Design And Synthesis Of Pd-based Catalysts And Their Three-way Catalytic Performance

Three-way catalytic technology has shown an obvious improvement comparison with decades ago,which showed a high contribution to control automobile emissions pollutions.However,facing the stringent emission regulations over the life of vehicles,there are remain some problems to solve for three-way catalysts in the applications.Especially,improvement the hydrothermal stability and reduction the cost of noble metals are two important assignments for application of three-way catalysts.Therefore,to solve these problems,we designed and systheszed a series of Pd based catalsyts in this study,and assessed the three-way catalytic activites and hydrothermal stability of them.Meanwhile,the catalytic properties of these catalysts were characterized by XRD,XPS,TEM,H₂-TPR,O₂-TPD and CO-DRIFT etc.methods.Moreover,the catalytic mechanism of the catalysts were studied by using the CO oxidation reaction probe.The reseach contents and corresponding progresses were shown as follows:?1?Shape-controlled Palladium?Pd?nanocrystals have been widely investigated as catalysts for various catalytic reactions.In this paper,we report the use of glucose,ascorbic acid,and citric acid as reducing agents for the shape-controlled synthesis of Pd nanocrystals.The so-prepared Pd nanocrystals were loaded on CeO₂ to form the Pd/CeO₂ catalysts.The Pd/CeO₂ catalyst produced with Pd nanocrystals?Pd/CeO₂-g?that were reduced by glucose showed higher catalytic activities than those generated by using ascorbic acid or citric acid.Additionally,Transmission electron microscopy,X-ray diffraction,Temperature programmed reduction and X-ray photoelectron spectroscopy results showed that Pd/CeO₂-g catalysts,which have the exposed high-energy surface?110?crystallographic facets,possessed large amounts of oxygen vacancies,which could improve the catalytic activity.Therefore,this work provides a new alternative to reduce the usage of noble metal in catalytic reaction:indeed,glucose can be employed as a reducing agent to prepare?110?facets Pd nanocrystals,which could be used to synthesise Pd/CeO₂ catalyst with excellent catalytic properties.?2?The hydrothermal stability and catalytic activity of Pd@Ce_0.5Zr_0.5O₂/Al₂O₃catalyst with a core-shell structure were investigated for automobile three-way reactions and compared with those of Pd/Al₂O₃,Pd@CeO₂/Al₂O₃,and Pd@ZrO₂/Al₂O₃catalysts.TEM,HRTEM,and EDS mapping analyses showed that the core-shell structure of Pd@Ce_0.5Zr_0.5O₂ nanoparticles was intact after the hydrothermal treatment at 1050?C for 5 h.Meanwhile,CO-DRIFT results suggested that the interface of Pd core and Ce_0.5Zr_0.5O₂ shell acted as the active sites in the reaction of three-way catalysts?TWCs?.Additionally,XPS,FT-IR,and CO-DRIFT analyses demonstrated that a large amount of OH groups were present on the surface of Pd@Ce_0.5Zr_0.5O₂/Al₂O₃ catalyst,which could accelerate the decomposition of carbonate species and reduce the activation energy of the catalytic reaction.This was an important reason for the Pd@Ce_0.5Zr_0.5O₂/Al₂O₃ catalyst to keep the high catalytic activity after aged at high temperature.?3?The moisture treated Pd@CeO₂/Al₂O₃ and Pd/CeO₂/Al₂O₃ catalysts were prepared for automobile three-way reactions.Compared to the Pd/CeO₂/Al₂O₃,the Pd@CeO₂/Al₂O₃ catalyst with the core-shell structure showed better three-way catalytic activities.TEM images and XPS spectra showed that more amount of PdO₂were existed on the interface of Pd and CeO₂ over the Pd@CeO₂ nanoparticles.FT-IR spectra analysis demonstrated that the hydroperoxyl?*OOH?groups were generated on the surface of Pd@CeO₂ nanoparticles.CO-DRIFT measurement suggested that CO adsorbed on*OOH species contributed to the formation of CO₂ and intermediates*COOH,which was a main factor for moisture treatment Pd@CeO₂/Al₂O₃ catalyst to show high catalytic activities.The results of this work revealed the significant role of moisture and core-shell interface co-promoting effect on three-way reactions over Pd@CeO₂/Al₂O₃ catalyst,which might be applied to other catalytic reactions.?4?The MgO-doped hexagonal boron nitride?MgO-h-BN?nanomaterials were designed as a support for Pd catalysts for CO oxidation reaction.The catalytic properties of Pd/MgO?5?-h-BN catalyst was investigated and compared with Pd/h-BN catalyst.Transmission electron microscopy?TEM?results suggested that the PdO nanoparticles with small grain sizes?ca.8 nm?were effectively dispersed on the surface of MgO,and the MgO were deposited on the h-BN nanosheets in the Pd/MgO?5?-h-BN catalyst.X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?analyses revealed that a small amount of Mg²⁺embedded into the B vacancies of h-BN nanosheets in the Pd/MgO?5?-h-BN catalyst.More significantly,a large amount of adsorbed oxygen appeared near the B vacancies of Pd/MgO?5?-h-BN catalyst.The experiments outcomes of diffuse reflectance fourier transform spectra analysis of CO?CO-DRIFT?indicated that the carbonate/formate species were easily decomposed to CO₂ on the Pd/MgO?5?-h-BN catalyst than Pd/h-BN catalyst,which might be a reason why the Pd/MgO?5?-h-BN catalyst had high CO oxidation activity.According to above analyses,we provide a new insight for the MgO-h-BN nanomaterials for Pd catalysts for application in CO oxidation reaction.