The nature of electrophilic oxygen: Insights from periodic density functional theory investigations

Abstract

Increasing demand of ethylene oxide and the cost of versatile chemical ethene has been a driving force for understanding mechanism of epoxidation to develop highly selective catalytic process.Direct epoxidation is a proposed mechanism which in theory provides 100% selectivity. A key aspect of this mechanism is an electrophilic oxygen (O ele ) species forming on the Ag surface. In the past two and half decades, large number of theoretical and experimental investigations have tried to elucidate formation of O ele on Ag surface with little success. Equipped with this rich literature on Ag-O interactions, we investigate the same using periodic DFT calculations to further understand how silver surface and oxygen interact with each other from a chemical standpoint. Based on energetics, Löwdin charges, topologies and pdos data described in this study, we scrutinize the established notions of O ele . Our study provides no evidence in support of O ele being an atomic species nor a diatomic molecular species. In fact, a triatomic molecular species described in this work bears multiple signatures which are very convincing evidence for considering it as the most sought for electrophilic entity.

Active Site of Catalytic Ethene Epoxidation: Machine-Learning Global Pathway Sampling Rules Out the Metal Sites

“…With the recent advance in near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), the Schlögl group observed various oxygen species on Ag(111) at low O 2 pressures. The electrophilic O with the O 1s XPS signal at about 530 eV was suggested to be the O species for EO production as it appears along with the EO production. − These electrophilic O species were suggested to be from the impurity SO 4 , , and also argued to be from O 3 species . Due to the very low EO conversion and selectivity under low-pressure conditions, these understandings, such as on SO 4 impurities, should not be simply transferred to industrial EO production.…”

Structural Inhibition of Silver Surface Oxidation

“…Since the 1970s, significant effort has gone into the characterization of oxidized silver (Ag) surfaces to determine the oxygen species present and their chemical behavior. − This effort is largely motivated by the industrial relevance of partial oxidation reactions over silver catalysts, particularly the industrial transformation of ethylene to ethylene oxide and methanol to formaldehyde at temperatures above 500 K. − While consensus has emerged regarding many details of the mechanisms for these reactions, − the exact nature of the reactive oxygenaceous species is still under debate. In particular, the importance of surface reconstructions and defects are active areas of research, where the goal is to use an atomic-level understanding to further develop the heterogeneously catalyzed chemistry of silver. ,,− …”

Formation and Behavior of Carbonates on Ag(110) in the Presence of Ethylene and Oxygen

“…In contrast to the obvious chemical form of O nucl (surface oxide), there is no broad consensus for the chemical form of O elec . A number of oxygen species have been proposed as candidates for the O elec species: adsorbed atomic oxygen, subsurface oxygen, ozone, and so on. Jones and co-workers reported from detailed experimental and theoretical investigations that the O elec species are covalently bound oxygen species. , Recently, it has been evidenced that adsorbed sulfate (SO 4 ) originating from impurity sulfur is one of the O elec species. , …”