CO adsorption on the multiple-site Ru(1121) surface: The role of bonding competition

摘要

The chemisorption and dissociation of CO on Ru(1121) were investigated by using high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy. Three different adsorption states of CO can be distinguished. The most strongly bound beta-state, characterized by a C-O stretch frequency of 166 mV, is attributed to CO adsorbed in a fourfold hoollow site of the Ru(1121) unit cell. This state occurs only at low total coverage and dissociates at T > 300 K. A more weakly bound state is alpha_1-CO with a stretch frequency of 240-255 meV, attributed to CO on-top bonded to first and second layer Ru atoms. This species converts to beta-CO at moderate total coverage and T > 360 K, increasing the amount of dissociated CO. The alpha_1-CO species dominates thecoverage regime up to 1.5 ML. The alpha-CO species is most weakly bound in the coverage range up to 2 ML and is characterized by a stretch frequency of 220 me V. It is proposed to be located in twofold bridge sites. The ratio of on-top to bridge bonded CO is equl to three at saturation. The finite existence range for beta-CO is rationalized by a bond competition effect, due to neighboring alpha_1-CO species destabilizing the beta-state at incresing coveratge. Consequently beta-CO converts to alpha_1-CO under these conditions. A decrease of the beta-state coverage via dissociation of CO may initiate the reverse process of alpha_1-to beta-CO conversion.