Reduced CO2 on a polycrystalline Rh electrode in acid solution: electrochemical and in situ IR reflectance spectroscopic studies1

PERGAMON

Electrochimica Acta 44 (1998) 1369±1378

Reduced CO2 on a polycrystalline Rh electrode in acid solution: electrochemical and in situ IR re¯ectance spectroscopic studiesp M. C. AreÂvalo a, *, C. Gomis-Bas a, F. Hahn b a

Departamento de QuõÂmica FõÂsica, Universidad de La Laguna. C/ AstrofõÂsico Fco., SaÂnchez s/n, 38200 La Laguna, Tenerife, Spain b Universite de Poitiers, UMR CNRS No. 6503, Equipe Electrocatalyse, 40 Av. du recteur Pineau, 86022 Poitiers, France Received 10 October 1997

Abstract Comparative studies of the electrochemical behaviour of CO2 on a polycrystalline Rh electrode in acid solution by electrochemical and spectroelectrochemical measurements are reported. Aqueous solutions of H2SO4, HClO4 and CF3SO3H have been used. In order to produce r-CO2 adsorbates, di€erent adsorption potentials (E ad) in the Hatom electrosorption potential range and in the hydrogen evolution region (HER) were chosen. The values of charge density of adsorbate oxidation (q ox) depend on E ad, on the solution composition and on the adsorption time (t ad). At constant E ad and t ad, q ox increases in the following order: H2SO4 < CF3SO3H < HClO4 in agreement with the adsorption strength of anions. The same sequence was observed in the oxidation peak potentials of the adsorbates produced. To analyzed the in¯uence of r-CO2 adsorbates in the HER, polarization curves have also been recorded. Studies have been carried out with in situ infrared re¯ectance spectroscopy. Three techniques were employed: SNIFTIRS, SPAIRS and potential steps. Several main IR bands are observed and associated with CO2 and CO (COB bridge-bonded and COL linearly bonded). Other bands are also observed between 1000±1800 cm ÿ 1 which show the existence of other adsorbed species as well as around 1000±1100 and 2800±3000 cm ÿ 1. This observation leads to the supposition that another more reduced species is formed from CO2. # 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: Reduced CO2; Rhodium electrode; Acid solutions; Cyclic voltammetry; Re¯ectance infrared spectroscopy; Electrocatalysis

1. Introduction The adsorption of CO2 on catalytic electrodes as reduced CO2 (r-CO2) is favoured in the H-adsorbing region of the metal. The formation of adsorbates requires the interaction between CO2 molecules and Hadatoms. The adsorbates play an important role as

The authors want to dedicate this work to Professor Bernard Beden, both for his teaching skills and in recognition of his contribution to the development of in situ spectroelectrochemistry. * Corresponding author. p

precursors in CO2 electroreduction and as probable intermediates in mechanisms of electrooxidation of simple organic molecules. The electrochemical reduction of CO2 has been studied extensively on various metal electrodes in aqueous solution from fundamental and practical points of view. Recently a review has been reported in which a great number of papers published about this subject have been summarized [1]. The electrochemical behaviour of CO2 has been highly investigated on di€erent Pt electrodes ( [2±4] and references cited therein) with the e€ects of the surface structures and of the solution composition as main subjects. Also, there are some references on Pd

0013-4686/98/$ - see front matter # 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 2 5 9 - X

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electrodes ( [5], [6] and references cited therein). However, not many exist on Rh electrodes [7±9]. In this work, we report comparative studies of the electrochemical behaviour of CO2 on a polycrystalline Rh electrode in acid solution to demonstrate the role of the adsorbed anions on the r-CO2 electroformation. Voltammetric techniques and FTIR re¯ectance spectroscopy have been employed. Aqueous solutions of H2SO4, HClO4 and CF3SO3H have been used. Most preceding studies have been carried out in H2SO4 medium [7±9]. Not much work has been published in HClO4 solution [9] and not even one using CF3SO3H as base electrolyte. Results are also compared to those previously obtained on di€erent Pt electrodes [3]. 2. Experimental The electrochemical measurements were performed in a three-electrode ¯ow electrochemical cell [10]. In most of experiments, the working electrode was a polycrystalline Rh plate encased in a glass holder which o€ered a bright metal active plane (Goodfellow, 99.9% purity). A Pt electrode with the geometric area ca. 1 cm2 as counter electrode and a hydrogen reference electrode (rhe) in the solution were used. The base electrolyte solutions were prepared from 70% HClO4, 98% H2SO4 (Merck, p.a.) and CF3SO3H (TFMSA, Aldrich >99%) in Milli-Q* water. The CO2 and CO were obtained as has been previously described [10, 11]. The experiments were performed at room temperature under either Ar (99.999% purity), CO2 or CO saturation depending on the type of experiment. The solution was saturated with CO2 at open circuit for 25 min, the rest potential being ca. 0.8 V. At this potential, it is possible to avoid the adsorption of CO2 as well the reduction of ClO4ÿ in the case of HClO4 solutions ( [12] and references cited therein). Then, a voltammetric scan was run before the electrode potential was stepped to a pre-set adsorption potential (E ad) and held there for a prescribed time (t ad), which was subsequently changed from 30 to 1200 s. The potential was then stepped to 0.320 V whereupon the electrolyte solution was replaced with fresh electrolyte base until the current was near zero (ca. 300 s). Next, a voltammetric scan beginning in negative direction from 0.02 to 1.20 V at di€erent scan rates was run to obtain the adsorbate oxidation charge (Q ad). The values of Q ad were obtained by subtracting the voltammograms at t ad = 0. Di€erent values of E ad in the H-atom electrosorption potential range and in the hydrogen evolution region were chosen. The polarization curves were recorded between 0.030 and ÿ0.120 V at 0.5
10 ÿ 3 V s ÿ 1. Most experimental details of the in situ IR measurements have been

described elsewhere [3] with the following di€erences: a Fourier transform infrared spectrometer (Bruker IFS 66v) was equipped with a special re¯ectance device allowing observation of re¯ectance spectra at the electrode±electrolyte interface with the IR light beam passing entirely through a vacuum purged atmosphere. A Globar source and a nitrogen cooled mercury cadmium telluride (MCT) detector (Infrared Associates) were used. Instrument control, data acquisition and processing were performed using a personal computer (COMPAQ Prolinea 486) equipped with OPUS 2.3 software. A special three electrode spectroelectrochemical cell was designed with a CaF2 IR transparent window allowing the beam to pass through a thin electrolyte layer (a few mm) and to be re¯ected with an incidence angle of 658. Spectra were taken using the ppolarized light and recorded in the spectral region from 1000 to 3000 cm ÿ 1. The working electrode was a polycrystalline Rh disc (high purity metal from Johnson Matthey) polished with alumina of a particle size down to 0.05 mm and rinsed with Millipore Milli-Q* water. The surface cleanliness was checked by cyclic voltammetry in the base electrolyte solution. The reference electrode was also a reversible hydrogen electrode and the counter electrode was a platinum loop wire surrounding the syringe plunger at the top of which the working electrode is ®xed. Before each experiment, the purity of the spectroelectrochemical cell was tested. After that, at E ad for CO2, the working electrode was pushed against the CaF2 window and CO2 was introduced into the cell for 30 min. For the complete elimination of CO2, N2 was bubbled for 30 min in the solution in absence of CO2. Three types of IR re¯ectance experiments were carried out: Single Potential Alteration Infrared Re¯ectance Spectroscopy (SPAIRS), Subtractively Normalised Interfacial Fourier Transform Infrared Re¯ectance Spectroscopy (SNIFTIRS) and potential steps. In the SPAIRS technique [13] the electrode re¯ectivity Ri is recorded at potential Ei each 100 mV during the ®rst voltammetric sweep at a slow rate (usually 1  10 ÿ 3 V s ÿ 1). Each IR spectrum is the Fourier transform of the average of 128 coadded interferograms, which takes about 20 s, i.e. over ca. 20 mV. Spectra are normalised as relative re¯ectivity change DR = (Ri ÿ R ref)/R ref, where R ref is the re¯ectivity at a given reference potential E ref (usually the initial adsorption potential). In the SNIFTIRS techniques [14], 128 interferograms are accumulated, averaged and Fourier transformed at two electrode potentials E1 and E2, (equivalent modulation frequency of 0.025 Hz). This process is repeated 50 times before calculating the normalised spectra as the relative re¯ectivity change DR = (R2 ÿR1)/R1. The potential window E2 ÿ E1 is kept constant and moved all along the po-

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tential range of interest (usually from 0 V to 0.700 V/ RHE). With potential steps, 6400 interferograms are collected at each potential E1 and E2 and results are displayed as DR/R = (R E2 ÿR E1)/R E1. 3. Results and discussion 3.1. Voltammetric data For the sake of comparison, Fig. 1 shows cyclic voltammograms obtained in the presence of 5  10 ÿ 2 mol dm ÿ 3 H2SO4, HClO4 and TFMSA (1, 2 and 3 respectively). Voltammograms in H2SO4 and HClO4 are similar to those previously reported [15, 16], whereas the voltammogram in TFMSA is only similar to that obtained in H2SO4 solution in the positive going potential scan. In the negative going potential scan a positive shift in the hydrogen adsorption peak is observed for TFMSA, which is probably caused by a weak anion/surface interaction [17]. The adsorption of sulphate and perchlorate anions on Rh electrodes has been extensively studied [12, 18± 21] and the adsorption of TFMSA on mercury and platinum electrodes leads to a weak speci®c adsorption of CF3SO3ÿ as compared to sulphate anions [22±24]. This feature allowed us to compare CO2 electroreduction in the three acids. The adsorption of CO2 on Rh electrodes in acid media yielding r-CO2 adsorbates takes place in the Hatom electrosorption potential range. However this potential range depends on the composition of the solution. Fig. 2 shows the anodic stripping cyclic voltammograms of r-CO2 at di€erent scan rates, v, in 5  10 ÿ 1 mol dm ÿ 3 HClO4. The adsorbed CO2 oxidation current consists of an apparent single peak that largely overlaps the O-electro-adsorption potential range. The

Fig. 1. Cyclic voltammograms obtained at 0.1 V s ÿ 1 in di€erent base electrolytes (1) 0.05 M H2SO4, (2) 0.05 M HClO4 and (3) 0.05 M CF3SO3H.

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half width of this peak decreases as v is diminished. Voltammograms show a positive shift of the peak potential (Ep) as v increases from 0.01 to 0.2 V s ÿ 1 (dEp/ d log v = 0.060 V dec ÿ 1). A similar shift of Ep has also been observed on Pt electrodes under the same experimental conditions [25]. When the potential scan direction is reversed, the Rh oxide electroreduction current partially overlaps the H-electrosorption potential region and at a low v it is possible to observe a new cathodic current, which is observable only when the electrode surface is oxide-free. Similar behaviour was also observed by Rhee et al. [17] during the electrooxidation of chemisorbed CO on Rh(100) and these authors concluded that the cathodic current was due to the reduction of perchlorate anions. A similar e€ect can be also observed in the voltammogram of base electrolyte (Fig. 2a) in the course of the positive going potential scan. A negative current is produced in the double layer potential range indicating the occurrence of a cathodic process, which has also been attributed to the electroreduction of perchlorate ions [26]. These contributions are no longer observed when using 5  10 ÿ 2 mol dm ÿ 3 HClO4 with v = 0.1 V s ÿ 1. Accordingly for 5  10 ÿ 2 mol dm ÿ 3 HClO4, H2SO4 and TFMSA di€erent E ad in the range 0.120 V < E ad < ÿ0.100 V and t ad in the range 30 s < t ad < 1200 s was chosen. The adsorbate oxidation charge density, q ox, depends on both, E ad and t ad, and on the solution compositions. As E ad becomes more negative, q ox increases and Ep is shifted to more positive potential values. These e€ects are also observed with increasing t ad. The dependence of q ox, specially at the highest E ad values for the three acids is shown in Fig. 3, where the in¯uence of the solution composition is clearly seen. In HClO4 the electrosorption of CO2 begins at a higher potential than TFMSA and H2SO4 solutions. For H2SO4 the threshold E ad was about 0.120 V, in agreement with the data previously reported [7], although the experimental conditions were di€erent. The in¯uence of anions can be also seen in the electrooxidation process of adsorbed residues which starts at ca. 0.45 V for HClO4 and or TFMSA and at 0.5 V in H2SO4. When the E ad is 0 V, Ep is at 0.70 V in HClO4, 0.72 V in TFMSA and 0.74 V in H2SO4 (Fig. 4). The di€erence in Ep becomes more remarkable when the E ad is decreased and the degree of surface coverage by r-CO2 species, y(CO2), increases to reach the maximum value y(CO2) = 0.82. Di€erences in voltammetric response observed in the three acids can be explained as a competitive adsorption between adsorbable anions and H-adatoms electroformation. This is re¯ected in the reaction between CO2 molecules and H-adatoms yielding to produce r-CO2.

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Fig. 2. Anodic stripping cyclovoltammograms of r-CO2 adsorbates at di€erent v. E ad = 0.06 V. Dashed line corresponds to 0.5 M HClO4 without CO2.

The potential dependence of sulphate adsorption on polycrystalline Rh has been found to be as a bell shaped function [16] between ca. ÿ0.08 and 0.75 V (vs. SHE). E ad values chosen in this work are in this potential range. The sulphate adsorption has a maximum at ca. 0.32 V. On either side of the maximum adsorption value a slow decrease is observed. At ca. 120 V the adsorption drops by 19% in comparison to the maximum adsorption value, and at ca. 0.07 V a drop of 45% is observed. The negative shift of the value of E ad in aqueous H2SO4 to produce a valuable quantity of r-CO2 adsorbate could be related to this speci®c adsorption of the sulphate/bisulphate anions on Rh. The strong blockage of Rh adsorption sites by sulphate anions shifts H-electrosorption reactions towards more negative potentials. It is worth noting that the

threshold value of E ad is coincident with where the decrease of sulphate adsorption starts to take place. In the case of HClO4 solution, the absence of adsorbable anions means that the r-CO2 formation can take place at a higher potential. The behaviour in TFMSA is more similar to that observed in HClO4. This can be taken as an indirect indication of the weak adsorption of this anion on Rh electrodes, as has been also observed on Pt electrodes. In the electrooxidation process, the anion e€ect is seen by a shift of Ep towards more positive potential as the anion adsorption strength increases. This could be interpreted both as an anion e€ect on adsorbate structure or as a competitive reaction between the adsorbable anions and the electrosorbed OH-species resulting from the underpotential decomposition of

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Fig. 3. Electrooxidation charge density of r-CO2 adsorbates vs. E ad. t ad = 1200 s. (w) in 0.05 M H2SO4, (T) 0.05 M HClO4 and (q) 0.05 M CF3SO3H.

water. The latter could be related to the fact that about 20% of the surface is not covered by r-CO2 adsorbates. When the same experiments were carried out on differently oriented Pt electrodes [3, 25], no di€erences in the q ox values were found. This is in accordance with the fact that the sulphate anion is more strongly bound to Rh than to Pt [16]. Thus, the potential range for the electroreduction of CO2 to form r-CO2 on Rh polycrystalline electrode increases in the following order H2SO4 < TFMSA < HClO4 and for the electrooxida-

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tion of the adsorbed residues in the opposite order, in agreement with the adsorption strength of the anions. Though the electrooxidation process of CO is beyond the scope of this work, for the sake of comparison some experiments under the same experimental condition (v and t ad) in HClO4 and H2SO4 medium were realised with CO. Three E ad, (ÿ0.1, 0.0 and 0.1 V), were chosen and the q ox values were ca. 450 mC/ cm2 in both media, independent of E ad used. Fig. 5 shows the electrooxidation voltammograms corresponding to r-CO2 and CO adsorbates. A minor di€erence in the CO electrooxidation potential in both media is found. It is interesting to observe that the electrooxidation peak of r-CO2 adsorbates is found in a more negative potential oxidation range than where the oxidation of CO residues occurs. The coverage by CO(ad) is higher than by r-CO2 adsorbates. 3.2. Polarization curves To obtain more information about the electroformation of r-CO2 adsorbates, polarization curves were taken from 0.03 to ÿ0.12 V, i.e. in the potential range where the q ox values are higher. Figures 6a and 6b show the polarization curves in CO2-free solutions and in CO2-saturated solutions in H2SO4 and HClO4 respectively. The polarization curves in the CO2-free solution present the same behaviour in both media. In ®rst region, between 0.03 and ÿ0.02 V, the graph is a straight line with a slope of ÿ0.030 V/decade (corresponding to the

Fig. 4. Electrooxidation voltammograms of r-CO2 adsorbates in di€erent base electrolytes. E ad = 0.1 V, t ad = 1200 s and v = 0.1 V s ÿ 1. The inset shows the potential program used.

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Fig. 5. Comparison of anodic stripping voltammograms of CO(ad) and r-CO2 adsorbates in 0.05 M H2SO4 and in 0.05 M HClO4. E ad = 0.0 V, t ad = 1200 s and v = 0.1 V s ÿ 1.

hydrogen evolution reaction HER) and in the second linear region it has a slope of about ÿ0.122 V/decade from ERÿ 0.07 V, where the formation of H2 bubbles could have some in¯uence on the HER. Polarization curves run in the CO2-saturated solution show a decrease in the current over the entire potential range. A minor linear potential region is still observed (slope = ÿ0.030 V/decade) whose current density values are coincident with those in the CO2free solution. The current decrease starts at a slightly more positive potential in HClO4 than H2SO4 solutions, indicating that the hydrogen evolution reaction (HER) is partially inhibited. If it is assumed that the inhibition is caused by r-CO2 adsorbates blocking active site on the surface, this would appear when y(CO2) is ca. 0.8 in both media, in agreement with the results obtained from voltammetric data. These results are slightly di€erent to those recently reported [8], probably because they are obtained under a potential scan at 0.5  10 ÿ 3 V s ÿ 1. The r-CO2 electroformation on Rh electrodes is relatively slow and presumably the stationary conditions in our experiments were not attained. 3.3. Spectroscopic data

Fig. 6. Tafel plot (a) in 0.05 M H2SO4 and (b) 0.05 M HClO4 in presence and absence of CO2 in solution.

Three techniques are used: SNIFTIRS, SPAIRS and potential steps. These techniques allow the detection of the adsorbed species and the reaction products near

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the electrode surface, but SPAIRS is more convenient for the detection of species in solution (reaction products), whereas SNIFTIRS and potential steps are more adapted to the detection of adsorbed species. 3.3.1. SNIFTIRS results The SNIFTIRS technique is particularly useful for studying adsorbed species involving potential dependent absorption bands. Adsorbates can be detected and it is also possible to follow their evolution if it occurs during the potential modulation. A whole potential range from 0 to 0.6 V was studied using ppolarization (Fig. 7). Fig. 7 shows SNIFTIRS spectra after CO2 adsorption in the three acid media. Di€erent modulation potential limits were used. First, at low potential, several absorption bands of anions can be observed. ClO4ÿ , and CF3SO3ÿ bands can be e€ectively HSO4ÿ , SO2ÿ 4 detected in the 1000±1300 cm ÿ 1 wavenumber region. The ClO4ÿ anion absorption band appears at ca. 1117 cm ÿ 1. The mean anion in H2SO4 solution is HSO4ÿ

Fig. 7. SNIFTIRS of r-CO2 adsorbates in di€erent base electrolytes (1) 0.5 M HClO4, (2) 0.5 M H2SO4 and (3) 0.5 M CF3SO3H. p-polarization.

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and has absorption bands at ca. 1200 and 1050 cm ÿ 1. The SO2ÿ anion shows a band at ca. 1100 cm ÿ 1. 4 Finally, CF3SO3ÿ presents a broad absorption band covering the 1000±1350 cm ÿ 1 range which involves a peak multiplicity due to the symmetric and asymmetric stretching vibration of the S±O bonds and to the C±F bond stretch. At ca. 1640 cm ÿ 1 the band corresponding to interfacial water is also present. Other bands are also observed such as the CO2 absorption band (ca. 2345 cm ÿ 1) which is present in the three spectra, even at low potentials. Perhaps the CO2 is not totally reduced and this band therefore appears like a upward band. This remark depends on the media and is true for CF3SO3H medium. An upward band (2030 cm ÿ 1) and an asymmetric bipolar band centre (ca. 1895 cm ÿ 1) can be attributed to linearly adsorbed CO (COL) and bridge-bonded CO (COB) respectively. When the potential increases, both are bipolar and completely disappear from the higher potential limits, (0.3±0.6 V), indicating the oxidation of the adsorbate residues oxidation and the CO2 band appears (downward band) principally for CF3SO3H. New weak bands at ca. 1050 and 1250 cm ÿ 1 are detected in HClO4 solution as well as another band in the 2800±3000 cm ÿ 1 region but the signal to noise ratio is high. At the moment, it is dicult to say with security what the origin of these last bands is, but they may be due to the formation of methanol. Comparative spectra with CO and CO2 adsorbed at low potential in HClO4 solution are shown in Fig. 8. Both present the CO bands (COL and COB). However the ratio between COL and COB is higher in the COsolution than in CO2 solution. A shift has also been observed towards low wavenumber for r-CO2 adsorbates. This fact was also reported during the oxidation of HCOOH on rhodium electrode as observed by EMIRS [27]. At higher potentials, COads from gaseous CO dissolved in the solution is still present when rCO2 is already oxidised. 3.3.2. SPAIRS results This technique is more suitable for detecting reaction products. Only the preliminary results obtained in HClO4 media are presented here and Section 3.3.3. In Fig. 9, SPAIRS spectra are shown. The spectra are normalized and are calculated using (1/DE) (DR/R) with DE = Ei ÿ E ref. This kind of calculation leads to a noisy ®rst spectrum in comparison to the following ones. The bipolar COB band is present at lower potentials and becomes monopolar upward at 0.66 V, simultaneously with the emergence of a new very intense downward band at 2345 cm ÿ 1 corresponding to CO2 formation. This illustrates the complete oxidation of adsorbed residues at this potential. The CO2 band appears for potential greater than 0.57 V in agreement with the voltammetric results. Only CO2 is detected as

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Fig. 9. SPAIRS at various potentials of r-CO2 adsorbates in 0.5 M HClO4. E ad = 0.10 V, E ref = 0.12 V. p-polarization. Fig. 8. Comparative SNIFTIRS of CO(ad) and r-CO2 adsorbates in 0.5 M HClO4. p-polarization.

oxidation products. According to Leung and Weaver [28, 29], the extinction coecient for CO2 is about 1.5 that of CO. Thus, their band intensities should be di€erent if only CO and CO2 are present on the surface. 3.3.3. Potential step results This technique is similar to SNIFTIRS technique, but without potential modulation and averaging. With this procedure, the spectra were taken at di€erent potentials. The potential program is depicted in the inset of Fig. 10 and four potential steps were chosen. In the ®rst potential step (0 V) the r-CO2 electroformation occurs and in the latter (0.5 V) its oxidation begins to take place. In Fig. 10 three spectra are illustrated corresponding to several calculations. The spectrum (a) compares the r-CO2 at two potentials (0 and 0.300 V). A weak downward CO2 band and a bipolar COB can be observed along with two very intense downward bands in 1640 cm ÿ 1 and 1150 cm ÿ 1 produced by interfacial

water and perchlorate anions respectively. When the oxidation of r-CO2 occurs, (see spectrum (b)), at 0 and 0.500 V, new bands appear in 2800±3000, 1250±1500 and in 1050 cm ÿ 1 wavenumber regions, in addition to the COL band. These results show clearly that CO is not the only species formed during the electroreduction of CO2 at E = 0 V. Given the location of the absorption bands observed between 2800±3000 cm ÿ 1 (see spectra (b) and (c) in Fig. 10), the origin of these bands is clearly attributable to ±CH3 vibration, therefore the formation of methanol at this potential could be possible. Those bands between 1250 and 1500 cm ÿ 1 could be due to ±CH and ±C±O vibrations and the band at 1050 cm ÿ 1 to C±O of an alcohol (though these could be also attributed to carbonate or bicarbonate as it has been recently reported on Pt singlecrystal electrodes [30]). The standard electrode potential for the reaction CO2 …g† ‡ 6H‡ ‡ 6eÿ $ CH3 OH…aq† ‡ H2 O 0

…1†

is E = 0.016 V (vs. SHE) [31], which indicates that the formation of CH3OH could be thermodynamically possible. This reaction could be also supported by the

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Fig. 10. Spectra obtained during potential steps of reduced CO2 formed from CO2 saturated 0.05 M HClO4 solution. E1 is 0.0, E2 0.3, E3 0.0 and E4 0.5 V. (a) DR/R = (R E2 ÿR E1)/R E1, (b) DR/R = (R E4 ÿR E1)/R E1 and (c) DR/R = (R E4 ÿR E2)/R E2.

catalytic properties of rhodium which is employed as catalyst for the selective synthesis of a number of organic compounds from CO and H2. However further studies will be carried out such as an in situ analysis of reduction products to completely con®rm this reaction. 4. Conclusion The correlation of electrochemical and spectroelectrochemical data can provide more complete information about r-CO2 electroformation. From voltammetric results, the dependence of this process on solution composition is clearly seen. A shift

toward more negative values of E ad, as the adsorption strength of anions increases, is observed. In H2SO4 solutions a competitive adsorption equilibrium between ÿ + SO2ÿ ions would be involved to form 4 /HSO4 and H H-adatom on the electrode surface which causes the reaction of the CO2 and the H-adatoms to take place at more negative potentials. When the adsorption of anions is weaker the r-CO2 is possible at higher potentials. Some di€erences in the adsorbate structure could be assigned to anion e€ects as can be seen from SNIFTIRS results. In the three media, COL and COB bands are present, but with di€erent ratios, the former being weaker in H2SO4 and TMFSA than in HClO4.

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SPAIRS data show that COB is mainly present in HClO4. When these results are compared to SNIFTIRS spectra it is possible to conclude that the potential modulation process can induce changes in the adsorbates structures. The CO/CO2 intensity ratios (ca. 0.16) also indicates that other adsorbates contribute to the CO2 formed. This is in agreement with the q ox values obtained from the voltammetric data (an average value of 300 mC cm ÿ 2 for the three media). Potential jump spectroscopic results also con®rm the formation of more reduced products than CO during the CO2 electroreduction process. When these results are compared to those previously obtained on Pt electrodes one can conclude that the anion e€ect is more remarkable on Rh than Pt electrodes. The q ox values obtained from the electrooxidation of adsorbates formed at lower E ad could indicate the presence of other reduced species. Other measurements where potential steps and SPAIRS techniques are combined at more negative potentials in the three acidic media, are being realised in our laboratories. A more extensive presentation and discussion of these results will be made in a forthcoming publication. Acknowledgements Financial support for this work by the Gobierno de Canarias (Direccion General de Universidades e InvestigacioÂn) under Research Contract No. 93/040 is gratefully acknowledged. A part of this work was developed within the frame of the Accion Integrada Hispano-Francesa (1995) de los Ministerios de EducacioÂn y Ciencia y de Asuntos Exteriores of Spain. References [1] M. Jitaru, D.A. Lowy, M. Toma, B.C. Toma, I. Oniciu, J. Appl. Electrochem. 27 (1997) 875. [2] J. Sobkowski, A. Czerwinski, J. Phys. Chem. 89 (1985) 365. [3] M.C. AreÂvalo, C. Gomis-Bas, F. Hahn, B. Beden, A. AreÂvalo, A.J. Arvia, Electrochim. Acta 39 (1994) 793. [4] N. Hoshi, T. Suzuki, Y. Hori, Electrochim. Acta 41 (1996) 1647. [5] S. Taguchi, A. Aramata, M. Enyo, J. Electroanal. Chem. 372 (1994) 161.

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