Interaction of acetylene with the Pd(110)(1 × 2)−Cs surface: promotion of ethylene formation

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Interaction

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Surface

Science 306 (1994) 179-192

of acetylene with the Pd( llO)( 1 X 2) -Cs surface: promotion of ethylene formation T. Takaoka, T. Sekitani ‘, T. Aruga, M. Nishijima * Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received

31 August

1993; accepted

for publication

29 November

1993)

Abstract The interaction of acetylene with the Pd(llOX1 X 2)-Cs surface has been studied by the use of high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. For a small exposure (0.2 L) at 90 K, acetylene is chemisorbed. For a large exposure (2.5 L), physisorbed acetylene and vinylidene (CCH,) are formed in addition to chemisorbed acetylene. By heating to 135 K, physisorbed acetylene desorbs from the surface. By heating to 260 K, acetylene is converted to vinylidene. Ethylene desorption takes place at 315 K. After heating to 400 K, methylidyne and carbon are formed on the surface. Compared with the case for Pd(llOX1 x l), ethylene formation is markedly promoted on the Pd(llOX1 x 2)-Cs surface. The effects of the Cs modification are discussed. It is shown that the acetylene hydrogenation occurs via vinylidene. 1. Introduction Interactions of acetylene with well-defined transition metal surfaces have been the object of

many studies as a prototype for the interaction of hydrocarbons with catalysts. A number of studies have been performed for palladium surfaces [l131. For the Pd(ll1) surface, Gates and Kesmodel [2] and Kesmodel et al. [71 reported, by the use of high-resolution electron energy loss spectroscopy (EELS), that chemisorbed acetylene is in the threefold site (A-site) with the CC axis parallel to the surface at 150 K. In addition, they reported the formation of CCH,, CCH, and CCH from the thermal evolution of C,H, on Pd(ll1). Sev-

* Corresponding author. Fax: 81-75-751-2085. ’ Present address: Photon Factory, National Laboratory for High Energy Physics (KEK), Tsukuba-shi, Ibaraki 305, Japan. 0039-6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(93)E1002-H

Science

era1 groups [4-6,9] observed benzene as a desorbing product from the acetylene cyclotrimerization in addition to ethylene, acetylene and hydrogen by the use of thermal desorption spectroscopy (TDS). It has been proposed that benzene formation occurs via the C,H, intermediate [5,12,14171. For the Pd(100) surface, Kesmodel [3] and Kesmodel et al. [7] showed by the use of EELS that acetylene is adsorbed in a near sp3 hybridization state at room temperature and it is transformed to the CCH species at 450 K. Sheppard 1111 proposed that acetylene bonds to four Pd atoms in a di-a/di-r structure. Gentle and Muetterties [41 and Rucker et al. [93 reported that the desorption of benzene from cyclotrimerization is observed in addition to ethylene, acetylene and hydrogen for the Pd(100) surface. We previously studied the adsorbed state and thermal reaction of acetylene on Pd(llOX1 x 1) [131. For 0.5 L exposure (1 L = 10e6 Torr . s) at

B.V. All rights reserved

180

T Takaoka et al. /Surface Science 306 (1994) 179-192

90 K, acetylene is adsorbed in the pZ-site with its

C-C bond axis inclined to the surface plane; one of the CH group is hydrogen-bonded to the surface and gives the softened CH stretching vibration. Upon heating to 180 K, acetylene is dissociated into the CCH species and H adatoms. For a large exposure (5 L) at 90 K, acetylene is adsorbed in several states. A part of the C,H, admolecules are desorbed intact at 100 K. Upon heating to 200 K, the rest are mainly dissociated into the CCH species and H adatoms. The C,H, and &-HZ desorptions occur at 265 and 320 K, respectively. As the heating temperature is increased from 400 to 600 K, the CCH species are converted into C,H, species (X > 1; y = 0, 1) and y-H, desorption with multiple peaks is observed. The effects of alkali metals on transition metal catalysts have received considerable attention. Alkali metals are used as promoters for FischerTropsch synthesis. However, the influence of alkali metals on hydrocarbon surface chemistry is not well understood. The coadsorption of alkali metals and acetylene on well-defined transition metal surfaces has not been studied to our knowledge. In the present work, we have studied the adsorption and thermal reaction of acetylene on the Pd(llOX1 x 2)-Cs surface using in-situ combined techniques of high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. We have found that the acetylene hydrogenation process (with the formation of ethylene) is markedly promoted on the Cs-modified surface. The mechanism of acetylene hydrogenation is discussed.

2. Experimental Details of the experimental methods have been described elsewhere [18], and only a brief explanation is given here. 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 ~9~of 60” with respect to the surface normal were used. The electrons were scattered along the [liO] azimuth. A heat and quench method

was used for the temperature-dependent measurements: a sample was heated at a rate of 5 K/s up to a certain temperature, cooled to 90 K, and then the EELS measurements were made. The mass spectrometer was multiplexed, and the TDS measurements were made at a heating rate of 5 K/s. The clean Pd(llOX1 X 1) surface was carefully prepared by oxidation, Ar+ ion bombardment, annealing, and flashing cycles. The cesium was deposited using a SAES getter source. The Pd(llOX1 X 2)-Cs surface was formed by presaturation of the clean Pd(llOX1 x 1) surface with Cs atoms at 90 (or 300) K and subsequent heating to 800 K. It is known that the structure of the Pd(llOX1 x 2)-Cs surface is the missing-row structure in which every other [liO] row of the Pd(llOX1 x 1) surface is missing [19]. It is noted that the fractional Cs coverage, i.e. the number of Cs atoms per (unreconstructed) Pd surface atom, is 0.1 [19,20]. Research-grade C,H, (99.8 mol% purity), C,D, (99.5 at% D, MSD Isotopes, Canada), and H, (99.8 mol% purity) were used. Gases were introduced into the vacuum chamber through a 10 mm diameter gas doser. The acetylene pressure in the chamber was monitored by a nude-type Bayard-Alpert ion gauge, and calibrated by the ion gauge sensitivity factor of C,H, (2.0 relative to N,). The base pressure of the vacuum system was 4 x lo-‘I Torr. 3. Results 3.1. Low-energy

electron diffruction

As the Pd(llOX1 x 2)-Cs surface was exposed to C,H, at 90 K, an increase in the background intensity was observed. For 2.5 L exposure, the half-order spots were hidden in the background, and only the (1 x 1) spots were observed with background. By heating the Pd(llOX1 X 2)-Cs surface exposed to 2.5 L C,H, to 150 K, the half-order spots reappeared. This corresponds to the desorption of the physisorbed acetylene as described below. After heating to 500 K, only the (1 x 2) pattern was observed with background. No superstructure other than (1 X 2) was observed at 150-500 K.

T Takaoka et

3.2. Thermal desorption

al. /Surface

spectroscopy

TDS measurements after the Pd(llOX1 X 2)Cs surface was exposed to C,H, at 90 K showed that H,, C,H, and C,H, were the only desorption products; no methane, ethane or benzene was detected. Fig. 1 shows C,H, (mass 26), C,H, (mass 28) and H, (mass 2) TDS spectra from the Pd(llOX1 x 2)-Cs surface exposed to C,H, at 90 K. For 0.2 L exposure, the C,H, and C,H, desorptions do not occur; only the H, desorption occurs at 390 K (Fig. la). For 0.5 L exposure, in addition to the H, desorption, the C,H, desorption occurs at 315 K (Fig. lb). For 1.5 L exposure, the C,H, desorption appears at 135 K (Fig. Id). With increasing C,H, exposure, the amount of desorbing C,H, is increased. Pd(llO)-(2xl)H

Pd(llO)(lx2)-Cs + c2H2

(mass 2

p=5Kh

C2H4

‘32 (mass

I

26)

H2

(mass 2)

(mass 28)

135

A&wA..A / 0.08

(c) o&

X5

\

A 0.40

0.04 (b) 0.5 L

,-._

k 0.015

390 (a) 0.2 L )o

200

0

300

0.20 400 200 300

Temperature

400

500

(K)

Fig. 1. TDS spectra of C2H2 (mass 261, C,H, (mass 28) and H2 (mass 2) from Pd(llOX1 x 2bCs surfaces exposed to various amounts of C2H2 at 90 K. Inset, a TDS spectrum of H, from the Pd(ll0) surface exposed to 0.3 L H, at 90 K. The heating rate was 5 K/s.

Science

306 (1994) 179-l 92

181

In order to estimate the fractional H coverage, OH, corresponding to the amount of desorbing H,, we performed TDS measurements for the Pd(l10X2 x 1)-H surface; the results are shown in the inset of Fig. 1. The Pd(110)(2 X 1)-H surface was prepared by exposing the clean Pd(ll0) (1 x 1) surface to 0.3 L H, at 90 K. It is known that 0, of the Pd(l10X2 x 1)-H surface is 1 [21-241. The fractional H coverages corresponding to the H, desorption spectra for the Pd(llOX1 x 2)-Cs surface exposed to C,H, can be estimated by comparing the area intensities of the desorption spectra and that for the Pd(llO)(2 x 1)-H surface. The results are included in Fig. 1. The fractional C,H, coverages corresponding to the C,H, desorption spectra can be estimated by comparing the area intensities of the C,H, desorption spectra and that for the Pd(llO)c(2 x 2)-C,H, surface. The fractional C,H, coverage corresponding to the C,H, desorption spectrum for the Pd(llO)c(2 X 2)-C,H, surface is 0.12 [18]. The results are included in Fig. 1. For 0.2 L exposure, H, desorption is observed at 390 K (Fig. la). This desorption temperature is nearly the same as that for the & state (associated with H atoms near Cs adatoms) from the H-covered Pd(llO)(l x 2)-Cs surface [25]. Therefore, this desorption is attributed to H atoms located near Cs adatoms after the C,H, decomposition. With increasing acetylene exposure, the 390 K peak is decreased in intensity and a new desorption peak is observed at 345 K (Fig. lb). The decrease in the 390 K peak intensity is attributed to the site blocking by the carbon atoms (which is the decomposition product of C,H,) near the Cs adatoms. This desorption temperature (345 K) is nearly the same as that for the pi state (associated with H atoms located distant from Cs adatoms) from the H-covered Pd(llO)(1 X 2)-Cs surface. Therefore, the desorption is assigned to H atoms located distant from Cs adatoms. For exposures > 1.5 L, the H, desorption becomes broader (Fig. Id). This is attributed to the decomposition of vinylidene and methylidyne, which will be discussed later. For 2.5 L exposure, H, desorption from the C,H,-exposed Pd(llO)(l X 2)-Cs surface corre-

T. Takaoka et al. /Surface

182

,..*

0.1

0

0.2 8

0.3

Science 306 (1994) 179-192

face corresponds to OCZH, = 0.08 (Fig. le). Therefore, it is estimated that the fractional coverage of chemisorbed acetylene OCzH, on the Pd(llOX1 X 2)-Cs surface at 90 K is 0.34. C,H, desorption is observed at 315 K. This temperature is higher than the C,H, desorption temperature from the C,H,-exposed Pd(llO)(1 x 2)-Cs surface (270 and N 290 K) [26]. This indicates that the C,H, desorption is reactionrate limited. The fraction of chemisorbed C,H, that is converted into C,H, is shown in Fig. 2 as a function of the fractional coverage of the chemisorbed C*H*.

0.4

CZHZ

Fig. 2. The fraction of the chemisorbed C,H, which is converted into C,H, as a function of the fractional coverage of chemisorbed C,H,.

sponds to 0, = 0.35 (Fig. le), or to the fractional C,H, coverage OCzH, of 0.18. C,H, desorption from the C,H,-exposed Pd(llOX1 x 2)-Cs sur-

3.3. Electron energy loss spectroscopy Figs. 3(a)-(c) show EELS spectra in the specular mode as the Pd(llOX1 x 2)-Cs surface is exPd(llO)(lx2)-Cs

+ 0.2 L C2H2

380

Pd(llO)(lx2)-Cs

+ GH,

(qD2)

570

(c)

CaH,

2.5

L

0

1000

2000

3000

ENERGY LOSS (cm-l) 0

1000

2000

3000

ENERGY LOSS (cm-‘) Fig. 3. EELS spectra in the specular mode as the Pd(llO)(1 x2)-Cs surface is exposed to an increasing amount of C,H, fC,D,) at 90 K. E, = 4 eV.

Fig. 4. EELS spectra in the specular mode of the Pd(llO)(1 x2)-Cs surface exposed to 0.2 L C,H, at 90 K and of the same surface subsequently heated to high temperatures. The heating rate was 5 K/s. All spectra were recorded at 90 K. E,=4eV.

T. Takaoka et al. /Surface

posed to increasing amounts of C,H, at 90 K. In Fig. 3, the loss peak intensities are normalized by the elastic peak intensity. For 0.2 L exposure, losses are observed at 530, 670, 880, 1050, 1350 and 2965 cm-’ (Fig. 3a). For 0.5 L exposure, a new loss peak is observed at 1520 cm-’ (Fig. 3b). For 2.5 L exposure, the loss peaks are markedly different. Loss peaks are observed at 525, 655, 785, 1080, N 1400, 1965, 2965, 3150 and 3360 cm-’ (Fig. 3~). We also performed off-specular measurements. As the off-specular angle A0 is increased, the intensities of all losses, except for those at 2965, 3150 and 3360 cm-‘, are reduced. Thus, all losses, except for those at 2965, 3150 and 3360 cm-‘, are mainly dipole-excited [27].

‘d(llO)(lx2)-Cs

+ 2.5 L GH, (CzDz)

(~1 500 K

183

Science 306 (1994) 179-192

Pd( 1 lO)( 1x2)-Cs

260 K

ENERGY LOSS (cm-‘) Fig. 6. EELS spectra in the specular mode of Pd(llOX1 X 2)Cs surfaces (a) exposed to 0.75 L C,D, at 90 K and subsequently heated to 260 K, (b) pre-exposed to 0.75 L C,D, and post-exposed to 1 L H, at 90 K and subsequently heated to 260 K. The heating rate was 5 K/s. All spectra were recorded at 90 K.

(e) 300 K 1000

-33 Cd)

(92

260 K

(c) 260 K 2965 533 (b) 150 K

2975

(a) 90 K 3150 s , xl000 0

1000

ENERGY

2000

LOSS

3000

(cm-‘)

Fig. 5. EELS spectra in the specular mode of the (1 X2)-Cs surface exposed to 2.5 L C2H2 (C,D,) at of the same surface subsequently heated to high tures. The heating rate was 5 K/s. All spectra were at 90 K.

Pd(llO)90 K and temperarecorded

Fig. 3(d) shows an EELS spectrum as the Pd(llOX1 x 2)-Cs surface is exposed to 2.5 L C,D, at 90 K. Loss peaks are observed at 515, 570, 900, 1070, 1305, 1470, 1745, 2200, 2375 and 2680 cm-‘. Fig. 4 shows EELS spectra in the specular mode of the Pd(llOX1 X 2)-Cs surface exposed to 0.2 L C,H, at 90 K and of the same surface subsequently heated to high temperatures at the rate of 5 K/s. All spectra were recorded at 90 K. Fig. 4(a) shows an EELS spectrum of the Pd(llOX1 x 2)-Cs surface exposed to 0.2 L C,H, at 90 K, and is identical to that shown in Fig. 3(a). After heating to 260 K, no new loss peak is observed, although the peak intensities are changed somewhat (Fig. 4~). After heating to 350 K, except for the loss peak associated with the CO contamination, only the 380 cm-’ loss peak is observed with a large intensity (Fig. 4d). Fig. 5 shows EELS spectra in the specular mode of the Pd(llOX1 X 2)-Cs surface exposed to 2.5 L C,H, at 90 K and of the same surface

184

T. Takaoka et al. /Surface

subsequently heated to high temperatures at the rate of 5 K/s. All the spectra were recorded at 90 K. Fig. 5(a) shows an EELS spectrum of the Pd(llO)(l x 2)-Cs surface exposed to 2.5 L C,H, at 90 K, and is identical to that shown in Fig. 3(c). The spectrum is markedly different after heating to 150 K (Fig. 5b): losses are observed at 525,860, 1060, 1370, 1520 and 2975 cm-r. After heating to 260 K, only the 520, 860, 1520 and 2965 cm-’ peaks are observed. The corresponding EELS spectrum for C,D, is shown in Fig. 5(d). All the loss peaks are reduced in intensity after heating to 300 K (Fig. 5e). After heating to 400 K, the EELS spectrum is changed: the 520, 860 and 1520 cm-’ loss peaks disappear, and loss peaks are observed at 380, 870 and 2950 cm-’ (Fig. Sf). After heating to 500 K, only the 380 cm-’ loss peak is observed (Fig. 5g). Fig. 6(a) shows an EELS spectrum after the Pd(llOX1 x 2)-Cs surface is exposed to 0.75 L C,D, at 90 K (OCZo, = 0.30) and subsequently

Pd( 1 lO)( 1x2)-Cs + GH,

260 K

:lOOO

.lOOO

(c) 2.5 L

1520

\

w

-

I

(b) 0.5 L

I

L x3333

(a) 0.2 L

L

IL

Is \

0

1000

ENERGY

2000

3000

LOSS (cm-‘)

Fig. 7. EELS spectra in the specular mode of Pd(llOX1 X 2)Cs surfaces exposed to various amounts of C,H, at 90 K and subsequently heated to 260 K: (a) 0.2 L, (b) 0.5 L; (c) 2.5 L. The heating rate was 5 K/s. All spectra were recorded at 90 K.

Science 306 (1994) 179-192

heated to 260 K. Fig. 6(b) shows an EELS spectrum after the Pd(llOX1 x 2)-Cs surface is preexposed to 0.75 L C,D, and post-exposed to 1 L H, at 90 K and subsequently heated to 260 K. The TDS result shows that the post-adsorbed H atoms correspond to 0, = 0.20. Fig. 7 shows a comparison of the EELS spectra for the Pd(llOX1 X 2)-Cs surface exposed to various amounts of C,H, at 90 K and subsequently heated to 260 K. For the 0.2 L exposure, the 1350 cm-’ loss peak is observed. For the 0.5 L exposure, the 1520 cm-’ loss peak appears. For the 2.5 L exposure, the 1350 cm-’ loss peak disappears and only the 1520 cm-’ peak is observed.

4. Discussion 4.1. Adsorbed state of acetylene at 90 K

The EELS peaks (Fig. 3) can be assigned by comparison with the vibrational (loss) energies of free molecules [28] and of acetylene chemisorbed on transition metal surfaces [l-3,7,8,11,13,29-48], and also by examining the energy ratios vJvo for C,H, on Pd(llOX1 x 2)-Cs and the deuterated counterparts. In the EELS spectrum of the Pd(llOX1 X 2)Cs surface exposed to 0.2 L C,H, at 90 K (Fig. 3a), the 1350 cm-’ loss is assigned to the CC stretching mode by examining the loss energy ratio vn/vo for C,H, and the deuterated counterpart. Similarly, the 530 cm-’ loss is ascribed to the PdC stretching mode. The 2965 cm-’ loss is ascribed to the CH stretching mode. The remaining losses can be attributed to the CH bending modes. The 670, 880 and 1050 cm-’ losses are assigned to the symmetric out-of-plane CH bending (p,) mode, symmetric in-plane CH bending (6,) mode, and the asymmetric in-plane CH bending (S,,) mode, respectively. The CC and CH stretching energies at 1350 and 2965 cm-‘, respectively, indicate that the rehybridization state of the chemisorbed acetylene is sp - 2s [181. Two structural models can be considered for the adsorbed acetylene: (1) Acetylene is located in the p2,-site with its

T. Takaoka et al. /Surface

CCH plane nearly perpendicular to the surface similar to the structure of acetylene chemisorbed on the Pd(llOX1 x 1) surface [131 for two reasons. First, the loss energies observed for Pd(llOX1 x 2)-Cs are similar to those for Pd(llOX1 x 1) [13], and second, the intensity of the (dipole-active) in-plane CH bending (8,) mode is stronger than that of the out-of-plane CH bending (p,) mode [27]. Table 1 summarizes the loss energies and their assignments for acetylene on various transition metal surfaces. The fact that the CC stretching energy for Pd(llOX1 X 2)-Cs is 120 cm-’ higher than that for Pd(llOX1 X 1) [13] indicates that the CC bond is stronger for acetylene on Pd(llOX1 x 2)-Cs, and that acetylene is weakly chemisorbed on Pd(llOX1 x 2)-Cs. The bond of acetylene to the metal surface is understood mainly by the r donation to the metal and back-donation to the r* orbitals [60], and it is known that 7r (r *> orbitals of acetylene are of bonding (antibonding) character with respect to the CC bond. Thus, it is considered that, as the CC bond becomes stronger, the electron occupation of the r (r *) orbitals is higher (lower), and therefore the bond to the metal surface is weaker. Moreover, the intensity of the CC stretching mode relative to those of the CH bending modes is stronger than that for Pd(llOX1 X 1). This may indicate that the CC axis of acetylene is nearer to the surface normal than that of acetylene on Pd(llOX1 X 1). A structural model of acetylene in the pu,-site is shown in Fig. 8(a). (2) Acetylene is adsorbed in the A-site [8,11,40]. The spectrum for the Pd(llOX1 X 2)-Cs surface exposed to 0.2 L C,H, is assigned to acetylene in the A-site rather than to acetylene in the p+ite,

185

Science 306 (1994) 179-192

0

Pd

C

0

H

Fig. 8. Structural models of acetylene adsorbed in the (a) p,-site; (b) A-site.

referring to the grouping by Jakob et al. [40] of C,H, adsorbed on different metal surfaces in the diagram of ecu versus vcc. The fact that the intensity of the out-of-plane CH bending (p,) mode of acetylene is small, as shown in Fig. 3(a), although that of acetylene in the A-site on various metals is known to be large [8,111, can be explained as follows. It is known that the structure of the Pd(llOX1 x 2)-Cs surface is the missing-row structure [19], and that the embryo Pd(ll1) face is formed. The embryo Pd(ll1) face is inclined to the (macroscopic) Pd(ll0) surface. The intensity of the out-of-plane CH bending (p,) mode is small if acetylene is adsorbed in the A-site on this embryo Pd(lll) surface with its

Table 1 Loss energies (cm-‘) and their assignments for acetylene on various metal surfaces Mode

Pd(llOX1 x 2)-Cs

Pd(ll0) 1131

Ni(ll0) [35]

Cu(ll0) 1371

Pd(lll)

CH stretch

2965 (2230)

2992 (2249)

3010 (2245)

1350 (1325) 1050 (900) 880

3015 (2170) 2900 1305 (1200)

2900 (2190)

CC stretch CH bend (S,,) CH bend (6,) CH bend (p,,) CH bend (p,) MC stretch

2985 (2250) 2820 (2130) 1230 (1200) 1070 (910) 925 (705)

1305 (1280) 1140 (930) 940 (680)

1402 (1359)

1310 (1260)

1034 (850) 872 (621) 673 (511) 500

985 (730)

670 530

700 (500) 460 (460) 360 (360)

890 (700) 745 675 470 370

640 (510) 470 (400) (300)

[l]

Pt(ll1) [481

770 (570) 570 340 (310)

186

T. Takaoka et al. /Surface

CCH plane nearly perpendicular to the Pd(ll0) surface. The fact that the CC stretching energy for the Pd(llOX1 x 2)-G surface is 120 cm-’ higher than that for the Pd(llOX1 x 1) surface is compatible with the results of Jakob et al. [40] that the CC stretching energy of acetylene in the A-site is higher than that of acetylene in the p2-site because the numbers of r-bonds are roughly one for the former and two for the latter, and the strength of the CC bond is stronger for the former. The CC axis of acetylene may also be inclined to the surface, considering the strength of the CC stretching mode. A possible structural model of acetylene in the A-site is shown in Fig. S(b). It is noted that one of the CH bonds is hydrogen-bonded on Pd(llOX1 x l), but the hydrogen bonding is not observed on Pd(llO)(l X 2)-cs [ 131. For 0.5 L exposure, a new loss peak .is observed at 1520 cm- * (Fig. 3b). This is associated with vinylidene, as will be described later. For 2.5 L C,H, (C,D,) exposure, energies of the observed losses at 655 (515), 785 (570), 1965 (1745), 3150 (2375) and 3360 (2680) cm-’ (Figs. 3c and d) are near the vibrational energies for gaseous acetylene. These losses are assigned to physisorbed acetylene. Table 2 summarizes the loss (vibrational) energies and their assignments for physisorbed C,H, (C,D,) on Pd(llOX1 X 2)Cs and for gaseous C,H, (C,D,). The remaining losses are mostly ascribed to chemisorbed acetylene and vinylidene, as will be discussed later. The 2965 (2200) cm- ’ loss is assigned to the CH (CD) stretching mode of acetylene and vinylidene. The 1305 and the 1470 cm-’ losses for C,D, are assigned to the CC stretching modes of Table 2 Loss (vibrational) energies (cm-‘) and their assignments for physisorbed acetylene on Pd(llOX1 X 2)-Cs and gaseous acetylene Mode

Pd(llOX1 x 2)-Cs

Gas 1281

CH CH CC CH CH

C,H, (C,D,) 3360 (2680) 3150 (2375) 1965 (1745) 785 (570) 655 (515)

C,H, (C,D,) 3374 (2701) 3289 (2439) 1974 (1762) 730 (537) 612 (505)

stretch (v,,) stretch (v,) stretch bend bend

Science 306 (1994) 179-192

acetylene and vinylidene, respectively. The broad loss at N 1400 cn-’ for C,H, is ascribed to the overlap of the CC stretching modes for acetylene and vinylidene. The 1080 (900) cm-’ loss is assigned to the asymmetric in-plane CH (CD) bending (a,,) mode of acetylene. It is noted that the 1070 cm-’ loss for C,D, is assigned to the combination band of the 570 and 515 cm-’ losses and/or the overtone of the 570 cm-’ loss. The N 1400 cm-’ loss for C,H, may involve contributions of the combination band of the 785 and the 655 cm-’ losses and/or the overtone of the 785 cm- ’ loss. 4.2, Thermal reaction of acetylene on the Pd(llO)(l x 21-G surface Acetylene remains on the surface after heating the Pd(llOX1 x 2)-Cs surface (exposed to 0.2 L C,H, at 90 K) to 260 K (Fig. 4~). After heating to 350 K, there is the 380 cm-i loss peak which is attributed to the PdC stretching mode (Fig. 4d). Very weak loss peaks that can be assigned to the hydrogen adatoms seem to be observed at 7001000 cm-’ [25]. Thus, for a small exposure (0.2 L), chemisorbed acetylene is simply decomposed into C and H adatoms and no stable intermediates are observed in the thermal decomposition process. After heating the Pd(llOX1 X 2)-Cs surface (exposed to 2.5 L C,H, at 90 K) to 150 K, the EELS spectrum is markedly changed due to the desorption of physisorbed acetylene (Fig. 5b). The 1520 cm-’ loss is increased in intensity relative to the 1370 cm-’ loss intensity. The losses are attributed to vinylidene (described below) and acetylene, respectively. After heating to 260 K, the 1060 and 1370 cm-’ losses disappear, and the 860 and 1520 cm-’ losses are increased in intensity (Fig. 5~). These results indicate that the chemisorbed acetylene with the CC stretching energy at 1370 cm- ’ is converted to a new chemisorbed species with the CC stretching energy at 1520 cm-‘. An interesting feature of this spectrum is the large intensity of the (dipole-active) CC stretching mode. This suggests that the CC bond axis is nearly perpendicular to the surface [27]. In addition, the value of the CC stretch-

r._

_----

CH stretch CH bend MC

Mode

-

2980 _ 800 390

2

_

CzH4

2950 870 380

455 (435)

(700)

(2180) (2290) (1010) (1350)

u

2940 925 470

C2H4

-.

3050 870 310

C2Hz


Fe(ll0) 1291

El

_

_

3015 795

C2H2

I301

,,.

-

-3

2980 790

CzH2

_

[321

Ni(ll1)

(CH) on Pd(llOX1

420

900

1437

1420 1130

-

Ru(001)

.,I

3010 810 465

C2H4

[531

-

3100 850

C2H4

WI

Pt(ll1)

and various

470 (445)

990 (775)

1405 (1025) 1145 (1160)

_,_I

3041 850 417

1586 988 973

3032 3094

-L>

_

compounds

.”

2994 895 426

m-d

(/Q-CHXRu(CO)sl [551

and in organometallic

_

compounds

Ru2(CO),(q5-C,H,)2(/A=CH,) [5l1

and in organometahic

(~&WKO(CO)~I~ [541

metal surfaces,

2986 3047 1470 (1040) 1331(1363) 1051(877) 963 (752) 811 311

Os,(CO)&-H),-

metal surfaces,

1421 (p3-q2-C=CH2) [511

2950 (2195)

320 K

C2H2

and various

R.1 Rh(lll)

X2)-G

x 2)-Cs

Pd(lll) 250 K

C2H2

2986

[481

on Pd(llOX1

2970

Pt(ll1) 350 K

C2H2

(CCH,)

Fe(ll1)

for methylidyne

1581

Pt(100) [501

C2H4

for vinylidene

Pd(100)

and their assignments

C2H2

.,

(cm-‘)

Pd(ll0) 1261 (1 x 2)-cs

.--

energies

520 (490)

860 (610)

1520 (1450)

2985 3050 1435 1435 965 895

Ru(OO01)-0 350 K

2965 (2220)

C2H4

Pd(llOX1 260 K

[491

‘) and their assignments

x2)-Cs

(cm

C2H2

energies

Pd(ll0) (1 x 2)-Cs

Table 4 Loss (vibrational)

CH, stretch (v,) CH, stretch (v,,) CH, bend (6) CC stretch CH, bend (p,) CH, bend (p,) CH, bend (p,) MC stretch

Mode

Table 3 Loss (vibrational)

i 8

188

T. Takaoka et al. /Surface

ing energy (1520 cm-‘) is higher than those observed for chemisorbed acetylene on metal surfaces (1100-1410 cm-‘) [l-3,7,8,11,13,29-48]. These indicate the existence of a chemisorbed species whose hybridization state -is sp - * with the CC axis nearly perpendicular to the surface [18]. The spectrum in Fig. 5(c) is similar to the vibrational spectra of vinylidene observed on various metal surfaces or in organometallic compounds. Table 3 summarizes the loss (vibrational) energies and their assignments of vinylidene on various transition metal surfaces and in organometallic compounds. Consequently, the observed vibrational spectrum after 260 K heating is assigned to the vinylidene (C=CH,) species: the 1520 cm-’ loss is assigned to the CC stretching mode; the 2965 cm- ’ loss to the CH, stretching mode; and the 860 cm-’ loss to the CH, wagging (p,) mode. It is noted that the CH, scissors (6) mode is not observed. Other candidates such as CCH (ethynyl), CHCH, (vinyl) and C,H, (ethylene) are not supported, for the following reasons. EELS spectra for CCH and C,H, observed on various metal surfaces including Pd(llO)(l X 1) [18] are different from those shown in Fig. 5(c), especially in terms of the intensity and vibrational energy of the CC stretching mode. The existence of CHCH, is accompanied by the decomposition products of C,H, such as C, CH, CCH, etc., because C,H, has to be supplied with H (produced by C,H, decomposition) for the formation of CHCH,. However, the existence of both CHCH, and the decomposition products of C,H, is not shown in Fig. 5(c). Fig. 9 shows structures of vinylidene included in organometallic compounds [51] (Figs. 9a, b), and the structural models of vinylidene on Pd(llOX1 X 2)-Cs (Figs. 9c, d). As the CH, wagging (p,) mode is dipole-active in the present case, models in which the CH, wagging mode is totally symmetric [27] are probable. Thus, it seems that the model shown in Fig. 9(c) is probable and the model shown in Fig. 9(d) is not. But the model analogous to that shown in Fig. 9(d) is also possible if vinylidene is bonded to the (secondlayer) Pd atoms in the embryo Pd(ll1) face with its CC axis inclined to the Pd(llO)(l X 2)-Cs surface. It is noted that the CC stretching energy of

Science 306 (1994) 179-192

(b)

Cd)

M

CI

Fig. 9. Structures of vinylidene included in the organometallic compounds: (a) OS,(CO),(~-H),(CL,-~*-CCH,) 1511, (b) Ru,(CO),(~~~-C,H,),(~-~H~) [Sl]; (cl and (d) corresponding structural models of vinylidene chemisorbed on the Pd(llOX1 x2)-Cs surface. Open and hatched circles represent the first- and second-layer Pd atoms, respectively.

vinylidene on the Pd(llOX1 x 2)-Cs surface is nearer to that of vinylidene shown in Fig. 9(b) (to which the model shown in Fig. 9(d) refers) rather than of vinylidene shown in Fig. 9(a) (to which the model shown in Fig. 9(c) refers); see Table 3. In the vinylidene spectrum shown in Fig. 5(c), the CH, scissors (6) mode is not observed, as mentioned above; the absence of this mode might be attributed to the overlap of the scissors mode with the CC stretching mode. But, in the spectrum of deuterated vinylidene (CCD,) shown in Fig. 5(d), a loss peak assigned to the CD, scissors mode, which is expected at N 1100 cm-‘, is not observed. Therefore, the possibility of an overlap of the CH, scissors mode with the CC stretching mode is ruled out. Thus, the dynamic dipole moment associated with the CH, scissors mode is considered to be small. After heating to 400 K, losses are observed at 380, 870 and 2950 cm-’ (Fig. 5f). From the similarity between this spectrum and those of methylidyne (CH) on various transition metal surfaces and in organometallic compounds, it is concluded that methylidyne is adsorbed on the surface (Table 4). The 380, 870 and 2950 cm- ’ losses are attributed to the PdC stretching mode, CH bending mode and CH stretching mode of methyli-

T Takaoka et al. /Surface Science 306 (1994) 179-l 92

dyne, respectively. As the ethylene desorption occurs below 400 K, as shown in the corresponding TDS spectrum (Fig. le), the ratio of the H/C concentration on the surface is less than 1. Therefore, the carbon adatoms are also adsorbed on the surface in addition to methylidyne. Hydrogen atoms produced in the thermal decomposition process of vinylidene do not remain on the surface at 400 K as the desorption is reaction-rate limited (Section 3.2). After heating to 500 K, only the 380 cm-’ loss is observed (Fig. 5g). This loss is attributed to the carbon adatoms. This indicates that methylidyne is decomposed, and that carbon adatoms remain on the Pd(llOX1 X 2)-Cs surface at 500 K. 4.3. thylidene formation on the Pd(llO)(l x 2)-G surface As described above, we observed vinylidene in the thermal reaction process. Three mechanisms are possible for the vinylidene formation: (1) the dehydrogenation-hydrogenation mechanism, in which acetylene is dehydrogenated to ethynyl first, then ethynyl is hydrogenated to vinylidene; (2) the hydrogenation-dehydrogenation mechanism, in which acetylene is hydrogenated to vinyl first, then vinyl is dehydrogenated to vinylidene; and (3) the intramolecular shift of a hydrogen atom. If vinylidene formation occurs by the hydrogenation-dehydrogenation or dehydrogenation-hydrogenation mechanisms, the CCHD species would be observed for the Pd(llOX1 X 2)-Cs surface which is exposed to C,D, and H, at 90 K and subsequently heated to 260 K, since both mechanisms (1) and (2) include the CD bond breaking and the surface H-atom addition. The vibrational energy of the CHD wagging mode is estimated to be _ 700-800 cm-’ from a comparison of the wagging mode of CCH, on Pd(llO)(1 X 2)-Cs and those of CCH, and CCHD included in the organometallic compounds [51]. However, the loss peak associated with the CCHD wagging mode is not observed in the experiments as shown in Fig. 6. In addition, we measured EELS spectra of the Pd(llOX1 x 2)-Cs surface exposed to a mixture of C,H, and C,D, at 90 K and subsequently heated to 260 K. But, the loss

189

peak associated with the CCHD wagging mode was not observed. These results suggest that the conversion from acetylene to vinylidene occurs via intramolecular hydrogen shift. It is noted that a broad loss peak observed at 800-1000 cm-’ in Fig. 6(b) is assigned to the bending and/or stretching modes of post-adsorbed H atoms [25]. Acetylene does not convert to vinylidene for a small exposure (Fig. 7a). With increasing acetylene exposure, the conversion occurs (Fig. 7b). For a large exposure, all acetylene is converted to vinylidene (Fig. 7~). These results suggest that the interaction between C,H, admolecules is important for vinylidene formation. Thus, it is considered that vinylidene formation occurs in the region where the local coverage of acetylene is high. As for the reason why vinylidene is stabilized on the Pd(llOX1 X 2)-Cs surface, we make two propositions, as follows: (1) Vinylidene is stabilized on the embryo (111) face that is formed on the Pd(llOX1 X 2kCs reconstructed surface (structural factor). It is known that the structure of the Pd(llOX1 x 2)-Cs surface is the missing-row structure [19], and that the embryo Pd(ll1) face is formed. Gates and Kesmodel [2] showed using EELS that vinylidene is formed on the Pd(lll) surface exposed to C,H, at 150 K and subsequently heated to 250 K. Moreover, Tysoe et al. [5] showed that vinylidene is formed on the Pd(lll) surface considering the orbital symmetries of adspecies by the use of ultraviolet photoelectron spectroscopy (UPS). From these results, it is considered that vinylidene is stabilized on the Pd(ll1) surface. Consequently, we propose the stabilization of vinylidene on the embryo (111) face which is formed on the Pd(llOX1 x 2)-Cs surface. (2) Vinylidene is stabilized on the electron-rich Cs-modified surface (electronic factor). Due to the difference in the electronegativities of Pd and Cs (2.2 and 0.8, respectively), electrons are transferred from Cs adatoms to the Pd surface. This electron transfer can induce the change in the electron density of states near the Fermi level [56] or the local electrostatic field effect [57]. On the other hand, Parmeter et al. [58] has shown that vinylidene is formed on Ru(OOOl)-

190

T. Takaoka et al. /Surface

p(2 x 2)-O and Ru(OOOl)p(l X 21-O surfaces exposed to C,H, at 80 K and subsequently heated to 350 K. Since the 0 atom has an electronwithdrawing character, the O-modified surface is electron-poor, opposite to the case of the Csmodified electron-rich surface. Thus, vinylidene formation on the O-modified Ru(OO01) surface is not compatible with proposition (2) described above, and we consider that vinylidene is stabilized on the Pd(llOX1 x 2)-Cs surface mainly by proposition (1). It is emphasized, however, that the above argument is rather primitive, and vinylidene must be stabilized on Pd(llOX1 x 2)Cs by the delicate balance between both structural and electronic factors. 4.4. Ethylene formation via vinylidene From the TDS measurements, ethylene desorption occurs at 315 K (Fig. le>. From the EELS measurements, vinylidene is observed at this temperature (Figs. 5c-e). We performed detailed EELS measurements in the 260-350 K range for high acetylene exposures, but, except for vinylidene (and methylidyne), any hydrocarbon intermediates were not detected. Accompanied by ethylene desorption, the loss peak intensity associated with vinylidene is reduced. In addition, the coverage dependence of the vinylidene formation, described in Section 4.3, coincides with that of the ethylene yield, as will be described later in this section. These results indicate that ethylene formation occurs via vinylidene species, as has also been reported for the Pd011) surface in a study using UPS [5]. We performed TDS measurements after the Pd(llOX1 X 2)-Cs surface pre-exposed to 0.75 L C,D, was heated to 260 K and post-exposed to 1 L H, at 90 K. As the C,D, pre-exposure is small (for 0.75 L exposure, the amount of ethylene thermally desorbed is a half of the maximum), desorption of the H-added ethylene (C,D,H) is observed. For 2.5 L C,D, pre-exposure, the Pd (110X1 x 2)-Cs surface is covered with vinylidene after heating to 260 K, and the H-added ethylene is not observed, because the amount of hydrogen (H) adsorbed by the post-exposure is negligible. The hydrogenation of acetylene was theoretically studied by Nakatsuji et al. 1591. They re-

Science 306 (I 994) 179-l 92

ported that the two-step mechanism via vinyl species is preferable to the one-step simultaneous mechanism for C,H, and H coadsorbed on the Pd surface. The present study shows that ethylene formation on the Pd(llOX1 x 2)-Cs surface exposed to C,H2 occurs via a mechanism involving vinylidene as an intermediate species. The detailed mechanism of ethylene formation will be examined in further theoretical and experimental work. We plotted in Fig. 2 the fraction of the chemisorbed C,H, that is converted into C,H, as a function of the fractional C,H, coverage @C,H,. For @C,H, < 0.22, C,H, desorption is not observed, while for OC-.uZ> 0.22, C,H, desorption increases in proportion to the increase in the C,H, coverage. Although it is difficult to interpret this relation quantitatively, it should be emphasized that the OCIH, dependence of the ethylene yield coincides with that of the amount of vinylidene formed. 4.5. Comparison of acetylene on Pd(ll0) (1 x 1) and Pd (110) (1 X 2) -Cs surfaces Prior to the present study, we studied the interaction of acetylene with the Pd(llOX1 x 1) surface [13]. In this section we compare acetylene on the clean Pd(llOX1 x 1) and Pd(llOXl x 2)Cs surfaces. The saturation coverage of chemisorbed acetylene on the Pd(llOX1 x 2)-Cs surface at 90 K is less than half of that on the clean Pd(llOX1 x 1) surface (OC2u2 = 0.34 and 0.77, respectively). This is attributed mainly to the existence of the (1 X 2) structure. Every other [liO] row of the Pd(llO)(1 X 1) surface is missing on the Pd(llOX1 x 2)Cs surface according to the missing-row model [19,20]. The CC stretching energy of chemisorbed acetylene on Pd(llOX1 X 2)-Cs is higher than that on Pd(llOX1 x 1) (1350 and 1230 cm-‘, respectively). This indicates that acetylene is weakly adsorbed on the Pd(llOX1 X 2)-Cs surface, as discussed in section 4.1. As described above, electrons are transferred from Cs adatoms to the Pd surface by the difference in the electronegativities of Cs and Pd. This electron transfer can

T Takaoka et al. /Surface

191

Science 306 (1994) 179-192

reduce the ability of the Pd surface to accept electrons of acetylene. Therefore, it is considered that chemisorbed acetylene is weakly adsorbed on the Pd(llOX1 x 2)-Cs surface due to the reduced donation of electrons from acetylene to the Pd surface [60]. Ethylene formation is greatly promoted on the Pd(llOX1 x 2)-Cs surface. On Pd(llOX1 X 0, only a few percent of the chemisorbed acetylene is hydrogenated to ethylene (OC2u4 N 0.01). On Pd(llOX1 x 2)-Cs, - l/4 of the chemisorbed acetylene is hydrogenated to ethylene (OCZH, = 0.08). This promotion of ethylene formation is related to the difference of the reaction path. On the Pd(llOX1 x 1) surface, at the desorption temperature of ethylene (265 K), there are C2H2, CCH and H on the surface. On the Pd(llO)(1 x 2)-Cs surface, there is mainly CCH, (which is not observed on the Pd(llOX1 X 1) surface) at the corresponding temperature (315 K). From these results, we propose that ethylene is formed via only vinyl on the Pd(llOX1 X 1) surface and via vinylidene on the Pd(llOX1 X 2)-Cs surface. Consequently, the promotion of ethylene formation on the Pd(llOX1 X 2)-Cs surface is attributed to the stabilization of vinylidene. In the process of the thermal decomposition, although CC bond scission occurs at N 500 K on the Pd(llOX1 X 1) surface, it occurs below 400 K on the Pd(llOX1 x 2)-Cs surface. In addition, complete decomposition occurs at 600 K on the Pd(llOX1 X 1) surface, and at 500 K on the Pd(llOX1 x 2)-Cs surface. It is interesting that CC bond scission and complete decomposition are promoted on Pd(llOX1 X 2)-Cs, although acetylene is chemisorbed weakly at low temperatures. This tendency has also been observed for ethylene 1261 and benzene [61] adsorbed on Pd(llOX1 X 2)--Cs, and is attributed to the electron-rich Cs-modified surface in which electrons near the Fermi level can occupy (after thermal activation) the antibonding orbitals of the CC and CH bonds.

lene with the Pd(llOX1 x 2)-Cs surface. Some of the important results are as follows: 1. For a small exposure (0.2 L) at 90 K, acetylene is adsorbed in the &A-site. After heating to 350 K, acetylene is completely decomposed, and carbon and hydrogen adatoms are formed on Pd(llOX1 x 2)-Cs. 2. For a large exposure (2.5 L) at 90 K, physisorbed acetylene and vinylidene exist in addition to acetylene in the &A-site. By heating to 135 K, physisorbed acetylene is desorbed from the surface. By heating to 260 K, acetylene is converted to vinylidene. The vinylidene species undergoes dehydrogenation and hydrogenation concomitantly at 315 K to form gaseous ethylene. After heating to 400 K, carbon adatoms and methylidyne exist on the surface. After heating to 500 K, carbon adatoms remain on the surface. 3. Compared with the case for Pd(llOX1 X 11, ethylene formation is markedly promoted on Pd(llOX1 X 2)-Cs. Although the saturation coverage of chemisorbed acetylene on the Pd(llO)(1 X 2)-Cs surface at 90 K is less than half of that on the clean Pd(llOX1 x 1) surface (ec,n, = 0.34 and 0.77, respectively), the amount of ethylene thermally desorbed from Pd(llOX1 X 2)-Cs is N 8 times larger than that from Pd(ll0). 4. The interaction between C,H, admolecules is important for the vinylidene formation. 5. The promoted ethylene formation is attributed mainly to the formation of vinylidene on the embryo (111) face of the (1 X 2)-Cs reconstructed surface (structural factor).

5. Summary

7. References

A combined vibrational EELS and TDS study has been performed on the interaction of acety-

6. Acknowledgments This work was supported in part by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and by a Grant-in-Aid from the Nippon Sheet Glass Foundation for Materials Science.

[ll J.A. Gates 4281.

and L.L. Kesmodel,

J. Chem.

Phys. 76 (1982)

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