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Kinetic, isotopic, and infrared studies on well-defined dispersed Pt clusters are combined here with first-principle theoretical methods on model cluster surfaces to probe the mechanism and structural requirements for CO oxidation catalysis at conditions typical of its industrial practice. CO oxidation turnover rates and the dynamics and thermodynamics of adsorption-desorption processes on cluster surfaces saturated with chemisorbed CO were measured on 1-20 nm Pt clusters under conditions of strict kinetic control. Turnover rates are proportional to O(2) pressure and inversely proportional to CO pressure, consistent with kinetically relevant irreversible O(2) activation steps on vacant sites present within saturated CO monolayers. These conclusions are consistent with the lack of isotopic scrambling in C(16)O-(18)O(2)-(16)O(2) reactions, and with infrared bands for chemisorbed CO that did not change within a CO pressure range that strongly influenced CO oxidation turnover rates. Density functional theory estimates of rate and equilibrium constants show that the kinetically relevant O(2) activation steps involve direct O(2)* (or O(2)) reactions with CO* to form reactive O*-O-C*=O intermediates that decompose to form CO(2) and chemisorbed O*, instead of unassisted activation steps involving molecular adsorption and subsequent dissociation of O(2). These CO-assisted O(2) dissociation pathways avoid the higher barriers imposed by the spin-forbidden transitions required for unassisted O(2) dissociation on surfaces saturated with chemisorbed CO. Measured rate parameters for CO oxidation were independent of Pt cluster size; these parameters depend on the ratio of rate constants for O(2) reactions with CO* and CO adsorption equilibrium constants, which reflect the respective activation barriers and reaction enthalpies for these two steps. Infrared spectra during isotopic displacement and thermal desorption with (12)CO-(13)CO mixtures showed that the binding, dynamics, and thermodynamics of CO chemisorbed at saturation coverages do not depend on Pt cluster size in a range that strongly affects the coordination of Pt atoms exposed at cluster surfaces. These data and their theoretical and mechanistic interpretations indicate that the remarkable structure insensitivity observed for CO oxidation reactions reflects average CO binding properties that are essentially independent of cluster size. Theoretical estimates of rate and equilibrium constants for surface reactions and CO adsorption show that both parameters increase as the coordination of exposed Pt atoms decreases in Pt(201) cluster surfaces; such compensation dampens but does not eliminate coordination and cluster size effects on measured rate constants. The structural features and intrinsic non-uniformity of cluster surfaces weaken when CO forms saturated monolayers on such surfaces, apparently because surfaces and adsorbates restructure to balance CO surface binding and CO-CO interaction energies.

Density functional theory (DFT) and infrared spectroscopy results are combined with mechanism-based rate equations to assess the structure and thermodynamics of chemisorbed CO (CO") and its activation during Fischer-Tropsch synthesis (FTS). CO" binding becomes weaker with increasing coverage on Ru(0001) and Ru201 clusters, but such decreases in binding energy occur at higher coverages on Ru201 clusters than on Ru(0001) surfaces (CO"/Ru = 1.55 to 0.75); such differences appear to reflect weaker repulsive interactions on the curved surfaces prevalent on small Ru201 clusters. Ru201 clusters achieve stable supramonolayer coverages (CO"/Ru > 1) by forming geminal dicarbonyls at low-coordination corner/edge atoms. CO" infrared spectra on Ru/SiO2 (~7 nm diameter) detect mobile adlayers that anneal into denser structures at saturation. Mechanism-based FTS rate equations give activation energies that reflect the CO"-saturated surfaces prevalent during catalysis. DFT-derived barriers show that CO" predominantly reacts at (111) terraces via H-assisted reactions, consistent with measured effects of H2 and CO pressures and cluster size effects on rates and O-rejection selectivities. Barriers are much higher for unassisted CO" dissociation on (111) terraces and low-coordination atoms, including step-edge sites previously proposed as active sites for CO" dissociation during FTS. DFT-derived barriers indicate that unassisted CO" dissociation is irreversible, making such steps inconsistent with measured rates. The modest activation barriers of H-assisted CO" dissociation paths remove a requirement for special low-coordination sites for unassisted CO" activation, which is inconsistent with higher rates on larger clusters. These conclusions seem generally applicable to Co, Fe, and Ru catalysts, which show similar FTS rate equations and cluster size effects. This study also demonstrates the feasibility and relevance of DFT treatments on the curved and crowded cluster surfaces where catalysis occurs. [ABSTRACT FROM AUTHOR]