| The abatement of the nitric oxides (NOx) is becoming one of important challenges since NOx are the source of severe eco-environmental problems and harmful to human health. Differing from the NOx flue gas from the power station by burning fossil fuels such as coal, oil and natural gas, the NOx industrial waste gas exhausts with low quantity but high local NOx concentration leading to severe pollution to local environment, so is urgent to be treated now. Generally, the NOx industrial exhausted gas is discharged under the following conditions of:(1) high oxygen content (-20%(V)); (2) high vapor content which can reach the saturated one in the gas; (3) low NOx oxidation degree (defined as the ratio of the concentration of NO2 and that of NOx in the gas), even lower than 10%; (4) ambient temperature and normal pressure; (5) dramatical fluctuation of NOx concentration due to the batch or non-continuous operation mode of the previous production units. According to these features, the NH3-SCR (Selective Catalytic Reduction) method is not suitable to the removal of NOx from the industrial process since SCR process needs high temperature and excellent control of the addition of NH3 content, while alkaline solution absorption process operating at normal temperature and normal pressure may be more suitable because the absorption process is insensitive to the fluctuation of NOx content and even can recover nitrite and nitrate salts. However, the low NO solubility in aqueous solution could lead to low NOx removal efficiency for this wet scrubbing process. So the process of NO gas phase catalytic oxidation and alkaline solution absorption was proposed, and then investigated in detail in this paper. Some conclusions have been drawn as following:1. Study on the absorption process of NOx by the alkaline solution including NaOH solution without and with the addition of the reductants or the oxidants(1) The addition of the reductants (including urea, ammonium sulfite, sodium sulfite, sodium sulfide or sodium hyposulfite, etc.) or the oxidants (including hydrogen peroxide or sodium hypochlorite, etc.) promote the absorption of NOx, but the NOx removal efficiency is insensitive to the concentration of NaOH and the additives (including the reductants and the oxidants). With the increase of the inlet NOx concentration, the removal efficiency of NOx will increase. The increase of the gas phase resident time will make for the removal efficiency of NOx especially that of NO2.(2) Among the operation conditions, the inlet NOx oxidation degree is the most significant one which effects the NOx removal, and exists the optimal value of 50～70%when using the NaOH solution or the urea-NaOH solution as the absorption solvents, but the higher inlet NOx oxidation degree will lead to the higher NOx removal efficiency when using the oxidant-NaOH solutions or the reductant-NaOH solutions (except urea additive). So, the key of the high NOx removal efficiency will result from the increase of the inlet NOx oxidation degree.(3) The relative rate of NOx removal by the alkaline solution absorption process can be sorted as followed:the rate of NO2 removal through the reaction of NO2 and (NH4)2SO3> the rate of NOx removal through the reaction of NaOH and N2O3 equilibrated with NO and NO2> the rate of NO2 removal through the reaction of NO2 and NaOH or H2O2.2. The study on the NO oxidation at ambient temperature by oxygen in the NOx waste gas on the catalysts including the coconut-based activated carbon (AC) and the modified activated carbon (MAC), the micro porous-based molecular sieve (high silica H-and Na-ZSM-5, pure silicaβ-ype molecular sieve)(1) With the increase of the reaction temperature, the NO conversion in the NO catalytic oxidation decreases under the dry NOx waste gas, but exists maximum value under the humid NOx waste gas at the temperature of 50～70℃over MAC and 20°C over ZSM-5 molecular sieve, while decreases monotonously over pure silicaβ-type molecular sieve during the range of the reaction temperature from 10°C to 90°C.(2) The increase of the inlet concentrations of NO and O2 and the space time benefits the NO conversion.(3) The vapor inhibits the oxidation of NO on all the investigated catalysts, but the extent of inhibitory effect depends on the nature of the catalysts. The effect of vapor on the NO oxidation on the AC (MAC) may be the most serious one due to its wide pore size distribution, especially the exist of the mesopore. Nevertheless, there is another situation to the molecular sieve, that is, the high hydrophobic surface can be obtained on the molecular sieve with high Si/Al ratio, which leads to less adsorption to vapor. So, the higher NO conversion under the wet NOx gas will be obtained on the molecular sieve with the higher Si/Al ratio.(4) The catalytic activity of the high silica ZSM-5 molecular sieve and the pure silicaβ-type molecular sieve demonstrates high stability both in the dry NOx gas and in the saturated wet NOx gas during the long periodic stability test, which means their potential industrial application in the NOx removal process.(5) The micro porous channels guarantee the catalytic activity of ACs and the molecular sieves to NO oxidation at ambient temperature, and the high catalytic activity can be obtained over the catalysts with high micro porous specific superficial area and volume.(6) The characterizations of TPD/TPSR and in-situ DRIFTS of the adsorbed NOx species on the high silica Na-ZSM-5 and the pure silicaβ-type molecular sieve reveal the reaction mechanisms of NO catalytic oxidation at ambient temperature. The molecular sieves exhibit the high catalytic activity through the active adsorption sites of the hydroxyl group (-OH) and cation (such as Na+). There is no adsorbing oxygen and only weakly-adsorbing NO2 can be detected on the surface of the molecular sieve, while NO can adsorb strongly on it. The adsorbing species of NO depends on the types of the active sites, e.g., the adsorbing NO3 (i.e., OONO) specie mainly exists in cation such as Na+, while the adsorbing (NO)2 (i.e., ONNO) specie may be in the hydroxyl group (-OH). Based on these, the catalytic reaction mechanism of NO oxidation at ambient temperature on the high silica Na-ZSM-5 molecular sieve can be speculated as following:NO+σ?NO·σNO·σ+O2?NO3·σNO3·σ+NO·σ?N2O4+2σ?2NO2+2σWhile the reaction mechanism on the pure silicaβ-type molecular sieve may be shown as:NO+σ?NO·σNO·σ?(NO·σ)2(NO·σ)2+O2?2NO2+2σ?N2O4+2σAll above investigations show that the NOx oxidation degree can efficiently be increased through the NO oxidation by O2 at the ambient temperature under the wet NOx waste gas using the high silica ZSM-5 or the pure silicaβ-type molecular sieve as the catalysts, and the chemical absorption of NOx can be enhanced by the addition of the reductant or the oxidant into the alkaline solution as the absorption solvent. Furthermore, the combination of the above two technologies synergically will lead to high NOx removal efficiency of the wet scrubbing process for NOx abatement, and the feasibility of the process of NO gas phase catalytic oxidation and alkaline solution absorption for the NOx removal from the industrial exhausted gas is then proved.
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