日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Although it has been reported that zeolite catalysts have high activity for the selective catalytic reduction of NO by hydrocarbon, unique reaction field of zeolite micropore can also have negative effect of diffusion resistance. In such diesel exhaust with using large molecular size hydrocarbon as a reductant, de-NO_x activity was hindered by the diffusion determined by geometrical configuration between the diffusing molecule and zeolite pore, i.e., geometry-limited diffusion. In the case of small molecule, on the other hand, it was found that the excessive adsorptive interaction between diffusing molecule and exchanged cation led to significant decrease in diffusivity, which resulted in the decrease in de-NO_x activity. In this case, the diffusion was controlled by the adsorptive interaction, i.e., adsorpion-controlled diffusion, whose mechanism was entirely different from the geometry-limited diffusion. Thus, zeolite diffusion was not only a physical phenomenon such as geometry-limited diffusion but also a chemical phenomenon influenced by the interaction between exchanged cation and diffusing molecule.