Identifying influential parameters of octahedrally coordinated cations in spinel ZnMnxCo2-xO4 Oxides for CO oxidation

Abstract

Increasing CO emission from urban and industrial development as well as motor vehicles creates a major concern to the growing population of Singapore. In order to curb this problem, noble metal catalysts are used to convert this harmful gas to safer gas by CO oxidation using heterogeneous catalysis process. However, noble metals are expensive and limited.

Transition metal oxides are one of the potential alternatives to noble metal catalysts for CO oxidation. Among these earth abundant oxide catalysts, cobalt or manganese based spinel oxides have gained consistent interests due to their promising activity. It has been found that the octahedral sites in spinel oxides are responsible for their catalytic performance by using simple spinels with two metal cations. However, the parameters of the octahedrally coordinated cations for CO oxidation are still not widely recognised or understood.

A series of ZnMnxCo2-xO4 (x=0~2.0) spinel oxides are investigated on the activity of CO oxidation with regards to the variation of compositional ratio of octahedrally occupied Mn to Co. It is found that ZnMn0.2Co1.8O4 with a compositional ratio of x = 0.2 has the highest catalytic activity. Both Mn and Co contribute to the activity, with Mn cations as the dominant active species when they co-exist. The presence of Mn4+ and Mn3+ in a proper ratio is essential for achieving high CO oxidation activity and it can be rationalized by the moderate oxygen adsorption during the CO oxidation, which facilitate the O vacancy refilling. The eg occupancy and average outer electron number (davg) of Mn cations, which are the dictations of how the electronic structure influences the adsorption of oxygen and reaction intermediators, respectively, are proposed to serve as the activity descriptors. This study may provide a new insight into the understanding of the activity of transition metal spinel oxides for CO oxidation.