Ammonia activation over catalysts for the selective catalytic reduction of NO_x and the selective catalytic oxidation of NH_3. An FT-IR study

摘要

The adsorption and transformation of ammonia over V_2O_5, V_2O_5/TiO_2, V_2O_5-WO_3/TiO_2 and CuO/TiO_2 systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH_2 amide species or to its dimeric form N_2H_4, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N_2~- and azide anions N_3~- are further observed over CuO/TiO_2. The comparison of the infrared spectra of the species arising from both NH_3 and N_2H_4 adsorbed over CuO/TiO_2 strongly suggest that N_2H_4 is an intermediate in NH_3 oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH_2 species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V_2O_5-based systems, but not over CuO/TiO_2, in spite of the high SCR and SCO activity of this catalyst. Consequently Broensted acidity is not necessary for the SCR activity.