CoO_x/sulphated-ZrO_2 and CoSO_4/ZrO_2 as catalysts for the abatement of NO with C_3H_6 in the presence of excess O_2

摘要

Cobalt sulphated-zirconia catalysts were prepared by two different methods: (i) impregnation of ZrO_2 with aqueous solutions of CoSO_4, and (ii) impregnation of sulphate-ZrO_2 with toluence solutions of Co (acetylacetonate)_2. Samples were characterized by UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared (FTIR) spectroscopy. NO reduction with C_3H_6 in the presence of excess O_2 was studied in a flow apparatus fed by a reactant mixture of NO:C_3H_6:O_2 = 4000:2000:20,000 ppm in He. DRS and XPS results showed that the presence of sulphates prevented the formation of Co_3O_4. XPS indicated the presence of Co(II) alone. Irrespective of the preparation method. FTIR showed that catalysts with the same sulpahte content had the same covalent sulphates. As the sulphatecontent in the sample increased, FTIR with CO or NO showed that the Lewis acid strength of Co(II) increased, whereas the reducibility and heterogeneity of Co(II) decreased. Sulphated samples with a cobalt content higher than 2 atomm~2 were far more active and selective than the corresponding unsulphated CoO_x/ZrO_2. The presence of sulphates had a weaker effect on the catalytic behaviour of samples with lower Co content (< 2 atomsm~2). We conclude that cobalt and sulpahte cooperate in determining the catalytic activity and selectivity of cobalt sulphated-ZrO_2 catalysts.