The Influence of a Chemisorbed Layer of Carbon Monoxide on

M. V. C. Sastri, T. S. Viswanathan, and T. S. Nagarjunan. J. Phys. Chem. , 1959, 63 (4), pp 518–521. DOI: 10.1021/j150574a015. Publication Date: Apr...
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M. V. C. SASTRI,T. S. VISWANATHAN AND T. S. NAGARTUNAN

Vol. 63

THE INFLUENCE OF A CHEMISORBED LAYER OF CARBON MONOXIDE ON SUBSEQUENT PHYSICAL ADSORPTION1 B Y M. V.

c. S A S T R I , T. s. VISWANATHAN AND T. s. NAGARJUNAN

Department of Applied Chemistry, Indian Institute of Technology, Kharaypur, India Received Julu 18, 1968

Experiments reported in this paper show that, on cobalt catalysts, a part of the carbon monoxide chemisorbed at liquid nitrogen' temperature is removed by evacuation a t -78". Thus two types of chemisorption of CO arevisualked, one resisting desorption at -78" and the other desorbing at -78". The former type suppresses the subsequent low temperature physisorption of nitrogen and carbon monoxide whilst the latter does not. The suppression is attributed to the positive charge gained by the metal surface a8 a result of CO chemisorption. These observations also show that low temperature CO chemisorption methods cannot be used, in all cases, to determine the extent of metal surface.

Introduction The extent of the exposed metallic component on the surface of iron, cobalt and nickel has been estimated by several workers by the chemisorption of carbon monoxide a t liquid nitrogen temperature. Since the total adsorption of carbon monoxide a t this temperature comprises of physisorption as well as chemisorption, the amount of chemisorbed CO is estimated as the difference between the total CO adsorption and the physisorption of the same gas, assuming that the chemisorbed carbon monoxide does not affect the subsequent CO physisorption on the chemisorbed film. To determine the amount of physisorption two methods have been used. In the first method proposed originally by Emmett and Brunauer,2 the total CO adsorption is determined at -194', followed by evacuation a t -78'. Assuming that this evacuation removes only the physisorbed gas leaving t,he chemisorbed layer intact, the CO adsorption isotherm is redetermined on this surface at -194' so that the second isotherm gives the amount of CO physisorbed. Finding that in certain cases evacuation a t - 78' removed part of the chemisorbed CO, Emmett and Skau3 later suggested that the difference between CO and N2 isotherms a t the same low temperature and relative pressure might be taken as the amount of CO chemisorbed, since the physical adsorption of CO and of N2 are almost identical on most surfaces. This method later on was extensively used by A n d e r ~ o n . ~The assumption in this method is that the adsorption characteristics of the bare surface and of the chemisorbed film are the same. Recently however some doubt regarding the validity of the above assumption arose from the results of Stone and Tiley6 who reported that the presence of a chemisorbed layer of carbon monoxide on cuprous oxide suppressed the subsequent physisorption of nitrogen or krypton. Besides, Joy and Dorlinge found that on a fueed iron Fischer-Tropsch catalyst, a part of the carbon monoxide chemisorbed at - 194' was removed by ( 1 ) Part of the work described in this paper is abstracted from the Ph.D. thesis of T.S.V.submitted to the Madras University in January, 1955. (2) P. H.Emmett and 8. Brunsuer, J . Am. Chcm. Soc., 67, 1754 (1935): 69, 310, 1553 (1937). (3) P. H. Emmett and N. Skau, ibid.. 65, 1029 (1943). (4) R. B. Anderaon, W. X.Hall and L. J. E. Hofer. ibid., TO, 2465 (1948). (5) F. 8. Stone and P. F. Tiley, Nature, 167, 654 (1951); 168, 434 (1951). (6)A. S, Joy and T. A. Doding, ibid., 168, 433 (1951).

evacuation at -78". This enabled them to differentiate between two types of chemisorption of carbon monoxide, the first resisting desorption during evacu: tion at -78' and the other removed during this e racuation. This differentiation between strong and weak chemisorption of CO is arbitrary but useful. They found that the presence of strongly chemisorbed CO on the iron catalyst did not affect the subsequent physisorption of nitrogen. Similar results have been obtained by Srikant' on iron synthetic ammonia catalyst. On the other hand, Sastri and Srinivasans observed that on Co catalysts physisorption of Nz was suppressed in presence of chemisorbed CO. From these observations it would appear that the suppression effect is specific to the metal in question. The present paper reports a more detailed investigation of the effect on cobalt catalysts particularly with reference t o the relative effects of the so-called strongly and weakly chemisorbed carbon monoxide. Incidentally, the reliability of the method under review for the estimation of carbon monoxide chemisorbed on metal surfaces hsla been re-examined.

Experimental The catalyst used in the present investigations was of the following composition: cobalt: thoria: kieselguhr 100: 18: 200, the surface area of kieselguhr used being 25.6 m.a/g. The catalyst was prepared from the corresponding nitrates by the simultaneous addition of kieselguhr and potassium carbonate. The catalyst was reduced by a rapid stream of carefully purified (described later) hydrogen till no more moisture could be detected in the exit gas by a liquid nitrogen trap.0 The reduction temperature was 350". The results reported in this paper refer to adsorption by about 1.5 g. of unreduced catalyst. The a paratus for the measurement of adsorption was essential& the same as that described in an earlier paperlo with an arrangement to permit continuous flow of gas through the catalyst tube whenever necessary without removing the catalyst tube. The gases used in the adsorption experiment were purified as follows. Hydrogen, used for reduction, was prepared electrolytically and purified by passage through (a) metallic co per a t 400" and (h) ''Deoxo" purifier of Messrs. Baker Co., London, which is an alumina-supported platinum catalyst active a t room temperatures. Subsequently the gas was passed over anhydrous magnesium perchlorate and finally over silica gel cooled in liquid nitrogen to remove completely water vapor and gases heavier than hydrogen. Carbon monoxide was prepared by adding formic acid

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(7) H. Srikant, Ph D. thesis accepted by the Madras University. in 1954 (8) M. V. C. Sastri and V. Srinivasan. Current Scz.. 23, 154 (1954). (9) J. T. Kummerand P. H . Emmett, T H r s JOURNAL, 65, 337 (1951). (10) J. C. Ghosh, M. V. C. Sastri and T. S. Vlswanathan, "Symposium on Contact Catalysis," Bull. N a t l . Inst. Sea., 1957 (in press).

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INFLUENCE OF A CHEMISORBED LAYEROF CARBON MONOXIDE

dropwise to 85% phos horic acid at 170" and removing acid spray and traces of C& by KOH. The gas was purified b passage over hot copper followed by U tubes of potasg pellets and anhydrous ma nesium perchlorate, respectively. Tank nitrogen was purifed by passage over hot copper and anhydrous magnesium perchlorate. Tank helium of purity 99.5% was passed over hot copper, anhydrous magnesium perchlorate and finally through an activated charcoal trap cooled in liquid nitrogen. After reduction by hydrogen, the catalyst was evacuated at the reduction tern erature for 8 hours to remove adsorbed hydrogen, begre nitrogen and carbon monoxide adsorptions a t low temperature were carried out. After each CO adsorption run the catalyst was evacuated at 100" for two hours to remove as much of the carbon monoxide as possible. It was then treated with flowing hydrogen a t 250" for two hours and later evacuated a t the reduction temperature for 8 hours. This procedure was found to give a reproducible catalyst surface. The temperature of the liquid nitrogen bath was measured by means of an oxygen vapor pressure thermometer. Solid carbon dioxide acetone and melting methylcyclohexane baths were used to obtain temperatures of - 78" and - 122", respectively, which were read by a pentane-in-glass thermometer. The adsorption results in each series of experiments were closely reproducible.

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Sequence of Experiments --/02 Influence of Strongly Chemisorbed CO on Physical Adsorption.-Experiments were carried RELATIVE PRESSURE 100. out on cobalt-thoria-kieselguhr catalyst to investiFig. 1.-Suppression of physical adsorption of nitrogen. gate (a) the different types of chemisorption of CO on the catalyst surface and (b) their influence on sented below for one sample of cobalt-thoria subsequent physical adsorption of Nz and CO. kieselguhr catalyst. The sequence of the experiments was as Results follows. The results are represented graphically in Fig. 1, (1) Adsorption of nitrogen at -194' was determined on the bare surface of the thoroughly the numbers on the curves denoting the respective evacuated catalyst. The catalyst later was evacu- operations. It is seen that the repeat carbon ated for two hours a t 200' to remove the ad- monoxide isotherm (3) falls in between the carbon sorbed nitrogen. monoxide isotherm on bare surface (2) and the nitrogen isotherm (1). The difference between (2) Adsorption of carbon monoxide at -194' on the bare surface obtained after step (1). (2) and (3) gives the amount of CO strongly chemi(3) The catalyst was evacuated a t -78' for sorbed whilst the amount weakly chemisorbed is one hour, after the adsorption in step (2). The roughly given by the difference between (3) and catalyst then was cooled down t o -194' and the (1). Hence low-temperature chemisorption of CO adsorption of carbon monoxide on the surface consists of (a) strongly chemisorbed CO resisting determined at this temperature. desorption a t - 78' and (b) weakly chemisorbed Also it is seen that (4) After the CO adsorption in step (3), the CO being desorbed a t -78'. catalyst was evacuated a t -78' for one hour. It due to the presence of a film of strongly chemithen was cooled back to - 194' and the adsorption sorbed CO, the physisorption of nitrogen is suppressed, the extent of suppression being given by the of nitrogen on the surface determined. (5) After the carbon monoxide adsorption in difference between (1)and (4). step (2), the catalyst was evacuated a t -122' for To determine the influence of weakly chemione hour. It then was cooled back to -194' and sorbed CO on subsequent physisorption, the system the adsorption of carbon monoxide on this surface after initial adsorption of CO a t - 194' was evacumeasured at this temperature. ated a t -122' to leave behind a film of weakly (6) After the adsorption in step ( 5 ) , the cata- chemisorbed CO and then the Nz isotherm was lyst was evacuated a t - 122' for one hour. It was determined at -194' (Fig. 1). The results show then cooled back to -194' and nit.rogen adsorbed that the presence of weakly chemisorbed CO does a t this temperature determined. not affect the subsequent physisorption of nitrogen. The catalyst then was evacuated and cleared of Since the activation energy for low temperature all the chemisorbed carbon monoxide as described CO chemisorption is very low, the activation energy before, and the entire cycle of operations noted for the desorption of the gas will be determined above was repeated to check the reproducibility predominantly by the heat. of adsorption. It may of the results. be estimated very roughly that gas adsorbed with These experiments also were performed on a a heat below 10 kcal. per mole would be removed catalyst of the same composition prepared with a by evacuation a t -78' (Stone and Tiley give the calcined kieselguhr sample having a surface area figure of 12 kcal. as the maximum limit of the heat of only 1.44 m.2/g. and with a cobalt-thoriaof adsorption of a gas removed by evacuation at magnesia-kieselguhr catalyst. The results ob- -78') ; while at - 122' any gas adsorbed with a tained with these were very similar to thoee pre- heat less than 7 kcal. is removed during evacuation.

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M. V. C. SASTRI, T. S. VISWANATHAN AND T. S. NAGAWUNAN

Thus the presence of carbon monoxide chemisorbed with heat between 7 and 10 kcal. and presumably any gas chemisorbed with heat less than 7 kcal. also, is without effect on physisorption.

Discussion The results show that less nitrogen is adsorbed in presence of a strongly chemisorbed film of CO left over at -78’ than on the bare surface. Weaker chemisorption of CO which could be removed by evacuation a t -78’ did not produce any increase in the suppression. Since this work was completed, Srinivasan” has reported that on reduced cobalt oxide powder also, the physisorption of nitrogen is similarly suppressed in the presence of chemisorbed carbon monoxide. Before attributing this suppression exclusively to the presence of the chemisorbed CO film i t is necessary to examine if any (molecular) chemisorption of nitrogen occurred on the cobalt surface a t -194’ (besides physisorption). Because if it did occur, the observed suppression would then be due, at least in part, to the strongly chemisorbed CO, occupying sites which were otherwise available for nitrogen chemisorption. However, it would be dif€icult to account for more than twofifths of the observed suppression (ca. 2.2 cc.) of nitrogen adsorption in this way, since the amount of strong CO chemisorption is only about 0.8 cc. A simple experiment indicated the low order of energy associated with the adsorption of nitrogen. When, after determining the nitrogen adsorption isotherm on the bare Co surface a t -195’ the gas was pumped off for 1 hour a t -78’ and the nitrogen isotherm redetermined at 194’ the previous isotherm was reproduced perfectly, thereby proving that the whole of the nitrogen adsorbed a t low temperature was removed by the short evacuation In other words, no part of the nitrogen a t -78’. adsorption was associated with an energy high enough (Le., m.8 kcal.) to make it resist desorption a t -78’. These observations, though not conclusive in eliminating altogether the possibility of Nzchemisorption occurring on the bare cobalt surface a t -194’ show clearly enough that this can a t best be only a minor factor in the over-all mppression observed. The bulk of the suppression must be interpreted in terms of the influence of the strong CO-chemisorption on the physical adsorption of nitrogen over the bare surface and over the chemisorbed CO molecules. This is the purport of the following discussion. Physisorption of non-polar gases on metals is caused by the co-operation of the two factors: (i) non-polar van der Waals forces arising out of interaction of induced’dipoles in the metal with the (continually changing) inducing dipoles of the gas molecules; (ii) the polarization of the molecules by the metal. The metal surface acquires a positive charge with electrons projecting a short distance outside the surface. The approaching molecules are polarized by the metal surface with the positive end pointing away from the surface; it is assumed that the negative layer of electrons does not polarize the molecules.

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(11) V . Srinivassn. Proc. Ind. Acad. Sci., XLVI, Sea. A, 120 (1957).

Vol. 63

The observed suppression of the physisorption isotherm is tantamount to decrease in the monolayer volume V,, which by definition is the product of the surface area ( A ) and the limiting value of adsorption per unit area. The suppression effect could therefore be due to either or both of the following causes: (i) decrease in the total area A , say by “blocking” of the pores and also perhaps by localized chemisorption a t the pore mouths (“poisoning” of the pore mouths) preventing the twodimensional migration of the physisorbed molecules along the pore walls into the interior of the pores; (ii) decrease in the value of v0, representing the density of packing a t saturation. As already mentioned, during physisorption, non-polar gas molecules are polarized with their positive ends pointing awty from the surface. Due to this polarization, the magnitude of the attraction forces betweell the adsorbed molecules are less, resulting in a decrease in the two-dimensional van der Waals constant az; sometimes even a negative value for a2 is obtained.12 In many cases of chemisorption normal covalent bonds are formed where an electron of the adsorbed atom and one of the metal form a pair. Since these bonds are partly ionic in character, the adatoms form dipoles on the surface of the metal, the direction of the dipoles depending upon the ionization potential of the adatom and the electron work function of the metal. In the case of CO chemisorbed over cobalt surface, the negative charges of the dipoles point away from the surface with a resultant decrease in the number of conduction electrons. The dipoles so formed tend to repel each other so that the chemisorbed layer is incomplete with patches of bare metal surface remaining exposed. Under such conditions, a molecule physisorbed subsequently comes directly into contact with the metal surface possessing enhanced positive charge. As a result, the polarization of the physisorbed molecule is more pronounced, t,hereby increasing the magnitude of the repulsive forces in the physisorbed layer. Similar results have been obtained by Mignolet,l* who observed that in presence of a chemisorbed film of hydrogen on nickel, the physisorption of nitrogen was suppressed t o a large extent with a simultaneous increase in the surface potential of the physisorbed film. Therefore the molecules in the physisorbed layer are now more loosely packed so that vo is less than that on the bare surface. These arguments hold good in the case of a polar molecule (like CO) also. Regarding the method suggested by Emmett and Skau, the difference between CO and Nz isotherms a t the same low temperature and relative pressure would give a lower estimate of the metal surface, since the suppression of physisorption of CO by the chemisorbed CO has not been considered. Moreover, since the extent of suppression in physisorption of CO will be different from that of Nz, it would be difficult to determine one with the help of the other. Reference may now be made to the observed difference in behavior of strongly and weakly (12) (13)

J. € de I. Boer, Adu. i n Catalysis, -11, 38 (1956). J. C. P. Mignolet, Disc. Farodov sot?.,8, 106 (1950).

COMPUTING COMBUSTION EQUILIBRIA ON A HIGH SPEEDDIGITAL COMPUTER

April, 1959

chemisorbed CO toward subsequent physisorption. The initial rapid chemisorption of CO increases the electron work function of the metal considerably so that the subsequent chemisorption of CO occurs with less heat of adsorption. It is probable that the heat of chemisorption may be of the same order as that of physical adsorption. Therefore at - 1 9 4 O , CO adsorbed subsequently will be bound to the surface in a quasi-chemical way without significantly contributing to the magnitude of the positive charge on the metal. A further assumption that the extent of physisorption is the same over the weakly and strongly chemisorbed CO gives an explanation for the result that the presence of weakly chemisorbed CO does not add any more to the suppression already caused by the strong CO chemisorption. Since the transition from physisorption to chemisorption is continuous and the heat of chemisorption a t high coverages may be as low as that of physisorption, it is not possible to visualize any

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temperature a t which only the physisorbed molecules are removed by evacuation. Nor is there any theoretical justification for assuming tacitly that the amount of such weak chemisorption is inconsiderable in comparison with the amount held more strongly. Strictly speaking, therefore, it would not be possible to estimate accurately the true extent of the CO-chemisorption by the technique of evacuation a t any temperature higher than that of liquid nitrogen and readsorption a t the same temperature. Evacuation a t liquid nitrogen temperature will not be any better, because it is bound to leave behind a considerable quantity of the physisorbed gas. I n view of the uncertainties caused on the one hand by the suppression effect and on the other hand by the desorbability of the chemisorbed gas over a range of low temperatures, neither of the procedures suggested in the paper of Emmett and Skau can be expected to give an accurate estimate of the surface metal sites on cobalt catalysts.

A METHOD OF SUCCESSIVE APPROXIMATIONS FOR COMPUTING COMBUSTION EQUILIBRIA ON A HIGH SPEED DIGITAL COMPUTER1 BY D. S. VILLARS Contributionfrom the Research Department,

U.S. Naval Ordnance Test Station, China Lake, California

Receiued JuZy 18, 1968

A rocedure has been developed for ra idly solving complicated thermodynamic equilibria by a flexible iteration method whici can be readily extended to inclug additional chemical elements. Changes in composition are computed for only one reaction at a time, neglectin the interaction of such changes in composition on the other equilibria. Current values of concentrations are used to calcaate back all equilibrium constants. For the next computation the program selects that reaction showin the greatest discrepancy between calculated and given equilibrium constant. Discrepancies are then recalculated andranother determination made of the reaction showing the greatest discrepancy. The process is repeated until the maximum discrepancy is reduced to a value less than an error, E, specified as a parameter of the problem. Speed of convergence may be maximized by expressing all species in terms of components existing in the largest concentrations at equilibrium. A subiteration procedure is utilized for solving the individual e uations. This converges upon the solution by halving successive tentative intervals. For errors of 0.001% the JPN equilhum at 2 atmos heres is computable with 28 iterations for 2500OK. and 43 for 4000’K. A composition involving excess solid carbon has feen computed with 14 iterations for 2500OK. and 20 for 4000’K. Each iteration involves about 0.2 second machine time.

Introduction proportion to the number of moles at equilibrium of It is well known2that the specific impulse Ispof a each of the different species present, it is highly depropellant is calculable from flame temperature T,, sirable to have an easy method of calculating the the average ratio of specific heats y , and the average molecular weight M of the gaseous mixture =i

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( l / g ) [ ( R T e / W I ~ Y / ( Y1 ) ) {1

- ( P ~ / P & v - ~ ) / v ]]’’*

(A)

if equilibrium is frozen during expansion through the nozzle, or from the change in enthalpy a t constant entropy AHs, on adiabatic expansion between chamber and exit 1.p

(-2AHs/M)’h/g

(B)

if the reaction mixture continually re-equilibrates during expansion. In the above equations g is the acceleration of gravity, R the molar gas constant, P e the external pressure, and Pc the chamber pressure. Since the above averages are weighted in (1) Preaented in part before the Division of Physioal and Inorganic Chemistry 131st Meetine of the American Chemical Society, Miami, April 12, 1957. (2) 6. 8. Penner, “Chemistry Problema in Jet Propubion,” Pergsmon Press. New York, N. Y., 1957, see Chapter 14.

equilibrium composition. I n order to discuss a concrete problem let us consider the typical propellant, JPN,* which is a double base ballistite consisting of carbon, hydrogen, oxygen and nitrogen. At flame temperatures in a rocket chamber six equilibria produce 10 reaction species in appreciable concentrations. To solve for 10 mole fractions one requires 10 equations. Four of the 10 necessary equations are atom balances in the four elements and are linear. However, the six equilibrium constant equations are quite non-linear, several of them being cubic and others quadratic. The only practical way of solving such a system of equations is by successive approximations. Such procedures are likely to be very laborious for desk machine calculations but with the recent availability of high speed digital computers which thrive on (3) R. N. Wimpreas. “Internal Ballistics of Solid-Fuel Rockets.” McGraw-Hill Book Co., New York, N. Y., 1950, Table 2-1, p. 4.