Volatile organic compounds (VOCs) are a group of highly toxic and highly polluting chemicals. At present, the pollution problems of VOCs are receiving more and more attention from all countries. China has promulgated the "Integrated Emission Standard of Air Pollutants" and formulated strict air emission standards for all kinds of organic pollutants. VOCs pollution control methods at home and abroad are mainly adsorption method, absorption method, biological treatment technology, membrane separation technology, direct combustion, catalytic combustion and so on. Among them, the catalytic combustion method is a highly efficient clean combustion technology, the main catalyst to make the organic waste gas at a low temperature conditions, full combustion. Compared with other treatment technologies, catalytic combustion has significant advantages: low light-off temperature, low energy consumption, high treatment efficiency and no secondary pollution, making it one of the most promising VOCs treatment methods.
Efficient catalytic combustion catalyst is the key core of catalytic combustion technology. The catalyst with bulk carrier as skeleton matrix is ​​called the regular structural catalyst, also called monolithic catalyst. Due to the special pore structure, these catalysts improve the material transfer on the catalytic reaction bed, improve the catalytic efficiency, reduce the pressure and reduce the operation cost, and get more and more in the heterogeneous catalytic reaction in the petrochemical industry, fine chemical industry, etc. Wide range of applications.
1 monolith catalyst
The monolithic catalyst is mainly composed of four components: active component, cocatalyst, dispersed support and matrix. The monolithic catalyst has a unique skeleton structure with a large number of macro-scale hollow channels. The internal channels are all straight from one end to the other. The framework matrix acts as a support and is called a first carrier. The dispersed support is usually in the form of a coating Coated on the framework matrix, referred to as the second support. In order to increase the specific surface area of ​​the first support, reduce the amount of the active components and distribute the active component more evenly on the support, the coating material generally has a high surface area, Such as Al2O3. The framework matrix and the dispersed support together constitute the carrier of the monolithic catalyst; the active component is a basic substance for catalysis, such as Pt, Pd, Cu and the like; and the cocatalyst is to improve the activity and selectivity of the procatalyst and improve the heat resistance of the catalyst and Life, etc. A substance component, the active component and the cocatalyst are supported on a carrier to form a monolith catalyst.
1.1 Monolithic catalyst and the comparison of the traditional granular catalyst
Monolithic catalysts have great advantages over granular catalysts in heat transfer, mass transfer and pressure drop performance. (1) For heat transfer, monolithic catalyst has many channels, thin wall and no radial fluid transmission between parallel channels, which can be regarded as radial thermal insulation. Due to its high surface area, the heat transfer to the surface is very fast. The quality of the pellet catalyst to be heated is relatively large, eventually resulting in a long catalyst ignition time. (2) In the mass transfer, the coated monolith catalyst can disperse the active component on the surface. On the other hand, due to its high geometric surface area, it shortens the diffusion of the reactants into the active center, In comparison, the internal diffusion distance is approximately one order of magnitude less and the monolithic catalyst conversion is independent of the catalytic reactor shape. (3) A prominent feature of the monolithic catalyst is the reduced pressure, the turbulent flow of the air in any packed bed of particles, and the through-passage of the monolithic catalyst reduces the flow resistance. At the same space velocity and bed height, the monolithic catalyst Pressure drop is 2 to 3 orders of magnitude lower than the granular catalyst bed. In addition monolithic catalyst industrial amplification compared to the granular catalyst is simple.
1.2 skeleton matrix carrier classification and characteristics
According to different structures, the skeleton matrix mainly has honeycomb type, foam type and cross flow type. The honeycomb carrier is the most studied and the most widely used one. At present, the honeycomb carrier can be mainly divided into a honeycomb ceramic carrier and a metal honeycomb carrier according to the material thereof.
1.2.1 honeycomb ceramic carrier
The main carrier materials are grass bluestone, corundum, TiO2 and so on. Honeycomb ceramic carrier is a honeycomb porous ceramic, commonly used specifications are 100cell / in2, 200cell / in2, 300cell / in2 and 400celt / in2, carrier properties shown in Table l.
1.2.2 metal honeycomb carrier
Metal honeycomb carrier is a new type of metal honeycomb composite material, the general material is stainless steel or stainless steel alloy, commonly used specifications are 400cell / in2,500cell / in2 and 600cell / in2, the carrier properties shown in Table l. Metallic honeycomb supports have many excellent properties and are used in many fields. So far, honeycomb alloy substrates with excellent oxidation resistance such as Fe-Cr-Al have been widely used in automobile exhaust purifiers.
1.2.3 Other carriers
In addition to honeycomb ceramic and metal honeycomb carriers, there are also monolithic carriers such as carbon (silicon) fiber carriers, wire mesh and metal foam metallurgy porous materials.
Table 1 commonly used carrier properties
2 noble metal monolith catalyst
Precious metals have long been used in the field of catalytic combustion, and noble metal catalysts show higher catalytic activity than non-noble metals. Currently used precious metals Pd, Pd, Rh, the most widely used in VOCs catalytic combustion of Pd and Pt [10]. Due to its volatility, high temperature sintering and high cost, noble metals are often supported on high surface area carriers to increase their dispersion, reduce their dosage, and to a certain extent improve the stability of precious metals.
2.1 One-component precious metal monolith catalyst
At present, many researchers coated the cordierite with a large specific surface area of ​​the coating carrier, such as modified r ~ Al2O3, and then impregnated single precious metal active components. Silica fiber has good chemical inertness and high specific surface area. Cao et al. Applied Al2O3 film on strong silicon-rich fiber to study the low-temperature oxidation of VOCs on Pt / Al2O3 / Si fiber catalyst. Ethanol, benzene, toluene and xylene , Their T10 (the temperature at which the conversion of the reactants reaches 10%) is 110 ° C, 190 ° C and 220 ° C, respectively, of which T90 (the temperature at which the conversion of the reactants reaches 90%) only needs 170 ° C, T90 toluene to reach 280 ℃. Li Dongxu Modified Al 2 O 3 coating by adding alkaline earth metal, the coating of Pt / Mg-Si-Al 2 O 3 / honeycomb ceramic catalyst was prepared and bonded firmly to the surface of ceramic support. And this catalyst has good high temperature resistance and high organic Exhaust gas purification rate, after operating 1000h still maintain more than 90% of the purification rate. Wang Chunyong Preparation of Pd / Ce-Zr ~ Al2O3 / cordierite honeycomb ceramic catalyst with acetone as the experimental object to evaluate its catalytic activity showed that the additives Ce, Zr not only improve the thermal stability of Al2O3, and add additives The T90 of the post-catalyst to acetone was 230 ° C, while the T90 of no additive was 280 ° C. Barbara et al prepared monolith catalysts Pd / Al2O3-SiO2 and Pd / Al2O3-ZrSiO4 on a metal support. The results showed that the catalyst aging temperature was 1060 ℃ and the methane conversion temperature was T99. (Temperatures at which the conversion of the reactants reached 99%) were 447 ° C and 415 ° C, respectively.
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