Charge Transfer and Catalysis at the Metal-Support Interface
Kinetic, electronic, and spectroscopic characterization of model Pt-support systems are used to demonstrate the relationship between charge transfer and catalytic activity and selectivity. The results show that charge flow controls the activity and selectivity of supported metal catalysts.
Using a Pt/n-Si Schottky junction, it is possible to externally control the electronic properties at the Pt/Si interface by applying bias or by exciting charge carriers with visible light. It is found that this device can control the rate of CO oxidation on Pt. Results show that negative charge on the Pt increases the reaction rate while positive charge on the Pt decreases the reaction rate. This is the first time that a solid-state device has been used to externally control the rate of a chemical reaction as determined by directly measuring the product yield. Similar experiments were performed for the H2 oxidation reaction and analogous results were obtained.
Similarly, electronic control of catalyst performance can be achieved by substrate doping. It is shown that F doping can tune the electronic structure of TiO2. When F is incorporated into TiO2, the electrical conductivity is dramatically enhanced because F acts as an n-type dopant. It is shown that this highly n-type TiO2 acts as an electronically active support for Pt catalysts, increasing the rate of CO oxidation by electronically activating surface O. It is further shown that for a multipath reaction, the electronic structure of TiO2 also controls the reaction selectivity. The electronic activity of highly n-type TiO2 serving as a Pt support for methanol oxidation selectively enhances the production of formaldehyde relative to CO2.
Finally, sum frequency generation (SFG) vibrational spectroscopy is used to probe the molecular nature of strong metal-support interactions (SMSI). 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. SFG spectra reveal 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.
This dissertation builds on extensive existing knowledge of metal-support interactions in heterogeneous catalysis. The results show the prominent role of charge transfer at catalytic interfaces to determine catalytic activity and selectivity. Further, this research demonstrates the possibility of selectively driving catalytic chemistry by controlling charge flow and presents solid-state devices and doped supports as novel methods for obtaining electronic control over catalytic reaction kinetics.