Stability of Intermediate States for Ethylene Epoxidation on Ag Cu Alloy Catalyst: A First-Principles Investigation

Ethylene oxide, obtained through partial oxidation of ethylene, is an important compound for the chemical industry, because it is widely used as an intermediate for the production of chemicals such as ethylene glycol, which finds applications as an antifreeze and as a precursor to polymers. The oxidation of ethylene can lead either to the desired product (ethylene oxide, EO) or to the formation of acetaldehyde (Ac), which is readily converted to carbon dioxide, with the latter being the thermodynamically favored pathway. The optimal catalyst for this reaction should therefore selectively promote the formation of EOwhile avoiding total combustion. In the chemical industry the catalyst of choice for this process is silver. While pure silver can reach a selectivity toward the production of EO of about 40 50%, the inclusion of alkali and chlorine promoters can increase the selectivity of silver up to about 80%. Considerable efforts in the past few decades have been devoted to the understanding, at the atomic level, of the mechanism of metal-catalyzed ethylene epoxidation, because this could provide insights into the properties of the catalysts thatmore strongly affect their selectivity. Campbell suggested the possibility of a common intermediate for both the pathway leading to EO and the one leading to total oxidation. Through temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS), Linic and Barteau have identified an oxametallacycle (OMC) intermediate on silver catalysts, where ethylene is bonded with one carbon atom to a surface metal atom and with the other carbon atom to oxygen, and proposed that an OMC is the common intermediate for both reaction paths. Several other intermediates have also been identified: Greely and Marvikakis, through density functional theory (DFT) calculations, found that ethylenedioxy (EDO) could be present at the surface of silver catalyst under conditions of temperature and pressure compatible with the ones used experimentally. Recently Linic and Barteau, through a combined theoretical and experimental work have shown that alloying silver with other metals, and in particular with copper, can lead to a substantial increase of the selectivity toward the formation of ethylene oxide. However, while many studies have been devoted to the understanding of the mechanism of ethylene epoxidation process on monometallic catalysts, 16 the mechanism of the reaction on the bimetallic catalysts still remains unclear. Barteau and co-workers suggested that the mechanism found for monometallic systems holds also in the case of the Ag-based alloys they investigated. They considered a catalyst model in which a surface alloy forms on top of Ag(111), obtained by replacing one out of four silver atoms with another metal atom. While this assumption could be reasonable in ultra-high-vacuum conditions, recent theoretical and experimental works involving some of us 19 have shown that such a structure is not predicted to be stable in an oxygen ethylene environment at temperatures and pressures relevant for industrial applications. Copper, on the other hand, segregates to the surface and forms stable, thin oxide-like films. These studies show that temperature and pressure significantly