Identification at the Single Molecule Level of C 2 H x Moieties Derived from Acetylene on the Pt(111) Surface

Identification at the Single Molecule Level of C2Hx Moieties Derivedfrom Acetylene on the Pt(111) SurfaceTomonari Okada,† Yousoo Kim,*,† Michael Trenary,*,‡ and Maki Kawai*,§

†RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan‡Department of Chemistry, 845 W. Taylor Street, University of Illinois at Chicago, Chicago, Illinois 60607, United States§Department of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan

ABSTRACT: A low-temperature scanning tunneling micro-scope operated at 4.7 K was used to observe individualmolecules produced from the thermal decomposition andhydrogenation reactions of acetylene on the Pt(111) surface.Acetylene molecules observed on the surface followingadsorption at 50 K are seen to persist up to room temperatureat the same time as two other moieties are observed to form.One moiety greatly increases in amount when the acetylene iscoadsorbed with hydrogen and is attributed to the vinylspecies, HCCH2, in agreement with a recent study usingreflection absorption infrared spectroscopy. A third speciesobserved at room temperature is identified as vinylidene,CCH2. The identification of these species is aided by usingvoltage pulses from the STM to further decompose them into species containing fewer atoms. Ethylidyne, CCH3, is identifiedafter heating the surface to 400 K and was confirmed by comparison to results obtained following ethylene adsorption. Exposureof the surface held at 800 K to acetylene produced C2 molecules on the surface, which could be subsequently hydrogenated toethylidyne. The hydrogenation of residual surface carbon also leads to the formation of ethylidyne, suggesting that the residualcarbon was in the form of C2 molecules rather than carbon atoms.

1. INTRODUCTION

The identification and characterization of surface intermediatesis a key part of the determination of the mechanisms ofheterogeneously catalyzed reactions. Even for the relativelysimple case of C2Hx surface intermediates that form on thePt(111) surface from the reactions of acetylene or ethylenewith and without coadsorbed hydrogen, numerous possiblesurface intermediates have been proposed, yet clear exper-imental identification of some of these species has been elusive.The identify of these intermediates is relevant to the moreapplied problem of the selective hydrogenation of acetylene toethylene, which has been studied for many years.1,2 One aspectof this topic that has recently received attention is the role ofsubsurface hydrogen in hydrocarbon reactions over Pdnanoclusters and the influence of surface carbon in facilitatingthe exchange of hydrogen between the Pd bulk and surface.3,4

Although hydrogen is not absorbed into the bulk of Pt as it is inPd, subsurface H at a low concentration of 0.29% has beendetected in studies of Pt(111).5 It is possible that thissubsurface hydrogen has a special role in hydrogenationreactions on Pt(111), such as it does on Ni(111).6 Becauseof the low concentration of subsurface hydrogen, its role in thesurface chemistry of C2Hx species on Pt(111) has generally notbeen considered in past studies nor is it considered in the low-temperature scanning tunneling microscopy (LT-STM) studiespresented here.

The stability of various C2Hx moieties on Pt(111) has alsobeen the subject of many computational studies.7−14 Forexample, Jacob and Goddard9 computed the geometry andenergy of 21 different arrangements of two carbon and up to sixhydrogen atoms on a Pt35 model of the Pt(111) surface. In thepresent study we have used a low-temperature scanningtunneling microscope (LT-STM) to observe at the singlemolecule level the different species that form primarily fromacetylene, and also to some extent from ethylene, on thePt(111) surface. When interpreted in the context ofinformation provided in previous studies using experimentaltechniques that probe surface reactions at the monolayer level,the single molecule results provide new insights into thisimportant set of reactions.Avery15 used high-resolution electron energy loss spectros-

copy (HREELS) to characterize the thermal evolution ofacetylene on the Pt(111) surface over the temperature range of85−420 K. He concluded that acetylene initially bonds in a μ3-η2 configuration and that this form is stable up to 300 K. In thetemperature range of 330−400 K the adsorbed acetylenedisproportionates to form ethylidyne, CCH3, and presumablyethynyl, CCH, although the latter was not detected

Received: August 2, 2012Revised: August 7, 2012Published: August 7, 2012

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© 2012 American Chemical Society 18372 dx.doi.org/10.1021/jp307676f | J. Phys. Chem. C 2012, 116, 18372−18381

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spectroscopically. Similar spectra were observed in an earlierHREELS study.16 A later study using sum frequency generation(SFG) of acetylene on Pt(111) over the temperature range of125−340 K resolved multiple peaks in the C−H stretch regionand concluded that in addition to μ3-η

2-acetylene the followingwere also produced: tilted μ3-η

2 and upright μ forms ofvinylidene (CCH2), vinyl (HCCH2), di-σ-bonded ethylene,ethylidene (CHCH3), and ethylidyne.17 A RAIRS study ofacetylene on Pt(111) from 85 to 400 K also detected multipleC−H stretch peaks that evolved with temperature, but inaddition reported spectra over the wavenumber range from1000 to 3400 cm−1 for deuterium and 13C-substitutedacetylene.18 For the most part, Deng et al.18 attempted tofollow the same assignments proposed by Cremer et al.17

Despite the extensive vibrational data obtained, there wasconsiderable ambiguity as to the assignment of all the peaks toparticular surface moieties. A principal objective of the presentstudy is to use the capabilities of LT-STM to establish in aquantitative manner the number of different surface speciesformed from acetylene on Pt(111), which should permitreconciliation of some of the ambiguities present in theprevious vibrational studies. Also, recent computational studiesof the mechanism of ethylidyne formation from ethylene onPt(111) provide new information on the stability of variousC2Hx species on this surface.11,12

Both acetylene and ethylene produce the same stableintermediate, ethylidyne, CCH3, at temperatures from 260 to400 K, with slightly higher temperatures required to completethe conversion from acetylene. The relative ease of producingethylidyne is consistent with calculations showing that it is themost stable C2Hx moiety on Pt(111). Since the same product isproduced from both acetylene and ethylene, it is plausible thatthe same surface intermediate precedes ethylidyne formation inthe two cases. In contrast to ethylidyne formation fromacetylene on Pt(111), the mechanism starting from ethylenehas been the subject of many studies, with CCH2 (vinyl-idene),19 CHCH2 (vinyl),20 CHCH3 (ethylidene),21,22 andCH2CH3 (ethyl)23 intermediates proposed. In a series ofpapers, Zaera and co-workers have considered the merits ofeach of the proposed mechanisms.24−30 An SFG study observedthe rise and fall of a peak at 2957 cm−1 as di-σ-bonded ethylenewas converted to ethylidyne with the authors arguing that themost reasonable assignment was to the νs(CH3) stretch ofethylidene.21 In a subsequent RAIRS study peaks wereobserved at 2960 and 1387 cm−1 with the two peaks followingthe same temperature dependence, which were thereforeassigned to the νs(CH3) and δs(CH3) modes of ethylidene.22

Despite this spectroscopic evidence for ethylidyne formationvia an ethylidene intermediate, calculations show that there is avery large barrier for the intramolecular 1,2-H transfer to formethylidene from ethylene.11,12 A mechanism in which ethyl-idene is formed not directly from ethylene, but via a vinylintermediate, has therefore been proposed. Such a mechanismhas also been suggested for ethylidyne formation fromacetylene on Pd(111).31−33 Theoretical studies of surfacereaction mechanisms typically focus on the stabilities of surfaceintermediates and the reaction barriers that separate one moietyfrom another. Information on how the coverages of varioussurface species evolve with time at a given temperature can begained through kinetic Monte Carlo simulations, an approachthat was used recently for the formation of ethylidyne fromethylene on the Pd(111) and Pt(111) surfaces.12 Thesimulations were done in the presence of 1 torr of gas phase

ethylene, which had the effect of maintaining very lowcoverages of hydrogen, which was quickly removed from thesurface by reaction with ethylene to form ethane. The negligiblehydrogen coverages favored dehydrogenation over hydro-genation steps. The kMC results implied that ethylidyneforms from ethylene on Pt(111) via vinyl and vinylideneintermediates, with the rate-limiting step being the hydro-genation of vinylidene. Most of the experimental surfacescience studies of these mechanisms have been performedunder ultrahigh vacuum conditions. In the case of ethylidyneformation from acetylene, the coverage of hydrogen should alsobe low and the intermediates are likely to be the same ascalculated. The LT-STM images presented here are generallyconsistent with a mechanism by which ethylidyne is formed byway of vinyl and vinylidene. As excess surface hydrogen ispresent when ethylidyne is formed from ethylene, stepsinvolving hydrogenation become more likely. This is consistentwith our observations reported here of a different intermediate,possibly ethylidene, in the formation of ethylidyne fromethylene.Vibrational spectroscopy continues to be one of the principal

means of establishing the identity of surface intermediates.While vibrational spectra have a high degree of chemicalspecificity, with each surface moiety having a unique set offrequencies, in practice associating observed vibrational featuresto a particular surface species is often ambiguous. Thelimitations of vibrational spectra include the fact that aparticular surface species may have only a few, if any, peaksthat are of sufficient intensity to be observable in theexperimentally accessible spectral range, even if the vibrationaltransitions are fully allowed by the selection rules. In addition,vibrational peaks have widely varying intensities for differentspecies and for the various normal modes within a particularadsorbate, so that quantifying the relative coverages of differentspecies is generally not possible with vibrational spectroscopyalone. In contrast, when STM is used at temperatures lowenough to permit observation of single molecules, directinformation on relative coverages is revealed. This is becauseeach species generally has a different appearance, or a differentresponse to the tip, either during scanning or as the result ofdeliberate application of voltage pulses. However, althoughindividual molecules and their adsorption sites are observablewith the LT-STM, their chemical identity is not readilyderivable from the images. The complementary nature ofsurface vibrational spectroscopy for monolayer coverages andLT-STM for observations of single molecules is illustrated bythe results presented here on identification of C2Hx surfaceintermediates on Pt(111).

2. EXPERIMENTAL METHODSThe experiments were performed with a low-temperature STM(LT-STM, Omicron GmbH) housed in an ultrahigh vacuumchamber (base pressure: 3 × 10−11 torr). Acetylene andethylene were purchased from Taiyo Nippon Sanso withquoted purities of greater than 99.9% by volume and were usedwithout further purification. The Pt(111) surface was cleanedby repeated cycles of Ar+ sputtering followed by annealing to1100 K, which resulted in relatively clean, atomically resolvedSTM images. All STM results were obtained at a sample bias of0.1 V and 1.0 nA, unless otherwise noted, with electrochemi-cally etched tungsten tips while the surfaces were at 4.7 K. Thesample does not have an attached thermocouple so temper-atures were based on length of time after it was removed from

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the STM stage, which over a period of about 45 min brings thesample up to room temperature. Temperature versus timecalibration curves were established for other samples withattached thermocouples. Because of this procedure, it was notonly difficult to reproducibly attain precise temperaturesbetween 4.7 and 300 K, but it was also difficult to anneal thesample between 300 and 700 K, with the latter temperaturebeing the lowest that could be read with an optical pyrometer.

3. RESULTS3.1. Thermal Chemistry of Acetylene on Pt(111).

Figure 1 shows STM images of the Pt(111) surface before

(a) and after (b) exposure to acetylene. The image of thenominally clean surface in Figure 1a reveals at least three typesof impurities at a total coverage estimated to be 7% of amonolayer. This impurity level is somewhat higher than usuallyquoted for “clean” Pt(111) surfaces prepared by methodssimilar to those used here. A surface judged free ofcontamination by techniques such as Auger electron spectros-copy or X-ray photoelectron spectroscopy is usually assumed tohave up to a few percent of a monolayer of contamination. Asone of the goals of the present study is to compare LT-STMresults with results from other studies where comparable levelsof contamination were also presumably present, the contam-ination evident in Figure 1a does not present any particularproblem. Given that carbon is the most persistent contaminantdetectable on Pt(111) surfaces, the two types of contaminantslabeled A and B are presumed to be two forms of carbon.Subsequent images described below support this conclusion.The even darker objects in the image labeled “impurity” arepresent at a much lower coverage and could be due to any of anumber of different elements often detected on Pt surfaces,such as Ca or Si. We assume that they have no influence on theresults or conclusions reached in this study.Following exposure to enough acetylene at a surface

temperature of ∼50 K to achieve a coverage of approximately0.01 ML, the image in Figure 1b was obtained revealingindividual acetylene molecules as the pale white protrusions.Figure 2 is an expanded view of the adsorbed acetylenemolecules that clearly shows that the protrusion is accompaniedby a depression on one side, directly revealing the asymmetry ofthe adsorbate−substrate complex. Three different orientationsof the molecules are also evident in the image, consistent withthe symmetry of the surface. The appearance of the acetylenemolecules is quite similar to what was observed on a Pd(111)

surface,34 and it can be assumed that acetylene occupies thesame adsorption site on the two surfaces. On Pd(111), whereboth the molecule and the Pd lattice were observable in thesame image, the protrusion is centered over a threefold hollowsite with the depression straddling an atop site. This isconsistent with the idea of σ bonding of the two carbon atomsto two metal atoms causing the CCH angles to decrease from180°, combined with the molecular plane tilting toward a thirdmetal atom to achieve π bonding. The results thus confirm thedi-σ/π bonding in a μ3-η

2 configuration for the adsorbedmolecule proposed in the earliest vibrational studies ofacetylene on Pt(111).15,16,35 The detailed structure obtainedfrom DFT calculations also supports the μ3-η

2 configurationand indicates a CCH angle of 126°.9 From the assumedadsorption site for acetylene on Pt(111), the adsorption sites ofthe other species produced from acetylene could be deduced,without direct observation of the Pt-atom lattice.Figure 3 shows the results of annealing a low coverage of

acetylene on Pt(111) to 250 K. In this image, the acetylene

molecules are still visible, but two additional types ofprotrusions are also clearly seen. The largest protrusion, onlyone of which is apparent in the image, is not reproduciblyobserved and is therefore attributed to an impurity molecule.Also apparent in the image are about half as many acetylenemolecules as seen in the image of Figure 1b. Figure 4 shows anexpanded view of the image of Figure 3 with a superimposedlattice of Pt atom positions established from the position of theacetylene molecules. Again ignoring the one brightest object,there are clearly two different species in addition to acetylenemolecules, which are the dimmest objects in these images. The

Figure 1. STM images (10 × 10 nm) of (a) clean Pt(111) and (b)after exposing the surface at 50 K to enough acetylene to achieve acoverage of approximately 0.01 ML. The nominally clean surfaceshows various impurities, with those marked A and B the mostprevalent and assumed to be due to carbon.

Figure 2. STM image (3 × 3 nm) showing that adsorbed acetylenemolecules appear as protrusions accompanied by depressions on oneside. For the molecules shown, the depressions have differentorientations, reflecting the underlying symmetry of the substrate.

Figure 3. STM image (10 × 10 nm) of the acetylene-exposed surfaceafter it was annealed to 250 K. In addition to adsorbed acetylene, twoadditional types of protrusions are visible in the image.



two species are both centered over threefold hollow sites andappear round in shape without the depressions associated withacetylene. Through the detailed arguments presented below, weidentify the brighter of these objects as vinyl (CHCH2) and thedimmer one as vinylidene (CCH2). In this particular image,vinyl is more abundant than vinylidene. However, in otherimages following a nominal 250 K anneal, vinylidene is moreabundant. In another image taken after annealing the surface asecond time, the vinyl-to-vinylidene ratio increases. Thevariability in the relative number of these two species reflectsthe difficulty in reproducing the temperature. The results afterannealing to 250 K imply, however, that vinylidene is anintermediate in the formation of vinyl from acetylene. Afterannealing to an even higher temperature of 300 K, the image inFigure 5 was obtained, in which vinyl is now the predominant

species. When the annealing takes place in the presence of anambient of 2 × 10−10 torr of hydrogen, the surface is coveredessentially exclusively by vinyl, as shown in Figure 6.The expectation that acetylene will eventually convert to

ethylidyne is supported by the image of Figure 7, where thesurface with adsorbed acetylene was annealed to 400 K. In thiscase, an entirely different set of objects are observed, which arebrighter than acetylene, but not as bright as vinylidene or vinyl,and are identified as ethylidyne molecules. The two very brightobjects are either impurities or possibly ethylidyne decom-position products. The identification of ethylidyne in Figure 7 issupported by other images obtained after annealing anethylene-covered surface to 300 K. From other studies, it is

known that ethylidyne can be formed from ethylene at slightlylower temperatures than from acetylene, and this is confirmedby our observations where ethylidyne is always observed for a300 K anneal in the case of ethylene, regardless of the time atthat temperature. Although ethylidyne appears the same in theimages, regardless of whether acetylene or ethylene is initiallyadsorbed, vinyl and vinylidene were not observed followingethylene adsorption.The previous vibrational studies of acetylene on Pt(111)

allow ready interpretation of the STM results in Figures 1−7.In both the SFG study of Cremer et al.17 and the RAIRS studyof Deng et al.,18 multiple closely spaced C−H stretch peakswere observed before the conversion of acetylene to ethylidynewas complete. Furthermore, annealing the acetylene layer in thepresence of hydrogen was found by RAIRS to lead to a greatlysimplified C−H stretch region dominated by a peak at 2988cm−1, which was attributed to the acetylene hydrogenationproduct, vinyl, CHCH2. Based on the differences in the spectrawhen C2H2 and C2D2 were coadsorbed with hydrogen anddeuterium, it was concluded that vinylidene, CCH2, is anintermediate in the formation of vinyl from acetylene, aconclusion fully consistent with the LT-STM images shownhere.

3.2. Tip-Induced Reactions. In addition to observing thechemical transformations that occur at different temperatures,voltage pulses can be used to induce reactions that do not occurthermally. This can further aid the identification of surfaceintermediates in this case as it can be safely assumed that forthe voltages used, the reactions lead either to reconfiguration ofthe bonding of a given adsorbate, such as due to isomerization,movement to a different type of site, or hopping to anequivalent site, or to the degradation of a given surface species

Figure 4. STM image (5 × 5 nm) of the acetylene-exposed surfaceafter it was annealed to 250 K along with a superimposed grid of Ptatom positions. The positioning of the Pt lattice is based on theadsorption site of the adsorbed acetylene molecules. The brightestobject in the image is an impurity unrelated to acetylene or its reactionproducts.

Figure 5. STM image (10 × 10 nm) of the acetylene-exposed surfaceafter it was annealed to 300 K.

Figure 6. STM image (10 × 10 nm) of the acetylene-exposed surfaceafter it was annealed to 250 K in a background of H2(g) at 2 × 10−10

torr.

Figure 7. STM image (10 × 10 nm) of the acetylene-exposed surfaceafter it was annealed to 400 K.



into a simpler one. Figure 8 shows images before and afterapplication of a 3.0 V pulse from the tip to the acetylene

molecules labeled “J”. Two of the molecules have beentransformed into a different species labeled “X”, while thethird acetylene molecule has been moved to a different butequivalent site. The X molecule already present in the upperimage had been produced by an earlier 3.0 V pulse. The Xmolecules have a slight but distinct oval shape and occupy anadsorption site different from the other species with the longaxis of the oval perpendicular to the line between two nearest-neighbor Pt atoms. Although the identity of the X molecules isnot clear, a possible candidate is an acetylene molecule rotatedfrom its equilibrium geometry. Because this species is onlyobserved as a tip-induced reaction product, it is not part of thethermal chemistry of acetylene on Pt(111).Of more relevance to the thermal chemistry is the behavior

of vinyl and vinylidene in response to voltage pulses as shownin Figure 9. The left panel shows four vinylidene molecules,marked “Z”, and one vinyl molecule, marked “E”. An Xmolecule is also present in the left panel, having been producedby a previous voltage pulse from the tip. After application of thepulse, the E molecule is clearly transformed into a Z molecule,showing that the pulse can abstract an H atom from vinyl toform vinylidene. The reverse reaction, the tip-induced trans-formation of vinylidene into vinyl, is never observed, addingadditional support to the identity of these two species. Theimage also shows that Z is transformed into a new species,labeled “I”, which appears as a weak protrusion accompanied bya depression to one side. Although this species is somewhatsimilar to the appearance of the μ3-η

2-acetylene, its protrusion

is lower, and it is therefore assigned to a species with one lesshydrogen atom than vinylidene, namely ethynyl, CCH. Otherimages show that CCH can be further degraded into a speciesthat appears quite similar to the black dots seen in all of theimages, which are attributed to residual carbon.

3.3. Properties of C2 Molecules and C Atoms. As tip-induced reactions are capable of producing objects that appearquite similar to the carbon impurities, experiments wereconducted to further explore the properties of surface carbon.A RAIRS study by Deng et al.36 showed that acetyleneexposures to the Pt(111) surface at a temperature of 750 Kproduced surface carbon that could then be hydrogenated toproduce mainly ethylidyne, along with some ethynyl. Througha combination of RAIRS intensities, hydrogen thermaldesorption, and Auger electron spectroscopy, it was foundthat essentially all of the acetylene-derived carbon could behydrogenated to either ethylidyne or ethynyl. The results weresimilar to those of an earlier study where HREELS was used toidentify the species formed from the hydrogenation of surfacecarbon that was deposited in the form of carbon atoms fromthe gas phase.37 In that case, ethylidyne was also observed,indicating that carbon−carbon coupling followed by ethylidyneformation had occurred. Figure 10 shows a 10 × 10 nm image

taken after exposing a carbon-covered surface to 0.6 L ofhydrogen. Prior to hydrogen exposure, 0.2 L of acetylene wasdosed onto the surface at a temperature of approximately 800K. The image reveals that a small fraction of the surface carbonhas reacted to form a CxHy species, which has the appearance ofethylidyne. The reason that such a small fraction of the carbonis hydrogenated in this case is probably because the hydrogenexposure was so much less than that used by Deng et al.36

Figure 8. STM images (5.0 × 2.5 nm) before (upper) and after(lower) application of 3.0 V pulses to two acetylene molecules, markedJ in the upper image, which convert them to the molecules labeled X.The X molecule in the upper image had been produced from an earlier3.0 V pulse to an acetylene molecule.

Figure 9. STM images (3.0 × 5.0 nm) before (left) and after (right) application of 3.0 V pulses to the molecules, marked Z, X, and E in the leftimage. The structural models of these species include a possible rotated acetylene molecule, X.

Figure 10. STM image (10 × 10 nm) obtained with 0.5 V and 1.0 nAafter exposing a surface covered with C2 molecules to 0.6 L of H2. TheC2-covered surface was obtained by exposing the Pt(111) crystal toacetylene at 800 K.



Although the appearance of ethylidyne in the images followinghydrogen exposure supports the presence of C2 molecules onthe surface, C2 itself was difficult to characterize with STM.Images obtained before and after exposing the surface at 800 Kto 0.2 L of acetylene showed no pronounced changes. As notedearlier, the nominally clean surface shows depressions that areassumed to be due to carbon contamination. These depressionsare not of uniform depth, so that they appear in the images asdots of varying degrees of darkness. The acetylene-derivedcarbon appears as the lightest of these dots, which raises thepossibility that some of the residual contamination on thesurface is in the form of C2 molecules. This is confirmed by theseries of images in Figure 11, which shows a comparison of thebias dependence of images obtained following room temper-ature exposure of H2 to a nominally clean surface with a set ofimages obtained following room temperature exposure to C2D4,which is known to produce mainly ethylidyne. In the images at0.2 V, the molecules in the top row are seen as brightprotrusions, which become dimmer as the bias is increased to1.0 V. In the image at 2.5 V, the molecules appear as weak ringsor halos. In the lower set of images, the more abundantmolecules are presumed to be ethylidyne, which changes itsappearance in parallel with the molecules in the upper row. Thebias dependence of the less abundant molecules in the lower set

of images is completely different, confirming that they areneither ethylene nor ethylidyne. We therefore assume that theyare ethylidene. The line profiles also shown in Figure 11 furtherindicate that the molecules in the upper and lower rows ofimages are the same. These results thus provide good evidencethat the residual surface carbon is hydrogenated to ethylidyne,establishing that the carbon was in the form of C2 molecules.Somewhat similar results were reported for the hydrogenationof carbon on the Ru(0001) surface, where the productappeared as a bright spot surrounded by a dark ring at a biasof 9 mV and a current of 495 pA.38 In that case, the carbon wasassumed to be in the form of individual atoms, and thehydrogenation product was assumed to be CH. For the upperrow of images in Figure 11 to be attributed to CH wouldrequire that CH and CCH3 have the same variation inappearance with bias voltage, which seems unlikely. In additionto producing what were presumed to be C2 molecules fromacetylene exposures at 800 K, the tip was used to dissociatewhat were presumed to be CCH moieties. However, theproduct of that process appeared as a dark and elongated objectwith the long axis perpendicular to the line connecting twonearest-neighbor Pt atoms, which is the same site occupied bythe X molecules described above. Another feature of the roundand deep depressions at threefold hollow sites, which are

Figure 11. (a) Series of 6.3 × 6.3 nm STM images obtained at bias voltages of 0.2, 1.0, and 2.5 V of a surface covered with ethylidyne (lower threeimages) and a surface that had been exposed to H2 at room temperature to hydrogenate residual surface carbon (upper three images). Theethylidyne-covered surface was prepared by exposing the Pt(111) surface to C2D4 at room temperature followed by a second annealing to roomtemperature. (b) Line profiles of the molecules indicated by the arrows in (a) for the upper (left) and lower (right) set of images in (a).



assumed to be due to carbons atoms, is that they are irreversiblytransformed into much less dark dots (shallower depressions)in response to 1 V pulses from the tip. If the objects are indeedcarbon atoms, then the only explanation for the change is thatthe voltage pulse drives atoms at the surface to subsurface sites.Regardless of the exact interpretation of the results, they clearlyreveal that there are several different forms of pure carbon onthe surface.

4. DISCUSSIONOne of the most valuable new insights provided by the LT-STM results presented here is on the number of distinctmoieties that form on the Pt(111) surface following thedeposition and annealing of acetylene. One species that is notobserved to form from acetylene is ethylene. In experimentswhere the surface is directly exposed to ethylene, both di-σ- andπ-bonded forms of the adsorbed molecule are observed,39 butthe characteristics of these surface species do not match thoseproduced from acetylene. Our conclusion that ethylene doesnot form, but that vinyl and vinylidene do, is somewhatsurprising as once vinyl is present the pathways to vinylidene orethylene are roughly equally favorable; CHCH2 → CCH2 + His calculated11 to be exothermic by 15 kJ/mol with an activationenergy of 53 kJ/mol whereas CHCH2 + H → CH2CH2 isexothermic by 25 kJ/mol with a barrier of 56 kJ/mol.14 Theseenergies were calculated for a coverage of 1/9 of a monolayer,which although low is still considerably higher than the ∼0.01ML used in the LT-STM experiments. The calculated energiesare highly dependent on coverage, with some reactions that areexothermic at one coverage becoming endothermic at anothercoverage. For example, at 1/3 of a monolayer the CHCH2 →CCH2 + H reaction has a ΔE of +32 kJ/mol, compared to −15kJ/mol at 1/9 of a monolayer.11 This suggests that an exactcorrespondence can only be expected if the experiments andcalculations pertain to exactly the same coverages.The absence of ethylene in the LT-STM images necessitates

a reconsideration of some previously assigned vibrational peaks.Thus a C−H stretch vibration observed with SFG17 at around2900 cm−1 and by RAIRS18 at 2918 cm−1 that was attributed todi-σ-bonded ethylene, which is known to give a strong C−Hstretch vibration at 2904 cm−1,22,40 needs to be reassigned. Aplausible alternative is the CH stretch of the CH end of vinyl,CHCH2. There are several arguments in favor of thisreassignment. First, in the study of Deng et al. the peak at2918 cm−1 grew in intensity, together with the one at 2988cm−1 attributed to vinyl, after annealing acetylene coadsorbedwith hydrogen to 250 K.18 Second, the 2918 cm−1 peakdisappeared while the 2988 cm−1 peak remained followingannealing to 250 K a surface covered with both acetylene anddeuterium. It was proposed that vinyl was formed via a CCH2intermediate, so that while coadsorption with H would produceCHCH2, coadsorption with D would produce CDCH2. Theanalogous behavior was observed when C2D2 was annealed to250 K where a 2242 cm−1 peak was observed in the presence ofeither coadsorbed H or D, indicating that CHCD2 and CDCD2were formed, but not CHCDH or CDCDH. This result ishighly supportive of assigning the lower wavenumber peak tothe CH stretch of vinyl, ν(CH), and the higher one to thesymmetric CH2 stretch, νsym(CH2). The asymmetric CH2stretch of vinyl would not be allowed for a μ3-η

2 structure,where the local symmetry would be Cs. Third, the positions ofthe two peaks match very well the values reported for vinyl inthe HOs3(CHCH2)(CO)10 complex at 2998 and 2920 cm−1,

with these two vibrations assigned to νsym(CH2) and ν(CH),respectively.41 Furthermore, the 2998 cm−1 peak was describedas being more intense than the 2920 cm−1 peak, whichis the same intensity relationship seen for the 2988 and 2918cm− 1 peaks in the RAIRS exper iment . 1 8 In a[Ru2(CO)3(C5H5)2(CHCH2)]BF4 complex, the correspond-ing values for vinyl are 2981 and 2904 cm−1.42 Thus, assigningthe two peaks at 2988 and 2918 cm−1 to one species, vinyl,reconciles the RAIRS and LT-STM results and is wellsupported by previous spectroscopic studies of vinyl.This assignment of the 2918 cm−1 peak depends critically on

it being due to ν(CH), rather than to νsym(CH2), of vinyl. Yetin other studies the opposite assignment is made. Based on anormal-coordinate analysis using empirical force constants ofthe vinyl fragment in the Os3 and Ru2 complexes referred toabove, Evans and McNulty42 argued that νsym(CH2) < ν(CH).The same order was reported in a recent DFT study of vinyl onPt(111).11 We carried out DFT calculations of vinyl attached toa planar Pt6 model of Pt(111) and also obtained the orderνsym(CH2) < ν(CH), which was also found for vinyl on thelarger Pt35 cluster model of Pt(111).43 The frequenciesdetermined from DFT calculations are, like the normal-modecalculations of Evans and McNulty,42 based on the harmonicapproximation and do not take into account anharmonicity,which can be quite large for CH stretch vibrations. Thus, if theanharmonic shifts are different for νsym(CH2) and ν(CH),assignments based on a normal-mode analysis, whether usingempirical or ab initio force constants, might easily lead to thewrong assignment of these two modes of vinyl. Despitecalculations based on the harmonic approximation that indicateνsym(CH2) < ν(CH), there is sufficient evidence to concludethat the opposite order is correct. In support of this conclusionwas a study44 of vinyl in the HOs3(CHCH2)(CO)10 complexin which intensities were used to argue that the assignment ofAndrews et al.41 (2998 cm−1νsym(CH2); 2920 cm−1ν(CH)), were preferred over the opposite assignment ofEvans and McNulty.42

The mechanism of ethylidyne formation on transition metalsurfaces has been the subject of many previous studies, and so itis important to assess the extent to which the LT-STM resultspresented here add additional insight into this important issue.The previous vibrational studies of acetylene on Pt(111) allshow that it forms ethylidyne on the surface after heating totemperatures in the range of 300−400 K. The additionalhydrogen needed to form CCH3 from HCCH is presumablysupplied from the background or by dissociation of acetylene toform CCH. In particular, RAIRS showed that after annealing to400 K, the only peaks in the spectrum were those of ethylidyne,whereas additional peaks due to one or more intermediateswere still visible after a 350 K anneal.18 The LT-STM resultsalso show that heating to above room temperature wasnecessary to produce a new species, not seen at roomtemperature or below. This additional species is identified asethylidyne, a conclusion supported by comparison with resultsobtained following ethylene adsorption. Unfortunately, becauseof experimental limitations it was difficult to anneal the sampleto temperatures above room temperature in a controlled andreproducible manner. For this reason, it was not possible toestablish with the LT-STM if additional intermediates, beyondvinyl and vinylidene seen at lower temperatures, develop priorto the formation of ethylidyne from acetylene.In contrast to the conversion of acetylene to ethylidyne,

ethylidyne formation from ethylene on Pt(111) has been the



subject of many studies. The recent kMC study12 considers theprevious experimental and theoretical work on the mechanismand reaches several useful conclusions. First, the barriers for1,2-hydrogen shifts, such as in the isomerization of ethylene(CH2CH2) to ethylidene (CHCH3) or of vinyl (CHCH2) toethylidyne (CCH3), are so high that these steps can be ruledout. The barrier for the 1,2-H shift to form vinylidene directlyfrom acetylene on Pt(111) apparently was not calculated, but abarrier of 119 kJ/mol was obtained on Pd(111),45 which is lowenough to make the C2H2 → CCH2 isomerization plausible.Second, for low hydrogen coverages, vinyl dehydrogenation tovinylidene is faster than hydrogenation to ethylidene. Third, thebarrier for the dehydrogenation of ethylidene to ethylidyne isvery low so that a stable ethylidene intermediate is unlikely toaccumulate on the surface. Under the conditions considered inthe kMC study, ethylene loses hydrogen to form vinyl, whichloses another hydrogen to form vinylidene, and the rate-limiting step to ethylidyne formation is then hydrogenation ofvinylidene. Each step involves transfer of a single hydrogenatom to or from the Pt surface. The kMC study also providescoverages of the various species versus temperature, showingthat vinylidene, but not vinyl or ethylidene, is present atsignificant coverages under the conditions considered. Theyalso find that hydrogenation of vinylidene to vinyl is 2 orders ofmagnitude faster than hydrogenation of vinylidene to ethyl-idyne. However, the latter reaction is irreversible, so thatethylidyne can eventually become the dominant species on thesurface. At lower temperatures, vinyl and vinylidene can bereadily interconverted, a finding consistent with our LT-STMobservations. Unlike in the kMC simulations, in UHV studiesof ethylidyne formation following adsorption of ethylene, thecoverage of the subsequently formed vinyl will equal thecoverage of hydrogen, and a high hydrogen coverage wouldfavor ethylidene formation.The spectroscopic evidence for the different mechanisms of

ethylidyne formation is ambiguous. Ethylidene was identifiedfrom observation of a C−H stretch at 2957 cm−1 with SFG,which rose and fell in intensity as the CH stretch attributed todi-σ-ethylene decreased and before the CH stretch ofethylidyne fully developed, behavior that would be expectedof an intermediate.11 A RAIRS peak observed at 2960 cm−1 wasalso attributed to ethylidene, as was an accompanying peak at1387 cm−1, assigned to the δs(CH3) mode.12 A comparison ofthe RAIRS results for ethylene and acetylene on Pt(111)reveals that the same set of peaks of one or more intermediatesare observed in both cases at 1387−1389, 1444, and 2960−2964 cm−1, suggesting in retrospect that there is a commonintermediate in the formation of ethylidyne from eitherethylene or acetylene. All these peaks could be due toethylidene as similar values were observed in a (μ2-CHCH3)-Os2(CO)8 complex at 1369, 1447, and 2950 cm−1, assigned toδs(CH3), δas(CH3), and νs(CH3), respectively, of ethylidene.

46

This interpretation would then require alternative assignmentsof the minor peaks observed with RAIRS in the conversion ofethylene to ethylidyne, which were attributed to tiltedethylidyne molecules. Thus, the 1444 cm−1 peak was attributedto the δas(CH3) mode of ethylidyne, as it was seen togetherwith peaks at 975 and 2939 cm−1, attributed to ρ(CH3) andνas(CH3). These three modes would be allowed for a titledethylidyne molecule, but not for the one with the CC bondoriented along the surface normal. However, the 1444 cm−1

peak seen in the formation of ethylidyne from acetylenecompletely disappeared after a 400 K anneal, indicating that it

was not associated with ethylidyne. Ambiguity arises from thefact that some of the peaks assigned to ethylidene can also beplausibly assigned to vinylidene. In Os3(CO)9(μ-H)2(μ3-η

2-CCH2),

42 νsym(CH2) is at 2986 and a mixed δ(CH2) +ν(CC) mode is at 1470 cm−1, which are close to the observedRAIRS peaks at 2960−2964 and 1444 cm−1, but the overallassignment to ethylidene is still better. A RAIRS peak observedat 3034 cm−1 in the conversion of acetylene to ethylidyne, butnot in the conversion of ethylene to ethylidyne, matches theνasym(CH2) mode of vinylidene at 3047 cm

−1, but it should notbe IR-allowed for a surface μ3-η

2-CCH2 structure. Thus if thepeaks associated with the immediate precursor to ethylidyneformation are assigned to a single species, ethylidene is the bestmatch.Like the spectroscopic studies, the LT-STM images also do

not provide clear evidence for the mechanism of ethylidyneformation from acetylene, which takes place at temperaturessomewhat higher than easily achieved in these experiments.The key question is whether an additional species, namelyethylidene, would be observed with LT-STM after annealing totemperatures between those where vinyl and ethylidyne areobserved. Unfortunately, such experiments were not possible inthe present study. In the case of ethylene adsorption at roomtemperature, a species distinct from ethylidyne and distinctfrom any of the species observed following acetyleneadsorption was observed in Figure 11 and is likely to beethylidene, but further characterization would be necessary toconfirm this. A key difference in forming ethylidyne fromethylene versus from acetylene is that there is excess hydrogenin the former case but a deficiency in hydrogen in the latter. Ahigher hydrogen coverage would favor ethylidene formation. Adifference between the LT-STM and RAIRS experiments is inthe initial hydrocarbon coverage. The identification of ethyl-idene following acetylene adsorption with RAIRS but not withLT-STM could be due to the much higher coverages used inthe former experiments. A DFT study of acetylene conversionto ethylidyne on Pt(111) noted that an ethylidene intermediatewas favored at higher coverages.13

In addition to the products of acetylene isomerization(vinylidene) and acetylene hydrogenation (vinyl and ethyl-idyne), the LT-STM results provide some information on theproducts of acetylene dissociation to ethynyl (CCH) andacetylide (C2). Although the formation of ethynyl (CCH)would provide the third H atom needed to form ethylidyne(CCH3) from acetylene (C2H2), ethynyl was not observed withLT-STM from the thermal decomposition of acetylene.However, the product of the tip-induced reaction of vinylidenewas assumed to be ethynyl. In RAIRS studies,22,47 ethynyl wasidentified from a weak CH stretch at 3037 cm−1 afterethylidyne dissociation at 500 K, with the ethylidyne producedfrom ethylene. Since the precursor to ethylidyne formationshould not influence its subsequent decomposition chemistry, a500 K anneal following acetylene adsorption should alsoproduce ethynyl. It was not, however, possible to do thisexperiment in the LT-STM system. A procedure used toprepare C2 on Pt(111) by exposure to acetylene at a surfacetemperature of 800 K was followed.36 The C2 moleculesprepared in this way appeared as shallow depressions and weredistinct from the darker dots attributed to carbon impuritiesthat were always visible in the images, even for the nominallyclean surface. In agreement with previous RAIRS and HREELSstudies,36,37 the C2 molecules could be hydrogenated to CCH3,as established from a comparison with ethylidyne produced



from ethylene. A surprising result was that ethylidyne was alsoproduced by hydrogen exposure to the nominally clean surface,implying that a large fraction of the residual surface carbon wasin the form of C2 molecules. This in turn implies that C2 has ahigh stability relative to dissociation to carbon atoms oragglomeration into larger carbon particles or into unreactivegraphite. The DFT calculations of Jacob and Goddard clearlyindicated that C2 is less stable than two carbon atoms.9

However, an earlier estimate using a combination ofexperimental and calculated energies concluded that dissocia-tion of C2 into C atoms on Pt(111) would be endothermic.48

The observation by Smirnov et al.,37 that a high coverage ofcarbon atoms deposited onto Pt(111) from the gas phasewould result in C−C bond formation to make C2, also suggeststhat C2 is more stable than isolated adsorbed C atoms.37 Fromtheir DFT calculations, Basaran et al.14 concluded that the C2→ 2C reaction is exothermic with ΔE = −46 kJ/mol onPt(111) but with a very high barrier of 135 kJ/mol, whereas it isstrongly endothermic on Ni(111) with ΔE = 71 kJ/mol. Thehigh barrier on Pt(111) for the dissociation of C2 suggests thatit might be kinetically, if not thermodynamically, stable.Spectroscopic analysis of carbon on nickel surfaces concludedthat species such as C2, or C3 were prevalent before graphiteformation at higher temperatures.49 There is evidence thatisolated carbon atoms can exist on Pt(111) and can behydrogenated to CH species as indicated by vibrationalspectroscopy.37,47 This was also found to be the case onRu(0001), where the C atoms produced from either ethyleneor CO dissociation could be hydrogenated to CH, as revealedby a C−H stretch at 2950 cm−1, unaccompanied by lossfeatures at the lower wavenumbers expected for other C2Hxspecies.50 The carbon atoms observed with LT-STM onRu(0001) by segregation from the bulk are also readilyhydrogenated to CH, and there is no doubt in that case that thereactive carbon species is not C2.

51 The most surprising aspectof the LT-STM results presented here on the hydrogenation ofresidual carbon is that only a single species, ethylidyne, wasobserved, rather than a mixture of both CH and CCH3, aswould be expected based on the previous work. The LT-STMresults indicate that by observing the products of hydrogenationreactions, the technique can provide a potentially powerful wayto investigate the nature of carbon on transition metal surfaces,and further studies along these lines are clearly warranted.

5. CONCLUSIONSThe LT-STM results presented here show that acetyleneadsorbs on the Pt(111) surface at 50 K in a di-σ/π fashion asproposed in earlier studies and as observed in LT-STM studiesof acetylene on the Pd(111) surface. This form of acetylene isobserved even after annealing the sample to room temperature.Annealing to temperatures of 250−300 K produces two newspecies, one of which completely covers the surface in responseto hydrogen coadsorption and is therefore identified as ahydrogenation product. As previous vibrational studiesidentified the hydrogenation product as vinyl, the sameassignment is made here, with the other species identified asvinylidene. Identification of vinyl and vinylidene is supportedby the observation that the pulses from the STM tip couldconvert the species identified as vinyl to the one identified asvinylidene, but the reverse reaction did not occur. Ethylidynewas observed after acetylene adsorption upon heating totemperatures in the range of 300−450 K. An intermediatespecies between vinyl and ethylidyne was not observed.

Ethylidyne was also observed as a product of the hydrogenationof surface carbon, indicating that the carbon was in the form ofC2 molecules on the surface.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (M.T.), [email protected] (Y.K.),[email protected] (M.K.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSM.T. acknowledges support by a grant from the U.S. NationalScience Foundation under grant CHE-1012201. We thank Dr.Timo Jacob for providing us with additional unpublishedcomputational results for C2Hx species on Pt35 and ProfessorMarcus Wilde for providing additional information on hisresearch.

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Identification at the Single Molecule Level of C 2 H x Moieties Derived from Acetylene on the Pt(111) Surface