J. Phys. Chem. 1988, 92, 2711-2714
2711
High-Resolution Electron Energy Loss Spectroscopic Study of CO, Adsorbed on Re( 0001) M. Asscher,* Department of Physical Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, Jerusalem 91 904, Israel
C.-T. Kao, and G . A. Somorjai Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, Berkeley, California 94720 (Received: February 8, 1988)
The adsorption geometry and dissociation of C 0 2on Re(0001) were studied by high-resolutionelectron energy loss spectroscopy (HREELS) over the crystal temperature range of 85-150 K. It was found that C 0 2adsorbs as an undistorted linear molecule with its axis parallel to the surface up to a crystal temperature of 120 5 K. It is characterized by vibrational losses that are very similar to the frequencies of the gas-phase molecule. In the temperature range of 120-135 K a fraction of the adsorbed molecules desorb and the rest transform to an intermediate tentatively identified as a bent C02- which dissociates to CO, 0, at 135 K and above. Similar results are obtained on an oxidized rhenium surface. The effect of post dosing of oxygen on the vibrational losses of CO adsorbed at 300 K was studied as a model for the COz fragments on the surface and was found to be negligible.
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+
Introduction The interaction and primary chemical reactions of C 0 2on clean metal surfaces have received increased attention in recent years. The adsorption and dissociation of carbon dioxide were shown to be surface structure sensitive on several different metal surfaces. The Fe( 1 10) surface was reported to be inactive with respect to both adsorption and dissociation of C02' at low pressures and temperatures while both a stepped Fe( 1 10) and the more open Fe( 111) planes were shown to adsorb and dissociate C 0 2 . Some dissociation on Fe( 110) was reported in a study at 300 K2. On Ni( 1lo), angle-resolved UPS and electron energy loss vibrational spectroscopy were employed to show that at 80 K C 0 2 is molecularly adsorbed, while above 140 K a bent and possibly negatively charged species is formede3 This ionic form leads to dissociation to CO, and 0, above a crystal temperature of 230 K.3 The observations support the results of ab initio valence bond calculations of transition-metal complexes with coordinated C 0 2 ligands which predict the bent ionic form of C02on Ni surface^.^ The interaction and reactivity of C 0 2on rhodium surfaces were extensively studied,%1°with indications for a large surface structure specificity.lo The A g ( l l 0 ) surface was reported to be inactive with respect to C 0 2 adsorption a t 100 K based on HREELS measurement." It can adsorb carbon dioxide only following oxygen preadsorption, which enhances both the molecular adsorption and carbonate formation. In contrast to the interaction of COSwith other smooth metal surfaces, a high dissociation yield was reported for low coverage C 0 2 on Re(0001)12a t temperatures below 150 K. In this letter we present a high-resolution electron energy loss study of COz on Re(0001) in the temperature range of 83-150 K, from which it is concluded that C 0 2 is molecularly bound to ( 1 ) Behner, H.; Spiess, W.; Wedler, G.; Borgmann, D. Surf. Sci. 1986,175, 276. (2) Yoshida, K.; Somorjai, G. A. Surf. Sci. 1978, 75, 46. ( 3 ) Bartm, B.; Freund, H.-J.;Kuhlenbeck, H.;Neumann, M.; Lindner, H.; Mllller, K. Surf. Sci. 1987, 179, 59. (4) Freund, H.-J.; Messmer, R. P. Surf. Sci. 1986, 172, 1. ( 5 ) Dubois, L. H.; Somorjai, G . A. Surf. Sci. 1983, 128, L231. (6) Weinberg, W. H. Surf. Sci. 1983, 128, L224. (7) Solymosi, F.; Kiss, J. Surf. Sci. 1985, 149, 17. ( 8 ) Solymosi, F.; Erdohelyi, A,; Kocsis, M. J . Carol. 1980, 65, 428. (9) Solymosi, F.; Erdohelyi, A. J. Coral. 1981, 70, 451. (10) Hendrickx, H. A. C. M.; Jongenelis, A. P. J. M.; Nieuwenhuys, B. E. Surf. Sci. 1985, 154, 503. ( 1 1 ) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Chem. Phys. Lett. 1982, 89, 48. (12) Peled, H.; Asscher, M. Surf. Sci. 1987, 183, 201.
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the surface with its axis parallel to the surface up to 120 f 5 K. The vibrational spectrum is very similar to that of gas-phase COP At 135 f 5 K and above, a fraction of the C 0 2dissociates to CO, and 0,.
Experimental Section The experiments described below were performed in an ultra-high-vacuum (UHV) chamber with a base pressure of 3 X Torr, equipped with typical cleaning (Ar' sputtering) and surface analysis tools Auger spectroscopy and low energy electron diffraction (LEED). In addition, a high-resolution electron energy loss spectrometer is utilized for vibrational analysis of adsorbates. The apparatus was previously described in detail." The sample was spark erosion cut from a 99.99% pure single crystal rod, oriented within l o of the (0001) plane by Laue back-reflection and then polished on both sides. It was spot welded on both sides to Ta wires which are then spot welded to the sample holder. The sample can be cooled to 80 K and resistively heated to 1400 K. Sample cleaning is achieved by both Ar+ sputtering and cycles of oxygen treatments (3 X lo-' Torr of O2 for 5 min with the sample at 800-1200 K) followed by hydrogen treatment (6 X Torr of H2for 5 min, with the sample at 800-1200 K). The carbon dioxide is 99% pure and was further purified by several freezepumpthaw cycles. The TPD spectra from the same crystal were taken in a separate chamber, which was previously described.l2 Results and Discussion The temperature programmed desorption spectrum of molecular carbon dioxide adsorbed on Re(0001) at 85 K is shown in Figure 1. The major desorption peak at mass 45 (for 13C02)is at 130 K,19 as was previously reported.12 A fraction of the adsorbed molecules undergo dissociation to CO, and 0, while increasing the crystal temperature. The desorption of the CO fragments is recorded a t mass 29 (I3CO) and is shown also in Figure 1. High-resolution electron energy loss spectroscopy (HREELS) was utilized to monitor the surface species vibration as a function of C 0 2 coverage and temperature. The HREEL spectrum obtained following the exposure of 0.8 langmuir (1 langmuir = lo4 Torr-s) of l2CO2on Re(0001) at 83 K (corresponding to 0.250,, where 0, is the saturation C 0 2 coverage) is shown in Figure 2a. The most intense loss feature is seen at 650 20 cm-', with relatively weak modes at 1600 cm-', at 1935 f 20 cm-', and a t 2350 f 20 cm-I. In the inset of Figure 2, the 10' off-specular
*
(13) Crowell, E. J. Ph.D. Thesis, University of California Berkeley, Berkeley, 1984.
0 1988 American Chemical Society
2712 The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
Letters
TABLE I: HREEIS Vibrational Frequencies and Mode Assignment of Molecular CO, Adsorbed on Various Metal Surfaces d(O=C=O) 661 660 670 650
co2 (gas)b CO,-Ag(llO) COz-Ni(1 10) COZ-Re(0001)
d (C=C=O)"
1286 1280
u,(O=c=O)
v,(O=C=O) 1388 1390 1390
ref
2349 2350 2350 2350
1290
14 11 3 this work
"Overtone of the bending mode. bThe frequencies of the gas-phase CO, obtained by infrared spectroscopy are also listed for comparison (in cm-I). 790 6 650 Cm-I
I\
MASS 29
'
80
I
200
x 46
I
I
I
400
600
800
650cm-I
lusI,
I
I000
l2CC
Temperature IK) Figure 1. Temperature programmed desorption spectra of 'jCO2 (mass 45) and ')CO (mass 29) following the exposure of 1 langmuir of I3CO2 on Re(0001) at 85 K.
-
spectrum at similar COzexposure is also presented for comparison. Not only the intensity at 1600 cm-' is enhanced relative to all other transitions in Figure 2a, but also a new vibrational mode a t 1285 cm-' can be distinctly observed. Following a short annealing of the sample to 120 K (Figure 2b), most of the 650-cm-' peak and that at 2350 cm-' have disappeared. The vibrational mode at 1600 cm-' persists and is somewhat enhanced. A similar procedure has been followed on an oxygen-precovered Re(0001) surface. The oxygen treatment of the surface (30 langmuirs of O2 with the sample at 800 K) improved the overall reflectivity of the 5-eV primary electrons; therefore, spectra with better signal-to-noise could be obtained. In Figure 2c, the spectra obtained following the exposure of 2 langmuirs of COS to the oxygen-precovered Re(0001) surface are shown. Loss features similar to those obtained on the clean surface are seen. The additional peak at 1290 f 20 cm-I, which is probably too weak to resolve in Figure 2a, can be clearly seen. On this surface the peak, previously observed at 1600 cm-I, is shifted to 1650 cm-'. Again, following a short (- 3 s) annealing to 120 K, the peaks at 1290 and 2350 an-'have disappeared. A new vibrational mode at 1230 cm-' shows up at the same time. The small peaks at 1935 and 2020 cm-' in Figure 2, observed before and after annealing on the clean and oxidized surfaces, respectively, belong to residual CO, which originates from the background of the UHV system. If we compare the major observed vibrational frequencies in Figure 2a,c to those of the linear gas-phase COz molecules,14 a remarkable similarity in the peak positions can be seen. The gas-phase vibrational transitions are at 667 (bending), 1286 (overtone of the bending mode), 1388 (symmetric stretch), and 2349 cm-' (asymmetric stretch). The HREELS peaks a t around 650, 1290, and 2350 cm-' can be clearly assigned as bending, overtone, and asymmetric stretch modes, respectively. The symmetric stretch at around 1390 cm-' is probably too weak to resolve from the background intensity. Similar vibrational spectra of C 0 2 adsorbed on Ag(ll0)" and Ni(110)3 below 100 K were also reported. These can all be attributed to a linear molecular C 0 2 lying parallel to the surface. As we do not observe a significant shift in the Vibrational frequencies, we believe the major surface species at 83-120 K is predominantly weakly adsorbed molecular (14) Herzberg, G. Molecular Spectra and Molecular Structure; Princeton University Press: Princton, NJ, 1945; Vol. 2.
b
j l r c m - 3lK '
0
' 236pm-1
a
1000 2000 3000 ENERGY LOSS ( c m - I )
Figure 2. Specular high-resolution electron energy loss (HREEL) spectra of I2CO2adsorbed on Re(0001) at 85 K. (a) 0.8 langmuir of C 0 2 on clean Re(0001) surface. (b) The same as (a), but following a short annealing of the sample to 120 K. (c) 2 langmuirs of C 0 2 adsorbed on an oxidized Re(0001) surface at 85 K. (d) As in (c), but following a short annealing to 120 K. The inset is a HREEL spectrum taken at loo off-specular for 0.8 langmuir of C 0 2 adsorbed on Re(0001) at 85 K.
COz. The assignments of the HREEL spectra for such a weakly perturbed surface C 0 2species and the gas-phase C 0 2IR spectrum are listed in Table I. We also noticed that the most intense mode is the S(O=C=O) bending mode at 650 cm-I for C 0 2 adsorption on Re(0001) at 83 K. This observation is also c h i s t e n t with that expected for a linear molecule adsorbed parallel to the surface, since the dynamic dipole moment of this bending mode is the most likely one t o have a component perpendicular t o t h e surface. According to the surface dipole selection rule, the vibrational transition of this mode will be strongly enhanced relative to other modes near the specular position.20 (15) Sakurai, M.;Obano, T.; Tuzi, Y . J . Vac. Sci. Technol. A 1987, 5(4), 431. (16) Ducros, R.; Tardy, B.; Bertolini, J. C. Surf. Sci. 1983, 128, L219. (17) Carley, A. F.; Gallagher, D. E.; Roberts, M. W. Sut$ Sci. 1987, 183, L263. (18) Froitzheim, H.;Ibach, H.; Lewald, S.Surf. Sci. 1977, 63, 56. (19) The current TPD was done with a more accurate temperature control, and the peak desorption was found to shift from 145 K12 to 130 K at a heating rate of 10.0 f 0.5 K/s. (20) Ibach, H.;Mills, D. L. EIectron Energy LOSSSpectroscopy and Surface Vibrations; Academic: New York, 1982.
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2713
Letters TABLE Ik HREELS Vibrational Frequencies and Mode Assignment of CO,-Swies on N i ( l l 0 ) and Re(0001) Surfaces a(O=C=O) u,(O=C=O) u,(O=C=O) ref
COF-Ni( 110) C02--Re(0001)
750 650
1130 1230
Li+C02-'
799
1330
1620 1600-1 650 -1570
I
3 this work 23
The vibrational frequencies as obtained by IR spectroscopy of LicC02in an argon are also listed for comparison (in cm-I).
The observed extra modes at 1600 cm-' on clean and at 1230 and 1650 cm-I on oxygen-precovered Re(0001) surface are clearly due to the presence of new surface species. Similar peaks have also been observed for C 0 2 adsorption on supported metal or metal oxide surfaces21,22 and were attributed to the formation of bent COT species. Two major peaks around 1320 and 1570 cm-' were normally which were observed over the frequency range of 1200-1800 an-], assigned as symmetric and asymmetric stretch, respectively. On the Ni(ll0) surface, two strong HREELS modes at 750 and 1130 cm-I and one weak mode at 1620 cm-I (asymmetric stretch) were seen for C 0 2 adsorption at 140 K.3 A bent C 0 2 molecular structure with C2, symmetry was proposed on the basis of the observed spectra and angle-resolved UPS studies. This surface species was suggested to be a precursor for C 0 2 dissociation to CO, and 0, on a N i ( l l 0 ) surface at 230 K. Following such an assignment, the modes at 1230 and 1650 cm-l in Figure 2d on the oxygen-precovered surface can be assigned as symmetric and asymmetric stretch vibrations, respectively. The mode at 1600 cm-I in Figure 2a,b can be assigned as asymmetric stretch mode. As we only detected one strong mode below 1000 cm-' over the whole temperature range for C 0 2 adsorption, the bending mode of such a C02-.species is probably very close to 650 cm-I. An analogous metallic Li+C02- has been synthesized in an Ar matrix, and the IR spectroscopy showed three major modes at 799, 1330, and 1570 cm-1.23 Clearly, the major difference in all these systems described above is a small change in the peak position and relative intensity of both bending and symmetric stretching modes. This could be due to the change of the surface bending geometry of the COz- species as compared to the COzion in the argon matrix. We summarize the assignment of such a new surface C02- species in Table I1 and will discuss the significance of such species to C 0 2 dissociation in the following paragraph. The losses observed after annealing the surface to 120 K indicate that most of the adsorbed linear C 0 2 has been removed, as seen by the disappearance of the 2350 and 1290-cm-l modes and intensity drop of the 650-cm-l mode. This is consistent with the onset of the molecular desorption peak shown in Figure 1. As was suggested before,I2 a competition between desorption and dissociation is likely to occur near the desorption temperature. The removal of the losses believed to pertain to the linear molecular species, however, has not yet generated the typical C O fragment stretch near 2000 cm-I, a clear indication that C 0 2 does not dissociate at 120 K. As we discussed previously, we believe that the extra modes at 1230 and 1650 cm-I for the oxygen-precovered and 1600 cm-' for the clean Re(0001) surface are indications of surface C 0 2 - species formation. This surface species can be a precursor for C 0 2 dissociation to CO, 0, by further annealing to 135 K or above, as shown in Figure 3 for an initial C 0 2 coverage of 0.20,. The mode a t 1970 cm-I, assigned as C-0 stretch frequency, is indicative of a low coverage CO on the Re(0001) surface.I6 The typical M-CO stretch at 430 & 20 cm-l and a M-O stretch at 595 20 cm-I are also observed (Figure 3). We conclude that C 0 2 partially dissociates to CO, 0, in the temperature range of 125-135 f 5 K on Re(0001), and that this is
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+
*
+
(21) Little, L. H. Infrared Spectra of Adsorbed Species; Academic: London, 1966. (22) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (23) Kafafi, Z. H.; Hauge, R. H.; Billups, W. E.; Margrave, J. L. J . Am. Chem. SOC.1983, 105, 3886.
I IO00 ENERGY
0
I
I
2000
3000
LOSS ( c m - ' I
Figure 3. HREEL spectrum of 1 langmuir of I2CO2on Re(0001) adsorbed at 85 K, following an annealing to 135 K, the temperature at which the spectrum was taken
70cm
1 1000
0
ENERGY
I 2 000
I
300C
LOSS ( c m - ' 1
Figure 4. HREEL spectra of CO on Re(0001) at 300 K. (a) 1.5 langmuirs of CO; (b) 1.5 langmuirs of CO and then 1 langmuir of 02;(c) 1.5 langmuirs of CO and then 10 langmuirs of 02.
a competing process with molecular C 0 2 desorption. The results discussed above imply that the stability of the precursor to C 0 2 dissociation on Re(0001) is limited to a relatively narrow temperature range of 115-1 30 f 5 K. This is compared with an intermediate that was found on N i ( l l 0 ) above 100 K,3 which is reported to dissociate also to CO, + 0,, but is stable on the surface up to 230 K.3 A carbonate intermediate that was recently proposed to be the precursor for C 0 2 decomposition on aluminum surfaces at low temperatures," on the other hand, was also shown to be unstable and to dissociate already above 120 K. From the above HREELS studies we could not estimate quantitatively the dissociation probability as was previously reported from a TPD study.I2 Finally we looked for a possible effect of the oxygen atom, coadsorbed with C O due to the C 0 2 dissociation event, on the CO stretch frequency. In Figure 4, the energy loss spectra of Re(0001) at 300 K following an exposure to 1.5 langmuirs of C O (Figure 4a) and those in which oxygen was subsequently adsorbed on the surface following exposures of 1 (Figure 4b) and 10 langmuirs (Figure 4c) are shown. The observed losses at 2010 f 15 cm-' and at 405 15 cm-' are similar to losses reported previously for CO on Re(0001)'6 for the CO stretch and the M-CO stretch, respectively, at the appropriate coverage. A
*
2714
J . Phys. Chem. 1988, 92, 2714-2716
shoulder near 550 cm-l is seen to grow with O2 exposure and is due to the Re-0 vibration. A very small shift in frequency is, however, observed at the C O stretch from 2025 cm-' for clean C O to 2008 cm-' for CO in the presence of 1 langmuir of O2 to 2000 cm-l after exposure to 10 langmuirs of 02.The small effect of oxygen on the C O stretch frequency implies that oxygen and CO occupy different surface sites and that the lateral interactions between them do not affect the CO-metal bond significantly. This observation is very different from the effect oxygen had on C O adsorbed on W(lOO),'* where displacement of the a-CO and the appearance of a new loss feature were reported at oxygen exposures as low as 0.3 langmuir and above.18 In conclusion, we found C 0 2 to adsorb molecularly on Re(0001) surface at 85 K, with energy losses that are very similar to the linear molecule in the gas phase. We conclude that the molecule lies linearly parallel to the surface at this temperature. Upon annealing the surface to 120 K, the molecular losses disappear and an intermediate, tentatively identified as C 0 2 - and charac-
terized by asymmetric stretch vibrations at 1600 and 1650 cm-I on the clean and oxidized surfaces, respectively, and symmetric stretch at 1230 cm-I on oxidized surfaces emerge. At this temperature a fraction of the adsorbed CO, desorbs and the other fraction transforms into the precursor to dissociation which is stable up to 135 K. Above 135 K, the precursor dissociates and from this temperature and above only the CO, and 0, fragments are found. The presence of coadsorbed oxygen was found to have only a negligible effect on the stretch frequencies of CO, indicating a relatively weak lateral interaction between 0, and CO,.
Acknowledgment. This research is supported by the US.-Israel Binational Science Foundation. Partial support by the Fritz Haber Research Center for Molecular Dynamics and the DOE is gratefully acknowledged. The Fritz Haber Research Center is supported by the Minerva Gesellschaft fur die Forschung, mbH, Miinchen, BRD. C.-T.K. gratefully acknowledges a B.P.-America Fellowship support for graduate research.
Ab Initio Calculations of the Hyperflne Coupling Constants in F,- Uslng Numerical Wave Functions K. W. Richman and E. A. McCullough, Jr.* Department of Chemistry and Biochemistry, Utah State University 0300, Logan, Utah 84322 (Received: February 16. 1988)
Numerical wave functions are employed to calculate the (I9F) hyperfine coupling constants, Ai, and Adip,in F2-(*2,+). Calculations were performed at the restricted open-shell Hartree-Fock, spin-unrestricted Hartree-Fock, and restricted single-excitation configuration interaction levels of approximation. Results are compared with two previous ab initio studies as well as with neon matrix electron spin resonance results.
Introduction There has recently been some controversy concerning the results of ab initio calculations of the (I9F) gas-phase isotropic hyperfine coupling constant' in F2-(2&+) and their comparison with experimental results. Knight and co-workers2 have performed an ESR study of the isolated FC ion trapped in a neon matrix. These authors concluded that the hyperfine parameters of F2- in this matrix should agree closely with those of the gas-phase ion in its 2Xu+ ground state. However, a recent ab initio study by Nguyen and Ha3 of the gas-phase isotropic hyperfine splitting constant of F2- resulted in a value (520 f 50 G) that is considerably different from the neon matrix value (280.2 G), indicating the possibility of a large perturbation due to the inert gas medium. Using a similar approach as that of Nguyen and Ha, Carmichae14 has calculated the hyperfine parameters of F2-and has concluded that the matrix effects in neon are much smaller than those predicted by Nguyen and Ha. Nguyen and Ha3 considered single and double excitations from a restricted open-shell Hartree-Fock wave function (RCISD) using a doubler basis plus diffuse and polarization functions (DZ+(d)). The excitations were from the valence space only and the configurations were selected on the basis of their contribution to the total energy using a threshold of 10" hartree. These authors also performed a series of calculations at the spin-unrestricted Hartree-Fock (SUHF) level using a variety of extended split-valence (1) Weltner, W. Magnetic A~omsand Molecules;Van Nostrand Reinhold: New York, 1983. (2) Knight, L. B.; Earl, E. A,; Ligon, A. R.; Cobranchi, D. P. J . Chem. Phys. 1986, 85, 1228. (3) Nguyen, M. T.; Ha, T.-K. J . Phys. Chem. 1987, 91, 1703. (4) Carmichael, I. J . Phys. Chem. 1987, 91, 6443.
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basis sets. They noted that the value of the spin density remained almost constant in the range of basis sets employed and therefore concluded that the value of the spin density would not be significantly altered by extending their double-{ basis in the (RCISD) calculation. Carmichae14 employed a triple-{ plus polarization (TZP) basis set and investigated the effects of all single and double excitations from the SUHF wave function (UCISD). In addition, he reported the results of the UCISD calculation using a double-{ plus polarization (DZP) basis as well as results from restricted calculations, namely ROHF, restricted single excitation CI (RSECI), and RCISD using the D Z and DZP basis sets. His value of the isotropic hyperfine parameter in the RCISD/DZP calculation (285 G) is strikingly different from that predicted by Nquyen and Ha and in much better agreement with experiment. We present here values of the isotropic and anisotropic hyperfine parameters of F2-(2B,+) calculated by using numerical wave functions. Calculations were carried out at the ROHF and SUHF levels of approximation. In addition, we present the results of a numerical RSECI calculation. Since such calculations are free of basis set errors, these results should help determine if basis set error is a source of the large difference in the results of the two previously mentioned calculations. All calculations were performed using our partial-wave multiconfiguration self-consistent field (PWMCSCF) methodG5The bond length used for F2-is 1.931 8,. This is the bond length used in the RCISD/DZ+(d) calculation of Nguyen and Ha and is 0.012 8, longer than that used by Carmichael. To obtain the numerical singles CI calculation, we proceed as follows. Consider, ( 5 ) McCullough, E. A. Compur. Phys. Rep. 1986, 4, 265.
0 1988 American Chemical Society
