Reaction kinetic measurements on single crystal catalysts: Methanol decomposition on Ni(111)

Abstract

A simple apparatus and methodology are presented for reaction kinetic measurements over clean single crystal catalysts in ultrahigh vacuum. Three useful quantities are simultaneously measured in one isothermal experiment: (1) steady-state reaction rate constant ( f R SS), (2) surface coverage of stable intermediates at steady state ( θ SS(CO), θ SS(H)), and (3) zero coverage reactive sticking coefficient ( S R 0) of the reactant. The decomposition of methanol to CO(g) and H 2(g) on Ni(111) is used as a model reaction. The observed dependence of steady-state decomposition rate on temperature is modeled using S R 0(T) and the fraction of empty surface sites. This “site blocking” model qualitatively fits the reaction kinetics from 300 to 500 K. Previously, measurements of S R 0 revealed a deuterium kinetic isotope effect (KIE) in the reaction of methanol with a clean Ni(111) surface (S. M. Gates, J.N. Russell, Jr., and J. T. Yates, Jr., Surface Sci. 146, 199 (1984). CH 3OH and CD 3OH are 1.5 times more reactive at zero coverage than CH 3OD (300–400 K). This observation is now extended to steady-state reaction conditions (SS). The ratio f R SS ( O- H)/ f R SS (O-D) has a maximum value of 2.2 at 363 K and decreases monotonically to about 1 at 500 K. The kinetic isotope effect is also observed from measurements of θ SS(CO), the steady-state CO surface coverage. Addition of products (CO or H 2) to the steady-state reaction decreases the decomposition rate by decreasing the number of empty surface sites.