Wet air oxidation is the liquid phase oxidation of organics at elevated temperatures (125-320℃) and pressures (0.5-20 MPa) using molecular oxygen as oxidant. Wet oxidation is suitable for the treatment of the wastewater which is too concentrated and/or toxic to be treated with biological approach because any organic compound could be ideally mineralized to the desired end products by wet oxidation. Using the proper catalysts for the wet oxidation system not only moderates the severity of the reaction conditions but also makes it possible to treat only a single pollutant, or a group of similar pollutants, out of a complex mixture of pollutants.
In this study, wet air oxidation of phenol was investigated over the noble metal catalysts or the metal oxide catalysts.
First, wet oxidation of phenol over alumina supported Pt catalysts was investigated. The Ce addition effect on metal dispersion, reducibility of catalysts, and catalytic activity of wet oxidation of phenol was observed for two different Pt precursors. $Pt/Al_2O_3$ made from $H_2PtCl_6$ showed the higher activity for the wet oxidation of phenol than that from $Pt(NH_3)_4Cl_6$, which could be explained with the Pt surface area: the Pt surface area of the former catalyst is higher than that of the latter one. Cerium addition lowered the catalytic activity of $Pt/Al_2O_3$ catalysts from $H_2PtCl_6$ while it improved the activity of $Pt/Al_2O_3$ catalysts from $Pt(NH_3)_4Cl_6$. The former could be explained by the decrease of the platinum surface area with addition of cerium while the latter could not be explained by platinum surface area because of little change in platinum surface area. The increase of activity of $Pt/Al_2O_3$ catalysts from $Pt(NH_3)_4Cl_6$ could be explained with the enhanced surface reducibility by the interactions between platinum and cerium.
There were carbonaceous deposits on the $Pt/Al_2O_3$ catalyst surfaces after wet oxidation reaction, and the chemical species of the deposited carbon seemed to be mainly aromatics; especially some phenolics.
$Al_2O_3$ supported transition metal oxide catalysts were prepared and tested for the wet oxidation of phenol. Cu loaded catalysts showed the highest catalytic activity which could be explained with the surface reducibility of the catalysts. The blank test decomposed phenol to a certain degree, however, Cu loaded catalysts remarkably enhanced the deep oxidation of phenol. Ce was co-impregnated with the transition metal oxide to enhance the activity of the transition metal catalysts. Ce seemed to enhance the activity of the supported transition metal oxide catalysts except for $Cu/Al_2O_3$.
There were carbonaceous deposits on the used supported transition metal oxide catalysts for the wet oxidation of phenol. The carbonaceous deposits on the catalyst surface were confirmed with TPO-MS and quantified with EA. The amount and the nature of the carbonaceous deposits varied according to the catalysts. Mn loaded catalysts showed the highest amount of carbonaceous deposits. Ce addition to the transition metal oxide catalysts also increased the amount of carbonaceous deposits on the catalysts which resulted partly in the improvement of the phenol conversion. The nature of the carbonaceous deposits was investigated with $N_2$ adsorption results, NMR spectrum and FTIR spectrum. These carbonaceous deposits were thought to have pores which resulted in the decrease of the pore volume and the average pore diameter and the increase of the surface area of the catalyst. The natures of the carbonaceous deposits were almost aromatic and the FTIR peaks from the aliphatic carbon stretching appeared only for the Cu loaded catalysts. The NMR and FTIR experiment showed that the carbonaceous deposits have mainly the aromatic nature and have some oxygen-bearing groups such as carboxylic acids and alcohols.
$CuO/Al_2O_3$ catalysts with different copper loadings, 1-25 wt%, were prepared and characterized by $N_2$ adsorption, TPR, $N_2O$ reactive frontal chromatography, XRD, EXAFS, ESR, and XPS experiments.
From the catalyst characterization results, the chemical state of copper in the $CuO/Al_2O_3$ catalysts could be summarized as follows: isolated $Cu^{2+}$ ions for Cu1; highly dispersed $Cu^{2+}$ cluster for Cu5 and Cu7; bulk CuO for Cu10 ~ Cu25. Cu7 showed the maximum copper metal surface area. The activity of the catalysts was increased as the copper loading increased to 7 wt%, and the activities of the catalysts with higher loading, above 7 wt% remained the same. Therefore, the optimum of copper loading for the wet oxidation of phenol over $CuOx/Al_2O_3$ was 7 wt% in this study. The intermediates/products of phenol wet oxidation were identified with the GC/MS results of the extracts from the treated phenol solutions.
Carbonaceous deposit of the used catalysts decreased as the copper loading increased, indicating that the carbonaceous deposits are mainly on the $Al_2O_3$ not on the copper.
