HRTEM characterization of the nanostructural features formed in highly active Ni–Nb–O catalysts for ethane ODH

In this work, we report an in-depth structural characterization of pure NiO and Ni–Nb–O mixed oxide catalysts (Nb/Ni = 0–0.25), highly active and selective materials for ethylene production via ethane ODH, using high resolution transmission electron microscopy (HRTEM) coupled with energy dispersive X-ray analysis (EDS). This study led to the identification and investigation of the nanostructural features formed in the Nb-doped NiO catalysts and their relation with the excellent catalytic functionality of the Ni–Nb–O materials. It was found that low-temperature treatment of pure NiO leads to the formation of a non-stoichiometric oxidic phase with characteristic structural defects due to cationic deficiency, as demonstrated by microscopy results. On the Nb-doped oxides, two distinct structural phases, formed via the reaction of the Nb cations with the cationic vacancies, were identified: a NiO phase having Nb cations incorporated in the host lattice, which retains its initial cubic structure (Ni–Nb solid solution), and a highly distorted Nb-rich phase, precursor for the formation of the mixed NiNb2O6 crystal compound. The reduction of the structural defects in NiO via their interaction with the niobium ions was correlated with the extremely high selectivity of the Ni–Nb–O catalysts to ethylene in the ethane ODH reaction, since these vacancies lead to the formation of strong oxidizing electrophilic oxygen species (O−), responsible for the total oxidation of ethane to CO2.