The adsorption of water vapour on a microporous carbon black

15

Powder Technclogy, 7 (1973) 15-19 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

The Adsorption S. J. GREGG,

of Water

S. NASHED*

Vapour

on a Microporous

Carbon

Black

and M. T. MALIKe

School of Chemistry, Btunel University, Kbtgston Lane, Uxbridge, MiaWese_x (Gt. Britakz) (Received March

1, 1972)

Swnmary Tile adsorption

of water vapour

on a microporous

carbon

black has been studied by measurement at 2PC of (a) the a&orption isotherm, (b) the heat of immersion in water of samples charged with dijJerent amounts of water vapour. The two sets of measurements were carried out on the carbon black both before and after the micropores had been. filled with n-nonane vapour. It was thus possible, in effect, to isolate the adsorptive behaviour of the micropores. As expected. the heat of aa5orption in the micropores was greater than on an open surface, but the isotherm of adsorption in the micropores remained convex to thepressure axis in the low pressure region; the carbon black remained “‘hydrophobic” despite the enhancement of the a&orption field within the micropores.

INTRODUCTION

It is widely recognised that in micropores, i.e. pores which do not exceed a few molecular diameters in width, the force of interaction of adsorbate molecules with the solid is greater than it is on an open surface. of the same substance’-4_ The exact value of the width w at which the effect first becomes appreciable varies from one adsorbate to another according to molecular size and polarisability, but for many common vapours such as nitrogen is around 20 A. The greater interaction manifests itself in an enhanced heat of adsorption and correspondingly if the isotherm on the open surface is of Type II or Type IV (BDDT classification5) in a very steep rise from the origin: the isotherm has a * Present address: Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt. * Present address: Government Superior Science College, M&an, West Pakistan.

sharp “knee”, or alternatively expressed, it becomes more concave to the pressure axis in the low pressure region. If on the open surface it is of Type III, i.e. is convex to the pressure axis, then the heat of adsorption is close to the latent heat of condensation of the vapour ; and one might expect the isotherm to become less ccnvex, or even actually concave, to the pressure axis when the vapour is adsorbed on a microporous sample of the same substance. An example of this effect is provided by carbon tetrachloride adsorbed on silica6. On a flame-produced silica composed of impervious particles, or on a mesoporous silica gel, the isotherm is of Type Ii1 (for mesopores 20 A < WC 500 A), but on a microporous silica it is of Type I (Langmuir typej. Now it is well known that the isotherm of water vapour on non-porous carbon is of Type III, the heat of condensation being close to the latent heat of condensation; this is because the interaction of water molecules with a carbon surface involves only dispersion forces, except on those parts of the surface where polar groups, notably chemisorbed OH or 0, are present. To these active sites water molecules become attached by hydrogen bonds, and the characteristic tetrahedral structure of water, or something like it, can spread out from these nuclei by a cooperative process of hydrogen bonding. Accordingly it was thought of interest to discover whether in the carbon-water system the presence of micropores would cause a changeover from a convex to a concave isotherm, i.e. from hydrophobic to hydrophilic behaviour. An indication that this might be so had been obtained earlier with a graphitised carbon, where with increasing “burn-off in air, which presumab!y induced micropores, the water vapour isotherm changed from a linear to a concave form’. This carbon, however, was somewhat ill-defined, having been made by graphitisation at 3OOb”C of a mixture of coke particles with a pitch binder. It was therefore decided to carry out a more

i6 systematic study using a rather better-defined absorbent. uiz. a carbon black in which the presence of micropores had been explicitly demonstrated by the method of tilling them with nonanes. The adsorption isotherm, as well as the heat of adsorption, of water vapour on the carbon, was determined both before and after its micropores had been filled with ?I-nonane. It was thus possible, in effect, to add or remove the micropores at will and thus to establish their influence on the adsorptive behaviour. The heat of adsorption AH, was determined indirectly by measuring the heat of immersion g, in liquid water of samples of carbon charged from the vapour phase with different amounts 1x2of adsorbed water. From the slope, at a given value of 11%of the curve of qw against nz, one can calculate immediately9 the corresponding value of the net heat of adsorption (AH, - L) (L=latent heat of condensation).

The isotherms of water vapour were determined with the aid of a recording vacuum microbalance (Cdhn RG electrobalance). The balance was connected to a dosing system, to a mercury manometer and a transducer for measuring the equilibrium pressure of water vapour, and to a manifold for precharging ampoules with water or nonane vapour. Successive doses of water vapour were introduced to the sample on the balance in order to determine points on the adsorption branch of the isotherm, and doses were correspondingly withdrawn by pumping to obtain points on the desorption branch. For pre-adsorption of nonane, the technique was slightly modified from that described earlier’, in that the outgassed sample was held at room temperature rather than at 77X while it was open to the reservoir of liquid nonane, likewise at room temperature; after a period of 30 minutes the weight became constant, and the sample was then opened to the pumps until (after 5 hours) the rate of loss of weight fell below 0.1 mg/h. In order to charge the samples with water vapour in preparation for the heat of immersion measurements, the manifold already referred to was used; it had six outlets, three for attaching standard ampoules through O-ring joints, two for attaching reservoirs containing liquid water and liquid nonane respectively, and one to serve as a receiver when these liquids were subjected to repeated redistillation

S. J. GREGG,

S. NASHED,

MmT. MALIK

to free them from dissolved gases. Three ampoules, each containing a weighed amount of carbon black, were outgassed together, and for runs with water vapour only, one ampoule was then sealed off and weighed. The dry weight of the sample was obtained from the difference in weights before and after outgassing and was used to calculate by simple proportion the dry weight of the other two samples. The samples were then charged with different amounts of water vapour at definite vapour pressures, by maintaining the water reservoir in succession at different controlled temperatures. For the runs with samples pre-charged with nonane three samples were again outgassed together, and one sample sealed off and weighed to obtain the dry weight, just as before. The remaining samples were then charged with nonane in the manner already described, and one sample was sealed off and weighed to obtain the weight of adsorbed nonane. The remaining sample was then exposed to a controlled pressure of water vapour; it was sealed off and weighed to obtain the total weight of nonane plus water, so that the weight of adsorbed water vapour could be obtained by difference. Determination of the heat of immersion in water was carried out with a simple diathermal calorimeterr’ based on the design of Zettlemoyer and his co-workers”. It consisted of a Dewar flask of glass with a lid of tufnol which carried supports for an ampoule holder, a thermister (FX(F)), a heater coil (k 10 ohms) for calibration, and a stirrer driven by a synchronous motor at 150 r-p-m. The stirrer incorporated a device for breaking the ampoule. The ampoules, of a standard shape, size and thickness, were blown in a specially constructed mould of graphiteI’. The heat of bulb breakage was 0.1 cal. The thermistor was connected into a simple Wheatstone bridge arrangement and the out-ofbalance current was registered directly as a deflection of the sensitive galvanometer (H. Tinsley, Type SR4,. sensitivity-, 485 mm/d). While less sensitive than a null method, this arrangement permits of very rapid readings, a facility which contributes significantly to the overall precision attainable in practice. After a bulb breakage the temperature continued to rise due to liberation of heat of immersion for around two minutes and then settled down again to the slow rate of rise produced by the stirring. Thermal calibrations, which preceded and followed the bulk breakage, agreed within 22%.

WATER

ADSORPTION

ON CARBON

17

BLACK

Materials The carbon black (“Mogul”, from the Cabot Corporation) was stirred in air at 500°C in order to create micropores. Both the n-nonane (from Phillips Petroleum Co., U.S.A.) and the water (de-ionised) we1.e freed from dissolved gases by repeated distillation in caczm using themanifold and reservoirs already referred to.

RESULTS

the isotherm of adsorption within the micropores themselves. If for the moment the peculiar form of its hysteresis loop be ignored, isotherm C has the shape (Type V) typical of the adsorption of water vapour on charcoals or on carbons of negligible external

area:

a

characteristic

feature

of

such

AND DISCUSSION

The isotherms of water vapour (Fig. 1) obtained before (B) and after (A) filling the micropores w?th nonane, both exhibit hysteresis, the loop of A being more regular in shape and smaller than that of B.

0

40 Prnount

I

80 adsorbed /mg

120 g-’

--is-

Fi_e 2 Heat of immersion q_ in water at 25°C of Mogul carbon black pm-charged with different amounts of water vapour. For curve A the micropores had been previously tilled. and for curve I3 they had not been so filled. with r,-nonane.

Relative

pressure

Fip 1. Adsorption isotherms of water vapour at 79C on oxidised Mogul carbon black, (A) after, (B) before, filling ofthe micropores with n-nonane. Isotherm C, which was obtained by subtractionof ordinates of A from the corresponding ones of B, represents

adsorption 0. 0, -a-,

in the micropores desorption.

only.

0.

0. --,

adsorption;

Isotheml A, which represents adsorp:ion on the outer surface of the carbon particles, is strikingly similar 53 that reported by Walker and Janovic13 for the adsorption of water vapour on graphon which had been oxidisecl in air to 38% burn-o& Since isotherm B represents adsorption both on the outer surface and within micropores, isotherm C, obtained by subtracting the ordinate values of A from the corresponding ones of B, may be taken as

0’

0.2

I

Relet-we

1

0.4 press”re

1

0.6

0.6

Fig. 3. Differential net heat of adsorption (AH, - L.), calculated from Fig. 2, of water vapour as a function OSthe relative pressure p/p0 at which samples had been pre-charged with water vapourFor curve A the micropores had been previously filled. and for curve B they had not been so filled with n-nonane.

13 isothelrms is the upward sweep, which in the present case commences ar&md 0.2-0.3 pO. The curves of the heat of immersion qw against the mass adsorbed are given. in Fig. 2, and the derived curves of the net heat of adsorption appear in Fig. 3: this quantity being plotted not against m but, following Kiselev, against the relative pressure p/p,, (p=equilibrium pressure of vapour, pO= saturated vapour pressure). As is seen, the net heat of adsorption is not greatly in excess of zero for the nonane-filled sample; this result is to be expected for adsorption of water vapour on a non-porous carbon, and accords with the Type III character of the isotherm. It corresponds to adsorption on the exterior surface of the carbon black. For the sample which is free of nonane, the net heat of adsorption is relatively large at low coverages, but falls sharply in the region corresponding to the upward sweep in isotherms B and C. Following the accepted explanation of the isotherms of water vapour on carbon14-‘5, the early part of the isotherms (e.g. ab of curve B) represents the primary adsorption on tire active sites already referred to, and the upward sweep (e.g. bc of curve B) the growth and coalescence of the clusters nucleated by these sites. Thus for the micropores the heat of adsorption is much higher in the region corresponding to adsorption on active sites than it is on the corresponding sites on an open surface; but once the cooperative process of the growth and merging of clusters has begun, the enhancement of the heat of adsorption within micropores is not very marked. We are led therefore to the conclusion that, within micropores, the bonding ofwater molecules to active sites is considerably stronger than it is to sites on an open surface. A reasonable explanation is that part of the micropore system consists of slit-like constrictions having a width around 2~ (o=diameter of a water molecule), so that a molecule of water is bound not only to an active site but also, by further hydrogen bonding to a molecule on the opposite wall. It is significant, however, that the higher heat of adsorption along branch cd does not lead to a distortion, in the sense of a relative enhancement in adsorption at low pressures, such as is found to accompany an increased heat of adsorption in the case of Type II isotherms3*lo_ Thus the ratios-& the ordinates of isotherms B and A respectively are 1.85, 1.952.05 and 2.09 for p/p0 values of 0_04,0.08, 0.12 and 0.16 respectively; such enhancement as does exist is, indeed, found at higher rather than

S. J. GREGG,

S. NASHED,

M. T. MALIK

lower pressures. The absence of the expected distortion implies that the higher heat of adsorption in this region is compensated by a lower entropy, corresponding to a more structured arrangement, of the water in these narrow micropores (c$ Table 1). T4BLE

1

Values of entropy change AS,(=AH,IT--K In p/p,,) for adsorption of water vapour when the micropores are (i) accessible (isotkrm B), (ii) not accessible (isotherm A) to water vapour

P/PO

AS. (isotherm B) AS. (isotherm A)

0.1

0.2

0.4

0.6

-6.3 +2.6

-25 + 1.4

-1.5 + 0.6

-1.2 +0.2

Thus, even in this microporous carbon, water remains an abnormal adsorbate, and the overlap of adsorption fields from opposite walls is not sufficiently intense to produce a concavity in the isotherm. It now remains to discuss hysteresis (Fig. 1). The isotherms of nitrogen .on two other samples of the b!ack which had been oxidised in a closely similar manner did not show hysteresis loops, but only a slight hysteresis which extended over the whole range of the isotherm. (Unfortunately, owing to lack of time, nitrogen isotherms could not be measured on exactly the same batch of materiaI as that employed in the present experiments with water.) It is therefore somewhat unlikely that the hysteresis in either of the isotherms A and B originates in a classical capillary condensation in mesopores. A possible alternative explanation is that the mechanism of growth and coalescence of clusters is not exactly reversible, so that the chemical potential for a given uptake on the adsorption branch of the loop is different from that on the desorption branch. CONCLUSION

The presence of micropores in the oxidised carbon black used in the present study leads to a marked increase in the heat of adsorption of water vapour in the region near the origin of the isotherm, but it does not bring about the expected change from Type III to Type II or I character (i.e. from being convex, to being concave, to the pressure axis) in this region. Since a change of the expected kind does occur with the graphitised coke referred to in the

WATER

ADSORPTION

ON CARBON

19

BLACK

Introduction, it would seem that the micropores present in the oxidised coke graphite are even finer than those in the oxidised carbcn black. Presumably the divergence in behaviour of the two materials on oxidation reflects a difference in morphology resulting from their difference in origin.

ACKNOWLEDGEMENTS

Cordial thanks for the grant of study leave are offered by one of us (S-N) to the Ain Shams University, Cairo, and by another (M-T-M.) to the Government of West Pakistan_ We are grateful to Dr. W. R Smith for the supply of carbon black.

REFERENCES 1 S J- Gregg and K. S. W. Sing Adsorption, Surface Area and Porosity, Academic Press, London, 1967, Chap. 4.

2 M. M. Dubinin, Quart_ Rev, London, 9 (1955) 101. 3 M. M. Dubinin and E. E Polstyanov, Russ. J. Phys. Chem, 40 (1966) 631. 4 k V. Kiselev, TheStructure and Properties ofPorous Materials,

Butterworths, London, 1958, p. 195. 5 S. Bronauer, I_ S. Den&g, W. S. Deming and E. Teller. J. Am Chem. Sm., 62 (1940) 1723. 6 P. A Cutting PhD. thesis, Brunei University, 1970. 7 S. J. Gregg, F. M. W. Olds and R F. S. Tyson, Third Conference on Industrial Carbon and Graphite, Academic Press. London, 1970. 8 S. J. Gregg and J. F. Langford, Trans. Farodoy Sot, 65 (1969) 1394. 9 A. C. Zettlemoyer and K. S. Narayan, in E. A. Flood (ea.), The Solid-Gas Interfnce. Vol. 1, Marcel Dekker, New York. 1966.

10 W. Diano, Ph.D. thesis, Exeter University, 1969. 11 A_ C Zettlemoyer, G. J.Young J. J. Chessick and F. H. Healey. J. Phys. Chem, 57 (1953) 649. 12 S. Sunner and I. Wadsy Acta Chem Scond., 13 (1958) 97. 13 P. L. Walker and J. Janovic, J. CoIZoid Interface SC& 28 (1968) 449. 14 A. V. Kiselev, Proc Second Inrem Congr_ Surfzce Activity,

Vol. 2, Butterworth& London, 1957, p. 218. 54 (1950) 784. 15 C Pierce and R N. Smith, J. Phys.