Catalytic Activity Variation with the Crystal Structure Change of PdCu Nanocatalysts for Rechargeable Lithium-Oxygen Batteries

Abstract

Palladium-copper (Pd-Cu) alloys have two representative crystal structures; one is body-centered cubic (bcc), the other is face-centered cubic (fcc) even though both Pd and Cu originally have fcc crystal structures. The fcc Pd-Cu alloys have disordered structure within which Pd and Cu atoms are positioned randomly in fcc lattice, whereas the bcc Pd-Cu alloys have ordered structure which consists of alternative layers with either Pd or Cu atoms. In order to properly design the optimum Pd-Cu alloys, its physical and chemical characteristics have been extensively controlled.Pd-Cu alloys have been widely utilized as a catalyst for the hydrogenation of carbonyl compounds, the reduction of nitrates and nitrites in water, the oxidation of formic acid in direct formic acid fuel cell, the membrane for hydrogen separation and so on. In particular, bcc Pd-Cu alloys have occasionally shown superior performance to fcc Pd-Cu alloys. As for the hydrogenation of acetylene, bcc Pd-Cu alloy have better ethylene selectivity than fcc Pd-Cu alloys, because isolated Pd atoms on the surface of bcc Pd-Cu alloys enhance selectivity. Most of bcc Pd-Cu alloys have been synthesized by annealing fcc Pd-Cu alloys for structural transformation and particle growth. Hence, the same size of fcc and bcc Pd-Cu alloys is quite difficult to be obtained because the crystallites larger than 20 nm favour the ordered bcc structure with lower symmetry. Thus, bcc Pd-Cu alloys in nanoscale have been rarely reported, and fcc and bcc Pd-Cu alloys have never been properly compared until now. In this study, we successfully synthesized fcc and bcc Pd-Cu alloys in nanoscale through a simple polyol process. For the comparison between these in catalytic activity, we adopted fcc and bcc Pd-Cu alloys as cathode catalysts for lithium-oxygen batteries. This work would firstly report the comparison between the catalytic activities of fcc and bcc Pd-Cu alloys in nanoscale.