Electrode IRRAS

K. and Enyo, M. (1989) Surface species produced on Pt electrodes during HCHO oxidation in sulfuric add solution as studied by infrared reflection-absorption spectroscopy (IRRAS) and differential electrochemical mass spectroscopy (OEMS)./. Electroanal. Chem., 258, 219-225. 

Kunimatsu K, Golden WG, Seki H, Philpott MR. 1985a. Carbon monoxide adsorption on a platinum electrode studied by polarization modulated FT-IRRAS. 1. Co Adsorbed in the double-layer potential region and its oxidation in acids. Langmuir 1 245 -250. 

Kunimatsu K, Aramata A, Nakajima N, Kita H. 1986. Infrared spectra of carbon monoxide adsorbed on a smooth gold electrode Part II. EMIRS and polarization-modulated IRRAS study of the adsorbed CO layer in acidic and alkaline solutions. J Electroanal Chem 207 293-307. 

Polarisation modulation infrared rejiection-absorption spectroscopy (PM-IRRAS or JRRAS). Potential modulation IR studies rely on switching the potential at a reflective electrode between rest and active states, generating difference spectra. However, the EMIRS technique has several drawbacks the relatively fast potential modulation requires that only fast and reversible electrochemical process are investigated the absorption due to irreversibly chemisorbed species would be gradually eliminated by the rapid perturbation. Secondly, there is some concern that rapid modulation between two potentials may, to some extent, in itself induce reactions to occur. 

Figure 2.47 (a) 1RRAS spectra of CO adsorbed on Pt in 1.0 M HCI04 saturated with CO. The electrode potential was held constant at (i) 50 mV vs. NHE, (ii) 250 mV, (iii) 450 mV and (iv) 650 mV. (b) Difference spectra resulting from subtraction of the IRRAS spectra (iii) and (i). (c) EMIRS spectrum of Pt electrode in CO-saturated 1 M HCIO modulated between + 50mV and 450mV. (d) A schematic representation of spectra at two potentials which could produce an EMIRS spectrum similar to that shown in (c). The IRRAS spectra in (a) rule out this possibility. After Russell et al, (1982). Copyright 1982 American Chemical Society,... 

Figure 2.4 FT-IRRAS spectrum of the electrode/electrolyte interface for CO adsorbed qn a smooth platinum electrode in 0.5 M H2SOA at 0.4 V vs. NHE. From W.G. Golden, K. Kunimatsu and H. Seki, J. Phys. Chem., 88 (1984) 1275. Copyright 1984 American Chemical Society.

A Pt electrode in acidic aqueous methanol was cleaned of adsorbed methanol fragments by pulsing the potential to 1.4 V vs. RHE for a few seconds prior to stepping the potential to various values between 0.4 V and 0.05 V for 15min, after which IRRAS spectra were collected. The spectra are shown in Figure 3.34. As can be seen from the figure, the frequency shift is... 

IRRAS spectrum collected at 0.4 V. In this way the build-up of C=Onds with adsorption time was monitored, as shown in Figure 3.36(b). From Figures 3.36(a) and (b) it is clear that the deactivation of the platinum electrode is closely related to the increase in C=Oads. 

Electrochemical infrared spectroscopy can be used on all kinds of electrodes and for all substances that are IR active. It is particularly useful for the identification of reaction intermediates, and has been used extensively for the elucidation of the mechanisms of technologically important reactions. A case in point is the oxidation of methanol on platinum, where linearly bonded = C = O (i.e., CO bonded to one Pt atom) has been identified as an intermediate Figs. 15.7 and 15.8 show EMIRS [6c] and IRRAS [8] spectra of this species. Near 2070 cm-1 the EMIRS spectrum shows the typical form produced by a peak that shifts with potential. This shift can be followed in the IRRAS spectrum... 

Figure 15.8 IRRAS spectrum of the C-0 stretching band of linear CO derived from CH3OH at various electrode potentials. Data taken from Ref. 8.

The present conference paper provides a discussion of some representative findings from our recent studies on these topics, with the aim of comparing and contrasting some of the distinctive properties of SERS and IRRAS as applied to fundamental interfacial electrochemistry. We limit the presentation here to a brief overview further details can be found in the references cited. All electrode potentials quoted here are with respect to the saturated calomel electrode (SCE). 

The combination of surface enhanced Raman scattering (SERS) and infrared reflection absorption spectroscopy (IRRAS) provides an effective in-situ approach for studying the electrode-electrolyte interface. The extreme sensitivity to surface species of SERS is well known. By using polarization modulation of the infrared beam for IRRAS, the complete band shape is obtained without modulating the electrode potential. 

Figure 2. PM-IRRAS (left) and SERS (right) spectra for Ag electrode in 0.01M cyanide solution for various potentials, -0.4V to -1.4V (Ag/AgCl).

Figure 9. PM-IRRAS spectra for Ag electrode in 0.03 M azide in 0.1 M Na01O4. These are obtained by taking the difference of the spectra taken at the specified potential and at -0.95 V (Ag/AgCl). (Reprinted with permission from ref. 50. Copyright 1988 American Institute of Physics.)...

In Figure 1 we show the PM-IRRAS spectra for a Pt electrode exposed to saturated C0/H S0 solutions which contain various concentrations of different organic nitriles. For comparison, we have also included a spectrum recorded in saturated CO/H SO with no added nitrile. The adsorption step was accomplished by pulling the electrode back into the bulk solution and cycling the potential from 0.55 V(SHE) up to 1.15 V, down to 0.0 V, and back to 0.55 V. The spectra were recorded after re-positioning the electrode against the cell window while the potential was held at 0.55 V. 

Adsorption kinetics. We can also study the adsorption kinetics of the nitrile component. This is illustrated by the IRRAS spectra shown in Figure 3, which demonstrate the influence of electrode potential on the competitive adsorption of CO and CjH CN. Curves a and b show the control experiments, in which spectra were recorded-at different potentials in saturated CO electrolyte with no nitrile added. A saturated CO layer is produced in both cases, but the frequency is different at the two potentials i.e., v(CO) 2085 cm at 0.55V, vs. v(C0) 2070 cm at 0.05 V. The magnitude of this shift is in agreement with the potential dependence of v(C0) discussed above. 

Figure 3. PM-IRRAS spectra of CO adsorbed on Pt in 0.5 M I SO + 0.2 M CjH CN solutions as a function of electrode potential during the adsorption step (see text).

Fig. 37. Schematic comparison between ATR-IR and IR reflection absorption (IRRAS) spectroscopy. In IRRAS, the sample (e.g., an electrode) is typically metallic and reflects the incident IR radiation. The IR beam has to pass through a liquid film twice.

Fig. 18. IRRA spectra of CO adsorbed on Au electrodes in 0.1 M HCIO4 at (a) 0.1 V at low CO coverage (b) -0.2 V at low CO coverage (c) -0.2 V at high CO coverage (all potentials referred to SCE). (After [61]). Reprinted by permission of Elsevier Science.

Fig. 32. FT-IRRAS spectra for adsorbed CN on polycrystalline Ag (a) and Au (b) electrodes in 0.1 M KCN+0.5M K2SO4. Adsorption potentials as indicated. (Taken from [111]). Reprinted by permission of Elsevier Science...

Fig. 33. FT-IRRAS spectra of the adsorbed CN on a polycrystalline Cu electrode at different adsorption potentials. Experimental conditions as indicated in the figure. (After [116]). Reprinted by permission of the Electrochemical Society.

Fig. 34. PM-IRRAS spectra of adsorbed CN on a polycrystalline Pt electrode in KCN +1 M NaClO solutions at -0.5 V vs. SHE. The CN concentrations are indicated. Note the formation of bridge-bonded CN at low CN concentrations. (After [120]). Reprinted by permission of Elsevier Science.

The adsorption of cyanide on Pd electrodes was studied by using a combination of polarization and potential modulation (FT-IRRAS and SNIFTIRS) [124]. The reason for this combination is to enhance the surface signal, since two FT-IRRAS spectra taken at two different potentials were ratioed to obtain the SNIFTIRS spectrum. Despite this effort, a solution band at 2135 cm" (also observed with s-polar-ized light) persists in the spectrum (Fig. 36). The spectrum in this Figure taken with p-polarized radiation presents, however, a strong band at 1980 cm", which was attributed to a bridged-bonded cyanide ion, and a weak band at 2065 cm" assigned to linearly adsorbed C-down CN". 

Fig. 36. Spectra for polycrystalline Pd electrode in 0.1 M NaC104 +25mM NaCN. The solid line is a combination of SNIF-TIRS and IRRAS spectra. Two SNIFTIRS spectra at -0.9 V (reference potential) and -1-0.7 V vs. Ag/AgCl are ratioed to obtain the spectrum shown. The dotted spectrum was obtained by a SNIFTIRS with s-polar-ized radiation. (After [124]). Reprinted by permission of Journal of Chemical Physics AIR...

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