Evolution of Microstructure during the Thermal Activation of Chromium-Promoted Tin(IV) Oxide Catalysts: An FT-IR, FT-Raman, XRD, TEM, and XANES/EXAFS Study

The chemical transformations occurring during the thermal activation of chromium-promoted tin(IV) oxide catalysts have been investigated by vibrational spectroscopy (FT-IR and FT-Raman), powder X-ray diffraction, transmission electron microscopy, and extended X-ray absorption fine structure and near-edge structure. Three methods of catalyst preparation have been employed: impregnation of SnO2 using aqueous CrO3 solutions, impregnation of SnO2 using aqueous chromium(III) nitrate solutions, and coprecipitation from aqueous solutions containing both tin(IV) and chromium(III) ions. The freshly prepared gel catalyst materials comprise small (ca. 1−2 nm) particles of hydrous tin(IV) oxide, on the surface of which are sorbed chromate(VI) anions, {Cr(H2O)6 3+} cations, or polymeric γ-CrOOH depending on the preparative route. In all three cases, however, calcination at 573 K results in the formation of the mixed-valence chromium compound Cr5O12. At higher calcination temperatures Cr2O3 is formed, which becomes more crystalline the higher the temperature. Concurrently, the size of the tin(IV) oxide particles increases, only slowly initially (ca. ×2 by 673 K and ca. ×4 by 873 K), but sintering to very large particles occurs at higher temperatures. No incorporation of chromium into the tin(IV) oxide lattice occurs even at high temperature.