Unraveling mechanistic aspects of the total oxidation of methane over Mn, Ni and Cu spinel cobaltites via in situ electrical conductivity measurements

Abstract

Mn, Ni and Cu cobaltite spinels were synthesized by coprecipitation, characterized and evaluated in the total oxidation of methane. To gain a deeper insight into their catalytic behavior, their electrical conductivity was studied as a function of temperature and oxygen partial pressure, and was followed with time during sequential exposures to different gases in conditions close to those used for the catalytic tests. The Ni cobaltite is in a metallic conductivity state and cannot be reduced in the reaction conditions, suggesting that it functions via a suprafacial mechanism. The Cu cobaltite behaves as a p-type semiconductor, but it is not reduced under the reaction mixture at the reaction temperature of 350 °C, suggesting that, at least in these conditions, the suprafacial mechanism is operating. The Mn cobaltite has an n-type semiconducting character and is reduced in the reaction conditions, suggesting that a heterogeneous redox mechanism is involved.