Charge Transfer and Support Effects in Heterogenous Catalysis

The kinetic, electronic and spectroscopic properties of two-dimensional oxide-supported/ncatalysts were investigated in order to understand the role of charge transfer in catalysis./nPt/TiO2 nanodiodes were fabricated and used as catalysts for hydrogen oxidation. During the reaction, the current through the diode, as well as its I-V curve, were monitored, while gas chromatography was used to measure the reaction rate. The current and the turnover rate were found to have the same temperature dependence, indicating that hydrogen oxidation leads to the non-adiabatic excitation of electrons in Pt. A fraction of these electrons have enough energy to ballistically transport through Pt and overcome the Schottky barrier at the interface with TiO2. The yield for this phenomenon is on the order of 10-4 electrons per product molecule formed, similar to what has been observed for CO oxidation and for the adsorption of many different molecules./nThe same Pt/TiO2 system was used to compare currents in hydrogen oxidation and deuterium oxidation. The current through the diode under deuterium oxidation was found to be greater than under hydrogen oxidation by a factor of three. Weighted by the difference in turnover frequencies for the two isotopes, this would imply a chemicurrent yield 5 times greater for D2 compared to H2, contrary to what is expected given the higher mass of D2. Reversible changes in the rectification factor of the diode are observed when switching between D2 and H2. These changes are a likely cause for the differences in current between the two isotopes./nIn the nanodiode experiments, surface chemistry leads to charge flow, suggesting the possibility of creating charge flow to tune surface chemistry. This was done first by exposing a Pt/Si diode to visible light while using it as a catalyst for H2 oxidation. Absorption of the light in the Si, combined with the band bending at the interface, gives rise to a steady-state flow of hot holes to the surface. This leads to a decrease in turnover on the surface, an effect which is enhanced when a reverse bias is applied to the diode./nSimilar experiments were carried out for CO oxidation. On Pt/Si diodes, the reaction rate/nwas found to increase when a forward bias was applied. When the diode was exposed to visible light and a reverse bias was applied, the rate was instead decreased. This implies that a flow of negative charges to the surface increases turnover, while positive charges decrease it./nCharge flow in an oxide supported metal catalyst can be modified even without designing/nthe catalyst as a solid state electronic device. This was done by doping stoichiometric and nonstoichiometric TiO2 films with F, and using the resulting oxides as supports for Pt films. In the case of stoichiometric TiO2, F was found to act as an n-type dopant, creating a population of filled electronic states just below the conduction band, and dramatically increasing the conductivity of the oxide film. The electrons in those states can transfer to surface O, activating it for reaction with CO, and leading to increased turnover for CO oxidation. This reinforces the hypothesis that CO oxidation is activated by a flow of negative charges to the surface./nThe same set of catalysts was used for methanol oxidation. The electronic properties of the/nTiO2 films again correlated with the turnover rates, but also with selectivity. With stoichiometric TiO2 as the support, F-doping caused an increase in selectivity toward the formation of partial oxidation products, formaldehyde and methyl formate, versus the total oxidation product, CO2. With non-stoichiometric TiO2, F-doping had the reverse effect./nAmbient Pressure X-Ray Photoelectron Spectroscopy was used to investigate this F-doping/neffect in reaction conditions. In O2 alone, and in CO oxidation conditions, the O1s spectrum/nshowed a high binding energy peak that correlated in intensity with the activity of the different films: for stoichiometric films, the peak decreased in intensity with F-doping, while for nonstoichiometric films, the opposite was observed. No such changes were visible in the C1s spectrum, confirming the role of O activation in the reaction./nThis thesis adds to the body of knowledge on the importance of charge transfer at the/nmetal-oxide interface in shaping the reactivity of heterogeneous catalysts, and provides/nexamples of how this can be the basis for new methods to tune reactivity.

Advisor: Somorjai, Gabor A