Automobile Exhaust Purification The Low Ignition Catalyst Research

The urban atmospheric pollution control has been listed as the important strategic task of China's sustainable development in the 21st century. Automobile exhaust emissions are the main sources of air pollution in cities. China's automobile exhaust emission regulations have rapidly developed from GBⅠto GBⅣsince 2000. More stringent emission regulations make higher demands on emission control technique. Three-way catalyst(TWC) is always the research emphasis as the core components of vehicle emission control system. Started from China's GBⅢ/GBⅣ, the cold start emission level of vehicle were required to test. When starting a cold engine for 30s～60s,60%～80% uncombusted hydrocarbons (HCs) are exhausted because of its fuel-rich running. But the traditional underfloor TWC cannot meet its light-off temperature to effectively treat emission pollution in 30s～60s. Therefore, the research on rapid light-off catalyst technique during the cold start becomes the hot investigation on the more stringent emission regulations.A great deal of research results demonstrated that, the rapid light-off catalyst technique is not just the research of catalyst formula and catalytic activity. Catalyst light-off characteristic research is a system engineering which often depends on many complex factors, such as carrier performance, catalyst formula, preparation process and structure design. The research was supported by the National High Technology Research and Development Program (863) of China, Yunnan Province Reserve Talented Person of Academic and Technical Leader Program. In allusion to China automobile industry urgent needs to GBⅣcatalyst which has rapid light-off characteristics, this research was based on the rapid light-off catalyst technique to explore the relationship between carrier properties, washcoat material property and the precious metal property with light-off characteristics. The key influencing factors for rapid light-off characteristics of catalyst were systematically investigated. The low-cost GBIVcatalyst which contained precious metals of 30g/ft3～35g/ft3(or 1.06g/L～1.2g/L) have been developed successfully. Since the systematic research for rapid light-off catalyst is less in our country, this research contents and low-cost targets in this thesis have innovation and advancement. Main research contents include the five parts, such as analyzing the relationship between cell structure of honeycomb ceramic substrate with light-off characteristics of catalyst, relationship research between pore structure of alumina with light-off characteristics of catalyst, the research of impact of washcoat material on light-off characteristics of catalyst, the cooperative effect research of washcoat with the precious metals on light-off characteristics of catalyst, and catalyst preparation.1. Substrate researchThis paper calculated and verified the relationship between the cell density, volume, backpressure of three types of substrates with different cell densities (400cpsi,600cpsi,900cpsi) and light-off factor(LOF). The results tested on engine bench demonstrated that, (1) Higher cell density of substrate could improve the light-off performance and thermal transfer characteristics of catalysts. Especially, the higher cell density has superior compensation effect for degradation of the surface area and catalytic activity caused by high temperature aging. The light-off performance of catalyst mainly depended on the cooperation effect between the cell density of substrate and the precious metals loading. The higher cell density of substrate benefited to rapid light-off when the precious metals loading were less than 1.0g/L. For the normal cell density substrate of 400cpsi, the increase of the precious metals loading to improve light-off activity was most effective when the precious metals are more than 1.0g/L. (2) The effect of increasing cell density of substrate on air-fuel ratio characteristic of catalyst was stronger than increasing precious metals loading. It was effective measurement that higher cell density enlarged the operating window of catalyst. (3) For the same precious metals loading, the increase of substrate volume benefited to obtain lower light-off temperature than the increase of cell density of substrate. (4) The backpressure test results of catalyst with 400cpsi substrate showed that, Hs decreased when the coating rate was more than 45%. Therefore, the coating rate of 45% was optimal for rapid light-off of catalyst with 400cpsi substrate.2. Research on the pore structure of aluminaThree kinds ofγ-Al2O3 with middle pores were prepared to the series of modifying alumina of AlBaLa oxide. The ratio of modifying element was La/Ba=1. The effect of (La+Ba) contents on the pore structure of AlLaBa oxide was observed. The screened modifying alumina loaded Pd, Rh. The relationship among the modifying agent content, pore structure and the precious metals in the modifying alumina with light-off characteristics were observed. Al2O3-02 was the optimal alumina raw material by the characteristics of BET, XRD, H2-TPR.2.5%La and 2.5% Ba were added in Al2O3-02 to form 2.5%La2.5%Ba/Al2O3-02 (abbreviate:GAL), which have optimal surface area, pore volume and thermal stability. Pd and Rh were loaded on GAL which has calcined at different temperatures. When the calcine temperature was lower than 950℃, GAL loaded Rh3+ had optimal dispersity. When the calcine temperature was higher than 950℃, Pd/Rh/GAL has optimal reduction activity at lower temperature, and the low temperature reduction activity of Pd/GAL was better than Rh/GAL。3. Research on the washcoat materalsThe Ce-Zr-Al complex oxides (CZA) were physically mixed into GAL. The catalytic activity of CZA and CZA/GAL was examined. Pd and Rh were loaded on the different GAL/CZA hybrid. The effect of mixing ratio of GAL and CZA on the catalytic reduction behavior, oxygen storage capacity (OSC) and surface area of Pd/Rh catalyst were examined by H2-TPR, BET, etc. High temperature aging was used to study the light-off characteristic and thermal stability of catalyst. New washcoat material of GAL0.4CZA0.6 has optimal stability and cooperativity between surface area and OSC.The fast aging condition and OSC stability of the above Pd/Rh catalyst were investigated on engine bench. The fast aging condition of catalyst screening was obtained when the inlet temperature was 830℃, the light-off activity and OSC of catalyst decreased fastest during the aging time of 0h～20h. The air-to-fuel ratio (A/F) decreased fastest during the aging time of 0h～10h. The above condition could be used to quickly examine catalyst durability.4. Reciprocity research between the washcoat and the precious metalsFour contents and ratios of palladium nitrate and rhodium nitrate were loaded on GAL0.4CZA0.6.The catalytic reduction behavior and OSC of the samples were examined by H2-TPR, the precious metal dispersity characteristics. The thermal aging method was used to investigate the light-off activity and thermal stability. Further discussion of the reciprocity mechanism between the washcoat and the precious metals were carried out. The test results showed that, the fresh catalyst loaded higher Rh content had the better light-off performance. After high temperature aging, the light-off activity of the catalyst loaded higher Pd/Rh content was optimal. The catalytic material P03R (the ratio of Pd and Rh was 3:1) had optimal precious metal dispersity and catalytic activity. The precious metal dispersity of P03R was 35% higher than P33R (the ratio of Pd and Rh was 33:1). The catalyst SAM7 based on P03R had superior performance.5. Catalyst Preparation with lower temperature lightTo study the effect of the preparation process of Pd/Rh with GAL0.4CZA0.6 and the localized loading design of the precious metals on catalyst light-off activity. The effect of two kinds of the preparation processes of Pd/Rh with GAL0.4CZA0.6 on catalytic activity was compared. Furthermore, the catalytic performances of eight sets of single-stage with two-block catalysts were investigated. Through the engine bench and vehicle test results, the excellent performance of the catalyst prepared by the technique A (immersion method) was shown. The single-stage catalyst SAM 10 (the precious metal content:1.15g/L@9:1) prepared by Pd/Rh/GALo.4CZA0.6 and the sigle-stage with two block catalyst SAM11(the precious metal content in front block was 1.6g/L@19:1, and the precious metal content in rear block was 0.7@9:1) had met GBⅣ. SAM10 and SAM11 had the superior light-off performance. The emission results of CO, HCs, NOx were better than the old catalyst. One of the best emission control performances on vehicle was SAM11. The lower precious metal content met 1.15g/L@14:1. This indicated that the single-stage catalyst matched vehicle has got a good emission control effect through optimizing washcoat performance. The single-stage with two block catalyst has more superior light-off performance and emission control capacity after optimizing the process for the precious metal localized loading. The precious metal content of catalyst SAM11 was 60% less than the foreign original catalyst. The precious metal content of catalyst SAM11 was 28% less than our company's old catalyst. Therefore, the new GBⅣcatalyst could offer the higher ratio of performance and price, and stronger competitive advantage.In allusion to the technical difficulty of cold-start emission control in GBIV implementation, the thesis was based on developing rapid light-off catalyst technique. It presented experiment method, experiment results, experiment conclusion and analytic view. The catalyst design idea was based on the combined numerical calculation, sample preparation, characterization, reaction mechanism analyses, simulation and verification. The GBIV catalyst product contained the precious metals of 1.15g/L which have high value of market popularization were successfully developed. Many rapid light-off techniques for effectively control exhaust gases during the cold-start were put forward. The catalyst light-off technique is a systemic engineering. Only combined technique of catalytic material, catalyst, system integration and vehicle matching can promote technique innovation of rapid light-off catalyst. It is very important to research the correlation between the above techniques. The systemic investigation idea has some innovation as follows:(1) The innovation in the aspect of substrate applied technology can offer reference and guidance of substrate selection for satisfying the stricter regulation; (2) The catalyst preparation technique of localized loading has advanced its property; (3) New washcoat material of alumina with Ce-Zr-Al oxides and preparation technique has novelty and advanced property; (4) The precious metal decreasing technique would make GBⅣcatalyst product have competitive advantage and market popularization value. This research has suggested a series of technical views, new material and new techniques to serve for catalyst researchers. The research achievements can offer the technical supports for China's catalyst product technique progress and product marketization. It is of great significance to promote emission control level of China's automobile industry and environmental protection development.