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

摘要

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 form 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_2O_3 at higher calcination temperatures. Both Cr(III)- and Cr(VI)-promoted catalyst materials exhibit the 250L?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_5O_(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 with Cr~(3+), corroborating the formation of the mixed-valence chromium phase Cr_5O_(12). For the material calcined at 673 K, the spectra indicate the presence of some Cr_5O_(12) together with amorphous Cr_2O_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_5O_(12). For all three types of catalyst, Cr_2O_3 is the sole chromium oxide species present after calcination at calcination temperatures.