(A) study on the characteristics of transition metal oxide catalysts for wet air oxidation of refractory organic pollutants

Abstract

Industrial wastewaters contain hazardous and refractory organic pollutants which are inappropriate for conventional biological treatment. Wet air oxidation (WAO) is the liquid phase oxidation of organic compounds at elevated temperatures (125-320$\degC$) and pressures (0.5-20 MPa) using a gaseous source of oxygen (either pure oxygen or air) as the oxidant. WAO is appropriate to treat wastewater which is too dilute to incinerate and too concentrated and/or toxic to be treated with biological approach. Application of the proper catalysts for WAO process, i.e. catalytic wet air oxidation (CWAO), not only reduces the severity of reaction conditions but also decomposes even refractory pollutants. Although the homogenous catalysts, e.g. dissolved copper salts, are effective, additional separation step is necessary to remove the metal ions from the treated effluent. The development of active heterogeneous catalysts has received a great attention. Generally, the noble metal catalysts show excellent activity but they are vulnerable to poisons and expensive, affecting tremendously the economics of corresponding processes. The main objective of this work was to investigate the applicability and effectiveness of transition metal (Mn, Fe, Co, Ni, and Cu) oxide catalysts for the CWAO of organic pollutants in wastewater. The $TiO_2$ supported transition metal oxide catalysts were prepared and their activities were compared for the CWAO of phenol. Among the $TiO_2$ supported catalysts, the Cu catalyst not only showed the highest catalytic activity due to the surface reducibility of the catalyst but also the highest selectivity to $CO_2$ due to the deep oxidation of phenol. The leached copper ions were detected and should contribute to the higher phenol conversion over the Cu catalyst via homogeneous catalysis. Although the Mn catalyst showed the high phenol conversion, the phenol was mainly converted into carbonaceous deposits on the surface of catalyst, which is one of th...