Testing site size requirements in chemisorption: Experiment and theory

作者：Charles T. Campbell, Mark T. Paffett, Arthur F. Voter · 发表于：Journal of Vacuum Science & Technology A Vacuum Surfaces and Films · 年份：1986 · DOI：10.1116/1.573608 · 被引用次数：23 · 研究领域：Catalytic Processes in Materials Science、nanoparticles nucleation surface interactions、Surface Chemistry and Catalysis

‘‘Ensemble requirements’’ in surface chemistry refer to the number and shape of contiguous empty sites necessary for chemisorption of a particular molecule. Ensemble effects can play a major role in directing the course of surface reactions, leading, for example, to dramatic changes in catalytic selectivity when the active metal component is diluted upon alloying with an inert metal. We will review here fundamental surface science studies that have attempted to isolate and probe this site size requirement by diluting active sites on a single crystal surface with an inert metal overlayer. In these studies, initial sticking probability and saturation coverage are measured as a function of diluent metal coverage. We will emphasize recent results in our lab on the interaction of simple molecules (CO, H2, O2) with Cu-, Ag-, and Bi-dosed Pt(111). These show that the group IB metals (Cu, Ag, Au) are poor diluents since they cluster into two-dimensional islands. We find that adsorbed Bi spreads out uniformly over the surface and effectively tests ensemble sizes. Theoretical models based upon Monte Carlo simulations will be summarized which, when compared to such data, allow more accurate determination of ensemble sizes. These models predict uptake curves that deviate strongly from the commonly used (1−θ)A law (A=number of sites in ensemble), which is valid only for an array of isolated ensembles.