J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 609-615 Voltammetric and Subtractively Normalized Interfacial FTlR Study of the Adsorption and Oxidation of L( +)=Ascorbic Acid on Pt Electrodes in Acid Medium: Effect of Bi Adatoms Miguel A. Climent,t Antonio Rodes, Maria J. Valls, Juan M. Perez, Juan M. Feliu and Antonio Aldaz Departamento de Quimica Fisica , Universidad de Alicante, Apartado 99,03080 Alicante, Spain The adsorption and oxidation of L(+)-ascorbic acid on bare and Bi-covered Pt electrodes in a sulfuric acid medium have been studied by cyclic voltammetry and in situ FTlR spectroscopy. The latter technique has allowed the direct detection of CO,,, from ascorbic acid adsorption, both at opencircuit and controlled potential, for the bare Pt surface. Under these latter conditions both linearly bonded and multi-bonded CO were detected.The existence of other strongly adsorbed species which are oxidized in the oxygen adsorption region has been demonstrated by cyclic voltammetry. The formation of all these adsorbed species is completely suppressed on the Bi-covered Pt surface under UPD conditions. Ascorbic acid oxidation was monitored by following the varia- tion of the IR spectra in the presence and in the absence of Bi3+ in solution. The electrocatalytic effect of the 6i adlayer can be explained mainly through the suppression of any dissociative adsorption step leading to the formation of strongly adsorbed species. Vitamin C, L(+)-ascorbic acid (AA), is a compound that takes part in many important life processes.Some of its bio- chemical functions, although they are not yet known precisely, are clearly related to its reductor character: AA is very easily oxidized, chemically or electrochemically, to dehydro-L( +)-ascorbic acid (DHA). This latter species undergoes a hydra- tion reaction leading to the hydrated product (HDHA). The electrochemical oxidation of AA on platinum has been studied less than that on or gold7p8 electrodes. The reversible, non-destructive, adsorption of AA at a Pt surface, partially blocking the hydrogen adsorption sites, was first reported by Brezina et aL9 These researchers suggested that this adsorption is responsible for the high overvoltage of the AA oxidation reaction on Pt, that proceeds in acid media via a (2H+, 2e-) irreversible process giving DHA; the adsorbed species would be oxidized in the potential range of surface oxide formation.Aldaz et a1." pointed out the forma- tion of a radical intermediate during the electrochemical oxi- dation of ascorbate on Pt and other electrodes. Karabinas et a1.,l1 on the basis of kinetic parameters, proposed a mecha- nism for the electro-oxidation of AA on Pt, suggesting the presence of anionic adsorbed intermediates ;the desorption of the product (DHA) would be the rate-determining step. Recently," the structure-sensitive character of AA electro-chemical oxidation has been demonstrated by using Pt single-crystal electrodes. Electrochemical evidence has also been given for the existence of spontaneous dissociative adsorption of AA on a Pt(100) surface.Under these condi- tions a strongly adsorbed species is formed, that is oxida- tively stripped at the same potential as CO (0.78 V).12 Therefore the oxidation of AA on Pt electrodes would consist of two pathways: direct oxidation to DHA without adsorp- tion, and another mechanism leading to CO, as a final product through dissociative adsorption of AA and forma- tion of CO as an adsorbed intermediate.', Formation of the same species on a polycrystalline Pt electrode has been con- firmed spectroscopically in an in situ Fourier-transform infra- red reflection-absorption spectroscopy (FTIRRAS) study by Xing et al.' The catalytic effects of several metal ions on the oxidation of AA on Pt electrodes were first reported by Takamura and t Permanent address: Department de Ingeniena de la Con-struccion, Obras Publicas e Infraestructura Urbana, Universidad de Alicante, Alicante, Spain.Sakam0t0.l~ These metal ions were adsorbed in under-potential deposition (UPD) conditions. It has been shown that single-crystal Pt(h, k, r) surfaces modified by irreversibly adsorbed Bi catalyse the oxidation of AA, changing the mechanism to a structure-insensitive process.12 This effect was attributed to the suppression of the dissociative adsorp- tion pathway.', The oxidation of AA on gold electrodes is also a structure-insensitive process and does not produce adsorbed CO.lS The aim of this work is to study the adsorption and oxida- tion of AA on polyoriented Pt bare electrodes and on the corresponding Bi-covered Pt surfaces, by cyclic voltammetry and SNIFTIRS (subtractively normalized interfacial Fourier- transform infrared spectroscopy).The study will be focused on the effect of the dissociative adsorption pathway on the overall AA electrooxidation mechanism. Experimental Solutions were prepared by dissolving the appropriate amount of L(+)-ascorbic acid (Merck p.a.) in 0.1 mol dm-3 H,SO, (Merck suprapur) as test electrolyte. Ultrapure water was supplied by a Millipore Milli-Q system. Deaeration of solutions was accomplished by argon bubbling. Spherical single-crystal platinum electrodes (polyoriented surface) were used for electrochemical experiments. Before each experiment the platinum samples were cleaned by heating in a gas-oxygen flame.They were then cooled in air, protected with a water droplet in equilibrium with air and transferred to the ~11.'~Other experiments were performed with a polycrystalline electrode extensively cycled between 0.06 and 1.4 V to compare these results with those obtained in the spectroelectrochemical cell, in which a polycrystalline electrode was employed. The potentials were measured against a reversible hydrogen electrode (RHE). Open-circuit AA adsorption experiments were performed following the experimental procedure described by Clavilier and Sun," without air exposure of the adsorbed species, as described below. The Bi-modified Pt electrodes were obtained by irreversible adsorption from a Bi3+ solution prepared by dissolving Bi,O, (Merck p.a.) in 0.1 mol dm-3 H2S0,.The clean elec- trodes were immersed in the Bi3+-containing solution for 30-120 s rinsed with water and immersed in the test solution, free of Bi3+ ions. Thus the adatoms were present only on the electrode surface. Alternatively, experiments were carried out in Bi3+-containing solutions, in a classical UPD pro- cedure.l4 The spectrometer employed was a Nicolet 5PC instrument with a liquid-nitrogen-cooled MCT detector. A glass external-reflection cell with a CaF, window was used for the spectroelectrochemical experiments. The bulk polycrystalline platinum working electrode mounted on a glass rod was pol- ished mechanically with successively smaller grades (1 .O, 0.3 and 0.05 pm) of alumina, and further steam washed.This electrode was then cycled between 0.06 and 1.6 V us. RHE until a stationary blank voltammogram was obtained. A platinum counter electrode was used; a saturated mercury(1) sulfate electrode was used as reference, but all measured potentials were converted to the RHE scale. The cell com- partment of the spectrometer was purged throughout the experiment by flowing clean air, free of CO, and H,O. The incident angle at the electrode surface was close to 60" with respect to the surface normal. Two types of FTIR spectra were obtained: (a) absolute spectra in which the reference spectrum was recorded with a 0.1 mol dm-3 H2S04 solution as test electrolyte.The sample spectrum was taken after adding the appropriate amount of AA. In both cases open-circuit conditions were employed. (b) Potential difference spectra were obtained by recording refer- ence and sample at different potentials with the same AA solution. Typically 920 spectra (approximately 2 scans s-', 8 cm-' resolution), were recorded at each potential with the voltage being switched every 92 scans. In order to monitor the AA oxidation process FTIR mea- surements were taken during slow potential sweeps (1 mV s-') controlled by software. This allows the acquisition of 99 interferograms in 50 s, corresponding to a potential window of 50 mV. The IR spectra obtained from these interferograms were ascribed to the centre of the potential interval where they were recorded.The first spectrum of the series was taken as the reference for the whole set. Results and Discussion Electrochemical Experiments Fig. l(a) shows the voltammetric behaviour of a spherical single-crystal Pt electrode in contact with a 0.1 mol dm-3 H2S04 solution during the first potential cycle including adsorption of an oxygen monolayer. The voltammograms corresponding to the first oxidation cycle of AA on the same f3 Fig. 1 Voltammograms of a spherical single-crystal Pt electrode in 0.1 mol dm-3 H,SO, + X mol dm-' AA solutions. X =(a),0, (b) 5.6 x (c) 4.7 x Sweep rate, I) = 50 mV s-'. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 electrode, at 5.6 x lop4 and 4.7 x lo-, mol dm-3 concen- trations, are also shown in Fig.l(b) and (c). Before these volt- ammetric curves were obtained the flame-treated electrode was placed in contact with the solution at 0.5 V. This poten- tial was held for 30 s in order to allow adsorption of AA from solution and then the potential was cycled between 0.06 and 0.5 V to determine the degree of suppression of hydrogen adsorption &,, (Sb = 1 -S,3, due to AA adsorption. The Sb values obtained were 0.57 and 0.66 for the low and high AA concentrations, respectively, in good agreement with pre- viously reported result^.^ It may be appreciated from Fig. 1 that AA adsorption takes place approximately to the same extent on all crystallographic orientations present on a spher- ical single-crystal Pt electrode.On a spherical single-crystal Pt electrode AA oxidation starts at potentials slightly higher than 0.5 V, i.e. before surface oxide formation [Fig. l(b)]. An additional surface process is observed at 0.79 V identical to that reported for the case of AA oxidation on Pt(100).12 The main AA oxidation peak on a spherical single-crystal Pt electrode appears at 0.9 and 1.0 V for the 5.6 x and 4.7 x lo-, mol dm-3 AA concentrations. Note, that in Fig. l(c), the higher oxidation density current is observed in the negative-going sweep rather than the positive-going sweep, at potentials below 0.8 V. This behaviour was also observed with Pt( 11 1) and Pt( 110) electrodes.12 This fact suggests the releasing of active sites upon oxidation of some adsorbed inhibiting species at high potentials.In order to isolate the irreversibly adsorbed species formed on the surface of the spherical single-crystal Pt electrode, open-circuit AA adsorption experiments l7 were performed. In Fig. 2 the various steps required for the electrochemical characterization of adsorbed species are reported : (a) the hydrogen adsorption-desorption profile of the bare Pt elec-trode in contact with the test electrolyte. (b) The hydrogen adsorption-desorption profile of the surface saturated with the adsorbed species, with 0.06 and 0.5 V as potential limits. This curve was recorded after putting the electrode in contact, for 1 min, with a 0.1 mol dm-3 AA-0.1 mol dm-3 H2S04 solution and then transferring to the test electrolyte without air exposure.(c) The oxidative stripping of the adsorbed species which leads to voltammetric profile with two marked peaks at 0.735 and 0.795 V. (d) The recovery of the hydrogen adsorption capability after oxidation of the adsorbed species. A comparison of the voltammograms shown in Fig. 2 with those reported for the dissociative adsorption of formic acid and methanol on platinum electrode^,'^ strongly suggests that the same irreversibly adsorbed species are formed in the adsorption processes of AA and of the smaller organic mol- ecules. From the potential range where this oxidative strip- ping takes place [see Fig. 2(c)], the adsorbed species can be identified as CO. The existence of two oxidation peaks at 0.735 and 0.795 V can be related to the structural heter- ogeneity of the electrode surface.As stated for formic acid and methan01,'~ the first peak can be ascribed to the strip- ping of the species adsorbed on (111) or (110) sites and the second to the same process on (100) sites. Quantitative analysis of the voltammetric results of Fig. 2 has been performed following ref. 17. The percentage of hydrogen sites blocked by the adsorbed species, S,, the per- centage of hydrogen sites recovered after its oxidative strip- ping, s,,and the number of electrons, n, transferred per Pt surface site during the latter process are 62%, 84% and 1.67 respectively. The n value obtained is consistent with the for- mation of both linearly bonded and bridge-bonded CO on the electrode surface.However the rather low value obtained for S,, in contrast with the 96% found for Pt(100)," suggests J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ! I, I'80 I' NI E 4O > E/V vs. RHE -80 Fig. 2 Voltammograms obtained with a spherical single-crystal Pt electrode in the different steps of a dissociative adsorption experi-ment with AA. (a) Voltammogram of bare electrode. (b) Hydrogen adsorption-desorption profile of the surface saturated with the che-misorbed species. (c) Oxidation of the chemisorbed species. (d) Hydrogen adsortion-desorption after oxidation of the chemisorbed species. Test electrolyte: 0.1 mol dm-3 H,SO,. u = 50 mV s-'. Immersion potential 0.5 V.that other species are adsorbed on the polyoriented Pt surface.These species need higher potentials for oxidation. Another open-circuit adsorption and isolation experiment was performed with a polycrystalline Pt electrode extensively cycled between 0.06 and 1.4 V. Under these conditions no additional changes of the electrode surface structure are expected during the stripping of the adsorbed species which oxidizes in the platinum oxide region." At the same time the surface structure conditions achieved in this way are similar to those reached in the FTIR experiments. Fig. 3 shows the results obtained. It may be appreciated from curve (c) that oxidation of two types of isolated adsorbed species occurs: 611 CO-like species that are stripped from the surface in a poten-tial range 0.6-0.9 V and other species that are oxidized at potentials higher than 0.95 V.The voltammetric curve corre-sponding to oxidation of adsorbed CO on this perturbed electrode is less resolved than the one corresponding to the polyoriented Pt surface (Fig. 2), showing the effect of the breaking of long-range ordered domains after extensive elec-trochemical cycling. The comparison of curves (a) and (d)in Fig. 2 and 3 shows clearly that a higher S, value is obtained in the latter case. The oxidation at potentials where the Pt surface is covered by oxygen is typical behaviour of unsaturated molecules that are adsorbed on Pt and, in principle, are able to form surface n-complexes" (e.g.ethene,22acetylenez3and acetonez4).The electrochemical results reported here do not allow determi-nation of the nature and origin of the adsorbed species that are oxidized in the Pt oxide region. They may be formed in the same dissociative adsorption process of AA leading to COads,or they may be the result of a different interaction between the AA molecule and the electrode surface (e.g. a different spatial orientation). However, the effect of the surface anisotropy in promoting the formation of such adsorbed species is clear. This can be seen by comparing the S, values with Pt(100) and polyoriented Pt after stripping the adsorbed species that are oxidized below 0.9 V (see preceding paragraphs). Ascorbic Acid Oxidation on a Bi-couered Pt Electrode The voltammograms shown in Fig.4 were obtained with a spherical single-crystal Pt electrode modified by irreversibly adsorbed Bi at high coverage, OBi = 0.86, (OBj = 1 -OH). Curve (a)corresponds to the Bi-modified electrode in contact with the test electrolyte, in the absence of AA. In this curve, the voltammetric peaks appearing at 0.63 and 0.9 V corre-spond to surface processes related to the presence of the Bi adlayer.' * Those at the higher potentials involve a partial desorption of the adlayer with a releasing of hydrogen adsorption sites. AA oxidation on a Bi-modified (irreversibly adsorbed) spherical single-crystal Pt electrode produces a single wave with peak potential at 0.615 and 0.68 V for the 8.1 x and 6.8 x mol dm-3 AA concentrations, respectively, Fig.4(b) and (c). These peak potentials are in good agreement with those obtained for AA oxidation on the three Pt basal planes highly covered with Bi. This confirms the structure-(C) --* i 2 mA cm-2 ju -100 t Fig. 4 Voltammograms of a spherical single-crystal Pt electrode Fig. 3 Voltammograms corresponding to the dissociative adsorp-highly covered with irreversibly adsorbed Bi (OBi = 0.86), in 0.1 mol tion of AA on a polycrystalline Pt electrode extensively cycled dm-3 H,SO, + X mol dm-3AA solutions. X = (a) 0,(b)8.1 x loe4 between 0.06 and 1.4 V. Same conditions as in Fig. 2. (c) 6.4 x lo-'. u = 50 mV s-'. insensitive character of the AA oxidation under these condi- tions.12 A comparison of curves (b) and (c) in Fig.1 and 4 shows that the AA electrocatalysis produced by the Bi adlayer is substantiated in a 0.3 V decrease of the peak potentials and a slight increase of the peak current densities for the Bi- modified electrodes. Nevertheless, it can be appreciated that the electrocatalytic effect of Bi is less effective in the potential region of the foot of the AA oxidation wave: this electro- chemical reaction starts at a potential only slightly higher (0.05-0.1 V) on the Pt bare electrodes than on the corre- sponding Bi-modified Pt surface. Note that the AA oxidation wave in Fig. 4(c) is practically identical to that obtained with a polycrystalline Pt electrode in contact with a Bi3 +-containing solution. This indicates that the electrocatalytic effect of Bi adatoms toward AA oxi-dation is independent of the way the adlayer is obtained: irre- versible adsorption or UPD from solution, provided that a high Bi coverage is reached on the Pt surface. Open-circuit AA adsorption experiments have also been performed on Bi-modified Pt electrodes.Fig. 5, solid line, shows the first and second voltammetric cycles obtained in the test electrolyte with a spherical single-crystal Pt electrode highly covered with irreversibly adsorbed Bi. The upper potential limits were 0.75 and 1.4 V, respectively. The broken line in the same figure shows the behaviour of the same elec- trode newly covered with Bi, and put in contact with an AA solution (same conditions as in the experiments reported in Fig.2 and 3). It can be deduced from the electrochemical behaviour shown in Fig. 5 that no irreversibly adsorbed species have been formed on the Bi-modified Pt surface since both experiments led to the same voltammetric curves. The small differences in current density can be attributed to the slightly different Bi coverages reached in each experiment. In this way it can be stated that AA does not undergo sponta- neous dissociative adsorption, and consequently CO is not produced on the Bi-modified Pt-polyoriented electrode. Spectroelectrochemical Experiments Fig. 6 shows the in situ FTIR absolute spectrum obtained with the polycrystalline Pt electrode in contact with a 0.05 rnol dm-3 AA-0.1 mol dm-3 H2S04 solution at open-circuit, with the incident beam in the p polarization plane.This spectrum represents the difference between those obtained with the aforementioned solution and with the test electrolyte. It is interesting to note that the spectrum shown in Fig. 6 has been collected under the same conditions, i.e. at open-circuit, as those used for the isolation of the strongly a 'r E/V vs. RHE -1 00 -t Fig. 5 Solid line: voltammogram of a spherical single-crystal Pt electrode highly covered with irreversibly adsorbed Bi (near monolayer) in 0.1 rnol dm-3 H,SO,. Broken line: similar experi- ment with a newly Bi-covered surface, but after contacting the elec- trode with an AA + H,SO, solution. D = 50 mV s-'. Immersion potential: 0.1 V. (See text for other conditions.) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2200 2000 1800 1600 1400 1200 1000 waven u m ber/cm -' Fig. 6 Absolute FTIR spectrum obtained with a polycrystalline Pt electrode in contact with a 0.1 rnol dm-3 H,SO, + 0.05 rnol dm-3 AA solution. Reference spectrum recorded with the test electrolyte. Open-circuit conditions, p polarization. adsorbed intermediates in the experiments reported in Fig. 2 and 3. In Fig. 6 a band appears, centred at 2050 cm-', that corresponds to on-top adsorbed CO. As expected this band is not observed when p-polarized is substituted by s-polarized radiation (spectrum not shown). The remaining bands, that are common to the spectra obtained with p and s polariza-tions are due to species in solution.The strong band centred at 1765 cm-' can be assigned to the C-0 stretching vibra- tion of AA in sol~tion,~~~~~ while that at around 1700 cm-' can be originated from C=O and/or C-C stretching contri- butions of AA or related anions.25 The strongly coupled 0-H bending and the CH, deformation modes of AA, produce the very broad band observed between 1500 and 1250 cm-'. Finally, the bands below 1250 cm-'are assigned to C-0 and C-C stretching modes.26 Fig. 7 shows potential difference FTIR spectra obtained with a polycrystalline Pt electrode in contact with a 0.05 mol dm-j AA-0.1 rnol dm-3 H2S04 solution. Spectrum (a)was obtained without Bi3+ in solution, while spectra (b)and (c) were recorded in the presence of 1 x and 2 x mol dm-3 Bi3+ , respectively.In these spectra reference and sample potentials are within the potential region where the I 2200 2100 2000 1900 1800 wavenumber/cm -' Fig. 7 Potential difference spectra obtained with a polycrystalline Pt electrode in contact with a 0.1 mol dm-j H,SO, + 0.05 mol dm-3 AA solution + X rnol dm-3 Bi3+ solution. X =(a) 0, (b) 1 x lo-,, (c) 2 x Sample potential: 0.435 V, reference poten- tial: 0.055 V US. RHE. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 bare platinum electrode is blocked by the adsorbed species coming from AA (see Fig. 1). On the other hand, in this potential range, the electrode surface is expected to be covered with a Bi adlayer in the Bi3+ containing solution (see Fig. 4 and 5). Spectrum (a) shows a bipolar band, centred at around 2050 cm-', characteristic of linearly adsorbed CO in acid medium.27 This feature was observed for AA concentra-tions down to 3 x mol dm-3.As expected, the decrease of AA concentration produces a lower CO coverage of the electrode, giving rise to a signal shifting toward lower wave- number^,^^.^^ and a decrease in the CO band intensity. It is interesting to note in Fig. 7(a) the presence of a band at ca. 1850 cm-', characteristic of multi-bonded CO. As can be observed in Fig. 7(b) and (c), the presence of Bi3+ ions in solution inhibits the formation of CO on the Pt surface. When the Bi3+ concentration reaches 2 x mol dm-3 the CO signal cannot be detected. This result is in agreement with the voltammetric experiments shown in Fig.4 and 5. Fig. 8 shows the voltammetric curves obtained in the spec- troelectrochemical cell: (a)is the blank voltammogram, (b)is the voltammetric curve in presence of 0.05 mol dmm3 AA and (c) corresponds to the same AA solution to which 2 x mol dm -Bi3+ has been added. Both curves (b)and (c)were obtained with the Pt electrode pushed against the CaF, window, just after collecting spectra (a) and (c) in Fig. 7. These AA oxidation voltammetric profiles are comparable to those shown in Fig. l(c) and qc), taking into account the different mass-transfer conditions existing in a thin-layer spectroelectrochemical cell (Fig. 8) and in an ordinary elec- trochemical cell (Fig. 1 and 4). , I' N a t Fig.8 Voltammograms obtained with the polycrystalline Pt elec- trode in the spectroelectrochemical cell. (a) 0.1 rnol dm-3 H,SO, solution, u = 100 mV s-l, electrode separated from the CaF, window. (b) 0.1 mol dm-3 H,SO, + 0.05 rnol dm-j AA solution, u = 1 mV s-', electrode pressed against window. (c) Same conditions and solution of (b) but with the addition of 2 x rnol dm-3 Bi3+. 613 Fig. 9(a) shows a set of potential difference spectra obtained with the polycrystalline Pt electrode during the potential sweep shown in Fig. 8(b).The reference was the first spectrum of the series that corresponded to a potential of 0.06 V. The spectrum corresponding to 0.31 V shows clearly the bipolar band centred at 2050 cm-', characteristic of lin- early bonded CO in acid medium.The 0.51 and 0.56 V spectra, recorded in the potential region at the foot of the AA oxidation voltammetric wave, show the simultaneous appear- ance of two negative-going absorption bands around 1770 and 1700 cm-', respectively, and a positive-going band at 1800 cm-'. The negative-going bands are related to the dis- appearance of the electrochemical oxidation reactant (AA), while the positive-going one can be attributed to the C=O stretching of the final oxidation product, in its hydrated bicyclic form (HDHA), as indicated by Xing et al.' Another negative-going absorption band can be seen near 1400 an-' due to AA consumption. Several positive-going bands appear below 1300 cm-'. This zone of the spectrum, which contains the strong absorption band of bisulfate anions at 1200 cm-', is highly mixed, thus preventing a clear assignment.The oxidation of adsorbed CO takes place at potentials positive to the beginning of AA oxidation. This can be moni- tored by the decrease of intensity of the 2050 cm-' bipolar band and by the appearance of the CO, absorption band at 2345 cm-'. The latter is first observed in the 0.66 V spec-trum, [Fig. 9(a)], in agreement with the CO oxidation volt- ammetric experiment shown in Fig. 2(c). Fig. 1qa) represents a plot of the integrated intensity of the CO, band (2345 cm-') in the spectra obtained in the poten- tial sweep in Fig. 8(b). It is observed that CO, production starts at approximately 0.6 V. The CO, band intensity remains almost constant in the potential range between 0.7 and 1.0 V approximately, and increases for potentials higher than 1.0 V.This evidences a change either in the kinetics or in the mechanism of the C0,-forming pathways. CO, can be produced from oxidation of the adsorbed CO and from further oxidation of the AA oxidation product^'^ and resi- dues of the AA dissociative adsorption on the Pt surface. No spectroscopic evidence of the presence and/or nature of the adsorbed species that oxidize in the Pt oxide potential region, (Fig. 3), can be drawn from the spectra obtained in this work. Fig. 9(b) shows a set of potential difference spectra similar to that reported in Fig. 9(a) but in a Bi3+-containing AA solution. These spectra were collected during the potential sweep shown in Fig.8(c). In the same way, Fig. 1qb) rep-resents the plot of the integrated intensity of the CO, band in this spectra series. All bands in the spectra in Fig. 9(a) are qualitatively similar, except obviously the CO signal, that is absent in the spectra corresponding to the Bi-covered poly- crystalline Pt (see Fig. 7). A comparison of the spectra corre- sponding to potentials up to 0.81 V allows spectroscopic confirmation of the following points: (a) AA oxidation starts at potentials slightly lower on the Bi-covered Pt electrode than on the bare Pt surface. (b) The direct AA oxidation pathway on both electrodes produces neither CO nor C02 in the potential range 0.4-0.83 V. The only C0,-forming pathway that works in the potential range mentioned above is the oxidation of CO formed by AA dissociative adsorption.The CO, production in the AA oxidation mechanism on the Bi-covered polycrystalline Pt electrode starts at approx- imately 0.86 V, as can be seen in Fig. 9(b) and lqb). For potentials higher than 1.1 V the CO, absorption band is more intense in the Fig. 9(b) spectra than in the Fig. 9(a) spectra, see Fig. 10. The Bi adlayer starts to be desorbed elec- trochemically at potentials around 0.9 V [see Fig. 4(a) and 5); so the CO, formation via AA dissociative adsorption is J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I 2500 2000 1500 1000 2500 2000 1500 1000 wavenumber/cm-' wavenumber/cm-' Fig. 9 (a) Set of SNIFTIR spectra collected during the potential sweep of Fig.8(b).Reference spectrum corresponds to 0.06 V. See text for details. (6) Spectra taken during the potential sweep of Fig. 8(c).The numbers on the spectra correspond to E/V us. RHE. 1.2q5 1.0 -0.8-> 4-.-ln d 0.6-.-0.4-0.2 -0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 E/V vs. RHE Fig. 10 Plots of the integrated intensity of the CO, band (arbitrary units) in the spectra series recorded during the potential sweeps of Fig. 8. (a) AA solution without Bi [corresponds to potential sweep of Fig. 8(b) and to 9(a) spectra series]. (b) AA-Bi3+ solution [corresponds to potential sweep of Fig. 8(c) and to 9(b) spectra series]. partially opened at this potential for the uncovered regions of the Pt electrode.Nevertheless, the higher CO, production rate observed at potentials higher than 1.0 V in Fig. lqb), as compared with that in curve (a), suggests the existence of another C0,-forming pathway that performs better in the presence of bismuth. In this way Takamura and Saka-mot~,'~~in an NMR analysis of the AA oxidation products on polycrystalline Pt with and without Bi3+ ions in solution found, in the former case, some new signals that were ascribed to degradation products of DHA : 2,3-diketo-~-gulonic acid (DKG) and its enol form (KGA); these products would be oxidized to CO,. So it may be postulated that the presence of Bi adatoms on Pt or Bi3+ ions in solution promote the further degradation of primary products of the AA oxidation. Conclusions The results obtained in this work confirm that the high over- voltage of AA oxidation on polycrystalline Pt electrodes is due to the presence of irreversibly adsorbed species: linearly bonded and multi-bonded CO that is formed through AA spontaneous dissociative adsorption and other species, pro- duced also when contacting the Pt surface with an AA acidic solution at open-circuit.These latter species, whose nature and origin is not yet known, oxidize in the Pt oxide region. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 615 As previously suggested for Pt single-crystal electrodes,’ * the electrocatalytic effect, towards AA oxidation, of the Bi adlayer formed on the polycrystalline Pt surface is due to the suppression of any irreversible adsorption step, i.e.to the ability of the Bi-covered electrode to keep all active sites free 9 10 11 12 M. Brezina, J. Koryta, T. Loucka, D. Marsikova and J. Pradac, J. Electroanal. Chem., 1972,40, 13. A. Aldaz and A. M. Alquie, J. Electroanal. Chem., 1973,47, 532. P. Karabinas and D. Jannakoudakis, J. Electroanal. Chem., 1984,160,159. M. J. Valls, J. M. Feliu, A. Aldaz, M. A. Climent and J. Clavilier, for AA oxidation. Nevertheless, a small electronic effect of Bi J. Electroanal. Chem., 1989, 260,237. cannot be ruled out since AA oxidation starts, on the Bi- covered polycrystalline Pt electrode, at potentials slightly lower than on the corresponding bare Pt surface. A detailed study concerning the effect of Bi coverage on the electro- catalysis of AA oxidation would be necessary to clarify this point.Further work is in progress. 13 14 15 16 X. Xing, I. T. Bae, M. Shao and C. C. Liu, J. Electroanal. Chem., 1993,346,309. (a)K. Takamura and M. Sakamoto, Chem. Pharm. Bull. (Tokyo), 1979, 27, 254; (b) K. Takamura and M. Sakamoto, J. Electro-anal. Chem., 1980, 113, 273. X.Xing, M. Shao, M. W. Hsiao, R. R. Adzic and C. C. Liu, J. Electroanal. Chem., 1992,339, 2 1 1. J. Clavilier, R. Faure, G. Guinet and R. Durand, J. Electroanal. The financial support of the DGICYT through contracts PB90-0560 and CE9 1.O001 is gratefully acknowledged. The authors thank the Conselleria d’Educaci6 i Ciencia de la Generalitat Valenciana for the funds for the FTIR facility 17 18 19 Chem., 1980,107,205.J. Clavilier and S.G. Sun, J. Electroanal. Chem., 1986, 199,471. J. Clavilier, J. M. Feliu and A. Aldaz, J. Electroanal. Chem., 1988,243,419. S. G. 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