THE REACTIVITY OF HIGH OXYGEN COVERAGES ON PD(110) IN CATALYTIC CO OXIDATION

The catalytic oxidation of CO on a Pd(110) surface has been investigated in CO titration experiments and under steady-state conditions in the 10(-4) Torr range. The reaction was followed by means of video-LEED, work function and rate measurements. The titration experiments with the oxygen-saturated Pd(110) surface revealed a change in the reaction mechanism from a uniform reduction of the oxygen coverage at high temperature (T almost-equal-to 500 K) to a hole-eating mechanism at low temperature (T < 400 K). High oxygen coverages were found to have an inhibiting effect on the rate of CO2 Production. This effect, which presumably is associated with the formation of subsurface oxygen, can be attributed to a lowering of the CO residence time caused by a decrease in the adsorption energy of CO and to a reduction of the CO sticking coefficient. Under steady-state conditions this inhibiting effect leads to the appearance of a hysteresis in the high rate branch of the kinetics of catalytic CO oxidation upon cyclic variation of the CO pressure.