1136
Ind. Eng. Chem. Res. 1998, 37, 1136-1139
Wet-Oxidation of a Model Domestic Wastewater on a Ru/Mn/Ce Composite Catalyst Seiichiro Imamura,* Yasuhiro Okumura, Takashi Nishio, and Kazunori Utani Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Yasuyuki Matsumura Osaka National Research Institute, AIST, 1-8-31 Midorigaoka, Ikeda 563, Japan
Catalytic wet-oxidation of a model domestic wastewater was carried out, and improvement of the performance of the previously developed Mn/Ce and Ru/Ce catalysts was attempted. Combination of both catalysts resulted in an increase in the catalytic activity; Ru/Mn/Ce (1/9), which had an atomic ratio of Mn to Ce of 1/9 and 3 wt % Ru, exhibited the highest activity. The activity of the Ru/Mn/Ce catalysts was independent of their redox property as examined by the decomposition of hydrogen peroxide. The possibility of further improvement of the catalytic activity was discussed. Introduction Wet-oxidation is carried out under high air pressure at elevated temperatures to decompose organic pollutants contained in wastewaters. This process has been successfully applied for the treatment of wastewaters discharged from petrochemical industries that are contaminated with toxic or refractory substances inappropriate for biological treatment (Keckler et al., 1993). Energy recovery is possible when wet-oxidation is applied to highly contaminated wastewaters (Versar, 1989). Useful organic compounds are also recovered by the oxidation of waste biomass (McGinnis et al., 1983). Thus, wet-oxidation has a highly promising future from the standpoint of both environmental- and energyrelated issues, as indicated by the excellent review of Mishra et al. (1995). However, because the process requires relatively severe reaction conditions, use of effective catalysts is desirable. Copper is the only effective homogeneous catalyst that is currently being used practically (Tagashira et al., 1976; Imamura et al., 1982a). However, copper must be recovered after its use to prevent the contamination of the receiving water by toxic copper. Thus, there have been many attempts to develop active heterogeneous catalysts (Katzer et al., 1976; Sadana, 1979; Ito et al., 1989; de Leitenburg et al., 1996; Gallezot et al., 1997). One of the authors has also been engaged in the development of active wet-oxidation catalysts. Among the various catalysts examined, Mn/Ce composite oxide was active for the treatment of various compounds such as ammonia, poly(ethylene glycol), pyridine, acetic acid, etc. (Imamura et al., 1985 and 1986). Also, a Ru/Ce catalyst was designed that is highly active for the decomposition of oxygen-containing compounds such as formaldehyde and formic acid (Imamura et al., 1988). All the work just cited, including that carried out in our laboratory, was aimed at purifying industrial wastewaters, and there seems to have been no work that investigates the application of wet-oxidation for the * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: 81-75-724-7580. Phone: 81-75-724-7534.
treatment of domestic wastewaters. This paper concerns the design of effective catalysts for domestic wastewater treatment. For this purpose, combination of the assets of the previously developed promising Mn/ Ce and Ru/Ce catalysts was attempted. Experimental Section Materials and Catalysts Preparation. Manganese(II) nitrate, cerium(III) nitrate, ruthenium(III) chloride, γ-alumina, and other reagents were used as obtained commercially (Nacalai Tesque Company; G. R. grade). The reagents to prepare the model domestic wastewater were provided by Matsushita Electric Works Ltd, and its composition (ppm) used in the reaction was dextrin, 91.8; peptone, 196.2; yeast extract, 196.2; beef extract, 223.8; NaCl, 20.1; MgSO4, 12.0; KH2PO4, 55.8; and KCl, 40.2. The total organic carbon (TOC) contained in this model wastewater was 315 ppm. The Mn/Ce composite oxides were prepared by coprecipitation from a mixed aqueous solution of manganese(II) nitrate and cerium(III) nitrate with known atomic ratios, followed by drying at 80 °C overnight and subsequent calcination in air at 773 K for 3 h. The forms of the catalyst were assigned as Mn2O3 for Mn and CeO2 for Ce by X-ray diffraction analysis. One of these composite catalysts that had a Mn-to-Ce atomic ratio of 1 was designated as Mn/Ce (1/1). The Co/Bi composite oxide with an atomic ratio of Co-to-Bi of 5 [Co/Bi (5/1); BET surface area of 29.8 m2/g] was prepared by the same procedure except that the metal salts were dissolved in concentrated nitric acid (Imamura et al., 1982b). Ruthenium was supported on Mn/Ce composite oxides (or on Mn and Ce single component oxides) as follows. Ruthenium (III) chloride (3 wt % loading as Ru metal) and formaldehyde (10 molar ratio to Ru) were dissolved in deionized water in the presence of Mn/Ce composite oxide that had previously been dried at 353 K overnight, and the solution was heated to 363 K. Sodium hydroxide (3 N) was added until the pH of the solution was 12, and the solution was stirred for 1 h. After the solution was cooled to room temperature, the solid portion was filtered and washed several times with deionized water until the pH of the solution was
