Chemical Engineering Seminar: “Prevalence of Bimolecular Reactions for Activating Bonds on Noble Metal Catalysts”
Catalysis will continue to shape the chemical and energy industries as we attempt to shift from petroleum-based feedstocks towards renewable resources. Understanding catalysis at the molecular level requires a combination of catalyst characterization, as well as kinetic, isotopic, and theoretical studies to elucidate reaction mechanisms and develop structure-function relationships. Any relevance of theoretical investigations of catalysts depends upon faithful models of the catalytic environment (e.g. metal particles, metal-support interfaces, co-adsorbed species, solvent) present during steady-state catalysis. Here, we examine the activations of strong chemical bonds found in O2, NO, and CO – necessary steps in oxidation chemistry (of CO and alkanols), reduction of toxic NO to environmentally-inert N2, and formation of large-chain hydrocarbons from CO-H2 mixtures (Fischer Tropsch synthesis, FTS). Direct dissociations of these species have large activation barriers on low-index surfaces of supported metal catalysts, which are prevalent on large (>3 nm) clusters. These kinetic hurdles can represent the noble nature of the metal catalyst (O2 activation on Au), high coverages of co-adsorbed intermediates (high coverages of chemisorbed NO on Pt during NO-H2 reactions), and/or the strength of the chemical bonds (CO activation on Ru or Co). Previous experimental and theoretical studies of single-crystal surfaces have demonstrated that defect sites, which are under-coordinated metal atoms, significantly lower direct dissociation barriers on nearly bare surfaces. We show, however, that such sites are often blocked by strongly-bound adsorbates (e.g. chemisorbed CO) and on crowded surfaces, bonds instead activate via bimolecular reactions that weaken the bond prior to cleavage. These case studies demonstrate the necessity of understanding the nature of the catalyst through kinetic and spectroscopic studies in order to model reactions at conditions relevant to practical catalysis—by considering the impacts of co-adsorbed species (which may be present at near saturation coverages), particle morphology (to assess any contributions of defect sites), and solvent (when present). These studies also offer valuable and transferable insights into chemistry on metal surfaces, which can ultimately lead to the development of new materials for the conversion of biomass and natural gas derived feedstocks into value-added fuels and chemicals.