Reaction Studies of the Selective Reduction of NO by Various Hydrocarbons

Abstract

A number of factors affecting the behavior of ZSM-5 and Ferrierite zeolites have been studied and implications concerning mechanism have been deduced. Increasing the loading of the ZSM-5 with Co2+ is shown to increase both the selective catalytic reduction (SCR) of NO and hydrocarbon combustion activities, but not to alter the selectivity under a given set of reaction conditions. The latter was nearly constant at all temperatures and valued at unity up to 673 K, where it precipitously fell at higher temperature. Selectivity was also a function of space velocity and of the NO/C4H10 ratio. Both SCR and combustion followed similar patterns with different charge balancing cations (including H) with a few notable exceptions. The shifts in temperature required to obtain a given conversion of NO to N2 were relatively minor compared with those required for constant combustion. NO and NO2 were equivalent in SCR in the presence of excess O2. In the absence of O2, NO2 was the better oxidant at low temperature although NO became quite effective above 800 K. The tabulated data at various temperatures showed a strong molecular sieve effect in the SCR reaction when neononane was employed, but not for the combustion reaction. None of the other molecules employed showed this behavior. Plots of conversion of NO to N2 vs conversion of hydrocarbon to CO2 correlated the data for all hydrocarbons, regardless of size, on single curves, except for neononane over Co- and CH4 over CuZSM-5. Even HZSM-5 behaved in this way. A further interesting molecular sieve effect was found when reactions over CoFer (Ferrierite) were compared with those over CuZSM-5.