083
??OTES
for the heat of vaporization at 25",' the heat of formation of gaseous dimethoxyborane ib - 139.2 kcal. mole-l. The heat of disproportioiiatioii, ieactioii I , may then be calculated to be - 16 8 kea1 for the liquid phase reaction and -20.5 kcnl. for the vapor phase. The energy of dissociation of the B-H bond has been estimated previously from the heat of formation of BH3 to be 91.2 kcal. Since the heat of formation of BH3 nas based on an indirect series of measurements, a check on the B-H bond energy would be of value. Yqinq the data of Charnley, Results and Discussion Skiiiner and Smith5 corrected for the more recent The average oherved heat of hydrolysis, as heat of sublimation of boron8 of 135 kcal., the mean given in Table 11, refers to the over-all reaction diisociation energy of the B-OCH, bond may be calculated to be 116.5 kcal. From this and the HB(OCH,)r(l) (n 3)H20(1) --+ heat of formation of dimethoxyborane, the disH,(g) (HyBOs(aq.) 2CHsOH)(nq.).nH?O(l) (2) sociation energy of the B-H bond 13 calculated 111 the hydrolysis reactions, n mas niade sufficiently to be 92.5 kcal. This result is in good agreement large to approach infinite dilution ( n > 1000). njth that ohtaiiied from BHB. In deriviiig the heat of formation, therefore, these (8) Thermochemistry and Thermodynamics of Sonw Boron Com.tandard heats of formation of the products a t pounds, NBS Report KO 6252. infinite dilution were used
reaction being checked by measurement of evolved hydrogen and by titration of the final solution for boric acid. As a separate check 011 the state of the reaction products, a portion of the final solution from one of the rum was analyzed for methyl borate by infrared spectrometry. KOlines of methyl borate were observed. Hydrolysis of a sample of methyl tlorate also went to completion under the same conditions as 1 he dimethovyborane hydrolysis. The heat of hydrolysis of ihe methyl borate sample was observed to be -4.65 kcal. inole-' as caompared to the value of -4.615 reported by I:harnley, Skinner and Smith.6 -411 results are in terms of the defined calorie (4.1840 absolute joules). S o correction was applied for the small :tmount of water vapoiizrd from thr r:Llorimrtpr with the ~~volvcd hycirogen.
+
+ +
+
C"aOH(aq, m ) AHP = -58.77' H,BOa(aq, a ) AHfo = -256 92'
PHYSICAL ADSORPTIOS O S CHEMISORBED FILMS BY D. S.M ~ c I ? EAR\ D H. H. TOBIN
TABLE 1 C.4LORIMETER C.4LIBR.4lIoN Energy input, joules
Mass H20, g.
190.0 190.0 190.0 190.0 190.0 190.0
U22.5 1427.3 1439.5 1444.8 1426.6 1407.2
Enefgy equiv., cal./ohm
Gulf Research & D e v e l o p m e n t C o m p a n y , Pzttsbwrgh SO, Pa.
2274.2 2289.5 2295.2 2271.8 2291.4 2286.4
It is generally considered' that the preqence of a layer of chemisorbed gas does not interfere with the measurement of the surface area of solids by the BET gas adsorption method provided that there is no blocking of adsorbent pores by the chemisorbate. There are in the literature, however, several indications that exceptions to this statement may exist. Thus Stone and Tiley2found that the chemisorption of carbon monoxide by a copper oxide film decreased the krypton B E T monolayer volume by some 20%; oxygen, on the other hand, had no effect. I n contrast to these results, Joy and Dorli11g3 reported that the chemisorption of carbon monoxide by an iron Fischer-Tropsch catalyst did not influence the subqequent physical adqorption of nitrogen. Teichner and Morrison4 have found that the prior chemisorption of either carbon monoxide or oxygen by nickel oxide reduces the B E T Vm value for argon. More recently, it ha5 been shoiln by Sastri, Viswanathsn and Sagarjunan5 that the chemisorption of carbon monoxide on cobalt catalysts decreases the subsequent phyFical adsorption of nitrogen. Theqe authors claim, although they do not demonstrate, that this decrease ii tantamount to a suppreqsion of the monolayer T olume. I n general, while it TT ould appear from the preceding discusiioii thst a chemisorbed film may influence subcequent physical adsorptioii, a question
2284.8 h3.9
Mean energy equiv. Stand. dev. of mean
TABLE I1 HYDROLYSIS i\'fE.4SVREhlENTS hfass sample, g.
0.7123
.i o 2 0
,2885 ,2967 ,6396 .58X ,9208
Mean obed. heat Std. dev. of mean
-Aaobsd,
kcal. mole
21 23 24 23 23 24 24
55 88 44
93 99 63 24
24.24 $0.12
The resulting heat of formation of liquid dimethoxyborane is - 145.3 kcal. molecl. The uncertainty of this value is estimated from the uncertainties of the calibration and hydroljT,-is measurements to he *1.5 kcal. From the reported value of 6.138 kcal. mole-' ( 5 ) T . Ctiarnley, H. A . Skinner and .4. B. Smith, J . Am. Cliem. Soc., 74, 2288 (lg52). ( 6 ) "Selected Values of Chemical Thermodynamic Properties." K B S Circular 500. (7) Thermodynamic Properties of Some Boron Compounds, NBS Report No. 4943.
Xecezued December 6 , 1.969
(1) P H. E m m e t t Cata1)sis" \ o l I ed b> P I3 Fnimett Reinliold Pub1 Corp , New York, N. Y 1954 p 2 1 ( 2 ) F S Stone and P F. Tiley, .\aLwe 167, 064 (1933) ( 3 ) A 5 Joy a n d r A Dorling z b z d , 168,433 11951) (4) 5 J Teichner a n a J Vorrlson 1 r a m i'oraday SOC 61, 901 (1965). ( 5 ) M. V. C. Sastri, T S. k i s a a n a t h a n a n a T S NsgarJunan THIB JOURNAL, 63,518 (1959)
A84
P
160-
140
T'ol. 64
NOTES
I-
2oP, O%i-++
d,
A
d5
RELArivE
d,
PRESSURE.
d7
d,
d,
Fig. 1.-Argon adsorption on chromia surfaces a t - 195". Open and solid syinbols represent adsorption and desorption, respectively: 0, clean surface; A, oxygen surface; . carbon dioxide surface; 0,carbon monoxide surface; oxygen carbon monovitle surface; 0,carbon dioxide carbon moiioxitle surface.
+
iisetl previously6 and vias st,ahilized by alternate oxidation and retluction at 5 O O O . 6 7 7 Before use the chromia was pretreat,ed by oxidation in a stream of osygeii for three hours a t 500°, atmospheric pressurr, and a gascous hourly spare velocity of 5000, reduction in hydrogen for six hours under the same conditions, and finally evacuation for 16 hours a t 500". An argon isotherm then was determined in the usual falihion a t -195" and the physically adsorbed argon removed by evacuation for one hour a t -78". A chemisorbed phase then was introduced by exposing the adsorbent t o scveral millimeters pressure of the chemisorbate for an hour After a t -195" and then evacuating for an hour at, -78'. recooling the adsorbent to -195", another argon isotherm was measured. At this point either the original surface was regenerated by repeating the pretreatment or a second chemisorhate int,roduced by a repetition of the procedure just, described and another argon isotherm obtained. In hetcrmining the argon isotherms adsorption points were taken only after the pressure had remained constant for a t least 15 minutes; in some instances points were checked by cqiiilibration for periods as long as 16 hours. The liquid vapor pressure of argons wa: used to compute relative pressures and a value of 16.9 A.2 was employed for the crosssectional area of a physically adsorbed argon atom.g Upon completion of the series of experiments described in this note the entire sequence was repeated: the results of the two sets of measiirements were in essential agreement. Despite the several regenerative treatments required, no :\ppreriable change in adsorbent surface :ire:% w:ts observed tliiring the course of the nork.
Results and Discussion The chemisorbates employed in the present study were carbon monoxide, oxygen and carbon dioxide ; as shown earlier6 these were chemisorbed to the extent of 0.14, 0.10 and 0.12 cc. (STP)/m.*, respectively. In addit,ion, the oxygen covered-surface chemisorbed 0.03 cc. (STTc.l)/m.2 of carbon monoxide while the carbon dioxide-covered surface chemisorbed either 0.06 cc.(STP)/m.? of oxygen or 0.04 cc. (STP)/m.? of carbon monoxide. It was possible, therefore, to study the physical adsorption of argon on chemisorbed layers of the three individual gases or on mixed layers of oxygen and carbon monoxide, oxygen and carbon dioxide, or carhon dioxide and carbon monoxide. The argon isotherms obtained on the clean, reduced surface and on the same surface after introduction of various chemisorbed layers are shown in Fig. 1; the corresponding BET plots are given in Fig. 2 and a summary of all the B E T parameters in Table I. For the sake of graphical clarity the argon
PIPO.
Fig. 2.-BET plots of argon adsorption on chromia surfaces a t 195": 0, clean surface; A, oxygen surface; u, carbon dioxide surface; G, carbon monoxide surface; V, oxygen carbon monoxide surface; 0, carbon dioxide RET carbon monoxide :;urface.
-
+
+
remains as t o the concurrent effect upon the parameters of the BET equation. Obviously this is a point of some importance because of the widespread use of the B E T formulation in determining surface aieas. The purpose of the present note is t o descrilie some observations of the effects of chemisorption upon the physical adsorption of argon by a chromia surface and to discuss these effects in terms of the B E T theory.
TABLE I ARGONh D S O R P T I O N PCRFACESAT - l05O
PAR.4METERS FOR
O N C]HROXI.3
Vm
cc.
Siirfnce
(STP)/g.
C
Clean 0 . 1 0 cc. (STP) O?/ni.2 . I 2 ec. (STP) C02/m.2 .14 cc. (STP) CO/m.2 .12 cc. (STP) CO, 0.06 cc. (STP) 0 2 / m . 2 . I 2 cc. (STI') GO, 0.04 cc. (STP) CO/m.2 .IO C C . (STP) CO 0 . 0 3 cc. (STP) O,/ni.*
4.9
107 131 67 11 40 43
+ + +
4.8 4 !I
5.0 4 0
4.2 4.5
40
Experimental The adslxption measurements were carried out in a standard volumetric gas adsorption apparatus; a description of this equipment and of the gases used may be found in an earlier publication.6 The chromia was the same sample
isotherm on the surface containing hot,h carbon dioxide and oxygen has been omitted from the
(6) D. S. CfacIvei a n d H. H. Tohin, Symposium on Theoretical Aspects of Heterogeneous Catalysis. Diviaion of Colloid Chemistry, A.C.S N e e t i i g , September. lY50; Pnper No. 6, to be pubhshed.
, Physica, 17, S i 6 (1Y,51). (9) €1. L. I'ickering and IT. C . Eckstroin. J. A n i . C h m ~ SOC., 74, 4778 (1982).
-
( 7 ) S. W. Wrller and S. E. Voltz, J . Am. C h ~ m .SOC.,1 6 , 4695 (1954).
I. Clark, F. Din. J. Robb. A. hlichelr, T. 1V;nxsPnne.r ani1 T.
figures; this isotherm was quite similar to that for the surface containing only carbon dioxide but, was displaced slightly lower with respect to the ordinate. It is readily apparent from Fig. 1 that the physical adsorption of argon on the clean, reduced chromia surface is anomalous in that a fairly sudden rise or step in the volume adRorbed takes place starting a t a relative pressure of about 0.2. This is reflected in the corresponding B E T plot of Fig. 2 by a tendency of the plot to be concave with respect to the pressure coordinate. The general shape of the isotherm is similar to those found on certain moderately graphitized carbon blacks where the presence of steps in the isotherms has been considered as (aharac.teristic8 of relatively homogeneous surfaces for physical adsorption. l o In the present case, n-hile an oxide such as chromia n-odd ordinarily be expected to exhibit a fairly heterogeneous wrface, it is conceivable that the stabilization of the surfaw by repeated oxidation and reduction a t elevated temperatures could have resulted i n a more honiogeneous surface than is usual. On this hasis the effect of chemisorbing oxygen or carbon dioxide n o d d be to introduce sufficient heterogeiieity so as to eliminate the stepwise feature of the isotherm. A s niay be seen in Fig. 1 such chemisorption appeared to have just this effect; the argon isotherms on the oxygen-covered and the carbon dioxide-covered surface have the usual sigmoidal shape anti seem to show iiormal B E T behavior. In the case of homogeneous surfaces it has been suggested” that data a t fairly low relative preisures (i.e., in the vicinity of a statistical monolayer) be employed in the B E T equation. Accordingly, the portion of the BET plot of Fig. 2 for the clean, reduced surface up t o a relative pressure of 0.14 vas used to calculate the corresponding R E T parameters uf Table I; the T,i value so obtained agrees quite I\ ell ith the values derived from the iiormal B E T plots of the oxygen-covered aiid carbon dioxide-covered burface‘.. This agreemelit n ould ceem to I alidate the use of the Iow pressure region to obtain the areti of the clean surface. The chemiwrbed carbon monoxide which covered essentially the entire surface6had a much more drastic effect 011 iubsequent physical adsorption than did the other rhemiwrbates. In Fig. 1 it is apparent that the carl~onmonoxide layer not only removed the stepit ise character of the isotherm but alqo caused a. large decrease in the amount of argon physically adsorbed a t any given relative pressure; in particular, it qhould be noted that the sharplydefined hend 111 the vicinity of a monolayer is missing. Deqpite thece changes, hon-e.rw, a B E T plot of the data, shoivn in Fig. 2 , seems fairly linear abo\re 0.10 relative pressure and indicates a T’, in good agreemelit with the earlier values. The priiicipal effect of the chemiwrption, iii terms of the 13ET formulatioii. 11 a s to decrease the C-value by a twtor of Ahout 20. This low C-.i-alue is indicative of a low heat of physical adsorption and it is thereby apparent that the argon interaction with the chemisorbed film was much weaker than the inter(10) \ I
I 1 I’a,ll, \
\ \ . 1) & haeHcr and \\
R Sruitli THISI o i
\ % I 67 4tiCJ I O i i )
(11)
1 ) S A l , u I \ w rrid I’ FI L n l w e t t ? h i d , 6 0 , 524 ( 1 Y i b )
R-
action with the clean surface. In general, the effect reported here of the carbon monoxide chemisorption was quite similar t o the effect, described by Singleton and Halsey, l 2 of physically adsorbing a layer of xenon on carbon black or silver iodide prior to the argon adsorption. It might be thought that the lowering of the argon isotherm by the carbon monoxide was caused, a t least in some part, by a blocking of adsorbent pores by the chemisorbate. Several points would seem to argue against this interpretation. First of all, the B E T monolayer volume obtained with argon appears to indicate that the entire surface was acressible to the argon atoms. Secondly, no evidence could be obtained that any small pores were present in the adsorbent; this is indicated by the lack of any appreciable hysteresis iii the isotherms of Fig. l . I 3 Finally, it seems unlikely that any pores blocked by carbon monoxide mould not also be blocked by chemisorbed carbon dioxide. It may be seen in Fig. 1 that when two cheniisorbed species were present the argon isotherms xere definitely modified in comparison to the clean surface isotherm, although in no case was the effect as pronounred as that found when the surface was completely covered with carbon monoxide. I n each instance of dual chemisorption the B E T plots were linear over the L‘usua1’7 range of relative preswres (ie., 0.05 to 0.35). Despite this linearity, however, the data in Table I show that when one of the chemisorbates was carbon monoxide the iiioiiolayer volume was somewhat lower than the expected value. Thus while the V , value for the surface containing both carbon dioxide and oxygen was the same as for the clean surface, substituting carbon monoxide for the oxygen reduced the appareii t V,, by some 14%. I n summary, carbon monoxide as the only chemisorbate studied which seemed to have a significant effect on the RET surface area; further, this effect was not found when the entire surface n a s covered with carbon monoxide. While a quantitative explanation for this behavior cannot be offered at this time, it is suggested that the presence of adsorption sites characterized by widely separated C-values (ie., heats of adsorption) may be responsible. Thus the chemisorption of carbon dioxide (high C-value) and carbon monoxide (low C-value) may produce a dual surface of the iiature discusqed by Walker and Zettlemoyer.14 As these workers have shown, it is possible in such a case to obtain a V , value somewhat smaller than the true value. A similar explanation has been advanced by Stone and Tiley? to account for their results: it niay be noted that in this latter work aiid in that of Teichiier and R1orrison4 the carbon moiioxide coverages were less than unity. Fin:Llly. it should he remarked that in the present c a h ~of chromia, the depression in the physical adsorption isotherms upon carbon nioiioxide chemisorptioii did not appear to have been caused by a change in the deiisity (12) J H Singleton and G. D Halsey, qbzd.. 6 8 , ,330(1954). (13) The desorption points In these isotherms nrre obtained after the relative pressure had been increased to a value greater than 0.98 (based on the solid phase saturation pressure for argon). (14) W. C. R‘alher and .I C. Zettlrmo)er, T H IJ~O L R ~ A L 62, . 4i ( 1 9 1x1.
686
1-01. 64
SOTES
of packing in the physically adsorbed phase as postulated by other workers5 for the case of a metallic surface.
lIERRITT
lf.BIRECY
AXD
2
LORENG. HEPLER'
C o n t r i b u t i o n ,from C s b h Chemical L a b o r a t o r y , U n i v e m i t y 0.1 V i r g i n i a , Charlottesaille, Va. Received December 9 , 1960
Part,ly tiecause of recent interest in perchlorates as cornpoileiits of solid propellants, the heats of solution lmve been determined
+ + +
liClO,(c) = K+(aq) C104-(aq) I
