Electrochemical Oxidative Coupling of Methane: Deciphering the Exceptional Properties of BaMg_0.33Nb_0.67–xFe_xO_3−δ for Enhanced Electrocatalysis and Durable Operation

Abstract

Perovskite metal oxides of the form A(B x M 1−x )O 3−δ are of interest to the electrocatalysis and heterogeneous communities due to their tunable electrical and catalytic properties. However, they suffer poor chemical stability under highly reducing and coke-forming conditions. Doped barium niobates show remarkable chemical stability during electrochemical oxidative coupling of methane (E-OCM) measurements. The reason for their chemical stability is not well understood. We i n v e s t i g a t e d t h e h i g h -t e m p e r a t u r e r e d u c t i v e s t a b i l i t y o f BaMg 0.33 Nb 0.67−x Fe x O 3−δ (BMNF), using a variety of methods. BMNF perovskites were exposed to methane-rich conditions in an E-OCM setup and in temperature-programmed reaction (TPR) measurements. X-ray powder diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy measurements on BMNF before and after methane exposure reveal that BMNF perovskites retain their crystal structure. Iron doping may increase their electronic conductivity, while changes in Nb 4+/5+ oxidation states provide chemical stability in reducing environments. For comparison, Fe nanoparticles supported on BMN perovskites (BMN-01Fe) were synthesized/analyzed. The lattice-doped Fe in BMNF did not show any exsolution while the surface-bound Fe in BMN-01Fe agglomerated to form nanoparticles under reducing conditions. Our research demonstrates that multivalence, transition metal oxide perovskite redox stability results from several complex factors and is not easily predicted from the behavior of the binary oxides.