EXAFS electrocatalysts

Alonso-Vante N, Malakhov IV, Nikitenko SG, Savinova ER, Kochubey DI (2002) The structure analysis of the active centers of Ru-containing electrocatalysts for the oxygen reduction. An in situ EXAFS study. Electrochim Acta 47 3807-3814... 

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... 

XAS has been successfully employed in the characterization of a number of catalysts used in low temperature fuel cells. Analysis of the XANES region has enabled determination of the oxidation state of metal atoms in the catalyst or, in the case of Pt, the d band vacancy per atom, while analysis of the EXAFS has proved to be a valuable structural tool. However, the principal advantage of XAS is that it can be used in situ, in a flooded half-cell or true fuel cell environment. While the number of publications has been limited thus far, the increased availability of synchrotron radiation sources, improvements in beam lines brought about by the development of third generation sources, and the development of more readily used analysis software should increase the accessibility of the method. It is hoped that this review will enable the nonexpert to understand both the power and limitations of XAS in characterizing fuel cell electrocatalysts. 

Maniguet, S. EXAFS studies of carbon supported fuel cells electrocatalysts. Ph.D. Thesis, University of Southampton, 2002. 

With these preliminary theoretical results in mind, a set of three Co-based porphyrin electrocatalysts (from CoTMPP precursors pyrolized at 600, 700, and 800°C) were examined in situ (room temperature de-aerated IM TFMSA) at beamline X-llA at the NSLS. Standard EXAFS analysis was of limited use in determining the stmctural parameters due to the disordered nature of... 

I.V. Malakov, S. Nikitenko, E. Savinova, D. Kochubey, N. Alonso-Vante, In situ EXAFS study to probe active centers of Ru chalcogenide electrocatalysts during oxygen reduction reaction. Phys. Chem. B. 2002, 106(7), 1670-1676. 

A prevailing view of carbon-supported metal nanoparticles is that they are in a form of cubo-octahedral or icosohedral structures.One such model for the active electrocatalyst with submonolayer Pt coverage, a cubo-octahedral Ru particle with Pt islands on its surface, is shown as an inset in Fig. 18b. Pt atoms are in 2D islands as expected from EXAFS measurements and the Ru nanoparticles are supported on a high surface area Vulcan XC-72 carbon. 

Characterize new electrocatalysts by high-resolution transmission electron microscopy (HRTEM), in-situ fourier transform infrared (FTIR) spectroscopy and extended x-ray absorption fine structure (EXAFS) techniques. 

As well, EXAFS has been used to confirm that several series of carbon supported PtCo catalysts prepared by SEA are completely alloyed. The development of these alloys for fiael cell electrocatalysts, along with carbon supported Pt/Mo and Pt/Ru, is presently under way. 

In order to get answers to these questions, the ability to better characterize catalysts and electrocatalysts in situ under actual reactor or cell operating conditions (i.e., operando conditions) with element specificity and surface sensitivity is crucial. However, there are very few techniques that lend themselves to the rigorous requirements in electrochemical and in particular fuel cell studies (Fig. 1). With respect to structure, in-situ X-ray diffraction (XRD) could be the method of choice, but it has severe limitations for very small particles. Fourier transform infra red (FTTR), " and optical sum frequency generation (SFG) directly reveal the adsorption sites of such probe molecules as CO," but cannot provide much information on the adsorption of 0 and OH. To follow both structure and adsorbates at once (i.e., with extended X-ray absorption fine stmcture (EXAFS) and X-ray absorption near edge stmc-ture (XANES), respectively), only X-ray absorption spectroscopy (XAS) has proven to be an appropriate technique. This statement is supported by the comparatively large number of in situ XAS studies that have been published during the last decade. 16,17,18,19,20,21,22,23,24,25 highly Versatile, since in situ measme-... 

Figure 14. Structural models estimated from EXAFS analysis for the indicated electrocatalysts prepared as deseribed in Ref 57. The PtRu Mix catalyst was prepared by mechanically mixing Pt/C and Ru/C powder, with some atoms of the other element thereby being deposited on each cluster surface.

Finally, combined Ap XANES and EXAFS studies have recently been reported on other more complex electrocatalysts, including RuS," "" RhSe, Au/SnOx, Pt/NbOx, " and Pt/TPPTP/C (TPPTP = triphenyl phosphine triphosphate) " catalysts. The Ap XANES has also been used to study the effects of anion adsorption on Pt including bisulfate " and Cl,"" differentiate the various oxidation states of S dming the electrochemical oxidation of S on Pt ", and identify acetaldehyde-like intermediates during ethanol oxidation on Pt. In all of these studies, unprecedented new details on the coverage and binding sites of adsorbates in situ have been revealed. Further, similar XAS studies in gas phase cat-... 

Common techniques for the characterization of the electrocatalysts include High Resolution Electron Microscopy (HRTEM), Extended X-ray Absorption Fine Stracture Spectroscopy (EXAFS), Energy Dispersive Spectroscopy (EDS), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), Near Edge X-ray Absorption Spectroscopy (XANES), X-Ray Powder Diffraction (XRPD), Infrared and Raman Spectroscopy (IR, RS). 

The element-specific electronic properties, e.g., Pt 5d-band state, of Ptmu electrocatalysts and the local environment, such as bond distance and coordination number of individual component, were obtained using X-ray absorption nearedge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies, respectively. 

Figure 3 shows the in situ EXAFS of Pt L3 edge from the PtMu/Pd/C electrocatalyst in 1 M HCIO4... 

Page, T., Johnson, R., Hormes, J., Noding, S., and Rambabu, B. (2000) A study of methanol electro-oxidation reactions in carbon membrane electrodes and structural properties of Pt alloy electrocatalysts by EXAFS. Journal of Electroanalytical Chemistry, 485 (1), 34-41. 

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