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Infrared reflection absorption spectra of carbon monoxide adsorbed on single crystal electrodes, Pd(111) and Pd(100)
90
Surface Science 227 (1990) 90-96 born-Holl~d
1~~~ APRON ABS~~ION SPECTRA OF CARBON MONO~DE ADSORBED ON SINGLE CRYSTAL ELECTRODES, Pd( 111) AND Pd( 100) Kenjiro YOSHIOKA, and Masatoki IT0
Fusao KITAMU~,
~itsu~
TAKEDA,
Machiko TA~HASHI
Department of Chemistry, Fact&y of Science and Technology, Keio University, Kohoku-ku, Yokohama 223, Japan Received 18 July 1989; accepted for publication 9 November 1989
Carbon monoxide (CO) adsorption on Fd(100) and Pd(ll1) single crystal electrode surfaces in an aqueous acid solution was studied by FT-WAS. A surface preparation technique of well-ordered single crystal palladium electrodes was established by using i~ine-moating and ~e~-qu~c~~8 of the electrodes On Pd(lll), b~d~~-~nd~ (1~-1~6~ cm-‘) and three-fold (lo-189~ cm -‘) CO was detected, while only the former species was observed on Pd(100). The structural changes in the CO overlayer were induced by the electrode potential and/or the coverage of CO on the electrode surfaces. The results were compared with those from previous UHV studies.
It is of fundamental importance to study the changes in the morphology of electrode surfaces which are subjected to various electrochemical treatments [l-4]. Also, elucidation of molecular motion at the surfaces is essential in revealing the kinetics of adsorption, desorption, and surface chemical reactions [5,6]. In parallel with the progress in rJHV research, Fourier transform infrared spectroscopy (FT-IR), surface Raman spectroscopies as well as scanning tunneling microscopy have been developed actively, because these techniques enable in-situ elucidation of adsorbed molecules on substrate atoms. The use and application of single crystals is a requisite to the understanding of electrochemical surface science on a microscopic, molecular level. Therefore, it is quite important to develop a nonvacuum, well-defined electrode surface preparation technique and to characterize the surfaces at a molecular level. In this regard, the pioneering contribution by Clavilier et al. [7] has been widely recognized. Several preparation methods [g$] such as anneal-quenc~ng and iodine-covering procedures as well as electrochemical activation have been reported to give clean platinum single crystal 0039~6028/90/$03.50 @ Elsevier Science Publishers B.V. (forth-Holland)
surfaces. The usefulness of these methods has already been justified by LEED/AES [3] and spectroelectrochemical characterizations [ 11 of vacuum-prepared surfaces and by LEED,/AES verifications of the immersed electrodes. In contrast to platinum electrodes, no extensive treatments of p~la~ium single crystal electrode surfaces, or surface characterization of immersed palladium electrodes have been reported so far, because of the difficulty of surface characterization of Pd single crystal electrodes. Moreover, repetitive cycling of Pd single crystal electrodes in the potential range of oxide fo~ation and reduction produces a reconstruction of the metal surface [lo]. Recently, Chiercbie and Mayer [ll] showed that the single crystal Pd{l~) and Pd(ll1) surfaces could be prepared by using the super-jet electropolishing technique. However, this method did not give satisfactory results from a spectroelectrochemical point of view, We adopted a modified method based on well developed anneal-quen~hing and iodine-coating techniques. In the present paper, we report on the infrared reflection absorption spectroscopy (IRAS) verification of the cleanliness and order of Pd(lO0) and Pd(ll1) electrode surfaces, and describe the spectral changes of adsorbed carbon monoxide (CO}
K. Yoshioka et al. / CO adsorption on Pd(ll1)
as a function of electrode potential and CO coverages. The results are compared with those of UHV-characterized Pd surfaces [12,13].
and Pd(lO0) single crystal electrodes
V equipped with a MCT resolution was 8 cm-‘.
detector.
91
The spectral
3. Results 2. Experimental 3.1. Cyclic voltammograms The Teflon thin layer spectroelectrochemical cell as well as the glassware were cleaned in chromic acid prior to each day’s experiments. The palladium single crystals used were disks of about 2 mm thick and 8 mm in diameter, which have (100) or (111) orientations (Metal Crystals, Ltd., UK). A palladium wire of 1 mm diameter was spot-welded to one end of the disk, and the other face was used for the measurements. Details of the electrochemical IR cell were described elsewhere [14]. Single crystal Pd electrodes were mechanically polished with progressively finer alumina up to 0.06 pm. In order to remove the rough surface layers left by mechanical polishing, the electrode was electropolished in a non-aqueous bath containing 0.5 M LiCl and 0.2 M Mg(ClO,), dissolved in methanol. After rinsing in water, iodine was adsorbed from a 1 mM KI aqueous solution. The iodine-coated crystal was transferred to the furnace. After annealing at 1000° C for several minutes, the crystal was quenched rapidly in pure water. These processes were necessary before well-defined voltammograms and subsequent reproducible IRAS data could be obtained. The surface treatments adopted in this work eliminate the need of preactivation of the Pd electrodes by cycling into positive potentials. CO was introduced by bubbling the gas into the electrochemical cell. The CO coverages on the Pd electrode were controlled by partial oxidation of the electrode by positive potential cycles up to +0.4 V (versus Ag/AgCl) for variable times. The electrode potential was controlled with a potentiostat (2001, Toho Technical Research) and a function generator. The reference electrode was KC1 saturated Ag/AgCl, and the counter electrode was a platinum foil. All solutions were prepared with ultrapure water supplied by a Milli-Q system (Millipore, Inc.) and super special grade sulfuric acid (Wako Pure Chemical Ind.). The Fourier transform infrared spectrometer used was a Biorad 40
(CV)
To test the cleanliness and order of the Pd crystals at various stages in the preparation procedures, we employed the underpotential electrodeposition (UPD) of copper atoms on Pd(ll1) or Pd(lOO) surfaces and each surface was subjected to characterization by cyclic voltammograms. To perform the experiments, each surface was immersed in a 1 mM CuSO, + 5 mM H,SO, + 250 mM Na,SO, solution. The potential was then scanned in the negative direction at 10 mV/s until the onset of copper bulk deposition. At this point the scan direction was reversed and scanning was stopped at a potential before copper oxidation. The system Cu/Pd( hkl) has the advantage that desorption of Cu atoms due to stripping occurs before the onset of oxide formation. Typical voltammograms on Pd(lOO) and Pd(ll1) are shown in fig. 1. The surfaces prepared without the I-coating procedure gave similar voltammograms, but substantially different IRAS spectra from those prepared by the present treatment. Polycrystalline Pd or insufficiently electropolished surfaces gave completely different CV and IRAS results. The copper UPD peak is very sensitive to the order of the crystal surface. Well-resolved UPD peaks, in fig. 1, are indicative of a high degree of crystal surface order. Chierchie and Mayer [ll] suggested that on both (111) and (100) Pd surfaces, Cu atoms adsorbed epitaxially with the Pd substrates. Also, data [15] from vapor-phase deposition under UHV indicate that Cu grows epitaxially with the Pd lattice and no alloy is formed. However, it is found that UPD Cu adatoms do not adsorb epitaxially on Pd electrodes, as will be described later. 3.2. CO/Pd(lOO) Fig. 2a shows the electrode potential dependence of the IRAS spectra of CO adsorbed on a
92
2o1
(a)
-200w
E(V)
Fig. 1. Cyclic voltammograms for the system Cu*+/Pd( hkl) in 1~10~~ M CuSO.,+5~10-~ M H,SO,+0.25 M Na,SO, solution. (a) Pd(100) and (b) Pd(ll1) electrodes. Scan rate was 50 mV/s. The electrode potential was referred to KC1 saturated Ag/AgCl.
2000 Wavenumber
1800 (cm-’ 1
Pd(lOO) surface at saturation coverage in a 0.25 M Na,SO, solution. The measurements were carried out stepwise starting at -0.4 towards more positive potentials. The absorption peak at 1942 cm-’ was shifted to a higher frequency with a positive potential change. The spectral change was completely reversible with the electrode potential. The intensity of the band is almost constant throughout the potential region -0.4 to +0.4 V. The intensity decrease at +0.6 V is due to partial oxidation of CO. The frequency shift caused by the potential change is explained by a change in the degree of backdonation of metal electrons into CO 2n* orbitals [5,16]. Fig. 2b shows the CO coverage (T,) dependence of the IRAS spectra at -0.4 V. T, values are defined as relative coverages of CO on the surface and they differ from the absolute conventional values (&,). The spectrum at saturation coverage (& = 1.0) corresponds to that in fig. 2a. At saturation coverage, the peak at 1942 cm-l is sharper compared with those of the lower coverages. With a decrease in coverage of CO, the band position moves to a lower frequency. At
2200
2doo Wavenumber
1600 (cm“)
Fig. 2. (a) IRA!3 of CO adsorbed on Pd(100) as a function of electrode potential. The spectra were measured at saturation coverage of CO. (b) IRAS of CO adsorbed on Pd(lO0) as a function of CO coverage. The spectra were measured at a fixed electrode potential of E= -0.4 v.
93
K. Yoshioka et ai. / CO aatsotption on P&l I I) and Pd(I 00) single crystal elemodes
lower coverages, an additional band occurs as shoulders, resulting in band broadening. At the lowest CO coverage, the band appears at 1898 cm-‘. Thus, the coverage dependent frequency shifts from 1898 to 1942 cm-i (44 cm-‘) at a constant electrode potential of - 0.4 V. The coverage dependent frequency shifts at other electrode potentials were almost similar to those at - 0.4 V. 3.3. CO/Pd(lil) From the IRAS spectra of saturated CO ,on Pd(ll1) electrode surface in 0.25 M aqueous Na,SO, solution, two kinds of absorption bands, centered at 1925 and 1882 cm-‘, appeared at -0.4 V. IRAS measurements on the electrode potential change were examined only at saturation coverage of CO. The relative intensity of these two absorption bands was not changed by the positive potential sweep until +0.4 V, but the peaks move to higher frequencies with 30 cm-‘/V at this coverage. Coverage dependence of IRAS spectra from a Pd(ll1) surface at a fixed electrode potential was also investigated. Fig. 3a shows the coverage dependence of IRAS spectra at - 0.4 V. The band at
a
I
2200
I
0.2 %
1840
2000
Wavenumber
higher frequency appears predominantly at high coverages. The intensity of the band decreases more steeply than the band at lower frequency with the reduction of the total coverage. Finally, only the band at lower frequency (1840 cm-i) remains at very low coverage. During these intensity changes, frequencies of both bands move to lower frequencies with a similar amount (42 cm-‘) as found on the Pd(lOO) surface. The changes in intensities are depicted more clearly in fig. 3b in which the integrated intensities of both bands were plotted against the total integrated intensity (T,). The intensities of both bands decrease linearly with the decrease in total coverage. The reduction rates of both species are similar until around T, = 0.3, where the intensity of the higher frequency band (0) starts to reduce more steeply and that of the lower frequency band (0) increases slightly and then decreases until To= 0. We examined the To dependence at another electrode potential and obtained similar results for the intensity changes. Note that a clear discontinuity can be seen near Cr,= 0.3, which suggests the occurrence of a surface structural change in the adsorbed layer at this coverage.
T3
1800 (cm-l 1
Oo:50 Total
coverage
Fig. 3. (a) IRAS of CO adsorbed on Pd(ll1) as a function of CO coverage. The spectra were.measured at a fixed electrode potential of E = -0.4 V. (b) The change of the fractional coverages of two-fold (0) and three-fold (0) CO as a function of the total coverage.
94
K. Yoshioka et al. / CO adsorption on Pd(l I I) and Pd(100) single crystal electrodes
4. Discussion 4.1. Adsorption surfaces
sites of CO on Pd(100) and Pd(ll1)
The Pd(100) surface offers a unique advantage to study the structure of adsorbed CO because only the two-fold bridge site can be occupied irrespective of CO coverages. In the IRAS of CO on Pd(lOO) from the UHV study, the band starts to appear at 1900 cm-‘, and develops in intensity with a coverage dependent frequency shift [12,13]. The present result from the Pd(lOO) electrode system reproduces quite well the IRAS of the UHV system, except for the absence of the unusual high C-O stretch around 1990 cm-’ due to uniaxial compression at large coverage found in the UHV system [12]. The appearance of a single band is indicative of a single adsorption site. Also the appearance of the narrow bandwidth at full coverage in fig. 2a means that well-ordered two-fold bridge CO is formed on the Pd(100) electrode surface as seen in the c(4 x 2) structure in the UHV system [13]. The bridge species on Pd(lOO) is so stable that no interconversion of bridge to on-top species due to electrode potential change occurs, in contrast with CO on the Pt(lOO) electrode 1161. The broadening of the band associated with the decrease in coverage is explained by disordering of adsorbed CO (131. The two kinds of absorption bands of CO on the Pd(ll1) electrode surface, are assignable to CO stretch vibration bands in two-fold and threefold hollow sites for the following reasons. (1) The
frequencies of both bands are in good agreement with the respective values of bridge and three-fold CO vibrations from the corresponding UHV studies [13]. (2) It is impossible to assign the lower frequency band to bridge-bonded species since the two frequencies are excessively apart (- 50 cm-‘). (3) It is also unreasonable to assign the band at lower frequency to CO molecules adsorbed at defect sites, since the absorption intensity is too large. The intensities of both bands are almost comparable, as shown in fig. 3b. 4.2. Comparison
with the UHV study
The frequency values of CO on Pd(lOO) and Pd(ll1) surfaces in the UHV work and the present electrode results were listed in table 1. A single species (bridge-bond) on Pd(100) and two kinds of species (bridge and three-fold) observed on Pd(ll1) in the UHV system [13] are also observed on the present single crystal electrode surfaces. Furthermore, the frequencies of those species on electrode surfaces are in good agreement with the values from the UHV results if we compare those with the frequencies from positive potential conditions. This suggests that the electronic energy level of UHV surfaces corresponds to a level at a more positive potential than PZC. Coverage dependent intensity changes of each band from UHV surfaces are also reproduced by partial oxidation on the present electrode surfaces. However, some important differences are found in both electrode and UHV results. On Pd(lOO)
Table 1 Comparison of the CO frequency values on single crystal Pd(100) and Pd(ll1) surfaces in the UHV (300 K) work and the present electrode results (V versus Ag/AgCl) Pd(ll1)
Pd(100)
UHV Electrode - 0.4 V + 0.2 v a) b, ‘) d,
Bridge CO Three-fold Bridge CO Bridge CO
Ref.
Y a) (cm-‘)
Y (cm-‘)
Ref.
Y a) (cm-‘)
Y ‘) (cm-‘)
(121
1964 1942 1960
1895 =) 1898 d,
[I31
1948 1925 1946
1813-1836 1840
at saturation coverage. CO. at T, = 0.05. at T, = 0.07.
K. Yoshioka et al. / CO adsorption on Pd(l II) and Pd(lO0) single crystal electrodes
under UHV conditions, large exposure of CO leads to an unusual compression of the c(4 X 2) structure and/or appearance of on-top CO species [13] at low temperature (100 K), whereas no such species appears on the electrode surfaces. The (111) surface for a variety of transmission metals shows ordering for adsorbed CO into a (6 x fi)R30 o structure [13,17]. The formation of a (6 x fi)R30° structure is also found for CO on Pd(ll1) at a coverage less than flc, = 0.3, and a c(4 x 2) structure at &co = 0.5 is created by continuous compression of the (fi X fi)R30 o unit cell. Therefore, the species at the three-fold hollow site at low coverage in the present Pd(ll1) electrode surface might form such a (0 x fi)R30“ structure as found on UHV surfaces. At higher coverage both two-fold and three-fold species co-exist on the Pd(ll1) electrode surface in a wide potential range. The remarkable structure change at To= 0.3 means that bridge-bonded CO easily desorbs but not three-fold CO, or a part of the bridge CO is changed into three-fold CO at about this coverage. The desorption of three-fold CO occurs only after complete disappearance of the bridge CO. On a Pd(ll1) surface under UHV condition, three-fold CO species move into bridge sites at higher coverages. However, the three-fold CO species still exists along with the bridge species on a Pd(ll1) electrode surface at negative potentials. The Pd(ll1) electrode surface at cathodic conditions stabilizes three-fold species (feasibility of back donation). Therefore the electronic state of the surface under UHV condition corresponds to the state of the electrode surface at positive potentials as described above. When annealing the Pd(lOO) electrode without iodine coating, a new absorption band appeared at - 1845 cm-’ at low coverages which is not assignable to bridge-bonded CO. The band can reasonably be assigned to three-fold CO adsorbed on the Pd(ll1) surface from the frequency. The additional band at lower frequency completely disappeared by depositing iodine before annealing, resulting in a single species of bridge-bonded CO. It is known that annealing (100) surfaces of fee metals under atmospheric conditions yields (111) facets. Therefore, Pd(ll1) facets occur on Pd(lOO) during the annealing process. The adsorbed iodine
95
layer on Pd(lOO) could protect the facet formation during annealing at elevated temperatures. We have examined the CO adsorption on copper adatoms deposited (UPD) on Pd(100) or Pd(ll1) surfaces. The coverage of copper (0 = 0.1-0.3) was estimated from the total oxidation charge by the linear potential sweep method, thus the palladium atoms are fairly exposed on the surface. IRAS on these surfaces showed an absorption of on-top CO (2110 cm-‘) on Cu atoms in addition to the usual absorptions of multi-bonded CO on Pd atoms. The higher frequency of CO adsorbed on copper (2110 cm-‘) suggests that copper atoms do not form a low index surface on Pd electrodes but form high index surfaces (island structure) from a comparison with the frequencies of the UHV results [18]. This conclusion differs from the result by Chierchie and Mayer [ll].
5. Conclusion We have established the preparation procedures of atomically smoothed, well-oriented single crystal Pd(lOO) and Pd(ll1) electrode surfaces by using combined techniques of I-coating and anneal-quenching. The intensity and the frequency behavior of CO adsorbed on these electrode surfaces are roughly similar to the results from corresponding Pd surfaces under UHV conditions. The large stability of multi-bonded CO species at negative potentials is ascribed to the favorable back-donation of Pd d-electrons to CO 2s* orbitals.
References [l] A.T. Hubbard, Chem. Rev. 88 (1988) 642. [2] F.T. Wagner and P.N. Ross, Jr., J. Electroanal. Chem. 150 (1983) 141. [3] M. Wasberg, L. Palaikis, S. Waller, M. Kamrath and A. Wieckowski, J. Electroanal. Chem. 256 (1988) 51. [4] D. Aberdam, R. Durand, R. Fame and F. El-Omar, Surf. Sci. 171 (1986) 303. [5] K. Ashley and S. Pons, Chem. Rev. 88 (1988) 673. [6] B.N.J. Perrson and A.M. Bradshaw, Surf. Sci. 213 (1989) 49. [7] J. Clavilier, R. Faure, G. Guinet and R. Durand, J. Electroanal. Chem. 107 (1980) 205.
96
K. Yoshioka et al. / CO adsorption on Pd(lll)
[8] D. &raw&i, L. Rice, M. Hourani and A. Wieckowski, J. Electroanal. Chem. 230 (1987) 221. [9] D. Armand and J. Clavilier, J. Electroanal. Chem. 225 (1987) 205. [lo] P.C. Andricacos and P.N. Ross, J. Electroanal. Chem. 167 (1984) 301. [ll] T. Chierchie and C. Mayer, Electrochim. Acta 33 (1988) 341. [12] A. Ortega, F.M. Hoffmann and A.M. Bradshaw, Surf. Sci. 119 (1982) 79. [13] A.M. Bradshaw and F.M. Hoffmann, Surf. Sci. 72 (1978) 513.
and Pd(100) single crystal electrodes [14] F. Kitamura, M. Takahashi and M. Ito, Chem. Phys. Lett. 142 (1987) 318. [15] H. Asonen, C. Barnes, A. Salokatue and A. Vuoristo, Appl. Surf. Sci. 22/23 (1985) 556. [16] F. Kitamura, M. Takahashi and M. Ito, J. Phys. Chem. 92 (1988) 3320. [17] F. Kitamura, M. Takahashi and M. Ito, Surf. Sci. 223 (1989) 493. [18] J. Pritchard, T. Catterick and R.K. Gupta, Surf. Sci. 53 (1975) 1.
Surface Science 227 (1990) 90-96 born-Holl~d
1~~~ APRON ABS~~ION SPECTRA OF CARBON MONO~DE ADSORBED ON SINGLE CRYSTAL ELECTRODES, Pd( 111) AND Pd( 100) Kenjiro YOSHIOKA, and Masatoki IT0
Fusao KITAMU~,
~itsu~
TAKEDA,
Machiko TA~HASHI
Department of Chemistry, Fact&y of Science and Technology, Keio University, Kohoku-ku, Yokohama 223, Japan Received 18 July 1989; accepted for publication 9 November 1989
Carbon monoxide (CO) adsorption on Fd(100) and Pd(ll1) single crystal electrode surfaces in an aqueous acid solution was studied by FT-WAS. A surface preparation technique of well-ordered single crystal palladium electrodes was established by using i~ine-moating and ~e~-qu~c~~8 of the electrodes On Pd(lll), b~d~~-~nd~ (1~-1~6~ cm-‘) and three-fold (lo-189~ cm -‘) CO was detected, while only the former species was observed on Pd(100). The structural changes in the CO overlayer were induced by the electrode potential and/or the coverage of CO on the electrode surfaces. The results were compared with those from previous UHV studies.
It is of fundamental importance to study the changes in the morphology of electrode surfaces which are subjected to various electrochemical treatments [l-4]. Also, elucidation of molecular motion at the surfaces is essential in revealing the kinetics of adsorption, desorption, and surface chemical reactions [5,6]. In parallel with the progress in rJHV research, Fourier transform infrared spectroscopy (FT-IR), surface Raman spectroscopies as well as scanning tunneling microscopy have been developed actively, because these techniques enable in-situ elucidation of adsorbed molecules on substrate atoms. The use and application of single crystals is a requisite to the understanding of electrochemical surface science on a microscopic, molecular level. Therefore, it is quite important to develop a nonvacuum, well-defined electrode surface preparation technique and to characterize the surfaces at a molecular level. In this regard, the pioneering contribution by Clavilier et al. [7] has been widely recognized. Several preparation methods [g$] such as anneal-quenc~ng and iodine-covering procedures as well as electrochemical activation have been reported to give clean platinum single crystal 0039~6028/90/$03.50 @ Elsevier Science Publishers B.V. (forth-Holland)
surfaces. The usefulness of these methods has already been justified by LEED/AES [3] and spectroelectrochemical characterizations [ 11 of vacuum-prepared surfaces and by LEED,/AES verifications of the immersed electrodes. In contrast to platinum electrodes, no extensive treatments of p~la~ium single crystal electrode surfaces, or surface characterization of immersed palladium electrodes have been reported so far, because of the difficulty of surface characterization of Pd single crystal electrodes. Moreover, repetitive cycling of Pd single crystal electrodes in the potential range of oxide fo~ation and reduction produces a reconstruction of the metal surface [lo]. Recently, Chiercbie and Mayer [ll] showed that the single crystal Pd{l~) and Pd(ll1) surfaces could be prepared by using the super-jet electropolishing technique. However, this method did not give satisfactory results from a spectroelectrochemical point of view, We adopted a modified method based on well developed anneal-quen~hing and iodine-coating techniques. In the present paper, we report on the infrared reflection absorption spectroscopy (IRAS) verification of the cleanliness and order of Pd(lO0) and Pd(ll1) electrode surfaces, and describe the spectral changes of adsorbed carbon monoxide (CO}
K. Yoshioka et al. / CO adsorption on Pd(ll1)
as a function of electrode potential and CO coverages. The results are compared with those of UHV-characterized Pd surfaces [12,13].
and Pd(lO0) single crystal electrodes
V equipped with a MCT resolution was 8 cm-‘.
detector.
91
The spectral
3. Results 2. Experimental 3.1. Cyclic voltammograms The Teflon thin layer spectroelectrochemical cell as well as the glassware were cleaned in chromic acid prior to each day’s experiments. The palladium single crystals used were disks of about 2 mm thick and 8 mm in diameter, which have (100) or (111) orientations (Metal Crystals, Ltd., UK). A palladium wire of 1 mm diameter was spot-welded to one end of the disk, and the other face was used for the measurements. Details of the electrochemical IR cell were described elsewhere [14]. Single crystal Pd electrodes were mechanically polished with progressively finer alumina up to 0.06 pm. In order to remove the rough surface layers left by mechanical polishing, the electrode was electropolished in a non-aqueous bath containing 0.5 M LiCl and 0.2 M Mg(ClO,), dissolved in methanol. After rinsing in water, iodine was adsorbed from a 1 mM KI aqueous solution. The iodine-coated crystal was transferred to the furnace. After annealing at 1000° C for several minutes, the crystal was quenched rapidly in pure water. These processes were necessary before well-defined voltammograms and subsequent reproducible IRAS data could be obtained. The surface treatments adopted in this work eliminate the need of preactivation of the Pd electrodes by cycling into positive potentials. CO was introduced by bubbling the gas into the electrochemical cell. The CO coverages on the Pd electrode were controlled by partial oxidation of the electrode by positive potential cycles up to +0.4 V (versus Ag/AgCl) for variable times. The electrode potential was controlled with a potentiostat (2001, Toho Technical Research) and a function generator. The reference electrode was KC1 saturated Ag/AgCl, and the counter electrode was a platinum foil. All solutions were prepared with ultrapure water supplied by a Milli-Q system (Millipore, Inc.) and super special grade sulfuric acid (Wako Pure Chemical Ind.). The Fourier transform infrared spectrometer used was a Biorad 40
(CV)
To test the cleanliness and order of the Pd crystals at various stages in the preparation procedures, we employed the underpotential electrodeposition (UPD) of copper atoms on Pd(ll1) or Pd(lOO) surfaces and each surface was subjected to characterization by cyclic voltammograms. To perform the experiments, each surface was immersed in a 1 mM CuSO, + 5 mM H,SO, + 250 mM Na,SO, solution. The potential was then scanned in the negative direction at 10 mV/s until the onset of copper bulk deposition. At this point the scan direction was reversed and scanning was stopped at a potential before copper oxidation. The system Cu/Pd( hkl) has the advantage that desorption of Cu atoms due to stripping occurs before the onset of oxide formation. Typical voltammograms on Pd(lOO) and Pd(ll1) are shown in fig. 1. The surfaces prepared without the I-coating procedure gave similar voltammograms, but substantially different IRAS spectra from those prepared by the present treatment. Polycrystalline Pd or insufficiently electropolished surfaces gave completely different CV and IRAS results. The copper UPD peak is very sensitive to the order of the crystal surface. Well-resolved UPD peaks, in fig. 1, are indicative of a high degree of crystal surface order. Chierchie and Mayer [ll] suggested that on both (111) and (100) Pd surfaces, Cu atoms adsorbed epitaxially with the Pd substrates. Also, data [15] from vapor-phase deposition under UHV indicate that Cu grows epitaxially with the Pd lattice and no alloy is formed. However, it is found that UPD Cu adatoms do not adsorb epitaxially on Pd electrodes, as will be described later. 3.2. CO/Pd(lOO) Fig. 2a shows the electrode potential dependence of the IRAS spectra of CO adsorbed on a
92
2o1
(a)
-200w
E(V)
Fig. 1. Cyclic voltammograms for the system Cu*+/Pd( hkl) in 1~10~~ M CuSO.,+5~10-~ M H,SO,+0.25 M Na,SO, solution. (a) Pd(100) and (b) Pd(ll1) electrodes. Scan rate was 50 mV/s. The electrode potential was referred to KC1 saturated Ag/AgCl.
2000 Wavenumber
1800 (cm-’ 1
Pd(lOO) surface at saturation coverage in a 0.25 M Na,SO, solution. The measurements were carried out stepwise starting at -0.4 towards more positive potentials. The absorption peak at 1942 cm-’ was shifted to a higher frequency with a positive potential change. The spectral change was completely reversible with the electrode potential. The intensity of the band is almost constant throughout the potential region -0.4 to +0.4 V. The intensity decrease at +0.6 V is due to partial oxidation of CO. The frequency shift caused by the potential change is explained by a change in the degree of backdonation of metal electrons into CO 2n* orbitals [5,16]. Fig. 2b shows the CO coverage (T,) dependence of the IRAS spectra at -0.4 V. T, values are defined as relative coverages of CO on the surface and they differ from the absolute conventional values (&,). The spectrum at saturation coverage (& = 1.0) corresponds to that in fig. 2a. At saturation coverage, the peak at 1942 cm-l is sharper compared with those of the lower coverages. With a decrease in coverage of CO, the band position moves to a lower frequency. At
2200
2doo Wavenumber
1600 (cm“)
Fig. 2. (a) IRA!3 of CO adsorbed on Pd(100) as a function of electrode potential. The spectra were measured at saturation coverage of CO. (b) IRAS of CO adsorbed on Pd(lO0) as a function of CO coverage. The spectra were measured at a fixed electrode potential of E= -0.4 v.
93
K. Yoshioka et ai. / CO aatsotption on P&l I I) and Pd(I 00) single crystal elemodes
lower coverages, an additional band occurs as shoulders, resulting in band broadening. At the lowest CO coverage, the band appears at 1898 cm-‘. Thus, the coverage dependent frequency shifts from 1898 to 1942 cm-i (44 cm-‘) at a constant electrode potential of - 0.4 V. The coverage dependent frequency shifts at other electrode potentials were almost similar to those at - 0.4 V. 3.3. CO/Pd(lil) From the IRAS spectra of saturated CO ,on Pd(ll1) electrode surface in 0.25 M aqueous Na,SO, solution, two kinds of absorption bands, centered at 1925 and 1882 cm-‘, appeared at -0.4 V. IRAS measurements on the electrode potential change were examined only at saturation coverage of CO. The relative intensity of these two absorption bands was not changed by the positive potential sweep until +0.4 V, but the peaks move to higher frequencies with 30 cm-‘/V at this coverage. Coverage dependence of IRAS spectra from a Pd(ll1) surface at a fixed electrode potential was also investigated. Fig. 3a shows the coverage dependence of IRAS spectra at - 0.4 V. The band at
a
I
2200
I
0.2 %
1840
2000
Wavenumber
higher frequency appears predominantly at high coverages. The intensity of the band decreases more steeply than the band at lower frequency with the reduction of the total coverage. Finally, only the band at lower frequency (1840 cm-i) remains at very low coverage. During these intensity changes, frequencies of both bands move to lower frequencies with a similar amount (42 cm-‘) as found on the Pd(lOO) surface. The changes in intensities are depicted more clearly in fig. 3b in which the integrated intensities of both bands were plotted against the total integrated intensity (T,). The intensities of both bands decrease linearly with the decrease in total coverage. The reduction rates of both species are similar until around T, = 0.3, where the intensity of the higher frequency band (0) starts to reduce more steeply and that of the lower frequency band (0) increases slightly and then decreases until To= 0. We examined the To dependence at another electrode potential and obtained similar results for the intensity changes. Note that a clear discontinuity can be seen near Cr,= 0.3, which suggests the occurrence of a surface structural change in the adsorbed layer at this coverage.
T3
1800 (cm-l 1
Oo:50 Total
coverage
Fig. 3. (a) IRAS of CO adsorbed on Pd(ll1) as a function of CO coverage. The spectra were.measured at a fixed electrode potential of E = -0.4 V. (b) The change of the fractional coverages of two-fold (0) and three-fold (0) CO as a function of the total coverage.
94
K. Yoshioka et al. / CO adsorption on Pd(l I I) and Pd(100) single crystal electrodes
4. Discussion 4.1. Adsorption surfaces
sites of CO on Pd(100) and Pd(ll1)
The Pd(100) surface offers a unique advantage to study the structure of adsorbed CO because only the two-fold bridge site can be occupied irrespective of CO coverages. In the IRAS of CO on Pd(lOO) from the UHV study, the band starts to appear at 1900 cm-‘, and develops in intensity with a coverage dependent frequency shift [12,13]. The present result from the Pd(lOO) electrode system reproduces quite well the IRAS of the UHV system, except for the absence of the unusual high C-O stretch around 1990 cm-’ due to uniaxial compression at large coverage found in the UHV system [12]. The appearance of a single band is indicative of a single adsorption site. Also the appearance of the narrow bandwidth at full coverage in fig. 2a means that well-ordered two-fold bridge CO is formed on the Pd(100) electrode surface as seen in the c(4 x 2) structure in the UHV system [13]. The bridge species on Pd(lOO) is so stable that no interconversion of bridge to on-top species due to electrode potential change occurs, in contrast with CO on the Pt(lOO) electrode 1161. The broadening of the band associated with the decrease in coverage is explained by disordering of adsorbed CO (131. The two kinds of absorption bands of CO on the Pd(ll1) electrode surface, are assignable to CO stretch vibration bands in two-fold and threefold hollow sites for the following reasons. (1) The
frequencies of both bands are in good agreement with the respective values of bridge and three-fold CO vibrations from the corresponding UHV studies [13]. (2) It is impossible to assign the lower frequency band to bridge-bonded species since the two frequencies are excessively apart (- 50 cm-‘). (3) It is also unreasonable to assign the band at lower frequency to CO molecules adsorbed at defect sites, since the absorption intensity is too large. The intensities of both bands are almost comparable, as shown in fig. 3b. 4.2. Comparison
with the UHV study
The frequency values of CO on Pd(lOO) and Pd(ll1) surfaces in the UHV work and the present electrode results were listed in table 1. A single species (bridge-bond) on Pd(100) and two kinds of species (bridge and three-fold) observed on Pd(ll1) in the UHV system [13] are also observed on the present single crystal electrode surfaces. Furthermore, the frequencies of those species on electrode surfaces are in good agreement with the values from the UHV results if we compare those with the frequencies from positive potential conditions. This suggests that the electronic energy level of UHV surfaces corresponds to a level at a more positive potential than PZC. Coverage dependent intensity changes of each band from UHV surfaces are also reproduced by partial oxidation on the present electrode surfaces. However, some important differences are found in both electrode and UHV results. On Pd(lOO)
Table 1 Comparison of the CO frequency values on single crystal Pd(100) and Pd(ll1) surfaces in the UHV (300 K) work and the present electrode results (V versus Ag/AgCl) Pd(ll1)
Pd(100)
UHV Electrode - 0.4 V + 0.2 v a) b, ‘) d,
Bridge CO Three-fold Bridge CO Bridge CO
Ref.
Y a) (cm-‘)
Y (cm-‘)
Ref.
Y a) (cm-‘)
Y ‘) (cm-‘)
(121
1964 1942 1960
1895 =) 1898 d,
[I31
1948 1925 1946
1813-1836 1840
at saturation coverage. CO. at T, = 0.05. at T, = 0.07.
K. Yoshioka et al. / CO adsorption on Pd(l II) and Pd(lO0) single crystal electrodes
under UHV conditions, large exposure of CO leads to an unusual compression of the c(4 X 2) structure and/or appearance of on-top CO species [13] at low temperature (100 K), whereas no such species appears on the electrode surfaces. The (111) surface for a variety of transmission metals shows ordering for adsorbed CO into a (6 x fi)R30 o structure [13,17]. The formation of a (6 x fi)R30° structure is also found for CO on Pd(ll1) at a coverage less than flc, = 0.3, and a c(4 x 2) structure at &co = 0.5 is created by continuous compression of the (fi X fi)R30 o unit cell. Therefore, the species at the three-fold hollow site at low coverage in the present Pd(ll1) electrode surface might form such a (0 x fi)R30“ structure as found on UHV surfaces. At higher coverage both two-fold and three-fold species co-exist on the Pd(ll1) electrode surface in a wide potential range. The remarkable structure change at To= 0.3 means that bridge-bonded CO easily desorbs but not three-fold CO, or a part of the bridge CO is changed into three-fold CO at about this coverage. The desorption of three-fold CO occurs only after complete disappearance of the bridge CO. On a Pd(ll1) surface under UHV condition, three-fold CO species move into bridge sites at higher coverages. However, the three-fold CO species still exists along with the bridge species on a Pd(ll1) electrode surface at negative potentials. The Pd(ll1) electrode surface at cathodic conditions stabilizes three-fold species (feasibility of back donation). Therefore the electronic state of the surface under UHV condition corresponds to the state of the electrode surface at positive potentials as described above. When annealing the Pd(lOO) electrode without iodine coating, a new absorption band appeared at - 1845 cm-’ at low coverages which is not assignable to bridge-bonded CO. The band can reasonably be assigned to three-fold CO adsorbed on the Pd(ll1) surface from the frequency. The additional band at lower frequency completely disappeared by depositing iodine before annealing, resulting in a single species of bridge-bonded CO. It is known that annealing (100) surfaces of fee metals under atmospheric conditions yields (111) facets. Therefore, Pd(ll1) facets occur on Pd(lOO) during the annealing process. The adsorbed iodine
95
layer on Pd(lOO) could protect the facet formation during annealing at elevated temperatures. We have examined the CO adsorption on copper adatoms deposited (UPD) on Pd(100) or Pd(ll1) surfaces. The coverage of copper (0 = 0.1-0.3) was estimated from the total oxidation charge by the linear potential sweep method, thus the palladium atoms are fairly exposed on the surface. IRAS on these surfaces showed an absorption of on-top CO (2110 cm-‘) on Cu atoms in addition to the usual absorptions of multi-bonded CO on Pd atoms. The higher frequency of CO adsorbed on copper (2110 cm-‘) suggests that copper atoms do not form a low index surface on Pd electrodes but form high index surfaces (island structure) from a comparison with the frequencies of the UHV results [18]. This conclusion differs from the result by Chierchie and Mayer [ll].
5. Conclusion We have established the preparation procedures of atomically smoothed, well-oriented single crystal Pd(lOO) and Pd(ll1) electrode surfaces by using combined techniques of I-coating and anneal-quenching. The intensity and the frequency behavior of CO adsorbed on these electrode surfaces are roughly similar to the results from corresponding Pd surfaces under UHV conditions. The large stability of multi-bonded CO species at negative potentials is ascribed to the favorable back-donation of Pd d-electrons to CO 2s* orbitals.
References [l] A.T. Hubbard, Chem. Rev. 88 (1988) 642. [2] F.T. Wagner and P.N. Ross, Jr., J. Electroanal. Chem. 150 (1983) 141. [3] M. Wasberg, L. Palaikis, S. Waller, M. Kamrath and A. Wieckowski, J. Electroanal. Chem. 256 (1988) 51. [4] D. Aberdam, R. Durand, R. Fame and F. El-Omar, Surf. Sci. 171 (1986) 303. [5] K. Ashley and S. Pons, Chem. Rev. 88 (1988) 673. [6] B.N.J. Perrson and A.M. Bradshaw, Surf. Sci. 213 (1989) 49. [7] J. Clavilier, R. Faure, G. Guinet and R. Durand, J. Electroanal. Chem. 107 (1980) 205.
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K. Yoshioka et al. / CO adsorption on Pd(lll)
[8] D. &raw&i, L. Rice, M. Hourani and A. Wieckowski, J. Electroanal. Chem. 230 (1987) 221. [9] D. Armand and J. Clavilier, J. Electroanal. Chem. 225 (1987) 205. [lo] P.C. Andricacos and P.N. Ross, J. Electroanal. Chem. 167 (1984) 301. [ll] T. Chierchie and C. Mayer, Electrochim. Acta 33 (1988) 341. [12] A. Ortega, F.M. Hoffmann and A.M. Bradshaw, Surf. Sci. 119 (1982) 79. [13] A.M. Bradshaw and F.M. Hoffmann, Surf. Sci. 72 (1978) 513.
and Pd(100) single crystal electrodes [14] F. Kitamura, M. Takahashi and M. Ito, Chem. Phys. Lett. 142 (1987) 318. [15] H. Asonen, C. Barnes, A. Salokatue and A. Vuoristo, Appl. Surf. Sci. 22/23 (1985) 556. [16] F. Kitamura, M. Takahashi and M. Ito, J. Phys. Chem. 92 (1988) 3320. [17] F. Kitamura, M. Takahashi and M. Ito, Surf. Sci. 223 (1989) 493. [18] J. Pritchard, T. Catterick and R.K. Gupta, Surf. Sci. 53 (1975) 1.