Study On Catalytic Oxidation Of Nitric Oxide (NO) From NO_x Industrial Waste Gas By Pure-silica Zeolite-β

The waste gases from the fine chemical and pharmaceutical industries were emited with high NO mole fraction in NO_x under room temperature and normal pressure and contained much oxygen and vapor. Because of the characteristics of these waste gases, catalytic oxidation-liquid absorption process is proposed to remove NO_x in the waste gases. An effective catalyst which has water-resistent ability and activity at low temperature for NO oxidation to NO₂ using O₂ as the oxidation agent which can provide the optimal oxidation degree of NO_x（NO₂/NO_x）, i.e. 50%⁶0% for the liquid absorption is the key point for the process. According to the theory analysis and experiments, the pure-silica zeolite-βcatalyst was chosen to realize NO oxidation into NO₂, and a series of researches were made as following:The tests of the catalyst’s activity showed that the ordered structure and micoporous surface area played an import role in NO catalytic oxidation. The pure-silica zeolite-βand H-ZSM-5 gave high catalytic activity for NO oxidation at low temperature, but the pure-silica zeolite-βwhich has larger micoporous surface area gave better catalytic activity than H-ZSM-5. Neither the SiO₂ with amorphous structure nor the pure-silica MCM-41 and SBA-15 with large pore diameter and ordered structure gave catalytic activity for NO oxidation. In humid NO_x gas, the catalytic activity of pure-silica zeolite-βis evidently higher than H-ZSM-5. The high Si/Al2 ratio can provide highly hydrophobic surface, which is benefit for NO oxidation in humid gas.The effects of the reaction conditions on NO oxidation showed that NO oxidation conversion decreased significantly with the increasing of the reaction temperature, for both dry and wet NO_x gas. The increase in the reaction temperature reduced the inhibition of the NO catalytic oxidation due to high relative humidity. The NO conversion increased with the increasing of NO concentration and the decreasing space velocity. The results of 240h stability test at 30℃demonstrated that, with an space velocity of 7200h-1, the NO oxidation conversion reached 67% and 56% when the relative humidity of the feed gas was 0% and 100%, respectively. The results indicated that the pure-silica zeolite-βcatalyst had high catalytic stability in the saturated wet gas for NO oxidation and it converted the waste gas into an optimum composition for alkali absorption of NO_x.NO_x species adsorbed on pure-silica zeolite-βafter NO adsorption and oxidation were studied by means of TPD and In-situ DRIFTS to obtain the reaction mechanism. NO, （NO）2, NO₂ were found to be reaction intermediates during adsorption of NO_x on pure-silica zeolite-β. According to the result of TPD and In-situ DRIFTS, a reaction mechanism was proposed for NO oxidation by pure-silica zeolite-β: （?）...