Correlating Heat of Adsorption of CO to Reaction Selectivity: Geometric Effects vs Electronic Effects in Neopentane Isomerization over Pt and Pd Catalysts

Silica-supported Pt and Pd nanoparticles from 1 to 10 nm in diameter were evaluated for neopentane conversion (hydrogenolysis and isomerization). Characterization of the catalysts was conducted utilizing scanning transmission electron microscopy (STEM), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO, X-ray absorption spectroscopy (XAS), and isothermal calorimetry of CO adsorption to determine how geometric or electronic structure effects can explain changes in reactivity. Isomerization selectivity of Pt was much higher than Pd for all particle sizes. There is a pronounced effect of particle size on selectivity, with the highest isomerization selectivity achieved over catalysts containing the largest particle size for both Pt (57%) and Pd (26%) catalysts. For both Pd and Pt catalysts, DRIFTS showed a decrease in the ratio of linear-to-bridge bonded CO with particle size, while isothermal calorimetry of CO adsorption shows that both Pt and Pd enthalpies of adsorption decrease with increasing particle size. The isomerization selectivity was found to correlate inversely with the strength of CO adsorption for all catalysts suggesting that the chemisorption energy and not the particle size, coordination geometry, or ensemble size is the most important factor for increasing the isomerization selectivity.