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

Abstract

Research directed toward understanding the dynamical factors governing the adsorption of molecules typically involved in heterogeneous catalytic processes has been continued at Harvard. Adsorption is the first step in any catalytic process, and predictions of rates of adsorption are fundamental to calculations of rates of catalytic reactions. Often activation of the bonds within the molecule proceed via a molecular precursor, and the rate of activation is determined by competitive bond activation and desorption of this precursor. Thus predictive capabilities for the adsorption probabilities of hydrocarbons become important for understanding the rates of simple reactions involving alkanes, such as reforming. In this work we have focused on the dynamics of molecular adsorption of low molecular weight alkanes on single crystal surfaces of platinum group metals in order to understand how different physical properties of the metals affect different trapping (adsorption) probabilities of the alkanes. The overall objective of these studies was to make a quantitative comparison of the molecular 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 the reactivity of the alkanes. This work built on previous studies with platinum and palladium single crystal surfaces for which we were able to apply molecular dynamic simulations to quantitatively predict trapping probabilities of low molecular weight alkanes for palladium directly from measured values of the trapping of ethane (only) on Pt(111). The trapping probabilities for the alkanes are higher on Pd(111) due to a larger dissipation of energy to lattice vibrations upon collision, suggesting a general scaling of trapping with the lattice force constant for different metal surfaces, provided the surface structure is not too different. In this grant period we have thus focused on the dynamics of molecular adsorption of low molecular weight alkanes on single crystal surfaces of platinum, palladium, copper, silver and nickel in order to assess this scaling directly. We observe that the trapping of each of the alkanes studied decreases with the lattice stiffness and mass of the metal atom, but not as strongly as predicted by the lattice for constant itself. These observations are in general agreement with the expectations of molecular dynamic simulations, but further theoretical work is needed to understand the trends in detail.