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 SnO 2 using aqueous CrO 3 solutions, impregnation of SnO 2 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(H 2 O) 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 Cr 5 O 12 . At higher calcination temperatures Cr 2 O 3 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.