Spectroscopic Characterization of Vinyl Formed from Acetylene on Pt

The surface intermediates formed following the adsorption of acetylene (C2H2) on Pt(111) were identified and characterized with reflection absorption ...
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J. Phys. Chem. C 2007, 111, 1459-1466

1459

Spectroscopic Characterization of Vinyl Formed from Acetylene on Pt(111) Rongping Deng, James Jones, and Michael Trenary* Department of Chemistry, UniVersity of Illinois at Chicago, 854 West Taylor Street, Chicago, Illinois 60607-7061 ReceiVed: September 2, 2006; In Final Form: NoVember 2, 2006

The surface intermediates formed following the adsorption of acetylene (C2H2) on Pt(111) were identified and characterized with reflection absorption infrared spectroscopy (RAIRS). It is found that isomerization of acetylene to vinylidene (CCH2), followed by hydrogenation to vinyl (CHCH2), precedes the formation of ethylidyne (CCH3). When hydrogen and acetylene are coadsorbed on the surface, di-σ-bonded ethylene (C2H4) and vinyl (CHCH2) intermediates form at about 250 K. Vinyl is identified by its CH2 symmetric stretch at 2988 cm-1 and CC stretch at 1280 cm-1. The isomerization of acetylene is indicated by the formation of only one vinyl isotopomer, CDCH2, from the reaction of surface deuterium with C2H2. Vinyl converts to ethylidene, ethylidyne, and a high-temperature form of vinylidene at temperatures above 300 K. Ethylidyne isotopomers formed from H/D exchange when C2D2 is adsorbed on the H preadsorbed surface and when C2H2 is adsorbed on the D preadsorbed surface are identified by their RAIR spectra.

1. Introduction Acetylene (C2H2) adsorption and reaction on transition metal surfaces under ultrahigh-vacuum conditions have been extensively studied over the past 2 decades. By employing surfacesensitive techniques such as high-resolution electron energy loss spectroscopy (HREELS),1-9 X-ray photoelectron spectroscopy (XPS),7,10 reflection absorption infrared spectroscopy (RAIRS),3,11 low-energy electron diffraction (LEED),12-16 scanning tunnelingmicroscopy(STM),17-19 andotheranalysistechniques,20-30 such studies have attempted to provide a molecular-level understanding of hydrocarbon reactions on transition metal surfaces and hence to provide insights into the chemical reactions that underlie important heterogeneous catalytic processes. It has been well-established that acetylene adsorbed at low temperatures converts to ethylidyne (CCH3) upon warming to room temperature or above on the Pt(111) surface1,28 as well as on other transition metal surfaces5,8,25,27 under ultrahighvacuum conditions. Possible intermediates involved in this reaction, such as vinylidene (CCH2) and vinyl (CHCH2), have not been well-characterized. The high resolution of RAIRS, combined with the high sensitivity the technique now has in the C-H stretch region, makes it possible to more definitively characterize surface hydrocarbon species.31 These features of RAIRS as well as the shifts that are observed upon substitution of H with D and of 12C with 13C make more definitive spectroscopic assignments possible and hence make it worthwhile to reexamine certain previously examined systems. The vinyl species has the same stoichiometry as ethylidyne but is less stable on the surface. Previous experimental studies of vinyl iodide decomposition on Pt(111) indicated that vinyl converts to ethylidyne.32-34 An experimental study suggested that vinyl could be an intermediate in the hydrogenation of acetylene and vinylidene on the Pd(111) surface.11 An analysis of the energetics of acetylene hydrogenation on Ni(111) by surface-bound H and bulk H proposed that vinyl formation is * To whom correspondence should be addressed: E-mail: mtrenary@ uic.edu.

the rate-determining step in the formation of ethylidyne.9 These previous studies suggest that vinyl may be a stable intermediate in the formation of ethylidyne from acetylene on Pt(111). The vibrational signature of vinyl on transition metal surfaces has remained elusive. Studies of the thermal decomposition of vinyl halides on Pt(111)32,34,35 and the photo-decomposition of 1,1,2-trichloroethane36 did not provide consistent spectroscopic identification for a surface vinyl species, in part because interpretation of the spectroscopic data depends on knowing the bonding configuration of the vinyl species. There are two possible pathways for vinyl formation from the hydrogenation of acetylene; vinyl may form by the simple addition of a hydrogen atom to one of the carbon atoms, or acetylene may first isomerize to vinylidene in which both hydrogen atoms are bonded to one carbon atom followed by addition of a third hydrogen atom to the other carbon atom. Here we report on the definitive identification of vinyl formation following the adsorption of acetylene on Pt(111). Strong indirect evidence is presented showing that vinyl is formed from acetylene by way of a vinylidene intermediate. 2. Experimental Section The experiments were performed in an ultrahigh-vacuum system that has been described in detail elsewhere.37 The base pressure in the chamber during these experiments was under 1 × 10-10 Torr. Temperature-programmed desorption (TPD) data were obtained at a heating rate of 2 K/s using a Balzers QMG112 quadrupole mass spectrometer. The RAIR spectra were obtained with a Bruker IFS-66v FTIR. To obtain optimum sensitivity, an InSb detector with a tungsten source was used in the 2200-4000 cm-1 region, and a mercury-cadmiumtelluride (MCT) detector with a SiC source was used for the 800-2200 cm-1 region. Each RAIR spectrum was recorded with a resolution of 2 cm-1 and 1024 scans. The clean Pt(111) crystal was exposed to acetylene and hydrogen by back-filling the chamber. The crystal preparation and cleaning procedures38 followed standard methods. Isotopically labeled acetylene (13C2H2, 99% 13C) was purchased from Sigma-Aldrich, and

10.1021/jp065710r CCC: $37.00 © 2007 American Chemical Society Published on Web 12/30/2006

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Figure 1. RAIR spectra of acetylene thermal conversion on Pt(111). (a) The surface was exposed to 1.0 L of acetylene at 85 K, followed by annealing to the specified temperatures for 60 s. All spectra were recorded at 85 K. (b) Spectra for the 210, 250, and 300 K anneals are plotted with an expanded scale in the 2850-3100 cm-1 range.

deuterated acetylene (C2D2, 99% D) was purchased from Cambridge Isotope Laboratories, while unlabeled acetylene (99.6%) was purchased from BOC Gases. Each acetylene isotopomer was further purified by several freeze-pump-thaw cycles before use. Deuterium (99.5% D2) was purchased from Union Carbide, Inc., and hydrogen (99.9999%) was purchased from Matheson Gas Products and were used without further purification. 3. Results 3.1. Acetylene on Pt(111). Figure 1 shows a series of RAIR spectra from 1000 to 3400 cm-1 in the upper panel with an expanded view of the C-H stretch region in the lower panel obtained after exposing the surface to 1.0 L of acetylene at 85 K and then annealing to the indicated temperatures. The 85 K spectrum shows too many peaks to be due to a single form of adsorbed acetylene. Instead, the C-H stretch at 2996 cm-1 is assigned to di-σ/π-bonded acetylene, as identified in previous vibrational spectroscopic studies.1-3 The C-H stretch at 3198 cm-1 is for acetylene weakly bonded to the surface, either a π-bonded form or simply a physisorbed species. The sharp peak at 3338 cm-1 is the C-H stretch of multilayer acetylene, with a frequency that is very close to the gas-phase value. The peak

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Figure 2. RAIR spectra of the acetylene reaction on Pt(111) with hydrogen pre-adsorption. (a) The top spectrum was recorded after the surface was exposed to 1.0 L of hydrogen and then 1.0 L of acetylene at 85 K. The rest of the spectra were recorded at 85 K after the surface was annealed at the specified temperatures for 60 s. (b) Spectra for the 210, 250, and 300 K anneals with an expanded scale in the 28503100 cm-1 range.

at 1362 cm-1 has the same exposure dependence as the one at 3198 cm-1 and is therefore associated with the weakly bound acetylene. The 1362 cm-1 peak is close to the strong IR band of gas-phase acetylene at 1328 cm-1, which is a combination band of the symmetric and asymmetric bending modes with fundamentals at 612 and 729 cm-1, respectively.39 After heating the surface to 210 K for 60 s, the spectrum shows only a multiplet peak consisting of resolvable components at 2982, 2993, and 3001 cm-1. The 210 K anneal not only desorbs the multilayer and physisorbed acetylene but also causes reactions or structural rearrangement of the di-σ/π-bonded acetylene. After annealing the surface to 300 K and above, the spectra undergo significant changes. The peaks at 1126, 1338, 2796, and 2887 cm-1 are assigned to the C-C stretch, CH3 symmetric bending, CH3 asymmetric bending overtone, and CH3 symmetric stretch, respectively, of the well-known CCH3 species. The assignments of other features in the spectra are difficult from the results of Figure 1 alone and require the information provided by the experiments described below. 3.2. Acetylene on Pt(111) with H and D Preadsorption. Figure 2 shows spectra analogous to those of Figure 1 except

Vinyl Formed from Acetylene on Pt(111)

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1461

Figure 3. TPD of ethylene after the Pt(111) surface had been (a) exposed to 1.0 L of acetylene after a 1.0 L H2 exposure and (b) exposed only to 1.0 L of acetylene.

Figure 5. RAIR spectra of the acetylene-13C reaction on Pt(111). (ac) the surface was exposed to 1.0 L of 13C2H2 and annealed to the specified temperature for 60 s. (d-f) The surface was exposed to 1.0 L of H2 and then 1.0 L of 13C2H2 and annealed to the specified temperature for 60 s. All spectra were recorded at 85 K.

Figure 4. RAIR spectra of the acetylene reaction on Pt(111) with deuterium pre-adsorption. The top spectrum was recorded after the surface was exposed to 2.0 L of deuterium and then to 1.0 L of acetylene at 85 K. The rest of the spectra were recorded at 85 K after the surface had been annealed at the specified temperatures for 60 s.

in this case the surface was dosed with 1.0 L of H2 followed by 1.0 L of C2H2 at 85 K. After the surface was annealed to 210 K, the spectrum shows a multiplet peak around 3000 cm-1 that is almost identical to what is seen in the 210 K spectrum of Figure 1. However, annealing to higher temperatures yields significantly different spectra. A sharp and intense peak at 2988 cm-1 and two weak peaks at 1280 and 2918 cm-1 appear after the 250 K anneal, but the one at 2918 cm-1 is absent in the 300 K spectrum. From the spectrum and thermal stability of di-σ-bonded ethylene (C2H4) on Pt(111), we assign the 2918 cm-1 peak to this species. This assignment is also supported by the TPD results in Figure 3, which shows that, with hydrogen preadsorption, about twice as much ethylene desorbs from the surface as when the same amount of acetylene is exposed to the clean surface. The peak at 2988 cm-1 correlates with the

very weak peak at 1280 cm-1 and indicates the formation of an intermediate associated with acetylene hydrogenation, as these peaks are not apparent in Figure 1. This intermediate decomposes at temperatures above 300 K, followed by the formation of essentially the same species as that formed in the absence of preadsorbed hydrogen, as seen by a comparison of Figures 1 and 2. Figure 4 shows spectra analogous to those of Figure 2, but with the surface exposed to 2.0 L of D2 before the 1.0 L acetylene exposure at 85 K. Interestingly, after the 250 K anneal, a C-H stretch peak appears at 2988 cm-1, just as in Figure 2. This indicates that the intermediate possesses the same CHx functionality in both the non-deuterated and partially deuterated species. We do not see the C-H stretch at 2918 cm-1 for the di-σ-bonded ethylene, indicating that this peak is absent in partially deuterated ethylene, which is presumably of the form CHDCHD or CH2CD2. In addition to the peaks due to ethylidyne, the 400 K spectrum shows peaks at 1249 and 2902 cm-1, which are assigned to the deformation and the C-H stretch modes of partially deuterated ethylidyne (CCH2D). 3.3. 13C2H2 on Pt(111). Figure 5 shows the RAIR spectra for 13C2H2 adsorbed on clean Pt(111) (Figure 5a-c) and hydrogen predosed Pt(111) (Figure 5d-f). Although these spectra show the small shifts expected for 13C substitution, the exact amount of the shift would be different for each species and in some cases can be an important tool in spectral assignments. The C-H stretch of 13C2H4, a hydrogenation product, shifts from 2918 to 2905 cm-1. The peaks observed at 1280 and 2988 cm-1 in Figures 1, 2, and 4 shift to 1233 and 2978 cm-1, respectively, in Figure 5. For ethylidyne, the C-C stretch shifts from 1126 to 1084 cm-1, in agreement with the expected shift for a harmonic oscillator ν(13C-13C) ) x12/13ν(12C-12C) ) 1082 cm-1.

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Figure 6. RAIR spectra of the C2D2 reaction on Pt(111). The surface was exposed to 0.5 L of acetylene at 85 K, followed by annealing to the specified temperature for 60 s.

The δs(CH3) and νs(CH3) modes of ethylidyne shift from 1338 and 2887 cm-1 to 1331 and 2882 cm-1, respectively. The peak at 2964 cm-1 seen in Figure 1 shifts to 2956 cm-1 in Figure 5. With 13C substitution, the sharp peaks seen at 1391 cm-1 in Figure 1 and at 1389 cm-1 in Figure 2, become a broad peak with unresolved components at 1388 and 1378 cm-1. This reveals that two different species coincidentally had the same frequencies for the C-H deformation modes for the 12C isotopomers but that these modes undergo different shifts with 13C substitution. 3.4. C2D2 on Pt(111). RAIR spectra for the adsorption and reactions of deuterated acetylene (C2D2) on clean and D and H precovered Pt(111) are shown in Figures 6-8. The peaks at 2246 and 2411 cm-1 in the 85 K spectra correspond to the peaks at 2996 and 3198 cm-1 in Figure 1 for C2H2 and are the C-D stretches of the di-σ/π-bonded and weakly adsorbed C2D2, respectively. In the case of C2D2 adsorbed on the clean surface as shown in Figure 6, the 250 K spectrum shows a multiplet peak around 2246 cm-1, which is similar to the 250 K spectrum in Figure 1. The 350 K spectrum shows distinct peaks at 1310 and 1156 cm-1. The latter peak increases in intensity after heating the surface to 400 K, and is assigned to the C-C stretch of deuterated ethylidyne, which also has a C-D symmetric stretch at 2037 cm-1. The 1310 cm-1 peak in Figure 6 correlates with the one at 1391 cm-1 in Figure 1. The relatively small shift with deuterium substitution precludes this peak being primarily a CH2 or CH3 deformation mode. On the other hand, the absence of a deuterated analogue in Figure 6 to the 1444 cm-1 peak of Figure 1 indicates that it has shifted to a low frequency and lost intensity, strongly implying that it is due to a C-H deformation mode. The peak at 2169 cm-1 in the 300 K spectrum best correlates with the one at 2964 cm-1 in Figure 1. The positive ∆R/R peak at ∼2050 cm-1 in Figures 6-8 is due to displacement of CO that had adsorbed from the background on the nominally clean surface used for the reference spectrum. Since the intrinsic intensity of the C-O stretch is so large, the small size of this feature implies an insignificantly low initial CO coverage.

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Figure 7. RAIR spectra of the acetylene reaction on Pt(111) with D2 pre-adsorption. The top spectrum was recorded after the surface was exposed to 2.0 L of D2 and then 0.5 L of C2D2 at 85 K. The rest of the spectra were recorded at 85 K after the surface was annealed at the specified temperatures for 60 s.

Figure 8. RAIR spectra of the acetylene reaction on Pt(111) with H2 pre-adsorption. The top spectrum was recorded after the surface was exposed to 1.0 L of H2 and then 0.5 L of C2D2 at 85 K. The rest of the spectra were recorded at 85 K after the surface was annealed at the specified temperatures for 60 s.

On the D preadsorbed surface, the adsorbed C2D2 reacts with the surface deuterium upon annealing to 250 K, which produces an intermediate with a sharp C-D stretch peak at 2242 cm-1 as shown in Figure 7. This peak is also seen for H preadsorption as shown in Figure 8. Just as comparison of the 250 K spectra in Figures 2 and 4 indicated that the intermediate that was formed through hydrogenation had the same CH2 group regardless of whether the surface was preexposed to H2 or D2, so does

Vinyl Formed from Acetylene on Pt(111)

Figure 9. Hydrogen desorption from the acetylene- and ethylenecovered Pt(111) surfaces. The adsorption layers were prepared by exposing the clean surface to 2.0 L of acetylene and ethylene, respectively, at 85 K.

the comparison of the 250 K spectra in Figures 7 and 8 show that the intermediate has the same CD2 group regardless of whether the surface is preexposed to H2 or D2. Also seen in Figure 7 for the 250 K spectrum is a peak at 2140 cm-1 that is assigned to di-σ-bonded C2D4 in agreement with earlier RAIRS40 and EELS41 studies of C2D4 on Pt(111). The 400 K spectrum in Figure 8 shows numerous peaks in the range of 1000-1500 cm-1 compared with the 400 K spectrum in Figure 7. As expected, hydrogenation and H/D exchange in the case of H coadsorbed with C2D2 produces several ethylidyne isotopomers. 4. Discussion Both adsorbed acetylene and ethylene molecules convert into ethylidyne on Pt(111) when the surface is heated to room temperature. However, the surface reactions of these two species below room temperature are quite different. This difference is clearly demonstrated by the hydrogen TPD measurements shown in Figure 9. For ethylene, the hydrogen desorption peak at 294 K represents a dehydrogenation process prior to ethylidyne formation, whereas with acetylene no hydrogen desorption is observed below 300 K. Therefore, any surface intermediates produced from acetylene below 300 K would be formed through isomerization, disproportionation, or hydrogenation reactions. Hydrogen desorption above 400 K in both cases is due to the dehydrogenation of ethylidyne and its dissociation products. Density functional theory (DFT) calculations for vinylidene, the CCH2 isomer of acetylene, bonded to the surface in a threelayer Pt cluster model show that µ3-η2-CCH2 is the most stable species among possible C2H2 isomers.42,43 Its C-C double bond is inclined to the surface by about 40°. Peaks observed with RAIRS at 1440 and 1306 cm-1 were assigned to the γ(CH2)/ ν(CC) modes of vinylidene formed from the thermal decomposition of vinyl iodide on Pt(111) at 130-170 K.34 The vibrational frequencies of vinylidene in the organometalic cluster Os3(CO)9(µ-H)2(µ3-η2-CCH2) occur at 3047, 2986, 1470, 1331, 1051, 961, and 811 cm-1 and are assigned to νas(CH2), νs(CH2), γ(CH2), ν(CC), F(CH2), w(CH2), and τ(CH2), respectively.44 An SFG study proposed that µ3-η2-CCH2 forms from acetylene on Pt(111) at temperatures below 200 K.28 The C-H stretch of this species may be within the multiplet at ∼2993 cm-1 in the 210 K spectrum in Figure 1. The deformation modes and the CC stretch are not observed at this temperature, probably due to a low coverage and the inclined bonding configuration of this species.

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1463 Hydrogenation of acetylene clearly occurs as evidenced by the observation of ethylene desorption in the TPD results shown in Figure 3 and by the observation of di-σ-bonded ethylene in the 250 K RAIR spectrum in Figure 2. The presence of a second hydrogenation product is indicated by the C-H stretch at 2988 cm-1 and the very weak peak at 1280 cm-1, which are assigned to the vinyl species. Although a 2988 cm-1 peak may be present within the multiplet peak in the C-H stretch region in Figure 1, the intensity of this peak is significantly enhanced when additional hydrogen is available on the surface, as shown in Figure 2. The experimental characterization of the vinyl species has proven difficult in previous studies. Although Zaera and Bernstein used TPD and RAIRS to show that vinyl iodide decomposes on Pt(111) to ethylidyne through the vinyl intermediate, they did not report vibrational frequencies for adsorbed vinyl.34 In an HREELS study of vinyl iodide on Pt(111), White and co-workers assigned peaks to vinyl at 2920 cm-1 (both the ν(CH) and νs(CH2) stretches), 1600 cm-1 (ν(CdC) stretch), 1380 cm-1 (γ(CH2) scissor), 1255 cm-1 (F(CH) rocking), and 690 and 955 cm-1 (w(CH) and w(CH2) waging modes).32 Vinyl formation on Pt(111) from the photo-decomposition of 1,1,2trichloroethane was reported in a HREELS experiment, but with different frequency assignments: 2980 cm-1 (νas(CH2)); 1450 cm-1 (ν(CdC)); 1400 cm-1 (γ(CH2)); 1255 cm-1 (F(CH2)); 930 cm-1 (w(CH2)).36 The assignment of peaks at 1600 and 1450 cm-1 to ν(CdC) in the above studies assumes that vinyl has a CdC double bond, which in turn implies that the molecule is bonded to the metal with a single σ bond through the C atom at the CH site. An analogue of this structure in the organometalic cluster HOs3(CHdCH2)(CO)10 was reported earlier, with the following IR frequencies for CHdCH2: 3052 (νas(CH2)), 2998 (νs(CH2)), 2920 (ν(CH)), 1476 and 1310 (ν(CdC)/γ(CH2)), 1266 (F(CH)), 990 cm-1 (w(CH2)).45 The HREELS peaks36 are close to our RAIRS peaks at 2988, 1444, 1391, and 1280 cm-1. However, the temperature dependence of the RAIRS spectra as well as the observed isotope shifts clearly shows that only the peaks at 2988 and 1280 cm-1 are due to vinyl. The assignment of surface vibrational spectra on the basis of the analogous organometallic complexes is generally quite useful, but it does require that the surface species and organometallic species have similar structures and bonding interactions with the metal. However, DFT calculations do not support a structure for vinyl on Pt(111) with a CdC double bond.42,43 Instead, these calculations show that a stable vinyl species on Pt(111) is located above the 3-fold hollow site with one C atom bonded to two Pt atoms and the second C atom bonded to the third Pt atom. In this structure, the C-C bond length is 1.476 Å, which is very close to the C-C bond length of 1.489 Å for ethylidyne and significantly differs from the CdC bond length of 1.393 Å for µ3-η2-CCH2. This implies that the CC bond order is close to one. In the calculated structure, the surface vinyl has Cs symmetry with the C-C bond inclined at an angle of 75° to the surface normal. According to this configuration, the intensity of ν(CC) would be very weak if observable, and νas(CH2) would not be allowed by symmetry. These facts combined with the observed frequency shifts with 13C and D substitution allow us to confidently assign the peak at 2988 cm-1 to νs(CH2) and the very weak peak at 1280 cm-1 to ν(CC) of vinyl. The vinyl peaks at 2988 and 1280 cm-1 are seen at the same positions regardless of whether the surface is preexposed to H2 or D2. This then indicates that, of the two possible 1,D-vinyl isomers, CHCHD and CDCH2, only CDCH2 is formed. The deuterium substitution on the R-C site does not affect the stretch

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TABLE 1: Vibrational Frequency (cm-1) Assignment for Vinyl on Pt(111) mode assignment νas(CH2) νs(CH2) νs(CD2) ν(CH) ν(CC) -CHCH2 -CDCD2 γ(CH2) F(CH2) δ(CH) w(CH2) νs(CD2) νas(CD2) a

HREELSa

HREELSb

SFGc

RAIRSd

2990

2988 2242

2980 2920 2920 1600

1450

1380

1400 1255 870 930 2165 2275

1255 955

1280 1251

2242

32 b

Thermal decomposition from C2H3I. Photo-decomposition from CHCl2CH2Cl.36 c Thermal evolution of C2H2.28 d This work.

frequency of the νs(CH2) mode at the β-C site. Similarly, the reactions of C2D2 with H produces only the CHCD2 isotopomer, which has the same value of 2242 cm-1 for νs(CD2) as seen for CDCD2. According to the calculated CHCH2 bonding configuration,43 the νs(CH2) peak at 2988 cm-1 should be more intense than the other two C-H stretch modes of vinyl. The weak peak at 1280 cm-1 shifts to 1234 cm-1 in the experiment with 13C2H2, as shown in Figure 5d, which well-matches the estimated shift for a harmonic oscillator: ν(13C-13C) ) x12/13ν(12C-12C) ) 1230 cm-1. The 250 K spectra in Figures 7 and 8 also show very weak peaks at ∼1251 cm-1 for the C-C stretch for CDCD2 and CHCD2. These frequency assignments together with the assignments from other studies are listed in Table 1. The formation of only the CDCH2 isotopomer in the deuterium co-adsorption experiment indicates vinyl is formed by the hydrogenation of vinylidene rather than by the direct hydrogenation of acetylene. The DFT calculations show that µ3-η2-vinylidene is more stable than the di-σ/π-bonded acetylene on Pt(111).42,43 An earlier HREELS study also concluded that acetylene is hydrogenated to ethylidyne via vinylidene on Pt(111).2 These studies provide evidence of a reaction pathway in which acetylene isomerizes to vinylidene prior to hydrogenation reactions on Pt(111). Isomerization prior to hydrogenation also occurs for ethylene on Pt(111) in which a 1,2-H shift reaction to form ethylidene precedes ethylidyne formation.35,46-48 These two examples seem to suggest that intramolecular hydrogen transfer reactions have lower activation energies allowing isomerization to occur at relatively low temperatures. The decomposition of vinyl halides has been used in previous attempts to characterize vinyl on transition metal surfaces.32,34-36 Comparison with our results indicates that vinyl is more stable in the presence of co-adsorbed halide atoms; whereas the peaks due to vinyl are most intense at 250 K in our spectra and are completely absent after heating to 400 K, vinyl produced from vinyl halides was reported to be stable up to 450 K.32,36 We observe various isotopomers of ethylidyne in these experiments, and the identification of these isotopomers can provide insights into the ethylidyne formation mechanism. We have summarized assignments for all of the peaks that we observe for the four possible CCHxD3-x (x ) 0, 1, 2, 3) isotopomers in Table 2 as well as assignments proposed in earlier studies.47,49-51 The reaction of C2H2 with D gives rise to an intense peak at 1249 cm-1 in Figure 4 due to δ(CH2D) of CCH2D but no peak at 1270 cm-1 due to δ(CHD2) of CCHD2, as seen in Figure 8 from the reaction of C2D2 with H. Unlike the vinyl species, which is formed at lower temperatures, deuterated ethylidyne evidently undergoes H exchange reactions

with background hydrogen. For this reason, the CCH2D and even the CCH3 isotopomers are seen in the spectra of Figure 8. The CCH2D isotopomer of ethylidyne is the one that would be expected from the CDCH2 isotopomer of vinyl, and its presence as indicated by the intense peaks at 1249 and 2902 cm-1 in Figure 4 further supports the idea that the first step in the acetylene reaction is isomerization to vinylidene. Vinylidene (µ3-η2-CCH2) is the most stable C2H2 isomer according to DFT calculations42,43 and our results imply that acetylene isomerizes to vinylidene prior to the formation of vinyl. Therefore, the vibrational features of this species should be present in spectra obtained at temperatures below where vinyl is observed, i.e., for annealing temperatures of 250 K and below. However, no features attributable to a vinylidene species are observed below 300 K. The RAIR peaks of the µ3-η2-CCH2 species may be too weak to observe due to its inclined bonding configuration on the surface. The µ3-η2-CCH2 group in an organometallic cluster has its ν(CC) and γ(CH2) modes at 1331 and 1470 cm-1, respectively.44 We do not observe these modes at low temperature. However, annealing to higher temperatures and the formation of other co-adsorption species may cause a change to its bonding configuration. In the 300 and 350 K spectra in Figures 1 and 2, we see peaks at 1391 and 1444 cm-1, which are at reasonable values for the ν(CC) and γ(CH2) modes of vinylidene. From the results of the 13C substitution experiment shown in Figure 5 in which a broad peak with unresolved components at 1378 and 1388 cm-1 is seen, it seems that two species contribute to the 1391 cm-1 peak observed in Figure 1. Neither of these components shifts by as large a fraction as the CC stretch modes of vinyl and ethylidyne do with 13C substitution. However, we expect two internal coordinates of vinylidene, ν(CC) and γ(CH2), to make large contributions to the modes in the 1200-1400 cm-1 region, and this could lead to a smaller shift for the nominal ν(CC) stretch mode than is seen for a purer ν(CC) mode, such as in ethylidyne. Thus, we assign the peak at 1378 cm-1 in Figure 5 to the ν(CC) mode of vinylidene and the one at 1388 cm-1 to a mode of a different surface moiety, the δs(CH3) mode of ethylidene. The best evidence from our study for a vinylidene species is the peak at 1310 cm-1 in Figures 6-8. The fact that the 1391 cm-1 peak only shifts to 1310 cm-1 with deuterium substitution rules out assigning this peak to any sort of C-H deformation mode, such as a δ(CH3) mode of ethylidene or ethylidyne or the γ(CH2) mode of vinylidene. It must therefore be a CC stretch of a molecule with a CC bond of order midway between one and two. The δs(CH3) mode of ethylidene (13CH13CH3) is associated with the 1388 cm-1 peak in Figure 5, and also coincidently occurs at 1391 cm-1 in Figure 1 for the CHCH3 species. The deuterated CDCD3 isotopomer would have a δs(CD3) mode below 1200 cm-1 and, presumably like the δs(CD3) mode of CCD3, may also have an intensity too low to observe. The only objection to assigning the 1444 cm-1 peak to γ(CH2) of vinylidene is the fact that it seems to correlate better in terms of its thermal stability with the peak at 2964 cm-1, close to a peak at 2960 cm-1 previously assigned to νas(CH3) of ethylidene.52 The 1444 cm-1 peak is also at a reasonable value for the δas(CH3) mode of either ethylidene or ethylidyne, but δas(CH3) should not be allowed for an upright ethylidyne species, whereas ethylidene is known to be highly titled on the surface. As the correlation with the 2964 cm-1 peak was not so clear in an earlier study of ethylidene,52 a 1444 cm-1 peak seen there was assigned to δas(CH3) of ethylidyne molecules that were tilted with respect to the surface normal. Obviously, arguments

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TABLE 2: Vibrational Frequency (cm-1) Assignment for Ethylidyne Isotopomers on Pt(111) mode assignment

CH(3-x)DxC-Co3(CO)9 (x ) 0-2)a

Pt(111)b

2910

2916

2901

2898

2902

2888

2885

2887

ν(CH) -CCHD2 νs(CH2) -CCH2D νs(CH3) -CCH3 νs(CD3) -CCD3 δs(CH3) -CCH3 δs(CH2) -CCH2D δ(CH) -CCHD2 ν(CC) -CCH3 -CCH2D -CCHD2 -CCD3 a

Pt(111)c

2167

2080

1356

Pt(111)e

2037

1335

1339

1338

1245

1247

1249

1264

1245

1247

1270

1161

1120 1107 1120 1147

1120 1120 1120 1150

1126 1128 1158 1156

1182 b

Pt(111)d

c

d

1160

e

Reference 50. Reference 51. References 47 and 49. Reference 41. This work.

can be made for either assignment of the 1444 cm-1 peak, and the results presented here do not provide definitive proof of either one. The evidence we have presented seems to indicate the presence of two forms of vinylidene. The various isotopomers of vinyl that we observe suggest a low-temperature form of vinylidene that serves as a precursor to vinyl formation. However, there is no direct spectroscopic evidence for this form. There is spectroscopic evidence for a vinylidene species at temperatures not only above where vinyl forms but also above where it dissociates. In the SFG study of Cremer et al.,28 two forms of vinylidene were proposed, a µ3-η2-CCH2 species with the CdC axis at a large angle from the surface normal and a µ-vinylidene species with the CdC axis along the surface normal. Their sole basis for the identification was the observa-

Figure 10. Acetylene reaction pathways on Pt(111).

tion of an SFG peak assigned to νs(CH2) of vinylidene at 3033 cm-1, which was observed at temperatures similar to our peak at 3034 cm-1. We previously assigned a peak at 3037 cm-1 to ethynyl (CCH) on Pt(111), but it was stable up to 550 K.53 This makes the assignment of the 3034 cm-1 peak to the ethynyl species unlikely. This conclusion along with the correlation of the 1391 and 3034 cm-1 peaks with temperature, supports assignment of the latter peak to νs(CH2) of vinylidene. We would then have to assume that the γ(CH2) mode of vinylidene is too weak to observe, a reasonable assumption given the weakness of the two peaks we do assign to this species. As we assume that acetylene isomerizes to µ3-η2-CCH2 prior to vinyl formation, the higher temperature form of vinylidene could be the µ-vinylidene species proposed by Cremer et al.28

1466 J. Phys. Chem. C, Vol. 111, No. 3, 2007 5. Summary The reaction scheme in Figure 10 summarizes the transformations that lead from adsorbed acetylene to ethylidyne. The first step is isomerization of acetylene to µ3-η2-vinylidene. This species can be hydrogenated to either di-σ-bonded ethylene or to vinyl. The formation of the latter two species is enhanced through the co-adsorption of hydrogen. Vinyl can lead directly to ethylidyne through intramolecular H transfer, or it can be hydrogenated to ethylidene, which then leads to ethylidyne. The nature of the vinyl and ethylidyne isotopomers formed is the main evidence for the presence of µ3-η2-vinylidene as no vibrational features attributable to this species were observed. Vinyl can also be converted to µ-vinylidene at temperatures above 300 K. However, similar amounts of ethylidyne are formed without hydrogen co-adsorption, indicating that some acetylene decomposition must occur to supply the needed hydrogen. Although the additional vinyl observed with hydrogen co-adsorption might be expected to lead to additional ethylidyne, this is balanced by the additional ethylene that desorbs and is unavailable for ethylidyne formation. The reaction scheme and intermediates we have identified are essentially the same as proposed by Cremer et al.28 on the basis of a much more limited set of data. Unlike their SFG study in which only the C-H stretch region was accessible, we have used RAIRS to examine the entire mid-IR region, which provided additional information for assigning the spectra. In addition, by using isotopic substitutions with both 13C and D, we were able to reach more definitive assignments for almost all of the observed vibrational features. However, as is typical of such studies, many of the modes for several of the species do not give rise to vibrations with sufficient intensity to be observed, even if allowed by the surface dipole selection rule. Therefore, even with the more extensive information available here, some of the assignments remain tentative. Acknowledgment. This work is supported by the National Science Foundation under Grant CHE-0135561. References and Notes (1) Avery, N. R. Langmuir 1988, 4, 445. (2) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978, 15, 407. (3) Sheppard, N. Annu. ReV. Phys. Chem. 1988, 39, 589. (4) Gates, J. A.; Kesmodel, L. L. J. Chem. Phys. 1982, 76, 4281. (5) Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1983, 124, 68. (6) Kesmodel, L. L.; Waddill, G. D. Gates, J. A. Surf. Sci. 1984, 138, 464. (7) Marinova, Ts.; Kostov, K. Surf. Sci. 1987, 181, 573. (8) Jungwirthova´, I.; Kesmodel, L. L. J. Phys. Chem. B 2001, 105, 674. (9) Haug, K. L.; Buergi, T.; Gostein, M.; Trautman, T. R.; Ceyer, S. T. J. Phys. Chem. B 2001, 105, 11480. (10) Freyer, N.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1983, 125, 327. (11) Azad, S.; Kaltchev, M.; Stacchiola, D.; Wu, G.; Tysoe, W. T. J. Phys. Chem. B 2000, 104, 3107.

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