Study On Catalytic Oxidation Of NO From NO_x Industrial Waste Gas By ZSM-5 Molecular Sieves

More than 90% of NO_x is NO in the waste gases from the fine chemical, pharmaceutical and nitric acid industries. An effective catalyst 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%, is the key point for the liquid absorption process using the alkaline （such as NaOH, Na₂CO₃） aqueous solutions to remove NO_x. Because these waste gases are emited under room temperature and normal pressure with high vapor content, it is needed that the NO oxidation catalyst has water-resistent ability and activity at low temperature. According to the theory analysis and experiments, the silica-rich ZSM-5 molecular sieve catalyst 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 H-ZSM-5 and Na-ZSM-5 gave high catalytic activity for NO oxidation at low temperature, even in presence of vapor. Molecular sieve H-ZSM-5 with a high Si/Al₂ ratio can be highly hydrophobic, which is benefit for NO oxidation in humid gas.The effects of the operation conditions on NO oxidation showed that NO oxidation conversion in dry gas decreased monotonously as reaction temperature increased, and low temperature was favorable for NO oxidation as well as water vapor adsorption. Therefore, in humid gas the optimal reaction temperature for NO oxidation was 20℃, which was close to the exhausted temperature of NO_x industry waste. The NO conversion increased with the increasing of NO and O₂ concentration and space time. A 200h stability test was conducted at 30℃, with an inlet NO concentration of 0.08% and a space time of 0.5s. The NO oxidation conversion reached 60% and 50%, when the relative humidity was 50% and 100%, respectively. ZSM-5 catalyst demonstrated high stability in saturated wet gas and it converted the waste gas into an optimum composition for alkali absorption of NO_x.With the O₂ concentration far beyond NO, the intrinsic kinetics of NO oxidation was studied, providing that the effects of internal and external diffusion in the ZSM-5 catalyst pellet is negligible. The X^W/F_A0 data and NO partial pressure at different temperatures were measured to evaluate the reaction rate -r_A in an isothermal integral reactor. The NO oxidation kinetic equation is determined by non-linear regression to beNO_x species adsorbed on ZSM-5 after NO adsorption and oxidation were studied by means of TPD and In-situ DRIFTS to obtain the reaction mechanism. A proper mechanism was proposed for NO oxidation:which can explained the dynamic diffusion-reaction process of NO oxidation over ZSM-5 molecular sieves well.