Tailoring Surface Properties of CeO₂ for Atomic Insights into its Catalytic Reactions

Abstract

Heterogeneous catalysis relies on the adsorption and activation of reactants at active sites on the surface of solids. By engineering the coordination structure of these active sites, the activation of reactants can be regulated. Various methods, such as doping, coating, loading, chemical deposition, and surface control, have been employed to manipulate the coordination structure of active sites and establish a reliable structure-activity relationship. Among these methods, controlling the exposed surfaces of catalysts allows for the creation of distinct and uniform coordination structures, enabling a more accurate understanding of the true structure-activity relationship in reactions.

CeO₂ is a versatile material with unique properties that make it suitable for various catalytic applications. CeO₂ surfaces exhibit different crystallographic orientations, and the surface Ce sites enclosed by distinct coordination structures, defined by coordination number and local geometry, interact differently with reactants. This, combined with the high structural uniformity, facilitates in-depth and precise mechanistic studies of reactant adsorption and activation. In this study, CeO₂ with three low-index surfaces was used as a platform for a systematic investigation of three different catalytic reactions as follows:

(1) (DMC synthesis) The direct synthesis of dimethyl carbonate (DMC) from CO₂ and methanol offers a green alternative to conventional methods using toxic chemicals. Despite extensive research on CeO₂-based catalysts, the structural factors influencing their DMC activity remain unclear. In this study, CeO₂ catalysts with well-defined Ce coordination structures were investigated. Our findings reveal that the activation of methanol by surface Ce sites is crucial for DMC production, rather than CO₂. The configuration of surface methoxy species, determined by Ce coordination structures, significantly impacts their reactivity towards CO₂. Head-to-head terminal methoxy species exhibit faster conversion of CO₂ to methyl carbonate (MC) compared to their atilt counterparts, resulting in higher DMC activity. Bridging methoxy species, on the other hand, are too stable to react with CO₂. The established structure-activity relationship in this study provides guidance for the design of CeO₂-based catalysts in other reactions involving methanol activation.

(2) (2-Cp hydrolysis) The direct synthesis of DMC from CO₂ and methanol offers a promising green alternative to conventional methods using toxic chemicals. However, the yield is limited by the equilibrium. Coupling this reaction with 2-cyanopyridine (2-Cp) hydrolysis over CeO₂-based catalysts was found to boost DMC yield by removing water. A recent study has revealed that methanol is the key species being activated by surface Ce sites to produce DMC. The reactivity of surface methoxy species towards CO₂ varies greatly with their configuration, which is determined by the Ce coordination structures. A similar challenge remains in understanding the CeO₂ surface feature governing the hydrolysis of 2-Cp to 2-picolinamide (2-PA). Herein, CeO₂ nanocrystallites with well-defined (111), (110), and (100) surfaces were used to study the effects of Ce coordination structures and their arrangements on this reaction and the coupled DMC synthesis. We found that the synergistic adsorption of 2-Cp via cyano-N and pyridine-N on (111) and (110) surfaces enables nucleophilic addition of lattice oxygen, converting cyano-N to imino-N with a stronger Lewis basicity and facilitating hydrolysis. The (111) surface outperforms the (110) surface due to its unique coordination structure and arrangement of Ce sites, which enable more 2-Cp activation and facilitate easier 2-PA desorption. In contrast, this synergistic 2-Cp adsorption/activation is not allowed on the (100) surface, leading to low activity through the typical catalytic pathway. These findings provide insights into the design of CeO₂-based catalysts for CO₂ conversion with other alcohols and amines using 2-Cp as a dehydrant.

(3) (Aniline oxidation) The development of solid catalysts for efficient H₂O₂ synthesis has received significant attention, but optimizing its utilization and product selectivity in specific reactions remains an understudied area. Designing solid catalysts for H₂O₂ activation in aniline oxidation to produce valuable nitrosobenzene and azoxybenzene has been a subject of interest in the past decade. However, literature reports often require distinct catalysts to achieve the desired products, making it challenging to provide comprehensive design guidelines. In this study, we demonstrate that regulating H₂O₂ activation on pristine CeO₂ surfaces with different crystal orientations allows for facile production of the target compounds in this reaction. The presence of bridging peroxo species on the (100) surface results in non-selective oxidation of H₂O₂ and aniline, leading to poor H₂O₂ utilization and low nitrosobenzene yield. Conversely, the formation of side-on peroxo species on the (110) surface exhibits some selectivity towards aniline, leading to significantly higher nitrosobenzene yield and improved H₂O₂ utilization. Surprisingly, the end-on peroxo species on the (111) surface remains inert to H₂O₂ in solution but can stoichiometrically convert aniline to phenylhydroxylamine (Ph-NHOH), a key intermediate for azoxybenzene production. The structure-selectivity correlation established in this study can guide the rational design of catalysts with high H₂O₂ utilization and product selectivity in other oxidation reactions.

Date of Award	8 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yung-kang PENG (Supervisor)