Experimental and kinetic modeling study of NO oxidation: Comparison of Fe and Cu-zeolite catalysts

Abstract

NO oxidation is considered as the rate determining step of the standard selective catalytic reduction (SCR) reaction (NO+NH3+O2) on metal-exchanged zeolite catalysts. A comprehensive experimental and kinetic modeling study of NO oxidation reaction was carried out and a comparison made between Fe-ZSM-5, Cu-ZSM-5 and recently developed small pore Cu-chabazite catalysts. Steady state integral measurements of NO oxidation show Fe-zeolite to be a more active NO oxidation catalyst than Cu-zeolite. Both NO2 temperature programmed desorption and decomposition experiments show that the NO2 is more strongly bound on the Cu-chabazite compared to Fe-ZSM-5. Increasing amounts of NO2 in the NOx feed decrease the NO oxidation activity on both the catalysts. Differential kinetic studies show a positive order rate dependence of NO oxidation reaction with respect to both NO (∼1) and O2 (∼0.5) while showing a negative order dependence with respect to NO2 (−0.5 to −1), which confirms that the NO2 inhibits the NO oxidation reaction. NO2 inhibition on NO oxidation was found to be more severe on Cu-chabazite (∼ −1 order) compared to Fe-ZSM-5 (∼ −0.5 order). The overall activation energy of NO oxidation was found to be 39kJ/mol on Fe-ZSM-5 compared to 56kJ/mol on Cu-chabazite. The presence of feed water strongly inhibits the NO oxidation reaction. Transient NO2 uptake experiments reveal nitrate formation on both catalysts. Catalysts pretreatments by different oxidizing/reducing agents (O2, NO2, H2 and NH3) did not affect steady state NO oxidation results. Experiments with monolith catalysts having the same washcoat volume but different washcoat thicknesses rule out the presence of washcoat diffusion limitations for NO oxidation. A global kinetic model is developed which accurately predicts the experimentally observed reaction orders, NO conversions and NO2 and water inhibition effects for a wide temperature range and different feed conditions. The kinetic model accounts for the experimentally observed instability of nitrates in the presence of feed NO and shows that the steady state NO oxidation data can be predicted with a global kinetic model assuming the surface reaction between adsorbed O atoms and gaseous NO as the rate determining step.