In Situ X‐Ray Absorption Fine Structure Study on Structure Transformation and Electronic State of Various Pt Particles on Carbon Electrode

Abstract

The structure and electronic state of fine Pt particles on the carbon electrode were investigated by XANES and extended x-ray absorption fine structure (EXAFS) spectroscopies. The white line intensities were processed into the parameter representing the density of unoccupied 5d state. They were described as kinds of voltammograms. It was different between the cathodic sweep and the anodic sweep even in the same potential. The small particle electrode showed large hysteresis of the d-density change. These changes by potential sweep are discussed in relation to the structure of the particle on the basis of the parameters of EXAFS. The large change in the small particle was explained by the ratio of the surface atoms in the whole particle. The thickness of the oxide layer and lattice disorder in subsurface was estimated from these structural parameters.Metal particles often change their physicochemical characteristics when the particle size becomes smaller. The effect in electric double layer has attracted attention recently because of the development of fuel-cell electrodes. Major contributions to exploring the particle size effect were the direct measurement of catalytic activities. One main interest has been the ORR (oxygen reduction reaction) activity per Pt atom when the particle becomes smaller. It is still unclear whether the activity increases with particle size decrease or whether it has an optimum value.1 The catalytic performance of the dispersed electrode often is considered to be closely related with the structure of the particle. 2 Additionally, the charge-transfer rate may be affected by the electronic state of the Pt particle or the surface. For these reasons, spectroscopic studies on dispersed Pt electrodes are required for the understanding of the particle size effect on the metal structure in double layer in addition to helping to develop commercial electrodes with good activities.X-ray absorption spectroscopy (XAS) has been widely used for in situ studies of electrochemical interfaces. 3' 4 It has also offered a deeper understanding of the state of the Pt particle on carbon electrodes. 5-7 The physicochemical parameters given by EXAFS and x-ray absorption nearend structure (XANES) can describe the dynamic transformation of fine particles as well as the structure and the electronic state in the double-layer region. In a previous paper 7 we investigated the relationship of the density of d state with parameters for the first coordination shell of Pt, the electrolyte, and functional groups on the support. The density of unoccupied d state was related to the roughness of the Pt particle lattice, although no Pt-O bond was observed. The reduction/oxidation of the surface functional groups on the support had no effect on the density of d states.The evidence of the oxidation and details of the oxidation process may be obtained by XAS when the particle size is smaller. In this sense, the extent of the oxidation near the surface and of the lattice roughening should be further confirmed by changing the part...

Probing the Structure of Operating Fuel Cell Cathode Catalysts using XAS

“…Figure 3 a shows that the white line intensity for the Pt catalyst under nitrogen atmosphere increases at potentials above 0.7 V. Increasing the potential in 0.1 V increments to 1.0 V results in a progressive increase in white line intensity corresponding to increasing platinum oxide formation. Increasing white line intensity with increasing potential has previously been observed for Pt/C as electrode layers in three electrode electrochemical cells with liquid electrolyte (23,24). Changing the potential to OCV under oxygen (1.08 V) results in a further increase in white line intensity.…”

Electrochemically Codeposited Platinum/Molybdenum Oxide Electrode for Catalytic Oxidation of Methanol in Acid Solution

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Effects of composition on structure and activity of PtRu/C catalysts

“…In terms of experimental techniques, EXAFS has been shown to be a powerful tool for characterising fuel cell catalysts. [24][25][26][27][28][29][30][31][32] EXAFS is element specific. Therefore, by probing both the Pt L III and Ru K edges, the average environment around each atom type can be determined.…”