284116 Building and Controlling Structure-Function Relationships of Titania Photocatalysts
Inexpensive titania and sol-gel derived mixed oxides are attractive for photocatalytic water remediation and other reactions where total mineralization is the goal. However, these materials are somewhat notoriously sensitive to synthesis and treatment conditions such that developing useful synthesis-structure-function relationships is challenging. In addition, these materials have seen little use as selective oxidation catalysts, and the Holy Grail in this area – photocatalytic CO2 reduction – remains inefficient and very poorly understood.
This talk presents two aspects of our work on controlling TiO2-based photocatalytic surfaces. In the first area, existing TiO2 photocatalyst particles (e.g. P25) are modified by the addition of 0.1-1.0 nm of Al2O3 via atomic layer deposition. Wherase the native TiO2 surface displays no reactant selectivity, we have shown that partially covering the surface makes the catalyst selective for the oxidation of the less hindered alcohol in the system. For example, in 1:1 mixtures of benzyl alcohol and trimethylbenzyl alcohol, 10:1 selectivities can be achieved for the production of benzaldehyde. The most selective catalysts are those where part of the TiO2 surface was partially covered with single-molecule masks during the Al2O3 deposition. This synthesis method will be described in more detail.
The second area of work builds off the interplay between a need for crystalline domains of TiO2 to absorb light, but the orthogonal need to have low-coordinate Ti at abundant interfaces to act as catalytic sites. Rather than rely on sol-gel synthesis of TiO2-SiO2 materials, we sought a controlled synthesis to systematically probe structure-function relationships between Ti coordination, titania interfaces, electronic descriptors, and reactivity in photocatalysis, particularly photocatalytic reduction of CO2. TiOX species were deposited on pre-formed SiO2 particles by grafting one or more cycles of Ti(OEt)4, Cp2TiCl2, and other precursors, followed by calcination. These surface densities correspond to 2% of a theoretical monolayer to 2 monolayers. A special emphasis has been placed on materials at low loadings where UV-visible spectra change over a very narrow range of composition. Similar to the technique above, we have also grafted Ti-containing molecular templates, followed by deposition of TiO2 around the template, resulting in both high TiO2 loadings and high interfacial area. We have probed these materials by X-ray absorption near edge spectroscopy, diffuse-reflectance UV-visible spectroscopy, and X-ray diffraction. Reactions include dye decomposition, benzyl alcohol photocatalytic oxidation as above, and preliminary results in CO2 photoreduction. Our results show a clear trend of decreasing coordination with decreasing Ti surface density through XANES and DRUV-Vis. The photo-oxidation rates increase with increasing Ti loading and light-absorption capacity, although reactivity per Ti atom decreases. At similar surface densities, there are significant differences in reactivity, crystallinity, and DRUV-Vis edge energies between materials made with Ti(OEt)4 and Cp2TiCl2 precursors. Some crystallinity is required for high rates of product formation.