Examination of Property-Reactivity Relationships of Early Transition Metal Carbides and Nitrides as Catalysts and Catalytic Supports

The search for fuel sources that provide alternatives to crude oil has been a continuing priority. The research described investigates material property-reactivity relationships that influence the catalytic performance of early transition-metal carbides and nitrides for reactions involved in the conversion of biomass to liquid transportation fuels, namely selective hydrogenation of oxygenates, water-gas shift, and Fischer-Tropsch synthesis. In particular, this work focuses on understanding the role of three main physical properties on catalyst chemistry, composition, structure, and catalytic reactivity: hydrogen adsorption sites; surface redox chemistry; and strong metal-catalytic support interactions. Experimental and computational results indicate that molybdenum nitride contains hydrogen adsorption sites both on the surface (NH) and in the subsurface layers (MoH). The relative density of surface versus subsurface hydrogen was found to have a direct effect on the hydrogenation of crotonaldehyde, with materials consisting of 95% subsurface hydrogen yielding ~93% selectivity to crotyl alcohol (C=O hydrogenation). Another study compared the synthesis, structural and compositional properties, and water gas shift activities of catalysts produced by depositing platinum onto unpassivated and passivated molybdenum carbide. Passivation (controlled surface oxidation) had a profound effect on the character of interactions between the Pt and support resulting in deleterious effects on Pt loadings, structures, and water gas shift rates. In particular, the unpassivated support adsorbed 3 times as much Pt and exhibited water-gas shift turnover frequencies nearly 4 times higher than those for the passivated support. These differences were determined to result from redox chemistry occurring between molybdenum carbide and the depositing Pt. Lastly, the presence of strong interactions, facilitated at high temperatures, between a molybdenum carbide support and deposited early transition metals was probed using Fischer-Tropsch synthesis. Ruthenium and cobalt were found to be inactive when supported on molybdenum carbide but not on silica, a typical Fischer-Tropsch support. By avoiding high temperature treatments, addition of ruthenium to molybdenum carbide resulted in turnover frequencies 18 times higher than those of bare molybdenum carbide alone and 4 times that of ruthenium on silicon dioxide. These relationships between material properties and reactivity will help inform the design and synthesis for specific applications of these catalysts.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135765/1/wyvrattb_1.pd