Interaction and reaction of coadsorbed NO and CO on a Rh(100) single crystal surface

In order to assess the possibility to follow surface reactions in a quantitative way by vibrational spectroscopy, a combination of temperature programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) has been used to study the decomposition of NO and the reaction between NO and CO on Rh(100). NO adsorbs in two configurations: in an almost parallel position at coverages below 0.18 ML and, in addition, in an upright position, probably on a bridge site, at all coverages. Coadsorbing NO and CO has only a minor influence on NO binding, whereas CO shifts gradually from top toward the bridge site under the influence of NO. Combining TP-RAIRS with TPRS during the reaction between CO and NO enabled us to simultaneously study site occupation and obtain qualitative surface coverages and desorption rates. At low surface coverages, NO dissociation is observed at lower temperatures than CO(2) formation. Near saturation, NO dissociation becomes blocked and shifts up in temperature. NO dissociation occurs simultaneously with CO(2) formation. To decompose NO, free surface sites have to be generated through surface diffusion or desorption of some CO. During NO decomposition, the formed oxygen atoms react with CO to form CO(2), creating more empty sites. This may lead to an explosive surface reaction.