ALUMINA-SUPPORTED MANGANESE OXIDE CATALYSTS .2. SURFACE CHARACTERIZATION AND ADSORPTION OF AMMONIA AND NITRIC-OXIDE

Alumina-supported manganese oxide catalysts (2-8.4 wt% Mn), prepared f rom manganese acetate, have been characterized by in situ infrared (IR ) spectroscopy and temperature-programmed reaction and desorption (TPR D), in relation to the selective catalytic reduction (SCR) of NO with NH,. Two Lewis acid-type coordinatively unsaturated Mn ions are presen t on the catalyst surface, most likely in the 3+ oxidation state. The Mn catalyst does not show Bronsted acidity other than that of the supp ort. The Mn dispersion amounts to at least 20-30% The molecular intera ction with ammonia is relatively strong. No ammonia oxidation is obser ved if oxygen is absent. The interaction with NO is very weak, althoug h strongly bonded oxidized species can also be formed in the presence of oxygen, resulting in NO2, nitrito, and nitrate groups. These specie s decompose giving back NO gas. IR spectra of NH3-NO coadsorption sugg est that Mn3+ species can bind both one NO and one NH, molecule. In th e absence of oxygen reaction between NO and NH3 is observed in the IR cell in the temperature range 300-423 K. In the presence of oxygen the reaction occurs to completion already at 325 K, provided ammonia is p readsorbed. Oxygen has several roles: it oxidizes the catalyst, favori ng NO adsorption; it permits hydrogen abstraction from adsorbed ammoni a, thereby activating it for reaction with NO; and it can oxidize gas- phase NO to NO2. Hydrogen abstraction that has proceeded too far resul ts in the formation of N2O, which occurs at higher temperatures and lo wers the reaction selectivity for SCR. (C) 1994 Academic Press, Inc.