(607c) Ethylene Epoxidation over Shape-Selective Silver-Based Catalysts

In recent years, scientists have developed a variety of synthesis approaches to control morphology and size of nanoparticles. Improving the catalytic activity of a given metal by tuning its size and controlling its shape, results in the exposure of different crystallographic orientation on the surface of the nanoparticles^1,2. Ethylene epoxidation is one of the instances that highlights the importance of the shape and size of nanoparticles. Experimental research showed silver(Ag) nanowires and silver nanospheres dominantly include Ag(100) and Ag(111), respectively². Computational studies demonstrated that Ag(100) faces have greater EO selectivity because of their lower apparent activation energy³.

This work implemented the modified polyol method and wetness impregnation to synthesize silver nanowires and nanospheres supported on aluminum oxide, respectively. The EO selectivity of these catalysts was evaluated in a reactor at constant conversion and varying oxygen to ethylene ratios. EO selectivity is raised by increasing the oxygen to ethylene ratio because if involving additional types of catalytic sites. In addition, silver nanowires exhibited greater EO selectivity than silver nanospheres due to their larger concentration of (100) silver facets compared to (111). The EO selectivity was further improved by using common promoters for ethylene epoxidations, such as cesium. Adding promoters at ppm level is crucial so they do not inhibit silver's active sites.

1. Sangaru, S. S. et al. Surface Composition of Silver Nanocubes and Their Influence on Morphological Stabilization and Catalytic Performance in Ethylene Epoxidation. ACS Appl. Mater. Interfaces 7, 28568–28576 (2015).
2. Christopher, P. & Linic, S. Shape- and size-specific chemistry of Ag nanostructures in catalytic ethylene epoxidation. ChemCatChem 2, 78–83 (2010).
3. Huš, M. & Hellman, A. Ethylene Epoxidation on Ag(100), Ag(110), and Ag(111): A Joint Ab Initio and Kinetic Monte Carlo Study and Comparison with Experiments. ACS Catal. 9, 1183–1196 (2019).