Preparation of electrocatalysts by reduction of precursors with sodium citrate

Abstract

In this work synthesis of Pt/C catalysts by reduction of H₂PtCl₆ with sodium citrate has been investigated. The strong pH-dependence of citrate as a reducing and stabilizing agent has been explored, and an optimum pH range for production of well dispersed catalysts is proposed. To achieve stabilizing and reducing conditions, the presence of both citrate anions and protonated citrates are required. This is achieved in an intermediate pH range between pK_a2 and pK_a3 (4.76 and 6.4) of citric acid, where both C₆H₅O₇³⁻ (denoted CA³⁻) and C₆H₇O₆⁻ (denoted H₂CA⁻) are present. At pH 5.3–5.4 a catalyst with particles around 3 nm was thus successfully prepared. At high pH (∼12) the reduction of Pt is limited, whereas at low pH reduction is fast, but the stabilizing ability of the citrate in solution is poor resulting in large cubic Pt particles. CO-stripping voltammetry indicate that Pt(111) faces are the dominating crystal plane in the nanoparticles formed when citrate anions are used as stabilizing agent. This effect is presumably caused by the distance between oxygen groups in citrate correlating well with the Pt–Pt distance on (111) faces.