Interaction of Tetrachloroethylene with Pd(100) Studied by High-Resolution X-ray Photoemission...

Interaction of Tetrachloroethylene with Pd(100) Studied by High-Resolution X-rayPhotoemission Spectroscopy

Ken T. Park and Kamil Klier*Zettlemoyer Center for Surface Studies and Department of Chemistry, Lehigh UniVersity,Bethlehem, PennsylVania 18015

Chuan Bao Wang and Wei Xian ZhangDepartment of CiVil Engineering, Lehigh UniVersity, Bethlehem, PennsylVania 18015

ReceiVed: April 1, 1997; In Final Form: May 6, 1997X

Adsorption and reaction of tetrachloroethylene (C2Cl4) on a clean Pd(100) surface have been investigated atroom and cryogenic temperatures. The 300 K saturation of Pd(100) with C2Cl4 gas gave rise to a well-ordered p(2× 2) overlayer structure. High-resolution X-ray photoemission spectroscopy established that theC-Cl bonds in the p(2× 2)C2Cl4 overlayer were dissociated while retaining the stoichiometry 2C: 4Cl, andthe amounts of carbon and chlorine on the surface were 0.125 and 0.25 monolayer (ML), respectively. At131 K, the exposure of the clean Pd(100) surface to C2Cl4 resulted in predominantly molecular adsorption,evidenced by the binding energies (BEs) of the Cl 2p and C 1s core levels. A detailed core level scan in theCl 2p region revealed two satellite Cl peaks: one shifted from the molecular C2Cl4 peak by∆BE ) -2.7 eVand the other by∆BE ) -1.4 eV, corresponding to atomic Cl and partially dissociated C2Cl4 species,respectively. As the temperature increased, the partially dissociated C2Cl4 gradually converted to adsorbedCl atoms untilT ) 291 K, at which temperature all Cl on the surface formed atomic Cl of 0.25 ML. Thecarbon species, while present in stoichiometric amounts, did not give rise to additional structural features,but they indirectly affected the Cl ordering in forcing the p(2× 2) structure, which does not form upondissociative adsorption of elemental chlorine.

1. Introduction

The interaction of halogens or halogen-containing hydrocar-bons with transition metal surfaces is of interest in heterogeneouscatalysis and in abatement of chlorohydrocarbons. Because oftheir high electron affinities and relatively weak bonding in theirparent molecules, halogens and halogen-substituted hydrocar-bons display high reactivity and behave as poisons or promoterson transition metal catalysts, altering the surface adsorption andreaction properties for subsequent reactants. In the case ofchlorine, these properties have been exploited in the selectivepoisoning of Ag ethylene epoxidation catalysts1 and the redis-persion of Pt reforming catalysts.2

The use of halogens to modify a catalytic process has beenalso applied to the oxidation of methane using palladiumcatalysts. Cullis et al.3 and later Mann and Dosi4 reported thatpalladium catalysts in the presence of halogenated hydrocarbonsproduce partial oxidation reaction products such as formaldehydeand carbon monoxide in contrast to the well-known activity ofPd for complete combustion of methane to carbon dioxide andwater in the absence of any promoters.5 To understand themechanisms of modifying the Pd surface reactivity as well asthe role of chlorine adatoms in controlling the activity andselectivity of partial oxidation of methane, Wang et al.6 studiedthe adsorption of dichloromethane (CH2Cl2) and its interactionwith oxygen on the single-crystal Pd(100) surface. In this study,the authors observed that CH2Cl2 adsorbs dissociatively at roomtemperature and forms partially ordered overlayer structure atsaturation, as evidenced by streaks at half-integer spots in thelow-energy electron diffraction (LEED) pattern. Furthermore,the partially ordered Cl adatoms left on the surface after theremoval of carbon through oxidation cycles decreased the

activation energy of oxygen desorption by as much as 10 kcal/mol, suggesting the modification of oxygen reactivity due tointermediate range (3-4 Pd-to-Pd distances) lateral interactionsbetween the adsorbed Cl and O atoms. Although the Wang etal. study demonstrated the use of CH2Cl2 as the Cl ensemble-forming agent and the influence of chlorine adatoms onsubsequent oxygen reactivity, it was deemed necessary toexplore other chlorohydrocarbons as the ensemble-formingagents and understand their basic chemical interaction with thepalladium surface in order to determine the general patterns ofadsorption of halogen-substituted hydrocarbons, the state of thecarbon and chlorine adatoms, stoichiometry, structure, andthermal stability of the halogen adatoms on palladium surfaces.In our continuing effort to examine the basic interactions of

halogen-substituted hydrocarbons with single-crystal Pd surfacesand their influence on reactants, we have investigated theinteraction of tetrachloroethylene (C2Cl4, TCE) with the Pd-(100) surface using high-resolution X-ray photoemission spec-troscopy (HRXPS) and LEED for the following reasons: (1)this simple haloalkene contains no hydrogen, and the surfacechemistry could be different from the previously studied CH2-Cl2, particularly in terms of C-Cl bond dissociation; (2) theC2Cl4 molecule is nearly square planar (2.88× 3.19 Å) withD2h symmetry,7 and its resemblance to the square mesh of thePd(100) surface lattice makes it an interesting choice as anensemble-forming reactant; (3) C2Cl4 is widely used as anindustrial solvent, and its interaction with transition metalsurfaces and decomposition may be useful in environmentalchemistry.8,9

2. Experimental Section

The experiments were carried out in an ultrahigh-vacuum(UHV) system, which houses the SCIENTA ESCA-300 HRXPSX Abstract published inAdVance ACS Abstracts,June 15, 1997.

5420 J. Phys. Chem. B1997,101,5420-5428

S1089-5647(97)01139-5 CCC: $14.00 © 1997 American Chemical Society

spectrometer.10 A two-speed rotating anode of special UHVdesign generates unpolarized, monochromatized Al KR radiation(hν ) 1486.8 eV) of up to ca 8 kW. The photoexcited electronsare detected by a 300 mm mean radius hemispherical energyanalyzer with variable slit widths. The newly acquired auto-mated sample manipulator/goniometer (Seiko Instruments)allows three degrees of translational motion (x, y, z) with 3 µmrepeatability as well as two degrees of rotational motion (polarangleθ, rotationφ) with 0.2° repeatability. In addition to thehigh-precision motion, the manipulator is equipped with aresistively heated hot and a liquid nitrogen-cooled stage for insitu temperature-dependent XPS study at temperatures rangingfrom 100 to 900 K. The sample temperature during the XPSdata acquisition is measured by an alumel-chromel thermo-couple probe, which is attached to the bottom of the stub.The Pd(100) single crystal was first aligned visually by two

small marks scratched along the⟨010⟩ direction previouslydetermined using Laue diffraction. Then, more precise in situsample alignment was carried out using the X-ray photoelectrondiffraction (XPD) maxima atθ ) 45° along the⟨010⟩ direction,10where the polar angleθ was measured from surface normal.The polar angle XPS scans were obtained at 1° intervals from-8° to 82°. At each angular position, XPS spectra of variouscore level regions were recorded before moving to the nextangular position. The binding energies in the XPS spectra werereferenced using the position of the Ag 3d5/2 core level at 368.25eV as well as the Pd 3d5/2 level at 335.05 eV from the cleanPd(100) single-crystal surface. Detailed information of thesample geometry and the SCIENTA ESCA-300 HRXPS spec-trometer are available elsewhere.11,12

The cleanliness and surface order of the Pd(100) single crystalwere verified using a LEED optics and HRXPS. An XPSsurvey scan revealed that a major impurity of the Pd(100) singlecrystal was carbon. The initial procedure of cleaning impuritiesincluding carbon involved a few cycles of Ar ion sputteringfollowed by annealing for a short period time using an electronbeam heater in the preparation chamber. Although the aboveprocedure was able to remove most impurities, this method ofcleaning usually left ca 0.1 ML of residual carbon on the surface.However, the complete removal of any residual carbon wasachieved by the oxidation of the residual carbonsheating thePd surface to approximately 800 K in a partial oxygenatmosphere (P ) 5 × 10-7 mbar) for 5 min, as previouslydescribed by Simmons et al.13 A typical clean Pd(100) surfacefree of impurities produced a sharp p(1× 1) LEED patternshown in Figure 1a.Following the characterization of the clean Pd(100) surface,

high-purity C2Cl4 (Aldrich, HPLC grade 99.9+%) in a glassbottle was admitted into the ultrahigh-vacuum chamber througha precision variable leak valve. Prior to the experiments, anumber of freeze-thaw cycles were employed to removeambient gas trapped in the glass bottle. For room-temperatureadsorption study, the Pd(100) surface was exposed to C2Cl4 atthe constant pressure of 1× 10-6 Torr in the preparationchamber.For low-temperature adsorption of C2Cl4, the clean Pd(100)

surface was first cooled toT ) 131 K using liquid nitrogen inthe analysis chamber for about an hour. Despite precautions,the XPS scan of the surface after cooling indicated a smallamount of carbon (≈0.06 ML) adsorbed on the surface.However, it is likely that this carbon does not spread but ratherit forms clusters leaving the most of the Pd surface free fromthe impurities, as supported by previous studies14,15 as well asthe fact that the C-C bonding energy is larger than Pd-Cbonding energy.16 The C2Cl4 gas was admitted to the Pd(100)

surface via a doser at ca 2.5 cm distance and 2.0× 10-8 Torr.Because the pressure reading was made with an ion gaugelocated distant from the Pd surface, the actual exposure of C2-Cl4 gas in terms of langmuirs was not known. However, theamount of adsorbed chlorine on the surface was determined bycomparing the intensity of the Cl 2p peak with that from thec(2 × 2) Cl/Pd(100) surface.6 After determining the Clcoverage, the amount of carbon was calculated using the XPSintensity ratio of the C 1s to the Cl 2p core levels from thesaturated p(2× 2) C2Cl4/Pd(100) surface. A more detaileddescription of quantification of the amount of surface adsorbatesis presented in section 4.1.1.

For the study of the temperature dependence of the interactionbetween the adsorbed C2Cl4 and the Pd(100) surface, thetemperature of the substrate was further raised from 131 to 291K. The continuous increase in the sample temperature wasachieved by blowing in N2 gas into the coldfinger and thuswarming the cold stage. The average heating rate was constantat 1.46 K/min, except during the initial warming to 147 K, forwhich it was much slower due to the evaporation of residualliquid nitrogen inside the coldfinger. To monitor the temper-ature dependence of the core level XPS, the Cl 2p and C 1score level scans were taken at selected temperatures:T) 147,173, 199, 223, 249, and 291 K with∆T ≈ (7.3 K, where∆T

Figure 1. 1. LEED pattern obtained from (a) a clean Pd(100) and (b)p(2 × 2) C2Cl4/Pd(100) at the electron energy E) 56.1 eV afterexposure of 77 langmuirs of C2Cl4 at room temperature. The (0,0) spotswere blocked off by a miniature electron gun assembly of the LEEDoptics.

Interaction of Tetrachloroethylene with Pd(100) J. Phys. Chem. B, Vol. 101, No. 27, 19975421

mainly resulted from the continuous increase in the sampletemperature during the ca. 10 min data acquisition of for theXPS scan.

3. Results

3.1. Room-Temperature Chemisorption of C2Cl4 on Pd-(100). Exposure of the clean Pd(100) surface to 77 langmuirsof C2Cl4 (1× 10-6 Torr× 100 s) at room temperature resultedin sharp LEED spots at (0,(1/2), ((1/2, 0), and ((1/2, (1/2)(Figure 1b). The measurements of the angular positions of theLEED maxima and the kinetic energy of the incident electronbeam yielded a value of 5.64 Å for the new lattice spacing ofthe C2Cl4-covered Pd(100) surface. The calculated valuecorresponded to twice that of the clean Pd(100) surface unitcell within 2%, showing that the observed LEED patternrepresents the p(2× 2) overlayer structure. The XPS surveyscan of the p(2× 2) C2Cl4/Pd(100) surface revealed the presenceof carbon and chlorine on the Pd(100) surface after the exposureto C2Cl4. Further exposure of the sample to 1× 10-6 Torr ofC2Cl4 for 200 s, corresponding to additional 154 langmuirs ofC2Cl4, did not increase the photoelectron intensities from eitherchlorine or carbon, indicating the saturation coverage had beenachieved.Both the Cl 2p and C 1s core level spectra from the p(2× 2)

C2Cl4/Pd(100) surface were examined in detail at the exit angleθ ) 70° (Figure 2a,b). The HRXPS scan in the Cl 2p corelevel region showed a well-resolved doublet of Cl 2p3/2 and Cl2p1/2 lines with the Cl 2p3/2 peak at 198.04 eV. The spin-orbit splitting between the 2p3/2 and 2p1/2 levels was 1.63 eV,in good agreement with the reported value of 1.67 eV for gas-phase C2Cl4.17 The C 1s core level scan from the p(2× 2)C2Cl4/Pd(100) surface (dots in Figure 2b) also showed a narrow,symmetric peak at 284.33 eV with full width at half-maximum(fwhm) of 0.54 eV. The stoichiometry between carbon andchlorine adatoms on the p(2× 2) C2Cl4/Pd(100) surface was

determined from the intensities of the C 1s and the Cl 2p corelevels and their Scofield photoionization cross sections.18 Theintensities of both the C 1s and the Cl 2p emissions weredetermined by integrating the area under the peaks after thesubtraction of linear background. After correcting for theirScofield photoionization cross sections, the measured intensityratio between the Cl 2p and the C 1s core levels was found asCl:C ) 2.04:1, which is the expected atomic ratio of chlorineto carbon from stoichiometric C2Cl4 within experimental error.It was also observed that the adsorbate stoichiometry as wellas the Cl 2p and the C 1s HRXPS spectra of the formed p(2×2) C2Cl4 overlayer remained unchanged upon 100 langmuirsof oxygen exposure and subsequent heating to 600 K, in sharpcontrast to the previously reported oxidation of surface carbonin the more open, partially disordered CH2Cl2/Pd(100) structure.6

The effect of the C2Cl4 adsorption on the Pd(100) surfacewas studied through the surface core level shifts (SCLS) of thePd 3d core level. For the clean Pd(100) surface, the Pd 3d corelevel emission spectrum along surface normal (bottom curvein Figure 3) showed the sharp 3d5/2 and 3d3/2 spin-orbit doubletat 335.05 and 340.32 eV, respectively, in good agreement withthe reported positions for the 3d lines for Pd bulk atoms.11,19

At the polar angleθ ) 70°, the Pd 3d core level spectraexhibited broadening on the lower binding energy side (topcurve in Figure 3), which had been previously interpreted asthe SCLS of the clean Pd surface atoms of the (100) surface.19

After exposure to C2Cl4 at room temperature, the Pd 3d corelevel spectrum atθ ) 70° revealed a new component in boththe Pd 3d5/2 and Pd 3d3/2 peaks, shifted by 0.49 eV toward higherbinding energy with respect to the peak positions of the bulkcomponents (middle curve in Figure 3). In addition to theemergence of the new surface component, the intensity of thephotoemission lines from the Pd substrate including the Pd 3dcore level was substantially attenuated due to the screening effectof the p(2× 2) C2Cl4 overlayer.The polar angle dependence of various core level intensities

both from the Pd substrate and the overlayer was also examined.For a quantitative determination of the photoelectron intensity

Figure 2. HRXPS spectra of (a) the Cl 2p core levels from p(2× 2)C2Cl4/Pd(100) (dots) and c(2× 2) Cl/Pd(100) (dotted line) and (b) theC 1s core levels from p(2× 2) C2Cl4/Pd(100) (dots) and (2x2 ×x2)R45 CO/Pd(100) (dotted line). All the HRXPS spectra were takenat θ ) 70° for higher surface sensitiviy.

Figure 3. HRXPS spectra of the Pd 3d core level taken atθ ) 70°before (top) and after (middle) the exposure of the clean Pd(100) surfaceto C2Cl4 forming p(2× 2) C2Cl4/Pd(100). Also displayed is the HRXPSspectrum of the Pd 3d core level emission along surface normal (bottom)to better display the bulk component. The lines shown with theexperimental Pd 3d spectrum in the middle curve are the results of thedecomposition into the bulk and the surface component using a Voigtfunction and a Shirley background (see ref 11).

5422 J. Phys. Chem. B, Vol. 101, No. 27, 1997 Park et al.

from Pd, the total area under the Pd 3d5/2 peak was integratedwithout resolving it into two peaks (middle curve in Figure 3).Theθ-dependence of the Pd 3d5/2 core level intensity along the⟨010⟩ azimuth (Figure 4a) exhibited the forward focusingdominant X-ray photoelectron diffraction (XPD) maxima, whichwere essentially identical with those from clean Pd(100)surface.10,20 Neither these changes in the peak positions of themajor XPD maxima atθ ) 0°, 20°, 45°, and 70° nor any newextra diffraction maxima possibly resulting from the scatteringoff the adatoms were observed. In contrast to the large intensityanisotropy observed in the polar angle dependence of the Pd3d5/2 core level, both the C 1s and the Cl 2p core level intensitiesdisplayed smoothly rising, isotropic angular profiles as the polarangle increased to grazing polar angles (Figure 4b,c). Theapparent intensity maxima nearθ ) 80° observed in the angle-resolved XPS (ARXPS) data of the C 1s and the Cl 2p corelevels were caused by the sharply diminishing photoemissionintensity due to the instrument response function at grazing polarangles. A detailed discussion about the effect of the instrumentresponse function on ARXPS data was presented elsewhere.11

3.2. Low-Temperature Adsorption of C2Cl4 on Pd(100).The Cl 2p core level scan after the exposure of the Pd(100)surface to C2Cl4 gas at 131 K showed two distinct Clcomponents: a dominant Cl 2p3/2 line located at 200.31 eV andthe other much smaller but clearly visible Cl 2p3/2 peak at 197.65eV with their 2p1/2 satellites shifted by 1.63 eV to higher bindingenergy (Figure 5a). As the Pd substrate was warmed, theevolution of the Cl 2p core level spectra was monitored at thefollowing temperatures:T ) 147, 173, 199, 223, 249, and 291K. Up to T ) 199 K, both Cl 2p components displayed nosignificant changes in either their intensities or positions. At199 K, the Cl 2p core level scan showed a large reduction inthe intensity of the Cl component on the higher binding energyside (Figure 5a). On the contrary, the intensity of the lowerbinding energy Cl component at 199 K was about the same asbefore warm-up. When the sample was heated to 223 K, theCl component at the higher binding energy further decreasedwhile the Cl peaks on low binding energy exhibited a slightincrease. This trend of the large reduction of the higher bindingenergy Cl peak continued as the sample temperature was furtherraised to 291 K, at which the higher binding energy Cl peakscompletely disappeared. The C 1s core level spectra were alsomonitored at the same temperatures. Figure 5b displays the C1s core level spectra21 taken at selected temperatures:T) 131,199, and 291 K. As in the Cl 2p core level, the C 1s core

region at 131 K showed one dominant peak at 286.74 eV anda barely visible peak at 284.3 eV. No changes in the intensitiesand positions were observed up to 199 K, at which temperaturethe C 1s higher binding energy peak at 286.74 eV substantiallydecreased. Upon further heating, the C 1s peak on high bindingenergy continued to decrease whereas the C 1s peak at 284.3eV gradually increased.

4. Discussion

The results presented here provide an unambiguous evidencethat the C2Cl4 molecules are dissociated on the Pd(100) surfaceat 200 K and above into a well-defined surface structure whichretains the 2C:4Cl stoichiometry and has both elements C andCl on the surface rather than penetrating inside the metal. Acomparison with the previously reported dissociation of CH2-Cl2 on Pd(100)6 leads to the conclusion that hydrogen in themolecular structure of the chlorocarbon is not necessary for theC-Cl bond dissociation on Pd(100) at room temperature. Theordered p(2× 2) C2Cl4/Pd(100) structure observed here is notformed from either the partially disordered CH2Cl2/Pd(100) orthe c(2× 2) Cl/Pd(100) structure,6 thus proving that the carbonof the dissociatively chemisorbed tetrachloroethylene plays animportant role in the lateral ordering of the chlorine ensembles.A detailed analysis of the experimental results that leads to theabove conclusion is presented in the following paragraphs.4.1. Room-Temperature Adsorption Study of the p(2×

2) C2Cl4/Pd(100). 4.1.1. Quantification of Adsorbed Carbonand Chlorine. The chlorine coverage in the p(2× 2) C2Cl4/Pd(100) structure was calculated by comparing the Cl 2p corelevel intensity with that from the well-ordered c(2× 2) Cloverlayer, which is formed at room temperature upon saturationwith Cl2 and corresponds to 0.5 monolayer (ML) of stableatomic Cl on a clean Pd(100) surface.6 The unit 1 ML is definedas the surface density of the Pd atoms of the (100) surface, i.e.,1.32 × 1015 atoms/cm2. A direct comparison of the Cl 2pintensity between the p(2× 2) C2Cl4 and the c(2× 2) Cloverlayers on the same Pd(100) surface yielded the intensityratio of 0.49 by integrating peaks in Figure 2a. Consequently,the number of Cl atoms in the p(2× 2) C2Cl4 overlayer isdetermined as 3.3× 1014/cm2 or 0.25 ML. For the amount ofcarbon of the p(2× 2) C2Cl4 overlayer on the Pd(100) surface,the measured intensity ratio of 2.04 between the Cl 2p and the

Figure 4. Polar angle dependence of (a) the Pd 3d5/2, (b) the C 1s,and (c) the Cl 2p core level intensities from the C2Cl4/Pd(100) surfacealong ⟨010⟩ azimuth. The solid line in (a) is a cubic spline fit to thedata points. The dotted lines in (b) and (c) are the theoretical angledependence of the two core level emission (see section 4.1.3).

Figure 5. HRXPS spectra of (a) the Cl 2p core level in C2Cl4/Pd-(100) at the sample temperatures of 131, 147, 173, 199, 223, 249, and291 K and (b) the C 1s core level atT ) 131, 199, and 291 K. All theHRXPS spectra were taken atθ ) 70° for higher surface sensitiviy.


C 1s core levels after correcting for the Scofield cross sectiondirectly yields the surface C coverage of 0.12 ML or 1.6×1014/cm2.22

4.1.2. Chemical State of the Cl and C Species in theAdsorbate.The observed binding energies (BEs) for the Cl 2pand C 1s core levels from the p(2× 2) C2Cl4/Pd(100)demonstrate that the room-temperature adsorption of C2Cl4 isdissociative. The Cl 2p3/2 peak from the p(2× 2) C2Cl4/Pd-(100) surface has the BE of 198.04 eV, which is close to (albeitnot identical with) that from the c(2× 2) Cl/Pd(100) surface(Figure 2a), 197.49 eV, and far removed from the Cl 2p3/2 peakof undissociated C2Cl4 at 200.31 eV. The measured Cl 2p corelevel position is also consistent with the value of 198.5 eV forthe unresolved Cl 2p doublet28 reported by Mason and Texter29

in their study of the room-temperature adsorption of C2Cl4 onthe Fe(111) surface, which readily dissociates the C-Cl bondsand forms FeCl. In comparison with the Cl 2p3/2 peak position,200.2 eV, in the molecularly adsorbed C2Cl4 on the Pt(111)and the Pt(110) surfaces at 95 K,30 the position of the Cl 2p3/2core level from the p(2× 2) C2Cl4/Pd(100) surface representsa core level shift of 2.3 eV toward lower binding energy, clearlyexcluding the possibility that the chlorine in the p(2× 2) C2-Cl4 overlayer is in the form of molecular C2Cl4. The C 1s corelevel spectrum from the p(2× 2) C2Cl4/Pd(100) surface alsoyields a consistent picture of dissociated C2Cl4 moleculesadsorbed on the Pd(100) surface at room temperature. The C1s BE of 284.33 eV in the p(2× 2) C2Cl4/Pd(100) structure isin sharp contrast to the reported C 1s BE of 286.7 eV formolecularly adsorbed C2Cl4 on the Pt(111) surface.30 The C-Clbond dissociation is evident by the large C 1s core level shift,but the nature of the carbon-carbon bond in the p(2× 2) C2-Cl4/Pd(100) structure is more difficult to be determined due tothe fact that the correlation between the C 1s core level shiftsand carbon-carbon bonds is subtle in various carbonaceoussystems. Yet, it is noted that the observed position of the C 1score level in the p(2× 2) C2Cl4/Pd(100) structure, 284.33 eV,is nearly identical with the C 1s BE measured for dissociatedC2H2 and C2H4 on Pt(111), 284.3 eV,27 and within the expectedrange of the C 1s positions for graphitic carbon, 284.3-284.5eV.31 Both our observed and the quoted C 1s BEs are welloutside the range for the carbidic carbon, 281-283 eV,29,31bya large shift of>1.3 eV to higher binding energies. This isalso in agreement with the calculation of relative energies ofcarbon pairs and dissociated carbon atoms below and with earlierresults on other Pd surfaces14 which show that on Pd surfacethe carbon-carbon bond is stronger than the carbon-palladiumbond, favoring association rather than dissociation of carbonspecies. In tetrachloroethylene the CdC bond is already presentand its dissociation to a surface carbide is energetically notfavored.4.1.3. Surface Structure of the p(2× 2) C2Cl4/Pd(100)

OVerlayer. A smoothly rising, isotropic profile observed fromthe angular dependence of the C 1s and the Cl 2p core levels(Figure 4b,c) is characteristic of an angular variation of thephotoelectron intensity from a thin overlayer of atomic thickness.The intensity of photoelectrons traveling a distancel in solidattenuates exponentially according to exp(-l/λ) whereλ is theinelastic mean free path for the particular solid.32 In the limitof a continuum for the solid, the polar angle dependence ofphotoelectron intensity due to the inelastic scattering from anemitting layer of the nominal thicknessz0 is11,33

whereI0 is the photoelectron intensity from the top layer and

R(θ) is the instrument response function. For a thin emittinglayer (z0 e 1 Å) and high kinetic energy photoelectrons (λ g15 Å), the quantityz0/(λ cosθ) is less than unity, except forθvalues near 90°, and eq 1 can be expanded to yieldI(θ) )I0z0R(θ)/cosθ. For smaller polar angles, the instrument responsefunction is nearly constant;11 then the angle dependence isroughly 1/cosθ, which is consistent with theθ dependence ofboth the C 1s and Cl 2p photoelectron intensities observed fromthe p(2 × 2) C2Cl4/Pd(100) surface. Using 20 Å for thecalculated values ofλ for both the C 1s and the Cl 2pphotoelectrons in eq 1,34 the best fit for theθ dependence ofboth the C 1s and Cl 2p photoelectron intensities yields theeffective thicknessz0 ≈ 0.2 Å, qualitatively representing well-dispersed submonolayer surface carbon.The isotropic polar angle dependence of the C 1s core level

intensity from the dissociated p(2× 2) C2Cl4/Pd(100) overlayerfurther implies no ordered orientation of the CdC and C-Clbond axes on the Pd(100) surface. Figure 6 shows a comparisonof the polar angle dependences of the C 1s photoemissionintensity between the p(2× 2) C2Cl4/Pd(100) (open circles)and the (2x2 × x2)R45 CO (full circles) overlayers. TheARXPS data from the (2x2 × x2)R45 CO/Pd(100) surfaceshow a maximum along surface normal (θ ) 0°) in addition tothe smoothly increasing background intensity. The observedmaximum results from the forward focusing enhancement ofthe C 1s photoelectron intensity by the oxygen atom along theupright CO bond axis with C down toward the metal surface.Such intensity maxima from forward scattering along themolecular axis have been previously observed in many otheradsorbate systems including CO on other transition metalsurfaces,35,36 CH3O on Cu(110),36 and N2 on Ni(100).37 Forethylenic molecules on the transition metal surfaces, Wesner etal.38 reported the C 1s intensity enhancement along surfacenormal due to the upright C-C bond of ethylidyne after thewell-characterized transition of adsorbed ethylene to ethylidyneon Pt(111).39 Therefore, the absence of such a forward focusing

I(θ) ) I0λR(θ)[1- exp(-z0

λ cosθ)] (1)

Figure 6. Polar angle dependence of the C 1s line intensities from (a)p(2 × 2) C2Cl4/Pd(100) and (b) (2x2 × x2)R45 CO/Pd(100). Thedotted line represents theoretical isotropic intensity profile calculatedfor submonolayer (nominal thickness of 0.3 Å) to simulate the ARXPSdata from p(2× 2) C2Cl4/Pd(100). Also, an illustration of the differencein the adsorbate geometry between the above systemts is shown below.


maximum in the present C 1s ARXPS data indicates no orientedC-C bond axis analogous to ethylidyne as well as C-Cl bondaxes of carbene, going out of the p(2× 2) C2Cl4/Pd(100) surfaceplane.Combining the results of the LEED, HRXPS, and ARXPS

data, we present a model for the surface chlorine and carbonadatoms in a p(2× 2) overlayer on the Pd(100) surface as shownin Figure 7. Because the present study cannot determine theexact adsorption sites for chlorine and carbon except for thep(2 × 2) periodicity, chlorine atoms are placed in the 4-foldhollow sites, where Cl atoms are found to adsorb on the (100)faces of Ni,40Ag,22,41Cu,43 and Pd.6 The positions of the carbonatoms are chosen to be randomly oriented within every othersurface unit cell resembling the shape of molecular C2Cl4, thevan der Waals envelope of which is also shown in relation tothe underlying Pd(100) surface. Therefore, in this surfacearrangement, the atomic Cl forms the usual p(2× 2) overlayerstructure of 0.25 ML, and the carbon atoms are randomlyoriented in every other surface unit cell, satisfying the 0.125ML carbon coverage and the observed p(2× 2) LEED pattern.Although the model presented assumes the ideally terminatedPd(100) surface, it is also possible to have the Pd sur-face reconstructed by the highly electronegative Cl atomsin the 4-fold sites such that the p(2× 2) LEED patternmainly results from the shifts of four Pd atoms nearest to theadsorbed Cl.Based on the C 1s core level binding energy (see section

4.1.2), the model for surface chlorine and carbon adatoms inFigure 7 presents unbroken C-C bonds in several orientations.To investigate the stability of the C-C bonds in the proposedp(2 × 2) C2Cl4/Pd(100) structure, semiempirical quantummechanical calculations using the extended Hu¨ckel (EH) pack-age from Quantum Chemistry Program Exchange44 wereperformed. For the calculations, a cluster of 61 Pd atoms,consisting of 36 surface and 25 subsurface atoms to simulatethe Pd(100) face was chosen. The large size of the cluster wasnecessary to minimize the edge effect. Four Cl atoms wereadsorbed at the 4-fold sites in the p(2× 2) arrangement, andthe distance from Cl to the surface Pd plane was fixed at 1.66

Å, which is the average experimental distance of Cl from Ag-(100) found in literature.42 Two C atoms were placed in thefollowing four possible C geometries (the bottom of Figure 7):(1) C atoms were positioned on the 2-fold bridge sitesrepresenting the broken C-C bond; (2)-(4) the C pairs werecentered at the 4-fold hole with the C-C bond axis parallel tothe surface plane. The distance between the two C atoms wasvaried from 1.543 to 1.337 to 1.205 Å, representing single,double, and triple bonds, respectively.45 The results of the EHcalculations showed that the two dissociated carbon atoms onthe bridge sites are the least stable among the four cases ofadsorption geometry. Taking the two C atoms at the bridgesites as reference, the energies for the C-C, CdC, and CtCwere calculated lower by 2.7, 3.9, and 4.8 eV, respectively.Furthermore, the total EH energy decreased as the C-C bonddistance decreased, indicating no driving force for dissociatingthe C-C bond to result in atomic carbon, but rather strengthen-ing of the C-C bond as the C-Cl bond dissociated.4.2. Low-Temperature Adsorption Study. 4.2.1. Adsorp-

tion of C2Cl4 at 131 K and Its Interaction with Pd(100).Afterexposure of the Pd(100) surface to C2Cl4 at 131 K, the totalamount of chlorine on the surface was determined to be 0.62ML by comparing the intensity of the Cl 2p core level to thatin c(2 × 2) Cl/Pd(100). The two well-resolved Cl 2pcomponents, each of which is split into a 2p3/2 and a 2p1/2 peak(Figure 5a), indicate the presence of two different kinds ofchlorine on the surface. The dominant Cl contribution at 200.31eV with its position of the Cl 2p3/2 peak is in good agreementwith the previously reported values of 200.2 eV from molecularC2Cl4 at 95 K,30 as well as 200.5 eV from multilayer CH3Cl at100 K.46 Thus, we attribute the observed Cl 2p3/2 peak at 200.31eV to molecular C2Cl4. The other Cl 2p component at 197.65eV is observed approximately at the same position as that inthe p(2× 2) C2Cl4 overlayer at room temperature (Figure 2a)and hence is assigned to atomic Cl from dissociated C2Cl4. TheC 1s core level spectrum also shows two types of carbon speciesat 131 K (Figure 5b). The first at 286.74 eV is nearly identicalwith the reported value of 286.7 eV for molecular C2Cl430

representing highly oxidized carbon. The second broad peak

Figure 7. Schematic representation of a possible surface arrangement for p(2× 2) C2Cl4/Pd(100). The surface Pd atoms, the p(2× 2) Cl atoms,and the randomly oriented carbon pairs are shown as dark, intermediate, and small spheres, respectively, on the ideally terminated Pd(100) surface.Also shown is an undissociated C2Cl4 molecule with Cl as large light spheres.


centered near 284.3 eV is consistent with carbon from thedissociated C2Cl4 observed at room temperature adsorption asdiscussed in section 4.1.1.A more careful examination of the Cl 2p core level spectra

suggests that there may be another Cl species present on thePd(100) surface at low temperatures. A hint for the possiblythird Cl species can be first gleaned from the fact that the relativeheights of the 2p3/2 and 2p1/2 peaks on the low binding energyfor the atomic Cl are roughly one-to-one whereas those at highbinding energy for molecular C2Cl4 shows the intensity ratioof 2:1 (Figure 5), expected from the ratio of the degeneraciesbetweenj ) 3/2 and j ) 1/2 states. Although the observeddeviation in the relative peak heights of the spin-orbitcomponents of atomic Cl could have simply resulted from theoverlap with the intense Cl 2p3/2 peak of molecular C2Cl4, thefact that it persists even after most molecular C2Cl4 hasdisappeared at 223 K strongly indicates the presence of a thirdCl species. To examine the possibility of the new Cl componentand furthermore study the Cl components quantitatively, theCl 2p core level at 131 K was fitted with Voigt functions on alinear background intensity. During the fitting procedure, it wasapparent that a reasonable fit to the experimental data pointscould not be achieved with two pairs of Cl 2p peaks alone ifthe criterion for the intensity ratio of 2 between the 2p3/2 and2p1/2 levels was taken into account. Therefore, the Cl 2p corelevel spectrum taken at 131 K was fitted using three Cl pairs ofa 2p3/2 and a 2p1/2 peaks with the following constraints: (1) thespin-orbit splitting for 2p3/2 and 2p1/2 levels was fixed at 1.63eV as determined in our room-temperature adsorption (section3.1 and Figure 2a); (2) the intensity ratio between 2p3/2 and2p1/2 peaks was kept constant at 2; and (3) the fitting parametersfor the 2p3/2 and 2p1/2 levels were varied together for each Clcomponent. The results of the best fit to the experimental Cl2p core level spectrum and the parameters used in the fittingprocedure are presented in Figure 8 and Table 1, respectively.The results indicate three Cl components at the followingpositions: one Cl 2p3/2 peak representing Cl in molecularC2Cl4 at 200.31 eV (a), another at 198.88 eV (b), and the third

Cl component for dissociated C2Cl4 at 197.65 eV (c). At thispoint, we tentatively assign the second Cl componentb to apartially dissociated C2Cl4 species. The relative intensitiesamong the three Cl species indicate that on the Pd(100) surfaceat 131 K, 70% of Cl is in molecular C2Cl4 while the remaining30% of Cl is in the form of either atomic Cl or partiallydissociated C2Cl4 species.4.2.2. Interaction of Adsorbed C2Cl4 with the Pd(100)

Surface between 131 and 291 K.The intensities of each Clspecies including the molecular C2Cl4 (a), partially dissociatedC2Cl4 (b), and the atomic Cl (c) were further followed as afunction of the substrate temperature in Figure 9, which clearlyshows three temperature regions. BelowT ) 173 K, themajority of adsorbed C2Cl4 overlayer is in molecular form. Forall the dissociated C2Cl4 species, the amount of partiallydissociated C2Cl4 speciesb steadily decreases while that ofatomic Cl speciesc increases approximately by the sameamount, suggesting a slow conversion from the partiallydissociated C2Cl4 to completely dissociated atomic Cl. As thetemperature increases from 173 to 223 K, the total Cl coverageon the Pd(100) surface decreases from 0.62 to 0.29 ML. Theobserved decrease is mainly due to the desorption of molecularC2Cl4, and the amount of Cl of molecular C2Cl4 decreases fromca. 0.44 to 0.02 ML atT ) 223 K (or ca. 0.11 ML of C2Cl4 to≈0.005 ML).47 The onset of the desorption of molecularC2Cl4 is at ca. 173 K, and the maximum desorption rate is at187 K. The activation energy for the desorption is estimatedto be 11 kcal/mol using the maximum desorption temperaturevalue,48 and the obtained activation energy is in good agreementwith the value of 12.7 kcal/mol, for molecular desorption fromthe Fe(110) surface49 and is slightly higher than the enthalpyof vaporization of C2Cl4(l), 9.2 kcal/mol.50

Between 173 and 223 K, the amount of all dissociated C2Cl4species (b and c in Figure 9) increases only by ca 0.07 MLfrom 0.19 to 0.26 ML, and the signal from atomic Cl continuesto increase from 0.14 to 0.20 ML. Thus, the observed increasein the Cl signal from all the dissociated C2Cl4 species is mostlydue to the increase in the amount of atomic Cl. It is likely thatthe excess amount results from the dissociation of molecularC2Cl4 accompanying the major desorption observed during thetemperature increments. No decrease in the amount of thepartially dissociated C2Cl4 species occurs as the temperature is

Figure 8. Decomposition of the Cl 2p HRXPS core level spectrumfor C2Cl4/Pd(100) atT ) 131 K. The fitting parameters are listed inTable 1.

TABLE 1: Decomposed Cl 2p Core Level Spectrum atT )131 K Using Voigt Functions; Only Results for the Cl 2p3/2Peak Are Listed Below

Cl species position (eV) fwhm (eV) asym factor mixa areab

a 200.31 0.856 0.13 0.68 0.70b 198.88 0.846 0.17 0.0 0.13c 197.65 0.989 0.31 0.90 0.17

aMix is the ratio of Gaussian to Lorentzian; 0 for Gaussian and 1for Lorentzian.bNormalized to 1.

Figure 9. Intensities of each Cl species identified as molecular C2Cl4(circle with dotted line,a) and all dissociated Cl (square, cross withsolid line,b + c) as a function of temperature. Further decompositionof dissociated Cl species into “partially dissociated” C2Cl4 (b) andatomic Cl (c) is also shown.


increased, and this suggests that molecular C2Cl4 initiallydecomposes into the partially dissociated C2Cl4 species, whichsubsequently dissociate into atomic Cl. As the temperature risesabove 223 K, the amount of atomic Cl increases at the expenseof the partially dissociated C2Cl4 species up to 291 K, at whichall the partially dissociated C2Cl4 species disappears. In theroom temperature chemisorption, the dissociation is complete,and no molecular precursors or partially dissociated species areobserved, as also described in section 4.1.2.4.2.3. The Cl 2p Binding Energy Shifts.The dissociation

of TCE on the palladium surface takes place directly at roomtemperature to give rise to the p(2× 2) structure with the BEof Cl 2p3/2 equal to 198.04 eV. However, the dissociation occursstepwise when the TCE is first physically adsorbed at 130 Kand then the temperature gradually raised to 300 K. Thisprocess is clearly one in which chlorine moves from an organicto an inorganic-bound state, as reflected in the BE shifts shownin Table 2. Starting from the Cl-CdC- bond in the physicallyadsorbed TCE and ending with the Cl-Pd bond in thedissociated p(2× 2) structure, the BE shifts of Cl 2p3/2 are inthe direction expected from an increased polarization of its bondwith a concomitant increase in ionicity of the Cl-CdC- toCl-Pd bond. The bond ionicitiesI(A-B) calculated from thePauling electronegativities (3.0 for Cl, 2.5 for C, and 2.2 forPd) suffice to provide a qualitative guide for the observed trends,an approach first employed by Siegbahn et al.51 in setting upcorrelations between the BEs and calculated charges:I(Cl-C)) 0.06 andI(Cl-Pd)) 0.15. Within this range, the Cl 2p3/2BEs vary by up to 2.8 eV (cf. Table 2) and therefore are quitesensitive to relatively small changes of the bond ionicity. Inlater work, the effective atomic charges were calculated bysemiempirical quantum mechanical methods and led to satisfac-tory correlations with the BE shifts, particularly for sulfur andnitrogen compounds.52 An account for final state effectsincluded polarization of the delocalizedπ-systems,53 but theBE vs semiempirical charge correlation has proven useful todate for a series of compounds in which the BE shifts arerelatively large54 and which do not involve transition metals.

In our case a transition metal is present and the behavior of itscore level shifts (e.g., Pd 3d) is quite complex.11 The metaleffects are indeed expected to play a role in the BE shifts ofthe Cl species, and it is those effects that may be responsiblefor alteration of the trends emerging from the simple BE vscharge correlation picture. Nevertheless, experience shows thata defined BE is a signature of a distinct chemical species, andboth the comparison with reference compounds and the BE vscharge correlations permit to identify the nature of the bondinginvolved.With this approach in mind, the observed Cl 2p BEs are

interpreted as follows. There are three distinct species of TCEadsorbed on Pd(100): (a) physically adsorbed, undissociatedTCE, present at low temperatures and desorbing around 180K, with the Cl 2p BE close to that of TCE physically adsorbedon Pt(111) and organic chlorohydrocarbons55 (Table 2); (b)“partially dissociated” TCE, present also at low temperatures,with the Cl 2p BE intermediate between the organic andinorganic chlorine species (Table 2), which is a precursor ofthe fully dissociated TCE and converts to it below roomtemperature; (c) fully dissociated TCE that forms the p(2× 2)structure at room temperature, with Cl 2p BE close to thatoccurring in the c(2× 2)Cl/Pd(100) overlayer formed upondissociative chemisorption of elemental chlorine (Table 2). Thefact that the Cl 2p BE in the p(2× 2)TCE/Pd(100) overlayer(speciesc) is higher (by 0.54 eV) than in the c(2× 2)Cl/Pd-(100) structure indicates that the effective negative charge onchlorine in the speciesc is lower (by ca. 0.02e) than in the c(2× 2) structure. This can be attributed to an inductive, electron-withdrawing, effect of the neighboring carbon, Cr Pdr Cl.A similar but larger effect occurs in the “partially dissociated”speciesb, in which the Cl 2p BE corresponds to bond ionicityI ) 0.11, close to an average of the ionicitiesI(Cl-Pd) andI(Cl-C). Thus, the “partially dissociated” speciesb wouldcorrespond to a complex in which the Cl-C bond is stretchedbut not broken while the Cl-Pd bond is being formed. It isinteresting that such a species is associated with a distinct Cl2p BE and survives a rather large temperature range. In thesearch for organometallic analogues to the “partially dissociated”TCE speciesb observed here, we note that a large number ofPd complexes exist with halogen, alkane, alkene, or alkyneligands, either mononuclear or bridged,56-59 in which thebridging ligands ought to be “π-donors”.56 Suchπ-donors arefrequently halogens coordinated to two neighboring Pd atoms.Therefore, our partially dissociated speciesb may be one inwhich the Cl atoms assume a 2-fold coordination before theymove to the 4-fold hole.

Conclusions

The room-temperature chemisorption of C2Cl4 gives rise toa well-defined p(2× 2) C2Cl4/Pd(100) overlayer structure ofdissociated, stoichiometric carbon pairs and chlorine adatoms.The p(2× 2) structure is forced by the presence of carbon, aschlorine adatoms form the well-known c(2× 2) structure. Thep(2× 2) C2Cl4/Pd(100) overlayer does not allow the subsequentchemisorption of oxygen and the oxidation of the carbon pairs.The amounts of carbon and chlorine on surface were determinedas 0.125 and 0.25 ML, respectively. The exposure of a cooledPd(100) surface to tetrachloroethylene atT ) 131 K showedpredominantly molecular C2Cl4 as established by the XPSbinding energies of the Cl 2p and C 1s core levels. In addition,the high-resolution XPS in the Cl 2p core region revealed twosatellite Cl peaks: one shifted from the molecular C2Cl4 peakby ∆BE ) -2.7 eV and the other by∆BE ) -1.4 eV,corresponding to atomic Cl and partially dissociated C2Cl4

TABLE 2: Cl 2p 3/2 XPS Binding Energies in VariousSpecies Relevant to Those Occurring in the Adsorption andDecomposition of Tetrachloroethylene (TCE) on Pd(100)

speciesBE of Cl2p3/2 (eV) reference

PdCl2 198.2-198.8 31c(2× 2) Cl/Pd(100) 197.50 present workp(2× 2) Cl/Pd(100) from TCE

decomposed at RT198.04 present work

Cl/Fe(111) from TCE decomposed at RT 198.5a 29CdC‚‚‚Cl/Pd(100) from TCE “partially

dissociated” at low temperature198.88 present work

C2Cl4 physically adsorbed on Pd(100) 200. 31 present workC2Cl4 physically adsorbed on Pt(111) 200.2 30

Cln

200.44 52

Cl

n

200.50 52

-O-CH2-CH2-Cl 200.53 52

C C

Cl

n

200.63 52

(-CHCl-CH2-)n 200.64 52(-CCl2-CH2-)n 200.78 52C2Cl4(g) 207.11b 17

a See the footnote 28.


species, respectively. When the temperature was increased, thepartially dissociated C2Cl4 gradually converted to atomic Cl.By the time the temperature reached 291 K, the dissociation ofC2Cl4 on the Pd(100) was complete.

Acknowledgment. We are thankful to Dr. Alfred Miller atthe SCIENTA ESCA laboratory for the time allocation andtechnical assistance rendered during this experiment. We arealso grateful to Prof. Gary Simmons for stimulating discussions.This work was supported by the Department of Energy BasicEnergy Sciences Grant DE-FG02-86ER13580.

References and Notes

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(22) In principle, the amount of carbon on the p(2× 2) C2Cl4/Pd(100)surface can be obtained independent of the results from Cl, that is, usingthe C 1s core level intensity from the surface of a known carbon coverage.For instance, the C 1s core level intensity of p(2× 2) C2Cl4/Pd(100) canbe calibrated against that from the (2x2 × x2)R45 CO/Pd(100) surface,for which it is well-established that 0.5 ML of molecular carbon monoxideadsorbs on the bridge site of the surface.23,24 However, when using theintensity of the C 1s core level from the (2x2 × x2)R45 CO/Pd(100)surface, extra care must be put into in estimating the C 1s intensityaccurately. In Figure 2b, the C 1s HRXPS core level spectrum from thep(2× 2) C2Cl4 overlayer is compared to that from the (2x2 × x2)R45CO overlayer on the same Pd(100) surface. Judging from the peak heightand width, the C 1s intensity from the p(2× 2) C2Cl4/Pd(100) surfaceappears to be only half of that from the (2x2 × x2)R45 CO/Pd(100)surface, not the expected one-fourth, contradicting the result from the Cl2p core level intensity (Figure 2a). The apparent discrepancy results fromthe fact that the C 1s core level intensity from the CO molecules issignificantly underestimated because a large portion, up to 50%, of the C1s core level intensity is lost into the inelastic background on high bindingenergy as the shake-up or shake-off broadening.25 Such an apparent errorby a factor of 2 was similarly reported by Bonzel and co-workers26,27 intheir attempt to calculate the amount of carbon from the p(2× 2) C2H4/Pt(111) surface using the C 1s level intensity from the c(4× 2) CO overlayeron the same Pt(111) surface. For a reliable quantification of carbon usingthe C 1s signal from CO, Griffiths et al.26 concluded that a significant portionof the inelastic tail, which may extend to several electronvolts on higherbinding energy must be included to account for the intensity lost into theshake-up region.

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