The kinetics of CO dissociation on Ru(001): Time-resolved vibrational spectroscopy at elevated pressures

Abstract

Time-resolved Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) has been utilized to measure the kinetics of CO dissociation on a Ru(001) surface at elevated pressures (10−3 to 10 Torr) and temperatures (500–700 K). The reaction of CO with Ru(001) is found to be a nonsteady state and results in CO disproportionation, i.e., 2CO→C+CO2. The decrease in total CO coverage follows first order kinetics and exhibits Arrhenius behavior with an activation energy of 20.6 kcal and a preexponential factor of 102 s−1. Comparison of the overall reaction rate with that of CO2 formation (O+CO→CO2) confirms that CO dissociation is the rate-limiting step in the disproportionation reaction. The in situ reaction rate constant exhibits a weak dependence on CO pressure (<first order). However, the determination of local CO coverages during reaction reveals a linear dependence of the dissociation rate with CO coverage. This confirms that the chemisorbed state of the molecule is a precursor to dissociation and that a high pressure is required to maintain a steady state surface coverage of CO at reaction temperature. In situ vibrational spectra demonstrate the formation of carbon islands under reaction conditions which prevent further CO adsorption and result in a decrease in total CO coverage at constant local CO coverage. Post-reaction spectroscopy confirms the formation of two-dimensional islands of carbon whose reactivity toward oxidation is found to be between that of amorphous carbon and three-dimensional graphite.