The catalytic activity of CoOx/sulphated-ZrO2 for the NO abatement with C3H6 in the presence of O2: the dependence of activity and selectivity on the sulphate content

Cobalt sulphated-zirconia catalysts were prepared by two different methods: (i) impregnation of ZrO2 with aqueous solutions of CoSO4, and (ii) impregnation of sulphated-ZrO2 with toluene 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 C3H6 in the presence of excess O2 was studied in a flow apparatus fed by a reactant mixture of NO:C3H6:O2 = 4000:2000:20000 ppm in He. DRS and XPS results showed that the presence of sulphates prevented the formation of Co3O4. XPS indicated the presence of Co(II) alone. Irrespective of the preparation method, FTIR showed that catalysts with the same sulphate content had the same covalent sulphates. As the sulphate content 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 atoms nm-2 were far more active and selective than the corresponding unsulphated CoOx/ZrO2. The presence of sulphates had a weaker effect on the catalytic behaviour of samples with lower Co content (< 2 atoms nm-2). We conclude that cobalt and sulphate cooperate in determining the catalytic activity and selectivity of cobalt sulphated-ZrO2 catalysts.