Alloyed Molybdenum Enables Efficient Alcohol Hydrodeoxygenation with Supported Bimetallic Catalysts

Bimetallic heterogeneous catalysts combining group 9 metals (Rh, Ir) or group 10 metals (Ni, Pd, Pt) with Mo on a silica-based support have been synthesized via surface organometallic chemistry and assessed in their catalytic activity for the hydrodeoxygenation (HDO) of alcohols with particular emphasis on the structural evolution of the catalysts and the role of Mo. The investigation was conducted with an air-free approach to isolate any sample alterations exclusively to those caused by the reaction. Structural analysis was performed using a combination of (S)TEM, IR, and XAS. It was found that Ir-Mo/SiO2, Rh-Mo/SiO2, and Pt-Mo/SiO2 display high activity for primary, secondary, and tertiary alcohol deoxygenation, while Pd-Mo/SiO2 only effectively catalyses tertiary alcohol deoxygenation. All other combinations as well as the corresponding monometallic materials do not display the same activity. Using X-ray absorption spectroscopy, the chemical states of M (M = Ni, Rh, Pd, Ir, or Pt) and Mo in the catalysts could be elucidated both before and after the HDO reaction, revealing M to be in metallic state. Mo K-edge X-ray absorption near-edge structure (XANES) spectra showed variations in the amount of Mo(0), Mo(IV) and Mo(VI) depending on the metal counterpart in the fresh catalysts, while complete conversion of Mo(VI) to lower oxidation states was demonstrated in the spent catalysts. For Rh, Pd, Ir, and Pt alloyed nanoparticle formation (M-Mo) was identified by the presence of M-Mo paths, confirmed via combined extended X-ray absorption fine structure (EXAFS) fits of the Mo K-edge and the corresponding M K- or L3-edge. Alloying is further supported by changes in the electronic structure of the nanoparticles supported on SiO2, as revealed by changes in CO-adsorbed IR spectroscopy. On the other hand, Ni-Mo/SiO2 did not exhibit an alloyed structure that was stable under reaction conditions, paralleling the ob-served inactivity for HDO. These findings provide new insights into the structure-activity relationship of Mo-based bimetal-lic catalysts, highlighting the critical role of metal alloying and oxidation state changes in selective alcohol deoxygenation, where reduced Mo species are key to efficiently activate C–O bonds.