Cu/MCM-41 for selective catalytic NO reduction with NH3—comparison of different Cu-loading methods

Abstract

Copper supported on MCM-41, through the template- and the conventional hydroxyl group-ion exchange and incipient-wetness impregnation, were employed for selective catalytic NO reduction with NH3 within a temperature range of 200–450°C. Cu/MCM-41 catalysts with a low Cu content (ca. 0.7wt.%) from the template- and hydroxyl group-ion exchange methods, i.e., TMCu and HMCu, respectively, showed high activities in the NO reduction. Further increase of the Cu content did not obviously improve the activity of the catalysts. HMCu has a higher activity than TMCu, especially at temperatures lower than 350°C. The Cu species on HMCu exhibited stronger tendencies towards reduction and oxidation, indicating that both reduction and oxidation of the active species controlled the NO reduction rate. The Mars–van Krevelen kinetic model gave a satisfactory simulation of the experimental data, which supported the argument that the NO reduction was governed by a cyclic redox of the Cu species. The reaction orders with respect to NO and NH3 were of a fraction (between 0.5 and unity) and of ca. 0, respectively. The activation energies were similar for reactions over these two catalysts, whereas the reactions over HMCu were seen to have larger values of the frequency factor, a parameter closely related to the intrinsic chemical structure. XANES spectra reflected that the catalysts were mainly composed of CuO. The coordination number of CuO analyzed by EXAFS reflected that HMCu had a slightly larger content of CuI, which has been suggested to facilitate NO attack on active sites.