Spectroscopy electrocatalyst

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

Reaction products can also be identified by in situ infrared reflectance spectroscopy (Fourier transform infrared reflectance spectroscopy, FTIRS) used as single potential alteration infrared reflectance spectroscopy (SPAIRS). This method is suitable not only for obtaining information on adsorbed products (see below), but also for observing infrared (IR) absorption bands due to the products immediately after their formation in the vicinity of the electrode surface. It is thus easy to follow the production of CO2 versus the oxidation potential and to compare the behavior of different electrocatalysts. 

Friedrich KA, Henglein F, Slimming U, Unkauf W. 2001. In-situ vibrational spectroscopy on Pt electrocatalysts. Electrochim Acta 47 689-694. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

Synchotron based techniques, such as surface X-ray scattering (SXS) and X-ray absorption spectroscopy (XAS), have found increased use in characterization of electrocatalysts during electrochemical reactions.37 These techniques, which can be used for characterization of surface structures, require intricate cell designs that can provide realistic electrochemical conditions while acquiring spectra. Several examples of the use of XAS and EXAFS in non-precious metal cathode catalysts can be found in the literature.38 2... 

Mukerjee, S. and Urian, R.C., Bifunctionality in Pt alloy nanocluster electrocatalysts for enhanced methanol oxidation and CO tolerance in PEM fuel cells electrochemical and in situ synchrotron spectroscopy, Electrochim. Acta, 47, 3219, 2002. 

Characterization of Alloy Electrocatalysts by Combined Low-Energy Ion Scattering Spectroscopy and Electrochemistry... 

In order to improve the fuel utilization in a Direct Alcohol Fuel Cell (DAFC) it is important to investigate the reaction mechanism and to develop active electrocatalysts able to activate each reaction path. The elncidation of the reaction mechanism, thus, needs to combine pnre electrochemical methods (cyclic voltammetry, rotating disc electrodes, etc.) with other physicochemical methods, such as in situ spectroscopic methods (infrared and UV-VIS" reflectance spectroscopy, or mass spectroscopy such as EQCM, DEMS " ), or radiochemical methods to monitor the adsorbed intermediates and on line chromatographic techniques"" to analyze qnantitatively the reaction products and by-products. 

As indicated above, the Ap technique has been applied to several other phenomena involving Pt-based electrocatalysts. The first report of Ap applied to operating Pt electrocatalysts was based on Hads at anodic potentials. The nature of Ha on Pt, and its contribution to the effective double layer, had long been a matter of debate. " Ap analysis of Pt Lmn XANES showed the H to be highly delocalized, and hopping between one-fold and three-fold (fee) sites on the Pt surface. While prior research had pointed to such activity, the realistic extent in respect to potential was murky due to the nature of the analytical techniques (e.g., IR spectroscopy, UHV studies, etc.) employed. The study by Teliska et al., ... 

Pt-based electrocatalysts have proven to be ideally suited to the Ap analysis primarily because of the extensive morphological characterizations (X-ray diffraction, single crystal electrochemical evaluations, UHV spectroscopies, etc.) performed over the past decades. In contrast, chalcogenide electrocatalysts are comprised of nanoscale amorphous clusters making a detailed analysis of the strac-ture/property relationships inherently difficult. In light of these considerations, we have recently applied the Ap technique to a novel mixed-phase chalcogenide electrocatalyst (RhxSy, commercially available from A-TEX, Inc). Rh Sy shows remarkable per-... 

Mukerjee, S. In-situ x-ray absorption spectroscopy of carbon-supported Pt and Pt-alloy electrocatalysts correlation of electrocatalytic activity with particle size and alloying, Wiley VCH, 2003. 

Lambert reviews the role of alkali additives on metal films and nanoparticles in electrochemical and chemical behavior modihcations. Metal-support interactions is the subject of the chapter by Arico and coauthors for applications in low temperature fuel cell electrocatalysts, and Haruta and Tsubota look at the structure and size effect of supported noble metal catalysts in low temperature CO oxidation. Promotion of catalytic activity and the importance of spillover are discussed by Vayenas and coworkers in a very interesting chapter, followed by Verykios s examination of support effects and catalytic performance of nanoparticles. In situ infrared spectroscopy studies of platinum group metals at the electrode-electrolyte interface are reviewed by Sun. Watanabe discusses the design of electrocatalysts for fuel cells, and Coq and Figueras address the question of particle size and support effects on catalytic properties of metallic and bimetallic catalysts. 

Part IV describes recent breakthroughs in the use of advanced experimental techniques for the in situ study of nanoparticle catalysts and electrocatalysts, including X-ray absorption spectroscopy, NMR, and STM. 

In-Situ X-Ray Absorption Spectroscopy of Carbon-Supported Pt and Pt-Alloy Electrocatalysts Correlation of Electrocatalytic Activity with Particle Size and Alloying... 

XPS or AES is extensively used not only to indicate the cleanliness of the sample before transfer, but also to indicate the presence of adsorbates and their oxidation states following electrochemical experiments and transfer back into the UHV environment. In the case of model platinum-based electrocatalysts, the electron spectroscopies have been used to estimate the coverage of the adsorbate metal atoms or the alloy composition. In the case of alloys, or the nucleation and growth of metal adsorbate structures, the techniques give only the mean concentrations averaged over a depth determined by the inelastic mean free path of the emitted electrons. Adsorption and reaction at surfaces often depend on the... 

In the context of supported electrocatalysts typically used in the current state-of-the-art PEMFCs, the in-situ XAS spectroscopy has three important functions. 

Model electrodes with a dehned mesoscopic structure can be generated by a variety of means, e.g., electrodeposition, adsorption from colloidal solutions, and vapor deposition and on a variety of substrates. Such electrodes have relatively well-dehned physico-chemical properties that differ signihcantly from those of the bulk phase. The present work analyzes the application of in-situ STM (scanning tunneling microscopy) and ETIR (Eourier Transformed infrared) spectroscopy in determining the mesoscopic structural properties of these electrodes and the potential effect of these properties on the reactivity of the fuel cell model catalysts. Special attention is paid to the structure and catalytic behavior of supported metal clusters, which are seen as model systems for technical electrocatalysts. 

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