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An infrared study of the electrosorption of isotopic mixtures of CO on Pt
165
J. Elecrroanal. Chem., 256 (1988) 165-180 Elsevier Sequoia S.A., Lausarme - Printed
in The Netherlands
An infrared study of the electrosorption of isotopic mixtures of CO on Pt A. Bewick and M. Razaq Department
of Chemtstty,
Southampton
University, Southampton,
SO9 5NH (Great Britain)
J.W. Russell Department (Received
of Chemistry 12 November
Oakland University, Rochester, MI 48063 (V.S.A) 1987; in revised form 25 July 1988)
ABSTRACT platinum electrodes from 1 M Infrared spectra of I2 CO + I3 CO mixtures adsorbed on polycrystalline H,S04 solutions have been obtained by EMIRS electrochemically modulated infrared spectroscopy. The ratios at constant saturation coverage and fixed carbon-oxygen stretch spectra for variable “CO/“CO ‘*CO/“CO ratios at high coverage show a single bipolar EMIRS band indicative of a highly coupled system. Lower coverage mixed isotopic samples with greater than 30% “CO produced spectra with two bipolar bands. A simple three parameter model for calculation of infrared band contours was used to assess the effects from adsorbate-adsorbate coupling and from changes in the principal force constant. The different potential and coverage dependences of the position of the absorption band for the observed spectra show that the former is due entirely to a change in the principal force constant and the latter is caused by changes in the coupling.
INTRODUCTION
The adsorption of carbon monoxide on polycrystalline and single crystal platinum has been studied by several spectroscopic techniques including reflection adsorption infrared spectroscopy [l-5] and more recently by electrochemically modulated infrared spectroscopy, EMIRS [6-S] and by FT-IRIUS [9,10]. EMIRS permits the detection of the infrared spectrum of submonolayer quantities of adsorbed species in the presence of a strongly absorbing medium such as water. The necessary sensitivity is achieved by using a modulation of the electrode potential as a pseudo-chopper which yields difference spectra for species at the electrode/solution interface which are affected by the potential. Total discrimination against absorption from bulk solution or atmospheric species is achieved. Through a variation in the limits selected for the electrode potential modulation, EMIRS allows a fine tuning of the electronegativity of the metal atoms in the surface. This additional
166
physical parameter may serve as a sensitive probe for phenomena dependent upon the surface concentration of electrons in the metal. The present EMIRS study utilizes both the variation of the electrode potential and a variation of the concentration ratio of r2C0 to 13C0 to investigate several possible sources which may contribute to the bonding of CO to Pt. The importance of understanding the bonding of CO to catalytic metals is increased by the recognition of the predominance of CO as a primary poison during the electrocatalytic oxidation of many simple organic molecules considered potential fuel cell feedstocks [11,12]. Infrared spectra from samples with different coverages and from mixed r2C0 + l3 CO samples with different isotopic ratios have served as the primary spectroscopic data for testing models for the bonding of CO to platinum. All studies have shown a shift in the CO stretching mode toward higher wavenumber as CO coverage increases. Mixed isotope experiments have revealed an intensity stealing from the lower wavenumber to the higher wavenumber band such that for equal mixtures the high wavenumber band has nearly all the observed intensity. Blyholder [13] attributed the coverage shift to a decrease in dm* back bonding into the T * CO orbital due to a competition for metal electrons with an increase in coverage. Hammaker et al. [14] developed a dipole-dipole coupling model to account for the coverage shift. Moskovitz and Hulse [15] showed that the simple dipole coupling model required a change in the dipole moment during the CO stretching motion, S,u/GQ, that was much larger than the value determined experimentally from infrared intensity measurements. They also showed that the modification in the local electric field produced by other adsorbed molecules produced a shift toward higher wavenumbers with coverage which was too small to fit the experimental results. Moskovitz and Hulse suggested that a more significant source for the coverage shift arises from vibrational coupling via a through-metal interaction between molecules adsorbed on neighbouring metal atoms. Scheffler [16] showed that a dipole coupling model could account for the observed frequency shift with change in coverage if the screening by the metal conduction electrons is properly considered in calculating the contributions from the dipole’s own image and from the images of the other dipoles. Crossley and Ring [5] using the dipole coupling model which included image and screening effects showed that the entire coverage shift could be obtained at constant total CO coverage by varying the percentage of i2C0 through addition of 13C0. They also showed that the integrated intensities from reflection absorption infrared spectra, when used to evaluate the vibrational component of the polarisability used in the Scheffler 1161formula for the coverage shift, gave shifts in agreement with their experimental results. More recently, Persson and Ryberg [17] have treated dipole coupling in isotopic mixtures using the coherent phase approximation. This approach has been compared with the simple vibrational coupling model and the calculated results were shown to be quite similar [lg]. Thus at present it appears that both a dipole coupling model and a through-metal vibrational coupling model can explain the coverage dependence of the frequency of the CO stretching mode for CO adsorbed on Pt.
167
Spectroscopic measurements using electrodes at controlled potential are able to make a unique contribution to discussion of the origin of coupling effects which is not available to non-electrochemical methods. This arises because the observed vibrational frequency of the adsorbed CO, v, is determined by both the potential of the metal, E, and by the CO coverage, 8. The coefficients (Zlv/aE), and (&/%?), can be measured separately in el~tr~he~c~ experiments. The former is expected to be due chiefly to changes of the effective principal force constant, f,. Any secondary contribution due to adsorbate interactions can be found by determining (G/aE), at several values of 8. On the other hand, the coverage dependence is produced by mechanisms which alter either the interaction force constant, f,,, alone or a combination of fi and f,,. Thus only the electrochemical method is able to measure the effects of changing f, on its own. In view of this, it is to be expected that measurements of the potential and coverage dependences of the spectra from mixed ‘*CO f I3 CO samples will give new insight into the origin of the coupling between the adsorbed molecules. The coverage and potential dependence of the frequency of the stretching mode for linearly bonded ‘*CO on Pt has been studied using both the potential modulation EMIRS method and the polarization modulation, IRRAS, method [7,8]. Over the potential range 50 mV to 650 mV (SHE) the wavenumber shift with potential was shown to be + 35 cm-‘/V for 1 M H,SO, and + 25 cm-‘/V for 1 M HClO,. The sign of the potential dependence indicates a strengthening of the CO bond as the electrode potential is made more positive. Such behaviour is consistent with explanations for CO bonding to Pt which include terms dependent upon the availability of metal electrons.
EXPERIMENTAL
Samples for the mixed isotope experiments were prepared from Air Products “CO and BOC 13C0 with BDH Aristar grade H,SO, and triply distilled water. 25 ml of 1 M H$O, solution was placed in a 100 ml round bottom flask equipped with a stopcock and joint for attachment to a vacuum line and with a ground glass inner joint through which the sample could be poured into the spectroelectrochemical cell. This 100 ml mixing vessel was placed on the vacuum line and the sample degassed. A mixture of “CO and 13C0 of the desired ratio which had been previously mixed in a 11 bulb on the vacuum line was admitted into the space in the mixing vessel above the frozen solution. A partial pressure of 200 Torr of CO was found to be adequate for the 25 ml of acid solution to produce a CO concentration in solution which would be sufficient to give saturation CO coverage on the electrode. CO saturation of the acid solution for the 200 Torr pressure was achieved by shaking the vessel for 24 h on a mechanical shaker. The sample was transferred rapidly to a cleaned sp~tr~le~tr~he~c~ cell which was i~ediately sealed with the working electrode and glass caps on the gas inlet and outlet ports. EMIRS spectra were obtained using the instrumental and experimental procedures previously described [19,20]. Two distinct types of sampling procedure were
168
used. In the first case a solution prepared with only 13C0 was placed in the cell and EMIRS spectra obtained for several choices of modulation potential. The potential range 50 to 450 mV (SHE) was found to produce the strongest EMIRS band. With the electrode positioned firmly against the window, a series of spectra were run using the 50 to 450 mV modulation as “CO was slowly bubbled through the bulk solution. Althou~ 13C0 was expected to be displaced rapidly from the bulk solution by “CO it took a period of several hours to achieve the replacement of I3%cls) by 12COcads).This particular method for obtaining mixed isotopic spectra was thought to permit the variation of the isotopic ratio while maintaining constant saturation surface coverage. For all spectra in this series corresponding to samples with mixed isotopic surface species the ratio of surface concentrations could not be dete~ned and in fact was changing slowly as spectra were scanned. The second sampling method utilized samples prepared with fixed ratios of ‘*CO to 13C0 of the following compositions: 100/O, 70/30, 50/50, 30/70, 10/90, and O/100. For each sample EMIRS spectra were obtained for the spectral region of 1800 cm-’ to 2100 cm-’ using a series of values for the potential modulation limits. These modulation limits were taken in the following sequence: 50-250 mV, 50-350 mV, 50-450 mV, 50-550 mV, 50-600 mV, 3~-500 mV, 300-550 mV, and 3~-6~ mV. This standard sequence for the 30/70 ‘*CO + i3 CO sample was followed by additional spectra taken with modulation limits of: 50-250 mV, 50-350 mV, and 50-450 mV. All potentials are specified with respect to the standard hydrogen electrode. The initial spectra in each series with the less positive potential limit fixed at 50 mV were thought to be due to samples with constant surface coverage until the more positive potential limit reached 600 mV. It has been shown previously that at potentials more positive than 550 mV, the coverage of CO on platinum is reduced by partial oxidation to CO, which desorbs [6]. With the electrode positioned against the window it has been shown that it takes times of the order of 1 h to reestablish saturation CO coverage due to the slow diffusion of CO from bulk solution to the thin layer between the electrode and the window (81. The series of scans using a 300 mV lower positive potential limit were for samples which had reduced coverage after partial oxidation of CO during the 50 to 600 mV scan. The 300 mV value was selected to prevent the reduction of CO, back to CO which has been shown to occur only in the presence of Hcadsj [21]. The additional three spectra for the 30/70 12C0 + l3 CO sample recorded after the 300 to 600 mV scan show the characteristics of a low coverage sample. RESULTS AND DISCUSSION
EMIRS
measurements Figure 1 shows the EMXRS spectra for six scans obtained with a constant saturation CO coverage and with different ratios of surface ~ncentrations of i* C%Kis) and 13C0 (adsj.Curves A and F represent saturation coverage for r3C0 and I2CO respectively. Assuming ‘*CO and ‘3C0 have the same solubilities at the same partial pressures, all intermediate spectra, B-E, are for bulk solutions containing at
169
2000
2050 Wavenumber
2100
I
cm"'
Fig. 1. EMIRS spectra of CO on Pt electrode in 1 M H,SO,. Spectra obtained at constant saturation coverage of CO with the ratio of “CO/‘*CO decreasing from A to F. A represents 1008 13C0 and F 100% ‘*CO. 50 mV to 450 mV modulation used for all spectra. Arrows show + 2 x 10m3 to - 2 x 10m3 AR/R range.
least as much dissolved CO as the initial i3C0 sample. Due to the strength of binding of CO to platinum we see no reason to assume CO molecules are desorbed without replacement by other CO molecules when the bulk solution is saturated with CO. The appearance of only a single bipolar EMIRS band for all samples in Fig. 1 and the initial rapid decrease in intensity and subsequent steady increase in intensity of the EMIRS band for mixed isotopic samples as i2C0 replaces 13C0, will need to be explained by any model for CO bonding to platinum. Figure 2 shows EMIRS spectra typical of the results obtained for samples containing any of the isotopic mixtures studied and for all modulation ranges which did not take the potential so far positive as to result in a reduction of coverage via partial oxidation of CO. The obvious feature in all these spectra is a bipolar band in the wavenumber region for linearly bonded CO. This band has a positive lower wavenumber component with a broader tail and a sharper and more intense high wavenumber negative component. The growth in intensity of this band and its shift toward higher wavenumber as the potential modulation range is increased by expanding the more positive potential limit is similar to results reported for studies of i2C0 [6]. The bipolar structure on the l2 CO spectra was attributed to an infrared
170
1800
1900 Wavenumber
2000
2100
/cm-’
Fig. 2. EMIRS spectra of samples containing “CO and 13C0 in 1 M H,SO, with compositions: (A) 100% “CO, (B) 70% ‘*CO. (C) 50% 12C0, (D) 10% “CO and (E) 100% 13C0. Arrows show +2~10-~ to - 2 x IO- 3 AR/R range, 50 mV to 450 mV modulation used for all spectra.
band which shifts towards higher wavenumber as the electrode is more positively polarised. This conclusion from earlier EMIRS studies has been substantiated by direct observation of the potential dependence of the infrared band by the IRRAS technique [7-lo]. The bipolar band shown in Figs. 2 and 3D shows a definite shift towards higher wavenumber as the percentage of ‘*CO on the platinum surface increases. The wavenumbers of the zero crossing points of the bipolar bands for spectra obtained using 50 to 450 mV modulation for the various isotopic samples are given in Table 1. The spectrum of the 30/70 ‘*CO f l3 CO sample with modulation limits of 50 to 250 mV shown in Fig. 3D is similar to the spectra in Figs. 1 and 2. The remaining spectra in Fig. 3 were obtained after polarising the electrode to sufficiently positive potentials to reduce the CO coverage in a gradual fashion by its partial oxidation. These spectra are characteristic of lower coverage conditions as observed by a decline in band intensity and a shift of the band toward lower wavenumber.
171
I
1800
1
I
I
I
2000
1900 Wavenumber
I
2100
/cm-’
Fig. 3. EMIRS spectra for 30% “CO+ 708 13C0 sample on Pt electrode in 1 M H,SO,,. Spectrum D was obtained first using 50 to 250 mV modulation with near saturation coverage of CO. The other spectra are for lower coverage samples produced by the partial oxidation of CO. The respective modulation ranges are: (A) 300 to 500 mV, (B) 50 to 450 mV and (C) 50 to 250 mV. See text for discussion of band shapes and relative intensities. Arrows show the AR/R ranges as follows: (A) +0.1~10-~ to -0.1x10-3;(B),(C)and(D) +1X10m3 to -1X10U3.
However, the most outstanding feature of the spectra shown in Figs. 3A-3C is the appearance of two bipolar bands rather than one. Figure 3C, for which the same potential modulation range was used as for the spectrum in Fig. 3D, shows all three properties: the development of a second bipolar band, the shift of the band to lower wavenumber, and the decrease in intensity. Comparison of Figs. 3B and 3C shows that the double bipolar feature behaves qualitatively in the same way as the single bipolar band, shifting towards higher wavenumber and growing in intensity when the more positive potential limit is increased. The dependence of the amplitudes of the bands upon coverage, lower coverage giving smaller bands, is seen clearly from a comparison of the spectra in Figs. 2 and 3. In Fig. 3, the shift of the double bipolar band to higher wavenumbers as the potential is made more positive is also apparent. A major deduction can be made from the potential dependence of the spectra for various values of coverage and for a range of isotopic mixtures: the electrode potential changes only the frequency of the observed spectral bands without altering either their number or relative intensities whereas the coverage can produce changes
172 TABLE
1
CO stretching mode of adsorbed t2 CO + I3 CO mixtures at high surface coverage H,SO,. Variation of the observed position of the EMIRS band and calculated band with isotopic composition Percentage
Zero crossing
‘2co
(EMIRS
band,
modulation)/cm100 70 50 30 10 0
2070 2065 2060 2047 2022 2022
Calculated
position 50-450
I
mV
band position
(90% coverage,
on a Pt electrode in 1 M position of the infrared
’
f, = 16.6 mdyn/A,
f,, = 0.15 mdyn/A,
A1,,,
=14 cm-‘)/cm-t
2076 2072 2066 2057 2032 2031
a These f, and f,, values were selected to fit IRRAS data at 400 mV. A reduced appropriate for an average over the 50-450 mV range, such as 16.50 mdyn/A, would approximately -6 cm-t. 1 A = 0.1 nm.
f, value more shift the bands
in all three characteristics; thus the origins of (~v/&?Z)~ and (&/&9), are almost certainly different. The appearance of the double bipolar feature upon partial oxidation of CO and consequent reduction in coverage does not occur for all isotopic mixtures. Figure 4 shows spectra for each of the other fixed composition samples over the same modulation range used for Fig. 3A. These spectra were all obtained immediately following spectral scans using 50 to 600 mV modulation. All 50 to 600 mV spectra showed the intensity drop and shift to lower wavenumber expected for lower coverage samples. The spectra shown in Figs. 4 and 3A show that the double bipolar band is not observed under low coverage conditions for samples containing a single CO carbon isotope or for mixed isotopic samples whose ‘sC0 content is 30% or less. Spectra shown in Figs. l-4 display two basic patterns for EMIRS bands in the 1950-2100 cm-’ region. A single bipolar band appears for all high coverage samples but a second bipolar band is observed at low coverage for mixed isotopic samples containing more than 30% 13C0. Since it is firmly established that the bipolar band for ‘*CO at saturation coverage on platinum arises due to a potential dependence of the frequency of the CO stretching mode, it is reasonable to attribute the double bipolar EMIRS feature to two infrared bands which are both potential dependent. Infrared bands of the shapes shown in Figs. 5A and 5C would produce the EMIRS difference spectra shown in Figs. 5B and 5D respectively if the position of the infrared band was shifted towards higher wavenumber by 3 cm-’ incremental steps. The simulated EMIRS bands in Figs. 5B and 5D resemble closely the observed contours of bands in Figs. l-4. In a study of the infrared spectra of mixed ‘*CO + l3 CO samples adsorbed on Pt (111) and Pt (OOl), Crossley and Ring [5] observed an intensity stealing by the higher frequency band. In the most simplistic description of coupling between two CO molecules adsorbed on adjacent metal atoms, Moskovitz and Hulse [15] describe
173
-t--Q-. la00
1900
2000
2100
Wavenumberlcm-'
Fig. 4. EMIRS spectra of low coverage samples of 12CO+ 13C0 mixtures of fixed composition obtained wth 300 to 500 mV modulation of Pt electrode in 1 M H,SO,. Compositions of samples are: (A) 100% 12C0, (B) 70% ‘*CO, (C) 50% ‘*CO, (D) 10% 12C0 and (E) 100% 13C0. Arrows show +1 x 1O-3 to - 1 x 10m3 AR/R ranges. See text for discussion of line shapes and intensities.
the intensity stealing by the near superposition of the strong infrared surface-active, in-phase ‘2CO-‘2C0 and ‘2CO-‘3C0 modes with the weaker, in-phase 13CO-‘3C0 mode and forbidden out-of-phase ‘2CO-‘3C0 mode at the lower frequency. Crossley and Ring point out that in the dipole coupling model, the positive coupling constant necessary to produce the observed intensity stealing requires coupling of oppositely oriented dipoles. They suggest such orientations for CO molecules on bridged and linear sites. Similar intensity stealing, as observed for isotopic mixtures, is thought responsible for observation of only the higher frequency linear CO component of a linear CO-bridged CO couple. Unlike the high coverage EMIRS spectra, the reflection absorption infrared spectra of Crossley and Ring do show two distinct bands for isotopic mixtures with 50% or more 13C0. Either coupling between CO molecules adsorbed on a platinum electrode in contact with an aqueous
174
lB.0
0.0
2000
-7
2100 UAVENUPBER
I
UA~ENU~~BER
cd
I
cm-’
B 10.
-6. 0. UAVENUHBER
C
Icm-’
li~vt34~ii~~~
Icm-’
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Fig. 5. (A) Calculated infrared band contour for 30/70 ‘2CO+‘3C0. Sample at 90% coverage on a surface 75% Pt (111) and 25% Pt (100) with effective CO stretching constant of 16.600 mdyn/A and mteraction force constant of 0.150 mdyn/A. Band contours were calculated assuming a full wtdth at half height for a Lorenzian fine at the position of each of the coilective CO modes. Intensity units are arbitrary. (B) EMIRS bands that would result from shifting the position of the infrared band in 3 cm-’ increments toward higher wavenumbers to represent the band at the more positrve potential wtth the band positioned as in (A) at the less positive potential. The electrode reflectivity is assumed flat over this range as was observed with those in (A). Since there IS no atmospheric or solution band, R is flat and taken as 1. (C) Calculated infrared band contour as in (A) for the sample at 40% coverage. (D) EMIRS bands corresponding to the infrared band in (C) calculated as in (B).
electrolyte is stronger than coupling between CO molecules adsorbed from the gas phase onto Pt (111) or Pt (001) or there is a higher coverage for the solution experiments. The EMIRS observation of two bipolar bands at lower coverage for mixed isotopic samples might result from a reduction in coupling between adsorbed molecules and samples with a high percentage of r2C0 might have sufficient intensity stealing to make the lower frequency component not observable. Model calculations
A simple three parameter vibrational force field model, similar to the one used by Moskovitz and Hulse [Is], was applied to estimate the magnitude of the coupling term necessary to fit the observed spectra. All parameters were deter~ned empirically from the spectral data. The advantage of such a mode1 is that, by giving alternative physical interpretations to the parameters, it can be used to describe either vibrational coupling or dipole coupling. The three parameters are an effective
175
CO stretching force constant (f,), a CO-CO interaction constant (f,,), and a line width for a CO infrared band (AV,,,). Calculations were made for CO molecules oriented perpendicular to the metal surface on both a six nearest neighbour Pt (111) surface and a four nearest neighbour Pt (100) surface. It was thought that these two calculations would show a pattern which could be used to make deductions for a polycrystalline surface. One hundred CO molecules were placed randomly on one of the Pt surfaces which was expanded from a 10 X 10 size to give the desired degree of coverage. Periodic boundary conditions were used such that the block containing 100 adsorbed CO molecules represented a section of an infinite two dimensional surface. The principal force constant in the vibrational coupling model is an effective CO stretching force constant whose value differs from that for gas phase CO due to interaction of CO with the metal surface and with solution species. In the dipole coupling case the principal force constant represents all interaction forces between the adsorbed CO molecule and the metal plus interaction forces with solution species. CO metal interactions would include donation of CO electrons directly to the metal, image interactions with the metal, and a d-electron back bonding from the metal to CO. The magnitude of the principal force constant, f, = 16.000 mdyn/A (= aJ Am2), is dete~ned from the frequency that an isolated CO molecule would have when adsorbed from solution onto the electrode. At the potential of zero charge this frequency is estimated to be 2030 cm-’ since this is the location where a band first appears in EMIRS and IRRAS spectra of acidic solutions upon initial addition of CO. It corresponds to the zero coverage value, 2064 cm-“, suggested by Shigeishi and King [l] for CO on platinum when a vacuum rather than a solution exists next to the CO, The shift to 2064 cm-’ from the value for an isolated CO molecule, 2143 cm-‘, has been attributed to metal-CO interaction weakening the CO bond; the further shift to 2030 cm-’ is evidence for additional weakening by interaction with solution species. The magnitude of the coupling constant, f,, = 0,150 mdyn/A, is chosen to produce a high coverage value of 2075 cm -’ for adsorbed “CO molecules, In the vibrational coupling model, this term is simply the coupling force constant between adjacent CO molecules due to coupling of their vibrational motions via the metal surface. For the dipole coupling case, the interaction term represents the sum of contributions by direct dipole-dipole coupling and interaction with images of other dipoles. With the randomly positioned samples and the above choices of f, and f,,. the frequencies and intensities of all the collective modes of the 100 molecules were determined through diagonalization of the Wilson F matrix as outlined by Moskovitz and Hulse [15]. To establish a total infrared band contour, a full width at half height of 14 cm-’ was applied to a Lorenzian line at each calculated frequency. This is the value produced by the formula given by Moskovitz and Hulse ]lS], although they state they used a 7 cm- ’ full width. Crossley and King [5] estimate a band width for CO on platinum of 9 cm-’ and Kunimatsu et al. [9] report the same value for electrosorbed CO. However, the calculations we report will not be appreciably affected by the choice of bandwidth within this range.
176
jll,~ :l~~etch 0 2000
2100
UAVENUMBER/Cm-’
A
0
2000
2100
WAVENUMBER
/cm-’
UAVENUMBER E
WAVENUHBER
/Cm-
C
B
/cm-
VAVENUMBER
/cm-’
F
Fig. 6. Calculated infrared absorption for ‘2CO+‘3C0 samples for 90% coverage on Pt (111) ( -) and Pt (100) (- - -) with effective CO stretching force constant of 16.600 mdyn/A and interaction force constant of 0.150 mdyn/.k. Elgenvaiues and intensities at 1 cm-’ Intervals are shown by solid vertical lines for Pt (111) surface. Band contours were calculated using a Lorentian line shape with 14 cm-’ full width at half height. (A) 100% “CO. (8) 70% “CO, (C) 50% “CO, (D) 30% “CO, (E) 10% ‘*CO and (F) 100% 13C0.
Figure 6 shows the calculated infrared band contours for the six 12C0 + I3 CO mixtures studied using the parameters given above. The solid curve gives the contour expected on a Pt (111) surface at 90% coverage and the dashed curve the contour on a Pt (100) surface at the same coverage. For the Pt (111) calculation the frequency of the infrared modes and their intensities for the 100 molecule collection are indicated by the solid vertical lines. Using the same force constants for both low area surfaces, one observes a greater resolution of the band into two components for the Pt (100) surface where coupling is limited to four nearest neighbours. With a polycrystalline surface one might expect to build a band contour from contributions from various low area components. From the two cases shown in Fig. 6 one would expect to observe a broadening of the low wavenumber side of the band for Pt (111). Figures 5A and 5C were calculated by taking the sum of the contours for the 50/50 ‘*CO + l3 CO bands for Pt (111) and Pt (100) with a 3 : 1 weighting. Table 1 compares the calculated peak frequencies for the various isotopic mixtures with the observed zero crossing points between the positive and negative components of the EMIRS bands using 50 to 450 mV modulation data. Agreement could be improved between calculated and observed frequencies by use of a lower value for f,. The
117
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mixtures for 40% coverage on Pt (111) and Pt Fig. 7. Calculated infrared absorption for ‘2CO+‘3C0 (100) surfaces. All other terms have meanings and values shown in Fig. 6.
calculated values thus correspond to a more positive potential than the 250 mV average for this modulation range. At lower coverage the coupling is less effective and the intensity stealing into the higher frequency component is thus reduced. Figure 7 shows the same type of band contour for a surface with 40% coverage as was shown in Fig. 6 for 90% coverage. From the intensities and band positions of the lower coverage EMIRS bands, it is estimated that the coverages used were less than half the high coverage values. In Table 2, the calculated peak positions for the low coverage bands are compared with the zero crossing frequencies for the two bipolar bands observed with 300 to 500 mV modulation. The agreement for the higher frequency band shows the 16.600 mdyn/A principal force constant corresponds better to a 400 mV electrode potential. The poorer agreement for the lower frequency component is expected due to its overlap with the higher band. The general trends for larger frequency changes between 50% and 70% 13C0 than between 70% and 90% “CO, the direction for movement of the high and low frequency components of the double bipolar bands as the isotopic ratio is changed, and the absence of a double band for spectra of between those observed and pure i2C0 or pure 13C0 show excellent agreement calculated. The calculations certainly serve to establish that the observation of two bipolar bands for these mixed isotopic samples originates from the reduced coupling at lower coverage. The calculated band contours also show, qualitatively, the intensity pattern found
178 TABLE 2 CO stretching mode of adsorbed ‘2CO+‘3C0 mixtures at low surface coverage on a Pt electrode in 1 M H,SO,. Variation of the observed posttion of EMIRS bands and calculated position of the infrared band with rsotopic compositton Percentage ‘2CO
Zero crossing positions (EMIRS bands, 300-500 mV modulation)/cm- ’
Calculated band positions (40% coverage, ,f, = 16.6 mdyn/A. f,, = 0.15 mdyn/A, A%,,,, = 14 cm-‘),/cm-’
100 70 50 30 10 0
2042 2060 = 1982,2049 1987,2027 1993,2026 2020
2050 X981,2043 1993,2052 2001f 2031 2010, 2029 2006
a The experimental procedure used did not allow preparation of low coverage samples with identical surface coverages. This sample appears due to its positton and intensity to have a higher coverage than the other samples in this table.
in the experimental spectra. In moving from 100% i3C0 to 100% 12C0 there is a rapid drop in the intensity of the EMIRS difference peak followed by a steady rise to the “‘CO maximum value. The largest shift in frequency for the band maxima occurs at low i3C0 composition due to the strength of the coupling between adsorbed molecules. One possible explanation for the potential dependent shift of the CO stretching frequency is that the availability of metal electrons for back bonding into the II* CO orbital is strongly potential dependent. As the electrode is polarised more positively, the electron concentration decreases, which should reduce back bonding and strengthen the CO bond. The band shift with coverage could be similarly attributed to increased competition for metal electrons as the number of nearest neighbours increases. One could potentially test such a model by using the observed potential dependence of the band as a guide. Such a test is suggested in Fig. 8. In these calculations the principal force constant was made to increase as the number of nearest neighbours increases. The two examples shown in Figs. 8A-C and 8D-F have respectively 44% and 67% of the total coverage shift attributed to a change in the principal force constant. To maintain a close fit to the measured total shift with coverage, the interaction constant must be decreased if f, is increased. For both calculations shown in Fig. 8 with coverage dependent force constants, the reduced coupling produces contours at 90% coverage with even more resolution of separate bands than the coverage independent force constant model gave at 40% coverage. The observed EMIRS spectra require therefore the highest possible coupling constant consistent with the observed coverage shift, i.e. the total shift with coverage needs to be produced by the fii term. A more definitive conclusion would be possible from a study using single crystal electrodes. We also note that an altemative mechanism involving a first order Stark effect might well explain the potential dependent frequency shift [22,23]
179
WAVEtd.JMBER /cm-’
UAVENUHBER
UAVENWBER
D
/cm-’
C
A
/cm-’
,,AVENUNEER
E
/cm-’
UAVENUMSER
/cm-’
F
Fig. 8. Calculated infrared absorption for ‘2CO+‘3C0 samples at 90% coverage on Pt (111) assuming coverage dependent force constants. If a, and n, are the number of occupied nearest neigbbour sites for atoms i and j, the force constants are: (A), (B), (C) f, = 16.600+0.065~,, f,, = 0.150 [I - n, + n, -2/201; (D), (E), (F) f, =16.600-O.O92n,, f,, = 0.085 [I -(n, + n, -2),‘201. (A). (D) 70% ‘*Co; 0% (E) 505% ‘*CO; (C), (F) 30% ‘*CO.
CONCLUSIONS
(1) The appearance of only a single bipolar EMIRS band for all isotopic mixtures of i2C0 and 13C0 studied when the CO surface coverage is high and the wavenumber position of the band for various isotopic mixtures demonstrate strong coupling between the adsorbed molecules, i.e. there is a considerable intensity stealing by the higher wavenumber modes. Lack of observation of a lower wavenumber component for samples with 50 to 70% t3C0 as was observed in reflection absorption infrared spectra for CO isotopic mixtures adsorbed on Pt (111) and Pt (001) surfaces may indicate stronger coupling for CO adsorbed from the solution phase or it may be due to differences in the extent of island formation between the single crystal and the comparatively rough polycrystalline surface. (2) The infrared absorption band may be modelled for all isotopic mixtures studied by a simple three parameter vibrational coupling model with an effective stretching force constant of 16.600 mdyn/& interaction constant of 0.150 mdyn/& and an individual infrared band full width at half height of 14 cm-‘. The wavenumber dependence upon electrode potential of the CO stretching mode is represented by a change in the value of the principal force constants, a change of 0.100 mdyn/A shifting the band 6 cm-‘.
180
(3) Varying the electrode potential over a range of 600 mV produces substantial changes in the adsorbate-metal bond but any concomitant variations in the coupling interaction between adsorbed molecules are undetectable. The different potential and coverage dependences for the band position indicate that the increase in wavenumber with coverage is due entirely to an, increase in the coupling between adsorbate molecules whereas the increase in wavenumber with potential is due entirely to an increase in the principal force constant. ACKNOWLEDGEMENTS
We wish to acknowledge Dr. A. Rest of South~pton University for supplying the ‘3C0 sample and for assistance in use of his vacuum line for preparing mixed isotopic samples. Discussions of vibrational coupling models with the late Professor John Overend of the University of Minnesota were most helpful. Professor Overend and Mark Severson kindly provided the portion of the computer program used to calculate the frequencies and intensities for a Pt (111) surface. Support for this work from the Office of Naval Research, Was~ngton, and from the Science Research Council is gratefully acknowledged. J.W.R. wishes to acknowledge the Oakland University Faculty Research Fund for a Summer Fellowship. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
R.A. Shigeishi and D.A. King, Surf. Sci., 58 (1976) 379. K. Horn and J. Pritchard, 3. Phys., 38 (1977) C4, 164. H. Krebs and H. Luth, Appl. Phys., 14 (1977) 337. A. Crossley and D.A. Kmg, Surf. Sci., 68 (1977) 528. A. Crossley and D.A. King, Surf. Sci., 95 (1980) 131. B. Beden, A. Bewick, K. Kunimatsu and C. Lamy, J. Electroanal. Chem., 142 (1982) 345. J.W. Russell, J. Overend, K. Scanlon, M. Severson and A. Bewick, J. Phys. Chem., 86 (1982) 3066. J.W. Russell, M. Severson, K. Scanlon, J. Gverend and A. Bewick, J. Phys. Chem., 87 (1983) 293. K. Kunimatsu, W.G. Golden, H. Seki and M.R. Philpott, Langmuir, 1 (1985) 245. K. Kunimatsu, H. Seki, W.G. Golden, J.G. Gordon and M.R. Philpott, Langmuir, 2 (1986) 464. B. Beden, C. Lamy, A. Bewick and K. Kunimatsu, J. Electroanal. Chem., 121 (1981) 343. A. Bewick in A.F. Silva (Ed.), Trends in Interfacial Electrochemistry, Nato AS1 Vol. C, Reidel. Dordrecht, 1986, p. 179. G. Blyholder, J. Phys. Chem., 68 (1964) 2772. R.A. Hammaker, S.A. Francis and R.P. E&hens, Spectrochim. Acta, 21 (1965) 1295. M. Moskovitz and J.E. H&e, Surf. Sci., 78 (1978) 397. M. Scheffler, Surf. Sci., 81 (1979) 562. B.N.J. Persson and R. Ryberg, Phys. Rev. B, 24 (1981) 6954. M.W. Severson, W.J. Tornquist and J. Overend, J. Phys. Chem., 88 (1984) 469. A. Bewick, K. Kunimatsu, J. Robinson and J.W. Russell, J. Electroanal. Chem., 119 (1981) 175. A. Bewick, K, Kunimatsu, B.S. Pons and J.W. Russell, J. Electroanal. Chem., 160 (1984) 47. B. Beden, A. Bewick, M. Razaq and J. Weber, J. Electroanal. Chem., 139 (1982) 203. D.K. Lambert, Solid State Commun., 51 (1984) 297. W. Muller and P. Bagus, J. Electron Spectrosc. Relat. Phenom., 38 (1986) 103.
J. Elecrroanal. Chem., 256 (1988) 165-180 Elsevier Sequoia S.A., Lausarme - Printed
in The Netherlands
An infrared study of the electrosorption of isotopic mixtures of CO on Pt A. Bewick and M. Razaq Department
of Chemtstty,
Southampton
University, Southampton,
SO9 5NH (Great Britain)
J.W. Russell Department (Received
of Chemistry 12 November
Oakland University, Rochester, MI 48063 (V.S.A) 1987; in revised form 25 July 1988)
ABSTRACT platinum electrodes from 1 M Infrared spectra of I2 CO + I3 CO mixtures adsorbed on polycrystalline H,S04 solutions have been obtained by EMIRS electrochemically modulated infrared spectroscopy. The ratios at constant saturation coverage and fixed carbon-oxygen stretch spectra for variable “CO/“CO ‘*CO/“CO ratios at high coverage show a single bipolar EMIRS band indicative of a highly coupled system. Lower coverage mixed isotopic samples with greater than 30% “CO produced spectra with two bipolar bands. A simple three parameter model for calculation of infrared band contours was used to assess the effects from adsorbate-adsorbate coupling and from changes in the principal force constant. The different potential and coverage dependences of the position of the absorption band for the observed spectra show that the former is due entirely to a change in the principal force constant and the latter is caused by changes in the coupling.
INTRODUCTION
The adsorption of carbon monoxide on polycrystalline and single crystal platinum has been studied by several spectroscopic techniques including reflection adsorption infrared spectroscopy [l-5] and more recently by electrochemically modulated infrared spectroscopy, EMIRS [6-S] and by FT-IRIUS [9,10]. EMIRS permits the detection of the infrared spectrum of submonolayer quantities of adsorbed species in the presence of a strongly absorbing medium such as water. The necessary sensitivity is achieved by using a modulation of the electrode potential as a pseudo-chopper which yields difference spectra for species at the electrode/solution interface which are affected by the potential. Total discrimination against absorption from bulk solution or atmospheric species is achieved. Through a variation in the limits selected for the electrode potential modulation, EMIRS allows a fine tuning of the electronegativity of the metal atoms in the surface. This additional
166
physical parameter may serve as a sensitive probe for phenomena dependent upon the surface concentration of electrons in the metal. The present EMIRS study utilizes both the variation of the electrode potential and a variation of the concentration ratio of r2C0 to 13C0 to investigate several possible sources which may contribute to the bonding of CO to Pt. The importance of understanding the bonding of CO to catalytic metals is increased by the recognition of the predominance of CO as a primary poison during the electrocatalytic oxidation of many simple organic molecules considered potential fuel cell feedstocks [11,12]. Infrared spectra from samples with different coverages and from mixed r2C0 + l3 CO samples with different isotopic ratios have served as the primary spectroscopic data for testing models for the bonding of CO to platinum. All studies have shown a shift in the CO stretching mode toward higher wavenumber as CO coverage increases. Mixed isotope experiments have revealed an intensity stealing from the lower wavenumber to the higher wavenumber band such that for equal mixtures the high wavenumber band has nearly all the observed intensity. Blyholder [13] attributed the coverage shift to a decrease in dm* back bonding into the T * CO orbital due to a competition for metal electrons with an increase in coverage. Hammaker et al. [14] developed a dipole-dipole coupling model to account for the coverage shift. Moskovitz and Hulse [15] showed that the simple dipole coupling model required a change in the dipole moment during the CO stretching motion, S,u/GQ, that was much larger than the value determined experimentally from infrared intensity measurements. They also showed that the modification in the local electric field produced by other adsorbed molecules produced a shift toward higher wavenumbers with coverage which was too small to fit the experimental results. Moskovitz and Hulse suggested that a more significant source for the coverage shift arises from vibrational coupling via a through-metal interaction between molecules adsorbed on neighbouring metal atoms. Scheffler [16] showed that a dipole coupling model could account for the observed frequency shift with change in coverage if the screening by the metal conduction electrons is properly considered in calculating the contributions from the dipole’s own image and from the images of the other dipoles. Crossley and Ring [5] using the dipole coupling model which included image and screening effects showed that the entire coverage shift could be obtained at constant total CO coverage by varying the percentage of i2C0 through addition of 13C0. They also showed that the integrated intensities from reflection absorption infrared spectra, when used to evaluate the vibrational component of the polarisability used in the Scheffler 1161formula for the coverage shift, gave shifts in agreement with their experimental results. More recently, Persson and Ryberg [17] have treated dipole coupling in isotopic mixtures using the coherent phase approximation. This approach has been compared with the simple vibrational coupling model and the calculated results were shown to be quite similar [lg]. Thus at present it appears that both a dipole coupling model and a through-metal vibrational coupling model can explain the coverage dependence of the frequency of the CO stretching mode for CO adsorbed on Pt.
167
Spectroscopic measurements using electrodes at controlled potential are able to make a unique contribution to discussion of the origin of coupling effects which is not available to non-electrochemical methods. This arises because the observed vibrational frequency of the adsorbed CO, v, is determined by both the potential of the metal, E, and by the CO coverage, 8. The coefficients (Zlv/aE), and (&/%?), can be measured separately in el~tr~he~c~ experiments. The former is expected to be due chiefly to changes of the effective principal force constant, f,. Any secondary contribution due to adsorbate interactions can be found by determining (G/aE), at several values of 8. On the other hand, the coverage dependence is produced by mechanisms which alter either the interaction force constant, f,,, alone or a combination of fi and f,,. Thus only the electrochemical method is able to measure the effects of changing f, on its own. In view of this, it is to be expected that measurements of the potential and coverage dependences of the spectra from mixed ‘*CO f I3 CO samples will give new insight into the origin of the coupling between the adsorbed molecules. The coverage and potential dependence of the frequency of the stretching mode for linearly bonded ‘*CO on Pt has been studied using both the potential modulation EMIRS method and the polarization modulation, IRRAS, method [7,8]. Over the potential range 50 mV to 650 mV (SHE) the wavenumber shift with potential was shown to be + 35 cm-‘/V for 1 M H,SO, and + 25 cm-‘/V for 1 M HClO,. The sign of the potential dependence indicates a strengthening of the CO bond as the electrode potential is made more positive. Such behaviour is consistent with explanations for CO bonding to Pt which include terms dependent upon the availability of metal electrons.
EXPERIMENTAL
Samples for the mixed isotope experiments were prepared from Air Products “CO and BOC 13C0 with BDH Aristar grade H,SO, and triply distilled water. 25 ml of 1 M H$O, solution was placed in a 100 ml round bottom flask equipped with a stopcock and joint for attachment to a vacuum line and with a ground glass inner joint through which the sample could be poured into the spectroelectrochemical cell. This 100 ml mixing vessel was placed on the vacuum line and the sample degassed. A mixture of “CO and 13C0 of the desired ratio which had been previously mixed in a 11 bulb on the vacuum line was admitted into the space in the mixing vessel above the frozen solution. A partial pressure of 200 Torr of CO was found to be adequate for the 25 ml of acid solution to produce a CO concentration in solution which would be sufficient to give saturation CO coverage on the electrode. CO saturation of the acid solution for the 200 Torr pressure was achieved by shaking the vessel for 24 h on a mechanical shaker. The sample was transferred rapidly to a cleaned sp~tr~le~tr~he~c~ cell which was i~ediately sealed with the working electrode and glass caps on the gas inlet and outlet ports. EMIRS spectra were obtained using the instrumental and experimental procedures previously described [19,20]. Two distinct types of sampling procedure were
168
used. In the first case a solution prepared with only 13C0 was placed in the cell and EMIRS spectra obtained for several choices of modulation potential. The potential range 50 to 450 mV (SHE) was found to produce the strongest EMIRS band. With the electrode positioned firmly against the window, a series of spectra were run using the 50 to 450 mV modulation as “CO was slowly bubbled through the bulk solution. Althou~ 13C0 was expected to be displaced rapidly from the bulk solution by “CO it took a period of several hours to achieve the replacement of I3%cls) by 12COcads).This particular method for obtaining mixed isotopic spectra was thought to permit the variation of the isotopic ratio while maintaining constant saturation surface coverage. For all spectra in this series corresponding to samples with mixed isotopic surface species the ratio of surface concentrations could not be dete~ned and in fact was changing slowly as spectra were scanned. The second sampling method utilized samples prepared with fixed ratios of ‘*CO to 13C0 of the following compositions: 100/O, 70/30, 50/50, 30/70, 10/90, and O/100. For each sample EMIRS spectra were obtained for the spectral region of 1800 cm-’ to 2100 cm-’ using a series of values for the potential modulation limits. These modulation limits were taken in the following sequence: 50-250 mV, 50-350 mV, 50-450 mV, 50-550 mV, 50-600 mV, 3~-500 mV, 300-550 mV, and 3~-6~ mV. This standard sequence for the 30/70 ‘*CO + i3 CO sample was followed by additional spectra taken with modulation limits of: 50-250 mV, 50-350 mV, and 50-450 mV. All potentials are specified with respect to the standard hydrogen electrode. The initial spectra in each series with the less positive potential limit fixed at 50 mV were thought to be due to samples with constant surface coverage until the more positive potential limit reached 600 mV. It has been shown previously that at potentials more positive than 550 mV, the coverage of CO on platinum is reduced by partial oxidation to CO, which desorbs [6]. With the electrode positioned against the window it has been shown that it takes times of the order of 1 h to reestablish saturation CO coverage due to the slow diffusion of CO from bulk solution to the thin layer between the electrode and the window (81. The series of scans using a 300 mV lower positive potential limit were for samples which had reduced coverage after partial oxidation of CO during the 50 to 600 mV scan. The 300 mV value was selected to prevent the reduction of CO, back to CO which has been shown to occur only in the presence of Hcadsj [21]. The additional three spectra for the 30/70 12C0 + l3 CO sample recorded after the 300 to 600 mV scan show the characteristics of a low coverage sample. RESULTS AND DISCUSSION
EMIRS
measurements Figure 1 shows the EMXRS spectra for six scans obtained with a constant saturation CO coverage and with different ratios of surface ~ncentrations of i* C%Kis) and 13C0 (adsj.Curves A and F represent saturation coverage for r3C0 and I2CO respectively. Assuming ‘*CO and ‘3C0 have the same solubilities at the same partial pressures, all intermediate spectra, B-E, are for bulk solutions containing at
169
2000
2050 Wavenumber
2100
I
cm"'
Fig. 1. EMIRS spectra of CO on Pt electrode in 1 M H,SO,. Spectra obtained at constant saturation coverage of CO with the ratio of “CO/‘*CO decreasing from A to F. A represents 1008 13C0 and F 100% ‘*CO. 50 mV to 450 mV modulation used for all spectra. Arrows show + 2 x 10m3 to - 2 x 10m3 AR/R range.
least as much dissolved CO as the initial i3C0 sample. Due to the strength of binding of CO to platinum we see no reason to assume CO molecules are desorbed without replacement by other CO molecules when the bulk solution is saturated with CO. The appearance of only a single bipolar EMIRS band for all samples in Fig. 1 and the initial rapid decrease in intensity and subsequent steady increase in intensity of the EMIRS band for mixed isotopic samples as i2C0 replaces 13C0, will need to be explained by any model for CO bonding to platinum. Figure 2 shows EMIRS spectra typical of the results obtained for samples containing any of the isotopic mixtures studied and for all modulation ranges which did not take the potential so far positive as to result in a reduction of coverage via partial oxidation of CO. The obvious feature in all these spectra is a bipolar band in the wavenumber region for linearly bonded CO. This band has a positive lower wavenumber component with a broader tail and a sharper and more intense high wavenumber negative component. The growth in intensity of this band and its shift toward higher wavenumber as the potential modulation range is increased by expanding the more positive potential limit is similar to results reported for studies of i2C0 [6]. The bipolar structure on the l2 CO spectra was attributed to an infrared
170
1800
1900 Wavenumber
2000
2100
/cm-’
Fig. 2. EMIRS spectra of samples containing “CO and 13C0 in 1 M H,SO, with compositions: (A) 100% “CO, (B) 70% ‘*CO. (C) 50% 12C0, (D) 10% “CO and (E) 100% 13C0. Arrows show +2~10-~ to - 2 x IO- 3 AR/R range, 50 mV to 450 mV modulation used for all spectra.
band which shifts towards higher wavenumber as the electrode is more positively polarised. This conclusion from earlier EMIRS studies has been substantiated by direct observation of the potential dependence of the infrared band by the IRRAS technique [7-lo]. The bipolar band shown in Figs. 2 and 3D shows a definite shift towards higher wavenumber as the percentage of ‘*CO on the platinum surface increases. The wavenumbers of the zero crossing points of the bipolar bands for spectra obtained using 50 to 450 mV modulation for the various isotopic samples are given in Table 1. The spectrum of the 30/70 ‘*CO f l3 CO sample with modulation limits of 50 to 250 mV shown in Fig. 3D is similar to the spectra in Figs. 1 and 2. The remaining spectra in Fig. 3 were obtained after polarising the electrode to sufficiently positive potentials to reduce the CO coverage in a gradual fashion by its partial oxidation. These spectra are characteristic of lower coverage conditions as observed by a decline in band intensity and a shift of the band toward lower wavenumber.
171
I
1800
1
I
I
I
2000
1900 Wavenumber
I
2100
/cm-’
Fig. 3. EMIRS spectra for 30% “CO+ 708 13C0 sample on Pt electrode in 1 M H,SO,,. Spectrum D was obtained first using 50 to 250 mV modulation with near saturation coverage of CO. The other spectra are for lower coverage samples produced by the partial oxidation of CO. The respective modulation ranges are: (A) 300 to 500 mV, (B) 50 to 450 mV and (C) 50 to 250 mV. See text for discussion of band shapes and relative intensities. Arrows show the AR/R ranges as follows: (A) +0.1~10-~ to -0.1x10-3;(B),(C)and(D) +1X10m3 to -1X10U3.
However, the most outstanding feature of the spectra shown in Figs. 3A-3C is the appearance of two bipolar bands rather than one. Figure 3C, for which the same potential modulation range was used as for the spectrum in Fig. 3D, shows all three properties: the development of a second bipolar band, the shift of the band to lower wavenumber, and the decrease in intensity. Comparison of Figs. 3B and 3C shows that the double bipolar feature behaves qualitatively in the same way as the single bipolar band, shifting towards higher wavenumber and growing in intensity when the more positive potential limit is increased. The dependence of the amplitudes of the bands upon coverage, lower coverage giving smaller bands, is seen clearly from a comparison of the spectra in Figs. 2 and 3. In Fig. 3, the shift of the double bipolar band to higher wavenumbers as the potential is made more positive is also apparent. A major deduction can be made from the potential dependence of the spectra for various values of coverage and for a range of isotopic mixtures: the electrode potential changes only the frequency of the observed spectral bands without altering either their number or relative intensities whereas the coverage can produce changes
172 TABLE
1
CO stretching mode of adsorbed t2 CO + I3 CO mixtures at high surface coverage H,SO,. Variation of the observed position of the EMIRS band and calculated band with isotopic composition Percentage
Zero crossing
‘2co
(EMIRS
band,
modulation)/cm100 70 50 30 10 0
2070 2065 2060 2047 2022 2022
Calculated
position 50-450
I
mV
band position
(90% coverage,
on a Pt electrode in 1 M position of the infrared
’
f, = 16.6 mdyn/A,
f,, = 0.15 mdyn/A,
A1,,,
=14 cm-‘)/cm-t
2076 2072 2066 2057 2032 2031
a These f, and f,, values were selected to fit IRRAS data at 400 mV. A reduced appropriate for an average over the 50-450 mV range, such as 16.50 mdyn/A, would approximately -6 cm-t. 1 A = 0.1 nm.
f, value more shift the bands
in all three characteristics; thus the origins of (~v/&?Z)~ and (&/&9), are almost certainly different. The appearance of the double bipolar feature upon partial oxidation of CO and consequent reduction in coverage does not occur for all isotopic mixtures. Figure 4 shows spectra for each of the other fixed composition samples over the same modulation range used for Fig. 3A. These spectra were all obtained immediately following spectral scans using 50 to 600 mV modulation. All 50 to 600 mV spectra showed the intensity drop and shift to lower wavenumber expected for lower coverage samples. The spectra shown in Figs. 4 and 3A show that the double bipolar band is not observed under low coverage conditions for samples containing a single CO carbon isotope or for mixed isotopic samples whose ‘sC0 content is 30% or less. Spectra shown in Figs. l-4 display two basic patterns for EMIRS bands in the 1950-2100 cm-’ region. A single bipolar band appears for all high coverage samples but a second bipolar band is observed at low coverage for mixed isotopic samples containing more than 30% 13C0. Since it is firmly established that the bipolar band for ‘*CO at saturation coverage on platinum arises due to a potential dependence of the frequency of the CO stretching mode, it is reasonable to attribute the double bipolar EMIRS feature to two infrared bands which are both potential dependent. Infrared bands of the shapes shown in Figs. 5A and 5C would produce the EMIRS difference spectra shown in Figs. 5B and 5D respectively if the position of the infrared band was shifted towards higher wavenumber by 3 cm-’ incremental steps. The simulated EMIRS bands in Figs. 5B and 5D resemble closely the observed contours of bands in Figs. l-4. In a study of the infrared spectra of mixed ‘*CO + l3 CO samples adsorbed on Pt (111) and Pt (OOl), Crossley and Ring [5] observed an intensity stealing by the higher frequency band. In the most simplistic description of coupling between two CO molecules adsorbed on adjacent metal atoms, Moskovitz and Hulse [15] describe
173
-t--Q-. la00
1900
2000
2100
Wavenumberlcm-'
Fig. 4. EMIRS spectra of low coverage samples of 12CO+ 13C0 mixtures of fixed composition obtained wth 300 to 500 mV modulation of Pt electrode in 1 M H,SO,. Compositions of samples are: (A) 100% 12C0, (B) 70% ‘*CO, (C) 50% ‘*CO, (D) 10% 12C0 and (E) 100% 13C0. Arrows show +1 x 1O-3 to - 1 x 10m3 AR/R ranges. See text for discussion of line shapes and intensities.
the intensity stealing by the near superposition of the strong infrared surface-active, in-phase ‘2CO-‘2C0 and ‘2CO-‘3C0 modes with the weaker, in-phase 13CO-‘3C0 mode and forbidden out-of-phase ‘2CO-‘3C0 mode at the lower frequency. Crossley and Ring point out that in the dipole coupling model, the positive coupling constant necessary to produce the observed intensity stealing requires coupling of oppositely oriented dipoles. They suggest such orientations for CO molecules on bridged and linear sites. Similar intensity stealing, as observed for isotopic mixtures, is thought responsible for observation of only the higher frequency linear CO component of a linear CO-bridged CO couple. Unlike the high coverage EMIRS spectra, the reflection absorption infrared spectra of Crossley and Ring do show two distinct bands for isotopic mixtures with 50% or more 13C0. Either coupling between CO molecules adsorbed on a platinum electrode in contact with an aqueous
174
lB.0
0.0
2000
-7
2100 UAVENUPBER
I
UA~ENU~~BER
cd
I
cm-’
B 10.
-6. 0. UAVENUHBER
C
Icm-’
li~vt34~ii~~~
Icm-’
II
Fig. 5. (A) Calculated infrared band contour for 30/70 ‘2CO+‘3C0. Sample at 90% coverage on a surface 75% Pt (111) and 25% Pt (100) with effective CO stretching constant of 16.600 mdyn/A and mteraction force constant of 0.150 mdyn/A. Band contours were calculated assuming a full wtdth at half height for a Lorenzian fine at the position of each of the coilective CO modes. Intensity units are arbitrary. (B) EMIRS bands that would result from shifting the position of the infrared band in 3 cm-’ increments toward higher wavenumbers to represent the band at the more positrve potential wtth the band positioned as in (A) at the less positive potential. The electrode reflectivity is assumed flat over this range as was observed with those in (A). Since there IS no atmospheric or solution band, R is flat and taken as 1. (C) Calculated infrared band contour as in (A) for the sample at 40% coverage. (D) EMIRS bands corresponding to the infrared band in (C) calculated as in (B).
electrolyte is stronger than coupling between CO molecules adsorbed from the gas phase onto Pt (111) or Pt (001) or there is a higher coverage for the solution experiments. The EMIRS observation of two bipolar bands at lower coverage for mixed isotopic samples might result from a reduction in coupling between adsorbed molecules and samples with a high percentage of r2C0 might have sufficient intensity stealing to make the lower frequency component not observable. Model calculations
A simple three parameter vibrational force field model, similar to the one used by Moskovitz and Hulse [Is], was applied to estimate the magnitude of the coupling term necessary to fit the observed spectra. All parameters were deter~ned empirically from the spectral data. The advantage of such a mode1 is that, by giving alternative physical interpretations to the parameters, it can be used to describe either vibrational coupling or dipole coupling. The three parameters are an effective
175
CO stretching force constant (f,), a CO-CO interaction constant (f,,), and a line width for a CO infrared band (AV,,,). Calculations were made for CO molecules oriented perpendicular to the metal surface on both a six nearest neighbour Pt (111) surface and a four nearest neighbour Pt (100) surface. It was thought that these two calculations would show a pattern which could be used to make deductions for a polycrystalline surface. One hundred CO molecules were placed randomly on one of the Pt surfaces which was expanded from a 10 X 10 size to give the desired degree of coverage. Periodic boundary conditions were used such that the block containing 100 adsorbed CO molecules represented a section of an infinite two dimensional surface. The principal force constant in the vibrational coupling model is an effective CO stretching force constant whose value differs from that for gas phase CO due to interaction of CO with the metal surface and with solution species. In the dipole coupling case the principal force constant represents all interaction forces between the adsorbed CO molecule and the metal plus interaction forces with solution species. CO metal interactions would include donation of CO electrons directly to the metal, image interactions with the metal, and a d-electron back bonding from the metal to CO. The magnitude of the principal force constant, f, = 16.000 mdyn/A (= aJ Am2), is dete~ned from the frequency that an isolated CO molecule would have when adsorbed from solution onto the electrode. At the potential of zero charge this frequency is estimated to be 2030 cm-’ since this is the location where a band first appears in EMIRS and IRRAS spectra of acidic solutions upon initial addition of CO. It corresponds to the zero coverage value, 2064 cm-“, suggested by Shigeishi and King [l] for CO on platinum when a vacuum rather than a solution exists next to the CO, The shift to 2064 cm-’ from the value for an isolated CO molecule, 2143 cm-‘, has been attributed to metal-CO interaction weakening the CO bond; the further shift to 2030 cm-’ is evidence for additional weakening by interaction with solution species. The magnitude of the coupling constant, f,, = 0,150 mdyn/A, is chosen to produce a high coverage value of 2075 cm -’ for adsorbed “CO molecules, In the vibrational coupling model, this term is simply the coupling force constant between adjacent CO molecules due to coupling of their vibrational motions via the metal surface. For the dipole coupling case, the interaction term represents the sum of contributions by direct dipole-dipole coupling and interaction with images of other dipoles. With the randomly positioned samples and the above choices of f, and f,,. the frequencies and intensities of all the collective modes of the 100 molecules were determined through diagonalization of the Wilson F matrix as outlined by Moskovitz and Hulse [15]. To establish a total infrared band contour, a full width at half height of 14 cm-’ was applied to a Lorenzian line at each calculated frequency. This is the value produced by the formula given by Moskovitz and Hulse ]lS], although they state they used a 7 cm- ’ full width. Crossley and King [5] estimate a band width for CO on platinum of 9 cm-’ and Kunimatsu et al. [9] report the same value for electrosorbed CO. However, the calculations we report will not be appreciably affected by the choice of bandwidth within this range.
176
jll,~ :l~~etch 0 2000
2100
UAVENUMBER/Cm-’
A
0
2000
2100
WAVENUMBER
/cm-’
UAVENUMBER E
WAVENUHBER
/Cm-
C
B
/cm-
VAVENUMBER
/cm-’
F
Fig. 6. Calculated infrared absorption for ‘2CO+‘3C0 samples for 90% coverage on Pt (111) ( -) and Pt (100) (- - -) with effective CO stretching force constant of 16.600 mdyn/A and interaction force constant of 0.150 mdyn/.k. Elgenvaiues and intensities at 1 cm-’ Intervals are shown by solid vertical lines for Pt (111) surface. Band contours were calculated using a Lorentian line shape with 14 cm-’ full width at half height. (A) 100% “CO. (8) 70% “CO, (C) 50% “CO, (D) 30% “CO, (E) 10% ‘*CO and (F) 100% 13C0.
Figure 6 shows the calculated infrared band contours for the six 12C0 + I3 CO mixtures studied using the parameters given above. The solid curve gives the contour expected on a Pt (111) surface at 90% coverage and the dashed curve the contour on a Pt (100) surface at the same coverage. For the Pt (111) calculation the frequency of the infrared modes and their intensities for the 100 molecule collection are indicated by the solid vertical lines. Using the same force constants for both low area surfaces, one observes a greater resolution of the band into two components for the Pt (100) surface where coupling is limited to four nearest neighbours. With a polycrystalline surface one might expect to build a band contour from contributions from various low area components. From the two cases shown in Fig. 6 one would expect to observe a broadening of the low wavenumber side of the band for Pt (111). Figures 5A and 5C were calculated by taking the sum of the contours for the 50/50 ‘*CO + l3 CO bands for Pt (111) and Pt (100) with a 3 : 1 weighting. Table 1 compares the calculated peak frequencies for the various isotopic mixtures with the observed zero crossing points between the positive and negative components of the EMIRS bands using 50 to 450 mV modulation data. Agreement could be improved between calculated and observed frequencies by use of a lower value for f,. The
117
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UAVENlJNt3ER
WAVENUMBER I cm-’
UAVENUHBER/cm-’
I
/cm-’
B
A
d:. !’
I
/
s
II
AL 1
2 1.00’
UAVENUMBER
/cm-’
0
I \
I
-\
il Ilk
1
2100
2000
WAVENUMBER /cm-’
E
/’ ,i
0
2100
2000
UAYENWBER
/ cm-’
F
mixtures for 40% coverage on Pt (111) and Pt Fig. 7. Calculated infrared absorption for ‘2CO+‘3C0 (100) surfaces. All other terms have meanings and values shown in Fig. 6.
calculated values thus correspond to a more positive potential than the 250 mV average for this modulation range. At lower coverage the coupling is less effective and the intensity stealing into the higher frequency component is thus reduced. Figure 7 shows the same type of band contour for a surface with 40% coverage as was shown in Fig. 6 for 90% coverage. From the intensities and band positions of the lower coverage EMIRS bands, it is estimated that the coverages used were less than half the high coverage values. In Table 2, the calculated peak positions for the low coverage bands are compared with the zero crossing frequencies for the two bipolar bands observed with 300 to 500 mV modulation. The agreement for the higher frequency band shows the 16.600 mdyn/A principal force constant corresponds better to a 400 mV electrode potential. The poorer agreement for the lower frequency component is expected due to its overlap with the higher band. The general trends for larger frequency changes between 50% and 70% 13C0 than between 70% and 90% “CO, the direction for movement of the high and low frequency components of the double bipolar bands as the isotopic ratio is changed, and the absence of a double band for spectra of between those observed and pure i2C0 or pure 13C0 show excellent agreement calculated. The calculations certainly serve to establish that the observation of two bipolar bands for these mixed isotopic samples originates from the reduced coupling at lower coverage. The calculated band contours also show, qualitatively, the intensity pattern found
178 TABLE 2 CO stretching mode of adsorbed ‘2CO+‘3C0 mixtures at low surface coverage on a Pt electrode in 1 M H,SO,. Variation of the observed posttion of EMIRS bands and calculated position of the infrared band with rsotopic compositton Percentage ‘2CO
Zero crossing positions (EMIRS bands, 300-500 mV modulation)/cm- ’
Calculated band positions (40% coverage, ,f, = 16.6 mdyn/A. f,, = 0.15 mdyn/A, A%,,,, = 14 cm-‘),/cm-’
100 70 50 30 10 0
2042 2060 = 1982,2049 1987,2027 1993,2026 2020
2050 X981,2043 1993,2052 2001f 2031 2010, 2029 2006
a The experimental procedure used did not allow preparation of low coverage samples with identical surface coverages. This sample appears due to its positton and intensity to have a higher coverage than the other samples in this table.
in the experimental spectra. In moving from 100% i3C0 to 100% 12C0 there is a rapid drop in the intensity of the EMIRS difference peak followed by a steady rise to the “‘CO maximum value. The largest shift in frequency for the band maxima occurs at low i3C0 composition due to the strength of the coupling between adsorbed molecules. One possible explanation for the potential dependent shift of the CO stretching frequency is that the availability of metal electrons for back bonding into the II* CO orbital is strongly potential dependent. As the electrode is polarised more positively, the electron concentration decreases, which should reduce back bonding and strengthen the CO bond. The band shift with coverage could be similarly attributed to increased competition for metal electrons as the number of nearest neighbours increases. One could potentially test such a model by using the observed potential dependence of the band as a guide. Such a test is suggested in Fig. 8. In these calculations the principal force constant was made to increase as the number of nearest neighbours increases. The two examples shown in Figs. 8A-C and 8D-F have respectively 44% and 67% of the total coverage shift attributed to a change in the principal force constant. To maintain a close fit to the measured total shift with coverage, the interaction constant must be decreased if f, is increased. For both calculations shown in Fig. 8 with coverage dependent force constants, the reduced coupling produces contours at 90% coverage with even more resolution of separate bands than the coverage independent force constant model gave at 40% coverage. The observed EMIRS spectra require therefore the highest possible coupling constant consistent with the observed coverage shift, i.e. the total shift with coverage needs to be produced by the fii term. A more definitive conclusion would be possible from a study using single crystal electrodes. We also note that an altemative mechanism involving a first order Stark effect might well explain the potential dependent frequency shift [22,23]
179
WAVEtd.JMBER /cm-’
UAVENUHBER
UAVENWBER
D
/cm-’
C
A
/cm-’
,,AVENUNEER
E
/cm-’
UAVENUMSER
/cm-’
F
Fig. 8. Calculated infrared absorption for ‘2CO+‘3C0 samples at 90% coverage on Pt (111) assuming coverage dependent force constants. If a, and n, are the number of occupied nearest neigbbour sites for atoms i and j, the force constants are: (A), (B), (C) f, = 16.600+0.065~,, f,, = 0.150 [I - n, + n, -2/201; (D), (E), (F) f, =16.600-O.O92n,, f,, = 0.085 [I -(n, + n, -2),‘201. (A). (D) 70% ‘*Co; 0% (E) 505% ‘*CO; (C), (F) 30% ‘*CO.
CONCLUSIONS
(1) The appearance of only a single bipolar EMIRS band for all isotopic mixtures of i2C0 and 13C0 studied when the CO surface coverage is high and the wavenumber position of the band for various isotopic mixtures demonstrate strong coupling between the adsorbed molecules, i.e. there is a considerable intensity stealing by the higher wavenumber modes. Lack of observation of a lower wavenumber component for samples with 50 to 70% t3C0 as was observed in reflection absorption infrared spectra for CO isotopic mixtures adsorbed on Pt (111) and Pt (001) surfaces may indicate stronger coupling for CO adsorbed from the solution phase or it may be due to differences in the extent of island formation between the single crystal and the comparatively rough polycrystalline surface. (2) The infrared absorption band may be modelled for all isotopic mixtures studied by a simple three parameter vibrational coupling model with an effective stretching force constant of 16.600 mdyn/& interaction constant of 0.150 mdyn/& and an individual infrared band full width at half height of 14 cm-‘. The wavenumber dependence upon electrode potential of the CO stretching mode is represented by a change in the value of the principal force constants, a change of 0.100 mdyn/A shifting the band 6 cm-‘.
180
(3) Varying the electrode potential over a range of 600 mV produces substantial changes in the adsorbate-metal bond but any concomitant variations in the coupling interaction between adsorbed molecules are undetectable. The different potential and coverage dependences for the band position indicate that the increase in wavenumber with coverage is due entirely to an, increase in the coupling between adsorbate molecules whereas the increase in wavenumber with potential is due entirely to an increase in the principal force constant. ACKNOWLEDGEMENTS
We wish to acknowledge Dr. A. Rest of South~pton University for supplying the ‘3C0 sample and for assistance in use of his vacuum line for preparing mixed isotopic samples. Discussions of vibrational coupling models with the late Professor John Overend of the University of Minnesota were most helpful. Professor Overend and Mark Severson kindly provided the portion of the computer program used to calculate the frequencies and intensities for a Pt (111) surface. Support for this work from the Office of Naval Research, Was~ngton, and from the Science Research Council is gratefully acknowledged. J.W.R. wishes to acknowledge the Oakland University Faculty Research Fund for a Summer Fellowship. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
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