Characterization of SnO2/Al2O3 Lean NOx Catalysts

SnO2/Al2O3 prepared with SnCl2 impregnation onto sol–gel Al2O3 and calcined at 800°C is one of the most active catalysts for lean NOx reduction with propene under high partial pressures of O2 (15%) and H2O (10%) and space velocity (30,000 h−1). An unusual feature of this catalyst is the independence of maximum NO conversion over a wide range of Sn contents (1 to 10 wt% Sn). This catalyst is, however, sensitive to other preparation variables such as the nature of Sn precursor and the calcination temperature. X-ray photoelectron spectroscopic results indicate that Sn is present in the +4 oxidation state irrespective of Sn loadings. X-ray diffraction, temperature-programmed desorption, and temperature-programmed reduction results indicate a wide range of oxo–tin cluster sizes as well as different ratios of amorphous and crystalline SnO2 present in samples with different Sn loadings. The Sn(IV) in the amorphous SnO2 fraction can be reduced to Sn(II) below 450°C in the TPR experiments. In fact, all of the SnO2/Al2O3 catalysts have very similar H2 uptake features below 450°C. In view of the fact that they also have very similar catalytic properties, it appears likely that easily reducible amorphous SnO2 is an active phase for lean NOx catalysis in these samples.