The catalytic activity of CuSO4/ZrO2 for the selective catalytic reduction of NOx with NH3 in the presence of excess O2

Abstract

Sulphated ZrO2 catalysts were prepared by (i) impregnation of ZrO2 with aqueous solutions of (NH4)2SO4, Na2SO4, or CuSO4, or (ii) sulphation of CuOx/ZrO2 via gas-phase (2770ppm SO2 and 1%O2, in He). Samples were characterised by means of electron spin resonance (ESR), X-ray diffraction (XRD), diffuse reflectance (DRS), Fourier transformed infrared (FT-IR) (using CO, NO and NH3 as probes), and TPD of NH3. The selective catalytic reduction (SCR) of NO with NH3 in the presence of excess O2, and the NH3 oxidation with O2 were studied in a flow apparatus fed by reactant mixtures of various composition (NO:NH3:O2=0–700:700:36,000ppm, in He).Characterisation and catalysis results indicated that impregnation with CuSO4 or sulphation via gas-phase yielded nearly identical catalysts. Covalent sulphates of various nuclearity formed on both sulphated ZrO2 and copper-containing sulphated ZrO2. Sulphated ZrO2 catalysts were only slightly more active than unsulphated ZrO2. As the CuSO4-content in the CuSO4/ZrO2 catalysts increased, (i) the (polynuclear)/(mononuclear) sulphate ratio, (ii) the concentration and strength of Lewis acid sites, (iii) the concentration of Brönsted acid sites, and (iv) the SCR activity increased in parallel. In the temperature region 473–600K, a range at which CuSO4/ZrO2 yielded nearly 100% SCR selectivity, NO molecules converted to N2 per second per Cu-site were nearly independent of the Cu-content, up to 2.3nm−2 of CuSO4 molecules.CuSO4/ZrO2 were much more selective than the relevant unsulphated CuOx/ZrO2. The higher selectivity of CuSO4/ZrO2 depended on their lower activity in the NH3 oxidation reaction.