Elaboration by high-energy ball milling of copper/palladium composite materials – characterization and electrocatalytic activity for the reduction of nitrate in alkaline medium

In this study, an original approach was explored to decorate copper particles with palladium and well-defined bimetallic copper/palladium powders were elaborated through a two-step ball milling procedure. First, copper powder was milled with previously determined optimal conditions (ball-to-powder mass ratio of 2, milling duration of 6 h under argon) in order to obtain spherical nanocrystalline copper particles with an average diameter of 800 μm. Then, an additional milling in presence of 1 at.% of palladium powder was performed, leading to the formation of Cu–Pd composite materials. Palladium surface concentrations from 3 to 62 at.% were obtained by varying both the ball-to-powder mass ratio (2:1 or 10:1) and the milling duration (from 5 to 30 min). Scanning electron microscopy, optical microscopy, X-ray photoelectron spectroscopy and X-ray diffraction analyses confirmed that the more intense the milling is, the easier the palladium diffuses into the copper matrix and smaller the palladium concentration on copper particles is. Cyclic voltammetry and electrolysis experiments showed that palladium inclusions on copper improve greatly the electrocatalytic activity for nitrate reduction in alkaline media. The key role of Pd in the Pd–Cu composite electrodes is to accelerate the reduction of nitrite, formed by the electrochemical reduction of nitrate on Cu sites. Also different nitrate electroreduction behaviors were observed at copper and copper–palladium electrodes leading to the preferential formation of nitrite or ammonia depending on the applied potential and the Pd surface concentration.