Rare earth elements have a special outer electronic structure, its coordination number determines the variability of their "reserve chemical bond" or "residual atomic value" effect, and this ability is the catalyst must have. Therefore, rare earth elements not only have their own catalytic activity, but also can be used as additives or cocatalysts to improve the catalytic performance of other catalysts, which is one of the fields of heavy rare earth use. The current application of rare earth catalysts include the following aspects: internal combustion engine exhaust purification; industrial exhaust and living environment purification; catalytic combustion; fuel cells; low value alkane utilization.
First, the rare earth catalyst in the car exhaust purification research and application
Rare earth catalyst for automobile exhaust purification, good effect, stable and reliable quality, low price, is a developing rare earth application market. Since the 1980s, rare earth catalysts have gradually entered the market. At present, the demand for automobile exhaust gas purification catalysts in the United States is rapidly increasing, becoming the largest market for rare earth applications. In 1996, the United States used 12,000 tons of rare earth (REO) in this market, accounting for 46% of the total amount of rare earth in that year. In the early 1980s, Japan and Europe and other countries to improve the catalyst technology, the rare earth into the catalyst, reducing costs and improve performance, greatly promoted the application of rare earth catalysts. The amount of rare earth used in Japan's automobile exhaust gas purification catalyst was 420 tons in 1996; the rare earth consumption in Europe is about 1,000 tons in this market. As early as the 1970s, scientists in our country started researching and developing automobile exhaust gas purification catalysts. After more than 20 years of hard work, applied research has been at the forefront of the world and basically meets the requirements of industrialization. In particular, technically it adopts a different technical route from abroad, that is, the main research and development focuses on the technology of pure rare earth catalysts or the introduction of a small amount of precious metals on the basis of rare earths, which is formed during the research and development of rare earth catalysts for purifying automobile tail gas A scientific research achievement with Chinese characteristics and world advanced level. Automobile exhaust gas purification catalyst production will be the next century, one of the largest catalyst industry, will also be one of the major users of rare earth. Research and development of natural gas buses and diesel vehicles to purify high performance and strong anti-sulfur rare earth oxide catalysts; develop the preparation technology of new Euro Ⅱ and Euro Ⅲ standard new vehicle exhaust gas purification catalysts; develop natural gas to catalyze high temperature combustion of rare earth catalysts, Non-polluting emissions; development of industrial sources of emissions of toxic, harmful, organic pollutants such as gas purification catalyst. In 2002, the field of environment includes catalysts for automobiles and industrial sources for sensors, and the amount of rare earths is about 500 tons. At present, rare earth catalysts produced in Shanghai, Beijing and Wuxi have entered the market and are gradually being applied to automobile exhaust purifiers. Their application technologies have reached the world advanced level in the 1990s. Has been more than 1,000 cars on the practical application, achieved significant purification effect. Doing a good job of application of rare earth in this aspect not only develops new application fields but also alleviates the air pollution in our cities and is of great significance to the realization of sustainable economic and social development. The early 21st century, rare earth catalyst in the automotive exhaust purification will be a big leap forward, and become a new rare earth consumption hot spots. Rare earth - precious metal composite catalyst development will be promising. It is estimated that in 2005 China will need 800 to 10 million sets of catalytic converters for automobile tail gas purification and will consume about 1,500 tons of rare earths. This set of data in 2010 will reach 12 million sets and 1,800 tons.
In China's large and medium-sized cities, especially in Beijing, Shanghai, Guangzhou and other places, automobile exhaust has become a major source of air pollution. In Beijing's total air pollutant emissions, an average of 83% of carbon monoxide, 74% of hydrocarbons and 41% of nitrogen oxides are generated by automobile exhaust emissions. To solve the automobile exhaust pollution, so far, recognized at home and abroad is the most effective way to deal with the EFI installation of exhaust gas purifier, purifier and the core technology is the catalyst. In foreign countries, precious metals are mainly used as catalysts, but even in the case of precious metal catalysts, a certain amount of rare earths are required to stabilize the high temperature phase of alumina in the dispersed layer of the substrate.
Since Libby proposed rare earth-containing catalysts used in automobile exhaust purification since the unique catalytic properties of rare earth oxides catalyst has attracted the attention of researchers. The paramagnetic properties of rare earth oxides, lattice oxygen mobility, cation variable price and surface alkalinity are intrinsically linked to many catalysis. With further research, rare earth oxides can not only be used as catalyst carriers and additives, but also found that adding rare earth oxides in the catalyst can improve the thermal stability and mechanical strength of the high surface area coating and improve the dispersion of the precious metal active components Degree, anti-poisoning and durability performance, improve the catalyst oxygen storage capacity and anti-oxidation properties.
China since the 1970s launched a rare-earth catalyst for car exhaust purification research, the use of rare earth partially or fully replace the scarce resources of precious metals for automobile exhaust purification research level in the forefront of the world, with perovskite rare earth composite oxide (PTO) replaces the noble metal completely or partially to act as the active component of the catalyst to catalytically reduce CO, HC, NO. The developed vehicle exhaust catalyst has good activity, good thermal stability, a certain ability of anti-sulfur, lead poisoning, life test has reached 50,000 km or more, basically have the conditions to industrialization. However, the study on the catalytic properties of rare earth oxides and their understanding of the laws of catalytic chemistry are still relatively superficial. Without a detailed and in-depth study, the understanding of the surface properties and catalytic properties of rare earth oxides is far less than that of noble metals or even transition metal oxides So meticulous, in-depth.
Our government implemented the new exhaust emission standard GB14761-1999 on January 1, 2000. All vehicles can only achieve the new emission standard if they are equipped with three-way catalytic converter by EFI. With the rapid development of China's automobile industry, as well as the increasing emphasis on environmental protection by relevant government departments and the public, the market of automobile exhaust emission purification technologies and products that suit China's national conditions has begun to take shape.
Table 1 is China's car ownership in the next few years and purifier market demand forecast. Obviously, the start of this part of the market will drive the development of rare earth research and application areas in China.
Table 1 is China's car ownership in the next few years and purifier market demand forecast
At present, there are more than 100 manufacturers of automobile exhaust gas purifiers in our country, and a few enterprises have reached an annual production capacity of over 300,000 sets. The performance and quality of their products can basically meet the current technical requirements for tail gas emission control in China. However, the influx of foreign purifier enterprises into China's auto market, the purifier industry in our country has formed a huge potential threat.
In addition to automobiles, China has been the world's largest motorcycle manufacturer since 1999, with more than 10 million motorcycles produced. At present, motorcycles exported to developed countries require the installation of an exhaust gas purifier. Some large and medium-sized cities in China have already started to request the treatment of exhaust pollution of motorcycles. This is also an important aspect of the application of rare earth catalytic materials.
In the exhaust gas pollution control of diesel vehicles, at present mainly relies on the installation of an oxidation purifier to oxidize and purify the soot and some gas pollutants emitted by the diesel vehicles. This is also an important area of application of rare earth catalytic materials.
Since 2002, the output of fixed small-sized fuel-powered engines in our country has also increased rapidly. Currently mainly used for household engines, garden lawn mowers, small irrigation equipment, water power equipment. In 2003, China exported only over 1,500,000 small-fuel-powered engines, and some manufacturers have demanded the installation of decontamination devices. In 2003, the output of automobile exhaust purifier produced in China reached 3.2 million sets. Including catalysts, carriers and oxygen sensors consumed by all types of rare earth, rare earth consumption of 910 tons. It is estimated that by 2005, the market demand of China's automobile exhaust purifier will exceed 5.5 million sets, and the consumption of rare earths will reach 1,560 tons.
Second, the rare earth catalyst in the industrial emissions, living environment purification research and application
China is a major producer of chemicals and can produce more than 37,000 chemicals (8% of which are poisonous chemicals). With the rapid industrialization in our country, toxic and harmful exhaust emissions from industrial production (such as petrochemical, footwear, leather, paint and coatings industries) and exhaust gas from the use of these chemicals have become one of the major sources of pollution in cities, particularly Is the emission of volatile organic compounds (VOCs). The rare earth catalytic materials used for industrial organic waste gas pollution control and living environment purification is one of the driving forces to promote the catalytic application of rare earth.
Development of economical and practical industrial sources Purification of toxic and hazardous pollutants Control technology is one of the most active areas of research in recent years. Catalytic combustion has the advantages of low operating temperature, high purification efficiency, no auxiliary fuel, and less secondary pollutants. It has been considered as the most effective and the most promising purification technology. Catalytic oxidation is mainly adapted to the purification of organic waste gas above the high concentration. High-performance oxidation catalyst is the key to purification technology. Rare earth catalytic materials have shown more and more obvious development and application prospects because of their unique catalytic oxidation properties. Many successful examples have been applied in this field. In 1997, the sales volume of VOCs purification catalyst in the United States reached about 1 billion U.S. dollars. The average annual growth rate of 20% to 25% is the fastest growth in the field of environmental protection catalysts.
Modern day life, 80% to 90% of the time spent indoors. Inhalation of airborne contaminants in large quantities can trigger a series of conditions that affect human health. Survey shows that 68% of modern diseases are related to indoor air pollution. Therefore, to purify the living environment and improve indoor air quality has become an urgent need of residents. Indoor air pollution is characterized by many pollutants, low concentration and poor self-purification. Therefore, purification of indoor air is much more difficult than catalytic purification of industrial exhaust gas. It involves the coupling of photocatalytic oxidation and room-temperature catalytic oxidation at room temperature.
Rare earths have complex energy level structure and spectral characteristics. Doped doping modification of nano-TiO2 can effectively improve the photocatalytic efficiency and is one of the most promising technologies to solve the problem of visible light utilization. The rare earth low temperature oxidation catalyst can catalyze the elimination of CO, O3 and other harmful gases at room temperature. Through the synergistic effect with the photocatalyst, it is one of the best ways to purify the living environment at room temperature. There are many successful examples about the catalytic combustion of organic waste gas in our country, but the research on the purification of low concentration VOCs is seldom reported. Sensitization of the photocatalyst is one of the hot spots in the field of photocatalysis. Rare earth element modification to improve the sensitization efficiency of the titanium dioxide photocatalyst has been reported in recent years, but no breakthrough results have been obtained. The current research also involves less research on the interaction between the atomic energy level of rare earth doping atoms and the energy level of the nano-TiO2 semiconductor, and lacks the research on the relationship between the rare earth speciation and photocatalytic performance.
Due to its good catalytic performance, unique low temperature activity and superior resistance to poisoning, rare earth catalytic materials have shown more and more superior development and application prospects in organic waste gas treatment. Among them, the rare earth composite mesoporous catalytic material has the characteristics of large surface area, suitable pore size distribution and stable structure, and has become one of the most promising catalytic materials for purifying industrial organic waste gas. In addition, nano-level design to develop advanced rare earth catalytic materials, can reduce the 90% of the amount of precious metals can still guarantee the catalytic purification efficiency doubled.
Third, the research and application of rare earth in catalytic combustion
Flame burning plays an extremely important role in the evolution of mankind and the development of human civilization. Modern scientific research shows that the traditional flame combustion method with low thermal efficiency and serious pollution has restricted the development of our economy. The fundamental solution to the inefficiencies and high emissions from flame combustion is catalytic combustion. Compared with the normal combustion, catalytic combustion has the advantages of high combustion efficiency, stable combustion, ultra-low emission of pollutants (such as CO, NOx and incomplete combustion), which is the focus of catalytic combustion research in various countries over the past 20 years the reason. Rare earth high temperature combustion catalyst has the advantages of cheap price, raw material availability, process stability, good purification effect, long service life, etc., and has certain application prospect in high-temperature catalytic combustion. Development of rare earth catalysts, the development of advanced international advanced catalytic combustion technology to improve China's traditional combustion method is in line with China's national conditions and development path.
There are two types of catalysts currently studied: one is a precious metal catalyst, which has high activity and certain stability, but the precious metal is easy to sinter and evaporate, and is easily poisoned by sulfur, lead and the like, Expensive, has not yet industrialized; the other is mainly concentrated in the rare earth, alkaline earth substituted perovskite oxide, hexaaluminate catalyst research. Catalytic combustion of the catalyst is the basic requirement of good low temperature activity and high temperature thermal stability. The experimental results show that rare earth combustion catalyst has high thermal stability, but the light-off activity is relatively poor.
At present, a large number of foreign studies have reported that just entering the catalytic burner research phase, there is still some distance away from the industrialization. Our country's research in this area is still in its infancy. At present, catalytic combustion has not been widely used, mainly due to the performance of catalytic materials can not meet the requirements. The excellent activity and stability of the rare earth catalytic materials are one of the ways to promote the large-scale practical application of catalytic combustion of natural gas and other fossil fuels.
China has abundant natural gas resources with a total resource of 38 trillion cubic meters and proven reserves of 1.53 trillion cubic meters. With the construction of national key projects such as the east-west gas transmission, the development of natural gas in the East China Sea and the five natural gas bases in the "West, East, South, and Middle East", solving the serious environmental pollution caused by coal burning in major cities along the route, the effective utilization of natural gas is our country Currently urgent need to solve the problem. Currently widely used are natural gas flame combustion furnace. According to statistics, the market demand in 2001 was more than 15 million units, increasing by 30% from 2002 to 2005. However, all the flame-burning furnaces currently on the market are "flame-burning and direct-exhaust-emission" products, all of which have the substantial problem of low thermal efficiency and serious pollution of flame combustion. Therefore, natural gas and other low-carbon alkane catalytic burner market prospects are very good.
Fourth, the research and application of rare earth in the use of low-carbon alkanes
One of the keys to replacing some of the oil resources by liquefaction of low-carbon alkanes is the preparation of inexpensive syngas to reduce the cost of liquefaction products. The main process for the synthesis of syngas from the catalytic conversion of low-carbon alkanes include steam reforming, steam reforming and second-stage furnace oxidation, catalytic partial oxidation and carbon dioxide reforming. In steam reforming syngas, water vapor usage is much higher than stoichiometry and water vaporization requires a large amount of energy consumption in order to suppress the high temperature sintering of coke and active metal Ni grains. Practice has proved that rare earth additives have good anti-Ni grain sintering and carbon deposition.
In addition to the use of low-carbon alkanes to make syngas, the use of olefins to the dehydrogenation is an effective way to utilize them. The process has been achieved industrialization, direct dehydrogenation catalyst mainly platinum and chromium aluminum two major categories, the fatal weakness of these catalysts is easy coke inactivation, short life expectancy, the catalyst every few minutes or ten minutes to be renewable once Moreover, the conversion rate of alkanes is not high. In recent years, rare earth-based additives for the chrome-aluminum catalyst, and achieved good results. In the catalyst after the introduction of rare earth, within two hours to maintain the activity and selectivity, after repeated regeneration dozens of times, its performance as ever. The performance of the catalyst has also been significantly improved. The conversion of propane is greater than 50% and the selectivity of propylene is greater than 90%. This is the best catalyst reported so far. Similar to the case of a catalyst for syngas with lower alkanes, the study on the performance improvement of rare earths is still preliminary. However, there is no report about the difference between the action mechanism of other rare earth additives and the synergistic effect of the multiple mixed rare earth additives.
Fifth, the research and application of rare earth in fuel cells
Solid oxide fuel cells (SOFCs) are attracting more and more attention due to their characteristics of high power generation efficiency, low pollution and sustainable power generation, and are praised as green energy in the 21st century. Research and development of rare earths in SOFCs mainly focus on the following aspects: development of cathode materials such as strontium-doped lanthanum manganite (LSM); development of anode materials such as nickel-YSZ cermet; bipolar plate materials, Such as calcium or strontium doped lanthanum chromite perovskite material (LCC): La1-xCaxCrO3; but more on electrolyte materials such as YSZ (yttria doped zirconia), doped lanthanum gallate , Thorium oxide, cerium oxide, rare earth perovskite composite oxide and the like.
So far, the oxygen ion conductors that may be used for SOFC are mainly ZrO2-based, fluorocarbon-based, CeO2-based, Bi2O3-based and perovskite-type LaGaO3-based materials. Finding new fine solid oxide electrolytes is still one of the key tasks for promoting the practical application of SOFC in the new century. Since Cook, Ishihara et al. Reported on the high conductivity of LaGaO3-based perovskite oxides, the research on perovskite-type solid electrolytes has received extensive attention. ABO3 not only has a stable crystal structure, but also has a strong tolerance to the change of ion radius at A and B sites, and can introduce a large amount of oxygen vacancies into the structure by doping with low-valent metal ions. Perovskite type oxides can contain a large amount of oxygen vacancies and have a high conductivity. For example, the conductivity of La0.8Sr0.2Ga0.83Mg0.17O3 at 800 ° C is 0.17 S / cm, which is an oxygen ion conductance The highest rate of ceramic materials. Rare earth oxides have good ionic and electronic conductivity, which plays an irreplaceable role in improving the performance of solid oxide fuel cells. By choosing a suitable oxide composition, the ionic conductivity of the electrode material can be increased, reducing the activation energy of oxygen reduction. By studying the composition, the relationship between structure and conductivity, and the morphology of doped ions, we designed and synthesized a new type of composite rare earth oxides with high electrocatalytic activity and high electrical conductivity. These materials are currently used in solid oxide fuel cells Research hot spots, but also rare earth in this area is an important application.
Sixth, the rare earth catalyst in the coking sewage purification application
Coal high temperature and temperature retort, gas purification and chemical products produced in the refining process of sewage is a highly complex, high concentration, toxic and difficult to deal with industrial wastewater. Not only contains a large amount of inorganic substances, but also contains a large amount of organic substances, including a large number of aromatic hydrocarbons and polycyclic aromatic hydrocarbons (PAH) such as BaP. At present, most of the coking wastewater treatment uses ordinary biochemical methods. Although this method can effectively remove phenol and cyanide to meet emission standards, it is also effective in removing COD (chemical oxygen demand). However, with the continuous deepening of people's understanding of the environment, the environmental protection requirements of the state are also becoming more stringent and it is hard to meet the new requirements. Catalytic wet oxidation is a modern purification technology for treating coking wastewater. The method is that the wastewater is kept in a liquid state under high temperature and pressure, and air is introduced into the wastewater. A novel and efficient two-component catalyst (precious metal and rare earth element) Pollutants are thoroughly oxidized and decomposed to convert them into harmless substances, so that the wastewater can be purified in depth.
Seven, rare earth catalyst in the flue gas desulfurization application
In recent years, due to the massive exploitation of coal and direct combustion, the content of SO2 in the atmosphere has been on the rise. The phenomenon of "acid rain" has occurred from time to time. According to statistics, the sulfur dioxide released to the atmosphere in 1995 reached 23.7 million tons in the world The first one. Therefore, to solve the problem of desulphurization of coal-fired flue gas has become a hot issue in all countries. Japan with rare earth catalytic gasification of coal research, with lanthanum nitrate, cerium nitrate, samarium nitrate load on the coal gasification rate than the previous use of sodium nitrate was significantly improved. Rare earth desulfurization agent prone to desulfurization reaction temperature range is wide, for 150 ~ 200 ℃, and the actual flue gas temperature (160 ℃) more consistent, and its desulfurization efficiency of up to 90%; desulfurization agent can also be recycled and reused Therefore, the rare-earth desulfurizer is suitable for SO2 removal in the flue gas.
Article from NdFeB Industry Network