Environmental (scanning and transmission) electron microscopy contribution to the study of silver-based catalysts during ethylene epoxidation

Ethylene oxide (EO) is a chemical intermediate of paramount importance and it is one of the most produced petrochemicals. It is used as a precursor for the further production of added value molecules, including plastics, polyester and ethylene glycol [1]. The most widely used catalytic process for the production of EO is the partial oxidation (epoxidation) of ethylene with air (or oxygen) on Ag/α-Al2O3 catalysts at low reaction temperatures (220 – 280 °C) but at high pressures (10-30 Bar) [2]. In the absence of any chemical promoters, the selectivity towards EO is in the range of 50%; to enhance the efficiency of the epoxidation process, the addition of chlorinated hydrocarbons (e.g. dichloroethene or vinyl chloride) is known to improve the selectivity to values ~90 %. However, chlorinated promoters are not environmentally friendly, and their eventual accumulation leads to a significant poisoning of the catalyst.Following an alternative approach, Stoukides and Vayenas reported the promotional effect of O2- ions on an Ag catalyst film on Ag//YSZ (catalyst-electrode//solid electrolyte) electrochemical catalysts for ethylene epoxidation [3]. It was found that pumping O2- ions towards the Ag electrode via electrical polarization allowed to dramatically enhance the selectivity and yield of EO production at 400 °C and atmospheric pressure.The idea of this study is to take advantage of the main features of both the electrochemical catalysts (i.e., the possibility to in situ supply O2- promoters) and the conventional industrial catalysts (i.e., higher metallic dispersion and accessibility to active sites). We studied the ethylene epoxidation reaction on Ag/YSZ composites supported on dense yttria-stabilized zirconia (YSZ) solid electrolytes at atmospheric pressure and in the absence of chlorinated promoters. The experiments performed showed a dynamic performance, where the activity evolved with time in an unprecedented manner. Before reaching a “steady-state” (with EO selectivity lower than 10 %) the system passed through an “optimized state”, where the EO selectivity was ~ 40-50 %. The pretreatment of the catalyst under oxidizing and reducing atmospheres, the influence of the oxide support (Gd-doped CeO2 and α-Al2O3) and the impact of the catalyst loading were studied. In addition, in situ electron scanning and transmission microscopy observations were performed on the Ag/YSZ composite. All these experiments suggest that the “optimized-state” takes place due to the production of small silver clusters formed after the evaporation, decomposition and further condensation of AgxO species from the bulk of the composite to the YSZ powder. The activity of these Ag clusters species would be dramatically enhanced due to the promotional effect of the O2- ions supplied by the YSZ support, due to metal-support interactions. However, once the YSZ support is saturated with Ag clusters, they start to agglomerate, leading to bigger nanoparticles with lower selectivity towards the desired EO product. This study clearly demonstrates therefore the possibility to use active catalytic supports (e.g. YSZ) to drastically enhance, in a significant manner, the performance of Ag catalysts for the ethylene epoxidation reaction without the addition of non-environmentally-friendly chlorinated promoters (commonly used for industrial applications). Also, to the best of our knowledge, the use of Ag/YSZ catalysts (without electrical polarization) has never been reported for ethylene epoxidation.This work is part of the “EPOX” project funded by the French National Research Agency, ANR-2015-CE07-0026.References1.Özbek, M.O.; van Santen, R.A. Catal. Lett. 2013, 143, 131–141.2.Chongterdtoonskul, A.; Schwank, J.W.; Chavadej, S. J. Mol. Catal. A-Chem., 2012, 358, 58–66.3.Stoukides, M.; Vayenas, C.G. J. Catal. 1981, 70, 137–146.