(205b) From Clusters to Single Atoms: Site Requirements for Carbon Monoxide Oxidation on Transition Metal and Metal Oxide Catalysts

CO oxidation is a simple reaction that occurs effectively on transition metal and metal oxide with practical significance for exhaust emission control in both the stationary and mobile sources. This reaction, however, may occur via multiple pathways and involve various reactive oxygen species, depending on the active site structures, the binding of adsorbates, and the presence of other species (e.g., H₂O), the latter may act as a co-catalyst. Here, we probe the mechanistic requirements for CO oxidation on the variety of metal (Group VIII and Ib) and metal oxide clusters (Group VIIb) and provide a generalized mechanistic picture that describes this reaction on crowded metal cluster surfaces covered with chemisorbed CO, crowded metal surfaces covered with chemisorbed oxygen adatoms, to single atom active centers. We capture these effects of coverages on the catalytic pathways and their kinetic manifestation and connect them to changes in the catalytic relevance of the elementary step. On Group VIII metal particles with a larger difference between the heat of CO relative to that of oxygen adsorption (e.g., Pt), CO adsorbs significantly more strongly than oxygen adatoms, occupying sites and thus retarding the overall reaction rates. Switching from Group VIII metal to one that has a lower heat of adsorption difference in CO to O adsorption such as Ag leads to a change in the most abundant surface intermediates from CO* to O* and thus to a transition in the rate dependence. On such surfaces, the relative coverages of O*-to-CO* are set by the operating [O₂]-to-[CO]² ratio. The coverage effects are eliminated, when metal-metal coordination decreases to zero for isolated, single atoms. On these single atom sites, the lattice cations and anions on the oxides involve in CO oxidation catalysis. Irrespective of the identity of the active site structures found on particles or single atoms, the identity of the kinetically relevant step remains the same. This step involves the kinetically relevant reactions between adsorbed molecular O₂* and CO*. Described within this talk is the method to alter the site environment and its effects on CO oxidation kinetics.