Room temperature epoxidation of ethylene over delafossite-based AgNiO 2 nanoparticles.

A mixed oxide of silver and nickel AgNiO 2 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 AgNiO 2 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 AgNiO 2 oxide, silver state can be considered as an intermediate between Ag 2 O and Ag 0 ( i.e. Ag δ + -like), while nickel is characterized by signs of a deeply oxidized state (Ni 3+ -like). The interaction of AgNiO 2 with C 2 H 4 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-C 2 H 4 data, a large number of weakly bound oxygen species were found on the pristine AgNiO 2 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 AgNiO 2 increased, the appearance of Ag 0 particles was first observed below 200 °C followed by the complete destruction of the delafossite structure at higher temperatures.