Alumina-Supported Manganese Oxide Catalysts: II. Surface Characterization and Adsorption of Ammonia and Nitric Oxide

Abstract

Alumina-supported manganese oxide catalysts (2-8.4 wt% Mn), prepared from manganese acetate, have been characterized by in situ infrared (IR) spectroscopy and temperature-programmed reaction and desorption (TPRD), in relation to the selective catalytic reduction (SCR) of NO with NH3. Two Lewis acid-type coordinatively unsaturated Mn ions are present on the catalyst surface, most likely in the 3+ oxidation state. The Mn catalyst does not show Brønsted acidity other than that of the support. The Mn dispersion amounts to at least 20-30% The molecular interaction with ammonia is relatively strong. No ammonia oxidation is observed if oxygen is absent. The interaction with NO is very weak, although strongly bonded oxidized species can also be formed in the presence of oxygen, resulting in NO2, nitrito, and nitrate groups. These species decompose giving back NO gas. IR spectra of NH3-NO coadsorption suggest that Mn3+ species can bind both one NO and one NH3 molecule. In the 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 preadsorbed. Oxygen has several roles: it oxidizes the catalyst, favoring NO adsorption; it permits hydrogen abstraction from adsorbed ammonia, thereby activating it for reaction with NO; and it can oxidize gas-phase NO to NO2. Hydrogen abstraction that has proceeded too far results in the formation of N2O, which occurs at higher temperatures and lowers the reaction selectivity for SCR.