Research status and development prospect of catalysts for automobile exhaust
Environmental problem is a global problem, which needs the efforts of everyone in the world to solve. With the continuous development of world economy, science and technology and social civilization, people's material needs are increasing day by day. Automobile is the most popular means of transportation in modern society, especially in recent years, there are more and more private cars, which have brought many problems, among which environmental problems can not be ignored. The environmental pollution caused by the use of automobiles mainly includes noise pollution and air pollution caused by exhaust emissions. In China, automobile exhaust purification is the most effective way to solve the exhaust pollution. The pollutants emitted by automobiles mainly come from internal combustion engines, and their harmful components include carbon monoxide (CO), hydrocarbons (CH), nitrogen oxides (NOx), sulfur and hydrogen compounds and ozone, among which CO, HC and NOx are the main air pollution components in automobile pollution control. Automobile exhaust is very harmful to human health, so it has become an urgent task to control automobile exhaust pollution.
1 Method of purifying automobile exhaust
As early as the mid-196s, foreign countries have already researched and developed automobile pollution control technology, and now it has reached the practical stage. The research shows that these harmful gases can be purified by improving the performance and production technology of catalyst and its carrier, improving the combustion technology of automobile internal combustion engine and treating the exhaust system of three-way catalyst. Automobile exhaust pollution control can be divided into two technologies: in-machine and out-of-machine. In-engine purification is mainly to improve the quality of fuel and improve the combustion conditions of fuel in the engine, and reduce the generation of pollutants as much as possible; The main way of off-board purification is to install a catalytic purifier. Treating harmful gases is the most effective way to purify off-board tail gas, and catalyst is the key to the purification effect. Therefore, it is one of the best measures to control automobile exhaust emissions to develop practical and efficient catalysts for automobile exhaust purification.
the purpose of catalytic purification of automobile exhaust is to oxidize harmful CO and HC into CO2 and H2O, and reduce NOx into N2. Because the chemical composition of automobile exhaust is very complex, its conversion rate is related not only to the activity of the catalyst, but also to whether the reaction gas is oxidation gas or reduction gas, so the catalyst can be divided into two parts in function: oxidation type and reduction type. The oxidation catalyst mainly catalyzes the oxidation reaction of CO and HC, The related reactions are as follows:
2co+O2 → 2co
4co+5o2 → 4co+2h2o
2no+2co → 2co+N2
HC+NO2 → CO2+H2O
HC+Co → N2+CO2+H2O
3No+H2O. :
2no+co → N2+CO2
2no+H2 → N2+2h2o
2no+HC → N2+H2O+CO2
The reaction of NO and H2 not only produces nontoxic N2 and H2O, but also has undesirable side effects:
2no+5h2 → 2nh3+H2O < Later, due to the improvement of the engine, a chemical environment that can make the two functions compatible was realized; Due to the improvement of catalyst preparation technology, two active centers * * * of oxidation and reduction exist on the same catalyst, and finally TWC(three-way catalyst) appears. At present, the most commonly used catalyst is honeycomb catalyst, and the carrier is ceramic honeycomb, which is coated with alumina with high specific surface area, and then impregnated with active components. Therefore, the catalyst for automobile exhaust purification is mainly composed of carrier, coating and active substance.
2 research situation at home and abroad
2.1 research situation abroad
2.1.1 oxidation catalyst
In the middle and late 197s, the automobile emission regulations only required to control the emission of CO and CH, and the engine has not used the carburetor open-loop system, so it can not be automatically adjusted with the change of working conditions because the A/F ratio is mechanically fixed to the theoretical value. In this state, by adjusting the A/F ratio, Precious metal catalysts were mainly used in this period, with platinum and palladium as active components. It is usually used in the alloy state of platinum and palladium = 7: 3, and the total load is about .12%. Precious metal catalysts have a fatal weakness, that is, they are afraid of lead poisoning. Therefore, in order to effectively use precious metal catalysts, it is necessary to change the structure of fuel oil and implement lead-free gasoline.
2.1.2 bimetallic catalyst
from the late 197s to the mid-198s, with the introduction of NOx emission control by EPA in the United States, the oxidation catalyst could not meet the requirements. Three-way bimetallic catalysts of platinum and rhodium appeared. From the end of 197s to the beginning of 198s, a double-bed platinum and rhodium catalyst appeared. The redox reaction of the catalyst was carried out in stages, with reduced honeycomb catalyst used in the first stage, oxidized honeycomb catalyst used in the second stage and air supplemented in the middle of the two stages. This arrangement can make the reduction reaction and oxidation reaction be carried out in a chemical atmosphere that is beneficial to itself, but this kind of catalyst has complex structure and troublesome operation, and NOx may be oxidized again after reduction. From 198 to 1985, Pt-Rh three-way catalyst began to be used in the closed-loop device of EFI, and the conversion rate of CO, CH and NOx could reach above 8-9% by controlling the A/F within the window. The total Pt-Rh loading of typical catalysts is .1-.15%, and alkaline earth and rare earth elements are added to the Pt: RH = 5: 1 coating to stabilize the catalyst structure and cooperate with precious metals to produce excellent oxygen storage function. However, at high temperature, Rh reacts with Al2O3 and CeO2 _ 2 in the surface coating, which leads to the reduction activity of the catalyst for NOx in reducing atmosphere.
2.1.3 Three-metal catalyst
From the mid-198s to the early 199s, a new generation of Pt-Rh-Pd three-way catalyst was used. This generation of catalyst is equivalent to placing a standard Pt-Rh catalyst on a Pd catalyst. In this structure, palladium has better thermal stability in the inner layer; Rhodium in the outer layer is more conducive to the reduction of NOx; Platinum plays an active coordinating role between palladium and rhodium. Therefore, the performance of the catalyst has been obviously improved. With the improvement of gasoline quality, the service life of the catalyst is greatly prolonged, and the total amount of precious metals in each liter of catalyst has dropped to .6-.8g. According to reports, after being used for 16, kilometers, the conversion rate of Tri-Metal catalyst developed by Engelhard can still reach 85% of CO, 9% of HC and 95% of NOx, which obviously meets higher environmental protection requirements.
2.1.4 Three-way Palladium Catalyst
At the end of 198s, Ford Company introduced the Three-way Palladium Catalyst, which requires alumina and rare earth oxides to form an organic complex with transition metal oxides, and palladium plays a leading role in it. By adopting special measures, the material has a specific structure, so that its activity at high temperature can be stabilized. The experimental results show that the catalytic activity of Pd-based catalytic materials alone is still good under the thermal shock of 12℃. At present, this catalyst is still under further development. Englhard company developed a double-layer Pd-based catalytic material. The bottom layer is composed of Pd and Ce, and the top layer is composed of Pd dispersed on the coating. Cheap metal oxides are added to both layers to stabilize and improve the activity of Pd. The top layer provides low-temperature catalytic activity; Pd-Ce layer provides high oxygen storage capacity to ensure high temperature catalytic activity. Pd is active for the simultaneous conversion of HC, CO and NO in the temperature range of 423-823℃.
2.1.5 NOx storage-reduction ternary catalytic material
This catalytic material is composed of precious metals, alkali metals or alkaline earth metals and rare earth oxides. The basic principle is that NOx is first oxidized on precious metals under oxygen-enriched conditions, and then reacts with NOx storage to form nitrate. When burning in theoretical ratio or rich combustion condition, nitrate decomposes to form NOx, and then NOx reacts with CO, H2 and HC to be reduced to N2. The research shows that the storage capacity of NOx is related to the concentration of oxygen. With the increase of oxygen concentration, NOx storage capacity is improved. When the oxygen concentration reaches more than 1%, the NOx storage capacity is basically unchanged. In addition, HC selective reduction catalytic materials also have good catalytic activity under oxygen-enriched conditions.
2.2 domestic research status
China's automobile exhaust pollution control began in the mid-198s. Universities and institutes in China have done a lot of preliminary basic research work on automobile exhaust pollution control, and researched and developed effective products that can meet China's national conditions, making contributions to reducing automobile exhaust.
2.2.1 Research status of non-noble metal catalysts
Many researchers in China studied automobile exhaust purification catalysts with mixed oxides such as non-noble metal and rare earth as active components around 199. Through the reasonable collocation of components, especially rare earth elements, it can produce synergistic effect, and has good catalytic activity and certain three-way performance.
the study of perovskite-type catalysts containing rare earth is a hot topic in the field of automobile exhaust catalysts. Chinese researchers have done a lot of research in this field. For example, in 1988, Wang Dao et al. prepared a series of La (Cu, Mn, Co) O3/LaAlO3 catalysts loaded with calcium chloride by impregnation method, and the experimental study showed that their activities were high. In 1993, Xu Kaili and others also developed a perovskite-type catalyst for purifying diesel engine exhaust, which is superior to Pt noble metal catalysts in activity and has strong SO2 resistance and carbon deposition resistance. Gu Qishun and others developed HR-1 catalyst with ceramic honeycomb coated with activated alumina as carrier and rare earth composite oxide as active component. Then rare earth elements are added to stabilize the alumina coating structure, which is a good three-way catalyst. In 21, Han Qiaofeng and others prepared perovskite LaMnO3 nano-materials by PFG method, and loaded them on cordierite carriers coated with Al2O3 as catalysts for purifying automobile exhaust. It was found that the active components of nanocrystals had good dispersion, small particle size and large surface area, and their catalytic efficiency for automobile exhaust was better than that prepared by solution.
2.2.2 Research status of precious metal catalysts
In view of the high price and shortage of precious metal catalysts Pt and Rh, Pd is a cheaper and more abundant precious metal, so Pd is used to replace or partially replace Pt and Rh. Domestic researchers have carried out research with Pd as the main active component and devoted themselves to improving the preparation process, adding additives and replacing some precious metals with non-precious metals to reduce the amount of precious metals. The most commonly used promoters of palladium-containing catalysts are rare earth oxides, alkaline earth metal oxides and transition metal oxides. Huang Chuanrong's research on the activity and thermal stability of La-Co-Ce-Pd catalyst shows that the enrichment of rare earth elements, La and Ce on the catalyst surface and the existence of activated alumina coating play a role in dispersing, isolating and stabilizing other active components, especially precious metal Pd, making it difficult to migrate, coal caking and lose, thus ensuring the good thermal stability of the catalyst. Guo Qinghua et al. added Ce to the intermediate coating of Pd-containing catalyst, and Ba also played a role in dispersing, isolating and stabilizing the structure of Pd components, thus improving the thermal stability of the catalyst. In addition, there are also studies on Rh and Ag catalysts. Although the supported Pd catalyst has high catalytic activity and good low temperature activity, it has poor anti-sintering and anti-sulfur poisoning ability, especially for NOx purification performance.
3 structural composition of automobile exhaust purification catalyst
Automobile catalyst mainly consists of four parts: carrier, coating with high specific surface, active components and additives.
3.1 carrier
Only when the catalytically active components are supported on a carrier with high specific surface can they play a good role, and the choice of carrier has a great influence on the catalyst activity. The early carriers were particles made of activated alumina, magnesium silicate and diatomite, which had a large surface area and were easy to use, but they had some disadvantages such as large pressure drop and heat capacity, poor heat resistance, low strength and easy to break, so they were gradually replaced by honeycomb ceramic carriers after the 198s. Honeycomb ceramic carrier, also known as monolithic carrier, is composed of many thin-walled parallel small channels, which has the advantages of small airflow resistance, large geometric surface and no wear. Cordierite carrier is widely used as the carrier of automobile exhaust catalyst because of its low thermal expansion coefficient and outstanding thermal shock resistance. At present, 95% of the carrier of automobile catalyst is honeycomb cordierite ceramic, which has the advantages of easy access to raw materials, low cost and good overall performance. Another monolithic carrier is a monolithic alloy carrier made by pressing Ni-Cr, Fe-Cr-Al or Fe-Mo-W alloy into corrugated shape, which has higher thermal stability than ceramic honeycomb carrier. At present, this metal carrier is mainly used in countries with strict requirements on automobile exhaust emission, such as Japan and the United States. The use of metal carrier is very beneficial to reduce the automobile exhaust resistance, obviously improve the power performance, improve the exhaust gas purification efficiency, and prolong the service life of the purifier.
3.2 coating with high specific surface (also called second carrier)
The active coating is attached to the surface of the carrier, and its function is to provide a large surface area for attaching precious metals or other catalytic components. The specific surface of cordierite carrier is low, generally only about 1m 2 /g, so a coating with high specific surface should be coated. The coating material is usually γ-Al2O3, which has strong adsorption capacity and large specific surface area. However, at high temperature, it will change into α-Al2O3, and the specific surface area will decrease. In order to suppress the phase transition of Al2O3, rare earth elements such as Ce, La, Ba, Sr and Zr or alkaline earth oxides are usually added as additives.
3.3 active components
the active components of the tail gas catalyst can be divided into two types: precious metals and non-precious metals.
Pt, Rh and Pd are the most commonly used precious metals. Pt component mainly plays the role of oxidizing CO and HC in the catalyst, and it has a certain reduction ability to NO, but its effect is not as good as Rh when the concentration of CO is high or SO 2 exists. Rh component is the main component of catalytic reduction of NOx, and the only reduction product N2 is obtained in the presence of oxygen. In the absence of oxygen, NH3 is the main reduction product at low temperature, and N 2 is the main reduction product at high temperature. In addition, Rh also plays an important role in the oxidation of CO and the steam reforming reaction of hydrocarbons, and the antitoxic type of Rh is worse than that of Pt. Pd component is mainly used to convert CO and hydrocarbons, but its effect on saturated hydrocarbons is slightly poor, and its ability to resist Pb and S poisoning is poor. It is easy to sinter at high temperature and form an alloy with lead, but it has high thermal stability and good light-off property. In the three-way catalyst for automobile exhaust, the functions of various components are coordinated with each other. Non-noble metal active components are mainly transition element oxides and their spinel and perovskite composite oxides.