Furfuraldehyde Hydrogenation on Titanium Oxide-Supported Platinum Nanoparticles Studied by Sum Frequency Generation Vibrational Spectroscopy: Acid-Base Catalysis Explains the Molecular Origin of Strong Metal-Support Interactions

This work describes a molecular-level investigation of strong metal-support interactions (SMSI) in Pt/TiO2 catalysts using sum frequency generation (SFG) vibrational spectroscopy. This is the first time that SFG has been used to probe the highly selective oxide-metal interface during catalytic reaction, and the results demonstrate that charge transfer from TiO2 on a Pt/TiO2 catalyst controls the product distribution of furfuraldehyde hydrogenation by an acid-base mechanism. Pt nanoparticles supported on TiO2 and SiO2 are used as catalysts for furfuraldehyde hydrogenation. As synthesized, the Pt nanoparticles are encapsulated in a layer of poly(vinylpyrrolidone) (PVP). The presence of PVP prevents interaction of the Pt nanoparticles with their support, so identical turnover rates and reaction selectivity is observed regardless of the supporting oxide. However, removal of the PVP with UV light results in a 50-fold enhancement in the formation of furfuryl alcohol by Pt supported on TiO2, while no change is observed for the kinetics of Pt supported on SiO2. SFG vibrational spectroscopy reveals that a furfuryl-oxy intermediate forms on TiO2 as a result of a charge transfer interaction. This furfuryl-oxy intermediate is a highly active and selective precursor to furfuryl alcohol, and spectral analysis shows that the Pt/TiO2 interface is required primarily for H spillover. Density functional calculations predict that O-vacancies on the TiO2 surface activate the formation of the furfuryl-oxy intermediate via an electron transfer to furfuraldehyde, drawing a strong analogy between SMSI and acid-base catalysis.