Insights Into The Reaction Mechanism Of Catalytic Wet Air Oxidation Of Ammonia

As the strict regulations and laws of environment putting into effect,more and more attention has been paid to the treatment of waste water that contains nitrogenous compounds,typically ammonia.Catalytic wet air oxidation(CWAO),an efficient and highly selective technology which can directly convert ammonia to nitrogen,has been becoming an attractive alternative in such a waste water treatment.However,the application of CWAO of ammonia is limited to its harsh operating conditions.In order to make the CWAO processes economical feasible and sustainable,more effective catalysts which can work under relative mild reaction conditions at the same time with high activity and stability are urgently needed.Besides,the investigation of reaction mechanism for CWAO of ammonia to N2 is conducive to designing and preparing effective catalysts for this process.We have reported that the bimetallic Ru-Cu/C catalyst prepared by chemical reduction method exhibited excellent activity for CWAO of ammonia at rather mild conditions.But,the stability of the catalyst for CWAO is still not ensure and the reaction mechanism is not thoroughly investigated.Based on these,this paper aims to explore the stability of Ru-Cu/C catalyst and the reaction mechanism of CWAO of ammonia.The main results are as follows.The bimetallic Ru-Cu/C catalyst possesses high activity and excellent stability at relative mild conditions(150?),the conversion of ammonia could be maintained at a level that was higher than 80%in five consecutive runs.Combined with various characterizations including ICP,XRD,TEM and BET,the results reveal that Ru and Cu are well dispersed.NO metal leaching and change of phase structure are observed after multiple reaction cycles.The mechanism for CWAO of ammonia to N2 is extensively studied over Ru-Cu/C catalyst by altering the initial pH,reaction temperature and atmosphere.It is found that four types of reactions occur during the process of ammonia oxidation over Ru-Cu catalyst,which are selective oxidation of ammonia to N2,over-oxidation of ammonia to nitrite,the disproportionation reaction between NH4+ and NV2-and the oxidation of HNO2 to nitric acid.All these reactions are greatly affected by the pH evolution during the CWAO process,duo to the varying concentration of NH4+,NH3,NO2-and HNO2.The reactivity of the surface activated oxygen species may be responsible for the evolutions of pH,the reactions of N2 formation and the distributions of various nitrogen species of over-oxidation reaction.New pathways of ammonia oxidation in aqueous phase were proposed according to the experimental results.In order to ascertain the reaction pathway,extra base or acid was introduced during the reaction of CWAO of ammonia.The observed phenomenon further confirms the proposed mechanism.