Pt-based Catalysts For Lean-burn Diesel Engine Exhaust Gas Purification

Diesel engine has been widely promotion and application on a global scale because of its good fuel economy, excellent performance and lower CO2emissions. Solid particles (Soot) and CO, HC, NOx are the main pollutant in diesel engine emissions. Because diesel engine is operated in the lean-burn condition, the traditional three-way catalysts (TWCs) could not effectively play a catalytic purification, and the emission control between Soot and NOx has a trade-off relationship in diesel engine, so in general, the after-treatment system of diesel engine need to adopt a variety of technology. NOx selective catalytic reduction (SCR) and particle capture (DPF) become the research and application focus for heavy duty diesel exhaust purification. By contrast, the research about eliminating the light-duty diesel exhaust which the main pollutants are soot and HC compounds is less. For the key problem of light diesel exhaust emission control technology, namely, how to improve the low temperature activity of Soot combustion and HC adsorption.The Pt-based catalysts that rare earth and titanium are respectively the main composition in the carrier for soot combustion and HC adsorption of molecular sieve catalyst were studied in this dissertation. The preparation of catalysts, reaction performance, structure characteristics, reaction mechanism and application were also studied and discussed. The main research content is as follows:Firstly, because the traditional soot removing precious metal catalyst has the characteristics of expensive and easy to poisoning, new soot eliminate catalysts were developed and the active component is mainly rare earth (CeO2), transition metal (TiO2) and small amounts of precious metals (Pt)The aspects of soot eliminate aspects, firstly, the ratio among ternary complex oxides Y2O3-CeO2-TiO2was fixed, through changing the precipitation sequence in the synthesis process to prepare a series of supports to study the soot eliminate performance under NO atmosphere, and the mechanical mixing of supports were also prepared to compare. It is found that the diffraction intensity of CeO2and TiO2vary greatly with different precipitation sequence. The catalysts possess good thermal stability when the intensity of the former slightly less than the latter, while the former is far less than the latter, the thermal stability of catalysts is poor.The too weak or strong interaction among Y2O3-CeO2-TiO2is badly for thermal stability and activity of catalysts. Parameters that affect the catalytic activity arranged by importance is CeO2deposited later> the combination between Y and CeO2> the SBET of catalysts. According to the results of O2-TPD, XPS and activity, It is deduced that the content of chemisorbed surface active oxygen species (O2-, O") is more, the soot eliminate performance under NO atmosphere is better. The results of In-situ DRIFT show that the bidentate nitrates, adsorbed NO2and bridging nitrates are the active intermediate species.Secondly, a series of CeO2-LnOx(Ln=La, Y, Pr, Nd:the loading of LnOx is10wt%) were prepared by co-precipitation, and the soot eliminate performance under NO atmosphere was studied carefully. The results could be summarized:(1) The doping of LnOx could inhibit the grain growth of CeO2, greatly increase specific surface area and the content of chemisorbed surface active oxygen species (O2-,O-), but it has little effect on the Pt dispersion in catalysts;(ii) The greater the surface area is, the more orderly the pore size distribution is, the smaller the grains of CeO2are, and the more surface active oxygen species the catalyst has, then the better eliminating performance under NO atmosphere is;(iii) Combination the results of In-situ DRIFTs and activity, the active intermediate NOx species by importance order is chelating bidentate nitrates>bridging bidentate nitrates> mondenate nitrates, in addition, the more intermediate species the catalyst has, the better the activity is.The synthesis of high specific surface area support CeO2-TiO2-Al2O3has also been studied. Through study sedimentation behavior of the three precursor (Ce, Ti and Al), it is found that the thermal stability of support is depended on the order of precipitation. Furthermore, the effect of pH of Al and Ce-Ti, crystallization temperature, and the type of template are also been studied. The results show that the supports possess poor thermal stability when the strong intensity of TiO2appears only in XRD spectra, however the strong intensity of both TiO2and CeO2implies that the supports possess good thermal stability. Further, a series of catalysts Pt/CeO2-TiO2-Al2O3were prepared using the supports which prepared by optimized method. By changing the pH of Pt loading process, the soot eliminate performance under NO atmosphere was also studied carefully. The results show that the lower pH of Pt loading process, the catalysts possess bigger Pt particle size, larger amount of chemisorbed surface active oxygen species (O2-, O-) and better activity. Combination the results of In-situ DRIFT and catalytic activity, it is concluded that bidentate nitrates and adsorbed NO2are the active intermediate species. Different concentrations of H2O and SO2resistant test were carried out on the P3catalyst, it is found that the performance changed little before and after poisoning, which implies that the stability of catalyst is good and possesses prospect for industrial applications.As the temperature of lean-burn diesel engine was low and the HC performance for oxide supported Pt based catalyst was not good, the further research work about HC adsorption material which used under the cold start condition is an important part of this paper. The conclusion are following:Comparison of the surface area of the three commercial zeolites HY, HZSM-5and Hβ after heat treatment and hydrothermal treatment, it found that the heat resistance and hydrothermal performance of HP are relatively quite good and may play unique advantages in gas purification. The experiments of Ce modified Hβ were also been studied. Combined with the industrial production practice, it ultimately determined that the optimal parameters for Ce3+ion concentration is0.025mol/L, the pH of ion exchange is6, the temperature of ion exchange solution is50℃and the exchange time is2. In addition, after determining the appropriate conditions for the modification of Hβ with Ce, the precious metal load conditions on modified molecular sieve material were studied. It found that the PH adjustment is crucial to the catalytic oxidation activity of methane in the process of loading, of which the pH6is the best. Furthermore, the elimination performance of the catalyst which prepared using different molecular sieve loaded precious metal on soot were been studied, the results show that the performance of Ce modified samples is much better than the performance of the unmodified sample, and the catalyst performance of samples which PH6is the best. The improvement of catalyst performance is due to the strong interaction between the Ce-Pt and zeolite Hp with Pt that makes the amount of β reactive oxygen species in the catalyst changed, the more the oxygen species are, the stronger the activity is.The catalytic activity of three catalysts Pt/modified zeolite, Pt/unmodified zeolite and Pt/Al2O3were evaluated on the engine bench, it found that the nature of materials has a great effect on the catalytic activity. The catalyst prepared by Ce modified Hβ has the greatest activity, the largest air-fuel ratio window and oxygen storage capacity. It is relative with the outer surface dispersion of Pt and the redox performance. Compared with the catalysts prepared by the unmodified Hβ, the ignition temperature of CO and THC of the catalyst prepared by modified zeolite reduces by23℃; moreover, compared with Pt/Al2O3, the T50of CO and HC were reduced by44℃and42℃respectively.In the last parts of study, the monolithic catalysts have been prepared, combined the conclusion of previous research work and the experience in the application of the diesel exhaust catalyst.Compared with the catalyst from foreign manufacturers and the traditional active coating material catalysts which prepared under the same condition, it is found that the overall performances of traditional catalyst are not as good as the catalyst produced by foreign manufacturers, however, new development catalysts not only have better conversion efficiency for CO, HC than the same type catalyst, but also in PM eliminate. Therefore, the company’s technical level had been improved and the company’s brand image had been maintained through the applied base research.