Pt-Based Zeolites For The Selective Catalytic Reduction Of Nitrogen Oxides By Hydrogen

Selective catalytic reduction of NOx by hydrogen（H₂-SCR）is one of the essential reserve technologies for the purification of nitrogen oxides（NOx）from hydrogen internal combustion engines.The core of H₂-SCR technology is the H₂-SCR catalyst.Developing H₂-SCR catalysts with high activity,high selectivity,wide temperature window,and low cost is essential.It is a promising H₂-SCR catalyst with high synthetic efficiency,low cost,large specific surface area,and rich,acidic aluminum-rich carrier.This study aimed to develop Pt/KFI zeolite catalysts for H₂-SCR with high activity and N₂ selectivity.The catalyst skeleton structure was characterized by employing isothermal nitrogen adsorption and desorption experiments（BET）,X-ray diffraction experiments（XRD）,transmission electron microscopy（TEM）,and X-ray absorption fine spectroscopy（XAFS）,and X-ray photoelectron spectroscopy（XPS）.The active sites of the catalyst were identified,and the mechanism of the H₂-SCR reaction was investigated using Fourier transform in situ infrared spectroscopy（in situ DRIFTS）:（1）A series of Pt-loaded Ptx/KFI zeolites were prepared for the H₂-SCR reaction.H₂ can simultaneously reduce NOx to N₂ and N₂O over Pty/KFI catalysts.the presence of O₂ inhibits the conversion of NOX,but a large amount of NH₃ is produced without oxygen.For Pt/KFI catalysts,only 0.05 wt.%Pt loading of Pt/KFI can A further reduction in Pt loading allows the reaction temperature window to be shifted towards higher temperatures.Given this,the temperature window of the H₂-SCR catalyst can be widened by optimizing the combination of Pt/KFI catalysts with different Pt loadings,and the preferred Pt_0.01Pt_0.03Pt_0.05/KFI combination catalyst achieves a NOx conversion efficiency above 90%at 1%H₂ in the 170-230℃ range.（2）The H₂-SCR reaction mechanism of the Pt/KFI catalyst was revealed,which follows the E-R mechanism.NOx is first adsorbed on the active site to form nitrate,followed by the rapid formation of reaction intermediates from H₂ to active nitrate,producing N₂ and H₂O.The primary function of O₂ in the reaction process is to re-life the active site.Differences in the Pt loading result in differences in the reaction mechanism:at low loading,Pt^δ+ species are the main active sites in the H₂-SCR reaction of Pt/KFI catalysts,whereas high loading of metallic Pt⁰ promotes the reaction of H₂ and O₂,resulting in a deficit of reductant for NOx.（3）Further modification by doping with rare earth elements（La,Ce）effectively promotes the high temperature NOx conversion efficiency and widens the temperature window of the Pt/KFI catalyst,with NOx conversion efficiency above 80%in the range of 140-220℃.The lack of high temperature efficiency of the Pt/KFI catalyst is due to insufficient reductant H₂ as a result of the hydrogen-oxygen competition reaction.The introduction of rare earth metals promotes the adsorption and activation of NOx,inhibits the high temperature hydrogen-oxygen competition reaction and allows more H₂ to reduce the adsorbed nitrate,thus increasing the high temperature activity of Pt/KFI and widening the active temperature window.