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Thermal evolution of acetylene and ethylene on Pd(111)
68
Surface Science 124 (1983) 68-86 North-Holland Publishing Company
THERMAL J.A. GATES Department
Received
EVOLUTION
OF ACETYLENE
AND ETHYLENE
ON Pd( 111)
and L.L. KESMODEL
of Physics,
Indiana
University, Bloomington, Indiana
28 June 1982; accepted
for publication
19 October
47405,
USA
1982
The thermal evolution of acetylene and ethylene and their deuterated counterparts on a palladium (I 11) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150-500 K. Analysis of the vibrational spectra indicates that chemisorbed acetylene evolves at 300 K in the presence of surface hydrogen to mainly ethylidyne, SC-CH,, and a small amount of residual acetylene. Spectra obtained with and without preadsorbed intermediate in the reaction. Chemisorbed ethylene hydrogen provide evidence for a ,’ C=CH, also evolves to ethylidyne after heating from 150 to 300 K but much of the ethylene desorbs. The high temperature (400-500 K) behavior of C,H, and CsH, involves formation of a CH species. Although a small amount of the CH species may be formed from the dehydrogenation of ethyhdyne, it is found that carbon-carbon bond scission of acetylene near 400 K is the dominant mechanism in CH formation.
1. Introduction In this paper we report and interpret fairly extensive high-resolution electron energy loss (EELS) spectra concerning the thermal evolution of acetylene (C,H,) and ethylene (C,H,) on a Pd(l11) surface in the temperature range 150-500 K. In earlier work we have discussed the chemisorption of C,H, [l] and C,H, [2] at 150 K as well as the adsorption and reaction of C,H, at 300 K [3]. Here we combine and extend these results to a much larger temperature range. These studies were partially motivated by a desire to extend earlier work on the closely related metals nickel, platinum and rhodium and to perhaps identify general trends. EELS studies of C,H, and C,H, on Ni( 111) and Pt( 111) [4-81 have been carried out by Ibach, Demuth and their colleagues, whereas Rh( 111) was studied by Dubois, Castner and Somojai [9]. A particularly interesting general result to emerge has been the formation of a rather stable hydrocarbon near room temperature from both C,H, and C, H, in the presence of hydrogen on the (111) surfaces of platinum, palladium and rhodium but not on nickel. The first EELS spectra of this hydrocarbon species was obtained by Ibach and co-workers [6] who identified the adsorbate as ethylidene (> CH-CH,). However, a subsequent LEED analysis and rein0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(lI1)
69
terpretation of the vibrational spectra was given by Kesmodel, Dubois and Somorjai [lo] who proposed ethylidyne (fC-CH,) as the stable species. A crucial aspect of the latter work was the proposal that the organometallic cluster CH,CCo,(CO), be used as a model compound in interpreting the EELS spectra. In fact a refined vibrational analysis of this cobalt compound by Sheppard and colleagues [ll] has recently served to strengthen the ethylidyne interpretation. This model has also gained support from theoretical molecular orbital studies [ 12-141 employing extended Hiickel calculations. Ibach has recently endorsed the ethylidyne model and has reviewed the evidence for the formation of this species on Pt( 111) [ 151. In the present work the formation of ethylidyne on Pd(ll1) near 300 K also plays a major role. The formation of this species from room temperature C,H, adsorption Pd(ll1) was reported earlier [3]. Here, we also find that C,H, in the presence of surface hydrogen evolves near 300 K to mainly ethylidyne. However, in general we find that the thermal evolution of both C,H, and C,H, on Pd( 111) are characterized at various temperatures by relatively complex EELS spectra which show mixed phases of reacted and unreacted molecules and intermediate species. Warming chemisorbed C,H, from 150 to 300 K, for example, results in a mixed phase of low temperature acetylenic species, ethylidyne and a CCH, intermediate. The high temperature (400-500 K) behavior of the molecules involves formation of surface CH species, which have also been seen on Ni, Pt and Rh [4,15,16]. It appears that on Pd(ll1) the CH species is formed predominantly from carbon-carbon bond scission of acetylene near 400 K rather than from ethylidyne dehydrogenation.
2. Experimental Vibrational spectra were obtained using the high-resolution electron spectrometer described elsewhere [17]. Resolution (FWHM) was maintained at 75-90 cm-i throughout the entire frequency range investigated (o-3300 cm-‘). Data acquisition was accomplished with signal averaging by a LSI-11 microcomputer. The spectrometer is housed in an ion-pumped stainless-steel vacuum chamber (base pressure - lo- lo Torr) which is also equipped for ion-sputtering, LEED, AES and quadrupole mass spectroscopy. The Pd( 111) sample was oriented to within k f ’ and polished as described previously [3]. Routine cleaning involved ion-bombardment and annealing to - 6OO’C. AES was used to check surface cleanliness. A tungsten filament mounted behind the sample was used for heating, and the temperature was monitored with a chromel-alumel thermocouple mounted next to the crystal on the sample holder. In order to assure that the crystal surface reached the desired temperature in an experiment, the thermocouple reading was held constant for 5 min before cooling down.
70
J.A. Gates, L.L. Kesmodel / Acetylene and ethylene on Pd(l I I)
Gases used were purified C,H, (99.6%) and CZH, (99.5%) from Matheson and C,D, and C,D, from Merck and Co. A dry ice-cooled trap was used to eliminate any acetone impurity in the acetylene. Purity was checked using the mass spectrometer. Dosing was accomplished through a leak valve into the main vacuum chamber. Pressure readings during acetylene and ethylene, exposures were divided by 2.0 and 2.3, respectively, according to Varian ionization gauge sensitivity tables.
3. Results and interpretation 3.1. 1.50-300 K In this section we will present results concerning the temperature dependent behavior of acetylene, ethylene, and their deuterated counterparts adsorbed on Pd( 111) in the range 150-300 K. For acetylene adsorption, the low temperature species partially evolves into a different species upon heating above 200 K. If surface hydrogen is available, the dominant species at 250 K is ethylidyne (fC-CH,), the same species as previously identified on Pd( 111) after room
Pd (1111+3L
C,H,
150K
0
600
1600
2400
3200
Fig. 1. Vibrational spectra obtained during the conversion of chemisorbed heated from 150 to 250 and 300 K. Scale changes are given in absolute necessarily relate to intensity relative to the elastic beam [19].
acetylene on Pd( 111) intensity and do not
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene an Pd(l I I)
cooled
71
in IO-7 T 0,
300K
,‘I
0
I
800
I
I
I
I
1600 2400 energy loss (cm*‘1
I
I
320C
Fig. 2. Vibrational spectra obtained during the conversion of chemisorbed deuterated acetylene on Pd( 111) heated from 150 to 250 and 300 K. Before adsorption, the sample was cooled in a background of IO-’ Torr deuterium. Scale changes are given in absolute intensity and do not necessarily relate to intensity relative to the elastic beam [ 191.
temperature exposure to ethylene [3]. Chemisorbed ethylene at 150 K mostly desorbs but partially transforms at about 300 K to the ethylidyne species. 3.1.1. Acetylene We will first briefly review the low temperature species formed during acetylene adsorption on Pd( 111). A spectrum obtained after a 3 L exposure to C,H, (C,D,) at 150 K is shown at the bottom of fig. 1 (fig. 2). The mode frequencies and assignments were determined elsewhere [ 11. We note here that the carbon-carbon stretching mode at 1402 cm-’ and the carbon-hydrogen stretching mode at 2988 cm-’ imply a C-C bond strength between a double and a single bond. The surface geometry of this species has been postulated to be close to a three-fold site with the C-C axis parallel to the surface. Fig. 1 shows the transformation of the vibrational spectrum with temperature [ 191. The clean surface was exposed to 3 L (10m6 Torr s) acetylene at 150 K and heated successively to 250 and 300 K for 5 min. All spectra were
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(Il I)
Pd(lll) cooled in IO-‘T 3L C2H2 150K Eo=2.9eV
1332
I-,
-2650 A
‘,I
0
I
I
800 energy
I
1600 loss
I
2400
I
I
3200
km-‘)
Fig. 3. Transformation of the vibrational spectrum of chemisorbed acetylene on Pd( 1II) heated from 150 to 250 and 300 K. The clean surface was cooled to 150 K in a background of lo-’ Torr hydrogen before C,H, adsorption. Scale changes are given in absolute intensity and do not necessarily relate to intensity relative to the elastic beam [ 191.
recorded after cooling the sample to 150 K. In an initial attempt to heat the low temperature deuterated analog, there appeared to be a great deal of H-D exchange (with residual surface hydrogen) as evidenced by a very large number of peaks and the presence of a peak in the C-H stretching region near 2900 cm -I. We were able to eliminate this problem by cooling the sample in a background of lo-’ Torr Dz (- 45 min) after cleaning. Results are illustrated in fig. 2. In fig. 3 we present analogous spectra for C,H, adsorption obtained of by following the above procedure using H,. We note here the preadsorption hydrogen does not significantly change the vibrational mode frequencies of the low temperature species (bottom of fig. 3). The peaks at 677, 870 and 2988 cm-’ have increased intensity and there is no change in the mode at 1402 cm-‘. From the similarity between the 300 K spectrum in fig. 1 and the 250 K spectrum in fig. 3, it is clear that preadsorbed hydrogen aids the transforma-
13
J.A. Gales, L.L. Kesmodel / Acetylene and ethylene on Pd(l1 I)
Table I Frequencies (cm- ‘) and assignments of vibrational modes observed for C,H, on Pd( 11 I) at 150, 250 and 300 K; corresponding values from C,D, adsorption are shown in parentheses, with isotope shift ratios below; acetylene was adsorbed at 150 K; values are shown for adsorption onto both clean and hydrogen-covered (H,/C,H,) surfaces; where values are not given, the corresponding mode was not observed (only high intensity modes have been tabulated) Assignment
150 K
250 K
300K
C,H, C2H2
Acetylene
v,-~
intermediate
Ethylidyne v,--, and CCH, mtermedrate
Ethylidyne
v,-~~
2986 2941
vcH,
2921
(2206) 1.34
vcu, 1.33
vCH/vCD
intermediate
vcc
1437
kHivCD
Acetylene
vcc
vCH/yCD
Ethylidyne
Ethyhdyne
8,,,(sym)
1314
1439
1333
1333
(988) 1.35 a,-c
1093 (1127) 0.97
vCH/vCD
Acetylene pc,,(asym) and ethyhdyne pCH, a)
(621)
vCH/vCD
1.40
Acetylene
1443 (1372) I .05
p&sym)
(Z) _
1402 (1359) 1.03
vCH/vCD
vCH/vCD
H,/C,H,
_
2988
vCH/vCD
CCH,
C2H2
(2249) 1.33
vCH/vCD
CCH,
H2/C2H2
870
673 (511) 1.32
861
885
1098
875
(-800, 657) 1.2-1.3 703
714 (505) 1.41
1332 (986) 1.35 1098 (1126) 0.98 878 (749, 645) 1.2-1.3
714
127 (513) 1.42
‘) This peak is broad at or above 250 K indicating the presence of two modes. The low frequency mode ( - 840 at 300 K) is a bending mode of the acetylenic species. Later in this paper, the high frequency component will be assigned to a rocking mode of the ethylidyne species (see table 3).
74
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(l I I)
tion of adsorbed acetylene. If C,H, is adsorbed on a clean surface and then heated in a background of H, or exposed to H, at 250 or 300 K results were similar to those without any hydrogen. This implies that exposure to atomic (but not molecular) hydrogen at 300 K is necessary for the formation of the ethylidyne species which requires excess hydrogen relative to acetylene. In our case, hydrogen is an impurity on the sample as evidenced by a M-H vibrational mode at - 450 cm-’ in spectra obtained after cleaning and cooling the sample. Therefore even without adding hydrogen we do observe some transformation to the species requiring hydrogen. In the following paragraphs we will assign and identify the surface species associated with all major peaks of figs. l-3. We have found that at 250 and 300 K three different phases coexist on the surface. Our results for the thermal evolution of the low temperature species are summarized in table 1. Here we show vibrational frequencies along with species assignments and deuteration ratios. Three peaks (2900, 1332 and 1098
r
1
Pd(lll)+3L
C2H4
150K E,=Z
Pd(llll E,=2 9eV
9eV
co
150
I
800 energy
/
1
1
2400 1600 1055 (cm-‘1
1
1
3200
_
: 0
Fig. 4. Comparison of vibrational spectra of the room temperature species on Pd( 11 I) resulting from ethylene and acetylene adsorption. Ethylene was adsorbed and the spectrum recorded at 300 K. Acetylene was adsorbed at 150 K.on a surface cooled in a background of IO-’ Torr hydrogen. The surface species was heated to 300 K and then cooled to 150 K to record the spectrum. Scale changes are given in absolute intensity and do not necessarily relate to intensity relative to the elastic beam [ 191.
[ 191.
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(l I I)
15
species for four reasons. First, these cm- ‘) were assigned to the ethylidyne peaks have their maximum relative intensities at higher temperatures where ethylidyne is known to form from room temperature ethylene adsorption on Pd( 111). The relative intensities of these peaks are larger on samples with preadsorbed hydrogen (fig. 3). The corresponding peaks in the deuterated spectra (middle and top of fig. 2) at 986, 1126 and 2183 cm-’ agree well with the ethylidyne assignment. Finally, from a comparison of the ethylidyne spectra obtained from ethylene and the top spectrum of fig. 3 shown in fig. 4 these three peaks correspond quite closely. Differences include the intensity of the peaks at 1332 and 2900 cm-’ and the frequency difference between 1098 and 1080 cm-‘. Modes below 1000 cm-’ are assigned to different species in the two systems. Also, ethylidyne is much harder to form from ethylene than from acetylene. As discussed below, the low temperature C,H, species mostly desorbs when heated and large exposures are required at room temperature. That there are probably fewer ethylidyne molecules resulting from ethylene adsorption is also supported by LEED data. The acetylene-derived system forms a clear (0 x fi)R30° pattern whereas this same pattern is much more diffuse for adsorbed ethylene. We conclude that the apparent discrepancies in the three “ethylidyne peaks” in fig. 4 can easily be explained by differences in the molecular environment. As a final note, the increased intensity of the SCH, mode at 1334 cm-’ in the acetylene-derived system allows for the observation of a Fermi-resonance enhanced overtone at 2650 cm-’ in fig. 4. Peaks at - 1437 and 2986 cm-’ are largest in the 250 K spectrum of fig. 1. Since these peaks are diminished in spectra obtained with preadsorbed hydrogen (fig. 3) they are assigned to an intermediate species which has fewer than three hydrogen atoms per molecule. Adsorption of C,D, and subsequent heating to 250 K results in the spectrum at the center of fig. 2. Given previous assignments to the ethylidyne species, the peak at 1372 cm-’ must correspond to that at 1443 cm- ’ for C,H,, making it a C-C stretching vibration. A vcn value of 2986 cm-’ confirms that this intermediate species is nearly sp* hybridized. We will associate this intermediate with a species of the form CCH, below. The assignment of observed peaks below 1000 cm-’ was more complicated than the others. We have concluded that the low temperature mode at 673 cm -’ (bottom of fig. 1) shifts rather continuously up in frequency and down in intensity with increasing ethylidyne population. Spectra obtained after heating the low temperature species to 200 K show a peak between 673 cm-’ at 150 K and 703 cm-’ at 250 K in both frequency (686 cm-‘) and intensity. The analogous mode (5 11 cm-’ at 150 K) in the deuterated species decreases in intensity but remains at about the same frequency with increasing temperature. Since this mode has been identified as one which is excited via the long-range dipole interaction, we can estimate relative amounts of the low temperature species from changes in its intensity relative to the elastic peak. From this, we
76
J.A. Gates, L.L. Kesmodel / Acetylene and ethylene on Pd(lll)
conclude that about one-third of the saturation coverage (0 = 0.33 estimated from LEED data [l]), or about 10% of a monolayer of the low temperature acetylenic species persists at 300 K. Finally we turn to the peaks appearing near 870 cm-’ in the 250 and 300 K spectra. These peaks become very broad at 300 K without and at 250 K with preadsorbed hydrogen, indicating the existence of two peaks. Without prior exposure to H, the frequency decreases continuously until the sample is heated above 250 K. We believe that this is a shift in the low temperature mode due to the changing environment on the surface. The frequency suddenly increases at the same temperature at which the peak broadens. We have approximately fit two gaussian peaks into the broadened peak at 300 K giving frequencies of 842 and 923 cm-‘. The former value agrees well with an extrapolated frequency versus temperature curve obtained from lower temperature spectra before broadening. As discussed in the next section, the 923 cm- ’ peak can be related to ethylidyne formation. Corresponding peaks in the deuterated spectra are at 650 and 749 cm-’ (fig. 2). Since the latter peak appears only at 300 K (although it may be retated to the 250 K peak at - 800 cm- ‘, which was hard to locate accurately in our spectra) we correlate it with the high frequency component ( - 920 cm-‘) of the 870 cm-’ doublet of the C,H, + Pd( 111) system. The peak at - 650 cm-’ then corresponds to the low temperature species. 3.1.2. Ethylene The thermal evolution of low temperature chemisorbed ethylene in the range 150-300 K is much different than that of acetylene although the dominant room temperature species is the same in both cases. A typical low temperature spectrum is shown at the bottom of fig. 5. Frequency values and assignments have been determined previously. The values of vcc (1502 cm- ‘) and vc- (2996 cm- ‘) imply that the C-C bond remains close to a double bond in the adsorbed state. The observed +n softening has led to a proposed surface geometry involving a tilted carbon-carbon axis 121. The top spectrum of fig. 4 is typical of those obtained after room temperature exposure of Pd( 1I 1) to ethylene. Modes at 1080, 1334 and 2900 cm-’ have been associated with the ethylidyne species (x-CH,). The peak at 780 cm-’ is not always present at 300 K. In the next section we will assign this mode to a higher temperature species. There has been some uncertainty as to whether the peak at 914 cm-’ belongs to ethylidyne [3]. We will also discuss this question in a later section. Heating the low temperature species to 300 K results in the top spectrum of fig. 5. Most of the chemisorbed molecules desorb as seen by the decreased intensity at 911 cm-’ and the increased intensity of the CO impurity peak. The appearance of a peak at 1334 cm- ’ implies that some of the remaining molecules are transformed into ethylidyne. We have heated chemisorbed
J.A. Gates, L.L. Kesmodel / Acetylene and ethylene on Pd(ll1)
Pd (111)+3L
II
C2H2
500K
430K
I
0
600 energy
I
1600
loss
I
2400
I
I
3200
I
(cm-‘)
Fig. 6. Transformation of chemisorbed acetylene on Pd(ll1) from 300 to 500 K. Spectra were all obtained at 150 K. Scale changes are given in absolute intensity and do not necessarily relate to intensity relative to the elastic beam [19].
ethylene to temperatures between 150 and 300 K, and have found no evidence for an intermediate species in this case. 3.2. 300-500
K
Above 300 K we find that yet another phase is formed on the surface. We have heated the acetylenic species formed from adsorption of acetylene at 150 K as well as the room temperature species formed from exposure to ethylene to 500 K. In both cases, we identify the surface species at 500 K as CH. In this section we will discuss results leading to this assignment. Our data also reveal information concerning the possible mechanism in the formation of the CH species.
78
J.A. Gales, L.L.
I
Kesmodel / Acetylene and ethylene on Pd(l I I)
1
I
,
I
792 Pd(lll) cooled in IO-‘T 3L C2H, 150 K Eo=2.9eV
x250
H,
500
0
800 energy
1600 loss
2400
K
3200
(cm-‘)
Fig. 7. Vibrational spectra of chemisorbed acetylene on Pd( 111)heated to 300,400 and 500 K. The clean sample was cooled in lo-’ Torr hydrogen before C,H, exposure. All spectra were recorded at 150 K. Scale changes are given in absolute intensity and are not necessarily related to intensity relative to the elastic beam [19].
Table 2 Comparison of frequencies (cm- ‘) assigned to the CH species on Pd( 11 l), Ni( 11 l), Pt( 11 l), and Rh( 111) with the cluster compound (CO),Co,(CH); corresponding modes in the deuterated species are shown in parentheses Assignment
(CQXo,(CW VII
Pd(lll)
Ni(ll1)
Pt(ll1)
Rh(ll1)
141
[201
1161
“CH
3041
3002
2980
(QD)
(2258)
(2242)
(2160)
6 (G)
850
162
790
(680)
(548)
(550)
- 3100
3025 (2260)
850 _
770 (545)
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(I 1 I)
Pd (Ill) cooled 3L
in IO-‘T
79
D2
C,Dz
150K E,=2
9eV
2242
0
600 energy
1600 loss
2400
3200
(cm-‘)
Fig. 8. Vibrational spectra of chemisorbed deuterated acetylene on Pd(l11) heated to 300, 400 and 500 K. The clean sample was cooled in lo-’ Torr deuterium before C,D, exposure. Scale changes are given in absolute intensity and are not necessarily related to intensity relative to the elastic beam [ 191.
3.2. I. Acetylene Heating Pd(ll1) with chemisorbed acetylene to 300, 400, 430 and 500 K produces spectra as shown in fig. 6 [19]. To obtain spectra as in fig. 7 we followed the same procedure with acetylene adsorbed at 150 K after the clean crystal was cooled in hydrogen. Results using C,D, on a sample cooled in D, are shown in fig. 8. The bottom spectra of figs. 6-8 are reproduced from figs. 1-3, respectively. Note that there is nearly twice as much of the ethylidyne species present at 300 K in the system with preadsorbed hydrogen as without (bottom of figs. 6 and 7). The ethylidyne species is present up to about 400 K. As shown in fig. 6, it has already disappeared by 430 K. At 500 K, there are only two major peaks 762 and 3002 cm-’ (top of figs. 6 and 7). We identify these as vibrational modes of a CH species similar to that observed on Ni(ll1) [4] and Pt(ll1) [15,20] under similar conditions. We have obtained several spectra, both onand off-specular, to confirm that there are no vibrational modes indicative of another species. In particular, we found no peak in the 1300-1450 cm-’ region or in the 900-1050 cm-’ region for the deuterated species which would be
J. A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(Il I)
Pd(lll)+2OL 300K E,=2.9eV
C2H4
300K
i 0
,
,
I
800 energy
,
1600 loss
I
f
2400
,
,
3xX
(cm-‘)
Fig. 9. Thermal evolution of the species formed from room temperature ethylene adsorption on Pd( 1I 1) to 400 and 500 K. All spectra were obtained at 300 K. Scale changes are given in absolute intensity and are not necessarily related to intensity relative to the elastic beam [ 191.
assigned to the scissor mode of a CH, group. There is also no mode which can be assigned to a C-C stretching vibration. The assignment of 762 cm- ’ to S,, in 500 K spectra is in agreement with results on Nit11 1) [4], Pt( 111) [ 15,20], Rh(ll1) [16], and with the cluster compound (CO)~(Co)~(CH) [21] as shown in table 2. From our off-specular data we have determined that the peak at 762 cm-’ is dipole-enhanced. This means that this species does not have the high C& symmetry found in the cluster compound. But a range of other geometries, including some with a perpendicular CH axis are consistent with our data. There are several interesting features of spectra obtained after heating to 400 K. Representative examples are shown in the centers of figs. 6, 7 and 8. The CH bending mode has appeared at 749 cm-’ with (753 cm-’ without) preadsorbed hydrogen and at 534 cm-’ in the deuterated spectrum. The peak in the CH stretching region appears to be a composite of the ZS.-~,at 2900 cm-’ and the +u at 3002 cm -I. There is a shoulder on the Sc, peak in the vicinity of 860 cm-’ which appears only at 400 K. This could be a CH bending mode resulting from CH in a different bonding geometry. It seems unlikely that this
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(l I I)
81
is the asymmetric bending mode for the same species since it does not appear at 500 K with on- or off-specular scattering. We do not observe any other modes unique to the 400 K spectrum which would imply the existence of a species other than CH. Finally, based on trends in frequency and relative intensity, we do not connect this shoulder with the broad peak near 870 cm-’ in the 300 K spectrum. The CCH, (CCD,) peaks at 1327 cm-’ (982 cm-‘) and at 1098 cm-’ (1126 cm-‘) in the 400 K spectra have about the same intensity relative to the elastic peak as at 300 K. Since these are dipole-enhanced modes, we infer that the ethylidyne species is stable up to 400 K. We note also that the peak at - 1439 cm-’ belonging to the intermediate CCH, species discussed above is also still present at 400 K if the clean sample was not cooled in a hydrogen background. Therefore, the CH species present at 400 K must have been formed primarily from the low temperature HCCH species rather than ethylidyne. Further support for this view was obtained from room temperature adsorption of C,H, onto clean Pd( 111). We found a prominent mode near 740 cm-’ with a shoulder around 860 cm- ‘. In addition, there are somewhat smaller ethylidyne modes at 1328 and 1100 cm-’ with evidence of CCH, in a shoulder on the 1328 cm- ’ peak. There is no indicaton of the low temperature species although a small amount could be masked by the S,, peak at 740 cm-‘. We conclude that in the case of room temperature adsorption, most of the acetylene dissociates but that some of it uses excess hydrogen to form ethylidyne. Heating the low temperature species from 400 to 500 K results in only a small of increase in the intensity of the S,, mode and the complete disappearance the CCH, species. Our postulate that the CH species is formed from the low temperature species will be further discussed and supported in connection with the ethylene-derived system below. 3.2.2. Ethylene The behavior of adsorbed ethylene above 300 K also indicates that the ethylidyne species is not primarily responsible for the formation of the high temperature CH species. We have already mentioned that heating low temperature chemisorbed ethylene causes most of it to desorb with partial transformation of the remaining adsorbate to the ethylidyne species (fig. 4). We obtained the spectra of fig. 9 by exposing the clean surface to C,H, at room temperature and then heating to 400 and 500 K. The familiar ethylidyne peaks at 1080, 1334 and 2900 cm-’ as well as the broad peak at 914 cm-’ are readily apparent at the bottom of fig. 9. The peak at 780 cm-’ has variable intensity relative to the other peaks. It and a very weak mode in the vcu region near - 2960 cm-’ are the only peaks visible (except for the CO impurity peak) after heating to 400 K. Heating to 500 K leaves only a weak mode near 796 cm-‘. Judging from the relative intensity of the 1334 cm-’ mode at the bottom of
82
J.A.
Gale*, L. L. Kesmodel / Acetylene and ethylene on Pd(l I I)
fig. 9, approximately half as much ethylidyne is present as from heating the low temperature acetylenic species to 300 K (bottom of figs. 7 and 8). As illustrated by comparison of figs. 7 and 8 with fig. 9, the CH species modes are much weaker in the ethylene-derived system than in the acetylene-derived system. The difference is quite dramatic at 500 K. We should, of course, consider that spectra from the two systems were obtained at different temperatures. If C,H, is adsorbed and a spectrum acquired at 300 K as for C2H, in fig. 9, the amounts of s--CH, are approximately the same. The difference is in the relative amounts of the high temperature CH species. For the acetylenederived system, the a,-., mode at 740 cm-’ is the largest peak in the spectrum which is clearly not the case for C,H, adsorption at the bottom of fig. 9. We are led to conclude, then, that the formation of the CH species is different after adsorption of the different molecules even though ethylidyne forms with approximately equal probabilities in both cases. Therefore, the CH species cannot primarily be formed from ethylidyne and must occur upon adsorption (at 300 K) or by decomposition of the few temperature ~he~sorbed species.
4. Discussion Our results yield information concerning the reaction of C,H, and C,H, on Pd( 111) at temperatures above 150 K. In the last section, we identified three species, other than the low temperature phase, present in one or both of these
Table 3 Comparison of modes assigned to ethylidyne formed on Pd( 111)from ethylene adsorption with corresponding modes in the cluster compound (CO)&o,(CCH,) Assignment
“CH,
%kI/%D &tJsym) %%i/%Il
pee QH/4D
(CO),Co,(CCH,) W%Co,(CCW) 2930 (2192) 1.34
2888 (-) -
1356 i 1002) 1.35 1163 (1182) 0.98
C,H, GD,)
Hz/C,H, @‘Z&W
2900 (2181) 1.33
2900 (2183) 1.33
1334
1332
(-)
(986) 1.35
1080 (1120) 0.96
1098 (1126) 0.98
PCH,
loo4
914
923 a)
(-)
ycn/vco
(828) 1.21
(749) I .23
‘) Approximated
by fitting two curves into the broad
peak
- 870 cm-’
at 300 K.
and acetylene
[ 1I]
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(1 I I)
83
systems. An intermediate CCH, species formed from the low temperature acetylenic phase above 200 K will be further discussed here in terms of its role in ethylidyne formation. We will also present new evidence related to the assignments of the ethylidyne modes on Pd( 111). The difference between the ability of the ethylene- and acetylene-derived systems to form the high temperature CH species leads to information about the origin of this species. Finally, our results will be compared with results on Ni( 111) [4,8,22], Pt( 111) [ 10,15,20], and Rh(ll1) [9]. From spectra obtained after heating chemisorbed acetylene to 250 K, we . ‘dentified a vcc mode at 1437 cm-’ and a vcu, mode at 2986 cm-’ (fig. 1) which are of reduced intensity if the clean surface was exposed to hydrogen before acetylene (fig. 3). We associated these peaks with an intermediate species containing no more than two hydrogen atoms. In view of the formation of ethylidyne in the presence of excess hydrogen, we tentatively identify this having a structure analogous to that observed in an species as CCH,, organometallic cluster containing CCH, and three osmium atoms [23]. The unsaturated nature of the carbon-carbon bond leaves it vulnerable to the addition of a hydrogen atom to form ethylidyne. This latter process occurs readily as evidenced by the dominance of the ethylidyne species at 250 K on a surface with preadsorbed hydrogen. This suggests that the palladium surface activates the transfer of hydrogen atoms from one carbon atom to the other, and that this is a step in the formation of ethylidyne [lo]. Ibach and Lehwald [6] have reported a spectrum of the species formed on Pt( 111) by heating the low temperature acetylenic species to 350 K without adding hydrogen. Although this spectrum is similar to that of the intermediate species on Pd( 111) at 250 K, information concerning either intermediate temperatures or the deuterated analog is not given for platinum. Therefore, it is difficult to make an accurate comparison with our results. The ethylidyne species forms from ethylene adsorption on Pt(ll1) [lo], Pt( 100) [24], and Rh(ll1) [9] and from acetylene adsorption on Pt(ll1) [ 151 and Rh(ll1) [9] in the presence of atomic hydrogen. We have previously reported ethylidyne on Pd( 111) resulting from room temperature adsorption of ethylene [3]. The present work with acetylene has shed new light on our earlier results. In particular, the mode at 914 cm-’ was not positively assigned. Possibly related spectral features were reported at 880 cm-’ on Rh(ll1) [9] and at 900 cm-’ on Pt( 111) [6]. The latter peak has since been shown not to be an essential part of the ethylidyne spectrum on Pt( 111) [ 151. Skinner et al. [ 1 l] reported a ,e,-u, mode in the cluster compound (CO),(Co),(CCH,) at 1004 cm- ’ but suggested that 900 cm-’ is too low to be the corresponding mode in the surface species. They proposed rather that this mode may be due to adsorption of hydrogen occurring in the conversion from C,H, to \$-CH,. Our work with acetylene transformation on Pd( 111) also indicates a mode at - 923 cm-’ associated with ethylidyne formation. In addition, we have
84
J.A. Gores, L. L.
Kesmodel / Acetylene and ethylene on Pd(l I I)
observed a corresponding mode in the deuterated species at 749 cm-‘. The then, is 1.23, in close agreement with 1.21 reported for pcH1 value of +n/~cn, in the cluster compound. We believe that a mode at 914-923 cm-’ is, indeed, a genuine feature of the ethylidyne spectrum on Pd( 111). While we cannot make this claim with absolute certainty, we have not observed any conditions under which ethylidyne is present and the peak at 914-923 cm-’ is either greatly diminished or not present. In table 3 we present the frequencies and assignments of vibrational modes associated with F-CH, in the cluster compound (CO),Co,(CCH,) and in surface species obtained from ethylene and acetylene in the presence of hydrogen on Pd( 111). We have already discussed formation of the CH species separately for C,H, and C,H, adsorption. In the former case, it was shown that this species is formed primarily from the low temperature acetylenic species at - 400 K rather than from the ethylidyne species. Above 400 K, there is some evidence, from a small increase in the Sc, mode intensity, that ethylidyne may form CH, but this is not conclusive. Room temperature adsorption of acetylene and ethylene resulted in comparable amounts of ethylidyne but the acetyleneTable 4 Comparison of the thermal evolution of acetylene and ethylene adsorbed on the (111)surfaces of Ni [S], Pd and Pt [ 10,15,20]; results are based on heating the low temperature chemisorbed species: the final high temperature decomposition on all surfaces is believed to be a carbonaceous overlayer
Ni(ll1)
Pd (111)
Cz”2
m+ C2”2
Pt (111)
C2H2
Ni(ll1)
C2H4
Pd(ll1)
Pt(ll1)
‘) Acetylene
C2”2
400K+
:C=CH2
(-H
+”
l
L
m
C2H4+
\;C-CH
fC-CH3
+
m
a C2H4
conversion
C2H4t
C2H4t
to CH on Ni(
+
+
p
CH
450K small yield
+
CH
400K small yield
l
CH
450K small yield
b
CH
C2H2
Lnnx
C2H4
400K 3 small yield
,
CHa)
C2H4 + SC-CH3
:-C-C”)
111)has been demonstrated
for high initial coverage
IS].
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(ll1)
85
derived spectrum had a much larger Sc, p eak. Heating the ethylene-derived system leaves only very weak CH modes. From these results, we conclude that the palladium surface directly breaks the carbon-carbon bond in acetylene. In the case of ethylene the conversion to CH is a more complex and less efficient process. Finally, the transformation of ethylidyne to CH gives at best a small yield. Finally, it is interesting to compare the thermal evolution of acetylene and ethylene adsorption on Ni( 111) [4,8,22], Pd( 11 l), and Pt( 111) [ 10,15,20]. We find, as shown in table 4, that our results on palladium are intermediate to nickel and platinum. Chemisorbed acetylene is stable up to 400 K on Ni( 111) [8] but converts exclusively to ethylidyne in the presence of hydrogen on Pt( 111) [6,15] by 300 K. On Pd, we observe both acetylene and ethylidyne at 300 K. Ethylene largely desorbs from all surfaces by 300 K. The remaining surface species is then C,H, on nickel [8,22], ethylidyne on platinum [IO], and ethylidyne and ethylene on palladium. Room temperature exposure of Ni( 111) to acetylene results in two surface phases [25]. The second phase, identified as CH, forms after saturation of the initial chemisorbed C,H, phase. Heating to 400 K causes a complete transformation to CH [8]. EELS [20] has shown CH resulting from ethylidyne on Pt at 450 K but with a small yield. As discussed above, CH appears on Pd( 111) near 400 K. Most of this species is formed from C,H, with at best a small fraction from \+--CH,, consistent with both nickel and platinum.
Acknowledgements This work was supported by the Office of Naval Research. J.A.G. is pleased to acknowledge a fellowship from the American Vacuum Society.
References [I] [2] [3] [4] [5] [6] [7] [S] [9] [IO] [I l]
J.A. Gates and L.L. Kesmodel, J. Chem. Phys. 76 (1982) 4281. J.A. Gates and L.L. Kesmodel, Surface Sci. 120 (1982) L461. L.L. Kesmodel and J.A. Gates, Surface Sci. 111 (1981) L747. J.E. Demuth and H. Ibach, Surface Sci. 78 (1978) L238. J.E. Demuth and H. Ibach, Surface Sci. 85 (1979) 365. H. Ibach and S. Lehwald, J. Vacuum Sci. Technol. 15 (1978) 407; H. Ibach, H. Hopster and B. Sexton, Appl. Surface Sci. 1 (1977) 1. H. Ibach and S. Lehwald, J. Vacuum. Sci. Technol. 18 (1981) 625. S. Lehwald and H. Ibach, Surface Sci. 89 (1979) 425. L.H. Dubois, D.G. Castner and G.A. Somojai, J. Chem. Phys. 72 (1980) 5234. L.L. Kesmodel, L.H. Dubois and G.A. Somojai, Chem. Phys. Letters 56 (1978) 267; J. Chem. Phys. 70 (1979) 2180. P. Skinner, M.W. Howard, I.A. Oxton, S.F.A. Kettle, D.B. Powell and N. Sheppard, J. Chem. Sot. Faraday Trans. II, 77 (1981) 1203.
86 [ 121 [ 131 [ 141 [15] [16]
[ 171 [18] [ 191
[20] [Zl] [22] [23] [24] [25]
J.A. Gates, L.L. Kesmodel / Acetylene and ethylene on Pd(l II) R.C. Baetzold, Chem. Phys. 38 (1979) 313. A. Gavezotti and M. Simonetta, Surface Sci. 99 (1980) 453. A.B. Anderson and A.T. Hubbard, Surface Sci. 99 (1980) 384. H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982) p. 326. L.H. Dubois and G.A. Somorjai, in: Am. Chem. Sot. Symp. Ser.; Eds. A.T. Bell and M.L. Hair (Am. Chem. Sot., Washington, DC, 1980). L.L. Kesmodel, J.A. Gates and Y.W. Chung, Phys. Rev. B23 (1981) 489. J.E. Demuth, H. Ibach and S. Lehwald, Phys. Rev. Letters 40 (1978) 1044. It should be pointed out that scale changes given in figs. l-9 are based on absolute intensity. The elastic peak shown is that of the lowest spectrum in each case. These scale changes may be misleading since the surface reflectivity and, therefore, the elastic beam count rate are not constant as a function of the adsorbate. In this paper, all intensities referred to are those relative to the elastic beam. We have made the assumption that the relative intensity of a dipole-enhanced mode is roughly proportional to coverage. A.M. Baro and H. lbach, J. Chem. Phys. 74 (1981) 4194. M.W. Howard, S.F. Kettle, I.A. Oxton, D.B. Powell, N. Sheppard and P. Skinner, J. Chem. Sot. Faraday Trans. II, 77 (198 1) 397. J.E. Demuth, Surface Sci. 76 (1978) L603. A.J. Deeming and M. Underhill, JCS Chem. Commun. (1973) 277; JCS Dalton Trans. (1974) 1415. H. Ibach, in: Proc. Conf. on Vibrations in Adsorbed Layers, Jtilich, 1978. J.E. Demuth, Surface Sci. 69 (1977) 365.
Surface Science 124 (1983) 68-86 North-Holland Publishing Company
THERMAL J.A. GATES Department
Received
EVOLUTION
OF ACETYLENE
AND ETHYLENE
ON Pd( 111)
and L.L. KESMODEL
of Physics,
Indiana
University, Bloomington, Indiana
28 June 1982; accepted
for publication
19 October
47405,
USA
1982
The thermal evolution of acetylene and ethylene and their deuterated counterparts on a palladium (I 11) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150-500 K. Analysis of the vibrational spectra indicates that chemisorbed acetylene evolves at 300 K in the presence of surface hydrogen to mainly ethylidyne, SC-CH,, and a small amount of residual acetylene. Spectra obtained with and without preadsorbed intermediate in the reaction. Chemisorbed ethylene hydrogen provide evidence for a ,’ C=CH, also evolves to ethylidyne after heating from 150 to 300 K but much of the ethylene desorbs. The high temperature (400-500 K) behavior of C,H, and CsH, involves formation of a CH species. Although a small amount of the CH species may be formed from the dehydrogenation of ethyhdyne, it is found that carbon-carbon bond scission of acetylene near 400 K is the dominant mechanism in CH formation.
1. Introduction In this paper we report and interpret fairly extensive high-resolution electron energy loss (EELS) spectra concerning the thermal evolution of acetylene (C,H,) and ethylene (C,H,) on a Pd(l11) surface in the temperature range 150-500 K. In earlier work we have discussed the chemisorption of C,H, [l] and C,H, [2] at 150 K as well as the adsorption and reaction of C,H, at 300 K [3]. Here we combine and extend these results to a much larger temperature range. These studies were partially motivated by a desire to extend earlier work on the closely related metals nickel, platinum and rhodium and to perhaps identify general trends. EELS studies of C,H, and C,H, on Ni( 111) and Pt( 111) [4-81 have been carried out by Ibach, Demuth and their colleagues, whereas Rh( 111) was studied by Dubois, Castner and Somojai [9]. A particularly interesting general result to emerge has been the formation of a rather stable hydrocarbon near room temperature from both C,H, and C, H, in the presence of hydrogen on the (111) surfaces of platinum, palladium and rhodium but not on nickel. The first EELS spectra of this hydrocarbon species was obtained by Ibach and co-workers [6] who identified the adsorbate as ethylidene (> CH-CH,). However, a subsequent LEED analysis and rein0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(lI1)
69
terpretation of the vibrational spectra was given by Kesmodel, Dubois and Somorjai [lo] who proposed ethylidyne (fC-CH,) as the stable species. A crucial aspect of the latter work was the proposal that the organometallic cluster CH,CCo,(CO), be used as a model compound in interpreting the EELS spectra. In fact a refined vibrational analysis of this cobalt compound by Sheppard and colleagues [ll] has recently served to strengthen the ethylidyne interpretation. This model has also gained support from theoretical molecular orbital studies [ 12-141 employing extended Hiickel calculations. Ibach has recently endorsed the ethylidyne model and has reviewed the evidence for the formation of this species on Pt( 111) [ 151. In the present work the formation of ethylidyne on Pd(ll1) near 300 K also plays a major role. The formation of this species from room temperature C,H, adsorption Pd(ll1) was reported earlier [3]. Here, we also find that C,H, in the presence of surface hydrogen evolves near 300 K to mainly ethylidyne. However, in general we find that the thermal evolution of both C,H, and C,H, on Pd( 111) are characterized at various temperatures by relatively complex EELS spectra which show mixed phases of reacted and unreacted molecules and intermediate species. Warming chemisorbed C,H, from 150 to 300 K, for example, results in a mixed phase of low temperature acetylenic species, ethylidyne and a CCH, intermediate. The high temperature (400-500 K) behavior of the molecules involves formation of surface CH species, which have also been seen on Ni, Pt and Rh [4,15,16]. It appears that on Pd(ll1) the CH species is formed predominantly from carbon-carbon bond scission of acetylene near 400 K rather than from ethylidyne dehydrogenation.
2. Experimental Vibrational spectra were obtained using the high-resolution electron spectrometer described elsewhere [17]. Resolution (FWHM) was maintained at 75-90 cm-i throughout the entire frequency range investigated (o-3300 cm-‘). Data acquisition was accomplished with signal averaging by a LSI-11 microcomputer. The spectrometer is housed in an ion-pumped stainless-steel vacuum chamber (base pressure - lo- lo Torr) which is also equipped for ion-sputtering, LEED, AES and quadrupole mass spectroscopy. The Pd( 111) sample was oriented to within k f ’ and polished as described previously [3]. Routine cleaning involved ion-bombardment and annealing to - 6OO’C. AES was used to check surface cleanliness. A tungsten filament mounted behind the sample was used for heating, and the temperature was monitored with a chromel-alumel thermocouple mounted next to the crystal on the sample holder. In order to assure that the crystal surface reached the desired temperature in an experiment, the thermocouple reading was held constant for 5 min before cooling down.
70
J.A. Gates, L.L. Kesmodel / Acetylene and ethylene on Pd(l I I)
Gases used were purified C,H, (99.6%) and CZH, (99.5%) from Matheson and C,D, and C,D, from Merck and Co. A dry ice-cooled trap was used to eliminate any acetone impurity in the acetylene. Purity was checked using the mass spectrometer. Dosing was accomplished through a leak valve into the main vacuum chamber. Pressure readings during acetylene and ethylene, exposures were divided by 2.0 and 2.3, respectively, according to Varian ionization gauge sensitivity tables.
3. Results and interpretation 3.1. 1.50-300 K In this section we will present results concerning the temperature dependent behavior of acetylene, ethylene, and their deuterated counterparts adsorbed on Pd( 111) in the range 150-300 K. For acetylene adsorption, the low temperature species partially evolves into a different species upon heating above 200 K. If surface hydrogen is available, the dominant species at 250 K is ethylidyne (fC-CH,), the same species as previously identified on Pd( 111) after room
Pd (1111+3L
C,H,
150K
0
600
1600
2400
3200
Fig. 1. Vibrational spectra obtained during the conversion of chemisorbed heated from 150 to 250 and 300 K. Scale changes are given in absolute necessarily relate to intensity relative to the elastic beam [19].
acetylene on Pd( 111) intensity and do not
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene an Pd(l I I)
cooled
71
in IO-7 T 0,
300K
,‘I
0
I
800
I
I
I
I
1600 2400 energy loss (cm*‘1
I
I
320C
Fig. 2. Vibrational spectra obtained during the conversion of chemisorbed deuterated acetylene on Pd( 111) heated from 150 to 250 and 300 K. Before adsorption, the sample was cooled in a background of IO-’ Torr deuterium. Scale changes are given in absolute intensity and do not necessarily relate to intensity relative to the elastic beam [ 191.
temperature exposure to ethylene [3]. Chemisorbed ethylene at 150 K mostly desorbs but partially transforms at about 300 K to the ethylidyne species. 3.1.1. Acetylene We will first briefly review the low temperature species formed during acetylene adsorption on Pd( 111). A spectrum obtained after a 3 L exposure to C,H, (C,D,) at 150 K is shown at the bottom of fig. 1 (fig. 2). The mode frequencies and assignments were determined elsewhere [ 11. We note here that the carbon-carbon stretching mode at 1402 cm-’ and the carbon-hydrogen stretching mode at 2988 cm-’ imply a C-C bond strength between a double and a single bond. The surface geometry of this species has been postulated to be close to a three-fold site with the C-C axis parallel to the surface. Fig. 1 shows the transformation of the vibrational spectrum with temperature [ 191. The clean surface was exposed to 3 L (10m6 Torr s) acetylene at 150 K and heated successively to 250 and 300 K for 5 min. All spectra were
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(Il I)
Pd(lll) cooled in IO-‘T 3L C2H2 150K Eo=2.9eV
1332
I-,
-2650 A
‘,I
0
I
I
800 energy
I
1600 loss
I
2400
I
I
3200
km-‘)
Fig. 3. Transformation of the vibrational spectrum of chemisorbed acetylene on Pd( 1II) heated from 150 to 250 and 300 K. The clean surface was cooled to 150 K in a background of lo-’ Torr hydrogen before C,H, adsorption. Scale changes are given in absolute intensity and do not necessarily relate to intensity relative to the elastic beam [ 191.
recorded after cooling the sample to 150 K. In an initial attempt to heat the low temperature deuterated analog, there appeared to be a great deal of H-D exchange (with residual surface hydrogen) as evidenced by a very large number of peaks and the presence of a peak in the C-H stretching region near 2900 cm -I. We were able to eliminate this problem by cooling the sample in a background of lo-’ Torr Dz (- 45 min) after cleaning. Results are illustrated in fig. 2. In fig. 3 we present analogous spectra for C,H, adsorption obtained of by following the above procedure using H,. We note here the preadsorption hydrogen does not significantly change the vibrational mode frequencies of the low temperature species (bottom of fig. 3). The peaks at 677, 870 and 2988 cm-’ have increased intensity and there is no change in the mode at 1402 cm-‘. From the similarity between the 300 K spectrum in fig. 1 and the 250 K spectrum in fig. 3, it is clear that preadsorbed hydrogen aids the transforma-
13
J.A. Gales, L.L. Kesmodel / Acetylene and ethylene on Pd(l1 I)
Table I Frequencies (cm- ‘) and assignments of vibrational modes observed for C,H, on Pd( 11 I) at 150, 250 and 300 K; corresponding values from C,D, adsorption are shown in parentheses, with isotope shift ratios below; acetylene was adsorbed at 150 K; values are shown for adsorption onto both clean and hydrogen-covered (H,/C,H,) surfaces; where values are not given, the corresponding mode was not observed (only high intensity modes have been tabulated) Assignment
150 K
250 K
300K
C,H, C2H2
Acetylene
v,-~
intermediate
Ethylidyne v,--, and CCH, mtermedrate
Ethylidyne
v,-~~
2986 2941
vcH,
2921
(2206) 1.34
vcu, 1.33
vCH/vCD
intermediate
vcc
1437
kHivCD
Acetylene
vcc
vCH/yCD
Ethylidyne
Ethyhdyne
8,,,(sym)
1314
1439
1333
1333
(988) 1.35 a,-c
1093 (1127) 0.97
vCH/vCD
Acetylene pc,,(asym) and ethyhdyne pCH, a)
(621)
vCH/vCD
1.40
Acetylene
1443 (1372) I .05
p&sym)
(Z) _
1402 (1359) 1.03
vCH/vCD
vCH/vCD
H,/C,H,
_
2988
vCH/vCD
CCH,
C2H2
(2249) 1.33
vCH/vCD
CCH,
H2/C2H2
870
673 (511) 1.32
861
885
1098
875
(-800, 657) 1.2-1.3 703
714 (505) 1.41
1332 (986) 1.35 1098 (1126) 0.98 878 (749, 645) 1.2-1.3
714
127 (513) 1.42
‘) This peak is broad at or above 250 K indicating the presence of two modes. The low frequency mode ( - 840 at 300 K) is a bending mode of the acetylenic species. Later in this paper, the high frequency component will be assigned to a rocking mode of the ethylidyne species (see table 3).
74
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(l I I)
tion of adsorbed acetylene. If C,H, is adsorbed on a clean surface and then heated in a background of H, or exposed to H, at 250 or 300 K results were similar to those without any hydrogen. This implies that exposure to atomic (but not molecular) hydrogen at 300 K is necessary for the formation of the ethylidyne species which requires excess hydrogen relative to acetylene. In our case, hydrogen is an impurity on the sample as evidenced by a M-H vibrational mode at - 450 cm-’ in spectra obtained after cleaning and cooling the sample. Therefore even without adding hydrogen we do observe some transformation to the species requiring hydrogen. In the following paragraphs we will assign and identify the surface species associated with all major peaks of figs. l-3. We have found that at 250 and 300 K three different phases coexist on the surface. Our results for the thermal evolution of the low temperature species are summarized in table 1. Here we show vibrational frequencies along with species assignments and deuteration ratios. Three peaks (2900, 1332 and 1098
r
1
Pd(lll)+3L
C2H4
150K E,=Z
Pd(llll E,=2 9eV
9eV
co
150
I
800 energy
/
1
1
2400 1600 1055 (cm-‘1
1
1
3200
_
: 0
Fig. 4. Comparison of vibrational spectra of the room temperature species on Pd( 11 I) resulting from ethylene and acetylene adsorption. Ethylene was adsorbed and the spectrum recorded at 300 K. Acetylene was adsorbed at 150 K.on a surface cooled in a background of IO-’ Torr hydrogen. The surface species was heated to 300 K and then cooled to 150 K to record the spectrum. Scale changes are given in absolute intensity and do not necessarily relate to intensity relative to the elastic beam [ 191.
[ 191.
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(l I I)
15
species for four reasons. First, these cm- ‘) were assigned to the ethylidyne peaks have their maximum relative intensities at higher temperatures where ethylidyne is known to form from room temperature ethylene adsorption on Pd( 111). The relative intensities of these peaks are larger on samples with preadsorbed hydrogen (fig. 3). The corresponding peaks in the deuterated spectra (middle and top of fig. 2) at 986, 1126 and 2183 cm-’ agree well with the ethylidyne assignment. Finally, from a comparison of the ethylidyne spectra obtained from ethylene and the top spectrum of fig. 3 shown in fig. 4 these three peaks correspond quite closely. Differences include the intensity of the peaks at 1332 and 2900 cm-’ and the frequency difference between 1098 and 1080 cm-‘. Modes below 1000 cm-’ are assigned to different species in the two systems. Also, ethylidyne is much harder to form from ethylene than from acetylene. As discussed below, the low temperature C,H, species mostly desorbs when heated and large exposures are required at room temperature. That there are probably fewer ethylidyne molecules resulting from ethylene adsorption is also supported by LEED data. The acetylene-derived system forms a clear (0 x fi)R30° pattern whereas this same pattern is much more diffuse for adsorbed ethylene. We conclude that the apparent discrepancies in the three “ethylidyne peaks” in fig. 4 can easily be explained by differences in the molecular environment. As a final note, the increased intensity of the SCH, mode at 1334 cm-’ in the acetylene-derived system allows for the observation of a Fermi-resonance enhanced overtone at 2650 cm-’ in fig. 4. Peaks at - 1437 and 2986 cm-’ are largest in the 250 K spectrum of fig. 1. Since these peaks are diminished in spectra obtained with preadsorbed hydrogen (fig. 3) they are assigned to an intermediate species which has fewer than three hydrogen atoms per molecule. Adsorption of C,D, and subsequent heating to 250 K results in the spectrum at the center of fig. 2. Given previous assignments to the ethylidyne species, the peak at 1372 cm-’ must correspond to that at 1443 cm- ’ for C,H,, making it a C-C stretching vibration. A vcn value of 2986 cm-’ confirms that this intermediate species is nearly sp* hybridized. We will associate this intermediate with a species of the form CCH, below. The assignment of observed peaks below 1000 cm-’ was more complicated than the others. We have concluded that the low temperature mode at 673 cm -’ (bottom of fig. 1) shifts rather continuously up in frequency and down in intensity with increasing ethylidyne population. Spectra obtained after heating the low temperature species to 200 K show a peak between 673 cm-’ at 150 K and 703 cm-’ at 250 K in both frequency (686 cm-‘) and intensity. The analogous mode (5 11 cm-’ at 150 K) in the deuterated species decreases in intensity but remains at about the same frequency with increasing temperature. Since this mode has been identified as one which is excited via the long-range dipole interaction, we can estimate relative amounts of the low temperature species from changes in its intensity relative to the elastic peak. From this, we
76
J.A. Gates, L.L. Kesmodel / Acetylene and ethylene on Pd(lll)
conclude that about one-third of the saturation coverage (0 = 0.33 estimated from LEED data [l]), or about 10% of a monolayer of the low temperature acetylenic species persists at 300 K. Finally we turn to the peaks appearing near 870 cm-’ in the 250 and 300 K spectra. These peaks become very broad at 300 K without and at 250 K with preadsorbed hydrogen, indicating the existence of two peaks. Without prior exposure to H, the frequency decreases continuously until the sample is heated above 250 K. We believe that this is a shift in the low temperature mode due to the changing environment on the surface. The frequency suddenly increases at the same temperature at which the peak broadens. We have approximately fit two gaussian peaks into the broadened peak at 300 K giving frequencies of 842 and 923 cm-‘. The former value agrees well with an extrapolated frequency versus temperature curve obtained from lower temperature spectra before broadening. As discussed in the next section, the 923 cm- ’ peak can be related to ethylidyne formation. Corresponding peaks in the deuterated spectra are at 650 and 749 cm-’ (fig. 2). Since the latter peak appears only at 300 K (although it may be retated to the 250 K peak at - 800 cm- ‘, which was hard to locate accurately in our spectra) we correlate it with the high frequency component ( - 920 cm-‘) of the 870 cm-’ doublet of the C,H, + Pd( 111) system. The peak at - 650 cm-’ then corresponds to the low temperature species. 3.1.2. Ethylene The thermal evolution of low temperature chemisorbed ethylene in the range 150-300 K is much different than that of acetylene although the dominant room temperature species is the same in both cases. A typical low temperature spectrum is shown at the bottom of fig. 5. Frequency values and assignments have been determined previously. The values of vcc (1502 cm- ‘) and vc- (2996 cm- ‘) imply that the C-C bond remains close to a double bond in the adsorbed state. The observed +n softening has led to a proposed surface geometry involving a tilted carbon-carbon axis 121. The top spectrum of fig. 4 is typical of those obtained after room temperature exposure of Pd( 1I 1) to ethylene. Modes at 1080, 1334 and 2900 cm-’ have been associated with the ethylidyne species (x-CH,). The peak at 780 cm-’ is not always present at 300 K. In the next section we will assign this mode to a higher temperature species. There has been some uncertainty as to whether the peak at 914 cm-’ belongs to ethylidyne [3]. We will also discuss this question in a later section. Heating the low temperature species to 300 K results in the top spectrum of fig. 5. Most of the chemisorbed molecules desorb as seen by the decreased intensity at 911 cm-’ and the increased intensity of the CO impurity peak. The appearance of a peak at 1334 cm- ’ implies that some of the remaining molecules are transformed into ethylidyne. We have heated chemisorbed
J.A. Gates, L.L. Kesmodel / Acetylene and ethylene on Pd(ll1)
Pd (111)+3L
II
C2H2
500K
430K
I
0
600 energy
I
1600
loss
I
2400
I
I
3200
I
(cm-‘)
Fig. 6. Transformation of chemisorbed acetylene on Pd(ll1) from 300 to 500 K. Spectra were all obtained at 150 K. Scale changes are given in absolute intensity and do not necessarily relate to intensity relative to the elastic beam [19].
ethylene to temperatures between 150 and 300 K, and have found no evidence for an intermediate species in this case. 3.2. 300-500
K
Above 300 K we find that yet another phase is formed on the surface. We have heated the acetylenic species formed from adsorption of acetylene at 150 K as well as the room temperature species formed from exposure to ethylene to 500 K. In both cases, we identify the surface species at 500 K as CH. In this section we will discuss results leading to this assignment. Our data also reveal information concerning the possible mechanism in the formation of the CH species.
78
J.A. Gales, L.L.
I
Kesmodel / Acetylene and ethylene on Pd(l I I)
1
I
,
I
792 Pd(lll) cooled in IO-‘T 3L C2H, 150 K Eo=2.9eV
x250
H,
500
0
800 energy
1600 loss
2400
K
3200
(cm-‘)
Fig. 7. Vibrational spectra of chemisorbed acetylene on Pd( 111)heated to 300,400 and 500 K. The clean sample was cooled in lo-’ Torr hydrogen before C,H, exposure. All spectra were recorded at 150 K. Scale changes are given in absolute intensity and are not necessarily related to intensity relative to the elastic beam [19].
Table 2 Comparison of frequencies (cm- ‘) assigned to the CH species on Pd( 11 l), Ni( 11 l), Pt( 11 l), and Rh( 111) with the cluster compound (CO),Co,(CH); corresponding modes in the deuterated species are shown in parentheses Assignment
(CQXo,(CW VII
Pd(lll)
Ni(ll1)
Pt(ll1)
Rh(ll1)
141
[201
1161
“CH
3041
3002
2980
(QD)
(2258)
(2242)
(2160)
6 (G)
850
162
790
(680)
(548)
(550)
- 3100
3025 (2260)
850 _
770 (545)
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(I 1 I)
Pd (Ill) cooled 3L
in IO-‘T
79
D2
C,Dz
150K E,=2
9eV
2242
0
600 energy
1600 loss
2400
3200
(cm-‘)
Fig. 8. Vibrational spectra of chemisorbed deuterated acetylene on Pd(l11) heated to 300, 400 and 500 K. The clean sample was cooled in lo-’ Torr deuterium before C,D, exposure. Scale changes are given in absolute intensity and are not necessarily related to intensity relative to the elastic beam [ 191.
3.2. I. Acetylene Heating Pd(ll1) with chemisorbed acetylene to 300, 400, 430 and 500 K produces spectra as shown in fig. 6 [19]. To obtain spectra as in fig. 7 we followed the same procedure with acetylene adsorbed at 150 K after the clean crystal was cooled in hydrogen. Results using C,D, on a sample cooled in D, are shown in fig. 8. The bottom spectra of figs. 6-8 are reproduced from figs. 1-3, respectively. Note that there is nearly twice as much of the ethylidyne species present at 300 K in the system with preadsorbed hydrogen as without (bottom of figs. 6 and 7). The ethylidyne species is present up to about 400 K. As shown in fig. 6, it has already disappeared by 430 K. At 500 K, there are only two major peaks 762 and 3002 cm-’ (top of figs. 6 and 7). We identify these as vibrational modes of a CH species similar to that observed on Ni(ll1) [4] and Pt(ll1) [15,20] under similar conditions. We have obtained several spectra, both onand off-specular, to confirm that there are no vibrational modes indicative of another species. In particular, we found no peak in the 1300-1450 cm-’ region or in the 900-1050 cm-’ region for the deuterated species which would be
J. A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(Il I)
Pd(lll)+2OL 300K E,=2.9eV
C2H4
300K
i 0
,
,
I
800 energy
,
1600 loss
I
f
2400
,
,
3xX
(cm-‘)
Fig. 9. Thermal evolution of the species formed from room temperature ethylene adsorption on Pd( 1I 1) to 400 and 500 K. All spectra were obtained at 300 K. Scale changes are given in absolute intensity and are not necessarily related to intensity relative to the elastic beam [ 191.
assigned to the scissor mode of a CH, group. There is also no mode which can be assigned to a C-C stretching vibration. The assignment of 762 cm- ’ to S,, in 500 K spectra is in agreement with results on Nit11 1) [4], Pt( 111) [ 15,20], Rh(ll1) [16], and with the cluster compound (CO)~(Co)~(CH) [21] as shown in table 2. From our off-specular data we have determined that the peak at 762 cm-’ is dipole-enhanced. This means that this species does not have the high C& symmetry found in the cluster compound. But a range of other geometries, including some with a perpendicular CH axis are consistent with our data. There are several interesting features of spectra obtained after heating to 400 K. Representative examples are shown in the centers of figs. 6, 7 and 8. The CH bending mode has appeared at 749 cm-’ with (753 cm-’ without) preadsorbed hydrogen and at 534 cm-’ in the deuterated spectrum. The peak in the CH stretching region appears to be a composite of the ZS.-~,at 2900 cm-’ and the +u at 3002 cm -I. There is a shoulder on the Sc, peak in the vicinity of 860 cm-’ which appears only at 400 K. This could be a CH bending mode resulting from CH in a different bonding geometry. It seems unlikely that this
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(l I I)
81
is the asymmetric bending mode for the same species since it does not appear at 500 K with on- or off-specular scattering. We do not observe any other modes unique to the 400 K spectrum which would imply the existence of a species other than CH. Finally, based on trends in frequency and relative intensity, we do not connect this shoulder with the broad peak near 870 cm-’ in the 300 K spectrum. The CCH, (CCD,) peaks at 1327 cm-’ (982 cm-‘) and at 1098 cm-’ (1126 cm-‘) in the 400 K spectra have about the same intensity relative to the elastic peak as at 300 K. Since these are dipole-enhanced modes, we infer that the ethylidyne species is stable up to 400 K. We note also that the peak at - 1439 cm-’ belonging to the intermediate CCH, species discussed above is also still present at 400 K if the clean sample was not cooled in a hydrogen background. Therefore, the CH species present at 400 K must have been formed primarily from the low temperature HCCH species rather than ethylidyne. Further support for this view was obtained from room temperature adsorption of C,H, onto clean Pd( 111). We found a prominent mode near 740 cm-’ with a shoulder around 860 cm- ‘. In addition, there are somewhat smaller ethylidyne modes at 1328 and 1100 cm-’ with evidence of CCH, in a shoulder on the 1328 cm- ’ peak. There is no indicaton of the low temperature species although a small amount could be masked by the S,, peak at 740 cm-‘. We conclude that in the case of room temperature adsorption, most of the acetylene dissociates but that some of it uses excess hydrogen to form ethylidyne. Heating the low temperature species from 400 to 500 K results in only a small of increase in the intensity of the S,, mode and the complete disappearance the CCH, species. Our postulate that the CH species is formed from the low temperature species will be further discussed and supported in connection with the ethylene-derived system below. 3.2.2. Ethylene The behavior of adsorbed ethylene above 300 K also indicates that the ethylidyne species is not primarily responsible for the formation of the high temperature CH species. We have already mentioned that heating low temperature chemisorbed ethylene causes most of it to desorb with partial transformation of the remaining adsorbate to the ethylidyne species (fig. 4). We obtained the spectra of fig. 9 by exposing the clean surface to C,H, at room temperature and then heating to 400 and 500 K. The familiar ethylidyne peaks at 1080, 1334 and 2900 cm-’ as well as the broad peak at 914 cm-’ are readily apparent at the bottom of fig. 9. The peak at 780 cm-’ has variable intensity relative to the other peaks. It and a very weak mode in the vcu region near - 2960 cm-’ are the only peaks visible (except for the CO impurity peak) after heating to 400 K. Heating to 500 K leaves only a weak mode near 796 cm-‘. Judging from the relative intensity of the 1334 cm-’ mode at the bottom of
82
J.A.
Gale*, L. L. Kesmodel / Acetylene and ethylene on Pd(l I I)
fig. 9, approximately half as much ethylidyne is present as from heating the low temperature acetylenic species to 300 K (bottom of figs. 7 and 8). As illustrated by comparison of figs. 7 and 8 with fig. 9, the CH species modes are much weaker in the ethylene-derived system than in the acetylene-derived system. The difference is quite dramatic at 500 K. We should, of course, consider that spectra from the two systems were obtained at different temperatures. If C,H, is adsorbed and a spectrum acquired at 300 K as for C2H, in fig. 9, the amounts of s--CH, are approximately the same. The difference is in the relative amounts of the high temperature CH species. For the acetylenederived system, the a,-., mode at 740 cm-’ is the largest peak in the spectrum which is clearly not the case for C,H, adsorption at the bottom of fig. 9. We are led to conclude, then, that the formation of the CH species is different after adsorption of the different molecules even though ethylidyne forms with approximately equal probabilities in both cases. Therefore, the CH species cannot primarily be formed from ethylidyne and must occur upon adsorption (at 300 K) or by decomposition of the few temperature ~he~sorbed species.
4. Discussion Our results yield information concerning the reaction of C,H, and C,H, on Pd( 111) at temperatures above 150 K. In the last section, we identified three species, other than the low temperature phase, present in one or both of these
Table 3 Comparison of modes assigned to ethylidyne formed on Pd( 111)from ethylene adsorption with corresponding modes in the cluster compound (CO)&o,(CCH,) Assignment
“CH,
%kI/%D &tJsym) %%i/%Il
pee QH/4D
(CO),Co,(CCH,) W%Co,(CCW) 2930 (2192) 1.34
2888 (-) -
1356 i 1002) 1.35 1163 (1182) 0.98
C,H, GD,)
Hz/C,H, @‘Z&W
2900 (2181) 1.33
2900 (2183) 1.33
1334
1332
(-)
(986) 1.35
1080 (1120) 0.96
1098 (1126) 0.98
PCH,
loo4
914
923 a)
(-)
ycn/vco
(828) 1.21
(749) I .23
‘) Approximated
by fitting two curves into the broad
peak
- 870 cm-’
at 300 K.
and acetylene
[ 1I]
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(1 I I)
83
systems. An intermediate CCH, species formed from the low temperature acetylenic phase above 200 K will be further discussed here in terms of its role in ethylidyne formation. We will also present new evidence related to the assignments of the ethylidyne modes on Pd( 111). The difference between the ability of the ethylene- and acetylene-derived systems to form the high temperature CH species leads to information about the origin of this species. Finally, our results will be compared with results on Ni( 111) [4,8,22], Pt( 111) [ 10,15,20], and Rh(ll1) [9]. From spectra obtained after heating chemisorbed acetylene to 250 K, we . ‘dentified a vcc mode at 1437 cm-’ and a vcu, mode at 2986 cm-’ (fig. 1) which are of reduced intensity if the clean surface was exposed to hydrogen before acetylene (fig. 3). We associated these peaks with an intermediate species containing no more than two hydrogen atoms. In view of the formation of ethylidyne in the presence of excess hydrogen, we tentatively identify this having a structure analogous to that observed in an species as CCH,, organometallic cluster containing CCH, and three osmium atoms [23]. The unsaturated nature of the carbon-carbon bond leaves it vulnerable to the addition of a hydrogen atom to form ethylidyne. This latter process occurs readily as evidenced by the dominance of the ethylidyne species at 250 K on a surface with preadsorbed hydrogen. This suggests that the palladium surface activates the transfer of hydrogen atoms from one carbon atom to the other, and that this is a step in the formation of ethylidyne [lo]. Ibach and Lehwald [6] have reported a spectrum of the species formed on Pt( 111) by heating the low temperature acetylenic species to 350 K without adding hydrogen. Although this spectrum is similar to that of the intermediate species on Pd( 111) at 250 K, information concerning either intermediate temperatures or the deuterated analog is not given for platinum. Therefore, it is difficult to make an accurate comparison with our results. The ethylidyne species forms from ethylene adsorption on Pt(ll1) [lo], Pt( 100) [24], and Rh(ll1) [9] and from acetylene adsorption on Pt(ll1) [ 151 and Rh(ll1) [9] in the presence of atomic hydrogen. We have previously reported ethylidyne on Pd( 111) resulting from room temperature adsorption of ethylene [3]. The present work with acetylene has shed new light on our earlier results. In particular, the mode at 914 cm-’ was not positively assigned. Possibly related spectral features were reported at 880 cm-’ on Rh(ll1) [9] and at 900 cm-’ on Pt( 111) [6]. The latter peak has since been shown not to be an essential part of the ethylidyne spectrum on Pt( 111) [ 151. Skinner et al. [ 1 l] reported a ,e,-u, mode in the cluster compound (CO),(Co),(CCH,) at 1004 cm- ’ but suggested that 900 cm-’ is too low to be the corresponding mode in the surface species. They proposed rather that this mode may be due to adsorption of hydrogen occurring in the conversion from C,H, to \$-CH,. Our work with acetylene transformation on Pd( 111) also indicates a mode at - 923 cm-’ associated with ethylidyne formation. In addition, we have
84
J.A. Gores, L. L.
Kesmodel / Acetylene and ethylene on Pd(l I I)
observed a corresponding mode in the deuterated species at 749 cm-‘. The then, is 1.23, in close agreement with 1.21 reported for pcH1 value of +n/~cn, in the cluster compound. We believe that a mode at 914-923 cm-’ is, indeed, a genuine feature of the ethylidyne spectrum on Pd( 111). While we cannot make this claim with absolute certainty, we have not observed any conditions under which ethylidyne is present and the peak at 914-923 cm-’ is either greatly diminished or not present. In table 3 we present the frequencies and assignments of vibrational modes associated with F-CH, in the cluster compound (CO),Co,(CCH,) and in surface species obtained from ethylene and acetylene in the presence of hydrogen on Pd( 111). We have already discussed formation of the CH species separately for C,H, and C,H, adsorption. In the former case, it was shown that this species is formed primarily from the low temperature acetylenic species at - 400 K rather than from the ethylidyne species. Above 400 K, there is some evidence, from a small increase in the Sc, mode intensity, that ethylidyne may form CH, but this is not conclusive. Room temperature adsorption of acetylene and ethylene resulted in comparable amounts of ethylidyne but the acetyleneTable 4 Comparison of the thermal evolution of acetylene and ethylene adsorbed on the (111)surfaces of Ni [S], Pd and Pt [ 10,15,20]; results are based on heating the low temperature chemisorbed species: the final high temperature decomposition on all surfaces is believed to be a carbonaceous overlayer
Ni(ll1)
Pd (111)
Cz”2
m+ C2”2
Pt (111)
C2H2
Ni(ll1)
C2H4
Pd(ll1)
Pt(ll1)
‘) Acetylene
C2”2
400K+
:C=CH2
(-H
+”
l
L
m
C2H4+
\;C-CH
fC-CH3
+
m
a C2H4
conversion
C2H4t
C2H4t
to CH on Ni(
+
+
p
CH
450K small yield
+
CH
400K small yield
l
CH
450K small yield
b
CH
C2H2
Lnnx
C2H4
400K 3 small yield
,
CHa)
C2H4 + SC-CH3
:-C-C”)
111)has been demonstrated
for high initial coverage
IS].
J.A. Gates, L. L. Kesmodel / Acetylene and ethylene on Pd(ll1)
85
derived spectrum had a much larger Sc, p eak. Heating the ethylene-derived system leaves only very weak CH modes. From these results, we conclude that the palladium surface directly breaks the carbon-carbon bond in acetylene. In the case of ethylene the conversion to CH is a more complex and less efficient process. Finally, the transformation of ethylidyne to CH gives at best a small yield. Finally, it is interesting to compare the thermal evolution of acetylene and ethylene adsorption on Ni( 111) [4,8,22], Pd( 11 l), and Pt( 111) [ 10,15,20]. We find, as shown in table 4, that our results on palladium are intermediate to nickel and platinum. Chemisorbed acetylene is stable up to 400 K on Ni( 111) [8] but converts exclusively to ethylidyne in the presence of hydrogen on Pt( 111) [6,15] by 300 K. On Pd, we observe both acetylene and ethylidyne at 300 K. Ethylene largely desorbs from all surfaces by 300 K. The remaining surface species is then C,H, on nickel [8,22], ethylidyne on platinum [IO], and ethylidyne and ethylene on palladium. Room temperature exposure of Ni( 111) to acetylene results in two surface phases [25]. The second phase, identified as CH, forms after saturation of the initial chemisorbed C,H, phase. Heating to 400 K causes a complete transformation to CH [8]. EELS [20] has shown CH resulting from ethylidyne on Pt at 450 K but with a small yield. As discussed above, CH appears on Pd( 111) near 400 K. Most of this species is formed from C,H, with at best a small fraction from \+--CH,, consistent with both nickel and platinum.
Acknowledgements This work was supported by the Office of Naval Research. J.A.G. is pleased to acknowledge a fellowship from the American Vacuum Society.
References [I] [2] [3] [4] [5] [6] [7] [S] [9] [IO] [I l]
J.A. Gates and L.L. Kesmodel, J. Chem. Phys. 76 (1982) 4281. J.A. Gates and L.L. Kesmodel, Surface Sci. 120 (1982) L461. L.L. Kesmodel and J.A. Gates, Surface Sci. 111 (1981) L747. J.E. Demuth and H. Ibach, Surface Sci. 78 (1978) L238. J.E. Demuth and H. Ibach, Surface Sci. 85 (1979) 365. H. Ibach and S. Lehwald, J. Vacuum Sci. Technol. 15 (1978) 407; H. Ibach, H. Hopster and B. Sexton, Appl. Surface Sci. 1 (1977) 1. H. Ibach and S. Lehwald, J. Vacuum. Sci. Technol. 18 (1981) 625. S. Lehwald and H. Ibach, Surface Sci. 89 (1979) 425. L.H. Dubois, D.G. Castner and G.A. Somojai, J. Chem. Phys. 72 (1980) 5234. L.L. Kesmodel, L.H. Dubois and G.A. Somojai, Chem. Phys. Letters 56 (1978) 267; J. Chem. Phys. 70 (1979) 2180. P. Skinner, M.W. Howard, I.A. Oxton, S.F.A. Kettle, D.B. Powell and N. Sheppard, J. Chem. Sot. Faraday Trans. II, 77 (1981) 1203.
86 [ 121 [ 131 [ 141 [15] [16]
[ 171 [18] [ 191
[20] [Zl] [22] [23] [24] [25]
J.A. Gates, L.L. Kesmodel / Acetylene and ethylene on Pd(l II) R.C. Baetzold, Chem. Phys. 38 (1979) 313. A. Gavezotti and M. Simonetta, Surface Sci. 99 (1980) 453. A.B. Anderson and A.T. Hubbard, Surface Sci. 99 (1980) 384. H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982) p. 326. L.H. Dubois and G.A. Somorjai, in: Am. Chem. Sot. Symp. Ser.; Eds. A.T. Bell and M.L. Hair (Am. Chem. Sot., Washington, DC, 1980). L.L. Kesmodel, J.A. Gates and Y.W. Chung, Phys. Rev. B23 (1981) 489. J.E. Demuth, H. Ibach and S. Lehwald, Phys. Rev. Letters 40 (1978) 1044. It should be pointed out that scale changes given in figs. l-9 are based on absolute intensity. The elastic peak shown is that of the lowest spectrum in each case. These scale changes may be misleading since the surface reflectivity and, therefore, the elastic beam count rate are not constant as a function of the adsorbate. In this paper, all intensities referred to are those relative to the elastic beam. We have made the assumption that the relative intensity of a dipole-enhanced mode is roughly proportional to coverage. A.M. Baro and H. lbach, J. Chem. Phys. 74 (1981) 4194. M.W. Howard, S.F. Kettle, I.A. Oxton, D.B. Powell, N. Sheppard and P. Skinner, J. Chem. Sot. Faraday Trans. II, 77 (198 1) 397. J.E. Demuth, Surface Sci. 76 (1978) L603. A.J. Deeming and M. Underhill, JCS Chem. Commun. (1973) 277; JCS Dalton Trans. (1974) 1415. H. Ibach, in: Proc. Conf. on Vibrations in Adsorbed Layers, Jtilich, 1978. J.E. Demuth, Surface Sci. 69 (1977) 365.