The Nature of the Chromium Species Formed during the Thermal Activation of Chromium-Promoted Tin(IV) Oxide Catalysts: An EPR and XPS Study

Abstract

Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the nature of chromium-promoted tin(IV) oxide catalyst materials prepared via three routes, by the impregnation of SnO2 using aqueous CrO3 or chromium(III) nitrate solutions and by coprecipitation from aqueous solutions containing both Sn(IV) and Cr(III) ions. The catalyst materials exhibit three different chromium signals (δ, γ, and β) in the EPR, depending on the treatment history of the catalysts. The δ-signal results from dispersed Cr3+ ions and appears in the spectra of the Cr(III)-promoted SnO2 catalyst at all temperatures and also in the spectra for the Cr(VI)-promoted catalyst after calcination. The β-signal detected for the uncalcined Cr(III)-promoted catalyst obtained by coprecipitation is most probably due to the hydrated γ-CrOOH phase but to Cr2O3 at higher calcination temperatures. Both Cr(III)- and Cr(VI)-promoted catalyst materials exhibit the γ-signal after calcination at temp...