Alexeev Oleg S, Li Fen, Amiridis Michael D, Gates Bruce C
Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, USA.
J Phys Chem B. 2005 Feb 17;109(6):2338-49. doi: 10.1021/jp048907x.
MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.
采用扩展X射线吸收精细结构光谱对负载于MgO、SiO₂和γ-Al₂O₃上的铂簇和颗粒（平均直径在11至45埃之间）以及负载于沸石上的Ir₄簇（直径约6埃）在H₂、O₂、乙烯、丙烯和乙烷存在的情况下，以及在烯烃加氢催化条件下进行了表征。结果表明，在各种气氛下，吸附质的存在对γ-Al₂O₃上较小的铂簇（11埃）的影响比对MgO或SiO₂上较大的铂颗粒（即平均直径大于约21埃的那些）的影响更为显著。当Pt/γ-Al₂O₃暴露于H₂时，铂的形态没有变化，尽管Pt-Pt键距增加。相反，当相同样品暴露于O₂时，发生了完全的氧化碎片化。在用H₂进行后续处理后，这个过程被逆转。暴露于烯烃会改变γ-Al₂O₃负载的铂簇的形态和电子密度（如X射线吸收近边光谱所示）。在室温下的烯烃加氢催化条件下，发现铂簇的电子性质和结构取决于反应物组成以及反应过程中涉及的分子的性质。反应物气体对较小的铱簇（Ir₄）的影响明显不那么显著，显然这是由于每个铱簇中的原子数量极少所致。
