The catalytic chemistry of nitromethane over Co-ZSM5 and other catalysts in connection with the methane-NOx SCR reaction

Abstract

The reaction of nitromethane over Co-ZSM5 has been studied with the aim of establishing the paths by which it can be convertedto 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 aluminosilicate 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.15 page(s