Newly Developed Flow Through Catalytic Honeycomb for Improving Future Emission Control

摘要

Development of a New Concept Honeycomb (NCH) substrate consisting of catalytic support, fibrous material, and binder was reported previously by our group [1]. This NCH substrate can provide double the amount of catalytic support compared with a conventional wash-coat type honeycomb catalyst and can widely disperse precious metal even after thermal aging. The NCH is expected to be applicable to not only DOC but also NSC or SCR reactions with NH_3. The strength of the NCH substrate was still insufficient for vehicle installation, however, and our efforts to improve the gas reaction performance and strength concurrently have been severely hampered. In this study we describe our attempts to solve this conflict by particularly focusing on one of the two pore structures that characterize the NCH substrate; that is, on macro-pores which range from 0.05μm to 1μm in size. First, a γ-Al_2O_3-based catalytic support was used to analyze how macro-pores in the NCH substrate contributed to the strength and CO/THC oxidation reaction. This analysis revealed, firstly, that the decrease of macro-pores inside the substrate wall helped to increase the strength in the γ-Al_2O_3-based catalytic support, and secondly, that the gas reaction performance did not depend upon the macro-pores. Elimination of macro-pores from the NCH substrate is one of several promising approaches to solve the above conflict. Secondly, to elucidate the cell structure dependence, the conversion efficiency was analyzed by focusing on the CO/THC oxidation reaction and on the SCR reaction to NH_3 using an Cu/zeolite catalytic support incorporating an ion-exchange. As a result, it was found that while parameters of cell structure such as wall thickness and GSA did not contribute to the CO/THC oxidation performance, the bulk density of NCH substrate strongly contributes to NH_3 adsorption amount and to NO conversion efficiency because of the increase of reaction site per unit volume of honeycomb in this experimental condition. NCH-SCR can widen the temperature window or decrease the catalyst volume compared with the conventional structure of SCR catalyst.