之间存在紧密相互作用。密度泛函理论（DFT）计算与该模型完全一致，并表明在水煤气变换反应条件下金的活性和稳定形态的一个重要方面是部分氧化的金（Audelta+）物种位于氧化物载体表面的铈阳离子空位处。研究发现，即使是低负载量的金催化剂（0.2%），在水煤气变换条件下离子金的比例也低于XANES的检测限（<5%）。得出的结论是，在水煤气变换反应条件下，金的活性形态包括与氧化物载体紧密接触的小金属金簇。
