Interaction of Tetrachloroethylene with Pd (100) Studied by High

Department of Civil Engineering, Lehigh University, Bethlehem, .... by Simmons et al.13 A typical clean Pd(100) surface free of impurities produced a ...
8 downloads 0 Views 278KB Size
Download PDF
5420

J. Phys. Chem. B 1997, 101, 5420-5428

Interaction of Tetrachloroethylene with Pd(100) Studied by High-Resolution X-ray Photoemission 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 Zhang Department 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 at room and cryogenic temperatures. The 300 K saturation of Pd(100) with C2Cl4 gas gave rise to a wellordered p(2 × 2) overlayer structure. High-resolution X-ray photoemission spectroscopy established that the C-Cl bonds in the p(2 × 2)C2Cl4 overlayer were dissociated while retaining the stoichiometry 2C: 4Cl, and the amounts of carbon and chlorine on the surface were 0.125 and 0.25 monolayer (ML), respectively. At 131 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 the Cl 2p region revealed two satellite Cl peaks: one shifted from the molecular C2Cl4 peak by ∆BE ) -2.7 eV and 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 adsorbed Cl atoms until T ) 291 K, at which temperature all Cl on the surface formed atomic Cl of 0.25 ML. The carbon 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 upon dissociative adsorption of elemental chlorine.

1. Introduction The interaction of halogens or halogen-containing hydrocarbons with transition metal surfaces is of interest in heterogeneous catalysis and in abatement of chlorohydrocarbons. Because of their high electron affinities and relatively weak bonding in their parent molecules, halogens and halogen-substituted hydrocarbons display high reactivity and behave as poisons or promoters on transition metal catalysts, altering the surface adsorption and reaction properties for subsequent reactants. In the case of chlorine, these properties have been exploited in the selective poisoning of Ag ethylene epoxidation catalysts1 and the redispersion of Pt reforming catalysts.2 The use of halogens to modify a catalytic process has been also applied to the oxidation of methane using palladium catalysts. Cullis et al.3 and later Mann and Dosi4 reported that palladium catalysts in the presence of halogenated hydrocarbons produce partial oxidation reaction products such as formaldehyde and carbon monoxide in contrast to the well-known activity of Pd for complete combustion of methane to carbon dioxide and water in the absence of any promoters.5 To understand the mechanisms of modifying the Pd surface reactivity as well as the role of chlorine adatoms in controlling the activity and selectivity of partial oxidation of methane, Wang et al.6 studied the adsorption of dichloromethane (CH2Cl2) and its interaction with oxygen on the single-crystal Pd(100) surface. In this study, the authors observed that CH2Cl2 adsorbs dissociatively at room temperature and forms partially ordered overlayer structure at saturation, as evidenced by streaks at half-integer spots in the low-energy electron diffraction (LEED) pattern. Furthermore, the partially ordered Cl adatoms left on the surface after the removal of carbon through oxidation cycles decreased the X

Abstract published in AdVance ACS Abstracts, June 15, 1997.

S1089-5647(97)01139-5 CCC: $14.00

activation energy of oxygen desorption by as much as 10 kcal/ mol, suggesting the modification of oxygen reactivity due to intermediate range (3-4 Pd-to-Pd distances) lateral interactions between the adsorbed Cl and O atoms. Although the Wang et al. study demonstrated the use of CH2Cl2 as the Cl ensembleforming agent and the influence of chlorine adatoms on subsequent oxygen reactivity, it was deemed necessary to explore other chlorohydrocarbons as the ensemble-forming agents and understand their basic chemical interaction with the palladium surface in order to determine the general patterns of adsorption of halogen-substituted hydrocarbons, the state of the carbon and chlorine adatoms, stoichiometry, structure, and thermal 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 surfaces and their influence on reactants, we have investigated the interaction of tetrachloroethylene (C2Cl4, TCE) with the Pd(100) surface using high-resolution X-ray photoemission spectroscopy (HRXPS) and LEED for the following reasons: (1) this simple haloalkene contains no hydrogen, and the surface chemistry could be different from the previously studied CH2Cl2, particularly in terms of C-Cl bond dissociation; (2) the C2Cl4 molecule is nearly square planar (2.88 × 3.19 Å) with D2h symmetry,7 and its resemblance to the square mesh of the Pd(100) surface lattice makes it an interesting choice as an ensemble-forming reactant; (3) C2Cl4 is widely used as an industrial solvent, and its interaction with transition metal surfaces and decomposition may be useful in environmental chemistry.8,9 2. Experimental Section The experiments were carried out in an ultrahigh-vacuum (UHV) system, which houses the SCIENTA ESCA-300 HRXPS © 1997 American Chemical Society

Interaction of Tetrachloroethylene with Pd(100) spectrometer.10 A two-speed rotating anode of special UHV design generates unpolarized, monochromatized Al KR radiation (hν ) 1486.8 eV) of up to ca 8 kW. The photoexcited electrons are detected by a 300 mm mean radius hemispherical energy analyzer with variable slit widths. The newly acquired automated sample manipulator/goniometer (Seiko Instruments) allows three degrees of translational motion (x, y, z) with 3 µm repeatability as well as two degrees of rotational motion (polar angle θ, rotation φ) with 0.2° repeatability. In addition to the high-precision motion, the manipulator is equipped with a resistively heated hot and a liquid nitrogen-cooled stage for in situ temperature-dependent XPS study at temperatures ranging from 100 to 900 K. The sample temperature during the XPS data acquisition is measured by an alumel-chromel thermocouple 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 previously determined using Laue diffraction. Then, more precise in situ sample alignment was carried out using the X-ray photoelectron diffraction (XPD) maxima at θ ) 45° along the 〈010〉 direction,10 where the polar angle θ 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 various core level regions were recorded before moving to the next angular position. The binding energies in the XPS spectra were referenced using the position of the Ag 3d5/2 core level at 368.25 eV as well as the Pd 3d5/2 level at 335.05 eV from the clean Pd(100) single-crystal surface. Detailed information of the sample geometry and the SCIENTA ESCA-300 HRXPS spectrometer are available elsewhere.11,12 The cleanliness and surface order of the Pd(100) single crystal were verified using a LEED optics and HRXPS. An XPS survey scan revealed that a major impurity of the Pd(100) single crystal was carbon. The initial procedure of cleaning impurities including carbon involved a few cycles of Ar ion sputtering followed by annealing for a short period time using an electron beam heater in the preparation chamber. Although the above procedure was able to remove most impurities, this method of cleaning usually left ca 0.1 ML of residual carbon on the surface. However, the complete removal of any residual carbon was achieved by the oxidation of the residual carbonsheating the Pd surface to approximately 800 K in a partial oxygen atmosphere (P ) 5 × 10-7 mbar) for 5 min, as previously described by Simmons et al.13 A typical clean Pd(100) surface free of impurities produced a sharp p(1 × 1) LEED pattern shown in Figure 1a. Following the characterization of the clean Pd(100) surface, high-purity C2Cl4 (Aldrich, HPLC grade 99.9+%) in a glass bottle was admitted into the ultrahigh-vacuum chamber through a precision variable leak valve. Prior to the experiments, a number of freeze-thaw cycles were employed to remove ambient gas trapped in the glass bottle. For room-temperature adsorption study, the Pd(100) surface was exposed to C2Cl4 at the constant pressure of 1 × 10-6 Torr in the preparation chamber. For low-temperature adsorption of C2Cl4, the clean Pd(100) surface was first cooled to T ) 131 K using liquid nitrogen in the analysis chamber for about an hour. Despite precautions, the XPS scan of the surface after cooling indicated a small amount of carbon (≈0.06 ML) adsorbed on the surface. However, it is likely that this carbon does not spread but rather it forms clusters leaving the most of the Pd surface free from the impurities, as supported by previous studies14,15 as well as the fact that the C-C bonding energy is larger than Pd-C bonding energy.16 The C2Cl4 gas was admitted to the Pd(100)

J. Phys. Chem. B, Vol. 101, No. 27, 1997 5421

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 after exposure of 77 langmuirs of C2Cl4 at room temperature. The (0,0) spots were blocked off by a miniature electron gun assembly of the LEED optics.

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 gauge located distant from the Pd surface, the actual exposure of C2Cl4 gas in terms of langmuirs was not known. However, the amount of adsorbed chlorine on the surface was determined by comparing the intensity of the Cl 2p peak with that from the c(2 × 2) Cl/Pd(100) surface.6 After determining the Cl coverage, the amount of carbon was calculated using the XPS intensity ratio of the C 1s to the Cl 2p core levels from the saturated p(2 × 2) C2Cl4/Pd(100) surface. A more detailed description of quantification of the amount of surface adsorbates is presented in section 4.1.1. For the study of the temperature dependence of the interaction between the adsorbed C2Cl4 and the Pd(100) surface, the temperature of the substrate was further raised from 131 to 291 K. The continuous increase in the sample temperature was achieved by blowing in N2 gas into the coldfinger and thus warming the cold stage. The average heating rate was constant at 1.46 K/min, except during the initial warming to 147 K, for which it was much slower due to the evaporation of residual liquid nitrogen inside the coldfinger. To monitor the temperature dependence of the core level XPS, the Cl 2p and C 1s core level scans were taken at selected temperatures: T ) 147, 173, 199, 223, 249, and 291 K with ∆T ≈ (7.3 K, where ∆T

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

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) the C 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 taken at θ ) 70° for higher surface sensitiviy.

mainly resulted from the continuous increase in the sample temperature during the ca. 10 min data acquisition of for the XPS scan. 3. Results 3.1. Room-Temperature Chemisorption of C2Cl4 on Pd(100). Exposure of the clean Pd(100) surface to 77 langmuirs of C2Cl4 (1 × 10-6 Torr × 100 s) at room temperature resulted in sharp LEED spots at (0, (1/2), ((1/2, 0), and ((1/2, (1/2) (Figure 1b). The measurements of the angular positions of the LEED maxima and the kinetic energy of the incident electron beam yielded a value of 5.64 Å for the new lattice spacing of the C2Cl4-covered Pd(100) surface. The calculated value corresponded to twice that of the clean Pd(100) surface unit cell within 2%, showing that the observed LEED pattern represents the p(2 × 2) overlayer structure. The XPS survey scan of the p(2 × 2) C2Cl4/Pd(100) surface revealed the presence of carbon and chlorine on the Pd(100) surface after the exposure to C2Cl4. Further exposure of the sample to 1 × 10-6 Torr of C2Cl4 for 200 s, corresponding to additional 154 langmuirs of C2Cl4, did not increase the photoelectron intensities from either chlorine or carbon, indicating the saturation coverage had been achieved. 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 angle θ ) 70° (Figure 2a,b). The HRXPS scan in the Cl 2p core level region showed a well-resolved doublet of Cl 2p3/2 and Cl 2p1/2 lines with the Cl 2p3/2 peak at 198.04 eV. The spinorbit 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 gasphase 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 and chlorine adatoms on the p(2 × 2) C2Cl4/Pd(100) surface was

Park et al.

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

determined from the intensities of the C 1s and the Cl 2p core levels and their Scofield photoionization cross sections.18 The intensities of both the C 1s and the Cl 2p emissions were determined by integrating the area under the peaks after the subtraction of linear background. After correcting for their Scofield photoionization cross sections, the measured intensity ratio between the Cl 2p and the C 1s core levels was found as Cl:C ) 2.04:1, which is the expected atomic ratio of chlorine to carbon from stoichiometric C2Cl4 within experimental error. It was also observed that the adsorbate stoichiometry as well as the Cl 2p and the C 1s HRXPS spectra of the formed p(2 × 2) C2Cl4 overlayer remained unchanged upon 100 langmuirs of oxygen exposure and subsequent heating to 600 K, in sharp contrast to the previously reported oxidation of surface carbon in the more open, partially disordered CH2Cl2/Pd(100) structure.6 The effect of the C2Cl4 adsorption on the Pd(100) surface was studied through the surface core level shifts (SCLS) of the Pd 3d core level. For the clean Pd(100) surface, the Pd 3d core level emission spectrum along surface normal (bottom curve in Figure 3) showed the sharp 3d5/2 and 3d3/2 spin-orbit doublet at 335.05 and 340.32 eV, respectively, in good agreement with the reported positions for the 3d lines for Pd bulk atoms.11,19 At the polar angle θ ) 70°, the Pd 3d core level spectra exhibited broadening on the lower binding energy side (top curve in Figure 3), which had been previously interpreted as the SCLS of the clean Pd surface atoms of the (100) surface.19 After exposure to C2Cl4 at room temperature, the Pd 3d core level spectrum at θ ) 70° revealed a new component in both the Pd 3d5/2 and Pd 3d3/2 peaks, shifted by 0.49 eV toward higher binding energy with respect to the peak positions of the bulk components (middle curve in Figure 3). In addition to the emergence of the new surface component, the intensity of the photoemission lines from the Pd substrate including the Pd 3d core level was substantially attenuated due to the screening effect of 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

Interaction of Tetrachloroethylene with Pd(100)

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) surface along 〈010〉 azimuth. The solid line in (a) is a cubic spline fit to the data points. The dotted lines in (b) and (c) are the theoretical angle dependence of the two core level emission (see section 4.1.3).

from Pd, the total area under the Pd 3d5/2 peak was integrated without resolving it into two peaks (middle curve in Figure 3). The θ-dependence of the Pd 3d5/2 core level intensity along the 〈010〉 azimuth (Figure 4a) exhibited the forward focusing dominant X-ray photoelectron diffraction (XPD) maxima, which were essentially identical with those from clean Pd(100) surface.10,20 Neither these changes in the peak positions of the major XPD maxima at θ ) 0°, 20°, 45°, and 70° nor any new extra diffraction maxima possibly resulting from the scattering off the adatoms were observed. In contrast to the large intensity anisotropy observed in the polar angle dependence of the Pd 3d5/2 core level, both the C 1s and the Cl 2p core level intensities displayed smoothly rising, isotropic angular profiles as the polar angle increased to grazing polar angles (Figure 4b,c). The apparent intensity maxima near θ ) 80° observed in the angleresolved XPS (ARXPS) data of the C 1s and the Cl 2p core levels were caused by the sharply diminishing photoemission intensity due to the instrument response function at grazing polar angles. A detailed discussion about the effect of the instrument response 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 Cl components: a dominant Cl 2p3/2 line located at 200.31 eV and the other much smaller but clearly visible Cl 2p3/2 peak at 197.65 eV with their 2p1/2 satellites shifted by 1.63 eV to higher binding energy (Figure 5a). As the Pd substrate was warmed, the evolution of the Cl 2p core level spectra was monitored at the following temperatures: T ) 147, 173, 199, 223, 249, and 291 K. Up to T ) 199 K, both Cl 2p components displayed no significant changes in either their intensities or positions. At 199 K, the Cl 2p core level scan showed a large reduction in the intensity of the Cl component on the higher binding energy side (Figure 5a). On the contrary, the intensity of the lower binding energy Cl component at 199 K was about the same as before warm-up. When the sample was heated to 223 K, the Cl component at the higher binding energy further decreased while the Cl peaks on low binding energy exhibited a slight increase. This trend of the large reduction of the higher binding energy Cl peak continued as the sample temperature was further raised to 291 K, at which the higher binding energy Cl peaks completely disappeared. The C 1s core level spectra were also monitored at the same temperatures. Figure 5b displays the C 1s core level spectra21 taken at selected temperatures: T ) 131, 199, and 291 K. As in the Cl 2p core level, the C 1s core

J. Phys. Chem. B, Vol. 101, No. 27, 1997 5423

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, and 291 K and (b) the C 1s core level at T ) 131, 199, and 291 K. All the HRXPS spectra were taken at θ ) 70° for higher surface sensitiviy.

region at 131 K showed one dominant peak at 286.74 eV and a barely visible peak at 284.3 eV. No changes in the intensities and positions were observed up to 199 K, at which temperature the C 1s higher binding energy peak at 286.74 eV substantially decreased. Upon further heating, the C 1s peak on high binding energy continued to decrease whereas the C 1s peak at 284.3 eV gradually increased. 4. Discussion The results presented here provide an unambiguous evidence that the C2Cl4 molecules are dissociated on the Pd(100) surface at 200 K and above into a well-defined surface structure which retains the 2C:4Cl stoichiometry and has both elements C and Cl on the surface rather than penetrating inside the metal. A comparison with the previously reported dissociation of CH2Cl2 on Pd(100)6 leads to the conclusion that hydrogen in the molecular structure of the chlorocarbon is not necessary for the C-Cl bond dissociation on Pd(100) at room temperature. The ordered p(2 × 2) C2Cl4/Pd(100) structure observed here is not formed from either the partially disordered CH2Cl2/Pd(100) or the c(2 × 2) Cl/Pd(100) structure,6 thus proving that the carbon of the dissociatively chemisorbed tetrachloroethylene plays an important role in the lateral ordering of the chlorine ensembles. A detailed analysis of the experimental results that leads to the above 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 Carbon and Chlorine. The chlorine coverage in the p(2 × 2) C2Cl4/ Pd(100) structure was calculated by comparing the Cl 2p core level intensity with that from the well-ordered c(2 × 2) Cl overlayer, which is formed at room temperature upon saturation with Cl2 and corresponds to 0.5 monolayer (ML) of stable atomic Cl on a clean Pd(100) surface.6 The unit 1 ML is defined as the surface density of the Pd atoms of the (100) surface, i.e., 1.32 × 1015 atoms/cm2. A direct comparison of the Cl 2p intensity between the p(2 × 2) C2Cl4 and the c(2 × 2) Cl overlayers on the same Pd(100) surface yielded the intensity ratio of 0.49 by integrating peaks in Figure 2a. Consequently, the number of Cl atoms in the p(2 × 2) C2Cl4 overlayer is determined as 3.3 × 1014/cm2 or 0.25 ML. For the amount of carbon 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

5424 J. Phys. Chem. B, Vol. 101, No. 27, 1997

Park et al.

C 1s core levels after correcting for the Scofield cross section directly 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 the Adsorbate. The observed binding energies (BEs) for the Cl 2p and C 1s core levels from the p(2 × 2) C2Cl4/Pd(100) demonstrate that the room-temperature adsorption of C2Cl4 is dissociative. 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 (albeit not 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 peak of undissociated C2Cl4 at 200.31 eV. The measured Cl 2p core level position is also consistent with the value of 198.5 eV for the unresolved Cl 2p doublet28 reported by Mason and Texter29 in their study of the room-temperature adsorption of C2Cl4 on the Fe(111) surface, which readily dissociates the C-Cl bonds and 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 2p3/2 core level from the p(2 × 2) C2Cl4/Pd(100) surface represents a core level shift of 2.3 eV toward lower binding energy, clearly excluding the possibility that the chlorine in the p(2 × 2) C2Cl4 overlayer is in the form of molecular C2Cl4. The C 1s core level spectrum from the p(2 × 2) C2Cl4/Pd(100) surface also yields a consistent picture of dissociated C2Cl4 molecules adsorbed on the Pd(100) surface at room temperature. The C 1s BE of 284.33 eV in the p(2 × 2) C2Cl4/Pd(100) structure is in sharp contrast to the reported C 1s BE of 286.7 eV for molecularly adsorbed C2Cl4 on the Pt(111) surface.30 The C-Cl bond dissociation is evident by the large C 1s core level shift, but the nature of the carbon-carbon bond in the p(2 × 2) C2Cl4/Pd(100) structure is more difficult to be determined due to the fact that the correlation between the C 1s core level shifts and carbon-carbon bonds is subtle in various carbonaceous systems. Yet, it is noted that the observed position of the C 1s core level in the p(2 × 2) C2Cl4/Pd(100) structure, 284.33 eV, is nearly identical with the C 1s BE measured for dissociated C2H2 and C2H4 on Pt(111), 284.3 eV,27 and within the expected range of the C 1s positions for graphitic carbon, 284.3-284.5 eV.31 Both our observed and the quoted C 1s BEs are well outside the range for the carbidic carbon, 281-283 eV,29,31 by a large shift of >1.3 eV to higher binding energies. This is also in agreement with the calculation of relative energies of carbon pairs and dissociated carbon atoms below and with earlier results on other Pd surfaces14 which show that on Pd surface the carbon-carbon bond is stronger than the carbon-palladium bond, favoring association rather than dissociation of carbon species. In tetrachloroethylene the CdC bond is already present and its dissociation to a surface carbide is energetically not favored. 4.1.3. Surface Structure of the p(2 × 2) C2Cl4/Pd(100) OVerlayer. A smoothly rising, isotropic profile observed from the angular dependence of the C 1s and the Cl 2p core levels (Figure 4b,c) is characteristic of an angular variation of the photoelectron intensity from a thin overlayer of atomic thickness. The intensity of photoelectrons traveling a distance l in solid attenuates exponentially according to exp(-l/λ) where λ is the inelastic mean free path for the particular solid.32 In the limit of a continuum for the solid, the polar angle dependence of photoelectron intensity due to the inelastic scattering from an emitting layer of the nominal thickness z0 is11,33

[

(

I(θ) ) I0λR(θ) 1 - exp -

)]

z0 λ cos θ

(1)

where I0 is the photoelectron intensity from the top layer and

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). The dotted line represents theoretical isotropic intensity profile calculated for submonolayer (nominal thickness of 0.3 Å) to simulate the ARXPS data from p(2 × 2) C2Cl4/Pd(100). Also, an illustration of the difference in the adsorbate geometry between the above systemts is shown below.

R(θ) is the instrument response function. For a thin emitting layer (z0 e 1 Å) and high kinetic energy photoelectrons (λ g 15 Å), the quantity z0/(λ cos θ) is less than unity, except for θ values near 90°, and eq 1 can be expanded to yield I(θ) ) I0z0R(θ)/cos θ. For smaller polar angles, the instrument response function is nearly constant;11 then the angle dependence is roughly 1/cos θ, which is consistent with the θ dependence of both the C 1s and Cl 2p photoelectron intensities observed from the p(2 × 2) C2Cl4/Pd(100) surface. Using 20 Å for the calculated values of λ for both the C 1s and the Cl 2p photoelectrons in eq 1,34 the best fit for the θ dependence of both the C 1s and Cl 2p photoelectron intensities yields the effective thickness z0 ≈ 0.2 Å, qualitatively representing welldispersed submonolayer surface carbon. The isotropic polar angle dependence of the C 1s core level intensity from the dissociated p(2 × 2) C2Cl4/Pd(100) overlayer further implies no ordered orientation of the CdC and C-Cl bond axes on the Pd(100) surface. Figure 6 shows a comparison of the polar angle dependences of the C 1s photoemission intensity between the p(2 × 2) C2Cl4/Pd(100) (open circles) and the (2x2 × x2)R45 CO (full circles) overlayers. The ARXPS data from the (2x2 × x2)R45 CO/Pd(100) surface show a maximum along surface normal (θ ) 0°) in addition to the smoothly increasing background intensity. The observed maximum results from the forward focusing enhancement of the C 1s photoelectron intensity by the oxygen atom along the upright CO bond axis with C down toward the metal surface. Such intensity maxima from forward scattering along the molecular axis have been previously observed in many other adsorbate systems including CO on other transition metal surfaces,35,36 CH3O on Cu(110),36 and N2 on Ni(100).37 For ethylenic molecules on the transition metal surfaces, Wesner et al.38 reported the C 1s intensity enhancement along surface normal due to the upright C-C bond of ethylidyne after the well-characterized transition of adsorbed ethylene to ethylidyne on Pt(111).39 Therefore, the absence of such a forward focusing

Interaction of Tetrachloroethylene with Pd(100)

J. Phys. Chem. B, Vol. 101, No. 27, 1997 5425

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.

maximum in the present C 1s ARXPS data indicates no oriented C-C bond axis analogous to ethylidyne as well as C-Cl bond axes of carbene, going out of the p(2 × 2) C2Cl4/Pd(100) surface plane. Combining the results of the LEED, HRXPS, and ARXPS data, we present a model for the surface chlorine and carbon adatoms in a p(2 × 2) overlayer on the Pd(100) surface as shown in Figure 7. Because the present study cannot determine the exact adsorption sites for chlorine and carbon except for the p(2 × 2) periodicity, chlorine atoms are placed in the 4-fold hollow sites, where Cl atoms are found to adsorb on the (100) faces of Ni,40 Ag,22,41 Cu,43 and Pd.6 The positions of the carbon atoms are chosen to be randomly oriented within every other surface unit cell resembling the shape of molecular C2Cl4, the van der Waals envelope of which is also shown in relation to the underlying Pd(100) surface. Therefore, in this surface arrangement, the atomic Cl forms the usual p(2 × 2) overlayer structure of 0.25 ML, and the carbon atoms are randomly oriented in every other surface unit cell, satisfying the 0.125 ML carbon coverage and the observed p(2 × 2) LEED pattern. Although the model presented assumes the ideally terminated Pd(100) surface, it is also possible to have the Pd surface reconstructed by the highly electronegative Cl atoms in the 4-fold sites such that the p(2 × 2) LEED pattern mainly results from the shifts of four Pd atoms nearest to the adsorbed Cl. Based on the C 1s core level binding energy (see section 4.1.2), the model for surface chlorine and carbon adatoms in Figure 7 presents unbroken C-C bonds in several orientations. To investigate the stability of the C-C bonds in the proposed p(2 × 2) C2Cl4/Pd(100) structure, semiempirical quantum mechanical calculations using the extended Hu¨ckel (EH) package from Quantum Chemistry Program Exchange44 were performed. For the calculations, a cluster of 61 Pd atoms, consisting of 36 surface and 25 subsurface atoms to simulate the Pd(100) face was chosen. The large size of the cluster was necessary to minimize the edge effect. Four Cl atoms were adsorbed at the 4-fold sites in the p(2 × 2) arrangement, and the 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 the following four possible C geometries (the bottom of Figure 7): (1) C atoms were positioned on the 2-fold bridge sites representing the broken C-C bond; (2)-(4) the C pairs were centered at the 4-fold hole with the C-C bond axis parallel to the surface plane. The distance between the two C atoms was varied from 1.543 to 1.337 to 1.205 Å, representing single, double, and triple bonds, respectively.45 The results of the EH calculations showed that the two dissociated carbon atoms on the bridge sites are the least stable among the four cases of adsorption geometry. Taking the two C atoms at the bridge sites as reference, the energies for the C-C, CdC, and CtC were calculated lower by 2.7, 3.9, and 4.8 eV, respectively. Furthermore, the total EH energy decreased as the C-C bond distance decreased, indicating no driving force for dissociating the C-C bond to result in atomic carbon, but rather strengthening of the C-C bond as the C-Cl bond dissociated. 4.2. Low-Temperature Adsorption Study. 4.2.1. Adsorption of C2Cl4 at 131 K and Its Interaction with Pd(100). After exposure of the Pd(100) surface to C2Cl4 at 131 K, the total amount of chlorine on the surface was determined to be 0.62 ML by comparing the intensity of the Cl 2p core level to that in c(2 × 2) Cl/Pd(100). The two well-resolved Cl 2p components, each of which is split into a 2p3/2 and a 2p1/2 peak (Figure 5a), indicate the presence of two different kinds of chlorine on the surface. The dominant Cl contribution at 200.31 eV with its position of the Cl 2p3/2 peak is in good agreement with the previously reported values of 200.2 eV from molecular C2Cl4 at 95 K,30 as well as 200.5 eV from multilayer CH3Cl at 100 K.46 Thus, we attribute the observed Cl 2p3/2 peak at 200.31 eV to molecular C2Cl4. The other Cl 2p component at 197.65 eV is observed approximately at the same position as that in the p(2 × 2) C2Cl4 overlayer at room temperature (Figure 2a) and hence is assigned to atomic Cl from dissociated C2Cl4. The C 1s core level spectrum also shows two types of carbon species at 131 K (Figure 5b). The first at 286.74 eV is nearly identical with the reported value of 286.7 eV for molecular C2Cl430 representing highly oxidized carbon. The second broad peak

5426 J. Phys. Chem. B, Vol. 101, No. 27, 1997

Park et al.

Figure 8. Decomposition of the Cl 2p HRXPS core level spectrum for C2Cl4/Pd(100) at T ) 131 K. The fitting parameters are listed in Table 1.

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

position (eV)

fwhm (eV)

asym factor

mixa

areab

a b c

200.31 198.88 197.65

0.856 0.846 0.989

0.13 0.17 0.31

0.68 0.0 0.90

0.70 0.13 0.17

a

Mix is the ratio of Gaussian to Lorentzian; 0 for Gaussian and 1 for Lorentzian. b Normalized to 1.

centered near 284.3 eV is consistent with carbon from the dissociated C2Cl4 observed at room temperature adsorption as discussed 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 the Pd(100) surface at low temperatures. A hint for the possibly third Cl species can be first gleaned from the fact that the relative heights of the 2p3/2 and 2p1/2 peaks on the low binding energy for the atomic Cl are roughly one-to-one whereas those at high binding energy for molecular C2Cl4 shows the intensity ratio of 2:1 (Figure 5), expected from the ratio of the degeneracies between j ) 3/2 and j ) 1/2 states. Although the observed deviation in the relative peak heights of the spin-orbit components of atomic Cl could have simply resulted from the overlap with the intense Cl 2p3/2 peak of molecular C2Cl4, the fact that it persists even after most molecular C2Cl4 has disappeared at 223 K strongly indicates the presence of a third Cl species. To examine the possibility of the new Cl component and furthermore study the Cl components quantitatively, the Cl 2p core level at 131 K was fitted with Voigt functions on a linear background intensity. During the fitting procedure, it was apparent that a reasonable fit to the experimental data points could not be achieved with two pairs of Cl 2p peaks alone if the criterion for the intensity ratio of 2 between the 2p3/2 and 2p1/2 levels was taken into account. Therefore, the Cl 2p core level spectrum taken at 131 K was fitted using three Cl pairs of a 2p3/2 and a 2p1/2 peaks with the following constraints: (1) the spin-orbit splitting for 2p3/2 and 2p1/2 levels was fixed at 1.63 eV as determined in our room-temperature adsorption (section 3.1 and Figure 2a); (2) the intensity ratio between 2p3/2 and 2p1/2 peaks was kept constant at 2; and (3) the fitting parameters for the 2p3/2 and 2p1/2 levels were varied together for each Cl component. The results of the best fit to the experimental Cl 2p core level spectrum and the parameters used in the fitting procedure are presented in Figure 8 and Table 1, respectively. The results indicate three Cl components at the following positions: one Cl 2p3/2 peak representing Cl in molecular C2Cl4 at 200.31 eV (a), another at 198.88 eV (b), and the third

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

Cl component for dissociated C2Cl4 at 197.65 eV (c). At this point, we tentatively assign the second Cl component b to a partially dissociated C2Cl4 species. The relative intensities among the three Cl species indicate that on the Pd(100) surface at 131 K, 70% of Cl is in molecular C2Cl4 while the remaining 30% of Cl is in the form of either atomic Cl or partially dissociated C2Cl4 species. 4.2.2. Interaction of Adsorbed C2Cl4 with the Pd(100) Surface between 131 and 291 K. The intensities of each Cl species including the molecular C2Cl4 (a), partially dissociated C2Cl4 (b), and the atomic Cl (c) were further followed as a function of the substrate temperature in Figure 9, which clearly shows three temperature regions. Below T ) 173 K, the majority of adsorbed C2Cl4 overlayer is in molecular form. For all the dissociated C2Cl4 species, the amount of partially dissociated C2Cl4 species b steadily decreases while that of atomic Cl species c increases approximately by the same amount, suggesting a slow conversion from the partially dissociated C2Cl4 to completely dissociated atomic Cl. As the temperature increases from 173 to 223 K, the total Cl coverage on the Pd(100) surface decreases from 0.62 to 0.29 ML. The observed decrease is mainly due to the desorption of molecular C2Cl4, and the amount of Cl of molecular C2Cl4 decreases from ca. 0.44 to 0.02 ML at T ) 223 K (or ca. 0.11 ML of C2Cl4 to ≈0.005 ML).47 The onset of the desorption of molecular C2Cl4 is at ca. 173 K, and the maximum desorption rate is at 187 K. The activation energy for the desorption is estimated to be 11 kcal/mol using the maximum desorption temperature value,48 and the obtained activation energy is in good agreement with the value of 12.7 kcal/mol, for molecular desorption from the Fe(110) surface49 and is slightly higher than the enthalpy of vaporization of C2Cl4(l), 9.2 kcal/mol.50 Between 173 and 223 K, the amount of all dissociated C2Cl4 species (b and c in Figure 9) increases only by ca 0.07 ML from 0.19 to 0.26 ML, and the signal from atomic Cl continues to increase from 0.14 to 0.20 ML. Thus, the observed increase in the Cl signal from all the dissociated C2Cl4 species is mostly due to the increase in the amount of atomic Cl. It is likely that the excess amount results from the dissociation of molecular C2Cl4 accompanying the major desorption observed during the temperature increments. No decrease in the amount of the partially dissociated C2Cl4 species occurs as the temperature is

Interaction of Tetrachloroethylene with Pd(100)

J. Phys. Chem. B, Vol. 101, No. 27, 1997 5427

TABLE 2: Cl 2p3/2 XPS Binding Energies in Various Species Relevant to Those Occurring in the Adsorption and Decomposition of Tetrachloroethylene (TCE) on Pd(100) species PdCl2 c(2 × 2) Cl/Pd(100) p(2 × 2) Cl/Pd(100) from TCE decomposed at RT Cl/Fe(111) from TCE decomposed at RT CdC‚‚‚Cl/Pd(100) from TCE “partially dissociated” at low temperature C2Cl4 physically adsorbed on Pd(100) C2Cl4 physically adsorbed on Pt(111)

BE of Cl 2p3/2 (eV)

reference

198.2-198.8 197.50 198.04

31 present work present work

198.5a 198.88

29 present work

200. 31 200.2 200.44

present work 30 52

200.50

52

200.53 200.63

52 52

200.64 200.78 207.11b

52 52 17

n

Cl

n

Cl

-O-CH2-CH2-Cl Cl C

C

n

(-CHCl-CH2-)n (-CCl2-CH2-)n C2Cl4(g) a

See the footnote 28.

increased, and this suggests that molecular C2Cl4 initially decomposes into the partially dissociated C2Cl4 species, which subsequently dissociate into atomic Cl. As the temperature rises above 223 K, the amount of atomic Cl increases at the expense of the partially dissociated C2Cl4 species up to 291 K, at which all the partially dissociated C2Cl4 species disappears. In the room temperature chemisorption, the dissociation is complete, and no molecular precursors or partially dissociated species are observed, 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 room temperature to give rise to the p(2 × 2) structure with the BE of Cl 2p3/2 equal to 198.04 eV. However, the dissociation occurs stepwise when the TCE is first physically adsorbed at 130 K and then the temperature gradually raised to 300 K. This process is clearly one in which chlorine moves from an organic to an inorganic-bound state, as reflected in the BE shifts shown in Table 2. Starting from the Cl-CdC- bond in the physically adsorbed TCE and ending with the Cl-Pd bond in the dissociated p(2 × 2) structure, the BE shifts of Cl 2p3/2 are in the direction expected from an increased polarization of its bond with a concomitant increase in ionicity of the Cl-CdC- to Cl-Pd bond. The bond ionicities I(A-B) calculated from the Pauling electronegativities (3.0 for Cl, 2.5 for C, and 2.2 for Pd) suffice to provide a qualitative guide for the observed trends, an approach first employed by Siegbahn et al.51 in setting up correlations between the BEs and calculated charges: I(Cl-C) ) 0.06 and I(Cl-Pd) ) 0.15. Within this range, the Cl 2p3/2 BEs vary by up to 2.8 eV (cf. Table 2) and therefore are quite sensitive to relatively small changes of the bond ionicity. In later work, the effective atomic charges were calculated by semiempirical quantum mechanical methods and led to satisfactory correlations with the BE shifts, particularly for sulfur and nitrogen compounds.52 An account for final state effects included polarization of the delocalized π-systems,53 but the BE vs semiempirical charge correlation has proven useful to date for a series of compounds in which the BE shifts are relatively large54 and which do not involve transition metals.

In our case a transition metal is present and the behavior of its core level shifts (e.g., Pd 3d) is quite complex.11 The metal effects are indeed expected to play a role in the BE shifts of the Cl species, and it is those effects that may be responsible for alteration of the trends emerging from the simple BE vs charge correlation picture. Nevertheless, experience shows that a defined BE is a signature of a distinct chemical species, and both the comparison with reference compounds and the BE vs charge correlations permit to identify the nature of the bonding involved. With this approach in mind, the observed Cl 2p BEs are interpreted as follows. There are three distinct species of TCE adsorbed on Pd(100): (a) physically adsorbed, undissociated TCE, present at low temperatures and desorbing around 180 K, with the Cl 2p BE close to that of TCE physically adsorbed on 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 and inorganic chlorine species (Table 2), which is a precursor of the fully dissociated TCE and converts to it below room temperature; (c) fully dissociated TCE that forms the p(2 × 2) structure at room temperature, with Cl 2p BE close to that occurring in the c(2 × 2)Cl/Pd(100) overlayer formed upon dissociative chemisorption of elemental chlorine (Table 2). The fact that the Cl 2p BE in the p(2 × 2)TCE/Pd(100) overlayer (species c) is higher (by 0.54 eV) than in the c(2 × 2)Cl/Pd(100) structure indicates that the effective negative charge on chlorine in the species c is lower (by ca. 0.02e) than in the c(2 × 2) structure. This can be attributed to an inductive, electronwithdrawing, effect of the neighboring carbon, C r Pd r Cl. A similar but larger effect occurs in the “partially dissociated” species b, in which the Cl 2p BE corresponds to bond ionicity I ) 0.11, close to an average of the ionicities I(Cl-Pd) and I(Cl-C). Thus, the “partially dissociated” species b would correspond to a complex in which the Cl-C bond is stretched but not broken while the Cl-Pd bond is being formed. It is interesting that such a species is associated with a distinct Cl 2p BE and survives a rather large temperature range. In the search for organometallic analogues to the “partially dissociated” TCE species b observed here, we note that a large number of Pd complexes exist with halogen, alkane, alkene, or alkyne ligands, either mononuclear or bridged,56-59 in which the bridging ligands ought to be “π-donors”.56 Such π-donors are frequently halogens coordinated to two neighboring Pd atoms. Therefore, our partially dissociated species b may be one in which the Cl atoms assume a 2-fold coordination before they move to the 4-fold hole. Conclusions The room-temperature chemisorption of C2Cl4 gives rise to a well-defined p(2 × 2) C2Cl4/Pd(100) overlayer structure of dissociated, stoichiometric carbon pairs and chlorine adatoms. The p(2 × 2) structure is forced by the presence of carbon, as chlorine adatoms form the well-known c(2 × 2) structure. The p(2 × 2) C2Cl4/Pd(100) overlayer does not allow the subsequent chemisorption of oxygen and the oxidation of the carbon pairs. The amounts of carbon and chlorine on surface were determined as 0.125 and 0.25 ML, respectively. The exposure of a cooled Pd(100) surface to tetrachloroethylene at T ) 131 K showed predominantly molecular C2Cl4 as established by the XPS binding energies of the Cl 2p and C 1s core levels. In addition, the high-resolution XPS in the Cl 2p core region revealed two satellite Cl peaks: one shifted from the molecular C2Cl4 peak by ∆BE ) -2.7 eV and the other by ∆BE ) -1.4 eV, corresponding to atomic Cl and partially dissociated C2Cl4

5428 J. Phys. Chem. B, Vol. 101, No. 27, 1997 species, respectively. When the temperature was increased, the partially dissociated C2Cl4 gradually converted to atomic Cl. By the time the temperature reached 291 K, the dissociation of C2Cl4 on the Pd(100) was complete. Acknowledgment. We are thankful to Dr. Alfred Miller at the SCIENTA ESCA laboratory for the time allocation and technical assistance rendered during this experiment. We are also grateful to Prof. Gary Simmons for stimulating discussions. This work was supported by the Department of Energy Basic Energy Sciences Grant DE-FG02-86ER13580. References and Notes (1) Hucknall, D. J. SelectiVe Oxidation of Hydrocarbons; Academic Press: London, 1974. (2) Twigg, M. V. In Catalysis and Chemical Processes; Pearce, R.; Patterson, W. R., Eds.; Leonard Hill: London, 1981. (3) Cullis, C. F.; Keene, D. E.; Trimm, D. L. J. Catal. 1970, 19, 378. (4) Mann, R. S.; Dosi, M. K. J. Chem. Technol. Biotechnol. 1979, 29, 467. (5) Anderson, R. B.; Stein, K. C.; Freeman, J. J.; Hofer, L. J. E. Ind. Eng. Chem. 1961, 53, 809. (6) Wang, Y.-N.; Marcos, J. A.; Simmons, G. W.; Klier, K. J. Phys. Chem. 1990, 94, 7597. (7) Interatomic Distances; Sutton, L. E., Ed.; The Chemical Society: London, 1958. (8) Wiedmann, T. O.; Gu¨thner, B.; Class, T. J.; Ballschmiter, K. EnViron. Sci. Technol. 1994, 28, 2321. (9) Dilling, W. L.; Tefertiller, N. B.; Kallos, G. J. EnViron. Sci. Technol. 1975, 9, 833. (10) Gu¨rer, E.; Klier, K. Phys. ReV. B 1992, 46, 4884. (11) Park, K. T.; Simmons, G. W.; Klier, K. Surf. Sci. 1996, 367, 307. (12) Scienta ESCA300 User’s Manual, Scienta, Uppsala. (13) Simmons, G. W.; Wang, Y.-N.; Marcos, J. A.; Klier, K. J. Phys. Chem. 1991, 95, 4522. (14) Wang, Y.-N.; Herman, R. G.; Klier, K. Surf. Sci. 1992, 279, 33. (15) Klier, K.; Hess, J. S.; Herman, R. G. J. Chem. Phys., submitted. (16) Van Santen, R. A.; De Koster, A.; Koerts, T. Catal. Lett. 1990, 7, 1. (17) Berndtsson, A.; Basilier, E.; Gelius, U.; Hedman, J.; Klasson, M.; Nilsson, R.; Nordling, C.; Svensson, S. Phys. Scr. 1975, 12, 235. (18) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1981, 8, 129. The photoionization cross sections of 1.00 for the C 1s and 2.285 for the Cl 2p (both 2p3/2 and 2p1/2) level were used in the calculation. (19) Nyholm, R.; Qvarford, M.; Andersen, J. N.; Sorensen, S. L.; Wigren, C. J. Phys.: Condens. Matter 1992, 4, 277. (20) Egelhoff Jr., W. F. Crit. ReV. Solid Mater. Sci. 1990, 16, 213. (21) To examine the C 1s signal from the adsorbed C2Cl4 only, the background C signal was subtracted from the C 1s core level spectra of the C2Cl4-covered Pd(100) surface in Figure 5b. (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, using the 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) can be 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 monoxide adsorbs on the bridge site of the surface.23,24 However, when using the intensity 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 intensity accurately. In Figure 2b, the C 1s HRXPS core level spectrum from the p(2 × 2) C2Cl4 overlayer is compared to that from the (2x2 × x2)R45 CO overlayer on the same Pd(100) surface. Judging from the peak height and width, the C 1s intensity from the p(2 × 2) C2Cl4/Pd(100) surface appears 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 Cl 2p core level intensity (Figure 2a). The apparent discrepancy results from the fact that the C 1s core level intensity from the CO molecules is significantly underestimated because a large portion, up to 50%, of the C 1s core level intensity is lost into the inelastic background on high binding energy as the shake-up or shake-off broadening.25 Such an apparent error by a factor of 2 was similarly reported by Bonzel and co-workers26,27 in their 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 overlayer on the same Pt(111) surface. For a reliable quantification of carbon using the C 1s signal from CO, Griffiths et al.26 concluded that a significant portion of the inelastic tail, which may extend to several electronvolts on higher binding energy must be included to account for the intensity lost into the shake-up region. (23) Bradshaw, A. M.; Hoffmann, F. M. Surf. Sci. 1978, 72, 513.

Park et al. (24) Behm, R. J.; Christmann, K.; Ertl, G.; Van Hove, M. A. J. Chem. Phys. 1980, 73, 2984. (25) Bjo¨rneholm, O.; Nilsson, A.; Zdansky, E. O. F.; Sandell, A.; Hernna¨s, B.; Tillborg, H. Phys. ReV. B 1992, 46, 10353 and references therein. (26) Griffiths, K.; Lennard, W. N.; Mitchell, I. V.; Norton, P. R.; Pirug, G.; Bonzel, H. P. Surf. Sci. Lett. 1993, 284, L389. (27) Freyer, N.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1983, 125, 327. (28) In this work, the Cl 2p spin-orbit splitting (∆ ) 1.6 eV) was not resolved, thus the quoted value refers to the position of the unresolved peak centroid. (29) Mason, R.; Texter, M. Proc. R. Soc. London 1977, A356, 47. (30) Cassuto, A.; Hugenschmidt, M. B.; Parent, Ph.; Laffon, C.; Tourillon, H. G. Surf. Sci. 1994, 310, 390. (31) Handbook of X-ray Photoelectron Spectroscopy; Chastin, J., Ed.; Perkin-Elmer: Eden Prairie, MN, 1992. (32) Practical Surface Analysis; Briggs, D.; Seah, M. P., Eds.; John Wiley & Sons: Chichester, 1983. (33) Electron Spectroscopy, Theory, Techniques, and Applications; Brundle, C. R., Baker, A. D., Eds.; Pergamon: Oxford, 1978. (34) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2. (35) Petersson, L. G.; Kono, S.; Hall, N. F. T.; Fadley, C. S.; Pendry, J. B. Phys. ReV. Lett. 1979, 42, 1545. (36) Holub-Krappe, E.; Prince, K. C.; Horn, K.; Woodruff, D. P. Surf. Sci. 1986, 173, 176. (37) Egelhoff Jr., W. F. Surf. Sci. 1984, 141, L324. (38) Wesner, D. A.; Coenen, F. P.; Bonzel, H. P. Phys. ReV. Lett. 1988, 60, 1045. (39) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: New York, 1994; p 417. (40) Yokoyama, T.; Takata, Y.; Ohta, T.; Funabashi, M.; Kitajima, Y.; Kuroda, H. Phys. ReV. B 1990, 42, 7000. (41) Zanazzi, E.; Jona, F.; Jepsen, D. W.; Marcus, P. M. Phys. ReV. B 1976, 14, 432. (42) Cardillo, M. J.; Becker, G. E.; Hamann, D. R.; Serri, J. A.; Whitman, L.; Mattheiss, L. F. Phys. ReV. B 1983, 28, 494. (43) Citrin, H.; Hammann, D. R.; Mattheiss, L. F.; Rowe, J. E. Phys. ReV. Lett. 1982, 49, 1712. (44) Whangbo, M.-H.; Evain, M.; Hughbanks, T.; Kertesz, M.; Wijeyesekera, S.; Wilker, C.; Zheng, C.; Hoffman, R. Extended Hu¨ ckel Molecular, Crystal and Properties Package; QPPE 571: Indiana University, IN, 1987. (45) For (1)-(4), the carbon pairs were 1.646, 1.531, 1.487, and 1.453 Å above the surface Pd plane, which were obtained using the C-Pd bond distance of 2.145 Å. (46) Zhou, X.-L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M. Surf. Sci. 1989, 219, 294. (47) For the quantification of molecular C2Cl4, 1 ML represents 0.33 × 1015 molecules/cm2 or a quarter of 1.32 × 1015 molecules/cm2. (48) King, D. A. CRC Crit. ReV. Solid Mater. Sci. 1978, 7, 167. (49) Smentkowski, V. S.; Cheng, C. C.; Yates Jr., J. T. Surf. Sci. 1989, 220, 307. (50) CRC Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; Chemical Rubber Co.: Cleveland, OH, 1983/1984; p c691. (51) ESCA, Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy; Siegbahn, K., Nordling, C., Fahlman, A., Nordberg, R., Hamrin, K., Lindgren, I., Lindberg, B. Nova Acta Regiae Societatis Scientiarum Upsaliensis Ser. IV Vol. 20, 1967. (52) Nordberg, R.; Albridge, R. G.; Bergmark, T.; Ericson, U.; Hedman, J.; Nordling, C.; Siegbahn, K.; Lindberg, B. J. Ark. Kemi 1968, 28, 257. (53) Marsili, M.; Gasteiger, J. J. Croat. Chem. Acta 1980, 53, 601. (54) Johansson, M.; Klier, K. Top. Catal. 1997, 4, 99. (55) High-Resolution XPS of Organic Polymers The Scienta ESCA300 Database; Beamson, G., Briggs, D., Eds.; John Wiley & Sons: Chichester, 1992. (56) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th Ed.; John Wiley & Sons: New York, 1988; pp 920-921. (57) Henry, P. Palladium Catalyzed Oxidation of Hydrocarbons; D. Reidel Publishing Company: Dordrecht, 1980. (58) ComprehensiVe Organometallic Chemistry The Synthesis, Reactions and Structures of Organometallic Compounds; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1982; Vol. 6. (59) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987.