(611d) Characterization of Model Vanadium Oxo Catalysts On Silicon Wafer Substrates

Supported vanadium oxides are active catalysts for a wide range of important chemical transformations, including oxidation of alcohols to aldehydes, oxidative dehydrogenation of alkanes to alkenes, ammoxidation of alkenes, and reduction of nitrous oxides [1-2]. Previous research has shown that activity for each of these reactions depends on the size of the metal oxo units: when VOx is deposited on high surface area supports, activity increases with increasing vanadium dispersion [3]. This suggests that VOx units with different nuclearity? that is, degree of oligomerization and connectivity to the support ? may show different catalytic behavior. However, the techniques typically used to analyze powdered samples have limited ability to provide information about the distribution of sizes of vanadium oxo clusters present in the catalyst, and how this distribution is influenced by the vanadium loading and choice of support material. To gain a deeper understanding of the degree of nanoscale aggregation and spatial distribution of these active species, we have developed a methodology for preparing two-dimensional model catalysts in which metal oxo units are dispersed on flat surfaces at concentrations ranging from 0.1 - 10 V/nm2. These model systems are then characterized using surface science techniques [4], with the goal of identifying how catalytic properties are affected by the size, distribution, connectivity, and support of metal oxo clusters. Samples are prepared by spin-coating silicon wafers with a solution containing a metal oxide precursor, such as Ti(OC2H5)4, followed by calcination to create a thin (1-3 nm) layer of support oxide. A second spin-coating using a vanadium-containing precursor, such as OV(C5H7O2)2, followed by a second calcination step, results in a model catalyst containing well-dispersed VOx units on a two-dimensional metal oxide support. Samples are then characterized using surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM), and tip-enhanced Raman spectroscopy (TERS). XPS characterization of a sample containing 9 V/nm2 on silica showed that all of the vanadium is present as V5+. ToF-SIMS data suggest that only a fraction of these V5+ oxo units are chemically bonded to the silica support. AFM images revealed that at this surface density, the vanadia is dispersed in 10-20 nm particles, and Raman spectra of these particles are consistent with V2O5. At lower vanadium surface densities, initial results show evidence for a variety of VOx species bonded to silica.

References

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