In Tae Bae

Heat-treated polyacrylonitrile-based catalysts for oxygen electroreduction

Polyacrylonitrile (PAN), mixed with Co(II) or Fe(II) salts and high-area carbon and then heat treated, has been found to yield very promising catalysts for O2 reduction in concentrated alkaline and acid solutions. The catalytic activities are comparable to those for the heat-treated corresponding transition metal macrocycles and polypyrrole black-based catalysts. The addition of the transition metal to the nitrogen-containing polymer, either before or after the heat treatment with carbon, is an important factor for good activity. The nitrile nitrogen of the PAN is probably retained and converted to pyridyl nitrogen during the heat treatment, and this nitrogen is believed to provide binding sites for the transition metal ions, which then act as catalytic sites for oxygen reduction to peroxide and its decomposition.

In situ infrared studies of glucose oxidation on platinum in an alkaline medium

Abstract The oxidation of α-D(+)-glucose on a platinum electrode in 0.1 M NaOH was investigated by in situ Fourier transform infrared reflection-absorption spectroscopy in the potential range, −0.76 to +0.46 V vs. Hg/HgO, OH−. The surface adsorbates were found to involve linear CO and bridged CO. The linear CO persisted on platinum in the entire potential range with gradual shifts to higher frequencies, ca. 70 cm−1/V while the bridged CO disappeared as the electrode potential reached −0.05 V. The oxidation products were car☐ylic acid and CO2. A potential excursion up to +0.46 V was found to cause a pH swing of more than 8 in the spectroelectrochemical thin layer due to the great extent of glucose oxidation. This was evident from the gluconic acid and CO2 peaks in the IR spectra.

In situ Fourier transform infrared reflection absorption spectroscopic studies of glucose oxidation on platinum in acid

Abstract Electrochemical oxidation of α-D( + )-glucose on a polycrystalline platinum electrode in 0.1 M HC1O4 was studied in situ by Fourier transform infrared reflection-absorption spectroscopy. The spectra showed unambiguously that the major adsorbate on the platinum surface during the glucose oxidation over the potential range −0.20 to +0.40 V vs. a saturated calomel reference electrode is linearly adsorbed CO. The strong adsorption of this species inhibits the electrode reaction. The oxidation products in the solution were found to be gluconolactone, carboxylic acids and CO2 whose formation occurred to the greatest extent in the platinum oxide region.

Underpotential Deposition of Aluminum and Alloy Formation on Polycrystalline Gold Electrodes from AlCl3 / EMIC Room‐Temperature Molten Salts

Interfacial processes occurring at polycrystalline gold in acidic 1-ethyl-3-methylimidazolium chloroaluminate melts at room temperature, at potentials more positive than aluminum bulk deposition, include both Al underpotential deposition and the formation of Al/Au alloys. This information complements that reported in the literature for Al electrodeposition on Au in AlCl{sub 3}/NaCl melts at temperatures of 200 C and higher. Cyclic voltammetry and chronopotentiometry suggest that at least two alloys form, and there is fast phase transformation between these intermetallic compounds at room temperature in the aluminum underpotential region. This transformation with thin gold films was slower than that in bulk gold electrodes.

Electro-oxidation of l-ascorbic acid on platinum in acid solutions: an in-situ FTIRRAS study

Abstract Studies of in-situ Fourier transform infrared reflection-absorption spectroscopy (FTIRRAS) showed that l -ascorbic acid (AA) undergoes a spontaneous dissociative or destructive adsorption at a Pt electrode in acid solutions at the hydrogen adsorption-desorption region with linearly bonded carbon monoxide (CO) L as the adsorbate. This dissociative adsorption process most likely takes place via an interaction between the side-chain of AA and the Pt surface. The overall electro-oxidation of AA at Pt in acid solutions may involve a contribution from its ethylene glycol (EG)-like side-chain portion as well as its lactone ring portion. This process may consist of three major stages: (1) dissociative adsorption via the EG-like side-chain to form (CO) L on Pt at the hydrogen adsorption-desorption region; (2) direct oxidation via the lactone ring to form dehydro- l -ascorbic acid (DHA) and its hydrated derivatives in the potential region from the onset of oxidation to the current peak; (3) the EG-like side-chain and (CO) L undergo further oxidation to form CO 2 as the final product when the potential is driven to a more positive region. This study indicates that AA undergoes a spontaneous dissociative adsorption at the Pt electrode in acid solution prior to its direct oxidation. This dissociative adsorption probably takes place via an interaction between the side-chain of the AA molecule and the Pt surface, with (CO) L as the adsorbate. The overall electro-oxidation of AA at Pt in acid solutions may consist of three major stages: (1) dissociative adsorption; (2) direct oxidation via the lactone ring to form DHA and its hydrated derivatives in the potential-region from the onset of oxidation to the current peak; (3) further oxidation via side-chain and (CO) L to form CO 2 as the final product when the potential is driven to a more positive region.

In Situ X‐Ray Absorption Fine Structure Measurements of LaNi5 Electrodes in Alkaline Electrolytes

In situ Ni K-edge and La{sub m}-edge x-ray absorption fine structure (XAFS) measurements of LaNi{sub 5} electrodes supported on hydrophobic carbon black-polytetrafluoroethylene (PTFE) layers were carried out in strongly alkaline electrolytes as a function of the state of charge. The Ni K-edge X-ray absorption near-edge structure (XANES) for the material in the discharged (and uncharged) and charged states were essentially identical to those reported earlier for crystalline LaNi{sub 5} before and after exposure to gas-phase hydrogen, respectively. The Ni K-edge extended X-ray absorption fine structure (EXAFS) analysis of the most prominent shell yielded average Ni-Ni nearest neighbor distances of 2.52 {+-} 0.02 {angstrom} for the charged compared to 2.47 {+-} 0.02 {angstrom} for the discharged and uncharged electrodes. For the La L{sub m}-edge XANES, the white line was found to be significantly more intense for the charged than for the uncharged LaNi{sub 5}, an effect consistent with an increase in the density of empty d-like states just above the Fermi level on hydrogen injection.

In Situ X‐Ray Absorption Fine Structure Studies of a Manganese Dioxide Electrode in a Rechargeable MnO2 / Zn Alkaline Battery Environment

Electronic and structural aspects of a MnO{sub 2} electrode in a rechargeable MnO{sub 2}/Zn battery environment have been investigated by in situ Mn K-edge x-ray absorption fine structure (XAFS). The relative amplitudes of the three major Fourier transform shells of the extended XAFS function of the rechargeable MnO{sub 2} electrode in the undischarged state were similar to those found for ramsdellite, a MnO{sub 2} polymorph with substantial corner-sharing linkages among the basic MnO{sub 6} octahedral units. The analyses of the background-subtracted pre-edge peaks and absorption edge regions for the nominally 1-e{sup {minus}} discharged electrode were consistent with Mn{sup 3+} as being the predominant constituent species, rather than a mixture of Mn{sup 4+} and Mn{sup 2+} sites. Furthermore, careful inspection of both the x-ray absorption near edge structure and EXAFS indicated that the full recharge of MnO{sub 2}, which had been previously discharged either by one or two equivalent electrons, generates a material with decreased corner-sharing linkages compared to the original undischarged MnO{sub 2}.

Solvent effects in metal underpotential deposition on single crystal metal surfaces

The underpotential deposition (upd) of Pb and Cd on Ag(111) and Au(111) single crystals grown on freshly cleaved mica has been examined in propylene carbonate (PC), water, tetrahydrofuran (THF), and dimethoxyethane (DME) by cyclic voltammetric techniques. Insight into the cleanliness of the single crystal metal electrode/non-aqueous electrolyte interfaces was gained from a comparison of the potential (E) dependence of the differential capacity (C) in PC solutions with that reported in aqueous media. The cyclic voltammetric features for the Pb/Ag(111) and Pb/Au(111) upd systems in PC were found to be very similar to those obtained in aqueous media. This behavior was unlike that observed for the upd of Cd on these single crystal substrates in the same two solvents, for which the differences in the voltammetric features, especially in the case of Ag(111), were quite substantial. This latter effect has been ascribed, in part, to the ability of Cd ad-ions to undergo partial discharge on the electrode surface, a factor that changes the solvation characteristics of the adsorbed species and thus modifies the energetics and kinetics of the upd process. For solvents of low dielectric constant, ie THF and DME, the same four upd/substrate systems yielded highly distorted cyclic voltammetry curves. This has been tentatively attributed to extensive ion pairing both at the interface and in the bulk electrolyte. The presence of ion-pairs in the solution phase was evidenced by the lifting of the degeneracy of the ν 4 mode of the perchlorate ion (as monitored by transmission FTIR measurements), for which the detailed nature of the band splitting was found to be metal-ion specific

In-situ infrared studies of the Cd-UPD mediated reduction of nitrate on gold

Underpotential-deposited cadmium on Ag, Au and Pt has been found to exhibit electrocatalytic properties for the reduction of nitrate in aqueous media [1,2]. Insight into the mechanism associated with this process has been recently obtained from experiments involving rotating ring-disk electrodes [2]. An unambiguous identification of reaction products, however, requires the use of in situ methods with molecular and atomic specificity, an area that has experienced extraordinary progress in recent years [3]. Particularly relevant to the study of the electrochemical properties of simple nitrogen containing compounds is the work of Bruckenstein and co-workers who pioneered the use of on-line mass spectrometric techniques for the detection and identification of gas phase products [4]. One of the main objectives of this work is to elucidate the mechanistic pathways associated with the reduction of nitrate on Au mediated by Cd-UPD by using in situ Fourier Transform Infrared Reflection Absorption Spectroscopy, FTIRRAS, for monitoring solution phase species. As will be shown, the results obtained have provided evidence that for [H+] < [NO;] the electrocatalytic process yields nitrite as the only detectable product. At least two well defined isosbestic points could be identified in plots of potential difference spectra as a function of the applied voltage. This indicates that to the level of sensitivity of in situ FTIRRAS the nitrate/nitrite conversion is quantitative. Additional evidence in support of this view was provided by the fact that the magnitude of the ratio of the integrated absorption bands of nitrate and nitrite was found to be constant over a wide potential region.

In Situ Real‐Time Pt LIII‐Edge X‐Ray Absorption Spectra of Carbon Monoxide Adsorbed on Platinum Particles Dispersed in High‐Area Carbon in Aqueous Electrolytes

Changes in the Pt L{sub III}-edge region of small Pt particles (4 to 5 nm diam) dispersed in a high-area carbon (XC72-R) induced by the adsorption of CO, were monitored in situ in aqueous 0.1 M HClO{sub 4} by measuring the fluorescence intensity at fixed x-ray energies during a voltammetric cycle. This real-time technique made it possible to detect a small increase in the normalized fluorescence (< 0.1) at the Pt L{sub III} main absorption edge (11,564 eV), following electro-oxidation of CO adsorbed at saturation coverages on the high-area Pt, derived from a shift in the edge toward lower energies. These in situ findings are in agreement with ex situ spectroscopic measurements of Pt surface core levels for CO adsorbed on Pt single crystals in ultrahigh-vacuum environments. Also described in this work are applications of this new in situ technique to the study of hydrogen adsorption-desorption and oxide formation and reduction on such high-area Pt/XC72-R electrodes in the same electrolyte. In situ Pt L{sub III}-edge studies of high-area Pt/high-area carbon electrodes in a fuel cell environment could, in principle, provide very valuable fundamental information of direct relevance to technological applications.