J. Phys. Chem. 1991,95,6657-6661
6657
Identification of Ethylene-Derived Species on A120,-Supported Rh, Ir, Pd, and Pt Catalysts by Infrared Spectroscopy Sheher B. Mohsin, Michael Trenary,* Department of Chemistry, M/C I 1 I , University of Illinois, Box 4348, Chicago, Illinois 60680
and Heinz J. Robota Allied-Signal Engineered Materials Research Center, 50 East Algonquin Road, Box 5016, Des Plaines, Illinois 60017 (Received: February 26, 1991; In Final Form: April 10, 1991)
We have used infrared spectroscopy to study ethylene adsorption on alumina-supported Rh, Ir, Pd, and Pt at and below room temperature. The adsorption of ethylene at room temperature gives a set of infrared bands indicating the presence of both r-bonded ethylene and ethylidyne on the surface of each metal, At lower temperatures di-a-bonded ethylene is formed which converts to ethylidyne as the temperature is raised. We have used the C-C stretch and the CH2 scissors mode of r-bonded ethylene as a measure of the bond strength between the molecule and the metal surface and find that the bond strength follows the sequence Rh > Pd, Ir > Pt. We have attempted to explain the difference in the bond strengths among these metals by using simple qualitative arguments.
Introduction Group VI11 transition metals catalyze a variety of hydrocarbon reactions. There are often variations in activity along a transition-metal series for a particular reaction under the same reaction conditions. The variation in activity depends on the strength of the chemisorption bond between the reactants and the metal surface and can be represented by a "volcano curve".' Such a curve graphically reveals that activity goes through a maximum across a row in the periodic table. The maximum occurs because an optimum bond strength between the reactants and the surface is required for maximum activity. The bond must be strong enough to produce high surface coverage of the reactant molecules but not so strong as to immobilize the reactants on the surface. There are many examples of reactions in the literature that exhibit the volcano behavior. The hydrogenation of ethylene is one of them. For metals that belong to the first transition series, the rate of ethylene hydrogenation is maximum for cobalt. For the second transition series, rhodium yields the maximum rate.2 The rate of olefin hydrogenation for isolated double bonds follows the sequence Rh > Pt > Pd > Ni > Ir > Co > Fe > Cu > Ag.) Ultimately, the kinetics of catalytic reactions are related to the details of the adsorbate-surface interactions. The frequencies of adsorbate vibrations, which can be directly probed with infrared spectroscopy, are in general quite sensitive to the nature of the adsorbate-surface bonding. In this study, we have used infrared spectroscopy to investigate the nature of ethylene adsorbed on transition-metal catalysts. The specific metals studied are Rh and Pd which belong to the 4d series and Ir and Pt which belong to the 5d series. These metals along with Ru and Os constitute the platinum group metals. We have used ethylene as a probe molecule to study changes in the adsorption properties of the metals as a function of their position in the periodic table. Our results indicate that the adsorption of ethylene at room temperature on all these metals results in the formation of r-bonded ethylene and ethylidyne. We have used the frequency of the C-C stretch and the CH2 scissors mode of the r-bonded ethylene species as a measure of the bond strength between the molecule and the metal surface and find that the bond strength follows the sequence Rh > Pd, Ir > Pt. The bond strength between ethylene and the metals increases from right to left across the group VI11 series and down (1 ) Balandin, A. A. Adu. Catal. Relat. Subj. 1969, 19, I . (2) Bond, G. C. Heterogeneous Catalysis. Principles and Applications; Clarendon Press: New York, 1987. (3) Bond, G . C. Catalysis by Metals; Academic Press: New York, 1962.
0022-3654/91/2095-6657$02.50/0
the group. We have attempted to explain the differences in activity among these metals by using simple qualitative arguments.
Experimental Section A general description of the experimental apparatus and the procedure used for preparing Pt/A1203 catalysts has been described previously! The IR spectra were obtained with the sample held in an evacuable stainless steel cell with two flange mounted CaF2 windows. The cell is based on a design published by Yates and co-~orkers.~,~ It consists of a double-sided flange modified to include a liquid nitrogen cooled sample holder and a chromel-alumel thermocouple feedthrough. The catalyst sample is held in a copper ring inside the cell. Two different types of metal/A1203 sample mounts were used. One consists of a 1in.-diameter CaF2 window with a thin layer of metal/Degussa 7-A1203sprayed onto half of the disk and a thin layer of pure Degussa 7-A1203on the other half. This procedure is described in ref 5. The other sample mount consists of a conventional pressed disk of catalyst pressed into a hole in a 0.002-in.-thick mica sheet which is then clamped tightly to the CaF2 disk. The sample can be cooled to low temperatures by passing cooled nitrogen gas through the copper tubes. The temperature of the sample is monitored by a chromel-alumel thermocouple attached to the copper ring. Good agreement between the temperatures of the copper ring and the center of the CaF2 disk was found by Beebe et ala5 However, by attaching a thermocouple directly to the catalyst with a small drop of Torr Seal, we have found substantial differences between the catalyst temperature and both the copper ring and the CaF2 disk. However, it is unclear whether the catalyst temperature measured in this way reflects the actual temperature of the metal particles. Because of this uncertainty, we do not quote actual temperatures for the catalyst when it is cooled below room temperature. However, it is clear that we can cool the catalyst to temperatures low enough to stabilize surface species that are not observed at room temperature. A detailed description of the method we used for preparing and characterizing Pt/Alz03catalysts appears in an earlier publicat i ~ n .Briefly, ~ the 3% Pt/A1203catalyst is prepared by impregnating Degussa 7-Al2O3 with chloroplatinic acid. The catalyst is then sprayed onto a CaF2 disk. The catalyst coated CaFz disk is calcined at 673 K in flowing air followed by reduction in flowing (4) Mohsin, S.B.; Trenary, M.; Robota, H. J. J. Phys. Chem. 1 9 0 , 92,
5229.
( 5 ) Beebe, T. P.; Gelin, P.; Yatcs, Jr., J. T. Sur/. Sd. 1984, 148, 526.
(6) Wang, H. P.; Yates, Jr., J. T. J . Phys. Chem. 1984, 88, 852.
0 1991 American Chemical Society
Mohsin et al.
6658 The Journal of Physical Chemistry, Vol. 95, No. 17, 199'1
Pd
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WAVENUMBERS 1204
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I
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,
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I
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,
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WAVENUMBERS
Figure 1. IR spectra of ethylene adsorbed on Rh/A1202, Ir/AlzOz,
Figure 2. IR spectra of ethylene adsorbed on Ir/AI2O3at room temperature. Spectrum A was recorded after exposing the sample to ethylene and then evacuatingto lo-' Torr. Spectrum B was obtained after adding hydrogen to the sample in spectrum A and evacuating it to IO-' Torr.
Pd/AI2Oz,and Pt/AIZOzat room temperature. hydrogen at 773 K. The 3% Ir/AI2O3was prepared by impregnating UOP yA1203with chloroiridic acid. The catalyst was dried at 423 K for 2 h, calcined in air at 573 K, and reduced in flowing hydrogen at 773 K. The 1.6% Rh/AI2O3 and 1.6% Pd/AIz03 catalysts were prepared by impregnating UOP 7-AI2O3with the corresponding metal chlorides. The catalysts were dried at 413 K, calcined in flowing air at 623 K, and reduced in flowing hydrogen at 673 K. A calcination temperature of about 473 K is known to give the highest dispersions for Rh/AIz03 and Pd/ A1203. A higher calcination temperature was used to prepare Rh/AI2O3and Pd/AIzO3because we wanted the average particle size in all these catalysts to remain the same a t approximately 19 A in diameter. By using catalysts with the same average particle size, it was possible to eliminate differences in the reactivity that were due to dispersion. This enables the assignment of differences in the adsorption properties of the catalysts to differences in the chemical properties of the metals. Also, the weight loadings were chosen to achieve comparable atomic loadings to allow semiquantitative comparisons of IR intensities among the metals. Following the treatment described above, the samples were mounted in the IR cell. After the cell was evacuated to lo-' Torr, the catalysts were reduced at 573-623 K in a static pressure of 400 Torr of hydrogen. The IR cell was evacuated at the reduction temperature and then cooled to room temperature. The Pt/A1203 catalysts were characterized by using XRD (X-ray diffraction), STEM (scanning transmission electron microscopy), and gas titration with CO. These three methods yield an average particle diameter of about 19 A. The Rh/AI2O3, Pd/AI2O3,and Ir/Al2O3catalysts were also characterized with XRD. No metal particles were detected with this method, indicating a particle size less than about 35 A. CO titration was used to characterize Pd/AI2O3and Ir/AI2O3. The dispersion obtained with this method was 0.62 for Ir/Alz03 and 0.60 for Pd/AI2O3. This would correspond to an average particle diameter of about 18 A. Extended X-ray absorption fine structure was used to characterize the Rh/AI2O3catalyst. An average particle size of 15-18 A was obtained by using this technique. Results Infrared spectra of ethylene adsorbed at room temperature on Rh, Ir, Pd, and Pt are shown in Figure 1 . The results of Figure 1 were obtained by first filling the cell with 600 mTorr of ethylene, evacuating to IOd Torr, and then recording the IR spectra. The bands seen in the figure are due to *-bonded ethylene and ethylidyne. We have discussed in detail the assignments of the bands in our earlier study of ethylene adsorption on Pt/A120,. Briefly, ethylidyne is characterized by an intense CH, symmetric bend at 1330-1 350 cm-I, the C-C stretch at 1 1 1 0 - 1 156 cm-l, and the symmetric C-H stretch at 2880-2890 cm-I, the asymmetric CH3 bend near 1400 cm-' and its first overtone near 2800 cm-I. The *-bonded ethylene species is indicated by the C-C stretch at 1185-1 240 cm-' and C-H stretches greater than 2955
2871
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WAVENUMBERS
Figure 3. 1R spectra of ethylene adsorbed on Pd/AIzO3at room temperature, under the same conditions as for Figure 2.
p 3073 1342
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Figure 4. IR spectra of ethylene adsorbed on Pt/A1203at room temperature, under the same conditions as for Figure 2.
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Figure 5. 1R spectra of ethylene adsorbed on Rh/A1203 below room temperature. Spectrum A was recorded after exposing the sample to ethylene and then evacuating to Torr. Spectrum B was obtained after adding hydrogen to the sample in spectrum A and evacuating it to torr.
cm-I. The exact values for each metal are listed in Table I. The two species can be readily distinguished by the strong difference
Ethylene-Derived Species on A1203-SupportedCatalysts
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6659
TABLE I: Frequencies (cm-I) for Ethylidyne and *-Bonded Ethylene on Rh, Ir, Pd, and Pt on A1203
Rh/A1203 Ir/A1203 Pd/AlzOp fi/A1203 a-bonded a-bonded r-bonded n-bonded ethylidyne ethylene ethylidyne ethylene ethylidyne ethylene ethylidyne ethylene C-H stretch region
2885 2939
2966 3080
2896 2941
overtone of asymmetric CH, bend C-C stretch region CH2 scissors and CH3 deformation region
2797 1110 1342 1408
1222 1509
2816 1156 1350 1418
in the rate at which they react with hydrogen to produce ethane. As we showed in an earlier paper,4 at 200 K *-bonded ethylene reacts much faster than ethylidyne. Figures 2-5 show the results obtained after adding hydrogen to *-bonded ethylene and ethylidyne covered surfaces and then evacuating the IR cell. The bands that disappear (or are strongly attenuated in the case of Rh/ A1203),indicated by the arrows, are due to n-bonded ethylene. When ethylene was adsorbed on Pt/A1203at temperatures well below 300 K, we detected the presence of di-u-bonded ethylene. Unlike ethylidyne and r-bonded ethylene, which have numerous distinct bands, only one band characteristic of di-a-bonded ethylene: a C-H stretch at 2912 cm-I, was observed. This band was observed only on the Pt/A1203catalyst. For each catalyst we monitored the development of the ethylene bands as a function of time at low temperatures. In each case the increase in the ethyldiyne bands was not accompanied by a corresponding decrease in the *-bonded ethylene bands. A corresponding decrease was observed4on Pt/A1203for the di-u-bonded ethylidyne band at 2912 cm-I. For the Pt catalyst the IR spectra unambiguously demonstrate that the di-u-bonded ethylene, but not the *-bonded ethylene, is converted to ethylidyne. For these reasons we assume that di-u-bonded ethylene and not *-bonded ethylene is the precursor to the ethylidyne species on the Rh, Ir, and Pd catalysts even though it is not directly observed with IR spectroscopy. The absence of the bands due to *-bonded ethylene on Rh/ A1203and their small intensity on Pd/A1203at room temperature is in contrast to their high intensity on Pt/A1203. We have found that for Pt/A1203 the concentration of the *-bonded ethylene species formed after room-temperature adsorption of ethylene depends upon the final evacuation temperature used during the preparation of the catalyst. The 623 K evacuation temperature used in the present study produces very stable *-bonded ethylene on Pt. However, when the Pt/A1203 catalyst is evacuated at a temperature of 563 K or lower, results similar to those for Rh/AI2O3are obtained. Very little or no r-bonded ethylene was observed when the catalyst is exposed to ethylene and evacuated. We have attributed this effect to the presence of a small amount of adsorbed hydrogen on the surface. In a separate publication’ we have discussed in detail the role the hydrogen remaining on the surface following a low pretreatment temperature has in reducing or eliminating the *-bonded ethylene observed with IR on Pt/A1203. We have found that this hydrogen alters the sites so as to give a weaker bond between Pt and *-bonded ethylene. Unlike hydrogen adsorbed a t room temperature, the hydrogen that destabilizes *-bonded ethylene is unreactive. This is demonstrated by the observation that once the catalyst has been prepared by either the low-temperature or high-temperature pretreatment, its behavior toward *-bonded ethylene is unaltered by subsequent treatments with H2 at room temperature. Once a particular IR measurement on adsorbed ethylene is completed, the catalyst can be regenerated by roomtemperature H2 exposure followed by evacuation. However, if ethylene forms a strong *-bond prior to this room-temperature exposure, it continues to do so following many cycles of roomtemperature exposure. Similarly, if the catalyst initially forms a weak r-bond to ethylene, many cycles of room-temperature H2 treatment do not alter this behavior. The hydrogen adsorbed at (7) Robota, H. J.; Mohsin, S. B.; Trenary, M. To be published.
2978
2871 2931
2966 3079
2887 2943
1239 1513
2803 1128 1342 1410
2773 1188 1504
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2955 2998 3018 3073 1204 1499
room temperature readily reacts with ethylene to form ethane, which can be detected by using both mass spectrometry and IR spectroscopy. However, if ethylene is added to a 563 K pretreated catalyst that has not been exposed to hydrogen a t room temperature, no ethane is detected. This demonstrates that the strongly bound H atoms resulting from the 563 K pretreatment are nonreactive. These observations on Pt/AI2O3 catalysts suggest that a pretreatment temperature higher than 623 K may be needed to produce stable *-bonded ethylene on Rh/AI2O3. Room-temperature adsorption of ethylene on Rh, Pd, and Pt supported on AI,O, has been studied by Beebe and Yatesa Their results differ from ours by a complete absence of bands due to r-bonded ethylene on all the metals studied. The only species they identified was ethylidyne. This difference is likely caused by their catalyst pretreatment temperature of 475 K. According to our studies,’ such a low pretreatment temperature would not yield stable 17bonded ethylene. Intense bands due to *-bonded ethylene on Rh/AI2O3 are observed when ethylene is adsorbed below room temperature. Figure 5 shows ethylene adsorption on Rh/AI2O3 below room temperature and the effect of adding hydrogen to the sample without changing the sample temperature. Bands due to *-bonded ethylene can be clearly seen in the figure which immediately decrease in intensity upon adding hydrogen. As noted in the Experimental Section, we do not have confidence in the accuracy of our temperature measurements below 300 K. However, we are clearly able to cool the sample enough to stabilize r-bonded ethylene which is not observed under the same conditions at room temperature. The frequencies and assignments of Rbonded ethylene and ethylidyne bands on Rh, Ir, Pd, and F’t are listed in Table I. Results similar to ours were obtained by Soma, who had studied the adsorption of ethylene at low temperatures on Rh, Pd, and Pt supported on A1203.9
Discussion IR studies of ethylene adsorption on supported metal catalysts are dominated by the observation of ethylidyne. Similarly, studies of ethylene bonding on clean single-crystal metal surfaces focus on either ethylidyne or its precursor, di-u-bonded ethylene. With the exception of Pd single-crystal surfaces,I2 *-bonded ethylene is most readily observed on single crystals in the presence of preadsorbed 0 or C atoms.I0 Thus, in the absence of substantial evidence for the importance of r-bonded ethylene on supported metal catalysts, its role in reaction pathways receives little attention. However, when considering reaction pathways of alkenes (of which ethylene is the prototype) on supported metal catalysts, it is important to identify all possible adsorbed species. Early IR investigations by Soma on Pt, Rh, and Pd catalysts supported on y-A1203found evidence for n-bonded ethylene on all of these metals9 More recently, our own detailed investigation of ethylene adsorption on Pt/yA1203 catalysts revealed that r-bonded ethylene is nearly as prevalent as e t h ~ l i d y n e . ~ This study has expanded on our previous investigations to include Pd, Rh, and Ir supported on y-A1203. As with supported Pt catalysts, *-bonded ethylene can be observed readily under the (8) Beebe, Jr., T. B.; Yates, Jr., J. T. J . Phys. Chem. 1987, 91, 254. (9) Soma, Y. J . Carol. 1979, 59, 239. (IO) Sheppard, N. Annu. Reo. Phys. Chem. 1988, 39, 589.
Mohsin et al.
6660 The Journal of Physical Chemistry, Vol. 95, No. 17, 199'1 appropriate conditions. However, the intensities of the *-bonded ethylene bands are typically weaker than on Pt. This is most notable Over supported Pd catalysts. Very weak ethylene-derived spectra on supported Pd catalysts have been previously noted.* Also, the stability of the metal-*-bonded complex can vary. This is most apparent on our Rh catalyst where room-temperature spectra reveal only weak absorption lines, while those obtained at lower temperatures are quite strong (compare Figures 1 and 5). These impediments to identifying *-bonded ethylene are likely responsible for its infrequent observation on supported metal catalysts. By taking advantage of the differences in hydrogenation rates between ethylidjne and *-bonded ethylene (seeFigures 2-5), we were able to clearly demonstrate the presence of *-bonded ethylene and to identify its spectral features. Ethylidyne is believed to form at sites of threefold symmetry on planar faces of the small crystallites present in supported catalysts. Formation of a *-bonded ethylene-metal complex should not be subject to such a restrictive geometrical requirement. Numerous inorganic complexes" exist where ethylene forms a *-bond to a single atom. Thus, disruption of a planar surface through the adsorption of foreign atoms, such as 0 and C, apparently suffices to block the threefold sites while still leaving single atom sites available for *-complex formation. In supported catalysts, the small crystallites will have many single atom sites along the boundaries where planar surfaces meet. From the measurements made in this investigation, it is not clear whether these edge sites or partially blocked planar sites are primarily responsible for *-complex formation. However, measurements in our laboratories on several Pt/y-Alz03catalysts of varying Pt particle size indicate that ethylidyne is favored relative to *-bonded ethylene as particle size increases. This implies that the edge sites, which are relatively more abundant on smaller particles, are those primarily responsible for the r-bonded ethylene-metal complexes observed in all of these supported catalysts. The bonding of *-bonded ethylene to transition metals is similar to the bonding of CO in that it involves both donation from a bonding orbital of the adsorbate and back-donation into an antibonding orbital. For ethylene, the donation of electrons is from the 2~-orbitalto the metal and back-donation of electrons from the metal to the 2~*-orbital.The greater the back-donation into the 2**-orbital, the greater is the bond strength between ethylene and the metal. Backdonation into the 2~*-orbitalmakes the C-C bond weaker, which reduces the vibrational frequency of this bond. Therefore, a smaller value for the frequency of the C-C stretch would indicate greater interaction between the metal and *-bonded ethylene. As can be seen in Table I, the C-C stretching frequency of *-bonded ethylene on these metals falls in the range 1180-1250 cm-l which seems to be low for the C-C stretch of a sp2-hybridized hydrocarbon. (The C-C stretch of gas-phase ethylene is about 1623 cm-I.) This is because the C-C stretch and the CHz scissors mode of n-bonded ethylene are strongly c o ~ p l e d . ' ~ JTherefore, ~l~ the C-C stretch of *-bonded ethylene by itself is not a good indication of the extent of interaction with the metal. In their investigation of ethylene adsorption on Pd single crystals, Stuve and MadixI3 proposed the use of a TU parameter to better characterize the ethylene-metal interactions. This parameter gives an indication of the degree of ethylene rehybridization upon adsorption or the extent of ethylene interaction with the metal. It is defined as
T-u Parameter
1;
171 1; 1 0.52
0.49
F m 6. Relationshipbetween the T U parameter defined in the text and the metal's position in the periodic table.
where 1623 and 1342 cm-I are the C-C stretch and CH2scissors modes of gas-phase ethylene. Band I refers to the higher frequency and band I1 to the lower frequency of the C-C stretch-CH2 scissors coupled pair. A normalization factor of 0.366 was used
so that TU is unity for CzH4BrZ,a model for di-a-bonded ethylene where the carbon atoms in ethylene are purely s$ hybridized. This formula gives a TU value of 0 for gas-phase ethylene and 0.38 for r-bonded ethylene in Zeise's salt. By use of this formula, the xu parameter is 0.44 for Rh, 0.52 for Ir, 0.40 for Pd, and 0.49 for Pt. Recently, Lapinski and Ekerdt have found that for r-bonded ethylene on Ni/AI2O3 the T U parameter is about 0.35." Figure 6 shows the arrangement of these metals in the periodic table and the TU parameter for r-bonded ethylene. The TU parameter for Ni/Al2O3has also been included in the figure and fits very well with the trends we observe. The higher the value of the Tu parameter, the greater is the interaction with the metal. It can be seen that the 'AU parameter increases down a group and increases from right to left across a period. It follows that the bond strength between ethylene and the metals increases from right to left across a series and down a group. The reason the 2r*-orbital of ethylene plays such an important role in the bonding of adsorbed ethylene is that this orbital is closest in energy to the Fermi level of the metals. For example, from the theoretical calculations carried out by Zheng et a1.,I5 the difference between the Fermi level of Pt and the 2worbital of ethylene is about 2.8 eV, whereas the difference between the Fermi level and the 2**-orbital of ethylene is only 0.8 eV. Therefore, the 2**-orbital would be expected to interact more strongly. The role of the Fermi level in the bonding of molecules to metal surfaces has been discussed in detail by Hoffmann and his group.15*'6The Fermi level for the transition metals rises from right to left across a period for Pt group metals and down the group. This makes electron transfer to the 2a*-orbital of ethylene easier. The ionization potential of an electron in the d orbital decreases from right to left across a period. This means that the ease with which electrons can be donated from the metal to the 2?r*-orbitalof ethylene increases in this sequence. All these factors contribute in making the bond between a-bonded ethylene and the metals stronger as one moves from right to left across a series and down the group. The vblcano behavior observed for olefin hydrogenation can be explained in light of these arguments. It was demonstrated in an earlier paper" that *-bonded ethylene and di-a-bonded ethylene species are the active species involved in the hydrogenation of ethylene. Since the bond strength between *-bonded ethylene and the metals increases from right to left across a series, we would expect an optimum bond strength somewhere in the middle of the series which corresponds to the maximum in the volcano plot. This maximum occurs at Co for the first transition series and at Rh for the second transition series? If comparison is made between the metals studied here, the rate The of hydrogenation follows the sequence Rh > Pt > Pd > fact that Rh has the highest activity in the 4d series is consistent with the fact that ethylene is very strongly bonded to Ir and weakly to Pd. It thus appears that the optimum ethylene-metal bond
(11) Mingos, D. M. P. Bonding of Unsaturated Organic Molecules to Transition Metals. In Comprehensive Organometallic Chemistry;Abel, E. W.,Ed.; Pergamon Press: New York, 1982; Vol. 3. (1 2) Hiraishi, J. Spectrochim. Acta, Part A 1969, 25, 749. (13) Stuve, E. M.; Madix, R. J. J . Phys. Chem. 1985.89, 3183.
(14) Lapinski. M. P.;Ekerdt, J. G. J . Phys. Chem., in press. (15) Zheng, C.; Apelog, Y.;Hoffmann, R.J . Am. Chem. Soc. 1988,IlO, 749. (16) Hoffmann, R.Reu. Mod. Phys. 1988,60, 601.
1623 - band I + 1342 - band 11)/o,366 1623 1342
~~~~~~
~
J. Phys. Chem. 1991, 95,6661-6666 strength for hydrogenation occurs for Rh and Pt. It is of interest to compare the trends in the stability of ethylene in organometallic complexes of different metals with our results. For metals with a d8 configuration in complexes of the formula [ ( ~ l e f i n ) ~ M C I ~the ] , 'stability ~ of ethylene increases down the group: NilI < PdlI < PtS. The stability also increases by lowering the formal charge on the metal: Ir' > Pt" in compounds of formula [ ( ~ l e f i n ) ~ M C I ] ~This . ' ~ is because back-donation is more facile if the formal charge on the metal is lower. These trends are similar to the trends observed in our experiments. A number of elementary steps are involved in the conversion of di-a-bonded ethylene to ethylidyne as it is unlikely that the conversion occurs in one concerted step. These steps should involve the breaking of a C-H bond and the transfer of a hydrogen atom from the a-carbon attached to the metal to the &carbon. From NEXAFS and thermal desorption experiments, ZaeraI8 has proposed that the slow step in the reaction is the breaking of the C-H bond. He has proposed a vinyl species (-CH=CH2) as the intermediate in the conversion of di-a-bonded ethylene to ethylidyne. Breaking of a C-H bond involves transfer of electrons from the a-orbital of C-H to the metal and back-donation of electrons from the metal to the a*srbital. This process is similar to the bonding of *-bonded ethylene explained above. Theoretical calculations have been carried out by Saillard and Hoffmann to understand C-H bond activation in methane on Ti and Ni sur-
6661
faces.19 The Fermi level of Ti is about 2 eV higher than in Ni. Since the a*-orbital of C-H is high in energy, C-H bond weakening will increase as the Fermi level increases. They have found that the electron density in the a*-orbital of C-H is higher for methane on Ti than on Ni. Therefore, the ability of the transition metals to break C-H bonds should increase from right to left across a series. If the breaking of a C-H bond is the ratelimiting step in the conversion of di-a-bonded ethylene to ethylidyne, one would expect the activation energy for ethylidyne formation to decrease from right to left across a series.
Conclwions Infrared spectra obtained upon ethylene adsorption on Pt, Pd, Rh, and Ir directly reveal the presence of both r-bonded ethylene and ethylidyne. At lower temperatures the presence of di-a-bonded ethylene is indirectly implicated by the formation of ethylidyne without an accompanying decrease in the amount of r-bonded ethylene. Using a presumed correlation between the frequencies of the C-C stretch and the CH2 modes of the *-bonded ethylene and the strength of the adsorbatesubstrate bond, we find that the bond strength increases from left to right across a series and down a group. This in turn correlates with the ability of the metal to donate electrons to the 2r*-orbital or *-bonded ethylene. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
(17) Carra, S.;Ugo, R. Inorg. Chim. Acta, Rev. 1967, I , 49. (18) Zaera, F. J . Am. Chem. Soc. 1987, 108, 3554.
(19) Saillard, J. Y.; Hoffmann, R. J . Am. Chem. Soc. 1986, 106,2006.
Free Energy Calculations on the Relative Solvation Free Energies of Benzene, Anisole, and 1,2,3-Trlmethoxybenzene: Theoretical and Experimental Analysis of Aromatic Methoxy Solvation Lee F. Kuyper, Robert N. Hunter, Wellcome Research Laboratories, Research Triangle Park, North Carolina 27709
David Ashton, Wellcome Research Laboratories, Beckenham, Kent BR3 3BS. England
Kenneth M. Merz, Jr.: and Peter A. Kollman* Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94143 (Received: February 27, 1989; In Final Form: January 10, 1991) We have carried out experimental determinationsof the free energy of solvation of anisole, 1,2-dimethoxybenzene(DMB), and 1,2,3-trimethoxybenzene (TMB) in water and perturbation free energy calculations on the relative aqueous solvation free energies of benzene, anisole, and TMB. The measured differences between the relative experimental free energies of solvation of benzene, anisole, DMB, and TMB support the concept of near additivity of aromatic methoxy group contributions to such gas phase to water transfer free energies. Calculated differences in solvation free energies were shown to be sensitive to the choice o f electrostaticcharge distribution model. Quantum mechanical electrostaticpotential fit charge models from STO-3G, 4-31G, and 6-31G* basis sets were compared for their ability to reproduce the relative free energies of solvation found experimentally. The 6-31G* charge model was the best in this regard and the STO-3G model was next in quality, but the 4-3 1G model significantly overestimated the effect of O-CH3 substitutionon solvation free energies. Models based on scaled 4-3 1G charges also produced reasonable results. Introduction
An understanding of molecular solvation is critical in studies of molecular association. Exciting recent developments in computer simulation methods are beginning to allow the calculation of free energies of solvation for a wide variety of molecules.14 These solvation free energies are interesting per se and are im'Current address: Department of Chemistry, Pennsylvania State University, University Park, PA.
0022-365419112095-6661$02.50/0
portant in determining the thermodynamicsof association between molecules. The methoxybenzenes show some interesting and unique solvation properties: their octanol-water partition coefficients suggest ( I ) Jorgenscn, W.; Ravimohan, C. J . Chem. Phys. 1985,83, 3050. (2) Bash, P.; Singh, U. C.; Langridge, R.; Kollman, P. A. Science 1987, 236. 564. (3) Fleischman, S. M.; Brooks 111, C. L. J. Chem. Phys. 1987,87, 3029. (4) Singh, U.C.; Brown, F. K.; Bash, P. A.; Kollman, P. A. J. Am. Chem. SOC.1987, 109, 1607.
0 1991 American Chemical Society
