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Atomic-Scale Imaging and Electronic Structure Determination of Catalytic Sites on Pd/Cu Near Surface Alloys Heather L. Tierney, Ashleigh E. Baber, and E. Charles H. Sykes* Department of Chemistry, Tufts UniVersity, Medford, Massachusetts 02155-5813 ReceiVed: NoVember 5, 2008; ReVised Manuscript ReceiVed: February 2, 2009
Water-gas shift chemistry provides a useful method for producing hydrogen from coal; however, fuel cell applications demand that this hydrogen be free of impurities. Due to their unique properties, Pd/Cu alloys represent an import class of materials used for H purification membranes and also serve as the active metals in many heterogeneous catalysts. Little is known about how Pd and Cu interact electronically in these mixed systems and there is debate in the literature over the direction of charge transfer between the two species. This study used the differential conductance (dI/dV) spectroscopy capabilities of a low-temperature scanning tunneling microscope (STM) to investigate the atomic-scale electronic structure of Pd/Cu surface alloys. dI/ dV spectroscopy gives a direct measure of the local density of states of surface sites with subnanometer precision. Results from this work demonstrate that individual, isolated Pd atoms in a Cu lattice are almost electronically identical to their host atoms. Over an energy range that spans 1 eV on either side of the Fermi level, the only significant electronic difference between isolated Pd and their host Cu atoms is that Pd atoms have a very slightly depleted electron density in the region of the Cu surface state maximum. Introduction Bimetallic alloys are currently being studied in great detail due to their desirable catalytic properties and multiple uses in hydrogen-related chemistry.1 H2 purity is crucial for applications such as fuel cells.2 Pd is highly selective for H separation; thus, Pd-rich membranes such as PdCu alloys are often employed for this application. These alloyed membranes are more resistant to contamination and are more robust than their pure Pd counterparts.3-5 PdCu alloys are also widely used in catalysis due to their reactivity and selectivity for reactions such as oxidation of CO and alkenes,6 hydrogenation of organic substances,7 reduction of NO to N28-10 and liquid-phase nitrate removal from drinking water.11 Due to the technological importance of these binary alloys, a wide variety of experimental and theoretical studies have been performed in order to elucidate the surface physics and chemistry of these systems.3,12-23 The bulk alloys exist in both ordered and disordered phases; however, at most catalytically relevant temperatures and compositions, the binary Pd-Cu system is a disordered fcc alloy.24-27 Surface studies have shown that the top atomic layer of Pd/Cu alloys is rich in Cu, whereas the nearsurface region (∼7 atomic layers) is rich in Pd.28 However, if there are strongly interacting adsorbates present, Pd is brought to the surface.21 Similar diffusion and exchange of surface/ subsurface atoms in other metal alloy systems has also been observed. As a reaction proceeded on the surface of a Pt50Rh50(100) alloy, changes in the alloy’s surface composition were related to the exact stoichiometry of the reactants.29 The excellent catalytic properties of Pd/Cu alloys have been discussed in terms of both ligand and ensemble effects.30-32 Ligand effects refer to the change in catalytic properties due to electronic interactions between the two elements of a bimetallic alloy. Ensemble effects refer to the spatial distribution of atomic sites that host reactants. Some reactions that require larger * Corresponding author. E-mail:
[email protected]. Phone: +1617-627-3773. Fax: +1-617-627-3443.
ensembles of reactive atoms to catalyze a transformation are halted when the active atom is monodispersed throughout an inert lattice. Surface studies have focused on elucidating both of these effects for the Pd/Cu system using a variety of techniques. Photoelectron spectroscopies have been used to investigate ligand effects in these alloys in order to quantify the charge transfer between the Cu and Pd atoms.12,13,15,20,22,23,33,34 X-ray photoelectron spectroscopy (XPS) measures the binding energy of electrons in core levels of the surface atoms in a sample and can, in certain instances, be used to quantify charge transfer between atoms or, in other words, determine oxidation states.35 However, XPS studies of Pd/Cu alloys have shown that the core levels of both Pd and Cu can shift either up or down in energy depending on the composition and annealing history of the alloy, and debate over the charge state of the atoms continues.13,20,23,33,36,37 Core level shifts in XPS occur not only from charge transfer but from other initial state effects including changes in coordination number, orbital rehybridization, and final state effects such as screening.20,37 Therefore, XPS cannot be used as a direct method for determining the electronic state of the constituents of an alloy. A few ultraviolet photoelectron spectroscopy (UPS) studies have also been performed on the Pd/Cu system;33,34 Hedman and co-workers used UPS to show that the higher the Pd content of the alloy, the more electron density exists just at and below the Fermi level (EF). The composition of Pd/Cu alloys is heterogeneous at the atomic-scale, and all of the techniques discussed previously are ensemble measurements that average over many µm2 of the surface. Therefore, important nanoscale detail about the surface composition and electronic structure may be lost due to averaging. While previous scanning tunneling microscopy (STM) studies27 have explained much of the structural information about this system, the electronic interplay between Pd and Cu has remained a mystery at the atomic level. The aim of the current work is to elucidate the local composition and electronic structure of Pd/Cu{111} with STM imaging and dI/dV spec-
10.1021/jp809766d CCC: $40.75 2009 American Chemical Society Published on Web 03/03/2009
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troscopy. These techniques allow the electronic structure of individual Pd atoms to be measured both in and under the Cu surface. Using exactly the same STM tip, dI/dV spectra of both the Cu and Pd atoms were measured and the results served as a direct comparison of the electronic structure of both sites without the possibility of any tip artifacts in the spectra. Also, as dI/dV spectra represent the local density of states (LDOS) as a function of energy, one can not only track the direction of charge transfer, but determine which energy levels are involved. Experimental Methods All experiments were performed in a low-temperature, ultrahigh-vacuum (LT-UHV) scanning tunneling microscope (Omicron).38 The very high stability of the microscope at 78 and 7 K allows for the study of a particular area of the surface for many hours and allows point spectroscopy to be performed with a minimum amount of z-drift (100) were taken over many days (>10) with multiple tip states (>10). The dI/dV spectra presented in the paper were collected on one day but are averaged over multiple Cu, surface, and subsurface Pd atoms within a given area. Changes in the STM tip state were very noticeable not only in the topographic images (which were taken concurrently with dI/dV spectra measurements), but also large spikes were observed in dI/dV spectra when a tip change occurred due to saturation of the signal from the lockin amplifier. In order to ensure that all data was recorded with the same STM tip state, dI/dV spectra were always taken multiple times alternatively above Pd and Cu atoms. If the tip had changed within the set of measurements, then the Pd spectra at the beginning and end of the set would differ and the data set would be discarded. The fact that dI/dV spectra of both the host Cu atoms and inserted Pd atoms can be measured with exactly the same tip allows a direct comparison of the electronic structure of both atomic sites with subnanometer resolution. These spectra were taken for surface and subsurface Pd on the Cu{111} crystal and compared to the spectra from Cu atoms in the alloyed surface (see Figure 4). At first glance all of the spectra look almost identical. Therefore, to first approximation, isolated Pd atoms are electronically identical to their Cu host. However, as all of the spectra in Figure 4 were taken using the same STM tip, the Cu background could be subtracted from the Pd spectra to reveal subtle differences in the LDOS of the Pd atoms as seen on the right of Figure 4. The largest difference in the Pd spectra can be seen near the surface state maxima of Cu{111}, which occurs at an energy of -0.45 eV.51 The background subtraction spectra reveal that both surface and subsurface Pd atoms have a depleted LDOS centered around -0.45 eV. As this energy corresponds to filled states, the Pd atoms therefore carry a slight positive charge. By
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Figure 4. dI/dV point spectra taken for 0.01 ML Pd/Cu{111}. Pd spectra for surface Pd vs subsurface Pd were compared with spectra from nearby Cu atoms. Subtraction of the Cu background spectra from the Pd spectra shows deviations from the bare Cu spectra near the surface state (about -0.45 eV for Cu).
Figure 5. Topographic and dI/dV images for Cu{111} and 0.01 ML Pd/Cu{111}. In the bare metal system, standing wave patterns are only seen emanating from the step edge. In the alloyed system, the Pd atoms in the Cu crystal are the most dominant scattering centers. (Imaging conditions: Vtip ) 0.1 V, 7 K, I ) 1.5 nA, for all images and the z-range ) 2-3 pm for the topography images.)
using the Wigner-Seitz radius of Cu (190 Å2/electron), and integrating the dI/dV curves over the energy range of the Cu{111} surface state, the charge transfer from Pd to Cu can be tentatively estimated.52 Using this method the charge transfer between Pd and Cu atoms was found to be 0.010 ( 0.001 efor surface Pd atoms and 0.004 ( 0.001 e- for subsurface Pd atoms. The direction of charge transfer is somewhat nonintuitive. Given the work functions of Cu (4.65 eV) and Pd (5.12 eV), one would naively expect the Pd to acquire charge as opposed to the reverse. As discussed in the introduction, the UPS work of Martensson et al.33 actually showed an increase in electron density just below the Fermi level as Pd/Cu alloys became increasingly Pd rich. In the present case the charge transfer is found to be from the Pd to the Cu atoms, as seen by the depleted electron density of the Pd in the subtracted differential conductance spectra (Figure 4). An important point to remember is that this study is of Pd in the dilute limit, and that the Cu{111} surface state electrons are clearly affected by the presence of Pd. For this reason dI/ dV imaging was used to spatially view the LDOS of the Pd/ Cu{111} surface at specific energies. Topography STM images
contain an average of the LDOS between the Fermi energy and the imaging voltage. dI/dV imaging allows for a smaller sliver of the LDOS to be measured and plotted spatially in an image. Figure 5 shows topography and dI/dV images of both a clean Cu{111} sample and one with 0.01 ML Pd. The Cu/Pd alloy sample has Pd atoms in both the surface and subsurface regions of the ascending step edges. The topographic image of a bare Cu{111} surface in the upper left side of Figure 5 shows electron standing waves emanating from a step edge. These standing waves are seen even more clearly in the corresponding dI/dV image because the color contrast associated with topographic features like the step edge is no longer present in the dI/dV image. Images in the lower half of Figure 5 show an equivalent area of Cu{111} but now with 0.01 ML of Pd incorporated into the surface and subsurface layers. This Pd incorporation leads to drastic differences both topographically and electronically as compared to the bare Cu{111}. The topography image shows that the upper terrace is no longer flat due to Pd incorporation. The dI/dV image reveals that the dominant scatterer is no longer the step edge; it is the isolated Pd atoms themselves. Such surface state electron scattering is a possible mechanism by which Pd atoms can be depleted of charge. However, this study shows that some degree of electron scattering can be observed over a fairly wide energy range (∼0.5 eV) and the fact that Pd atoms are depleted of charge in a much narrower range of energies (∼0.1 eV) suggests that electron scattering does not give rise to this charge transfer effect. We suggest that further modeling work is required to fully explain the subtle electronic differences between Pd and Cu we measure experimentally. Conclusions The most stable surface compositions of Pd/Cu alloys are composed of ∼100% Cu and, due to their larger size, any Pd atoms in the surface layer exist as isolated monomers surrounded by Cu atoms.26,27 Therefore, an understanding of the atomicscale structure and electronic properties of these isolated Pd atoms is crucial for understanding their reactivity. While previous studies utilizing a variety of spectroscopies have revealed the ensemble properties of Pd/Cu alloy systems, this work employed LT-STM imaging and differential conductance spectroscopy to elucidate the local surface composition and electronic structure of individual Pd and Cu atoms in a Pd/Cu{111} surface alloy. By using dI/dV spectroscopy, a direct comparison of the LDOS of isolated Pd atoms and their Cu host was drawn and the charge transfer between Pd and Cu over an energy range 1 eV above and below the Fermi level was quantified. This work provides insight into the electronic
7250 J. Phys. Chem. C, Vol. 113, No. 17, 2009 properties of an important catalytic surface and reveals that isolated Pd atoms in a Cu{111} surface are almost electronically identical to their Cu host, except for a very small charge depletion from the Pd near the peak in the Cu surface state. dI/dV imaging revealed that isolated Pd atoms serve as scattering centers for the surface state electrons of the Cu{111} surface. This work shows that STM spectroscopy is an ideal method for elucidating the atomic-scale electronic structure of metal alloys. Further research is aimed at extending the energy range of the dI/dV spectroscopy in order to elucidate the full d-band structure of these catalytically important alloys and relate the results to theory. Acknowledgment. is made to the Donors of the American Chemical Society Petroleum Research Fund (Grant No. 45256G5) and the NSF (Grant No. 0717978) for support of this research. A.E.B. and H.L.T. thank the U.S. Department of Education for GAANN fellowships. The authors thank Eric Heller and Don Eigler for useful discussions about the data. References and Notes (1) Chen, J. G.; Menning, C. A.; Zellner, M. B. Surf. Sci. Rep. 2008, 63, 201. (2) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K. I.; Iwashita, N. Chem. ReV. 2007, 107, 3904. (3) Kamakoti, P.; Morreale, B. D.; Ciocco, M. V.; Howard, B. H.; Killmeyer, R. P.; Cugini, A. V.; Sholl, D. S. Science 2005, 307, 569. (4) Ma, Y. H.; Akis, B. C.; Ayturk, M. E.; Guazzone, F.; Engwall, E. E.; Mardilovich, I. P. Ind. Eng. Chem. Res. 2004, 43, 2936. (5) Paglieri, S. N.; Way, J. D. Sep. Purif. Methods 2002, 31, 1. (6) Choi, K. I.; Vannice, M. A. J. Catal. 1991, 131, 36. (7) Anderson, J. A.; FernandezGarcia, M.; Haller, G. L. J. Catal. 1996, 164, 477. (8) Illas, F.; Lopez, N.; Ricart, J. M.; Clotet, A. J. Phys. Chem. B 1998, 102, 8017. (9) Andersen, T. H.; Li, Z.; Hoffmann, S. V.; Bech, L.; Onsgaard, J. J. Phys.-Condes. Matter 2002, 14, 7853. (10) Fernandez-Garcia, M.; Martinez-Arias, A.; Belver, C.; Anderson, J. A.; Conesa, J. C.; Soria, J. J. Catal. 2000, 190, 387. (11) Batista, J.; Pintar, A.; Ceh, M. Catal. Lett. 1997, 43, 79. (12) Andersen, T. H.; Bech, L.; Li, Z.; Hoffmann, S. V.; Onsgaard, J. Surf. Sci. 2004, 559, 111. (13) Cole, R. J.; Brooks, N. J.; Weightman, P. Phys. ReV. B 1997, 56, 12178. (14) Cole, R. J.; Brooks, N. J.; Weightman, P. Phys. ReV. Lett. 1997, 78, 3777. (15) Cole, R. J.; Brooks, N. J.; Weightman, P.; Francis, S. M.; Bowker, M. Surf. ReV. Lett. 1996, 3, 1763. (16) Fernandez-Garcia, M.; Conesa, J. C.; Clotet, A.; Ricart, J. M.; Lopez, N.; Illas, F. J. Phys. Chem. B 1998, 102, 141. (17) GandugliaPirovano, M. V.; Kudrnovsky, J.; Scheffler, M. Phys. ReV. Lett. 1997, 78, 1807. (18) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810. (19) Greeley, J.; Mavrikakis, M. J. Phys. Chem. B 2005, 109, 3460.
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JP809766D
