The nature of the chromium species formed during the thermal activation of chromium-promoted tin(IV) oxide catalysts: An EPR and XPS study

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 (delta, gamma, and beta) in the EPR, depending on the treatment history of the catalysts. The delta-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 beta-signal detected for the uncalcined Cr(III)-promoted catalyst obtained by coprecipitation is most probably due to the hydrated gamma-CrOOH phase but to Cr2O3 at higher calcination temperatures. Both Cr(III)- and Cr(VI)-promoted catalyst materials exhibit the gamma-signal after calcination at temperatures of greater than or equal to 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 Cr5O12 Photoreduction of Cr6+ to, Cr5+ by the X-ray flux occurs during collection of Cr 2p XPS data for Cr6+ compounds. XPS for Cr(VI)-promoted tin(IV) oxide catalysts dried at 333 K are consistent with surface-adsorbed Cr6+ species (monochromate, dichromate). Cr6+ is Still present after calcination at 573 K together with Cr3+, corroborating the formation of the mixed-valence chromium phase Cr5O12 For the material calcined at 673 K, the spectra indicate the presence of some Cr5O12 together with amorphous Cr2O3. Cr 2p XPS spectra for both Cr(III)-promoted catalyst materials dried at 333 K exhibit peaks corresponding to Cr3+ most probably as gamma-CrOOH. However, after calcination at 573 K, both types of material exhibit spectra consistent with the formation of some Cr6+, consistent with the presence of the mixed-valence oxide Cr5O12 For all three types of catalyst, Cr2O3 is the sole chromium oxide species present after calcination at calcination temperatures.