Copper Catalysts Supported On Novel Metal-Organic Framework For Selective Catalytic Reduction Of Nox With CO

Nitrogen oxides（NO_x）are an atmospheric polluting gas emitted during the combustion and utilization of fossil fuels.SCR-CO is an ideal flue gas denitration technology.Existing studies have shown that SCR-CO catalysts exhibit excellent catalytic performance under oxygen-free conditions.However,the reaction temperature is higher.Therefore,further lowering the temperature under aerobic conditions and maintaining the activity of the catalyst is the main goal of the next study.Due to its high catalytic activity,high N₂ selectivity and low reaction temperature,Cu is regarded as an ideal reactive metal in SCR-CO.Recent studies show that TiO₂is a highly active support,however,its specific surface area of TiO₂ is small,which is not conducive to the high dispersion of CuOx and to the low-temperature reduction performance of the catalyst.A material with excellent characteristics of traditional TiO₂ supports and a large surface area was required as a support.MOFs have a high specific surface area and highly open metal sites,as well as high porosity and tunable properties.MIL-125（Ti）is a crystalline titanium-based MOF composed of TiO₅（OH）octahedral cluster,1,4-benzenedicarboxylic acid linker bridged.Through pyrolysis to make it have the crystal structure of TiO₂ metal oxide and MIL-125（Ti）coexistence,the support has both the outstanding properties of a traditional TiO₂ support and the characteristics of a large surface area and high porosity.In this article,choosing MIL-125（Ti）as the support.Synthesis of Cu-MIL-125（Ti）was using impregnation.Investigating their SCR-CO denitration through experiments.Besides,using ICP,SEM,XRD,TGA,XPS,BET,and other characterization techniques to characterize the catalyst.The followings are the study’s primary findings:（1）It was thoroughly studied how Cu loading affected the catalytic activity of Cu-MIL-125.The results show that the prepared Cu-MIL-125 catalyst can catalyze the efficient reduction of NO by CO under aerobic conditions,the NO conversion rate of 6.2Cu-MIL-125 is 90%at325℃,and the selectivity of N₂ can reach 92%.When the reaction gas is C₃H₆ or C₃H₆+CO,there is still more than 85%NO conversion rate and more than 90%N₂ selectivity.The results of SEM and XRD showed that the catalyst showed the coexistence of TiO₂ structure and metal-organic framework structure,and the multiphase structure of the catalyst helped to improve the catalytic activity.According to N₂ adsorption isothermal experiments,MIL-125（Ti）is a micro-mesoporous material with a high surface area and high porosity.The results of XPS and H₂-TPR showed that Cu⁺and Cu²⁺contained in highly dispersed Cu may be factors affecting the activity of the catalyst,and the high content of adsorbed oxygen can enhance the catalyst activity,which is determined by the reduction of Cu²⁺to Cu⁺and then by the reduction of Cu⁺to Cu⁰.After exploring the thermal stability of the catalyst through TGA characterization,it is found that the catalyst has three weight loss stages,corresponding to the collapse of moisture,non-coordination organic linker,and structure.The activity is best when the water is removed from the connecting agent.Pyridine analysis can show that the catalyst’s active ingredient comes from L acid,which is due to the loss of its B acid after activating MOFs material.Sulfur dioxide had an obvious negative effect on the NO conversion.The results revealed that the NO conversion reduced by18%at 325℃ with 0.02%SO₂.In addition,the catalytic activity could not be recovered after removing SO₂,indicating that the influence of SO₂ on its catalytic performance was irreversible.The primary explanation may be that SO₂ prevented the adsorption of NO and formed sulfate instead of the original intermediate nitrate.（2）By changing the calcination temperature of Cu-MIL-125,the structure of the catalyst was adjusted to study the influence of the catalyst structure on the performance.The catalyst was calcined at 200℃,300℃,and 400℃,respectively.The results show that the catalyst with a calcination temperature of 300℃ can achieve the best NO removal efficiency under the same reaction conditions.The results of SEM and XRD showed that with the increase in calcination temperature,the metal-organic framework gradually collapsed to generate more TiO₂.The reason why the catalyst Cu-MIL-125（300）shows the best activity efficiency may be that the calcination temperature of 300℃ helps the catalyst to remove water and uncoordinated linker,making the catalyst pores blocked by the uncoordinated linker available,and help the oxidation of the catalyst active ingredients,The catalyst is in the coexistence of metal oxides and metal-organic framework,and calcination makes the catalyst pores accessible to enhance the catalytic activity of catalyst.The results of H₂-TPR and XPS revealed that the catalyst’s Cu⁺and Cu²⁺contents may affect the catalytic activity.Besides,the high amount of adsorbed oxygen may increase the catalystic activity.The direct reduction of Cu+to the element Cu underlies most of the catalytic activity of Cu-MIL-125（400）.According to the BET data,the surface area of Cu-MIL-125 reduced with an increase in calcination temperature.The three catalysts are mesoporous substances.（3）The in-situ infrared data demonstrated that NO was initially adsorbed on the catalyst surface to produce NO_x species,which prevented the adsorption of CO and the production of Cu+-CO species.There are a lot of oxygen vacancies on the surface of catalyst,and the molecular oxygen will first decompose in the oxygen vacancies and react with CO to generate CO₂during the reaction.Then,the Cu-O species are reduced to form the oxygen vacancies to decompose NO molecules into N+O.The final products of the reaction are N₂,CO,CO₂,and Cu are oxidized byO₂ to form Cu oxide.