Zeolite-based catalysts are known to be active in the selective catalytic reduction (SCR) of NO by hydrocarbons and to have some advantages in this reaction such as ion exchange property and strong adsorption ability. Contrary to this, the zeolite channel structure might have a negative effect such as a large diffusion resistance due to small zeolite channel. We reported that this reaction is, sometimes, limited by the intracrystalline diffusion. In this presentation, these results are summarized and discussed in the light of not only a conventional concept of restricted diffusion in zeolite, geometry-limited diffusion, but also a newly proposed concept of restricted diffusion, adsorption-controlled diffusion.
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