1779
Langmuir 1991, 7, 1779-1783
Adsorption Isotherms of Nitrogen at 77 and 87 K and n-Butane at 236,253, and 273 K onto Tungsten Disulfide M. A. Baiiares-Muiioz' and V. Sanchez Escribano Department of Inorganic Chemistry, Faculty of Chemical Sciences, University of Salamanca, 37008 Salamanca, Spain Received August 9,2990. I n Final Form: February 22, 1991
A study was made of the surface properties of tungsten disulfide from the corresponding adsorption isotherms of nitrogen at 77 and 87 K and n-butane at 236, 253, and 273 K. The shape and the values calculated from the parameter C of the BET equation show that such isotherms belong to type I1 of the BDDT classification. The adsorption isotherms, both of nitrogen at 77 and 87 K and of n-butane at 236, 253, and 273 K, show a cross-over point that practically coincides with the value calculated for the monolayer volume calculated by different methods. From the aforementioned isotherm, it was possible to calculate the isosteric heats and entropies of adsorption. 1. Introduction
The special layered structure of tungsten disulfide' confers on this compound certain physicochemical properties that are ideal for its application in certain technological operations. In recent years, works have been published2J that have shown its importance, among other applications, as an antifriction and lubricant material. There is also a considerable body of literature concerning the study of different catalytic p r o c e s ~ e sand ~ * its ~ behavior as a component for electrode^.^^^ However, and in spite of the important technological interest of the compound, few data have been published in works relating to the study of the surface properties of tungsten disulfide. With a view to gaining further insight into the surface properties of this compound, we were prompted to carry out a low-temperature gas adsorption study to obtain the corresponding adsorption isotherms of nitrogen and n-butane a t two and at three different temperatures, respectively, determining from these isotherms the isosteric heats and entropies of adsorption of both gases onto WS2. The existence of crossover points among the isotherms was observed. 2. Experimental Section The tungsten disulfide used as the adsorbent was a synthetic product from Germany (AlfaVentron)and was purchased in the form of a greasy, gray-black powder. The supplyingf iprovided somechemical data on the compound molar mass, 247.88 g mol-'; chemical composition, 73.8% tungsten and 26.2% sulfur. The adsorption isotherms were obtained by using a conventional precision volumetric system. Calibration of the different volumes of the adsorption apparatus was carried out by using helium (N-45)supplied, like the gases employed as adsorptive, by the Sociedad Castellana de Oxigeno. The gas buret was calibrated by weighing the corresponding volume of mercury previously distilled. Nitrogen N-52 (99.9992% vol) and n-butane N-25 (99.5% vol) were used as the adsorptive gases. (1)Le Bwp, F.;Mathiron, C.; Toesca, 5.;Delafwe, D.; Colson, J. C. Bull. Soc. Chrm. Fr. I S U , 11,3869-73. (2)Lebedrva, J. L.; Yukhno,T. P.; Mar'yakhina, E. J. Tsenie Iznas 1988,8,1060-7. (3)Miyake, Shojiro; Hirano, Motchiaa;Ohata, Kouhei; Kato, Umeko JunAatru 1988,M (l), 41-63. (4)Startaev, A. N.;Skuropat, 5. A.; Zaikovakii, V. I.; Moroz, E. M.; Ermakov, Yu, I. Kinet. Katal. 1988,29(2),398-406. (6) Belloloui,A.; Beywe, M.; Lacrois, M.; Moeoni, L.; Roubin, M.; Vrinat, M. Bull. SOC.Chim. Fr. 1987,1,18-20. (6)Kokai Tokkyo, Koho J. P., Jpn. 67,172,1982. (7)Grisheva,G. A.;Shulepona, N. Y.; Toetly, A. N.; Novikov, G. 1. Ser. Fir-Energ. Nabuk. 1982,66-87.
Determination of the degree of purity and crystalliiity of the tungsten disulfidewas performed by X-ray,applying the DebyeScherrertechnique. The corresponding diagrams were obtained by using a Siemens powder chamber and Cu Ka radiation, with a nickel filter. In order to discover the distributionand particle size, the granulometricstudywas conducted by followingthe Coulter technique on a Coulter-counter Z-M apparatuswhich employsan electrolyte medium standardized for the apparatus with an approximate composition of 1.5% sodium chloride, 4% sodium pyroeulfate, 6% sodium hydroxide, and 2% calcium chloride. The temperatures of 77 and 87 K at which the experimenta on the adsorption of nitrogen were carried out were chosen by observing the change in state (boiling point) of liquid nitrogen and argon, respectively; the gases were contained in a Dewar flask. The experimenta on the adsorption of n-butane at 273 K were also performed in a thermostatic bath containing melting ice. The remaining temperaturesof 253and 236 K were obtained by cooling a mixture of 72 % alcohol and 18%water in a Hetofrig cryostat. In all the adsorption experiments control and verification of temperature constancy were carried out by using a digital thermometer from L'Air Liquide, Model P-50, with a resolution of f0.1 K. The samplesof tungstendisulfidewere treated under a dynamic vacuum of 10-6Torr and 473 K over 14h in the adsorption system used subsequently to determine the corresponding adsorption isotherms. 3. Experimental Results The results obtained by X-ray diffraction, applying the Debye-Scherrer technique, are shown in Table I, together with the relative intensities, that are very similar to those reported in the literature for tungsten disulfide.8 The table also shows the values corresponding to the NelsonRiley functions which, by extrapolation, lead to the determination of the true value of the COparameter, which indicates the distance between two consecutive layers of the tungsten disulfide; the value calculated was 12.3 A. Figure 1 shows histograms of the data obtained with the Coulter technique. Analysis of these data gave a mean particle diameter of 6.6 p m together with a mode diameter, with a value of 9.8 pm. To obtain the adsorption isotherms of nitrogen and n-butane at the different temperaturesassayed on tungsten disulfide, several sets of adsorption experiments were carried out, with 5 to 10 experimental points each. Such (8) Joint CommiteeonPowderDifractionStandarde. SelectedPowder Diffraction Data for Minerals; Pennsylvania, 1974. (9)Nelson and Riley, Proc. Phys. SOC.London 1946,67, 160-77. 0 1991 American
Chemical Societv
Bailares- Muiloz and Sanchez Escribano
1780 Langmuir, Vol. 7, No. 8, 1991 Table I. Reading of the Debye-Scherrer Diagram tungsten disulfide JCPDS tungsten disulfide characterized d, A IlZOO fN-R d,A 1/10 hkl 6.04 vs 3.94 6.18 100 002 3.06 2.71 2.66 2.48 2.27 2.04 1.82 1.64 1.57 1.54 1.53 1.48 1.40 1.36 1.28 1.24 1.12
m m
1.89
m W 8 9
1.16
9
vw m 8
m vw W
vw vw vw
0.76
3.09 2.73 2.67 2.49 2.28 2.06 1.83 1.64 1.58 1.54 1.53 1.48 1.40 1.36 1.30 1.24 1.13
14 25 25 8 35 12 18 2 16 8 14 4 6 4 4 2 1
004 100 101 102 103 006
105 106 110 008 112 107 114 200 108 0010 1010
t Figure 2. Isotherms of nitrogen at 77 and 87 K.
vw 0 Key: vs, very strong; s, strong; m, medium; w, weak; vw,very weak.
1 0.1
0.2
0.4
0,3
0.5
5.6
0.7
0.8
I
0.9
P,*D
Figure 3. Adsorption-desorption isotherms of nitrogen at 77 K. P
Figure 1.
experimentsrevealed an excellent degree of reproducibility in the volume values of gas adsorbed, corresponding to similar equilibrium pressures obtained in different seta of experiments. In this way it was possible to define each of the isotherms obtained with great precision. With respect to the equilibrium pressure, corrections of temperature, capillary pressure, dilation of the mercury, the brass scale, etc., were applied; these were also applied to the deviation due to the nonideal behavior of the gases adsorbed at the corresponding temperatures.1° On obtaining the data of the adsorption of nitrogen at the temperatures of 77 and 87 K, respectively, at relatively low pressures, the correction due to the thermomolecular diffusion phenomenon was taken into account, using the Liang equationll with the modifications introduced by Bennet and Tompkins.12 In all cases, the volume of gas adsorbed under normal conditions was plotted against the relative equilibrium pressure. Figure 2 shows the adsorption isotherms of nitrogen at temperatures of 77 and 87 K, represented on a double logarithmic plot to show the zones of lowest relative pressure with greater clarity. Figure 3 shows the hysteresis loop formed by the adsorption-desorption isotherms of nitrogen obtained at 77 K. Figure 4 shows (10)Gregg,S.I.; Sing, K. 5.W. Adsorption Surface Area andPorosity, 2nd ed.; Academic Press: New York, 1982. (11)Chu Lian S.J. Appl. Phys. 1961,22,148; J. Appl. Phys. 1963, 57,910;Con. J. &em. 1966,33,279. (12)Bennett, M.5.;TompkinB, F. C. Tram. Faraday SOC.1967,55, 186.
t
1 8
2 1 0 1 ,
I
I
i
I
I
I
I
4
8 1 0 2
1
5
1111
>
Figure 4. Adsorption isotherms of n-butane at 236, 253, and 273 K.
the adsorption isotherms of n-butane obtained at temperatures of 273, 253, and 236 K, respectively. 4. Discussion
The sample of tungsten disulfide was analyzed by X-ray applying the Debye-Scherrer technique (Table I); this provided the corresponding diffractogram from whose study the spacing and relative intensities were obtained, together with a value of the spacing corresponding to the (0,0,2) line after determining the Nelson-Riley function. These were very similar to those reported in the literature for this compound. Accordingly, the adsorbent may be considered to have an extremely high degree of crystallinity. The results obtained in the granulometry study are shown in Figure 1. The histograms of frequency and
Langmuir, Vol. 7, No. 8, 1991 1781
Adsorption onto Tungsten Disulfide accumulated frequencyin volume, expressed in percentage against particle diameter in micrometers, point to the elevated degree of uniformity in the particle size distribution and the small dispersion with respect to the mean value. It can also be seen that the greatest contribution to the volume of the sample can be attributed to particles whose diameters lie within the 5.5 and 10.9 bm range. The granulometric analysis shows that tungsten disulfide has a sufficiently small particle size and values so close to the means that previous sieving of the adsorbent is unnecessary. From Figure 2, showing the adsorption isotherms of nitrogen at 77 and 87 K on a double logarithmic plot, it is easy to infer that the isotherms belong to type I1 of the BDDT classification, corresponding to processes of physical adsorption of gases onto nonporous or macroporous s01ids.I~The values calculated for the C parameter of the BET equation were 77 and 59, respectively, thus confirming that they clearly belong to type I1 of this classification. The hysteresis loop in the adsorption-desorption isotherm of nitrogen, Figure 3,belonging to type A according to the classification of de Boer,14 indicates the existence of cylindrical pores open at both ends. It should be noted that the adsorption isotherms of nitrogen onto tungsten disulfide at temperatures of 77 and 87 K (Figure 2) show a crossover point among each other when the relative pressure is 1.1 X W,corresponding to a volume of nitrogen adsorbed (STP)of 0.97mL&, which is almost the same as the volume adsorbed onto the first adsorption monolayer calculated by other methods: B point, BET, L6pez-Gonzhlez (L-G).'"" This kind of behavior when the adsorption isotherms are obtained at different temperatures, also reported in other research works,lgZOwas used in the present study to determine the volume of nitrogen adsorbed onto the first adsorption monolayer. When gas pressure approaches saturation pressure, volume increases rapidly and tends toward an asymptotic value as a result of its condensation onto the surface of the adsorbent. This is followed by the process of desorption, which differs from that of adsorption,l0thus leading to the phenomenon of hysteresis, as shown in Figure 3. The values calculated for the C parameter of the BET equation for the adsorption isotherms of n-butane were 38for the isotherm obtained at 236 K, 36 for that obtained at 253 K, and 33 for that obtained at 273 K; thus, these isotherms clearly belong to type I1 of the BDDT classification. Joint plotting of the adsorption isotherms of n-butane on a double logarithmic plot is shown in Figure 4. In the zone corresponding to low relative pressures, a first crossing-point of the isotherms is seen at arelative pressure of approximately 9 X 10-2,corresponding to an adsorbed volume of n-butane of 0.17 mL0g-I (STP), which is equivalent to a fraction coverage of 0.4. This phenomenon has been described by other authors for the case of adsorption isotherms of n-butane onto graphite oxidez3 and nitrogen on an "Agot" grade nuclear graphiteSz2 (13) Sing, K. S. W. Atre Appl. Chem. 1982,54, 2201-18. (14) De Boer, J. H.; Lippene, B. C. J. Catal. 1965,3, 38. (15) Emmett, P. H.; Brunauer, S. J. Am. Chem. SOC.1937,59,1553. (16) Brunauer, 5.;Emmet, P. H.; Teller, E. J. Am. Chem. SOC.1938, 60,309. (17) Upez-GonzAlez,J. de D. An. R.SOC.Eap. Fis. Quim., Ser. B 1956, R.52. 287. I Ldpez-GonzAlez,J. de D.; Carpenter, F. G.;Deitz, J. R. J. Phya.
It therefore seems that for absolute gas pressures above the value at which cross-over points become visible in the isotherms, a different adsorption mode begins, as shown by the change in slope observed in their plot. The change in slope correspondingto this crossover point is, relatively, very pronounced in the isotherm obtained at the lowest temperature; it later becomes less pronounced as the temperature employed for determining the isotherms is increased. The gradual decrease in slope value as the temperature for determining the isotherms increases could be interpreted as being due to the higher statistical distribution of the adsorbed molecules, possibly owing to their greater kinetic energy. When relative pressure reaches approximately 2 X a change occurs in the slope value; this is fairly pronounced in the case of the isotherm obtained at 236 K but slightly lower in the isotherm obtained at 253 K, corresponding to surface coverage fractions of 0.62and 0.57,respectively. In the case of the isotherm obtained at 273 K, the variation in the value of the slope is practically constant until relative pressures of around 10-l are reached. Similar changes in slope value, at similar coverage fractions, have been reported above when discussing the adsorption of nitrogen onto tungsten disulfide, although it should be noted that such changes in the slope value, in the case of nitrogen, occur at slightly larger coverage fractions than when n-butane is adsorbed, probably due to lateral interactions among the molecules of gas adsorbed, that would logically be larger in the case of n-butane in view of the particularly long shape of the molecule. In the joint representation of the three n-butane isotherms, it is also possible to observe the presence of a second crossing point, common to all, at a relative pressure of 1.4 X lO-l,which corresponds to an adsorption of n-butane of 0.46 mL.g-l (STP). This value is very close to that of the volume adsorbed onto the first adsorption monolayer. Similar behavior was also observed for the adsorption isotherms of nitrogen at temperatures of 77 and 87 K onto the same adsorbent and has also been mentioned by other authors, thus confirming that using the isotherms crossover method it is possible to determine the value of the volume of the first adsorption monolayer. Table I1 shows the values of molecular areas16t21*24 and the volume of the adsorption monolayer of nitrogen and n-butane at the different working temperatures determined by the point B, BET, L-G, and crossover point methods. The table also shows the values of the specific surface area of tungsten disulfide calculated from the corresponding monolayer volumes. The results obtained both regarding the monolayer volume and the specific surface area are in excellent agreement, which shows that the methods employed for their determination are quite suitable for this kind of adsorbent. Figure 5 shows the isosteric heats of adsorption, qat (addition of the differential heat of adsorption, q, and the latent heat of condensation, I), of n-butane and nitrogen, respectively, with respect to the fraction coverage (the values of qet greater than I mean that the energy of adsorption is greater than the energy of condensation of the liquid). It can be seen that the isosteric heat of the adsorption of n-butane displays an absolute maximum for a 6 value of 0.6,while in the case of the adsorption of nitrogen the maximum occurs at a fraction coverage of 0.8. A similar finding has been reported by other authorsz6 (21) Mc Clellan, A. L.; Harn Sberger, H. F. J. Colloid Interface Sci.
1967, 23, 517.
(22) Bafiares-Munoz, M. A.; Flores Gonzilez, L. V.; Martin Llorente, J. M. Carbon 1987,25,603-608.
BaAaree-Muitoz and Sanchez Escribano
1782 Langmuir, Vol. 7, No.8, 1991
Table 11. Molecular Areas (Aa), Monolayer Volumes (cma.q*,STP),and Values of Surface Areas (m'&) of WSr Determined from the Adsorption Isotherms of Nitrogen and r-Butene at Several Temperatures gas T,K area, A2 V,, cmg.g-1 S, mz.g-1 method nitrogen 77 16.2 0.93 4.05 point B 77 77
16.2 16.2 16.2 17.0 17.0 17.0 17.0 31.0 31.0 31.0 31.0 31.5 31.5 31.5 31.5 32.2 32.2 32.2 32.2
77
n-butane
0.5
87 87 87 87 236 236 236 236 253 253 253 253 273 273 273 273
0.94 0.98 0.96 0.94 0.96 0.97 0.96 0.47 0.46 0.48 0.46 0.47 0.47 0.49 0.46 0.47 0.46 0.48 0.46
BET
4.09 4.30 4.28 4.09 4.38 4.43 4.28 3.91 3.83 4.00 3.83 3.98 3.98 4.15 3.89 4.15 4.02 4.15 3.82
L-G crossing point
point B BET
L-G crossing point point B
BET L-G crossing point point B
BET L-G crossing point point
B
BET L-G crossing point
t I
0.5
1.c
1.5
R =VN,
Figure 1. Isosteric heat of adsorption, q,t, of n-butane (a) and nitrogen (b),onto WSZ,at 253 and 82 K,respectively. upon adsorbing n-butane onto an ultrapure mineralogical graphite. Maximum values for the isosteric heat of adsorption of 0.9 have also been found in the following cases: the adsorption of argon onto ultrapure mineralogical graphite;ls nitrogen and argon onto a graphite of mineral originm and onto an "Agot" grade nuclear gra~hite.~' The generally accepted interpretation of different authors who have found a maximum value of the isosteric (23) Lbpez-Gondez, J. de D.; BaAarw-Mufloz,M. A.; Ramirez Saenz, A. Quim. Ind. 1970,16,3-7. (24)Handbook of Chemistry and Physics, 66th ed.; C.R.C. Press: Cleveland, OH,1978. (25) Mpez-Gonzblez,J. de D.; Bafiares-Mufloz,M. A. An. Quim. 1964, 60B, 771.
0.5
1.0
!.5
20
R=V"
Figure 6. Molar entropy of adsorption of n-butane (a) and nitrogen (b) onto WSz, at 253 and 82 K,respectively. heat of adsorption close to a coverage fraction of 0.9 is that at this value of covered surface, close to the formation of the monolayer, lateral interactions occur among the molecules adsorbed, the maximum value in the heat of adsorption appearing before the first adsorption monolayer has been completely formed. In the present case, the appearance of the maximum values at relatively low fraction coverages (0 = 0.6) can also be attributed to lateral interactions among the n-butane molecules adsorbed, due to their particularly elongated shape, greater than in the case of the molecules of argon and nitrogen. After the maximum value in the isosteric heat of adsorption of n-butane and nitrogen has been reached, a sharp drop occurs in this value down to 0 = 1,corresponding to the complete formation of the first adsorption monolayer. Ita corresponding value on the ordinate axis is of the same order of magnitude as the latent heat of condensationof n-butane and nitrogen, respectively. This finding is in accordance with the BET theorylO in which it is considered that the adsorption heat should decrease rapidly, from its maximum value, until it becomes equal to the latent heat of condensation of the gas being adsorbed, at the point where the first monolayer adsorbed has been completed. The deviation in the experimental values that may occur may be due to the heterogeneity of the adsorbent or at the start of the formation of the second monolayer before the first monolayer has been completed. From the existence of crossing points between the adsorption isotherms of n-butane at 253 and 273 K and nitrogen at 77 and 87 K, it can be inferred that the isosteric heat of adsorption, obtained from the Clausius-Clapeyron equation, correspondingto these values of relative pressure, must be equal to zero. After this crossing point, the adsorption heats become negative since for the same volume of gas adsorbed, the relative pressure of the isotherm obtained at a higher temperature is lower than that obtained at lower temperature. Figure 5 shows that the values of the adsorption heats of n-butane and nitrogen onto WSz become lower than the latent heat of condensation, of the corresponding gas, starting from a fraction coverage, greater than unity for nitrogen and in the coverage ranges 0 . 4 and 1-2 for n-butane. These values of the adsorption heat, lower than the latent heat, show that once the first adsorption monolayer has been formed, the molecules that become emplaced in the second monolayer have a lower adsorption heat than the condensation heat of the adsorbate. The way in which this second layer of molecules is adsorbed means that the molecules are in a thermodynamic state that is intermediate between the state corresponding to the gaseous phase and the liquid phase in equilibrium with ita vapor.
Adsorption onto Tungsten Disulfide Figure 6 shows the isosteric entropies of adsorption of n-butane and nitrogen onto WS2 with respect to the fraction coverage,8. The entropy of adsorption of nitrogen gradually decreases until a 8 value of 0.5 is reach&, a shoulder at a fraction coverage of 0.4 can also be observed. Following this, a fairly pronounced decrease occurs until a minimum entropy value for a fraction of surface covered 8 = 0.9 is reached. From this point onward, close to the complete formation of the first adsorption monolayer, the entropy value increases parallel to the increase in the fraction coverage since the molecules have a greater degree of freedom. Similar minima, found before that corresponding to the formation of the f i i t adsorption monolayer, have been discussed in several works on adsorption onto graphite surfaces.2s*n In our case, this slow decreaae in entropy down to a value of 8 = 0.5 together with the (26) Mpez-Gonzlllez, J. de. D.;*ez Reinom, F.;Bailam-Mufioz, M. A.; Moreno Caetilla, C. An. Qurm. 1976,72,643. (27) BeAarea-Maoz,M. A.;Flores Go~uAlez,L.V.;Martin Llorente, J. M.Carbon 1988,26, 206211. (28)Beebe, R. A.; Young, D.M. J. Phys. Chem. ISM, 69,93. (29) Mpse-Godez, J. de D.;w e z Reinom, F.;Baftaree-Mu502, M. A.; Linarsa Solano, A. An. Qurm. 1976,72,673.
Langmuir, Vol. 7, No.8, 1991 1783 existence of a slight relative minimum at 8 = 0.4 can be attributed to the formation of a layer of gas adsorbed onto the more imperfect and active surface of WS2. In the variation of the values of the adsorption entropy of n-butane onto WS2, it can be seen that such values decrease very rapidly as the fraction of covered surface increases, until aminimum for a value of 8 = 0.7is reached. Later, the values increase parallel to the increase in the fraction coverage because as from the first adsorption monolayer the molecules have a greater degree of freedom. The shapeof these curves is qualitativelysimilar to those corresponding to processes of adsorption of n-butane and nitrogen onto a graphite sample.%p However, the entropy values throughout the process should not be overlooked. In this sense, it was observed that in the coverage ranges, 0 . 5 5 and 0.75-2.5 for butene and 0 . 5 0 and 0.90-2.5 for nitrogen, the molar entropy, of both gases, is greater than the molar entropy of the corresponding liquid. Accordingly, the molecules adsorbed in the second monolayer can be considered to be in a thermodynamic state that is intermediate between that corresponding to the gas and liquid in equilibrium with its vapor.
