Sum-frequency generation in situ study of CO adsorption and catalytic CO oxidation on rhodium at elevated pressures

Abstract

Optical IR-visible sum-frequency generation surface vibrational spectroscopy was employed for in situ detection of chemisorbed CO during CO adsorption and catalytic CO oxidation on a rhodium (111) catalyst in the substrate temperature range T s =300–8000 K. CO adsorption studies performed over 12 orders of magnitude in CO pressure ( p CO =10 −8 –1000 mbar) demonstrated the reversible molecular adsorption of CO up to a pressure of about 10 mbar. For CO pressures above 10 mbar, the onset of a new irreversible dissociative CO adsorption pathway could be observed already at a substrate temperature of T s =300 K. CO dissociation was found to result in the formation of carbon on the surface as the only detectable dissociation product, indicating that CO dissociation occurs via the Boudouard reaction: 2CO→C(s)+CO 2 . Further experiments were performed to investigate catalytic oxidation of CO under intermediate reagent pressure conditions ( p tot =20 mbar). The latter experiments were performed under laminar flow conditions in a well-defined stagnation point flow geometry to allow for comparison with numerical reactive flow simulations based on a detailed Langmuir-Hinshelwood (LH) surface reaction mechanism for CO oxidation, including molecular adsorption and desorption of CO and dissociative adsorption of oxygen as well as the formation of CO 2 through reaction of the adsorbed CO and O species. The good agreement obtained between the results of the CO oxidation experiment and the simulation indicates that for CO and O 2 partial pressures as typically present in the exhaust gas of spark-ignited internal combustion engines, the Rh(111)-catalyzed CO oxidation can be quantitatively described in the framework of a mean-field approach employing an LH reaction scheme along with kinetics data derived in surface science studies.