Research progress on the application of catalysts in environmental protection
1、Definition of environmental protection catalysts Environmental protection catalysts are catalysts used to protect and improve the surrounding environment by treating toxic and hazardous substances in a direct or indirect way to make them harmless or reduce them. In a broad sense, the scope of environmental protection catalysts can be considered as all catalysts that are beneficial to environmental protection, including catalytic synthesis processes that do not or do not produce harmful by-products; in a narrow sense, it is the type of catalysts involved in the improvement of greenhouse effect, ozone layer destruction, expansion of acid rain range and water pollution, etc.
Environmental catalysts are classified into direct and indirect. For example, the catalysts used to remove nitrogen oxides (NOX) from exhaust gases are direct; while the catalysts used to inhibit NOX production in the combustion process are indirect.
2. Research progress in the application of environmental catalysts
2.1 catalysts for lean combustion vehicles When diesel engines are operated under lean combustion conditions, the air-fuel ratio (ratio of air to fuel) of gasoline engines is greater than 17:1 or even higher. At this time, the power performance of the engine can be greatly improved and the emission of CO, hydrocarbon compounds and CO2 is reduced, however, the NOx emission is greatly increased. For the current popular triple-effect noble metal catalysts, such a high air-fuel ratio is beyond the normal operating range, so it cannot effectively improve the reduction of NOx.
Therefore, new catalysts should be developed for automotive applications that can improve NOx conversion under lean combustion conditions, and catalytic reduction of NOx under lean combustion conditions has attracted the interest of researchers. Once this catalyst is successfully studied, it will be widely used in vehicles with diesel engines and oil-poor gasoline engines.
2.2 Research on flue gas desulfurization The best method for flue gas desulfurization is the selective catalytic reduction of SO2 to elemental sulfur. This method not only eliminates the source of SO2 pollution in flue gas, but also recovers the product, i.e. solid elemental sulfur, which is not only easy to transport but also reusable. The current method of selective catalytic reduction of SO2 to elemental sulfur is mostly in the research stage. The problems are the interference of excess oxygen in the flue gas to the reduction process and the poisoning of the catalyst.
2.3 Catalytic oxidation treatment of high concentration hard-to-degrade organic wastewater With the development of pharmaceutical, chemical and dye industries, there are more and more high concentration hard-to-degrade wastewaters, which are characterized by high toxicity of pollutants, high concentration of pollutants, difficult to biodegrade; high content of inorganic salts. One of the most effective methods to treat such wastewater is chemical oxidation.
High efficiency wet catalytic oxidation technology is currently a popular research topic. This method can directly oxidize organic pollutants in water or oxidize large-molecule organic pollutants in water into small-molecule organic pollutants to improve the biochemical properties of wastewater. It can remove the organic pollutants in water better after treatment with biochemical method. This method often uses oxidants to increase the ability of catalytic oxidation of organic pollutants, the oxidants can be used: air, hydrogen peroxide, ozone, sodium hypochlorite and chlorine dioxide, etc. The key to this method is the development of efficient non-homogeneous oxidation catalysts.
2.4 Types of environmental catalysts and their utilization status There are various environmental problems on the earth, and the problems that are urgently wanted to be solved are: greenhouse effect, ozone layer destruction, expansion of acid rain, emission of heavy metals and other environmental pollutants, reduction of tropical rainforest and soil desertification. The first three of these problems are caused by chemical substances that are emitted into the atmosphere.
For example, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are related to the greenhouse effect, Freon and N2O destroy the ozone layer, and sulfur dioxide (SO2) and NOX are the main factors in the formation of acid rain and photochemical smog, and the solution to remove or reduce these pollutants is mainly through chemical methods. Since the amount of reactants involved in the emission process of these pollutants is low, the reaction temperature is either too high or too low, and the contact time between the reactants and the catalyst is particularly short, environmental protection catalysts require higher activity, selectivity and durability than other catalysts for chemical reactions, and are more difficult to produce.
2.5 New environmental catalysts
2.5.1 Silicate materials Natural clays such as montmorillonite have a molecular sieve-like structure and are good adsorbents for catalyst carriers and for treating heavy metal ions in sewage. It is widely used as a carrier for environmental protection catalysts such as automobile exhaust gas purification, flue gas desulfurization, denitrification and catalytic combustion of organic waste gas.
2.5.2 TiO2 is an N-type semiconductor with good photosensitive conductivity, often used as a catalyst carrier. Now TiO2 is widely used as photocatalyst and electrode catalyst, coated with active TiO2 self-cleaning glass, tiles, furniture, curtain cloth, in the sun and light under the irradiation of automatic catalysis and purification of indoor air.
2.5.3 The biocatalytic process is usually based on non-toxic and harmless biological materials as raw materials, which can be reacted at room temperature and pressure, and the process is simple. Biocatalysts are ideal green catalysts because of their high conversion rate, high specificity, low by-products and reusability.
2.5.4 Room temperature ionic liquid can be used as both an acid catalyst and a green solvent. With the advantages of easy production, low toxicity, low cost, non-combustibility and adjustable performance, they are predicted to be environmentally friendly catalysts with the potential to cause a chemical industrial revolution and good prospects for industrial applications.
