No abstract

This work describes a molecular-level investigation of strong metal-support interactions (SMSI) in Pt/TiO(2) 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 TiO(2) on a Pt/TiO(2) catalyst controls the product distribution of furfuraldehyde hydrogenation by an acid-base mechanism. Pt nanoparticles supported on TiO(2) and SiO(2) 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 TiO(2), while no change is observed for the kinetics of Pt supported on SiO(2). SFG vibrational spectroscopy reveals that a furfuryl-oxy intermediate forms on TiO(2) 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/TiO(2) interface is required primarily for H spillover. Density functional calculations predict that O-vacancies on the TiO(2) 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.

Ultrafast soft X-ray reflection-absorption spectroscopy as a probe for following surface charge carrier dynamics in catalytically-relevant functional materials.

The ability to observe charge localization in photocatalytic materials on the ultrafast time scale promises to reveal important correlations between excited state electronic structure and photochemical energy conversion. Of particular interest is the ability to determine hole localization in the hybridized valence band of transition metal oxide semiconductors. Using femtosecond extreme ultraviolet reflection absorption (XUV-RA) spectroscopy we directly observe the formation of photoexcited electrons and holes in FeO, CoO, and NiO occurring within the 100 fs instrument response. In each material, holes localize to the O 2p valence band states as probed at the O L-edge, while electrons localize to metal 3d conduction band states on this same time scale as probed at the metal M-edge. Chemical shifts at the O L-edge enable unambiguous comparison of metal-oxygen (M-O) bond covalency. Pump flux dependent measurements show that the exciton radius is on the order of a single M-O bond length, revealing a highly localized nature of exciton in each metal oxide studied.