On the mechanism of the selective catalytic reduction of NO with higher hydrocarbons over a silver/alumina catalyst

The possibility of forming nitrogen in the gas phase by reaction of activated forms of NOx with amines and ammonia as well as with other organic intermediates, which can be converted to amines and/or ammonia, was investigated. The activation of NO was carried out by passing NO together with oxygen and small amounts of hydrogen over an Ag/alumina catalyst. A special T-shaped reactor was used to produce activated forms of NOx and to feed model compounds of proposed intermediates to the gas phase after the catalyst. Nitrohexane, hexylisocyanate, heptanenitrile, hexylamine, and ammonia were used as model compounds. The T-reactor tests showed that both ammonia and hexylamine react in the gas phase with activated NOx species producing N2. Hydrogen was shown to have at least two main functions. First, hydrogen contributed to improved oxidation of all involved species, resulting in faster production of key intermediates. Secondly, hydrogen assisted in the formation of activated NOx species for the gas-phase reactions. Nitrohexane was transformed to NH3 in the presence of O2 over the Ag/alumina already at 250 °C and the amounts of NH3 produced increased in the presence of H2O. Hexylisocyante was hydrolyzed to amine and ammonia at 250 °C over the catalyst in the presence of O2 but only to ammonia at 400 °C. At 250 °C the conversion to amine and ammonia was almost doubled by the addition of H2O. Heptanenitrile was quite stable and only small amounts of NH3 were observed at 400 °C; however, nitriles may react directly with activated forms of NOx forming N2. The importance of gas-phase reactions as a part of the HC-SCR mechanism is emphasized. R-NO2, R-NCO, and R-CN are intermediates for the formation of amines and ammonia, which are consumed both on the surface of the catalyst and in the gas phase behind the catalyst bed by reactions with activated NOx species.