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.

The identities of photoexcited states in thin-film Co 3 O 4 and the ultrafast carrier relaxation dynamics of Co 3 O 4 are investigated with oxidation-state-specific pump−probe femtosecond core level spectroscopy. A thin-film sample is excited near the 2.8 eV optical absorption peak, and the resulting spectral changes at the 58.9 eV M 2,3edge of cobalt are probed in transient absorption with femtosecond highorder harmonic pulses generated by a Ti/sapphire laser. The initial transient state shows a significant 2 eV redshift in the absorption edge compared to the static ground state, which indicates a reduction of the cobalt valence charge. This is confirmed by a charge transfer multiplet spectral simulation, which finds the experimentally observed extreme ultraviolet (XUV) spectrum matches the specific O 2− (2p) → Co 3+ (e g ) charge-transfer transition, out of six possible excitation pathways involving Co 3+ and Co 2+ in the mixed-valence material. The initial transient state has a power-dependent amplitude decay (190 ± 10 fs at 13.2 mJ/cm 2 ) together with a slight redshift in spectral shape (535 ± 33 fs), which are ascribed to hot carrier relaxation to the band edge. The faster amplitude decay is possibly due to a decrease of charge carrier density via an Auger mechanism, as the decay rate increases when more excitation fluence is used. This study takes advantage of the oxidation-state-specificity of time-resolved XUV spectroscopy, further establishing the method as a new approach to measure ultrafast charge carrier dynamics in condensed-phase systems.

The hydrogenation of crotonaldehyde in the presence of supported platinum nanoparticles was used to determine how the interaction between the metal particles and their support can control catalytic performance. Using gas-phase catalytic reaction studies and in situ sum-frequency generation vibrational spectroscopy (SFG) to study Pt/TiO2 and Pt/SiO2 catalysts, a unique reaction pathway was identified for Pt/TiO2, which selectively produces alcohol products. The catalytic and spectroscopic data obtained for the Pt/SiO2 catalyst shows that SiO2 has no active role in this reaction. SFG spectra obtained for the Pt/TiO2 catalyst indicate the presence of a crotyl-oxy surface intermediate. By adsorption through the aldehyde oxygen atom to an O-vacancy site on the TiO2 surface, the C=O bond of crotonaldehyde is activated, by charge transfer, for hydrogenation. This intermediate reacts with spillover H provided by the Pt to produce crotyl alcohol.

No abstract

True n-type doping of titanium oxide without formation of midgap states would expand the use of metal oxides for charge-based devices. We demonstrate that plasma-assisted fluorine insertion passivates defect states and that fluorine acts as an n-type donor in titanium oxide. This enabled us to modify the Fermi level and transport properties of titanium oxide outside the limits of O vacancy doping. The origin of the electronic structure modification is explained by ab initio calculation.