Reversible Restructuring of Silver Particles during Ethylene Epoxidation

Abstract

The restructuring of a silver catalyst during ethylene epoxidation under industrially relevant conditions was investigated without and with vinyl chloride (VC) promotion. During non-VC-promoted ethylene epoxidation, the silver particles grow and voids are formed at the surface and in the bulk. Electron tomography highlighted the presence of voids below the Ag surface. A mechanism is proposed involving reconstruction of the silver lattice and defect sites induced by oxygen adsorption on the external surface and grain boundaries, which finally create pores. Promotion of the catalytic reaction by VC suppresses to a significant extent void formation. The use of VC also redisperses silver particles, initially grown during ethylene epoxidation without VC. This process is rapid as the average size decreased from 172 to 136 nm within 2 h. These insights emphasize the dynamic nature of the silver particles during the ongoing ethylene epoxidation reaction and indicate that particle size and morphology strongly depend on reaction conditions.

Non-noble metal single-atom catalysts with phosphotungstic acid (PTA) support: A theoretical study of ethylene epoxidation

“…Experimentally, the selectivity of ethylene oxide over clean Ag is about 40%-50%, and its selectivity can be increased to 90% by the accumulation of solid promoters such as alkali metals as well as Re and Mo [27][28][29]. Among numerous experimental studies, van Hoof et al [30] studied the ethylene epoxidation over Ag catalysts by adding selective species, and found that substantial impact of a small amount of impurity on a well-studied heterogeneous catalyzed reaction. These results point to the importance of adding fine-selected "impurity" on improving catalytic reaction performance for ethylene epoxidation.…”

“…Among the various metal oxide supports, polyoxometalate (POM) clusters are heteropolyacid oxoanions that posses considerable stability, well-defined structures, and interesting catalytic properies, thus being a promising support for developing SACs with high stability. The solid POMs possess a separate anionic structure, which is more like a distinct portion of the solid metal oxides [30][31][32][33][34][35][36][37][38][39][40][41]. For many years, POMs have been used as catalyst support materials due to the following advantages.…”

Ca–Ag compounds in ethylene epoxidation reaction

“…Depending on the coverage, chlorine can: (i) adsorb on surface or penetrate into subsurface at low coverages or (ii) completely poison (via blockage of oxygen chemisorption) the catalyst in case of high-concentration co-feeding [81,86,[88][89][90]. The scenario of formation of highly mobile AgCl species leading to the restructuring and redispersion of Ag particles was also considered in the literature [91]. Interestingly, the replacement of the alumina support (industrially used) by CuO leads to CuO acting as a sponge for low concentrations of chlorine and is accompanied with formation of CuO/ Cl derivatives [92].…”

Atom Probe Tomography for Catalysis Applications: A Review

“…Tomography techniques are particularly valuable to the study of both the surface and the bulk structure and composition of catalytic materials. Neutron tomography [1] and X-Ray microtomography [2][3][4] have only been used to a small extent to study catalytic materials due to their lower resolution, but there is growing interests in the study of catalysts by electron tomography [5][6][7] and atom probe tomography [8]. Atom probe tomography is part of the field emission techniques, which comprise the Field Emission Microscopy (FEM), Field Ion Microscopy (FIM) and Atom Probe Tomography (APT).…”