Construction And Process Characteristic Of New Catalyst Systems Of Heteropoly Compounds As Functional Material For The Adsorption-dessorption Of NO_x

NO_x is one of the main air pollutants .It is also the main precursor of photochemistry fog. It has been a hot point in the international environmental research field to control and treat NO_x pollution. Among various denitrification methods, catalytic decomposition and selective catalytic reduction (SCR) were adopted widely. SCR technology with ammonia as reducing agent has being widely used commercially for denitrification of flue gas. But ammonia which is hard to transport is liable to leak and is also corrosive to pipe line. Investment and running cost of SCR with NH₃ as reducing agent is high. Decomposition of NO_x is a simple method without reducing agent. Furthermore, the cost of investment and running of NO_x decomposition is low. But most catalysts used are active only in high temperature and may strongly be inhibited by oxygen and water in the flue gas. Hence, the development of new catalyst systems for NO_x conversion has becoming the focus of worldwide researches. Heteropoly compounds (HPCs) are functional material which are unharmful to the environment. Using heteropoly compounds as catalysts for NO_x decomposition could overcome the negative effects caused by O₂ and H₂O in the flue gas and is therefore promising in the treatment of NO_x.In this paper, many kinds of HPCs and supported HPCs were prepared and evaluated concerning their de-NO_x efficiencies, and new catalyst systems as functional material for adsorption-decomposotion of NO_x were constructed. The relationship between the catalyst structure and the denitrification performance was studied. The mechanism of the process of NO_x adsorption-decomposotion was revealed with the aid of modern analytical instruments.The paper is composed of three sections.In the first section, test conditions for the new catalyst systems of heteropoly compounds were confirmed primarily. In the presence of oxygen, the effects of space velocity, water vapor concentration and reaction temperature on NO_x convertion were discussed. The experimental results showed that the optimum test conditions are 4.2% (v/v) of water vapor concentration. 150℃of reaction temperature and 5298h^-1 of space velocity. Several intrinsic drawbacks existed in the conventional catalysts, including oxygen, water and active temperature inhibition, were overcomed effectively.In the second section, NO_x adsorption efficiencies of hetepoly compounds with Keggin structure prepared by means of ethanol-extraction were evaluated. The results showed that among the prepared HPCs, H₃PW₁₂O₄₀ (HPW) was the best adsorbent. Subsequently, HPW was impregnated on several supports including carbon nanotube (CNT), Titania (TiO₂), Cerium and zirconium oxides, Y-type zeolite, activated carbon, cordierite and activated soil to form corresponding catalyst systems. NO_x adsorption efficiencies of supported HPW were evaluated so as to find good supports and their corresponding catalyst systems. After that, the optimum catalyst systems including HPW/CNT, HPW/TiO₂ and HPW/CeO₂ were characterized by XRD, BET, IR and TEM. The results showed that Mixed HNO₃/H₂SO₄, with a volume ratio of 1:3. could produce functional groups such as COO- and C=O on the surface of CNT, which made the dispersion of CNT easier in the aqueous solution. At a HPW loading of 70%. the best adsorption efficiency of NO_x by HPW/CNT catalyst system, prepared using water as impregnation solvent and HNO₃/H₂SO₄ -modified CNT as support, could be achieved as 73.5%. The adsorption efficiency of HPW/TiO₂ can reach 62.8% at most. when the HPW loading was 20% and the calcination temperature of TiO₂ was 500℃.The adsorption efficiency of HPW/CeO₂ can reach 75.3% at most, when the HPW loading was 70%. NO_x adsorptive performence is mainly relevant to the pseudoliquidphase property of HPW supported on the supports and unsensitive to the catalyst surface.In the third section, the characteristic and mechanism of the process of NO_x adsorption-decomposition by HPW and HPW/CeO₂ were studied. The favourable adsorption conditions for HPW/CeO₂ are 8% of O₂, 4.2% of water vapor concentration, 170℃of reaction temperature and a space velocity within 2649h^-1 to 5298h^-1. The favourable adsorptian conditions for HPW/CeO₂ are nearly identical to the former. The saturated adsorption amounts of NO_x at 150℃of reaction temperature by HPW and HPW/CeO₂ were detected to be 50.5mg NO₂/g and 85.6 mg NO₂/g respectively. With the aid of IR and GC-MS, the NO_x-catalyst interaction mode and process effectiveness for NO_x conversion to N₂ were revealed.