Size contraction in Pt/C and PtRu/C commercial E-TEK electrocatalysts: An in situ X-ray diffraction study

High performance carbon supported Pt/C and PtRu/C (1:1) alloy electrocatalysts supplied by E-TEK are widely used as reference for fuel cell applications. A thorough in situ X-ray structural characterization of these catalysts, using a high-intensity, low-background Guinier system has been conducted. The diffraction patterns were simulated by model Debye functions after careful subtraction of support scattering and correction for the related angular factors. Size dependent contractions against the bulk lattice constants of 0.8% have been observed for both catalysts in the as received state (particle size of 2-3 nm) after previous surface oxide reduction at low temperature. The lattice contractions were alleviated after gas phase CO oxidation or the oxygen reduction reaction (ORR) at around 200 degrees C. Twinning defects, incorporated to the fcc-type model clusters, largely improved the fit in the case of the alloy catalyst. The Debye Function Analysis (DFA) used in this study yields the particles size distribution and provides an accurate value for the lattice constants. The average intraparticle atomic distance is a measure for the perfection of alloying in a binary system. Data from our study show that the PtRu/C particles are in a single phase state of perfect alloying, and the observed low value of the lattice constant (a = 3.832 angstrom) is due to a size-related cluster contraction.