THE ACTIVE-SITES OF MANGANESE-CONTAINING AND COBALT-CONTAINING CATALYSTS IN THE SELECTIVE GAS-PHASE REDUCTION OF NITROBENZENE

The active sites of the spinels Mn3O4 and Co3O4 in the selective reduc tion of nitrobenzene to nitrosobenzene have been investigated, by tota l and partial substitution of Mn-n+ (Co-n+) ions in tetrahedral positi on and/or Mn-n+ (Co-n+) ions in octahedral position by redox-inactive ions. Investigation of the catalytic activities of the totally substit uted manganese spinel ZnMn2O4 and that of the totally substituted coba lt spinels CoAl2O4 and ZnCo2O4 showed that the manganese and cobalt io ns in octahedral position were responsible for the activity of the rea ction. MnAl2O4, however, also showed some activity for the selective r eduction of nitrobenzene, which should be due to surface enrichment an d/or oxidative transfer of manganese from tetrahedral sites to octahed ral sites. XRD results of the partially substituted spinels MnxZn1-xAl 2O4 showed that there was a linear correlation between the manganese c oncentration expressed by x and the lattice constant. Moreover, each s ample showed segregation of Al2O3: again there was a linear correlatio n between x in a certain series of MnxZn1-xAl2O4 and the amount of fre e Al2O3. This points to an oxidative transfer of manganese from tetrah edral sites to octahedral sites. XPS showed that the concentration of manganese in the surface layer corresponded roughly to the value x, bu t the concentration of zinc in the surface layers was much smaller tha t expected. The catalytic activities of the MnxZn1-xAl2O(4) series of catalysts are larger than would be expected from the results on ZnMn2O 4 and ZnCo2O4. (C) 1994 Academic Press, Inc.