Compositional Dependence of Hydrodeoxygenation Pathway Selectivity for Ni2-xRhxP Nanoparticle Catalysts

Transition-metal phosphides (TMPs) are promising materials for biomass conversion processes, where their metal-acid bifunctionality provides active sites for the necessary variety of catalytic reactions (e.g., hydrogenation, hydrogenolysis, decarbonylation, and dehydration). Aiming to understand the catalytic performance of ternary TMP catalysts for the hydrodeoxygenation (HDO) reaction of biomass oxygenates, a synthetic protocol was developed herein to prepare a series of ternary Ni2-xRhxP nanoparticles (NPs), incorporating Rh into a parent Ni2P template. This solution synthesis method allowed for a series of NPs to be prepared having precisely controlled compositions with similar morphology and crystalline structure. Detailed characterization of this series of Ni2-xRhxP (x = 1) NPs revealed that Rh substituted into the parent hexagonal crystal lattice of Ni2P with concomitant expansion of the lattice. The influence of composition on the catalytic performance of silica-supported Ni2-xRhxP (x = 0 to 0.8, and cubic Rh2P) NPs was investigated through the HDO reaction of m-cresol. Whereas Rh2P was more selective for direct deoxygenation relative to Ni2P, increasing concentrations of Rh in Ni2-xRhxP resulted in a decreased selectivity to direct deoxygenation products (i.e., toluene, benzene, xylene), and an associated increased selectivity to hydrogenation products (i.e., methylcyclohexene). Through in situ high energy X-ray diffraction and density functional theory modeling, we identified OH* adsorption energy and surface-P sp-band center as effective descriptors for the observed shift in selectivities across this series of TMPs. Further, the calculated electronic-structure changes were found to exert greater influence over the observed product selectivity than the subtle geometric changes associated with lattice expansion. Identification of this structure-function relationship demonstrates that the controlled synthesis of TMPs enables an understanding of composition-dependent selectivity for the HDO reaction of phenolic molecules and this approach could be extended to other ternary TMP compositions for diverse catalytic applications.