Dispersed Small Nanoparticles Research Articles

Carbon supported Ag and Ag–Co catalysts tolerant to methanol and ethanol for the oxygen reduction reaction in alkaline media

Ag/C and Ag–Co/C catalysts with high activity towards the oxygen reduction reaction in alkaline media, different metal loadings and average sizes below 20 nm were synthesized by glycerol and borohydride reduction methods without stabilizers. Physicochemical characterization of the materials was performed by X-ray techniques (diffraction, energy dispersive and photoelectron spectroscopy) and transmission electron microscopy. Well dispersed small nanoparticles were obtained in all cases, mainly formed by Ag(0). For Ag–Co/C catalyst, it was observed that Co was not alloyed but presented as cobalt oxides. The electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solution was evaluated by cyclic voltammetry and rotating disk techniques. A four electron transfer mechanism was established although increasing Ag loading produces a decrease of this number, indicating that hydrogen peroxide produced as intermediate in a first two electron step was not completely reduced. Alcohol tolerance of the catalysts was also established in methanol and ethanol solutions. Materials were not active for the electro-oxidation of alcohols, although it was observed that both methanol and ethanol were adsorbed on the catalyst. Highest activity and alcohol tolerance was observed for the 60 wt.% Ag loading material. Also the introduction of Co produces an increase in the activity (higher ORR limiting currents) and in the alcohol tolerance. In comparison to Pt and Pd, Ag and Ag–Co present more appropriate activities for ORR when alcohol tolerance is considered, being good candidates for the use as catalysts in alkaline direct alcohol fuel cells.

Electrochemical Behavior of Organics Oxidation on Palladium-Based Nanocatalysts Synthesized from Bromide Anion Exchange

Palladium-based and gold nanomaterials were synthesized by the so-called “Bromide Anion Exchange” method. This soft and clean synthetic route leads to well dispersed small nanoparticles on carbon support. Accordingly, these nanoscale structures have a high active surface area like 81 m2 g-1 for the 20 wt.% Pd/C electrocatalyst. The CO stripping experiments performed in alkaline medium have highlighted the dependence of the CO adsorption mode on palladium. When used as anode for glycerol or glucose electrooxidation in alkaline medium, they exhibit good activities. Moreover, the bimetallic catalysts showed an important improvement of the kinetics at lower potentials when compared to Pd/C. Some added value products like glycerate or glycolate have been detected by chromatographic analysis. These nanomaterials are promising electrodes in fuel or hybrid biofuel cell devices in a cogeneration of high value chemicals and energy process