A revisit to carbon monoxide oxidation on Pd(111) surfaces

Carbon monoxide (CO) oxidation on Pd(111) surfaces has been studied by molecular beam methods with mixed molecular beams (CO + O2) between 400 and 900 K and a CO:O2 ratio of 7:1 to 1:10. A new aspect of the above reaction observed in the transient kinetics regime is the evidence for oxygen diffusion into Pd(111) subsurface layers and its significant influence toward CO oxidation at high temperatures (≥600 K). An overall influence of subsurface oxygen on the kinetics of the CO oxidation reaction is addressed. Interesting information derived from the above studies is the necessity to fill up the subsurface layers with oxygen atoms to a threshold coverage (θOsub), above which the reactive CO adsorption occurs on the surface with subsequent CO2 production. The above observation was demonstrated with CO-rich reactant compositions (CO + O2) above 600 K via instant oxygen adsorption on Pd surfaces; however, onset of CO adsorption as well as CO2 production occurs after a time delay. θOsub and the time delay in CO...