The Journal of
Physical Chemistry
0 Copyright, 1989, by the American Chemical Socieiy
VOLUME 93, NUMBER 11 JUNE 1, 1989
LETTERS Vibrational Study of COP- on K-Promoted Pt( 111) Z. M. Liu, Y. Zhou, F. Solymosi? and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: February 21, 1989)
The adsorption of C 0 2 on K-dosed Pt( 11 1) was studied by high-resolution electron energy loss and thermal desorption spectroscopy. Adsorption of C02produced measurable vibrational spectra at low K coverage (e, = 0.05) as well as for a multilayer of K (0, = 1.27). At a K coverage of OK = 0.49, intense losses were observed at 1600 20, 1340 f 20, and 820 f 20 cm-' tentatively attributed to the asymmetric stretching, symmetric stretching, and the bending modes of a bent C02species formed by the direct interaction of metallic potassium and COz. COz- may be stabilized in the form of a dimer or oxalate. Above 200 K, this surface species is transformed into carbonate and CO, which are characterized by loss features at 1440 f 20 and 1600 20 cm-', respectively. CO desorbs above 600 K (Tp = 640 K), while carbonate decomposes above 650 K ( T , = 674 and 790 K).
*
*
Introduction An increasing number of investigations have been concerned with the catalytic transformation of C 0 2 into more valuable compounds. An important observation in this field is that COz is the main source of carbon in the synthesis of methanol from H 2 - C D C 0 2 mixtures.'*2 The activation of C 0 2 on solid surfaces is a difficult problem, as it forms a stable carbonate on oxide and adsorbs in a linear unreactive form on most metal surfaces. Recently, it has been observed that potassium adlayers on Pd( 100) and Rh(l11) surfaces dramatically influence the bonding and reactivity of CO,, very likely through the formation of negatively charged C 0 2 specie^.^-^ A similar effect was exerted by Cs on a Cu( 1 10) surface.6 In the case of K/Ag( 11 l), the presence of oxygen adatoms was required for binding C02.' In the present work, high-resolution electron spectroscopy (HREELS), Auger electron spectroscopy (AES), and temperature-programmed desorption (TPD) are used to characterize COz *To whom correspondence should be addressed. ' Fulbright Scholar. Permanent address: Reaction Kinetics Research Group of the Hungarian Academy of Science and Institute of Solid State and Radiochemistry, University of Szeged, H-6701 Szeged, Hungary.
0022-3654/89/2093-4383$01 S O / O
+
K adlayers on Pt( 111) at low temperature and to evaluate surface complexes formed in reactions of these adlayers at higher temperatures. Experimental Section The high-resolution electron energy loss spectrometer used in this study has been previously described.* The experiments were performed in an ultra-high-vacuum chamber with a base pressure Torr, equipped with facilities for AES and TPD of 4 X (1) Rozovskii, A. Ya.; Lim, G.; Liberov, L. B.; Slivinski, E. V.; Loktev, S. M.; Kagan, Yu.B.; Bashkirov, A. N. Kinet. Catal. (Engl. Transl.) 1977.18, 691. (2) Chinchen, G. C.; Denney, P. J.; Parker, D. G.; Spencer,M. C.; Whan, D.Appl. Catal. 1987, 30, 333. (3) Berko, A.; Solymosi, F. Surf. Sci. 1986, 171, L498. Solymosi, F.; Berko, A. J. Catal. 1986, 101, 458. (4) Solymosi, F.; Bugyi, L. J. Chem. SOC.,Faraday Trans. 1 1987, 83, 2015. (5) Kiss, J.; Revesz, K.; Solymosi, F. Surf. Sci. 1988, 207, 36. (6) Rodriguez, J. A,; Clendening, W. D.; Campbell, C. T. J. Phys. Chem., in press. (7) Blass, P. M.; Zhou, X.-L.; White, J. M. J. Vac. Sci. Technol., in press. (8) Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 188, 206.
0 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 1 1 , 1989
Letters
TABLE I: Characteristic Fremencies (cm-') and Mode Assignments of Gaseous C02 and Negatively Charged Adsorbed CO? Swies" Pt( 1 1 1 ) + K - C o t K2-CO2 KLC02C02(g) Li-C02 Ni(110)-C02 Ni(110)-C02 Re(000 1 )-C02
.-
820 739
1340 1184
667 750 750 719 650
1388 1330 1130 1195 (1395) 1130
present study 14b 14b 18
1600 1342 1609 2349 1569 1620'
14a
16 15 17
1600
"Values for alkali metal-C02 systems refer to C, symmetry and to spectra taken in argon at 15 K. *OK = 0.49. cOff specular.
measurements as well as for cleaning (Ar+ sputtering). A commercial SAES getter was used to deposit K onto a Pt( 111) surface situated 3 cm from the K source. The deposition of K was performed at 120-1 30 K. The K coverage was determined by means of A E S 9 A coverage of Bk = 0.33 (0.33 K per surface Pt) was taken for the first saturated K layer.9 The carbon dioxide (99% pure) was further purified by several freeze-pumpthaw cycles. It was dosed through a 3-mm-diameter tube that terminated r l cm from the face of the sample. During dosing, the C 0 2 input Torr. was adjusted to raise the background pressure by 2.5 X
Results and Discussion Potassium-Free P t ( l l 1 ) . Exposure of a clean Pt(ll1) surface Torr at to C 0 2 up to 40 langmuirs at a pressure of 1 X 110-300 K produced no observable changes in AES and HREELS of Pt, and no desorbing gases were detected in subsequent TPD. This result is in harmony with the observations of Norton and Richards,Io*" who were able to measure the X-ray photoelectron spectrum of adsorbed C 0 2 only at 77 K. They concluded that C 0 2 is only physisorbed on a clean Pt film and the estimated heat of adsorption was f 1 0 kcal/mol. As regards the effect of preadsorbed oxygen on C 0 2 adsorption, we observed no promoting effect at 110 K. Potassium-Doped P t ( l l 1 ) . A completely different picture was obtained when preadsorbed potassium was present. Adsorption of C 0 2 produced measurable vibrational spectra at the lowest K coverage used (0, = 0.05), as well as for multilayers of K (0, = 1.27). Here, we focus on detailed measurements for a sample containing slightly more potassium than required for monolayer (0, = 0.49), where potassium exhibits a metallic character.12 The advantage of this sample is that it adsorbs CO extremely slowly (no losses were detected even after keeping the sample at 110 K for 60 min), so the background adsorption is negligible. Figures 1 and 2 illustrate the effects of C 0 2 exposure and temperature on the development of HREELS features. Deposition of potassium on a clean Pt( 1 1 1) surface produced only one loss at 190 cm-I, which is attributed to a K-substrate ~ i b r a t i 0 n . I ~ Adsorption of C 0 2 on this surface at 112 K gave additional losses at 441, 820, 1340, and 1600 cm-l. An increase in the C 0 2 exposure intensified all the losses, particularly at 1600 cm-', but their positions remained in the range of accuracy (A20 cm-I) of the determination of peak positions. This coverage-independent behavior may be an indication of a direct interaction between C 0 2 and K. Similar behavior was observed in the case of CO adsorption on Pt( 111) at high potassium coverage (0, = 0.44).13 The following losses are tentatively attributed to the asymmetric stretching (1 600 f 20 cm-I), symmetric stretching (1 340 f 20 cm-I), and the bending mode (820 f 20 cm-I) of a bent C 0 2 species formed by the direct interaction of metallic potassium and C02
K
+ C 0 2 ( g )= K+-C02-(a)
which may be stabilized in the form of a dimer or oxalate. The (9) Greenlief, C. M.; Radloff, P. L.; Akhter, S.; White, J. M. Surf.Sci. 1987, 186, 563. (10) Norton, P. R. Surf.Sci. 1974, 44, 624. (11) Norton, P. R.; Richards, P . J. Surf.Sci. 1975, 49, 567. (12)Kiskinova, M.;Pirug, G.; Bonzel, H. P. Surf Sci. 1983, 133, 321. (13) Pirug, G . ;Bonzel, H. P. Surf.Sci. 1988, 199, 371.
1600
0
500
I
I
I
1000
1500
2000
X30
20.0L
X30
1O.OL
X30
2.50L
X30
063L
X30
0161
ELECTRON ENERGY LOSS (cm-')
Figure 1. H R E E L spectra of adsorbed COSon Pt(l11) surface at OK = 0.49 at 112 K as a function of C 0 2 exposure time. Exposure times (seconds) are (a) 12.5, (b) 25, (c) 50, (d) 200, and (e) 400.
1
L
J' 0
500
1000
1500
X30
650K
X30
303K
X30
200K
X30
150K
X30
112K
2000
ELECTRON ENERGY LOSS (cm")
+
Figure 2. Spectral changes observed following warming of the C 0 2 K coadsorbed layer on Pt(l11) surface to higher temperatures. OK = 0.49. C 0 2 exposure = 20 langmuirs. (a) As dosed at 112 K (same as Figure le); the other spectra are taken after heating to the indicated temperature and recooling to take H R E E L spectrum. (b) 150 K, (c) 200 K, (d) 303 K, (e) 650 K, (f) 750 K, and (g) 850 K.
positions of the above stretches agree well with those for C 0 2 observed following the interaction of alkali metals and C 0 2 by matrix isolation infrared spectro~copy'~ (Table I). The C 0 2 species has also been detected by HREELS on more reactive Ni( 110)15*'6 and Re(OOOl)17surfaces. However, the concentration (14) (a) Kafafi, Z.H.; Hauge, R. H.; Billups, W. E.; Margrave, J. L. J . Am. Chem. SOC.1983, 105, 3886. (b) Kafafi, 2.H.; Hauge, R. H.; Billups, W. E.; Margrave, J. L. Inorg. Chem. 1984, 23, 171. (15) Linder, H.; Rupprecht, D.; Hammer, L.; Muller, K. J. Electron Spectrosc. Relat. Phenom. 1987, 44, 141. (16)Bartos, B.; Freund, H. J.; Kuhlenbeck, H.; Neumann, M.; Lindner, H.; Muller, K. Surf.Sci. 1987, 179, 59.
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4385
Letters of C02- species in these cases, considering the relative intensities of the vibrational modes, is apparently much less than on potassium-doped Pt. On the basis of a vibrational study of an 0-K coadsorbed layer on Ru(OOl),I9 the weak losses at 260 and 441 cm-I are attributed to K-0 vibrations. At higher C02exposures we also observed loss features at 660 and 2350 cm-I, which are assigned to the bending and asymmetric stretching modes of weakly adsorbed, and only slightly perturbed, C02.18 The symmetric stretch at around 1388 cm-I is probably too weak to resolve. It is interesting that the losses of weakly adsorbed C02did not occur after dosing a K-free Pt( 111) surface under the same conditions. Potassium also increases the extent of physical adsorption of C02on Rh(l1 l ) , an effect ascribed to the formation of negatively charged C 0 2 clusters of the form (C02),,-.6920 The formation of this cluster ion has been extensively studied in the gas phase, where n varies between 2 and 16.21,22 Upon heating the saturated layer, we observed the first spectral changes a t 200 K (Figure 2). This consisted of (1) complete elimination of peaks assigned to weakly adsorbed C02 (660 and 2340-2350 cm-I), (2) significant attenuation of the loss at 1340 cm-l, (3) the formation of a new intense feature at 1440 cm-', and (4) an intensification of the loss at 1600 cm-'. The weak shoulder at 1340 cm-I vanished slightly above 200 K, and the losses at 1440 and 1600 cm-' intensified further. It is important to point out that the intensities of 1440- and 1600-cm-' losses are higher by about 30% than those at 1340 and 1600 cm-l measured at 150 K, which makes it very likely that both losses belong to new surface species. In addition, several very weak losses were resolved in the low-frequency region at 780, 870, and 1020 cm-'. All these spectral changes suggest a significant rearrangement in the adsorbed layer which we attribute to the occurrence of the reaction 2C02-(a) = C03"(a)
+ CO(a)
The loss at 1440 cm-' is assigned to carbonate23and the feature at 1600 cm-l to the asymmetric stretch of C O perturbed by potassium. As regards the location of the C O vibration, we mention that preadsorbed potassium drastically lowers the position (17) Asscher, M.; Kao, C. T.; Somorjai, G. A. J . Phys. Chem. 1988, 92, 2711. (1 8) Herzberg, G. Molecular Spectra and Molecular Srrucrure; University Press: Princeton. 1945: Vol. 2. (19) DePaola,'R. A.i Hoffmann, F. M.; Heskett, D.; Plummer, E. W. J . Chem. Phys. 1987, 87, 1361. (20) Solymosi, F.; Kiss, J. To be published. Sattler, K.; Recknagel, E. Surf:Sci. (21) Knapp, M.; Kreisle, D.; Echt, 0.; 1985, 156, 3i3. 122) Stematovic. A,: Leiter. K.: Ritter. K.: Steohan. K.; Mark, T. D. J . Chem'Phvs. 1985. 83. 2942. (23) Nikamoto; K.' Infrared and Raman Spectra; Wiley: New York, 1986.
of C O losses, which, depending on the K and CO coverages, fall in the range 1496-1700 cm-1.'3*24925The origin of losses in the low-frequency part is not absolutely clear. The losses around 780 and 870 cm-' also appeared following CO adsorption on a Pt( 111) surface covered with a K mon01ayer.I~ These losses vanished, together with the loss feature at 1600 cm-I (Figure 2), strongly suggesting that they are connected with C O formed in the surface reaction. As regards the loss feature developed at 1024 cm-I, we mention that a 1007-cm-I peak is observed in the infrared spectrum of vaporized KzC03 and assigned to a KO, species.14b The above description is supported by the high stability of these spectral features: between 200 and 650 K, HREEL spectra indicated very little alteration in the coadsorbed layer. As we cannot expect such a high thermal stability from either COz- or C2042-anion, an alternative, carbonate, is required. An attenuation of the intense loss at 1600 cm-' starts above a t 650 K, and it is completely eliminated around 710 K together with weak features at 780 and 870 cm-I. In this temperature range the majority of potassium is also desorbed, generating free Pt sites on which C O may absorb from the background. A significant attenuation of the loss at 1440 cm-l occurs between 680 and 750 K, and it is completely absent above 800 K. However, even after this temperature treatment, very weak losses remain at 190,441, and 1000 cm-'. Supplementary TPD measurements revealed that C O ( T p = 640 K) and COz ( Tp = 674 and 790 K) desorbed in nearly equal amounts. The characteristic peak temperature of C O ( Tp = 640 K) agrees well with the peak temperature of CO desorption from 1 monolayer of K on P t ( l 1 l).9*24The evolution of C 0 2 points to the decomposition of carbonate-like species C032-(a) = 02-(a)
+ C02(g)
The peak temperature of C 0 2 desorption is in good agreement with those obtained on potassium-doped Pd(100) and Rh( 11 1) surfaces, where the formation of carbonate-like species was established by UPS and XPS.3-5 Adsorbed oxygen was detected by Auger electron spectroscopy after complete desorption of CO and C02. The existence of loss features a t 260, 441, and 1000 cm-I in the HREEL spectrum even after thermal treatment at 850 K supports the assignment of these losses to vibrations of K-O surface species. Acknowledgment. This work was supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences. (24) Crowell, J. E.; Garfunkel, E. L.; Somorjai, G. A. SUP$Sei. 1982,121, 303.
(25) Wesner, D. A.; Pirug, G.; Coenen, F. P.; Bonzel, H. P. Surf. Sci. 1986, 178, 608.
