Research On Synergy Of Combining Electrochemical Oxidation And Catalytic Wet Oxidation

Catalytic wet oxidation （CWO） has received considerable attention for the treatment of biorefractory waster, since many hazardous organic compounds have proved to be eliminated effectively to CO₂, H₂O and small molecules. Despite its success in laboratory applications, the main drawback of CWO, preventing it from a broad industrial application, consists in the severe reaction conditions of high operating temperature and pressure needed in the CWO and consequently the lack of equipment material with the suitable chemical and mechanical properties for performing in the severe reaction conditions. Since it has been shown that organic compounds oxidation occurs in the CWO system through a free-radical mechanism. Thus, making it operate at milder temperature and pressure without decrease in the formation efficiency of hydroxyl radicals, which has become one of greatest challenges to be faced by researchers these decades. On the other hand, electrochemical oxidation （ECO） has also drawn considable attention recently, and three-demensional electrode emerges as an attractive treatment in particular. So, we attempt to carry out an electrode to the reactor of CWO, then the catalysts worked for the CWO can serve as three-dimensional electrodes at the same time. Consequently, rapid and efficient catalytic wet oxidation is expected with the use of three-demensional electrolytic systems, and this combination system might strongly catalyses the conversion of pollutant organic compounds to innocuous compounds, such as CO₂ and water. The aim of the present research is to investigate the feasibility of synergy of CWO and three-dimensional electrochemical oxidation.A new fixed-bed reactor for the combination system was developed to investigate the feasibility of synergetic advanced oxidation. A cylindrical Ti/Ta-Ir electrode （8 mm in diameter, 300 mm in length）, coaxial to the fixed-bed-reactor inner wall （16 mm i.d.）, was used as the anode. The wall of fixed-bed reactor was used as the cathode to promote cathodic protection of the fixed-bed reactor from corrosion. The new-designed electroassisted fixed CWO reactor, which was characterised by the cocurent upflow of both gas and liquid phases, discharge hydrogen gas from cathodic evolution successfully. The reactor can be heated effectively and the precision of temperature control is about±1℃, and it can keep the pressure steady during the operation.The issue, which has become a major impeding factor of the research work of synergetic effect, is to explore the ability of supported catalysts with high catalytic activity and stability in both CWO reaction conditions and ECO reaction conditions. Therefore, a batch-wise three-dimensional electrochemical reactor was developed, in which a set of supports and active ingredients was studied. Various catalysts with different active metals, i.e. Cu, Zn, Ni, Ce, Fe, Co, Mn and Sn supported on 13X-type zeolite andγ-Al₂O₃ prepared by the incipient wetness impregnation were discussed. 13X-type zeolite might be not fit for support. Therefore, 13X zeolite was not used in the further experiments for a support material. Results showed that all electrocatalysts of single oxide supported onγ-Al₂O₃ could improve the degradation of phenol in varying degrees. The single oxide of Mn was quite powerful in its ability to induce complete mineralization as compared to other single oxides, while the Sn-containing particle electrode could efficently break down phenol, and made it possible to operate at lower cell potential.As discussed, the Sn-containing particle-electrode made it possible to operate at lower cell potential, whereas the Mn-containing particle-electrode had an inverse result. Therefore, a particle-electrode that employed Mn and Sn together seemed a worthwhile endeavour. So, a series of composite Mn/Sn/Sb metal oxides supported onγ-Al₂O₃ prepared by sol-gel method was used as electrocatalyst for the electrocatalytic oxidation of phenol. These particle electrodes were. Mn-Sn-Sb （2:1）/γ-Al₂O₃, which had a molar ratio of Mn to Sn of 2:1 and 0.1wt% Sb, was found to be the highest electrocatalytic active. It was observed that both disappearance of phenol and loss of total organic carbon （TOC） obtained for the Mn-Sn-Sb （2:1）/γ-Al₂O₃ were significantly more efficient than that obtained with the Ru/TiO₂ particle electrode used as reference. In addition, Mn-Sn-Sb （2:1）/γ-Al₂O₃ particle-electrode was found to be active even after the particle electrode was reused fifth, indicating high stability with the particle electrode. Calcination temperature on the physical characterization and the electroactivity was investigated. Morphologies and microstructures were characterized by BET specific surface area （BET）, X-ray diffraction （XRD）, scanning electron microscopy （SEM） and energy dispersive X-ray analysis （EDX）. After selection of active ingredient with high catalytic activity and stability in the bath-wise reactor, the Mn-Sn-Sb/γ-Al₂O₃ catalyst was further investigated and optimized in the setup for the combination system.Phenol conversion and TOC reduction from the solution containing phenol for three processes, catalytic wet oxidation, electrochemical oxidation and electroassisted catalytic wet oxidation were compared. Good electroassisted catalytic wet oxidation efficiency was obtained in the setup for the combination system even at mild conditions (t=130℃, P_O2=1.0 MPa) that the phenol conversion and TOC reduction were up to 94.0% and 88.4% after 27 min treatment, respectively. The result also showed that the rate constants of electroassisted catalytic wet oxidation are much greater than that of not only both catalytic wet oxidation and electrochemical oxidation process alone but also additive efficiencies of the latter processes, indicating an apparent synergetic effect between CWO and ECO processes. Almost all of the phenol loaded to the reactor was converted into non-passivating polymeric products, denoting a safe and easy method for the degradation of phenol. The effect of reaction temperature （120～150℃）, oxygen partial pressure （0.6～1.4 MPa）, current intensity （0.25～1.0 A）, initial phenol concentration （1500～3000 mg/L）, inlet pH value （3.33～9.50） and catalyst diameter （20 mesh＜D_p＜10 mesh） on synergetic effect were also investigated.The effluent of electroassisted CWO of phenol in the typical operational mode was analysed by means of GC-MS, the oxidation reaction paths of phenol was analysed preliminarily.