Steady-state oxidation of carbon monoxide over supported noble metals with particular reference to platinum

Activation energies, kinetic orders, and relative activities have been determined for the oxidation of CO by O₂ over five supported noble metals. With ruthenium drifts in reaction rate occurred for many hours following pressure and temperature changes but the initial responses were qualitatively similar to the simple behavior found for iridium, rhodium, and palladium (above 390 K), namely, kinetic orders near -1 and +1 in CO and O₂, respectively, and an activation energy near 100 kJ mol^-1. The platinum-catalyzed reaction had a much lower activation energy (56 kJ mol^-1), and the kinetic order was only slightly negative in CO with a possible tendency for these findings to change toward those found for the other metals at temperatures above 480 K. These results were compared with the predictions of a model which used parameters derived from measurements made under uhv conditions and/or with only one reactant present in the gas phase. The two reactions indicated were those oxygen molecularly adsorbed on a near complete carbon monoxide layer and between adsorbed oxygen atoms and gas-phase carbon monoxide molecules. The kinetic results which were common to each metal conformed to that expected for this latter reaction with the rate limited by CO desorption. Under these conditions the order of metal activity should be opposite to the order of heats of adsorption of carbon monoxide, and from the measurements available such a relationship did seem to hold.