Synthesis of heterogeneous catalysts with metal oxide overcoat for renewable catalysis applications

Metal oxide deposition is an emerging synthetic technique for designing heterogeneous catalysts. Depositing a nanoscale overcoat of metal oxide on heterogeneous catalysts can modify their structural and chemical characteristics. Many deposition approaches, especially for coating transition metal oxides, have relied on atomic layer deposition (ALD). However, ALD requires an expensive instrument, large excesses of precursors and several purging steps. In this thesis, I present a novel kinetically controlled sol-gel strategy to overcoat heterogeneous catalysts. Using non-hydrolytic sol-gel (NHSG) or chelation chemistry, the hydrolysis/condensation rates of alumina precursor can be well-controlled, which enables us to overcoat alumina on high-surface-area catalysts with high uniformity that approaches what can be seen in ALD processes. Specifically, the chelation method was used to synthesize a sintering resistant Al2O3@Cu/Al2O3 catalyst, whose nanoparticles remained well-dispersed after five cycles of liquid phase furfural hydrogenation. In comparison, the application of NHSG pathway was to create more Lewis acid-Pt interfacial sites on Al2O3@Pt/SBA-15 and this catalyst could convert lignin-derived 4-propylguaiacol to propyl cyclohexane with 87% selectivity via hydrodeoxygenation. The kinetic control can be extended to overcoat other transition metal oxides. Specifically, in Chapter 3 I describe the deposition of Nb2O5 on SBA-15, which results in a mesoporous solid acid catalyst. This Nb2O5@SBA-15 has a more thermally stable amorphous structure that prevents it from losing acidity during thermal regenerations at 500 °C. This niobia solid acid outperforms commercial niobia catalysts in both xylose dehydration and Friedel-Crafts alkylation due to its optimal Bronsted/Lewis acid ratio and favorable nanostructure. Lastly, I describe the synthesis of an atomically dispersed material using metal coordination complex. The metal-metal oxide interface of this material can be further engineered using sol-gel overcoating techniques. In contrast to the deposition on catalysts with nanoparticles, overcoating the material with atomically dispersed Pd before reductive treatment enabled us to limit the particle growth during thermal activation steps, yielding highly accessible, sinter-resistant Pd clusters less than 2 nm in diameter. Notably, engineering the Pd-ZrO2 interface of Pd/ZrO2 into inverted ZrO2-Pd interface using ZrO2 overcoat leads to an unprecedented 100% CO selectivity during CO2 hydrogenation. The results of this thesis show that sol-gel based overcoating is a promising, quick and straightforward strategy for designing robust and selective heterogeneous catalysts for a wide range of sustainable applications.