Catalytic oxidation of carbon monoxide on palladium: Effect of pressure

The turnover rate for CO oxidation on palladium increases by a factor of 7.4, at 445 K and an equimolar ratio of reactants, when the total pressure increases from 10{dollar}sp{lcub}-6{rcub}{dollar} to 10{dollar}sp2{dollar} mbar. This effect of pressure is contrary to that expected from previous kinetic studies that predict no effect of pressure on rate, with one exception. Additionally, the apparent activation energy changes from ca. 112 kJ mol{dollar}sp{lcub}-1{rcub}{dollar} at high pressure (10{dollar}sp2{dollar} mbar) to ca. 75 kJ mol{dollar}sp{lcub}-1{rcub}{dollar} at lower pressures (10{dollar}sp{lcub}-1{rcub}{dollar} and 10{dollar}sp{lcub}-6{rcub}{dollar} mbar), suggesting a change in reaction mechanism with pressure.; This effect of pressure is not expected from a uniform surface, but can be accounted for if the surface is assumed to be non-uniform. This is due to the surface coverage by CO changing from low to very high over a pressure range of eight orders of magnitude. In this unusual situation, the non-uniformity of the surface reveals itself. A commonly accepted model of a non-uniform surface based on observed kinetic and thermodynamic behavior, explains simply the effect of total pressure on both turnover rate and surface coverage by CO. At low pressure the steady state surface coverage by CO is determined by equal rates of adsorption of CO and reactive desorption of CO.