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The role of rare earth catalytic materials in automotive exhaust purification 

Release time :2024/01/06
The role of rare earth catalytic materials in automotive exhaust purification

        At present, catalysts widely developed and applied in automobile exhaust purification abroad are mainly composed of precious metals such as platinum (Pt) and rhodium (Rh). Currently, the commonly used platinum rhodium based precious metal ternary catalysts mainly purify HC and CO through the oxidation of Pt, and purify NOx through the reduction of Rh. Although this catalyst has advantages such as high activity, good purification effect, and long service life, its cost is also high, especially Pt, Rh, etc., which are limited by resources. In order to alleviate the contradiction between the supply and demand of Pt, especially Rh, palladium (Pd), which is relatively inexpensive, has been widely used. Catalysts composed of Pt, Rh, and Pd, as well as palladium catalysts, have been developed.

        People have found that catalysts made by replacing some precious metals with rare earths have low costs and can achieve satisfactory purification effects.

        The rare earth used in rare earth automotive exhaust purification catalysts is mainly a mixture of cerium oxide, praseodymium oxide, and lanthanum oxide, with cerium oxide being the key component. Due to the redox properties of cerium oxide, effective control of the composition of exhaust emissions can provide oxygen in a reducing atmosphere or consume oxygen in an oxidizing atmosphere. Cerium dioxide also plays a stabilizing role in noble metal atmospheres to maintain high catalytic activity of the catalyst. Therefore, the development of automotive exhaust purification agents with rare earth elements and precious metals is to take advantage of the advantages of rare earth elements to compensate for the shortcomings of precious metals, and produce practical automotive exhaust purification agents. Its characteristics include low price, good thermal stability, high activity, and long service life, making it highly favored in the field of automotive exhaust purification.

        The outer electronic structure of rare earth elements is similar, and the difference in catalytic performance between rare earth elements is relatively small. The overall catalytic activity is not comparable to the transition elements and precious metal elements in the outer electronic structure. In current practical industrial catalysts, rare earths are generally only used as co catalysts or as an active component in catalysts, and are rarely used as the main catalyst. As additives for precious metal catalysts, rare earths can improve and alter the performance of catalysts, and their role as additives is far greater than that of traditional alkali metal or alkaline earth metal elements. China's motor vehicle emissions are severe, but precious metals are scarce and rare earth resources are abundant. Therefore, the application of rare earth elements in motor vehicle exhaust treatment is widely used in China.

        In the exhaust purification catalysts for motor vehicles, the dilute mixture mainly has oxygen storage and catalytic effects. Adding it to the catalytic activity group can improve the catalyst's resistance to lead and sulfur poisoning, high temperature stability, and air fuel ratio working characteristics.


Application of Rare Earths in TWC

        The unique properties of rare earth oxides have long attracted widespread attention from catalyst researchers both domestically and internationally. However, so far, rare earth oxides are mostly used as catalyst carriers and promoters. The role of rare earths in catalysts mainly includes the following aspects.

1. Active ingredients of automotive exhaust purification catalysts

        The main harmful components in automobile exhaust are carbon oxides (Hc), carbon monoxide (CO), and nitrogen oxides (NO), and the chemical reactions in the purifier include oxidation and reduction reactions. Therefore, it is necessary to find a ternary catalyst that can simultaneously carry out both oxidation and reduction reactions, so that the catalyst can use the exhaust temperature and oxygen concentration in the air in the car exhaust pipe to simultaneously oxidize and reduce CO, HC, and NO in the exhaust gas, converting them into harmless substances C02, H2O, and N2. The research results on the catalytic activity of Ce and La rare earths indicate that the introduction of Ce02 significantly enhances the catalytic conversion activity of CO and NO. Therefore, rare earth oxides can be used to completely or partially replace precious metals as the active components of catalysts, catalyzing the reduction of Co, HC, and No.

2. Improve the anti poisoning ability of catalysts

        The Pb, S, P and other substances contained in the exhaust of motor vehicles are easily toxic to precious metal three-way catalysts. These substances generate chemical adsorption on the surface active sites of the catalyst, hindering the reaction and causing the catalyst to lose catalytic activity.

        Rare earth elements have the ability to resist sulfide poisoning because these toxic substances react with them to form stable phases, such as Ce203 reacting with sulfides to form stable C02 (S04) 3. In a reducing atmosphere, these sulfides are released and converted into H2S on Pt and Rh catalysts, which are discharged together with the exhaust gas (producing foul smelling H2S). The conversion effect of rare earth on sulfides gives rare earth containing catalysts strong anti poisoning ability.

        Research has shown that Ce02 has a certain sulfur storage effect on the S02 component in exhaust gas. When the car engine operates under lean combustion conditions, the following reactions occur: 6 Ce02+3S02- Ce2 (S04) 3+2C0203. The sulfur stored under rich combustion conditions will be released, thereby enhancing the catalyst's ability to resist S poisoning.

3. Improve the thermal stability and mechanical strength of catalysts

        Normally, the Ya-A1203 that forms the activated coating will transform into a-A1203 above 800 ℃, causing an increase in density and a decrease in surface area, resulting in the collapse of the pore structure. And at temperatures above 1200 ℃, the activated coating will detach from the carrier, increasing gas resistance and reducing catalytic activity.

        Adding Ce02 can stabilize the crystal structure of YA-A1203, keeping the activated coating stable at high temperatures and suppressing activity loss. Cerium oxide can still maintain a surface area of 60 m2 · g · 1 after several hours of treatment at 1473 K in a reducing or neutral atmosphere, indicating that Ce3+, mainly present in Ce A1203, hinders crystal growth and the transformation of alumina.

4. Automatic adjustment of air fuel ratio (improving oxygen storage capacity and catalyst activity)

(Based on the theoretical air-fuel ratio during the operation of a car engine, the composition of car exhaust gas undergoes periodic changes. By utilizing the selection characteristics, the substance that can reversibly adsorb and release oxygen in the exhaust gas is called an oxygen storage substance, and CeO has this effect.)

        Many studies have found that rare earth oxides such as ceria have the ability to store and release oxygen. Ce02 releases O2 in the oxygen poor zone, oxidizes C0 and HC, and stores O2 in the oxygen rich zone, thereby controlling the atmosphere fluctuations near precious metals and stabilizing the air-fuel ratio A/F near stoichiometric equilibrium, thereby expanding the air-fuel ratio window and maintaining the catalytic activity of the catalyst.

        Ce in Ce02 can change its oxidation state (conversion between Ce4+and Ce3+), and has excellent oxygen storage and release capabilities. Under lean/rich combustion conditions, it can store/release oxygen, thereby improving the conversion rate of the catalyst to CO, HC, and NO.

        When the engine momentarily becomes rich in oil and causes instantaneous oxygen deficiency in the exhaust gas, tetravalent Cc (CeO2) can be converted into trivalent Ce (Ce2O3), releasing O2. When the engine momentarily becomes poor in oil and causes instantaneous oxygen enrichment in the exhaust gas, Ce2O3 combines with O2 to convert into CeO2, which is the so-called oxygen reserve effect.

The reaction equation is as follows: 2 CeO2-- Ce2O3+1/2O2.)

5. The role of adjuvants

        Car exhaust contains about 10% water vapor, and Ce02 can promote water gas transfer reaction to produce reducing gas, which can improve the purification rate of CO under hypoxia. At the same time, H2 can be used in the reduction of NO, improving the purification rate of NO in the rich combustion zone. CO+H2O - CO2+H2

        In order to compensate for the insufficient ability of Pd in catalytic reduction of NO in Pd rich and all Pd catalysts, La203 was added to Pd. This Pd La catalyst is completely comparable in performance to Pt Rh catalyst.

6. Improve the catalytic activity of active coatings

        Adding CeO2 keeps the precious metal particles in the active coating dispersed, avoiding the reduction of catalytic lattice points caused by sintering and damaging the activity. In Pt/ γ Adding CeO2 to 2Al2O3, as CeO2 can γ Single layer dispersion on 2Al2O3 (maximum single layer dispersion amount is 01035 gCeO2Pg) γ 2Al2O3), changed γ The surface properties of 2Al2O3 improve the dispersion of Pt. When the CeO2 content is equal to or close to the dispersion threshold, the dispersion of Pt reaches its maximum. The dispersion threshold of CeO2 is its optimal dosage. Rh loses its activation effect in an oxidizing atmosphere above 600 ℃ due to the formation of a solid solution between Rh2O3 and Al2O3 generated by high-temperature oxidation. The presence of CeO2 will weaken the reaction between Rh and Al2O3, maintaining the activation effect of Rh. La2O3 can also prevent the growth of Pt ultrafine particles. Add CeO2 and La2O3 to PdP γ After 2Al2O3, it was found that the addition of CeO2 promoted the dispersion of Pd on the carrier and produced a synergistic reduction effect. Highly dispersed Pd and its interaction with CeO2 in Pd/ γ The interaction on 2Al2O3 is the key to the high activity of the catalyst.

        CeO2 is also an effective catalyst for hydrocarbon oxidation. When examining CO oxidation on Pt/CeO2, it was found that lattice oxygen at the interface between Pt and CeO2 plays an important role. CeO2 surface can generate low valence cerium and oxygen defects in vacuum or reducing atmosphere, exhibiting excellent catalytic performance and gas sensing function for O2 reduction, especially the ability to exchange charges and species with adsorbed molecules. CeO2 is prone to generate low valence cerium and oxygen vacancies under the action of hydrogen. Pt/CeO2 can absorb gas-phase hydrogen and release it again. At room temperature, partially reduced CeO2 adsorbs oxygen to form molecular ion oxygen species. Oxygen species can be partially desorbed and can be converted into lattice oxygen above 170 ℃ [4]. In addition, CeO2 has an impact on γ The modification of 2Al2O3 support is beneficial for the desorption and oxidation recovery of surface oxygen species on palladium catalysts, thereby promoting Pd/CeO22 γ The oxidation effect of 2Al2O3 catalyst .

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