Understanding the Role of EXAFS in Electrocatalysis: Insights into Pt/C Catalysts

Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy plays a crucial role in understanding the electronic and structural properties of catalysts, particularly in the field of electrocatalysis. In the case of linearly adsorbed carbon monoxide (CO) on platinum catalysts, the specific arrangement of platinum atoms significantly influences the scattering pathways observed in EXAFS. When CO is adsorbed in a collinear or near-collinear manner, the contributions of these multiple-scattering pathways enhance our ability to detect neighboring oxygen atoms in the EXAFS spectra.

Previous studies have demonstrated this effect with different catalytic systems, such as Os3(CO)12 on γ-Al2O3 and Pt2Ru4(CO)18 on the same support. The oxygen neighbors adjacent to the CO ligands were clearly observable at the Os and Pt absorption edges. However, in the case of the Pt/C electrocatalyst, the high density of platinum neighbors at similar distances tends to dominate the EXAFS data, effectively masking the signal from the oxygen neighbors. This underscores the complexity of interpreting EXAFS in systems with multiple scattering contributions.

Further investigations into the adsorption behavior of various species, including hydrogen (H), oxygen (O), and sulfate (SO4^2-), on Pt/C electrodes have been conducted using X-ray Absorption Near Edge Structure (XANES) analysis. By comparing the data at the Pt L2 and L3 absorption edges, researchers can derive a difference spectrum known as AS for antibonding states. The theoretical background of this method is rooted in the unique contributions of the L3 and L2 edges, which probe different electronic transitions.

The formation of antibonding states during adsorption can significantly influence the spectral features in XANES. Notably, when analyzing the difference between the spectra at the L3 and L2 edges, researchers can gain insights into the valence band density of states. This approach allows for the identification of changes in the electronic structure as a function of the applied potential, revealing valuable information about the dynamics of adsorption and the overall catalytic activity.

A sophisticated analysis of AS spectra has shown that the characteristic Fano-resonance lineshape observed in these spectra can vary depending on the adsorption state of the catalyst. For instance, the resonance energy associated with the adsorbed states shifts based on whether hydrogen or oxygen is present. This variation can provide insights into the metallic nature of the catalyst and its oxidation state, further enriching our understanding of electrocatalytic processes.

Overall, the integration of EXAFS and XANES techniques provides a powerful framework for probing the intricate interactions and electronic structures in Pt/C catalysts, paving the way for advancements in fuel cell technology and beyond.