CATALYTIC REDUCTION OF NO BY NH3 IN FLUE GASES OVER A NOVEL C11-V/AI2O3 CATALYST AT LOW TEMPERATURE

摘要

The combustion of fossil fuels to meet human energy requirements discharges a large quantity of pollutants into the atmospheric environment. Among these air pollutants, emission of nitric oxide (NO) and nitrogen dioxide (NO2), collectively called nitrogen oxides (NO_x), is considered a severe environmental problem because NO_x are major contributors to the photochemical smog formation, acid precipitation, global warming, ozone generation in the low troposphere and ozone layer depletion in the stratosphere . Over the past two decades, catalytic techniques have shown promises for the direct removal of NO and other NO_x from emission gases. In principle, the simple decomposition of NO into N2 and O2, a thermodynamically favorable reaction at low temperature, is desirable. However, the reaction is very slow and sufficiently active catalysts have not been discovered. As a result, CO, methane, propane and other hydrocarbons have been tested as reductants to reduce NO into N2. But these reactions require the catalysts made of noble metals such as Pt, Pd and Rh, high cost of facilities and still suffer from the low removal efficiencies at low temperature . More recently, the selective catalytic reduction (SCR), particularly those using NH3 to selectively reduce NO_x under the excessive oxygen condition, have been extensively studied and applied to eliminate NO_x from stationary sources such as power plants and automotive exhausts. Such techniques offer the advantages of low cost, high selectivity of NH3 reaction with NO in the presence of oxygen and the possibility for converting the noxious gas to one or more innocuous substances such as nitrogen gas and water, which are free of disposal problems. However, the reaction mechanism of selective catalytic reduction of nitrogen oxides by NH3 is not well understood yet because reactions are complicated with the different proportions of ammonia, nitric oxide and oxygen at different temperatures in the SCR system. The NH3-SCR technique still contains some severe disadvantages such as NH3-slip by unreacted and/or excess NH3 and high operation temperature and, thus, high cost of facilities.