Development of porous nano-structured materials with heterojunction structure and application to low temperature de-NOx catalysts

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The anion exchange property of hydrotalcite(HT),Mg_<0.74>Al_<0.26>(OH)_2(NO_3)_<0.26>, was studied for the preparation of porous catalysts containing highly dispersed noble metals. In the anion exchange using complexes, PtCl_6^<2-> and PdCl_4^<2->, partial replacement of the ligand with NO_3^- in the HT interlayer occurred as was revealed by XAFS analysis. On heating in a H_2 flow, the nitrate ligand would be reduced to evolve N_2 and the vacant site thus formed can subsequently be occupied by NO_3^- in the interlayer. Such a catalytic role of interlayer noble metals facilitates the removal of NO_3^- from the interlayer space and thus converts the layered structure into a porous framework. The Pt-HT catalyst allowed 82% NO_x conversion at 70℃, whereas a supported Pt/MgO required higher temperature 90℃ where 87% NO_x conversion was obtained. Similarly, Pd-HT catalyst allowed higher NO_x conversion at the lower temperature compared with a supported Pd/MgO catalyst. Thus, Pt-HT nanocompos … More ites were found to be active for the low temperature NO-H_2 reaction in the presence of excess O_2.On the other hand, the reduction of NO to N_2/N_2O in the presence of excess O_2 has been successfully achieved at 70℃ using an electrochemical cell of the type,0.1% NO,0-10% O_2,Pt |NAFION| Pt,H_2O. An H^+-conducting solid polymer electrolyte(SPE) plays a key role in evolving hydrogen on the Pt cathode, where the catalytic NO-H_2 takes place. It was revealed that the competitive H_2-O_2 reaction is suppressed because the Pt surface was covered with stable nitrate(NO_3) species, which blocks oxygen adsorption hereon. The inhibition of H_2-O_2 reaction becomes most efficient at 【less than or equal】100℃ in agreement with the optimal operation temperature range of SPE. The reduction efficiency of NO in an excess O_2 could be improved by packing 1 wt% Pt/ZSM-5 catalyst in the cathode room. The combination between the SPE cell and Pt catalysts can broadly be applied to novel low-temperature deNOx processes in a strongly oxidizing atmosphere. Less