Ap XANES Technique

Since the absorption p equals Po(l+x) the total signature Ap can be expressed as 

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XAS has traditionally been rather limited in gleaning the effective surface chemistry involved in catalysis (i.e., weakly-bound adsorbate interactions) due to its bulk-averaging nature. This limitation has been alleviated by the Ap XANES analysis technique pioneered by Koningsberger and Ramaker, and has turned XAS into a truly surface sensitive technique (see Fig. 2a). Only recently, the Ap XANES technique has been applied to the adsorption of H, O, and OH on Pt and Pt-M (M=Cr, Fe, Co, and Ni) cathodes in an electrochemical cell, as well as CO, O and OH on alloyed Pt-Ru electrodes. Results recently obtained have been successful in providing detailed binding site information for adsorbed CO simultaneously with OH, and even coverage levels... 

This review will give a short introduction to the EXAFS and Ap XANES techniques and then focus on various investigations in PEM fuel cells. The advantages and limitations of XAS in fuel cell research will be compared to other characterization techniques, and then investigations of specific Pt-Ru anode catalysts presented. In the light of these recent results, the suitability of the XAS approach to reveal fundamental steps in the reaction mechanisms will be discussed. 

In situ FTIR diffuse reflection spectroscopy (FTIR-DRS) has been routinely used to track various adsorbates, but the specific adsorption sites (e.g., on the Pt or Ru site) carmot be determined with this technique." Further it carmot be used to track OH and H adsorption, which are both of significant importance in electrochemical studies. To om knowledge, adsorbed OH has not yet been directly observed on any Pt anodes except via the Ap XANES technique. Thus, the evidence for OH formation had to come previously from the coincidence of the potential for the onset of water activation and the CO oxidation peak in CV cmves (e.g., see Fig. 6 below). 

The accuracy of the A a method depends on the availability of a good reference spectrum, most favourably a spectrum obtained from the blank, adsorbate free metallic snrface. Unfortunately, for in-situ measurements in a working fuel cell the situation is not so straightforward, as it is difficult to find an appropriate reference spectram where no species, or at least a controlled amount, are adsorbed on the surface. The specific measmes taken for methanol and reformate operation in a working fuel cell, i.e., when there is CO, H and 0[H] co-adsorption, are described below. More specifically, we illustrate the Ap XANES technique to follow the coverage of specific adsorbates on carbon snpported Pt and different Pt/Rn bimetallic systems in pnre H2, in simulated reformate (150 ppm CO in H2) and in direct methanol operation as a function of current in an operating PEM fuel cell. 

The latter feature renders the Ap XANES a suitable technique to unravel complex reaction mechanisms and synergetic actions, as they are often active in bimetalhc catalysts. One example might be the bifunctional mechanism in CO poisoning of Pt-Ru fuel cell anodes, which will be discussed in detail below. 

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., ... 

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Koningsberger DC, Prins R (eds) (1988) X-ray absorption Principles, applications, techniques of EXAFS, SEXAFS, and XANES. John Wiley Sons, New York Lindle DW, Cowan PL, Jach T, LaVilla RE, Deslattes RD, Perera RCC (1991) Polarized X-ray emission studies of methyl chloride and chlorofluoromethanes. Phys Rev A 43 2353-2362 Loo BW, Sauerwald IM, Hitchcock AP, Rothman SS (2001) A new sample preparation method for biological soft X-ray microscopy nitrogen based contrast and radiation tolerance properties of glycol methacrylate-embedded and sectioned tissue. J Microsc 204 69-86 Matsui F, Yeom HW, Matsuda I, Ohta T (2000) Adsorption and reaction of acetylene and ethylene on the Si(001)2xl surface. Phys Rev 62 5036-5044... 

Figure 2b shows two different XANES spectra. Here p(Ad/Pt) is the Pt Z3 edge spectrum in the presence of the adsorbate and p(Pt) the Z3 edge spectrum at some different potential or current, where the adsorbate coverage is either much smaller, or at least very different from the initial spectrum so that the effect of the adsorbate can be isolated. The Ap technique can therefore be considered a subtractive techrrique with the difference, Ap, given as ... 

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