Research On Ce-based SCR Catalyst With Core-shell Structure Assisted By Plasma For Reducing NO_x From Marine Exhaust

In view of the emission characteristics of marine diesel engine exhaust with wide temperature fluctuation,high sulfur and water content,the traditional V₂O₅-WO₃/Ti O₂catalyst is difficult to achieve the goal of efficient purification under low-temperature and high-sulfur.On the one hand,the environment-friendly SCR catalyst with excellent low-temperature and strong sulfur resistance was developed to reduce NO_xfrom marine exhaust.On the other hand,the non-thermal plasma was combined with NH₃-SCR to form NTP-SCR coupling process to further improve the denitration performance.Ce served as the active component due to its excellent oxygen storage and release properties,and the Zr,W,Mn and Fe were doped into it to explore its denitration performance,sulfur resistance,water resistance and catalytic stability,respectively.The Ce Mn Zr O_x was screened through component optimization due to its excellent low-temperature denitrification performance,and the physicochemical properties and reaction mechanism were analyzed by systematic characterization.In order to improve its sulfur resistance,water resistance and catalytic stability,the Ce-based SCR catalyst with core-shell structure had been successfully synthesized by chemical deposition,in which Ce Mn Zr O_x acted as active component and Ti O₂ served as protective layer.The thickness of Ti O₂ shell was controlled by adjusting the amount of tetrabutyl titanate,and the catalytic performance was evaluated.The structure-activity relationship was clarified by analyzing the physicochemical properties and reaction mechanism.The NO_x conversion of Ce-based SCR catalyst assisted by non-thermal plasma was investigated simulating the cold start and idle conditions of marine diesel engine.The effect of DBD reactor structure parameter on the denitration performance of NTP-SCR was revealed,and the reaction mechanism of influence factor was elucidated to realize the objective of system coordination and coupling synergy.The basic data and theoretical support were expected to provide for industrial applications,and the detailed research conclusions are as follows:（1）The Ce-based SCR catalysts were prepared by Sol-Gel method,and the denitration performance was as follows:Ce Mn Zr O_x>Ce Fe Zr O_x>Ce WZr O_x>V₂O₅-WO₃/Ti O₂>Ce Zr O_x in the temperature range of 150-250°C.The Ce Mn Zr O_xexhibited remarkable low-temperature denitration performance,but its sulfur resistance,water resistance and catalytic stability were poor.The SEM images showed that the microscopic morphology presented three-dimensional network structure,and the loose porous structure was conducive to the uniform dispersion of active sites.The XRD pattern presented Ce O₂ crystal phase,and Zr O_x,WO_x,Mn O_x and Fe O_x crystal phase diffraction peaks were not detected,which indicated that Zr,W,Mn and Fe might be embedded in Ce O₂ lattice in form of ions,but did not destroy the Ce O₂ crystal structure.The XPS results indicated that the content of Ce³⁺and O_αin Ce Mn Zr O_x was high,which was beneficial for the generation of oxygen vacancies.The NH₃-TPD results showed that the NH₃ desorption peak area of Ce Mn Zr O_x was larger than others,which indicated that the content of acid site was high.The H₂-TPR results explicated that the reduction peak area of Ce Mn Zr O_x was large,and the reduction peak shifted to the low-temperature range,indicating that Ce Mn Zr O_x was easily reduced at low temperature.The in situ FTIR results demonstrated that Ce Mn Zr O_x catalyst followed the E-R mechanism at 300°C.（2）The Ce-based SCR catalysts with core-shell structure were synthetized by chemical deposition method.The experimental result showed that the NO_x conversion of Ce Mn Zr O_x@Ti O₂（1:1.5）was higher than others,and its sulfur resistance,water resistance and catalytic stability were significantly improved compared to Ce Mn Zr O_x,which was more suitable for reducing NO_x from marine exhaust.The TEM images showed that Ce Mn Zr O_x@Ti O₂（1:1.5）catalyst existed in the form of spherical nanoparticles,and there was no agglomeration between the particles.The surface of Ce Mn Zr O_x@Ti O₂（1:1.5）presented clear core-shell boundary,and the thickness of Ti O₂ was about 20-30 nm.The XRD pattern presented Ce O₂ crystal phase,and Zr O_x,Mn O_x and Ti O₂ crystal phase diffraction peaks were not detected,which indicated that Zr and Mn might be embedded in Ce O₂ lattice in form of ions,and Ti O₂ might be highly dispersed on the surface of Ce Mn Zr O_x in form of amorphous.The XPS results indicated that the content of Ce³⁺,Mn⁴⁺and O_αin Ce Mn Zr O_x@Ti O₂（1:1.5）was high,which was beneficial to improve redox properties.The NH₃-TPD results showed that the NH₃ desorption peak area of Ce Mn Zr O_x@Ti O₂（1:1.5）catalyst was larger than others,which indicated that the content of acid site was high.The H₂-TPR results explicated that the reduction peak of Ce Mn Zr O_x@Ti O₂（1:1.5）shifted to the low-temperature range,indicating that Ce Mn Zr O_x@Ti O₂（1:1.5）was easily reduced at low temperature.The in situ FTIR results demonstrated that Ce Mn Zr O_x@Ti O₂（1:1.5）catalyst followed both L-H and E-R mechanisms at 300°C.（3）In the NTP-SCR system,the NO_x conversion was higher than that of copper and stainless steel when tungsten was used as high voltage electrode,and the NO_xconversion of screw tungsten was significantly higher than that of rod tungsten.The NO_x conversion was the highest when corundum was used as dielectric barrier material,followed by ceramics and quartz.The discharge characteristics were closely related to the dielectric constant.The NO_x conversion decreased with the increase of gas discharge gap at same discharge voltage.The effect of medium filler on NO_xconversion was as follows:mixed filler>γ-Al₂O₃>ZSM-5>empty tube when the discharge voltage exceeded 16 k V,and the NO_x conversion increased with the increase of reaction zone.The optimal structure parameters of DBD reactor were screw tungsten electrode,quartz dielectric tube,gas discharge gap of 12 mm,γ-Al₂O₃ and ZSM-5 mixed filler and reaction zone length of 300 mm,and the NO_x conversion of Ce Mn Zr O_x@Ti O₂（1:1.5）reached 61.2%when the discharge voltage was 18 k V.However,the NO_x conversion decreased when the discharge voltage exceeded 18 k V,and the reason might be that the reverse reaction occured on account of the excess of·O free radicals.（4）In the NTP-SCR system,the NO_x conversion decreased with the increase of gas hourly space velocity.The effect of non-thermal plasma on the improvement of NO_x conversion was obvious at low temperature.The NO_x conversion increased with the increase of C₃H₆ concentration,and the denitration performance of Ce Mn Zr O_xdecreased sharply with the increase of SO₂,while Ce Mn Zr O_x@Ti O₂（1:1.5）was less affected by SO₂.The denitration performance was improved within a range of H₂O content.However,the NO_x conversion decreased due to competitive adsorption and free electron annihilation if the H₂O content was too high.The Ce Mn Zr O_x@Ti O₂（1:1.5）exhibited strong catalytic stability even in the coexistence of SO₂ and H₂O,and the NO_x conversion reached 91.6%,which indicated that it was more suitable for reducing NO_x from marine exhaust.The NO_x conversion was closely related to the gas composition,which was subject to the comprehensive effect of dielectric barrier discharge and NH₃-SCR catalytic reaction.In the dielectric barrier discharge,gas-phase molecules were ionized,excited and dissociated to generate intermediate products,such as free radicals and active species,which could participate in NH₃-SCR reaction.Among them,H₂O was dissociated to form free radicals,such as·O,·OH and so on,which could oxidize NO to form NO₂ effectively,and the increase of NO₂/NO ratio in reaction atmosphere was beneficial to promote the"Fast SCR"reaction.