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

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 t he support in NH4OH followed by calcination at 600 degrees C. Temperat ure-programmed reduction (TPR) of a corresponding unsupported chromia showed peaks at 370 and 490 degrees C, due to reduction of surface chr omate groups; commercial alpha-Cr2O3 showed no hydrogen uptake. TPR me asurements on the chromia-coated supports, together with information f rom the literature, lead to the conclusion that reducible chromate gro ups are present on the surface in a variety of forms having different degrees of ease of reduction and hydrolysis, their numbers and structu res depending on Cr content and the nature of the support. H-2 uptakes are markedly less than expected on the basis of hydrazine titration o f surface excess oxygen. The reactivities of surface chromate groups m ay be understood on the basis of difference in the polarizability of m etal-oxygen bonds in the support.