Effect of Chemical Composition on the Nanoscale Ordering Transformations of Physical Mixtures of Pd and Cu Nanoparticles

Abstract

The nanoscale composition and structure of alloy catalysts affect their performance in heterogeneous catalysis. In particular, previous reports indicated that PdCu nanoparticles are more efficient as catalysts in fuel cell reactions than monometallic Pd catalysts. To understand the structural transformations of PdCu nanoalloys, real-time in situ synchrotron X-ray diffraction was used to examine the temperature-induced evolution of physical mixtures of Pd and Cu nanoparticles. Ex situ transmission electron microscopy measurements provide additional information about the size, phase, composition, and ordering of the nanoparticle mixtures. The results for PdCu mixtures of composition 1 : 1 and 3 : 1 supported on SiO2 are presented in detail here. The annealing procedure involved two stages: (a) isothermal annealing at 450°C and (b) ramped annealing from 450°C to 750°C, both in forming gas atmospheres. We found the ordered B2 phase to be formed at 450°C in all compositions studied. Ramped annealing of PdCu 1 : 1 mixtures from 450°C to 750°C leads to the transformation of the B2 phase into two different alloys, one rich in Cu and the other rich in Pd. This structural evolution bears the signature of spinodal decomposition and is different from that of PdCu bulk alloys. In PdCu 3 : 1 mixtures, the B2 phase dominates after isothermal annealing at 450°C, but a significant disordered alloy fcc phase is also formed. During annealing at 750°C, the disordered fcc phase grows at the expense of the B2 phase. These findings are important for the understanding of thermal activation of PdCu nanocatalysts for fuel cells.