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8462

J . Phys. Chem. 1992, 96, 8462-8468

Interaction of Ethylene with the H-Preadsorbed Pd( 110) Surface: Hydrogenation and H-D-Exchange Reactions T. Sekitani, T. Takaoka, M.Fujisawa, and M. Nishijima* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received: October 10, 1991)

Interaction of ethylene with the H-preadsorbed Pd( 110) surface has been studied by the use of thermal desorption spectroscopy and high-resolution electron energy loss spectroscopy. On the Pd( 110)(2 X 1)-H surface saturated with ethylene at 90 K, weakly-adsorbed ethylene exists in addition to the *-bonded ethylene. The fractional saturation coverage of ethylene is 0.58, which is nearly equal to that on the Pd( 110) clean surface. The amount of ethane which is thermally desorbed is about 15 times as large as that from the Pd( 110) clean surface. The ethane formation is promoted as the amount of preadsorbed hydrogen is increased. The H-D-exchange reaction for C2D4 postadsorption on the H-preadsorbed Pd( 110) surface is discussed. Experimental evidences are given which indicate that the ethane formation and H-D-exchange reaction occur predominantly via ethyl species. The interaction of hydrogen with the ethylene-preadsorbed Pd( 110) surface is briefly described.

I. Introduction The hydrogenation reaction of unsaturated hydrocarbons is important in the catalytic reforming of petroleum feedstocks. The ethylenehydrogen interactions on well-defined transition-metal surfaces have been studied both experimentally [Ru(0001),' Pt( 111),2-5etc.] and theoretically [Pt(111)6*7]as a prototype for the hydrogenation of olefin hydrocarbons. Zaera and Somorjai2 have proposed, from the study by using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), that the Pt( 111) surface, during hydrogenation at 300 K, is probably covered with ethylidyne. Hills et al.' reported, from the study by using TDS and high-resolution electron energy loss spectroscopy (EELS), that the preadsorbed hydrogen inhibits ethylene postadsorption on the Ru(0001) surface. Zaera4 examined the mechanisms for ethylene hydrogenation and H-D exchange on the Pt( 11 1) surface by the use of TDS. Berlowitz et al.5 studied the reaction of ethylene with the H-covered Pt( 111) surface by the use of TDS. They have reported that ethylene located near the preadsorbed H atoms is weakly-held; furthermore, the weakly-held ethylene is hydrogenated more easily than the strongly-held ethylene (which is located far from the adsorbed H atoms). As for transition-metal (1 10) surfaces, only a few works have been done [Pt(110) surf a ~ e , etc.]. ~ - ~ Yagasaki and Masel* studied the coadsorption of ethylene and hydrogen on the Pt(l10)(2 X 1) surface. They have reported the existence of a weakly-bound form of ethylene at 93 K on hydrogen-preexposed Pt( 110) which is more easily hydrogenated than either di-u- or *-bound ethylene formed on clean Pt( 110). They have also reported that hydrogen acts as a site blocker which prevents di-a-ethylene formation and inhibits ethylene decomposition. In the present work, we have studied, by using the in situ combined techniques of TDS and EELS, the interaction of ethylene with the H-preadsorbed Pd( 110) surface. It is well-known that palladium is a good catalyst for the hydrogenation reaction. A predominant mechanism for the ethylene hydrogenation and H-D exchange is discussed. The interaction of hydrogen with the C2H4-preadsorbed Pd( 1 10) surface was briefly studied. Comparison with the related previous works was also made. This work has been made as an extention of our previous studies.lOJ' On the Pd(ll0) clean surface,I0ethylene is ?r-bonded to the Pd(110) surface at 90 K. Above 300 K, C2H4admolecules 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( 110) surface. Compared with ethylene on the Pd( 110) clean surface, ethylene on the Pd( 1 lo)( 1 X 2)-Cs surface'' is more weakly-bonded to the Pd( 110) surface at 90 K. Nevertheless, the dehydrogenation and CC bond scission processes are promoted on the Pd( 1 lo)( 1 X 2)-Cs surface. 0022-365419212096-8462$03.00/0

11. Experimental Section Details of the experimental methods have been described elsewhere,'O and only a brief explanation is given below. For EELS measurements, a primary electron energy, E ,of 4 eV, an energy resolution of 40 cm-' ( 5 meV) (full wi&h at half-maximum), and an incidence angle, Bi, of 60° with respect to the surface normal were used. The electrons were scattered along the [1TO] azimuth. The heat-andquench 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 and cooled to 90 K, and then, the EELS measurements were made. The mass spectrometer was multiplexed, and the TDS measurements were carried out at a heating rate of 5 K/s. The Pd(ll0) clean surface was carefully prepared by oxidation, Ar+ ion bombardment, annealing, and flashing cycles. Research-grade C2H4 (99.8 mol 96 purity), C2D4 (99.5 atom % D, MSD Isotope, Canada, Ltd.), and H2 (99.8 mol 5% purity) were used. Gases were introduced into the vacuum chamber through a IO-mm-diameter gas doser. H2 and C2H4 gas pressures in the chamber were monitored by the use of a nudetype Bayard-Alpcrt ion gauge and were calibrated by the ion-gauge sensitivity factors (0.5 and 2.3 relative to N2, respectively). The base pressure of the vacuum system was 4 X lo-" Torr.

III. Results A. EdIyleDep' on the Pd(110)(2 X 1)-H M a w The Pd(l10)(2 X 1)-H surface was formed by the exposure of 0.3 langmuir of H2 (1 langmuir = lod Torr s) to the Pd(l10) clean surface at 90 K. It is noted that the corresponding 2 X 1 pattern observed by LEED has sharp but weak spots; the (in/2, 0) spots along the [ 1101 azimuth are absent.'* With the increase of the ethylene postexposure to the Pd( 110)(2 X 1)-H surface, the fractional-order spot intensity becomes weaker and the background intensity becomes stronger. For 5-langmuir exposure (saturation exposure), the fractional-order spots are completely hidden in the background. Figure la shows an EELS spectrum of the Pd(l10)(2 X 1)-H surface. Lossa are ob6erved at 515,800, and 980 cm-I. The 800and 980-cm-' losses are associated with the (in-plane) bending and stretchingvibrational modes of H atoms in the low-symmetry short-bridge sites (pseudo-3-fold sites), The 5 15-cm-' loss has not been observed previously, and we tentatively identify it as the (out-of-plane) bending mode of H atoms; our detailed measurements indicate that the 515-cm-I loss is not associated with C adatoms. Figure lb-f shows EELS spectra of the Pd(110)(2 x 1)-H surface exposed to 5 langmuir of C2H4 at 90 K and of the same surface subsequently heated to high temperatures. Figure l b shows an EELS spectrum of the Pd(1 10)(2 X 1)-H surface exposed to 5 langmuir of C2H4 at 90 K. 0 1992 American Chemical Society

The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8463

Ethylene and Pd( 110) Surface Interaction

f 520 K

m

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0

a $OK ( Z X I ) - H XlWO

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Figure 1. EELS spectra in the specular mode of the Pd(l10)(2 X 1)-H surface and of the same surface exposed to 5 langmuir of C2H4at 90 K and subsequently heated to high temperatures. The heating rate was 5 K/s. All spectra were recorded at 90 K. Ep = 4 eV. TABLE I: Observed Vibrational Energies ( c o i l ) md Their Assilplwats for C2H4(C2D4) Molecules on the Pd(110)(2 X 1)-H Surface at 90 K

assignment CH2(CD2)stretch CC stretch CH2(CD2)scissors CH2(CD2)twist CH2(CD2) wag PdC

C2H4 3060 1540 1270 1170 915 550 330

C2D4

1.36 1.10

970 675

1.21 1.36

290

1.14

CCH 2965 1260 940 665 485 370

m Zm, ENERGY LOSS (cm-')

0

m

Figure 2. EELS spectra in the specular mode of the Pd(l10)(2

X 1)-H surface exposed to 5 langmuir of C2D4 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, = 4 eV.

yH/yD

2250 1400

CCD 2220 -1220 755

mass 30

I

TABLE 11: Observed Vibrational Energies (cm-') and Their Assignments for the CCH(CcD) Spccies

assignment CH (CD) stretch CC stretch (in-plane) CH (CD) bend (out-of-plane) CH (CD) bend PdC s stretch PdC as stretch

I

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455 380

This spectrum is similar to that of the Pd( 110) clean surface exposed to 5 langmuir of C2H4at 90 K." The vibrational energies and their assignments for C2H4 (C2D4)on the Pd(l10)(2 X 1)-H surface are summarized in Table I. It is considered, by comparison with the study of ethylene on the Pd( 110) clean surface, that ethylene molecules are *-bonded to the Pd(l10)(2 X 1)-H surface and are located in the on-top sites of Pd( 110). In Figure lb, losses associated with the adsorbed hydrogen are not observed well. It is considered that the vibration associated with H atoms in the low-symmetry short-bridge sites is screened by the n-bonded ethylene admolecules. By heating to 160 K,all loss peaks (except for the -3060-cm-' peak) are decreased in intensity due to the desorption of weakly-bound ethylene, as will be discussed later (Figure IC). The intensity of the -3060-cm-' loss remains nearly unchanged, possibly because it is excited predominantly by the impact mechanism.1° The EELS spectrum is drastically changed after 350 K heating; losses are observed at 370, 485, 665, 940, 1260, and 2965 cm-'(Figure le). This spectrum is similar to that of ethynyl (CCH) formed by heating the C2H4-exposedPd( 110) surface to 350 K.Io For the Pd( 110) clean surface, the existence of ethynyl was confirmed by comparison of the corresponding EELS spectrum with those for various systems and also with the vibrational energies of a triosmium cluster, Os,(CO),(p H)(ps-q2-CCH). Moreover, the TDS results have indicated that the surface stoichiometry of ethynyl C:H is 2:1, which is indeed the case. The observed losses and their identifications are summarized in Table 11. Therefore, on the H-preexposed surface also, ethynyl is formed as a stable intermediate of the ethylene

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TEMPERATURE (K)

Figure 3. TDS spectra of (a) H2 (mass 2), (b) C2H4 (mass 27), and (c) C2Hs (mass 30) from the Pd( 110)(2 X 1)-H surface (solid curves) and Pd(ll0) clean surface (dashed curves) exposed to 5 langmuir of C2H4 at 90 K. The H2 desorption spectrum for the Pd( 110)(2 X 1)-H surface is shown in d. The heating rate was 5 K/s.

decomposition process. After heating to 520 K,only one peak is observed at 525 cm-'(Figure If). This spectrum indicates that ethynyl is completely decomposed and only carbon adatoms remain on the Pd( 110) surface. Thus, it is concluded that the thermal decomposition process, studied by EELS, of ethylene on the Pd(1 10)(2 X 1)-H surface is very similar to that on the Pd( 110) clean surface. Figure 3a-c shows TDS spectra after exposing the Pd( 110)(2 X 1)-H surface to 5 langmuir of C2H4. The desorption products were H2, C2&, and C2H6. No methane or benzene was detected. The C2H4 desorption (mass 27) is observed as a sharp peak at 150 K,as a peak at -220 K with the shoulder at -250 K,and as a hump at 320 K (solid curve, Figure 3b). It is noted that Figure 3b is corrected for the ethane contribution: we have subtracted the ethane-cracking contribution from the original mass 27 desorption spectrum to obtain the ethylene desorption spectrum. The C2H4 desorption peak at 150 K was not observed for the Pd(ll0) clean surface (dashed curve, Figure 3b).I0 This indicates

8464 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 that weakly-adsorbed ethylene exists on the Pd( 110)(2 X 1)-H surface owing to the modification of the electronic states of the Pd( 110) surface by the existence of the preadsorbed hydrogen. It is estimated, assuming first-order desorption kinetics, that the activation energy of desorption of the weakly-adsorbed ethylene is 8 kcal/mol (preexponential factor: 5 X ~ O " / S ) .The ~ ~ vibrational lossa associated with the weakly-adsorbed ethylene are not clearly observed; only a decrease in the loss intensity is observed by the desorption (compare parts b and c of Figure 1). This may partly be due to the low coverage. As will be described later, the coverage of the weakly-adsorbed ethylene is approximately onesixth of the total ethylene coverage. We are unable to distinguish whether the weakly-adsorbed ethylene is chemisorbed or physisorbed. (Perhaps, a study by the use of photoelectron spectroscopy will clarify this problem unambiguously.) We have previously found physisorbed ethylene (with the desorption temperature of 110 K; heat of desorption 2 kcal/mol) for ethylene on the Pd(1 lo)( 1 X 2)-Cs surface." It is interesting that weakly-adsorbed ethylene exists both on H- and Cs-preadsorbed Pd( 110) surfaces. The existence of weakly-chemisorbed ethylene on the Pd( 1lo)( 1 X 2)-Cs surface has been understood by assuming that ethylene is a r-donor molecule and that (electropositive) Cs prevents the r donation.l'*I5 On the other hand, the existence of the weakly-bound ethylene on the Pd( 110)(2 X 1)-H surface cannot be understood simply by assuming that ethylene is a r-donor molecule: the assumption implies that (electronegative) H enhances the r donation and, thus, predicts the existence of strongly-bound ethylene. Therefore, if the weakly-bound ethylene on Pd( 110)(2 X 1)-H is chemisorbed, we have to assume also the contribution of the backdonation of d electrons into the r* orbital of ethylene to understand the existence of the weakly-bound ethylene on the Pd(110)(2 X 1)-H surface. The H2 (mass 2) desorption is observed as peaks at 325 and 480 K and as a shoulder at -280 K (solid curve, Figure 3a). Compared with the H2desorption spectrum for the Pd(ll0) surface exposed to 5 langmuir of ethylene, the desorption peak at 325 K is increased in intensity, and a shoulder at -280 K appears (compare solid and dashed curves of Figure 3a). The Hz desorption at -280 and 325 K originates from the decomposition of C2H4admolecules and the contribution of the preadsorbed hydrogen. An increase in the H2 desorption intensity reflects the amount of preadsorbed hydrogen, as will be discussed later. The 480 K peak intensity is nearly the same as that for the Pd(ll0) clean surface. The 480 K peak originates from the decomposition of ethyny1,'O and thus, on the hydrogen-preadsorbed surface, the amount of ethynyl formed is nearly the same as that on the clean Pd(ll0) surface. The -280 K shoulder may be associated with H atoms in the subsurface sites of the bulklike Pd( 110) surface observed for the H-covered Pd(1 10) surface peak).1z16The existence of the -280 K shoulder is interpreted to indicate that the Pd( 110) surface is locally reconstructed, prior to the H2 desorption temperature, to form 1 X 2 patches.lz The ethane (mass 30) desorption is observed as a broad peak at 250 K. The amount of ethane formed on the Pd( 110)(2 X 1)-H surface is greatly increased in comparison with that on the Pd( 110) clean surface. It is noted, however, that ethane is still a minor product, as will be described later. A small desorption peak is observed at 3 15 K for the Pd( l 10) clean surface, as is shown by the dashed curve of Figure 3c. (In a previous paper, Nishijima et a1.I0 have reported that the ethane desorption is not observed, but by the improvement of the signal/noise ratio, our detailed measurements have shown that desorption actually takes place.) It is noted, for the Pd( 110) clean surface, that the CH bond scission of ethylene is required to form hydrogen adatoms used for the ethane formation, and thus, the ethane formation occurs at a relatively high temperature of 315 K. The ethane desorption intensity for the Pd(l10)(2 X I)-H surface is about 15 times as large as that for the Pd(ll0) clean surface. The saturation coverage of ethylene on the Pd( 110)(2 X 1)-H surface can be estimated by the TDS spectra. The fractional H coverage, OH (number of adsorbed H atoms per surface Pd atom), corresponding to the desorbing H2 can be estimated by comparing the area intensity of the H2 desorption spectrum (solid curve,

Sekitani et al. Figure 3a) and that for the Pd(l10)(2 X 1)-H surface (Figure 3d) on which = 1 or by comparison with our previous study of ethylene on the Pd( 110) clean surface.I0 The fractional H coverage corresponding to H2produced by ethylene decomposition which is followed by ethynyl formation is OH = 1.5 or = 0.38 (dashed curve, Figure 3a).1° It is noted that the difference in area intensities of the solid and dashed curves of Figure 3a corresponds nearly to OH = 1. The fractional C2H4 coverage corresponding to the C2H6 desorption is roughly estimated to be 0.02 by comparing the area intensity of the CzH6desorption spectrum (solid curve, Figure 3c) and that of the CzH4 desorption spectrum for the Pd( 110) clean surface (dashed curve, Figure 3b)I0 with the correction of the ionization probabilities and cracking pattern coefficients of ethylene and ethane. The ratio of the ionization probabilities of ethylene and ethane was assumed to be similar to that of their ion-gauge sensitivity factors (2.3/2.7). It is noted that the above estimate is not corrected for the difference in the transmission of the fragment ions in the mass spectrometer. The fractional C2H4 coverage corresponding to the C2H4 desorption is 0.18 from the comparison of ethylene desorption spectrum for the H-preadmbed surface (solid curve, Figure 3b) and that for the Pd(ll0) clean surface (dashed curve, Figure 3b).I0 It is noted that the fractional coverage of the weakly-adsorbed ethylene is 8ClH, 0.1. Thus, the fractional saturation coverage of C2H4 on the Pd( 110)(2 X 1)-H surface is estimated to be 0.58, which coincidentally agrees with the saturation coverage of ethylene on the Pd(ll0) clean surface.I0 Therefore, it is concluded that the H preadsorption (6, = 1) does not markedly block the ethylene postadsorption. This is compatible with the fact that C2H4 molecules are adsorbed in the on-top sites of the Pd( 110) surface which are not geometrically hindered by H atoms in the lowsymmetry short-bridge sites near the three-coordinated sites of Pd(ll0). We have also studied CzD4 adsorption on the Pd( 110)(2 X 1)-H surface in order to study the H-D-exchange reaction. Figure 2 shows the EELS spectra of the Pd(l10)(2 X 1)-H surface exposed to 5 langmuir of C2D4 at 90 K and of the same surface subsequently heated to high temperatures. Figure 2a shows an EELS spectrum of the Pd(l10)(2 X 1)-H surface exposed to 5 langmuir of C2D4. Thii spectrum is similar to that of the Pd(ll0) surface exposed to 5 langmuir of C2D4.I0The observed loss energies and their assignments are summarized in Table I. As the heating temperature is increased to 230 K, all the losses (except for the d in intensity (Figure 2c). By heating 225o-Cm-I loss) are d to 300 K, a new loss peak is observed at -2980 cm-I. This loss is attributed to the CH stretching mode of the C2D3Hand CCH admolecules which are formed by the exchange reaction (Figure 2d). It is noted that the 695- and 725-cm-I losses in Figure 2d are attributed (mainly) to the CD2 wagging mode of C2D4and (in-plane) CD bending mode of CCD (Table 11),respectively. By heating to 350 K, losses are observed at 380, 455, 755, 945, 1220,2220, and 2955 cm-I. The losses are d a t e d with the CCD and CCH admolecules (Table 11). The 945-cm-' loss is attributed to the (in-plane) CH bending mode. The 2220- and 2955-cm-' losses are attributed to the CD and CH stretching modes, raspectively. The area intensity of the 2955-cm-' loss is about one-third of that of the 2220-cm-' loss. It is considered that this roughly reflects the merage ratio of the CCH and CCD admolecules. [Thisargument is supported simply by examining the contribution of CCH and CCD in the H2,HD, and D2thermal desorption spectra for the Pd(l10)(2 X 1)-H surface exposed to C2D4(Figure 4a-c), which will be discussed later.] By heating to 520 K,only carbon adatoms remain on the Pd( 110) surface (Figure 2f). Figure 4 shows TDS spectra after exposing the Pd( 110)(2 X 1)-H surface to 5 langmuir of C2De The Dz (mass4) desorption is observed at 335 and 480 K;the shoulder is observed at -280 K (Figure 4c). The HD (mass 3) desorption is observed at 330 and 480 K, the shoulder is observed at 280 K (Figure 4b). The H2 (mass 2) desorption is observed at 325 K with the shoulder at -280 K;a small desorption peak is observed at -480 K (Figure 4a). The C2D4 (mass 32) desorption spectrum has a sharp

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-

The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8465

Ethylene and Pd( 1 10) Surface Interaction

l%(llOX2Xl)-H+ C2Ck

I

100

303 500 TEMPERATURE ( K )

Figure 5. TDS spectra of C2H4 (mass 27) from Pd(ll0) surfaces preexposed to various amounts of H2 and ptexposed to 5 langmuir of CzH4 at 90 K. The heating rate was 5 K/s.

mass 4

mass 2 loo

m

Joo

La)

m

600

TEMPERATLIRE(K)

mass 32

Figure 4. TDS spectra of (a) H2 (mass 2), (b) HD (mass 3), (c) D2 (mas 41, (4C A H (mass 311, (e) CzD4 (mas 321,(0C2DiH2 (" 34), (g) CzDd (mass 35), and (h) C2D6(mass 36) from the Pd(l10)(2 X 1)-H surface exposed to 5 langmUir of CzD4 at 90 K. The heating rate was 5 K/s.

peak at 150 K and a peak at -220 K with the high-temperature These peaks are correlated with the 150 and tail (Figure 4). -220 K peaks for the C2H4-exposedPd(110)(2 X 1)-H surface (Figure 3b), respectively. The C2D3H(mass 31) desorption is observed at -250 K (Figure 4d). This peak is correlated with the -250 K shoulder peak for the C2H4-exposedPd(llO)(2 X 1)-H surface (Figure 3b). It is noted that the C2D3Hdesorption spectrum (Figure 4d) is corrected for the contribution from the C2D4H2cracking. The cracking patterns for isotopically-sub stituted ethane molecules are obtained from the literature." C2D3His formed by the H-D exchange of C2D4. Figure 4d shows that the -250 K peak appears by the H-D-exchange reaction. Comparison of parts d and e of Figure 4 shows that the H-Dexchange reaction does not occur for the weakly-adsorbed ethylene. The C2D4H2(mass 34), C2D5H(mass 35), and C2D6(mass 36) desorptions are observed at -240,250, and 260 K,respectively (Figure 4f-h). The desorption temperature of C2D6 is 10 K higher than that of C2D5H;the desorption temperature of C~DSH is 10 K higher than that of C2D4H2.This isotope effect originates from the necessity of the CD bond scission in the cases of C2D6and C2D5Hformation and the difference in the mobility of H and D adatoms. The existence of the C2D5Hdesorption is interpreted to indicate that the ethane formation occurs stepwise: ethylme is fmt hydrogenated to the ethyl spacia, and in the second step, ethyl is hydrogenated to ethane. The desorption temperature of C2D4H2(-240 K,Figure 4f) is near that of C2D3H(-250 K, Figure 4d). This suggests that the C2D4H2and C2D3Hdesorptions are assoCiated with the same unstable intermediate. It is considered that this intermediate is the ethyl species. The C2D4 admolecules are first hydrogenated to C2D4H,and then some C2D4Hadmolecules are further hydrogenated to C2D4H2,while the others are dehydrogenated to C2D3H. Strictly speaking, the desorption temperature of C2D3H is higher than that of C2D4H2by 10 K. This indicates that the ethyl species is more easily hydrogenated to ethane than dehydrogenated to ethylene. It is noted, according to Zaera," that, on R(l1 l), the ethyl species is the intermediate for the ethylene hydrogenation and H-D-exchange reaction. For the Pd(ll0) clean surface, we have reported that the unstable intermediate which is associated with the H-D-cxchange reaction of ethylene is predominantly the vinyl species.1° With

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mass 31 d

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L

H

2

6C0 30-b

a-

01 ,

100

m

,

,

Xx)m

I

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m6m

TEMPERATURE ( K )

Figwe 6. TDS spectra of (a-d) C2D3H(mass 31) and (ti)C2D4(mass 32) from Pd( 110) surfaces prccxposcd to various amounts of H2and postexpod to 5 langmuir of C2D4 at 90 K. The heating rate was 5 K/s.

the existence of the preadsorbed hydrogen atoms, the H-D-exchange reaction is considered to occur predominantly via the ethyl intermediate. B. Etsyknc PasCdsorptionOII Pd( 110) Surface4 Rerdsorkd with VUiae Amounts of Hydrogen. We have also studied the ethylene postadsorption on Pd( 110) surfaces preadsorbed with various amounts of hydrogen. By exposure of 0.6 langmuir of H2 to the Pd( 110) clean surface at 90 K,a 1 X 2 LEED pattern is observed, which indicates the formation of the Pd(llO)(l X 2)-H surface.'* For further exposure, the Pd(ll0) surface exhibits a streaky 1 X 2 LEED pattern. By 5-langmuir ethylene pastcx~~thefractional-ordcrspotsdisappear,andthe~ intensity is increased. We have performed EELS measurements for various H2-preexposod surfaces, but the results obtained were similar to those for the Pd(l10)(2 X 1)-H surface (Figure 1). Figure 5 shows C2H4 (mass 27) desorption spectra after exposing 5 langmuir of C2H4(which is sufficient for the saturation coverage) on the W( 110) surfaces preadsorbed with various amounts of hydrogen. For 0.1 langmuir of H2 exposure, a new dasorption peak appears at 165 K,and the 300 K peak obsmed for the clean surface is decreaacd in intensity (compare parts a

8466 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992

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mass2

Sekitani et al.

+ i' :' v1

mass 30

h

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Z

tu b a

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Figure 9. TDS spectra of C2H6 (mass 30) from Pd(ll0) surfaces preexposed to various amounts of H2 and postexposed to 5 langmuir of C2H4at 90 K. The heating rate was 5 K/s. too

2co a0 m 500 MO TEMPERATURE (K)

F'igure 7. TDS spcctra of H2 (mass 2) from Pd( 110) surfaces preexposed to various amounts of H2 and postexposed to 5 langmuir of CzH4at 90 K. The heating rate was 5 K/s.

-!

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51

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v)

Z

mass 3

mass 2

1 c o 2 " 4 o o ~ K c

TEMPERATURE ( K )

Figure 8. TDS spectra of (a-d) H2 (mass 2), (e-h) HD (mass 3), and (i-m) D2 (mass 4) from Pd( 110) surfaces preexposed to various amounts of H2and postexpcsed to 5 langmuir of CIDI at 90 K. The heating rate was 5 K/s.

and b of Figure 5). For > 0.3 langmuir of H2 exposure, the desorption over -250 K is decreased in intensity, and instead, the 150 K desorption intensity is increased. Figure 7 shows H2 (mass 2) desorption spectra. The 325 K peak and the -280 K shoulder grow with the increase of the preadsorbed hydrogen. It is noted that the -280 K desorption, in the high-H2-exposure region, is associated with H atoms in the subsurface sites of the bulldike surface.12 The 480 K peak remains nearly unchanged. This indicates that the amount of ethynyl thermally formed is almost independent of the amount of hydrogen preexposure. It is noted that, for the Pd( 1 10) surface exposed to hydrogen, a new peak (a2peak) appears at 220 K for 50 langmuir of hydrogen exposure,16but the Corresponding desorption peak is not observed by ethylene postexposure. The mechanism is not understood well. It appears, in the high-exposure region, that, as the adsorption energy of C2H4 is larger than that of H2, the preadsorbed hydrogen is (partially) desorbed by the subsequent C2H4 adsorption (displacement desorption) at 90 K.

Figure 9 shows C2H6(mass 30) desorption spectra. For the Pd( 110) clean surface, a desorption peak is observed at 315 K. With increasing hydrogen preexposure, the desorption peak is shifted toward lower temperature, and for 50 langmuir of H2 preexposure, the peak intensity is -50 times as large as that for the clean Pd(ll0) surface. It is noted that the ethylene saturation coverage is nearly the same as that on the Pd(l10)(2 X 1)-H surface. These results indicate that ethylene hydrogenation is increasingly promoted by the increase of the amount of preadsorbed hydrogen. This is quite reasonable because the average distance between an ethylene admolecule and a hydrogen adatom is decreased by the increase of the preadsorbed hydrogen atoms. Spectra a-d and e-i of Figure 6 show C2DjH (mass 31) and C2D4(mass 32) thermal desorption spectra obtained after postexposing 5 langmuir of C2D4 on the Pd( 110) surface preexposed to various amounts of H2, respectively. For the Pd(ll0) clean surface, the C2D4 desorption is observed as a peak at 315 K and as a plateau between 150 and 260 K. With increasing H2 preexposure, new desorption peaks appear, and for 50 langmuir of H2 preexposure, the desorption peak is observed at 150 K with the high-temperatureshoulder. The C2D3Hdesorption is o M e d as a broad peak at 320 K for 0.1 langmuir of H2 preexposure. With increasing H2 preexposure, the desorption is observed at lower temperature, and for 50 langmuir of H2 preexposure, the desorption is observed at -220 K. In addition, the C2D3Hdesorption is increased in intensity. These results indicate that the H-D exchange of ethylene is promoted as the amount of preadsorbed hydrogen is increased, as is expected. It is noted, with increasing H2preexposure, that the CH stretching mode due to the H-D exchange between adsorbed H and C2D4 is observed at lower heating temperatures in the EELS measurements. Spectra a-d, e-h, and i-m of Figure 8 show H2 (mass 2), HD (mass 3,and D2 (mass 4) thermal desorption spectra for various amounts of H2 preexposure, respectively. The D2 desorption is observed at 335 and 480 K for the Pd( 110) clean surface (Figure 8i). With increasing H2 preexposure, both 335 and 480 K peaks are decreased in intensity, and the shoulder appears at -280 K. The decrease of the 335 and 480 K peak intensities is attributed to the increase of the H-D-exchange reaction; the increase of the -280 K peak intensity is ascribed to the increase of D adatoms formed by the H-D-exchange reaction. The HD desorption peak is observed at 330 K with a small peak at 480 K for 0.1 langmuir of H2 preexposure. With increasing H2 preexposure, the 330 K peak is increased in intensity, and the shoulder at -280 K and the 480 K peak grow. The 480 K peak is ascribed to the decomposition of CCD and CCH species. The CCH species is formed by the H-D-exchange reaction of an C2D4 admolecule. The H2 desorption peak is observed at 325 K for 0.1 langmuir of H2 preexposure. With increasing H2preexposure, the shoulder at -280 K and the 480 K peak appear. Spectra a-d, e-h, and i-m of Figure 10 show CZD4H2 (mass 34), C2DsH (mass 35), and C2D6(mass 36) thermal desorption spectra for various amounts of H2 preexposure, respactivcly. As 0.1 langmuir of H2 is preexposed, the C2D4H2desorption is ob-

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The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8467

Ethylene and Pd( 110) Surface Interaction

-I E

E- l

n

mass 27

t

L

Figure 10. TDS spectra of ( a d ) C2D4H2(mass 34), ( e h ) C2D~H (mass 3 9 , and (i-m) C2D6(mass 36) from Pd(ll0) surfaces preexposed to various amounts of H2and postexposed to 5 langmuir of C2D4at 90 K.

The heating rate was 5 K/s. served as a broad peak at -270 K (Figure loa), the C2D5H desorption is observed at -280 K (Figure lOe), and the C2D6 desorption is observed at -300 K (Figure lOj). With increasing Hz preexposure, the C2D4H2desorption is markedly increased in intensity; the C2D6 desorption is decreased in intensity. In addition, the C2D4H2,C2D5H,and C2D6 desorptions occur at lower temperatures. It is noted that the decrease of the C2D6 desorption temperature reflects the decrease in the temperature of the HD-exchange reaction. There is not much change in the intensity of the CzDsH desorption. This is due to the existence of two competing factors: the probability of H addition to CzD4is increased with increasing H2 preexposure, but the probability of D addition to C2D4 is decreased. For 50 langmuir of H2exposure, the CzD4H2desorption is observed at -200 K the C2DsH desorption at -235 K,and the C2D6 desorption at -240 K. The C2D6 desorption intensity is about '/=th of the C2D4H2desorption intensity, and the C2D5Hdesorption intensity is about 1/4th of the C2D4H2desorption intensity for 50 langmuir of H2 preexpure from the comparison of area intensities of the C2D6,C2DsH,and C2D4H2desorption spectra with the correction for the ionization probabilities and cracking pattern coefficients of C2D6, C2DSH, and C2D4H2. C. Hydrogen Postadsorption on the Ethylene-headsorbed Pd(ll0) Surface. We have briefly studied the interaction of hydrogen with the ethylenapreadsorbed Pd( 110) surface. Spectra a and b of Figure 11 show CzH4 (mass 27) and H2 (mass 2) thermal desorption spectra after 50-langmuir exposure of H2 on Pd( 110) preexposed to 5 langmuir of C2H4,respectively. The spectra are similar to those for 5 langmuir of C2H4exposure on Pd(ll0) with H2 preexposure of 0.1 langmuir (Figures 5b and 7b). The fractional coverage of the postadsorbed hydrogen is estimated to be eH = 0.5. On the other hand, for the Pd(ll0) clean surface, 50 langmuir of H2 corresponds to BH = 2.5. This indicates that the preadsorbed ethylene blocks the hydrogen postadsorption. This is attributed mainly to the blocking of sites for molecular hydrogen dissociation due to the occupation of the on-top sites of the Pd( 110) surface by the adsorbed ethylene. It is considered that the dissociation of molecular hydrogen (to form hydrogen adatoms) occurs in the on-top sites.'* It is noted, from the existence of the C2H4desorption peak at 160 K, that the

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8468 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992

the reaction of ethylene with hydrogen-covered Pt( 111) and Pt(1 10) surfaces, respectively. These research groups reported the existence of weakly-bonded (probably, r-bonded) ethylene which was related to the promoted ethane formation. The weakly-adsorbed ethylene formed on the H-preadsorbed Pd( 110) surface is not related to the ethane formation as discussed in section 111. It is noted that ethylene is di-a-bonded to the surface on clean Pt( 111); both di-u- and *-bonded ethylene coexist on clean Pt(1 lo), while ethylene is *-bonded on clean Pd(ll0). Zaera and Somorjai2have proposed that the Pt( 111) surface, during hydrogenation at 300 K, is probably covered with ethylidyne. Anderson and Choe6 have reported, from a theoretical calculation, that ethylidyne is an inactive spectator of the ethylene hydrogenation reaction. On the Pd( 1 10) surface, the ethylidyne species is not formed. Maurice and Minot’ investigated theoretically the mechanisms for hydrogenation on the Pt( 111) and Pt( 110) surfaces: both Langmuir-Hinshelwood type [involving C2H4(adsorbed) and H (adsorbed)] and Eley-Rideal type [involving C2H4 (gas) and H (adsorbed)] were examined. They have proposed that the Eley-Rideal mechanism is the most favorable reaction pathway. The Eley-Rideal mechanism was not studied in the present work. Interaction of ethylene with palladium surfaces has been studied by several g r o ~ p s . ~Ethylene ~ , ~ ~ is di-a-bonded to the Pd( 100) surface, while it is r-bonded to Pd( 111) and Pd( 1 10) surfaces. In the thermal decomposition process, ethylidyne is observed as a stable intermediate on the Pd( 111) surface at 300 K. Ethynyl is observed on Pd( 11l), Pd( 1lo), and Pd(100) surfaces at 400 K. Methylidyne is observed on Pd( 111) and Pd( 100) surfaces. It is noted that the adsorbed state of ethylene and the thermal decomposition process depend on the surface structure of the substrate metal. A more detailed study is needed to understand the surface structure specificity. Only ethynyl was observed as a stable intermediate on both clean Pd( 110) and H-preadsorbed Pd( 110). It is noted that, above -350 K, the ethylene decomposition processes on both clean Pd( 110) and H-preadsorbed Pd( 110) are the same simply because H adatoms are desorbed as H2 by -350 K. On the 0-modified Fe(ll1) surface,19 the inhibition of the CC bond scission was observed due to the decrease of the backdonation of d electrons into the r * orbital of ethylene. It has also been reported that, due to modification of the metal surface, the species of the stable intermediate in the thermal decomposition process is changed. Vinylidene is a stable intermediate on the 0-modified Ru(OOO1) surface, while ethynyl is a stable intermediateon clean Ru(0001).24 Both ethylidyne and ethylidene are the stable intermediates on K-modified Pt(l1 l), while ethylidene is not a stable intermediate on clean Pt( 11I).*’

v.

summry

A combined thermal desorption and vibrational EELS study has been performed on the interaction of ethylene with the hydrogen-preadsorbed Pd( 110) surface. Some of the important results are as follows: 1. In addition to r-bonded ethylene, weakly-adsorbed ethylene exists on the Pd( 110)(2 X 1)-H surface exposed to ethylene at 90 K. Only *-bonded ethylene exists on the Pd( 110) clean surface. 2. The fractional saturation coverage of ethylene on the Pd(1 10)(2 X 1)-H surface at 90 K is estimated to be OcZb = 0.58. The fractional coverage is nearly the same as that on the Pd( 110) clean surface. The site blocking due to the preadsorbed hydrogen does not occur for ethylene adsorption on the Pd( 110)(2 X 1)-H surface.

Sekitani et al. 3. The amount of ethane thermally desorbed from the Pd(110)(2 X 1)-H surface is -15 times as large as that from the Pd( 110) clean surface. 4. As the amount of preadsorbed hydrogen is increased, the ethane formation is promoted by the proximity effect. For 50 laagmuir of Hz preexposure, the amount of ethane is -50 times larger than that from the Pd( 110) clean surface. Ethane is desorbed at a lower temperature with an increase of the hydrogen coverage. 5 . The ethane formation occurs predominantly via the ethyl species. 6. The CzD4 adsorption study on the Pd(l10)(2 X 1)-H surface shows that the H-D-exchange reaction (to form C2D3H)occurs predominantly via the ethyl species. It is noted that, on the Pd( 110) clean surface, the reaction occurs predominantly via the vinyl species. 7. The H-D-exchange reaction is promoted with an increase of the preadsorbed hydrogen. 8. Hydrogen adsorption on the ethylene-preexposed Pd( 110) surface is hindered by the ethylene admolecules located in the on-top sites of Pd( 110).

Acknowledgment. This work was supported in part by the Grant-in-Aid for Scientific Research on Priority Areas (Research Program “Surface as a New Material”) from the Ministry of Education, Science and Culture and by the Grant-in-Aid from the Murata Science Foundation. Repbw NO. HzC=€Hz, 74-85-1; Pd, 7440-05-3; H m , 212248-7; HSCCHZ, 2025-56-1; Hz, 1333-74-0.

References .nd Notes (1) Hills, M. M.; Parmeter, J. E.; Weinberg, W. H. J . Am. Chem. SOC. 1986. 108,7215. (2) Zaera, F.; Somorjai, G.A. J . Am. Chem. Soc. 1984, 106, 2288. (3) Godbey, D.; Zaera, F.; Yeates, R.; Somorjai, G. A. Surf.Sci. 1986, 167, 150. (4) Zaera, F. J. Phys. Chem. 1990, 94, 5090. (5) Berlowitz, P.; Megiris, C.; Butt, J. B.; Kung, H. H. Lungmuir 1985, 1,206.

(6) Anderson, A. B.; Choc, S. J. J . Phys. Chem. 1989, 93,6145. (7) Maurice, V.; Minot, C. J . Phys. Chem. 1990, 94, 8579. (8) Yagasaki, E.; Masel, R. I. Sur/. Sci. 1990, 226, 51. (9) Yagasaki, E.; Backman, A. L.; Chen, B.; Masel, R. I. J. Vac. Sci. Technol. 1990, A8, 2616. (10) Nishijima, M.; Yoshinobu, J.; Sekitani, T.; Onchi, M. J. Chem. Phys. 1989,90, 5114. (1 1) Sekitani, T.; Yoshinobu, J.; Onchi, M.; Nishijima, M. J . Phys. Chem. 1990.91.6847. (12) Jo, M.; Kuwahara, Y.; Onchi, M.; Nishijima, M. Solid Sfate Commun. 1985,55,639. (13) Ellis, T. H.; Morin, M. Surf. Sci. 1989, 216, L351. (14) Chan, C. M.; Ark,R.; Weinberg. W. H. Appl. Surf.Sci. 1978, I , 360. (15) Windham. R. G.:Bartram. M. E.:. Kocl.. B. E. J . Phvs. Chem. 1988. 92, ‘2862. (16) Behm, R. J.; Penka, V.; Cattania, M.-G.;Christmann, K.; Ertl, G. J . Chem. Phys. 1983, 78,1486. (17) Amenomiya, Y.; Pottie, R. F. Can. J . Cham. 1986, 46, 1741. (18) Muscat, J. P. Surf.Sci. 1981, 110, 85. (19) Seip, U.; Tsai, M.-C.; KUppers, J.; Ertl, G. Surf.Sci. 1984,147,65. (20) Stuve, E. M.; Madix, R. J. Surf.Sci. 1985, 160, 293. (21) Kostov, K. L.; Marinova, Ts. S. Surf. Sci. 1987,184, 359. (22) Steininger, H.; Ibach, H.; Lehwald, S. Surf.Sci. 1982, 117, 685. (23) Caasuto, A.; Mane, M.; Hugenschmidt, M.; Dolle. P.; Jupille, J. Surf. Sci. 1990, 237, 63. (24) Hills, M. M.; Parmeter, J. E.; Weinberg, W. H. J . Am. Chem. Soc. 1987, 109,4224. (25) Gates, J. A.; Kesmodel, L. L. Surf.Sci. 1982,120, L461; 1983,124, 68. (26) Stuve, E. M.; Madix, R. J. J . Phys. Chem. 1985, 89, 105. (27) Windham, R. G.;Kocl, B. E. J. Phys. Chem. 1990, 94, 1489.