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 SnO 2 using aqueous CrO 3 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 Cr 3+ ions and appears in the spectra of the Cr(III)-promoted SnO 2 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 Cr 2 O 3 at higher calcination temperatures. Both Cr(III)- and Cr(VI)-promoted catalyst materials exhibit the γ-signal after calcination at temperatures of ≥573K attributed to mixed-valence trimers of the type “Cr(VI)−O−Cr(III)−O−Cr(VI)”, possibly indicating formation of the mixed-valence chromium oxide Cr 5 O 12 . Photoreduction of Cr 6+ to Cr 5+ by the X-ray flux occurs during collection of Cr 2p XPS data for Cr 6+ compounds. XPS for Cr(VI)-promoted tin(IV) oxide catalysts dried at 333 K are consistent with surface-adsorbed Cr 6+ species (monochromate, dichromate). Cr 6+ is still present after calcination at 573 K together with Cr 3+, corroborating the formation of the mixed-valence chromium phase Cr 5 O 12 . For the material calcined at 673 K, the spectra indicate the presence of some Cr 5 O 12 together with amorphous Cr 2 O 3 . Cr 2p XPS spectra for both Cr(III)-promoted catalyst materials dried at 333 K exhibit peaks corresponding to Cr 3+ most probably as γ-CrOOH. However, after calcination at 573 K, both types of material exhibit spectra consistent with the formation of some Cr 6+, consistent with the presence of the mixed-valence oxide Cr 5 O 12 . For all three types of catalyst, Cr 2 O 3 is the sole chromium oxide species present after calcination at calcination temperatures.