Keiji Kunimatsu

Infrared spectroscopic study of methanol and formic acid adsorbates on a platinum electrode: Part I. Comparison of the infrared absorption intensities of linear CO(a) derived from CO, CH3OH and HCOOH

Abstract The infrared absorption intensities of linear CO(a) derived from CO, CH3OH and HCOOH, respectively, on a platinum electrode in 0.5 M H2SO4 were determined as a function of the total adsorbate coverage. The intensity of the CO(a) derived from HCOOH almost coincides with, although slightly lower than, that of the CO(a) derived from CO throughout the coverage range lower than ca. 0.9. The intensity of the CO(a) derived from CH3OH almost coincides with that of CO(a) from CO only in the low coverage range. With increasing coverage from ca. 0.3, however, the intensity of the CO(a) from CH3OH becomes progressively lower than that of the CO(a) from CO. It has been confirmed, based on quantitative comparison of the infrared absorption intensities, that the linear CO(a) derived from CH3OH and HCOOH is in fact the predominant surface species on a smooth platinum electrode in acidic medium. However, the presence of minor surface species besides linear CO(a) has also been indicated from the intensity data.

Infrared spectroscopic study of methanol and formic acid absorbates on a platinum electrode: Part II. Role of the linear CO(a) derived from methanol and formic acid in the electrocatalytic oxidation of CH3OH and HCOOH

Abstract The formation and oxidation of the linear CO(a) from CH 3 OH and HCOOH on a platinum electrode in 0.5 M H 2 SO 4 has been studied by using in-situ polarization modulated infrared reflection-absorption spectroscopy as a function of the electrode potential. The formation from CH 3 OH is greatly reduced in the hydrogen region, while from HCOOH it occurs more readily in the hydrogen region than at 0.4 V (vs. RHE). Linear CO(a) is oxidized off the Pt surface in a narrow potential range around 0.5 V, after which the rate of the electrooxidation of CH 3 OH and HCOOH increases sharply. The poisoning effect of linear CO(a) was also confirmed by the rapid development of the infrared absorption intensity of the linear CO(a) band during continuous decrease of the electrocatalytic activity with time while the electrode was held at 0.4 and 0.2 V for methanol and formic acid, respectively. Reaction schemes to produce CO(a) from HCOOH and CH 3 OH in both the hydrogen region and the double-layer region are proposed.

Study of Pt Electrode/Nafion Ionomer Interface in HClO4 by In Situ Surface-Enhanced FTIR Spectroscopy

Pt electrode/Nafion interface has been characterized in HCIO 4 aqueous solutions using surface-enhanced infrared absorption spectroscopy (SEIRAS) with the aim of investigating the structure of the interface which plays a crucial role in the performance of polymer electrolyte fuel cells (PEFCs). A potential dependent band was found around 1100 cm - 1 and assigned to the symmetric vibration of the -SO - 3 groups of the ionomer membrane. Electric field driven orientation/adsorption of the ionomer membrane at the interface was suggested from the potential dependence of the band intensities of -SO - 3 groups. It was inferred that the -SO - 3 groups act like counterions at the Pt/ionomer interface to form the electric double layer.

Vibrational spectroscopy on platinum single-crystal electrodes: Part I. In-situ infrared spectroscopic studies of the adsorption and oxidation of CO on Pt (111) in sulphuric acid

Abstract Platinum single-crystal electrodes of 5 mm diameter were prepared for in situ infrared spectroscopic measurements by melting platinum wires. The linear potential sweep voltammograms of hydrogen adsorption/desorption on Pt (111), (110) and (100) in 0.5 M sulphuric acid are in excellent agreement with those observed on smaller platinum single-crystal surfaces. The adsorption and oxidation of CO on Pt (111) in 0.5 M sulphuric acid was studied by in situ polarization modulated infrared reflection absorption spectroscopy. The effects of the initial adsorption potential and surface reconstruction on the nature and oxidation mechanism of the adsorbed CO layer are reported.

Electrochemical oxidation of CO on Pt in acidic and alkaline solutions: Part I. voltammetric study on the adsorbed species and effects of aging and Sn(IV) pretreatment

Abstract CO oxidation on a Pt electrode was studied in terms of voltammograms, Tafel plots, surface species and the effects of pH (1–13), Sn(IV) treatment, aging, surface history and CO-adsorption potential. The electrochemical behaviour changes critically above or below a solution pH of ca. 11. The oxidation rate in acids is one to two orders of magnitude smaller than in alkaline solutions and its Tafel plot reveals a characteristic retardation region over the potential range from 0.5 to 0.8 V. The voltammetric oxidation of the surface species gives two peaks, I and II (I is minor), in acidic solution and only one (corresponding to I) in alkaline solution. Sn(IV) pretreatment excludes II in acidic solution and enhances CO oxidation by one to two orders of magnitude, resulting in a Tafel plot similar to that in alkaline solution. The aging in the hydrogen region, however, was found to have the same effect. CO adsorption on the aged surface in the hydrogen region or just after removal of the adsorbed hydrogen in acidic solution gives only I and a higher oxidation rate, whereas CO adsorption in the double-layer region after the reduction of surface oxygen gives I and II and a lower oxidation rate. Thus, the surface history and the CO-adsorption potential are the essentially important factors in controlling CO oxidation.

ATR-FTIR Study of Water in Nafion Membrane Combined with Proton Conductivity Measurements during Hydration/Dehydration Cycle

We have conducted combined time-resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) and proton conductivity measurements of Nafion NRE211 membrane during hydration/dehydration cycles at room temperature. Conductivity change was interpreted in terms of different states of water in the membrane based on its δ(HOH) vibrational spectra. It was found that hydration of a dry membrane leads first to complete dissociation of the sulfonic acid groups to liberate hydrated protons, which are isolated from each other and have δ(HOH) vibrational frequency around 1740 cm(-1). The initial hydration is not accompanied by a significant increase of the proton conductivity. Further hydration gives rise to a rapid increase of the conductivity in proportion to intensity of a new δ(HOH) band around 1630 cm(-1). This was interpreted in terms of formation of channels of weakly hydrogen-bonded water to combine the isolated hydrophilic domains containing hydrated protons and hydrated sulfonate ions produced during the initial stage of hydration. Upon dehydration, proton conductivity drops first very rapidly due to loss of the weakly hydrogen bonded water from the channels to leave hydrophilic domains isolated in the membrane. Dehydration of the protons proceeds very slowly after significant loss of the proton conductivity.

Infrared spectra of carbon monoxide adsorbed on a smooth gold electrode: Part II. Emirs and polarization-modulated irras stuidy of the adsorbed co layer in acidic and alkaline solutions

Infrared spectra of CO adsorbed on a gold electrode in 1 M HClo4, 0.2 M NaOH and 0.2 M (CH3)4NOH have confirmed that the adsorption is greatly enhanced in the alkaline solutions compared to the acidic solution. The enhanced adsorption is apparently assisted by OH− ions while there is little effect from the alkali metal ions. The adsorption reaches a maximum around −0.1 V (RHE). The CO stretching frequency at a constant potential shifts to lower frequencies by ca. 50–60 cm −1 with the decrease of CO coverage. The observed frequency shift with coverage is opposite in nature to that found in the gas phase for the Au/CO system. Oxidation of the CO layer established initially at –0.1 V, proceeds randomly in the adsorbed layer from ca. 0.7 V, giving rise to a steep decrease of the intensity of the CO stretching band with a concomitant large shift of the CO stretching frequency to lower wavenumbers. The potential dependence of the CO stretching frequency is linear, 64 cm−1/V, both in the acidic and alkaline solutions, between −1.1 V and +0.7 V(SHE). The infrared frequencies of the CO stretching vibration are lower that the frequencies observed by SERS (surface-enhanced Raman spectroscopy) by more than 50 cm−1

In-situ FT-IR spectroscopic study of bisulfate and sulfate adsorption on a platinum electrode: Part 2. Mildly acid and alkaline sodium sulfate solutions

Abstract Adsorption of sulfate/bisulfate ions and water molecules on a platinum electrode has been studied in a mildly acid solution (pH = 3.4) of 0.5 M Na 2 SO 4 and H 2 SO 4 and an alkaline solution (pH =11.5) of 0.5 M Na 2 SO 4 and NaOH by potential difference FT-IRRAS (Fourier transform infrared reflection absorption spectroscopy). In the acidified sodium sulfate solution, sulfate ions and water molecules are adsorbed on top of the adsorbed hydrogen layer at 0.05 V/RHE. As the potential becomes more positive they are desorbed and replaced by bisulfate ions. The desorption of the sulfate ions is completed m the oxygen region. The number of sulfate ions and water molecules replaced by one bisulfate ion has been found to be constant throughout the entire potential range. In contrast to pure sulfuric acid solutions, the asymmetric S-O stretching frequency of the adsorbed bisulfate ions shows much smaller potential dependence. In the alkaline sodium sulfate solution, adsorption of sulfate ions increases continuously from 0.05 V/RHE to more positive potentials and saturates in the oxygen region, and no desorption of sulfate ions has been observed. The sulfate ion adsorption is accompanied by adsorption of water molecules. Only a very weak bisulfate band has been detected in the oxygen region.

Adsorption/oxidation of CO on highly dispersed Pt catalyst studied by combined electrochemical and ATR-FTIRAS methods: oxidation of CO adsorbed on carbon-supported Pt catalyst and unsupported Pt black.

ATR-FTIRAS measurements combined with linear potential sweep voltammetry were conducted to investigate oxidation of CO adsorbed on a highly dispersed Pt catalyst supported on carbon black, Pt/C, and carbon-unsupported Pt black catalyst, Pt-B. Bands nu(CO) of atop- and bridge-bonded COs were resolved into those of COs adsorbed at terrace and step edge sites by curve-fitting analysis. At the high coverage near the saturation, a band around 1950-1960 cm(-1) assigned to asymmetric bridge-bonded CO, CO(B)(asym), was observed to develop on both Pt/C and Pt-B, which was the predominant type on the latter. Preferential oxidation of atop-CO adsorbed at the step edge site was commonly observed on both Pt/C and Pt-B during the potential sweep from 0.05 to 1.2 V. However, it has been found that CO(B)(asym) is the most reactive species. The high reactivity of the CO(B)(asym) on Pt/C and Pt-B is demonstrated for the first time in the present report. Adsorption of CO on the Pt/C and Pt-B resulted in growth of a sharp nu(OH) band around 3642-3645 cm(-1) which is assigned to non-hydrogen-bonded water molecules coadsorbed with CO. The nu(OH) band frequency exhibits a linear increase with potential with a Stark tuning rate of ca. 20 cm(-1)/V. Analysis of the potential dependence of this band in the CO oxidation potential region led us to conclude that this is the oxygen-containing species to oxidize adsorbed CO. Stark tuning rates of nu(CO) bands for the COs at the terrace and step edge sites on both Pt/C and Pt-B are almost independent of the adsorption sites for both atop- and bridge-bonded COs. However, CO(B)(asym) exhibits tuning rates of 41 cm-1/V and 37 cm-1/ V on Pt/C and Pt-B, respectively, which is in between the rates of atop and symmetric bridge-bonded COs.

Electrochemical oxidation of HCOONa on Pt in acidic solutions

Abstract The electrooxidation of HCOONa was carried out over a wide range of pH on Pt. HCOO − and its associated form of HCOOH do not show any difference in electrochemical behaviour. A voltammetric study demonstrates the formation of two kinds of poisoning species in the hydrogen (X 1 ) and double-layer (X 2 ) regions. Their dependences on the potential and pH were examined. Constant polarization measurements give the rate expression, i = k (α H + ) −0.43 exp(0.4 F φ n / RT ), independent of the concentrations of HCOO − and HCOOH. The rate-determining step is concluded to be HCOO − (a) → COO − (a)+H + + e − or HCOOH(a) → COOH(a)+H + + e − . The negative reaction order with respect to H + was explained through the retarding action of X 2 . The nature of X 1 and X 2 is discussed.