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. 150Å 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.