Tuning the Alloy Behavior and Particle Size of Ptm (M=Ni, Co) Nanoparticles By Using in Situ High-Temperature X-Ray Diffraction

Abstract

Alloying process has emerged as a promising strategy to tailor the chemical, physical and catalytic properties of metals. According to the phase diagram, the physical and chemical properties and behaviors of macroscopic alloys are well known. On the atomic and nano-scale, the formation mechanisms of nano-alloys are, yet, poorly understood since nanostructured alloys possess different segregation tendency and reaction enthalpies compared to those of bulk materials. In the last 20 years, bimetallic alloy nanoparticles have attracted great attention in different fields such as catalysis, seniors, medicine due to their unique optical, electronic, magnetic, morphological and catalytic properties. Partial substitution of Platinum by less noble metals like Nickel or Cobalt enables to decrease the material costs in combination with improved electrocatalytic activity for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFC).[1,2,3] However, the alloyed state and the homogeneity of bimetallic PtM (M=Ni, Co) nanoparticle electrocatalysts are crucial for the manufacturing. A simple and facile synthetic route to prepare Pt-based alloy nanoparticles is the wet impregnation approach followed by thermal annealing. The challenge is to control the alloy formation and particle growth of these bimetallic nanoparticles. Therefore, the implementation of in situ high-temperature X-ray diffraction (HT-XRD) is a powerful tool to simultaneously probe the changes in crystallographic phases and crystallite size of these nano-alloys during the thermal treatment.[4,5]We analyzed the formation dynamics and particle growth of carbon supported PtM (M = Co, Ni) alloy nanoparticles by using in situ HT-XRD. We developed a tool box how annealing control parameters such as heating rate, temperature and time influence the crystallographic phase structure, composition and crystallite size of the bimetallic nanoparticles. Afterwards, the alloy nanoparticles are tested for the ORR to correlate their structure, composition and particle size with the resulting catalytic properties.The HT-XRD studies on Pt-based alloy nanoparticles enable to improve the preparation of nano-alloys with desired composition, crystalline phase and particle size for PEMFC applications.