Annealing changes on the (100) surface of palladium and their effect on CO adsorption

Abstract

LEED beam profiles and intensities for an argon ion bombarded (100) Pd surface have been studied as a function of annealing to determine the minimum anneal which will remove the bombardment damage. The resulting surface has the same structure as the substrate (100) planes and readily chemisorbs CO to form a Pd(100) 2 × 4−45°-CO structure, with one CO molecule adsorbed to every two surface Pd atoms in an alternating bridged bond arrangement. This structure bears a close relationship to that of CO on (100) Ni. In the early stages of cleaning more extensive annealing resulted in the formation of a P2 × 2 structure followed by a stable C2 × 2 which could not be removed by heating. However, these structures could not be regenerated following a long sequence of alternate high temperature heating and argon ion bombardment, although changes in the intensities of the integral order beams still resulted from extended annealing. We attribute these changes to contamination by diffusion of a bulk impurity to the surface. CO does not adsorb on the contaminated surface. We conclude that argon ion bombardment can best be relied upon to produce a clean (100) Pd surface when the subsequent anneal is just sufficient to remove the sputtering damage.