For the methanol synthesis in many CO2 feedstocks, there are sulfur (S) impurities which can adsorb on Cu surfaces and block the active sites of the catalyst for the desired reaction and can result in catalyst deactivation. If the concentration of S is high enough inactive copper sulfide phases can form on the catalyst. However, in the RWGS there is significant hydrogen pressure which can destabilize the adsorbed S by forming H2S that desorbs form the surface of the Cu catalyst. The extent of destabilization depends on the chemical potentials of the sulfur, hydrogen and hydrogen sulfide which are determined by reaction conditions. Since Cu is known to be susceptible to S poisoning the adsorption of S on low Miller index facets of Cu surfaces has to be studied considering the thermodynamic equilibrium of facets with their environment.
The aim of this work is to combine the concept of atomistic thermodynamics with a computational approach (DFT) in order to understand the structure and stability of S poisoned Cu catalyst that is in equilibrium with its environment and to investigate the effect of hydrogen gas in destabilizing the adsorbed S. The adsorption properties of S on Cu(100), Cu(110) and Cu(111) were investigated in different adsorption sites and at coverages between 0.25ML and 1ML. The atomistic thermodynamic framework based on DFT was used for describing the S phase behavior on different Cu facets in the presence of hydrogen gas. Phase diagrams of S on each surface were constructed which showed S poisoning occurs even at ppm levels of H2S in the environment. The shapes of a Cu nanoparticle under various S atom chemical potentials, corresponding to various experimental conditions of temperature and pressure, were determined using the Wulff construction. We found that the crystal shape changes significantly from truncated octahedral at very low S chemical potentials to a shape dominated by (110) facets at high chemical potentials.