Unravelling the Catalytic Mechanism of CeO2 Nanozymes from the Perspective of Heterogeneous Catalysis

Abstract

Enzyme is an important and powerful biocatalyst with high reaction rate and substrate specificity. However, natural enzymes surfer from their shortcomings, such as high cost in preparation/storage, difficulties in recycling and reuse, and poor robustness under harsh conditions (i.e. narrow working pH and temperature range). To overcome these drawbacks, researchers made numerous efforts to mimic natural enzymes using nanocatalysts namely nanozymes. Among them, CeO₂ is one of the well reported transition metal oxides nanozymes with multiple enzymatic activities (e.g. peroxidase (POD), alkaline phosphatase (or phosphatase, ALP), catalase and oxidase).

From the perspective of heterogeneous catalysis, it is known that the interplay between substrates and active sites on the topmost surface of CeO₂ nanozymes determines its activity in enzyme-like reaction. The electron density of the active site is believed to affect both adsorption and activation of reactant molecules at the surface. Unfortunately, commercial X-ray photoelectron spectroscopy, which is often adopted for such characterization, is not sensitive enough to analyze the topmost surface of CeO₂ nanozymes. Most researchers fail to acknowledge this point during their catalytic correlation, leading to different interpretations in the literature in recent decades. Recent studies on pristine Cu₂O [Nat. Catal. 2019, 2, 889; Nat. Energy 2019, 4, 957] have clearly suggested that the electron density of surface Cu is facet dependent and plays a key role in CO₂ reduction.

Herein, it is shown that pristine CeO₂ can reach 40 times/211.8 times increase in ALP/POD-like activity ifthe exposed surface of CeO₂ is wisely selected. By using solid state NMR spectroscopy with a surface probe, the electron density of the surface Ce（i.e., the active site) is found to be facet dependent and the key factor dictating their enzyme-mimicking activities. Most importantly, the surface area of the CeO₂ morphologies is demonstrated to become a factor only if surface Ce can activate the adsorbed reactant molecules.

Date of Award	23 Feb 2022
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yung-kang PENG (Supervisor)