J. Phys. Chem. 1990, 94, 1154-1 157
1154
localized states, is modified at the mobility edge where D now depends on 7 as a power law with exponent LY = 0.57.
Acknowledgment. P.G.W. very much would like to thank Harry Drickamer for being such a wonderful role model for those striving to combine chemical and physical modes of thought.
D.A.E. and R.T.S. acknowledge useful conversations with Mark Friedrichs. This work was supported by NSF grant NSF-DMR 86-12860. Some of the computations were carried out at the National Center for Supercomputing Applications in Urbana, IL. We thank Karl Hess for providing time on his Ardent supercomputer.
Similaritles and Differences in the C-C Bond Scission of Ethylene and Acetylene on Supported I r and Pt Clusters Po-Kang Wang,+ Charles P. Slichter,*,t Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
and John H. Sinfelt Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: July 6. 1989)
We have used NMR to study the C-C bond scission of adsorbed acetylene and ethylene on supported Pt and Ir clusters. We have monitored the extent of C-C bond scission by measuring the I3C-I3C dipolar interaction. We have found that C-C bond scission of adsorbed ethylene has the same reaction path and activation energy (36 kcal/mol) on Pt and Ir. On the other hand, the predominant intermediate undergoing C-C scission in the case of adsorbed acetylene is different for the metals, and the activation energies are also different (37 kcal/mol on Ir and 53 kcal/mol on Pt). By measuring the I3C-'H dipolar interaction, we have found an extensive dehydrogenation of adsorbed acetylene prior to C-C bond scission on Pt clusters. The results are discussed in relation to Sinfelt's studies of the reaction kinetics of ethane hydrogenolysis and are shown to support the conclusions drawn by Sinfelt.
Introduction One of the most interesting phenomena in catalysis is the drastically different abilities of different metals to catalyze particular reactions. For instance, the rate of ethane hydrogenolysis (C2H6+ H2 2CH4) on Ir is about 6 orders of magnitude higher than that on Pt.I On the basis of the data of the reaction kinetics, Sinfelt concluded that the composition of the chemisorbed intermediate C2H, undergoing C-C bond scission depended on the metal.' By studying the temperature dependence of the reaction rate, he deduced barrier heights for breaking the C-C bond of 35, 36, and 54 kcal/mol for Os, Ir, and Pt, respectively. In this paper we report an N M R study of the C-C bond scission of adsorbed acetylene and ethylene on Pt and Ir clusters. (We include some data from acetylene on Os.) We contrast the similarities of the C-C bond scission of adsorbed ethylene on the two metals with the situation for acetylene in which the rate of C-C bond scission differs strongly between Ir and Pt. We show that the C-H bonds of the adsorbed acetylene break before scission on the C-C bond on Pt but not on Ir. We find that these results give a detailed confirmation of Sinfelt's earlier results on ethane hydrogenolysis by these metals. To set the present work in context, it is useful to keep in mind Sinfelt's work. For the various metals, he investigated how the rate of production of CH4 depended on the temperature and on the partial pressures of C2H6and H2. He concluded that the hydrogenolysis rate was limited by C-C scission in a chemisorbed species C2H,. We have previously determined the structure of adsorbed acetylene and ethylene on Pt, before and after C-C bond scission." The structure is determined by the N M R measurement of I3C-I3C and 13C-'H dipolar interactions and 13C line shapes. We have found that, before C-C scission, the structure of adsorbed ethylene on Pt is CCH3 (an ethylidyne species), the structure of the majority (about 3/4) of adsorbed acetylene on Pt is CCH,, and the structure of the minority (about 1/4) is HCCH. Recently,
-
'Present address: IBM Almaden Research Center, San Jose, CA 95120. *Also Department of Chemistry.
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our group5 has used deuterium N M R to confirm the formation of CCH3 from ethylene on Pt. After adsorbed ethylene or acetylene has been heated to 690 K, the C-C bond breaks and the remnants are mostly isolated carbon atoms, a small amount of methane, and h y d r ~ g e n .In ~ this paper we study the processes and the activation energies of C-C bond scission of adsorbed acetylene and ethylene. We compare the reaction on Pt with that on Ir and discuss the relation of our results to those of Sinfelt for the ethane hydrogenolysis reaction. Experimental Aspects Our samples are small Pt, Os, or Ir clusters supported on ?-alumina. The dispersions (percentage of metal atoms on the surface) are measured by hydrogen chemisorption. The dispersions of the Pt samples are 51% and 58%. The dispersion of the Os samples is 40%, and that of the Ir samples is 86%. The Pt and Ir samples are cleaned at 570 K under alternating flows of H 2 and O2 gas and then evacuated to lod Torr. The samples are cooled to room temperature under vacuum, I3C-enriched ethylene or acetylene gas is admitted, and the samples are sealed off in glass vials. Typical sample size is about 1 cm3. The ethylene coverages are 25% on Pt, 8% on Os, and 15% on Ir. The acetylene coverages are 25% and 50% on Pt, 28% on Os, and 25% on Ir. The low ethylene coverages on Os combined with its lower dispersion made studies of ethylene on Os substantially more difficult. Therefore, most of the data in this paper are concerned with Pt and Ir, though we report a bit for acetylene on Os. (1) Sinfelt, J. H. J. Catal. 1972, 27, 468; Prog. Solid State Chem. 1975,
IO, 5 5 . (2) Wang, P.-K.; Slichter, C. P.; Sinfelt, J. H. Phys. Rev. Lett. 1984, 53, 8 3 . Wang, P.-K. Ph.D. Thesis, University of Illinois, Urbana, IL, 1984. (3) Wang, P.-K.; Slichter, C. P.; Sinfelt, J. H. J . Phys. Chem. 1985, 89, 7606
"---I
(4) Wang, P.-K.; Ansermet, J-Ph.; Slichter, C. P.; Sinfelt, J. H. Phys. Rev. Lett. 1985, 55, 2733. ( 5 ) Zax. D. B.; Klua, - C. A.; Slichter, C. P.; Sinfelt, J. H. J . Phys. Chem. 1989, 93, 5009.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1155
C-C Bond Scission of Ethylene and Acetylene
In
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i 0
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200 400 600 800 1000 1200 1400 1600
r(,us) adsorption at room temperature (0)and after annealing to 391 (A),453 ( O ) , and 479 K (0). Solid lines are theoretical fits to the data. To study the reaction of adsorbed ethylene or acetylene, we have brought the samples through annealing cycles. In each cycle a sample, contained in the glass vial, is annealed in an oven held at an elevated temperature for 3 h, and then taken to 77 K for NMR structural determination. The oven temperature is then increased, and the cycle is repeated. In this way we can measure both the reaction path (the adsorbed species and the thermal evolution of its structure) and the activation energies.
Experimental Results Ethylene. We have used the so-called “slow beats” in the decay of I3C spin echoes for monitoring the extent of C-C scission after each annealing cycle.’ Figure 1 shows the slow-beat data of ethylene adsorbed on Pt. In the experiment we apply two rf pulses separated by a time interval r and observe a spin echo at a time T after the second pulse. The decay of the amplitudes of spin echoes versus the pulse separation r consists of two components. One is a simple exponential attributed to isolated 13Cnuclei. The other is an oscillating component, which also decays exponentially, attributed to ‘3C-13Cpairs. From the frequency of the oscillation as a function of T we obtain the C-C length. From the relative intensities of the two components we deduced the fraction of broken C-C bonds. We express this mathematically by saying the echo amplitude S ( t ) at time t = 27 is given by
+ (1 - a ) exp(-27/
A
T i ) (1)
where a is the fraction of 13C nuclei in I3C-I3C pairs, T2and T i are relaxation times, yc is the I3C gyromagnetic ratio, r is the C-C distance, 0 is the angle between one C-C bond and the external static magnetic field, and the notation ( )avg means we average the expression over the angle 0 for the powdered sample. There are thus four parameters, a, r, T2, and T i , to fit. If the C atoms were 100%I3C, a would tell one directly the fraction of C atoms that are bonded to another C atom. Since the isotopic abundance is however only 90%, there are some molecules that are I2C-l3C (or 13C-12C) pairs. Thus, though their C-C bond is intact, they do not produce beats. (Of course, we do not detect molecules made up of 12C-’2Cpairs.) The qualitative significance of the two terms is made clearer by examining Figure 1. The curve at room temperature largely arises from the first term, whereas the curve at 479 K is almost exclusively from the second term. Although the first term has an oscillatory behavior, the oscillation eventually dies out so that the curve becomes an exponential in T with time constant T2/2. The second term is an exponential (with time constant T2’/2) at all times including times near zero. Thus, when only the second term is present, if one fits the data at long times by a straight line, that line extrapolates back at T = 0 to the T = 0 data points. This is the situation for the data at 479 K. On the other hand, if one tried to approximate the room-temperature data, at long times
A 0
0
I
I
0
Figure 1. ‘)C spin echo amplitude of I3C2H4 on Pt at 77 K versus T after
S ( t ) / S ( O ) = a(cos ((y&/rJ) x (1 - 3 cos’ 8)r))avg exp(-2~/ T2)
c
b
400
200
Temperature
600 (K)
Figure 2. Fraction of broken C-C bonds of adsorbed ethylene on Pt (0) and on Ir (A)versus annealing temperature.
(beyond 800 I S ) by a straight line, it would extrapolate back to a T = 0 value of about 20, far below the experimental value of about 90. Thus, a good test for the amount of the second component that is present is first to fit the long-time data by a straight line and then examine how its intercept, when extrapolated to T = 0, compares with the actual data. Having gotten the best fit to the data, we can now take the measured a and from it deduce the fraction of C-C bonds that are intact and the fraction that are broken. Figure 2 shows the fraction of broken C-C bonds of adsorbed ethylene on Pt after each annealing cycle as deduced from values of a. We find that C-C scission takes place above 390 K and is complete at about 480 K. If we assume that the probability of breaking the C-C bonds per unit time is given by vo exp(-E/kBT) with vo of IO” Hz, from the temperature at which the adsorbed ethylene breaks up, we obtain an estimate of the activation energy, E, of 36 kcal/mol on Pt., The energy deduced is not significantly affected by the choice of yo. In Figure 1 we find that the frequency of the slow beats remains the same throughout the annealing steps. The fact that the slow-beat frequency is the same implies that after each anneal the bond length and the structure are the same. We therefore conclude that adsorbed ethylene remains as CCH3 prior to C-C bond scission. We have found the same C-C bond length (1.49 f 0.02 A), and therefore likely the same structure, after ethylene adsorption at room temperature on Os, Ir, and Pt. We have studied C-C bond scission of adsorbed ethylene on Ir and find that it also takes place at the same temperature (see Figure 2) as on Pt. From observing the frequency of the slow beats, we concluded that there is no structural change of adsorbed ethylene prior to C-C bond scission on Ir. Therefore, we concluded that adsorbed ethylene has the same reaction path and activation energy on Pt and Ir. Acetylene. On the contrary, there are striking differences between the C-C bond scission of adsorbed acetylene on Ir and on Pt. Figure 3 shows the fraction of broken C-C bonds versus annealing temperatures for acetylene on Pt and on Ir. We observe that most C-C bonds break at much higher temperatures on Pt than on Ir. If we take 470 and 670 K as the temperatures at which the C-C bond of adsorbed acetylene breaks on Ir and Pt, respectively, we obtain activation energies of 37 kcal/mol on Ir and 53 kcal/mol on Pt. We have found that, after adsorption at room temperature, the majority of adsorbed acetylene has the same C-C bond length (1.44 0.02 A) on Pt, Os, and Ir. Therefore, the majority of adsorbed acetylene starts out with the same structure (CCH2) on all three metals. For adsorbed acetylene on Ir after the various anneals we have observed no change in C-C bond length prior to C-C bond scission. For adsorbed acetylene on Pt, however, we have observed a smearing of the slow-beat pattern at and above 100 OC, well below the temperature at which C-C bonds break (see Figure 4). This shows itself formally in the value of T , of eq 1 needed to fit the
*
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The Journal of Physical Chemistry, Vol. 94, No. 3, 1990
Wang et al.
4 I
1
A
Room Temperoture 490 K 590 K
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Figure 3. Fraction of broken C C bonds of adsorbed acetylene on Pt (0) and on Ir (A)versus annealing temperature.
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’
Figure 5. SEDOR fraction of I3C2H2on Pt versus T’ after adsorption at room temperature (A)and after annealing to 490 (0)and 590 K (0). TABLE I: Barrier Energy (kcal/mol) for Various Processes C-C scission C-C scission rate-limiting step of metal of acetyleneo of ethylend ethane hydrogenolysisb os 37‘ 35 Ir 37 36 36 Pt 53 36 54
From N M R . From reaction kinetics. C Asubstantial fraction, but not all, C-C bonds break with this energy barrier.
Io”(!
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400
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Figure 4. 13C spin echo amplitude of 13C2D2on Pt at 77 K versus 7 after annealing to 325 (0),397 (O), 506 (A), and 561 K (0).Solid lines are theoretical fits to the data.
data. T2drops from 350 ps for the 325 K curve to about 200 ps for the 397 and 561 K curves of Figure 4. We believe the shortening of T2 for acetylene on Pt is not actually a faster relaxation but rather represents the formation of several surface species with different C-C bond lengths. Though each species would give a pattern similar to the 325 K curve, the positions of the peaks and valleys in the oscillation would differ among the species owing to their different bond lengths. Thus, though all would have a rapid drop for the first 100 ps, their oscillations at later times would not be in step, leading to a destructive interference compared to the case of only a single-bond length. We did not observe this kind of behavior with Ir. The existence of several C-C bond lengths for acetylene on Pt and thus several structures could easily arise from loss of hydrogen from some of the molecules. Therefore, we used IH-l3C spin echo double resonance (SEDOR) to investigate the extent to which H atoms were attached to C atoms. We have previously used SEDOR in the determination of the structure of acetylene and ethylene after adsorption at room temperature.24 We observe 13Cspin echoes by applying two I3C pulses. We then observe the reduction in the I3C spin echo amplitude when we apply an ‘H pulse between the two 13C pulses. Figure 5 shows the fractional reduction (so-called “SEDOR fraction”) versus the pulse separation T’ between the first I3C pulse and the IH pulse. It suffices to say that as f increases, the SEDOR fraction reaches a plateau, which is approximately the fraction of carbons bonded to hyd r o g e n ~ .From ~ Figure 5 we note that as adsorbed acetylene is annealed to higher temperatures, the fraction of carbons attached to hydrogens decreases. From earlier studies of acetylene on Pt we know that for samples annealed above 690 K the species produced after C-C bond scission is isolated carbon atoms: We shall assume initially for simplicity of discussion that even at lower temperatures the monocarbon fragments formed by C-C scission
contain no hydrogen. If we assume no carbon polymerization, the remaining species are dicarbon species. The dicarbon species can have (1) both carbons, (2) one carbon, or (3) neither carbon bonded to hydrogens. We can use the SEDOR data to give some estimate of the nature of the dicarbon species. Annealing to 490 K reduces the SEDOR fraction to 58% of its initial value, and annealing to 590 K reduces it to 38% of its initial value. This temperature is still well below the 670 K value at which the C-C bond scission becomes exceedingly rapid. One can then go through a calculation that includes both the isotopic abundance and some SEDOR calibration factors to find the spectra one expects to see under various assumed mixtures of species. If, for example, at 590 K the remaining pairs of C atoms were a mixture of C, and HCCH, 65% of the pairs would be (2,’s. If the pairs were a mixture of C2 and CCH, or C2 and CCH2, about 30% of the pairs would be C2(s. If all four species (C,, CCH, HCCH, CCH2) were present, the C i s would represent something between 30% and 65% of the pairs at 590 K. (If we included CH, CH,, or CH3 species, we would have an even larger fraction of pairs as C2) It is evident that acetylene on Pt loses much of its hydrogen before the C-C bond breaks at 690 K. (In contrast for acetylene on Ir, we found no loss of hydrogen before breaking the C-C bonds.) For acetylene on Pt, there is a gradual breaking of C-C bonds between 450 and 670 K followed by a precipitous breaking between 670 and 690 K. About half the C-C bonds are broken by 670 K, and the remaining half break above 670 K. A possible hypothesis is that the “gradual” temperature region represents a distribution in barrier energies such as might result from the presence of a number of species (e.g., CCH,, CCH, CCH3, HCCH) and that the “precipitous” temperature region corresponds to C-C breaking of a single species, perhaps C2. We discuss this further below. We have also followed C-C bond scission of acetylene on Os. There is a sharp break at 470 K as with Ir, implying a 37 kcal/mol barrier energy. However, the fraction of broken C-C bonds levels off well below 100%. We suspect a more complex process is taking place than on Ir. We have not yet investigated it.
Discussion In Table I we list the activation energy for C-C bond scission for several molecules on Os, Ir, and Pt, comparing data deduced by Sinfelt from reaction kinetics of ethane hydrogenolysis on Os, Ir, and Pt with NMR annealing data on Ir and Pt for acetylene
J . Phys. Chem. 1990, 94, 1157-1 164
and ethylene. We note first that the activation energies for C-C bond scission of acetylene are the same as the activations deduced from ethane hydrogenolysis kinetics. Sinfelt's conclusion that the rate-limiting step in ethane hydrogenolysis occurred was C-C scission for a C2H2species on Os and Ir, but a C2 species on Pt is qualitatively supported by the N M R evidence for substantial hydrogen loss from acetylene on Pt prior to scission and the close agreement between activation energies for the scission of C-C bonds in adsorbed acetylene and for ethane hydrogenolysis. Recall that there is a 6 order of magnitude difference in the rates of ethane hydrogenolysis on Pt and Ir. The N M R result that the adsorbed ethylene has the same energy barrier to C-C scission on Pt and Ir suggests that the species CCH3 is not an important intermediate in ethane hydrogenolysis on Pt. An important element in Sinfelt's analysis of ethane hydrogenolysis was the postulate that there was a quasi-equilibrium between the initial species (H2 and C2H,) and the species C2H, undergoing C-C scission. This implies that H is going back and forth between the initial and intermediate species. Thus, one expects some population of all the intermediate species. Obviously, the populations of the various species will depend on temperature. Thus, in our annealing experiments, during the cooling step, there may be substantial population shifts. Thus, the populations we record at 77 K may differ substantially from those at higher temperatures. However, to the extent that irreversible steps occur at the annealing temperature (e.g., C-C scission), our 77 K data are reliable. For this reason, the energies of Table I are meaningful, as is the fact that some C-C scission occurs for acetylene
1157
on Pt over a broad temperature range starting at about 350 K and extending to 650 K. Clearly, to study the reversible processes, one must do N M R measurements at reaction temperatures. Our colleague C. Klug is carrying out such studies. He has proposed that one source of C-C bond breaking on Pt below 650 K might be hydrogen-transfer reactions between adsorbed acetylene species leading to the formation of CCH3. This species can then undergo C-C bond scission as we know from Figures 1 and 2 at temperatures above 450 K. In fact, he already has some preliminary results using 2H N M R which show that CzD2coadsorbed with H on Pt at low temperatures forms CCD2H upon annealing. Thus, the existence of species formed by H loss from acetylene is accompanied by the possibility of formation of species containing more than two hydrogens, with the possibility of C-C bond breaking. An intriguing possibility suggested by the above analysis is that for Ir the near coincidence of the temperatures for C-C bond scission for acetylene and ethylene may be the result of a rapid conversion of acetylene to ethylidyne followed by C-C bond scission by a process we describe above. In conclusion, it seems clear the the N M R is observing phenomena that play a role in the kinetics of a catalytic reaction. Acknowledgment. This research was supported by the U S . Department of Energy, Division of Materials Research, under Contract No. DE-AC02-76ER01198, and by the Exxon Education and Research Foundation. Registry No. C2H4, 74-85-1; CCH2, 74-86-2.
High-pressure NMR Study of Transport and Relaxation in Complex Liquids of 2-Ethylhexyl Cyclohexanecarboxylate and 2-Ethylhexyl Benzoate J. Jonas,* S. T. Adamy, P. J. Grandinetti, Y. Masuda, S. J. Morris, D. M. Campbell, and Y. Li Department of Chemistry, School of Chemical Sciences, University of Illinois, Champaign-Urbana, Illinois 61801 (Received: July 1 1 , 1989)
The self-diffusion coefficients, densities, and shear viscosities of liquid 2-ethylhexyl cyclohexanecarboxylate (EHC) were measured as a function of pressure from 1 to 4500 bar within the temperature range from -20 to 8 0 'C. The Stokes-Einstein equation is applicable over 5 order of magnitude changes in self-diffusion and viscosity. The experimental data obtained are compared to those for the complex liquid of 2-ethylhexyl benzoate (EHB) in order to characterize the molecular structure effect of the replacement of the benzene ring with a saturated cyclohexyl ring. In particular, the low-temperature data suggest that conjugation of the phenyl ring with the ester group in EHB slows down diffusion and increases viscosity in comparison with EHC. Analysis in terms of the rough hard sphere model indicates a high degree of rotational-translational coupling which increases as density increases. By use of high-resolution, high-pressure NMR techniques the natural-abundance I3C spin-lattice relaxation times, T I ,and nuclear Overhauser enhancement were measured for each individual carbon in EHC and EHB over the same range of temperatures and pressures. An approximate analysis of the experimental T Idata indicates anisotropic reorientation with multiple internal rotations.
Introduction The results of systematic N M R experiments1V2on studies of liquids at high pressure have provided convincing evidence about the essential role of pressure as an experimental variable in the studies of the dynamic structure of liquids. In recent papers3v4 we presented the results of a 'H N M R study of self-diffusion in the complex liquid of 2-ethylhexyl benzoate (EHB), the structure of which is depicted in Figure 1. Selection of EHB for this study was motivated by a lack of understanding of the relationship ( 1 ) Jonas, J. Science 1982, 216, 1179. (2) Jonas, J. NATO ASI, Ser. C 1987, 197, 193. (3) Walker, N. A.; Lamb, D. M.; Jonas, J.; Dare-Edwards, M. P. J . Magn. Reson. 1987, 74, 580. (4) Walker, N. A.; Lamb, D. M.; Adamy, S.T.; Jonas, J. J . Phys. Chem. 1988, 92, 3675.
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between the molecular properties and bulk fluid properties of elastohydrodynamic (ehd) lubricant^,^ which operate under conditions of high pressure. In this respect EHB has been chosen as a model synthetic hydrocarbon based ehd lubricant, its molecular structure being complex enough to represent a real ehd fluid while still being simple enough to allow detailed investigation of its molecular dynamics. In our study4 we reported the selfdiffusion coefficients, densities, and viscosities of liquid EHB as a function of pressure from 1 to 4500 bar within the temperature range from -20 to 100 'C. The rough hard sphere (RHS) model analysis6 of the data indicated a high degree of coupling between the rotational and translational motions which increased as density (5) Dowson, D.; Hugginson, G. R. Elastohydrodynamic Lubrication; Pergamon Press: London, 1977. ( 6 ) Chandler, D. J . Chem. Phys. 1975, 62, 1358.
0 1990 American Chemical Society
