Progress Report on: Sulfur in Ethylene Epoxidation on Silver (SEES2)

Abstract

The primary goal of the “Sulfur in ethylene epoxidation on silver” (SEES2) project is to elucidate the mechanism(s) by which catalytic ethylene epoxidation occurs over silver surfaces. There is a particular focus on the role of sulfur. The program involves using density functional theory to predict stable surface phases under various conditions by way of ab initio atomistic thermodynamics. Once identified, the spectroscopic properties of candidate phases are computed to enable experimental verification. Minimum energy paths associated with the (re)formation and reaction of the identified phases are then computed to determine their possible roles in ethylene epoxidation. Through this approach we identified a novel $\text {Ag(SO}_4$) phase and showed it selectively transfers oxygen to ethylene to form the epoxide during temperature programed reaction. In the last year we have shifted the focus to the behavior of surface species under catalytic conditions, focusing on the 0 K minimum energy paths of the surface reaction network before moving on to finite temperature effects. In this effort we have identified additional phases and studied the competition between them. Of the studied species the novel $\text {SO}_4$ phase, where sulfur is present as S(V+), is the only silver one capable of selectively reacting with ethylene to form the epoxide. We further found the $\text {SO}_4$ species is rapidly regenerated through reaction with oxygen, which suppresses the coverage of an adsorbed $\text {SO}_3$ that appears to be selective in total oxidation. The presence of $\text {SO}_x$ species is also found to reduce the EO:AcH branching ratio associated with the reaction of ethylene with atomic oxygen on the unreconstructed Ag(111). Thus, it appears under conditions that are not artificially clean EO is produced in large part by oxygen transfer from the novel $\text {SO}_4$ phase. These new insights are only possible due to the use of various levels of parallelization to extend the scaling of our code on the Cray XC40 system Hazel Hen, which has allowed us to compute the minimum energy paths of a complex network of surface reactions.