CO OXIDATION ON PALLADIUM .1. A COMBINED KINETIC-INFRARED REFLECTION-ABSORPTION SPECTROSCOPIC STUDY OF PD(100)

The oxidation of carbon monoxide on a Pd(100) single-crystal catalyst was studied using an elevated-pressure IR cell/ultrahigh vacuum surfac e analysis system. The kinetics of the reaction was followed by monito ring the total pressure change during the course of the reaction. Simu ltaneously, the surface concentration of adsorbed CO was measured usin g IR reflection absorption spectroscopy. An activation energy of 29.4 +/- 0.3 kcal/mol was found between 500 and 575 K at a total pressure o f 1.50 Torr and CO/O2 = 2. The IR data obtained under reaction conditi ons show the CO coverage to vary between 0.45 and 0.55 monolayers (ML) . For this coverage regime, the CO heat of adsorption was determined t o be 29 +/- 2 kcal/mol, corresponding closely to the apparent activati on energy. Between 0.5 and 10.0 Torr, negative first-order CO and posi tive first-order oxygen pressure dependencies were measured, yielding a zero-order total pressure dependence of the reaction rate. No change in CO coverage was observed with a change in the oxygen pressure, whi le the CO coverage increased from 0.50 ML at P(CO) = 0.50 Torr to 0.55 ML at P(CO) = 8.0 Torr.