In situ sulphated CuOx/ZrO2 and CuOx/sulphated-ZrO2 as catalysts for the reduction of NOx with NH3 in the presence of excess O2

Sulphated catalysts containing the same amount of sulphates (2.6 SO4 nm-2) and a different amount of copper (0.3-3.9 Cu atoms nm-2), Cu/SZ, were prepared by impregnation of sulphated-ZrO2 with toluene solutions of Cu(acetylacetonate)2. Sulphated catalysts containing the same amount of copper (0.3 or 2.5 atoms nm-2) and a different amount of sulphates (up to 4.9 SO2 nm-2), Cu/ZSg, were prepared by sulphation of CuOx/ZrO2 (Cu/Z) via the gas-phase. Samples were characterised by Fourier transformed IR spectroscopy. The selective catalytic reduction of NO with NH3 in the presence of excess O2 (SCR reaction), the NH3 + O2 and the NO + O2 reactions were studied in a flow apparatus.
Activity and selectivity did not depend on the sulphation method used for catalyst preparation but depended on the amount of copper and sulphate, particularly on the sulphate/copper ratio.
As on Cu/Z, on Cu/SZ CuII was active for both SCR and NH3 + O2 reactions. The presence of covalent sulphates caused lower reducibility of CuII to CuI and higher Lewis acid strength of CuI in Cu/SZ than in Cu/Z.
For (i) SCR, (ii) NH3 + O2 and (iii) NO + O2, Cu/ZSg were less active than the parent Cu/Z. As the sulphate content in Cu/ZSg increased, the NO yield in the NH3 + O2 reaction markedly decreased, thus accounting for the increased selectivity in the SCR reaction. In CuOx/sulphated-ZrO2 copper ions were less prone reversibly to undergo the redox process CuII/CuI.
These findings provide new information on the role of copper and sulphate in determining the activity and selectivity for the SCR with NH3.