Investigation On Preparation And Performance Of The Highly Stable Pt-based Diesel Oxidation Catalysts

The main role of diesel oxidation catalyst（DOC）in the diesel vehicle after-treatment system is to completely oxidize carbon monoxide（CO）,hydrocarbon compounds（HC）and soluble organic compounds（SOF）to CO₂ and H₂O,as well as the oxidation of nitrogen monoxide（NO）to NO₂.With the upgraded emission regulations,it requires the low limit value（NO_x:0.4 g/KWh）of China VI and good durability（700000 km）,and the adopted world Harmonized Transient cycle（WHTC）emissions work at low temperature（below 200°C）.Thus,it is necessary for DOC with the excellent low temperature activity,high NO oxidation performance and high hydrothermal stability of DOC in practical application.In this paper,the particle size and chemical state of Pt modified by polymer are systematically developed to control the amount of active platinum center.On that basis,redox additives can strengthen the interaction of Pt with oxides to promote the formation of active oxygen and construct the multi-active centers for improving the catalytic performance of DOC.In order to improve the hydrothermal stability of DOC,the stable promoter is further developed and the preparation method are optimized.The main contents are as follows:1.Investigation about the effect of polyethylene glycol（PEG）on catalytic performance of Pt-based oxidation catalyst is employed.By complexing Pt²⁺and PEG with different molecular weight,it can effectively regulate the microchemical chemical state of Pt nanoparticles,and further affect the catalytic oxidation performance.The results of X-Ray Diffraction（XRD）,Transmission Electron Microscope（TEM）,Thermal Gravimetry（TG）,H₂-Temperature Programmed Reduction（H₂-TPR）,Ultraviolet Visible（UV-Vis）and Diffuse Reflection Infrared Fourier Transform spectra（DRIFTs）suggest that the PEG chain could have a size matching effect between the cavity of pseudo crown ether and Pt ions.And the formation of carbon layer via the"two-step roasting"method could inhibit the migration of Pt,which could improve the platinum dispersion and expose more active sites.The hydroxyl group on the polyethylene glycol interact with platinum species to inhibit the formation of platinum species with a high valence state of Pt.The introduction of PEG could improve the catalytic performance compared to blank sample.With the increased PEG molecular weight from 200 to 600,their catalytic performance shows the increased and then reduced trend.The PEG-modified catalyst with moderate molecular weight of 400 shows the better low temperature catalytic performance,due to the higher platinum dispersion and more metallic platinum.Its light-off temperature of CO/C₃H₆/NO are 203℃,216℃ and 239℃,respectively,with the NO maximum conversion of 52%.2.Investigation about the effect of Polyvinyl alcohol（PVA）on catalytic performance of Pt-based oxidation catalyst is employed.At the room temperature,PVA has the more hydroxyl group than PEG and its open chain could increase the interacted sites with platinum.To further improve the catalytic performance of Pt/Si O2-Al2O3,it is further to investigate the effect of polyvinyl alcohol（PVA）on the growth process and chemical state of Pt nanoparticles,as well as the catalytic activity.The results of XRD,TEM,TG,H₂-TPR,X-ray photoelectron spectroscopy（XPS）,UV-Vis and DRIFTs indicate that the interaction between the hydroxyl group on the side chain of PVA and Pt²⁺promote the formation of metallic platinum,and the spatial effect of PVA chain is conducive to disperse the platinum species.With an increased PVA addition amount（3～7 mass%）and reaction time（5～20 h）of PVA with Pt²⁺,the dispersion of platinum and the amount of metallic platinum has the increased and then decreased tendency,which is same as the varied trendency of catalytic performance.The addition of PVA could improve the catalytic performance compared to the blank sample.Among them,the catalysts with the PVA addition amount of 5 mass%,the degree of polymerizate of 1700 and the reaction time of 10 h,has the best catalytic performance,which has the temperature of 176°C,187°C and 214°C for the light-off temperature of CO/C₃H₆/NO,respectively,and the NO maximum conversion of65%.3.Investigation about the effect of manganese oxides on catalytic performance of Pt-based oxidation catalyst is employed.On the basis of PVA-modified Pt-based catalyst,manganese oxides are introduced to strengthen the interfacial interaction of redox manganese oxides with platinum species for promoting the generation of active oxygen species,the construction of the active oxygen centers,and the construction of the multi-active centers with platinum and active oxygen.The effect of different phases and surficial defect of redox manganese oxides on the catalytic performance are studied to reveal the interrelationship between the construction of multiple active sites and the catalytic performance.The results of XRD,TEM,H₂-TPR,O₂-Temperature Programmed Desorption（O₂-TPD）,Raman spectrum（Raman）,XPS and DRIFTs show that the interface of Pt with manganese oxide could promote the amount of exposed Pt sites and active oxygen species.Comparing to the mono-phase of manganese oxides,the catalysts modified with the equivalent ratio of Mn O2 and Mn2O3 has the higher interface reaction rate and turnover of frequency（TOF）value.The results of Electron Paramagnetic Resonance（EPR）,Raman,XRD,TEM and H2-TPR suggest that the construction of surficial defect on the manganese oxides could anchor platinum to improve the platinum dispersion.With the increased reduction temperature from 200°C to 300°C,the surficial defect concentration of manganese oxides shows the increased and then decreased tendency,which is consistent with their catalytic performance.The catalyst modified with manganese oxides reduced at250°C has the highest defected concentration,forming the more multi-active centers,promoting the formation of intermediate species and thus significantly improving the catalytic oxidation performance.The light-off temperature of CO/C₃H₆/NO could be reduced to 170°C,180°C and 185°C,and the NO maximum conversion is 76%。4.Investigation about the effect of manganese-based mullite on hydrothermal stability of Pt-based oxidation catalyst is employed.To improve the hydrothermal stability of catalysts,the stable manganese-based mullite is introduced as the promoter,whose formed temperature is over 800°C.And the effect of atmosphere treatment on the microscopic chemical state of multi-active centers and hydrothermal stability of Pt-based oxidation catalyst modified with manganese mullite and PVA are studied.The results of XRD,TEM,H₂-TPR,XPS and DRFITs show that the platinum particles grew rapidly（8.5 nm）after oxygen treatment at 800°C whose activity is obviously degraded.After oxidation（800°C）-reduction（250°C）cyclic treatment,the catalyst could be reconstructed and the interaction between Pt and YMn₂O₅ could be strengthened,which could delay the aggregation of Pt nanoparticles（7.5 nm）and expose more Pt sites.Increasing the surficial defect concentration leads to the formation of more active oxygen species,which is conducive to the formation of intermediate species of reaction molecules.After hydrothermal aging（800°C,10%H₂O）,the catalyst with oxidation-reduction cyclic treatment has a higher hydrothermal stability than the un-treated catalyst,which has the light-off temperature of NO at300°C decreased by around 23°C and the maximum NO conversion of 27%decreased by around 4%.5.Investigation about the effect of preparation methods on hydrothermal stability of Pt-based oxidation catalyst is adopted.To further improve the hydrothermal stability,the preparation method of YMn₂O₅ and PVA co-doped Pt-based oxidation catalyst via coprecipitation method,sol-gel method,impregnation method and electrostatic adsorption method are compared to reveal the effect of prepared methods on the stability of multi-active centers and catalytic performance.The results of XRD,H₂-TPR,O₂-TPR and DRFITs suggest that the preparation method can affect the interaction of Pt with YMn₂O₅,and greatly affect the amount of exposed platinum sites,resulting in the sequence of stability on catalysts prepared by different preparation methods as follows:Electrostatic adsorption method>Impregnation method>Sol-gel method>Coprecipitation method.The catalyst prepared by optimized electrostatic adsorption method could induce Pt²⁺to adsorb on the surface of mullite,with the bi-function of anchoring noble metal ions and generating active oxygen center in order to stabilize the multi-active centers.The catalyst after hydrothermal aging（800°C,10%H₂O）has the excellent stability that the light-off temperature of NO is 261°C and the NO maximum conversion is 38%.Compared with commercial catalysts after hydrothermal aging,the Pt Pd bimetallic catalyst via electrostatic adsorption method shows an excellent NO oxidation performance,which has the potential value in practical application.