The Journal of
Physical Chemistry
0 Copyrighi, 1985, by the American Chemical Society
VOLUME 89, NUMBER 15
JULY 18, 1985
LETTERS Use of the 7rc Parameter for Characterization of Rehybridization upon Adsorption on Metal Surfaces E. M. Stuvet and R. J. Madix* Department of Chemical Engineering, Stanford University, Stanford, California 94305 (Received: March 18, 1985)
A new parameter, called the TU parameter, is proposed as a measure of the extent of C2H4 rehybridization upon adsorption.
This parameter takes into account the vibrational coupling between the u(CC) and 6(CHz) modes of C2H4 The TU parameter ranges from zero for gaseous CzH4, to 0.38 for Zeise’s salt, taken as a model for *-bonded CzH4, to unity for C2H4Br2,taken as a model for di-&bonded C2H4 Values of the T U parameter calculated from the vibrational spectra of C2H4 adsorbed on clean and oxygen covered metal surfaces show good agreement with the *-bonded and di-u-bonded model compounds. Additionally those metal systems which show high values of TU dehydrogenate CzH4 whereas those with values less than Zeise’s salt bind CzH4 weakly and reversibly.
In this Letter, we introduce the TU parameter and show that it can be used to indicate the state of hybridization of adsorbed ethylene, correlating well with the reactivity of C2H4with the metal. The occasion for such a parameter arose from our previous studies of C2H4 adsorption and reaction on clean’ and oxygen covered2Pd( 100). Ethylene undergoes dehydrogenation following adsorption on clean Pd( 100). Preadsorbed oxygen inhibits dehydrogenation without completely blocking CzH4 adsorption, however. Vibrational spectroscopy has been used to show that di-a-bonded CzH4is stable on an initially clean Pd( 100) surface, whereas preadsorbed oxygen blocks CzH4 adsorption into the di-a-bonded form while still allowing *-bonded CzH4. (The two carbon atoms of C2H4 have approximately sp3 hybridization in the di-a-bonded form.) In general, we expect the reactivity of di-a-bonded C2H4 to be distinctly different from the *-bonded form, and thus, it is useful to have some measure of the state of adsorbed ethylene on metal surfaces. ‘Present address: Department of Chemical Engineering, BF-IO, University of Washington, Seattle, WA 98195.
Measurement of the vibrational spectrum of adsorbed C2H4 with high-resolution electron energy loss spectroscopy (EELS) is perhaps the most straightforward method for determining the state of hybridization due to the sensitivity of the C - C stretching frequency v(CC) to the hybridized state of the two carbon atoms. For example, u(CC) is 1974 cm-l for gaseous C2Hz, 1623 cm-’ for gaseous C2H4,and 993 cm-’for gaseous CzH6,which represent sp, spz, and sp3-hybridized systems, re~pectively.~For C2H4 adsorbed on metal surfaces in ultrahigh vacuum (UHV), u(CC) ranges from 1050 cm-’ on Pt( 111)4 to 1565 cm-I on oxygen covered Ag( 1 It has been customary to assign the 1050-cm-l band to adsorbed CzH4 with nearly sp3 hybridization (Le,, di-abonded), and the 1565-cm-’ band to CzH4with approximately (1) E. M. Stuve and R. J. Madix, J . Pbys. G e m . , 89, 105 (1985). (2) E. M. Stuve and R. J. Madix, Surf.Sci., in press. (3) G. Herzberg, “Molecular Spectra and Molecular Structure”, Vol. 11, Van Nostrand-Reinhold, New York, 1945. (4) H. Steininger, H. Ibach, and S. Lehwald, Surf.Sei., 117, 685 (1982). (5) C. Back, C. P. M. de Groot, and P. Biloen, Appl. Surf.Sci., 6, 256 (1980).
0022-365418512089-3183%01.50/0 0 1985 American Chemical Society
3184 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
spz hybridization, for which the n-bonded form is an appropriate model. It must be remembered, however, that the normal vibrational modes of a molecule involve motions of all the atoms in the molecule. In many instances these vibrational modes are uncoupled. That is, the normal vibrational mode involves motion of primarily only two or three atoms in the molecule. For these uncoupled modes, then, the observed vibrational frequency is said to be characteristic of the bond or bonds between the primary atoms of motion. This does not hold for C2H4 in which there is substantial coupling between u(CC) and the in-plane C H 2 scissoring mode 6(CHz). To understand this situation more fully, it is helpful to consider the u(CC) and 6(CHz) assignments for K[(CZH4)PtCl3](Zeise's salt), an organometallic complex in which CzH4 is n-bonded to a Pt atom. The assignment of the u(CC) and 6(CHz) modes in Zeise's salt has been the source of much discussion in the organometallic literature.68 The source of this controversy is that the v(CC) and 6(CHz) modes both have A I symmetry and are of approximately the same energy. This leads to vibrational coupling of these modes, which in turn results in a "mixing" of the two modes to form two new levels, neither of which represents the true u(CC) or 6(CH2) vibrations. Hiraishi6 assigned the two bands at 1243 and 1515 cm-I in Zeise's salt to u(CC) and 6(CHz) respectively, whereas Powell et al.7 and Hartleys preferred the opposite assignment. The vibrational coupling is much less of a problem for the deuterated form of Zeise's salt for which bands are observed at 962 and 1353 cm-1.6 The 6(CDz) fundamental in this case is of significantly lower frequency than that for u(CC) so the preferred assignments are u(CC) = 1353 cm-' and 6(CDz) = 962 cm-1.6 The isotope shifts (uH/uD) are then 1.57 for 6(CDz) and 0.92 for u(CC) according to the assignments of Hiraishi6 for K[(CzH4)PtC13]. These abnormal shifts can then be understood as a direct consequence of strong vibrational coupling in normal Zeise's salt vs. weak coupling in the deuterated form. In normal Zeise's salt the 6(CHz) frequency (1515 cm-l) is raised and the u(CC) frequency (1243 cm-I) is lowered due to vibrational coupling. In the deuterated form the large shift to lower frequency expected for 6(CDz) (to 962 cm-I) should substantially diminish the vibrational coupling and allow u(CC) to remain at a higher frequency of 1353 cm-' which is a more representative frequency for the C-C bond in Zeise's salt. (Of course, vibrational coupling still occurs in deuterated Zeise's salt and also tends to shift the u(CC) band.) It should be clear from the preceding discussion that one cannot speak of an unique 6(CD2) or u(CC) frequency for C2H+ In their study of olefins n-bonded to Pt and Ag, Powell et al.7 showed that both bands shifted to lower frequency with increased coordination of CzH4. They proposed that the sum of the percentage of lowering of these bands from the gas phase is a better measure of the degree of interaction. Borrowing from Powell, we propose a slightly different parameter for adsorption of ethylene on metals, called the n u parameter, defined as follows: n ~ ( C 2 H 4 )=
(
1623 -band I 1623
+
1342 -band 11)/o,366 1342
(1) where band I refers to the higher frequency and band I1 to the lower frequency of the u(CC)-G(CH2) pair. The numbers 1623 and 1342 are the u(CC) and 6(CHz) frequencies in cm-l for gaseous CzH4. The formula for CzD4 is 1515 -band I 981 -band I1 )/0.366 (2) n~(CzD4)= 98 1 1515
(
+
The n u parameter used here is the same as proposed by Powell et al.' except for the normalization factor of 0.366. This factor was chosen such that n u for C2H4Br2,a model for full di-u(6) J. Hiraishi, Spectrochim. Acta, Part A , 25A, 749 (1969). (7) D. B. Powell, J. G. V. Scott, and N. Sheppard, Spectrochim. Acta, Part A , 28A, 327 (1972). ( 8 ) F. R. Hartley in "Comprehensive Organometallic Chemistry", Vol. 6, G. Wilkinson, F. G. A. Stone, and E. W. A M , as. Pergamon. , Oxford, 1982, pp 654-656.
Letters
TABLE I: The T U Parameter for C2H,Adsorbed on Several Surfaces TU
system
ref
band I, cm-I
band 11, cm-'
parameter
C2H4Br2 Pt(ll1) Fe(ll1) Ru(001)" Ni( Ni(ll1) Pd( 100) Fe( 1 10) Pd( 111)" Ru(001) + 0 Zeise's salt Pd(100) t 0 Pt(ll1) t 0 Cu( 100) Ag(ll0) t 0 [Ag(CZH,)]BF, C2H4 gas
9 4 18 11 12 13 1 14 15 11 7 2 4 16 5
1420 1430 1385 1400 1390 1440 1455 1410 1502 (1355)' 1510 1515 (1340) (1370) 1560 1565 1579 1623
1019 1050 1115 1110 1130 1100 1135 1250 1229 (953) 1230 1243 (985) (970) 1290 1290 1320 1342
1.oo 0.92 0.86 0.85 0.83 0.80 0.78 0.55 0.43 (0.37) 0.42 0.38 (0.30) (0.27) 0.21 0.14 0.12 0
7 3
"Energy losses reassigned, see text. *Energy losses not assigned. Values in parentheses are for C2D4.
bonding, is unity (band I = 1420 cm-I, band I1 = 1019 cm-' 9 ) . The n u parameter therefore ranges from zero for gaseous CzH4, to 0.38 for Zeise's salt, to unity for CzH4Brz. Thus the larger the TU parameter, the greater the degree of rehybridization of ethylene. There is a slight drawback to the n u parameter in that n u for CzD4Brzis not unity, but 0.78 (band I = 1141 cm-', band I1 = 947 cm-' lo). As there is good agreement between the n u values of normal (0.38) and deuterated (0.34) Zeise's salt,7 we propose that n u values may be compared between CzH4 and C2D4 systems for T U less than about 0.4, but that for larger n u values, CzH4 systems should be compared with other C2H4 systems and CzD4 systems with other C2D4 systems. The nu parameters for molecular ethylene adsorption on single crystal metals in UHV are listed in Table I. Also shown are the frequencies used for the calculation of nu. The n u parameters for C2H4Br2,Zeise's salt, [Ag(C,H4)]BF4, and gaseous CzH4 are included for reference points. The continuous change in the n u parameter from zero to unity suggests that this parameter is a good quantity with which to access the degree of rehybridization of C2H4 It can be seen that the extent of rehybridization for CzH4 adsorbed on the clean group 8 metals19 is greater than in Zeise's salt, in agreement with previous claims of di-a-bonding of C2H4 to these surfaces. Only CzH4 on Pt( 111) has a value of n u similar to C2H4Brz,whereas the other systems fall at least 14% short. One could therefore conclude that CzH4 on P t ( l l 1 ) is the best example of di-a-bonded CzH4 and is almost completely rehybridized, whereas CzH4 on Fe(l1 l ) , Ru, Ni, and Pd( 100) does not show quite as much rehybridization. It is further evident from Table I that the extent of ethylene rehybridization on oxygen covered metal surfaces is much less than on their respective clean surfaces. The maximum T U value (9) T. Shimanouchi, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 39 Vol. 1 (1972).
(IO) J.'T. Neu and W. D. Gwinn, J . Chem. Phys., 18, 1642 (1950). Q.Broughton, and D. Menzel, Appl. Surf. Sci.,
(1 1) M. A. Barteau, J. 19, 92 (1984).
(12) S. Lehwald, H. Ibach, and H. Steininger, Surf. Sci. 117, 342 (1982). (13) S. Lehwald and H. Ibach, Surf. Sci. 89, 425 (1979). (14) W. Erley, A. M. Baro, and H. Ibach, Surf. Sci. 120, 273 (1982). (15) J. A. Gates and L. L. Kesmodel, Surf. Sci., 120, L461 (1982). (16) C. Nyberg, C. G. Tengstal, S.Andersson, and M. W. Holmes, Chem. Phys. Lett., 87, 87 (1982). (17) Barteau et al." designated CzH4adsorption on clean Ru(001) as di-u-bonded and on Ru(001) + 0 as a bonded. (18) U. Seip, M. C. Tsai, J. Kiippers and G. Ertl, Surf. Sci., 147, 65 (1984). (19) In this paper the periodic group notation is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e&, I11 3 and 13.)
-
J . Phys. Chem. 1985,89, 3185-3188 for C2H4 on an oxygen covered surface is 0.42 for Ru(OO1). Ethylene on oxygen covered Pd( 100) and Pt( 111) has x u values of 0.30 and 0.27, respectively. These x u values are sufficient to classify the adsorption of ethylene on oxygen covered surfaces as x-bonded. The lowest x u values are for the group 11 metals Cu(100) (0.21) and Ag(ll0) + 0 (0.14). Thesevalues imply that there is very little rehybridization and consequently a relatively weak bonding interaction between ethylene and these surfaces. The general trends in the xu parameter are in excellent agreement with the observed reactivities of C2H4 with these metals. Those with high values of n u dehydrogenate ethylene and form strong C2H4-metal bonds, whereas those with low values bind C2H4 weakly and reversibly.'s2 The capacity of the T U parameter to measure CzH4 rehybridization does not depend on the details of the structure of the C2H4 metal complex. For n-bonded C2H4 (nu I 0.4) the degree of rehybridization is small precisely because CzH4 interacts only weakly with the substrate via the n-electrons, regardless of the bond lengths. For strongly interacting C2H4 (nu 1 0.4) vibrational coupling between the external C2H,-metal vibrations (phonons) and the internal v(CC) and 6(CHz) modes should also be small since the external modes are generally less than 500 cm-' and far removed from the 1000-1450-cm-' range of the internal modes. In any case, it it important to remember that CzH4 rehybridization is reflective of the state of bonding of C2H4,and the n u parameter therefore is a measure of the bonding of adsorbed ethylene. The use of the x u parameter to characterize CzH4 adsorption suggests a reassignment of the bands previously assigned for CzH4 on Ru(001) and P d ( l l 1 ) . For Ru(001)," the assignments for di-u-bonded C2H4 were v(CC) = 1330 cm-' and 6(CH2) = 1400 cm-' for a n u value of 0.40. This value is very close to that for n-bonded CZH4 on Ru(001) + 0 of 0.42,"317 yet dehydrogenation on the clean surface implies stronger bonding. We propose the assignment of band I as 1400 cm-' and band I1 as 1110 cm-l on Ru(001) for a n u value of 0.85. Both assignments are tentative, however, since spectra for CzD4 on Ru(001) were not measured.
3185
It should also be noted that, for CzH4 on both Pd(100) and Ru(001), v(CH) increased on the oxygen covered surface, consistent with less rehybridization of ethylene upon adsorption. For Pd(lll),ls four bandswereobservedforC2H4at 1145, 1229, 1418, and 1502 cm-I. The assignments were v(CC) = 1502 cm-l and 6(CH2) = 1418 cm-' for a x u value of 0.05, which is almost a gas phase value. There were only two bands at 1335 and 953 cm-' for C2D4on Pd( 111) so the assignment of band I and band I1 is unambiguous; the n u value is 0.37, very close to that of Zeise's salt. If the isotope shifts between C2H4 and CzD4 on Pd( 11 1) are equated to those on Pd( 100) (1.58 and 0.93 for 6(CH2) and v(CC), respectively), then the assignments for CzH4 (C2D4) on Pd( 111) should be 6(CH2) = 1502 (953) cm-' and v(CC) = 1229 (1 355) cm-I. This gives a x u parameter of 0.43 for C2H4 on Pd( 111). This xu value implies that C2H4 is n-bonded on Pd( 111) which is unique when compared with di-a-bonding of CzH4 on the other clean transition metals in Table I. The origin of this apparent anomaly is unknown at this time.
Summary The x u parameter is proposed as a measure of the extent of C2H4 rehybridization upon adsorption. This parameter takes into account the vibrational coupling of the v(CC) and 6(CH2) modes and is therefore a more accurate probe of the C-C bond order than the v(CC) frequency alone. Values of the xu parameter range from zero for gaseous C2H4,to 0.38 for Zeise's salt, to unity for C2H4Br2.C2H4 adsorbed on Pt( 11 1) is a good example of di-ubonded CzH4 with its x u value of 0.92, whereas C2H4is n-bonded on oxygen covered, group 8 transition metals with n u values ranging from 0.27 to 0.43. Consideration of the x u parameter may also assist in the assignment of the vibrational frequencies of adsorbed C2H4. Acknowledgment. The authors gratefully acknowledge the support of the National Science Foundation (NSF-CPE 8320072). Registry No. C2H,, 74-85-1.
13C Magic Angle Spinning NMR Study of CO Adsorption on Ru-Exchanged Zeolite Y R. K. Shoemaker and T. M. Apple* Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0304 (Received: March 27, 1985; In Final Form: May 13, 1985)
Three types of adsorbed carbon monoxide are observed on Ru-Y zeolite by I3C magic angle spinning NMR: linear, bridged, and dicarbonyl CO. Samples exposed to CO at room temperature exhibit only linear and dicarbonyl species. At higher adsorption temperature bridged species are formed and a relative increase in dicarbonyl adsorption is observed. A smaller percentage of linear species is produced at higher temperature. The electronic environments of linearly bonded CO are more diverse than those of bridging and dicarbonyl moieties. COzis formed over Ru-Y zeolite upon initial exposure of the catalyst to CO at room temperature, apparently through reaction with unreduced metal oxide.
Introduction The adsorption of carbon monoxide on supported group 820 metals has received considerable attention due to the interest in catalytic hydrogenation of CO. Numerous studies of C O bonding to supported group 8 metals have been conducted using transmission IR,'-7 but few with 13C N M R have been A. C. Yang and C. W. Garland, J. Phys. Chem., 71, 1504 (1957). H. Arai and H. Tominaga, J . Catal., 43, 131 (1976). H. C. Yao and W. G. Rothschild, J . Chem. Phys., 68, 4774 (1978). J. T. Yates, Jr., T. M. Duncan, S. D . Worley, and R. W. Vaughas, J . Chem. Phys., 20, 1219 (1979). (5) J. T. Yates, Jr., T. M. Duncan, and R. W. Vaughan, J . Phys. Chem., (1) (2) (3) (4)
71, 15 (1979). ( 6 ) Y. Tanaka, T. Iizuka, and K. Tanabe, J. Chem. Soc., Faraday Trans. 1, 78, 2215 (1982). (7) B. L. Gustafson, Ph.D. dissertation, Texas A&M University, 1981.
0022-3654/85/2089-3185$01.50/0
Several I3C N M R studies have been conducted on metal carbonyls'O and supported metal It is generally accepted that three types of adsorbed CO exist on the surface of a supported-metal catalyst at or near room (8) T. M. Duncan, J. T. Yates, Jr., and R. W. Vaughan, J . Chem. Phys., 71, 3129 (1979). (9) T. M. Duncan, J. T. Yates, Jr., and R. W. Vaughan, J . Chem. Phys., 73, 975 (1980). (10) J. W. Gleeson and R. W. Vaughan, J . Chem. Phys., 78, 5384 (1983). (11) J. B. Nagy, M. van Eenoo, and E. G. Derouane, J . Catal., 58, 230 (1979). (12) E. G. Derouane, J. B. Nagy, and J. C. Vedrine, J . Catal., 46, 434 (1977). (13) W. M. Shirley, B. R. McGarvey, B. Mait, A. Brenner, and A . Cichowlas, J . Mol. Catal., 29, 259 (1985). (14) B. E. Hanson, G. W. Wagner, R. J. Davis, and E. Motell, Inorg. Chem., 23, 1635 (1984).
0 1985 American Chemical Society
