Surface diffusion of CO on Pt(111)

Abstract

The surface diffusion of CO, reaction of CO with O, and surface diffusion of NO on Pt(111) have been investigated by laser-induced thermal desorption (LITD) for coverages between 1.0 × 10 14 and 4.1 × 10 14 molecules cm 2 and temperatures between 320 and 360 K. The diffusion coefficients for CO displayed Arrhenius behavior, D = D 0 exp(− E diff / RT), with E diff remaining essentially coverage independent at 12.5 ± 1.0 kcal mol . The pre-exponential factor, D 0, was 0.9 ± 0.3 cm 2 s , only by a factor of two over the coverage range studied. Compared to other similar surfaces for which CO diffusion data exist, the surface diffusion of CO on Pt(111) is very slow; at 300 K, ~ 100 times slower than on Rh(111) and more than three times slower than on Ru(001). For desorption from coadsorbed CO and O on Pt(111) at low temperature, LITD shifted the selectivity significantly from the oxidation pathway favored in conventional temperature-programmed desorption (TPD) to the reactant desorption pathway, showing that CO and O can be desorbed without reaction at sufficiently high heating rates. In a variant of the standard hole-filling surface diffusion technique, LITD was also used to demonstrate that coupled surface diffusion and surface reaction can be studied with LITD. Separate attempts were made to measure the surface diffusion coefficient of NO on Pt(111) in a similar temperature and coverage regime. An upper bound of 5 × 10 −10 cm 2 s at 320 K was determined based on the detection limit of the experimental system.