Screening effects in surface infrared spectra of CO adsorbed on electrodes: application to isothermal desorption measurements

Analytica Chimica Acta 397 (1999) 53–59

Screening effects in surface infrared spectra of CO adsorbed on electrodes: application to isothermal desorption measurements C. Korzeniewski ∗ , J. Huang Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061 USA Received 3 September 1998; accepted 17 November 1998

Abstract Infrared spectra of CO adsorbed on platinum electrodes were examined for effects of dielectric screening. Screening of the incident infrared radiation field by the adlayer polarization typically results in non-linear relationships between adsorbed CO vibrational band intensities and CO surface coverages. It is shown that the solvent in the electrochemical environment can reduce the strong Beer’s law deviations observed for adsorbed CO in vacuum by maintaining the adlayer polarizability more constant with changes in CO surface coverage. The screening analysis was applied to correct adsorbed CO infrared peak absorbances in measurements of rate constants for CO desorption from electrodes. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Isothermal desorption measurements; Adlayer polarization; Chemisorption; Dielectric screening in infra red spectra

1. Introduction Infrared spectroscopy is an important ancillary technique in electrochemistry, because it can be used to identify species that participate in electrochemical processes. One of the most common areas for application of infrared spectroscopy in electrochemistry has been in the study of chemisorption [1–4]. In this regard, the bonding of halide [2,5] and organic [1] species on electrodes has been investigated, and carbon monoxide (CO) adsorption has received the most attention [4,6]. Interest in adsorbed CO stems from its importance as an intermediate and potent surface poison in many organic electrochemical oxidation reactions [7]. In addition, adsorbed CO has important physical prop∗ Corresponding author. Tel.: +1-806-742-3059; fax: +1-806-742-1289 E-mail address: [email protected] (C. Korzeniewski)

erties that have made it a good candidate for fundamental investigations of chemisorption on electrodes. First, adsorbed CO can be detected on electrodes down to low coverages, near 1/10 of a monolayer [6,8,9]. Second, the energy of the C–O stretching vibrational bands provides information about the adlayer structure and interfacial environment [4,8,10,11]. Finally, in aqueous solution CO surface coverages can be determined fairly reliably, which has made it possible to use infrared spectroscopy to study the structure of CO adlayers [8,10,12,13] and the interfacial environment during reaction of organic species that form adsorbed CO as an intermediate [14,15]. From an analytical standpoint, it has often been important to be aware of instances when the intensities of adsorbate vibrational bands do not reflect the coverage of these molecules on the surface [16–21]. The most striking departures from Beer’s law have been encountered in experiments with different iso-

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topes of an adsorbate [4,8,9,18,20,22]. When mixtures of 12 CO and 13 CO adsorb on a metal surface, for example, intensity shifts toward the highest energy band (the 12 CO band) associated with each surface-CO coordination geometry (linear, two-fold bridging, etc.) [16–19,22,23]. The response arises because the vibrational motions of molecules in the adlayer are coupled, typically by dipole-dipole interactions [16–19,22,24,25]. The intensity effects can be understood through consideration of the symmetry of the adlayer vibrational modes [18]. A related, but less well-studied influence upon adsorbate band intensities is dielectric screening [17,24,26–30]. This phenomenon is the screening of the incident infrared radiation electric field by polarizable electrons in the adsorbate layer. For CO on metal surfaces in vacuum, the screening effects increase as the CO surface coverage becomes greater [17,19,24]. A non-linear relationship between CO vibrational band intensity and surface coverage results, because the electric field of the incident radiation does not remain constant throughout the measurements [19]. The present report explores dielectric screening in infrared spectra of adsorbed CO on electrodes. In addition to screening by CO, adsorption of solvent at low CO coverages also influences the intensities of the C–O stretching mode vibrational bands. The solvent appears to reduce the changes in adlayer polarizibility that occur as the CO surface coverage varies. This solvent effect improves linearity in Beer’s law plots. Results are presented for the strong C–O stretching mode of linearly coordinated (atop) CO on platinum electrodes in acetonitrile and aqueous solutions.

2. Experimental section 2.1. Reagents High purity HPLC grade acetonitrile (Burdick & Jackson Brand, Baxter, Muskegon, MI) was dried in a suspension of calcium hydride (Aldrich, Milwaukee, WI) under nitrogen and distilled fresh prior to each use. Doubly recrystallized tetrabutylammonium tetrafluroborate (TBAF) was obtained from Chem-Biochem Research Inc. (Salt Lake City, UT). Carbon monoxide (Scott Specialty Gases, Plumsteadville, PA) was obtained in 99.8% purity. Perchlo-

ric acid (redistilled, 99.999% purity) was obtained from Aldrich. Aqueous solutions were prepared from distilled water that was further processed with a four-cartridge Nanopure II system. 2.2. Electrochemistry The working electrode consisted of a polycrystalline platinum disk (7.5 mm dia.) sealed into the end of a glass tube. The platinum surface was polished to a mirror finish with decreasing grades of abrasives down to 0.05 ␮m alumina. The glass tube also held a temperature sensor (AD590, Analog Devices, Norwood, MA) and heating element for thermostatted spectro-electrochemical experiments [31]. Before each experiment, the working electrode was polished mechanically with a slurry of 0.05 ␮m alumina in water followed by electro-polishing in 0.1 M HClO4 until defined hydrogen waves appeared. The electrode was rinsed with acetonitrile before transfer to the spectro-electrochemical cell. The infrared spectro-electrochemical cell was the standard three-electrode design constructed from Kel-F [21,32,33]. A platinum ring counter electrode and KCl saturated Ag/AgCl reference electrode were used in all experiments. The cell potential was controlled with a potentiostat (PAR Model 173) and waveform generator (Model PPRI, Hi-Tek Instruments, England). All potentials are reported with respect to the KCl saturated Ag/AgCl reference electrode. Before dosing the electrode with CO, the temperature of the spectro-electrochemical cell was brought to the desired value with a PID temperature controller (LFI-3551, Wavelength Electronics, Bozeman, MT). (A diode soldered between the temperature controller and one of the resistive heating wires ensured current flowed only during a heating cycle.) The CO adlayer was formed by bubbling CO into solution as described previously [34]. The cell was wrapped with a layer of aluminum foil followed by fiberglass insulation to maintain thermal stability throughout the experiments. 2.3. Infrared Spectroscopy Infrared spectra were obtained with a Mattson Instruments R/S-1 Fourier transform infrared spectrom-

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eter system (Madison, WI). After passing through the interferometer, the infrared beam was directed out of the instrument through the external beam port into a nitrogen purged chamber that contained the optics for reflectance measurements. A pair of a 60◦ off-axis parabolic reflectors (6.900 EFL, Mattson Instruments) was used for reflectance sampling. One reflector was placed immediately before the spectro-electrochemical cell and directed the infrared beam onto the working electrode surface through the spectro-electrochemical cell’s trapezoidal CaF2 window [21,32,33]. The second reflector was placed just after the cell to collect radiation reflected from the electrode surface. The beam was passed to the detector focusing mirror (75◦ off-axis parabolic reflector, 1.900 EFL, Mattson Instruments) and then onto a liquid nitrogen cooled mercury-cadmium-telluride (MCT) infrared detector (Model 997-0037, Bio-Rad, Cambridge, MA). Single beam infrared spectra were computed from the average of 256 interferograms collected at 2 cm–1 resolution. Data were acquired with WinFirst® software (Mattson Instruments) running on a 200 MHz pentium pro personal computer. A WinFirst® macro controlled the switching of the cell potential between sample and background values through a digital-to-analog converter board (DAC-02, ComputerBoards, Mansfield, MA).

3. Results and discussion

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Fig. 1. Time sequence of infrared spectra for CO adsorbed on a polycrystalline platinum electrode in 0.1 M TBAF in acetonitrile at 35◦ C. (A) Sample and reference single beam spectra were collected with the working electrode at 0.2 V and 0.0 V (vs. KCl saturated Ag/AgCl), respectively, at the indicated times. The displayed spectrum was computed by taking the negative of the logarithm (–log) of the ratio of sample and reference single beam spectra. (B) The corresponding sample single beam spectrum in panel A was ratioed to the background single beam spectrum obtained at the completion of CO desorption (after about 120 min). The displayed spectrum is the –log of the ratio.

3.1. Infrared spectroscopic measurements Fig. 1 shows infrared spectra of CO on a polycrystalline platinum electrode in acetonitrile solution. Spectra were recorded as a function of time with the spectro-electrochemical cell thermostatted at 35◦ C. Spectra in panel A of Fig. 1 were recorded by collecting a sample and a background single beam spectrum at 0.2 and 0.0 V, respectively, at the indicated times. The displayed spectrum was computed as the negative of the logarithm (–log) of the ratio of the sample and background single beam spectra [4,34–36]. The derivative-like bands that result are typical of adsorbed CO potential difference infrared spectra [4,34–36]. The CO surface coverage remains essentially constant

when the potential is switched between 0.0 and 0.2 V, but the C–O bond strength is different at the two potentials and leads to the observed wavenumber shifts [3,4,35,37]. The spectra in Fig. 1A allowed the CO adlayer coverage to be monitored at regular time intervals during the experiment. Panel B of Fig. 1 shows the bands that result when the corresponding sample spectrum in Fig. 1A is ratioed against the background single beam spectrum collected at the completion of CO desorption, after about 120 min. The use of a CO-free background spectrum allows the recovery of normal shaped absorbance bands. The bands in Fig. 1, which are centered 2080–2070 cm–1 , arise from C–O stretching modes of species coordinated to

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of the CO adlayer after spectra of the adsorbed layer have been acquired [8,41–43]. However, the oxidation of CO to CO2 is not possible in acetonitrile, because surface oxides that form readily in water are required for the reaction. Therefore, in non-aqueous solvents the adsorbed CO vibrational bands provide the only means to estimate the surface coverage, and non-linear absorbance/coverage dependencies need to be understood. To study the influence of screening on adsorbate vibrational bands, absorbance intensities (I) can be evaluated with the relationship [18,22,39]: I∝ Fig. 2. Plot of atop CO peak absorbance at 0.2 V as a function of time. All other parameters were the same as in Fig. 2.

a single platinum surface atom (atop CO) [4,22,34]. The maximum absorbance is nearly constant up to about 99 min and decays more rapidly thereafter. Fig. 2 shows the complete time dependence of the absorbance of the atop CO band in Fig. 1B. The absorbance change is initially slow and becomes more rapid after reaching about 90% of the initial value. We have observed similar responses for temperatures between 25–45◦ C. The time for the onset of rapid intensity loss becomes shorter as the temperature increases. Further, in our experiments using aqueous electrolyte solutions the CO adlayer has remained stable for more than 24 h at 25–45◦ C and 0.0–0.2 V. Therefore, we believe the type of response shown in Fig. 2 reflects a destabilizing influence of acetonitrile upon adsorbed CO. These results will be expanded upon elsewhere [31]. The present report examines the extent to which distortions caused by dielectric screening influence the ability to determine kinetic parameters from plots like the one in Fig. 2. 3.2. Possible effects of solvent dielectric screening Rate constants for CO desorption can be derived from the curve in Fig. 2 with knowledge of the relationship between the adsorbed CO spectral band intensities and the CO surface coverage. This relationship is often non-linear [19,38–40]. In aqueous solution, CO coverages on platinum electrodes can be established with infrared spectroscopy by measuring the CO2 evolved from the electrochemical oxidation

1 (1 + αe U˜ )2

(1)

In Eq. (1), α e is the adsorbate electronic polarizibility (α e ∼ = 2.5 Å3 for adsorbed CO [24,39,44]) and U˜ is the lattice sum term [18,22,24,45]. The U˜ term depends upon the adsorbate spatial positions and is proportional to d−3 , where d is the distance between adsorbates [18,19,22,24]. When there is a second polarizable species in the adlayer, such as solvent, the change in band intensity with CO surface coverage can be evaluated with Eq. (2) [17]: c1 (2) I∝ (1 + (c1 αe + c2 α 0 e )U˜ )2 In Eq. (2), c1 and c2 are the fractional coverages of CO and solvent, respectively, and α 0 e is the electronic polarizability of the solvent medium. In the present calculations α 0 e values of 4.4 Å3 , 1.5 Å3 , and 0 Å3 were employed for acetonitrile [46,47], water [30,46] and vacuum, respectively. The polarizability value for acetonitrile was calculated from the Lorenz-Lorentz formula and is in agreement with the value reported in [46] and with the mean polarizability values of CH3 X compounds reported in [47]. Also, it was assumed that each CO molecule removed from the surface would be replaced by a molecule of solvent; therefore, c1 + c2 = 1. The plots in Fig. 3 show the variation in atop CO peak absorbance for two packing densities of CO on platinum in the three environments mentioned previously: vacuum, water, and acetonitrile. Calculations were performed for CO in hexagonal lattices. √ In Fig. 3, the lattice constant was set to 4.8 Å (= ( 3√ × 2.77√Å) [48]), the value for CO on Pt(1 1 1) in the ( 3 × 3) R 30◦ structure. This structure is formed by CO on

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Fig. 3. Theoretical plots of the atop CO absorbance as a function of the fractional CO surface coverage for hexagonal CO lattices at two different packing densities on Pt(1 1 1) in the presence of acetonitrile (䊏, water (N), and vacuum (䉬). For comparison, the absorbance values were ratioed to the maximum absorbance at saturation coverage. The dashed line in each plot traces the linear Beer’s law response for the high coverage screening limit.

Pt(1 1 1) in ultra high vacuum (UHV) when the CO coverage relative to the substrate atoms is at or below 1/3 (θ = 0.33) [49,50]. The lattice sum, U˜ , was evaluated for an array of 9.98 × 105 molecules. Then, Eq. (2) was solved for band intensities at various CO surface coverages. Plots in Fig. 3 were constructed similarly, but using an adlayer lattice constant √ of 3.5 √ Å, appropriate for CO on Pt(1 1 1) in a ( 19 × 19) R 23.4◦ structure, which has been observed on platinum electrodes [22,51]. With a CO coverage of 13/19 [22,51], the adlayer studied in Fig. 3 has a higher packing density than the adlayer associated with the plots in Fig. 3. For the plots in Fig. 3, the upward curve in the trace for CO in vacuum is in good agreement with the experimental response [19,38–40]. The non-linear behavior results from screening of the incident electromagnetic radiation field by the adsorbed molecules. The screening, which is proportional to the electronic polarizibility of the adlayer (see Eqs. (1) and (2)), reduces

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the intensity of the incident infrared radiation at the surface. Therefore, the mean square electric field ratio /, where Er and Ei are the electric fields of the reflected and incident waves, respectively, is not linearly related to the CO surface coverage, because Ei , which is typically constant during an absorbance measurement, changes as the adlayer fills in. In vacuum, the polarizibility of the environment surrounding adsorbed CO molecules decreases as the CO surface coverage becomes smaller. However, when solution is present adsorbed CO molecules interact with solvent. Since the solvent polarizability can be greater or less than that of CO, the effect on the CO absorbance will vary. The polarizibility value for acetonitrile (4.4 Å3 ) is larger than for adsorbed CO (2.5 Å3 ) [24,46]. Therefore, screening is expected to be greater at low than high CO coverages when acetonitrile is present. Indeed, the plots in Fig. 3 for acetonitrile show the strongest deviations from linearity at low CO coverages. These solvent effects have also been observed experimentally [26,27,46]. In UHV, addition of acetonitrile to platinum containing a partial CO monolayer attenuates the adsorbed CO infrared band intensities [27,46]. Kizhakevariam and co-workers have demonstrated marked band attenuation by acetonitrile for CO coverages just below saturation on Pt(1 1 1) [46]. In contrast to acetonitrile, the electronic polarizibility of water (1.5 Å3 ) is less than for adsorbed CO. Hence, the plots in Fig. 3 for water show a slight positive deviation from the ideal linear response. Evidence for this type of behavior has also been seen experimentally (vide infra) [8,30]. It is interesting to compare the responses in Fig. 3 for solvent and vacuum. At low CO coverages in the presence of water, in particular, the CO molecules are surrounded by a medium with similar electronic polarizability. Thus, the spectral perturbations are not as great in water as in vacuum, where the electronic polarizability of the medium that separates CO molecules approachs zero at low CO coverages. It appears the solvents ’match’ the adsorbed CO index of refraction and help to keep the screening constant ( unchanging) as the CO surface coverage varies. Beer’s law plots have been constructed for adsorbed CO in aqueous solution [8,41,42]. Fig. 4 shows there is good agreement between the theoretical curve for the high density (θ max = 0.68) hexagonal CO adlattice and the experimental results for CO on Pt(1 1 1) in aqueous

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Fig. 4. Plot of atop CO absorbance versus fractional CO coverage for: (solid line) a hexagonal CO lattice with θ max = 0.68 on Pt(1 1 1), and (symbols) experimental normalized integrated absorbances from [8] for CO on Pt(1 1 1) at –0.25 V (vs. SCE) in 0.1 M HClO4 . The dashed line in the plot shows the linear Beer’s law response for the high coverage screening limit.

solution. It has come to light recently that the maximum coverage of CO in the experimental adlayer is likely about θ max ≈ 0.75 [22,51]. The correspondence between experiment and theory indicates the screening model may be useful for estimating non-linear effects in Beer’s law plots constructed from adsorbate vibrational bands. It is important to note that non-linearities in Beer’s law plots involving adsorbate vibrational bands also arise from coverage dependent phase transitions [17,19,38]. These are often observed as abrupt changes in the slope of Beer’s law plots and typically result from the redistribution of molecules among different coordination sites (i.e. atop versus bridging). In addition, linearity in Beer’s law plots can be improved upon the formation of CO islands [8,41]. The islands maintain the adsorbed CO electronic environment nearly constant down to low CO coverages [8,41,52,53]. This property has been valuable in the study of CO adlayer structure on electrodes [8,10]. 3.3. Kinetic parameters The plots in Fig. 3 indicate non-linearity in the Beer’s law relationship for adsorbed CO on platinum in acetonitrile is not as severe as would be expected

based on results from infrared measurements in UHV. Nevertheless, we used Eq. (2) to calculate θ /θ max values (assuming θ max ≈ 0.68) from the absorbances (normalized to Amax , the absorbance at the start of the experiment) in Fig. 2 for the rapidly decaying region of the plot. A desorption rate constant (zeroth order) of 5.7 × 10-4 monolayers s-1 (R2 = 0.993) was determined from these corrected values. Without correction, the desorption rate constant was found to be 7.4 × 10-4 monolayers s-1 (R2 = 0.995). At least for this limited example, screening does not appear to seriously influence the experimental absorbance values. This approach is being tested further with desorption plots from experiments at other temperatures and for higher order kinetic models.

4. Conclusions Relating adsorbed CO infrared band intensities to CO surface coverages requires consideration of screening effects. The departures from Beer’s law that arise can be studied with methods for analyzing adlayer vibrational modes [19,22,24]. Calculations indicate coadsorbed solvent may reduce the non-linearity that appears in Beer’s law plots for adsorbed CO in vacuum. Compared to vacuum, solvent in the adlayer can maintain the electronic environment, and hence the screening, more constant with changes in CO surface coverage. These results are consistent with earlier experimental findings [8,30,41,42].

Acknowledgements We thank Professor Mark Severson (Oakland University) for providing guidance with the simulations and Professor M.J. Weaver (Purdue University) for making references [30] and [53] available in advance of publication. We are grateful to the Office of Naval Research for support of this work.

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