TEMPERATURE-PROGRAMMED REDUCTION OF CALCINED CHROMIA-COATED ALUMINA AND SILICA CATALYSTS - PROBING CHROMIUM (VI)-OXYGEN SPECIES

摘要

Chromia-coated alumina and silica, containing 0.5, 5 and 10 wt% Cr2O3, have been prepared by adding Cr(NO3)(3) solution to a suspension of the support in NH4OH followed by calcination at 600 degrees C. Temperature-programmed reduction (TPR) of a corresponding unsupported chromia showed peaks at 370 and 490 degrees C, due to reduction of surface chromate groups; commercial alpha-Cr2O3 showed no hydrogen uptake. TPR measurements on the chromia-coated supports, together with information from the literature, lead to the conclusion that reducible chromate groups are present on the surface in a variety of forms having different degrees of ease of reduction and hydrolysis, their numbers and structures depending on Cr content and the nature of the support. H-2 uptakes are markedly less than expected on the basis of hydrazine titration of surface excess oxygen. The reactivities of surface chromate groups may be understood on the basis of difference in the polarizability of metal-oxygen bonds in the support. [References: 46]