Room temperature epoxidation of ethylene over delafossite-based AgNiO₂ nanoparticles

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-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.