Final Defense – Anika Auni
About the event
Speaker: Anika Auni
Group: Qiang (Jack) Zhang
Title: PROBING THE ROLE OF ZINC-ZIRCONIUM MIXED OXIDE AS SUPPORT IN SINGLE ATOM CATALYSIS
Abstract:
Single atom (SA) catalysis has emerged as a promising candidate in catalytic research, showcasing outstanding efficiency while minimizing the use of catalytic metals. The superior activity of single atoms stems primarily from their increased dispersion on support materials. Therefore, the appropriate choice of support is crucial, as single atoms have a strong tendency to agglomerate due to increasing surface energy and sintering. Metal oxides serve as promising supports for SA catalysts owing to their straightforward synthesis and excellent functional flexibility. However, relying on a single type of metal oxide as the support constrains maximum dispersion and catalytic performance. Combining different metal oxides offers more versatility with multiple anchoring/coordination sites and tunable SA-support interactions.
We prepared several ZnO-ZrO2 mixed oxide compositions using the co-precipitation method, which forms solid solutions due to their compatible crystal lattice coexistence. These samples were characterized via X-ray diffraction, X-ray fluorescence, gas adsorption, and other spectrometric techniques to investigate the limits of solid solution formation, allowing us to pinpoint the optimal ZnO-ZrO2 composition for the SA catalysis. In a novel approach, we anchored palladium (Pd) ions to our chosen mixed oxide via co-precipitation, resulting in a competition between Pd and Zn for the binding sites. Unlike conventional impregnation techniques, this method provided a more controlled incorporation of Pd ions into the support matrix. X-ray absorption confirmed the presence of individual Pd atoms on the ZnO-ZrO2 support surface. We employed this ZnO-ZrO2 supported Pd catalyst (0.06 wt%) for carbon-carbon coupling (Heck) reactions. Reaction conditions were optimized to achieve complete conversion (>99.9%), high yield, and exceptional selectivity using an environmentally friendly ethylene glycol-water solvent at a relatively low temperature (85°C) in the open air. The catalyst was recovered through centrifugation and reused in subsequent cycles with low Pd leaching (<5%).
We extended our studies to anchor another potential catalytic metal, Copper (Cu), to the optimized ZnO-ZrO2 support using the in-situ co-precipitation technique. Employing this ZnO-ZrO2 supported Cu catalyst (3.4 wt%) for the azide-alkyne Click reactions, we achieved excellent catalytic efficiency at room temperature utilizing the one-pot three-component synthesis method. We undertook a reaction parameter variation (e.g., solvent medium, substrate ratio, reaction time, etc.) study to identify the optimal conditions for choosing a greener solvent. Following the optimization, using a unique combination of ethylene glycol-water mix as the solvent media, we observed more than 99.9% conversion with high yield and selectivity under eco-friendly operating conditions. Moreover, with minimal Cu leaching, this catalyst was efficiently recycled through simple centrifugation, demonstrating its robustness and reusability. Our findings are especially relevant because of the versatile nature of both Heck and Click reaction mechanisms and their wide application in pharmaceuticals, fine chemicals, and next-generation biomarkers. In summary, we present a novel approach for the controlled loading of catalyst metals on the support by leveraging the tunability and versatility of mixed metal oxides and demonstrate the remarkable catalytic efficiencies in Heck and Click reactions.