INFLUENCE OF SU33STRATE STRUCTURE ON ADSORPTION
mental spectrum, and, the spurious peaks produced should not be confused with the real bands. A notable feature of the analysis is the rather large moduli of the cross-ring coupling constants; noninterconverting tetrasubstituted cyclobutanes6have a modulus of about 1 cps for this type of coupling. The constants are remaining in signs and magnitudes.
1557
Acknowledgments. We are indebted to the Science Research Council for providing an equipment grant. Also we wish to thank Mr. D. Collens of the University Computer Laboratory for his help.
(6)
C. H. Krauch, 5. Farid, and G . 0.Schenck, C h a . Ber., 99, 626
(1966).
The Influence of Substrate Structure on Adsorption. 11. Nitrogen and Benzene Adsorption on Characterized Silicas
by James W.Whalen Research Department, Field Research Laboratory, Mobil 02 Cbrporatwn, Dallas, Tezas (Received January 86,1966)
Precise gravimetric data extending down to 10" PIP0 have been obtained for nitrogen and benzene on two silica surfaces previously characterized by other techniques. When subjected to a BET treatment, such data provide a needed verification of constancy of nitrogen occupancy areas on surfaces of variable chemical composition. Relatively minor apparent surface area dependence on structure was found for nitrogen. Benzene does not form complete monolayers or statistically equivalent multilayers in the region of BET applicability. Interaction energy distributions derived for the adsorption processes occurring on representative surface structural states reflect interactions with oxide and hydroxyl surface domains. Variations in the form of the distribution function and the site interaction energies are consistent with surface structures and with the specific adsorbate interaction.
The Brunauer-EmmettrTeller (BET) isotherm equation in two-constant form
applied to the determination of surface area from nitrogen adsorption data has provided an indispensable basis for comparison of the surface properties of different substances in various states of subdivision and bulk structure. In such application the volume of adsorbed gas, V,, equivalent to monolayer coverage,
must be independent of the detailed structure of the surface. Variation in C (=exp(El - E L ) / R T )owing to variation in average first-layer adsorbate-adsorbent interaction energy (El) with surface structure must be accompanied by appropriate modification in the shape of the isotherm, ie., in the P - V , dependence. In particular it is required' that at monolayer coverage (J'alvrn = 1)
Volume 71 Number 6 May 1967 ~
1558
First-layer interaction energies for structural variations in the solid surface should be reflected both in C value variation, as obtained from the linear BET plot, and in the location of regions of linearity including the monolayer. In cases where surface structure can be modified without concomitant surface area changes, V , should be constant if the BET method is to serve as a basis for the calculation of surface area and constant cross section is to be assigned to the adsorbate molecule. On the basis of assumed constancy for the coverage assigned the nitrogen molecule, it is widely recognized2 that other adsorbates reflect variable coverage on different surfaces. Various authorssg4 have assigned real significance to apparent surface area changes of 10% or less, based on nitrogen BET treatment, in cases where the chemical nature of the adsorbent is known to be variable under the experimental conditions. The alternative explanation, that a t least a part of the discrepancy may be due to variability in nitrogen coverage or effective cross-sectional area, has not been given serious consideration. The P V , relationship for nitrogen adsorption is known to be dependent on the nature of the adsorbent surface as demonstrated by adsorption on silicas in various hydration state^,^ adsorption on presorbed layers on benzene or methyl alcohol,6 water,3 and on other surfaces of widely different structures.' There is, however, a paucity of data from which the constancy of Vm can be adequately demonstrated for variable C and fixed surface area and there are no experimental studies in the literature which demonstrate that surface areas derived from the BET treatment of nitrogen adsorption are independent of substrate structure for variable structure surfaces of constant area. In view of the importance of surface area determination to essentially all generalization in adsorbateadsorbent interaction studies and the rather widespread tendency to treat most adsorption systems within the BET framework, it is of interest to consider the cornpliance of the BET for both weakly and strongly interacting adsorbates on variable structure surfaces Of Constant area. The silica surface, known to contain oxide sites, both interacting and isolated hydroxyl groups>and n'olecular water, is particularly to such adsorbent requirements. Several structurally stable silica materials have been studied by methods which characterize the surfaces with respect to fine structure and energy.8-10 The adsorption of nitrogen and benzene on two such materials is reported. Experimental Section The silica materials, supplied by Mallinckrodt Chemical Works, are precipitated gels of high purity The Journal of Physical Chemistry
JAMESW . WHALEN
and were used as received except for outgassing. Previous designationse of Silica SL for Silicic Acid Special Luminescent and Silica SB for Silicic Acid Special Bulky are used in this discussion. The samples were outgassed in situ a t a selected temperature between 110 and 400" for 24 hr after the residual pressure in the adsorption system reached torr. Immersion heat work under these conditions has established surface structure reproducibility. The adsorption measurements were carried out in the greaseless McBain-Bakr quartz spring system utilized in prior work." Oil and mercury manometers were isolated from the adsorption system by a null pressure capacitance device. The oil manometer covered a pressure range equivalent to 25 mm; €he mercury manometer was used at higher pressures. Quartz springs with a 250-mg load limit and sensitivities of 1.125- and 1.065mm elongation/mg were employed. Reading accuracy was mm in spring length. Pressure measurement accuracy was mm in the lowpressure region and 1 2 X mm at pressures above 25 mm. Nitrogen was purified over a copper wire screen a t 450" followed by condensable trapping a t 77°K. Spectroscopic grade benzene was dried over Linde 5A Molecular Sieves activated a t 300" and distilled under vacuum into the adsorption reservoir. Benzene adsorption data were obtained at 25 f 0.05". Nitrogen adsorption was carried out a t 77"K, the liquid nitrogen temperature being monitored by oxygen gas thermometer. Results In light of past experience suggesting unusually restricted BET ranges for these materials, the nitrogen 331 (1958). (1)L. Meyer, z, physik. Chem. (Frankfurt), (2) F. Rouauerol. J. Rouauerol. and B. Imelik. Bull. Soc. Chim. Fiance, 636 i1964); R. T. Davis,'T. W. de Witt, and P. H. Emmett, J . Phys. Colloid Chem., 51, 1232 (1947); J. A. Singleton and G . D . ~ ~J . phys.l Chem., ~ 58, 330 ~ (1954). ~ , (3) w. H. Wade, ibid., 68, 1029 (1964). (4) A. W. Adamson, I. Ling, and S. K. Datta, Advances in Chemistry Series, No. 33,American Chemical Society, Washington, D. C., 1961, p 62. (5) M. R. Harris and K. 5. W. Sing, Chem, Ind. (London), 487 (1959). (6) A. I. Sarakov, M. M. Dubinin, and Y. F. Berezkiva, I t v . Akad. Nauk SSSR, Ser. Khim., 1165 (1963). (7) P. C. Carman and F . A. Raal, Tram. Faraday SOC.,49, 1465 (1953). (8) R. S. McDonald, J . Phys. Chem., 6 2 , 1168 (1958). (9) J. W.Whalen, Advances in Chemistry Series, No. 33, American Chemical Society, Washington, D. C., 1961,p 281. (10) S.Brunauer, D. L. Kantro, and C. H. Weise, Can. J . Chem., 34, 1483 (1956). .
(11) J. w. Whalen, J . PILUS. Chem., 65, 1676 (1961).
INFLUENCE OF SUBSTRATE STRUCTURE ON ADSORPTION
110
r
The relative pressure range for which rigorous linearity was observed in these studies for the adsorption of nitrogen on Silica SB was 0.009-0.06 PIPo. On Silica SL the range was somewhat greater, extending from 0.005 to 0.075 PIPo. The surface areas and average interaction energies are given in Table I. Little variation in either parameter was encountered for Silica SB. The surface area variation for Silica SL is significant, ranging from 593 to 646 m2/g with relatively constant interaction energies. The relative pressure values a t which adsorbed amounts correspond to the BET indicated monolayer values (Figures 1 and 2) emphasize the variation in isotherm character with surface structure. Typical BET plots are shown in Figure 3.
P/PD
Figure 1. Nitrogen adsorption on Silica SB. I.
*.
t
110. OUTGASSING
1
ZOQ.
'XI0
1559
Table I: BET Derived Surface Areas -Nitrogen Thermal treatment,
'
r4MP
"0
.02
D4
.06
.OB
.IO
.I2
.I\
.J6
O C
SilicaSB .Ib
.A
;.2
P/P,
Figure 2. Nitrogen adsorption on Silica SL.
investigation was carried out over the relative pressure to 0.2 as a function of surface hyrange PIP0 = dration for Silica SB and Silica SL. Adsorption data points are shown in Figures 1 and 2. For Silica SB (Figure 1) a significant decrease in the quantity adsorbed was obtained between 110 and 200" sample outgassing. The adsorption of nitrogen increases slightly following 300" thermal treatment and again following 400" treatment, almost coinciding in the latter case with the quantity adsorbed following 110" thermal treatment. For silica SL (Figure 2) there is no initial decrease in the quantity of adsorbed nitrogen as noted with Silica SB, but an increase over the 110-300" outgassing treatment range, corresponding to the behavior noted on the Silica SB material after removal of the molecular water. The final condensation between 300 and 400" is accompanied by an almost insignificant change in the amount of nitrogen adsorbed. Changes in shape of the adsorption isotherm are reflected as "crossing-over tendencies" for the 100-200 and 300400" surface preparations. The linear range in relative pressure for BET treatment was found to be quite restricted, resulting in surface area values slightly lower than those previously obtainedg*10in the higher, generalized range.
Silica SL
adsorptionE1 = Surface RT In C area, EL, m*/g kcal/mole
+
-Benzene
Surface area, m'/g
adsorptionEl = R T In C EL. kcal/mole
+
110 200 300 400
334 341 337 339
2.33 2.31 2.29 2.28
178 166
11.94 10.96
91
10.92
110 200 300 400
593 611 637 646
2.14 2.19 2.21 2.17
346 320
10.14 10.20
181
9.93
The adsorption of benzene on the same silica surfaces was carried out from relative pressures of approximately lo-* to near saturation. The adsorption-desorption curves are relatively uninformative in their entirety ; only the low-pressure data are shown in Figures 4 and I4r
P/
Po
Figure 3. Typical nitrogen BET plots.
Volume 71I Number 6 M a y 1967
JAMESW.WHALEN
1560
w
V-
E
I *'./
'!
.dZ
.04
.d,
d8
.I9
J2
A4
.:6
.;a'
.io
2 ;.
.$4
P/P,
Figure 4. Adsorption of benzene vapor at low pressures on Silica SB.
relative pressure range 0.04-0.24 with no significant variation in the range from sample to sample. The statistical monolayer, based on the nitrogen derived area and utilizing the 33-A2 coverage value for the benzene molecule, is not included in that range. Surface areas and average first-layer interaction energy values derived from the BET plots are summarized in Table I. The data are characterized by apparent decreases in surface area with increasing oxide surface structure and apparent surface area values which are quite low in comparison to the nitrogen derived values. Interaction energy values decrease with decreasing hydroxyl content and are, in general, two to four times the comparable values for the nitrogen-silica interaction. The relative pressure range over which BET model applicability should exist, on the basis of eq 2, is thus indicated to be even lower than that encountered for the nitrogen-silica interaction and far lower than the limits of the linear range actually encountered.
Discussion
L .A,
09
.02
.a4
.d8
.;a
.I: .A
.1'6
.A
A .A
24
P/ P
Figure 5. Adsorption of benzene vapor a t low pressure on Silica SL.
5 . The adsorption was reversible in this region, hysteresis being confined to relative pressures above 0.45. No significant variation in the form of the hysteresis loop was noted for the several sample preparations and direct comparison of the desorption curves for each data set showed only random point scatter well within experimental error. Utilizing a coverage of 33 A2/ molecule, as suggested by Livingston,12 three molecules per 100 A2 are required for monolayer coverage. On the basis of the nitrogen derived areas this coverage is not attained until the relative pressure exceeds 0.6. For Silica SI3 a low-pressure "limiting" adsorption value is attained at less than two adsorbed molecules per 100 A2. The adsorption of benzene by Silica SL is a more gradual function of pressure; no "limiting" adsorption at low pressure is indicated. For both materials benzene adsorption is a strongly decreasing function of the outgassing temperature. The 110' outgassed Silica SB sample containing bound water does not exhibit oxide character as was indicated by nitrogen adsorption (Figure 1). When the benzene adsorption data are subjected to BET treatment, linear plots are obtained within the The Journal of Physical Chemistry
The Silica SB surface has 2.2 water molecules and 7 hydroxyl groups per 100 A2 following 110" outgassing. Bound water is removed under 200" outgassing and, at higher temperatures, hydroxyl sites are condensed leaving a residual 0.4 isolated hydroxyl site a t 400" outga~sing.~Silica SL has no bound water; the 5.5 hydroxyl sites present after 110' outgassing are condensed over the 200-400" temperature range leaving a residual 0.4 isolated site.9 Nitrogen adsorption is seen from Figures 1 and 2 to show slight increases as the oxide content of the surface is increased. For Silica SB both the 110" outgassed surface containing bound water (associated in double hydrogen bonding with, and therefore shielding, hydroxyl sites) and the 400' outgassed surface exhibit maximum early stage adsorption. For Silica SL nitrogen adsorption increases throughout the range of hydroxyl group condensation. Regions of linearity in BET plots beginning a t very low relative pressures have been encountered in prior work for both nitrogen1$ and other adsorbates.14 Meyer' and others16 have demonstrated (eq 2) that such behavior is to be anticipated for high C value systems. The linear ranges encountered for nitrogen in this work (0.005-0.07 PIP,,) are in agreement with those demanded by considerations requiring applicability in the region encompassing the monolayer (Figures (12) H. K. Livingston, J. Colloid Sci., 4, 447 (1949). (13) D. S. McIver and P . H. Emmett, J . Phys. Chem., 60, 824 (1956). (14) M. L. Corrin, %%id., 59, 313 (1955). (15) G. L. Gains and P. Cannon, ibid., 64, 997 (1960).
INFLUENCE OF SUBSTRATE STRUCTURE ON ADSORPTION
1 and 2). Surface areas obtained in this range are slightly higher than those previously reportedsJO based on adsorption data in the higher generalized range. The BET refinement over the Langmuir equation is superfluous a t the low pressures over which the former is found to be applicable for these systems. Not surprisingly, a Langmuir treatment was found to yield comparable surface areas and a wider range data fit. Real surface area changes, even of the order indicated by the treatment of nitrogen adsorbed on Silica SL, do not occur. The approximately 10% increase in apparent surface area for that system is, however, far outside the experimental error of these measurements. The materials are structurally stable; Silica SL has been subjected to thermal treatment above 400" in preparation prior to these studies. Both sintering, which does not occur with these materials prior to 700",and possible pore dimension changes on dehydrationl'j lead to decreases, not increases, in surface area. The apparent increase in Silica SL surface area is almost certainly due to changes in surface structure and, while negligible by routine standards, is of significance in studies related to the comparison of interaction energies (e.g., heats of wetting) on various surfaces. The adsorption of benzene on these surfaces is strongly dependent on the outgassing temperature. Although the trend toward decreasing apparent surface area appears unquestionably due to surface dehydration, the very large initial discrepancy between nitrogen and benzene derived areas could be attributed to the presence of a micropore structure of less than 7-A pore diameter. Silica structures having a majority of the pores in the 4-5 A range are but there is no evidence that such porosity is significant in these samples. Integral energetic measurements would reflect the fractional surface involvement when expressed in energy per unit area based on total (nitrogen) area. In particular, if 50% of the surface area were unavailable to benzene, heats of wetting in benzene should be one-half those for nonporous particles of the same surface functionality. The immersion heat for quartz in benzene'* is 115 ergs/cm2, for Silica SB in the 200-400" surface state 100-115 ergs/cm2, and for Silica SL from 75 to 95 ergs/cm2.l9 In the latter case the lower immersion heats are again attributable to surface structure as evidenced by almost identical immersion heats for the two materials in cvclohexane. For the benzenesilica systems studied here it must be concluded that microporosity is not an important factor. Hovart and Sing20 have noted that benzene oc-
1561
cupancy areas ranging from 31 to 50 A2/molecule must be assigned on various silica gels and have questioned the relationship of the BET derived V , values to true monolayer capacity. In this work benzene crosssectional areas ranging from 62 to 119 A2/moleculeon Silica SB and from 50 to 119 A2/molecule on Silica SL would be required to obtain dbta consistent with the nitrogen surface areas. It is apparent that only particular energetic fractions of the surface are included in the BET monolayer value for benzene. In addition, the relative pressure range over which BET applicability is indicated by C values from Table I is far outside the linear range actually encountered. It must be concluded that apparent compliance of the benzene data to the BET model is fortuitous. The condensation of hydroxyl sites characteristic of these silicas can occur only between sites spaced within hydrogen-bonding distances. These spacing requirements (2.8-3.8 A) preclude the uniform distribution of such sites for all except the 400" outgassed
3rI
IIO*OUTGASSING
~ ( K C A L MOLE
Figure 6. Site energy distribution function us. site energy for nitrogen adsorbed on Silica SB. The distribution function f is defined as f = dF/dQ where F is the fraction of the surface involved in interactions having energy equal to or greater than Q. The Langmuir equation, with selected constanta u = occupancy area per molecule and 7 = characteristic adsorption time, is utilized to describe local isotherms over d&.m For nitrogen adsorption u = 16.2 A*/molecule and 7 = 10-'8 sec.
(16) J. H.deBoer and J. M, Vleeskins, Kondnkl.Ned. Akad. Wdenuchap. proc. Ser. B , 61, 86 (19%). (17) D. Dollimore and G . R. Heal, Trans. Faraday SOC.,59. 2386 (1g63)* (18) J. W.Whalen, unpublished data. (19) J. W. Whalen, J . Phyu. C h m . , 66,611 (1963). (20) D. M. Hovart and K. 9. W. Sing, J . Appl. Chem., 11, 313 (1961).
Volume 71, N u m b 6 May 1987
JAMES W. WHALEN
1562
samples. The functional nature of the surface is that of a grouping of hydroxyl sites within the oxide matrix. This conclusion is strongly supported by site energy distribution functions derived from the adsorption isotherms by the Adamson-Ling treatment21 and shown in Figures 6-9. For nitrogen the site energy distribution is characterized by two peak functions which vary in accord with the established oxidehydroxide character of the sur-
141
1
200° OUTGASSING
“F 1
IO
I
I
300’
i
\ \
Q ( K CAL / MOLE) Figure 9. Site energy distribution function us. site energy for benzene adsorbed on Silica SL. sec (see Figure 6). v = 33 A*, 7 =
Q( K C A L / MOLE ) Figure 7. Site energy distribution function us. site energy for nitrogen adsorbed on silica SL. u = 16.2 A2, 7 = 10-13 sec (see Figure 6).
16r
12-
I I I
I
200° OUTGASSING
I
10-
I
I
Figure 8. Site energy distribution function us. site energy for benzene adsorbed on Silica SB. sec (see Figure 6). u = 33 A2, 7 =
The Journal of Physical Chemistry
face. The interaction energies are sharply limited between 1.9 and 3.2 kcal for all surface states. Average interaction energies obtained from the BET model (Table I) of 2.2-2.3 kcal are in reasonable agreement with the lower limit of these calculated values in accordance with expectation. The 2.9-kcal peak occurring in both Figures 6 and 7 represents nitrogen interaction with strongly hydrogen-bonded hydroxyl site groupings characteristic of the highly hydrated surface. The 3.2-kcal peak reflects nitrogen interaction with (predominantly) less strongly bonded hydrogen pair sites. Two oxide states are reflected by these data, the first a t 1.9 kcal (Figure 6) associated with the strongly bound water on Silica SB and the second a t 2.3 kcal characteristic of the substrate oxide surface. The entire spectrum of interaction energies is confined to a 1.3-kcal region and therefore to a correspondingly limited pressure region on the adsorption isotherm. The relative insensitivity of nitrogen as an adsorbate to distinction between these dissimilar sites, although surprising in view of the dipole-quadrupole interaction between nitrogen and hydroxyl sites, effects apparent energetic homogeneity in isotherm measurements of routine point spacing. (21) A. W. Adamson and J. Ling, Advances in Chemistry Series, No. 33, American Chemical Society, Washington, D. C., 1961, p 61.
INFLUENCE OF SUBSTRATE STRUCTURE ON ADSORPTION
1563
In contrast to this behavior the spectrum of interaction energies for benzene with these surfaces is quite wide.22 A resolvable high-energy peak is encountered for benzene on the Silica SB samples (Figure 8). This value of 10.2-10.8 kcal, again depending on the density of hydroxyl sites, is in close agreement with the Table I BET interaction energies of 11-12 kcal/mole for benzene-Silica SB. Clearly in view of the absence of resolved high-energy peaks in the distribution function for Silica SL (Figure 9), the spectrum of interaction energies can be quite wide depending on hydroxyl group density, distribution, and hydrogen bonding between such groups. These considerations as well as the isotherm forms offer no support for the position23 of common isotherm character for benzene adsorption below regions of capillary condensation on silica surfaces of equal degrees of hydration.
strongly dependent on the detailed chemical nature of the surface, although no simple relationship exists between hydroxyl site content and benzene adsorption. Benzene does not form complete monolayers or statistically equivalent multilayers in the regions of BET applicability . Surface areas derived from nitrogen adsorption data using the BET (or Langmuir) equation appear slightly dependent on surface structure for some site distributions. From the standpoint of routine surface area measurement the variations for the surfaces are not important. For the wider variation in surface structure encountered in general application and from the standpoint of the increasing importance attached to comparison of properties of surfaces of widely different nature, the 10% variation encountered in this work is disturbing.
Conclusions
Aclcnowkdgment. The author expresses appreciation to Mobil Oil Corporation for permission to publish these results, to Conley Jenkins for assistance in many of the experimental measurements, and to Dr. T. W. Haas, who made a number of significant contributions.
The interaction of both nitrogen and benzene with the silica surface is dependent upon the substrate structure. Nitrogen interaction energies ranging from 1.9 to 2.3 kcal/mole predominate on the oxide surface. On hydroxyl surfaces, predominating interaction energies are in the 2.9-3.2-kcal/mole range. Although hydroxyl sites yielding benzene interaction energies of 1011 kcal/mole can predominate on silica surfaces, a wider spectrum of interaction energies may result from variation in hydroxyl site distribution. The adsorption of benzene on silica surfaces is
(22) The site energy distribution treatment requires data extending over several orders of ten in pressure and includes regions of the benzene adsorption isotherm bounded by the limits 10-4 < P/Po(2845) < 0.45 and lo-' < P/P0(2846) < 0.7. (23) D. P. Dobychin and T. F. Tsellinskaya, Zh. Fiz. Khim., 33, 204 (1959); A. V. Kiselev, Prm. Intern. Congr. Surface Adioity, &d, London, 1067, 179 (1957).
Volume 71, Number 6 Mau 1867
