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The Journal of Physical Chemistty, Vol. 83, No. 24, 1979
and oxygen vacancies to the surface. As a result, the bulk Cu'/va~&cy defects, which generate the described energy band overlapping with the ZnO band edge, are neutralized and the band structure of pure zinc oxide gradually reappears.
Acknowledgment. Partial support for this research was provided by Department of Energy Grants AER-7503776 and ET-78-S-01-3177. One of the authors (J.B.B.) acknowledges the receipt of an NSF Fellowship.
Pichat et at.
References and Notes (1) Herman, R. 0.; Klier, K.; Simmons, G. W.; Finn, B. P.; Bulko, J. 6.; Kobylinski, T. P. J . Catal. 1979, 56, 407. (2) Mehta, S.; Simmons, G. W.; Klier, K.; Herman, R. G. J. Cafai. 1979, 57. 339. Klier, K. Catal. Rev. 1967, 7 , 207. Klier, K. J . Opt. SOC.Am. 1972, 62, 882. Innes, W. B. In "Experimental Methods in Catalytic Research", R. B. Anderson, Ed.; Academic Press: New York, 1968; p 84. (6) Chapple, F. H.; Stone, F. S.Proc. Brit. Ceram. SOC.1964, 7 , 45. (7) Texter, J.; Strome, D. H.; Herman, R. G.; Klier, K. J . Phys. Chem. 1977, 87, 333.
Photocatalytic Oxidation of Propene over Various Oxides at 320 K. Selectivity Pierre Pichat, Jean-Marie Herrmann, Jean Disdler, and Marie-Noeile Mozzanega Institut de Recherches sur la Cataiyse, C.N.R.S., 69626 Vileur6anne Cgdex, France (Received Ju/y 9, 1979) Publication costs assisted by Centre National de la Recherche Scienfifique
Propene oxidation has been studied at 320 K over the following series of UV irradiated oxides: Ti02,Zr02, V205, ZnO, Sn02,Sb2O4, Ce02, WOS, and a Sn-0-Sb mixed oxide. None of these solids was active at this temperature in the absence of UV irradiation corresponding to an energy equal to or greater than its band gap. A rise in temperature had a negative effect on the photocatalytic activity. The quantum yields varied widely with the metal oxides; however, only the V205sample used was found photoinactive. The selectivity greatly depended on the catalyst. As for thermal catalysis, the selectivity to mild oxidation products increased with decreasing conversion (modified by varying light intensity). For low and equivalent conversion levels, total oxidation predominated for Ce02and TiOz,and, to a lesser extent, for Zr02and ZnO, whereas partial oxidation products only were obtained over SnOz, W03, and the Sn-0-Sb mixed oxide, and almost solely over Sb204. In addition to water, C 0 2 (but no CO), ethanal, acrolein, acetone and, in some cases, propene oxide and traces of propanal, were produced. Ethanal was generally formed preferentially to acrolein. For a given catalyst, the product distribution was influenced by the activation mode (UV light or increase in temperature). The results, which emphasize the important role of the catalyst sample in determining the selectivity even for the same activation mode of oxygen at near room temperature, are briefly discussed.
Introduction Although the photocatalytic properties of n-type semiconductors in oxidation reactions are well known, most studies dealt with TiOl or ZnO, and, to our knowledge, the influence of the photocatalyst on the selectivity has not been thoroughly investigated. It has only been briefly mentioned that Ti02,Zr02,Sn02,and W03 for isobutane oxidation1 and the same oxides plus ZnO and MooBfor ammonia oxidation2 exhibited photocatalytic activities which varied over a large range, whereas the reaction products remained the same. On the other hand, it has been reported that oxygen isotopic exchange around room temperature over UV-irradiated Ti02,3Zr02, ZnO, and Sn024proceeded only via the same mechanism which involved a surface dissociated oxygen species, whereas, generally, several mechanisms occur simultaneously in the case of thermally activated isotopic exchange. Propene oxidation has often been chosen to study the selectivity of mild oxidation catalysts. Furthermore, although alkenes have been considered as probable intermediates in the photocatalytic oxidation of alkanes and of aliphatic secondary and tertiary alcohol^,^ articles on their own photocatalytic oxidation are not very numerous. Ti02617and ZnO' have been reported to photocatalyze complete oxidation of propene; however, additional formation of methanal has also been pointed out for C3H6and C2H4 oxidations over UV-irradiated Ti02.8 Traces of COZ
were detected upon illumination of Sn02,whereas MgO, V2O5, Cu20, Y2O3, Zr02, Moo3, Sb2O3, GdzO3, Hf02,W03, and PbO were found photoinactive for propene and isobutene ~ x i d a t i o n .Total ~ oxidation of the two n-butenes was achieved over T i 0 2 as a photocatalyst.6 Isobutene yielded mainly acetone when the catalyst was Ti02,UV irradiated either a t room t e m p e r a t ~ r eor l ~in~ ~ the ~ 333-423 K temperature ra~~ge,~JO while methylacrolein was formed over ZnO, UV irradiated between 373 and 543 Ke7Finally, very recently,l' the photocatalytic oxidation of the 3methylbutenes over T i 0 2 a t room temperature has been interpreted in terms of molecular configurations. The present article reports a more precise analysis of the products of propene oxidation over a series of oxides (Ti02,Zr02, V205,ZnO, Sn02, Sb204,CeOz, W03, and a Sn-0-Sb mixed oxide) UV irradiated a t 320 K, in order to emphasize the role of the catalyst in the selectivity.
Experimental Section The catalysts, whose specific area and origin are included in Table I, were irradiated by a Philips SP 500-W mercury lamp through a water containing cell and an optical filter (300-400-nm transmission) so that the 365-nm (3.4-eV) Hg line was essentially used. In order to modify the conversion level of propene, the light intensity was weakened by using various calibrated metallic grids, which did not change the energy distribution of the photons nor the geometry of the
0022-3654/79/2083-3122$01.00/00 1979 American Chemical Society
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TABLE I: Sample Characteristics, Quantum Yields, and Selectivities for Propene Oxidation at 320 K specific area, quantum conversn, % % % % samples origin mz g-’ yield, % % X 10’ CO,= ethanal acrolein acetone 27 70 37 3 20 2.7 25 120 100 15 35
b
CeO, TiO, -1 ZrO, ZnO-1 Zn0-2 SbZO, Sn0,-1 Sn0,-2 Sn-0-Sb WO i VZO,
C
d
e
f
g g h +?>
i
J d
3.5 5.5 5 5.5 5.5 6 2.5 2 4 2.5 0
0.7 10 3 0.3 0.3 0.6 0.06 0.07 0.1 0.03 0
12.5 12 30.5 30 17 68 44 62 81.5 78
82 60 46 47 78 4 trk 0 tr 0
2 11 12.5 20 3.5 20 55 35 17.5 16
3.5 6.5 6 3 1.5 8 1 3
1 6
%
propene oxide tr 10.5 5 tr 0 0 0 tr 0 0
a Percentage of propene totally oxidized (corrected for ethanal). Reference 28. ‘,Degussa. Reference 1 5 . e Merck. Reference 29. g Reference 1 3 . Koch-Light. Atomic ratio Sb/(Sb + Sn) = 0.4. Reference 30. The CeO,, ZrO,, and tr denotes traces. In Tables 1-11 the propene conZ n 0 - 2 samples were received from the authors of the references cited. version can be obtained in mol s-’ cm-’ of catalytic bed by multiplying the percentage by xtj.3 f
J
TABLE 11: Effect of Temperature and UV Irradiation on the Selectivity of Propene Oxidation over the Ti0,-1 Sample
sample
temp, K
UV light energy: mW cm-2
1 2
320 448 455 320
0.6 0 48 2.4
3 4 a
Received by the catalyst.
%
conversn,
%
%
%
%
acetone
propene oxide
% X 10’
CO,b
ethanal
acrolein
propanal
5.5 5.5 15 20
60 58 60 73
12 33.5 22 10.5
11 -2 4 8.5
6.5 -1 8 5.5
10.5 0 0 2.5
tr -5 -6 tr
%
Percentage of propene totally oxidized (corrected for ethanal).
beam. The energy received by the catalyst (0.6-100 mW cm-2) was measured by a radiometer (United Detector Technology, Model 21 A) calibrated against a microcalorimeter.12 The quantum yields were calculated by taking into account these energy values, the emission spectrum of the UV lamp, the filter transmission, and, finally, the absorption curve of each catalyst, since it has been checked that the photocatalytic activities parallel these latter curves which were scanned with an Optica CF4 DR spectrophotometer provided with a Pulfrich integrating sphere (reflectance standard MgO). For the Sn-0-Sb sample no significant absorption curve was obtained on account of its yellowish grey color; it was assumed that this sample absorbs the same number of photons as SnOz since (i) its structure is mainly that of SnOz because of the partial dissolution of S b in this oxide,13 and (ii) for the wavelengths considered the spectra of S n 0 2and Sbz04are very close. Aldehydes and ketones absorb in the 230-340-nm region; in particular the absorption maxima are a t -280 nm for ethanal and acetone and a t 330 nm for acrolein. Consequently, under the present conditions where the greatest part of the photons corresponds to a lower energy (365 nm), the products of propene oxidation cannot be significantly transformed by the UV light. The fixed-bed differential flow photoreactor was the same as previously ~ s e d . ~ ~The ~ , temperature ~ , ~ J ~ of the catalysts, measured by a thermocouple in contact with the layer of powder, was maintained between 313 and 323 K. The activity, measured after the achievement of a stationary state, varied linearly as a function of the catalyst mass u p to a critical value which corresponded to a completely UV irradiated layer. In the present experiments, we made sure that the mass used was greater than the critical value of each oxide. In Tables I and 111, the quantum yields refer merely to the geometrical surface of UV irradiated catalyst. The total flow rate (16 cm3 min-l, C3H,/Oa/He = 41616 cm3 min-l) gave reaction rates out of the diffusion-controlled regime. Low conversion levels (0.02-3.5%, depending on the catalyst and on the UV light intensity) were
-
achieved as required in a differential reactor. The effluents were analyzed by gas chromatography. For organic compounds we used a flame ionization detector equipped with a column of Fractonitril I11 on Chromosorb PAW, 80-100 mesh, heated at 343 K. A catharometer was employed for inorganic gases: 02,N2and CO were separated on a zeolite 13X column a t 298 K, while COPand H20 were analyzed on two columns in series: Porapak Q and R a t 363 K.
Results and Discussion Effect of UV Light and Temperature. No propene oxidation occurred when the reactor was UV illuminated in the absence of any catalyst. This means that the reactions reported here are not photochemical and also that the reactor walls do not intervene at least in the formation of the primary products. No activity was observed when the catalysts were not UV irradiated a t 320 K. In the case of the Vz05sample, which is a relatively good oxidation catalyst, the first traces of acetone were detected above 340 K with our experimental device and a temperature of -390 K was required to form a significant amount of this compound. In the case of the TiOz-1 sample, which exhibits the highest photocatalytic properties, Table I1 (lines 1 and 2) shows that, for the same low conversion level obtained either by a UV illumination of weak intensity (0.6 mW ern-') or by a 128 K increase in temperature, the distribution in products of partial oxidation greatly depended upon the activation mode. The activation by photons and/or the decrease in temperature promoted the formations of propene oxide, acetone, and acrolein, whereas they reduced those of ethanal and propanal. Comparison of lines 2 and 3 of Table I1 shows that UV irradiation of T i 0 2 a t 455 K increased the conversion and the relative percentages of acrolein and acetone, whereas it decreased that of ethanal; however, no propene oxide was obtained probably because of its instability over TiOz at this temperature, which might explained the large growth in acetone percentage and the unvaried percentage of propanal. It may be seen from lines 3 and
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Pichat et al.
TABLE 111: Characteristics, Quantum Yields, and Selectivities of Anatase Samples for Propene Oxidation a t 320 K
sample 1 2 3 4
origin b C
d e
specific area, m2 g-'
quantum yield, %
conversn,
%
%
%
%
%
COZa
ethanal
acrolein
acetone
70 10.5 10 13
10 0.6 0.75 1
0.5 0.5 0.4 0.5
77 56 58 64
10 23 21.5 16.5
6 14 13 11
%
Percentage of propene totally oxidized (corrected for ethanal). Degussa. AT, (Thann and Mulhouse, France). e AT, (Thann and Mulhouse, France).
a
4 of Table 11, referring to close conversion levels, that, by raising the temperature, the percentages of ethanal and propanal increased, whereas that of acrolein decreased and propene oxide formation disappeared. In conclusion, the UV illumination and the decrease in temperature similarly influenced the distribution in partial oxidation products. On the other hand, the rise in temperature had a negative effect on the photocatalytic activity, as may be seen on comparing the UV light energy and conversion levels a t 320 and 455 K in Table 11. Since the conversion and UV light energy are proportional a t a given temperature (vide infra) a conversion some 28 times greater would have been obtained at 320 K for the UV energy employed at 455 K. An analogous temperature effect has been indicated for isobutane photocatalytic oxidation over Ti02.119 Quantum Yields. The quantum yields, listed in Tables I and 111, are the ratios of the number of propene molecules oxidized per second and per area unit of catalytic bed to the incident flux of photons absorbable by the same geometrical area of catalyst. They were obtained within an error of -20% because of various lacks of accuracy: (i) the number of propene molecules oxidized was calculated by summing the amounts of all the oxidation products as measured on the chromatograms and (ii) the calculation of the flux of photons (1.5 X 1015-2.5 X 10l6 cm-2) took into account several measurements (see Experimental Section). The quantum yields varied widely with the nature of the oxide: from 10% for the Ti02-1sample to 0.03% for the W 0 3 sample (Table I). The same value has been found by actinometry for the Ti02-1sample in the case of isobutane oxidation.11J4 It has been reported7 that Zr02 and W03 did not photocatalyze propene oxidation, while only traces of C 0 2 were formed over Sn02. The discrepancy with our work may be due to (i) a higher UV light intensity, (ii) a greater sensitivity of our chromatographic analysis, or (iii) the different methods of sample preparation (see the following paragraph). The Zr02, Sn02-2, and W03 samples were also found photoactive and the V205sample was photoinactive for oxidations of i-C4H101 and of NHp2 Results obtained in the cases of O2 isotopic exchange (Zr02, Sn02-2,V20,)4and of CO oxidation (Zr02, Vz05)16are also in agreement with the present data. The quantum yields (Table I) appear too weak, with the exception of the Ti02-l sample, for a practical use of the catalysts examined in propene oxidation. While no difference was found between the quantum yields of the two ZnO samples or of the two SnOz samples (Table I), the various specimens of TiOz exhibited distinct quantum yields (Table 111). These differences, which are not unexpected and have been pointed out for other photocatalytic r e a c t i ~ n s , ~ stress J ~ J ~ a difficulty in classifying the oxides according to their photocatalytic activities. Even comparison between allotropic varieties appears questionablelo since the photocatalytic activity is sensitive t o the texture3 as in the case of thermal cata1ysis.l' Selectivity. As for thermally catalyzed reactions, the comparison of the selectivities of various samples should
propene oxide 2 6.5 6.5 7.5
5
> 0.5
1 1 AT, (Thann and Mulhouse, France),
. a4
z 0
v)
K
w
> z
0 0
w 2 w n
Bn
I I lo
Figure 1. Percentage of propene conversion as a function of the relative UV light intensity, I / Io (the subscript zero refers to a nonattenuated beam), for the Ti02-l sample.
ethanal
.
c
-
acetone acrolein
el 0 PROPENE
propene oxide
'--eo
2 CONVERSION I %
Figure 2. Percentages of the products of propene oxidation as a function of conversion for the Ti02-l sample. The C02 percentage corresponds to the percentage of completely oxidized propene.
be made at equivalent conversion levels. We have checked that these conversion levels are proportional to UV light intensity. Figure 1 shows the results in the case of the Ti02-1sample, chosen because of its greater photocatalytic activity. The points are satisfactorily aligned by taking into account the lack of accuracy in the determination of the conversion level. The linearity also indicates that saturation of the catalyst by photons was not reached. Similar straight lines were obtained for other samples. Consequently, the quantum yields given in Tables I and I11 are significant, and variations in UV light intensity were used to adjust the conversion of the more photoactive oxides to the level of the less photoactive. As expected, on decreasing the conversion the selectivity for mild oxidation products increases. Figure 2, corresponding to the Ti02-1sample, presents a typical example. Moreover the percentages of the various products of mild oxidation may vary. As shown in Figure 2 somewhat parallel increases in ethanal and acrolein percentages were observed a t low conversion levels, whereas the increase in propene oxide percentage was more marked and, on the contrary, the percentage of acetone was less affected. Ef-
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979 3125
Photocatalytic Oxidation of Propene
fects of the same order of magnitude were found for the Zr02 sample, while the selectivities of the CeOz and ZnO-1 samples were less modified for the same conversion interval. The influence of the nature of the solid oxide upon the selectivity is much greater than that of UV light intensity. Table I gives the percentages of oxidation products for several oxides a t equivalent conversion levels. Total oxidation predominates over CeOz, Ti02, ZrOz, and ZnO. However, products of partial oxidation are also obtained, although it has been reported that TiOZ6p7and ZrOz7 yielded only COz. By contrast, Sn02,W03, and the mixed Sn-0-Sb oxide photocatalyze only partial oxidation, while Sb204yields a few percents of COz in addition to mild oxidation compounds. It has been reported7 that a SnOz sample yielded only traces of COz and no partial oxidation products. The photocatalytic properties of Ce02have only been briefly mentioned without examples.18 Some variations were also found for different samples of the same oxide. For instance, the formations of ethanal and acrolein did not range in the same order for the two S n 0 2 samples and total oxidation was preponderant over only one of the two ZnO samples (Table I). The TiOz samples (Table 111) also presented some differences and could be classified in two groups according to their origin. Among the products of partial oxidation, ethanal was generally formed preferentially to acrolein with the exception of the SnOz-l sample (Table I). In all cases, the percentage of acetone was small. Propene oxide was detected only in the case of the TiOz and ZrOz samples (Tables I and 111) and, as mentioned above, its relative percentage increased a t low conversion levels (Figure 2). These samples were the more photoactive; nevertheless the difference in quantum yields between the last three TiOz samples of Table I11 and the Sbz04sample was small and only a suspicion of a shoulder corresponding to propene oxide was observed in the chromatogram referring to propene oxidation over this latter catalyst. Traces of propanal were also found in some cases, but no CO was observed. Special chromatographic analysis showed that acids were not formed. Photocatalytic oxidation of organic compounds such as alkanes,14a l ~ o h o l s , ~and J ~ Jalkyl~~~~ toluenesz1 is also essentially limited to aldehydes and ketones as mild oxidation products. It is worth noting that the selectivities to acrolein of the Sn-0-Sb and Sn02-2 samples were inverted compared with those observed a t 653 K in the absence of UV light,13 while in both cases the Sbz04 sample presented an intermediate selectivity. More work is required to determine whether this was due to the effect of the decrease in temperature upon the adsorption equilibria of the products with respect to each catalyst or to some other cause. Preliminary experiments have been performed, with the Ti02-1sample only, to study the oxidation of the products of oxidation of propene under the same conditions. Acetone was stable as expected since it is, for instance, the main oxidation product of i s ~ b u t a n e . l ~Acrolein ~ ~ ~ ~ ~also J~ withstood oxidation, yielding only small amounts of COz. A substantial part of ethanal was transformed into COz. Propene oxide yielded all the other products including noticeable amounts of propanal. It may be noted that propanal was the major product, followed by CO C02, when a mixture of propene oxide and oxygen was contacted with supported molybdenum catalysts a t 693 KeZ2 In conclusion, Table I shows that the decrease in temperature, allowed by the activation by photons, does not improve the selectivity of the various catalysts to one particular product of propene oxidation. On the other
+
hand, oxygen isotopic exchange proceeds via the same mechanism irrespective of the UV irradiated samples (several TiOz,3Sn02-2,ZnO-1, or Zr02) and the activated arid dissociated oxygen species involved in this mechanism also appear to intervene in the photocatalytic oxidation of isobutane over Ti02.3,23We suggested that this oxygen species arises from the neutralization of adsorbed 0- ions by photoproduced holes.23 Thus, if the existence of the same kind of activated oxygen is assumed on the various oxides, it does not yield the same distribution of products in propene oxidation. This distribution depends on the oxide sample. Although the initial step, i.e., attack of an adsorbed hydrocarbon species by the activated oxygen, may be similar, the subsequent reactions might be quite different. In other words, as for thermal catalysis, no one single factor suffices to determine the selectivity; a concerted action of oxygen species and of the catalyst is involved. A “rake” mechanismz4may be relevant in agreement with the fact that all the oxidation products occurred simultaneously. The UV light does not influence the desorption equilibria of the products, at least for TiOz, since it has been found that COz and various alkenes, aldehydes, and ketones do not change the electrical photoconductivity of this Le., these compounds do not interact with the surface free electrons whose concentration is modified by the UV illumination. However, the decrease in temperature, which is required to obtain a better photonic activation, can affect these desorption equilibria. Nevertheless, UV irradiation a t a temperature sufficient for the formation of oxidation products in the dark has an effect on the selectivity (the conversion being increased). At this stage it would be premature to suggest a mechanism. However, it has been proposed26that the oxidation may proceed, a t relatively low temperatures, by water addition onto propene to give propan-2-01 which is then transformed into acetone. In the present case, no propan2-01 has been detected and the percentage of acetone produced is small in all cases. Consequently, this mechanism should be ruled out under the present conditions. On the other hand, water addition onto acrolein can yield 0-hydroxypropanal which easily dissociates into ethanal and methanal.27 Methanal would be readily oxidized to COz and H2O.I4 Our preliminary experiments showed that acrolein was relatively stable over UV irradiated TiOz a t 320 K and does not give rise to large amounts of ethanal, so that it seems the mechanism involving 0-hydroxypropanal can be disproved. On the other hand, the reaction of propene oxide with oxygen photocatalyzed by Ti02 yielded all the other oxidation products of propene (with, however, a much larger amount of propanal), and the relative percentage of this epoxide markedly increased a t low conversion levels (Figure 2). These facts seem to suggest that, in the case of TiOz,propene oxide might be the initial product arising from the attack of adsorbed propene by the activated and dissociated oxygen species mentioned above. Further work is required to determine the validity of this hypothesis. Finally, propene appears as a good model molecule to study photocatalytic oxidations and furthermore it seems that the results obtained might bring some help to interpret thermally catalyzed oxidations.
Conclusion This article shows that a large number of simple oxides were active for the photocatalytic oxidation of propene. The quantum yields differed greatly as a function of the catalyst and even according to the particular sample. Among the solids investigated only one TiOz sample had a quantum yield sufficient for a possible practical use. The formation of partial oxidation products mainly depended
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The Journal of Physical Chemistry, Vol. 83,
No. 24, 1979
Garfias
on the type of catalyst. In contrast with the behavior (9) M. Formenti, F. Juillet, and S. J. Teichner, Bull. SOC.Chim. Fr., 1031 (1976). found for thermal ~ a t a l y s i s , the ' ~ Sn-0-Sb mixed oxide (10) L. V. Lyashenko, G. I.Batalin, V. I. Stepanenko, and F. A. Yamexamined decreased the proportion of acrolein as compared pol'skaya, Ukr. Khim. Zh., 44, 1020 (1978). with the SnOz and Sb204samples. In all cases the oxida(11) N. Djeghri, ThBse, Lyon, France, 1978. (12) J.-L. Macqueron and A. Nouailhat, "Actes Colloque International tion was limited to the formation of aldehydes, ketones, CNRS", Marseille, France, 1965, CNRS, 1967, p 31. and epoxides, a t least for the small conversion levels ex(13) J.-M. Herrmann, J.C. Portefaix, M. Forissier, F. Figueras, and P. Pichat, amined. J . Chem. SOC.,Faraday Trans. 1 , 75, 1346 (1979). (14) N. Djeghri, M. Formenti, F. Juillet, and S. J. Teichner, FaradayDiscusS., The interest was also to show that no one single factor 58, 185 (1974). suffices to determine the selectivity for photocatalysis as (15) F. Juillet, F. Lecomte, H. Mozzanega, S. J. Teichner, A. Thevenet, for thermal catalysis. A mechanism cannot be proposed and P. Vergnon, Faraday Symp., 7, 57 (1973). on the basis of the present data, although in the case of (16) R. B. Cundall, B. Hulme, R. Rudham, and M. S. Saiim, J . Oil. Colour Chem. Assoc., 61, 351 (1978). TiOz the results appear consistent with the initial attack (17) J.-M. Herrmann, P. Vergnon, and S. J. Teichner, J . Catal., 37, 57 of adsorbed propene by an activated and dissociated oxy(1975). gen species to form propene oxide, in line with preceding (18) H. P. Boehm, Chem. Ing. Tech., 17, 716 (1974). I.~Bickley, G. Munuera, and F. S. Stone, J. Catal., 31, 398 (1973). (19) suggestions for the oxidation of other s ~ b s t r a t e s On . ~ ~ ~ ~(20) ~ ~J.R.~Cunningham, 9.K. Hodnett, and A. Walker, Proc. R . Irish Acad., the other hand, mechanisms proposed for propene oxida32, 411 (1977). tion a t relatively low temperatures can be ruled out in the (21) M.-N. Mozzanega, J.-M. Herrmann, and P. Pichat, TetrahedronLett., 2965 (1977). present case. (22) M. Che, F. Fgueras, M. Forissier, J. McAteer, M. Perrin, J . I . Portefaix, and H. Praliaud, Proc. Int. Congr. Catal., 6th, 1, 261 (1976). (23) J.-M. Herrmann, J. Disdier, M.-N. Mozzanega, and P. Pichat, J . Cafal., in press. (24) P. Boutry and R. Montarnal, C. R. Acad. Sci., Ser. C , 263, 1102 (1966). (25) J.-M. Herrmann, J. Disdier, and P. Pichat, "Proceedings of the 7th International Vacuum Congress and 3rd International Conference on Solid Surfaces", Vienne, Vol. 11, R. Dobrozemsky et ai., Ed., 1977, p 951. (26) Y. Mor0 Oka, S. Tan, and A. Ozaki, J . Catal., 12, 291 (1968). (27) F. Weiss, J. Marion, J. Metzger, and J.-M. Cognion, Kinet. Katal., 14, 45 (1973). (28) M. Guenin, Ann. Chim., 8, 147 (1973). (29) J.-P. Beaufils, J.-P. Bonnelle, and B. Gras, J . Chim. Phys., 62, 1005 (1965). (30) J.-E. Germain and R. Laugier, Bull. SOC.Chim. Fr., 541 (1972). (31) H. Mozzanega, J.-M. Herrmann, and P. Pichat, J. Phys. Chem., 83, 2251 (1979). (32) J.-M. Herrmann and P. Pichat, J . Chem. SOC.,Faraday Trans. 1 , in press.
Acknowledgment. The authors thank Dr. H. Praliaud for her advice on the UV reflectance spectra and Drs. M. Guenin, J.-L. Portefaix, and P. Vergnon for the gift of certain samples. References and Notes M. Formenti, ThBse, Lyon, France, 1974. H. Mozzanega, These CNAM, Lyon, France, 1975. H. Courbon, M. Formenti, and P. Pichat, J . Phys. Chem., 81, 550 (1977). H. Courbon and P. Pichat, C. R. Acad. Sci., Ser. C , 285, 171 (1977). A. Walker, M. Formenti, P. Meriaudeau, and S. J. Teichner, J. Catal., 50, 237 (1977). M. Formenti, F. Juillet, P. Meriaudeau, and S. J. Teichner, Chem. Techno/., 1, 680 (1971). L. V. Lyashenko, Y. B. Gorokhovat-skii, V. I. Stepanenko, and F. A. Yampol'skaya, Teor. Eksp. Khim., 13, 35 (1977). I. S. McLintock and M. Richtie, Trans. Farachy SOC.,61, 1007 (1965).
Solid Monolayers at the Water-Gas Interface F. J. Garflas Faculty of Chemistry, Universidad Nacional Autdnoma de M6xic0, M6xico 20, D.F. (Received March 27, 1979) Publication costs assisted by Consejo de €studios de Posgrado, U.N.A.M.
Due to the asymmetry of surface fields, it is proposed that the outermost layer of water molecules becomes highly struct'ired, leading to a polychair or polyboat surface network. When half of the water molecules of the surface layer are removed and replaced by fatty alcohol molecules, a condensed monolayer is produced. The film behavior of monolayers of fatty alcohols in water is mostly described in terms of three linear segments with two sharp discontinuities. The upper limit of the upper line corresponds to the collapse pressure and can be represented by a saturated polyboat network, whereas the phase transition at the upper discontinuity can be represented by the saturated polychair network. The saturated polychair and the saturated polyboat networks are assumed to mix ideally. The 1.025ratio of surface area at discontinuity to surface area at collapse for octadecanol suggests that the solid monolayer flattens the polyboat and polychair structures to produce an 0-0-C bond angle of 102.68". The virial theorem is applied to the saturated polyboat and polychair networks, and good estimates of surface pressure are made when a 9-6 Lennard-Jones potential is used, and when the midpoint of the C-C bonds of the hydrocarbon chain is considered as the center of interaction. The estimated surface energy of a water polychair network is 62 erg/cm2. Introduction In the determination of the 7i"A diagram of monolayers of fatty alcohols in water, Adam1 observed that the film behavior was mostly described in terms of three linear segments with two fairly sharp discontinuities. For octadecanol films a t 24.9 "C, the upper discontinuity appeared a t 13.3 dyn/cm, and the lower one a t approximately 1 dyn/cm. The upper line was steeper than the intermediate 0022-3654/79/2083-3126$01 .OO/O
line, with the bottom line having a very small slope. The upper limit of the upper line was the collapse pressure, that is, the highest pressure to which a monolayer was compressed without detectable expulsion of molecules to form a new phase. The phase corresponding to the highly incompressible, most closely packed condensed film is called ~ o l i d . ~The -~ more compressible intermediate line was referred to as 0 1979 American
Chemical Society
