Active site of substituted cobalt spinel oxide for selective oxidation of COH2. Part II Original Research Article

Substituted spinel oxides such as ZnxCo3−xO4 (x = 0–1), AlxCo3−xO4 (x = 0–2.5) and FexCo3−xO4 (x = 0–2.5) were synthesized to clarify the relation between the surface cobalt species and the oxidation activity/selectivity. These cobalt oxides of the spinel type contain various cobalt species, such as cobalt(II) in tetrahedral sites (CoTd2+), cobalt(III) in octahedral sites (CoOh3+) and cobalt(II) in octahedral sites (CoOh2+). From XRD it was concluded that the crystal structure of ZnxCo3−xO4, AlxCo3−xO4 and FexCo3−xO4 was that of the spinel oxide in the range of x = 0–1, 0–2, 0–2, respectively. CoTd2+ and CoOh3+ in the crystals are replaced by Zn2+. and Al3+, respectively while iron(III) replaces CoOh3+ in the range of x from zero to one, and CoTd2+ from one to two. Similar replacement of cations was confirmed at the surface from XPS of Co 2p level. Oxidation of COH2 mixed gas was conducted at 50–450°C on the substituted cobalt spinel oxides. The products were water and carbon dioxide. The substituted cobalt oxide showed high oxidation activity and CO oxidation selectivity only when CoOh3+ was observed on the surface, while it showed quite low activity and selectivity when little CoOh3+ was observed. It is concluded that the main factor which determined the activity and selectivity for CO oxidation in the presence of H2 is CoOh3+ on the surface of the catalyst. On the other hand, the lack of a quantitative correlation between the oxidation rate and the quantity of CoOh3+ in the range of x = 0–1.5 suggests that other secondary factors also contribute to the activity.