Effect of Water Vapor on the Selective Reduction of NO by Methane over Cobalt-Exchanged ZSM-5

The effect of water vapor on the reduction of NO with CH4 in excess O2 over a Co-ZSM-5 catalyst was studied. The presence of 2% water significantly decreases the NO conversion at T ≤ 450°C but has less of an effect at T ≥ 500°C. The selectivity of methane toward NO reduction was enhanced by the presence of water at low temperatures and is unchanged at high temperatures. This inhibition is reversible upon eliminating water from the system. The effect of water is dependent on the level of water added, space velocity, reaction temperature, and level of methane in the feed. The addition of water increases the empirical reaction order with respect to either CH4 or NO from a fractional order to first order. Temperature-programmed desorption studies show that the amount of NO adsorption on Co-ZSM-5 is significantly reduced if it is not fully dried, and the competitive adsorption between H2O and NO is probably the cause of the inhibition by water. In addition, Co-ZSM-5 is capable of removing NO and CO simultaneously in either a dry or wet feed.