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Surface Science
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CO adsorption and dissociation on Ru(0001) at elevated pressures☆
David E. Starr a,⁎, Hendrik Bluhm b
a Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USAb Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
☆ Notice: This manuscript has been co-authored by emAssociates, LLC under contract no. DE-AC02-98CH1088Energy. The publisher by accepting the manuscript for pthe United States Government retains a non-exclusivewide license to publish or reproduce the publishedallow others to do so, for United States Government pu⁎ Corresponding author at: Helmholtz-Zentrum Berl
GmbH, Institute for Silicon Photovoltaics, KekuléstrasFax: +49 30 806241333.
Wehave investigated the adsorption anddissociation of CO onRu(0001) surfaces at pressures fromultra-high vac-uum up to 1 Torr and temperatures from 293 to 575 K using Ambient Pressure X-ray Photoelectron Spectroscopy(AP-XPS). At CO pressures above ~10−6 Torr additional CO is adsorbed on the surface, leading to a CO coveragegreater than the saturation coverage (0.66 ML) observed under UHV conditions. Binding energy shifts of the O1s core level indicate that most of the additional adsorbed CO is located in bridge sites between two Ru atoms.At pressures above 10−2 Torr the coverage of CO saturates at ~0.88ML. Isobaricmeasurements at 0.04 Torr CO in-dicate that the bridge bonded CO is stable up to a temperature of ~350 K and desorbs entirely by ~400 K. Addition-al CO desorbs over the temperature range of ~450 K to ~485 K, decreasing the CO coverage to ~0.58 ML. Above520 K we observe the build-up of carbon on the surface which we attribute to the dissociation of CO. At 575 Kand 0.04 Torr CO the equivalent of ~3.8 ML of carbon is present on the Ru(0001) surface. Potential mechanismsfor the formation of this large amount of carbon on the surface will be discussed.
Over the past decade, synchrotron based Ambient Pressure X-rayPhotoelectron Spectroscopy (AP-XPS) has developed into a valuabletool for investigating the adsorption and reaction of gas phasemoleculesat elevated pressure conditions [1–5]. This is due to the combined capa-bilities of AP-XPS to be surface sensitive, quantitative and operateat pressures up to 5 Torr. Among the more fundamental questionsthat AP-XPS can answer is if and how the adsorbate-surface systemevolves as the gas pressure is increased beyond ultra-high vacuum(UHV) conditions. Particularly useful in this context are studies usingsingle crystal surfaces. By using single crystal surfaces, molecular-levelinformation (e.g. adsorption site occupation) can be retained but the ele-vatedpressure operating conditions of AP-XPS extend the thermodynam-ic phase-space that can be explored with photoelectron spectroscopy.
CO adsorption, dissociation and reaction on Ru(0001) have beenstudied extensively using UHV surface science techniques [6–16].This interest stems, in part, from Ru being an active catalyst for
ployees of Brookhaven Science6 with the U.S. Department ofublication acknowledges that, paid-up, irrevocable, world-form of this manuscript, orrposes.in für Materialien und Energiese 5, 12489 Berlin, Germany.
methanation and Fischer–Tropsch synthesis (F–T synthesis) ofhigher molecular weight hydrocarbons [17–29]. UHV studies of COadsorption on Ru(0001) have shown that for exposures above ~10 L(1 L=10−6 Torr • s) the coverage of CO saturates at ~0.66 ML at UHVconditions. XPS as well as infrared spectroscopy measurements haveshown that at UHV conditions CO adsorbs exclusively in a-top sites onthe Ru(0001) surface up to the UHV saturation coverage. There is evi-dence that for coverages larger than 0.33 ML some of the adsorbed COmay slightly shift out of its adsorption site directly above the Ru atom[8,10,13]. This suggests that for CO adsorption onRu(0001) the Ru surfaceis relatively homogeneous both electronically and geometrically, and thatadsorption structures are largely determined by CO–CO interactions. Con-trary to this, Jakob recently concluded from IR measurements of variousisotopic compositions of CO adsorbed on Ru(0001), that the saturatedCO layer must contain at least two inequivalent a-top adsorbed species[16]. Jakob suggested that a dense stripe phase arrangement of CO as pro-posed by Biberian and van Hovewould be consistent with the IR spectro-scopic data [30]. In this adsorption structure, all COmolecules are in a-topsites or slightly shifted out of the a-top sites and their inequivalence arisesfrom different numbers of neighboring CO molecules.
CO adsorption studies under UHV on other transition metal surfaces,such as Pt(111) and Ni(100), have indicated that CO adsorbs in multiplewell-defined sites (e.g., a-top, bridge, or hollow sites). XPS has beenused to quantify the amount of CO adsorbed in these various sites atUHV conditions [31–34]. These studies have also shown that the bindingenergies of the C1s andO1s peaks of adsorbedCO followan inverse trendwith the coordination of CO to the surface, i.e., BEa-top>BEbridge>BEhollow.
Previous temperature programmed desorption studies and vibra-tional spectroscopy measurements at UHV of CO adsorption and
dissociation on various Ru surfaces have shown that the CO dissocia-tion sites are likely under-coordinated Ru atoms, such as step edgeson single crystal surfaces [23,24]. Zubkov et al. used isotopically labeledCO to demonstrate that a desorption peak observed at temperaturesgreater than 500 K resulted from the recombinative desorption of COthat had dissociated on a stepped Ru(109) surface [23]. In contrast, thisdesorption peak was absent on Ru(0001), which led the authors to con-clude that the under-coordinated step sites on the Ru(109) surface werethe active site for CO dissociation.
Hoffman et al. have used infrared reflection absorption spectroscopy(IRAS) to study CO adsorption and reaction on Ru(0001) at elevatedpressures [35–38]. Many of these studies have addressed the adsorptionand reaction of CO in the presence of an additional gas, such as H2 or O2,which may modify the adsorption site for CO on the Ru(0001) surface[36–38]. For IRAS studies in a pure CO ambient at pressures from 10−3
to 10 Torr and temperatures of 500 to 700 K, Hoffmann et al. did notobserve any additional absorption bands, which implies the occupationof a single adsorption site by CO (i.e., a-top) at these conditions [35,38].Hoffmann et al. concluded that the reaction of CO on Ru(0001) occursvia disproportionation (2CO→C+CO2), which leads to the build-up ofcarbon islands on the surface. Using post-reaction CO titration measure-ments (which measures the amount of Ru not covered by carbon) thesame authors found that, following reaction at 700 K in 2.5 Torr of COfor 700 s, 80% of the surface was covered by carbon (i.e., 20% of the sur-face is not covered) [35]. If CO dissociation and disproportionation occurat step sites, this implies highmobility of carbon at these conditions. Thisis consistent with more recent work by Vendelbo et al. who observed atelevated CO pressures and temperatures (0.1 bar, >550 K) that stepblocking by carbon does not inhibit the uptake and spreading of carbonover the terraces [39].
In our current studywehave used AP-XPS to study CO adsorption anddissociation on Ru(0001) at pressures up to 1 Torr and temperatures upto 575 K. We find that at elevated pressures and temperatures below~400 K some of the adsorbed CO occupies a new adsorption site onRu(0001) that can be distinguished from a-top bound CO by its lower O1s binding energy. By comparing the relative binding energies of the O1s core levels of the a-top bound CO and this newCO species to previous-ly studied systems with multiple surface adsorption sites, we concludethat this new adsorption site for CO on Ru(0001) is likely a bridge site[31–34]. Thepresence of bridge-boundCOat elevatedpressures increasesthe total CO coverage beyond the observed UHV saturation coverage at300 K. We have also addressed the temperature dependence of the COcoverage and build-up of carbon on Ru(0001) at pressures above thoseneeded to saturate the bridge adsorption site. When heating theRu(0001) crystal in the presence of 0.04 Torr of CO, we observe a de-crease in CO coverage from 300 K to ~485 K which is followed by thebuild-up of significant amounts of carbon on the surface beginning at~520 K.
2. Experimental
The experiments were performed at beam line 11.0.2 of the Ad-vanced Light Source at Lawrence Berkeley National Laboratory usingthe Ambient Pressure Photoemission Spectroscopy end-station [3–5].The Ru(0001) single crystal was cleaned by repeated cycles of Ar+ ionbombardment (1 keV, PAr=5×10−6 Torr, 5 min) followed by flashannealing to 1350 °C using electron bombardment. Small amounts ofremaining carbon on the surface were removed by exposure to O2(g)(PO2=5×10−8 Torr for 100 s, b400 K) followed by flash heating invacuum to remove the adsorbed oxygen. Surface cleanlinesswas judgedby the lack of intensity in the O 1s region, the intensity of the surfacecore level shift peak of the Ru 3d5/2 level compared to the bulk Ru3d5/2 peak, and comparing the ratio of the Ru 3d3/2 peak to the Ru3d5/2 to that expected for clean Ru (carbon contamination is expectedto overlap with the Ru 3d3/2 peak). For the data presented in thispaper there are no indications of adsorbed oxygen or carbon on the
surface prior to exposure to CO(g) within the estimated detectionlimit of less than 2% of a monolayer unless otherwise noted in thetext. In the temperature dependent studies at elevated pressures thesample was heated using the filament of the electron bombardmentheater. A maximum temperature of ~600 K could be achieved usingthismethod at COpressures of 0.04 Torr. The design of the sample hold-er ensures that the Ru(0001) surface does not have line of site to thefilament and that any carbon or oxygen formed via cracking of CO onthe filament undergoes multiple wall collisions before reaching theRu(0001) surface. While this will help to mitigate the influence of thehotfilament on the build-up of carbon on the surfacewe cannot entirelyrule out potential contributions to the amount of surface carbon fromcracking CO on the filament.
During the elevated pressure studies CO was leaked into an ultra-high vacuum chamber (base pressure ~2×10−10 Torr) through a leakvalve after passing through a carbonyl filter composed of fine Cugranule heated and maintained at 495 K. We monitored for the pres-ence of Ni-carbonyl using the Ni 3p core level (BE ~66 eV). The pres-ence of Ni or Ni-carbonyl on the surface was never observed. Once COwas leaked into the chamber we monitored the C 1s and Ru 3d regionsusing 390 eV photons and the O 1s region using 638 eV photons. Thesephoton energies ensure the same probing depth for the C 1s, Ru 3d and O1s levels since the photoelectrons all have similar kinetic energies of~100 eV. The electron energy analyzer was a modified Specs Phoibos150 hemispherical analyzer with nine channeltron detectors [40]. Poten-tial damage from the X-ray beam was checked by periodically changingsample positions and checking for differences in the spectra. All spectrapresented here showed no signs of beam damage.
3. Results and discussion
3.1. CO/Ru(0001): coverage calibration
In Fig. 1 the C 1s, Ru 3d and O 1s regions are shown both (a) beforeand (b) after exposure of the Ru(0001) surface to 23 L of CO at 300 Kunder UHV conditions. The C 1s, Ru 3d region of the clean surface(Fig. 1a) shows the Ru 3d5/2 (BE ~280.0 eV) and Ru 3d3/2 (BE~284.1 eV) photoemission peaks. The Ru 3d5/2 peak is split into twopeaks, with the lower binding energy peak due to the surface core levelshift (SCLS) of the Ru 3d states of the surface atoms relative to the bulkRu atoms [41,42]. The O 1s region of the clean surface shows that the asprepared surface is free of any oxygen. In the C 1s, Ru 3d region there isa small peak located at 282.65 eV which is likely associated with a smallamount of residual carbon. Using the C:Ru 3d5/2 ratio for this peak andour calibration factor (see below) this peak amounts to ~0.02 ML of car-bon contamination on the surface. Upon exposure of the Ru(0001) sur-face to 23 L of CO, the C 1s, Ru 3d region shows an additional peak at abinding energy of 285.8 eV and attenuation of the SCLS peak of the Ru3d5/2. We have fitted the Ru 3d5/2 peak so that the intensity of the SCLSpeak attenuates to 0.34 its original intensity consistent with the adsorp-tion of 0.66 ML of a-top adsorbed CO. This was done so that the intensityof the SCLS peak can be used as an estimate of the amount of uncoveredCO in the temperature dependent studies (see below). The peak locatedat 285.8 eV can be assigned to adsorbed CO. TheO 1s region following ex-posure to 23 L of CO shows a single peak located at 531.9 eV due toadsorbed CO, consistent with previous work [13].
Previous studies have shown that exposures greater than ~10 L at300 K saturate the surfacewith CO located in Ru a-top sites at a coverageof 0.66ML [10,13]. The presence of a single peak in theO 1s spectrum in-dicates that CO is adsorbed predominantly in a single adsorption site. Inthe following we use the C:Ru 3d5/2 ratio (0.088) from the spectrumwith the known coverage of 0.66ML shown in Fig. 1b) as a coverage cal-ibration factor for the coverage of CO at elevated pressures. This has theadvantage that all experimental factors thatmay influence relative peakintensities (such as changes in photon flux, gas phase attenuation, oranalyzer transmission) are eliminated since both the C 1s and Ru 3d
COC 1s
Ru 3d3/2
Ru 3d5/2Bulk
Ru 3d5/2SCLS
COO 1s
b)b)
a)a)
Fig. 1. C 1s, Ru 3d spectra (left panel) and O 1s spectra (right panel) before a) and afterb) exposure to 23 L of CO at 300 K. The C 1s, Ru 3d spectra were taken with a photonenergy of 390 eV and the O 1s spectra with 638 eV. The experimental data are theblack dots, the solid blue lines are the background and individual peaks from fittingthe data, and the solid red line is the sum of the fitting results.
243D.E. Starr, H. Bluhm / Surface Science 608 (2013) 241–248
spectra are recorded in a single spectrum (the same photon energy andapproximately the same photoelectron kinetic energies). When usingthis ratio, however, further attenuation of the Ru 3d5/2 peak from CO ad-sorption at coverages greater than 0.66 ML is not accounted for andtherefore coverages may be slightly overestimated. The small amount(0.02 ML) of residual carbon on the surface may also slightly affect thecoverage calibration but not by more than 0.02 ML.
Ru3d, C 1s
Ru 3d5/2InelasticScattere
Gas Phase CO
Ru3d3/2CO
a)
b)
c)
d)
e)
Fig. 2. C 1s and Ru 3d spectra with increasing CO pressure at 300 K (hν=390 eV); a) UHV,data are the black dots, the solid blue lines are the background and individual peaks from fittregion from 282 eV to 287 eV, middle panel the full spectrum and the right panel from 278
3.2. CO/Ru(0001): pressure dependence from UHV to 1 Torr at 300 K
Fig. 2 shows the C 1s, Ru 3d region with increasing pressure of COfrom UHV to 0.5 Torr on a freshly cleaned Ru(0001) surface (i.e., not thesurface used for coverage calibration). In the left and right hand panelsof Fig. 2 the high BE and low BE portions of the spectra are presented tohighlight the changes in the C1s and Ru 3d5/2 parts of the spectra, respec-tively. The spectrum taken at 1.0×10−6 Torr (spectrum b) in Fig. 2) issimilar to the spectrum in Fig. 1 following exposure to 23 L of CO. Increas-ing the pressure to above 10−6 Torr, however, causes some notablechanges. First, the Ru 3d5/2 SCLS peak is more attenuated relative to thebulk Ru peak, indicating that the CO coverage has increased (see rightpanel of Fig. 2). In addition, there is an increase in intensity between theC 1s peak and theRu3d3/2 peak,which becomesmore prominentwith in-creasing pressure (see left panel of Fig. 2). This results in a broadening ofthe CO C 1s peak from 0.5 eV FWHM at 1.0×10−6 Torr to 0.8 eV FWHMat 0.5 Torr. Previous studies of CO adsorption on other surfaces haveestablished that the BE of the C 1s and O1s peaks of adsorbed CO followsan inverse trend with the coordination of CO to the surface [31–34]. Forexample, the C 1s binding energy for CO in a-top sites on Ni(100) is285.9 eV, whereas it is 285.5 eV and 285.1 eV for CO adsorbed in bridgeand hollow sites, respectively [33]. On Pt(111) the C 1s binding energyis 286.7 eV for CO adsorbed in a-top sites, and 286.0 eV when located inbridge sites [31]. The additional intensity at the low binding energy sideof the a-topCOC1s peak (see Fig. 2)may therefore be due to CO adsorbedin higher coordinated sites on the Ru(0001) surface.
When the pressure is increased to greater than ~0.04 Torr two ad-ditional peaks can be observed (see middle panel of Fig. 2, spectrume). The 290.3 eV peak is due to gas phase CO. The small peak locatedat 288.4 eV scales in intensity with the gas phase peak, indicating thatit is associated with the gas phase. In previous studies of Pd surfacesin the presence of hydrogen, additional peaks with BEs of ~12.7 eVabove the main peak in both C 1s and Pd 3d spectra were observedand attributed to inelastic scattering of the photo-emitted electronsby H2 molecules in the gas phase [43]. A similar explanation can beused in the present case. Electron impact studies of gas phase carbonmonoxide have observed a vibrationally broadened loss peak with itshighest intensity at 8.4 eV associated with the A1Π←X1Σ+ transition[44]. The peak at 288.4 eV matches the energy expected for inelasticscattering of Ru 3d5/2 photoemitted electrons (BE of 280.0 eV) by
allyd
Ru3d5/2Bulk
Ru3d5/2SCLS
a)
b)
c)
d)
e)
a)
b)
c)
d)
e)
b) 1.0×10−6 Torr, c) 1.0×10−4 Torr, d) 0.021 Torr, and e) 0.5 Torr. The experimentaling the data, and the solid red line is the sum of fitting the data. The left panel shows theeV to 282 eV.
CO due to this transition. The combination of the high intensity of theRu 3d5/2 peak and the elevated gas pressures makes this loss peak ob-servable. Similar inelastic scattering of the Ru 3d3/2 or C 1s (fromadsorbed CO) photoemitted electrons would give rise to peaks locat-ed outside the range of the spectra shown in Fig. 2. Additional peaksfrom inelastic scattering of the C 1s photoemitted electrons of theadsorbed CO would likely be too weak to be observed.
Fig. 3 showsO1s spectrawith increasingCOpressure. As in the case ofthe C 1s, Ru 3d spectra, the O 1s spectrum taken at 1.9×10−9 Torr CO issimilar to the O 1s spectrum taken after 23 L exposure of CO at UHV con-ditions. However, as the pressure is increased above ~10−6 Torr a shoul-der at the low binding energy side becomes visible. This shoulderincreases in intensity until ~10−2 Torr and can be fit with an additionalpeak located at 530.5 eV, which is 1.4 eV lower than the binding energyof the O 1s peak attributed to a-top adsorbed CO at 531.9 eV. The530.5 eV peak has not been previously observed in UHV studies of CO ad-sorption on Ru(0001). CO adsorption studies on Pt(111) and Ni(100),however, have found O 1s binding energy shifts of −1.7 eV betweena-top adsorbed CO and bridge bonded CO on Pt(111), −0.9 eV for CO/Ni(100) and−1.6 eV for CO/H/Ni(100) [31–34]. For the latter adsorptionsystem a shift of−2.5 eVwas observed for the binding energy differencebetween CO bound in a-top sites and hollow sites, i.e., a 1 eV larger shiftthan what we observe in this study. Therefore, comparison of O 1s bind-ing energy shifts of other CO/transition metal adsorption systems indi-cates that the shoulder observed in our spectra is likely associated witha bridge bonded CO on the Ru(0001) surface. At 300 K this species canonly be observed at elevated CO pressures. The increased intensity be-tween the C 1s and Ru 3d3/2 peaks in the C 1s, Ru 3d spectra is also
COa-top
CObridge
COGas Phase
Fig. 3. O 1s spectra with increasing CO pressure at 300 K (hν=638 eV). The experi-mental data are the black dots, the solid blue lines are the background and individualpeaks from fitting the data, and the solid red line is the sum of the fitting results.
consistent with CO bound at bridge sites at elevated pressures, althoughneither observation can entirely exclude the possibility of CO bound inhollow sites.
A peak in the O 1s spectrumwith a binding energy of 530.5 eV couldbe interpreted as chemisorbed oxygen on the Ru(0001) surfaceresulting from CO dissociation. This however is unlikely in the currentstudy. In general, previous studies have highlighted the role of under-coordinated Ru atoms for COdissociation (in particular step sites on sin-gle crystal surfaces) while terrace sites are inactive [23,24]. Our cover-age estimate of the species responsible for the peak located at 530.5 eVat pressures above ~10−2 Torr (0.18 ML, see below) is much higherthan the expected concentration of under-coordinated Ru on theRu(0001) surface. Further, theoretical calculations have shownthat the activation barrier for CO desorption from Ru(0001) terracesis lower than the barrier to dissociation indicating a preference forCO desorption over dissociation [27].
Using the calibration factor for the C 1s:Ru 3d5/2 peak area ratiothat was determined from the exposure to 23 L of CO at 300 K (seeabove) we will now quantify the coverage of the surface at elevatedCO pressures. Previous photoemission studies of CO adsorbed ontransition metal surfaces have shown the presence of shake-up satel-lites in the C 1s and O 1s spectra, which contribute significantly to thetotal intensity of the spectra [14,33]. In particular, satellites in the O1s spectra have been observed for CO adsorbed on Ru(0001) byFuggle et al., although satellite peaks in the corresponding C 1s spec-tra are not discussed in this study [11,14]. The procedure for the COcoverage estimation used here does not account for the area that maybe present due to satellite peaks. However, since satellite peaks tend toresult from excitations that are localized on the adsorbate, their intensityincreases proportionally to the amount of adsorbate [33]. Our coverageestimates are therefore reasonable since the area from the possible satel-lite peaks is neither included in our calibration factor nor in the peak areasat elevated pressures. In addition, the coverage is calculated from relativepeak intensities that are calibrated with a known coverage (relative to0.66 ML) within a single adsorption system (CO/Ru(0001)). As noted inprevious studies of CO adsorption on Ru(0001), this should provide accu-rate coverage estimates without the use of possible satellite peak intensi-ties [14].
Fig. 4a) shows the coverage of the Ru(0001) surface by CO as a func-tion of pressure, alongwith the fraction of the total O 1s intensity that iscontained in the additional peak located at 530.5 eV as a function ofpressure in Fig. 4b). At pressures greater than ~10−7 Torr, the coverageof CO increases above the UHV saturation coverage until it reaches acoverage of 0.88 ML at ~10−2 Torr, which is stable for pressures up to1 Torr. This observation is consistent with a recent kinetic analysis ofDFT calculated adsorption parameters for CO/Ru(0001). These calcula-tions (which were performed for a sample temperature of 500 K) indi-cated that a CO coverage of 0.886 ML should be reached at pressures of0.0076 Torr [45].
The relative area of the 530.5 eV O1s peak (i.e., the bridge-bondedspecies) follows the same trend with increasing pressure (see Fig. 4b).In fact, the bridge bonded CO accounts for nearly all the additional COon the surface above the UHV saturation coverage. The amount of addi-tional CO at 1 Torr (~0.22 ML) is 0.25 of the total coverage of CO(~0.88 ML), compared to ~0.2 of the bridge bonded O 1s fractionalpeak intensity. We note that our measurements provide coverages aver-aged over large portions of the surface. Therefore we cannot exclude thepossibility that the observed coverage of ~0.88 ML at 1 Torr CO is an av-erage over multiple surface phases of CO on the Ru(0001) surface, withlocal variations in the bridge bonded to a-top CO ratio.
Previous studies of CO adsorbed on single crystal metal surfaces atelevated CO pressures have found both similarities and differences toUHVmeasurements. Often it is assumed in UHV studies that decreasingthe temperature of the crystal in order to increase the coverage of an ad-sorbate accurately simulates the effects of elevated pressures. Usingscanning tunnelingmicroscopy Cernota et al. observed two new surface
a)
b)
UHV Saturation
Coverage
Fig. 4. a) Red dots, coverage estimates as a function of pressure using the C 1s:Ru 3d5/2
intensity ratio from the data in Fig. 2 calibrated with the C 1s:Ru 3d5/2 from the spectrain Fig. 1 with the known CO coverage of 0.66 ML. b) Blue dots show the fraction of totalO 1s intensity contained in the low binding energy shoulder as a function of pressurefrom the data in Fig. 3.
245D.E. Starr, H. Bluhm / Surface Science 608 (2013) 241–248
structures for CO adsorbed on Rh(111) at 300 K: a (2×1) phase and a(√7×√7)R19° phase, with coverages of 0.5 and 0.57 ML, respectively,at pressures between 10−8 and 10−4 Torr [46]. At pressures above5 Torr, a (2×2) structure was observed, which was assumed to be thesame structure as observed in low temperature STM measurements at240 K in UHV [47]. The (2×2) structure was stable up to 700 Torr andconsists of either pure or mixed three-fold hollow and a-top adsorbedCO. Cernota et al.'s study demonstrated that dense CO overlayers arepresent at metal surfaces at room temperature and elevated pressures,when the adsorbed CO is in equilibriumwith its gas phase, as is the casein the present study. Cernota et al.'s investigation, however, did not re-veal any new CO adsorption sites at elevated pressures; both three-foldhollow and a-top absorbed CO had been previously observed using lowpressure and temperature UHV studies. Similar studies of CO adsorptionon Pt(111) at 300 K and pressures up to 760 Torr found a continuousvariation in the coverage of CO with pressure [48]. The hexagonal (orquasi-hexagonal) structures observed in elevated pressure studies ofCO/Pt(111) are similar to those observed under UHV conditions, imply-ing that for CO/Pt(111) a true pressure gapmay not exist. Other studiesby the samegroup on COadsorption on Pt(110), NO on Pd(111) and hy-drogen on Cu(110) all showed that the adsorption structures at highpressures were identical to structures observed at lower pressuresand temperatures, thus also indicating that for these systems a pressuregap does not seem to exist [49].
Our observation of CO bound to bridge sites on the Ru(0001) surfaceclearly shows that there is a pressure gap in the CO/Ru(0001) systemsince the bridge adsorption site was not previously observed at lowerCO pressures and temperatures. The existence of this pressure gap
may not be surprising since previous UHV work had indicated a shiftof CO away from adsorption sites directly above Ru atoms for coverageabove 0.33 ML [6–10]. The CO adsorption structures observed in UHVhave been argued to be largely determined by CO–CO interactions onthe surface, implying electronic and geometric homogeneity of theRu(0001) surface. In contrast to this, more recently, Jakob concludedthat the adsorbed CO in the saturated layer at UHV must contain twoinequivalent a-top adsorbed CO molecules [16]. A structural modelthat is consistent with this conclusion is that provided by Biberianand van Hove and consists of a dense striped phase arrangement ofCO on the surface [30]. Such a structure may provide an explanationfor the additional adsorbed CO we observe at elevated pressures. InFig. 5 we reproduce the structural model of Biberian and van Hove(see also Fig. 24b) and d) of Ref. [30]). Fig. 5 shows both the “unrelaxed”and “relaxed” structural models for the 0.66 ML coverage of CO onRu(0001). In the relaxed structure CO has shifted laterally (i.e., parallelto the Ru surface, and perpendicular to the CO stripe direction (seeFig. 5)). Biberian and van Hove concluded that the relaxed structure(lower portion of Fig. 5) is more consistentwith the observed LEED pat-tern than the unrelaxed structure (the latter is included in Fig. 5 for clar-ity). In either structure it is evident that there are open bridge sites(blue circles in Fig. 5) on the surface for the adsorption of additionalCO. Adsorption of CO onto these sites increases the CO coverage to0.83 ML, very close to our observed coverage of ~0.88 ML. Again wenote that our coverage estimate is likely over-estimated (see above).Also, in the structure containing bridge bound CO (Fig. 5, middle) 1/5of the CO molecules are located in bridge sites while the other mole-cules are located in a-top sites or slightly shifted from a-top sites inthe relaxed structure. This is also very similar to our observed fractionof adsorbed CO in bridge sites, ~0.20 at PCO>0.01 Torr. It is likely that,once additional CO adsorbs into the bridge sites, further structural re-laxation occurs as indicated in the bottom, right structure of Fig. 5.A-top adsorbed CO relaxes parallel to the stripe direction (see Fig. 5)but remains in a-top or slightly shifted a-top sites.
Up to now CO has not been observed in a high symmetry site otherthan the a-top position on Ru(0001) [6–9]. Hoffmann et al. studied COadsorption and reaction on Ru(0001) at elevated pressures using IRAS[35–38]. In these studies, no CO adsorbed in a high symmetry siteother than a-topwas observed. However, the temperature range studiedin a pure COambientwas 500 to 700 K. Belowwe show that, in a pure COambient, the bridge bound CO begins to desorb at ~350 K and is fullydesorbed from the surface by 400 K. Therefore, our results are fully con-sistent with the studies by Hoffmann et al. [35,38].We point out, howev-er, that – althoughwe are likely probing a hitherto unexplored portion ofthe CO/Ru(0001) phase diagram in this study – this extension to higherCO pressures may still not probe the relevant phase for catalysis, partic-ularly FT-synthesis, since most catalytic reactions take place at evenhigher pressures and temperatures. We will address this point in moredetail in the following section.
3.3. CO/Ru(0001): temperature dependence to 575 K in 0.04 Torr CO(g)
Fig. 6 shows the behavior of the C 1s, Ru 3d and O 1s regions, aswell as the CO coverage (right panel), with increasing temperatureat a pressure of 0.04 Torr, which is above the estimated pressure re-quired for a saturation coverage of 0.88 ML CO (~0.01 Torr). The spec-tra show little change between 300 and 350 K, where the averagecoverage is 0.88±0.03 ML (the error is the standard deviation ofthe coverage estimates from 300 K to 350 K). The average fractionof the bridge bonded CO is 0.20±0.02 in this temperature range.When the temperature is increased above ~350 K, a decrease in theintensity around 285.2 eV, i.e., in the region between the a-top bond-ed CO and the Ru 3d3/2 peak, is observed (see left panel in Fig. 6). Con-current with this decrease there is a slight broadening of the Ru 3d5/2peak to lower binding energy, most likely due to an increase in inten-sity of the SCLS peak located at the low binding energy side of the Ru
0.66 ML
Unrelaxed
0.83 ML
RelaxedPCO
> 0.01 Torr
Further
Relaxation
Ru atomAdsorbed COwith θ = 0.66 ML
Additional COFor P ~ 0.01 Torr
PCO
> 0.01 Torr
Fig. 5. Unrelaxed and relaxed structural models for CO adsorbed in a dense stripe structure (the stripe direction is indicated by the red arrow) at a coverage of 0.66 ML on Ru(0001)from Biberian and van Hove [30]. The blue circles indicate bridge sites where additional CO can adsorb at elevated pressures. Adsorption at these sites increases the coverage of COon the surface to 0.83 ML. Possible further relaxation of the surface occurs as indicated in the lower right figure while preserving the same ratio of bridge to a-top adsorbed CO.
3d5/2 peak. Both observations are consistent with desorption of bridgebonded CO at T>350 K. This is further supported by the observedchanges in the O 1s region (see middle panel in Fig. 6), where the de-crease of the low binding energy shoulder with increasing temperaturealso indicates desorption of bridge bonded CO. By ~400 K most of thebridge bonded CO has desorbed from the surface. At temperaturesabove ~400 K, the CO adsorbed on the Ru(0001) surface is located near-ly exclusively in a-top sites. At T=400 K the coverage of CO is approx-imately 0.71±0.02 ML, close to the known UHV saturation coverage of0.66 ML and likely similar in structure. This coverage is stable up to~450 K. From ~450 K to ~485 K the CO coverage decreases further to~0.58±0.02 ML, similar to a coverage that has been observed in UHVstudies, and which was attributed to a (2√3×2√3) R 30° structure [8].In our experiments the 0.58 ML coverage of CO is stable up to ~520 K.Above that temperature a reduction of intensity of the CO C 1s peakand an increase in intensity and a broadening of the Ru 3d3/2 are ob-served. This is consistent with the build-up of carbon at the surfacesince the binding energies of the C 1s peaks of graphitic, amorphous,as well as carbidic-like carbon on transition metal surfaces are in thesame binding energy range as the Ru 3d3/2 level [50–52]. In addition,the O 1s spectra show a decrease in the total O 1s intensity above
Onset of carbon accumulation
CO C Ru3d3/2 Ru3d5/2
Ru3d5/2SCLS Peak
Onset of car
O 1sa-topBondedCO
Fig. 6. Left panel: C 1s, Ru 3d region as a function of temperature in 0.04 Torr CO, middle patemperature using the C 1s:Ru 3d5/2 ratio from the data in the right panel calibrated with ththin line in the left panel at ~269.6 eV marks the position of the Ru 3d5/2 SCLS peak of the satabove ~350 K.
520 K and a slight shift of the peak to lower binding energy to a valueconsistent with that of adsorbed oxygen on the surface [13]. Both ofthese observations are consistent with the onset of large amounts ofCO dissociation occurring at ~520 K.
In order to quantify the amount of carbon on the Ru(0001) surfacewe have fit the data from an average of 5 spectra over the tempera-ture range from 565 K to 585 K (see Fig. 7), using literature valuesfor the C 1s peak positions of graphitic, amorphous and carbide likecarbon [50–52] (Note that in the 300 K spectrum there is a smallamount – ~0.04 ML – of carbon contamination with a binding energyof about 283.2 eV). The quantification of the various carbon species isdifficult due to the aforementioned overlap of their C 1s photoemis-sion peaks with the Ru 3d3/2 peak. In order to estimate the carboncoverage, the area ratio of the Ru 3d3/2 peak to Ru 3d5/2 was fixedto the value for the clean Ru surface (2:3). We can tentatively identifyamorphous and graphitic carbon (binding energies 284.9 eV and284.2 eV respectively) and possibly the presence of a carbide-likespecies at 283.3 eV. Using the C:Ru coverage calibration factor fromthe 0.66 ML CO/Ru(0001) spectra and the total area for all C species,we estimate that at a temperature of 575 K the coverage of C on thesurface is 3.8 ML. Note that due to the overlap of the C 1s peaks of
bon accumulation
O1s
O 1sBridgeBondedCO
Onset of carbon accumulation
θ = 0.58 ± 0.02
θ = 0.71 ± 0.02
θ = 0.88 ± 0.03
nel: O 1s region as a function of temperature, and right panel: coverage as a function ofe C 1s:Ru 3d5/2 from the spectra in Fig. 1 with the known CO coverage of 0.66 ML. Theurated surface, helping to see the broadening of the SCLS peak to lower BE upon heating
CO
AmorphousCarbon
GraphiticCarbon
Carbide-likeCO
Ru 3d3/2
Ru 3d5/2Bulk
Ru 3d5/2SCLS
Fig. 7. C 1s, Ru 3d region in 0.04 Torr CO at 300 K (left panel) and 575 K (right panel). The black dots are the data, the solid blue lines are the background and individual peaks fromfitting the data, and the solid red lines are the sum of the fitting results. In the right panel the solid green lines are the peaks that have been added to account for the build-up ofcarbon on the surface.
247D.E. Starr, H. Bluhm / Surface Science 608 (2013) 241–248
the various types of carbon on the surface, their relative amounts aredifficult to quantify.
The fits of the Ru 3d5/2 peak indicate an increase in intensity of theSCLS peak at T>520 K compared to its intensity at 300 K in 0.04 TorrCO. This is consistent with an increase in exposed Ru with increasingtemperature. The ratio of the SCLS peak to the bulk Ru 3d5/2 peak indi-cates that ~0.28 ML of Ru is not covered by carbon at 575 K, comparedto only ~0.12ML at 300 K. This amount of exposed Ru is consistentwiththe results of Hoffmann et al. who observed ~0.20ML of exposed Ru fol-lowing reaction at 700 K in 2.5 Torr of CO for 700 s [35]. Since the totalcoverage of carbon on the surface exceeds a monolayer, the carbonmust either be in the form of patches of multi-layers of carbon, or inthe form of sub-surface carbon or possibly a combination of these.
In general, UHV experimental and theoretical studies agree thatCO dissociation occurs at under-coordinated Ru sites, specificallystep-edges on single crystal surfaces in the absence of hydrogen orCHx species [21–24]. Recent theoretical calculations by Inderwildiet al. have indicated that CO reaction with adsorbed CHx speciesfollowed by CO dissociation is possible on the terraces of some tran-sition metal surfaces [25–27]. In addition, Inderwildi et al. showedthat on Ru(0001) hydrogen may attack adsorbed CO and form a for-myl species, which then decomposes into adsorbed CH and O. Ourcoverage estimate of 3.8 ML at 575 K exceeds the expected carboncoverage from CO dissociation exclusively at step-edges. Therefore,our observations can be explained by the following possible scenarios:(1) the Ru(0001) terraces facilitate CO dissociation, (2) C-containingspecies are mobile on the Ru(0001) surface at temperatures of 520 to575 K following dissociation at step-edges, (3) CHx species form onthe surface from residual hydrogen in the chamber and CO reacts withthese species to form the adsorbed carbon species on the surface, or(4) residual hydrogen in the chamber attacks adsorbed CO leading toformyl intermediates, which then decompose to form adsorbed CHand O. Isotopic mixing has been observed in recombinative desorptionof CO from stepped Ru surfaces at ~500 K and attributed to CO dissoci-ation at step-edges [23]. Further it is generally agreed that the Ru(0001)terraces are not active for CO dissociation. Therefore scenario (1) seemsunlikely. However, our results do not allow us to distinguish betweenscenarios (2), (3) and (4). Based on calculations of Inderwildi et al., sce-nario (4) is more likely than scenario (3) on Ru(0001), however manytheoretical and experimental studies point to the role of steps being ac-tive for CO dissociation in the absence of hydrogen. If CO dissociation
occurs exclusively at the step edges, our observations suggest that thehigh coverage of C on the surface above 520 K is due to the mobility ofC-containing species on Ru(0001) at temperatures greater than 520 K.CO may dissociate at the step edges at a temperature lower than 520 Kbut this cannot be determined in the present study since this wouldlead to undetectably small amounts of carbon on the surface. However,above 520 K C-containing species must move away from the stepedges and open up the step sites for further CO dissociation. Such behav-ior is consistent with recent results fromVendelbo et al. [39]. In addition,previous theoretical calculations have indicated a C–C coupling reactionbarrier at Ru step edges of 1.05 eV [22] and 1.14 eV [21]. Possibly CO dis-sociates at step edges and C–C coupling occurs at T>520 K. C–C couplingmay cause aweakening of the bonding of the C-containing species to thestep edges and lead to its eventual detachment. The C-containing speciesthen become mobile, perhaps as polymeric species as suggested byVendelbo et al. [39]. Carbon mobility may have particular relevancefor C–C bond formation and hydrocarbon chain propagation inFT-synthesis, in particular demonstrating that terraces on Ru(0001)may also provide sites for hydrocarbon chain growth either throughfurther C–C coupling reactions or CO insertion or via formyl forma-tion as suggested by Inderwildi et al. [27].
4. Conclusion
In summary,we have investigated the adsorption of COonRu(0001)at pressures from UHV conditions up to 1 Torr and temperatures up to575 K using AP-XPS. At pressures above ~10−6 Torr the coverage of COincreases to above the UHV saturation coverage at 300 K. Relative O 1sbinding energies indicate that the additional CO is likely a bridge bond-ed CO. The amount of adsorbed CO saturates at pressures of ~10−2 Torrand remains stable up to at least 1 Torr;most of the additional CO beingadsorbed in the newly observed bridge bonded site. The increase in theamount of CO above the UHV saturation coverage as well as the obser-vation of a new bonding site for CO on Ru(0001) indicate that for theCO/Ru(0001) system a true pressure gap exists between UHV studiesand elevated pressure studies. This may have important consequencesfor applying the results from UHV studies to systems of catalytic inter-est, specifically Fischer–Tropsch catalysis using Ru.
We have also acquired AP-XPS spectra as a function of tempera-ture in 0.04 Torr CO. The results of these measurements indicatethat bridge bonded CO is stable on the surface up to a temperature
of ~350 K. At ~400 K, a coverage (0.71 ML) that is nearly equal to theUHV saturation coverage of 0.66 ML is observed. This coverage is stableup to temperatures of ~450 K, and likely similar to the UHV saturationcoverage structure. Desorption of additional CO from ~450 K to ~485 Kreduces the coverage to ~0.58 ML. Once a temperature of ~520 K isreached we observe significant build-up of carbon on the surface, indi-cating that significant amounts of CO begin to dissociate at ~520 K. Theamount of carbon on the surface at 575 K and 0.04 Torr CO is ~3.8 ML.Such large quantities of carbon on the surface indicate that eitherRu(0001) terraces are active for CO dissociation via the formation of aformyl intermediate by reactionwith residual hydrogen followed by dis-sociation, or C-containing species are mobile on the surface after CO hasbeen dissociated at the step sites on Ru(0001) with mobile carbon spe-cies as key factor in either scenario.
Acknowledgments
This work was made possible through the Center for FunctionalNanomaterials, Brookhaven National Laboratory, which is supported bythe U.S. Department of Energy, Office of Basic Energy Sciences, undercontract no. DE-AC02-98CH10886. The ALS and the MES beamline11.0.2 are supported by the Director, Office of Science, Office ofBasic Energy Sciences, Division of Chemical Sciences, Geosciencesand Biosciences and Materials Sciences Division of the US Depart-ment of Energy at the Lawrence Berkeley National Laboratoryunder contract no. DE-AC02-05CH11231.
References
[1] M. Salmeron, R. Schlögl, Surf. Sci. Rep. 63 (2008) 169.[2] H. Bluhm, M. Hävecker, A. Knop-Gericke, E. Kleimenov, R. Schlögl, D. Teschner, V.I.
Bukhtiyarov, D.F. Ogletree, M. Salmeron, J. Phys. Chem. B 108 (2004) 14340.[3] D.F. Ogletree, H. Bluhm, G. Lebedev, C.S. Fadley, Z. Hussain, M. Salmeron, Rev. Sci.
Instrum. 73 (2002) 3872.[4] H. Bluhm, J. Electron Spectrosc. Relat. Phenom. 177 (2010) 71.[5] D.F. Ogletree, H. Bluhm, E.L.D. Hebenstreit, M. Salmeron, Nucl. Instrum. Methods
Phys. Res., Sect. A 601 (2009) 151.[6] H. Pfnür, D. Menzel, F.M. Hoffmann, A. Ortega, A.M. Bradshaw, Surf. Sci. 93 (1980)
431.[7] E.D. Williams, W.H. Weinberg, A.C. Sobrero, J. Chem. Phys. 76 (1982) 1150.[8] G.E. Thomas, W.H. Weinberg, J. Chem. Phys. 70 (1979) 1437.[9] G. Michalk, W. Moritz, H. Pfnür, D. Menzel, Surf. Sci. 129 (1983) 92.
[10] H. Pfnür, D. Menzel, J. Chem. Phys. 79 (1983) 2400.
[11] H. Pfnür, P. Feulner, D. Menzel, J. Chem. Phys. 79 (1983) 4613.[12] E.D. Williams, W.H. Weinberg, Surf. Sci. 82 (1979) 93.[13] J.C. Fuggle, T.E. Madey, M. Steinkilberg, D. Menzel, Surf. Sci. 52 (1975) 521.[14] J.C. Fuggle, T.E. Madey, M. Steinkilberg, D. Menzel, Chem. Phys. Lett. 33 (1975) 233.[15] P. Jakob, Appl. Phys. A 75 (2002) 45.[16] P. Jakob, J. Chem. Phys. 120 (2004) 9286.[17] H. Schulz, Appl. Catal. A 186 (1999) 3.[18] A.Y. Khodakov, W. Chu, P. Fongarland, Chem. Rev. 107 (2007) 1692.[19] E. Igelsia, S.C. Reyes, R.J. Madon, J. Catal. 129 (1991) 238.[20] M.E. Dry, Catal. Today 6 (1990) 183.[21] J. Chen, Z.P. Liu, J. Am. Chem. Soc. 130 (2008) 7929.[22] Z.P. Liu, P. Hu, J. Am. Chem. Soc. 124 (2002) 11568.[23] T. Zubkov, G.A. Morgan, J.T. Yates, Chem. Phys. Lett. 362 (2002) 181.[24] E. Shincho, C. Egawa, S. Naito, K. Tamaru, Surf. Sci. 149 (1985) 1.[25] O.R. Inderwildi, D.A. King, S.J. Jenkins, Phys. Chem. Chem. Phys. 11 (2009) 11110.[26] O.R. Inderwildi, S.J. Jenkins, D.A. King, J. Phys. Chem. C 112 (2008) 1305.[27] O.R. Inderwildi, S.J. Jenkins, Chem. Soc. Rev. 37 (2008) 2274.[28] B.H. Davis, Fuel Process. Technol. 71 (2001) 157.[29] M.A. Vannice, J. Catal. 37 (1975) 449.[30] J.P. Biberian, M.A. van Hove, Surf. Sci. 138 (1984) 361.[31] O. Björneholm, A. Nilsson, H. Tillborg, P. Bennich, A. Sandell, B. Hernnäs, C. Puglia,
N. Mårtensson, Surf. Sci. 315 (1994) L983.[32] H. Antonsson, A. Nilsson, N. Mårtensson, I. Panas, P.E.M. Siegbahn, J. Electron
Spectrosc. Relat. Phenom. 54/55 (1990) 601.[33] H. Tillborg, A. Nilsson, N. Mårtensson, Surf. Sci. 273 (1992) 47.[34] A. Föhlisch, N. Wassdahl, J. Hasselström, O. Karis, D. Menzel, N. Mårtensson, A.
Nilsson, Phys. Rev. Lett. 81 (1998) 1730.[35] F.M. Hoffmann, J. Chem. Phys. 90 (1989) 2816.[36] F.M. Hoffmann, M.W. Weisel, C.H.F. Peden, J. Electron Spectrosc. Relat. Phenom.
54/55 (1990) 779.[37] F.M. Hoffmann, J.L. Robbins, J. Vac. Sci. Technol. A 5 (1987) 724.[38] F.M. Hoffmann, J.L. Robbins, J. Electron Spectrosc. Relat. Phenom. 45 (1987) 421.[39] S.B. Vendelbo, M. Johansson, D.J. Mowbray, M.P. Andersson, F. Abild-Pedersen, J.H.
Nielsen, J.K. Nørskov, I. Chorkendorff, Top. Catal. 53 (2010) 357.[40] Specs Surface Nano Analysis GmbH, Berlin, Germany.[41] P.J. Godowski, Z.S. Li, J. Bork, J. Onsgaard, Surf. Rev. Lett. 14 (2007) 911.[42] P.J. Godowski, J. Onsgaard, Z. Ryszka, Ł. Rok, Z.S. Li, Surf. Sci. 602 (2008) 465.[43] D. Teschner, A. Pestryakov, E. Kleimenov, M. Hävecker, H. Bluhm, H. Sauer, A.
Knop-Gericke, R. Schlögl, J. Catal. 230 (2005) 186.[44] V.D. Meyer, A. Skerbele, E.N. Lassettre, J. Chem. Phys. 43 (1965) 805.[45] E.D. German, M.J. Sheintuch, J. Phys. Chem. C 112 (2008) 14377.[46] P. Cernota, K. Rider, H.A. Yoon, M. Salmeron, G. Somorjai, Surf. Sci. 445 (2000)
249.[47] M. Gierer, A. Barbieri, M.A. van Hove, G.A. Somorjai, Surf. Sci. 391 (1997) 176.[48] S.R. Longwitz, J. Schnadt, E.K. Vestergaard, R.T. Vang, E. Lægsgaard, I. Stensgaard,
H. Brune, F. Besenbacher, J. Phys. Chem. B 108 (2004) 14497.[49] R.T. Vang, E. Lægsgaard, F. Besenbacher, Phys. Chem. Chem. Phys. 9 (2007) 3460.[50] A. Wiltner, Ch. Linsmeier, Phys. Status Solidi A 201 (2004) 881.[51] A. Wiltner, Ch. Linsmeier, T. Jacob, J. Chem. Phys. 129 (2008) 084704.[52] A. Wiltner, Ch. Linsmeier, Surf. Sci. 602 (2008) 3623.
