Interaction of CO with Palladium Supported on Oxidized Tungsten

Czech Republic, Charles University, Faculty of Mathematics and Physics, ... I. Jirka , J. Plšek , M. Vondráček , F. Šutara , V. Matolín , V. ...
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J. Phys. Chem. B 2006, 110, 23837-23844

23837

Interaction of CO with Palladium Supported on Oxidized Tungsten Ivan Jirka,† Jan Plsˇek,*,† Frantisˇek Sˇ utara,‡ Vladimı´r Matolı´n,‡ Vladimı´r Cha´ b,§ and Kevin C. Prince| Academy of Sciences of the Czech Republic, J. HeyroVsky´ Institute of Physical Chemistry, 182 23 Prague 8, Czech Republic, Charles UniVersity, Faculty of Mathematics and Physics, Department of Electronics and Vacuum Physics, 180 00 Prague, Czech Republic, Academy of Sciences of the Czech Republic, Institute of Physics, and Sincrotrone Trieste, BasoVizza (Trieste), I-34012 Italy ReceiVed: May 24, 2006; In Final Form: September 13, 2006

A model catalyst system, palladium on tungsten oxide, has been examined by temperature-programmed desorption and photoemission spectroscopy. The samples were prepared by evaporation of palladium onto an oxidized tungsten foil under ultrahigh vacuum conditions. Mostly three-dimensional (3-D) palladium (Pd) clusters were found to be present on oxidized tungsten (WOx) surfaces at room temperature. Upon annealing to 670 K, the palladium clusters are redispersed and decorated by the WOx surface layer. The nature of the WOx phase on top of the palladium clusters is dependent on the mode of oxidation of the tungsten foil prior to palladium deposition. Mainly W2+ species decorate palladium deposits on tungsten oxidized at room temperature, while mainly W4+ species are on top of palladium deposits on the surface oxidized at 1300 K. The appearance of a Pdn+-O-W4+ mixed oxide phase with n < 2 was observed on the oxidized tungsten surface. The substantial reduction (relative to nonannealed samples) of molecular CO coverage induced by annealing is discussed in terms of the changes in chemical composition and morphology of the outermost surface.

1. Introduction The chemical reactivity of gaseous molecules with metal deposits on the surface of metal oxides is strongly influenced by metal-support interactions.1,2 Changes in the electronic structure of the metal adatoms because of their dispersion and bonding with the surface atoms of the support, as well as changes of morphology of the metal adlayers, may take place. Photoelectron spectroscopy (PES) is often used to investigate chemical interactions of metals with the support surface. The appearance of new chemical surface species affects the positions and line shapes of the photoelectron spectra of the metal adatoms and the support. Use of synchrotron radiation (SR) for excitation of the photoelectrons is a considerable advantage in these studies. The kinetic energies of emitted photoelectrons can be adjusted to give maximum surface sensitivity by appropriately selecting the excitation energy, and the effects on the positions and line shapes of measured spectra of the metal-support interface can thus be substantially enhanced. Palladium dispersed on the surface of tungsten oxides is very promising for many catalytic processes such as skeletal rearrangements of hexanes and hexenes since it is very selective toward isomerization.2-4 The palladium-tungsten supported systems have also been tested for removal of carbon monoxide (CO), nitrogen monoxide (NO), and hydrocarbons from exhaust gases.5 The possible influence of the interaction of palladium with tungsten oxide (WOx) at elevated temperatures on the

strength of interaction of palladium with CO and NO was discussed in ref 6. The observed weakening of the Pd-CO (NO) bond was attributed to palladium-tungsten interaction. However, the thermally induced changes of adsorption properties of palladium dispersed on WOx may be alternatively explained by morphological changes of the Pd/WOx system that take place on its surface, for instance, decoration of palladium clusters by tungsten oxide species. This strong metal-support interaction (SMSI) was used to explain the reduced CO adsorption on palladium dispersed on several other oxides.7,8 Detailed investigation of the palladium-support interaction by SR PES with a real catalyst is extremely difficult, so that to understand the nature of the interaction of palladium deposits with the WOx support, one should prepare a model system. In this work, palladium was deposited on the surface of a polycrystalline oxidized tungsten foil under UHV conditions. This system represents a compromise between simplification of the investigated system essential for characterization of the nature of palladium-support interactions and the closeness of the model system to the real catalyst. Adsorption of CO before and after heat treatment of this sample was investigated by temperature-programmed desorption (TPD), and the palladiumsupport interaction was studied by SR PES. Substantial reduction of the CO coverage was observed for the samples annealed to 670 K. This effect is related to SMSI in this work, while direct palladium-tungsten chemical bonding is excluded. 2. Experimental Section

* Author to whom correspondence should be addressed. E-mail: [email protected]. † Academy of Sciences of the Czech Republic, J. Heyrovsky ´ Institute of Physical Chemistry. ‡ Charles University. § Academy of Sciences of the Czech Republic, Institute of Physics. | Sincrotrone Trieste.

The TPD experiments were carried out in the USU 4 apparatus (The Experimental Factory of Scientific Engineering, Moscow, Academy of Sciences, USSR) equipped with a quadrupole mass spectrometer (MASSTORR DX, VG, England). The SR PES experiments were carried out at the

10.1021/jp0631782 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006

23838 J. Phys. Chem. B, Vol. 110, No. 47, 2006 Materials Science Beamline at the Elettra synchrotron radiation source, Trieste, equipped with a hemispherical electron analyzer (SPECS model Phoibos, mean radius 150 mm). The base pressures during TPD were typically less than 1 × 10-9 mbar and were ∼2 × 10-10 mbar during the SR PES measurements. Tungsten foil (99.9%) from the same lot was used in both the TPD and SR PES experiments. The surface was cleaned before SR PES measurement by alternating Ar+ sputtering and flashing to 1300 K. This procedure enabled us to minimize contamination of the surface (C, O < ∼1 at. %). A standard procedure was used to outgas and clean the sample surface before CO adsorption in the TPD experiments (this procedure included repeated heating of the sample in oxygen and flashing to 2300 K). The tungsten surface was then oxidized in situ. Two modes of oxidation were used. First, the support was oxidized at room temperature by 8 langmuirs (L, 1 L ) 1.33 × 10-6 mbar.s) of oxygen (saturation coverage, sample denoted as WOx/W(RT)) and then was heated to 670 K. Alternatively, tungsten foil was oxidized at a high temperature (1300 K, 40 L of oxygen, sample denoted as WOx/W(1300)). The samples were heated resistively, and the temperature was monitored by means of a W-Ta thermocouple (in the TPD experiments) and by a chromelalumel thermocouple (in the SR PES experiments) attached to the rear of the foil. Palladium was deposited on the sample surfaces by evaporation from a wire wrapped on a tungsten filament. In the TPD experiments, the thickness of the palladium layers was controlled by a quartz crystal thickness monitor, model IL 150 (produced by Intellemetrics Ltd., United Kingdom). In the SR PES experiments, the thickness of the palladium layer was evaluated from the attenuation of the W 4f photoelectron line by the palladium deposits. Each TPD (SR PES) experiment was done on newly prepared Pd/WOx/W system. The palladium-containing samples were then annealed at 670 K for 3 min and were exposed to CO. Carbon monoxide was adsorbed at room temperature by exposing the sample surface to ∼40 L of CO (pCO ) 5 × 10-7 mbar for 60 s), expecting to saturate the palladium surface with CO.9 Partial pressures of the desorbed CO (AMU 28) or CO2 (AMU 44) molecules or Pd atoms (AMU 108) were measured by the quadrupole mass spectrometer. The heating rate during TPD measurements was 12 Ks-1 (73 Ks-1 in the case of Pd desorption). The photoelectrons were excited by light with energy 110 eV (∼2 × 1011 photons s-1; W 4f and O 2s and valence band (VB) spectra) and 420 eV (Pd 3d and C 1s spectra) from a bending magnet source. The light was dispersed using a grating monochromator.10 The analyzer was operated in the constant transmission mode, and binding energies were referred to the Fermi level. To fit the line shapes and intensities of W 4f and Pd 3d spectra, a damped nonlinear least-squares fitting procedure was used. A weighted sum of Gaussian and Lorentzian functions with an exponential tail11 was employed. Curve fitting of the Pd 3d spectra was done using asymmetric d doublets. Optimized asymmetry parameters of the Pd/WOx/W(1300) system were close to those obtained by optimization of asymmetry parameters of the Pd 3d spectrum of the palladium foil. Another line had to be added to fit the Pd 3d spectrum of the Pd/WOx/W(RT) sample (see the discussion below). Symmetric doublets were used for fitting of the annealed samples. The W 4f spectra were fitted using two asymmetric doublets of the lines for clean tungsten and symmetric 4f doublets for the oxidized tungsten species; a Shirley background was subtracted.12

Jirka et al.

Figure 1. Thermal desorption spectra of carbon monoxide from the Pd/WOx/W(RT) and Pd/WOx/W(1300 K) systems and its dependence on annealing temperature (Pd layer thickness ∼ 0.4 nm).

The W 4f line shape changes were further analyzed using difference spectra, calculated as follows: the W 4f spectra were normalized to unit height after first subtracting the Shirley background. To visualize the effect of oxidation, the normalized spectrum of clean tungsten was subtracted from the normalized spectrum of the oxidized sample. Difference spectra reflecting the W 4f line shape changes brought about by palladium deposition and subsequent annealing of the samples were calculated similarly: the normalized spectrum of the sample before palladium deposition (annealing) was subtracted from the normalized W 4f spectrum of the palladium-containing (annealed) sample. Chemisorption of CO on annealed Pd/WOx/W(RT) and Pd/ WOx/W(1300) samples induced rather small line shape changes in the valence band region, which were evaluated using difference spectra. The spectrum before CO adsorption (annealing) was subtracted from the spectrum after CO adsorption (annealing). As adsorption of CO affects overall intensities of photoelectron core lines negligibly, no renormalization of the spectra was required in the calculation of difference spectra. 3. Results 3.1. Temperature-Programmed Desorption of CO. The influence of thermal pretreatment of Pd/WOx/W(RT) and Pd/ WOx/W(1300) samples on adsorption of CO is evident from the results summarized in Figures 1 and 2. Figure 1 demonstrates the changes of CO adsorption of Pd layers ∼0.4 nm thick deposited on both types of oxidized tungsten supports because of annealing to several temperatures before CO adsorption. Characteristic features of TPD curves (i.e., their shape and peak temperatures) from nonannealed systems correspond to the data published for metal-oxide-supported palladium layers (Pd/ Al2O3,13,14 Pd/ Nb2O5 15) and for Pd(100),16 Pd(111),17 and Pd(110).18,19 The broad desorption peak observed in nonannealed systems indicates the presence of multiple adsorption sites (curve 1 in Figure 1). A significant decrease of desorption peak area because of annealing started already at 400 K (Pd/WOx/W(RT) sample) and at 500 K (Pd/WOx/W(1300) sample) (Figure 1). Annealing to higher temperatures (up to 700 K) caused a further decrease of desorption peak area of both samples (Figure 1). This effect was observed for various palladium layer thicknesses (0.1-2.3 nm, see Figure 2). The decrease of adsorption capacity induced

CO with Palladium Supported on Oxidized Tungsten

J. Phys. Chem. B, Vol. 110, No. 47, 2006 23839

Figure 2. Dependence of CO desorption peak area on Pd layer thickness d for Pd/WOx/W(RT) and Pd/WOx/W(1300 K) systems after evaporation and after annealing to 700 K.

by annealing was accompanied by a lower desorption temperature and thus a weaker CO chemisorption bond. The desorption peaks of samples annealed to 700 K are typically narrow (Tmax is in the interval 372-379 K), and their shape is independent of both the type of oxidation and the Pd layer thickness. In the range 0.1-2.3 nm of palladium layer thickness, the ratio between the peak areas from annealed and nonannealed systems was 4-10% for the Pd/WOx/W(RT) sample and 1-2% for the Pd/WOx/W(1300) sample. For the lowest palladium layer thickness (0.1 nm), the area of the desorption peak from samples preannealed to 700 K was comparable with that of the CO desorption peak from bare oxidized tungsten (Figure 2). However, with increasing palladium layer thickness, the area of desorption peak increased, and CO adsorption is therefore accompanied with Pd atoms even after annealing to 700 K. The decrease of desorption peaks of samples annealed up to 600 K (curves 1-4 in Figure 1) could be interpreted by a reduction of the possible adsorption sites, and lowering of the desorption temperature after heating to 700 K (compare curves 4 and 5 in Figure 1) also indicated weakening of CO-Pd bonds. 3.2. SR PES Results. Both molecular and dissociative CO chemisorption has been observed on palladium dispersed on a variety of oxides (see, for example, refs 13 and 20 and references therein). The effect of CO adsorption on the line shape of the valence band (VB) spectra of the samples annealed to 670 K is shown in the top of Figure 3A and 3B: negligible line shape changes were observed. This is consistent with the TPD results, where only residual CO adsorption took place, and again suggests that on this surface CO does not adsorb significantly. Limited molecular CO adsorption took place on the surface of the Pd/WOx/W(RT) sample as evidenced by the presence of lines of the 5σ + 1π and 4σ molecular orbitals of CO at Eb ) 8.1 and 11.2 eV in the photoelectron spectrum (see Figure 3A, curve ii). Thermal desorption of CO from the surface of the Pd/WOx/W(RT) sample was accompanied by minor desorption of the surface oxygen (see the decrease in intensity of the spectrum of the annealed sample in the region of O 2p photoelectron line at Eb ∼ 6.5 eV (see Figure 3A, curve iii). However, evolution of CO2 during this process was not observed in the TPD experiment. No molecular adsorption of CO was detectable by SR PES on the surface of the Pd/WOx/W(1300) sample. Observed line shape changes of the VB spectrum of this sample were insignificant (see Figure 3B, curve v).

Figure 3. The line shapes of the valence band (VB) spectra of the preannealed samples Pd/WOx/W(RT) (A) and Pd(WOx)/W(1300) (B). (i) VB spectrum of the sample Pd/WOx/W(RT) after adsorption of CO. (ii) Difference of VB spectrum of the sample Pd/WOx/W(RT) before and after CO adsorption. (iii) Difference of VB spectrum of the sample Pd/WOx/W(RT) before CO adsorption and after CO desorption at 670 K. (iv) VB spectrum of the sample Pd/WOx/W(1300) after CO adsorption. (v) Difference between the VB spectra before CO adsorption and after CO desorption at 670 K (sample Pd/WOx/W(1300)). The VB spectra before CO adsorption and after CO desorption are not shown for clarity.

Figure 4. W 4f spectra of samples (Pd)/WOx/W(RT) and (Pd)/WOx/ W(1300) before oxidation (a), after oxidation (b), after palladium deposition (c), and after annealing to 670 K (d).

The W 4f spectrum of a clean tungsten foil consisted of an asymmetric line at 31.36 eV accompanied by a low-energy shoulder, which is due to the unresolved surface core level shifted (SCLS) W 4f line (Figure 4). Similar shifts were already observed for W(111) and W(110) monocrystalline samples.21,22 The state of art of SCLS of various metals is summarized elsewhere.23 A fit of the W 4f lines gave a spin-orbit splitting of 2.16 eV (in agreement with published value24). Oxidation, palladium deposition, and subsequent annealing of the sample resulted in changes of intensities and line shape of the W 4f spectrum. The line shapes of the W 4f spectra are discussed

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Figure 5. W 4f spectra (open squares) and their fits (lines): clean tungsten foil (a) and tungsten foil oxidized at 1300 K (b) (sampleWOx/ W(1300)). Above each spectrum is placed its second derivatives. The dotted lines indicate positions of peaks.

below on the basis of difference spectra and the curve-fitting procedure employed. Typical fits of the W 4f spectra of tungsten foil before and after oxidation together with their second derivatives are depicted in Figure 5. The W 5p line is also included in the fit as it is separated by only ∼3 eV from the W 4f5/2 line and partially overlaps the 4f line of oxidized species (see results below). The W 5p spectral region was approximated by one peak in the fit. The value of the W 5p/W 4f intensity ratio obtained from the curve-fitting procedure (0.14 ( 0.03) was close to the ratio of the corresponding photoionization cross sections σ, σ(W 5p)/σ(W 4f) ) 0.1325 regardless of the treatment used.

Jirka et al. Oxidation caused reduction of the intensity of the surface W 4f line and appearance of lines in the high-energy region of W 4f spectra. According to the second derivatives of the spectra, two f doublets were needed to fit the W 4f spectrum of clean tungsten and another five f doublets were needed to fit the spectrum of the oxidized sample. The fits of W 4f spectra of the (Pd)/WOx/W(RT) and (Pd)/ WOx/W(1300) samples before and after palladium deposition and after annealing to 670 K are shown in Figure 6. The binding energies Eb of the W 4f7/2 lines and intensities of the W 4f lines of oxidized tungsten species obtained from the curve fitting are summarized in Table 1. It is apparent from a comparison of the estimated and published values of Eb of the W 4f7/2 line listed in Table 1 that the surfaces were covered by Wn+ oxide species with n e 4.26-29 No 4f lines of oxidized tungsten species are present in the W 4f spectra above ∼34 eV (Figure 6). The most straightforward comparison of our results with literary data can be done with published SR PES investigation of oxidized polycrystalline tungsten foil.29 The W 4f photoelectron spectrum of oxidized tungsten species were composed from the lines in the energy regions 31.98-32.1 eV and 32.7833.0 eV. These lines were assigned to W2+ and W4+ species, respectively. The lines #1 (Eb ∼ 31.7 eV) and #3 (Eb ∼ 32.3 eV) were distinguished in our spectra. The former one is related to the presence of monovalent tungsten cations,26 and the assignment of the latter one is less straightforward. We propose that this line is related to W2+ species. Absence of these lines in the spectrum of referenced study29 is explainable by much higher resolution used by us. The set of five 4f doublets with Eb values which do not differ by more than 0.07 eV in the two investigated samples can be used to describe the high-energy region of the W 4f spectra of the two samples before palladium deposition. The observed difference between these two sets of lines was accepted as the error accompanying the evaluation of Eb values. A sixth doublet had to be introduced to fit the spectra of palladium-containing samples (Figure 6). The two types of investigated supports differed in oxygen concentration, being higher in the WOx/W(1300) sample. This

Figure 6. The W 4f spectra (open squares) and their fits (lines) of the samples (Pd)/WOx/W(RT) (left column) and (Pd)/WOx/W(1300) (right column) before palladium deposition (a), after palladium deposition (b), and after annealing palladium containing samples to 670 K (c).

CO with Palladium Supported on Oxidized Tungsten

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TABLE 1: The Binding Energies (Eb) and Intensities (I) of the High-Energy Photoelectron Lines of W 4f7/2 Spectra of Oxidized Tungsten Species Measured for Samples (Pd)/WOx/W(RT) and (Pd)/WOx/W(1300)c line

Eb (ox)

I (ox)a

I (ox)b

Eb (Pd)

I (Pd)a

I (Pd)b

Eb (∆T)

I (∆T)a

I (∆T)b

1 2 3 4 5 6

31.71 (0.035) 31.99 (0.040) 32.29 (0.005) 32.65 (0.055) 33.07 (0.035)

0.061 0.191 0.112 0.124 0.062 0

0.074 0.39 0.176 0.126 0.161 0

31.70 (> 0.01) 31.96 (0.015) 32.28 (0.050) 32.88,a 32.60b 33.30,a 33.00b 33.50 (0.050)

0.089 0.206 0.177 0.126 0.068 0.059

0.080 0.495 0.291 0.096 0.176 0.050

31.73 (0.075) 32.00 (0.040) 32.28 (0.015) 32.67 (0.030) 33.08 (0.035) 33.56 (0.070)

0.036 0.297 0.181 0.194 0.096 0.023

0.195 0.236 0.275 0.144 0.289 0.224

a Pd/WOx/W(RT). b Pd/WOx/W(1300). c The W 4f7/2,5/2 spin-orbital splitting is 2.16 eV. Binding energies are the mean values calculated from the fits of both samples (numbers in brackets represent the corresponding statistical errors). The treatments employed, oxidation (ox), palladium deposition (Pd), and annealing (∆T), are shown.

Figure 7. Effect of oxidation of W foil and annealing of Pd/WOx/W systems on the line shape of W 4f spectra. Left column: The W 4f spectra of before (W 4f0) and after oxidation (W 4fox) of the sample W/WOx(RT) (A) and the sample W/WOx(1300) (B). Right column: The W 4f spectra before (W 4fPd) and after annealing (W 4f∆T) of the sample Pd/WOx/W(RT) (C) and Pd/WOx(1300) (D). The line shape changes of the W 4f spectra upon oxidation and annealing are visualized by pertinent difference spectra. Points: experimental data; lines: fitted curves.

effect was mainly due to increased concentration of oxygen bonded to W2+ surface species, that is, because of the increase of the W 4f line at Eb ∼ 32 eV (line #2 in Table 1), assigned to this tungsten oxidation state27,29 (see Figures 6 and 7 and Table 1). The effective thickness dox of the WOx layer, as calculated from the attenuation of the W 4f line of nonoxidized tungsten, was 0.20 nm (WOx/W(RT)) and 0.28 nm (WOx/ W(1300)). As follows from the intensity of the SCSL W 4f line (Figure 6), a substantial part of the surface of the WOx/ W(RT) (∼30%) sample remained nonoxidized while the surface of the WOx/W(1300) sample was almost completely oxidized. The difference W 4f spectra (Figure 7) were calculated from the experimental data (points in the figures) as well as from the envelopes of the fits of spectra (lines in the figures). The line shape changes of the difference spectra demonstrate that several tungsten oxide species are present on the surfaces upon oxidation and subsequent annealing of palladium-contain-

ing samples. The changes observed in difference spectra following palladium deposition were almost negligible (not shown in Figure 7). Annealing of the Pd/WOx/W(RT) sample increased the intensity of the difference spectrum at Eb ∼ 32 eV with a shoulder at ∼32.8 eV (Figure 7C), while the intensity of the difference spectrum of the Pd/WOx/W(1300) sample increased at Eb ∼ 33.4 eV with a low-energy shoulder at ∼32.9 eV (Figure 7D). The values of Eb and the intensities obtained by curve fitting of the Pd 3d spectra (Figure 8) are summarized in Table 2. A similar line shape of the Pd 3d spectrum was observed for palladium deposits on the clean tungsten surface (Pd/W30). The binding energy of the high-energy Pd 3d line is, however, higher in the latter spectrum. The spectra of the annealed Pd/WOx/ W(RT) and Pd/WOx/W(1300) samples were identical. They were fitted by one symmetrical doublet at Eb ) 335.50 eV (see Table 2).

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Jirka et al.

Figure 8. Effect of annealing of Pd/WOx/W systems on the line shape of Pd 3d spectra. Left column: sample Pd/WOx/W(RT) before annealing (a) and after annealing to 670 K (c). Right column: sample Pd/WOx/W(1300) before annealing (b) and after annealing to 670 K (d). Points: experimental data; lines: fitted curves.

TABLE 2: Results of the Curve Fitting of Pd 3d Spectra of the Samples Pd/W, Pd/WOx/W(RT), and Pd/WOx/W(1300)d sample

Eb (eV)a

Eb (eV)b

d(nm)

(I)∆T/(I)RT

(I)high/(I)low,a

(I)high/(I)low,b

Pd/Wc Pd/WOx/W(RT) Pd/WOx/W(1300)

334.95, 335.88 334.81, 335.34 334.92

335.32, 335.91 335.50 335.54

0.46 0.30 0.35

0.66 0.57 0.43

0.35 0.21

0.47

a Before annealing. b After annealing. c Reference 30. d (I)∆T/(I)RT is the ratio of intensity after annealing to before annealing, and (I)high/(I)low is the ratio of the intensity of the high- and low-energy Pd 3d lines.

4. Discussion A high coverage of molecular CO adsorbs at room temperature on three-dimensional (3D) palladium clusters,13-15 and the CO desorption is molecular and complete at 600 K. The present TPD data for nonannealed samples are in line with these findings. The substantial reduction of CO coverage on the surfaces of the annealed Pd/WOx/W(RT) and Pd/WOx/W(1300) samples, observed by TPD (Figures 1 and 2) as well as by SR PES (Figure 3), is related to chemical changes of the outermost surface of the sample. Information about these changes can be obtained from interpretation of the Eb values and the line shapes of Pd 3d and W 4f spectra. A substantial reduction of the quantity of CO adsorbed on annealed palladium overlayer on a clean polycrystalline tungsten surface was also demonstrated previously.30,31 Palladium is present on the surface of this sample in the form of twodimensional (2D) islands exhibiting no chemical interaction with the support and in the form of a palladium-tungsten bimetallic phase. The presence of two chemical states of palladium was indicated by two Pd 3d photoelectron lines at Eb ) 335.32 and 335.92 eV (after annealing) in the spectrum. The bimetallic Pd-W phase was present in this sample already before annealing (together with bulklike palladium deposits: the value of Eb of the low-energy Pd 3d line is equivalent to that of the

palladium foil). Annealing resulted in redispersion of the palladium 3D species and increased the concentration of the Pd-W bimetallic phase (see the upward shift of the low-energy Pd 3d line and the increased intensity of the high-energy Pd 3d line, Table 2). At room temperature, no CO adsorption took place on the Pd-W bimetallic phase or on the 2D palladium layers.30,31 The observed residual CO adsorption was attributed to chemisorption on bare tungsten surface areas. Palladium clusters of a single type are present on the Pd/ WOx/W(1300) surface before annealing. Only one Pd 3d doublet with Eb and parameters of asymmetry close to those estimated for the palladium foil (Table 2) was sufficient to fit the photoelectron spectrum. This line is thus attributed to bulklike 3D clusters. The two Pd 3d photoelectron lines present in the spectrum of Pd/WOx/W(RT), the more intense one at Eb ) 334.8 eV with bulk asymmetry parameters and the high-energy, symmetrical one of low intensity at Eb ) 335.3 eV (Figure 8, Table 2), can be explained as resulting from two modes of palladium/support interaction. The low-energy line is assigned, in analogy to the surface of the Pd/WOx/W(1300) sample, to palladium adatoms which only weakly interact with the support, forming 3D clusters. The high-energy line is attributed to palladium atoms which chemically interact with surface oxygen (see the discussion below). Pd-W alloy formation can be

CO with Palladium Supported on Oxidized Tungsten

Figure 9. Thermal desorption spectra of palladium from Pd/WOx/ W(RT) and Pd/WOx/W(1300 K) samples for different Pd layer coverages.

excluded in this case since the binding energy of the high-energy Pd 3d line is substantially lower than that of palladium atoms chemically bonded to tungsten (Table 2). The high palladiumtungsten oxide surface (WO3) interfacial energy underlies the appearance of 3-D palladium clusters, and cluster formation has been also observed by scanning tunneling microscopy (STM). 32 No alloying takes place also after annealing the samples with oxidized surfaces (see Tables 1 and 2). The above conclusion concerning the absence of a Pd-W bimetallic phase is in line with TPD of palladium atoms from oxidized tungsten surfaces (Figure 9). A set of desorption curves of Pd (AMU 108) for different coverages indicates the existence of only one desorption state. For Pd desorption from W(110), Schlenk and Bauer33 reported two desorption states, one resulting from Pd desorption from the first layer (high-temperature peaks in the range 1420-1480 K) and the other from Pd desorption of three-dimensional crystallites (low-temperature peaks in the range 1170-1250 K). On the basis of a comparison with our data, one can conclude that the interaction between Pd atoms and the oxidized support is weak, and Pd desorption occurs from crystallites. This is in agreement with the observed threedimensional growth mode of Pd deposits. From the line shapes of the Pd 3d photoelectron spectra, one can conclude that palladium is present in a single chemical state on both surfaces after annealing (Figure 8c, d). The Eb value of the Pd 3d5/2 line (335.5 eV for both samples, Table 2) is equal or close to the value of Eb(Pd 3d5/2) of oxidized Pd(111) at low temperatures (300 K, 523 K).34 This shows that oxidation of palladium clusters takes place on the surfaces of the Pd/WOx/ W(RT) and Pd/WOx/W(1300) samples. It is reasonable to assume that this effect is accompanied by redispersion of palladium clusters. This assumption is additionally supported by findings from a field emission study of annealed Pd layers on a tungsten tip oxidized by the same procedure as for the Pd/WOx/W(RT) sample.35 Annealing also causes a partial recovery of the intensity of W 4f photoelectron spectra (Figure 4). Oxidation of palladium clusters is thus accompanied by encapsulation of palladium deposits in the WOx surface layer.

J. Phys. Chem. B, Vol. 110, No. 47, 2006 23843 An analogous heat-induced encapsulation of palladium deposits was already observed in Pd/Al2O3/Al and Pd/NbOx/Nb systems.36,37 The aforementioned evidence concerning the appearance of a chemical palladium-oxygen interaction is in line with the discussion of the line shapes of the W 4f photoelectron spectra, from which the presence of a Pd-O-W mixed oxide phase in the studied samples was inferred. It follows from the values of Eb obtained by curve fitting of the W 4f spectra (Table 1) that the surfaces of the two samples contain the same set of Wn+Ox species with n e 4. The Eb values of the W 4f lines of the (Pd)/WOx/W(RT) and (Pd)/WOx/W(1300) samples are the same within experimental error (