Trends in the Surface and Catalytic Chemistry of Transition-Metal Ceramics in the Deoxygenation of a Woody Biomass Pyrolysis Model Compound

Abstract

The general surface chemical reactivity, surface reaction site nature, and van der Waals dispersion interaction capability of 10 transition metal ceramics were investigated in the catalytic reaction of guaiacol deoxygenation—a model compound for aromatics in woody biomass pyrolysis oil. A computational surface science approach has been applied to investigate Ti and Ni oxide, carbide, nitride, sulfide, and phosphide to ascertain the effect of element selection on surface and catalytic chemistry. The results indicated that systematic trends in surface chemistry are present in the transition-metal ceramics and that transition-metal phosphides present special balanced reactivity toward O, C, and H that results in their appreciable catalytic activity in deoxygenation reactions. The remaining ceramics were found to exhibit either too low or too high of reactivity toward oxygen, carbon, or hydrogen, which resulted in insurmountable thermodynamic and kinetic barriers for C–O bond cleavage or hydrogenation and the presence of surface poisons that could not be effectively removed. The surface chemical properties that allow for improved production of olefins and aromatic molecules in deoxygenation reactions over ceramic catalysts have been isolated as an electronic effect that limits carbon–surface bond formation, reduces C═C activation, and dramatically inhibits van der Waals dispersion interactions. These three effects greatly limit the unselective activation of unsaturated products circumventing overhydrogenation and hydrogen waste. Moderate systematic trends were discovered with respect to the bonding within the solid and the nature of the surface reactivity and chemical composition of the active surface reaction sites. Metal-rich Ni ceramics exhibited selectively hybridized bulk electronic structures that lead to Ni-like surface reactivity. More extensively hybridized electronic structure of the Ti ceramics led to an electronic effect that favored the enhanced reactivity of the p-block elements. Over a large number of ceramics, the p-block element played a critical, if not dominant, role in the surface chemistry.