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

The nature of the chemical transformations occurring during the thermal activation of chromium(III) and copper(II) doubly promoted tin(IV) oxide catalysts of three stoichiometries (Sn:Cr:Cu atom ratios 1:0.30:0.34, 1:0.30:0.13, and 1:0.13:0.27) have been investigated by a combination of FT-IR, powder X-ray diffraction, transmission electron microscopy, extended X-ray absorption fine structure and near edge structure, and X-ray photoelectron spectroscopy. The freshly prepared gel catalyst materials comprise small (ca. 1−2 nm) particles. Calcination results in a progressive increase in the size of the tin(IV) oxide particles, only slowly initially (ca. ×2 by 673 K, ca. ×10 by 873 K), but sintering to very large particles occurs at higher temperatures. No incorporation of chromium or copper into the tin(IV) oxide lattice occurs even at high temperature. Interaction of the two promoter metals at calcination temperatures of 573 K leads to the formation of copper(II) chromate(VI), CuCrO4. At temperatures ≥873 K the material with a Cu:Cr ratio of 1 comprises crystalline tin(IV) oxide and copper(II) chromite(III), CuCr2O4, only. CuCr2O4 is also formed in both the copper-rich and the chromium-rich materials, but in addition in these materials crystalline CuO and Cr2O3, respectively, are formed. Calcination of the copper-rich material at 1273 K results in a further tranformation to copper(I) chromite(III), Cu2Cr2O4.