Room temperature epoxidation of ethylene over delafossite-based AgNiO2 nanoparticles

Abstract

A mixed oxide of silver and nickel AgNiO₂ was obtained via co-precipitation in alkaline medium. This oxide demonstrates room temperature activity in the reaction of ethylene epoxidation with a high selectivity (up to 70%). Using the PDF method, it was found that the initial structure of AgNiO₂ contains stacking faults and silver vacancies, which cause the nonstoichiometry of the oxide (Ag/Ni < 1). It has been established that on the initial surface of AgNiO₂ oxide, silver state can be considered as an intermediate between Ag₂O and Ag⁰ (i.e. Ag^δ+-like), while nickel is characterized by signs of a deeply oxidized state (Ni³⁺-like). The interaction of AgNiO₂ with C₂H₄ at room temperature leads to the simultaneous removal of two oxygen species with E_b(O 1s) = 529.0 eV and 530.5 eV considered as nucleophilic and electrophilic oxygen states, respectively. Nucleophilic oxygen was attributed to the lattice oxygen (Ag–O–Ni), while the electrophilic species with epoxidation activity was associated with the weakly bound oxygen stabilized on the surface. According to the TPR-C₂H₄ data, a large number of weakly bound oxygen species were found on the pristine AgNiO₂ surface. The removal of such species at room temperature didn’t result in noticeable structural transformation of delafossite. As the temperature of ethylene oxidation over AgNiO₂ increased, the appearance of Ag⁰ particles was first observed below 200 °C followed by the complete destruction of the delafossite structure at higher temperatures.