(678c) Synthesis-Structure-Function Relations for Supported Oxides, Including Quantification of Accessible Metal Oxide Active Sites On Ti-SiO2 Systems
Quantitative determination of the number of active sites in supported catalysts is required for full understanding. Characterizing site dispersion in supported metals is typically carried out through techniques such as CO chemisorption, direct particle imaging, or fitting to EXAFS. [1-3] While strong acid sites or the reducible fraction of a supported oxide are also straightforward, [4, 5] quantification of weaker Lewis sites is not routine. Specifically, many supported oxide catalysts undergo significant structural changes as surface densities increase from statistically isolated sites to crystallites, but there is not necessarily parallel quantification of the number of surface-accessible sites.
Here, we provide synthesis-structure-function relationships for supported oxides that include the effect of several grafting precursors on electronic structure (DRUV-vis, XAS, XRD) and reactivity in H2O2 activation, photo-oxidation, and CO2 reduction for Fe, Ti, and other oxide domains supported on SiO2. Borrowing from literature on self-assembled monolayers, we use the specific adsorption of phosphonic acid on the active oxide and not SiO2 to estimate the number of surface accessible atoms. For example, the photocatalytic oxidation of benzyl alcohol is shown to have a rate of ~0.15 h-1 over SiO2-supported titanium oxides of different precursor, loadings, electronic structure and crystallinity, when counting only the sites which phosphonic acid can titrate. Time permitting, other supported oxides and reactions will be discussed, with an emphasis on systems known to exhibit strong structure sensitivity.
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