J. Phys. Chem. 1985,89, 1331-1334
1331
Adsorption and Oxidation of Ethylene at Gold Electrodes As Examined by Surface-Enhanced Raman Spectroscopy Mary L. Patterson and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: December 4, 1984)
The adsorption and oxidation of ethylene at gold electrodes has been studied by surface-enhanced Raman spectroscopy (SERS). Bands associated primarily with C-C stretching and symmetrical CH, bending vibrations were observed at 1535 and 1278 cm-I, respectively. A splitting of both peaks was observed that is apparently due to adsorption at two energetically different surface sites. Chloride preferentially displaces the weakly bound species at more positive potentials; adsorbed bromide displaces all adsorbed ethylene except at extreme negative potentials. Ethylene electrooxidation gives rise to new bands at 990 and 425 cm-' which are tentatively ascribed to a partially oxidized adsorbed intermediate.
Much activity has been directed toward understanding the varying degrees of heterogeneous catalysis exhibited by different metals toward the oxidation of ethylene, both in electrochemical' and gas-phase2 environments. The vibrational properties of adsorbed ethylene have also been studied extensively a t various metal-UHV interface^;^^^ however, no such information is available at electrode surfaces. We have been exploring the application of surface-enhanced Raman spectroscopy (SERS) as an in-situ structural probe of reactive adsorbates at metalsolution i n t e r f a ~ a . ~In. ~particular, SERS has been utilized to follow electrochemical redox transformations? Very recently, we have formulated a straightforward electrochemical roughening procedure that yields gold surfaces displaying intense and stable S E R S 7 Gold is a particularly suitable substrate for ethylene electrooxidation;' this therefore provides an intriguing model system with which to explore the virtues of SERS for monitoring electrocatalytic processes. We report here SER spectra for adsorbed ethylene at the gold-aqueous interface. Information on the electrooxidation mechanism is derived from SER spectral changes resulting from (1) (a) For a review, see Herlem, M.; Bobilliart, F.; Thiebault, A. In 'Encyclopedia of Electrochemistry of the Elements", Bard, A. J., Lund, H., Eds.; 1978; Vol XI, pp 23-26. (b) Dahms, H.; Bockris, J. O M . J. Electrochem. Soc. 1964,122,728. (c) Kuhn, A. T.; Wroblowa, H.; Bockris, J. O M . Trans. Faraday SOC.1967,63, 1458. (d) Hartley, T. N.; Price, D. J . Electrochem. Soc. 1970, 127,448. (e) Johnson, J. W.; Lai, S.C.; James, W. J. Electrochim. Acta 1971, 16, 1763. (f') Cwiklinski, C.; Perichon, J. Electrochim. Acta 1974, 29,297. (g) Semrau, G.; Heitbaum, J. In 'Proceedings of the Symposium on the Chemistry and Physics of Electrccatalysis"; McIntrye, J. D. E., Weaver, M. J., Yeager, E. B., Eds.;Electrochemical Society: Pennington, NJ, 1984; p 639. (2) For example (a) Kilty, P. A.; Sachtler, W. H. M. Catal. Rev. Sci. Eng. 1974, 20, 1. (b) Geenen, P. V.; Boss, H. J.; Pott, G. T. J. Catal. 1982, 77, 499.
(3) For example (a) Backx, C.; deGroot, C. P. M.; Biloen, P. Appl. Surf. Sci. 1980,6,256. (b) Nyberg, C.; Tengstal, C. G.; Anderson, S . ; Holmes, M.W. Chem. Phys. Lett. 1982,87,87. (c) Gates, J. A.; Kesmodel, L. L. Sui$ Sci. 1982, 120, L461. (d) Dubois, L. H.; Castner, D. G.; Somorjai, G. A. J . Chem. Phys. 1980, 72, 5234. (4) (a) Maskovits, M.; DiLella, D. P. Chem. Phys. Lett. 1980,73,500. (b) Manzel, K.;Schultze, W.; Moskovits, M. Chem. Phys. Lett. 1982, 85, 183. (c) Pockrand, I. Surf. Sci. 1983, 226, 192. (5) (a) Weaver, M. J.; Barz, F.; Gordon 11, J. G.; Philpott, M. R. Surf. Sci. 1983, 125, 409. (b) Barz, F.; Gordon 11, J. G.; Philpott, M.R.; Weaver, M. J. Chem. Phys. Lett. 1982,91, 291. (c) Hupp, J. T.; Larkin, D.; Weaver, M. J. Surf. Sci. 1983, 125, 429. (d) Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon 11, J. G.; Philpott, M. R. J . Electroanal. Chem. 1984, 260, 321. (e) Tadayyoni, M. A.; Farquharson, S.; Li, T. T.-T.; Weaver, M. J. J. Phys. Chem. 1984,88,4701. ( 6 ) (a) Farquharson, S.;Weaver, M. J.; Lay, P. A,; Magnuson, R. H.; Taube. H. J. Am. Chem. Soc. 1983,105,3350. (b) Farquharson, S.; Guyer, K. L.; Lay, P. A.; Magnuson, R. H.; Weaver, M. J. J. Am. Chem. Soc. 1984, 206, 5123. (c) Tadayyoni, M. A.; Farquharson, S.;Weaver, M. J. J . Chem. Phys. 1984, 80, 1363. (d) Farquharson, S.;Milner, D.; Tadayyoni, M.A.; Weaver, M. J. J. Electroanal. Chem. 1984, 178, 143. (7) Gao, P.; Patterson, M. L.; Tadayyoni, M. A.; Weaver, M.J. Langmuir 1985, I, 173.
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excursions to positive potentials where ethylene oxidation occurs.
Experimental Section Details of the Raman spectral measurements are essentially as given in ref 6b and 6e, employing a SPEX 1403 scanning spectrometer operated at a 5-cm-' band-pass. Raman excitation was provided by a Spectra Physics 165 Kr' laser operated at 647.1 nm. The gold surface (a 4-mm-diameter disk sealed into a Teflon sheath) was mechanically polished with 1.0- and 0.3-pm alumina, and rinsed with distilled water. It was then roughened by means of a succession of oxidation-reduction cycles (ORC) in 0.1 M KC1, by employing 500 mV s-' potential sweeps from -300 to +1100 mV vs. the saturated calomel electrode (SCE), with a cathodic peak current density of around 5 mA cm-2.7 The electrode was then rinsed and transferred to the SERS cell. Optimal SERS signals were typically obtained after ca. 20 such scans. This procedure yielded mildly roughened gold surfaces with electrochemical properties virtually identical with mechanically polished gold, and SERS intensities and signal stabilities that are typically greater than those obtained for electrochemically roughened silver? Ethylene (99.5%) was obtained from Matheson; solutions were also purged with argon to remove dissolved oxygen. All electrode potentials are quoted vs. the SCE, and all measurements were made at room temperature (23 f 1 " C ) . Results and Discussion SERS of Adsorbed Ethylene. A representative SER s p t r u m of adsorbed ethylene at a roughened gold electrode at 200 mV in an ethylene-saturated (ca. 5 mM) 1 M H s 0 4 solution is shown in Figure 1. This electrolyte was chosen since ethylene electrooxidation a t gold has been studied in this medium.' Similar, although weaker, spectra were obtained for roughened gold surfaces in contact with ca. 1 atm of ethylene gas a t room temperature. The bands around 1545 and 1278 cm-' are the only observable features from 150 to 3600 cm-' that are attributable to adsorbed ethylene and appear and disappear reversibly upon purging the solution with ethylene and argon, respectively. Assignments of these bands can be made by comparison with the normal Raman spectrum of gas-phase C2H4, and with EELS and SERS spectra of adsorbed CzH4 reported previously at silver and copper surfaces in UHV, as follows. A complete analysis of the vibrational spectra of gas-phase ethylene has been undertaken.* The only modes found in the 1100-1700-~m-~region are a C=C stretch at 1623 cm-I, v2 [v(C=C)], a symmetric CH, bending mode at 1342 cm-', v 3 ["symmetric scissors", 6,(CH2)], and an asymmetric CH, bending mode at 1443 cm-I, vlZ ["asymmetric scissors"]. Both v2 and v j are Raman-active only, being of A, symmetry, and v I 2 is infra(8) Herzberg, G., 'Molecular Spectra and Molecular Structure: Vol. 2, Infrared and Raman Spectra of Polyatomic Molecules"; Van Nostrand New York, 1945.
0 1985 American Chemical Society
Letters
1332 The Journal of Physical Chemistry, Vol. 89, No. 8, 1985
l
nr
1700
1600
1535
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1400 A i i (ern-' 1
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Figure 1. SER spectrum of ethylene adsorbed at electrochemically roughened gold at +200 mV vs. SCE in ethylene-saturated 1 M H2S04. Laser power (647.1 mm) was 25 mW, focussed to a ca. 2-mm-diameter spot.
red-active only. In an EELS study of ethylene at Cu( 110) at 180 K, Nyberg et al. assigned bands at 1557 and 1290 cm-' to v(C=C) and 6,(CHz), respecti~ely.~~ B a c k et al. studied ethylene a t oxygen-covered A g ( l l 0 ) at 110 K using EELS3a and also assigned bands at 1564 and 1323 cm-l to v(C=C) and 6,(CH2), respectively, and the same assignments were made by Moskovits and DiLella to SERS bands at 1585 and 1330 cm-' found for ethylene a t silver at 11 K under UHV condition^.^" Taken together, these results strongly suggest that the bands at 1545 and 1278 cm-' in Figure 1 are also due to v(C=C) and 6,(CH2), respectively. The bonding in bulk-phase olefin-metal complexes has been described as involving n electron donation from the C = C double bond, followed by back-donation from the metal d orbitals to T* antibonding orbitals in CZH4.9 Surface coordination likely involves similar bonding.I0 The net effect of surface bonding should therefore be reflected in a weakening of the C=€ bond, and hence a decrease of both v(C=C) and 6,(CH2) from the gas-phase frequencies since both have substantial C=C stretching character: in accordance with the present observations. This n bonding is strong a t some transition-metal surfaces having unfilled d orbitals.lob The anticipated weaker metal-ethylene interactions at gold, as for copper and silver surfaces, is consistent with the smaller (50-100 cm-') shifts in the vibrational modes upon adsorption at this metal. Adsorbed ethylene probably involves a C=C bond that is flat on the surface, with C-H bonds that are tilted away from the surface;" this lowers the symmetry from DB to at least C, since the molecule loses planarity. Under these conditions all modes will become Raman active; in fact a number of these additional modes are observed for SERS of C2H4 adsorbed on Ag under UHV conditions a t 11 K.4a Although one might anticipate the SERS intensity of v(C=C) to be low if this vibration is parallel to the surface,12 the prominent appearance of this band at gold may be due to nonplanarity of the adsorbate. In this case, 6,(CH2) has a vibrational component normal to the surface; since v(C=C) has substantial 6,(CHz) characteP this accounts for the appearance of the former. Figure 1 clearly shows that both the vibrational bands are composed of at least two peaks. This feature can be in principle be due to the presence of normally infrared- as well as Ramanactive modes, or to splitting of otherwise degenerate modes associated with differing surface interactions. The former possibility can be eliminated since, as noted above, vlZ is the only infrared mode in the relevant frequency region. Furthermore, neither v(C==C) nor 6,(CH2) are multiply degenerate in gas-phase ethylene.8 (9) Duddell, D. A. In "Spectroscopy and Structure of Molecular Complexes", Y a r w d , J., Ed.; Plenum: New York, 1973; p 427. (10) (a) Demuth, J. E. IBM J . Res. Develop. 1978, 22, 265. (b) Felter, T. E.; Weinberg, W. H. Surf. Sci 1981, 103, 265. (11) Nichols, H.; Hexter, R. M. J . Chem. Phys. 1981, 75, 3126. (12) Allen, C. S.; Van Duyne, R. P. Chem. Phys. Lert. 1979, 63, 455.
1600
1550
1500"1325
1275
J 1225
AB(cm-')
Figure 2. Effect of altering electrode potential on v ( C 4 ) and b,(CH2) bands of adsorbed ethylene: (a) -400 mV; (b) -200 mV; (c) 0 mV;(d) 200 mV. Other conditions as in Figure 1.
1278
c
I
? O b
I530
1540
1500'1325
1275
1225"300
250
200
150
AF (crn.'I
Figure 3. Effect of adsorbed chloride or bromide on v ( C 4 ) and 6,(CH,) bands of adsorbed ethylene at 200 mV. Electrolytes: (a) 1 M H2S04; (b) 1 M H2SO4 + 0.1 M KCI; (c) 1 M HzS04 + 0.1 M KBr, each saturated with ethylene.
Several lines of evidence suggest that the latter explanation is most likely correct. The first is gleaned from the potential-dependent bandshapes of the v(C==C) and 6,(CH2) modes as summarized in Figure 2. Note that the low-frequency component of both bands gains in intensity with respect to the high frequency portion with increasing positive potential. This suggeststhat there are two separate populations of adsorbed ethylene molecules, that associated with v(C=C) and 6,(CHz) centered at 1535 and 1278 cm-I, respectively, gaining intensity with respect to the population with modes at 1545 and 1288 cm-' as the electrode potential becomes more positive. It is intuitively reasonable that the IO-cm-' lower vibrational frequencies should be associated with more strongly bound ethylene. Evidence suggesting that these populations are associated with different crystallites on the gold surface can be found in an EELS study of formic acid adsorbed on Au(1 10) and Au( 111) at 100 K.I3 For example, u(C-H) and v(C-0) are 10 cm-l higher on A u ( l l 0 ) than on Au(l1 l ) , and v(C=O) is 15 cm-' higher on the former crystal plane. Further evidence favoring such an interpretation is derived from the effect of competitive adsorption with chloride and bromide, as summarized in Figure 3. Figure 3a shows the 150-300, 1225-1325, and 1500-1580-~m-~regions for the same conditions as Figures 1 and 2, whereas the spectra in Figure 3, b and c, result (13) Chtaib, M.;Thiry, P. A.; Delrue, J. P.; Pireaux, J. J.; Caudano, R. J . Electron Speclrosc. Related Phenom. 1983, 29, 293.
The Journal of Physical Chemistry, Vol. 89, No. 8, 1985 1333
Letters
I
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1851
1278 I
990 I
1278 e1
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Figure 4. Illustration of competitive adsorption between ethylene and bromide as a function of electrode potential: (a) -400 mV; (b) -200 mV, (c) 0 mV; (d) 200 mV; (e) 400 mV. Electrolyte contained 0.1 M KBr, other conditions as in Figure 1.
from adding 0.1 M KCl or KBr, respectively, at the same electrode potential, 200 mV. Both the 178- and 265-cm-' features in Figure 3a are also obtained in the absence of ethylene and are due to surfaceadsorbate vibrations associated with adsorbed HSO, and C1-, respectively.' (The latter presumably results from residual chloride remaining after the electrode pretreatment.) As expected, this chloride band is enhanced in Figure 3b, even though the HS04- band remains largely intact. Most significantly, chloride addition yields a selective intensity decrease in the high-frequency component of both v(C=C) and 6,(CH2), inferring that adsorbed chloride displaces preferentially the more weakly bound form of adsorbed ethylene. This result therefore strongly supports the assignment of these pairs of peaks to the presence of ethylene at least at two energetically distinct adsorption sites. Addition of 0.1 M KBr (Figure 3c) completely eliminates the spectral features associated with adsorbed ethylene and chloride and yields a band at 185 cm-' due to surfacebromide stretching.' This is expected since bromide is present at coverages approaching a monolayer under these conditions,14whereas only submonolayer adsorption of chloride is probably generated in Figure 3b. The ethylene SERS signals could be recovered even in the presence of bromide when the potential is made more negative. Figure 4 shows spectra in the low-frequency and 6,(CH2) regions as a function of electrode potential. At the most positive potentials (400,200 mV; Figure 4, e, d), only the 185-cm-l adsorbed bromide band is seen, whereas when the potential is made more negative (0 to -400 mV; Figure &a), this band disappears and is replaced by the 6,(CH2) mode along with a weak band around 230 cm-'. This last feature may be due to a surface-ethylene mode. These changes are reversed upon returning the potential to more positive values. In the absence of ethylene, the adsorbed bromide band is observed at potentials as negative as -700 mV.' These trends follow those expected from simple electrostatic considerations: ethylene should be able to more effectively compete for surface sites at potentials negative of the potential of zero charge (ca. -100 mV)I5 since bromide undergoes electrostatic repulsion from the surface under these conditions. It is important to note that reversibility of such potential-dependent adsorption-desorption equilibria is maintained over the extended periods (14) For example, Hamelin, A. J . Efectroanal. Chem. 1982, 142, 299. (15) (a) Clavilier, J.; Huong, N. V.J. Efectroanul. Chem. 1973, 41, 193. (b) Hamelin, A.; Lecoeur, J. Colfecr.Czech. Chem. Commun. 1971, 36, 714.
A C (crn-') Figure 5. Influence of ethylene electrooxidation on SER spectra. (a) Electrode potential was held at 200 mV. (b) The potential was stepped to 1000 mV and returned to 200 mV after passage of ca. 120 mC cm-* anodic charge, the spectrum being acquired immediately thereafter.
of time (1-2 h) necessary to gather a series of SER spectra with the scanning spectrometer. This temporal stability of the spectra is commonly seen at gold electrodes prepared by the above procedure and contrasts the irreversible signal decay typically observed at silver, especially at more negative potential^.^^^^ SERS of Ethylene Electrooxidation. The electrooxidation of ethylene at gold electrodes has been studied by a number of workers.' In strongly acid solution, partial oxidation occurs with acetaldehyde and ketones being the major However, the details of the electrooxidation mechanism are unclear. Anodic-cathodic cyclic voltammograms indicate that ethylene oxidation commences at around 850 mV and strongly inhibits anodic oxide film formation as evidenced by the absence of the surface oxide reduction peak seen with the blank electrolyte. Similar voltammograms were obtained at mechanically polished and roughened gold. Only very weak SER spectra were obtained with ethylene-saturated 1 M H2S04at potentials where significant anodic current flowed. Nevertheless, information on the species formed under these conditions was obtained by stepping the potential to around 800-1000 mV and returning to 200 mV after a measured amount of anodic charge, qa, was passed. Figure 5, a and b, illustrates the SER spectral changes induced in this manner. The bands at 990 and 425 cm-' (Figure 5b) that appear following this potential excursion become more intense as qa is increased up to 50 mC cm-2. These features are reproduced if 1 M HC104 is substituted for 1 M H2SO4 but do not appear in the absence of ethylene. They are entirely removed when 0.1 M KCl as well as 0.1 M KBr is added to the solution. Indeed, it has been shown that addition of bromide severely inhibits the kinetics of ethylene electrooxidation at gold.Id Examination of the various mechanisms for ethylene electrooxidation at gold in acidic solution reveals that the adsorbed intermediate CH3CH20has commonly been postulated.'b*e This surface species is quite consistent with the spectrum in Figure 5b. Thus, the frequency of the band at 990 cm-' suggests a C-C or C-0 single bond stretching vibration. The 425-cm-' band is more difficult to assign but could be associated with a bending mode or a surface-adsorbate vibration involving a light surface binding group such as carbon or possibly oxygen. The likelihood that the 990- and 425-cm-' bands do not result from aldehyde products was evidenced by the absence of a detectable SER spectrum for acetaldehyde at gold under the present experimental conditions. Although somewhat speculative, these data nevertheless illustrate the potential virtues of SERS as a tool for electrochemical reaction mechanism diagnosis as well as for elucidating the nature (16) Interestingly,it appears that metals such as gold, silver, and palladium which produce relatively small decreases in U ( C = C )for ~~ ~ ~ ethylene adsorbed yield products corresponding to partial oxidation,',*whereas transition metals such as platinum and rhodium which yield larger downshifts in u ( C = C ) ) ~ electrooxidize ethylene largely to C02.'
J . Phys. Chem. 1985,89, 1334-1336
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of adsorbatesurface interactions. Further work in this direction for both organic and inorganic electrode reactions, in particular by coupling SERS and rotating disk voltammetry,6dis underway in this laboratory.
Acknowledgment. This work is supported in part by the NSF
Materials Research Laboratory at h r d u e and the Air Force Office of Scientific Research. M.J.W. acknowledges a fellowship from the Alfred P. Sloan Foundation. Registry No. C2H4, 74-85-1; gold, 7440-57-5; chloride, 16887-00-6; bromide, 24959-67-9.
EXAFS: A New Approach to the Structure of Uranium Oxides Geoffrey C. Allen,* Paul A. Tempest, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Glos, GL13 9PB, U.K.
C. David Gamer, Ian Ross, Department of Chemistry, Manchester University, Manchester, M13 9PL, U.K.
and Deborah J. Jones Department of Chemistry, The University, Southampton, SO9 5NH, U.K. (Received: December 6, 1984)
In view of its importance in the chemistry of the solid state, crystal stoichiometry and hence the structure of inorganic compounds have recently attracted a great deal of interest. Anion excesf defect structures, though relatively unusual, are best exemplified by the commercially important system UOZ+*.Not surprisingly, much effort has been directed toward a full characterization of this structure and more than one defect cluster model described. Here, as the initial step in the use of synchrotron radiation to investigate defects in uranium oxides, we report for the first time EXAFS studies of stoichiometric U02, fi-U307,and a-U308.
Introduction The stable fluorite and orthorhombic crystal structures of U 0 2 and a-U308are known to tolerate gross departures from stoichiometry in forming the superstructures U02+*and U308-l.1-5 The high stability of the cation sublattice ensures that the predominant defects are oxygen interstitials in U02+*and oxygen vacancies in U30E1. A number of intermediate phases, such as U8OI9and U205of unknown crystal structure have been reported in the region of composition where neither pure distorted hyperstoichiometric U02nor pure distorted hptoichiometric U308 are easily formed. Large levels of disorder may be established by oxygen defect clustering, and understanding of the complex structure of these nonstoichiometric phases has been considerably improved by diffraction techniques.2v6 However, diffraction techniques provide information averaged over many unit cells, and details of localized defect structures have been sought by other techniques, including X-ray photoelectron spectr~scopy.~*~ Since the association of oxygen defects in clusters leads to localized changes in the length of the metal-oxygen bonds and the coordination number of the metal site@),’ we considered that valuable information could be obtained from the extended X-ray absorption fine structure (EXAFS)+” associated with an absorption edge of the metal. If x ( E ) is the fractional modulation of the total absorption coefficient r ( E ) above an edge
where po(E) is the background absorption coefficient in the absence of backscattering atoms then, in order to relate x ( E ) to structural parameters it is necessary to convert the energy E into the photoelectron wavevector via 112
k = [$(E
-Eo)]
In this expression E is the incident photon energy and Eo the threshold energy of that particular absorption edge. For the plane-wave approximation x ( k ) in k space is then given by
x ( k ) = C N p j ( k ) F , ( k ) exp(-2uj2k2) I
X
exp(-2rj/X(k)) sin (2kr1 + @,(k))/kr; (3)
Here Nl is the number of atoms in thejth coordination shell, u is the Debye-Waller factor, and X is the mean free path of the
electron. r, is the distance from the absorber of an atom in the jth coordination shell and aj the phase shift experienced by the photoelectron. F4 is the backscattering amplitude and S j ( k ) an amplitude reduction factor.
Results and Discussion
(1) Allen, G. C.; Tempest, P. A. J. Chem. Soc., Dalton Trans. 1982,2169. ( 2 ) Willis, B. T. M.J. Phys. Radium 1964, 25, 431. (3) Catlow, L. R. A. Proc. R . Soc. London, Ser. A 1977, 353, 533. (4) Greaves, C.; Fender, B. E. F. Acra Crystallogr., 1972, 28, 3609. (5) Loopstra, B. 0. Acra Crystallogr. 1964, 17, 651. (6) Belbeoch, B.; Piekarski, C.; Perio, P. Acro Crystallogr. 1%1,14,837. ( 7 ) Allen, G. C.; Tucker, P. M.;Tyler, J. W. J. Chem. Soc., Chem. Commun. 1981, 691. (8) Allen, G. C.; Tucker, P. M.;Tyler, J. W. Vacuum 1982, 32, 8, 481. (9) Sayers, D. E.; Lytle, F. W.; Stern, E. A. Adu. X-Ray Anal. 1970, 13,
Studies of silicate glasses containing uranium have recently been reported by the Argonne EXAFS groupI2 and U-0 distances derived for these amorphous materials. To initiate an EXAFS investigation of nonstoichiometry in crystalline uranium oxides, we have recorded the X-ray absorption spectra for U02,B-U307, and a-U308, three fundamental oxides in the above nonstoichiometric range. Samples were handled under dry nitrogen and incorporated in dry nujol to prevent air oxidation and to avoid problems of operator contamination. Under these conditions changes due to oxidation were most unlikely during the short time scale of the experiment. EXAFS data were recorded on the
(10) Stem, E. A. Sci. Am. 1976, 234, 96. (1 1) ‘EXAFS Spectroscopy: Techniques and Applications”, Teo, B. K., Joy, D. C., Eds., Plenum Press: New York, 1981.
(12) Knapp, G. S.;Veal, B. W.; Lam,D. J.; Paulikas, A. P.; Pan, H. K. Mater. Lett. 1984, 2, 253.
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0 1985 American Chemical Society
