Evolution of microstructure during the thermal processing of manganese-promoted tin(IV) oxide catalysts

The chemical transformations occurring during the thermal activation of manganese-promoted tin(IV) oxide catalysts have been investigated by FT-IR, powder X-ray diffraction, and nitrogen adsorption. Two methods of catalyst preparation have been employed: W coprecipitation from aqueous solutions containing both tin(IV) and manganese(II) ions and (ii) impregnation of SnO2 using aqueous manganese(II) nitrate or acetate solutions. The initial product obtained by coprecipitation comprises small (2-nm) particles of SnO2 together with much larger (57-nm) crystallites of MnSn(OH)(6). Thermal processing at 573 K causes decomposition of MnSn(OH)(6). No Mn-containing phase can be identified in the temperature range 573-873 K, but crystalline alpha-Mn2O3 is present after treatment at 1073 K and Mn3O4 after treatment at 1273 K. The impregnation route leaves adsorbed nitrate or acetate on the surface of the SnO2 particulate, but both adsorbates are decomposed by 573 K. For the nitrate-derived material, no Mn-containing phase can be identified in the temperature range 573-873 K, although crystalline gamma-Mn2O3 is present after treatment at 573 K, which is transformed to alpha-Mn2O3 by 873 K in the acetate-derived material. Both alpha-Mn2O3 and Mn3O4 are present after treatment at temperatures > 1073 K in both materials but in different amounts. Pore texture, surface area, and particle size evolution with temperature are similar in the three materials.