Carbon Monoxide Oxidation under Transient Conditions: A Fourier-Transform Infrared Transmission Spectroscopy Study

In contrast to the apparent simplicity of the overall reaction, oxidation of CO over Pt/A203 is known to present complex behavior. Self-sustained oscillations, hysteresis loops in conversion, and enormous rate enhancements under forced feed concentration cycling have been observed. In this study the dynamic behavior of the IR-active surface CO species was followed with a fast-scan Fourier-Transform Infrared (FTIR) during transient response and periodic operation. Depending on the conditions, up to four linearly adsorbed CO bands at 2123,2092,2073, and 2061 cm-*, and a bridge-bonded CO band at 1850 cm-i were observed. The time-averaged and transient intensities of the bands were found to be significantly different during periodic operation as compared to steady state. Linearly adsorbed CO on metallic platinum (2073 cm-‘) was found to have the fastest response to changes in the gas-phase concentration. It was also observed that CO adsorbed on platinum oxide is very stable and does not take part in the reaction to any appreciable extent. CO, production rates during periodic operation were found to correlate extremely well with the integrated total absorbance of the 2092-cm-r band, assigned to CO linearly adsorbed on a platinum atom sharing an oxygen with a neighbor. Based on the experimental observations a detailed mechanism of CO oxidation on platinum is proposed. The model can successfully explain the enormous rate enhancements observed during periodic operation. 8 198s Academic PESS, IK.