Application of Nano Rare Earth Oxide in Automobile Exhaust

As we all know, rare earth minerals in China are mainly composed of light rare earth components, of which lanthanum and cerium account for more than 60%. With the expansion of rare earth permanent magnet materials, rare earth luminescent materials, rare earth polishing powder and rare earth in metallurgical industry in China year by year, the demand for medium and heavy rare earth in domestic market is also increasing rapidly.It has caused a large backlog of high abundance light rare earths such as Ce, La and Pr, which leads to a serious imbalance between the exploitation and application of rare earth resources in China. It is found that light rare earth elements show good catalytic performance and efficacy in the chemical reaction process due to their unique 4f electron shell structure. Therefore,Using light rare earth as catalytic material is a good way for comprehensive utilization of rare earth resources. Catalyst is a kind of substance that can accelerate chemical reaction and is not consumed before and after reaction. Strengthening the basic research of rare earth catalysis can not only improve production efficiency, but also save resources and energy and reduce environmental pollution, which is in line with the strategic direction of sustainable development.

Why rare earth elements have catalytic activity?  

Rare earth elements have a special outer electronic structure (4f), which acts as the central atom of the complex and has various coordination numbers ranging from 6 to 12. The variability of coordination number of rare earth elements determines that they have “residual valence”. Because 4f has seven backup valence electron orbitals with bonding ability, it plays a role of “backup chemical bond” or “residual valence”.This ability is necessary for a formal catalyst. Therefore, rare earth elements not only have catalytic activity, but also can be used as additives or cocatalysts to improve the catalytic performance of catalysts, especially the anti-aging ability and anti-poisoning ability.

At present, the role of nano cerium oxide and nano lanthanum oxide in the treatment of automobile exhaust has become a new focus.

Harmful components in automobile exhaust mainly include CO, HC and NOx. The rare earth used in the rare earth automobile exhaust purification catalyst is mainly a mixture of cerium oxide, praseodymium oxide and lanthanum oxide. The rare earth automobile exhaust purification catalyst is composed of complex oxides of rare earth and cobalt, manganese and lead. It is a kind of ternary catalyst with perovskite, spinel type and structure, in which cerium oxide is the key component.Due to the redox characteristics of cerium oxide, the components of exhaust gas can be effectively controlled.

 

Automobile exhaust purification catalyst is mainly composed of honeycomb ceramic (or metal) carrier and surface activated coating. The activated coating is composed of large area γ-Al2O3, proper amount of oxide for stabilizing surface area and catalytically active metal dispersed in the coating. To reduce the consumption of expensive pt and RH, increase the consumption of cheaper Pd and reduce the cost of catalyst,On the premise of not reducing the performance of the automobile exhaust purification catalyst, a certain amount of CeO2 and La2O3 are commonly added into the activation coating of the commonly used Pt-Pd-Rh ternary catalyst to form a rare earth precious metal ternary catalyst with excellent catalytic effect. La2O3(UG-La01) and CeO2  were used as promoters to improve the performance of γ- Al2O3 supported noble metal catalysts. According to research, CeO2,The main mechanism of La2O3 in noble metal catalysts is as follows:

1. improve the catalytic activity of the active coating by adding CeO2 to keep the precious metal particles dispersed in the active coating, so as to avoid the reduction of catalytic lattice points and damage to the activity caused by sintering. Adding CeO2(UG-Ce01) into Pt/γ-Al2O3 can disperse on γ-Al2O3 in a single layer (the maximum amount of single-layer dispersion is 0.035g CeO2/g γ-Al2O3), which changes the surface properties of γ-Al2O3 and improves the dispersion degree of Pt.When CeO2 content is equal to or close to the dispersion threshold, the dispersion degree of Pt reaches the highest. The dispersion threshold of CeO2 is the best dosage of CeO2. In the oxidation atmosphere above 600℃, Rh loses its activation due to the formation of solid solution between Rh2O3 and Al2O3. The existence of CeO2 will weaken the reaction between Rh and Al2O3 and keep the activation of Rh. La2O3(UG-La01) can also prevent the growth of Pt ultrafine particles.Adding CeO2 and La2O3(UG-La01) to Pd/γ 2al2o3, it was found that the addition of CeO2 promoted the dispersion of Pd on the carrier and produced a synergistic reduction. The high dispersion of Pd and its interaction with CeO2 on Pd/γ2Al2O3 are the key to the high activity of the catalyst.

2. Auto-adjusted air-fuel ratio (aπ f) When the starting temperature of the automobile rises, or when the driving mode and speed change, the exhaust flow rate and exhaust gas composition change, which makes the working conditions of the automobile exhaust gas purification catalyst constantly change and affects its catalytic performance. It is necessary to adjust the π fuel ratio of air to the stoichiometric ratio of 1415~1416, so that the catalyst can give full play to its purification function.CeO2 is a variable valence oxide (Ce4 +ΠCe3+), which has the properties of N-type semiconductor, and has excellent oxygen storage and release capacity. When the A π F ratio changes, CeO2 can play an excellent role in dynamically adjusting the air-fuel ratio. That is, O2 is released when the fuel is surplus to help CO and hydrocarbon oxidize; In case of excess air, CeO2-x plays a reducing role and reacts with NOx to remove NOx from the exhaust gas to obtain CeO2.

3. Effect of cocatalyst When the mixture of aπ f is in stoichiometric ratio, besides the oxidation reaction of H2, CO, HC and the reduction reaction of NOx, CeO2 as cocatalyst can also accelerate the water gas migration and steam reforming reaction and reduce the content of CO and HC. La2O3 can improve the conversion rate in water gas migration reaction and hydrocarbon steam reforming reaction.The generated hydrogen is beneficial to NOx reduction. Adding La2O3 to Pd/ CeO2 -γ-Al2O3 for methanol decomposition, it was found that the addition of La2O3 inhibited the formation of by-product dimethyl ether and improved the catalytic activity of the catalyst. When the content of La2O3 is 10%, the catalyst has good activity and the methanol conversion reaches the maximum (about 91.4%). This shows that La2O3 has good dispersion on γ-Al2O3 carrier.Furthermore, it promoted the dispersion of CeO2 on γ2Al2O3 carrier and the reduction of bulk oxygen, further improved the dispersion of Pd and further enhanced the interaction between Pd and CeO2, thus improving the catalytic activity of the catalyst for methanol decomposition.

According to the characteristics of current environmental protection and new energy utilization process, China should develop high-performance rare earth catalytic materials with independent intellectual property rights, achieve efficient utilization of rare earth resources, promote technological innovation of rare earth catalytic materials, and realize leap-forward development of related high-tech industrial clusters such as rare earth, environment and new energy.

At present, the products supplied by the company include nano zirconia, nano titania, nano alumina, nano aluminum hydroxide, nano zinc oxide, nano silicon oxide, nano magnesium oxide, nano magnesium hydroxide, nano copper oxide, nano yttrium oxide, nano cerium oxide, nano lanthanum oxide, nano tungsten trioxide, nano ferroferric oxide, nano antibacterial agent and graphene.The product quality is stable, and it has been purchased in batches by multinational enterprises.

 

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