Ethylene Epoxidation on Ag: Identification of the Crucial Surface Intermediate by Experimental and Theoretical Investigation of its Electronic Structure

Abstract

An important issue regarding the mechanism of heterogeneous epoxidation of ethylene on silver (Ag) is related to the formation of CÀO bonds. [1][2][3] There are two possible ways for the CÀO bonds to form: It is conceivable that the dominant pathway would proceed through either a concerted addition of surface oxygen to the C = C bond of ethylene or by sequential formation of the two CÀO bonds by a creation of an asymmetric surface intermediate (Scheme 1).It has been shown previously in temperature-programmed desorption (TPD) experiments that a surface intermediate formed on Ag(111) upon adsorption of ethylene oxide (EO) at about 250 K under ultrahigh vacuum (UHV) conditions reacts at about 310 K to reform EO plus small amounts of other products, including ethylene.[4] High-resolution electron energy loss spectroscopy (HREELS) was used to identify this surface intermediate as the asymmetric structure shown in Scheme 1, namely a surface oxametallacycle.[4] This work Scheme 1. Possible intermediates in ethylene epoxidation on Ag: a) in the concerted addition of ethylene to oxygen and b) in the sequential formation of CÀO bonds (surface oxametallacycle).

Structurally Characterized Coinage‐Metal–Ethylene Complexes

“…[12,13] Silver-catalyzed oxidation of ethylene to ethylene oxide is a major industrial process. [74][75][76][77][78] Gold-based materials serve as excellent catalysts for the selective epoxidation of propene and other alkenes. [70,79,80] Despite the interest in them, isolable Cu I , Ag I , and Au I complexes bearing simple alkene ligands are somewhat limited.…”

Theory meets experiment: Gas-phase chemistry of coinage metals

“…As the critical point, the branching between the formation of ethylene oxide and acetaldehyde, the first intermediate on the way to the complete combustion can be identified. Density functional calculations reveal that the reaction pathways lead via a common intermediate represented by a metallaoxetane structure [199][200][201], namely the intermediate [Ag • c-AgOC 2 H 4 ] + (Fig. 8).…”

Principles of Heterogeneous Catalysis

“…The third pillar of studies at the Elucidation Level involves the use of DFT calculations to assess the structures, stabilities, and reactivity of species adsorbed onto the surface sites (with the sites being composed of clusters of atoms or as periodic slabs of atoms) [77][78][79][80][81]. These studies are used to help interpret the results obtained from spectroscopic studies of catalyst surfaces (e.g., to predict the vibrational spectra of species adsorbed in different orientations on different sites), to calculate heats of adsorption for various intermediates in a reaction mechanism (e.g., to predict which species are expected to be abundant on the catalyst under reaction conditions), to estimate the energy changes for possible steps in a reaction mechanism (thereby eliminating from further consideration steps with very positive energy changes), and to determine activation energy barriers for steps that are suspected as being kinetically significant in the reaction scheme.…”