A structure investigation of Pt-Co bimetallic catalysts fabricated by mechanical alloying

Three Pt-Co mixtures of composition Pt25Co75, Pt50Co50 and Pt75Co25 respectively, were synthesized using the high-energy ball milling technique of the elemental powders with a view to prepare catalysts for fuel cells. The kinetics of phase evolution, their structure and average microstructure properties were quantitatively investigated by X-ray powder diffraction with the Rietveld method. The results show that the ball milling technique is able to produce Pt-Co solid solutions soon after few minutes of mechanical treatment. Of the two polymorphs of cobalt the fcc allotrope appears to be involved preferentially in the early stage of alloying reaction with fcc platinum. For the three compositions, a sigmoidal equation based on an interdiffusion-controlled mechanism satisfactorily accounts for the evolution of the solid solution as a function of mechanical treatment time. A characteristic reaction time of 3–6 h is observed for the solid state transformation reaction with the milling conditions adopted in our reactor. In the case of the Pt25Co75 composition, a competitive-consecutive reaction is observed. Lattice parameters of the solid solutions after extended times of milling and related atomic volumes turn out to be slightly above the values ideally predicted on the basis of the Vegard’s law. For the Pt75Co25 composition the average crystallite size is reduced down to ca. 150 Å after 12 h when the lattice microstrain is also at a maximum, but further mechanical treatment increases the average crystal size value and to decrease the strain. Similar results are found for equiatomic and Co-rich compositions. Annealing of the alloyed equiatomic powders promotes a cubic-to-tetragonal transformation which is already operative at 600 °C. In fact, after this treatment two tetragonal phases are observed. Further thermal treatment and annealing at 700 °C induces peak sharpening of the diffraction patterns, due to ordering phenomena still in progress.