The effect of the Mo precursor on the nanostructure and activity of PtRuMo electrocatalysts for proton exchange membrane fuel cells

Carbon supported PtRuMo nanoparticles for CO and methanol electrooxidation have been prepared using different Mo precursors (MoCl5, (NH4)6Mo7O24 and MoO3). Electrocatalysts as well as support characterization has been carried out through various physicochemical (XRD, TEM, XPS, TPR and TXRF) and electrochemical techniques as cyclic voltammetries and current–time curves, applied in combination with differential electrochemical mass spectrometry (DEMS). MoO3 precursor was mainly used as representative of Mo6+ species in the electrocatalyst. The results of this work pointed out that irrespectively of the similar structure and particle size are obtained using MoCl5 and (NH4)6Mo7O24, the final composition and homogeneity of the ternary catalysts were found to depend markedly on these precursors. MoCl5 led to a dramatic loss of Mo, while (NH4)6Mo7O24 appeared to be more stable and handy for synthesis of PtRuMo nanoparticles. Once the catalysts have been stabilized in the electrode, similar Mo species, mainly Mo5+, are formed on the catalysts surface of all PtRuMo catalysts. This is indicative that similar metal–support and metal–metal interactions are likely developed in carbon supported PtRuMo nanoparticles at these potential conditions. The possible metal interactions that take place and the lower oxidation states of surface Mo species may be responsible of the high activity in CO and methanol electrooxidation as compared with binary and ternary catalysts obtained with MoO3. The higher activity observed using (NH4)6Mo7O24 as precursor could be finally attributed to the composition and a remarkable Ru–Mo interaction detected by TPR. Finally, the MoO3-loaded catalyst proves that high oxidation states of Mo not only result in high losses of metals during the stabilization of the system and lower activities but also affect negatively the development of Pt–Ru interactions.