NH3 and urea in the selective catalytic reduction of NOx over oxide-supported copper catalysts

The temperature-programmed activity of a series of oxide-supported (TiO2, Al2O3 and SiO2) Cu catalysts formed from two different Cu precursors (Cu(NO3)2 and CuSO4) for the selective catalytic reduction of NOx using solutions of urea as a reductant have been determined. These activities are compared to those found using NH3 as a reducing agent over the same catalysts in the presence of H2O and it is found that catalysts that are active for the selective reduction of NOx with NH3 are inactive for its reduction using solutions of urea. Poisoning of the surface by H2Oads is not responsible for all of this decrease in activity and it is postulated that the urea is not hydrolysing to form NH3 over the catalysts but rather is oxidising to form N2 or forming passivated layers of polymeric melamine complexes on the surface. The catalysts were characterised by temperature-programmed reduction while temperature-programmed desorption and oxidation of NH3 and temperature programmed decomposition of urea are used to characterise the interaction of both reductants with the various catalysts.