EXAFS, XPS and TEM Characterization of SnO2-Based Emission Control Catalysts

Abstract

ABSTRACTCatalysts formed by sorption of Cr(VI) on to tin(IV) oxide have been investigated by EXAFS, XPS and TEM. The latter shows that uncalcined catalysts comprise uniform, homogeneous particles of dimensions 2nm which increase to 4nm on calcination at 400°C. Calcination at higher temperatures results in an increase in particle size to 20nm × 30nm at 600°C, ca. 60nm at 800°C, and 90–100nm at 1000°C. XPS data show the evolution of chromium(III) species upon calcination and both chromium(VI) and chromium(III) species exist at all calcination temperatures. Chromium K-edge XANES also shows the presence of tetrahedral chromium(VI) species at all calcination temperatures, but the relative abundance decreases with increase in calcination temperature. Corresponding EXAFS data indicate that the uncalcined catalyst contains sorbed four-coordinated {Cr2O7} species whereas calcination produces six-coordinated {CrO6} species together with some residual tetrahedrally coordinated chromium. EXAFS data show that adsorbed {Cu(H2O)6} is the copper(II) species present prior to calcination of CuII-SnO2 catalysts.