Publication

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

Yuan-Peng Du
2020
Thèse EPFL
Résumé

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.

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Concepts associés (34)
Catalyse hétérogène
vignette|droite|Catalyseur monolytique utilisé pour l'oxydation de CO en En chimie, on parle de la catalyse hétérogène lorsque le catalyseur et les réactifs sont dans plusieurs phases. Généralement, le catalyseur est solide et les réactifs sont gazeux ou en solution aqueuse. La catalyse hétérogène est d'une importance primordiale dans de nombreux domaines de l'industrie chimique et le secteur de l'énergie. L'importance de la catalyse hétérogène est mise en évidence via les Prix Nobel pour Fritz Haber en 1918, Carl Bosch en 1931, Irving Langmuir en 1932 et Gerhard Ertl en 2007.
Atomic layer deposition
L’atomic layer deposition (ALD) est un procédé de dépôt de couches minces atomiques. Le principe consiste à exposer une surface successivement à différents précurseurs chimiques afin d'obtenir des couches ultra-minces. Il est utilisé dans l'industrie des semi-conducteurs. L'énorme avantage de l'ALD est de pouvoir faire une monocouche sur une surface présentant un très fort rapport d'aspect (des creux et des bosses). Notamment car la réaction de CVD se déroule directement à la surface, sur une monocouche de gaz précurseurs adsorbés.
Heterogeneous gold catalysis
Heterogeneous gold catalysis refers to the use of elemental gold as a heterogeneous catalyst. As in most heterogeneous catalysis, the metal is typically supported on metal oxide. Furthermore, as seen in other heterogeneous catalysts, activity increases with a decreasing diameter of supported gold clusters. Several industrially relevant processes are also observed such as H2 activation, Water-gas shift reaction, and hydrogenation. No gold-catalyzed reaction has been commercialized.
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