The reaction of nitromethane over Co-ZSM5 has been studied with the aim of establishing the paths by which it can be converted to N2 under the conditions of the methane-SCR reaction over this type of catalyst. When reacted alone it readily decomposes above 250°C to give CO2 and NH3 but the latter is further converted to N2 at temperatures above 360°C if NO and O2 are also present. The reaction of ammonia with NO and O2 is sufficiently fast to account for all N2 formed. The nitromethane reaction systems are stable above 300°C but below that deactivation sets in after a few hours with isocyanic acid (HNCO) eventually becoming the major nitrogen-containing product. If water is then added to the feed the HNCO is largely hydrolysed to NH3 and CO2 and conversion stabilises. Reaction in the presence of water at higher temperature restores the original activity. It is believed that HNCO is the initial decomposition product of nitromethane and deactivation under dry conditions is due to its deposition, possibly as a polymer such as cyanuric acid. Nitromethane also decomposes to CO2 and NH3 over alumina, H-ZSM5, and Na-ZSM5 but only the latter shows deactivation. There is even some conversion over silica but with HNCO observable from the beginning and produced in larger amounts than NH3 below 340°C. Experiments with deuterated nitromethane show that the reaction over Co-ZSM5 has only a small kinetic isotope effect. However, there is rapid H/D exchange between methyl groups and water, most likely via the hydroxyl groups of aci-nitromethane, the enol tautomer of nitromethane. Hydrogen cyanide is a significant minor product over all three zeolite systems at temperatures around 300°C. With Co-ZSM5 its concentration tracks that of ammonia during the course of deactivation and the subsequent enhancement of HNCO hydrolysis when water is added. Nitromethane reacts much faster with NO2 than with NO and O2 over all three zeolites with complete conversion at ≈220°C to give almost entirely N2 and CO2 with Co-ZSM5 and Na-ZSM5 but with CO and N2O formed as well over H-ZSM5. NO2 may act by removal of strongly adsorbed decomposition products (NH3 or HNCO) since the reaction of NH3 with NO2 is very fast with Co-ZSM5 and gives N2 as the dominant product. The overall findings can be explained in terms of a scheme involving dehydration/hydrolysis reactions, largely on the alumino-silicate surface, followed by conversion of nitrogen-containing species to N2 on the transition metal. A reaction scheme which can explain the observations has been developed and its implications with respect to the possible involvement of nitrocompounds in the corresponding SCR reactions of higher hydrocarbons over other catalysts is discussed.
|Number of pages||15|
|Journal||Journal of Catalysis|
|Publication status||Published - 1998|