Surface Regulation of Metal Oxides for Catalytic Insights into Amide Hydrolysis and H₂O₂ Activation

Abstract

Metal oxides play a vital role as catalysts in industrial applications, including amide hydrolysis and H₂O₂ activation. Surface properties significantly influence their catalytic performance. Surface regulation of certain catalysts (e.g. layer reduction of layered HNb₃O₈ materials) can enhance their activity and selectivity. This abstract summarizes the studies on bulk-to-nano regulation of layered HNb₃O₈ materials and their catalytic application in H₂O₂ activation and amide hydrolysis.

The result chapter 4 focuses on bulk-to-nano regulation of layered HNb₃O₈. HNb₃O₈ is a material composed of edge-shared NbO₆ layers linked by bridged protons. These structural protons on the surfaces of bulk HNb₃O₈ (denoted as b-HNb₃O₈) were shown to act as Brønsted acid (BA) sites. By reducing the materials' layer thickness from bulk (b-HNb₃O₈) to few (f-HNb₃O₈) and single (s-HNb₃O₈) layers, different Lewis acid (LA) strengths of exposed Nb sites are revealed, which depend on the distortion of NbO₆ units. This, together with bridged protons preserved on the surface of few/single layered samples, ensures that BA and LA sites are in close proximity.

The result chapter 5 discusses the activity of the three layered HNb₃O₈ samples in H₂O₂ activation. H₂O₂ is commonly used as an environmentally friendly oxidant, but its disproportionation by BA sites or redox metal sites leads to low H₂O₂ utilization. To address this, metals with low redox activity have been explored, as they form selective surface metal-peroxo species. However, the presence of BA sites for charge balance complicates the study of H₂O₂ activation pathways. Layered HNb₃O₈ samples with structurally preserved BA sites were employed to investigate this process. The exposure of Nb sites through bulk-to-nano regulation hindered H₂O₂ disproportionation promoted by BA sites. Highly distorted NbO_x with bidentate configuration exhibited excellent reactivity in alkene epoxidation and nearly stoichiometric H₂O₂ utilization.

The result chapter 6 delves into the realm of mimicking urease using the three HNb₃O₈ materials. Urea pollution is a growing environmental concern, and its removal via catalytic hydrolysis is challenging due to the resonance-stabilized amide bonds. In nature, this reaction is catalyzed by ureases in many soil bacteria. However, the remedy of this problem with natural enzymes is not feasible as they are easily denatured and require high costs for both preparation and storage. Given this, the development of nanomaterials bearing enzyme-like activity (nanozymes) with advantages such as low production cost, simple storage, and pH/thermal stability has attracted much attention over the past decade. As inspired by the mechanism of urease-catalyzed urea hydrolysis, the co-presence of LA and BA sites is imperative to proceed with this reaction. Layered HNb₃O₈ samples with intrinsic BA sites were adopted for investigation. Among the catalysts examined, single-layer HNb₃O₈ (s-HNb₃O₈) bearing strong LA and BA sites displays the best hydrolytic activity towards acetamide and urea. This sample with high thermal stability was found to outperform urease at temperatures higher than 50 °C. The acidity–activity correlation established in this study is believed to guide the future design of industrial catalysts to remediate urea pollution.

Date of Award	3 Apr 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yung-kang PENG (Supervisor)