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: (i) coprecipitation from aqueous solutions containing both tin(IV) and manganese(II) ions and (ii) impregnation of SnO 2 using aqueous manganese(II) nitrate or acetate solutions. The initial product obtained by coprecipitation comprises small (2-nm) particles of SnO 2 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 α-Mn 2 O 3 is present after treatment at 1073 K and Mn 3 O 4 after treatment at 1273 K. The impregnation route leaves adsorbed nitrate or acetate on the surface of the SnO 2 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 γ-Mn 2 O 3 is present after treatment at 573 K, which is transformed to α-Mn 2 O 3 by 873 K in the acetate-derived material. Both α-Mn 2 O 3 and Mn 3 O 4 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.