(378e) Hydrogen Spillover and Its Relation to Hydrogenation Catalysis

Hydrogen spillover entailing the transfer of H atoms from metal sites onto the catalyst support is of tremendous fundamental and technological interest for fields ranging from heterogeneous hydrogenation catalysis to hydrogen storage. Insights into the structural requirements, the kinetics of this process, the structure of the materials after hydrogen spillover as well as the contribution of spilled over H to catalysis are vastly lacking. Here, we provide answers to those questions with the development and investigation of molecularly defined solubilized polyoxometalate-supported single-atom catalysts demonstrating hydrogen spillover. Due to its solubility and molecular definedness, the single-atom catalyst after hydrogen spillover can be investigated by means of liquid phase characterization techniques and DFT calculations. This illuminates the stoichiometry of intermediate and final spillover species, as well as the electronic state of catalyst and the spilled over H. Hydrogenation kinetics for compounds with different reducible functional groups reveal the nature and the magnitude of the contribution of the spilled over H. By comparing hydrogenation rates of nitrobenzene, acetophenone, and vinyl acetate, a correlation between the nature of the reducible bond and the contribution of spilled over H becomes apparent. Poisoning the metallic site with benzyl mercaptan demonstrates that while the hydrogenation activity is immediately reduced to zero for C=C bonds, the hydrogenation activity for polar functional groups prevails. Not only is spilled over H essentially inactive for the C=C bond hydrogenation but initial turnover frequencies are lower when the catalyst underwent a larger extent of H spillover. Electrospray ionization mass spectrometry showed that the contribution of spilled over H either consisted of the formation of oxygen vacancies or the direct transfer of spilled over H. We further demonstrate that conclusions drawn about the hydrogen spillover on our polyoxometalate-supported single-atom catalysts are more generally applicable for example for the prototypical Pt/WO₃ hydrogen spillover catalyst.