2741
RADIOLYSIS OF ETHAXOL ADSORBED ON SILICA
test for the above explanation of the discrepancy in the rather large observed discrepancy with the present experimental dissociation energy of gaseous P N by U y , investigation in the corresponding reaction enthalpies. et al., and the present investigation. On the basis of the A possible explanation of the observed discrepancy may heat of formation of gaseous PN, AHf0298= 45.3 f 4.0 be found in the slow rate of the decomposition of PN(g) kcal/mol measured in the present investigation, and the into the n - 2 and the observed phosphorus species. heat of vaporizationof P&6(s), AH0298 = 282 f 20kcal/ Huffman, et al., have needed 100 hr t o reach equilibrium mol8the standard heat of formation for P&~(s),AHf02g8, for the decomposition of gaseous P N into the gaseous becomes -146 f 30 kcal/mol. This compares with molecules of the component elements a t about 1 atm the value of -230 f 20 given by Uy, et ~ l . that , ~ was of pressure and at temperatures that were comparable based on the higher value for the dissociation energy of to those used by Uy, et al. I n the present investigation PN(g) found by these latter authors. considerably higher temperatures have been used which Acknowledgments. The author wishes to express his certainly effected a much more rapid equilibration thanks to Professor G. Herzberg for valuable comments between the reactants. This argument would assume the measurement of the heat of vaporization of P ~ N ~ ( s ) and to Professor K. D. Carlson for making the manuscript by 0. M. Uy, F. J. Kohl, and K. D. Carlson (reaction: P3Ng(S) = 3PN(g) Nz(g)) by Uy, et al., available prior to its publication. He also thanks to be correct within the uncertainty limits given by the Professor E. D. Cater for a critical reading of this authors. manuscript, and Dr. E. Clementi for making him aware A calorimetric determination of the heat of formation of the possibility to obtain a value for the dissociation of PaNs(s), together with the heat of vaporization of energy of PN(g) from published ab initio calculations. PsNa(s) that was measured by Uy, et al., would provide a
+
Radiolysis of Ethanol Adsorbed on Silica'. by Lloyd Abrarnslb and A. 0. Allen Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973 (Received January 24, 1969)
Ethanol on high-surface silica (Cabosil) is mainly held by physical adsorption, but a few per cent is held irreversibly in a form shown by infrared absorption studies to consist of Si-O-CzHa and Si-0-H groups. When the system is irradiated by y rays, considerable decomposition of ethanol results from transfer of excitation energy from the silica. The major gaseous products (hydrogen, carbon monoxide, and methane) occur in a,bout the same ratio as in radiolysis of liquid ethanol and probably result from excitation of adsorbed ethanol molecules, but small-yields of ethane and ethylene vary in a peculiar manner which suggests that they arise from ethyl carbonium ions formed a t acid sites created by radiation on the silica surface.
A number of papers have a p ~ e a r e d ~on- ~the radiolysis of organic materials adsorbed on silica or other minerals which show that energy originally taken up from radiation by the solid can be transferred to the adsorbed molecules and bring about their decomposition. The mechanism by which this transfer occurs is not clear. It was thought3 that overlap in the wave functions of the adsorbed molecules and of excited electrons trapped in surface states would be sufficient to bring about the required energy transfer. As an alternative, it has been proposeds that the radiation might bring about transient alterations in the surface to produce sites of an acidic character similar to those existing in silica-alumina catalysts, which would be capable of bringing adsorbed molecules
into chemical reaction. Support for this hypothesis would seem to be given by the reported formationfiof (1) (a) Research performed under the auspices of the U. S. Atomic Energy Commission; (b) Pigments Department, Experimental Station, E. I. duPont de Nemours and Co., Wilmington, Del. (2) J. M. Caffrey, Jr., and A. 0. Allen, J . Phys. Chem., 62, 33 (1958). (3) J. W. Sutherland and A. 0. Allen, J . Amer. Chem. SOC.,83, 1040 (1961). (4) J. G. Rabe, B. Rabe, and A. 0. Allen, J . Phys. Chem., 70, 1098 (1966). (5) (a) C. Barter and C. D. Wagner, ibid., 68, 2381 (1964); (b) C. Barter and C. D. Wagner, ibid., 69, 491 (1965). (6) H. W. Kohn, {bid., 66, 1185 (1962). (7) E. A. Rojo and R. R. Hentz, ibid., 70, 2919 (1966). (8) V. I. Vladimirova, G. M. Zhabrova, B. M. Kadenatsi, V. B. Kazanskii, and G . B. Pariiskii, Dokl. Akad. Nauk S S S R , 164, 361 (1965).
Volume 73, Number 8 August 1969
2742
carbonium ion colors on irradiation of numerous aromatic compounds adsorbed on silica. The present work on y radiolysis of ethanol adsorbed on silica was undertaken to determine whether use of a more highly polar substrate than employed in previous investigations might shed some light on the mechanism of the energy transfer. Specifically, if the acid site mechanism was of importance, one would expect that dehydration reactions to give ethylene would become much more important in the adsorbed state radiolysis than they are in the normal radiolysis of liquid ethanol.
Experimental Section Silica used was Cabosil Grade ,11-5 silica spheres obtained from the Cabot Corporation and stated by the manufacturer to have a surface area of 200 m2g-1. Our determination of the surface area by the BET method agreed with the manufacturer's determination within 1%. For ease in handling, the loose powder was compressed a t 1000 psi into l-in. diameter wafers. The wafers were broken up into smaller pieces for weighing. Commercial Solvents Corp. 95% ethanol was thoroughly dried over calcium oxide and finally distilled over magnesium chips and iodine. The ethanol was transferred for degassing to a greaseless vacuum system employing Hoke bellows-sealed valves, where it was distilled under vacuum from CaO and stored under vacuum. Four-gram Cabosil samples were weighed into cylindrical vessels of fused quartz which were fitted with break-seals. After heating in air to 550" the containers were sealed to the grease-free vacuum system. The samples were outgassed at 550" and treated at temperature with oxygen to destroy any remaining organic matter that might be retained on the surface of the silica. Samples were then outgassed at 550" for about 70 hr to a residual pressure of 6 X Torr. Ethanol, measured as a liquid in a calibrated tube, was distilled onto the cooled silica samples which were then sealed off under vacuum at liquid nitrogen temperature. The weight of Cabosil taken initially in each sample was 4.07 g, which yielded a weight of 4.00 g of silica after outgassing. Samples were irradiated at about 23" with cobalt-60 y rays at a dose rate of 0.275 or 1.0 nlrad/hr. The fraction of ethanol decomposed by radiolysis was about 1% for most runs. After irradiation the ampoule was sealed to a vacuum line, the break-seal was opened, and the gaseous products were pumped through a series of liquid nitrogen traps to a McLeod gauge. Aliquots were analyzed in a Perkin-Elmer Model 154 C gas chromatograph using a 2-m column of silica gel a t room temperature with helium or argon as carrier gas. Amounts of gas were determined from peak areas calibrated by known samples. A light hydrocarbon fraction was then collected in the McLeod gauge by raising the temperature of the traps to -80"; this The Journal of Phusical Chemistry
LLOYD ABRAMS AND A. 0. ALLEN
-r"
2'O-'? -
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lo-'
'
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Figure 1. Hydrogen yield (moleoules/100 eV) from CabosiI without adsorbate, irradiated with y rays to 0.45 &{rad, as a function of outgassing temperature.
fraction was analyzed on the same column at a temperature of 100". No additional gas was obtained when the ampoule was heated to 150" for recovery of the ethanol. Heating to this temperature resulted in recovery of more than 90% of the ethanol, but the remaining few per cent was apparently adsorbed irreversibly and could not be recovered, even if the sample had not been irradiated. Measurements of infrared spectra were made by a Perkin-Elmer 521 recording spectrophotometer, equipped with a Model 621 interchange and a l-A globar source. The Cabosil samples were irradiated in cells made of fused quartz fitted with infrared transmitting windows made either of Infrasil (Amersil Co.) or Vitreosil (Thermal American Fused Quartz Co.) brands of purified silica. The windows had absorption bands in the region 3000 to 2100 cm-l necessitating the use of a reference cell in the recording of spectra. The windows were opaque below 2100 cm-I. In order to determine how much of the ethanol was actually adsorbed on the silica and how much remained in the vapor phase, the adsorption isotherm of ethanol (9) P. J.
Dyne and N. H. Sagert, Nature, 210, 1153 (1966).
2743
RADIOLYSIS OF ETHANOL ADSORBED ON SILICA 4.0 I
on Cabosil was determined. At one monolayer ~ ethanol is coverage at 24' the relative pressure p / p of 0.11 where p o is the vapor pressure of liquid ethanol. Less than 0.670 of the added ethanol was in the gas phase in our cells, a t any coverage. For the calculation of the number of monolayers adsorbed, we took Wade's valuelo of 25.8 A2 for the area per ethanol molecule. Using the BET method with ethanol vapor at room temperature we obtained areas ranging from 24.3 to 31.3 AC2. This determination was not very accurate and we use the literature value.
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Results When Cabosil. was irradiated in the absence of any added adsorbed material, a yield of hydrogen was obtained which decreased with increasing degassing temperature of the sample as shown in Figure 1. No other gases than hydrogen were seen (except some CO from a sample not preheated in oxygen). The hydrogen presumably arises from hydroxyl groups held on the silica surface. The number of OH groups per unit silica surface as a function of degassing temperature has been given by Borello, et a2." When G(H2) was plotted on log-log paper against the number of OH groups, a straight line was obtained which showed that G(H2) (at 0.45 Mrad) was proportional to the 3.5 power of the surface OH concentration. At a temperature of 550") the hydrogen yield is negligible compared to the yields of this gas produced during irradiation of a,dsorbed ethanol, and this outgassing temperature was used in all the ethanol runs recorded here. No appreciable quantity of gas was produced when ethanol was added to a preirradiated sample of Cabosil. Yields of hydrogen found at a total dose of 0.45 Mrad are shown in Figure 2 as a function of the coverage in monolayers of ethanol. The upper curve for Go(H2) shows the molecules of hydrogen formed per 100 eV absorbed by the whole system, silica plus ethanol. The lower curve marked "liquid" represents the yield that would be seen if only energy absorbed directly by the ethanol present were effective in producing hydrogen and if the specific hydrogen yield were the same in the adsorbed state as in the liquid ethanol (GL(H~)). The difference between the two curves, AG, may be taken to represent the yield of hydrogen resulting from energy transfer from the silica to the adsorbed ethanol layer. Energy is assumed to be absorbed by each phase in proportion to its electron density. Thus, AG is given by GO- GLE,where E is the electron fraction of ethanol in the mixture, and AG is equal to the number of molecules decomposed or produced by energy transfer from the silica, divided by the total dose to the entire system in hundreds of electron volts. A more logical unit would seem to be the molecules decomposed by energy transfer per unit energy absorbed in the silica alone. This quantity is given by AG/(l - e).
I
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0
4
e
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t
6 (MONOLAYERS)
8
1
1
IO
Figure 2. Upper curve, hydrogen yield from ethanol adsorbed on Cabosil, irradiated with y rays to 0.45 Mrad, in units of molecules/100 eV taken up by the entire system of ethanol plus silica, as a function of the surface coverage in monolayers of ethanol. Lower curve, G(H2)for liquid ethanol multiplied by E, the electron fraction of ethanol in the system.
e (MONOLAYERS) 10
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0.006 0.004
y
0.002 0,001
1
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e
1 0,3
\
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Figure 3. Product yields in the ethanol silica system. Ordinate, additional yield AG due to the presence of silica (this is the difference between the two curves of Figure 2 ) divided by the mole fraction of silica, 1 - E. Abscissa, amount of ethanol, given as electron fraction e (bottom) or number of monolayers on the silica surface (top).
(10) J. Barto, J. L. Durham, V. F. Baston, and W. H. Wade, J . Colloid Interface Sci., 2 2 , 491 (1966). (11) E. Borello, A. Zecchina, and C. Morterra, J. Phys. Cham., 71,
2938 (1967). Volume 73, Number 8 August 10130
2744 Figure 3 shows the yields of product gases a t exposures of 0.45 Mrad in terms of this quantity, A G / ( l - e), plotted on a logarithmic scale for convenience. The coverage is shown both as number of monolayers and as electron fraction of ethanol. For the calculation, values of GL for each product were taken from the work of Myron and Freeman.lZ From the yields of the major product, hydrogen, and also those of methane and carbon monoxide, it may be seen that the majority of the energy transfer occurs with only a single monolayer present. A slight increase in energy transfer is, however, indicated by the slow rise in the curves at coverages beyond the first monolayer, contrary to what was found in the case of pentaneeZ At high coverages the ratios of the yields of these three gases appear to be approaching those characteristic of the pure liquid, suggesting that the mechanism of the formation of the gases in the adsorbed layer and in the liquid is not very different. Quite different results are found, however, for the yields of ethane and ethylene. At the lowest coverage studied the yield of ethane is already high while that of ethylene is small. With increasing coverage, however, the yield of ethane falls rapidly as the yield of ethylene rises. The sum of the two yields is nearly constant below 4 monolayers, and one gets the impression of a competition between two processes for some Cz precursor. At coverages higher than 4 monolayers both yields fall, and the yield of ethane in fact becomes smaller than GLE,so that the value could not be represented on the present plot. Evidently not only must the mechanism for formation of these products in the adsorbed state be very different from that occurring in the liquid but the mechanism which holds in the liquid must be, to a great extent, suppressed in the adsorbed state. Changing the total dose over the range 0.2-2.5 RIrads had no significant effecton the yields of carbon monoxide and methane. An increase in dose resulted in considerable decrease of the ethylene yield at coverage of 1 monolayer, but there was no corresponding increase in the ethane yield. A different dose effect was found on the yield of hydrogen, which increased from 2.4 to 2.82 in AG as the dose increased from 0.2 to 1.2 Mrads, with no further increase at a higher dose. The results suggested that the hydrogen first formed was being taken up by the silica surface under the influence of the radiation. That this occurs was shown by measuring the pressure decrease when a sample of outgassed Cabosil was irradiated under a pressure of 0.3 Torr of hydrogen. The pressure, p , was found to decrease gradually with increasing dose D, according to the law PO - p = Samples irradiated in hydrogen atmosphere showed the growth of a band at 2284 cm-l, which is close to the position of the band assigned to the SiHz grouping by Low and Il!t~rterra.'~The same band at 2284 cm-1 appeared when the Cabosil was heated in hydrogen t o 750". This band was not seen to form The Journal of Physical Chemistry
LLOYDABRAMS AND A. 0. ALLEN I
f l
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0 Z
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cn Z
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FREQUENCY (cm" 1
Figure 4. Differential infrared absorption spectra of a sample of 1 monolayer of ethanol adsorbed on Cabosil, irradiated with y rays to 2.5 Mrads, and then pumped out at increasing temperatures: curve a, 100'; b, 160'; c, 260"; d, 400'; e, 475'; f, 550"; taken against a Cabosil sample that had been exposed neither to ethanol nor to y rays.
when ethanol was radiolyzed on a sample outgassed at 550") but did appear when ethanol was radiolyzed on a sample outgassed a t 750". A different band is formed by the adsorption of ethanol on silica. Figure 4 shows the differential spectrum of a Cabosil sample which had been outgassed at 550", had a single monolayer of ethanol put on, was irradiated to a dose of 2.5 Mrads, and was then pumped out at different temperatures. The spectra were all taken as differences from another sample of Cabosil that had been outgassed a t 550' but had not been exposed to ethanol. The high-frequency absorbance is due to OH stretching. At lower frequencies there is a characteristic group of six bands at 2886 (shoulder), 2907, 2938, 2952 (shoulder), and 2986 cm-l which is still noticeably present at 400" but disappears on outgassing a t higher temperatures. This spectrum was found to be matched almost exactly, with a shift of only six wave numbers, by the absorption spectrum of a thin layer of (12) J. J. J. Myron and G. R. Freeman, Can. J . Chem., 43, 381 (1966). (13) C . Morterra and M. J. D. bow, Chem. Commun., 204 (1968).
RAD~OLYSIS OF ETHANOL ADSORBEDON S ~ L ~ C A ethyl orthosilicate. It is thus assigned to the stretching vibrations in the grouping Si0-CzHs. The same spectrum was found a t about half the intensity when a sample of Cabosil was exposed to ethanol without radiation, and formation of this grouping no doubt accounts for the irreversible adsorption of ethanol mentioned above. This irreversible adsorption amounts to approximately 10% of the total adsorbed a t coverages below one monolayer; the amount is not increased by additional ethanol above one monolayer. On heating samples of irradiated adsorbed ethanol up to 500", yields of gaseous product were found which were several tim.es greater than the gas appearing at room temperature. These yields were not reproducible in either amount or composition and presumably arose from pyrolysis of the irreversibly adsorbed ethanol. Carbon dioxide was always a major component of this pyrolysis gas.
Discussion The Cz intermediate which is the precursor of ethane and ethylene is most plausibly taken to be the ethyl carbonium ion C2H6+. At low coverage this entity would be expected usually not to find another ethanol molecule in the vicinity to react with and would pick up an electron from the silica to form an ethyl radical, which would eventually react with an ethanol molecule by H abstraction to form ethane. At somewhat higher
2745 coverages the CzH6+would have a greater probability of donating a proton to a neighboring ethanol molecule to form ethylene. The ethane formed in the radiolysis of liquid ethanol, which is a more prominent product than with adsorbed ethanol, probably arises from ethyl radicals. Myron and Freeman12suggest the reaction of solvated electrons with acid cations CzH60Hz+as a possible source of ethyl radicals. This reaction would not be expected to occur in the adsorbed phase, where neither solvated electrons nor protonated ethanol molecules would probably be formed to a great degree. Instead, the acid character of the silica could result in the transient formation of the ethyl carbonium ions. Thus the Cz hydrocarbon yields suggest the participation of acid sites on the silica surface in the ethanol radiolysis. However, the yield of these products is very small and the larger yields of hydrogen and methane appear to be formed by processes similar to those occurring in the liquid. It seems probable, therefore, that at room temperature most of the energy transfer resulting in radiolysis does not involve particular sites on the surface but is merely a result of excitation of normal surface states on the silica which can interact with the adsorbed molecules. The reaction rate of acid sites with ethanol on silica-alumina catalysts increases rapidly with temperature, and we would expect an increase in G(CzH4) if radiolysis of ethanol a,dsorbed on silica were conducted at an elevated temperature.
Volume 73, Number 8 August 1960
