Source: China University of Science and Technology News

Recently, Associate Professor Geng Zhigang of the University of Science and Technology of China made a major breakthrough in the field of electrocatalytic oxidation of propylene to prepare propylene oxide. A series of AgPO4 catalysts with single exposed crystal face were prepared by controlled synthesis, and the regulatory mechanism of AgPO4 crystal face effect on the catalytic performance of propylene electrocatalytic oxidation to propylene oxide was investigated. The results are published in the journal Nature Communications under the title "Face-dependent electrooxidation of propylene into propylene oxide over Ag3PO4 crystals. Communications. 2022, 13, 932).

Propylene oxide is the third largest derivative of propylene in the propylene industry after polypropylene and acrylonitrile. As an important basic organic chemical synthesis raw material, propylene oxide is mostly used in the production of polyether polyols, propylene glycol, dimethyl carbonate, propylene glycol ether and various surfactants, flame retardants and emulsifiers. At present, the main industrial production methods of propylene oxide are chlorol, cooxidation and direct oxidation. However, the traditional industrial production methods of propylene oxide are faced with many problems such as high pollution and high energy consumption. Therefore, the development of an environmentally friendly, low-cost and high-yield propylene epoxidation process is of great significance for promoting the industrial production of propylene oxide and opening up the market application of propylene oxide. The electrocatalytic epoxidation process of propylene uses clean electric energy as the driving force, which can effectively reduce the consumption of fossil fuels and greenhouse gas emissions in the thermal catalytic process. At the same time, as the epoxidation process of propylene is exothermic reaction, the electrocatalytic epoxidation process of propylene at room temperature and pressure is expected to break through the thermodynamic limitation and greatly improve the epoxidation efficiency of propylene. So far, platinum and palladium-based catalysts have been reported to be used in the electrocatalytic oxidation of propylene to prepare propylene oxide. However, platinum and palladium-based catalysts can not only activate the carbon-carbon double bond (C=C) in propylene, but also activate the carbon-hydrogen bond (C-H) in the terminal methyl group, reducing the selectivity of the target product propylene oxide. The silver based catalyst has the effect of selective activation of C=C in propylene, so it can effectively improve the selectivity of the target product propylene oxide. However, the reported low catalytic activity of silver-based catalysts has greatly limited the development and application of electrocatalyzed propylene epoxidation. Therefore, the design of efficient silver based catalysts to improve the epoxidation activity of electrocatalysis of propylene has a driving effect on the transformation of propylene epoxidation industry.

Figure 1. Electrochemical catalytic oxidation of propylene to propylene oxide reaction path and reaction energy barrier on different crystal faces.

In this work, the researchers successfully prepared homogeneous AgPO4 cubes, AgPO4 dodecahedrons and AgPO4 tetrahedrons through fine controlled synthesis. Selective electron diffraction and X-ray crystal diffraction show that the exposed crystal faces of the AgPO4 cube, AgPO4 dodecahedron and AgPO4 tetrahedron are (100), (110) and (111), respectively. The evaluation of electrochemical catalytic performance of propylene shows that the AgPO4 cube with exposed (100) crystal faces has the highest catalytic activity. At an operating potential of 2.4 V relative to the standard hydrogen electrode, the yield of propylene oxide produced by the AgPO4 cube is as high as 5.3 gPO m-2 h-1, which is 1.6 times and 2.5 times of the yield of the AgPO4 tetrahedron on the exposed (100) crystal face and the AgPO4 tetrahedron on the exposed (111) crystal face, respectively. This value is much higher than the reported yield of propylene oxide prepared by silver-based catalysts (<0.01 gPO m−2 h−1). The mechanism of electrocatalytic oxidation of propylene to produce propylene oxide on different AgPO4 crystal faces was investigated by density functional theory simulation. Theoretical studies show that the coupling of adsorbed propylene and oxygen species *OH is the decisive step of the whole reaction, and the catalytic reaction occurring on the AgPO4 (100) crystal plane has the lowest decisive step energy barrier (FIG. 1). Further based on Bader charge analysis, the active site Ag on the AgPO4 (100) crystal plane has the strongest polarization effect on propylene, which is conducive to breaking the symmetry of C=C and promoting the formation of C-O. In addition, the weak adsorption of AgPO4 (100) crystal surface on propylene and oxygen species *OH increases the vibration amplitude and frequency of adsorbed species *OH to a certain extent, increases the collision probability between adsorbed propylene and oxygen species *OH, reduces the step energy barrier of determination speed, and improves the catalytic activity of propylene electrochemical catalytic oxidation to propylene oxide.

The research was supported by the National Natural Science Foundation of China. Associate Professor Geng Zhigang is the sole corresponding author of the paper, while doctoral students Ke Jingwen and Chi Mingfang and postdoctoral researcher Zhao Jiankang are co-first authors of the paper.

The thesis links: https://www.nature.com/articles/s41467-022-28516-0