A. Maltha et al., THE ACTIVE-SITES OF MANGANESE-CONTAINING AND COBALT-CONTAINING CATALYSTS IN THE SELECTIVE GAS-PHASE REDUCTION OF NITROBENZENE, Journal of catalysis, 149(2), 1994, pp. 356-363
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.