The Selective Species in Ethylene Epoxidation on Silver

Abstract

Silver's unique ability to selectively oxidize ethylene to ethylene oxide under an oxygen atmosphere has long been known. Today it is the foundation of ethylene oxide manufacturing. Yet, the mechanism of selective epoxide production is unknown. Here we use a combination of UHV and in situ experimental methods along with theory to show that the only species that has been shown to produce ethylene oxide, the so-called electrophilic oxygen appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed SO4 (SO4,ad). This adsorbate is part of a 2D Ag/SO4 phase, where the nonstoichiometric surface variant, with a formally S(V+) species, facilitates selective transfer of an oxygen atom to ethylene. Our results demonstrate the significant and surprising impact of a trace impurity on a well-studied heterogeneously catalyzed reaction.

Nanocatalysts Unravel the Selective State of Ag

“…Further, this might be the origin of a possible blocking of the reaction sites and explain the lowered X (O 2 ). The oxygen species desorbing in region II are not affected by the Ar pre‐treatments, consistent with its assignment to dissolved oxygen . The O 2 desorption behavior of the Ag15/ α ‐Al 2 O 3 −O 2 reference (pre‐treated in synthetic air) is entirely different to the Ag5/SiO 2 −O 2 sample.…”

“…From desorption temperature alone, we cannot determine if the oxygen desorbing in region I is also associated with oxygen on the unreconstructed surface, a species that may participate in EO formation . The desorption temperature for region II is in line with dissolved oxygen and/or electrophilic oxygen participating in EO formation during temperature programmed reaction . However, since the mentioned oxygen species were identified on model systems, their roles and relevance for supported Ag catalysts remains rather speculative and will not be further discussed.…”

“…When titrating with C 2 H 4 again, an enormous heat evolution due to a chemical reaction with the stored atomic oxygen is observed (>300 kJ ⋅ mol −1 ). Obviously, a small quantity of dissolved oxygen appears to be enough to activate O 2 and to form Ag δ+ O x to facilitate ethylene adsorption/reaction . As a consequence, purely metallic Ag 0 (without Ag d ‐/O p ‐hybridized states, “closed d ‐band”) is unable to activate the reactants .…”

“…Density functional theory (DFT) calculations were performed with the Quantum ESPRESSO package at the PBE level including dispersion corrections with the exchange‐hole dipole moment (XDM) model . Following earlier work, PAW datasets were taken from PS library and used with a kinetic energy cutoff of 30 Ry. A k ‐point mesh equivalent to (12×12) for the (1×1) Ag(111) surface unit cell was employed along with Marzari‐Vanderbilt smearing with a 0.02 Ry smearing parameter .…”

Ca–Ag compounds in ethylene epoxidation reaction

“…Accordingly, the catalyst should be considered as a dynamic part of the catalytic system, and its completely different behaviour under operation as opposed to under steady-state conditions needs to be taken into account. These observations act as driving forces for development of advanced in situ and operando techniques [2][3][4][5][6][7][8][9], which give an insight into the crystal structure, surface morphology and composition of the catalyst.…”