J . Phys. Chem. 1990, 94, 4269-4275
4269
Chemisorption and Thermal Decomposition of Acetylene on Pd( 110): Electron Energy Loss Spectroscopy and Thermal Desorption Studies J . Yoshinobu; T. Sekitani, M. Onchi, and M. Nishijima* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received: September 5, 1989; In Final Form: December I , 1989)
The adsorbed states of acetylene on Pd(l IO) at 90 K and its thermal decomposition in the temperature region up to 700 K have been investigated by using high-resolution electron energy loss spectroscopy and multiplexed thermal desorption spectroscopy. For a small exposure (0.5 langmuir) at 90 K, acetylene is chemisorbed molecularly in a single state (disorderedly) and is located in the p2-site with its C-C bond axis inclined to the surface plane; one of the CH groups is hydrogen-bonded to the surface and gives the softened CH stretching vibration. Upon heating to 180 K, acetylene (C2H2)is dissociated into ethynyl (CCH) species and H adatoms. For a large exposure ( 5 langmuirs) at 90 K, acetylene is adsorbed in several states. A part of the C2H2admolecules are desorbed intact at 100 K. Upon heating to 200 K, the rest are mainly dissociated into CCH species and H adatoms. The C2H4and P-H2desorptions occur at 265 and 320 K, respectively. As the heating temperature is increased from 400 to 600 K, the dehydrogenation occurs progressively: the C,H, species (x 2 I ; y = 0, 1) are formed, and the y-H, desorption with multiple peaks is observed.
I. Introduction The adsorption and thermal decomposition of acetylene on well-defined transition-metal surfaces have been of considerable interest not only from the scientific viewpoint but also in view of the technological applications. In particular, vibrational studies, using high-resolution electron energy loss spectroscopy (EELS), of acetylene adsorbed on single-crystalline transition-metal surfaces (Fe,l.2 Ni,3-9 Cu,8,10,11 Ru 12-14 Rh 15,16 Pd 8,17-20 Ag,21 w,22-24 Ir 25 Pt,26,27etc.) have elucidated both chemisorbed states of acetylene and mechanisms of the thermal decomposition. Recently, the interaction of acetylene with Si surfaces has also been studied by using EELS.28,29 The adsorbed state and thermal decomposition of acetylene on Pd( 1 IO) have been studied by a few research groups. Bandy et ale8have studied the adsorbed state of acetylene (in the saturation coverage region) at 1 I O K by EELS. They have proposed that acetylene is adsorbed in the p3-site and is a-bonded to two metal atoms and *-bonded to a third with the formation of the (di-a T ) bond. However, a more systematic and detailed (coverageand temperature-dependent) EELS study is needed in order to elucidate the surface chemistry of acetylene on Pd( 110). The thermal desorption spectroscopy (TDS) study of Gentle and Muetterties30 has shown that the desorption products from acetylene (C2D2)on Pd( 1 IO) adsorbed at 143 K are C2D2[desorption temperatures: 208 and 323 K] and D2 (358 and 473 K). They also reported that a small amount of benzene, which was formed on this surface at a low temperature, was detected by the chemical displacement with P(CH3), at 298 K. On the other hand, Rucker et aL3I have reported that the desorption products from acetylene (C,H,) on Pd(l IO) are C2H2 (175 K with a broad shoulder up to 500 K), C2H4 (265 K), benzene (265 and 425 K), and H2(broad peak at 495 K), which seems to be in contradiction to the TDS results of Gentle and M ~ e t t e r t i e s . ~ ~ In the present work, we have employed the in situ combined techniques of low-energy electron diffraction (LEED), EELS, and TDS in order to study the adsorption and thermal decomposition of acetylene on the Pd( 110) surface. The heat-and-quench method (see section 11) was employed for the temperature-dependent EELS measurements to correlate EELS spectra with the TDS spectra. The adsorbed states of acetylene at 90 K and thermal decomposition products/path are discussed in detail.
+
11. Experimental Section
The experiments were carried out by the use of an ultrahigh-vacuum chamber which housed a high-resolution electron
'
Present address: Surface Science Center, Department of Chemistry, University of Pittsburgh, PA 15260.
0022-3654/90/2094-4269$02.50/0
spectrometer for EELS, a four-grid retarding field analyzer with a normal-incidence electron gun for LEED, a cylindrical mirror analyzer for Auger electron spectroscopy (AES), and a quadrupole mass spectrometer for TDS. The high-resolution electron energy loss spectrometer used for the present study is a double-pass spectrometer with 127" cylindrical deflectors each for the monochromator and analyzer.32 For
(1) (2) (3) (4)
Erley, W.; Bar6, A. M.; Ibach, H. Surf. Sci. 1982, 120, 273. Seip, U.; Tsai, M.-C.; Kuppers, J.; Ertl, G. Surf.Sci. 1984, 147, 65. Demuth, J. E.; Ibach, H. Surf. Sci. 1979, 85, 365. Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. (5) Ibach, H.; Lehwald, S. J . Vac. Sci. Techno/. 1981, 18, 625. (6) DiNardo, N. J.; Demuth, J. E.; Avouris, Ph. Phys. Reu. B 1983, 27, 5832. (7) Stroscio, J. A.; Bare, S. R.; Ho, W. Surf. Sci. 1984, 148, 499. (8) Bandy, B. J.; Chesters, M. A.; Pemble, M. E.; McDougall, G. S.; Sheppard, N. Surf. Sci. 1984, 139, 87. (9) Zaera, F.; Hall, R. B. J . Phys. Chem. 1987, 91, 4318. (10) Avery, N. R. J . Am. Chem. SOC.1985, 107, 6711. (11) Marinova, Ts. S.; Stefanov, P. K. Surf. Sci. 1987, 191, 66. (12) Parmeter, J. E.; Hills, M. M.; Weinberg, W. H. J . Am. Chem. SOC. 1986, 108, 3563. (13) Parmeter, J. E.; Hills, M. M.; Weinberg, W. H. J . Am. Chem. SOC. 1987, 109, 72. (14) Jakob, P.; Cassuto, A.; Menzel, D. Surf. Sci. 1987, 187, 407. (15) Dubois, L. H.; Castner, D. G.; Somorjai, G. A. J . Chem. Phys. 1980, 72, 5234. (16) Mate, C. M.; Kao, C.-T.; Bent, B. E.; Somorjai, G. A. Surf. Sci. 1988, 197, 183. (17) Gates, J. A.; Kesmodel, L. L. J . Chem. Phys. 1982, 76, 4281. (18) Gates, J. A.; Kesmodel, L. L. Surf.Sci. 1983, 124, 68. (19) Kesmodel, L. L. J . Chem. Phys. 1983, 79, 4646. (20) Kesmodel, L. L.; Waddill, G. D.; Gates, J. A. Surf. Sci. 1984, 138, 464. (21) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1982, 123, 491. (22) Backx, C.; Feuerbacher, B.; Fitton, B.; Willis, R. F. Surf. Sci. 1977, 63, 193. (23) Backx, C.; Willis, R. F. Chem. Phys. Lett. 1978, 53, 471. (24) Hamilton, J. C.; Swanson, N.; Waclawski, B. J.; Celotta, R. J. J . Chem. Phys. 1981, 74, 4156. (25) Marinova, Ts. S.; Kostov, K. L. Surf.Sci. 1987, 181, 573. (26) Ibach, H.; Hopster, H.; Sexton, B. A. Appl. Surf. Sci. 1977, I , 1. (27) Ibach, H.; Lehwald, S. J . Vac. Sci. Techno/. 1978, 15, 407. (28) Yoshinobu, J.; Tsuda, H.; Onchi, M.; Nishijima, M. Chem. Phys. Lett. 1986, 130, 170. (29) Nishijima, M.; Yoshinobu, J.; Tsuda, H.; Onchi, M. Surf. Sci. 1987, 192, 383. (30) Gentle, T. M.; Muetterties, E. L. J . Phys. Chem. 1983, 87, 2469. (31) Rucker, T. G.; Logan, M. A,; Gentle, T. M.; Muetterties, E. L.; Somorjai, G. A. J . Phys. Chem. 1986, 90, 2703.
0 1990 American Chemical Society
Yoshinobu et al.
4270 The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
EELS measurements, a primary energy E of 4 eV, an energy resolution of 40 cm-' ( 5 meV) (full widtR at half-maximum, fwhm), and an incidence angle di of 60' with respect to the surface normal were used. The electron beam was scattered along the [ 1 TO] azimuth. The acceptance angle of the analyzer was 1 So (fwhm). The off-specular measurements were made by rotating the analyzer around the axis perpendicular to the incidence plane of the electron beam. The heat-and-quench method was used for the temperature-dependent measurements: the sample was heated at a heating rate p of 5 K/s up to a certain temperature, cooled to 90 K, and then the EELS (and LEED) measurements were made. For LEED, E, = 40-100 eV and an incident electron current I, of 5 X A were chosen. For AES, E,, = 2 keV and A were chosen. I, = 1 X TDS measurements were carried out with a quadrupole mass spectrometer whose ionizer was enclosed in a glass envelope with a 2-mm-diameter aperture.46 The aperture was located 2 mm from the crystal surface during the TDS measurements for the selective sampling of gases that desorbed directly from the central portion of the sample surface. The linear temperature ramp ( p = 5 K/s) was performed by using a home-built temperature controller which was programmed by an EPSON PC-286 computer. The mass spectrometer was multiplexed by the use of the PC-286. The Pd(l10) clean surface, which showed a ~ ( 1 x 1 )LEED pattern, was carefully prepared by the oxidation, Ar+ ion bombardment (600 eV, 10 pA/cm2, 30 min), annealing (800 K, 20 min), and flashing (1000 K, 2 min) cycles. Cleanness of the Pd( 1 IO) clean surface was checked by AES and EELS; no surface impurities were observed within the detection limit of AES, and, in particular, no peak associated with the Pd-C stretching vibration was observed in the EELS spectrum. The Pd sample was heated by electron bombardment from the rear and the temperature was measured by using a chromel-alumel thermocouple attached to the sample edge. The sample cooling was performed by contact of the sample mount (via an insulator) with the liquid N2 reservoir which was connected with the liquid N2 feed pipe. Research-grade C2H2(99.8 mol % purity), C2D2(99.5 atom % D, MSD Isotope, Canada, Ltd.), and H2 (99.8 mol % purity) were used. The acetylene gases were introduced into the vacuum chamber through a gas doser which produced a flux at the sample surface about 50 times the background acetylene flux. The sample surface was exposed to acetylene at a chamber pressure between Torr, which was monitored by using a 5X and 5 X nude-type Bayard-Alpert ion gauge and calibrated by the ion gauge sensitivity factor of C2H2 (2.0 relative to N2). The ultra-high-vacuum pumping system consisted of a 500 L s-' sputter-ion pump, a 2000 L s-' titanium cryo-sublimation pump, and a 160 L s-l turbomolecular pump. The base pressure of the vacuum system was 4 X IO-" Torr.
Ill. Results A. Low-Energy Electron Diffraction. Figure 1 a shows the p( 1 XI) LEED pattern of the Pd( 110) clean surface at 90 K together with the schematic representation ( E , = 64 eV). As the clean surface was exposed to acetylene at 90 K, only an increase of the background intensity was observed. For 5-langmuir exposure (saturation exposure), the integral-order spots became very weak ( 1 langmuir = lod Torr s). These results indicate that acetylene molecules are adsorbed disorderedly on the Pd( 110) surface at 90 K. When the Pd( 1 10) surface exposed to 5 langmuirs of acetylene (C2D2)was heated up to 200-350 K, a very diffuse ~ ( 2 x 2 pattern ) was observed as shown in Figure 1 b. A (1 X 1 ) LEED pattern with high background intensity was observed after heating at 400-500 K, and a relatively sharp (1 X 1) pattern was observed on heating up to 600 K. B. Electron Energy Loss Spectroscopy. Figure 2 shows EELS spectra as the Pd( 1 10) clean surface is exposed to an increasing amount of C2H2at 90 K. All the EELS spectra are normalized by the elastic peak intensities. Figure 2a shows the EELS (32) Nishijima, M.; Kubota, Y.; Kondo, K.; Yoshinobu, J.; Onchi, M. Rev. Sci. Instrum. 1987, 58, 307.
Figure 1. LEED patterns of (a) the Pd( 1 10) clean surface at 90 K, and (b) the Pd( 1 10) surface exposed to 5 langmuirs of C2D2 at 90 K and heated to 260 K. E, = 64 eV. Schematic representationsof the observed LEED patterns are also included. . __ -- __ - _-- - - - - - -.
i --
I _
2985 2820
I
0
loo0
29bQ
ENERGY L3SS (cm-')
Figure 2. EELS spectra of (a) the Pd( 1 10) surface exposed to 0.5 langmuir of C2H2at 90 K (Ad = 0 '); (b) 1 langmuir (Ad = 0'); (c) 5 langmuirs (Ad = O O ) ; (d) 5 langmuirs (Ad = 2O). E , = 4 eV. The inset shows the electron scattering geometry.
-
spectrum in the specular mode after 0.5 langmuir of C2H2exposure. Losses are observed at 360, 460, -700, 925, 1070, 1230, 2820, and 2985 cm-I. The 1900-cm-' loss is attributed to impurity CO molecules which were accumulated on the surface during the EELS measurements. For 1-1angmuir exposure, the -700-cm-' loss intensity is increased relative to the 925-cm-' loss intensity (Figure 2b). Figure 2, c and d, shows EELS spectra in the specular mode and the off-specular mode (Ad = 2'; see the inset of Figure 2 for the definition of the off-specular angle Ad) after 5 langmuirs of C2H2exposure, respectively. The relative intensity of the -930-cm-I loss (in the specular mode) is further decreased, and losses are observed at 320, -450,610,695,835, 935, 1070, 1250, 1390, 1590, -2900, and -3000 cm-I. Figure 3 shows EELS spectra of the Pd( 1 10) surface exposed to C2D2at 90 K. For 0.5-langmuir C2D2exposure, losses are observed at 360,460,500,705, -910, 1200,2130, and 2250 cm-I (Figure 3a). Figure 3, b and c, shows EELS spectra in the specular mode and off-specular mode (A8 = 2') after 5 langmuirs of C2D2exposure, respectively. Losses are observed at 320,380, 515, 600, 685, 885, 1230, 1380, 1520, and -2220 cm-I.
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-
Thermal Decomposition of Acetylene on Pd(l10)
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4271
I
I
Pd(ll0) 0.5L
1,
C2Dz
5:
I
0
13X
ENEKSY
2533
LCSS
icm-')
Figure 3. EELS spectra of (a) the P d ( l l 0 ) surface exposed to 0.5 langmuir of C2D2at 90 K (A0 = 0'); (b) 5 langmuirs (A0 = OD); (c) 5 langmuirs (A0 = 2 O ) . E , = 4 eV.
\
515
Figure 5. EELS spectra in the specular mode of the Pd(1 I O ) surface preexposed to 0.5 langmuir of C2Dzat 90 K and subsequently heated to the 160-500 K range. The heating rate was 5 K/s. All the spectra were recorded at 90 K.
Figures 4 and 5 show EELS spectra in the specular mode of the Pd( 110) surface preexposed to 0.5 langmuir C2H2and C2D2 at 90 K and subsequently heated to high temperatures at the heating rate of 5 K/s, respectively. All the spectra were recorded at 90 K. In Figures 4 and 5, the loss peak intensities are normalized by the elastic peak intensities. Figure 4a shows an EELS spectrum of the Pd( 110) surface exposed to 0.5 langmuir C2H2 at 90 It( and is identical with Figure 2a. As the heating temperature is increased, the 2820-cm-I loss is monotonically decreased in intensity (Figure 4a-c). The 2820-cm-l loss almost disappears by heating to 160 K (Figure 4d). As the heating temperature is further increased from 160 to 180 K, the loss intensities in the 30C-500-cm-' region are increased, and losses are observed at 380, 470, -680, 930, 1290, and 3000 cm-' (Figure 4e). After 300 K heating, the intensity of the 930-cm-I peak is decreased (Figure 4f). The spectra seem almost unchanged by heating to 300-400 K (Figure 4f,g). After heating up to 500 K, only one peak is observed at 515 cm-I (Figure 4h). Figure 5a shows an EELS spectrum of the Pd( 1 10) surface exposed to 0.5 langmuir of C2D2 at 90 K, and it is identical with Figure 3a. As the heating temperature is increased, the 2130-cm-l loss is decreased in intensity (Figure 5a-c), and it almost disappears at 190 K (Figure 5d). After 260-400 K heating, losses are observed at 380, 460, 520, 765, 960, -1250, and -2230 cm-l (Figure 5e,f). Only the 515-cm-' loss is observed on heating up to 500 K (Figure 5g). Figures 6 and 7 show EELS spectra of the P d ( l l 0 ) surface preexposed to 5 langmuirs of C2H2and C2D2at 90 K and subsequently heated to high temperatures at a heating rate of 5 K/s, respectively. Figure 6a shows an EELS spectrum of the Pd( 1 10) surface exposed to 5 langmuirs of C2H2at 90 K and is identical with Figure 2c. As the heating temperature is increased from 90 to 200 K, the losses at -700, 1590, and -2900 cm-' are decreased in intensity; on the other hand, the losses in the 300500-cm-I region and the -930-cm-l loss are increased in intensity (Figure 6a-d). On heating the surface to 200 K, the -2900-cm-' loss vanishes, and losses are observed at 350, 450, -710, 920, 1250, 1370, and 3000 cm-I (Figure 6d). Similar spectra are observed between 200 and 350 K (Figure 6d-g). After 400 K heating, the spectrum is drastically changed, and losses are observed at -460, 765,930, 1170, -1410, and 3000 cm-' (Figure 6h). As the heating temperature is increased from 400 to 500 K, the -460- and 930-cm-I losses are decreased in intensity
-
iiLl-_
~
0
ENERGY LOSS (cm-') Figure 4. EELS spectra in the specular mode of the Pd( 110) surface preexposed to 0.5 langmuir of C2H2at 90 K and subsequently heated to the 140-500 K range. The heating rate was 5 K/s. All the spectra were recorded at 90 K. Angle-dependent EELS spectra of the Pd(ll0) surface exposed t o 0.5-5 langmuirs of C2H2(C2D2)at 90 K were measured in detail. As A0 is increased, all the loss peak intensities are reduced
at a similar rate as the elastic peak intensity except for the 2800-3000 (2100-2300)-~m-~ peak intensities. Thus, it appears that, except for the 2800-3000 (2100-2300)-cm-' losses, the losses are excited predominantly by the dipole mechanism.33 ( 3 3 ) Ibach, H.;Mills, D. L. Electron Energy Loss Specrroscopy and Surface Vibrations; Academic: New York, 1982.
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4272 The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
Yoshinobu et al.
(d) --' (').&--,
(b) (a)
ENERGY LOSS
-__---
___-
160
(a4)
Figure 6. EELS spectra in the specular mode of the P d ( l l 0 ) surface preexposed to 5 langmuirs of C2H2at 90 K and subsequently heated to the 150-600 K range. The heating rate was 5 K/s. All the spectra were recorded at 90 K.
L,
r
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360-
L - --~ 075
\---460 ~
~
055
- O3
lL 051 021
300 TEMPERATURE ( K ) Figure 8. TDS spectra of (a)-(0 H 2 (mass 2), (g) C2H2(mass 26), and (h) C2H4(mass 27 fragment) for the Pd(l10) surfaces exposed to various amounts of C2H2at 90 K. The H2desorption spectrum for the Pd( l l 0 ) ( 2 X l ) - H surface IS shown in (I). The heating rate was 5 K/s. '
posed to 5 langmuirs of CzD2at 90 K, and is identical with Figure 3b. As the heating temperature is increased from 90 to 200 K, the losses around 500 cm-l and the 1520-cm-' loss are decreased in intensity; the -680-cm-' loss is increased in intensity (Figure 7a-c). At 200 K, losses are observed at 320,450, 530, 675, 885, 1340, and 2265 cm-' (Figure 7d). It is noted that the -1210, width of the -2200-cm-I peak is decreased after 200 K heating. Similar spectra are observed between 200 and 350 K (Figure 7d-f). Heating to 400 K changes the EELS spectrum drastically and losses are observed at 460, 555, 725, 810, 1100, 1320, and 2250 cm-' (Figure 7g). The 555-cm-l peak intensity is increased after 500 K heating (Figure 7h). Only one peak is observed at 515 cm-' after heating up to 600 K (Figure 79. C. Thermal Desorption Spectroscopy. Multiplexed TDS measurements showed that for a small C2H2 exposure (11.5 langmuirs), H, was the only desorption product; the H2, C2H2, and C2H4desorptions were observed for a large exposure ( 1 2 langmuirs). No methane, ethane, or benzene was detected. H2 (mass 2), CzH2 (mass 26), and C2H4 (mass 27 fragment) TDS spectra are shown in Figure 8. For 0.2 langmuir of C2Hzexposure, two H2 desorption peaks are observed at 360 K (labeled @)and 460 K (labeled y) as shown in Figure 8a. With increasing C2H2exposure, the area intensity of the @-peak is initially increased; the peak temperature is shifted toward lower temperatures, and it is observed at 320 K for 1.5 langmuirs of C2H2 exposure (Figure 8d). However, the @-peakintensity is decreased for large exposures (Figure 8e,f). On the other hand, the peak temperature of the y-H2 desorption is shifted toward higher temperatures with increasing CzH2exposure. For 5 langmuirs of C2Hzexposure, the y-H2 desorption peak is observed around 505 K and consists of the peaks at 410, 505, 530, and 565 K (Figure 8f). For a large C2Hz exposure (5 langmuirs), a sharp C2H2desorption peak is observed at 100 K (Figure 8g); the C2H4 desorption is observed at 265 K (Figure 8h). The C2H2desorption peak around 100 K has been observed for other transition-metal surfaces and attributed to the existence of physisorbed acetylene.'J2J6 We have not determined the coverage of the physisorbed acetylene; however, we consider it to be very small because only
-
- -
ENERGY LOSS (cm-9 Figure 7. EELS spectra in the specular mode of the Pd( 1 10) surface preexposed to 5 langmuirs of C2D2a t 90 K and subsequently heated to the 150-600 K range. The heating rate was 5 K/s. All the spectra were recorded at 90 K.
-
(Figure 6i,j), and a t 500 K, losses are observed at 785, 1170, 1410, and 3090 cm-l (Figure 6j). After the surface was heated up to 600 K, only one peak is observed at 515 cm-' (Figure 6k). Figure 7a shows an EELS spectrum of the Pd( 1 10) surface ex-
Thermal Decomposition of Acetylene on Pd( 1 IO)
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4213
TABLE I: Vibrational Energies ( c d ) and Their Assignments for 0.5 langmuir of Acetylene on Pd( 110) at 90 K (This Work) and for Acetylene on Ni(ll0)' and Cu(llO)lo
assignment C H (CD) stretch, vCH
CC stretch, vcc C H (CD) bend, d,, C H (CD) bend, 6, C H (CD) bend, pas C H (CD) bend, pa meta1-C metal-C
0.5 langmuir acetylene acetylene on acetylene on on P d ( l l 0 ) at 90 K Ni(ll0) Cu(ll0) C2H2 C2D2 U H / V D C2H2 C2D2 C2H2 C2D2 2985 2250 1.33 3015 2170 2900 2190 2820 2130 -1230 -1200 1070 -910 925
705
1.32 1.03 1.18 1.31
2900 1305 1200 1305 1280 1140 930 890
700
940
680
-
745 -700 460 360
500
1.40
675
640
510
460 360
1.00 1.00
470 370
470
400
a small change was observed in EELS spectra by the desorption. In order to estimate the fractional H coverage OH (the number of the H atoms per surface Pd atom) corresponding to the total amount of H2 desorbed by the C2H2 decomposition, we have performed TDS measurements for the Pd( 110)(2X I)-H surface, and the result is shown in Figure 8i. The Pd(l10)(2X I)-H surface was prepared by exposing the P d ( l l 0 ) clean surface to 0.3 langmuir of H2 at 90 K. It is assumed that OH of the Pd(1 10)(2XI)-H surface is 1.34-37 The fractional H coverages corresponding to the H2 desorption spectra for Pd( 110) exposed to C2H2can be estimated by comparing the area intensities of the H2 desorption spectra with that for the Pd(ll0)(2Xl)-H surface, and the results are included in Figure 8a-f. We also performed TDS measurements of the Pd( 1 10) surfaces exposed to C2D2,and obtained results similar to those for C2H2.
IV. Discussion A. Assignments of the EELS Peaks and the Chemisorbed State of Acetylene for Small Exposure (0.5 langmuir) at 90 K . Assignments of the EELS peaks (Figures 2a and 3a) can be made by comparison with the vibrational energies of free molecules38 and of acetylene chemisorbed on transition-metal surface^,^-^'-^^ and also by examining the energy ratios vH/vD for C2H2on Pd( 1 10) and the deuterated counterparts. The losses at 2820 (2130) and 2985 (2250) cm-l are ascribed to the CH(CD) stretching modes. The softening of the CH(CD) stretching vibration [2820 (2130) cm-' loss] is attributed to the H(D) atom of the CH(CD) group which is partially bonded to the Pd( 1 10) surface by hydrogen bonding, because hydrogenbonded C H stretching vibrations have been observed between -2600 and 2800 The 1230-cm-' loss for C2H2is related to the 1200-cm-' loss for C2Dz,and as the energy loss ratio is near unity, the 1230 ( 1 200)-cm-' loss can be ascribed to the CC stretching mode. The 360 (360)- and 460 (460)-cm-I losses are ascribed to the Pd-C stretching modes. The remaining losses can be attributed to the CH(CD) bending modes. Proposed assignments of the losses associated with 0.5 langmuir of acetylene on Pd(l1O) are summarized in Table I together with the data for acetylene on Ni(1 and Cu(1 10).Io The 925-cm-I loss for C2H2corresponds to the 705-cm-l loss for C2D2,because both of them are rather strong in intensity, and the energy ratio is 1.3 1. These losses may be ascribed to the
-
(a)
N
-
(34) Behm, R. J.; Penka, V.; Cattania, M.-G.; Christmann, K.; Ertl, G. J . Chem. Phys. 1983, 78, 7486. (35) Jo, M.; Kuwahara, Y.: Onchi, M.; Nishijima, M. Solid State Commun. 1985, 55, 639. (36) He, J.-W.; Norton, P. R. Surf.Sci. 1988, 195, L199. (37) He, J.-W.: Harrington, D. A.: Griffiths, K.; Norton, P. R. Surf, Sci. 1988, 198, 413. (38) Shimanouchi, T. "Table of Molecular Vibrational Frequencies"; Natl. Stand. Ref. Data Ser. Nail. Bur. Stand. 1972, 39, Vol. 1.
[@@lI
Figure 9. Proposed structural models of (a) C2H2on Pd( 1 IO) for a small exposure at 90 K and (b) CCH on Pd(l IO).
(in-plane) C H and CD bending modes (6,), respectively. The -700-cm-' loss may correspond to the 500-cm-' loss, and these are ascribed to the (out-of-plane) C H and CD bending modes (p,), respectively. The -910-cm-l loss observed for C2D2may be ascribed to the CD bending mode (6.J, and it corresponds, perhaps, to the shoulder at 1070 cm-l for C2H2. From the above discussion, it is concluded that acetylene is adsorbed as a molecule disorderedly (from LEED, section IIIA) on Pd(l10) at 90 K. The stretching energies of the CH(CD) group for gas-phase hydrocarbons are -3300, 3100, and 3000 (-2600, 2300, and 2200) cm-' for sp, sp2, and sp3 bonding of the carbon atom, respectively; the CC stretching energies are -2100, 1600, and 950 cm-' for CC triple, double, and single bonds, r e ~ p e c t i v e l y . ~ ~ , ~ ~ Thus, the CH(CD) stretching energy at 2985 (2250) cm-' and the CC stretching energy at 1230 (1 200) cm-I are interpreted to indicate that the rehybridization state of the chemisorbed acetylene is near sp3, and the CC bond length is estimated to be 1.5 '4.39 A structural model of acetylene on Pd( 1IO) for a small exposure is shown in Figure 9a. In this model, C2H2is chemisorbed in the pz site with a significant tilt of the CC bond axis; one of the H atoms of C2Hzis closer to the Pd surface, which accounts for the softened CH stretching mode at 2820 c d . Further, the CCH plane is nearly perpendicular to the surface, which makes the intensity of the dipole-active in-plane bending mode 6, much stronger than that of the out-of-plane C H bending mode ps. A similar model of the chemisorbed acetylene has been proposed for Ni( 110) by Demuth@and Stroscio et aL7 It is noted that the fractional acetylene coverage corresponding to the Pd( 110) surface exposed to 0.5 langmuir of acetylene at 90 K is 0.28 from the TDS results (Figure 8b). B . Assignments of the EELS Peaks and the Chemisorbed States of Acetylene for Large Exposure ( 5 langmuirs) at 90 K . The -2900 (---)- and -3000 (2220)-cm-' peaks are readily ascribed to the CH(CD) stretching modes [Figure 2c,d (Figure 3b,c)]. The 1590-cm-' loss for C2Hzcorresponds to the 1520-cm-I loss for CzDz, because the energy ratio is 1.05, and in general, the CH(CD) bending mode is not observed in this energy region.l-29s47Thus, the 1590 (1 520)-cm-' loss is ascribed to the CC stretching mode. The 1390 (1380)- and 1250 (1230)-cm-' losses are also ascribed to the CC stretching modes, as the energy ratios are 1. The 1250 (1 230)-cm-' loss may be associated with C2H2 (C2D2)in the p2 sites detected for small exposures. Therefore, it is concluded that there exist C2H2(CZD2) molecules in several adsorbed states on Pd( 110) for large exposures at 90 K and that the CC stretching modes of C2H2(C2Dz)in three dominant states
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(39) Nishijima, M.; Yoshinobu, J.; Sekitani, T.; Onchi, M. J . Chem. Phys. 1989, 90, 5 1 14.
(40) Demuth, J. E. Surf.Sci. 1980, 93, 127.
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are observed by EELS. Formation of the C2H2(C2D2)multilayer islands might be envisaged for high exposures, and they could be one reason for the multiple states observed. However, we have no clear evidence for this, because (1) EELS peaks characteristic of a multilayer'2 s e r e not observed and (2) little changes were observed in EF.LS spectra for exposures between 5 and 10 langmuirs. Thc 320 (320)- and -450 (380)-cm-' peaks may be ascribed to the carbon-Pd surface modes. The remaining losses at 610 ( - - - I . 69.5 (.515), 835 (600), 935 (685), and 1070 (885) cm-I are attributed to the C H (CD) bending modes. The fractional acetylene coverage corresponding to Pd( 1 IO) exposed to 5 langmuirs of acetylene is >0.75 from the TDS result (Figure 8f-h). C. Thermal Decomposition of Acetylene on Pd( I 1 0 )f o r Small Exposure (0.5 langmuir). As the Pd( 1 I O ) surface [exposed to 0.5 langmuir of C,H, (C,D,)] is heated to 160 (190) K, the 2820 (2 1 30)-Cn-1 loss intensity is monotonically decreased due to the scission of the CH bond which is hydrogen-bonded to the Pd( 1 IO) surface, and the intensities of losses in the 300-500-cm-' region are increased (Figures 4a-d and 5a-d). These results are ascribed to the dehydrogenation of C,H, (C2D2) in the p2-sites to form ethynyl [CCH(CCD)] species discussed below. The EELS spectra between 180 and 400 K heating (Figures 4e-g and 5d-f) can be attributed mainly to the ethynyl (CCH, CCD) species from the following arguments.j9 A structural model of the ethynyl species on Pd( I 10) is shown in Figure 9b. Presence of species with the CH2 (CD,) or CH3 (CD,) group can be denied, because the CH, (CD,) scissoring mode at 1400 (1070) cm-I and the CH, (CD,) symmetric deformation mode at 1350 (1050) cm-I are not observed. Thus. the 3000 ( -2230)-cm-1 loss can be safely assigned to the CH (CD) stretching mode. The -1290 (1250)-cm-' loss is ascribed to the CC stretching mode. The 930-cm-I loss for C,H, corresponds to the 765-cm-' loss for C2D2, and the 930 (765)-cm-] loss is ascribed to the (in-plane) CH (CD) bending mode. The -680 (520)cm-I loss may be attributed to the (out-of-plane) CH (CD) bending mode. The 470 (460)- and 380 (380)-cm-! losses may be attributed to the Pd-C vibrations. [For a detailed comparison of chemisorbed ethynyl species, see Table I11 in ref 39.1 As the hydrogen desorption is not observed below 260 K (Figure 8b), H (D) atoms formed by the dehydrogenation are chemisorbed on the Pd surface below 260 K. However, the corresponding H (D)-Pd vibrational peaks have not been identified clearly; they are, perhaps, overlapped with the peaks associated with the CH (CD) bending modes, since the vibrational peaks associated with H atoms on the Pd( I IO) surface lie at 750 and 980 cm-1.35It is noted that the 9 6 0 - ~ m -loss ~ observed for C2D2after 400 K heating (Figure 5f) may be ascribed to the Pd-H stretching/CH bending modes associated with H/hydrocarbon impuritie~.,~ The TDS result after 0.5 langmuir of C2H2exposure at 90 K (Figure 8 b) shows that the p- and 7-H2 desorption occur at 340 and 470 K, respectively. The P-H, desorption occurs by the recombination of H atcms formed by the dehydrogenation of the adsorbed acetylene into ethynyl species, and is desorption-rate limited. The 7-H2 desorption occurs by the decomposition of the ethynyl species39and is reaction-rate limited. The surface stoichiometry C:H after 400 K heating is 2:-0.5, as the area intensities of the @- and y-H, desorption peaks correspond to fractional H coverages OH of 0.4 and 0.15, respectively (Figure 8b). Thus. the TDS result seems to indicate, after 400 K heating, that C adatoms are formed in addition to the ethynyl species; Le., there exists a reaction path in which acetylene is completely dehydrogenated. [See also Figure 8a for 0.2 langmuir of C2H2 exposure, where the greater part of acetylene is dehydrogenated after 400 K heating.] It is estimated, from the area intensity of the -y-H, desorption, that the fractional ethynyl coverage after 400 K heating is 0.1 5 ; the fractional C coverage 0.25 (Figure 8b). Upon heating up to 500 K, only 5 15 (5 15)-cm-l loss is observed in the EELS spectrum [Figure 4h (Figure 5g)], and the 7 - H 2 desorption is completed (Figure 8b). These results indicate that ethynyl is completely decomposed and the CC bond is broken. We have no evidence for the formation of C, ( n L 2) species or
Yoshinobu et ai. a graphitic overlayer; the relatively sharp single peak at 5 15 ( 5 15) cm-' is interpreted to indicate that carbon adatoms are formed on the Pd( 1 I O ) surface. D. Thermal Decomposition of Acetylene on Pd( 1I O ) f o r Large Exposure ( 5 langmuirs). The thermal decomposition of acetylene on Pd( 110) for a large exposure (5 langmuirs) is rather complicated. There are multiple chemisorbed states of acetylene at 90 K as discussed in section IVB, and the TDS results show that the desorption products are H2, C2H2,and C2H4. With the increase of the heating temperature up to 200 K. the EELS spectra are gradually changed (Figures 6a-d and 7a-d). The changes are partly attributed to the desorption of a small amount of acetylene at 100 K (Figure 8g) but are mainly attributed to the thermally induced rearrangement of the adsorbed acetylene. It is noted that the C2H2(C,D,) species characterized by the 1590 (1520)-cm-l loss vanish on 200 K heating. The -2900-cm-I loss intensity for the 5 langmuirs of C2H2exposed Pd( 110) surface is monotonically decreased as a result of the scission of C H bond (Figure 6a-d). [The decrease of the width of the ,-2220-cm-I peak may be related to the CD bond scission (Figure 7a-d).] The EELS spectra in the 200-350 K range [Figure 6d-g (Figure 7d-f)l are mainly attributed to the CCH (CCD) species discussed in section IVC, and the assignments of the losses are as follows: 350 (320), 450 (450) cm-I, Pd-C stretch; -710 (530) cm-', out-of-plane CH (CD) bend; 920 (-680) cm-', in-plane C H (CD) bend; -1250 (----) cm-I, CC stretch; 3000 (2280) cm-l, C H (CD) stretch. The adsorbed species associated with the CC stretching energy of 1370 (1340) cm-' is not understood. The TDS results show that C2H4desorption occurs at 265 K (Figure 8h) and P-H, desorption at 320 K (Figure 8f). The fractional C,H4 coverage corresponding to the C2H, desorption is -0.01 ML (ML = monolayer) by comparison with our previous The mechanism of the C2H4formation is not understood well. Detailed EELS measurements were made in the 200-350 K range, but losses characteristic of the intermediate species were not detected. The mechanism of the P-H2 desorption is similar to the case for low exposures (section IVC). The area intensity of the P-H, desorption is decreased for large exposures (Figure 8d-f). These results are interpreted to indicate that, for a larger C2H2exposure, the active sites for acetylene dehydrogenation are blocked by the C2H2admolecules themselves. Moreover, it is noted that some of the surface hydrogen atoms formed by acetylene dehydrogenation is used to hydrogenate acetylene into ethylene as discussed above. After 400 K heating, the EELS spectra are drastically changed (Figures 6h and 7g). Assignments of the observed losses are -460 (460) cm-', Pd-C stretch; 765 ( 5 5 3 , 930 (725), --- (810) cm-I, CH(CD) bend; 1170 (-IlOO), -1410 (1320) cm-', CC stretch; 3000 (2250) cm-', CH(CD) stretch. The presence of species with the CH2 (CD,) or CH, (CD,) group can be denied, because we have not observed loss peaks derived from these groups. After SO0 K heating, the CH(CD) bending mode at 930 (725) cm-' is decreased in intensity as a result of the progressive dehydrogenation [Figure 6j (7h)l. On heating the surface to 600 K, a sharp peak is observed at 5 15 (5 15) cm-I which is ascribed to C adatoms [Figure 6k (7i)I. The H, desorption spectrum for 5 langmuirs of C,H, consists of several peaks between 400 and 600 K (Figure 8f). This is interpreted to indicate that C H bonds in different bonding states exist because these peaks arise from the decomposition-rate-limited reactions. It has been postulated that the multiple H, desorption peaks at high temperature are related to the formation of carbon chains containing C H group^.^^^'^ Therefore, combining the EELS and TDS results, it is concluded that, after heating at 400-600 K, the ethynyl species are converted into C,H,(C,D,) species ( x 2 1, y = 0 , l ) and y-H2(D2). Finally, it is noted that, during the thermal evolution of acetylene on Pd( 1 lo), we have not observed any losses attributable to benzene chemisorbed on Pd( 110). E. Comparison with Prerious Studies of Acetylene on Pd( 110). Bandy et aL8 have reported EELS spectra of P d ( l l 0 ) exposed to 10 langmuirs of acetylene (saturation exposure) at 110 K, which
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Thermal Decomposition of Acetylene on Pd( 1 10) may correspond to our spectra shown in Figures 2c and 3b. They have assumed that those spectra are derived from a single adsorbed state. From a detailed examination of the spectral shape, they have interpreted that the adsorbed state of acetylene is classified as "type B", which means that acetylene is adsorbed in the p3 site and is di-a-bonded to two metal atoms and a-bonded to a third metal a t ~ m . ~ .However, ~' as we have discussed in section IIIB, acetylene is not chemisorbed in a single state but in multiple states on Pd( 110) for large exposures at 90 K. Therefore, their interpretation may be erroneous. We have carried out multiplexed TDS measurements for various C2H2exposures. For a small exposure (11.5 langmuirs), hydrogen is the only desorption product (Figure 8a-d). For a large exposure (5 langmuirs), we have observed H2, C2H2, and C2H4 as the desorption products (Figure 8f-h). Benzene was specifically looked for but was not detected. These results are partly in contradiction to those of the previous ~ t u d i e s . ~Gentle ~ . ~ ' and MuettertiesMhave reported TDS spectra for C2D2on P d ( l l 0 ) with the fractional C2D2 coverage of -0.4. Those spectra may approximately correspond to our spectrum for 1 langmuir of C2H2exposure (Figure 8c). The hydrogen desorption spectrum reported by Gentle and Muetterties30 is quite similar to ours, and two desorption maxima are observed at -340 and 470 K. However, we have not detected desorbing acetylene as they have reported. On the other hand, Rucker et al.31have reported TDS spectra for 6 langmuirs of C2H2exposure, which may approximately correspond to those for our 5-langmuir exposure (Figure 8f-h). As for the H2 and C2H4 desorptions, we have obtained similar results, though our careful measurements have clearly shown that the hydrogen desorption peak around 500 K consists of several peaks (Figure 8f). However, they have also reported benzene desorption (at 265 and 425 K) and acetylene desorption (at 175 K with its tail extending to 500 K). We have not observed any benzene desorption, and the acetylene desorption was observed at 100 K. These contradictions may originate from the different experimental conditions employed. We have selectively measured gases desorbing directly from the central portion of the surface by the use of a glass envelope (section 11). Therefore, it is speculated, in the study of Rucker et al.,3' that the acetylene desorption at 175 K (with its tail) might have occurred at the sample support and that benzene might have been synthesized at the defect sites containing, e.g., the P d ( l l 1 ) facets. Rucker et al.31have reported that, among the Pd(l1 l ) , Pd(1 lo), and Pd( 100) surfaces, Pd( 1 1 1) is the most active surface for the acetylene cyclotrimerization to form benzene. Another reason for the discrepancies to earlier reports would be steps or other defects on those other crystal surfaces. F. Adsorption and Thermal Decomposition of Acetylene on Pd( I I I ) and Pd( 100). For comparison, the acetylene chemistry on P d ( l l 1 ) and Pd(100) which has been reported is briefly summarized. Kesmodel et al.173'8320studied the chemisorption of acetylene on Pd( 1 1 1) at 150 K and thermal decomposition using EELS. They have concluded that the chemisorbed acetylene is strongly rehybridized ( s P ~ .and ~ ) di-u a bonded at 150 K and that CCH, and CCH species are formed by the heat treatment ~ ~claimed that benzene-derived losses (300-450 K). M a r ~ h o nhas are observed on Pd( 1 1 1) at 153 K. On the other hand, Timbrell et al.43have examined the EELS spectra of this system in detail
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(41) Sheppard, N . J. Electron Spectrosc. Relat. Phenom. 1986, 38, 175. (42) Marchon, B. Surf. Sci. 1985, 162. 382.
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4275 and have concluded that there is no direct evidence for the formation of benzene at 130 K. It has been reported that the thermal desorption products from CzHzon Pd( 1 1 1 ) are H2, C2H2,CzH4, and C6H6.30'31'44'45It is noted that these TDS results are still inconsistent. On Pd(100), the chemisorbed acetylene is in a near-sp3 hybridization state at 300 K, and by 450 K heating, the ethynyl species (CCH) is formed as a stable intermediate.'9,20 The thermal desorption products are reported to be H,, C2H2,C2H4, and C6H6.30331 On the whole, the P d ( l l 0 ) surface is the most active surface for acetylene dehydrogenation among the Pd( 11 l), Pd( loo), and Pd( 1 10) surfaces, because acetylene on Pd( 110) is dehydrogenated into ethynyl by only -200 K heating (section IVC,D).
V. Summary A combined vibrational EELS and TDS study has been performed on the interaction of acetylene with the Pd( I 10) surface. Some of the important results are as follows: (1) For a small acetylene exposure (0.5 langmuir, which corresponds to the fractional acetylene coverage of 0.28) at 90 K, acetylene is adsorbed in the pz-site with the CC bond axis inclined to the surface parallel and with one of the CH group located closer to the surface by hydrogen bonding. A structural model is proposed in Figure 9a. By heating to 180 K, acetylene is decomposed into ethynyl species (Figure 9b) and hydrogen adatoms. At 340 K, the hydrogen adatoms are desorbed from the surface as P-H2. After heating at 500 K, the ethynyl species are completely decomposed and the carbon adatoms are formed on Pd( l 10); the decomposition is accompanied by the desorption of y H 2 . Most probably, there exists another reaction path in which acetylene is completely dehydrogenated before 400 K heating. (2) For a large exposure (5 langmuirs, which corresponds to a fractional acetylene coverage of >0.75), multiple adsorbed states of acetylene are formed at 90 K. By heating to 100 K, a small amount of acetylene is desorbed intact from the surface. By heating to 200 K, the remaining acetylene is mainly decomposed into ethynyl species and hydrogen adatoms. Ethylene desorption takes place at 265 K; the hydrogen adatoms are desorbed as P-H, at 320 K. By the increase of the heating temperature from 400 and 600 K, the ethynyl species are progressively dehydrogenated to form C,H, species (x 2 1, y = 0, 1); the dehydrogenation is accompanied by the y H 2 desorption with several peaks.
Acknowledgment. This work was supported in part by the 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 the Grant-in-Aid from the Foundation for Promotion of Material Science and Technology of Japan. One of us (J.Y.) gratefully acknowledges the fellowship from the Japan Society for the Promotion of Science for Junior Scientists (1988-1989). We thank Dr. M. A. Henderson for his insightful comments on this manuscript. Registry No. C2H2, 74-86-2; Pd, 7440-05-3; CCH, 2122-48-7. (43) Timbrell, P. Y.; Gellman, A. J.; Lambert, R. M.; Willis, R. F. Surf. Sci. 1988, 206, 339.
(44) Sesselmann, W.; Woratschek, B.; Ertl, G.; Kuppers, J.; Haberland, H . Surf.Sci. 1983, 130, 245. (45) Tysoe, W. T.;Nyberg, G. L.; Lambert, R. M . Surf.Sci. 1983, 135, 128. (46) Feulner, P.; Menzel, D. J. Vacuum Sci. Technol. 1980, 17, 662. (47) Sheppard, N. Annu. Reo. Phys. Chem. 1988, 39, 589.
