Preparation of electrocatalysts by reduction of precursors with sodium citrate

Abstract

In this work synthesis of Pt/C catalysts by reduction of H 2 PtCl 6 with sodium citrate has been investigated. The strong pHdependence 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 C 6 H 5 O 3-7 (denoted CA 3-) and C 6 H 7 O -6 (denoted H 2 CA -) 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.

Fast-Scan Cyclic Voltammetry Allows Determination of Electron-Transfer Kinetic Constants in Single Nanoparticle Collision

“…Polyacrylate may also change the electron density of these particles in a different manner than citrate . The 10 nm particles are mostly single crystalline cubic particles, possessing {100} crystal faces, which are known to have higher activities for hydrazine oxidation, shown in experiments with single crystal platinum electrodes, than the more abundant {111} crystal faces on typical citrate capped Pt NPs . However, recent experiments have shown the opposite trend where hydrazine oxidation is found to be better on high index, high defect crystal faces, which the 4 and 30 nm particles most likely have a very high density of defects and grain boundaries that could contribute to the activity.…”

Surfactant-Free Colloidal Syntheses of Precious Metal Nanoparticles for Improved Catalysts

“…Syntheses in high temperature, high pressure, or supercritical conditions can be considered surfactant-free but often do not lead to stable colloids and are also excluded . Following the definition proposed above for surfactant-free syntheses, the use of ions such as citrate, , oxalate, ascorbic acid, most ionic liquids, and anions such as amino acids or enzymes is ruled out. However, species such as acetate, − sulfate/phosphate, formaldehyde or formic acid, and certainly many others could be used but are also not considered further here to keep the focus on syntheses where the solvent is playing the triple role of reaction medium, reducing agent, and stabilizer.…”

Surfactant-free colloidal syntheses of precious metal nanoparticles for improved catalysts

“…90 Syntheses in high temperature, high pressure, supercritical conditions can be considered surfactant-free but often do not lead to stable colloids and are also excluded. 114 Following the definition proposed above for surfactant-free syntheses, the use of ions such as citrate, 115,116 oxalate, 117 ascorbic acid, 118 most ionic liquids 119 as well as anions such as amino-acids or enzymes 120 is ruled out. However species such as acetate [121][122][123] or sulfate / phosphate, 124 formaldehyde or formic acid, 125 could be used but are also not considered further here to keep the focus on syntheses where the solvent is playing the triple role of reaction medium, reducing agent and stabilizer.…”