Catalytic Wet Air Oxidation Of Ammonia Containing Waste Water Effect Of Weak Acid Anion And Synthesis Of Highly Efficient Ru-Co/TiO₂ Bimetallic Catalyst

The large amounts of industrial waste waters,contaminated by hazardous and toxic compounds together with ever tightening legislation,have challenged traditional wastewater treatment methods.Therefore,the development of discharge limits under,cost-effective and ecological waste water treatment has become an essential concern.Catalytic water phase technologies are interesting alternatives for traditional wastewater treatment.Among them catalytic wet air oxidation(CWAO)has been used successfully in the management of various industrial effluents.However,the development of an active and stable catalyst for the severe reaction conditions of CWAO has proved truly challenging.The aim of this thesis was to briefly study the wet air oxidation reaction of ammonia in the presence of suitable catalysts.In the first part of this study,screening studies of catalysts was performed by executing preliminary experiments for high catalytic performance of CWAO and metal-support effects was investigated by using three different kinds of noble metals(Pd,Ru,Pt)with different metal oxide and carbon material supports.The results of these preliminary experiments revealed that the catalytic performance of noble metal Ru was better as compared with Pd and TiO2 was appeared to be a thermally stable support among all other supports for this reaction,and thereby Ru/TiO2 catalyst emerged to be the most stable catalyst in this work.Moreover,three different transition metals Cu,Ni and Co was added with Ru to prepare bimetallic catalysts aiming to decrease the precious metal loading and to improve the catalytic activity and stability of wet air oxidation of ammonia over Ru/TiO2 catalyst.Ru-Co/TiO2 was found to be the most stable catalyst but selectivity to molecular nitrogen(N2)was still unsatisfactory.In the second part of this work,the effect of weak acid anions on the ammonia removal has been extensively studied for the process of selective catalytic wet air oxidation(CWAO)of ammonia to nitrogen.It was found that the introducing weak acid anions in the feed solution can effectively enhance the ammonia conversion and selectivity towards molecular nitrogen.The combination between the weak acid anions and H+ to produce corresponding weak acid molecules was responsible for such enhancement.Firstly,the H+ consumption of weak acid anions increased the NH3 concentration and thus the reactivity of ammonia oxidation was increased due to the shift of NH4+ equlibrium reaction(NH4+?NH3+ H+)to right to produce NH3.Secondly,the combination of H4+ with the weak acid anions leads to HNO2 ? NO2-+ H+ and this equilibrium shift:ed to right so HNO2-can increase the concentration of NO2-and thus boosts the disproportionation reaction between NH4+ and NO2-(NH4++NO2-?N2+2H2O)to produce molecular nitrogen,thus improving the selectivity of oxidation reaction to molecular nitrogen.In the third and final part of this investigation,the activity and selectivity of Ru/TiO2 and Ru-Co/TiO2 catalysts were intensively investigated.It was found that the activity and selectivity for both catalysts were greatly enhanced by introducing weak acid ions in the feed solution.Compared with monometallic catalysts,Ru/TiO2 and Co/TiO2,bimetallic Ru-Co/TiO2 catalyst showed better activity and selectivity to N2 in NH4Cl solution.In contrast,for weak acid(CH3COONH4)solution,the activity to produce molecular nitrogen was more for monometallic Ru/TiO2 catalyst in comparison with bimetallic Ru-Co/TiO2 catalyst.The results from inductively coupled plasma mass spectroscopy(ICP-MS)also exposed that the co-existence of Ru and Co can effectively prevent metals from leaching which greatly confirmed the stability of bimetallic catalyst.These catalysts was characterized by X-ray powder diffraction(XRD),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS)and hydrogen temperature-programmed reduction(H2-TPR)and the outcomes of these characterizations proved the strong interaction between Ru and Co over Ru-Co/TiO2 catalyst.This strong interaction between Ru and Co adjusted the oxygen activity and concentration of oxygen species on the surface of the catalysts which influenced the catalytic wet air oxidation of ammonia to nitrogen in two different environments.