Characterization of SnO₂/Al₂O₃ lean NO_x 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 NO_x 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 NO_x catalysis in these samples.