Selective Catalytic Reduction Of NO_x By Hydrogen Over Pt/HZSM-5

As is widely known,NOx causes photochemical smog,damages ozone and endangers human health.Consequently,there are number of de-NOx techniques have been investigated and reported in literatures.In recent years,selective catalytic reduction NOx by hydrogen?H₂-SCR?has been widely studied over precious metal loaded catalysts,because of its high NOx conversion at low temperatures.Although the H₂-SCR is a promising method for eliminating NOx in industry,there are still several problems to be urgently resolved.For instance,the utilization rate for the precious metal in the catalysts is still low;the selectivity of N2 is to be increased;the effective utilization rate for the reductant H₂ is much poor.For Pt-loaded zeolite catalysts,the Pt can primarily locate inside the zeolite channels or on the external surface of the zeolite crystal,according to the preparation method and conditions,which generally makes the performance of the catalyst quite different.However,up to date,there has been no literature differentiating the role of Pt that located different place in the zeolite for the H₂-SCR yet.Hence,two series of Pt/HZSM-5 catalysts that respectively have the character mentioned above in the Pt dispersion were designed for the investigation in this thesis.The Pt/HZSM-5 catalysts were prepared by impregnation of HZSM-5 zeolite with H₂PtCl6 or Pt?NO3?2.From the results of XRD,FTIR and XPS,it was confirmed that for the catalysts prepared with H₂ Pt Cl6 aqueous solution,Pt mainly located at the external surface of the zeolite,whereas for the catalysts prepared with Pt?NO3?2 aqueous solution,Pt in considerable amount entered into the zeolite channels.The Pt inside the HZSM-5 zeolite channels existed in the form of Pt2+ ion and hence could not activate H₂ under the reaction conditions.Nevertheless,it significantly suppressed the H₂ combustion on the catalyst by enhancing the NOx adsorption ability of the catalyst,and hence improved the effective utilization rate of H₂ and the N2 selectivity.To confirm the function of Pt2+ ions inside the zeolite channels,a catalyst of Pt being loaded on the HZSM-5-Al was studied.The corresponding support was obtained by exchanging the HZSM-5 in Al?NO3?3 aqueous solution.In the Pt/HZSM-5-Al catalyst,Pt mainly located at the external surface of the zeolite and Al3+ existed in the zeolite channels,which was supported by the characterization of FTIR.The catalytic performance of the Pt/HZSM-5-Al catalyst in the H₂-SCR results was quite like that of the Pt/HZSM-5 prepared with Pt?NO3?2.The results indicated that the Al3+ inside the zeolite channels also enhanced the NOx adsorption,improved the H₂ utilization efficiency and the N2 selectivity of the H₂-SCR.Thus,the thesis demonstrated that the Pt2+ inside the zeolite channels had only the ability to adsorb NOx,but not the ability to activate H₂.As the most important conclusion of the thesis,it is that the Pt should be support on the external surface of the zeolite,by using the precursor H₂PtCl6 and the zeolite support that containing some high valence ions inside its channels,so as to increase the utilization rate of the precious metal in the catalysts,effective utilization rate of H₂,as well as the N2 selectivity.