KINETICS OF HYDROGEN ABSORPTION BY CHEMICALLY MODIFIED Pd( 110) *

The kinetics of hydrogen absorption by Pd(ll0) have been measured as a function of impurity sulfur coverage on the metal surface. In contrast to results obtained on the clean surface in which the absorption process was found to be rate-limited by the diffusion of atomic hydrogen into the bulk, submonolayer coverages of sulfur cause the kinetics to become surface dominated with the dissociative chemisorption of molecular hydrogen now the rate-limiting step. This change was also evident in the measured energetics with the activation energy lowered from a clean surface value of 5.4 to 3.4 kcal/mol on a sulfur covered surface. This latter result may reflect an activation barrier for adsorption induced by the presence of the impurity sulfur, chemisorption being nonactivated on a clean surface. The involvement of a subsurface binding site for hydrogen may also explain the observed kinetics.