(544dg) A Fundamental Understanding of the Surface and Catalytic Chemistry of Transition Metal Ceramics in Deoxygenation
Transition metal ceramics -- oxides, carbides, nitrides, sulfides, phosphides, and selenides -- present interesting and unique surface chemistry that is of catalytic interest in many contemporary reactions. These materials can exhibit surface chemistry similar to noble metals or wholly new reactivity that can provide new avenues for chemical transformations. The aim of these studies is to understand how to promote the scission of a recalcitrant CâO bond and simultaneously preserve unsaturation or aromaticity in products via a systematic investigation of the surface and catalytic chemistry of transition metal ceramic catalysts. Utilizing deoxygenation reactions as a probe (i.e. guaiacol and phenol), we have investigated the innate surface chemistry of 1st row transition metal phosphides and Ti and Ni ceramics in promoting recalcitrant phenolic CâO bond cleavage and limiting the unselective activation and hydrogenation of C=C bonds for woody biomass upgrading applications. Reaction mechanism analysis showed that dramatically enhanced surface reactivity towards oxygen was responsible for driving C--O cleavage. Reduced surface chemical reactivity towards carbon and limited hydrogenation activity were also found to be critical in limiting C=C activation in the aromatic ring and unselective overhydrogenation. The more covalent bonding within the phosphides inhibited a correlation between kinetics and thermodynamics of the C--O cleavage step due to the energetics associated with electron density transfer to or from the catalyst surface. However, the need for greatly elevated surface reactivity towards oxygen remained clear. The kinetic barriers in the hydrogenation step tracked well with surface reactivity markers and d-band center of the phosphides. The less metallic electronic structure also contributed to electronically different hydrogen bonding to the surface and limited kinetics for hydrogenation that may favorably reduce unselective C=C hydrogenation. Surface reaction site nature also tracked with bonding within the ceramics and their metal-to-nonmetal ratio suggesting strong electronic effects in the manipulation of the metal reaction sites and the role of nonmetal sites in the reaction mechanism.