6474
J. Phys. Chem. B 1997, 101, 6474-6478
Temperature-Dependent Rearrangements and Reactions of Acetylene Adsorbed on Pt(111) Monitored in the Range 125-381 K by Sum Frequency Generation Paul S. Cremer, Xingcai Su, Y. Ron Shen, and Gabor A. Somorjai* Departments of Chemistry and Physics, UniVersity of California at Berkeley, Berkeley, California 94720, and Materials Sciences DiVisions, Lawrence Berkeley Laboratory, Berkeley, California 94720 ReceiVed: February 11, 1997; In Final Form: May 12, 1997X
The thermally induced rearrangements and reactions of acetylene on the (111) crystal face of platinum were observed by monitoring the vibrational spectra of surface adsorbates in the CH stretch range with infraredvisible sum frequency generation. It was found that adsorption of acetylene at 125 K led directly to the formation of an adsorbed vinylidene species (η2-µ3-CCH2). Upon annealing, a variety of different species were formed that included ethylidene (MdCHCH3), di-σ-bonded ethylene (M-CH2CH2-M), µ-vinylidene (MdCCH2), and ethylidyne (MtCCH3). The presence of species with C2H4 and C2H3 stoichiometry was strong evidence that a disproportionation reaction must occur. Above 381 K ethylidyne was the only observable species on the surface.
Introduction The adsorption and dehydrogenation of acetylene on Pt(111) was first studied over 20 years ago by low-energy electron diffraction. It was noted that if the clean Pt(111) surface was exposed to acetylene at 300 K and annealed to above room temperature, a (2 × 2) surface structure could be formed.1 Subsequently, ethylene was found to exhibit similar behavior. By comparing electron energy loss spectra (HREELS) with organometallic cluster analogs, it was determined that the species formed from the surface reactions of both acetylene and ethylene was ethylidyne, (MtCCH3).2 This species is sp3 hybridized and resides in the fcc 3-fold hollow site on the Pt(111) surface.3 The preponderance of spectroscopic evidence for the mechanism of ethylidyne formation from ethylene favors an ethylidene intermediate (Figure 1).4-6 The thermally induced rearrangements and reactions of acetylene/Pt(111) which produce ethylidyne have received much less attention because of the complexities of the process. Indeed, annealing in the absence of adsorbed hydrogen cannot lead to ethylidyne formation by a stoichiometric reaction. Instead, a disproportionation reaction is required, whereby a hydrogen poor species, probably ethynyl (M-CCH), must be formed simultaneously to provide the necessary hydrogen to produce ethylidyne (Figure 2). HREELS studies of acetylene dehydrogenation by Avery found that ethylidyne formed between 300 and 420 K, but no clear spectroscopic evidence could be found for the vinylidene (MdCCH2) species that was believed to be an important intermediate. Therefore, the data were interpreted as intact adsorption of acetylene followed by the formation of ethylidyne upon annealing.7 Thermal desorption mass spectroscopy indicated that ethylidyne formation from acetylene was not accompanied by hydrogen evolution. Detailed studies of the thermal desorption species from acetylene/Pt(111) by Megiris et al. show that the first hydrogen desorption peak occurs near 490 K,8 well above the 300-400 K temperature range where ethylidyne is formed. Some intact acetylene is found to desorb near 150 K, and small quantities of ethylene are evolved in three peaks at 250, 275, and 460 K. Evidence was also noted for a very small quantity * To whom correspondence should be addressed at the Department of Chemistry. X Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5647(97)00538-5 CCC: $14.00
Figure 1. Schematic of the decomposition of di-σ-ethylene to ethylidyne on Pt(111).
Figure 2. Disproportionation reaction of acetylene to ethynyl and ethylidyne on Pt(111).
of methane (a cracking product) near 310 K; however, no ethane or any higher hydrocarbons were found to desorb. A study of the thermally induced rearrangements and reactions of acetylene in the 125-381 K temperature range on the (111) surface of platinum was undertaken using SFG vibrational spectroscopy, and the results are presented in this paper. The vibrational spectra give strong evidence for the formation of several stable intermediates on the surface leading up to ethylidyne formation. These species include ethylidene (MdCHCH3) as well as η2-µ3- and µ-vinylidene (MdCdCH2) (Figure 3). Experimental Section Experiments were carried out in an ultrahigh-vacuum chamber with a base pressure below 1 × 10-10 Torr on a Pt(111) single crystal. The chamber was equipped with a retarding field analyzer for Auger electron spectroscopy and low-energy electron diffraction, a mass spectrometer, and an Ar+ gun for cleaning the sample. In addition, CaF2 windows on 23/4 in. © 1997 American Chemical Society
Acetylene Adsorbed on Pt(111)
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6475 (111) surface to a saturation coverage of acetylene (4 langmuirs) at 125 K, a scan of the CH stretch range of the vibrational spectrum was taken between 2800 and 3100 cm-1. The sample was then annealed through a series of predetermined temperatures for 60 s after each of which the crystal was allowed to cool to 125 K, and another vibrational spectrum was obtained. This process was repeated between 125 and 381 K. It was verified via control experiments that the results were identical for immediately annealing to a predetermined temperature and arriving at the same temperature over several heating cycles. Indeed, the spectra looked the same after for waiting 2 h before obtaining a spectrum as they did for immediately annealing to the predetermined temperature and taking the data. Further control experiments were performed to ensure that the laser radiation used was sufficiently weak as to avoid laser-induced heating or photochemistry. Results
Figure 3. Schematic representations of adsorbed acetylene, vinylidene, vinyl, ethylidene, and ethylidyne species.
flanges allowed infrared and visible light to pass in and out of the chamber. Vibrational spectra were obtained by employing sum frequency generation (SFG), a surface specific nonlinear optical spectroscopy. Details of the SFG-UHV instrumentation have been described previously.4 Briefly, the radiation for the experiment was produced by a passive-active mode-locked Nd: YAG laser at 1064 nm with a pulse width of 20 ps, a repetition rate of 20 Hz, and a pulse energy of 50 mJ. The 1064 nm beam was divided into two portions, the first of which was frequency doubled to 532 nm while the second could be angle tuned between 2600 and 4000 cm-1 using a LiNbO3 optical parametric generator/amplifier (OPG/OPA) stage. The infrared and visible beams were focused concentrically onto the platinum sample inside the UHV chamber, and the output radiation was sent through a monochromator to a photomultiplier tube where it was detected and stored on a personal computer. In order for a vibrational mode to give rise to an SFG signal, it must be both IR and Raman active. This requires centrosymmetry to be broken, which is necessarily the case at surfaces and interfaces but usually not the case for bulk samples or isotropic gas phases. Thus, SFG is a surface specific technique. Because SFG requires a change in dipole moment (IR selection rule), it is also subject to the same metal dipole selection rule as reflection infrared techniques on conducting surfaces. The metal dipole selection rule states that for a mode to be observed it must have a component of its dynamic dipole normal to the surface. This requirement arises because modes that are inplane with the surface are canceled by the image dipole formed inside the conductor. The acetylene used in this experiment was cleaned by passing it through a copper tube immersed in an acetone-dry ice bath in order to remove acetone and other contaminants. The purity was verified by the acetylene fragmentation pattern in the quadrupole mass spectrometer. After exposing the clean Pt-
Figure 4a shows the vibrational spectrum for a saturation exposure of acetylene on Pt(111) at 125 K. Four peaks are clearly visible in the vibrational spectrum at 2837, 2878, 2924, and 2981 cm-1 as well as some additional weak intensity near 3020 cm-1. The spectrum matches well to the infrared data taken for the corresponding osmium cluster analog, µ3-η2vinylidene, which has features of 3047, 2986, 2926, 2898, and 2855 cm-1.9,10 By contrast, the osmium cluster for acetylene gives rise to only two features: a 2990 cm-1 peak from the symmetric stretch of the molecule and a 2942 cm-1 from the asymmetric stretch. In this case only the higher frequency mode would provide a reasonable match to the present data. The spectral features remain unchanged upon heating to 140 K; however, annealing the sample between 150 and 300 K leads to a dramatic change in the vibrational spectrum (Figure 4b). This spectrum represents heating to 191 K, and no further change could be noted until the sample was heated above 300 K. Three peaks at 2960, 2976, and 2990 cm-1 dominate the CH stretch region, while some weak intensity near 2900 cm-1 can also be seen just above the noise. The 2976 cm-1 is most likely from vinylidene as in the previous spectrum, while the 2990 cm-1 intensity is probably due to the formation of µ3-η2vinyl by comparison with cluster analogs.9,10 This assignment can be made despite the overlapping of bands in vinyl and acetylene in the CH stretch region, because acetylene was already observed to be unstable upon adsorption from the gas phase at 125 K. This assignment is also consistent with vibrational data from vinyl groups adsorbed on Pt(111) below 140 K from vinyl iodide.4 In that case the strongest feature was also at 2990 cm-1. The 2960 cm-1 feature in Figure 4b is characteristic of an νas(CH3).4,11 Other possibilities such as vinyl can be ruled out on the basis of previous work with the corresponding iodide.4 For signal from the asymmetric stretch of a methyl group to be detectable, the species must be inclined with respect to the surface normal. This is because of the metal dipole selection rule, which states that for a mode to be surface active it must have some component of its dynamic dipole normal to the surface. All dipole components in the plane of the metal are canceled by an image dipole in the metal surface. The 2960 cm-1 is therefore assigned to an ethylidene species (MdCHCH3).4,11 An ethyl assignment would also be possible, but the lack of sufficient surface hydrogen makes this less likely. Further, the 2960 cm-1 feature was noted to survive to quite high temperatures (at least 356 K, Figure 4e), well above where ethyl is known to dehydrogenate. The weak intensity near 2900
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Figure 4. (a) SFG spectrum of a saturation coverage (4 langmuir) of acetylene adsorbed on Pt(111) at 125 K. The system was annealed to (b) 191, (c) 313, (d) 339, (e) 356, and (f) 381 K for 60 s and then allowed to cool below 150 K where an SFG can was performed.
cm-1 may be assigned to the νs(CH2) from di-σ-bonded ethylene which has been shown to give rise to a 2904 cm-1 feature on Pt(111).4 This assignment correlates well with the onset of
ethylene desorption observed from TPD data just above this temperature.8 It should be noted that although the intensity near 2900 cm-1 was weak it was consistently reproducible.
Acetylene Adsorbed on Pt(111)
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6477
Figure 5. Proposed pathway for acetylene decomposition to ethynyl and ethylidyne on Pt(111).
The next indication of significant reaction occurs near 313 K (Figure 4c). At this point the 2950-3000 cm-1 range simplifies with only a 2960 cm-1 shoulder which is assigned to ethylidene on the sharp 2976 cm-1 feature from vinylidene. There is a small new feature at 3033 cm-1, which is most readily assigned to the νs(CH2) of µ-vinylidene (Figure 3) by comparison with the frequency from the corresponding cluster analog.9 Again, some broad intensity can be seen near 2900 cm-1 which is probably from di-σ-bonded ethylene. Further heating to 339 K causes a dramatic drop in the intensity of the 2976 cm-1 feature, but the 2960 and 3033 cm-1 peaks are still present at somewhat increased strength (Figure 4d). Annealing to 356 K leads to the complete disappearance of the 2976 cm-1 feature accompanied by the formation of ethylidyne with its characteristic νs(CH3) showing up at 2884 cm-1 (Figure 4e).12 Under these conditions, ethylidyne coexists on the surface with ethylidene at 2960 cm-1 and µ-vinylidene at 3033 cm-1. Further annealing of the surface to 381 K causes the higher frequency features to disappear with the ethylidyne feature remaining (Figure 4f). The intensity of the ethylidyne feature is strengthened significantly with respect to the 356 K spectrum. The intensity of this feature represents a coverage of approximately 0.08 monolayer (ML) as judged from comparing the curve fitted peak strength from this spectrum with known calibrations for ethylidyne at saturation coverage (0.25 ML).4 Discussion Surface Stoichiometry and Hydrogen Production. The temperature-dependent dehydrogenation of acetylene on Pt(111) from 125 to 381 K gives rise to a rich variety of spectral features in the CH stretch range. The features in Figure 4b, such as νa(CH3) at 2960 cm-1 and the νs(CH2) of di-σ-bonded ethylene, indicate that significant CH bond breaking and forming already occur below 200 K. This is consistent with thermal desorption data by Megiris et al., which indicates the onset of ethylene desorption to be near 250 K.8 The transfer of hydrogen from one acetylene to another is responsible for the formation of these species as well as the ethylidyne which forms at higher temperatures. Indeed, control experiments demonstrated that gas phase hydrogen can be ruled out as making a significant contribution to the formation of ethylidyne under UHV (10-10 Torr) conditions.7 Therefore, ethynyl (M-CtCH) or other hydrogen-poor surface residues must be generated to account
for the presence of adsorbed species with higher hydrogen/ carbon ratios. Evidence for an ethynyl species cannot be directly obtained from the SFG spectra apparently because of the lack of a surface normal component of the ν(CH) dynamic dipole. This was confirmed by control scans in the 3156 cm-1 range (not shown) where ν(CH) signal from this species would be expected.9 However, Avery noted evidence for ethynyl during acetylene decomposition in the HREELS spectrum from a weak feature at 750 cm-1.7 Other possible surface residues include methylidyne (MtCH); however, the formation of this species requires breaking of the C-C bond. The TPD data does shows evidence for a very small amount of methane production,8 which would indicate carbon-carbon bond breakage (perhaps occurring at defect sites). This route, however, is probably not mechanistically significant in generating the substantial hydrogen quantities necessary to produce the coverages of hydrogenated species seen in these experiments. If we assume that the formation of ethynyl is the major source for surface hydrogen, then for every C2H4 moiety observed on the surface, two C2H species must be formed. Further, each ethylene that desorbs from the surface requires the formation of two ethynyl groups. This means that the overall stoichiometry on the surface is less than C2H2 even below 200 K. Indeed, it is quite reasonable to suspect that the presence of two adsorbed acetylene molecules does not directly lead to disproportionation into one ethylidyne and one ethynyl, but rather several ethynyl molecules are most likely formed for every ethylidyne produced. This ultimately explains the low coverage of ethylidyne (0.08 ML) finally observed. It should be noted that this number is significantly different than the 0.125 ML of ethylidyne that one might expect if a 1:1 disproportionation reaction occurred form an initial adsorption of 0.25 ML of acetylene. Proposed Reaction Mechanism of Adsorbed Acetylene Rearrangements and Reactions. Upon adsorption on Pt(111) at 125 K, acetylene most likely rearranges by undergoing a 1-2 shift to form the adsorbed vinylidene that is observed in Figure 4a. This is shown schematically in Figure 5. Upon annealing above 150 K evidence for vinyl and ethylidene species emerges. These could be formed by the addition of hydrogen at the R-carbon to form vinyl followed by a second hydrogen addition at the β-carbon to form ethylidene. In order for these reactions to occur, two molecules of ethynyl need to be produced for every ethylidene formed. The appearance of the 3033 cm-1
6478 J. Phys. Chem. B, Vol. 101, No. 33, 1997 peak is the first new feature that appears in the spectrum upon annealing above room temperature. The source of its formation may be the rehybridization of η2-µ3-vinylidene to form µ-vinylidene. Such a rehybridization might be required if the fcc 3-fold hollow site in which η2-µ3-vinylidene resides becomes irreversibly occupied by ethynyl. By 339 K only three species appear to be on the Pt(111) crystal surface: ethylidene, vinyl, and µ-vinylidene. Of these, it is most likely that the ethylidene is directly responsible for the formation of ethylidyne (Figure 5). An ethylidene pathway is favored by analogy with the case of di-σ-bonded ethylene. In that case ethylene was seen to convert to ethylidyne through an ethylidene intermediate on Pt(111) near 270 K, although no vinyl or µ-vinylidene species were seen to reside on the surface during the conversion.4 However, these species were observed during the thermal evolution of vinyl iodide to ethylidyne on Pt(111).4 Under these circumstances µ-vinylidene and vinyl were noted to thermally evolve to ethylidene rather than directly to ethylidyne. Hence, it is unlikely that they directly evolve to ethylidyne during acetylene dehydrogenation, but rather also go through an ethylidene intermediate (Figure 5). It should be noted that vinyl, ethylidene, and vinylidene are stable to much higher temperatures during acetylene thermal rearrangement (at least 300 K) than they are during the thermal evolution of di-σ-bonded ethylene and vinyl iodide.4,5 A plausible explanation for this may stem from the high concentration of dehydrogenated species (ethynyl) that block the crucial 3-fold hollow sites needed for thermal evolution. If this is the case, the overall reaction pathways of each of these species might be expected to be the same, but the barrier to reaction would be raised to a temperature necessary to move ethynyl out of the way in order to free up 3-fold hollow sites. It should also be noted that above 300 K evidence for the vinyl species no longer appears with that of vinylidene and ethylidene in the vibrational spectra (Figure 4c,d). Evidently, there is sufficient surface hydrogen present above this temperature for vinylidene species to be hydrogenated readily onto ethylidene once the vinyl
Cremer et al. intermediate is formed, hence keeping the overall surface concentration of vinyl groups low. Conclusion Ethylidyne is formed near 350 K on the surface of Pt(111) from the thermal rearrangement reactions of adsorbed acetylene. Even at low temperatures (below 200 K), C-H bonds are broken, and relatively hydrogen-rich C2H4 species such as ethylidene and di-σ-bonded ethylene are formed. Heating the system near 339 K yields a mixture of µ-vinylidene and ethylidene. Upon further heating above 350 K, ethylidyne is formed, most likely from the ethylidene species. The net reaction involves three acetylene molecules undergoing disproportionation to form one ethylidyne, two ethynyl molecules, and one surface hydrogen atom. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy, under Contract DE-AC03-76SF00098. References and Notes (1) Weinberg, W.; Deans, H.; Merrill, R. Surf. Sci. 1974, 41, 312. (2) Skinner, P.; Howard, M.; Oxton, I.; Kettle, S.; Powell, D.; Sheppard, N. J. Chem. Soc., Faraday Trans. 1978, 15, 407. (3) Starke, U.; Barbieri, A.; Materer, N.; Van hove, M.; Somorjai, G. Surf. Sci. 1993, 286, 1. (4) Cremer, P.; Stanner, S.; Niemantsverdriet, J.; Shen, Y.; Somorjai, G. Surf. Sci. 1995, 328, 111. (5) Zaera, F.; Bernstein, N. J. Am. Chem. Soc. 1994, 116, 4881. (6) Windham, R.; Koel, B. J. Phys. Chem. 1990, 94, 1489. (7) Avery, N. Langmuir 1988, 4, 445. (8) Megiris, C.; Berlowitz, P.; Butt, J.; Kung, H. Surf. Sci. 1985, 159, 184. (9) Evans, J.; McNulty, G. J. Chem Soc., Dalton Trans. 1983, 639. (10) Evans, J.; McNulty, G. J. Chem. Soc., Dalton Trans. 1984, 79. (11) Anson, C.; Sheppard, N.; Powell, D.; Norton, J.; Fischer, W.; Keiter, R.; Johnson, B.; Lewis, J.; Bhattacharraya, A.; Knox, S.; Turner, M. J. Am. Chem. Soc. 1994, 116, 3058. (12) Malik, I.; Agrawal, V.; Trenary, M. J. Chem. Phys. 1988, 89, 3861.
