Development Of Catalysts For CWAO Of Ammonia And The Effect Of Weak Acid On Reaction

The pollution of wastewater containing ammonia becomes more and more serious and harmless treatment of ammonia wastewater is crucial.Among the treatment methods,catalytic wet air oxidation(CWAO)is an effective technology for ammonia harmless treatment.In this work,CWAO is used to treat ammonia containing wastewater.The catalyst and the effect of weak acid on reaction are studied from two aspects:the ability of the catalyst to activate oxygen and the effect of pH on the reaction.The high efficiency bimetallic PdNi/Darco G60 catalyst is developed,which can work under mild conditions due to the synergistic effect of Pd and Ni.According to the effect of pH on the oxidation of ammonia,the weak acid ion is used to adjust the amount of H+ in the reaction process.The main achievements are obtained as follows:Firstly,a series of monometallic palladium catalysts are prepared to study the effect of different supports.The results show that Pd/Darco G60 which is prepared by chemical reduction method has the best catalytic performance among the catalysts.When the Pd loading is greater than 3%,the thermal treatment temperature has a great influence on the catalytic performance.Sulfur will reduce the catalyst activity,but just little.The addition of second metal component Ni on Pd/Darco G60 catalysts can improve the catalyst activity,and the best mass ratio of Pd:Ni is 2:1 based on the experimental results in this thesis.The ammonia decomposition is nearly 100%at 1400C and more than 80%at 120 0C for such a catalyst.These catalysts are characterized by XRD,CO-TPR,XPS and the results are revealed that there is interaction between Pd and Ni over PdNi/Darco G60 catalysts and the co-presence of Pd and Ni can effectively adjust the reactivity and coverage of oxygen species.Secondly,the addition of weak acid anions was investigated.The results illustrate that the present of weak acid can effectively reduce the dependence of pH on the reaction system,and improve the catalytic oxidation activity of ammonia and the selectivity to N2.The effect of weak acid ions is attributed to two asepcts:1)weak acid ions combined with H+ to generate weak acid,thus NH4+(?)NH3+H+ shifts to right because of H+ consumption.The increase of NH3 concentration is beneficial to the improvement of reaction activity.The formation of weak acid molecules also results in the HNO2(?)NO2-+H+ ionization equilibrium shifting to the right.The increase of NO2-is beneficial to NH4+NO2-=N2+2H2O.It will increase ammonia conversion and N2 selectivity.Thirdly,the addition of weak acid anions further improve the catalytic oxidation performance of PdNi/Darco G60 under mild reaction conditions.The addition of weak acid can significantly improve the N2 selectivity over Ru/TiO2 and RuCo/TiO2 catalysts.The difference of surface reactivity and coverage of oxygen species leads to the catalytic oxidation activity of RuCo/TiO2 in the solution of pH equal to 12 is better than that of Ru/TiO2,but lower than Ru/TiO2 in the solution of pH equal to 8.