ACTIVE-SITE OF SUBSTITUTED COBALT SPINEL OXIDE FOR SELECTIVE OXIDATION OF CO H-2 .2./

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/selecti vity. These cobalt oxides of the spinel type contain various cobalt sp ecies, such as cobalt(II) in tetrahedral sites (Co-Td(2+)), cobalt(III ) in octahedral sites (Co-Oh(3+)) and cobalt(II) in octahedral sites ( Co-Oh(2+)). From XRD it was concluded that the crystal structure of Zn xCo3-xO4, AlxCo3-xO4 and FexCo3-xO4 was that of the spinel oxide in th e range of x=0-1,0-2,0-2, respectively. Co-Td(2+) and Co-Oh(3+) in the crystals are replaced by Zn2+. and 2+ Al3+, respectively while iron(I II) replaces Co-Oh(3+) in the range of x from zero to one, and Co-Td(2 +) from one to two. Similar replacement of cations was confirmed at th e surface from XPS of Co 2p level. Oxidation of CO/H-2 mixed gas was c onducted at 50-450 degrees C on the substituted cobalt spinel oxides. The products were water and carbon dioxide. The substituted cobalt oxi de showed high oxidation activity and CO oxidation selectivity only wh en Co-Oh(3+) was observed on the surface, while it showed quite low ac tivity and selectivity when little Co-Oh(3+) was observed. It is concl uded that the main factor which determined the activity and selectivit y for CO oxidation in the presence of H-2 is Co-Oh(3+) on the surface of the catalyst. On the other hand, the lack of a quantitative correla tion between the oxidation rate and the quantity of Co-Oh(3+) in the r ange of x=0-1.5 suggests that other secondary factors also contribute to the activity.