Modelling Hydrogen Spillover on Reducible Metal Oxides using Transition Metal Hydrides as Well-Defined H▪ Donors

• Applicant: Professor Dr. Deven Estes
• Subject Area: Technical Chemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 533915215

Hydrogen Spillover, the transfer of protons and electrons from surfaces that activate H2 (typically metal nanoparticles) to ones that do not activate hydrogen (often support materials such as metal oxides or carbon), is a well-known phenomenon in heterogeneous catalysis that is known to play roles in many catalytic reactions. One such reaction, M/EO3 (M = Mo, W) and M/Nb2O5 catalyzed hydrodeoxygenation (HDO) of aromatic alcohols such as cresol, anisole, and phenol, is potentially important for processing lignin-derived biomass for use as a chemical feedstock. Hydrogen spillover is thought to increase the selectivity for the more valuable aromatic products from HDO by producing reduced metal active sites that hydrogenolyze the C-O bond but do not hydrogenate the aromatic rings. However, very little is known about the thermodynamics, kinetics, mechanism, and role of hydrogen spillover in catalysis, due to the ill-defined nature of these supported metal catalysts. In particular, the metal nanoparticles from which hydrogen is transferred consist of many different metal surfaces and defect sites with widely varying properties. In contrast, the well-defined nature of molecular metal complexes has allowed researchers to elucidate the mechanisms of proton-electron transfer (PET) reactions such that the thermodynamics and kinetics can now in many cases be predicted. In this proposal, we will replace these ill-defined metal nanoparticles with well-defined transition metal hydrides such as HV(CO)4dppe (1), CpCr(CO)3H (3), and HCo(CO)4 in order to measure the thermodynamics and kinetics of hydrogen spillover on MoO3, WO3, and Nb2O5 and understand the role of hydrogen spillover in the HDO of the lignin model compounds cresol, anisole, and phenol. Analyzing the reaction products of the metal hydrides with metal oxides for spectroscopic signatures of hydrogen spillover (including EPR, UV-Vis, and XAS) will be used to identify spillover reactivity. Titrating these metal oxides with metal hydride complexes will allow us to measure the thermodynamics of hydrogen spillover, while understanding of the mechanism of hydrogen spillover for these metal oxides will come through kinetic measurements (using IR and NMR spectroscopy). Finally, we can use the measurements above to predict the extent of hydrogen spillover for different combinations of metal nanoparticles and metal oxides and understand how this affects the activity and selectivity of HDO of the lignin model compounds. This will give us insight into both how hydrogen spillover works and its role in this catalytic reaction.

• DFG Programme: Research Grants