Reactivity of CO₂ on Clean and Hydrogen‐Covered Rhenium Single Crystal Surfaces

The adsorption and dissociation of CO₂ were studied on clean and H₂‐precovered Re(0001) and Re(1120) surfaces. Molecular CO₂ lies linear and parallel to the surface at crystal temperatures below 115 K, as indicated by high‐resolution electron energy loss spectroscopy. Upon heating the surface during temperature‐programmed desorption experiments, a fraction of the molecules dissociates. It is found that dissociation is more effective on the smooth Re(0001) surface than on the rough Re(1120) one. A model for a possible active site which may account for the observed structure sensitivity is discussed. At adsorption temperatures above 150 K, a sharp decrease in the dissociation probability is observed, which is explained in terms of a faster competing desorption process. H₂ preadsorbed on Re(0001) was found mostly to block the adsorption of molecular CO₂ and to destabilize the adsorbate. As a result, the dissociation of CO₂ decreases with increasing H₂ coverage.