Micropore diffusivities of NO and NH3 in Cu-ZSM-5 and their effect on NH3-SCR

Micropore diffusivities of NO and NH3 were experimentally measured for Cu-ZSM-5 using constant volumetric method at a temperature range from 373 K to 423 K. Intracrystalline diffusivities and effective diffusivities of NO were lower than those of NH3, and the activation energy for intracrystalline diffusivity of NO (21 kJ mol(-1)) was higher than that of NH3 (6.3 kJ mol(-1)). The intracrystalline diffusivity of NO in Cu-ZSM-5 catalysts at 373 K was independent of Cu content and Si/Al ratio of Cu-ZSM-5. We proposed an equation for calculation of effective diffusivity of NO in Cu-ZSM-5 which was validated by the experimental data. Kinetic studies for NH3-SCR and NH3 oxidation were carried out using Cu-ZSM-5 with different crystal thickness. The apparent reaction rate of NH3-SCR at 523 K depended on the zeolite crystal thickness; the rates by a small Cu-ZSM-5 (0.088 mu m) was 2.75 times higher than that by a large crystal catalyst (2.7 mu m), corresponding to the effectiveness factor of 0.36. The apparent reaction rate of NH3 oxidation did not depend on the zeolite crystal size. It is concluded that NH3-SCR is controlled by intracrystalline diffusion of NO, or in other words, the active sites in zeolite crystal are not fully utilized due to the NO diffusion limitation. The effective diffusivity of NO in Cu-ZSM-5 estimated by the catalytic study was close to that calculated using the NO diffusivity results, which supported our conclusion.