Dissociative chemisorption of CO₂ on a Re(0001) single crystal surface

The interaction of CO₂ with a clean Re(0001) single crystal surface was studied under ultrahigh vacuum conditions at a crystal temperature range of 80-1500 K. At low coverage a dissociation probability (CO₂(a)→CO(a)+O(a)) of 0.85±0.1 was determined by thermal programmed desorption studies. This probability is coverage dependent, decreasing to 0.35 at saturation CO₂ coverage. The dissociation occurs at a crystal temperature above, 140 K and competes with the desorption of molecular CO₂. This is implied by the decrease, by almost an order of magnitude, of the dissociation probability when CO₂ adsorption is at 310 K. The CO(a) and O(a) fragments form an ordered overlayer with a (2×2) LEED pattern and appear to grow in islands having this structure. Mass balance measurements indicate that at saturation the (2×2) pattern is formed by island of CO(a) and O(a) covering half of a monolayer. Only 0.1 of the CO(a) fragments undergo recombinative desorption, as isotopic labeling studies with ¹³CO₂ indicate. A possible dissociation mechanism and the effect that coadsorption of CO and oxygen following CO₂ dissociation may have on the interaction of the CO fragment with the metal and on the formation of ordered structure are discussed.