On the Electronic Efffect of the Sn Atoms Towards Noble Metal Adsorption Sites: The Influence of the Structure on the Glycerol Electrooxidation Reaction

Abstract

The searching for electrode materials with outstanding performance towards organic fuels electrooxidation reactions often is focused on polymetallic materials since they can exhibit a synergetic action resulting from electronic, geometric and/or bifunctional effects. However, as those effects are correlated, it is a difficult task to ascribe the improvement of the performance to a specific property and so to acquire a better understanding of the electrocatalytic process. It well known that Sn atoms exert a beneficial effect towards noble metal adsorption site that, on its turn, enhanced the kinetic of several organic fuels electrooxidation reactions. The enhancement is often suggested because of the electronic donor action of Sn atom; oxophilic action that provides the removal of intermediate species from the surface sites or a combination of these and other effects. The present work aims to elucidate the influence of the Sn atom present in the matrix of ordered intermetallic and alloy PtSn and PdSn (1:1 atomic ratio) nanoparticle materials towards the glycerol electrooxidation reaction in alkaline medium. Ordered intermetallic PtSn/C and PdSn/C nanoparticles were synthesized via modified polyol method and the same materials as alloys were synthesized via microemulsion route. All materials were obtained with similar particles diameters (~2nm) that assures the comparison of the electrode materials performances excluding the influence of the particle sizes. The materials were fully characterized by means of X-Ray Diffraction (XRD); Energy Dispersive Spectroscopy (EDS); High Resolution Transmission Electron Microscopy (HRTEM) and X-Ray Absorption Spectroscopy (XAS) techniques. Moreover, the electrochemical evaluation was performed by using cyclic voltammetry (CV) and chronoamperometry (CA) techniques and the identification of stable intermediates and products of the reactions were obtained from Mass Spectrometry (MS) technique. The electrochemical experiments were carried out in 0.5 mol.L-1 KOH electrolyte solution and all the potentials cited herein are referred to the Reversible Hydrogen Electrode (RHE). The obtained results have pointed to the best performance of the ordered intermetallic material PtSn/C. This best performance was attributed to the electronic donor effect of the Sn atom towards Pt adsorption site that optimized the adsorption of the glycerol molecule causing the dissociative adsorption of the fuel with the cleavage of the C-C bond. The electronic action of the Sn atom was confirmed by the XAS data. This phenomenon was reinforced by the identification of the following intermediates/products of the electrode reaction: formic acid (higher amount from the reaction taking place at ordered intermetallic PtSn/C); glyceric acid and glyceraldehyde (higher amount from the reaction taking place at alloy PtSn/C); glyceraldehyde/dihydroxyacetone (higher amount for ordered intermetallic PdSn/C) and tartronic acid (higher amount for alloy PdSn/C). The electrode reactions mechanisms were proposed for all systems pointing out to the possibility of using different materials for energy or high aggregate value chemicals production. FAPESP Proc. # 2013/05634-8 Figure 1