J. Phys. Chem. 1987, 91, 6241-6244 which made its application to the pentyl polyacid difficult and to the hexyl polyacid impo~sible.~ The fluorescence technique presented in this paper does not depend on the nature of the transition and is also applicable in the range where the polyacid is completely micellized. A comparison of the rate parameters with those obtained by others7,l5in similar luminescence quenching studies of sodium dodecyl sulfate micelles with the same probe and quencher shows that the values of ko and k , are of the same magnitude as ours. However, contrary to the experience with our systems, their values of A2 did not increase with quencher concentration but remained equal to ko. An explanation is therefore needed for the dependence of A2 on [Q] in our system. If we assume that transfer into the water medium is the only process by which a quencher leaves a micelle, then k+, the second-order rate constant for a quencher entering a micelle from the water phase, would be given by the equation
k+ = Kk(15) This leads to k+ being of the order of 10" M-' s- I for all our sample runs. However, this value is more than 1 order of magnitude larger than the value estimated for k+ by the Smoluchowsky equation." A more likely possibility would be a direct exchange of quencher between micelles for which several mechanisms have been prop~sed''-'~and which might be facilitated by the close (17) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. J. Phys. Chem. 1981,85, 1196.
6241
proximity of the micelles belonging to the same macromolecule. Such an intramolecular mechanism would not be possible for the sodium dodecyl sulfate systems. However, one would then expect that k-, as defined here, should increase with the intramolecular micelle concentration. While the higher value of k- at 6, = 1 in Table I11 might be considered in agreement with this expectation, the other k- data show no systematic increase with Om. Further work is needed to elucidate this mechanism. Conclusion This investigation gives direct evidence that a large hydrophobic polyacid molecule in its hypercoiled conformation consists of a large number of small intramolecular micelles. The aggregation number of these micelles is the same in the completely hypercoiled states and in the states consisting partly of micellar and partly of random coil conformations.
Acknowledgment. We are grateful to Dr. Linda Hade1 for her assistance in performing the dynamic luminescence experiments. We also wish to acknowledge a grant received in support of this work from the Charles and Johanna Busch Memorial Fund of Rutgers University. Registry No. R~(bpy),~+, 15158-62-0; 9-MeA, 779-02-2; DP-1700, 27966-67-2. (18) Lessner, E.; Teubner, M.; Kahlweit, M. J . Phys. Chem. 1981, 85, 3167. (19) Malliaris, A.; Lang, J.; Zana, R. J . Phys. Chem. 1986, 90, 655.
Kinetlcs and Mechanism of the Oxidation of Allyl Alcohol on A g ( l l 0 ) J. L. Solomon and R. J. Madix* Department of Chemical Engineerifig, Stanford University, Stanford, California 94305 (Received: May 4, 1987)
The adsorption and reaction of allyl alcohol on clean and atomic oxygen covered Ag( 110) has been investigatedwith temperature programmed reaction spectroscopy (TPRS). Allyl alcohol adsorbs and desorbs molecularly from the clean Ag( 110) surface. When the surface is covered with 0.1 monolayer (ML) of atomic oxygen, allyl alcohol forms an intermediate that reacts further to yield hydrogen, water, acrolein, and allyl alcohol at 310 K in a single rate-limiting first-order reaction. Increasing the oxygen coverage to 0.25 ML increases the amount of products formed as expected for activation of the allyl alcohol by the predosed oxygen. The results obtained are consistent with the formation of an alkoxy intermediate, which has also been observed for methanol and ethanol on this surface.
Introduction Significantly different surface reactivity may be observed in molecules containing more than one functional group than would be expected from independent studies of one or both functions alone. Thus, one group that interacts only weakly with a given surface can be held in proximity of the surface by a more strongly interacting function, possibly opening reaction channels of the weakly interacting group at elevated temperatures. The bifunctionality of allyl alcohol (H2C=CHCH20H) is revealed in the reaction of allyl alcohol with Pt, Ni, and Pd metal complexes. For example, it reacts with potassium tetrachloroplatinate(I1) (K2PtC14) to give (diallyl ether)PtC12.' The reaction of allyl alcohol with a mixture of bis( 1,5-cyclooctadienyl)nickel and triphenylphosphine (PPh,) at 30 OC causes the dismutation of allyl alcohol to yield C3Hs, Ni(CH2=CHCHO)(PPh3)2, and water in about a 1:l:l ratio.2 Allyl alcohol also reacts with Pd(PCy3), (PCy, = tricyclohexylphosphine) at 30 OC, causing condensation of the (1) Jones, R. J. Chem. SOC.A 1969, 12, 2477. (2) Yamamoto, T.; Ishizu, J.; Yamamoto, A. J. Am. Chem. SOC.1981, 103, 6863.
0022-3654/87/2091-6241S01.50/0 , I
I
-
allyl alcohol to yield diallyl ether and its coordination product, Pd(dially1 ether)(PCy3)., In each of the above reactions the 0-H bond has been activated, as evidenced by the formation of either acrolein or diallyl ether, and the double bond is involved in .rr bonding of the acrolein or diallyl ether molecule to the metal atom as shown in Figure 1. To the best of our knowledge, no studies of the bonding of allyl alcohol to the silver surface have been undertaken. Simple alcohols and unsaturated hydrocarbons, including methan01,~ethan01,~l-propanol,6 2-propanol,6 ethylene? propylene,8 and acetylene? do not react with the clean silver (1 10) surface. In the presence of adsorbed oxygen atoms, the hydroxyl proton of the alcohol reacts with the preadsorbed oxygen to yield (3) Yamamoto, T.; Akimoto, M.; Saito, 0.; Yamamoto, A. Organometallics 1986, 5, 1559. (4) Wachs, 1. E.; Madix, R. J. Surf. Sci. 1978, 76, 531. (5) Wachs, I. E.; Madix, R. J. Appl. Surf. Sci. 1978, I , 303. (6) Jorgensen, S. W.; Madix, R. J. Surf. Sci. 1983, 130, L291. (7) Barteau, M. A.; Madix, R.J. Surf. Sci. 1981, 103, L171. (8) Barteau, M. A.; Madix, R. J. J. Am. Chem. SOC.1983, 105, 344. (9) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1982, 123, 491.
0 1987 American Chemical Society
6242 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Solomon and Madix
0)
CHZ-CHCH20H / Ag ( 1 10) 230 K
b)
Ph3P
\
C)
CH2
/ ; : -
I
250.
I
350.
Temperature (K)
Pd
CY 3p
I
150.
y$ NCc” CH2
Figure 1. Metal complexes formed from the reaction of allyl alcohol with potassium tetrachloroplatinate(I1) (a) (ref l), with a mixture of bis(1,5-~yclooctadienyl)nickeland triphenylphosphine(PPh,) (b) (ref 2), and with P ~ ( P C Y ,(PCy, ) ~ = tricyclohexylphosphine) (c) (ref 3).
an alkoxy intermediate, RO(a), and an adsorbed hydroxyl group, OH(a). Furthermore, ethylene,’ propylene? and acetylene9exhibit a larger range of reactivity with the oxygen than is observed for the alcohols. Whereas ethylene does not react, propylene reacts to produce water, carbon dioxide, and surface carbon. Of the three, acetylene is the only one for which identifiable chemically bound intermediates have been isolated, namely, C,H(a) and C2(a). The pattern of reactivity observed in the simple alcoholse6 predict that the allyloxy (-OCH,CH=CH,; IUPAC name is 2-propenyloxy) intermediate will dehydrogenate to acrolein (CH2=CH2CHO) with poasible hydrogenation of the double bond to form propionaldehyde (CH3CH2CHO).Studies of the reaction over a silver sponge indeed report formation of acrolein,I0 and recent results in our laboratory of this reaction on C u ( l l 0 ) show abundant hydrogenation.” In this work we have examined the activation of the two functionalities on the Ag( 1 10) surface with temperature programmed reaction spectroscopy (TPRS)I2 to see if the stability of the allyloxy intermediate, expected from the reaction of allyl alcohol with surface oxygen, affords hydrogenation of the double bond. Experimental Section The TPRS experiments were performed in the stainless steel ultra-high-vacuum system described previ0us1y.l~ The UHV system was equipped with PHI Auger electronics, a cylindrical mirror analyzer, PHI four-grid LEED optics, an argon ion bombardment gun, and a UTI-IOOC quadrupole mass spectrometer. The silver crystal was cooled to 120 K by conduction through copper supports attached to a liquid nitrogen cooled reservoir. The crystal was radiatively heated from behind by using a tungsten filament with a heating rate of approximately 7 K/s. The temperature was monitored with a Chromel-Alumel thermocouple that was pressed into a hole on the edge of the crystal. By use of a computer, the mass spectrometer was multiplexed so that eight mass to charge ratios could be monitored simultaneously during (10) Imachi, M.; Cant, N. W.; Kuczkowski, R. L. J . Catal. 1982,75,404. (1 1 ) Brainard, R.; Peterson, C.; Madix, R. J., manuscript in preparation. (12) Madix, R. J. Science (Washington, D.C.) 1986, 233, 1159. (13) Madix, R. J. Surf. Sci. 1979, 89, 540.
Figure 2. TPD spectra for allyl alcohol ( m / e 57) desorbing from the clean Ag(ll0) surface. A heating rate of 7 K/s was used, and the allyl alcohol was dosed at 140 K. Curves shown are for 0.2-, 0.3-, 1.0-, and 3.0-langmuir doses of allyl alcohol, respectively.
a single TPRS experiment. Multiplexing of the mass spectrometer is essential to product identification in reactions of this complexity. The ionizer of the mass spectrometer was collimated so that interference due to desorption from the crystal supports and edges was suppressed. The silver surface was cleaned by argon bombardment followed by annealing to 735 K for 5 min. Sharp LEED spots were observed, confirming long-range order of the surface. The major surface contaminant, surface carbon, was removed from the Ag(ll0) surface by oxygen ~ 1 e a n i n g . l ~The surface was considered clean when the amount of carbon dioxide flashed from the surface was less than 10% of the amount of oxygen detected following a 300-langmuir exposure to O2 at 300 K. Reagent-grade allyl alcohol was dried over MgS04 and degassed by using five freeze-pumpthaw cycles. The oxygen and the allyl alcohol were dosed onto the single-crystal surface through separate stainless steel needle dosers. The atomic oxygen coverage was calibrated by using temperature programmed desorption (TPD). The peak area at saturation coverage was referenced to the known saturation coverage of 0.5 monolayer (ML).15 In the performance of experiments on the oxygen predosed surface, oxygen was dosed at 300 K so that only atomic oxygen was present on the surface even when the crystal was cooled.16 The crystal was then cooled to 150 K, and 1 langmuir of allyl alcohol was dosed, which resulted in multilayer formation. The surface was then annealed to 260 K to remove the molecular states and allowed to recool to 200 K. TPRS was then performed. Areas of individual TPRS peaks were calculated numerically to assist product identification. All doses are reported with uncorrected ion gauge readings. Results At the highest coverages studied allyl alcohol desorbed molecularly from the clean Ag(ll0) surface from states at 230 and 265 K (Figure 2), respectively. The 230 K multilayer state grew without limit up to a 3-langmuir exposure. The state at 265 K must be due to monolayer or submonolayer coverage. By use of the analysis of Redhead” for first-order kinetics and preexponential factor of loi3s-l, the desorption energy of the submonolayer state at the lowest coverage studied (T,= 255 K) was estimated to be 15.1 kcal/mol. An upward shift of the peak temperature is observed for the monolayer state, which may be indicative of (14) Barteau, M. A,; Bowker, M.; Madix, R. J. Surf. Sci. 1980, 94, 303. (15) Engelhardt, H. A,; Menzel, D. Surf. Sci. 1976, 57, 591. (16) Backx, C.; DeGroot, C. P. M.; Biloen, P. Surf Sci. 1981, 104, 300. (17) Redhead, P. A. Vacuum 1962, 12, 203.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Oxidation of Allyl Alcohol on A g ( l l 0 )
80.
-
60.
-
40.
-
20.
-
h
2
6243
C
3
200.
300.
400.
Temparoture (K)
Figure 3. TPR spectrum for a 1-langmuirdose at 140 K of allyl alcohol on the Ag(ll0) surface covered with 0.1 ML of O(a). After the allyl alcohol dose and prior to flashing, the surface was annealed to 260 K to remove the multilayer and monolayer states. A heating rate of 7 K/s was used. The curves shown correspond to allyl alcohol ( m / e 57), acrolein ( m / e 56), water ( m / e 18). and hydrogen ( m / e 2).
0(a) Coverage Variation 0 (a) coverage (ML)
Ea = 18.5
kcal
mo 1
0.035 0.012 0.0035
250.
300. 350. Temperature (K)
Figure 4. Change in m / e 56 signal as O(a) coverage was changed. The allyl alcohol dose was held constant at 1 langmuir and a heating rate of 7 K/s was used. The apparent activation energy, E,, was estimated from the peak temperature, assuming first-order kinetics and a preexponential factor of ioi3s-I.
attractive interactions within the adsorbed layer. On the oxygen predosed surface, in addition to these two molecular states, allyl alcohol, acrolein, water, and hydrogen were observed to form at 3 10 K. Carbon remains on the surface after the reaction. The amount of carbon was not quantified. Figure 3 shows the TPRS curves for the reaction of allyl alcohol with 0.1 ML of O(a) on the Ag(ll0) surface, following an anneal to 260 K to remove the multilayer and monolayer states. An analysis of the characteristic cracking patterns was performed to determine that the m l e 57 signal was due only to allyl alcohol and the mle 56 signal was due primarily to acrolein (see below). The effect of oxygen coverage on the m l e 56 signal is shown in Figure 4. A similar dependence is observed for m l e 57. As the O(a) coverage was increased from 0.0035to 0.25ML,the total amount of allyl alcohol and acrolein observed each increased (Figure 5). Because of the structural similarity of the four products expected, the cracking fractions of these molecules overlap significantly, and the individual contributions to the signals detected from each of the products must be distinguished. Table I lists the cracking fractions for the four possible products measured by
0.000
0. 125 0.250 O(a) Coverage (ML)
Figure 5. Amounts of allyl alcohol and acrolein produced for varying coverages of O(a). TABLE I: Cracking Fraction Distribution amt of products detected in each m / e channel relative to largest mle value mass to allyl charge ratio alcohol acrolein 1-propanol propionaldehyde 26 27 28 29 31 39 55 56 57 58 59 60
42 73 65 100 48 33 7 7 45 10
64 100 67 37
24 65 87 100 3 3
14 5 12 100
11 15 6 17 12 6
TABLE 11: Product Identification Using Cracking Fractions' area. V s exptl AA % of mean % % if 5% peak AA and ACR orig peak and std of 57 is m l e area. V s subtrd subtrd area dev PNAD 26 27 28 29 31 56 57 58
10.4 16.0 3.1 14.3 5.24 2.06 4.58 1.12
6.11 8.57 6.46 4.07 0.35 1.35
0.36 -0.41 0.45 0.75 0.35
3.5 -2.6 3.4 5.3 6.7
-3 f 6 -8 f 37 3f2 1f5 6f2
0.10
0.10
9.3
6f4
-4.5 -10 -22 -18 1.8 -44
"Example for m / e 26: 10.4 V s - (42/45)4.58 V s = 6.1 1 V s; 6.1 1 V s - (64/15)1.35 V s = 0.36 V s. AA = allyl alcohol = CH,=CHCH20H; ACR 3 acrolein aldehyde = CH3CH2CH0.
f
CH,=CHCHO; PNAD
propion-
TPD of condensed multilayers.l* Table I1 illustrates the data reduction to determine the products. The second column of the table lists the actual areas of the TPRS peaks for a single experiment. If we assume that only allyl alcohol contributes to the m l e 57 signal, corrections to the spectra for allyl alcohol and acrolein are straightforward using the data in Table I, and the reduction is shown in columns 3 and 4 of Table 11. The residual signal (column 5) shows that less than 10% of the original peak areas remain. Column 6 contains the mean percentage remaining (18) Peterson, C. Y . ,unpublished
results.
6244
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
t 0 Agcsi
H20
R
i
2 ROH CH22CHCH2
Figure 6. Catalytic cycle for the oxidation of allyl alcohol on silver. The extended silver surface is schematically designated by Ag(s), and reaction does not occur on a single silver atom.
and the standard deviation for three TPRS experiments monitoring the same set of masses. If, rather than attributing all the m / e 57 signal to allyl alcohol, it is assumed that 5% of the signal due to propionaldehyde and the remaining 95% is due to allyl alcohol, the results given in column 7 are obtained. Since, in general, the residual error in the reduction of the data is much larger by this assumption, we can conclude that very little, if any, propionaldehyde was produced in this reaction. The production of 1-propanol can also be eliminated because no m l e 60 signal was detected. Furthermore, 1-propanol and other similar hydrocarbons have many cracking fragments in the m l e 27-31 range, and if these products were produced, the residual areas in this range would not reduce to such small values. Thus, allyl alcohol and acrolein are the dominant reaction products of the intermediate produced via reaction of allyl alcohol with preadsorbed oxygen. Negligible hydrogenation of the double bond occurs.
Discussion It has been shown p r e v i o ~ s that l ~ ~methanol and ethanol react on the oxygen predosed surface to form alkoxy intermediates which further dehydrogenate upon heating to yield hydrogen, the corresponding aldehyde, and, by reversible hydrogenation, the parent alcohol. The results obtained here indicate that allyl alcohol is activated in analogous fashion to form an allyloxy intermediate which reacts via loss of the a hydrogen to yield acrolein, allyl alcohol, and hydrogen by recombination reactions. In addition, propionaldehyde and 1-propanol may be produced by hydrogenation of the double bond, if this bond is subject to attack by adsorbed hydrogen at 300 K. As noted above, hydrogenation of the double bond does not occur on Ag(ll0) in TPRS,even though reversible hydrogenation to the alcohol group of the bound allyloxy intermediate occurs. This result clearly indicates that stabilization of the double bond to 300 K in the vicinity of the surface does not lead to hydrogenation, whereas on Cu( 110) activation is observed." This behavior facilitates highly selective oxidation of allyl alcohol to acrolein. The increasing amounts of reaction product with increased oxygen precoverage (Figure 5) indicates that the allyl alcohol reacts with the O(a) as expected from rather general acid/base reactions of surface oxygen.I2 Furthermore, the constant peak temperature with increasing coverage indicates that the reaction of the surface intermediate is unimolecular, though the peak shape reflects the possibility of intermolecular interactions. Knowing the reaction is unimolecular and assuming a preexpontial factor of 1013s-l, an apparent activation energy of 18.5 kcal/mol was calculated. Both the evident reaction with oxygen and first-order kinetics of the rate-limiting step are consistent with the formation
Solomon and Madix of an allyloxy intermediate, particularly in view of the inertness of the double bond and the simultaneous evolution of allyl alcohol, acrolein, and Hz at 310 K. Exactly this pattern of reactivity is observed with other alcohol^.^" The water produced at 310 K can also be explained by the formation of an allyloxy intermediate. For methanol and ethanol, the alcohol, ROH, reacts with the predosed oxygen, O(a), to form an alkoxy intermediate, RO(a), and a hydroxyl group, OH(a). For allyl alcohol the allyloxy intermediate decomposes at 310 K to yield acrolein and allyl alcohol. A hydrogen atom is also liberated when acrolein is formed. The hydrogen atom can then combine with an allyloxy intermediate producing the allyl alcohol, with a hydroxyl group producing water or with another hydrogen atom producing molecular hydrogen. Since the observed peak temperature of 310 K is close to the reported peak temperature of 320 K for hydroxyl rec~mbination,'~ it is also possible that the water is a product of hydroxyl recombination. Therefore, the products observed at 310 K, acrolein, allyl alcohol, water, and hydrogen, are consistent with the formation an allyloxy intermediate produced via reaction of allyl alcohol with preadsorbed oxygen. Although the allyl alcohol molecule is bifunctional, the reaction center for oxidation on A g ( l l 0 ) is the alcohol linkage, since acrolein is the primary reaction product and no hydrogenation of the double bond occurs. It is possible, however, that the double bond is involved in the bonding of the intermediate to the surface. In the metal complexes previously menti~nedl-~ the unsaturated linkage of the allyl ether or of the acrolein is bonded to the metal center and remains intact even through reaction occurred at the alcohol function. Near-edge X-ray absorption fine structure measurements of the reaction intermediate giving rise to the products at 310 K on Ag(ll0) indicated that the double bond lies parallel to the plane of the surface without measurable change in the bond order at a surface temperature of 90 K,20 showing that the double bond interacts at least weakly with the surface. The double bond is not susceptible to hydrogen attack at 300 K, however, since none of the hydrogen released in formation of acrolein reacts in this way. The catalytic oxidation of allyl alcohol over a silver catalyst has been studied previously, and the results can be explained by the present findings on the (1 10) single-crystal surface. For example, acrolein was produced from the oxidation of allyl alcohol in the gas phase over a Ag wire.21 Allyl alcohol has also been oxidized by Ag2' in an acidic solution to acrolein with a 70% yield.22 Imachi et al.1° studied the oxidative dehydrogenation of allyl alcohol over a silver sponge catalyst. They reported a selectivity of 98.8% to acrolein with a total conversion of 96.6% in the presence of oxygen at a tempeature of 210 O C . In addition to acrolein, they reported the production of COz and HzO. The present study reveals that some carbon was left on the Agf 110) surface after the reaction. The COz production can be explained by oxygen in the feed reacting with surface carbon, producing COz. The present findings for Ag( 110) are then consistent with previous catalytic studies and suggest the catalytic cycle shown in Figure 6.
Acknowledgment. We gratefully acknowledge the support of the National Science Foundation (NSF CPE 83-20072) without which this study could not have been accomplished. Registry No. Ag, 7440-22-4; allyl alcohol, 107-18-6. (19) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Sur/. Sci. 1981, I l l , 11. (20) Solomon,J. L.; Madix, R. J.; Stohr, J. manuscript in preparation. (21) Morita, H.; Kawashima, E.; Nomura, K. Kanaruwu Daigaku Kogukubu Kiyo 1969, 5(4), 125. ( 2 2 ) Syper, L. Tetrahedron Lett. 1967, 42, 4193.
