TWC catalyst rare earth Power

The role of rare earth in catalyst mainly has the following aspects.

1. Active ingredient of automobile exhaust purification catalyst

The main harmful components in automobile exhaust are carbon oxides (Hc), carbon monoxide (CO) and nitrogen oxides (NO). Chemical reactions in purifiers include oxidation and reduction reactions. Therefore, it is necessary to find out a kind of ternary catalyst that can make oxidation and reduction reactions at the same time, so that the catalyst in the exhaust pipe of the automobile with the help of exhaust temperature and oxygen concentration in the air, the CO, HC and NO in the tail gas at the same time REDOX action, so that they can be converted into harmless substances C02, H20 and N2. The catalytic activities of Ce and La were studied. The results showed that the introduction of Ce02 significantly improved the catalytic conversion activities of CO and NO. Therefore, rare earth oxides can be used as active components of catalysts to reduce Co, HC and No completely or partially instead of noble metals.

2 Improve the anti - poisoning ability of catalyst

The substances such as Pb, S and P contained in motor vehicle exhaust are easy to poison the noble metal three-way catalyst. These substances produce chemisorption on the active surface of the catalyst, which hinders the progress of the reaction and makes the catalyst lose its catalytic activity. The resistance to sulfide poisoning is due to the formation of stable phase of these toxic substances, such as Ce203 and sulfide reaction to form stable C02(S04)3. In a reducing atmosphere, these sulfides are released and converted to H2S on Pt and Rh catalysts, which are expelled with exhaust gas (producing smelly H2S). The rare earth containing catalyst has strong resistance to poisoning due to the conversion of sulfides on the dilute surface. The results show that Ce02 has a certain sulfur storage effect on S02 component in tail gas. The following reaction occurs when the automobile engine works under the condition of poor combustion: 6 Ce02+3S02 - Ce2(S04)3+2C0203, the stored sulfur will be released under the condition of rich combustion, thus enhancing the anti-S poisoning ability of the catalyst.

3. Improve the thermal stability and mechanical strength of the catalyst

Ya-a1203, which usually constitutes the activated coating, will transform into A-A1203 above 800℃, which increases the density and decreases the surface area, resulting in the collapse of the pore structure. And above 1200℃, the activated coating will fall off from the support, so that the gas resistance increases and the catalytic activity decreases. Adding Ce02 can stabilize the crystal structure of YA-A1203, keep the activated coating stable at high temperature, and inhibit the activity loss. Cerium oxide retained a surface area of 60 m2·g. 1 after treatment at 1473 K for several hours in a reducing or neutral atmosphere, indicating that Ce3+, mainly in the presence of Ce A1203, hindered crystal growth and alumina transition.

4. Automatic adjustment of air-fuel ratio (oxygen storage capacity increases catalyst activity)

Around the theoretical air-fuel ratio of automobile engine operation, the composition of automobile exhaust gas will change periodically. Using the seed selection characteristics, the oxygen in the exhaust gas can be reversibly adsorbed and released of substances called oxygen storage substances, CeO has this role. Many studies have found that cerium oxide and other rare earth oxides have the ability to store and release oxygen. Ce02 releases 02 in the oxygen-poor region, oxidizes C0 and HC, and stores 02 in the oxygen-rich region, so as to control the atmosphere fluctuation near precious metals and stabilize the air-fuel ratio A/F near the stoichiometric equilibrium, which plays the role of expanding the air-fuel ratio window and maintaining the catalytic activity of catalysts. Ce in Ce02 can change the oxidation state (the transformation between Ce4+ and Ce3+), has excellent oxygen storage effect and oxygen release ability, can store/release oxygen under the conditions of poor combustion/rich combustion, so as to improve the conversion rate of CO, HC and NO. (When the engine is transient oil rich and the exhaust gas is transient hypoxia, the tetravalent Cc (CeO2) can become trivalent Ce(Ce2O3) and release O2. When the engine instantaneous depleted oil and cause the exhaust gas instantaneous oxygen-rich, Ce2O3 combined with O2 and converted into CeO2, this is the so-called oxygen reserve. The reaction equation is as follows :2 CeO2-- Ce2O3+1/2O2.

5. The role of accelerant

Automobile exhaust contains about l0% water vapor. Ce02 can promote the water-gas transfer reaction to produce reducing gas, which can improve the CO purification rate in the case of anoxia. At the same time, H2 can be used in the reduction of NO to improve the purification rate of NO in the combustion rich zone. In order to make up for the lack of Pd ability in catalytic reduction of NO in Pd rich and full Pd catalysts, La203 was added into Pd. This PD-LA catalyst was completely comparable to PT. Rh catalyst in performance.