Automobile Exhaust Purification Catalyst
1. Mechanism of Catalyst Action
The pollutants in automobile exhaust mainly include carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NO), sulfur dioxide (SO2), and particulate matter (lead compounds, carbon soot, etc.). Currently, there are two main methods for purifying automobile exhaust: internal purification and external purification. Internal purification involves changing the structure of the engine to promote complete combustion of fuel or to enable the re-combustion of some exhaust gases to reduce harmful substances. External purification mainly employs catalytic purification methods, which involve both oxidation reactions of CO and HC as well as reduction reactions of NO, using catalytic action to convert harmful substances in the exhaust into harmless CO, H2O, and N2. The catalytic purification methods currently used include catalytic oxidation, catalytic reduction-oxidation, and three-way catalytic purification. Three-way catalysts are widely used in various countries. The three-way catalyst mainly consists of three parts: the catalyst substrate, the active coating, and the catalytic active components. Additionally, to enhance the performance of the catalyst, small amounts of additives are often added to the active coating and catalytic active components, mainly rare earth oxides and alkali earth metal oxides, etc.
2. Carrier Function and Requirements
The use conditions of the three-way catalytic converter for automobile exhaust purification are quite harsh, including temperature variations ranging from -50°C to 950°C, severe impact and vibration from high-speed airflow, and a service life of up to 2 years or 160,000 kilometers. Moreover, the high activity and temperature resistance required for CO and HC oxidation and NOx reduction, as well as resistance to S and P poisoning, impose higher requirements on the catalyst. The composition of the catalyst, the compatibility between various components, and the properties of the active alumina used have important effects on the performance of the catalyst, especially the directly affect the activity and service life of the catalyst.
(1) Catalyst Composition
Three-way catalysts mainly consist of the catalyst substrate, active coating, and active components. Through special preparation processes and different distributions of active components with different ratios in the coating, the requirements for good catalytic performance at different positions in the automobile exhaust system, from cold start to high temperature, can be met.
Catalyst substrate: The substrate, also known as the support, mainly includes cordierite honeycomb ceramics, silicon carbide, metal honeycombs, corrugated sheets, etc. The substrate should meet the following requirements: high mechanical strength to withstand thermal shock and severe vibration of high-speed airflow; large external surface area and porosity to facilitate the adhesion and dispersion of the active coating; low coefficient of thermal expansion and high temperature resistance to prevent cracking and deformation leading to coating detachment due to drastic changes in operating temperature; high airflow permeability and high pressure drop resistance to avoid excessive engine power loss due to high exhaust resistance; low heat capacity and high thermal conductivity to rapidly increase the temperature during cold starts for catalytic action; and immunity to substances that may poison the catalyst without interacting with it.
Active coating: The coating should have strong adhesion to the substrate and a coefficient of thermal expansion similar to that of the substrate to prevent coating detachment due to temperature variations and thermal expansion and contraction of the substrate; good high-temperature stability to inhibit phase transformation or sintering at high temperatures; and certain tolerance to trace toxic substances such as Pb, S, and P to avoid poisoning the active components. In addition to active alumina, the coating material mainly includes rare earth composite oxides such as Ce and Zr, alkaline earth metals or alkali metals, and metal oxides such as Ba, Sr, and TiO2, added to improve the thermal stability of the coating material and enhance the catalyst's resistance to high temperatures, oxygen storage capacity, resistance to poisoning, dispersion of active components, and thermal stability.
Active components of the catalyst: The active components should have good high-temperature resistance, resistance to S, P poisoning, low ignition temperature, high catalytic activity, including high oxidation performance of CO and HC and high reduction performance of NOx, and good dispersion. Precious metal active components mainly include platinum, palladium, rhodium, and their combinations. Palladium and platinum have excellent catalytic activity for the oxidation of HC and CO, while rhodium has excellent catalytic activity for the reduction of NOx, and its low-temperature activity is better than that of palladium and platinum. With the tightening of automotive emission standards and the widespread implementation of Euro V standards, the emission requirements for NOx are becoming more stringent, and three-way catalytic converters generally contain varying amounts of rhodium.
Three-way catalytic converters for automobile exhaust purification can maintain good performance under harsh operating conditions, and the optimization combination of the catalyst substrate, active coating, and active components is crucial in addition to good preparation methods. Three-way catalysts are usually based on cordierite honeycomb ceramics or metal honeycombs, with active alumina carriers loaded with precious metal active components and rare earth composite oxides or alkali or alkaline earth metal oxides as additives, ground into a slurry as coating material, coated onto the substrate through a special process, and prepared by drying, calcination, and activation.
(2) Role and Influence of Alumina
The role of active alumina in three-way catalytic converters lies in serving as a carrier for precious metal active components to ensure their high dispersion and as a component of the coating material to provide a high specific surface area, maintain good adhesion and matching with the ceramic substrate, and prevent coating detachment and phase transformation. Currently, the most commonly used active alumina is AOS, which has a large specific surface area, moderate pore distribution, and good resistance to sintering. However, γ-Al2O3 is a metastable phase and is prone to phase transformation and sintering at high temperatures, leading to stable α-phase and coarsening of particles, resulting in a significant decrease in specific surface area, thereby affecting the dispersion of active metals on its surface and reducing the catalyst's performance or even deactivation. Moreover, in a high-temperature oxidizing atmosphere of 800~900°C, the γ-Al2O3 coating will react with the active component Rh to form non-active aluminum salts, also reducing the catalyst's activity.
To improve the high-temperature stability of the coating active alumina and prevent its agglomeration and phase transformation, the current common method in industry is to add non-precious metal elements such as rare earth or transition metals to γ-Al2O3. Rare earth elements have unfilled 4f electron shells, rich and unusual electron energy levels, and many excellent optical, electrical, magnetic, and nuclear properties, coupled with their very active chemical properties, they can form various novel materials with different categories, functions, and uses with other elements. The cations of rare earth elements have ion radii much larger than Al3+, which can raise the phase transformation temperature of γ-Al2O3, suppress the diffusion of O2- or Al3+, thereby improving the high-temperature sintering resistance of the coating active alumina, and maintaining its high specific surface area. Studies have shown that when stabilizing the structure of active alumina, rare earth elements such as La, Pr, Nd, and Ce, as well as alkaline earth metals Ba, Sr, and Ca, etc., can be added. The high-temperature sintering resistance of active alumina is related to the size of rare earth element ion radii to some extent, and the better stabilizing effect is achieved with larger ion radii. Therefore, La is the better modifier. La-modified active alumina will form a perovskite-type LaAlO3 on the surface, and the nucleated LaAlO3 will be fixed on the corners of the Al2O3 lattice, thereby improving the thermal stability and specific surface area of alumina and inhibiting its transformation into the α phase.
The role of rare earth catalytic materials in three-way catalytic converters for automobile exhaust purification, especially the oxygen storage and release function of cerium oxide in the catalyst. Cerium has two oxidation states, Ce4+ with an ion radius of 0.97 Å and Ce3+ with 1.03 Å. As the oxygen content in the reaction system alternates, Ce4+ and Ce3+ in the catalyst also alternate, i.e., when the oxygen content is high, Ce3+ is converted to Ce4+, and the catalyst adsorbs and stores more oxygen from the reaction system; when the oxygen content is low, Ce4+ is converted to Ce3+, and the catalyst releases more oxygen into the reaction system. The role of cerium oxide also includes stabilizing the specific surface area and pore structure of alumina carriers, maintaining the good dispersion of precious metal active components, improving the activity and resistance to sulfur and lead poisoning of the catalyst, etc.
High-performance alumina can increase the specific surface area of the catalyst and the dispersion of precious metal particles, ensure the high dispersion of precious metal particles, and significantly improve the high-temperature stability of alumina after modification by adding a certain amount of rare earth oxide or modifying the surface of alumina by rare earth oxide. After fully mixing and grinding precious metals with the above high-performance alumina carrier, rare earth oxide with high temperature and high oxygen storage performance, other additive components, and deionized water, then coating, drying, calcination, and activation are carried out, and the obtained three-way purification catalyst has excellent performance and can replace imported catalyst products. Alumina is the most widely used catalyst coating carrier, but the thermal stability problem of Al2O3 has long plagued people, especially in reaction environments with high temperatures and the presence of water vapor, γ-Al2O3 is prone to phase transformation and sintering, leading to the development of stable α-phase and particle coarsening, resulting in a significant decrease in specific surface area, becoming one of the important causes of deactivation of loaded catalysts. Therefore, research and development of alumina carrier materials with high-temperature stability and large specific surface area are key technologies for developing a new generation of automotive exhaust purification catalysts. In addition, China is a major producer of rare earths, and how to develop its advantages in rare earths and develop rare earth-based automotive exhaust purification three-way catalysts with better performance, replacing precious metal catalysts with inexpensive rare earths, will be a new development direction for automotive exhaust catalysts with broad prospects.
Other Coating-Type Catalysts
In addition to the first type of coating-type catalysts such as three-way catalytic converters for automobile exhaust purification, four-way catalytic converters for diesel vehicle exhaust purification, catalytic converters for industrial waste gas desulfurization and denitrification, and VOC conversion catalysts, there is a second type of coating-type catalysts such as precious metal Pt catalysts coated on fuel cell electrodes and nano-alumina coatings on lithium battery separators. Because the carriers for the Pt catalysts used in fuel cells are mainly carbon-based materials such as graphite, and the nano-alumina materials coated on lithium-ion battery separators.