Understanding EXAFS and Particle Size in Catalysts

Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is a powerful tool used to analyze the structural properties of materials, especially in the field of catalysis. A notable example is the investigation of platinum (Pt) catalysts supported on carbon (Pt/C). In comparing the EXAFS data from a Pt foil to that of the Pt/C electrode, researchers have observed reduced oscillation amplitudes and Fourier transform peaks at specific distances, highlighting the influence of particle size on coordination numbers.

The bulk structure of platinum metal is face-centered cubic (fcc), where each atom typically has twelve nearest neighbors in the first coordination shell. However, in smaller Pt particles, such as those found in catalysts, the average coordination number is diminished due to surface atoms lacking full neighbor interactions. Analytical formulas derived by Benfield allow for estimating the number of atoms and coordination numbers in clusters with complete shells, specifically for geometries like icosahedra and cuboctahedra, which can help interpret EXAFS parameters.

Despite these analytical frameworks, challenges arise when dealing with incomplete outer shells or variations from ideal geometries. In such cases, the average particle sizes predicted from first shell coordination numbers may underestimate the actual sizes observed through transmission electron microscopy (TEM). This discrepancy is significant, as EXAFS measurements reflect an average across all absorber atoms in a sample, including very small particles that might not be detectable by TEM.

The applied potential also plays a crucial role in influencing the structural characteristics of Pt/C catalysts as revealed in the X-ray Absorption Spectroscopy (XAS) spectra. As the potential varies, notable changes occur in the adsorption of hydrogen and the formation of oxides. For example, increasing the potential from 0.1 to 1.2 V leads to observable changes in the Fourier transform amplitudes, indicating shifts in the structural environment around the platinum atoms.

The changes observed in the EXAFS data are particularly pronounced between the potentials of 0.8 and 1.0 V. During this range, the oscillation amplitudes decrease, reflecting the dynamic nature of the catalyst's surface under varying electrochemical conditions. Structural parameters derived from least-squares fitting of the EXAFS data provide further insights into how potential affects atomic arrangements and coordination, which are essential for understanding catalyst performance.

Overall, the interplay between particle size, shell coordination, and applied potential significantly influences the behavior of Pt/C catalysts, emphasizing the importance of advanced spectroscopic techniques such as EXAFS in the ongoing quest for more efficient catalytic materials.