Low-temperature adsorption of oxygen on calcined chromia: IR spectroscopic and sorptiometric evidence for oxygen-assisted topochemical reduction of surface chromate species

The evolution with calcination of chromia samples of catalytic interest, prepared from gels with or without added ammonium nitrate, has been studied by thermogravimetry (TG) and differential thermal analysis (DTA). The low-temperature (77 K) adsorption of oxygen on alumina- and silica-supported samples of the former type has revealed the presence of monolayer and multilayer types of irreversibly adsorbed species. Low-temperature in situ infrared (IR) spectra show that the adsorption of oxygen is associated with the removal of a multiplicity of absorptions between 1100 and 730 cm−1. Probable types of surface species removed are CrO groups of Cr(VI)–O surface species, dichromates and chromates together with possibly peroxide; (O2)2−, the latter being associated with prominent bands near 800 cm−1. Room-temperature ex situ infrared spectra on an alumina-supported sample shows at lower frequencies the presence of the growth of α-Cr2O3 surface species after oxygen adsorption.