MnO2-supported catalytic bodies for selective reduction of NO with NH3: Influence of NO2 and H2O

Catalytic bodies based on MnO2, TiO2 and sepiolite showed an improved behavior for selective reduction of NOx with NH3 at low temperatures. In this work, the influence of Mn %wt and NO2 and/or water vapor presence was assessed. The materials were prepared by wet equilibrium impregnation of the pre-shaped support. All the catalysts exhibited the characteristic diffraction peaks of TiO2-anatase, hexagonal MnO2 and anhydrous sepiolite. According to SEM-EDX, MnO2 was preferentially deposited over TiO2. The optimum nominal content of Mn was 7.5 %wt, what may be related to an appropriate particle dispersion that confers the suitable properties: lower acid strength, and better redox properties with improved reducibility. This catalyst was then evaluated under NO2 and/or H2O presence. NO2 improved the catalytic response and the best result was obtained with 1:1 NO2:NO ratio showing 90 % NOx conversion and 97 % N-2 selectivity at very low temperature, 110 degrees C. A higher amount of NH4NO3 seems to be formed with the increase in the NO2/NO ratio causing a modification of N-2 selectivity. Although water presence affected the performance, mainly at low temperatures, the catalyst showed good resistance to water vapor at temperatures above 180 degrees C maintaining good activity (90 % of NO conversion and 96 % of N-2 selectivity). When NO2 was co-added to the feed, a beneficial effect was again observed with 91 % of NOx and 95 % of N-2 selectivity converted at 143 degrees C. At low temperatures, deactivation has to do with a competitive effect of H2O with NOx towards the adsorption centers, which decreases the concentration of nitrate species adsorbed on the surface. However, the adsorption of ammonia was less affected. 7.5 Mn catalyst is a highly efficient system to work in real conditions, where there is water in gas stream. In this situation, co-feeding NO2 improves activity.