254
J. Phys. Chem. 1987, 91, 254-257
hopping between these rings should be fast as seen for the reduced RuL3"+ type complex. An Arrhenius plot (In line width vs. 1/T) does give a straight line with an activation energy of 520 cm-' (Table 11). The two-electron product, [ R ~ ( b p z - ) ~ b p (Figure y]~ 3b), gives an ESR spectrum with hfs (and no line broadening). The spectrum of this species is very similar to that of the reduced mono bpz complex, thus the coupling of the spins is small as expected. The three-electron reduction product, [Ru(bpz-),(bpy-)]-, gives an ESR spectrum with hyperfine structure (no line broadening) but the structure is less resolved than that of the two-electron species as expected if now a Ru-bpy- spectrum is superimposed upon the Ru-bpz- spectrum. The spectra for the mono and bis bpz complexes are in sharp contrast to those for the reduced [ R ~ ( b p z ) ~species. ]~+ N o hyperfine structure is observed for the one- and two-electron reduced species but a temperature-dependent line broadening is measured (Table 11). The three-electron reduced product, for which hopping should not occur, unfortunately gives a very weak signal; consequently, no hfs can be observed. Comparison of the ESR spectra of the two-electron reduction product, [R~(bpz-)~bpy]~, and the [Ru(bp~-)(bpy)~]+ does provide a clue to the absence of hfs for the three-electron reduction product of [ R ~ ( b p y ) ~and ] ~ +other RuL3"+ species. In both of these bpz complexes the barrier to electron hopping is high so line broadening should not occur; but the hfs for the two-electron bis bpz- species is much less well resolved than that of the mono bpz-, due likely to the inexact superposition of the two bpz- spectra in the bis complex. Therefore resolution of hfs for the three-electron reduction products of R U L , ~ would + be critically dependent upon the exact superposition of the spectra for three nearly identical
L- species. Thus residual g factor or hfs anisotropy or a small g shift (0.5 G ) would result in the loss of the hfs. That the -54 OC spectrum of the [Ru(bpz-)(bpy),]+ does not give hfs (because of such an anisotropy effect) while the room temperature spectrum of this species is well resolved (Figure 2) is verification of this hypothesis for the loss of hyperfine structure. The identification of the conditions necessary to obtain hyperfine splitting will enable systematic structural perturbations. Ultimately, the desire is to adjust AElj2to be sufficiently large to observe hfs at low temperature but sufficiently small to facilitate electron hopping at high temperature. The one-electron [Ru(pq-)(bpy),]+ complex (pq = 2,2'-pyridylquinoline) does show both a region of temperature-independent line width as well as a higher temperature line broadening region.19 Unfortunately, the asymmetry of the ligand precludes any observation of hfs in the no line broadening region. The use of ENDOR techniques may enable more rapid determination of the spin density and the related dynamics processes. N M R techniques20 can also be useful but the necessity for determination of proton, nitrogen, and ruthenium signals makes N M R an expensive and slow approach to spin density determination for these complexes.
Acknowledgment. Support by the National Science Foundation (Grant No. CHE-8507901) and the Swiss National Science Foundation (support of J.N.G.) is gratefully acknowledged. (19) Vess, T. M.; Tait, C. D.; DeArmond, M. K.; Hanck, K. W.; Wertz, D. W., unpublished results. (20) Ohsawa, Y.; DeArmond, M. K.; Hanck, K. W.; Moreland, C. G.J . Am. Chem. Soc. 1985, 107, 5383.
Infrared Spectroscopic Investigation of the Ethylene Chemisorption Reaction on Supported Metallic Catalyst Surfaces: Ethylldyne Formation on Pt, Rh, Pd, and Ru Supported on Alumina Thomas P. Beebe, Jr., and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: September 12, 1986)
The reaction of ethylene with Pt, Rh, Pd, and Ru supported on alumina has been studied at 300 K by transmission infrared spectroscopy. Small metal particles of all four elements form the ethylidyne species, =CCH3,and the vibrational frequencies of these species are in close agreement with those reported for ethylidyne adsorbed on the close-packed faces of single crystals of these same metals. Our Pt and Rh surfaces produce ethylidyne IR bands of higher intensity than Pd and Ru. This is consistent with the presence of smaller Pt and Rh average particle sizes, where quantitative CO adsorption measurements indicate an approximately 2- to 3-fold greater chemisorption capacity of Pt and Rh surfaces than Pd and Ru surfaces. In addition, Pt and Rh surfaces exhibit relatively strong ethylidyne asymmetric modes [v,,(CH,) and 6,,(CH3)], indicating that the surface-dipole selection rule is not strictly in operation on these surfaces. This weakening of the selection rule for Pt and Rh surfaces is consistent with the small average particle sizes for Pt and Rh (19 and 28 A, respectively) compared to larger average particle sizes for Pd and Ru (84 and 66 A, respectively).
Introduction There has been much interest recently in the characterization of reactions between small hydrocarbon molecules and the surfaces of the platinum group metals (Pt, Pd, Rh, Ru, etc.). In the past 4 years, it has been shown that ethylene adsorption on these surfaces often yields the ethylidyne species, =CCH3. Ethylidyne has been shown to form upon reaction of C2H4with P t ( l 1 l),] Rh( 11l),, Pd( 11l),, Ru(OOO~),~ and, most recently, Pd/A1203,5 (1) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982, 227, 685. (2) Koel, B. E.; Bent, B. E.; Somorjai, G. A. Surf. Sei. 1984, 246, 211. (3) Kesmodel, L. L.; Gates, J. A. Surf. Sci. 1981, 211, L747. (4) Barteau, M. A.; Broughton, J. Q.; Menzel, D. Appl. Surf. Sci. 1984, 19, 92.
0022-3654/87/2091-0254%01.50/0
Pt/A&O3,6 and Pt/Si02 and Pd/Si02.7,8 In addition, the surface capacity for ethylidyne has been postulated to reflect the abundance of Pd( 111) sites on Pd/Al,O, catalyst^.^ It has been suggested that on P t ( l l 1 ) the related species, ethylidene, = CHCH,, is a crucial hydrogen-transfer agent or "co-catalyst" in ( 5 ) Beebe, Jr., T. P.; Albert, M. R.; Yates, Jr., J. T. J . Catal. 1985, 96,
1.
(6) Wang, P-K.; Slichter, C. P.; Sinfelt, J. H. J . Phys. Chem. 1985, 89, 3606. (7) Bandy, B.J.; Chesters, M. A,; James, D. I.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Philos. Trans R . SOC.London, A 1986, 318, 141. (8) Sheppard, N.; James, D. I.; Lesiunas, A.; Prentice, J. D. Commun. Dep. Chem. (Bulg. Acad. Sci.) 1984, 17, 95. (9) Beebe, Jr., T. P.; Yates, Jr., J. T. Surf. Sci. 1986, 173, L606.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 255
Letters
TABLE I: Ethylidyne Vibrational Frequencies (in Wavenumber, cm-') on Alumina-Supported and Single Crystals of Pt, Rh, Ru, Pd, and a Cobalt Organometallic Complex mode Pt/A1203" . Pt( 1 1 Rh/A1203" Rh(l1 l)c Ru/A1203" Ru(OOO1)d Pd/A1203"*e Pd( 1 1 1)f CH3CCo3(C0)# 3000 2930 2920 not seen 2944 2950 2940 va(CH3) 2900 (br) 2910 2888 2880 2888 2890 2886 2887 v,(CH3) 1450 1 397h 1400 1420 not seen 1407 1420 1420 1411 6,(CH3) 1340 6,(CH3) 1345 1350 1344 1337 1333 1334 1356
{::::
6,(CC)
1129
1130
1120
1121
1131 (br)
" This work. Reference 1. e Reference 2. Reference 4. e References 5 and I
I
1120
1090
1080
10. /Reference 3. gReference 13.
1163 Extremely weak.
I
I
3 . 0 0 2 Abs
4
(cresoiution (FWHM)
W z u
2799 I
m a
cc
am
k - 1 ; -.._-
3000
2900
3 Wovenumber I..
2800
2700
Wavenumber (cm" 1
Figure 1. Infrared spectra in the C-H stretching region for ethylene exposed to alumina-supported Pt, Rh, Pd, and Ru. Exposures were made at T = 300 K. Spectra were obtained under vacuum (P< 1 X lo4 Torr), at T = 300 K. Data acquisition time was 61 s/cm-l.
the ethylene hydrogenation reaction. lo Recent studies from our laboratory have shown that this is not the case for the ethylene hydrogenation reaction on Pd/A1203.' Quite to the contrary, we find that the presence or absence of ethylidyne on these Pd/A1203surfaces has no effect on the measured rates of ethylene hydrogenation. In the present Letter, we compare results for the C2H4 reaction with alumina-supported Pt, Rh, Pd, and Ru and show that all four metals convert ethylene to a surface species exhibiting the characteristic ethylidyne vibrational modes. Experimental Section Sample preparation by impregnation of alumina with nitrate or chloride salts of the metal of interest is described in detail elsewhere.12 All studies were done on 10 wt % metal/A1203 surfaces weighing 0.028 f 0.003 g. The metallic/A1203 deposits were decomposed under vacuum at 473 f 3 K for 112 h, followed by multiple treatments with O2 a t 300 K followed by multiple reductions in 400 Torr of H2 at 473 K. IR spectra were recorded with a grating IR spectrometer (Perkin-Elmer PE-783) a t a resolution of 5.6-cm-' fwhm. The spectra were signal-averaged for integration times in the range of 40-82 s/cm-'. Signal-to-noise degrades very quickly in the 1000-cm-' region due to the strong lattice absorption of IR radiation by the alumina support. In the case of all four surfaces, ethhylidyne has been formed by exposure of C2H4(g)to the surface at 300 K. Spectra were then recorded under vacuum ( P d 1 X lo-* Torr), a t 300 K. Results and Discussion ZR Assignments. Transmission IR spectra in the 3050-2700cm-' range are shown in Figure 1 for alumina-supported Pt, Rh, Pd, and Ru. The assignment of these features to the ethylidyne surface species is straightforward and is based on the close agreement of the observed IR frequencies with those reported (10)Zaera, F.; Somorjai, G. A. J . Am. Chem. SOC.1984, 106, 2288. (1 1) Beebe, Jr., T. P.; Yates, Jr., J. T. J . Am. Chem. Soc. 1986,108,663. (12)Yates, Jr., J. T.; Duncan, T. M.; Vaughan, R. W. J . Chem. Phys. 1979,71, 3908.
(cm-')
Figure 2. Hydrogenation experiments for the C2H4/Pt/Al2O3system at 300 K. Spectra were recorded under vacuum (P < 1 X Torr) following addition of small quantities of H2(g): (a) 1.5 X lo" molecules of H2; (b) an additional -5.3 X 10'' molecules of H,; (c) an additional -1.1 X 1OI8 molecules of H2.
-
previously by electron energy loss spectroscopy (EELS) for ethylidyne adsorbed on the close-packed single-crystal planes of these same metals. Table I shows these IR frequencies, as well as the vibrational frequencies, for a cobalt organometalliccomplex containing the ethylidyne ligand,13 which has been of great value in making the vibrational assignments in the EELS studies. The mode at ~ 2 9 4 0cm-' is assigned to the asymmetric C-H stretching vibration of ethylidyne, while the feature at ~ 2 8 8 5cm-' is assigned to the ethylidyne symmetric C-H stretching vibration. The presence of vibrational modes at -2800 cm-', as well as the broadness exhibited in the peak at -2940 cm-', is indicative of the presence of species other than ethylidyne. Mixtures of species on supported metallic surfaces have been proposed by others, as We have conducted experiments to determine whether this is so. Different species will almost certainly have different reactivities toward 1*14 Experiments involving incremental addition to H2(g) at 300 K to Pt/A1203 surfaces prepared as in Figure 1 are presented in Figure 2. The spectra of Figure 2 may be divided into at least two sets of bands designated by shading in that figure. The first set, nshaded (Figure 2), is attributed to (in order to descending wave umber) va(CH3), v,(CH3), 6,(CH3), and 6,(CH3) in ethylidyne.: These bands decrease steadily in integrated intensity with H 4 g ) addition. The second set, designed by cross-hatching in Figyre 2, exhibits behavior different from that of the set of bqnds attributed to ethylidyne, showing a more constant intensity behavior during hydrogenation. Similar behavior is observed for these ethylenederived species on Rh/A1203 surfaces. This behavior therefore supports the assignment of all four unsh'aded bands (Figure 2) to the same surface species, for which there is little doubt, based on the symmetric deformation and C-C stretching modes, that this species is ethylidyne. This then represents the first assignment of v,(CH,) and 6,(CH3) for ethylidyne on supported metal surfaces. The factor of 2-3 greater intensity of ethylidyne adsorbed on Pt and Rh than on Ru and especially Pd is at least partially the consequence of the different chemisorption capacities of the various surfaces. Table I1 summarizes the data obtained in quantitative H2.591
K
~
~~
(13) Skinner, P.; Howard, M. W.; Oxton, I. A.; Kettle, S. F. A.; Powell, D. B.; Sheppard, N. J . Chem. Soc., Faraday Trans. 2 1981,77, 1203. (14)Soma, Y. J . Catal. 1982,75,267.
256
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The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
TABLE II: Summarv of Ouantitative CO Chemisorption Measurements iv&/mass samplea Pt/A1203(10%) Rh/AI203 (10%) Pd/AI,O, (10%) Ru/A1209 (1 0%)
mass, g
@%,b molecules x loL8
0.0278 0.0322 0.026 1 0.0332
3.74 5.6 1.63 2.36
@,&/Nmetalr (molecules X mol/mol 1016)/mg 13.45 0.436 17.39 0.297 6.25 0.107 7.11 0.120
normalized
dispersion: %
avd particle diameters, 8,
2.15 2.78 1.oo 1.14
58 40 14 16
19 28 84 66
"All samples are 10 wt % metal on alumina; reduction temperature 473 f 3 K. *Obtained by extrapolating to PCO= 0 on the isotherm of flo = 0.7K2' dAssumptions: that of footnote c; an equal distribution of adsorbed vs. Pco. 'Surface to volume ratio; assumption @:T/Nsurfacc metal the three main low index planes is present on the particles; and that the particles are spherical. I
I
I
,-I129
A
1411
fo 0,
V
4
E 0
I'\
010A b s
t-resolution (FWHM)
Pt
d lO-'Torr
V
C 0
n L. v)
n
Rh
a
\
-
