Synthesis of a Copper-Containing Catalyst for Electrocatalytic. Oxidation of Volatile Organic Compounds. Yu. V. Tsarev, A. E. Dmitriev, and V. V. Kostrov.
Russian Journal of Applied Chemistry, Vol. 75, No. 7, 2002, pp. 111731119. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 7, 2002, pp. 113531137 Original Russian Text Copyright C 2002 by Tsarev, Dmitriev, Kostrov.
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ENVIRONMENTAL PROBLEMS ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ OF CHEMISTRY AND TECHNOLOGY
Synthesis of a Copper-Containing Catalyst for Electrocatalytic Oxidation of Volatile Organic Compounds Yu. V. Tsarev, A. E. Dmitriev, and V. V. Kostrov Ivanovo State University of Chemical Engineering, Ivanovo, Russia
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Received December 11, 2001; in final form, April 2002
Abstract The synthesis of a copper-containing catalyst for electrocatalytic oxidation of acetone in exhaust gases was studied. Data on the electrocatalytic oxidation of acetone were analyzed in relation to the CuO content.
Treatment of waste gases to remove volatile organic compounds (VOCs) is an urgent problem of environmental protection. The largest amounts of exhaust VOCs are discharged by oil refining and oil production plants (502 and 202 thousand tons annually), chemical plants, and machine-building plants (89.45 and 43.28 thousand tons annually) [1]. The traditional methods for treatment of waste gases to remove VOCs are thermal afterburning, catalytic oxidation, sorption, and sorption-catalytic treatment [2]. Along with traditional methods, the oxidation of VOCs in barrier discharge plasma became a common method [3]. The thermal afterburning and sorption treatment of VOCs are rather expensive processes. Sorption-catalytic and catalytic treatment (including VOC oxidation) in the non-steady-state mode [4] involve smaller service expenditures. Platinum or less expensive oxide catalysts are used in oxidation. As oxide catalysts are used copper(II) and manganese(IV) oxides [5], and as supports, alumina, titania, and other oxides. In some studies [6, 7], the support was activated carbon. In this study, the main attention is given to the electrocatalytic treatment of hydrocarboncontaining gases on a copper oxide catalyst supported by carbon black. As support we used PM-750 carbon black. The catalysts were obtained from a mixture of carbon black and copper hydroxycarbonate, to which a plasticizer (10% polyvinyl alcohol solution) was subsequently added. The mixture with the optimal moisture content was pressed into 0.531-mm granules, which were then calcined in a CO2 flow at 350oC. The model gas mixture contained air and acetone (the concentration of the latter was 132 g m33). The acetone concentration in a gas3air mixture was determined nephelometrically on a KFK-2MP photoelec-
trocolorimeter at l = 320 nm [8]. The degree of the acetone conversion in the catalyst samples synthesized was determined in an isothermal plug-flow reactor. An ac voltage was applied to the catalyst bed. The total specific surface area of the catalyst was measured on a Tsvet-211 sorption meter [9]. The surface area of the catalysts was determined by selective low-temperature sorption of helium in pulsed mode at 3136oC. The X-ray phase analysis of the catalyst samples was done on a DRON3UM1 diffractometer using a CuKa X-ray tube (l = 0.154 nm). The interplanar spacings were calculated on a PC. The compounds were identified by comparison of the interplanar spacings with the reference data [10]. The phase composition of the catalyst samples as influenced by the preparation conditions was evaluated from the intensity of the strongest reflections. The diffraction angle 2q was calculated from the relation l = 2d cos q. The electrocatalytic oxidation products were absorbed with hexane and analyzed on a Saturn2000R gas chromatograph3mass spectrometer. The contribution of the acetone sorption on a catalyst to the total process of the electrocatalytic treatment was estimated from the dependence of the degree of the acetone adsorption on time (Fig. 1). It was shown that, at saturations longer than 15 min, the degree of adsorption is virtually zero. Therefore, in determining the degree of acetone oxidation on a catalyst, after the acetone-containing gas is passed through a catalyst bed for 1 h, the sorption can be neglected. The optimal content of copper oxide in the catalyst samples was determined from the plots of the catalytic activity vs. the CuO percentage in the catalyst for
1070-4272/02/7507-1117$27.00 C 2002 MAIK [Nauka/Interperiodica]
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TSAREV et al. A, %
t, min Fig. 1. Degree A of acetone adsorption on the catalyst vs. time t.
a, %
(a)
W, W cm33 a, %
(b)
CuO, wt % Fig. 2. Degree a of acetone conversion vs. (a) the supplied specific power of electric current W and (b) CuO content. (a) CuO content (wt %): (1) 0, (2) 10, (3) 20, (4) 30, (5) 40, and (6) 60. (b) W (W cm33): (1) 1.0, (2) 2.0, and (3) 3.0.
different specific loads of the electric current (Figs. 2a, 2b). The experimental data show that, with the specific load on the catalyst bed increased from 0.6 to 3 W cm33, the maximal degree of acetone conversion is observed for catalysts containing 40 and 60% copper oxide, respectively. At high specific loads of the electric current (more than 4 W cm33), the content of the active component in the catalyst only slightly affects the degree of acetone conversion. The specific surface area of the catalyst samples was determined in the course of the experiment (283
35 m2 g31). The experimental data show that the specific surface areas of the sample calcined at 350oC and of that calcined at 700oC are similar (28 m2 g31), i.e., factors other than sintering of the catalyst are responsible for the decreased catalytic activity of the samples calcined at high temperatures. Most probably, the high specific surface area of the catalyst sample containing 60% CuO develops owing to decomposition of (CuOH)2CO3 whose content in the catalyst is large. The specific surface area of the catalyst sample without copper oxide is equal to that of the initial carbon black (35 m2 g31), i.e., it is affected by mixing insignificantly. The experimental data show that, for the specific powers of the supplied electric current less than 2 W cm33, the degree of acetone conversion on the catalyst decreases with increasing calcination temperature (Fig. 3). Thus, we can conclude that the CuO and Cu2O phases enhance the effect of the electric current power on the electrocatalytic oxidation of acetone. The metallic copper phase does not affect electrocatalytic oxidation of acetone. At the powers of the supplied electric current higher than 2 W cm33, the calcination temperature does not affect the degree of acetone conversion on a catalyst. This may be due to the fact that, at high specific powers of supplied electric current, the electrical contribution is dominating. The phase composition of the catalyst samples was determined by X-ray phase analysis. The test samples containing 20% active component were calcined at 350, 500, and 700oC. The diffraction pattern is shown in Fig. 4. A comparison of the experimental diffraction patterns with the reference data for various compounds revealed crystalline CuO (2 q = 62o) or Cu2O (2q = 36o) in the samples synthesized. With increasing calcination temperature, the amount of the crystalline phase decreases, which is manifested as a decrease in the intensity of the strongest reflections attributable to these compounds. Copper oxides undergo reduction, which occurs on the catalyst at high temperature and results in the formation of phase C of metallic copper (2q 43o, 51o). The diffraction patterns show that the reflections of metallic copper grow in intensity with temperature. The catalyst support, carbon black A, exists in the catalysts in amorphous or weakly crystalline state, which is revealed as a halo corresponding to the diffraction angle 2q = 25o. CONCLUSIONS (1) A study of the conversion of acetone-containing gas in relation to the content of copper(II) oxide
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SYNTHESIS OF A COPPER-CONTAINING CATALYST
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peratures and result in the formation of metallic copper, inactive in acetone oxidation.
a, %
(3) The calcination temperature does not affect the specific surface area of the catalyst samples. REFERENCES
T, oC Fig. 3. Degree of acetone conversion a vs. calcination temperature T. W (W cm33): (1) 4.0, (2) 2.0, and (3) 0.6.
2q, deg Fig. 4. Diffraction patterns of the catalyst sample containing 20% CuO and calcined at different temperatures. (I /I0) Relative intensity and (2q) Bragg angle. Calcination temperature (oC): (1) 350, (2) 500, and (3) 700. (A) Carbon black, (B) CuO (Cu2O), and (C) metallic copper.
in the catalyst samples showed that the catalyst containing 40 3 60% copper oxide is the most efficient in electrocatalytic oxidation of acetone. (2) The X-ray phase analysis showed that the decrease in the catalytic activity of samples with increasing calcination temperature is due to the reduction processes, which occur on the catalyst at high tem-
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