George, (Miss) Susan

Sastri, M. V. C.

Viswanathan, B.

English

NOPR

Indian Journal of ChemistryVol. 15A, April 1977, pp. 285·287Kinetics of Oxidation of Carbon Monoxide on Lanthanum Cobaltite(Miss) SUSAN GEORGE*, B, VISWANATHAN & M. V~C. SASTRItDepartment of Chemistry, Indian Institute of Technology. Madras 600036Received 30 August 1976; accepted 25 October 1976Adsorption of CO and 0, has been studied on a lanthanum cobaltite catalyst in a standardvolumetric adsorption apparatus. It is found that the catalyst does not adsorb either CO or O2separately but adsorbs both the gases from a mixture of the two. The kinetics of the oxidationof CO over LaCoO. has been Investtgated in an all ~lass recirculatory type of reactor. Thereaction is found to follow the kinetic equation for a total order equal to one but the initialpartial orders for both CO and O. are zero. Hence, it is suggested that the reaction occursbetween the adsorbed species, the catalyst surface being continuously saturated by the reactantsand inhibited by the carbon dioxide produced.FOLLOWING the original suggestion by Libby!that La.coOa can be a good catalyst forautomobile exhaust pollution control, varioussubstituted and unsubstituted rare earth transitionmetal oxides having the perovskite structure havebeen shown to be good catalysts for the oxidationof carbon monoxide and hydrocarbonss+, Extensivestudies' have been carried out in evaluating theperovskite type oxides as oxidation catalysts undervarious experimental conditions like (i) varyingthe composition of the catalyst, (ii) varying thethermal treatment of the oxides and (iii) in presenceof S02 and sulphur-free environment. As a partof the effort undertaken to evaluate the kineticparameters of the CO oxidation over various catalystsunder different conditions, a study of the oxidationof C-o on LaCo03 has been carried out.Materials and MethodsLaCo03 catalyst was prepared by heating lantha-num oxalate and cobalt oxalate at 9000 for 23 hr.Adsorption measurements were carried out in astandard volumetric adsorption apparatus usinga Fischer gas partitioner model 29 (Fischer HamiltonCo., USA) for the in, situ analysis of gas mixtures.The kinetics of oxidation of CO Were studied inan all glass recirculatory type of reactor describedelsewhere".ResultsIt was found that though the catalyst did notadsorb, even minutely, either carbon monoxide oroxygen separately, it adsorbed both the componentstogether when exposed to a mixture of carbonmonoxide and oxygen. The mixed gas adsorptionisotherms at non-reaction temperatures are givenin Figs. 1 and 2.The kinetics of oxidation of CO were studied underthe following conditions: (i) at constant total initialpressures, either at 60 or 100 torr, but varying the=Present address: Lecturer, Department of Chemistry,Women's Christian College. Madras 600006.t Present address: Head. Materials Science Research Centre,Indian Institute of Technology. Madras.c,~z000'"04:w 0·5::I:::>a>0·1rn w ~PARIIAL PRESSURE ICM)Fig. 1 - Adsorption isotherms of CO on LaCoO. fromCO: O2 mixture (2: 1)04c,z 0.)8ci'"~ 0.210 15PARTIAL PRESSURE ICM)Fig. 2 - Adsorption isotherms of O2 on LaCo03 from CO: O2mixture (2: 1)tem~erature; and (ii) at constant temperature butvarYlll? the. c0!llposition of the gas mixtures.Typical kinetic plots at various temperatures atan initial total pressure of 60 torr are given in Fig. 3and the corresponding plots at 1900 for varying285INDIAN ]. CHEM., VOL. 15A, APRIL 1977280°C 2 BOOC51.2N 4S?1·0Q.a"3;;0.8 '3z 0·6QV1crw>z8 0·4~TABLE 1 - KINETIC DATA FOR OXIDATION OF CO ONLaCo03'.2 Mixture Total Pco po, Initial(CO: O2) initial (mm Hg) (mm Hg) ratepressure (mm Hgl(in mm Hg) min)'·0TEMP. 19000.8 1: 1 61·0 30·5 30·5 0·0242: 1 62·0 41·3 20·7 0·0213: 1 59·2 44·4 14·8 0·022z0iQ 0·6 TEMP. 1500w>1 : 59·6 28·8 28·8 0·028z8 2: 1 61·1 40·7 20·4 0·0300·4 -3ICO: \023: 1 59·0 44·3 14·7 0·030~G2CO: '02(3eD :102 TIi:MP. 28000 A lCO : 30229·45 0·035I: 1 58·9 29·45fEMPERATURE Igoe 2: 1 60·0 40·0 20·0 0·0363: 1 59·4 44·6 14·8 0·03810 20 30 40 SO soTIME ,MINUTES I10 6030 40 50TIME (MINUTES)Fig. 3 - Percentage conversion at different temperaturesat an initial pressure of 60 torr20Fig. 4 - Percentage conversion at different CO: O2 mixturesat constant temperaturecompositions of the gas mixtures are presented inFig. 4.It is seen from Fig. 5 that the experimental datafollow the kinetic equation for the total order equalto one, i.e, log Po/P varies linearly with i, where pis the total pressure at time t and Po is the initialtotal pressure. The partialorders with respect tothe concentration of carbon monoxide and oxygenin non-stoichiometric mixtures are then established,by fitting the data in the equationdPco.ldt=kP~oP'b2; x+Y= 1286~o 50 eoTIME (MINUTES)Fig. 5 - First order kinetic plots at different temperatures10 20 30These experiments were carried out at varioustemperatures on the same catalyst and the valuesof the initial rates are given in Table 1.Excess of either carbon monoxide or oxygen hadno influence on the initial rate, indicating that theinitial (zero time) partial orders are zero for boththe reactants. Hence, the total order of oneexhibited with respect to time, is an apparent order.Table 2 gives the values of the initial experimentalrates for different total pressures of the stoichio-metric mixtures. If the order one, during the re-action, were a true one, the initial rate should beproportional to the initial total pressure accordingto (dPco,ldt)t~o=kpo. . ..The "a lues in Table 2 show that the init ial rateis a constant, unaffected by the initial total pressure.The true order at time zero is determined byGEORGE et al.: OXIDATION OF CO ON LANTHANUM COBALTITETABLE 2 - INITIAL RATES AT 90° FOR STOICHIOMETRICMIXTURESPo (mm Hg) Initial ex peri-mental rate(mm Hg/min)0·0750·080·0850·07549·061·7105·5200·6plotting the logarithm of the initial rate of Table 2as a function of the logarithm of the initial pressurePo. The slope of this line gives a value m=O·1 ~ 0for the initial true order. But this value doesnot arise out- of a compensation between twopartial orders of opposite sign.The kinetic data obtained at 1900 for stoichio-metric mixtures by varying the initial total pressurefrom 3 to 15 ern Hg were ana lysed by plotting logPo/P vs t. From the initial slopes of these plots thevalues of the rate constant (k = slope/2·303) werededuced. The logarithm of the rate constant wasthen plotted against Iagarithm of Po and its slopegave a value of n = 1 and this can be expressed bythe equation k = k'/P~, where k' is the true rateconstant, independent of the initial conditions. Theinstantaneous rate at any moment for the first orderkinetics is therefore given bydPco,ldt=kp=k' PIP~and, in particular at zero time, the total order is m.The explanation for a catalytic reaction exhibitinginitially and during the process the total and partialorders = zero, is that the reaction occurs betweenthe adsorbed species and the catalyst surface isbeing continously saturated by the reactants. Thiscondition is applicable for zero time only. At anyTABLE 3 - EFFECT OF CO. REMOVAL ON THE INITIALRATESTemp.°CInitial rate withtrap (mm Hg/min)0·040·090·05Initial rate withouttrap (mm Hg/min)0·020·040·036190240280definite time t, despite the surface saturation, therate decreases according to the first order lawinstead of remaining constant. This could be dueto the carbon dioxide produced inhibiting the rateof the reaction, as seen from the results for the re-action with and without a cold trap at liquid nitrogentemperature to remove the CO2 formed (Table 3).Acknowledgemen tThis work forms part of a research projectsupported by the Department of Science andTechnology, Government of India, New Delhi.References1. LIBBY, W. F., Science, 171 (1971), 499.2. PARRAVANO, G., J. Am. chem. Soc., 75 (1953), 1497.3. VOORHOEVE, R. J. H., REMEIKA, J. P., FREELAND, P. E.& MATTHION, B. T., Science, 177 (1972), 353.4. JOHNSON, D. W. & GALLAGHER, P. K., Thermochim, Acta,7 (1973), 303.5. JOHNSON, D. W., GALLAGHER, J. A., VOGEL, E. M. &SCHREY, E., Proc. fourth international con]. on thermalanalysis, Budapest (Hungary), 1974.6. SORENSON, S. C., WRONKIEWICZ, J. A., SIS, L. B. &WIRTZ, G. P., J. Am. ceram. Soc., 53 (1974), 446;GALLAGHER,P. K., JOHNSON, D. W., REMEIKA, J. P.,SCHREY, E., TRUMBILE, L. E., VOGEL, E. M. & VOOR-HOEVE, R. J. H., Mat. Res. Bull., 10 (1975), 529.YOA, YUNG-FANG, J. Catalysis, 36 (1975), 266.7. KRISHNAMURTHY, K. R., VISWANATHAN, B. & SASTRI,M. V. C., Proc. Indian Natn Sci. Acad. (Communicated).287

Kinetics of Oxidation of Carbon Monoxide on Lanthanum Cobaltite

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Kinetics of Oxidation of Carbon Monoxide on Lanthanum Cobaltite

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