Reaction Processes on Catalytically Active Surfaces

Abstract

In this article results of studies are reported in which in-situ infrared-visible sum-frequency generation (IR-VIS SFG) surface vibrational spectroscopy was combined with density functional theory (DFT) ab initio computations to investigate the adsorption of carbon monoxide (CO) on rhodium (Rh) catalyst surfaces at elevated substrate temperatures (Ts ≥ 300K) over a wide pressure-range (pco = 10−8−1000 mbar). The experimental studies demonstrated the reversible molecular adsorption of CO up to a pressure of ca. 10 mbar. For higher CO pressures, however, the onset of a new irreversible dissociative CO adsorption pathway could be observed already at a substrate temperature of Ts=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(ad) + CO2. To rationalize the experimental findings DFT studies were performed the results of which suggest the possibility of a novel low-temperature/high-pressure CO/Rh(111) dissociation pathway where a gas-phase CO(g) molecule reacts with a CO molecule chemisorbed on the catalyst surface, CO(ad), to yield gas-phase CO2 (g) and surface carbon C(ad) through an Eley-Rideal mechanism.