64: Organic ligand-mediated surface modification of Au/TiO2 and its impact on H2 adsorption properties

Abstract: Catalytic H2 activation is a key step in multiple chemical transformations for energy utilization and storage, including hydrogenation, dehydrogenation, hydrodesulfurization, methanol synthesis, and the water gas shift reaction. Most well-known hydrogenation catalysts (e.g., Pd, Pt, Ir) exhibit strong H2 adsorption activity through dissociative chemisorption. Hydrogen adsorption on extended Au surfaces is thermodynamically unfavorable, so the fundamentals of H2 activation on supported Au catalysts have received less attention and are poorly understood. On Au/TiO2, H2 adsorbs weakly at the metal-support interface (MSI), yielding H-atom equivalents on the Au and support. This is followed by rapid H-atom transfer to the support, resulting in two weakly coupled proton-electron pairs on TiO2. The electrons injected into the support surface conduction band gives rise to an observable IR signal, provides a rare opportunity to probe the kinetics and thermodynamics of H2 adsorption and activation using in-situ FTIR and chemisorption. We report here the electronic/steric effects induced by modifying Au/TiO2 with PPh3 ligands on the kinetics and thermodynamics of H2 adsorption. PPh3 was deposited onto the catalyst using a solution adsorption method, monitored using UV-Vis absorbance spectroscopy. The UV-Vis absorbance and 31P NMR data suggest that PPh3 is predominantly absorbed on TiO2. Kinetic measurements revealed a sharp drop in H2 activation rates with PPh3, without affecting the activation parameters. Thermodynamic analysis indicated that both the adsorption equilibrium constant (Kp) and the total amount of H2 adsorbed decreased with PPh3. A van’t Hoff analysis of the same revealed an increase in ΔHapp from -1 to 12 kJ/molH, and ΔSapp increased from 15 to 40 kJ/molH. This can be attributed to PPh3 displacing strongly adsorbed water at Ti+4 Lewis acidic sites, thus perturbing the proton-exchange equilibria on the TiO2 surface. As a result, the overall surface entropy of TiO2 changes, which is reflected in the calculated ΔHapp and ΔSapp values. We also investigated how co-adsorbates, such as carbonates, affect H2 adsorption properties. These results provide valuable insights into the mechanistic details of H2 adsorption and spillover on Au/TiO2, which can be leveraged to engineer catalysts for enhanced H2 adsorption activity by tailoring the electronic and support environment of Au/TiO2.