Infrared absorption spectroscopic study of the CO/O2 reaction over a Pd/SiO2 catalyst

Abstract

Infrared spectroscopy was used to monitor the extent of CO chemisorption of a Pd/SiO₂ catalyst under flowing reaction conditions involving CO and O₂ as reagents. Different ratios of CO:O₂ were used with pressure (in Torr) 2:2, 20:20, 30:30, 2:20, 20:2, 10:5 and 21:16. Spectra were investigated between room temperature and 100 or 200 °C. At room temperature in all cases the surface was largely covered with CO. On raising the temperature the adsorbed CO had mainly disappeared at 125 and 75 °C, respectively, for the 2:2 and 2:20 CO:O₂ mixtures. The results for high coverage of CO correlate with a reaction regime with the rate equation depending on p_O2p^–1_CO. For high pressures of CO in the reacting mixture strong absorption bands from adsorbed species persisted up to 150–200 °C. For the 30:30 mixture an abrupt and complete removal of the CO bands occurred above 150 °C and at the same time the reaction became self-heating. In this regime the rate is known to depend upon p_CO and to be independent of p_O2. It is considered to relate to a constant coverage of the catalyst by chemisorbed oxygen.