Methanol Oxidation on Pd-Ru/C Nanocatalysts: Role of Electronic Properties and FTIR Studies

In this work, methanol oxidation in alkaline solution was studied on carbon-supported Pd-Ru nanocatalysts of nominal Pd:Ru atomic composition 1:1. The synthesis of the catalysts involved the preparation of nanoparticles in colloidal state by a modified polyol method using two reducers. The nanoparticles were separated from the reaction medium and thoroughly cleaned by a sequence of centrifugation and redispersion steps. After that, the particles were supported on high surface area carbon powder (Vulcan XC-72, Cabot). The total metal load was 20 wt%. The carbon-supported catalyst obtained was repeatedly washed with dilute solution of KOH in ethanol, dilute solution of H₂SO₄, and hot water. After filtration, the catalyst was dried at 80^o C. One sample of Pd/C prepared by the same method was used as reference material. Portions of the as-prepared Pd-Ru/C sample were thermally treated at 200 and 400^oC during 4 hours in argon, aiming to verify if the degree of alloying could be increased. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).

Cyclic voltammetry (CV) in solutions devoid of alcohol was used to evaluate the general electrochemical behavior. The CV curves showed a quite thick double layer region indicating the presence of Ru species. The electrocatalytic activities towards methanol oxidation were studied by linear potential sweeps and chronoamperometry. Oxidation of adsorbed CO was also measured. The charges of CO stripping curves allowed determining the Pd electrochemically active areas that were used for calculating current densities.

It was observed that the catalytic activity was slightly improved after heat treatment at 200^oC, while the treatment at higher temperature had a detrimental effect. Measurements of X-ray absorption spectroscopy were carried out around the Pd L3 edge (3173 eV) in order to evaluate the relevance of electronic effects. Despite the fact that these experiments could only be made in high vacuum conditions, it was found that current densities of methanol oxidation measured at 0.5 V vs RHE can be correlated with the Pd 4d band vacancy. It was observed that the methanol oxidation current density increases as the Pd 4d band vacancy becomes larger, in a way similar to what was observed for methanol oxidation in acid solution on Pt-Ru catalysts [1,2].

In situ Fourier transform infrared spectroscopy (in situ FTIR) experiments were performed in order to determine the distribution of reaction products as a function of the applied potential. In a general manner, data showed the formation of formate and the presence of small amounts of adsorbed CO between 0.3 and 0.7 V. The formation of CO₂was only observed above 0.7 V as result of the acidification of the thin layer of solution in contact with the catalyst that is caused by the production of protons during methanol oxidation.

Acknowledgments

Thanks are due to the Brazilian Agencies FAPESP (2014/12255-6; 2013/50206-4; 2013/01822-4) and CNPq (407143/2013-0) for financial support and fellowships. Thanks are also due to the Brazilian Synchrotron Light Laboratory (LNLS) for assisting XAS measurements.

References

[1] D.R.M. Godoi, J. Perez, H.M. Villullas. J. Phys. Chem. C 113 (2009) 8518.

[2] D.R.M. Godoi, H.M. Villullas. Langmuir 28 (2012) 164.