Oxidation of Carbon Monoxide Over a Rh/A1203 Catalyst Gonul Glindilz Chemical Engineering Department, Ege University, 35 100 Bornova - lzmir, Turkey Rhodium has an important role in the three-way control of automobile exhaust emissions. The kinetics of CO oxidation on polycrystalline and supported-Rh catalysts has received considerable attention in recent years. The results on supported- Rh catalysts indicate that the oxidation of CO proceeds via dissociative oxidation by its own oxygen, as well as via direct oxidation by gas-phase oxygen with a Langmuir-Hinshelwood type reaction.'-' The present work considers the effect of product CO;! on the CO oxidation using an impregnation-type Rh catalyst I 0.2 0.4 I 0.6 I 0.8 I 1.0 Figure 1 Conversion-space time curves for CO oxidation (10 mol % CO in air) Developments in Chemical Engineering, Vol. 1, No. 1, Page 52 Oxidation of carbon monoxide 53 Table I Experimental data for the inhibition effect of C02. X YCO 0.0750 0.0750 0.0750 0.0750 0.0750 0.0750 0.0750 0.0458 0.0458 0.0458 0.0458 0.0458 0.0458 0.0458 0.0458 mk/vo a 1.o 1.o 0.4 0.4 0.2 0.2 0.2 1.o .o 1 0.4 0.4 0.2 0.2 0.1 0.1 (YCO,>O - 0.002 18 - 0.00098 - 0.00041 0.00055 - 0.00235 - 0.00088 - 0.00050 - 0.00025 (mol %) 3.67 2.76 2.08 1.72 1.20 1.07 0.67 11.11 8.89 6.30 4.93 3.78 1.56 2.1 1 0.96 r (moVgcathx104) 4.12 3.10 5.84 4.83 6.74 5.99 3.74 7.49 5.99 10.67 8.30 12.73 5.24 14.23 6.44 mk/vo in g s cmp3. mk = 0.5 g; T = 423 K). a Nomenclature = amount of the catalyst (g) mk r = rate of reaction (mol g-' cat. h-') T = reaction temperature (K) = volumetric flow rate (cm3 s-l) vo mk/vo = space time (g s cm-3) x = conversion (mol%) y c o = mole fraction of CO in the inlet stream yco, = mol fraction of COz in the exit stream (ycoJ0 = mol fraction of C 0 2 in the inlet stream at atmospheric pressure. Details of catalyst preparation and the experimental set-up are presented elsewhere. 9 Oxidation of C O with oxygen in air over 0.5 g of catalyst (mk) was studied for CO-air mixtures containing 2.5, 4.58, 7.5 and 10 mol% CO. A temperature range of 403-443 K and a space time (mk/vo) range of 0.07-1.00 g s cm-3 were used. The conversions and rates of reaction (r) were calculated from the experimental Goniil Giindiiz 54 T=423 K 20 16 t n V 12 9 X f di[8 \ 9 0 E .L - Qa .4 1 2 3 47 Mole Fraction of C02 in exit Strcam(y co., x1 6)- L Figure 2 Dependence of reaction rate on mole fraction of C02 in exit stream. data. Experimental problems arose because the catalyst showed continuous activity changes, and a steady value for conversion was obtained after a long reaction time. For example, a conversion-space time curve required more than three days. The plot of conversion versus space time in Figure 1 (for 10% CO in air) shows that the conversion to C 0 2 increases as the space time or temperature increases. Figure 2 illustrates the dependence of reaction rate on the mole fraction of product C 0 2 in the exit stream (yco,) for different CO mole fractions in the inlet stream (yco), and for different volumetric flow rates (v,). It appears that on the rhodium catalyst, reaction rate is inhibited not only by the reactant CO but also by the product C02. Reaction rate increases as yco and yco, decrease. In order to show this inhibition effect of C 0 2 on the oxidation rate of CO more clearly, carbon dioxide was directly added to the feed. Oxidation experiments were carried out by varying the fraction of C 0 2 in the feed stream (from zero to Oxidation of carbon monoxide 55 a given value), while holding constant the fractions of CO and oxygen, the temperature and the flow rate. Table 1 presents these experimental data for CO-air mixtures containing 4.58 and 7.5 mol% CO, two runs were performed for each mk/v, value. The first run is the oxidation of CO to C02 without C02 in the feed. In the second run, the same experiment was repeated at the same conditions, but with C02 in the feed. The comparison of these experiments for each set of mk/vo values demonstrates the inhibiting effect of carbon dioxide on the oxidation rate of CO. Although the data obtained without CO;? in the feed in Table 1 and the results in Figure 2 are for identical conditions, different numerical values have been obtained for the oxidation rates. This difference is due to the activity change in the catalyst. As the two experiments in each set (with and without C02 in the feed) in Table 1 were carried out consecutively, the decrease in rate observed shows the relative inhibition effect of C02 on the oxidation rate of CO. However, these are only preliminary results and further experimental work is required. References 1 Campbell, C.T. and White, J.M. 1978. The adsorption, desorption and reactions of CO and CO 2 on Rh. J. Catal, 54, 289-302. 2 Campbell, C.T., Shi, S.K.and White, J.M. 1979a. Kinetics of oxygen titration by carbon monoxide on rhodium. J. Physical Chem., 83(17), 2255-2259. 3 Campbell, C.T., Shi, S.K. and White, J.M. 1979b. Carbon monoxide oxidation reaction over Rh. J. Vac. Sci. Tech., 16(2), 609407. 4 Campbell, C.T., Shi, S.K. and White, J.M. 1979c. The Langmuir Hinshelwood reaction between oxygen and CO on Rh. App. Sur. Sci.,2,382-396. 5 Matsushima, T. 1980. Tracer studies of the reaction paths of the CO oxidation over polycrystailine palladium and rhodium. J. Catal., 64, 38-50. 6 Kim, Y.,Shi, S.K. and White, J.M. 1980. Oxygen inhibition of CO oxidation on polycrystalline Rh. J. Catul., 61, 374-377. 7 Oh, S.H., Fisher, G.B., Carpenter, J.E. and Goodman, D.W. 1986. Comparative kinetic studies of CO-02 and CO-NO reactions over single crystal and supported rhodium catalyst. J. Cutul., 100, 350-376. 8 Cho, B.K. and Stock, C.J. 1989. Dissociation and oxidation of carbon monoxide over Rh/Ai203 catalysts. J. Catal., 117, 202-217. 9 Gundilz (Tufan), 0.and Lintz, H.G. 1983. Oxidation of carbon monoxide by air over a supported rhodium catalyst. Proc. Vth. Int. Symp. Heterogeneous Catalysis, Vurnu, Part 11, pp.79-84. Received: 23 September 1991; Accepted: 25 May 1992.