J. Phys. Chem. 1996, 100, 19215-19217
19215
In Situ Core-Electron Spectroscopy of Carbon Monoxide Adsorbed on High-Area Platinum in an Acid Electrolyte In Tae Bae and Daniel A. Scherson* Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106-7078 ReceiVed: June 20, 1996; In Final Form: October 4, 1996X
Modifications in the electronic density of states of small Pt particles (4-5 nm diameter) dispersed in higharea carbon (XC72-R) induced by the presence of adsorbed CO were examined by in situ Pt-LIII-edge X-ray absorption near edge structure (XANES) in a 1.0 M H3PO4 solution. An increase in the intensity of the in situ Pt-LIII-edge XANES was observed at energies a few electronvolts positive of the white line maximum (11 567 eV), after CO was bubbled in the solution, with the electrode polarized at potentials in the range in which CO undergoes irreversible adsorption on Pt. A well-defined peak centered at 11 570 eV could be clearly identified upon subtracting these spectra from those acquired in that same potential region prior to CO adsorption. This feature has been ascribed to transitions from the Pt 2p3/2 orbital to unoccupied states above the Fermi level localized predominantly on CO.
Introduction A detailed characterization of adsorbate-substrate interactions may be regarded as crucial to the further understanding of a variety of interfacial phenomena and, in particular, catalysis and electrocatalysis. Techniques have been developed over the past two to three decades to examine various aspects of the vibrational, electronic, and structural properties of these systems at the gas-solid1 and, more recently, liquid-solid interfaces.2 Of considerable concern is a description of the changes in the electronic structure of both the adsorbate and the substrate induced by the formation of the interfacial bond, and, more specifically, an identification of the densities and symmetries of the occupied and unoccupied states near the Fermi level. Information regarding these energy states can be obtained via the analysis of photoemission and inverse photoemission spectroscopic data acquired in ultrahigh vacuum environments, as illustrated by a growing number of papers published in the literature.3 The rather recent advent of tunable, high-intensity X-ray sources have opened new prospects for the study of these systems by enabling the excitation of electronic transitions originating from states localized in the core of the adsorbate atoms.4 A primary advantage of these emerging techniques is that X-rays penetrate rather deeply into matter, including thin layers of solid and liquid materials, making it possible to conduct experiments in which the adsorbate-substrate system is immersed or buried in high-pressure or condensed media.5 Unfortunately, and because of the same reasons, methodologies based on the detection of X-rays, such as fluorescence-type measurements, lack surface specificity, except for flat surfaces under conditions of very high incidence angles at which the beam undergoes total external reflection.6 An alternate approach that overcomes some of these limitations involves the use of substrate materials in highly dispersed form to enhance surface, as opposed to bulk, contributions to the observed signal. This is the case, for example, of Pt particles dispersed in high-area carbon, which are used in the fabrication of gas-permeable electrodes of the type used in fuel cells or in metal oxide supports for catalytic applications.5a Although the surface of these particles is seldom well characterized, X-rayX
Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)01840-0 CCC: $12.00
based techniques provide ideal conditions for the study of fundamental aspects of reactions of relevance to electrocatalysis in an environment that closely resembles that found in a real device. This short communication presents in situ Pt-LIII-edge X-ray absorption near edge structure (XANES) for carbon monoxide adsorbed on high-area platinum dispersed in a high-area carbon support in 1 M H3PO4 as a function of the applied potential. Phosphoric acid was selected for these studies because of its importance in fuel cell applications. As will be shown, a comparison of the in situ XAFS recorded in the presence and in the absence of CO revealed a clearly defined feature ascribed to electronic transitions from the 2p3/2 to unoccupied orbitals localized primarily on the adsorbate. Experimental Section High-area Pt (average diameter ca. 4-5 nm) dispersed in high-area XC72-R carbon (1 mg, 20% Pt/C Johnson Matthey) was thoroughly mixed with an emulsified Teflon suspension (20% Teflon, Dupont, Wilmington, DE) using a spatula, and the resulting pastelike material was then spread onto a hydrophobic carbon layer (5 × 10 mm, Eltech System) over an area 3 × 10 mm (A in Figure 1). At energies about the LIII-edge of Pt (ca. 12 keV), the amount of Pt in the final electrode corresponds to ca. 0.08 absorption lengths; hence, spectral distortions due to thickness effects for data collected in the fluorescence mode (see below) may be assumed to be negligible. External electrical connection to the electrode was made with a fine gold wire (0.1 mm, Johnson Matthey, D in Figure 1) pressed between two sheets of hydrophobic carbon. The remaining section of the gold wire was covered with Kapton tape (not shown in the figure) to avoid contact with the electrolyte. The electrode was mounted on a glass cell specifically designed for in situ XANES in the fluorescence mode shown schematically in Figure 1. A dynamic hydrogen electrode (DHE, I in the figure) connected via a stopcock and a Luggin capillary to the main cell compartment was used as a reference electrode. Experiments were performed at room temperature in 1.0 M phosphoric acid prepared by diluting the as-received concentrated orthophosphoric acid (Fisher, ACS Certified) with pyrodistilled water. XANES were acquired at beam line X-23A2 © 1996 American Chemical Society
19216 J. Phys. Chem., Vol. 100, No. 50, 1996
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Figure 2. Cyclic voltammograms of a high-area Pt/XC72-R Teflonbonded electrode in 1 M H3PO4 obtained in the cell shown in Figure 1 before (solid line) and after (dotted line) adsorption of CO. Scan rate: 2 mV/s. Figure 1. Schematic diagram of the electrochemical cell for in situ Pt-LIII-edge XANES measurements of highly dispersed Pt in the fluorescence mode: A, Hydrophobic carbon layer; B, Pt/XC-72R/Teflon electrode; C, kapton tape; D, gold wire; E, glass cell body; F, gas outlet; G, gas inlet; H, Pt foil counterelectrode; I, dynamic hydrogen electrode. The two diagrams below the cell represent front (left) and side (right) views of the electrode assembly.
at the National Synchrotron Light Source (Brookhaven National Laboratory, Upton, NY) working at an energy of 2.56 GeV, with a ring current of 100-200 mA. The light was monochromatized with a set of Si(311) crystals with the slits adjusted to 1 mm (vertical) and 7 mm (horizontal). Data were recorded in the range 11364-11764 eV in steps of 2, 0.4, and 1 eV in the regions -200 to -50, -50 to 70, and 70 to 200 eV, respectively, as measured relative to the Pt-LIII-edge at 11 564 eV. A PIN diode covered with an Al foil filter and powered by a high-voltage battery (300 V) was used to detect the fluorescent X-rays. In situ XANES were recorded at five different potentials in the range 0.075 and 0.800 V vs DHE, first in a nitrogen-purged 1 M H3PO4 solution and after CO was bubbled for 2 h at precisely the same potentials. On the basis of prior experience with this type of highly convoluted electrodes, this period of time is sufficient to achieve full CO surface saturation. Results and Discussion Figure 2 shows cyclic voltammograms of a high-area Pt/ XC72-R Teflon-bonded electrode in 1 M H3PO4 obtained in the cell shown in Figure 1 before (solid line) and after (dotted line) adsorption of CO. As is well-known, the presence of CO on the Pt surface blocks sites for hydrogen adsorption, as clearly evidenced by the much reduced pseudocapacitive current in the range 0.0-0.4 V. The prominent peaks in the region 0.5-1.0 V are ascribed to the oxidation of adsorbed CO and possibly solution phase CO trapped in the pores of the electrode. Figure 3 displays in situ Pt-LIII-edge fluorescence XAS spectra recorded before (solid lines) and after CO was bubbled into the 1 M H3PO4 solution (dashed lines) at potentials of 0.075, 0.175, 0.400, and 0.800 V. The energy scale in this figure has been referenced to the LIII-edge of Pt, 11 564 eV, ∆E ) 0. In all cases, a clear increase in the signal intensity was observed at energies of about ∆E ) 6 eV. The changes induced by the adsorption of CO can be better visualized by subtracting spectra collected in the presence of CO from those in the absence of CO at the same potential. As shown in Figure 4, this spectral manipulation revealed a
Figure 3. In situ Pt-LIII-edge fluorescence XAS spectra obtained before (solid lines) and after CO was bubbled into the 1 M H3PO4 solution (dashed lines) at the indicated potentials vs DHE. The energy has been referred to the LIII-edge of Pt, i.e. 11 564 eV.
prominent peak centered at ∆E ) 5-6 eV for potentials more negative than 0.80 V vs DHE, i.e. the range in which CO is irreversibly adsorbed on the Pt surface (see dotted line in Figure 2). This feature, however, disappeared in the difference spectrum at 0.80 V, a potential sufficiently positive for the oxidation of adsorbed CO to ensue. This supports the view that the voltammetric feature centered at 0.8 V in Figure 2 is associated with the electrooxidation of adsorbed CO and that those above this potential are due to CO trapped in the electrode pores. The curves in Figure 4 reflect differences in the XANES between a Pt surface at the same potential, covered in one case by CO and by adsorbed species derived from the (CO-free) electrolyte in the other, predominantly atomic hydrogen at 0.075 V and H2PO4- and/or HPO42- in the more positive range. This effect may account for changes in the shape of the main peak centered at ∆E of ca. 5-6 eV, as well as for more subtle spectral modifications as a function of potential, and will be further
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Figure 4. Difference XANES spectra obtained by subtracting the in situ XANES in the absence of CO from that in the presence of CO shown in Figure 3 at the indicated potentials. Also included in this figure is the difference spectra at 0.8 V vs DHE, a potential at which CO is electrooxidized on Pt in this electrolyte (see also caption of Figure 3).
explored by performing measurements in electrolytes containing other anions such as sulfuric and perchloric acid. A clearly defined feature centered at about 4-5 eV above the Fermi level has also been reported in the inverse photoemission spectroscopy (IPS) spectra of CO adsorbed on Pt(110) in ultrahigh vacuum7,8 and attributed to transitions to empty electronic energy levels localized primarily on the adsorbate.9 Considerable care must be exercised, however, when comparing XAS with IPS data because of the so-called final state rule, that is, the spectral shape is determined by the density of states in the final state. For example, a difference of about 2 eV has been found between the energies associated with the 2π resonances in the C 1s XAS spectrum of Ni/Cu(100) and the IPS spectrum of CO adsorbed on Ni.10 In contrast, the Ni 2p XAS spectrum of CO/Ni/Cu(100), which would probe the same 2π-3d states, reveals a CO-induced 2π-derived feature at an energy of 3 eV above the core ionization threshold and, therefore, much higher than that observed in the corresponding C 1s XAS spectrum of CO/Ni/Cu(100). This implies that the perturbation due to the core hole in the case of the Ni excitation is not nearly as large as in the case of C. Support for this model was provided by comparing these data with the IPS of NO (instead of CO) on Ni and the IPS of CO adsorbed on Cu (instead of Ni), which yielded bands at almost the same energies as their Z-1 (C and Ni) counterparts. It may thus be concluded
J. Phys. Chem., Vol. 100, No. 50, 1996 19217 that in the case of Pt, the final state effect would be minimal and that therefore the qualitative comparison of the in situ XAS data presented in our paper and ex situ IPS data is fully warranted. However, the lack of a common reference point for the energy between the in situ XAS (ionization potential) and IPS (Fermi level) experiments precludes a more quantitative comparison between the results obtained with these techniques. On the basis of the evidence presented, it is reasonable to assign the peak in the potential difference in situ XAS spectra in the 0.075-0.60 V vs DHE to transitions between the 2p3/2 level of Pt and unoccupied states of adsorbed CO (2π and perhaps 5σ). This provides the first illustration of in situ coreelectron spectroscopy of a species adsorbed on a metal surface in an electrochemical environment. Qualitatively similar results to those shown in Figure 3 were found upon adding methanol to a CO-free 1 M H3PO4 solution. This may not be surprising, as methanol is known to undergo dissociative chemisorption on Pt to yield adsorbed CO as one of the products. It is expected that the further development of in situ electroncore spectroscopy in electrochemical environments may provide valuable new insights into the effects of the applied potential on the nature of adsorbate-substrate chemical bonds and therefore contribute to a better understanding of electrocatalytic processes. Acknowledgment. This work was supported by the Advanced Research Projects Agency, ONR Contract No. N0001492-J-1848. Additional support was provided by NSF Grant No. 9207885. Valuable discussions with Dr. J. Gordon from IBM, Almaden, and Prof. A. Hitchcock of McMaster University are gratefully acknowledged. References and Notes (1) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge University Press: Cambridge, 1986. (2) See, for example: Scherson, D. A.; Yeager, E. B. In InVestigations of Surfaces and Interfaces; Rossiter, B. W., Baetzold, R. C., Eds.; Physical Methods in Chemistry, 2nd ed. Vol IXB; John Wiley & Sons Inc., New York, 1993; p 543. (3) See, for example: Johnson, P. D. In Angle-resolVed Photoemission; Kevan, S. D., Ed.; Studies in Surface Science and Catalysis 74; Elsevier: Amsterdam, 1992; p 509. (4) Somers, J. S.; Lindner, Th.; Surman, M.; Bradshaw, A. M.; Williams, G. P.; McConville, C. F.; Woodruff, D. P. Surf. Sci. 1987, 183, 576. (5) (a) Sinfelt, J. H.; Meitzner, G. D. Acc. Chem. Res. 1993, 26, 1. (b) For applications of synchrotron radiation to electrochemistry, see: Abruna, H. D. In Electrochemical Interfaces: Modern Techniques for insitu Interface Characterization; Abruna, H. D., Ed.; VCH Publishers: New York, 1991. (c) See also: Toney, M. F.; McBreen, J. Interface 1993, 2, 22. (6) Toney, M. F.; Melroy, O. R. Cf. ref 5b, p 79. (7) Ferrer, S.; Frank, K. H.; Reihl, B. Surf. Sci. 1985, 162, 264. (8) Bertel, E.; Memmel, N.; Rangelov, G.; Bischler, U. Chem. Phys. 1993, 177, 337. (9) Wong, Y.-T.; Hoffman, R. J. Phys. Chem. 1991, 95, 859. (10) Nillson, A.; Martensson, N. Physica B 1995, 208/209, 19.
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