Xuekun Xing

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.

The Electrocatalytic Reduction of Nitrate Mediated by Underpotential‐Deposited Cadmium on Gold and Silver Electrodes in Acid Media

This paper reports rotating ring-disk electrode techniques employed for the investigation of the electrocatalytic reduction of nitrate induced by underpotential-deposited cadmium on Au and Ag surfaces. Based on Alberys kinetic analysis it has been concluded that for concentrations of nitrate in the millimolar range, the reaction products depend on the {ital p}H of the solution. Specifically, for {ital p}H = 3, the reaction yields predominantly nitrite, whereas for {ital p}H = 1, the process proceeds beyond the nitrite stage to generate products which do not undergo oxidation on a Au ring electrode polarized at potentials as high as 1.0V {ital vs.} SCE. Additional evidence in support of these results was provided by exhaustive bulk electrolysis experiments. The lack of linearity of some of Alberys diagnostic plots could be accounted for quantitatively by a numerical integration of the differential equations which govern disproportionation-type reactions at rotating ring electrodes. Good agreement was found between theory and experiment using independently determined values for the rate constant of disproportionation of NO{sub 2} in solution.

Structural effects in electrocatalysis: electro-oxidation of l-ascorbic acid on single-crystal gold surfaces

Abstract The electro-oxidation of l -ascorbic acid is examined on three low index planes of gold single crystals in various aqueous electrolytes. The overall oxidation process in the double-layer potential region of gold consists of two major stages. The first stage is structure insensitive and is not affected significantly by anions of different specific adsorption ability. This stage corresponds to an outer-sphere two-electron transfer process with the formation of dehydro- l -ascorbic acid and/or its hydration product as the primary product. The second stage is structure sensitive and is affected significantly by the anions in solution. This stage is believed to correspond to the further oxidation of dehydro- l -ascorbic acid, which is an inner-sphere process.

In Situ, Time-Resolved Raman Spectromicrotopography of an Operating Lithium-Ion Battery

A Raman microscope has been coupled to a computer-controlled, two-dimensional linear translator attached to a custom-designed, sealed optical chamber to allow in situ acquisition of space-, and time-resolved spectra of an operating thin graphite/LiCoO2 Li-ion battery. This unique arrangement made it possible to collect continuously series of Raman spectra from a sharply defined edge of the battery exposing the anode (A), separator (S), and cathode (C), during charge and discharge, while the device was moved back and forth under the fixed focused laser beam along an axis normal to the layered A/S/C plane. Clear spectral evidence was obtained for changes in the amount of Li(+) within particles of graphite in the anode, and, to a lesser extent, of LiCoO2 in the cathode, during battery discharge both as a function of position and time. Analysis of time-resolved Raman spectro-micro-topography (SMT) measurements of the type described in this work are expected to open new prospects for assessing the validity of theoretical models aimed at simulating the flow of Li(+) within Li-ion batteries under operating conditions.

Electrocatalytic properties of metal adatoms in a potential range negative to Nernstian bulk deposition

Abstract The formation of submonolayers of Cu adatoms on polycrystalline Au in 0.1 M HClO4 solutions containing only perchlorate anions has been found to proceed at unusually slow rates. This has made it possible to examine the electrocatalytic activity of submonolayer coverages of Cu adatoms exhibiting UPD like properties at potentials more negative than Cu bulk deposition. Rotating ring disk measurements have provided evidence that such layers show electrocatalytic properties for the reduction of nitrate. This process proceeds by a parallel mechanism yielding nitrite as one of the intermediates.

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.

Electrochemical oxidation of dihydronicotinadmide adenine dinucleotide (NADH) on single crystal gold electrodes

The electro-oxidation of dihydronicotinamide adenine dinucleotide (NADH) on three low-index basal single crystal gold faces was studied in a phosphate buffer solution. The reaction was found highly structure-sensitive; the Au(100) and Au(111) faces showed a high electrocatalytic activity, while the Au(110) face was much less active. The high electrocatalytic effect was attributed to the oxidation-mediator ability of the partially discharged OH(1 − r)− species on the gold surface. The structural effect of the gold faces on the electro-oxidation of NADH may result from the difference in the adsorption of NAD+, via its pyridine moiety as the adsorption site, on the different gold faces. The possibility of developing a highly sensitive NADH sensor based on the electrochemical faceting technique is suggested.

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.