Akira Shichi

Catalyst effectiveness factor of cobalt-exchanged mordenites for the selective catalytic reduction of NO with hydrocarbons

The influence of intracrystalline diffusion on the selective catalytic reduction of NO was examined by using cobalt-exchanged mordenites with different crystal sizes. In the NO-CH4-O2 reaction, the reaction rate increased with cobalt exchange level, but the rate over a large crystal mordenite was slightly lower than that over a small crystal mordenite. In the NO-C3H8-O2 reaction, where the reaction rate was higher than that of the NO-CH4-O2 reaction, the effect of crystal size was more significant. Furthermore, the reaction rate over the small crystal increased with increasing cobalt exchange level, whereas the rate over the large crystal decreased with increasing cobalt exchange level. The effectiveness factor calculated from these results was below unity, indicating that the volume of zeolite crystal was not fully utilized because of the diffusion limitations.

Effective diffusivities of lighter hydrocarbons in Cu- and Co-MFI-type zeolite catalysts

Abstract Intracrystalline and effective diffusivities of ethane, propane, ethylene and propylene were measured for several kinds of MFI-type zeolite, such as silicalite-1, H-MFI, Cu- and Co-MFI-type zeolites at temperatures ranging from 323 to 823 K . The intracrystalline diffusivity represents the mobility of molecules within zeolite crystals, and was found to be reduced to 1/100 for paraffins and 1/1000 for olefins by the existence of active sites (proton, Cu and Co ions), as compared with silicalite-1. The effective diffusivity was evaluated by multiplying the intracrystalline diffusivity by a partition factor. Although the intracrystalline diffusivity of olefins was about 1/10 to 1/100 of that of paraffins, the effective diffusivity of olefins was almost equal to that of paraffins. The calculated effective diffusivity of ethylene within Cu-MFI was in good agreement with those evaluated by the kinetic analysis of the catalytic decomposition of NO with ethylene over Cu-MFI catalyst.

Influence of hydrocarbon molecular size on the selective catalytic reduction of NO by hydrocarbons over Cu-MFI zeolite

The influence of hydrocarbon molecular size on the HC-SCR activity was investigated over Cu-MFI zeolites having different crystal sizes. In the case of n-hexane as a reductant, the catalytic activity did not depend on the zeolite crystal size. In the case of 2,2-dimethylbutane, however, the observed reaction rate depended on the zeolite crystal size, indicating that the reaction was controlled by intracrystalline diffusivity. It was verified that the catalytic activity for the SCR of NO by a larger hydrocarbon over Cu-MFI zeolite was restricted by geometry-limited diffusion depending on the hydrocarbon molecular size and the zeolite pore size.

Factors controlling catalytic activity of H-form zeolites for the selective reduction of NO with CH4

The selective reduction of NO with CH 4 in the presence of excess O 2 over H-ZSM-5 and H-mordenites was investigated, and the factors controlling catalytic activity were discussed. The activity for the reduction of NO into N 2 was independent of crystal size and pellet size, indicating that the diffusion in zeolite channel and macro-pore has negligible effect on the catalytic activity. The catalytic activity proportionally increased with the acid amount, which strongly indicates that the acid amount is the controlling factor for this reaction. The activity was also dependent on the type of zeolites, which may be due to the difference in the acid strength, but not in the pore structure.

30-O-03-Effect of carbon number in hydrocarbon reductant on the selective catalytic reduction of NO over cation-exchanged MFI zeolites

Publisher Summary This chapter discusses the effect of carbon number in n -alkane over copper (Cu)-, silver (Ag)-, and cobalt (Co)-MFI zeolites to examine the possibility of using higher hydrocarbon as a reductant for the selective catalytic reduction of nitric oxide (NO). On all the catalysts, NO conversion shows a volcano-type correlation to the carbon number in n -alkane, although the carbon number at maximum NO conversion varied with the exchanged cation species and the presence of water vapor. The NO conversion increased with increasing carbon number while the deactivation by carbonaceous deposition and strongly adsorbed species became significant. The catalyst deactivation could be suppressed by the presence of water vapor and by periodic selective catalytic reduction (SCR) reaction operation.

The zeolite micropore as a unique reaction field for the selective catalytic reduction of NO by hydrocarbon

Zeolite micropores offer unique reaction fields for catalytic reactions based on narrow pores, a wide variety of channel structure, ion exchangeability and strong solid acids. As a negative effect, narrow pore and channels may suppress the diffusivity of reactant molecules and reduce the catalyst effectiveness factor. This article focuses on the influence of the intracrystalline unique reaction fields of zeolite on the selective catalytic reduction of NO by hydrocarbons (HC-SCR) under extreme conditions, i.e. high space-velocity ratio and low content of reactants in exhausts. Zeolite diffusivity in the HC-SCR is affected by two types of diffusion: geometry-limited diffusion, which is determined by the relative size of diffusing molecule and zeolite pore-opening, and adsorption-controlled diffusion, which is determined by the strong interaction between diffusing molecule and intracrystalline cation.