ADSORPTION, ACTIVATION, AND OXIDATION OF AMMONIA OVER SCR CATALYSTS

The catalytic activity in the reduction of NO by ammonia in the presen ce 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 conve rsion near 520 K (CuO-TiO2) and 600 K (MgO-Fe2O3). At higher temperatu res, NO conversion is reduced because of the competitive NH3 oxidation by O-2 to NOx and N-2. The adsorption and transformation of ammonia o ver these systems has been investigated by FT-IR spectroscopy in vacuu m and in contact with oxygen and NO. In all cases ammonia is first coo rdinated over Lewis sites and later undergoes hydrogen abstraction giv ing rise either to NH2 amide species or to its dimeric form N2H4, hydr azine, detected over CuO-TiO2 and gamma-Fe2O3. Other species tentative ly identified as imide NH, nitroxyl HNO, nitrogen anions N-2(-) and az ide anions N-3(-) are produced further. In the presence of NO3- coordi nated ammonia rapidly disappears, the above intermediates are not foun d, and water is produced, showing that the SCR reaction occurred. Ammo nia protonation to ammonium ion is not detected at all over these syst ems. It seems consequently demonstrated that Bronsted acidity is not n ecessary for the appearance of SCR activity. A comparison with the pre viously-published data on V2O5-TiO2-based systems is done and mechanis ms of ammonia oxidation by oxygen and by NO are proposed. (C) 1995 Aca demic Press, Inc.