Electrooxidation of H2/CO on carbon-supported PtRu-MoOx nanoparticles for polymer electrolyte fuel cells

Ternary PtRu-MoO_x catalysts with various Mo compositions have been investigated as anode electrocatalytic materials for a polymer electrolyte fuel cell fed with H₂/CO mixtures. Electrocatalysts have been prepared using a highly reproducible two step method, which allowed good control over the composition and particle size. All the prepared catalysts record a total metal loading close to 30 wt%, and a Mo load of 0, 1 and 3 wt%, supported on carbon Vulcan XC-72R, keeping the Pt/Ru atomic ratio constant. The incorporation of different amounts of Mo over the PtRu system does not modify structural characteristics such as particle size and crystal phases. However, the surface composition depends largely on the amount of Mo. An increase in the Mo loading to 3 wt% resulted in a decrease of the Pt surface area. The in situ FTIR technique has been used to investigate the CO oxidation process. The extent of CO poisoning was found to be lower for the trimetallic catalysts than for the binary catalyst at a potential below 0.25 V. The fuel cell performance was evaluated at 80 °C in a PEMFC fed with H₂/CO (10 ppm). Polarization curves for the catalysts show that activity depends heavily on composition, with catalysts with a small amount of Mo (1 wt%) displaying the highest CO tolerance. An increase in Mo loading (3 wt%) decreases activity of the PtRuMo, although it also reduces CO poisoning. The presence of Mo⁵⁺ species must be crucial for reducing the saturation coverage of irreversibly adsorbed CO on Pt surface atoms at very low potentials. However, the surface metal ratio of Pt/Mo (wt%) must be higher than 4, in order to keep the enough surface bare platinum sites, which are required for the dissociative adsorption of molecular H₂.