J . Phys. Chem. 1990, 94, 6847-6852 sorption or another crystalsolution interaction) according to the metal-anion bond strength. They state that the weaker the bond is, the closer will its position be to midgap. Therefore, we believe that these midgap interface states are formed due to the relatively weak Cu-S bonds (as compared to the Cd-S bonds). Thus the observed increase in SRV is due to the increase in u resulting from the redistribution of the interface states closer to midgap. It is instructive to compare the effect on SRV of the solution-deposited Cu and the evaporated Cu. Based on Figure 4, and the above discussion, submonolayer coverages of evaporated and solution deposited Cu resulted in similar values of SRV. We have no clear evidence for the formation of Cu-S compound in the case of evaporation. Brucker and Brillsonz3 have also shown that evaporation of Cu on cleaved CdS under UHV conditions does not change the Cd/S surface concentration ratio up to about IO-A Cu coverage. They suggested that the unchanged ratio indicates that no Cu-S chemical reaction takes place similarly to Au/CdS interfaces and unlike reactive metals ( e g , AI or Ti) interfaces with CdS. Our results show that the SRV values are essentially the same for both deposition methods. The difference in bonding does not in our opinion rule out a common origin for the SRV increase. In the solution case, the increase is due to surface states formed as a result of Cu-S bonds formation, while the evaporated Cu leads to surface states formation due to weaker bonding and lattice disruption. Although the effect on the SRV values is essentially the same, the energy position of these states may be different. These energy positions may be identified by surface photovoltage measurements on Cu/CdS interfaces. In the past24we reported on the measurement of the SRV value on CdS surfaces covered with Au. We suggested that the measured increase in the SRV (23) Brucker, C. F.; Brillson, L. J. J . VUC.Sci. Technol. 1981, 19, 617. (24) Huppert. D.:Evenor, M.; Shapira, Y. J . Vac. Sci. Technol. 1984, A2, 532.
6847
was caused by the CdS surface disruption by the unreactive Au, which resulted in formation of midgap interface states. On the basis of similar SRV dependence we suggest that in the Au case the formation of midgap interface states is predominantly a result of the CdS lattice disruption and defect formation. It seems that, in the Cu case, the interface recombination centers are a result of a combined effect of weak bonding and lattice disruption.
Summary and Conclusions We have studied the effect of Cu coverage on the surface recombination velocity of CdS. It is found that very dilute CuSO., solutions produce controllable submonolayers of Cu-S compound and increase the value of SRV dramatically. SRV increases by almost 1 order of magnitude at an estimated Cu coverage of 1/ 10 monolayer. It continues to be a strong function of coverage up to monolayer where it saturates at values above 3 X lo5 cm/s. The Cu-S compound formation in such dilute solutions is due to its extremely low solubility in water relative to CdS. These conclusions were supported both by AES and AAS measurements. The SPS measurements have supplied the evidence that compound evolution led to midgap surface-state formation. We propose that weak bonding and lattice defects formed in the Cu-S/CdS interface are responsible for these states. Similar SRV values were obtained by UHV-deposited Cu. It is suggested that the increased SRV in this case is due to similar reasons. Cuinduced weak bonding and lattice defects create midgap interface states, which contribute to increase of SRV. This points to a direction in which SRV could be systematically controlled.
-
Acknowledgment. Y.R. gratefully acknowledges the financial support of a grant from the Gordon Center for Energy Research at Tel Aviv University. We are grateful for the support of the Krantzberg Institute, The Israeli National Academy of Sciences, and the U S . AID-CDR Program. Registry No. Cu, 7440-50-8; CdS, 1306-23-6.
Chemisorption and Thermal Decomposition of Ethylene on a Pd( 1lo)( 1X2)-Cs Surface: Electron Energy Loss Spectroscopy and Thermal Desorption Studies T. Sekitani, J. Yoshinobu,+ M. Onchi,$ and M. Nishijima* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received: February 2, 1990; In Final Form: April 24, 1990)
The adsorbed state of ethylene on the Pd( 1 lo)( IX2)-Cs surface and its thermal decomposition have been studied by the use of high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. For a small exposure (0.1 langmuir) at YO K, ethylene is a-bonded to the Pd( 1 lo)( 1 X2)-Cs surface. For a large exposure (2 langmuirs), a small amount of physisorbed ethylene exists in addition to T-bonded ethylene. The saturation coverage corresponds to 0.23 C2H4 molecules per Pd atom of the unreconstructed Pd( 1 IO) surface. The physisorbed ethylene is desorbed by heating to 1 IO K. A part of the *-bonded ethylene is desorbed intact at 200 K, and additionally at 270 K by a recombinative process. By heating to 300 K, the residual C2H4admolecules are decomposed, and methylidyne (CH) species are formed. Only carbon adatoms remain on the Pd( 1 IO)( 1X2)-Cs surface by heating to 500 K. These results are compared with those for the Pd( 1 IO) clean surface, and the effects of Cs adatoms on the surface reactions are discussed.
I. Introduction sorption and thermal decomposition of ethylene on the Pd(l IO) clean surface using electron energy loss spectroscopy (EELS), The adsorption and thermal decomposition of ethylene on well-defined surfaces (Fe,l.2 Ni,3-5Cu,6 Ru?v8 Rh?JO Pd,11-16*2w22 Ag," lr,'* Pt,I9 etc.) have been an object of many studies as a (1) Erley, W.; Bar6, A . M.; Ibach, H. Surf. Sci. 1982, 120, 273. prototype for the interaction of olefin hydrocarbons with catalysts. (2) Seip, U.; Tsai, M.-C.; Kiippers, J.; Ertl, G. Surf. Sci. 1984, 147, 65. (3) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. The ethylene-Pd( 1 IO) surface interaction has been studied by a (4) Stroscio, J . A.; Bare, S. R.; Ho, W. Surf. Sci. 1984, 148, 499. few research groups.13,2w22Recently, we have studied the ad( 5 ) Zaera, F.; Hall, R. B. Surf. Sci. 1987, 180, 1 . Present address: Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260. 'Professor Emeritus. +
0022-3654/90/2094-6847$02.50/0
(6) Nyberg, C.; Tengstil, C . G.; Andersson, S . ; Holmes, M. W. Chem. Phys. Lett. 1982, 87. 87. (7) Hills, M. M.; Parmeter, J. E.; Mullins, C. B.; Weinberg, W . H. J . Am. Chem. Soc. 1986, 108, 3554.
0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS).I6 Ethylene is *-bonded to the Pd( 1IO) clean surface at 90 K. At above 300 K, CzH4admolecules are dehydrogenated, and ethynyl (CCH) species are formed. By heating to 450-520 K , ethynyl is decomposed, and only carbon adatoms remain on the Pd( 1 IO) surface; the decomposition is accompanied by H2 desorption. It is well-known that alkali-metal-induced (1 X2) reconstruction . ~ present ~ occurs on the Pd( 1 IO) s u r f a ~ e . ~In~the work, we have studied, using in situ combined techniques of EELS and TDS, the interaction of ethylene with the Pd( 1 IO)( 1 X2)-Cs surface. The adsorbed state of ethylene at 90 K and thermal decomposition mechanisms were studied. The effects of alkali-metal (Cs) atoms on the surface chemistry of ethylene on Pd(ll0) were examined. It is noted that coadsorption experiments of ethylene and alkali metals on transition-metal surfaces have been made by a few research groups [Pt( 1 1 I)-K25-27]. 11. Experimental Section Details of the experimental methods are described elsewhere,16 and only a brief explanation is given below. For EELS measurements, a primary energy E , of 4 eV, an energy resolution of 40 cm-' (5 meV) (full width at half-maximum), and an incidence angle Bi of 60° with respect to the surface normal were used. The electrons were scattered along the [ 1 101 azimuth. Heat-and-quench method was used for the temperature-dependent measurements: a sample was heated at a heating rate of 5 K / s up to a certain temperature, cooled to 90 K, and then the EELS measurements were made. TDS measurements were multiplexed and were carried out at a heating rate of 5 K/s. The Pd( 110) clean surface was carefully prepared by oxidation, Ar+ ion bombardment, annealing, and flashing cycles. Cesium was deposited from an SAES getter source. The Pd( 1 lo)( 1 X 2)-Cs surface was formed by presaturation of the Pd( 1 IO) clean surface with Cs atoms at 90 (or 300) K and subsequent heating to 800 K [see ref 24 for the structure of the Pd( 1 I O ) ( 1 X2)-Cs surface]. I t is noted that the fractional Cs coverage is -0.1.23324 Research-grade C2H4 (99.8 mol % purity), C2D4(99.5 atom % D, MSD Isotope, Canada, Ltd.), and H, (99.8 mol % purity) were used. Ethylene was introduced into the vacuum chamber through a gas doser which produced a flux at the sample surface about 50 times the background ethylene flux. Ethylene pressure in the chamber was monitored by the use of a nude-type Bayard-Alpert ion gauge, and calibrated by the ion-gauge sensitivity factor of C2H4 (2.3 relative to VI). Base pressure of the vacuum system was 4 X lo-" Torr.
Sekitani et al.
+ CEH4
4803Ck
kl ~110)I1X2)-Cs
90K
Jl
I
i
,355
Figure 1. E E L S spectra as the Pd(llO)(lX2)-Cssurface a t 90 K is exposed to (a) 0.1 langmuir of C2H4 (A0 = 0'); (b) 0.3 langmuir (A0 = OO); (c) 2 langmuirs (A0 = OO); (d)2 langmuirs (A0 = 4O). E, = 4 eV. The inset shows the electron scattering geometry.
(8) Hills, M. M.; Parmeter, J . E.; Weinberg. W. H. J . Am. Chem. SOC. 1987, 109. 4224.
(9) Dubois. L. H.; Castner, D. G.; Somorjai, G. A. J . Chem. Phys. 1980, 72, 5234. (IO) Slavin, A. J.; Bent, B. E.; Kao, C.-T.; Somorjai, G. A. Surf. Sci. 1988, 206, 124. ( I I ) Gates, J . A.; Kesmodel, L. L. Surf. Sci. 1982, 120, L461. (12) Gates, J . A.; Kesmodel, L. L. Surf. Sci. 1983, 124, 68. (13) Chesters, M. A.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Appl. Surf. Sci. 1985, 22 f 23, 369. (14) Stuve, E. M.; Madix, R. J . J . Phys. Chem. 1985, 89, 105. (15) Stuve, E. M.; Madix, R. J . Surf. Sci. 1985, 160, 293. (16) Nishijima. M.; Yoshinobu, J.; Sekitani, T.; Onchi, M. J . Chem. Phys. 1989, 90, 5 1 14. (17) Backx, C.; de Groot, C.P. M.; Biloen, P. Appl. Surf. Sci. 1980, 6, 256. (18) Marinova, Ts. S.;Chakarov, D. V . Surf. Sci. 1987, 192, 275. (19) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982, 117, 685. (20) Bandy, B. J.; Chesters, M. A.; James, D.1.; McDougall, G. S.; Pemble, M. E.; Sheppard, N . Philos. Trans. R. SOC.A 1986, 318, 141. (21) Gentle. T. M.; Muetterties, E. L. J . Phys. Chem. 1983, 87, 2469. (22) Rucker. T. G . ; Logan, M. A,; Gentle, T. M.; Muetterties, E. L.; Somorjai, G . A . J . Phys. Chem. 1986, 90,2703. (23) Barnes, C. J.; Ding.M. Q.; Lindroos, M.; Diehl, R. D.; King, D. A . Surf. Sci. 1985, 162, 59. (24) Barnes, C . J.; Lindroos, M.; King, D. A. Surf. Sci. 1988, 201, 108. (25) Zhou, X.-L.; Zhu, X.-Y.; White, J . M. Surf. Sci. 1988, 193, 387. (26) Windham, R. G.; Bartram, M. E.; Koel, B. E. J . VUC.Sci. Techno/. A 1981, 5 , 457. (27) Windham, R. G.; Bartram, M. E.: Koel, B. E. J . Phys. Chem. 1988, 92, 2862
_
0
i
1222
,
n +
i " l L
.
.___ ~~
3300
FNERGV LaS5 ICm-') Figure 2. E E L S spectra in the specular mode of the Pd( 1 IO)( 1 X2)-Cs surface preexposed to 2 langmuirs of C2H4 a t 90 K and subsequently heated to high temperatures. The heating rate was 5 K/s. All spectra were recorded a t 90 K. 111. Results A. Low-Energy Electron Diffraction. As the Pd( 1 lo)( 1X 2 ) X s
surface was exposed to C2H4 at 90 K, only an increase of the background intensity was observed. This indicates that C2H4
Decomposition of Ethylene on a Pd( 1 IO)( 1 X2)-Cs Surface
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6849
110
2
645
3
/I(
MASS 2 7
, Oo3
(b)
m
cr
a
'
(ai
04L
0 1L
la X O
300
403
500
TEM PERATURE (KJ Figure 4. TDS spectra of (a-d) C2H4(mass 27) and (e) H2 (mass 2) as
2
the Pd( 1 IO)( 1 X2)-Cs surface is exposed to various amounts of C2H4 at 90 K. The heating rate was 5 K/s.
LU
0 J ,l -L -
3
1"
2000
t;
ENERGV LOSSk m-'1
,h
Figure 3. EELS spectra in the specular mode of the Pd(l lO)(lX2)-Cs surface preexposed to 2 langmuirs of C2D4 a t 90 K and subsequently heated to high temperatures. The heating rate was 5 K/s. All spectra were recorded at 90 K.
molecules are adsorbed disorderedly on the Pd( 1 IO)( 1 X2)-Cs surface at 90 K. By heating the C2H4-exposedPd( 1lo)( 1X2)-Cs surface (up to 500 K), no remarkable change was observed, and only a (1 X2) pattern was observed with background. B . Electron Energy Loss Spectroscopy. Figure la-c shows EELS spectra in the specular mode (A0 = 0'; see the inset of Figure 1 ) as the Pd( 1 IO)( 1X2)-Cs surface is exposed to an increasing amount of C2H4 at 90 K. For 0.1 langmuir of C2H4 exposure (1 langmuir = IO" Torr s), losses are observed at 875, 1 1 IO, and -2945 cm-' (Figure 1a). For 0.3-langmuir exposure, loss intensities are increased (Figure Ib). For 2 2 langmuirs of C2H4 exposure (saturation exposure), new losses appear, and losses are observed at 365,760,875,980, 1 120, 1240, 1340, 15 15,2930, and 3060 cm-l. Figure I C shows an EELS spectrum in the specular mode for 2 langmuir exposure. Figure Id shows a corresponding EELS spectrum in the off-specular mode (A0 = 4O). As A0 is increased, intensities of all losses, except for the 2930- and 3060-cm-' losses, seem decreased. Thus, all losses, except for the 2930- and 3060-cm-' losses, are mainly dipoleexcited.28 Figure 2 shows EELS spectra of the Pd( 1 IO)( 1 X2)-Cs surface preexposed to 2 langmuirs of C2H4 at 90 K and subsequently heated to high temperatures. All spectra were recorded at 90 K, and loss-peak intensities are normalized by the elastic peak intensities. Figure 2a is identical with Figure IC. On heating to 180 K, the 760-, 980-, 1340-, and 3060-cm-' losses seem decreased in intensity (Figure 2b). On heating to 230 K, losses are observed at 390,865, 1 105 and 2920 cm-I; a shoulder is observed at 1200 cm-I, and a weak loss at 1500 cm-l (Figure 2c). On heating to 300 K, the elastic peak markedly decreased (1.1 X lo4 counts/s), and the spectrum is changed: losses are observed at 1040, and -2980 cm-' (Figure 2d). O n heating 390, -800, to 500 K, only the 390-cm-' loss is observed (Figure 2e). Figure 3 shows EELS spectra of the Pd( 1 IO)( 1X2)-Cs surface preexposed to 2 langmuirs of C2D4 at 90 K and subsequently
-
-
-
(28) Ibach, H.; Mills, D. L. Electron Energy Loss Specrroscopy and Surface Vibrations; Academic: New York, 1982.
i
MASS 32
21
loo Kx7
403 500 TEMPERATURE (KJ Figure 5. TDS spectra of (a) C2D4 (mass 32) and (b) D2 (mass 4) as the Pd(lIO)(IX2)-Cs surface is exposed to 2 langmuirs of C2D4a t 90 K. The heating rate was 5 K/s.
3oc)
heated to high temperatures. Figure 3a shows an EELS spectrum of the Pd( 1 lo)( 1 X2)-Cs surface exposed to 2 langmuirs of C2D4 at 90 K. Losses are observed at 355, 530, 630, 735, 915, 1210, 1335, 2125, and 2215 cm-I. Similarly to the case of C2H4 on Pd(1 IO)( 1X2)-Cs, all losses, except for the 2125- and 2215-cm-I losses, are mainly dipole-excited. By heating to 180 K, the 735 cm-I loss is decreased in intensity (Figure 3b). On heating to 230 K, the 530-cm-' loss disappears, and the 1335-cm-' loss is decreased in intensity; losses are observed at 390, 645, 905, 1220, 1335, 2140, and 2210 cm-I (Figure 3c). On heating to 300 K, the elastic peak intensity is decreased, and the spectrum is drastically changed. Losses are observed at 400, 620, 745, and -2230 cm-l (Figure 3d). Only the 390-cm-l loss is observed by heating up to 500 K. C . Thermal Desorption Spectroscopy. TDS measurements after the Pd( 1 IO)( 1 X2)-Cs surface was exposed to ethylene at 90 K showed that C2H4(C2D4) and H2 (Dz) were the only desorption products; no methane, acetylene, ethane, or benzene was detected. C2H4 (mass 27 fragment) and H2 (mass 2) TDS spectra for the Pd( 1 IO)( 1X2)-Cs surface exposed to C2H4 are shown in Figure 4. For a small exposure (-0.1 langmuir), only H 2 desorption occurs (Figure 4a). With increasing exposure, in addition to H2 desorption, C2H4 desorption appears first at 270 K and -290 K (shoulder) (Figure 4b), then at 200 K (Figure 4c), and at 110 K (for 20.6-langmuir exposure, Figure 4d). The H 2 desorption peak is observed at -340 K (shoulder) and 395 K (Figure 4e). Figure 5 shows TDS spectra for the Pd( 1 IO)( 1 X2)-Cs surface exposed to 2 langmuirs of C2D4 at 90 K. C2D4 desorption (mass 32) is observed at 110, 210, and 280 K; D2 desorption (mass 4) at -400 K. In order to examine the isotope mixing effect in the ethylene desorption, we have also measured TDS spectra for the Pd( 1 IO)( 1 X2)-Cs surface exposed to a mixture of C2H4 and C2D4. The C2H4-,D, desorption (x = 0-4) was observed in the 260-300
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The Journal of Physical Chemistry, Vol. 94. No. 17. 1990
TABLE I: Vibrational Energies (cm-’) and Their Assignments for CzH, (CzD,) on the Pd(ll0) ( l X 2 ) C s Surface at 90 K and Clean Pd(ll0) at 90 K and for Zeise’s Salt Together with the Energy Ratios u H / v D ethylene on clean Pd(l Zeise’s sait33J4 ethylene on Pd( 1 I O ) ( I X2)-Cs assignment CH2(CD2) as stretch
CIH, 3060
C2D4
2215
I .38
C H 2 ( C D 2 )s stretch
2930
2125
1.38
C C stretch CH2(CD2) as scissors CH2(CD2) s scissors CH,(CD2) twist CHACD2) wag
1515 1340 1240 1120 980 875
I335
1.13
915 735 630
uH/’D
1.22 1.33 1.39
C H 2 ( C D 2 )rock 760 MC M C as stretch M C s stretch
530 365
355
I .03
MC
K range, which indicates that C2H4 (C2D4) desorption at 270 (280) K occurs by a recombinative process. The isotope mixing effect was not observed for C2H4 (C2D4) desorbing at 1 I O ( I I O ) and 200 (210) K, which indicates that these ethylene molecules are desorbed intact from the surface. In order to estimate the fractional H coverage OH corresponding to the total amount of desorbing H,, we performed TDS measurements for the Pd( I10)(2Xl)-H surface. It is considered that OH = 1 for the Pd(l10)(2xl)-H surface.2e32 The fractional H coverages corresponding to H, desorption spectra for the Pd( 1 IO)( 1 x2)-Cs surface exposed to C2H4 can be estimated by comparing the area intensities for the H2 desorption spectra and that for the Pd( 110)(2XI)-H surface. Thus, H2 desorption from the C2H4-saturated Pd( 1 lo)( 1 X2)-Cs surface corresponds to OH = 0.5 (Figure 4e), or to the fractional C2H4 coverage of 0.13. The C2H4 coverage corresponding to C2H4 desorption spectra can be estimated by comparing the area intensities for the C2H4 desorption spectra and that for the Pd( 1 10)c(2X2)-C2H4 surface (OCIH3 = 0.5),16 and the results are included in Figure 4. The fractional C2H4 coverage corresponding to the C2H4 desorption spectrum for the C2H4-saturatedPd( 1 IO)( 1X2)-Cs surface is 0.1 [OCIH, 0.03 for the 110 K peak; 0.03 (200 K); 0.03 (270 K)] (Figure 4d). Therefore, it is estimated that the saturation coverage of ethylene on the Pd( 1 IO)( 1 X2)-Cs surface at 90 K corresponds to 6C1H4= 0.23.
-
IV. Discussion A . Assignmenfs ofthe EELS Peaks. Vibrational spectra for C2H4 and C2D4 on the Pd( 1 I O ) ( 1 X2)-Cs surface at 90 K are shown in Figures 1 and 3a, respectively, and are similar to those on the Pd( 1.10) clean surface. Assignments of the observed EELS peaks can be made by comparison with the vibrational energies of Zeise’s salt-do (-d4),33,34 gaseous C2H4 ( C Z D ~ and ) , ~ ethylene ~ on clean Pd( 1 10)13316 and on other metal s ~ r f a c e s , l - ’ ~and , ’ ~ by examining the loss-energy ratios uH/vD for C2H4 on Pd( 110)( 1 X2)-Cs and the deuterated counterpart. Table I summarizes vibrational energies and their assignments for C2H4 (C2D4) on the Pd( 1 IO)( 1 X2)-Cs surface, and on clean Pd( 1 I O ) , and for
(29) Behm, R. J.; Perka, V.; Cattania, M.-G.; Christmann, K.; Ertl, G . J. Chem. Phys. 1983, 78, 1486. (30) Jo, M.: Kuwahara, Y . ;Onchi. M.; Nishijima, M. Solid State Commun. 1985, 55, 639. (31) He, J.-W.; Norton, P. R. Surf. Sci. 1988, 195, L199. (32) He, J.-W.; Harrington. D. A.; Griffiths, K.; Norton, P. R. Surf. Sci. 1988, 198, 413. (33) Hiraishi, J. Spectrochim. Acta A 1969, 25, 749. (34) Powell, D. B.; Scott, J. G. V.; Sheppard, N. Spectrochim. Acta A 1972, 28, 321. (35) Shimanouchi, T. Tables of Molecular Vibrational Frequencies: Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. 1972, 39, Vol. I . (36) Sheppard, N.Annu. Reo. Phys. Chem. 1988. 39, 5 8 9 .
pHIUD
C2D4
C2H4
2350
1.31
3000
2265
1.32
1520 1410 1235 I I30 1040 900 865 780 530 380 335 280
1370 I040 960 920 810 665
1.1 1 I .36 I .29 I .23 I .28 1.35
530 370 335 275
1 .oo
1.03 1
.oo
do 3094 3079 3013 2988 1515 1426 1243 1180 I010 975 84 I 720
d4 2349 2331 2224 2193 1353 1059 962 982 812 757 597 525
493 405
45 1 385
yH/’D
1.32 1.32 I .35 I .36
1.12 1.35 1.29 1.20 I .24 1.29 1.41 1.37 1.09 1.05
I .02
(3)
‘E:
Figure 6. Proposed structural models of (a) C2H, on the Pd(1 lO)(lX 2)-Cs surface at 90 K, and (b) C H on Pd(l IO)(] XZ)-Cs. Arrows show the direction of tilt of the C2H4 molecule.
Zeise’s salt together with the energy ratios vH/vD. It is considered that the 1210-cm-’ loss for C2D4 on Pd(1 IO)( 1X2)-Cs corresponds to the 1240-cm-I loss for C2D4 on clean Pd( 1 10): the 12 10-cm-I loss may be derived from the Fermi resonance between the overtone of the 630-cm-’ loss and the C C stretching vibration at 1335 cnr1.16 B. Adsorbed State of Ethylene at 90 K . TDS measurements show that, for a small exposure (0.1 langmuir), ethylene is chemisorbed and that, for a large exposure (2 langmuirs), a small amount of physisorbed ethylene (with the desorption temperature of I10 K) exists in addition to chemisorbed ethylene. It is estimated, assuming first-order desorption kinetics, that the activation energy of desorption of physisorbed ethylene is -2 kcal/mol (prefactor 3 X I04/s).37 On the other hand, EELS spectral change due to the desorption of physisorbed ethylene is small (compare Figures 2, a and b, or 3, a and b). Thus, the EELS peaks (Figures 2a and 3a) are attributed predominantly to chemisorbed ethylene. The C H 2 (CD,) symmetric stretching energy [2930 (2125) cm-’1 and CC stretching energy [ 15 15 ( 1 335) cm-l] indicate that the rehybridization state of chemisorbed ethylene on the Pd( 1 IO)(lX2)-Cs surface is -sp2 and that ethylene is *-bonded to the Pd( 1 IO)( 1X2)-Cs surface.16 All vibrational modes we have detected, except for the CH, (CD,) stretching modes, seem mainly dipole-excited. Thus, we tentatively consider, using the surface-normal dipole selection rule,2s that ethylene on the Pd( 1 IO)( IX2)-Cs surface has a CI symmetry. The C H 2 (CD,) symmetric stretching energy [2930 (2125) cm-l] indicates that ethylene is not hydrogen-bonded to the Pd( 1 lo)( 1X2)-Cs surface.28 Therefore, roughly speaking, the C C bond axis is not
-
( 3 7 ) Chan. C.M.; Aris. R.; Weinberg. W. H. Appl. Surf. Sci. 1978, I , 360.
Decomposition of Ethylene on a Pd(l lO)(lX2)-Cs Surface
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6851
TABLE 11: Vibrational Energies (cm-') Observed of Methvlidvne (CH) on Various Transition-Metal Surfaces and for Oreanometallic Comwunds
C H stretch C H bend MC
2960 3050 850 -350
3015
2980
3010
3002
2940
3100
3041
2994
795
790
810
762 307
925 470
850
850 417
895 426
parallel, but is perpendicular to the [ IT01 azimuth. A possible structural model of chemisorbed ethylene on the Pd( 1 IO)( 1X2)-Cs surface at 90 K is shown in Figure 6a. C. Thermal Decomposition of Ethylene on the Pd(llO)(lX 2)-Cs Surface for Large Exposure (2 langmuirs). Drastic change is not observed in EELS spectra by heating to 180 K (Figures 2b and 3b). This indicates that the adsorbed state of ethylene is not markedly changed by I80 K heating. Small spectral changes and the decrease in number of observed losses [note, e.g., the decay of the 760-cm-' loss (Figure 2b)l may partly be attributed to the increase of ethylene molecules of higher point-group symmetry by the desorption of physisorbed ethylene. Some ethylene is desorbed by heating to 200 K (Figure 4d). It is estimated, assuming the first-order desorption, that the activation energy of the ethylene desorption is -9 kcal/mol (prefactor 1 X 109/s).37 By heating to 230 K , relative intensities of the CC stretching mode at 1500 ( I 335) cm-I and C H 2 scissors mode at 1200 (---) cm-' are decreased, but no new EELS peaks are observed (Figures 2c and 3c). Recombinative desorption of some C2H4 occurs at 270 K (Figure 4d). I t is considered that, by heating to 270 K, a part of the C2H4 molecules are decomposed to form unstable vinyl (CHCH2) species and H adatoms, which are subsequently recombined to form desorbed C2H4.I6 For C2D4, ethylene desorption occurs IO K higher than for C2H4. This is attributed to the isotope effect associated with the dehydrogenation mechanism of the adsorbed ethylene and/or the mobility of hydrogen atoms. The shoulder at -290 K (Figure 4d) is ascribed to the recombinative desorption of ethylene located distant from Cs adatoms. For clean Pd( 1 I O ) , the recombinative desorption of ethylene occurs at 300 K.I6 EELS spectra are drastically changed (Figures 2d and 3d) on heating to 300 K. Losses are observed at 390 (400), -800 (620), 1040 (745), and -2980 (2230) cm-l. The C C bond is broken as the C C stretching mode [-1500 (1335) cm-I] vanishes. Presence of species with the C H 2 (CD,) or CH3 (CD,) group can be ruled out because the C H 2 (CD,) scissors mode at -1400 (1070) cm-I and CH, (CD,) symmetric deformation mode at 1350 ( 1050) cm-' are not o b ~ e r v e d . ~ Therefore, *.~~ it is considered that methylidyne (CH, CD) species exist on the Pd(1 IO)( 1X2)-Cs surface by heating to 300 K. Table I1 summarizes vibrational energies observed of methylidyne on various surf a c e ~ , ~ , ~ , and ~ , ~for, 'organometallic ~ , ~ ~ . ~ ~ compounds [ (p3-CH)[ C O ( C O ) ~ ] (p3-CH)[Ru(CO)3]3].3g ~,~* The -1040 (745)-cm-l loss is at a too high energy for the CH (CD) bending mode. Thus, the 1040 (745)-cm-' loss may be attributed to the PdH (PdD) stretching mode; the -800 (620)-cm-I loss the C H (CD) bending mode/PdH (PdD) bending mode associated with methylidyne/ H(D) adatom. The PdH stretching mode and PdH bending mode are observed at IO I O and 805 cm-' on the Pd( 1 IO)( 1X2)-Cs surface, respectively.@ The -2980 (2230)-cm-I loss is attributed to the CH (CD) stretching mode of methylidyne. The 390 (400)-cm-I loss is ascribed to the Pd-methylidyne stretching mode and to the PdC stretching mode associated with C adatoms to be discussed in section IVD. A structural model of methylidyne on the Pd( 1 IO)( 1 XZ)-Cs surface is shown in Figure 6b. We performed detailed EELS measurements in the 230-300 K range, but, except for methylidyne, other hydrocarbon intermediates were not detected. TDS results show that H2 desorption does not occur by 300 K heating. Thus, H atoms produced by the C H bond
-
-
-
-
-
(38) Howard, M. W.; Kettle, S.F.; Oxton, I. A,; Powell, D. B.; Sheppard, N.; Skinner, P. J. Chem. Soc., Faraday Trans. 2 1981, 77, 397. (39) Oxton, 1. A . Rea. Inorg. Chem. 1982, 4, I . (40) Nishijima, M.; Yoshinobu, J.; Sekitani, T.; Onchi, M. Phys. Rea. B 1989, 40, 1308.
scission are bonded to the Pd( 1 IO)( 1 X2)-Cs surface. TDS results show that H 2 desorption occurs at -340 K (shoulder) and 395 K (Figure 4e). As the desorption temperatures are the same as those for the p2' state (associated with H atoms located distant from Cs adatoms) and p3 state (associated with H atoms near Cs adatoms) from the H-covered Pd( 1 IO)( 1X2)-Cs surface, r e s p e ~ t i v e l yit, ~is~ considered that H 2 desorption from the C2H4-exposed Pd( 1 lo)( 1 X2)-Cs surface is desorption-rate ~ is limited. On heating to 500 K , only the 390 ( 3 9 0 ) - ~ m -loss observed (Figures 2e and 3e). This indicates that C adatoms exist on the Pd( 1 IO)( 1X2)-Cs surface. Thermal decomposition of ethylene for a small exposure (0.1 langmuir) was not studied in detail as the EELS peak intensity was small. However, it is considered that, except for the absence of ethylene desorption, the decomposition mechanism is similar to that for a large exposure (2 langmuirs). D. Comparison with Ethylene Adsorption on the P d ( l l 0 ) Clean Surface. We studied the interaction of ethylene with the Pd( 1 IO) clean surface prior to the present study,I6and comparisons of ethylene adsorption on the Pd( 1 IO) clean and Pd( I IO)( 1 X2)-Cs surfaces are discussed below. Ethylene on Pd( 1 lo)( 1 X2)-Cs is more weakly bonded to the surface as compared with ethylene on clean Pd( 1 IO). The C2H4 desorption temperatures for Pd( 1 IO)( 1 X2)-Cs are 110, 200 and 270 K, whereas those for clean Pd(ll0) -200 and 300 K. In particular, physisorbed ethylene exists on Pd( 1 IO)( 1 X2)-Cs. The fraction of C2H4 admolecules formed on Pd( 1 lo)( 1 X2)-Cs at 90 K (by saturation exposure) which are desorbed as C2H4is 44%; the fraction for clean Pd( 110) 35%. These results may be qualitatively understood by the following argument: Due to the difference of electronegativities between Pd and Cs (2.2 and 0.8, respectively), electrons are transferred from Cs adatoms to the Pd surface and the Pd surface becomes "electron-rich", which decreases the ability of the Pd surface atoms to accept *-electrons of ethylene. It is considered that the bonding between ethylene and Pd occurs mainly via the n-electron donation.27 Electron transfer from Cs adatoms to the Pd surface can be induced by a "long-range" electronic interaction, e.g., the through-metal change in the electron density of states near the substrate Fermi level4I or the local electrostatic field effect.42 On the whole, the dehydrogenation process is promoted on the Pd( 1 IO)( 1 X2)-Cs surface. The recombinative C2H4 desorption occurs at 270 K for Pd( 1 lo)( 1X2)-Cs, but at 300 K for clean Pd( 110). Complete C2H4 dehydrogenation occurs below 400 K for Pd(l10)(1 X2)-Cs but at 480 K for clean Pd(1 IO). It is considered that the CH bond scission occurs via hydrogen-bonding interaction of an H atom of an adspecies with the Pd surface and/or the interaction of a C atom with Pd. Thus, for example, the recombinative C2H4 desorption may be promoted on Pd(1 IO)( 1x2)-Cs because n-bonded ethylene is more weakly bonded to the Pd surface and its thermally excited rotational/translational motion is less hindered. The C C bond scission process is promoted on the Pd( 110)(1 X2)-Cs surface. At 300 K, methylidyne (CH) is formed on Pd( 1 I O ) ( 1 X2)-Cs, while on clean Pd( 1 IO), ethynyl (CCH) is formed as a reaction intermediate. Ethynyl on clean Pd( 1 IO) is a stable intermediate up to the decomposition temperature of 450-520 K. These results are interpreted as follows: On Pd( 1 lo)( 1 X2)-Cs, ethynyl is an unstable intermediate and, immediately after the formation of ethynyl, the CC bond scission occurs and ethynyl species is decomposed into methylidyne and C ada(41) Feibelman, P. J.; Hamann, D. R. Phys. Reu. Lerr. 1984, 52, 61. (42) Narskov, J. K.; Holloway, S.; Lang, N. D. S u r . Sci. 1984, 65, 137.
J . Phys. Chem. 1990, 94, 6852-6854
6852
toms. The C C bond scission is promoted on Pd( 1 IO)( 1X2)-Cs because this surface is electron-rich, and thus, the r* antibonding orbital associated with the CC bond of ethynyl species may easily be filled with electrons. The saturation C2H4 coverage on the Pd( 1 IO)( 1 X2)-Cs surface at 90 K is less than half of that on the Pd(l10) clean surface = 0.23 and 0.58, respectively). This is attributed mainly to the existence of the ( 1 X2) structure. Every other [ IT01 row of the Pd(l I O ) surface is missing on the Pd(l IO)(lX2)-Cs surface according to the missing-row m ~ d e I . ~ ~ - * ~ V. Summary A combined vibrational EELS, TDS, and LEED study has been performed on the interaction of ethylene with the Pd( 1 lo)( 1 X 2)-Cs surface. Some of the important results are as follows: I . At 90 K, ethylene is predominantly *-bonded to the Pd( 1 IO)( 1 X2)-Cs surface. A small amount of physisorbed ethylene exists for a large exposure (2 langmuirs). The fractional ethylene coverage is 0.23 at the saturation.
2. Physisorbed C2H4(C2D4) is desorbed by heating to 110 ( 1 10) K. A part of *-bonded C2H4(C2D4)is desorbed intact at 200 (210) K, and additionally at 270 (280) K by the recombinative process. On heating to 300 K, methylidyne (CH, CD) species, and H (D) and C adatoms are formed. On heating up to 500 K, only carbon adatoms exist on the Pd( 1 lo)( 1 X2)-Cs surface. 3. Compared with ethylene on the P d ( l l 0 ) clean surface, ethylene on the Pd( 1 IO)( 1 X2)-Cs surface is more weakly bonded to the surface at 90 K. The dehydrogenation and CC bond scission processes are promoted on the Pd( 1 IO)( 1 X2)-Cs surface. The CC bond scission occurs at 300 K. Effects of Cs adatoms on the surface reactions are discussed in section IVD. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Areas (Research Program “Surfaces as New Materials”) from the Ministry of Education, Science and Culture, and by a Grant-in-Aid from the Foundation for Promotion of Material Science and Technology of Japan.
Correlation of Photoelectron Yields and Photodissociation Rates of CH,CI on Pt( 111) and Carbon-Covered Pt( 111) Sam K. Jo and John M. White* Department of Chemistry. The University of Texas at Austin, Austin, Texas 78712 (Received: February 16, 1990)
Photoelectron yields, as a function of coverage, were measured during UV irradiation (
