AbstractThe understanding of solid–gas interactions has been greatly advanced over the past decade on account of the availability of high-resolution transmission electron microscopes (TEMs) equipped with differentially pumped environmental cells. The operational pressures in these differentially pumped environmental TEM (DP-ETEM) instruments are generally limited up to 20 mbar. Yet, many industrial catalytic reactions are operated at pressures equal or higher than 1 bar—50 times higher than that in the DP-ETEM. This poses limitations forin situstudy of gas reactions through ETEM and advances are needed to extendin situTEM study of gas reactions to the higher pressure range. Here, we present a first series of experiments using a gas flow membrane cell TEM holder that allows a pressure up to 4 bar. The built-in membrane heaters enable reactions at a temperature of 95–400°C with flowing reactive gases. We demonstrate that, using a conventional thermionic TEM, 2 Å atomic fringes can be resolved with the presence of 1 bar O₂gases in an environmental cell and we show real-time observation of the Kirkendall effect during oxidation of cobalt nanocatalysts.