Adsorption, activation, and oxidation of ammonia over SCR catalysts

The catalytic activity in the reduction of NO by ammonia in the presence of oxygen (SCR process) is reported for CuO-TiO2 and for different catalysts belonging to the MgO-Fe2O3 system. These materials show high activity even at relatively low temperatures, with a maximum NO conversion near 520 K (CuO-TiO2) and 600 K (MgO-Fe2O3). At higher temperatures, NO conversion is reduced because of the competitive NH3 oxidation by O-2 to NOx and N-2. The adsorption and transformation of ammonia over these systems has been investigated by FT-IR spectroscopy in vacuum and in contact with oxygen and NO. In all cases ammonia is first coordinated over Lewis sites and later undergoes hydrogen abstraction giving rise either to NH2 amide species or to its dimeric form N2H4, hydrazine, detected over CuO-TiO2 and gamma-Fe2O3. Other species tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N-2(-) and azide anions N-3(-) are produced further. In the presence of NO3- coordinated ammonia rapidly disappears, the above intermediates are not found, and water is produced, showing that the SCR reaction occurred. Ammonia protonation to ammonium ion is not detected at all over these systems. It seems consequently demonstrated that Bronsted acidity is not necessary for the appearance of SCR activity. A comparison with the previously-published data on V2O5-TiO2-based systems is done and mechanisms of ammonia oxidation by oxygen and by NO are proposed. (C) 1995 Academic Press, Inc.