Suprapur

The following chemicals were used for platinum
nanoparticles (Pt NPs) synthesis and flocculation: hexa-chloroplatinic
(IV) acid hexahydrate (H₂PtCl₆·6H₂O, 99.9%, Alfa Aesar), ethylene glycol (EG, 99%, Alfa Aesar), sodium
hydroxide (NaOH, 99.99%, Merck), 37% hydrochloric acid (HCl, Suprapur,
Merck), and acetone (for HPLC, VWR Chemicals BDH). Vulcan XC72R carbon
black was employed as support for the metal nanoparticle deposition.
37% hydrochloric acid (HCl, Suprapur, Merck) and 65% nitric acid (HNO₃, Suprapur, Merck) were used for the dissolution of the metal
nanoparticles for inductively coupled plasma mass spectrometry analysis.
Isopropanol (IPA, for HPLC, VWR Chemicals BDH), Milli-Q water (resistivity
> 18.2 MΩ·cm, total organic carbon (TOC) < 5 ppb),
and Nafion (D1021, 10 wt %, Fuel Cell Store) were used for catalyst
ink preparation. 70% perchloric acid (HClO₄, Suprapur,
Merck) and formic acid (FA, ≥95%, Sigma-Aldrich) were used
for the preparation of the electrolyte and reactant during the electrochemical
measurements. Gas diffusion layers (GDL) with microporous layer (H23C8,
Freudenberg) and without microporous layer (H23, Freudenberg) as well
as Nafion membranes (Nafion 117, Fuel Cell Store) were used for the
catalyst layer assemblies. Ar (99.999%, Air Liquide) and CO (99.97%,
Air Liquide) were used for the electrochemical measurements.

The Zn (II), Pb (II), Cd (II) and Hg (II) solutions were obtained from certified standard solutions at 1000 mg/L (Fluka Trace Select). Three different stocks solutions of Zn, Pb and Cd were prepared from cascade dilutions in order to obtain final concentrations of 10⁻⁴ M, 10⁻⁵ M and 10⁻⁶ M.
The ionic strength was set using a solution of 1 M NaNO₃ prepared from solid powder (Sigma-Aldrich, supra-pur, Saint Louis, MO, USA). The pH was adjusted with 1 M NaOH standard (Merck titripur^®, Darmstadt, Germany) or 1 M HNO₃ diluted from a 65% (Merck suprapur) solution.
For the working electrode pretreatment, a cleaning solution with 0.2 M H₂SO₄ (Sigma-Aldrich, p.a, Saint Louis, MO, USA). A second cleaning step in a solution containing 1 M NH₄CH₃COO from the salt (Sigma Aldrich p.a) diluted in 0.5 M HCl (Merck, p.a, Darmstadt, Germany) was also carried out. To prepare the solution for the redissolution of the mercury film, 0.1 M NH₄SCN from the salt (Sigma Aldrich, p.a) was used. Ultrapure milli-Q water (resistivity 18.2 MΩ cm, Elga, labwater, High Wycombe, UK) was employed in all the experiments. Nitrogen (>99.999% pure) for the solution purging was purchased from Air liquid, France.