J. phys. Chem. 1980, 8 4 , 2779-2782
chemical saturation (of course moderated by the characteristics of the combining elements themselves, such as C1, Br, F, etc.) and at a minimum or nonexistant when chemically unsaturated. One more point requiring mention is the possible effect of the graphite substrate on an amorphous surface layer. That is, is the attraction to water which produces spreading, due to the sum of forces exerted on the water by the crystalline graphite substrate and the superficial amorphous layer, or to the amorphous layer alone. If the substrate does have an effect, it would, of necessity, be due to long-range forces. Previous calculations have indicated, however, that a layer such as that produced here by ion bombardment can be expected to completely screen the long-range forces of any substrate.18 The behavior of the amorphous carbon layer produced here by ion bombardment of graphite can therefore be generalized to hold true for all equivalent amorphous carbon layers regardless of the nature of the underlying bulk material.
Conclusions 1. Electron-bombardment heating in ultrahigh vacuum of ZYB oriented graphite (Union Carbide) yields a surface with a LEED pattern and a water contact angle of 38 f 3O. Disordering of this surface through controlled ion bombardment destroys the LEED pattern and gradually lowers the contact angle to 0' while retaining the mirrorlike appearance of the surface. 2. The contact angle of water on a clean, perfectly ordered graphite (O001) surface lies in the range of 42 f.'7 3. The contact angle of water on a clean, highly disordered, elemental carbon surface is 0'. 4. Comparison of the estimated contact angle of water on perfect graphite (0001) with theoretical and experimental determinations of the surface energy of the latter yields results consistent with the hypothesis that the
2779
graphite (0001)/H20 interaction is mainly through dispersion forces, with a possible small hydrogen-bonding contribution. 5. The hydrophobicity of surface carbon tends to increase with its covalent chemical saturation, e.g., hydrocarbons > graphite > clean ion-bombarded carbon. Acknowledgment. I thank Max. G. LagaUy for valuable discussions on interpretation of the LEED patterns.
References and Notes (1) Presented In part at the 177th Natlonal Meeting of the American Chemlcal soclety,"onolukr, HI, Aprll 1979. (2) Part 4 M. E. Schradw, J. phys. chem.79, 2508 (1975). (3) F. M. Fowkes and W. D. Harkhs, J. Am. Chem. Soc., 62, 3377 (1940). (4) I. Morcos, J . cdkld Interface Scl., 34,469 (1970). (5) I. Morcos, J . Chem. phys. 57, 1801 (1972). (6) B. R. b y , presented at the 163rd Natknal Meeting of the Amerlcen chemical soclety, Aprll 1972. (7) M. E. Tadros, P. Hu,and A. W. A d a m , J . W I n t e r l e c e Scl., 49, 184 (1974). (8) M. E. Schradw, "Swface Contamlnatlon: Gene&$, Detectton, and Control", Vol. 2,K. L. M a l . Ed., Plenum, New York, 1979. (9) W. A. Z h n , A&. Chem. Ser., No. 43,1 (l963),and references threln.
(10) L. A. Qmalco and I?.J. Goal, J. phys. Chem., 61,904 (1957). (11) F. M. Fowkes, J. phys. Chem., 0 ,382 (1962). (12) F. M. Fowkes, I d . €ng. Chem., 56, 40 (1984). (13) E. M. Ltfshltz, Zh. Eksp. Tew. Rz., 29,94 (1955);Sov. phys. J. ( ~ n g lmensr.), . 2, 73 (1956). (14) I. E. DzyakshlnskY, E. M. Ltfshltz, and L. P. Pttaevsktl, A&. phys. 10, 165 (1959). (15) V. A. Parseglan, Trans. Fera&y Soc..62,848 (1966). (16) V. A. Parwglan, Sclence, 156, 939 (1967). and B. W. NFsram, &h(Lada?), 224,1197 (1999). (17) V. A. (18) V. A. perseglan, Q. H. W e b , end M. E. schreder, J. W Z n t e i i & @ Scl., 61,356 (1977). (19) R. D. Brennan, J . chem. phys., 20, 40 (1952). (20) L. A. CYifalco and R. A. Lad, J . Chem. phys., 25, 593 (1956). (21) A. D. Crowd, J. Chem. phys., 29,446 (1958). (22) P. J. Bryant, P. L. Qutshal, and L. H. Taylor, Weer, 7, 118 (1984). (23) F. A. Pumam and 1. Fort, Jr., J. phys. chem., 79, 459 (1975). (24) 1.Fat, Jr., and V. P. Toan, presented at the 177th N a b 1 k h t h g of the American Chemical Society, AprR 1979.
Interaction between Hydroxides of Alkali Metals and Acid Centers on the Surface of Alumina R. Fledorow' and I. 0. Dalla Lana' Depertment of chemlcal Em-,
U n m of A m , Edmonton, Cam& T60 206 (Receki: ApU 7, 1080)
The interaction between lithium and sodium hydroxides and alumina surface centers was studied by using IR spectroscopy. The results indicate that the amount of ypicoline (a base capable of selective interaction with Lewis-acid centers) decreases significantly as a result of introduction of sodium cations and less so after the introduction of lithium cations. It is concluded that alkali metal ions can react not only with Bronsted-acid centers but also with Lewis-acid centers. Furthermore, the evidence suggests a greater than stoichiometric interaction, possibly from steric coverage effects, when the ionic radius of the alkali metal ion exceeds that of lithium ions.
Introduction The deposition of sodium hydroxide upon the surface of y-alumina changes both the surface properties and the catalytic activity'+ of the alumina. These changes have been attributed to ion exchange between the sodium ions and the acidic OH groups of the alumina surfa~e;~ however, the possibility of interaction between the alkali metal and
coordinatively unsaturated A13+ cations cannot be discounted. According to Pines and Haag,6 sodium hydroxide can react with Lewis-acid sites on the surface of alumina to form the following complex:
'Institute of Chemistry, A. Mickiewicz University, Poznan, Poland.
contrary to Roas and Bennett6who were unable to observe detectable amounts of such a complex. In addition, Da-
0022-3654/8O/2084-2779$0 1 .OO/O
-AI
I I
+
NaOH
-
0 1980 American Chemical Society
I I
[-AI-OH]
-
No.
i
2780
Fledorow and Dalla Lana
The Journal of physical Chemistry, Vol. 84, No. 21, 1980
vydov and Shchekochikhin,' from studies of IR spectra of deuterated ammonia adsorbed on alumina containing 1.6 wt ?% NaOH, have concluded that impregnation of alumina with sodium hydroxide leads to blocking of the Bronsted-acid centers by sodium ions but has no significant influence on the Lewis-acid centers. Gasanova et al.8 suggest that the absence of poisoning effects of NaOH on the Lewis-acid sites results from using relatively small quantities of the base in the previous study and, furthermore, affmthat the effect is quite clear when larger quantities of alkali (3-5.3 wt ?% KOH) are deposited on y-alumina. According to Bremer et al.? who studied Na+-containing alumina with IR spectroscopy and measured ita acidity and its catalytic activity for several reactions, both Bronstedand Lewis-acid centers are poisoned by sodium ions. Again, contrarily, Chukin,lobasing his view on interpretation of IR spectra of ammonia adsorbed on NaOH-impregnated alumina, concluded that only Bronsted-acid centers were poisoned. An entirely different opinion, presented by Marczewski and Malinowski," was that sodium hydroxide can both decrease and increase the acidity of alumina depending on the concentration of sodium added to the alumina. These authors claim that even the strong acid centers with Ho= -5.6 (which for alumina can only be Lewis centers) increase in number with increasing NaOH content up to 0.8 wt ?% NaOH. For the weaker acid centers, another maximum in acidity appears at 4 wt ?% NaOH. Only at NaOH concentrationsexceeding this value was a continuous decrease of surface acidity observed until complete disappearance a t 40 wt ?% NaOH. Thus,these experiences reflect conflicting views on the role of alkali metal ions in changing the acidity of the alumina surface, Since the influence of alkali metal ions on Lewis-acid centers greatly affects the chemical interaction between aliphatic alcohols and the surface of alumina, and the interpretation of carbonium ion mechanisms for other reactions, this study was directed at this specific problem. The technique, which is applied herein to pure, NaOH-, and LiOH-impregnated aluininas, is based upon results reported by Kno~inger.'~J~ Under appropriate conditions, the adsorption of y-picoline on alumina is confined to Lewis-acid centers; thus, 1R spectra of adsorbed y-picoline on various alkali metal-poisoned aluminas should reveal whether the Lewis-acid sites are affected by sodium or lithium hydroxides.
Experimental Procedure and Results Alon (a high-purity alumina once manufactured by Cabot Corp.) was used in this study. Alumina samples containing 2 or 5 wt % of both NaOH and LiOH were prepared by impregnating the Alon with the appropriate amounts of hydroxide solutions, drying at 120 OC for 16 h, and then crushing the resulting particles in an agate mortar. The powders so obtained were compacted into IR-transparent wafers by compression of 120 mg of powder a t 1.45Mg/cm2 in a 2.5-cm diameter die. Wafers for each of the powders (Alon, 2% NaOH/Alon, 5% NaOH/Alon, 2% LiOH/Alon, and 5% LiOH/Alon) were evacuated at 400 "C to 0.133 mPa, and then heated at 400 OC and 40 kPa of oxygen for 2 h within a conventional IR cell. After reevacuating and reheating with oxygen again for 2 h, the cell was evacuated overnight at 400 "C. The following day, the cell was cooled to either 150 or 300 "C and exposed to about 1.33 kPa of y-picoline vapor (obtained from Eastman Kodak Co.) for 0.5 h. Then, the cell was evacuated for 5 h at either 150 or 300 "C and cooled to room temperature, and an IR transmission spectrum was recorded by using a Perkin-Elmer Model 621 spectrophotometer.
itm
/6m ism
lux)
*a
IW cm'
Flgure 1. IR spectra of ypicollne a d s u b d on various aluminas: (A) basehe, (6) pwe Akm alumha, (C) 2% W A k n , @) 5% uoH/Abn, (E) 2% NaOH/Akn, (F) 5 % NaOH/Abn.
To measure the effect of the NaOH- or LiOH-treated alumina upon adsorption of y-picoline on Lewis-acid centers, the coordinativesensitive IR band intensities at 1632, 1510, 1233, and 1215 cm-' for picoline on alumina and their counterpart intensities on the treated aluminas are listed in Table I. These and other IR bands which were recorded in the 1200-1700-cm-' spectral region are shown in Figure 1. In the temperature range used in this study, 150-300 "C, y-picoline acts as a base which interacts selectively with coordinatively unsaturated A13+ions (Lewis-acid centers). Earlier,12y-picoline was shown to react Specifically via chemisorptive bonds to electron-acceptorsites (Lewis-acid centers) and not at all via H bridging between OH centers and y-picoline at temperatures above 45 OC. In effect, the y-picoline exhibits the aame steric requirements of pyridine but, being a stronger base, it can also react with the weaker Lewis-acid centers. Therefore, IR spectroscopy may be used to detect such weak-acid sites which might remain
Alkall Hydroxide-Alumina Surface Center Reactions
TABLE I: Influences of Alkali Metal Hydroxide Treatment of Alon and Adsorption Temperature upon IR Absorption of yPicoline absorbance band, at
IR
sample ~
~~
Alon
2% LiOH/Alon
5% LiOH/Alon
2% NaOH/Alon
5% NaOH/Alon
cm-’ 150°C 1632 0.75 1510 0.16 1233 0.10 1215 0.05 0.57 1632 1510 0.12 1233 0.07 1215 0.03 1632 0.34 1510 0.08 1233 0.03 1215 0.01 1632 0.39 1510 0.08 1233 0.05 1215 0.02 1632 0.12 1510 0.03 1233 0.01 1215
IR
absorbance
band,
at
Initial S~ODOS
cm-’ 300°C 1637 0.39 1514 0.10 1237 0.04 1217 0.01 1637 0.22 1514 0.06 1237 0.02 1217 0.01 1637 0.02 1514 1237 1217 1637 0.01 1514 1237 1217 1637 1514 1237 1217
even after treatment of the alumina with larger amounts of NaOH. Under such circumstances, one may suppose that the alkali metal hydroxides react with both Bronsted-acid as well as Lewis-acid centers on alumina However, the stronger Bronsted-acid centers, whether neutralized or not, should not affect the interpretation of IR bands in the 1200-1700-~m-~region of the spectrum. The question of how much of the alkali metal hydroxide added to the alumina remains to react with Lewis-acid centers after elimination of Bronsted-acid centers may be uncertain. An additional uncertainty, which is applicable to all studies with alkali-metal-impregnated aluminas, arises from possible interaction with ions in the lattice of the host y-alumina due to leaching effects during the impregnation procedure.
Discussion The IR spectra for adsorbed y-picoline, more clearly listed in Table I, show that each of the four absorption bands, 1632, 1510, 1233, and 1215 cm-’, shift slightly toward higher frequencies with the corresponding new bands appearing at 1637,1514,1237, and 1217 cm-I, respectively, for all wafers tested when the adsorption temperature is increased from 150 to 300 “C.This shift had been reported by Knazinger and Stolz.’2 The decreasing absorbances for y-picoline when the alkali metal hydroxide addition increases, especially for sodium hydroxide, demonstrate that chemisorption of y-picoline is very strongly reduced in the following order: pure Alon < 2% LiOH/Alon < 5 % LiOH/Alon = 2% NaOH/Alon < 5% NaOH/Alon. Increasing the temperature from 150 to 300 “C causes a complete disappearance of y-picoline bands from the spectrum of 5 % NaOH/Alon, nearly complete suppression of y-picoline chemisorption on the surface of 2% NaOH/Alon and 5% LiOH/Alon (only t r a m of the most intense band at 1637 cm-’ remain), and a major decrease but not total disappearance of absorbances of all four bands for 2% LiOH/Alon and pure alumina (Alon). To examine whether the interactions between LiOH or NaOH and Lewis-acid centers are stoichiometrically equivalent, the hydroxide contents, expressed as mol per
gmol Hydroxide/100 g Alon
Fburr 2. Absorbance of y-pioolhe as function of alkali content of Ah.
100 g of Alon, have been plotted against the absorbances of the strongest coordinatively sensitive bands, 1632 cm-’ at 150 O C or 1637 cm-I at 300 “C,in Figure 2. Although the data are limited, they clearly suggest that the elimination of Lewis-acid centers by reaction with lithium hydroxide is very nearly linear up to 0.10 mo1/100 g. For the removal of an equivalent number of moles of surface hydroxyls by reaction with NaOH, far less NaOH is required. This may be seen by using values from Figure 2 very tentatively. To obtain equivalent absorbance reductions, 0.75 to 0.58 and 0.39 to 0.24 for lithium hydroxide at 150 and 300 “C, respectively, the amounts of NaOH required are less than one-fourth of the LiOH consumed. More accurately, a comparison of “initial slopes” shows that approximately 4X and an estimated 12X greater “covering capacity” is exhibited by NaOH at 150 and 300 “C,respectively. If the linearity of the LiOH function demonstrates some stoichiometric relationship between lithium ions and Lewis-acid centers, then the greater interaction between sodium ions and Lewis-acid centers likely involves an additional mechanism for eliminating Lewis-acid sites. This effect becomes more pronounced at elevated temperatures. The consumption of NaOH or LiOH by Bronsted-acid centers, as mentioned in the preceding section, would, presumably, affect the “gmol hydroxide/ 100 g Alon” values on the abscissa scale of Figure 2 (equivalent to a translation along that axis), but does not influence the absorbance values and, thus, the shape of the curves plotted. For example, the initial slopes of the various curves would not be affected by such uncertainties. The linearity of the adsorptive capacity of the LiOH/ Alon function suggests that lithium ions exhibit a “one to one” relation with Lewis-acid centers at low LiOH contents. This could be interpreted to be in agreement with the base-exchange and dehydration at acid sites on silica alumina catalysis proposed by Danforth“ and confirmed
2782
Ths Journal of Fhydcal Chemki?y, Vd. 84, No. 21, 1980
by Flockhart and Pink.I6 In terms of typical Lewis-acid sites, Le., considering only one fully accessible type of A13+ surface cation, poisoning with LiOH may be represented by 0
0
0-AI
I I 0
site on catalyst
0-AI
I
0
0
OH2
I
0
hydrated site
I0-AI-OH I 0
L:
exchanged site
I-
0-AI-OH
Ll+
I
O
exchanged and calcined
These results clearly suggest that Lewis-acid sites are influenced by the addition of alkali metal hydroxides to the surface of y-alumina. Since the type of alkali metal involved can presumably result in greater than stoichiometric equivalence of alkali-Lewis-acidcenter interaction, a steric factor may be involved for alkali metals with ionic radii larger than that of lithium ions. The ionic radii of sodium, lithium, and aluminum ions are of the order 0.97,0.68,and 0.51 A, respectively, thus a NaOH-Lewis-acid interaction conceivably could block access of y-picoline to adjacent accessible Lewis-acid centers as well. To explain the conflicting views concerning alkali metal hydroxide interaction with alumina, one must clearly appreciate the roles of chemisorptive sites. For example, Chukin’s conclusionloconcerning the alleged absence of interaction between Lewis-acid centers and NaOH, inferred from IR spectroscopic studies of adsorption of ammonia on alumina, may not be correct. Ammonia does not adsorb selectively on Lewis-acid centers.lg Furthermore, sodium ions on the surface may also bond coordinatively with ammonia as in the case of zeolites containing sodium ions. Knozinger reviews poisoning of oxide surfaces13 and cautions that special care is required in evaluating the role of Lewis-acid centers. Nitrogen bases such as pyridine or ammonia will simultaneously form coordination compounds with Lewis-acid centers and bond with exchangeable cations. The resulting IR bands frequently overlap and prevent discimination on the basis of IR evidence. To illustrate, pyridine coordinatively bonded on Lewis-acid centers is responsible for the 1452-cm-’ absorption band, while the same base coordinatively bonded on beryllium cations (beryllium form of mordenite) is responsible for a similar band at 1452.8 cm-l.17 Ammonia, in such circumstances, generates much broader bands making discrimination between the two bands even more difficult. This study also refutes the viewe that the poisoning effect of NaOH on Lewis-acid centers is only noticeable when large amounts of the base are deposted on alumina. In some parallel workla on properties of alumina poisoned with NaOH, amounts of NaOH as low as 0.1 w t % drastically decreased the catalytic activity of alumina for the skeletal isomerization of cyclohexene. Above 0.3%NaOH, this reaction did not occur at all, even though the above reaction requires strong acid centers and on alumina these centers are the Lewis-acid centers? The contents of NaOH used in the above work are considerably lower than those used in the study by Davydov and Shchekochikhin’ and yet the poisoning of Lewis-acid centers is clearly evident. Finally, a more detailed explanation of the relative poisoning effects of NaOH and LiOH is suggested in terms
Fiedorow and Dalla Lana
of existing models of the surface of y-al~mina.~~” Stronger Lewis-acid centers consist of more than one over-exposed coordinately unsaturated AI3+ ions. For instance, triplet vacancies occurring in the boundaries between “regular oxide domain regions”l9provide unusual exposure of the aluminum ions in the underlying layer and so constitute strong electron-acceptor sites. The number of such easily accessible triplet vacancies, expressed as a fraction of total number of all types of sites on the surface, appears to be s~bstantial.’~ Such triplet vacancies may accommodate, of course, more lithium than sodium ions, e.g., because of the larger size of the sodium ions. As a result, fewer sodium ions will result in the same blocking of Lewis-acid centers observed for a larger number of lithium ions. Quantitatively,it was observed that the 2 wt % LiOH was insufficient to prevent adsorption of y-picoline on remaining Lewis-acid centers. On the other hand, the 2 % NaOH nearly completely suppressed adsorption of y-picoline at 300 OC, and its blocking effect was approximately equivalent to the 5% LiOH. Conclusion On the basis of the premise that y-picoline will adsorb only on Lewis-acid centers on the surface of y-alumina, this work reveals (1)that the addition of metal alkali hydroxides to alumina will eliminate Lewis-acid centers; (2) that the poisoning effect upon Lewis-acid centers will increase with increasing ionic radius of the metal alkali ion, in the order Li+ < Na+, and presumably, < K+. (3) To explain the above effect, one must invoke a steric factor (in terms of acceptable models of the surface of y-alumina). (4)A basis for resolving a number of cited conflicting viewpoints in the literature now seems to be available.
Acknowledgment. The financial support of the National Science and Engineering Research Council of Canada is gratefully acknowledged. References and Notes (1) A. V. De0 and I. G. Dab Lana, J . Phys. Chem., 73,716 (1969). (2) A. V. Deo, T. T. Chuang, and I. G. D a b Lana, J. Phys. chem.,75, 234 (1971). (3) T. T. Chuang and I. (3. D a b Lana, J. Chem. W., Farahy Trans. 7, 68, 773 (1972). (4)A. V. Deo, I. 0. Dab Lana, and H. W. Habgood, J . Catel., 21,270 (1971). (5) H. Pines and W. 0. Haag, J. Am. Chem. Soc., 82, 2471 (1960). (6) R. A. Ross and D. E. R. Bennett, J. Cab/.,8, 289 (1967). (7) A. A. hwand YU. M. s h c h e k m , m. c a ~(~ng. . rmnsr.), 10, 130 (1989). (8) N. I. Gasanova, A. E. Usovskll, and T. G. Akharov, KIM?. Catel. (€ng/.Trans/.),17,929 (1976). (9) H. B r e w , K. H. Steinberg, and K. D. Wendlandt, 2.Anorg. A/&. Chern., 366, 130 (1989). (10) G. D. Chukh, J . Stnrct. Chem. (€ng/.Trans/.), 17, 99 (1976). (11) M. Marcrewskl and S. Malhowskl, Bul. Acad. pokn. Sd., Ses. Sd. Chlm., 24, l(1978). (12) H. Kdrhger and H. St&, 5%.6msmp3S. Phys. ch9f7L 75,1055 (1971). (13) H. Kn6rlnget, A&. Catel., 25, 184 (1976). (14) J. D. Danfath, “Actes 2iBm Cagreas Intematbnal Cataiyse“, Vd. 1, Technlped Paris, 1961,p 1330. (15) 6. D. Fkxkhart and R. C. Plnk. J. Cab/.,4, 90 (1965). (16) M. L. Hak, “InfraredSpectroscopy in Surface Chemistry”, Marcel Dekker, New York, 1987,p 146. (17) H. G. Karge, A&. Chem. Ser., No. 40,584 (1977). (18) R. Fledorow, W. Przystajko, M. Sopa, and I. 0. Dah Lana, ROC. Int. Cow.Catel., 71h, communlcation accepted for publlcetlon.
(19) J. B. Ped, J. phys. Chem., (IS,220 (1985). (20) H. Kn6and P. Ratnasamy, atel.Rev.--. Em,, 17,31(1978).
