The Dynamics of Adsorption on Clean and Adsorbate-Modified Transition Metal and Metal Oxide Surfaces

Madix, Robert J.

Stanford University

关键词: Lattice Vibrations;    Platinum;    Dissociation;    Nickel;    Energy Losses;   

DOI  :  10.2172/1001764
RP-ID  :  DOE/ER/15511-1 - Final Report
RP-ID  :  FG02-04ER15511
RP-ID  :  1001764

美国|英语

来源: UNT Digital Library

【 摘 要 】

Research directed toward understanding the dynamical factors governing the adsorption of molecules typically involved in heterogeneous catalytic processes has been continued. Adsorption is the first step in any catalytic process, and predictions of rates of adsorption are fundamental to calculations of rates of catalytic reactions. Dissociative adsorption can occur either directly upon impact with the surface or as the result of the migration of a temporarily trapped species across the surface. Alkane activation exhibits both of these pathways for reaction on metal surfaces. We have focused on the dynamics of dissociative adsorption of low molecular weight alkanes on single crystal surfaces of platinum group metals. The overall objective of these studies was to make a quantitative comparison of the dissociation probabilities of C{sub 2}-C{sub 4} alkanes on different metals in order to assess the effects of the structures of the different alkanes and the intrinsic differences of the metals on reactivity. First, an unusual and somewhat unexpected difference is observed in the reactivity of linear and branched alkanes. Further, the reactivity of each alkane is significantly higher on Pt(111) than on Pd(111). It has also been observed that the trapping probabilities for the alkanes are higher on Pd(111) due to a larger dissipation of energy to lattice vibrations upon collision, suggesting that energy dissipation in the reaction coordinate into phonons may be involved in dissociative adsorption. We have thus focused on the dynamics of dissociative adsorption of low molecular weight alkanes on single crystal surfaces of platinum, palladium and nickel in order to assess the role of energy dissipation from the incident molecule and the differences of the reactivity of the different metals. We observe that the reactivity of each of the alkanes studied to date differs by only a small amount. On the contrary, due to the dissipation of incident translational energy to lattice vibrations upon collision, the trapping probabilities for the alkanes differ appreciably for the three surfaces. There thus appears to be no correlation between the large differences observed in the trapping probabilities and the reactivity of the surfaces for direct collisional activation of the alkanes, suggesting that no significant amount of the incident kinetic energy of the alkane is dissipated from the reaction coordinate into lattice vibrations. Further, the differences in reactivities of different alkanes on these surfaces appears to originate from steric differences in the accessibility of C-H bonds during the collisional encounter with the surface.