Rational tuning towards B-sites (B = Mn, Co, Al) on CoB2O4 binary oxide for efficient selective catalytic oxidation of ammonia

Abstract

NH3 catalytic oxidation technology (NH3-SCO) is promising for the direct conversion of NH3 to N2 and H2O. Currently, balancing its NH3 catalytic oxidation activity and N2 selectivity remains a challenge. Herein, the redox properties and the surface oxygen vacancies of CoB2O4 were modulated to enhance the activity of the catalyst by using three elements (Co, Mn, Al) as the B-site elements of the CoB2O4 spinel. Eventually, CoMn2O4 synthesized with Mn as the B-site element possessed more surface oxygen vacancies and generated Mn3+-Mn4+ pairs that could facilitate the redox properties compared with CoAl2O4 and CoCo2O4 due to the interaction between Co and Mn, which in turn enhanced its catalytic activity together and enabled the complete conversion of NH3 at 150 °C. Additionally, using Mn as the B-site element of CoB2O4 also facilitated the generation of the Bronsted acid sites on the catalyst surface, which was beneficial to its N2 selectivity (65 % at 150 °C). In situ DRIFTs indicated the existence of two reaction mechanisms over CoMn2O4: the i-SCR mechanism with bidentate nitrate as the primary reactive intermediate species and the hydrazine mechanisms with –NH2 and N2H4 as the main intermediate species.