Research status and development prospect of automobile exhaust catalyst
Environmental problem is a global problem, which needs the efforts of everyone in the world to solve. With the continuous development of world economy, 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 brings many problems, among which environmental problems can not be ignored. The environmental pollution caused by automobile use mainly includes noise pollution and air pollution caused by exhaust emissions. In China, automobile exhaust purification is the most effective way to solve 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 (nitrogen oxides), sulfur and hydrogen compounds and ozone, among which CO, HC and nitrogen oxides are the main air pollution components in automobile pollution control. Automobile exhaust is very harmful to human health, so controlling automobile exhaust pollution has become a top priority.
1 automobile exhaust purification method
As early as the mid-1960s, foreign countries have 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 catalysts and their carriers, improving the combustion technology of automobile internal combustion engines and treating the exhaust system of three-way catalysts. 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, so as to minimize the generation of pollutants; The main way of purification outside the vehicle is to install a catalytic purifier. Dealing with harmful gases is the most effective way to purify the exhaust gas outside the vehicle, and the catalyst is the key to the purification effect. Therefore, it is one of the best measures to control automobile exhaust emission 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 nitrogen oxides into N2. Because the chemical composition of automobile exhaust is very complex, its conversion rate is not only related to the activity of the catalyst, but also related 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, and the related reactions are as follows:
2CO+O2→2CO2
4HC+5O2→4CO2+2H2O
2NO+2CO→2CO2+N2
HC+NO2→CO2+H2O
Hydrocarbon+carbon monoxide →N2+ carbon dioxide +H2O
3NO+2NH3→2N2+3H2O
2h 3→N2+3H2O
The reduction catalyst mainly catalyzes the reduction reaction of nitrogen oxides;
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
2NO+H2→N2O+2H2O
Because these two reactions need different chemical environments, the early catalysts separated them. 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, there are two active centers of oxidation and reduction on the same catalyst, and TWC (three-way catalyst) finally 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 status at home and abroad
2. 1 research status abroad
2. 1. 1 oxidation catalyst
In the middle and late 1970s, the automobile emission regulations only required the control of CO and CH emissions, and the engine did not adopt the carburetor open-loop system. Because the A/F ratio is mechanically fixed on the theoretical value, it cannot be automatically adjusted with the change of working conditions. In this state, by adjusting the A/F ratio to about 15 and installing an oxidation catalyst in an oxygen-enriched state, the conversion rate of CO and HC can reach 90%, but the conversion rate of nitrogen oxides is relatively low. During this period, precious metal catalysts were mainly used, with platinum and palladium as active components. It is usually used in the alloy state of platinum and palladium = 7: 3, and the total loading is about 0. 12%. Precious metal catalysts have a fatal weakness, that is, fear of lead poisoning. Therefore, in order to effectively use precious metal catalysts, it is necessary to change the structure of fuel oil and implement unleaded gasoline.
2. 1.2 bimetallic catalyst
From the late 1970s to the mid-1980s, with the United States EPA proposed to control the emission of nitrogen oxides, the oxidation catalyst could not meet the requirements. Platinum-rhodium ternary bimetallic catalyst appeared. From the late 1970s to the early 1980s, double-bed platinum-rhodium catalysts appeared. The redox reaction of the catalyst is carried out in stages. In the first stage, the reduced honeycomb catalyst is used, in the second stage, the oxidized honeycomb catalyst is used, and air is supplemented in the middle of the two stages. This arrangement can make the reduction reaction and oxidation reaction in a favorable chemical atmosphere, but this catalyst has complex structure and troublesome operation, and nitrogen oxides may be oxidized again after reduction. 1980- 1985 Pt-Rh three-way catalyst was used in the closed-loop device of EFI. By controlling the A/F within the window, the conversion rate of CO, CH and nitrogen oxides can reach above 80-90%. The total loading of Pt-Rh of typical catalyst is 0. 1-0. 15%. Adding alkaline earth and rare earth elements into the coating of Pt: RH = 5: 1 stabilizes the catalyst structure and works synergistically with precious metals to produce excellent oxygen storage function. However, at high temperature, Rh reacts with Al2O3 and CeO2 in the surface coating, which leads to the reduction activity of the catalyst for nitrogen oxides in reducing atmosphere.
2. 1.3 Trimetal Catalyst
From the mid-1980s to the early 1990s, a new generation of Pt-Rh-Pd three-way catalyst was used. This generation of catalyst is equivalent to putting standard Pt-Rh catalyst on Pd catalyst. In this structure, palladium has better thermal stability in the inner layer; The outer rhodium is more conducive to the reduction of nitrogen oxides; Platinum plays an active coordination 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 is reduced to 0.6-0.8g It is reported that the conversion rate of the three-metal catalyst developed by engelhard can still reach 85% of CO, 90% of HC and 95% of nitrogen oxides after being used for/kloc-0.6 million kilometers, which obviously meets the higher environmental protection requirements.
2. 1.4 three-way palladium catalyst
In the late 1980s, Ford Company introduced a three-way palladium catalyst, which required alumina and rare earth oxides to form organic complexes with transition metal oxides, among which palladium played a leading role. By taking special measures, the material has a specific structure, which can stabilize its activity at high temperature. The experimental results show that the palladium-based catalytic materials alone still have good catalytic activity under the thermal shock of 65438 0200℃. At present, this catalyst is still under further development. Englhard Company has developed a double-layer palladium-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 the two 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. In the range of 423-823℃, Pd is active for the simultaneous transformation of HC, CO and NO.
2. 1.5 NOx storage reduction type three-way catalytic material
This catalytic material consists of precious metals, alkali metals or alkaline earth metals and rare earth oxides. The basic principle is that nitrogen oxides are first oxidized on precious metals under oxygen-enriched conditions, and then react with nitrogen oxides to generate nitrates. When burning according to the theoretical ratio or rich combustion conditions, nitrate decomposes to form nitrogen oxides, and then the nitrogen oxides react with carbon monoxide, H2 and hydrocarbons to reduce to N2. Studies show that the storage capacity of nitrogen oxides is related to the concentration of oxygen. With the increase of oxygen concentration, the storage capacity of nitrogen oxides increases. When the oxygen concentration reaches above 65438 0%, the storage amount of nitrogen oxides is basically unchanged. In addition, HC selective reduction catalytic materials also have good catalytic activity under oxygen-enriched conditions.
2.2 Domestic research status
The control of automobile exhaust pollution in China began in the mid-1980s. Universities and research institutes in China have done a lot of preliminary basic research work on automobile exhaust pollution control, and developed effective products in line with China's national conditions, contributing to reducing automobile exhaust emissions.
2.2. 1 research status of non-noble metal catalysts
Many domestic researchers have studied automobile exhaust purification catalysts with mixed oxides such as non-precious metals and rare earths as active components around 1990. Through the reasonable collocation of components, especially rare earth elements, synergistic effect can be produced, which has good catalytic activity and certain three-way performance.
The study of perovskite-type catalysts containing rare earths is a hot spot in the field of automobile exhaust catalysts. Researchers in China 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 by impregnation method, and the experimental study showed that their activities were high. 1993, Xu Kaili and others also developed a perovskite-type catalyst for purifying diesel engine exhaust gas, which is superior to Pt noble metal catalyst 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. 200 1 Han et al. prepared perovskite-type LaMnO3 nano-materials and loaded them on Al2O3-coated cordierite carriers as catalysts for purifying automobile exhaust. It is found that nanocrystalline active components have good dispersibility, small particle size and large specific surface area, and their catalytic efficiency for automobile exhaust is better than that prepared by solution method.
2.2.2 Research status of precious metal catalysts
In view of the high price of precious metal catalysts Pt and Rh and the shortage of resources, Pd is a relatively cheap and abundant precious metal. Pd is used to replace or partially replace Pt and Rh. Domestic researchers have carried out research on Pd as the main active component, and are committed to improving the preparation process, adding additives, replacing some precious metals with non-precious metals and reducing 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 study 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 active alumina coating play a role in dispersing, isolating and stabilizing other active components, especially precious metal Pd, which makes it difficult to migrate, agglomerate and lose coal, thus ensuring the good thermal stability of the catalyst. Guo Qinghua and others added Ce to the intermediate coating containing Pd 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, rhodium and silver catalysts are also studied. Although supported Pd catalyst has high catalytic activity and good low-temperature activity, its anti-sintering and anti-sulfur poisoning ability is poor, especially its purification performance for nitrogen oxides.
3. Structure composition of automobile exhaust purification catalyst
Automobile catalyst is mainly composed of carrier, high specific surface area coating, active components and additives.
3. 1 vector
Only when the catalytically active components are supported on the carriers with high specific surface area can they play a good role, and the choice of carriers has a great influence on the catalyst activity. The early carriers were particles made of activated alumina, magnesium silicate and diatomaceous earth, which had large specific surface area and were easy to use. However, they had the disadvantages of large pressure drop and heat capacity, poor heat resistance, low strength and easy to break, and were gradually replaced by honeycomb ceramic carriers after the 1980s. 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 area 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 ceramics, which has the advantages of easily available raw materials, low cost and good comprehensive performance. Another monolithic carrier is a monolithic alloy carrier made by pressing nickel-chromium, iron-chromium-aluminum or iron-molybdenum-tungsten alloy into corrugated shape, which has higher thermal stability than ceramic honeycomb carrier. At present, this metal carrier is mainly used in Japan, the United States and other countries with strict requirements on automobile exhaust emissions. The use of metal carrier is very beneficial to reduce automobile exhaust resistance, obviously improve power performance, improve tail gas purification efficiency and prolong the service life of purifier.
3.2 High specific surface coating (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 area of cordierite carrier is relatively low, generally only about 1m 2 /g, so it is necessary to coat a coating with high specific surface area. The coating material is usually γ-Al2O3, which has strong adsorption capacity and large specific surface area. However, it will be transformed into α-Al2O3 at high temperature, 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 metal oxides are usually added as additives.
3.3 Active components
The active components of tail gas catalyst can be divided into precious metals and non-precious metals.
Platinum, rhodium and palladium are the most commonly used precious metals. Pt component mainly plays the role of oxidizing CO and HC in the catalyst, and has a certain reduction ability to NO, but the effect is not as good as Rh when CO concentration is high or SO 2 exists. Rh component is the main component of catalytic reduction of nitrogen oxides, and the only reduction product N2 is obtained in the presence of oxygen. Under anaerobic conditions, 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 of hydrocarbons, and the antitoxic type of Rh is worse than that of Pt. Pd component is mainly used to transform 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-precious metal active components are mainly transition element oxides and their spinel and perovskite composite oxides. However, due to the poor heat resistance, low activity and high light-off temperature of single-component oxides, their use is limited, and multi-component formulations and appropriate preparation techniques are generally used.
3.4 Auxiliary equipment
The promoter itself is an additive with no catalytic effect or low activity, which can greatly improve the activity, selectivity and life of the catalyst. CeO2 is the most important promoter of automobile exhaust purification catalyst, and its main functions are: storing and releasing oxygen; Improve the dispersion of precious metals and inhibit the formation of inactive solid solution between precious metal particles and Al2O 3; Improve the anti-poisoning ability of the catalyst; Increase the thermal stability of the catalyst, etc. Summers and Orson studied the interaction between cerium and precious metals. In the fresh Pd and Pt noble metal catalysts supported on Al2O3, the surface dispersion of Pt decreases with the increase of CeO2 content. However, the surface dispersibility of Pd has nothing to do with the loading of CeO2.
4 development direction of automobile exhaust purification catalyst
4. Catalytic conversion of nitrogen oxides under1lean combustion conditions
Only when the air-fuel ratio of the engine is close to the stoichiometric ratio (14.7/ 1) and unleaded gasoline is used, the noble metal three-way catalyst can effectively purify CO, HC and NO X ... When the air-fuel ratio is lower than 14.7, it is in a rich combustion zone, and the catalyst has high reduction and low oxidation, and the purification of CO and HC is incomplete. However, when it is higher than 14.7, it is in a lean combustion zone, and the oxygen content in the tail gas is large, while the contents of CO and HC are very low. The catalyst has high oxidation and low reduction, and can not effectively reduce nitrogen oxides. Therefore, the development of a new type of automobile exhaust purification catalyst under lean combustion conditions has become a research hotspot. Once the catalyst is successfully studied, it will be widely used in diesel engines and vehicles equipped with lean gasoline engines. In order to reduce the concentration of nitrogen oxides, which have been discharged into the atmosphere, especially in areas with poor atmospheric diffusion conditions, such as streets and tunnels in big cities, it has been proposed to use the high oxidation ability and reduction ability of TiO _ 2 photocatalysis to mix TiO _ 2 in building materials, coat it on the external wall of buildings, and then convert nitrogen oxides into NO3- under the conditions of O2 and H2O***, because this highly active TiO _ 2 molecular sieve catalyst with four coordination structures is injected.
4.2 Development of non-precious metal catalysts
Precious metal ternary catalyst is a popular catalyst for automobile exhaust purification at present. However, precious metals are expensive, easy to be poisoned by lead, sulfur and phosphorus, and may also cause secondary pollution to the environment, such as N2O gas, which is one of the main greenhouse gases. Therefore, it has become an inevitable trend to find new catalytic materials to partially or completely replace precious metals. The price of non-precious metal catalyst is much lower than that of precious metal, but its catalytic activity is lower than that of precious metal, so it needs to be made into a special structure and interact with various metal components to improve its activity. At present, rare earth elements are the most studied active components, but because the performance of rare earth catalysts is not as good as that of precious metal catalysts, such as activity and stability, many technical problems need to be solved. Rare earth catalysts mainly include perovskite, special lanthanum complexes, special cerium complexes, rare earth metals containing copper and nickel and nitrates. Secondly, the catalysts are mainly transition metals, among which CuO, MnO2 _ 2 and CO2 _ O3 have higher oxidation activity for CO, while NiO and Cr2O3 have better reduction activity for nitrogen oxides. Therefore, it is necessary to prepare three-way catalyst with compound formula.
5 Current problems and solutions
The use of catalysts to purify automobile exhaust gas has effectively improved the pollution of exhaust gas to the atmosphere, but many problems have also been exposed in practice, which need further study and exploration.
(1) Catalytic conversion rate: At present, most catalysts have good high-temperature activity and poor low-temperature activity, which greatly inhibits their performance.
(2) Catalyst failure: including thermal failure and poisoning failure. Up to now, automobile exhaust catalyst has not been properly solved. The thermal deterioration of the catalyst at high temperature and the poisoning of sulfur, phosphorus and lead greatly shorten the service life of the catalyst.
(3) Cold start problem: 60%-80% of toxic gases in automobile exhaust are produced within two minutes of cold start. In order to effectively treat the exhaust gas at this stage, it is necessary to improve the low-temperature activity of the catalyst and improve the low-temperature catalytic conversion of the exhaust gas.
(4) Cost: At present, most of the catalysts widely used in automobiles are precious metals or precious metals doped with other metal oxides, and their cost is still high.
At present, the practical catalysts for tail gas purification are nothing more than precious metal catalysts (oxidation catalysts and three-way catalysts) and rare earth catalysts. However, the precious metal catalyst does not have the conditions to be popularized and used in China at this stage, mainly because of its high price, which requires the use of unleaded gasoline and the corresponding electronic fuel injection system and other automotive technological transformation. Practice has proved that rare earth catalyst has good purification effect on CO and HC and strong resistance to lead poisoning, which can meet the existing automobile emission standards. In particular, China's rare earth resources are extremely rich and cheap, and it is the first choice catalyst suitable for China's national conditions at this stage. Therefore, it is imperative to carry out the research on exhaust gas purification catalysts with rare earth metals as the main component and a small amount of precious metals or transition metals, and the prospects are broad. The key point is to make a breakthrough in the following three aspects:
(1) Using the principle of combinatorial chemistry, we designed the catalyst with the best catalytic activity, developed new materials and improved the utilization rate of precious metals.
(2) Develop a three-way catalyst with clay minerals as the carrier, improve the high-temperature resistance of the catalyst, reduce the production cost at the same time, and open the way for the industrialization of the catalytic purifier.
(3) Study the non-noble metal catalytic material system, with a view to partially or completely replacing noble metal catalysts.
6 conclusion
With the rapid development of automobile industry, the pollution of automobile exhaust to the environment is becoming more and more serious. All countries in the world have formulated strict automobile exhaust emission standards and adopted automobile exhaust purification catalysts, which greatly reduced the air pollution in cities. However, noble metal catalysts are expensive and their application conditions are limited. Therefore, the development of catalysts and non-noble metal catalysts used under lean combustion conditions has become a research hotspot at present, and the development prospect of automobile exhaust purification catalysts is very broad.