INHIBITING AND DEACTIVATING EFFECTS OF WATER ON THE SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE WITH AMMONIA OVER MNOX AL2O3/

The effect of water on the selective catalytic reduction (SCR) of nitr ic oxide with ammonia over alumina supported with 2-15 wt.-% manganese oxide was investigated in the temperature range 385-600 K, with the e mphasis on the low side of this temperature window. Studies on the eff ect of 1-5 vol.-% water vapour on the SCR reaction rate and selectivit y were combined with TPD experiments to reveal the influence of water on the adsorption of the single SCR reactants. It turned out that the activity decrease due to water addition can be divided into a reversib le inhibition and an irreversible deactivation. Inhibition is caused b y molecular adsorption of water. TPD studies showed that water can ads orb competitively with both ammonia and nitric oxide. Additional kinet ic experiments revealed that adsorbed ammonia is present in excess on the catalyst surface, even in the presence of water. Reduced nitric ox ide adsorption is responsible for the observed reversible decrease in the reaction rate; the fractional reaction order changes from 0.79 in the absence of water to 1.07 in its presence. Deactivation is probably due to the dissociative adsorption of water, resulting in the formati on of additional surface hydroxyls. As the amount of surface hydroxyls formed is limited to a saturation level, the deactivating effect on t he catalyst is limited too. The additional hydroxyls condense and deso rb in the temperature range 525-775 K, resulting in a lower degree of deactivation at higher temperature. A high temperature treatment at 77 5 K results in a complete regeneration. The amount of surface hydroxyl s formed per unit surface area decreases at increasing MnOx-loading. T he selectivity to the production of nitrogen is enhanced significantly by the presence of gas phase water.