Supported catalysts have been widely used in petrochemistry, exhaust gas treatment, new energy batteries and other aspects closely related to life, due to their robust interfacial structure and facile separation for reuse. In supported catalysts, there are unavoidable interactions between metal nanoparticles and supports, which could involve charge transfer via interfaces, changes in particle morphology and chemical composition, and the formation of encapsulated structures after the migration of supports. These factors jointly affect the adsorption and conversion processes of the reactants. Therefore, the interfacial effect is a crucial factor in determining the catalytic performance of supported catalysts.
Among various interfacial effects, the electronic effect is the most significant for the regulation of catalytic performance. The interfacial electronic structure is closely related to the adsorption strength of surface species, which directly affects the catalytic activity and selectivity. Previous studies have disclosed that the catalytic activity and the adsorption strength of key intermediates on the catalyst surfaces usually display a volcanic relationship, while only moderate adsorption strength can achieve the highest catalytic performance. However, how to precisely control the interfacial electronic effect by chemical means, so that the adsorption strength of important intermediates can match the demand of different reactions, is a major sticking point in catalysis research; at the same time, it has been not clear what is the important intermediate for complex reactions. Therefore, it is urgent to develop precise interfacial regulation strategies, followed by resolving the structure-performance relationship of complex reactions and guiding the design of catalytic materials for key energy-related reactions such as CO2 conversion and hydrogen generation.
Recently, Yawen Zhang group developed two methods to precisely control the interfacial electronic structure based on supported CeO2 nanomaterials, including the electrochemically induced interfacial modulation strategy and the ammonia heat-treated interfacial modulation strategy. They have successfully achieved enhanced metal-support electronic interaction and weakened metal-support electronic interaction, respectively (Fig. 1).
Fig. 1 The illustration of modulation strategies: (a) electrochemically induced interfacial modulation strategy; (b) ammonia heat-treated interfacial modulation strategy.
The electrochemically induced strategy mainly utilized the strong electron-obtained properties of Au3+ species. After loading Au(OH)3 species on the surface of CeO2 nanorods, the reduction of Au3+ species could also induce the reduction of CeO2 carrier in the subsequent electrochemical pretreatment process, thereby enhancing metal-support interaction. The acquired catalysts of Au-CeO2-DP was used in the catalysis of electrocatalytic CO2 reduction and exhibited CO Faradaic efficiencies of over 95% under a wide potential range of −0.7 to −1.0 V (Fig. 2). Benefiting from the control of the interfacial structure, its Au mass current density at −0.7 V was 5.8 times higher than that of Au-CeO2-FR catalyst obtained by the traditional NaBH4 reduction, which was also at a leading level compared with the results in the literature. Research results showed that the enhanced metal-support interaction enabled Au nanoparticles to exhibit a positive valence state on surface atoms, while generating abundant oxygen vacancies over the Au-CeO2 interface. The change of the interfacial electronic structure improved the adsorption stability of CO2 molecules and accelerated the generation of important carboxylate intermediates, thus improving the catalytic performance of the CO2 electroreduction reaction.
Fig. 2 The electrocatalytic CO2 reduction performance of Au-CeO2-DP catalyst: (a) LSV curves; (b) CO Faradic efficiency; (c) Au mass current density; (d) stability test.
The ammonia heat-treated strategy introduced N doping into CeO2 nanostructures by NH3 treatment process. The N doping could seal oxygen vacancies and then weaken the interaction between CeO2 nanostructures and surface metal species. The Co-CeO2 catalysts obtained by this synthetic strategy were used in the water-gas shift reaction for hydrogen production. It was found that the catalytic activity gradually promoted with the increase of the NH3 treatment temperature (Fig. 3). Compared with the literature results, the weakened interfacial electronic interaction greatly improved the catalytic efficiency of Co sites, making it possible for Co-based catalysts to be used in industrial production. The improvement of catalytic activity mainly came from two aspects. On the one hand, due to the weakened metal-support interaction, the average valence state of Co species decreased under reaction conditions, following that the increase of metallic Co sites could stabilize CO adsorption and accelerate the generation of carboxylate intermediates. On the other hand, N species were unstable under the reaction conditions. The remove of N species was conducive to the generation of oxygen vacancies, thereby enhancing the H2O activation ability over activated interface. Both worked together to improve the catalytic performance of the water-gas shift reaction.
Fig. 3 The water-gas shift reaction performance of Co-CeO2 catalysts: (a) reaction activities; (b) reaction rates.
This series of works on metal-support interactions of ceria supported nanocatalysts have recently been published by the journal of ACS Catalysis (2022, 12, 923−934 & 2022, 12, 11942−11954), in which, Xiaochen Sun, a Ph.D. student at the College of Chemistry and Molecular Engineering, Peking University, is the first author, and Prof. Yawen Zhang from the College of Chemistry and Molecular Engineering, Peking University, is the corresponding author. Prof. Haichao Liu and his research group from Peking University have also made substantial contributions to this research in resolving the catalytic mechanisms. These studies on catalytic interfacial effect provide new ideas for the development of stable and highly active supported catalysts for wide industrial applications.
This research was funded by the National Natural Science Foundation of China, the National Key R&D Program of China and the Beijing National Laboratory for Molecular Sciences. This work was also strongly supported by the research group of Prof. Chunhua Yan and Prof. Lingdong Sun of Peking University.
Original link: https://doi.org/10.1021/acscatal.1c05503
https://doi.org/10.1021/acscatal.2c03664
