Chemisorption of CO and Mechanism of CO Oxidation on Supported Platinum Nanoclusters

Abstract

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 O₂ pressure and inversely proportional to CO pressure, consistent with kinetically relevant irreversible O₂ activation steps on vacant sites present within saturated CO monolayers. These conclusions are consistent with the lack of isotopic scrambling in C¹⁶O−¹⁸O₂−¹⁶O₂ 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 O₂ activation steps involve direct O₂* (or O₂) reactions with CO* to form reactive O*−O−C*═O intermediates that decompose to form CO₂ and chemisorbed O*, instead of unassisted activation steps involving molecular adsorption and subsequent dissociation of O₂. These CO-assisted O₂ dissociation pathways avoid the higher barriers imposed by the spin-forbidden transitions required for unassisted O₂ 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 O₂ 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 ¹²CO−¹³CO 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 Pt₂₀₁ 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.