Article pubs.acs.org/Langmuir
Surface Investigation of Intermetallic PdGa(1̅ 1̅ 1̅)
Dirk Rosenthal,*,†,⊥ Roland Widmer,‡,⊥ Ronald Wagner,† Peter Gille,§ Marc Armbrüster,∥ Yuri Grin,∥ Robert Schlögl,† and Oliver Gröning‡ †
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany nanotech@surfaces Laboratory, EMPA, Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland § Crystallography Section, Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Theresienstrasse 41, 80333 Munich, Germany ∥ Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Strasse 40, 01187 Dresden, Germany ‡
S Supporting Information *
ABSTRACT: The intermetallic PdGa is a highly selective and potent catalyst in the semihydrogenation of acetylene, which is attributed to the surface stability and isolated Pd atom ensembles. In this context PdGa single crystals of form B with (1̅ 1̅ 1̅) orientation were investigated by means of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), X-ray photoelectron diffraction (XPD), and lowenergy electron diffraction (LEED) to study the electronic and geometric properties of this surface. UPS and thermal desorption spectroscopy (TDS) were used to probe the chemisorption behavior of CO. The PdGa(1̅ 1̅ 1̅) surface exhibits a (1 × 1) LEED and a pronounced XPD pattern indicating an unreconstructed bulktruncated surface. Low-temperature STM reveals a smooth surface with a (1 × 1) unit cell. No segregation occurs, and no impurities are detected by XPS. The electronic structure and the CO adsorption properties reveal PdGa(1̅ 1̅ 1̅) to be a bulk-truncated intermetallic compound with Pd−Ga partial covalent bonding.
1. INTRODUCTION The electronic and catalytic behavior of bimetallic systems has been of great interest since the 1950s.1 This fundamental interest in bimetallic systems stems from the unique “ensemble” and “ligand” effects introduced by Sachtler2 and Sinfelt and Ponec in the 1970s. With the development of surface science, it was also possible to perform in-depth studies of the electronic and ad/desorption properties of bimetallic surfaces.1,3,4 One major result was the strong binding energy shift of a metallic monolayer on another metallic substrate. The interpretation of those shifts within the initial state model leads to the idea of intermetallic bonding;5 consistently these systems were called surface intermetallic compounds.4 Recently, the bulk intermetallic compound PdGa (FeSi structure type, Hellner and Laves6) was identified as a highly selective catalyst for the semihydrogenation of acetylene in ethylenean important process step for the production of polyethylene.7−10 In contrast to the commonly used disordered substitutional alloys (solid solutions of the elements or intermediate phases with concentrational homogeneity ranges), e.g., PdxAg1−x,11 intermetallic PdGa crystallizes in an ordered crystal structure and has a very narrow homogeneity range.12 According to the IUPAC recommendations in 200513 and due to the consistency with the previous literature, the intermetallic compound is named © 2012 American Chemical Society
PdGa instead of GaPd. The theoretically predicted covalent bonding within the compound7 should lead to increased bulk stability, and hence, a stable and ordered surface is expected. Such ordered surfaces are ideal candidates to study the catalytic activity of a well-defined “ensemble” configuration depending on the surface orientation and termination. The stability of the bulk compound under the reaction conditions has already been shown by a large variety of in situ methods, where, e.g., the formation of the hydrides, segregation, decomposition, or subsurface chemistry could be excluded.9,14 These studies comprised in situ X-ray photoelectron spectroscopy (in situ XPS) in the millibar pressure range, but also ambient pressure investigations by, e.g., prompt γ activation analysis (PGAA) and in situ X-ray diffraction (XRD).9,14 All studies were performed on unsupported material. This strategy allows reduction of the materials gap between the reactor and subsequent ultrahigh vacuum (UHV) studies to the use of polycrystalline material in the reactor and single crystals in UHV studies. So far, the knowledge of the adsorption properties of polycrystalline PdGa is limited to CO chemisorption studies in Received: December 21, 2011 Revised: April 1, 2012 Published: April 20, 2012 6848
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2. EXPERIMENTAL SECTION
the millibar range using FT-IR. Here, only one band, showing a fine structure, is detected7,14 at 2048 cm−1. Comparison to literature data on pure Pd15,16 led to the assignment of the signal to CO adsorbed on top of Pd atoms within PdGa. The absence of dipole−dipole interactions with partial pressure variations was interpreted as a direct consequence of isolated adsorption sites on the surface. However, the possibility of CO adsorbed solely on Ga or Pd−Ga mixed bridge sites could not be excluded. To the best of our knowledge, no study exists for CO adsorption on Ga. Photoemission studies on the adsorption of rare gases, H2, and N2 on polycrystalline gallium and gallium films on Ni(110) resulted in the broad, structureless signal between the Fermi energy EF and a binding energy of 3 eV with a maximum around 1 eV for the valence band of polycrystalline gallium.17,18 Modern methods of surface science have proven to be able to determine the relation between the structure and composition of surfaces and the reaction rate and selectivity for specific chemical processes.19 This was achieved by investigating both single-crystal model catalysts and real catalyst systems using a combination of UHV surface science methods and highpressure kinetic techniques. Through the usage of model systems, it was possible to bridge the so-called materials and pressure gap20 which exists between surface science and applied catalysis, and a wide variety of processes have been investigated.21−23 As already mentioned earlier, a large variety of “high-pressure” techniques have already been applied to investigate intermetallic Pd−Ga compounds. These techniques give a good insight into the overall catalytic properties,8,9 but they do not yield a detailed apprehension of the local atomic arrangement on the surface, the electronic structure, and the adsorption sites, which is crucial to understand the atomistic catalytic reaction mechanisms. To address these questions for the intermetallic Pd−Ga compounds, we focus in the present paper first on the structural characterization of the (1̅ 1̅ 1)̅ surface by means of UHV techniques. Although an early surface science study of PdGa(110) exists,24 only the recent availability of large and well-characterized single crystals of PdGa12 now enables in-depth studies using the before-mentioned surface science techniques. Surface cleanliness and the expected binding energy shifts will be investigated by XPS. The surface structure will be in the first step characterized by low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). Atomically resolved scanning tunneling microscopy (STM) measurements will allow the verification of a locally ordered surface exposing only one ensemble configuration. Ultraviolet photoelectron spectroscopy (UPS) and thermal desorption spectroscopy (TDS) experiments after CO adsorption and comparison with corresponding experiments on Pd(111) will provide insight into the electronic structure and adsorption behavior. These investigations will deepen the understanding of the surface structure and of the elemental processes on the surface, thus helping to clarify hitherto unresolved questions, such as the differences in adsorption behavior of PdGa in comparison to Pd and disordered substitutional alloys such as PdxAg1−x. This study is the first of a series investigating different lowindexed surfaces of PdGa by XPS, UPS, XPD, LEED, and STM. Complementary temperature-programmed desorption studies on small probe molecules enable a comparison to the properties of elemental Pd.
The single crystalline samples used in this study originate from one large crystal of PdGa. This crystal was grown by the Czochralski method.12 The resulting PdGa crystal is of form B according to ref 25 but where the nomenclature of “form A” and “form B” is accidentally interchanged (see section 4.1). Orientation, cutting, and polishing of the samples is described in ref 12. The orientation of the polished (1̅ 1̅ 1̅) surface of the crystals was verified by reflective energydispersive X-ray diffraction (D2 Cryso, Bruker AXS, Rh X-ray tube) to be within 0.3°. UHV measurements were conducted at EMPA, Duebendorf, and at Fritz-Haber-Institut der Max-Planck-Gesellschaft (FHI), Berlin. In Berlin, the sample was mounted on a sapphire by a tungsten wire, which was pressed into small slits. A K-type thermocouple was placed in one slit between the tungsten wire and the sample. Additionally, the temperature during annealing could be controlled by a pyrometer (Kleiber). This pyrometer was calibrated against the pyrometer (Raytek Marathon MM2ML) used in Duebendorf. The temperature readings are the same within 10 K in the range from 820 to 970 K. The surface of the sample was prepared by several sputtering−annealing cycles (sputtering, Ar+, 1 keV; annealing, several minutes at 870 K) until a sharp 1 × 1 LEED pattern was obtained. If not differently stated, all experiments were performed at surfaces annealed at 870 K. Additionally, a lower annealing temperature (670 K) was chosen to mimic typical temperatures used in the semihydrogenation reaction.8 In addition, this low annealing temperature still yields a sharp 1 × 1 LEED pattern. The XPS, UPS, LEED, and TDS experiments were performed in a UHV system (base pressure < 2 × 10−10 mbar) equipped with a hemispherical energy analyzer with a multichanneltron detector (Phoibos 150, MCD 9) and an X-ray gun (XR 50) (both SPECS), a UV lamp (HIS13, Omicron), an Omicron LEED instrument, and a differentially pumped QMS 200 quadrupole mass spectrometer (QMS; Pfeiffer Vacuum). TDS was performed by placing the sample at ∼1 mm from the entrance aperture (1 mm in diameter) of the differentially pumped QMS. The QMS signal was calibrated for different CO pressures in the chamber. Quantification by integration of the TDS signal was carried out via a procedure already described in the literature.26 Briefly, the pressure in the chamber is linear with the QMS signal from the UHV base pressure to the 10−6 mbar range. From kinetic gas theory, the amount of gas molecules flying through the entrance aperture per time unit is calculated. If the different ionization probabilities for different thermal velocities are taken into account, a direct correlation of the amount of desorbing gas with the QMS signal per time unit is established. Additionally, we exploited the CO saturation coverage (0.7 monolayer (ML)) at room temperature on Pt(111), which was investigated, e.g., by Ertl and co-workers,27 for comparison of the corresponding TDS signal. The integral of the TDS CO signal on Pt(111) fits within 15% with the one calculated by kinetic gas theory. All XP spectra were recorded with fixed analyzer transmission at room temperature using nonmonochromatized Mg Kα or Al Kα radiation at a pass energy of 20 eV, leading to fwhm < 1.1 eV for Ag 3d5/2. The binding energy scale of the system was calibrated using an energy for Au 4f7/2 of 84.0 eV and an energy for Cu 2p3/2 of 932.7 eV from foiled samples. The angle between the PdGa normal and the analyzer was 30°, and that between the X-ray gun and the PdGa normal was 54°. UPS was conducted using He I and He II radiation in normal emission. The incident beam was in the plane of [1̅ 1̅ 1̅] and [1̅ 1̅ 2] and 45° to both vectors, which is well approximated by [4 4 1̅]. In Duebendorf, the sample preparation consists also of several sputtering−annealing cycles. The annealing temperature was 870 K. The LEED and XPS/XPD investigations were carried out in an Omicron ESCA system equipped with a VSW 125 HR electron analyzer, operating at a base pressure of 5 × 10−11 mbar. A nonmonochromatized Al Kα X-ray source of 1486.6 eV photon energy was employed for the presented XPS and XPD measurements. The surface sensitivity of these methods is due to the limited meanfree path of the photoelectrons in the substrate, which is on the order 6849
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of 2 nm (the vector [111] within the unit cell has a length of 0.85 nm) for the photoelectron energies used in our XPD study.28 XPD patterns were measured over the complete hemisphere above the sample surface: Core-level photoelectron intensities were collected on a dense grid of polar emission angles (θ) from normal emission (0°) to 90° grazing emission and over the full 360° azimuthal angle (φ) range. The XPD patterns in the figures are represented in a stereographic projection of the scattered photoelectron intensity in real space, where the outer ring delimiting the XPD patterns indicates a polar emission angle of 90°, i.e., emission along the surface plane. The center of the patterns corresponds to emission along the surface normal. The photoelectron intensity is given in a linear gray scale representation with white corresponding to maximum intensity. To enhance the signal-to-noise ratio, the data shown have been azimuthally averaged according to a 3-fold symmetry, which all the {111} and {1̅ 1̅ 1̅} planes of the space group P213 possess, and therefore, no information is lost in this averaging procedure. Background subtraction of the experimental data was performed using multipole expansion with spherical harmonics Yl,m(θ,φ) up to l = 64 and m = 64 and removal of all components with m = 0. Model calculations to simulate the XPD patterns were performed using the single scattering cluster (SSC) model29 for bulk-truncated surface structures. Polarization of the Pd− Ga bonds was considered to generate the phase shift input files for the SSC calculations. In combination with the chemically sensitive XPD, which has proven to be a powerful tool for structural investigations of the near-surface region,29 the SSC calculations allow us to gain insight into structural aspects of such systems. STM measurements were performed with an Omicron variabletemperature (VT) scanning tunneling microscope at 77 K with a base pressure below 3 × 10−11 mbar. STM imaging was performed by using a mechanically cut Pt/Ir tip, and the STM data were analyzed using WSxM software.30
Figure 1. PdGa:B(1̅ 1̅ 1̅) Pd1 and PdGa:B(111) Pd1 surface terminations with marked surface unit cells depicted above a side view along [0,1̅,1] with the PdGa:B unit cell. In this side view, both Pd1 surface terminations are marked. The lower one is PdGa:B(1̅ 1̅ 1̅) Pd1, and the upper one is PdGa:B(111) Pd1. Additionally, a cut of the slab is shown with PdGa:B(1̅ 1̅ 1̅) Ga1 and PdGa:B(111) Pd3 as resulting surface terminations. The atomic radii used for the spacefilling models are 0.14 nm for Pd and 0.13 nm for Ga.
of the outward normal of the plane, i.e., [111], and a parameter δ “that specifies the distance from the dividing plane to the nearest plane through lattice points within the half-crystal”. The outermost three layers of the four terminations can have stoichiometries of Pd:Ga = 4:3, 3:4, 4:1, and 1:4. To obtain the stoichiometry of the Pd1 (111) termination, one has to start with the Pd1 layer and count along the [111] direction: 1Pd + 1Ga + 3Pd yields Pd4Ga1. This is easily verified in Figure 1. The resulting values for both enantiomorphic forms and the composition of the resulting surfaces are given in Table S1 in the Supporting Information. 3.2. X-ray Photoelectron Spectroscopy. XP spectra were measured after introduction of the sample into the chamber and after the sputtering−annealing cycles. Examples of both spectra are shown in Figure 2. After introduction, the crystal
3. RESULTS 3.1. Surface Crystallography of PdGa: Polarity, Enantiomorphism, and Resulting Surface Termination. PdGa crystallizes in the space group P213 (No. 198), and the unit cell contains four Pd and four Ga atoms, while the cell parameter varies (a = 0.489695(6)−0.489557(6) nm) depending on the Pd:Ga ratio.12 This structure is polar along the ⟨111⟩ directions, and two enantiomorphic forms have to be considered. Form A is defined by the atomic positions of Ga (x = y = z = 0.84284 (4a)) and Pd (x = y = z = 0.14234 (4a)). On the other hand, enantiomorphic form B is defined by inversion. Detailed insight is given in the Supporting Information, where in Figure S1 the unit cells of both enantiomorphic forms are shown. Relevant for the PdGa(1̅ 1̅ 1)̅ surface investigated in this paper is the slab representation shown in Figure 1. Considering the polarity and the two enantiomorphs, we have 16 possible (1 × 1) terminations for surfaces along the [111] and [1̅ 1̅ 1̅] directions. Note that for each of the eight terminations for one enantiomorph exists a mirrored termination of the other enantiomorph (for comparison see Figure 1 and the Supporting Information, Figure S2). For an easy discrimination, we propose the following nomenclature for these 16 terminations: The chemical formula of the compound is followed by the enantiomorphic form A or B and the Miller index of the surface (hkl). The nomenclature becomes unique by stating the number of the outermost atoms of the surface in the unit cell. For instance, form B of PdGa with the (1̅ 1̅ 1̅) surface and the outermost layer termination with one palladium atom is named PdGa:B(1̅ 1̅ 1)̅ Pd1 and is shown in Figure 1. Another definition for the surface termination nomenclature follows the recommendation from Landolt−Börnstein:31 the different surface terminations can also be defined by the (hkl)
Figure 2. XPS overview spectra of PdGa as introduced in the UHV system and after sputtering−annealing cycles (Al Kα).
exhibits only weak signals of Pd and Ga but strong signals of the contaminants carbon and oxygen. After sputtering− annealing cycles, the absence of contaminants is only verifiable in the case of carbon with Al Kα radiation, while the O 1s signal overlaps with strong signals from PdGa. The possibly remaining amount of oxygen was not detectable even with the help of 6850
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difference spectra (not shown). The binding energy of Pd 3p3/2 is 533.4 eV, that of Pd 3d5/2 is 336.3 eV, that of Ga 2p3/2 is 1116.7 eV, and that of Ga 3p3/2 is 104.3 eV. Compared to literature values, the results show no shift in binding energy for Ga 2p3/2 (1116.7 eV32), but a higher binding energy shift of 1.2 eV for Pd 3d5/2 (metallic Pd, 335.1 eV33). 3.3. Surface Structure by LEED, XPD, and SSC. After several sputtering and annealing cycles, a sharp (1 × 1) LEED image was observed in an energy range from 10 to ca. 450 eV. Figure 3 shows the results of the LEED investigations on the
Figure 4. SSC calculations of (a) PdGa(1̅ 1̅ 1̅) and (b) PdGa(111) for the Pd 3d5/2 core level and the corresponding XPD patterns of (c) PdGa(1̅ 1̅ 1̅) and (d) PdGa(111), revealing the different chiralities. Figure 3. LEED (1 × 1) patterns at 40 eV (a) and 105 eV (b) beam energy.
respectively, of Figure 4) are not superimposable. The chirality can be revealed by identifying a pinwheel-like structure formed by the bright feature labeled A at about θ = 36−58°, the orientation of the small triangle labeled B, and the relative position of the intense forward-focusing maxima labeled C at about θ = 57°. The absence of mirror symmetry implied by the handiness of the pinwheel structure shows the chirality of the system, which, in turn, implicates the necessity to distinguish two groups of the (111) and (1̅ 1̅ 1̅) surfaces of the PdGa enantiomorphs: forms A and B as introduced in section 3.1. Single-crystal X-ray diffraction analysis revealed that the crystal under investigation is of form B (see section 4.1); accordingly, form B was used for the SSC calculations. The experimentally obtained XPD patterns (Figures 4c,d and S3c,d) are then compared with those obtained by SSC calculations (Figures 4a,b and S3a,b). Despite the relative simplicity of the SSC model as compared to multiple scattering theory, the calculated XPD patterns exhibit a good agreement of the single intensity features, the overall appearance, and the orientation. From the SSC calculations, the PdGa surface orientation can be unambiguously determined to be (1̅ 1̅ 1̅) for the sample presented in Figures 4c and S3c and to be (111) for the sample presented in Figures 4d and S3d. As mentioned above, the handiness of the surface as well as the chemical identity of the surface termination plays an important role in chiral recognition and catalytic activity. To reveal possible segregation phenomena, the experimental Pd 3d5/2 and Ga 3p3/2 intensity distributions were azimuthally averaged and normalized to the same intensity at normal emission, yielding the polar-angle-dependent intensity distributions shown in Figure 5. It is apparent that the relative intensity ratio of Pd to Ga does not diverge at large emission angles (besides some effects of the structure factor), and therefore, no segregation is observed, which is a further affirmation of a bulktruncated surface termination. 3.4. STM Characterization. Figure 6 presents the results of the STM investigations of the PdGa:B(1̅ 1̅ 1̅) surface prepared as described in the Experimental Section. The large-scale STM images as shown in Figure 6a reveal large atomically flat terraces of various widths separated by a unique step height. The latter is equal to 284 pm as indicated in the profile line (Figure 6b) along the red line drawn in Figure 6a, which corresponds very well to one-third of the XRD lattice
clean PdGa:B(1̅ 1̅ 1̅) surface at two particular primary electron energies (40 and 105 eV). The experimental LEED patterns in Figure 3 exhibit clear 3-fold rotational symmetry and reveal the surface unit cell diffraction spot corresponding to 0.69 nm in real space, which is in perfect agreement with bulk XRD experiments.12 It is worth noting that the usually symmetryequivalent higher order spots arising in the LEED patterns show a very distinct intensity variation compared to each other. These intensity variations are related to the structure factor of the unit cell and show the lack of inversion symmetry. As LEED is a surface-sensitive technique, one can conclude from these results that the preparation of a well-defined terraced surface could be obtained by the procedure described in the Experimental Section and that the surface is close to bulk termination, in the sense that no reconstruction is taking place. Additionally, the low background intensity indicates the absence of segregation. To further elucidate the nature of surface termination, we used element-selective X-ray photoelectron diffraction to obtain data on the surface orientation and termination. From kinetic energy and intensity considerations, the Pd 3d5/2 and Ga 3p3/2 emission lines were measured, as they are well separated from other elements and appear at rather high kinetic energies (see Figure 2). The selection of emission lines with a high kinetic energy is important to have an XPD pattern dominated by forward focusing. This in turn validates the SSC approach to simulate the XPD pattern for comparison with experiments. Figure 4 shows the Pd 3d5/2 XPD patterns of PdGa(1̅ 1̅ 1̅) and PdGa(111) in comparison with the corresponding SSC calculations. The Ga 3p3/2 XPD patterns of PdGa(1̅ 1̅ 1)̅ and PdGa(111) are shown for comparison in the Supporting Information (Figure S3). The patterns reveal a clear 3-fold rotational symmetry as expected for the {111} and {1̅ 1̅ 1̅} surfaces in the P213 space group. Additionally, the patterns also show a characteristic handiness stemming from the lack of inversion symmetry. The XPD patterns exhibit a very rich fine structure, which results from a well-defined crystallographic order. There are only minor differences between the Pd 3d5/2 and the Ga 3p3/2 XPD patterns of the same surface orientation, but because of their chiral nature, the corresponding XPD pattern of the opposite surfaces (left and right in parts c and d, 6851
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surfaces and additionally on Pd(111) for comparison. The TDS series for the well-defined surface (as shown by STM in Figure 6) prepared at 870 K is displayed in Figure 7. The spectra are
Figure 5. Normalized angle dependence of the Pd 3d5/2 (blue) and the Ga 3p3/2 (red) core-level photoemission intensities derived from the XPD data displayed in Figure 4: from polar emission angle (θ) 0° (normal emission) to 90° grazing emission and averaged over the full 360° azimuthal angle (φ) range.
Figure 7. Thermal desorption spectrum series after adsorption of CO at 85 K on PdGa:B(1̅ 1̅ 1)̅ annealed at 870 K (heating rate 1.5 K/s). The spectrum series starts with 0.3 langmuir and ends with saturation coverage after 50 langmuirs. The two dark-colored spectra correspond to dosages of 1 and 50 langmuirs as indicated.
parameter along the [111] direction. As shown above, crystallographically identical layers are found in the bulk structure after one-third of the unit cell by passing the sequence of planes in the [111] direction. This means that out of four possible terminations of the (1̅ 1̅ 1̅) surface, only one is stable after this preparation condition (annealing at 870 K). Highresolution STM images reveal that all investigated terraces show the same 3-fold surface structure with a nearest neighbor distance of 0.697(7) nm as shown in Figure 6c, which is further evidenced by the fast Fourier transform (FFT) inset and which is in agreement with the bulk model periodicity12 within the {111} planes. The STM image of the PdGa:B(1̅ 1̅ 1̅) surface in Figure 6c shows some 30 pm high depressions and protrusions which are located at well-defined lattice sites. It is however remarkable that the triangular shape of the protrusions reveals the intrinsic 3-fold rotational symmetry of the {1̅ 1̅ 1̅} surfaces in the space group P213. Additional STM experiments indicate mobility of surface atoms already at room temperature, which needs further investigation. 3.5. Thermal Desorption Spectroscopy. CO adsorption was carried out on three differently prepared PdGa:B(1̅ 1̅ 1̅)
dominated by a major component (γ state) at 215 K with a first-order desorption behavior. Around a dosage of 1 langmuir, a new state, namely, β, at lower temperature begins to grow as a shoulder accompanied by two states α1,2 at very low temperature. Intensity calibration and integration of the γ and β states yield one CO molecule per PdGa:B(1̅ 1̅ 1)̅ unit cell (0.4153 nm2), which is 2.4 CO molecules/nm2 as compared to 11.5 CO molecules/nm2 in full coverage for Pd(111) (see below). It should be stated here that, in all CO desorption experiments on PdGa:B(1̅ 1̅ 1)̅ , no CO desorption occurs above room temperature (investigated up to 500 K), and no change of the (1 × 1) LEED pattern was observed under any conditions. To mimic the temperatures during the semihydrogenation reaction,8 the surface was prepared at a lower temperature (670 K). To allow insight into this surface morphology, large-scale STM images were recorded (shown in the Supporting
Figure 6. STM investigations: (a) large-scale STM image (100 × 100 nm2) of the highly terraced surface area with a drawn red line, US = −1 V, IT = 0.5 nA, (b) profile along the red line in (a), (c) high-resolution STM image (20 × 20 nm2) of atomic arrangement, with the FFT inset demonstrating the (1̅ 1̅ 1̅) symmetry properties, US = −2.5 V, IT = 0.5 nA. 6852
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Information) and compared with those of the surface prepared at 870 K (see above). Again, a series of TDS spectra were obtained (not shown here), and the saturation coverage after 50 langmuirs of CO is shown in the summary of all different preparation procedures in Figure 8. The integration yields 5−6
Figure 9. Two He II UPS spectra of clean PdGa:B(1̅ 1̅ 1̅) (annealing at 870 and 670 K) at room temperature. Additionally, the He II spectrum of clean Pd(111) is depicted for comparison. For all spectra the centroid of the valence band is indicated (1, 2.8, and 3.2 eV). The two PdGa:B(1̅ 1̅ 1̅) spectra are offset for clarity, and the color code corresponds to that in Figure 8.
both annealing temperatures, PdGa exhibits several features in the valence band between 0.5 and 5 eV with binding energy maxima around 2.7 and 3.1 eV. In comparison, the valence band for Pd(111) is quite narrow with nearly the highest intensity around the Fermi edge, which compares very well with the literature.35 Calculations of the bulk electronic density of states reveal a strong domination of the valence band by Pd 4d states, while gallium states are also always present in a minor part,7 which is consistent with the already mentioned early study of PdGa(110)24 and the photoemission studies on polycrystalline gallium and gallium films on Ni(110).17,18
Figure 8. Thermal desorption spectra after adsorption of CO at 85 K on PdGa:B(1̅ 1̅ 1)̅ annealed at 670 K (heating rate 1.4 K/s, saturation coverage 50 langmuirs), intermediate 800 K (1.5 K/s, 25 langmuirs), and 870 K (1.5 K/s, 50 langmuirs). Additionally, CO adsorption was carried out on Pd(111) for comparison (heating rate 1.4 K/s, 27 langmuirs, adsorption temperature 85 K). The desorption range for the α, β, and both γ ranges are depicted. The spectra are offset for clarity.
CO molecules per PdGa:B(1̅ 1̅ 1̅) unit cell at saturation coverage (12−14.5 CO molecules/nm2). The high temperature desorption state γ is significantly shifted to higher temperatures and contains 1.3 CO molecules per unit cell, while the α states (3 CO molecules per unit cell) and β states (1.7 CO molecules per unit cell) remain at their positions. The high-temperature γ state from the preparation at 670 K could also be found as a very small signal in the TDS for the high-temperature preparation. TDS from intermediate-temperature preparation (800 K) clearly shows both γ states. For comparison, CO desorption was also measured by TDS on the Pd(111) surface (Figure 8). The saturation coverage for CO adsorption at 87 K on Pd(111) is 0.75 ML.34 This is consistent with our TDS calibration. From the absence of CO desorption above room temperature for PdGa:B(1̅ 1̅ 1)̅ in Figure 8, it is clear that no elemental Pd segregated to the surface as already discussed in Figure 5. 3.6. Ultraviolet Photoelectron Spectroscopy. He II UPS was employed to investigate the electronic structure of PdGa and the change of the valence band compared to that of Pd(111) (Figure 9). The He II spectrum of clean PdGa:B(1̅ 1̅ 1̅) differs strongly from that of Pd(111) and additionally depends on the annealing temperature. The centroid of the valence band (note that the usually used name “d-band center” is incorrect here, because of the additional 4p electron of gallium) is considerably shifted to higher binding energy compared to that of Pd(111) and even more for the sample annealed at higher temperature (870 K). The intensity at the Fermi edge is strongly lowered compared to that of Pd(111) but still well pronounced, indicating the metallic character of PdGa. For
4. DISCUSSION 4.1. Surface Stability: Segregation versus Bulk Truncation with Different Terminations. The crystal structure of PdGa is rather complex compared to a closepacked metal structure; it is enantiomorphic and polar. Therefore, it is especially important to rely on the structural information given in section 3.1 to identify the enantiomorphic form of PdGa. The space group P213 does not allow an origin shift from (000) to (1/4 1/4 1/4) as done in Armbrüster et al.,25 because of the different symmetry properties of these points. Such an origin shift would result in a transformation of one enantiomorphic form into the other.36 Although singlecrystal XRD was useful to discriminate the enantiomorphic forms and the surface orientation, the surface science method XPD is needed to unambiguously discriminate the surface orientation independent of the enantiomorphic form to be either (111) or (1̅ 1̅ 1)̅ . In addition, the (1 × 1) LEED results clearly show a bulklike nonreconstructed surface termination for the (1̅ 1̅ 1̅) surface. Verbeek et al.24 mentioned that another low-indexed surface, namely, PdGa(110), is unreconstructed by showing a (1 × 1) reflection high-energy electron diffraction pattern. Segregation phenomena are well investigated for substitutional alloys such as PdCu37 or PdAg,38 where it occurs. For PdGa:B(1̅ 1̅ 1̅), the high surface sensitivity of XPD at grazing angles already points to the absence of surface segregation. The absence of changes in the (1 × 1) LEED pattern is also a further hint that no segregation occurs. In contrast in the PdCu system, segregation leads to a very few 6853
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unit cell and a first-order desorption behavior and the formation of a surface at 670 K with five CO molecules adsorbed per unit cell. The surface at intermediate preparation temperature (800 K) clearly shows the possibility of mixed terminations due to the two coexisting high-temperature γ states. The 5−6 CO molecules per surface unit cell correspond to 0.8−1.0 ML of CO on Pd(111), hence a very dense CO layer. Even if one accepts a 20% error in the quantification, still five CO molecules exist as the minimum per surface unit cell. Hence, CO adsorption on gallium is in principle required on a fully ordered, bulk-terminated surface. The five or six CO molecules also require the on-top adsorption of three CO molecules at the Pd3 or the Ga3 ensemble. As already discussed in section 4.1, the α state may arise from CO desorption from gallium in analogy to CO desorption from aluminum. Although we cannot exclude that the high-temperature desorption states β and γ belong to CO at gallium and the α state belongs to CO on Pd, the shift in the CO desorption temperature of 350 K from CO on Pd(111) to CO on Pd in PdGa would be the largest ever reported in the literature (235 K for Pd on Ta(110)46). Therefore, we assume at least the highest desorption temperature states γ to originate from Pd and α states from Ga. A prediction for the different terminations at different annealing temperatures is feasible with these assumptions; namely, for the on-top adsorption geometry of CO on the Pd3 or Ga3 ensembles, the α and γ states are due to CO on gallium and palladium, respectively. The termination of the 870 K annealing temperature with one CO molecule per unit cell must be CO adsorption on the Pd1 ensemble, or the Pd3 ensemble must at least be sterically hindered to yield only one CO molecule per unit cell. These requirements are fulfilled for the PdGa:B(1̅ 1̅ 1̅) Pd1 and PdGa:B(1̅ 1̅ 1̅) Ga1 terminations only (see Figure 2). Consequently, the γ state at higher desorption temperature corresponds to desorption from the Pd3 ensemble without exposed Pd1, i.e., the PdGa:B(1̅ 1̅ 1̅) Ga3 and PdGa:B(1̅ 1̅ 1̅) Pd3 terminations. Both terminations expose seven or five atoms per surface unit cell, respectively, within the first three layers and six or four atoms, respectively, within the first two layers in line with the high amount of desorbed CO from the surface prepared at 670 K. The ratio of the intensity for the α, β, and γ states points to a coexistence of at least these two terminations. This picture is strongly supported by the comparably rough surface at a 670 K annealing temperature observed by STM (see Figure S4 in the Supporting Information). 4.3. Electronic Structure. The photoemission spectrum of clean PdGa:B(1̅ 1̅ 1̅) is remarkable. In a raw view, one could interpret the valence band as superposition of the states of Pd(111) and the broad, featureless states for gallium17,18 as was done for, i.e., Cu3Pt.47 However, there exists a considerable difference: the states at the Fermi level are depleted compared to those of Pd(111), and the electrons are shifted to states with higher binding energy. The coincidence of peak maxima near the Fermi level of PdGa:B(1̅ 1̅ 1)̅ and Pd(111) might suggest an influence of Pd−Pd interaction in PdGa; however, Verbeek et al.24 suggested a correlation with antibonding states of PdGa. Interestingly, the centroid of the PdGa valence band is at higher binding energy for the high-temperature-annealed termination, whereas the CO desorption of the γ state occurs at lower temperature. This might be an artifact, if one considers the valence band of the low-temperature-annealed surface (670 K)
different LEED patterns (PdCu(110)39). It should be pointed out that whereas PdCu and PdAg are substitutional alloys, PdGa presents an intermetallic compound from which the absence of surface reconstruction and segregation can be understood. A very sensitive tool for surface segregation is CO adsorption followed by TDS.37,40−42 In all of these cases in the literature, the TDS spectra are very sensitive to the amount and ensemble configuration of Pd on the surface. Segregation of Pd to the surface without strong electronic perturbation will yield CO desorption temperatures of elemental Pd together with the typical long tail at lower temperatures due to the different adsorption geometries at higher CO coverage.43 Strong electronic perturbations of a hypothetic Pd segregation layer could indeed lead to much lower CO desorption temperatures.5 These strong electronic perturbations are usually accompanied by a shift of the Pd 3d core levels to higher binding energy.5 XPS of PdGa:B(1̅ 1̅ 1)̅ reveals a single Pd 3d5/2 signal which indeed is shifted by a higher binding energy of 1.2 eV compared to that of pure palladium. This shift is even slightly higher than for polycrystalline PdGa samples.14 Consequently, the surface palladium atoms are shifted equally in binding energy compared to the bulk palladium atoms, and the interpretation of Pd surface atoms as situated in bulk-truncated PdGa positions is very likely. Since nothing is known about CO TDS from gallium, we refer to CO TDS from the Al(110) surface;44 here the desorption temperature is around 125 K (heating rate 1.3 K/s), comparable to that of the α states of PdGa. Again strong electronic perturbations could lead to an increase of the CO desorption temperature in TDS experiments accompanied by an XPS core-level shift toward lower binding energy,5 but the Ga 2p3/2 core level is not shifted. Thus, the α desorption states in CO TDS could indeed stem from gallium. Additionally, the different desorption spectra for different preparation temperatures could point to Ga segregation phenomena, but the (1 × 1) LEED pattern and the XPS results make the occurrence of different surface terminations and hence different ensemble configurations more likely. The XPD and LEED results (for the high-temperature preparation) still left four possible bulklike surface terminations open. From STM, we can deduce that the surface terminations for the 870 K prepared surface did not differ from terrace to terrace and the only observed step height is exactly 1/3 of the unit cell, leading to a sequence of only one surface termination, which has to be considered. High-resolution STM results confirm the LEED results that the surface unit cell is the one expected from continuation of the bulk. The assignment of contrast in STM with real height is ambiguous due to electronic effects. The case of rutile TiO2(110) with the geometrically lower Ti atoms but brighter contrast is well-known.45 The few bright spots within the majority layer might be interpreted as partial elemental exchange, i.e., in the possible surface termination PdGa:B(1̅ 1̅ 1̅) Pd1 the exchange of Pd1 atoms with Ga atoms. This picture is usually found in the PdxAg1−x alloy with Pd having the brighter contrast.38 Another possibility is the presence of adsorbates (CO or H2, which are always present in UHV systems) at these positions. 4.2. Surface Termination. CO TDS should in principle be able to discriminate between the four possible terminations due to their different near surface stoichiometries. The results reveal the formation of one termination at 870 K with one CO per 6854
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Notes
to consist of at least two different terminations as suggested by STM (see Figure S4 in the Supporting Information). Comparably, in a very recent publication, it was shown that the annealing of Sn overlayers on Pt(110) leads from a rough surface alloy with Sn islands to an ordered surface alloy.48 This change of surface morphology also leads to a shift of the valence band centroid. The shift of the valence band centroid is consistent with the shift to higher binding energy of the Pd 3d core level and the considerable lower desorption temperature of CO.1 The same observations were made for Pd adlayer systems, e.g., Pd on Ta(110).46,49,50 For this behavior, several reasons are discussed in the literature, including rehybridization of the Pd 4d → 5s,5p levels as well as charge transfer51−53 and “covalent bonds”.1,54 Maciejewski et al. pointed out that a heteronuclear covalent bond is always polar and the term “charge transfer” might be semantic.55 All of these explanations correlate with the theoretical result of localized electron density between Pd and Ga describing an intermetallic covalent bonding situation for PdGa.7 Therefore, in analogy to surface alloys, intermetallic Pd−Ga bonds are formed within the probing depth of XPS; hence, further experimental evidence has shown that PdGa is an intermetallic compound and not an alloy (as, e.g., PdxAg1−x).
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.R. and R.Wi. thank Julia Dshemuchadse for discussions, and R.Wi. thanks the Swiss National Science Foundation (Contract 200021-129511) for financial support.
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(1) Rodriguez, J. A. Bimetallic Model Catalysts. In Handbook of Heterogeneous Catalysis; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2008. (2) Sachtler, W. M. H. Chemisorption Complexes on Alloy Surfaces. Catal. Rev.Sci. Eng. 1976, 14, 193−210. (3) Campbell, C. T. Bimetallic Surface-Chemistry. Annu. Rev. Phys. Chem. 1990, 41, 775−837. (4) Rodriguez, J. A. Physical and Chemical Properties of Bimetallic Surfaces. Surf. Sci. Rep. 1996, 24, 225−287. (5) Rodriguez, J. A.; Goodman, D. W. The Nature of the Metal Metal Bond in Bimetallic Surfaces. Science 1992, 257, 897−903. (6) Hellner, E.; Laves, F. Kristallchemie des In und Ga in Legierungen mit einigen Ü bergangselementen (Ni, Pd, Pt, Cu, Ag und Au). Z. Naturforsch., A: Phys. Sci. 1947, 2, 177−183. (7) Kovnir, K.; Armbrüster, M.; Teschner, D.; Venkov, T. V.; Jentoft, F. C.; Knop-Gericke, A.; Grin, Y.; Schlögl, R. A New Approach to Well-Defined, Stable and Site-Isolated Catalysts. Sci. Technol. Adv. Mater. 2007, 8, 420−427. (8) Osswald, J.; Kovnir, K.; Armbrüster, M.; Giedigkeit, R.; Jentoft, R. E.; Wild, U.; Grin, Y.; Schlögl, R. Palladium-Gallium Intermetallic Compounds for the Selective Hydrogenation of AcetylenePart II: Surface Characterization and Catalytic Performance. J. Catal. 2008, 258, 219−227. (9) Osswald, J.; Giedigkeit, R.; Jentoft, R. E.; Armbrüster, M.; Girgsdies, F.; Kovnir, K.; Ressler, T.; Grin, Y.; Schlögl, R. PalladiumGallium Intermetallic Compounds for the Selective Hydrogenation of AcetylenePart I: Preparation and Structural Investigation under Reaction Conditions. J. Catal. 2008, 258, 210−218. (10) Armbrüster, M.; Kovnir, K.; Behrens, M.; Teschner, D.; Grin, Y.; Schlögl, R. Pd-Ga Intermetallic Compounds as Highly Selective Semihydrogenation Catalysts. J. Am. Chem. Soc. 2010, 132, 14745− 14747. (11) Johnson, M. M.; Walker, D. W.; Nowack, G. P. U.S. Patent 4404124, 1983. (12) Gille, P.; Ziemer, T.; Schmidt, M.; Kovnir, K.; Burkhardt, U.; Armbrüster, M. Growth of Large PdGa Single Crystals from the Melt. Intermetallics 2010, 18, 1663−1668. (13) Connelly, N. G.; Damhus, T.; Hartshorn, R. M.; Hutton, A. T. Nomenclature of Inorganic ChemistryIUPAC Recommendations 2005; The Royal Society of Chemistry: Cambridge, U.K., 2005. (14) Kovnir, K.; Armbrüster, M.; Teschner, D.; Venkov, T. V.; Szentmiklosi, L.; Jentoft, F. C.; Knop-Gericke, A.; Grin, Y.; Schlögl, R. In Situ Surface Characterization of the Intermetallic Compound PdGaA Highly Selective Hydrogenation Catalyst. Surf. Sci. 2009, 603, 1784−1792. (15) Gelin, P.; Siedle, A. R.; Yates, J. T. Stoichiometric Adsorbate Species Interconversion Processes in the Chemisorbed LayerAn Infrared Study of the CO/Pd System. J. Phys. Chem. 1984, 88, 2978− 2985. (16) Li, M. S.; Shen, J. Y. Microcalorimetric and Infrared Spectroscopic Studies of CO and C2H4 Adsorption on Pd/SiO2 and Pd-Ag/SiO2 Catalysts. Mater. Chem. Phys. 2001, 68, 204−209. (17) Jacobi, K.; Hsu, Y. P.; Rotermund, H. H. Photoemission from Ne, Ar, Kr and Xe Layers on Ni(110) and Ga Films. Surf. Sci. 1982, 114, 683−691. (18) Jacobi, K. Work-Function Changes and Photoemission FinalState Relaxation of Ne, Ar, Kr, Xe, H-2, and N-2 on Gallium. Surf. Sci. 1987, 192, 499−506.
5. CONCLUSION The PdGa:B(1̅ 1̅ 1̅) surface can exhibit four bulk-truncated terminations named Pd1, Ga1, Pd3, and Ga3. No segregation is observed over a wide temperature range, but different surface terminations occur for different annealing temperatures. From TDS and high-resolution STM, the Pd1 termination is likely for a preparation temperature of 870 K. The Ga3 and Pd3 terminations seem to dominate at lower preparation temperatures. The strong shift to higher binding energy in the XPS core-level Pd 3d5/2 (1.2 eV) as well as in the centroid of the valence band reveals in analogy to bimetallic surface compounds an intermetallic Pd−Ga partial covalent bonding. Hence, PdGa is a bulk intermetallic compound instead of an alloy. This is supported by the very low desorption temperature of CO (below 250 K at a heating rate of 1.5 K/s). While the high-temperature preparation (870 K) yields a well-defined surface termination with an isolated Pd atom ensemble, mobility of the surface atoms even at room temperature is still observed.
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ASSOCIATED CONTENT
S Supporting Information *
Figures showing the two enantiomorphs of PdGa, PdGa:A(1̅ 1̅ 1)̅ Pd1 and PdGa:A(111) Pd1 surface terminations, SSC calculations of PdGa:B(1̅ 1̅ 1̅) and PdGa:B(111) and the corresponding XRD patterns, and large-scale STM images of PdGa:B(1̅ 1̅ 1̅) surfaces and a table listing the nomenclature, distance from the nearest plane through lattice points, and composition of the outermost three layers for PdGa(111) surfaces of both enantiomorphic forms. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +49 30 8413 4523. Fax: +49 30 8413 4401. Author Contributions ⊥
These authors contributed equally to this work. 6855
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