Synthesis Strategies for Understanding Ceria-Supported Transition Metal Oxide Catalysts for NO reduction

Savereide, Louisa

doi:https://doi.org/10.21985/n2-xqba-pz22

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Synthesis Strategies for Understanding Ceria-Supported Transition Metal Oxide Catalysts for NO reduction

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Supported transition metal oxides are an important class of catalysts with a wide range of industrially relevant applications. However, commonly used synthesis techniques to prepare these catalysts often result in a complex mixture of surface species. This inhomogeneity makes it difficult to understand what specific structures might be responsible for high catalytic activity, selectivity, or stability. In this work, a number of synthesis strategies are employed to allow for greater control of what types of surface structures are formed by first-row transition metal oxides on silica or ceria supports. These surface species are then correlated to catalyst activity in reducing and oxidizing reactions. First, we explored the effect of doping the catalyst surface with promoters for two different systems. In the first system, we looked at the possibility of using high throughput methods to quickly screen potential promoters of silica-supported copper oxide systems for the selective oxidation of propylene to acrolein. It was found that vanadium not only increases catalyst reactivity, but also at high temperatures helps stabilize sites selective to producing acrolein. In the second system, we found that while stoichiometric addition of sodium to ceria-supported iron oxide catalysts promotes catalyst activity for the reduction of NO by CO, at higher sodium and iron loadings activity decreases, suggesting the importance of ceria-iron interfacial sites. Next, we investigated the effect of morphology of nanocrystalline oxides on the dispersion, oxidation state, and catalytic activity of supported cobalt oxide also for the catalytic reduction of NO by CO. From x-ray absorption spectroscopy (XAS), it was found that the ability of a support to maintain Co in a partially reduced state under reaction conditions after initial calcination is linearly correlated with NO reduction activity. Furthermore, this stabilization appears to be correlated with the high number of defects on the ceria nanorods observed via Raman spectroscopy. Finally, we simultaneously systematically tuned two synthesis handles at once: ceria morphology and metal precursor chelating ligand. We discovered that the beneficial effects of a defected surface and a bulky chelating ligand were additive, and that a catalyst made using the optimal ceria morphology (nanorods) and chelating ligand (acetyl acetonate) was nearly three times as active as a catalyst prepared a commercial ceria support and cobalt nitrate. Furthermore, through XAS studies we found evidence that cobalt coordinates to defects in the ceria surface as soon as it is deposited on the surface, which may be one reason that it stays more dispersed during calcination compared to cobalt on a non-defected ceria surface. These insights may lead to new generations of selective catalysts based on nanoceria-supported oxides for emissions abatement and other important reactions

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