The mechanism of nitrogen oxides reduction by hydrocarbons and in other systems

Three aspects of the catalytic reduction of NO_x have been investigated with particular emphasis on Cu-ZSM5. The reaction using methane shows a large kinetic isotope effect which is absent with isobutane. Thus, while abstraction of hydrogen from methane is rate determining, a different step plays that role with higher hydrocarbons. With Cu-ZSM5 and Co-ZSM5 the equilibration step, NO to NO₂, is incomplete below 400°C in the presence and absence of methane. However, conversion of NO₂ to NO is near complete when using isobutane over Cu-ZSM5, possibly due to a process which also maintains the catalyst free of deposited material. Measurements of the further reactions of isocyanic acid, HNCO, over Cu-ZSM5 at 270°C show that its rate of hydrolysis to CO₂ and NH₃ is fast while that with NO to give N₂ and N₂O is absent.