Abstract:
Co sub(3) O sub(4) with spinel structure shows CO oxidation activity at very low temperature under dry condition. This study aims at finding the origin of the unique catalytic activity of Co species in Co sub(3) O sub(4) based oxides. Although, Octahedral site Co sup(3+) species has been reported to be active in Co sub(3) O sub(4) based catalyst, there is no solid explanation as to why Co is so special as compared with other metal like Fe having similar redox states. In this study, mainly, three model spinel catalysts including MnCo sub(2)O sub(4), MnFe sub(2) O sub(4) and CoCr sub(2) O sub(4) have been chosen. The detailed analysis of bulk and crystal surface structure, surface properties of the catalysts and redox properties of the active metals have been performed to understand the unusual catalytic activity. Low-temperature CO oxidation activity decreases in the following order: MnCo sub(2)O sub(4) greater than MnFe sub(2) O sub(4) greater than CoCr sub(2) O sub(4). It indicates that Co sup(2+) species in tetrahedral site (in CoCr sub(2) O sub(4)) remains inactive for low-temperature catalytic activity, while Co sup(3+) in octahedral site (in MnCo sub(2)O sub(4)) is active in Co sub(3) O sub(4) based catalysts. This ressult is corroborated with CoFe sub(2) O sub(4) which shows higher activity than CoCr sub(2) O sub(4), as it has partial occupation of octahedral site. Fe, being a weak redox metal, does not show low-temperature activity, although, crystallite facets of MnCo sub(2) O sub(4) and MnFe sub(2) O sub(4) catalysts are predominantly exposed in (100) and (110) lattice planes, which contain quite similar concentration of Co sup(3+) and Fe sup(3+) species in both. Intensity of redox peak for CO oxidation involving Co sup(3+)/Co sup(2+) couple in MnCo sub(2) O sub(4) indicates a highly favorable reaction, while a non-responsive behavior of Co species is observed in CoCr sub(2) O sub(4). As expectedly, MnFe sub(2) O sub(4) is proven to be a weak, giving a much lower intensity of electrochemical CO oxidation. Both CO- and H sub(2) -TPR indicate much higher reducibility of Co species in MnCo sub(2)O sub(4) as compared with Co species in CoCr sub(2) O sub(4) or Fe in MnFe sub(2) O sub(4).
