Selective Catalytic Reduction Of Nitrogen Oxide By H₂ On Noble Metal Catalysts In The Presence Of Excess Oxygen

The control of nitrogen oxides（NO_x）is a hot research topic.Hydrogen selective catalytic reduction（H₂-SCR）of NO_x has many advantages such as good low-temperature activity,clean and pollution-free products.Therefore,it has great application value and broad development prospects.Both Pt-based and Pd-based catalysts exhibited high activity in the H₂-SCR reaction.In this research,Pt-based catalysts were first studied.Pt/TiO₂ was doped with precious metal Ir to prepare Pt-Ir/TiO₂ bimetallic catalysts.The promotion mechanism of Ir on Pt/TiO₂ was revealed.In order to compare with Pt-based catalysts,the performance of Pd-based catalysts in H₂-SCR reaction was investigated.The activity of Pd-based catalysts was improved by regulating support morphology,doping with additives and modifying the support.The physical/chemical structures of the catalysts were deeply analyzed by a series of characterization methods,and the reaction mechanism of H₂-SCR was explored by Fourier Diffuse Reflectance Infrared Spectroscopy（In situ DRIFTS）.The main research results of the present research are as follows:（1）For Pt-based TiO₂ catalysts,when the total mass fraction of noble metals was 0.5%and the mass ratio of Pt:Ir was 1:1,the activity of Pt₁Ir₁/Ticatalyst was the highest.The NO_x conversion was as high as 99%at 150°C.XPS results showed that the electrons on Pt₁Ir₁/Ticatalysts were transferred from Ti⁴⁺to Pt²⁺and Ir^δ+,generating Pt⁰ and Ir⁰,both of which were favorable for H₂-SCR reaction.In situ DRIFTS results showed that the kinds and adsorption amount of Pt₁Ir₁/Tiwere significantly increased,and still retained high NO_x adsorption ability in high temperature region.Most active intermediates and NH₃ adsorbed on Lewis acid sites were generated on Pt₁Ir₁/Ticatalyst,which promoted the NO_x reduction reaction.（2）TiO₂ nanowires were prepared by hydrothermal method,and 0.5 wt%Pd（Pd/TiO₂-NWs）was supported on the TiO₂ nanowires by impregnation method.Compared with Pd based TiO₂ nanoparticles（Pd/TiO₂-P）,the catalytic activity of Pd/TiO₂-NWs was significantly improved,and the NO_xconversion reached 82.3%at 150°C.At 125-350°C,the NO_x conversion over Pd/TiO₂-NWs catalyst was consistently maintained above 60%.The catalyst still maintained a good nanowire structure after the reaction,indicating that it had good stability.XRD results showed that the Pd/TiO₂-P catalyst was a mixed crystal structure composed of anatase phase and rutile phase,while Pd/TiO₂-NWs catalyst was completely anatase phase.XPS results showed that 44.1%of Pd O was reduced to Pd⁰ on Pd/TiO₂-NWs catalyst,and the amount of oxygen adsorbed on the surface of Pd/TiO₂-NWs was significantly increased.In situ DRIFTS showed that more NO_x species were adsorbed on Pd/TiO₂-NWs catalyst,and more active intermediates such as NH₃ adsorbed on Lewis acid sites were generated at the same time,which promoted the H₂-SCR reaction.（3）The Pd-Co/TiO₂ catalyst was prepared with TiO₂ as the support.When the mass fractions of Pd and Co were both 0.5%,the activity of Pd0.5Co0.5/Tiwas the highest.At 125-300°C,the NO_x conversion of Pd0.5Co0.5/Ticatalyst was higher than 80%;at 150°C,the NO_x conversion rate was as high as 97%,and its N₂ selectivity was also significantly improved.Pd-Co/TiO₂ catalyst has good resistance to high temperature,water and CO poisoning.XPS results showed that the addition of Co promoted the formation of Pd⁰;meanwhile,the amount of·H was significantly improved,thereby improving the catalytic activity of Pd-Co/TiO₂.Compared with Pd0.5/Tiand Co0.5/Ti,Pd0.5Co0.5/Tiadsorbed the largest amount of NO_x and active intermediates,and generated NH₃ adsorbed on Lewis acid sites.The above factors are all conducive to the improvement of catalyst activity.（4）Zr_aTi_b supports with different Zr:Tiratios were prepared by hydrothermal method,and 0.5 wt%Pd was supported on Zr_aTi_b support by the impregnation method.When the mole ratio of Zr:Tiwas 1:2,the activity of prepared Pd/Zr₁Ti₂ catalysts was the highest.At 125-300°C,the NO_xconversion rate of Pd/Zr₁Ti₂ were higher than 90%,and the temperature window was significantly broadened.Pd/Zr₁Ti₂ catalyst showed good hydrothermal stability.In the presence of impurity gases H₂O,CO and CO₂,Pd/Zr₁Ti₂ catalyst can still maintain high activity.The results of XRD and Raman spectroscopy showed that Zr-Tisolid solution was formed in Pd/Zr₁Ti₂catalyst,and there was a strong interaction between Zr and Ti.According to XPS characterization,85.8%of Pd O on Pd/Zr₁Ti₂ catalyst was reduced to Pd⁰,and the amount of surface oxygen was also significantly increased,which were beneficial to the H₂-SCR reaction.The amount of NO_x adsorbed on Pd/Zr₁Ti₂ catalyst was significantly increased,and more active intermediates were generated,which improved the activity of the catalyst.