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XANES study of underpotential deposited copper on carbon-supported platinum
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J. Electroanal. Chem., 307 (1991) 229-240 Elsevier Sequoia S.A., Lausanne
XANES study of underpotential deposited copper on carbon-supported platinum J. McBreen Department
of Applied Science, Brookhaven National Laboratory,
Upton, NY I I973 (USA)
W.E. O’Grady Code 6170, Naval Research Lnboratoty,
Washingion, DC 20375-5000 (USA)
G. Tourillon, E. Dartyge and A. Fontaine L (IRE - CNRS, 91405 Orsay Cedex (France) (Received 6 November 1990)
Abstract Underpotential deposited (UPD) copper on carbon-supported platinum was investigated in-situ in 0.5 M H,SO, at 0.05 V (SCE) by XANES (X-ray Absorption Near Edge Structure). By taking XANES spectra at the Cu K-edge and the Pt L,,,- edge it was possible to determine the valence state of the copper and observe modifications in the electronic structure of the platinum on adsorption of copper. The XANES for UPD copper shows that the adsorbed copper had an oxidation state close to Cu+. XANES features indicate strongly a tetrahedral coordination for the adsorbed Cu species. A reduction in intensity of the white tine in the platinum XANES is consistent with a partial filling of empty Pt d-band vacancies on adsorption of copper. Thus, UPD species can modify the electronic structure of a platinum catalyst. The adsorbed Cuf species are apparently associated with HSO; ions.
INTRODUCTION
Underpotential deposited (UPD) copper on platinum has been studied extensively, for thirty five years, since Breiter et al. [l] first showed that copper atoms adsorbed on platinum interfered with hydrogen adsorption. Copper adsorption, in the UPD region, was later confirmed by DeGeiso and Rogers using radiotracer measurements [2]. They found indications that the anion affected the adsorption process. After that, several new electrochemical techniques were applied to the Pt/Cu2+ system [3-121. Analytical techniques were also developed to determine the adsorption rate of the metal atoms [13-181. To reconcile the current with the adsorption rate the concept of electrosorption valence (y) was introduced [19]. A 0022-0728/91/$03.50
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detailed study by Schultze [13] for the Pt/Cu ‘+ in 0.5 M H,SO, showed that y was 2 at low coverages and 1.4 at high coverages. This is the opposite to what is observed at the metal-vacuum interface and was thought to be due to interactions of the UPD layer, at higher coverages, with species in the outer Hehnholtz plane. The radiotracer work of Horanyi has shown clearly that copper adsorbed on platinum interacts strongly with both Cl- [20] and HSOC [21] ions, even in the presence of excess supporting HClO, electrolyte. He attempted to recalculate y, assuming no specific adsorption of ClOi ions. The discovery that even ClOi ions are adsorbed by UPD Cu on Au [22] casts doubt on the validity of this assumption. Horanyi also proposed several possibilities for co-adsorption of copper and sulfate ion pairs or the specific species. These are the adsorption of Cu*+ and SO:adsorption of HSOT or SOi- ions with either completely discharged or partially discharged Cu ‘+ ions. This complicates even the use of more direct methods for determination of y such as the quartz microbalance [23]. The question of the valence of adatoms can really be resolved only by spectroscopic measurements such as XANES. There are several indications that the nature of adsorbed copper species on platinum is dependent on coverage. The cyclic voltammogram reveals three oxidation peaks [9,24]. Potential step measurements on Pt(ll0) surfaces show two steps in the adsorption isotherm [25]. The most direct evidence for structural changes is a rapid change in optical constants, from in-situ differential reflectance spectra, at about 2/3 of a monolayer [26]. However, ex-situ XPS studies of UPD copper showed a -0.95 eV shift in the Cu2p,,, level that did not vary with coverage [24,27]. Even though the spectra for the UPD samples showed adsorbed H,O and SO:- the authors ruled-out the presence of either CuSO, or Cu,O. On this basis the authors ruled out the presence of Cu2+ species. Even though they could not rule out unequivocally the presence of Cu+ species they concluded that the monolayer structure is metallic in nature and that only a fractional charge transfer exists between the adsorbed copper and the platinum. In a comparative study of vapor deposited Cu films on Pt, an identical shift was found at low coverages. However, the binding energy approached that of bulk copper at average calculated thicknesses well below monolayer coverages [24]. This was ascribed to cluster formation. Later studies on vapor deposition of Cu on Pt(ll1) surfaces showed a shift of only -0.65 eV [28]. Furthermore, LEED observations showed a sharp (1 X 1) pattern, indicating an epitaxial copper monolayer and no evidence of cluster formation. Photoemission was used to follow the evolution of Cu3d states with increasing coverage. Below a coverage of 0.6, the Cu3d states are relatively localized. At higher coverages, Cu-Cu interactions become apparent, but bulk Cu properties were not seen until deposition of about three monolayers. Adsorbed Cu also lowered the work function of Pt. However an interpetation in terms of electron transfer to the Pt could not be made because of inconsistencies with Pt4f,,, core-level data. Instead the results were rationalized in terms of polarization of Cu4s electrons towards the platinum substrate or differences in the surface dipole at the Cu adlayer/vacuum interface as compared with the clean Pt/vacuum interface. Work with single crystal
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alloy showed strong PtSd-Cu3d. hybridization on (111) surfaces with ~t,.9*Cuo.oz large atomic coordination [29]. There is evidence from in-situ electroreflectance studies that the 3d, 4s and 4p levels of adsorbed Cu on Pt are shifted and broadened considerably in comparison to their counterparts in isolated atomic Cu [26]. The energy shifts were consistent with an electron transfer from the Cu atom to the Pt. Surface conductance has also been used as an in-situ technique to study UPD Cu on Pt [30]. It was found that the deposition of Cu in 1 M H,SO, increased the conductance of the Pt film. This was interpreted as due to a partial electron transfer from the adsorbed Cu to the Pt. When the conductivity change is compared with charge passed, then 0.65 electrons are transferred per atom of Cu. Kolb [31] has suggested that this value is too high and that other effects contribute to the increase in conductance. The conductance effects are different at low and high Cu coverages, indicating a change in adsorption with coverage. Like many UPD species [32,33], UPD Cu on Pt displays catalytic effects for many electrochemical reactions. It inhibits oxygen reduction [34,35], promotes the oxidation of carbon monoxide and formic acid [36], and enhances the reduction of ethylene [37]. It also inhibits oxidation of hydrogen [38]. Several mechanisms have been proposed for the enhancement of oxidation of small organic molecules by UPD layers on platinum [32,33,39-411. These include (i) modification of the electronic structure of the platinum, (ii) changes in the physical structure of the platinum, (iii) adsorption of oxygen containing species from the electrolyte on the UPD atoms, (iv) redox processes involving the UPD species and (v) blocking of the adsorption of poisons. In principle, X-ray adsorption spectroscopy (XAS) can identify the occurrence of the first four mechanisms. Thermal desorption studies on vapor-deposited copper on platinum indicate that Cu adatoms have very little effect on the energetics of hydrogen adsorption on Pt [42]. However, there is some evidence that adsorbed Cu can modify the bonding of CO to neighboring Pt sites [42], indicating some electronic effect on the Pt. In the case of the Pt + Cu alloy (111) surface discussed above it was found that the Cu lowered the Gibbs energy of desorption of CO from Pt sites [43]. In this case the effects can be clearly correlated with electronic factors. So far the application of XAS to UPD studies has been limited [44,45] and there are no reports of work on platinum. The most extensive work is a grazing incidence angle EXAFS study of UPD Cu on Au(ll1) on mica in 0.5 M H,SO, [46,47] and on Au(100) single crystals in 0.5 M NaSO, [48]. All of this work is for full monolayer [46-481 or bilayer [48] coverages. The EXAFS results on Au(ll1) indicate a strong interaction between UPD Cu and SOi- ions. This is not surprising in the light of Horanyi’s radiotracer results on platinum [21]. Polarization dependent XAS studies indicated that the SOi- ions were adsorbed on top of the UPD Cu atoms. The XANES data showed that the UPD species was not divalent Cu, and the authors concluded that the adsorbed species was metallic Cu [47]. The recent results of Tourillon et al. on Au(100) indicate that the Cu is in a valence state close to Cu+ [48]. These authors also concluded that the UPD Cu occupied a top position on the
232
Au and Cu was coordinated with four oxygen atoms from either sulfate or water species in the electrolyte. These results demonstrate the power of the XAS technique for studying UPD layers. With better detectors, that can utilize all the fluorescence X-rays, the time for doing these type of experiments could be greatly reduced. In early work it took up to 25 h to get reasonable data [47]. With an improved detector arrangement this has been reduced to 5 h [48]. One method to improve the signal to noise ratio in an absorption experiment is to increase the number of absorbers per unit volume. In NMR experiments on the platinum/electrolyte interfaces this has been done using either colloidal platinum [49] or fuel cell grade platinum black [50]. In the present work XAS measurements were made on UPD Cu on carbon-supported Pt in 0.5 M H,SO,. This can be done because Cu adsorbs specifically on the Pt and not on the carbon 1511. Measurements were made at both the Cu K-edge and the Pt L,,,-edge. With highly dispersed Pt about 50% of the Pt atoms are surface atoms. This permits the use of XANES to study the effect of adsorbed species on the electronic structure of the Pt [52,53]. EXPERIMENTAL
Electrodes The carbon-supported platinum electrodes were circular discs, 1.9 cm in diameter and 0.75 mm thick. These were made using fuel cell electrode fabrication techniques. Each electrode contained 64 mg of 10% Pt on carbon (Prototech Corp. fuel cell catalyst), 10 mg of carbon fibers and 9 mg of PTFE (DuPont Corp. Teflon T-301. The platinum loading was 2.2 mg/cm2 and the platinum surface area was about 140 m2/g. A full Cu monolayer of 2 X 1O-9 mol/cm2 Cu [31] on the Pt would yield a loading of 5.71 x 10e4 g of Cu/cm2 on the electrode. In an X-ray absorption experiment this would give a step height Apx = 0.1 at the Cu K-edge [54]. This is adequate for doing transmission XAS experiments. Cell
A schematic drawing of the cell is given in Fig. 1. The end plates of the cell were identical to those used in a spectroelectrochemical cell described earlier [55,56]. The center portion of the cell consisted of a machined acrylic plastic block (7.6 X 7.6 X 2.6 cm) with a machined cylindrical cavity (diam. = 3.8 cm). The center block had O-ring grooves to provide a leak free seal to the end plates. The working electrode was suspended at the center of the cavity by gold wires that were fed through epoxy seals. Two gold screen counter electrodes were placed against the cell windows on opposite sides of the working electrode. These had slits to allow passage of the X-ray beam. The cell had provisions for a nitrogen purge and addition and removal of electrolyte. Experimental procedure A typical run was done as follows. The working electrode was placed in a beaker, weighted down with a glass stopper and vacuum treated in 0.5 M H,SO, for 600 s.
233 W.E. Contact Wire
C.E.
I
I
Electrolyte Levels
Fig. 1. Schematic
of spectroelectrochemical
cell for XAS studies
The vacuum was released and the flooded electrode was mounted in the current collector wires and the cell assembled. The cell was filled with 30 ml of 0.5 M H,SO, and cyclic voltammograms were run at 1 mV/s in a potential envelope between -0.240 V and 1.0 V SCE. This was done to check complete wetting of the catalyst. The voltammograms usually stabilized after three cycles. After this the cell was drained and refilled with a 0.5 M H,SO, + 4 X 10e4 M CuSO, electrolyte. The cell was purged with nitrogen and the working electrode was held at a potential of 0.050 V (SCE) to deposit the UPD layer. The 30 ml of electrolyte contained 1.2 X lop5 M Cu’+. However, on the basis of the above calculations, 1.76 X lo-’ M Cu2+ are needed to form a monolayer. Excess Cu2+ was furnished by removing 20 ml of the electrolyte after 1 h and refilling with fresh deaerated electrolyte. After removal of the electrolyte the electrolyte level was still one quarter way up the working and auxiliary electrodes. This procedure was repeated twice. In the run reported here the electrode was held at 0.050 V for an additional 3h after the final electrolyte addition. Then 20 ml of the electrolyte were removed to lower the electrolyte level below that of the cell windows. Nitrogen purging was continued and XAS measurements were done both at the Cu K-edge and the Pt L,,,-edge. Two aspects of the experiment that had to be addressed were contributions to the X-ray absorption by species in the electrolyte and control of the UPD process after lowering the electrolyte. The electrode had a 40% porosity. So at the most it would contain 9.5~10~’ g/cm* of Cu2+ ions, a Apx of only 2.4 x 10e4. With a full monolayer coverage of Cu on the Pt the contribution of electrolyte species to the overall absorption step would be only 0.24%. The UPD process occurs under diffusion controlled conditions and good current distribution and electrolyte stirring are vital. However, once a UPD layer is formed it can be maintained by potential control even under conditions with poor primary current distribution. The polariza-
234
tion resistance (an/ai) for the process is high and in the steady state a~/i3i + co. Thus there is a condition of infinite throwing power. So the UPD layer can be maintained in the lowered electrolyte as long as there is no other Faradaic process with a lower steady state polarization resistance. This can be achieved if oxygen is carefully excluded from the cell. XAS measurements The XAS measurements were done on the dispersive EXAFS Beam Line at LURE-CNRS. The characteristics of this beam line have been described previously [57,58]. Apart from the aspect of time resolved measurements, the advantages of this beam line are lack of mechanical movement in the monochromator and fast measurement times. The lack of mechanical movement in the monochromator permits measurements of energy shifts at the adsorption edge with great accuracy [59]. The measurement time for each spectrum was only 4 s. In addition to in-situ measurements with the cell XAS data were obtained on Cu foil, Pt foil, Cu,O, CuO and 0.1 M CuSO,. RESULTS
AND DISCUSSION
XANES for copper standard materials Figure 2 shows a set of normalized XANES spectra for Cu foil, Cu,O and CuO. The data are essentially identical to those published recently [60] and indicate the
ENERGY/eV Fig. 2. Normalized XANES spectra for Cu foil ( ---),Cu,O(---)andCuO(-+-+-).Zero energy is taken as the first inflection in the XANES for Cu foil.
235
high resolution that can be obtained on the dispersive beam line. The edge for divalent Cu is shifted approximately 8 eV above the edge for Cu foil. The edge shift between Cu and Cu+ is small (about 1.5 eV) and this shift is evident mostly below the first peak in the XANES. Some slight changes in the Cu+ edge have been observed by Tranquada et al., depending on whether the sample was a solid or an ion in solution [60]. However, they concluded that the overriding factor determining edge positions is the absence or presence of 3d or 4s electrons. So the edge position can determine the valence. This has been borne out in the recent measurements of Kau et al. on 19 Cu+ and 40 Cu2+ compounds [61]. The edge positions for all Cu+ compounds were the same. Changes in Cu+ coordination introduced various peaks and shoulders in the XANES spectra. In-situ XANES at Cu K-edge Figure 3 shows a comparison of normalized XANES data for Cu, Cu,O and UPD Cu on carbon-supported platinum. The respective step heights for the UPD sample were 0.057 at the Cu K-edge and 0.26 at the Pt Lit,-edge. This indicates a Cu coverage of 0.57 on the Pt. This is probably a good estimate since the amount of Pt and Cu present was determined directly from the respective step heights in the XAS measurements. The surface area is based on a Pt particle size of 2 nm. An expanded view of the region below the first peak is shown in Fig. 3b. The absorption edge for the UPD Cu is clearly that for Cu+. The first peak was less well defined and was shifted to 1.5 eV higher than that for Cu,O. This is typical for Cu+ compounds with tetrahedral symmetry [61]. A small reproducible shoulder was observed, at lower energies, at the beginning of the edge. This structure is characteristic of tetrahedrally coordinated Cu+ compounds [61] and was also seen in the work of Tourillon et al. on Au(100) [48]. All of this suggests that UPD Cu is in the +1 state and that the coordination is tetrahedral. There are two possibilities for tetrahedral coordination of the copper. One is copper adsorption at a 3-fold hollow site on the Pt and coordination to a single oxygen from the sulfate species. The other is coordination to a single Pt atom together with coordination to three oxygens from the sulfate-. species. Determination of the exact coordination awaits a detailed EXAFS study. So far the results indicate that the atomic arrangements are probably similar to those suggested for UPD Cu on Au [46-481. In-situ XANES at Pt L,,, edge Figure 4 shows a comparison of the XANES for Pt foil and carbon-supported platinum with a layer of UPD Cu. With the adsorption of Cu there is a decrease in the area under the white line. This represents a change in the electronic properties of the Pt consistent with a partial filling of empty d-states on the adsorption of Cu [52]. Increases in the area under the white line are observed on adsorption of hydrogen from the gas phase [53] and in electrochemical experiments [62]. Adsorption of anions such as phosphate also increase the area under the white line [62]. In both cases there is a partial emptying of d-states in the Pt. On a partially covered surface at 0.050 V (SCE) there are most likely the opposing contributions of adsorbed
236
r :I
0.6 +
*i
0.4 i
:r
i
+i
ENERGYleV
i
(b) . / . . . . f ;
./
l*..
ENERGYfeV Fig. 3. (a) Normalized XANES for Cu foil (+ + +), Cu,O (. .) and UPD Cu (- - -) on carbon-supported Pt at 0.050 V (SCE); (b) an expanded view of lower part of the absorption edges.
hydrogen on one hand, and the adsorbed Cu on the other. Since the net effect is a reduction in area there is very strong evidence for electron transfer to the Pt on adsorption of Cu. Since this is a net effect of at least two contributions the partial
237
Fig. 4. Norrnaliied XANES for Pt foil (- - -) and carbon-supported Pt with UPD Cu at 0.050 V SCE (). filling per atom of Cu cannot be calculated, even by doing experiments at both the Pt L,, and L,,, edges [52]. The transfer of electrons to Pt does not agree with conclusions from work with vapor-deposited Cu on Pt [28]. However, the two cases are not really comparable. Vapor deposition yields only adsorbed Cu. The XANES at the Cu K-edge and preli~na~ EXAFS results [63] indicate that the Cu is also coordinated with SO:- ions. This could aid in the hybridization of the Cu3d and PtSd states. Modification in the d-electrons of Cu by adsorbed SOihas also recently been invoked to explain second harmonic generation (SHG) results on UPD Cu on Au [64]. The SHG results indicated reduced delocalization of Cu d-electrons, even at high coverages. The ejjecr
oj oxygen When oxygen was introduced into the cell during the XAS experiments several changes occurred rapidly. The Cu XANES shifted to the Cu2+ edge and was identical to that for a CuSO, electrolyte [60]. The area under the Pt XANES white line also increased. When the electrolyte level was raised there was a large cathodic current due to the redeposition of the UPD Cu. If oxygen was excluded carefully none of these effects were seen over a period of hours. Oxygen could be excluded by vigorous bubbling with purified nitrogen and the use of a long thin exit tube for the gas to prevent back diffusion of air, particularly during the electrolyte lowering operation.
238
The nature of the LJPD layer The idea of having other than an adsorbed metallic species, beyond very low coverages, has generally been dismissed on the basis of electrostatic arguments. Even though Horanyi had warned about specific co-adsorption of anions [21], and adsorbed anions were seen early in ex-situ XPS experiments [24], this was largely ignored till recent spectroscopic evidence confirmed the radiotracer results [22,46481. The present work also shows that there can be transfer of electrons to the substrate metal. The XANES data can be interpreted only in terms of the presence of cu+ species. The electroneutrality condition is fulfilled by co-adsorption of anions and transfer of electrons to the substrate metal. These results, and recent quartz microbalance data [65], show the great difficulty in interpreting electroanalytical experiments, such as cyclic voltammetry, when specific adsorption of ions is involved. In addition to the metalladsorbate interactions, cation adsorption appears to involve invariably coadsorption of anions [66] and there is now evidence that anion adsorption promotes co-adsorption of cations [67]. Even with the application of various in-situ and ex-situ spectroscopies the study of these systems presents formidable difficulties [68]. Unambiguous methods of determining what and how much is adsorbed are needed. Also needed are probes of the chemical environment of the adsorbed species and substrates. This will require a combination of in-situ techniques such as X-ray methods, radiotracer studies, scanning tunnelling microscopy and the application of cleverly chosen ex-situ methods. The present results also show that XAS can be used to study electronic and structural modifications of platinum catalysts by UPD species. Further work will be done on systems that are known to promote the oxidation of small organic molecules such as methanol. ACKNOWLEDGEMENTS
The authors acknowledge funds from a grant from the NATO Scientific Affairs Division (Grant No. 86/532). This work was performed under the auspices of the S.S. DOE under Contract No. DE-AC02-76CH00016. REFERENCES 1 2 3 4 5 6 7 8 9 10
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J. Electroanal. Chem., 307 (1991) 229-240 Elsevier Sequoia S.A., Lausanne
XANES study of underpotential deposited copper on carbon-supported platinum J. McBreen Department
of Applied Science, Brookhaven National Laboratory,
Upton, NY I I973 (USA)
W.E. O’Grady Code 6170, Naval Research Lnboratoty,
Washingion, DC 20375-5000 (USA)
G. Tourillon, E. Dartyge and A. Fontaine L (IRE - CNRS, 91405 Orsay Cedex (France) (Received 6 November 1990)
Abstract Underpotential deposited (UPD) copper on carbon-supported platinum was investigated in-situ in 0.5 M H,SO, at 0.05 V (SCE) by XANES (X-ray Absorption Near Edge Structure). By taking XANES spectra at the Cu K-edge and the Pt L,,,- edge it was possible to determine the valence state of the copper and observe modifications in the electronic structure of the platinum on adsorption of copper. The XANES for UPD copper shows that the adsorbed copper had an oxidation state close to Cu+. XANES features indicate strongly a tetrahedral coordination for the adsorbed Cu species. A reduction in intensity of the white tine in the platinum XANES is consistent with a partial filling of empty Pt d-band vacancies on adsorption of copper. Thus, UPD species can modify the electronic structure of a platinum catalyst. The adsorbed Cuf species are apparently associated with HSO; ions.
INTRODUCTION
Underpotential deposited (UPD) copper on platinum has been studied extensively, for thirty five years, since Breiter et al. [l] first showed that copper atoms adsorbed on platinum interfered with hydrogen adsorption. Copper adsorption, in the UPD region, was later confirmed by DeGeiso and Rogers using radiotracer measurements [2]. They found indications that the anion affected the adsorption process. After that, several new electrochemical techniques were applied to the Pt/Cu2+ system [3-121. Analytical techniques were also developed to determine the adsorption rate of the metal atoms [13-181. To reconcile the current with the adsorption rate the concept of electrosorption valence (y) was introduced [19]. A 0022-0728/91/$03.50
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detailed study by Schultze [13] for the Pt/Cu ‘+ in 0.5 M H,SO, showed that y was 2 at low coverages and 1.4 at high coverages. This is the opposite to what is observed at the metal-vacuum interface and was thought to be due to interactions of the UPD layer, at higher coverages, with species in the outer Hehnholtz plane. The radiotracer work of Horanyi has shown clearly that copper adsorbed on platinum interacts strongly with both Cl- [20] and HSOC [21] ions, even in the presence of excess supporting HClO, electrolyte. He attempted to recalculate y, assuming no specific adsorption of ClOi ions. The discovery that even ClOi ions are adsorbed by UPD Cu on Au [22] casts doubt on the validity of this assumption. Horanyi also proposed several possibilities for co-adsorption of copper and sulfate ion pairs or the specific species. These are the adsorption of Cu*+ and SO:adsorption of HSOT or SOi- ions with either completely discharged or partially discharged Cu ‘+ ions. This complicates even the use of more direct methods for determination of y such as the quartz microbalance [23]. The question of the valence of adatoms can really be resolved only by spectroscopic measurements such as XANES. There are several indications that the nature of adsorbed copper species on platinum is dependent on coverage. The cyclic voltammogram reveals three oxidation peaks [9,24]. Potential step measurements on Pt(ll0) surfaces show two steps in the adsorption isotherm [25]. The most direct evidence for structural changes is a rapid change in optical constants, from in-situ differential reflectance spectra, at about 2/3 of a monolayer [26]. However, ex-situ XPS studies of UPD copper showed a -0.95 eV shift in the Cu2p,,, level that did not vary with coverage [24,27]. Even though the spectra for the UPD samples showed adsorbed H,O and SO:- the authors ruled-out the presence of either CuSO, or Cu,O. On this basis the authors ruled out the presence of Cu2+ species. Even though they could not rule out unequivocally the presence of Cu+ species they concluded that the monolayer structure is metallic in nature and that only a fractional charge transfer exists between the adsorbed copper and the platinum. In a comparative study of vapor deposited Cu films on Pt, an identical shift was found at low coverages. However, the binding energy approached that of bulk copper at average calculated thicknesses well below monolayer coverages [24]. This was ascribed to cluster formation. Later studies on vapor deposition of Cu on Pt(ll1) surfaces showed a shift of only -0.65 eV [28]. Furthermore, LEED observations showed a sharp (1 X 1) pattern, indicating an epitaxial copper monolayer and no evidence of cluster formation. Photoemission was used to follow the evolution of Cu3d states with increasing coverage. Below a coverage of 0.6, the Cu3d states are relatively localized. At higher coverages, Cu-Cu interactions become apparent, but bulk Cu properties were not seen until deposition of about three monolayers. Adsorbed Cu also lowered the work function of Pt. However an interpetation in terms of electron transfer to the Pt could not be made because of inconsistencies with Pt4f,,, core-level data. Instead the results were rationalized in terms of polarization of Cu4s electrons towards the platinum substrate or differences in the surface dipole at the Cu adlayer/vacuum interface as compared with the clean Pt/vacuum interface. Work with single crystal
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alloy showed strong PtSd-Cu3d. hybridization on (111) surfaces with ~t,.9*Cuo.oz large atomic coordination [29]. There is evidence from in-situ electroreflectance studies that the 3d, 4s and 4p levels of adsorbed Cu on Pt are shifted and broadened considerably in comparison to their counterparts in isolated atomic Cu [26]. The energy shifts were consistent with an electron transfer from the Cu atom to the Pt. Surface conductance has also been used as an in-situ technique to study UPD Cu on Pt [30]. It was found that the deposition of Cu in 1 M H,SO, increased the conductance of the Pt film. This was interpreted as due to a partial electron transfer from the adsorbed Cu to the Pt. When the conductivity change is compared with charge passed, then 0.65 electrons are transferred per atom of Cu. Kolb [31] has suggested that this value is too high and that other effects contribute to the increase in conductance. The conductance effects are different at low and high Cu coverages, indicating a change in adsorption with coverage. Like many UPD species [32,33], UPD Cu on Pt displays catalytic effects for many electrochemical reactions. It inhibits oxygen reduction [34,35], promotes the oxidation of carbon monoxide and formic acid [36], and enhances the reduction of ethylene [37]. It also inhibits oxidation of hydrogen [38]. Several mechanisms have been proposed for the enhancement of oxidation of small organic molecules by UPD layers on platinum [32,33,39-411. These include (i) modification of the electronic structure of the platinum, (ii) changes in the physical structure of the platinum, (iii) adsorption of oxygen containing species from the electrolyte on the UPD atoms, (iv) redox processes involving the UPD species and (v) blocking of the adsorption of poisons. In principle, X-ray adsorption spectroscopy (XAS) can identify the occurrence of the first four mechanisms. Thermal desorption studies on vapor-deposited copper on platinum indicate that Cu adatoms have very little effect on the energetics of hydrogen adsorption on Pt [42]. However, there is some evidence that adsorbed Cu can modify the bonding of CO to neighboring Pt sites [42], indicating some electronic effect on the Pt. In the case of the Pt + Cu alloy (111) surface discussed above it was found that the Cu lowered the Gibbs energy of desorption of CO from Pt sites [43]. In this case the effects can be clearly correlated with electronic factors. So far the application of XAS to UPD studies has been limited [44,45] and there are no reports of work on platinum. The most extensive work is a grazing incidence angle EXAFS study of UPD Cu on Au(ll1) on mica in 0.5 M H,SO, [46,47] and on Au(100) single crystals in 0.5 M NaSO, [48]. All of this work is for full monolayer [46-481 or bilayer [48] coverages. The EXAFS results on Au(ll1) indicate a strong interaction between UPD Cu and SOi- ions. This is not surprising in the light of Horanyi’s radiotracer results on platinum [21]. Polarization dependent XAS studies indicated that the SOi- ions were adsorbed on top of the UPD Cu atoms. The XANES data showed that the UPD species was not divalent Cu, and the authors concluded that the adsorbed species was metallic Cu [47]. The recent results of Tourillon et al. on Au(100) indicate that the Cu is in a valence state close to Cu+ [48]. These authors also concluded that the UPD Cu occupied a top position on the
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Au and Cu was coordinated with four oxygen atoms from either sulfate or water species in the electrolyte. These results demonstrate the power of the XAS technique for studying UPD layers. With better detectors, that can utilize all the fluorescence X-rays, the time for doing these type of experiments could be greatly reduced. In early work it took up to 25 h to get reasonable data [47]. With an improved detector arrangement this has been reduced to 5 h [48]. One method to improve the signal to noise ratio in an absorption experiment is to increase the number of absorbers per unit volume. In NMR experiments on the platinum/electrolyte interfaces this has been done using either colloidal platinum [49] or fuel cell grade platinum black [50]. In the present work XAS measurements were made on UPD Cu on carbon-supported Pt in 0.5 M H,SO,. This can be done because Cu adsorbs specifically on the Pt and not on the carbon 1511. Measurements were made at both the Cu K-edge and the Pt L,,,-edge. With highly dispersed Pt about 50% of the Pt atoms are surface atoms. This permits the use of XANES to study the effect of adsorbed species on the electronic structure of the Pt [52,53]. EXPERIMENTAL
Electrodes The carbon-supported platinum electrodes were circular discs, 1.9 cm in diameter and 0.75 mm thick. These were made using fuel cell electrode fabrication techniques. Each electrode contained 64 mg of 10% Pt on carbon (Prototech Corp. fuel cell catalyst), 10 mg of carbon fibers and 9 mg of PTFE (DuPont Corp. Teflon T-301. The platinum loading was 2.2 mg/cm2 and the platinum surface area was about 140 m2/g. A full Cu monolayer of 2 X 1O-9 mol/cm2 Cu [31] on the Pt would yield a loading of 5.71 x 10e4 g of Cu/cm2 on the electrode. In an X-ray absorption experiment this would give a step height Apx = 0.1 at the Cu K-edge [54]. This is adequate for doing transmission XAS experiments. Cell
A schematic drawing of the cell is given in Fig. 1. The end plates of the cell were identical to those used in a spectroelectrochemical cell described earlier [55,56]. The center portion of the cell consisted of a machined acrylic plastic block (7.6 X 7.6 X 2.6 cm) with a machined cylindrical cavity (diam. = 3.8 cm). The center block had O-ring grooves to provide a leak free seal to the end plates. The working electrode was suspended at the center of the cavity by gold wires that were fed through epoxy seals. Two gold screen counter electrodes were placed against the cell windows on opposite sides of the working electrode. These had slits to allow passage of the X-ray beam. The cell had provisions for a nitrogen purge and addition and removal of electrolyte. Experimental procedure A typical run was done as follows. The working electrode was placed in a beaker, weighted down with a glass stopper and vacuum treated in 0.5 M H,SO, for 600 s.
233 W.E. Contact Wire
C.E.
I
I
Electrolyte Levels
Fig. 1. Schematic
of spectroelectrochemical
cell for XAS studies
The vacuum was released and the flooded electrode was mounted in the current collector wires and the cell assembled. The cell was filled with 30 ml of 0.5 M H,SO, and cyclic voltammograms were run at 1 mV/s in a potential envelope between -0.240 V and 1.0 V SCE. This was done to check complete wetting of the catalyst. The voltammograms usually stabilized after three cycles. After this the cell was drained and refilled with a 0.5 M H,SO, + 4 X 10e4 M CuSO, electrolyte. The cell was purged with nitrogen and the working electrode was held at a potential of 0.050 V (SCE) to deposit the UPD layer. The 30 ml of electrolyte contained 1.2 X lop5 M Cu’+. However, on the basis of the above calculations, 1.76 X lo-’ M Cu2+ are needed to form a monolayer. Excess Cu2+ was furnished by removing 20 ml of the electrolyte after 1 h and refilling with fresh deaerated electrolyte. After removal of the electrolyte the electrolyte level was still one quarter way up the working and auxiliary electrodes. This procedure was repeated twice. In the run reported here the electrode was held at 0.050 V for an additional 3h after the final electrolyte addition. Then 20 ml of the electrolyte were removed to lower the electrolyte level below that of the cell windows. Nitrogen purging was continued and XAS measurements were done both at the Cu K-edge and the Pt L,,,-edge. Two aspects of the experiment that had to be addressed were contributions to the X-ray absorption by species in the electrolyte and control of the UPD process after lowering the electrolyte. The electrode had a 40% porosity. So at the most it would contain 9.5~10~’ g/cm* of Cu2+ ions, a Apx of only 2.4 x 10e4. With a full monolayer coverage of Cu on the Pt the contribution of electrolyte species to the overall absorption step would be only 0.24%. The UPD process occurs under diffusion controlled conditions and good current distribution and electrolyte stirring are vital. However, once a UPD layer is formed it can be maintained by potential control even under conditions with poor primary current distribution. The polariza-
234
tion resistance (an/ai) for the process is high and in the steady state a~/i3i + co. Thus there is a condition of infinite throwing power. So the UPD layer can be maintained in the lowered electrolyte as long as there is no other Faradaic process with a lower steady state polarization resistance. This can be achieved if oxygen is carefully excluded from the cell. XAS measurements The XAS measurements were done on the dispersive EXAFS Beam Line at LURE-CNRS. The characteristics of this beam line have been described previously [57,58]. Apart from the aspect of time resolved measurements, the advantages of this beam line are lack of mechanical movement in the monochromator and fast measurement times. The lack of mechanical movement in the monochromator permits measurements of energy shifts at the adsorption edge with great accuracy [59]. The measurement time for each spectrum was only 4 s. In addition to in-situ measurements with the cell XAS data were obtained on Cu foil, Pt foil, Cu,O, CuO and 0.1 M CuSO,. RESULTS
AND DISCUSSION
XANES for copper standard materials Figure 2 shows a set of normalized XANES spectra for Cu foil, Cu,O and CuO. The data are essentially identical to those published recently [60] and indicate the
ENERGY/eV Fig. 2. Normalized XANES spectra for Cu foil ( ---),Cu,O(---)andCuO(-+-+-).Zero energy is taken as the first inflection in the XANES for Cu foil.
235
high resolution that can be obtained on the dispersive beam line. The edge for divalent Cu is shifted approximately 8 eV above the edge for Cu foil. The edge shift between Cu and Cu+ is small (about 1.5 eV) and this shift is evident mostly below the first peak in the XANES. Some slight changes in the Cu+ edge have been observed by Tranquada et al., depending on whether the sample was a solid or an ion in solution [60]. However, they concluded that the overriding factor determining edge positions is the absence or presence of 3d or 4s electrons. So the edge position can determine the valence. This has been borne out in the recent measurements of Kau et al. on 19 Cu+ and 40 Cu2+ compounds [61]. The edge positions for all Cu+ compounds were the same. Changes in Cu+ coordination introduced various peaks and shoulders in the XANES spectra. In-situ XANES at Cu K-edge Figure 3 shows a comparison of normalized XANES data for Cu, Cu,O and UPD Cu on carbon-supported platinum. The respective step heights for the UPD sample were 0.057 at the Cu K-edge and 0.26 at the Pt Lit,-edge. This indicates a Cu coverage of 0.57 on the Pt. This is probably a good estimate since the amount of Pt and Cu present was determined directly from the respective step heights in the XAS measurements. The surface area is based on a Pt particle size of 2 nm. An expanded view of the region below the first peak is shown in Fig. 3b. The absorption edge for the UPD Cu is clearly that for Cu+. The first peak was less well defined and was shifted to 1.5 eV higher than that for Cu,O. This is typical for Cu+ compounds with tetrahedral symmetry [61]. A small reproducible shoulder was observed, at lower energies, at the beginning of the edge. This structure is characteristic of tetrahedrally coordinated Cu+ compounds [61] and was also seen in the work of Tourillon et al. on Au(100) [48]. All of this suggests that UPD Cu is in the +1 state and that the coordination is tetrahedral. There are two possibilities for tetrahedral coordination of the copper. One is copper adsorption at a 3-fold hollow site on the Pt and coordination to a single oxygen from the sulfate species. The other is coordination to a single Pt atom together with coordination to three oxygens from the sulfate-. species. Determination of the exact coordination awaits a detailed EXAFS study. So far the results indicate that the atomic arrangements are probably similar to those suggested for UPD Cu on Au [46-481. In-situ XANES at Pt L,,, edge Figure 4 shows a comparison of the XANES for Pt foil and carbon-supported platinum with a layer of UPD Cu. With the adsorption of Cu there is a decrease in the area under the white line. This represents a change in the electronic properties of the Pt consistent with a partial filling of empty d-states on the adsorption of Cu [52]. Increases in the area under the white line are observed on adsorption of hydrogen from the gas phase [53] and in electrochemical experiments [62]. Adsorption of anions such as phosphate also increase the area under the white line [62]. In both cases there is a partial emptying of d-states in the Pt. On a partially covered surface at 0.050 V (SCE) there are most likely the opposing contributions of adsorbed
236
r :I
0.6 +
*i
0.4 i
:r
i
+i
ENERGYleV
i
(b) . / . . . . f ;
./
l*..
ENERGYfeV Fig. 3. (a) Normalized XANES for Cu foil (+ + +), Cu,O (. .) and UPD Cu (- - -) on carbon-supported Pt at 0.050 V (SCE); (b) an expanded view of lower part of the absorption edges.
hydrogen on one hand, and the adsorbed Cu on the other. Since the net effect is a reduction in area there is very strong evidence for electron transfer to the Pt on adsorption of Cu. Since this is a net effect of at least two contributions the partial
237
Fig. 4. Norrnaliied XANES for Pt foil (- - -) and carbon-supported Pt with UPD Cu at 0.050 V SCE (). filling per atom of Cu cannot be calculated, even by doing experiments at both the Pt L,, and L,,, edges [52]. The transfer of electrons to Pt does not agree with conclusions from work with vapor-deposited Cu on Pt [28]. However, the two cases are not really comparable. Vapor deposition yields only adsorbed Cu. The XANES at the Cu K-edge and preli~na~ EXAFS results [63] indicate that the Cu is also coordinated with SO:- ions. This could aid in the hybridization of the Cu3d and PtSd states. Modification in the d-electrons of Cu by adsorbed SOihas also recently been invoked to explain second harmonic generation (SHG) results on UPD Cu on Au [64]. The SHG results indicated reduced delocalization of Cu d-electrons, even at high coverages. The ejjecr
oj oxygen When oxygen was introduced into the cell during the XAS experiments several changes occurred rapidly. The Cu XANES shifted to the Cu2+ edge and was identical to that for a CuSO, electrolyte [60]. The area under the Pt XANES white line also increased. When the electrolyte level was raised there was a large cathodic current due to the redeposition of the UPD Cu. If oxygen was excluded carefully none of these effects were seen over a period of hours. Oxygen could be excluded by vigorous bubbling with purified nitrogen and the use of a long thin exit tube for the gas to prevent back diffusion of air, particularly during the electrolyte lowering operation.
238
The nature of the LJPD layer The idea of having other than an adsorbed metallic species, beyond very low coverages, has generally been dismissed on the basis of electrostatic arguments. Even though Horanyi had warned about specific co-adsorption of anions [21], and adsorbed anions were seen early in ex-situ XPS experiments [24], this was largely ignored till recent spectroscopic evidence confirmed the radiotracer results [22,46481. The present work also shows that there can be transfer of electrons to the substrate metal. The XANES data can be interpreted only in terms of the presence of cu+ species. The electroneutrality condition is fulfilled by co-adsorption of anions and transfer of electrons to the substrate metal. These results, and recent quartz microbalance data [65], show the great difficulty in interpreting electroanalytical experiments, such as cyclic voltammetry, when specific adsorption of ions is involved. In addition to the metalladsorbate interactions, cation adsorption appears to involve invariably coadsorption of anions [66] and there is now evidence that anion adsorption promotes co-adsorption of cations [67]. Even with the application of various in-situ and ex-situ spectroscopies the study of these systems presents formidable difficulties [68]. Unambiguous methods of determining what and how much is adsorbed are needed. Also needed are probes of the chemical environment of the adsorbed species and substrates. This will require a combination of in-situ techniques such as X-ray methods, radiotracer studies, scanning tunnelling microscopy and the application of cleverly chosen ex-situ methods. The present results also show that XAS can be used to study electronic and structural modifications of platinum catalysts by UPD species. Further work will be done on systems that are known to promote the oxidation of small organic molecules such as methanol. ACKNOWLEDGEMENTS
The authors acknowledge funds from a grant from the NATO Scientific Affairs Division (Grant No. 86/532). This work was performed under the auspices of the S.S. DOE under Contract No. DE-AC02-76CH00016. REFERENCES 1 2 3 4 5 6 7 8 9 10
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