Nanometric nickel exsolution in the hexagonal perovskite Ba8Ta6NiO24: Survey of the structural, magnetic and catalytic features

Abstract

Nickel nanoparticles that decorate the surface of a parent oxide are well known to be developed by some cubic-type perovskites from the exsolution of their intrinsic Ni2+ species in reducing conditions with subsequent catalytic properties. In this work, we show similar phenomenon in a hexagonal-type perovskite. The reducing treatment in ammonia of Ba8Ta6NiO24, a hexagonal perovskite-type oxide, leads to the exsolution of nickel nano-sized particles at the surface of the precursor leaving oxygen vacancies and partially depleted but rearranged Ni/Ta sites. Spectroscopic methods, X-ray diffraction, TEM and TGA were employed to investigate the Ni exsolution while TGA was used to investigate the thermal behavior and its reversibility. 50% of the initial nickel content transferred into Ni0 nanoparticles as determined from M(H) magnetization plots and TGA, in agreement with the concomitant creation of oxygen vacancies in the preserved Ni2+ coordination (50%). The cationic vacancies (Vc) involve face-sharing octahedral dimers in which Ni/Ta/Vc rearranged through the shared face window, but preserved a non-centrosymmetric cationic partition. Ni exsoluted oxides often show a catalytic activity; we present here an original example of a hexagonal perovskite with catalytic activity toward syngas production by dry reforming of methane with carbon dioxide.