Activation of Platinum-Based Centers through Modification with Metal Oxo Species toward Electrocatalytic Oxidation of Dimethyl Ether and Methanol

Abstract

We have considered nanonstructured metal oxides as promoters for platinum, palladium or bimetallic platinum-ruthenium nanoparticles to fabricate electrocatalytic systems capable of efficient oxidation of dimethyl ether, a potentially new fuel of importance to low temperature fuel cells. The research has been performed in comparison to methanol. We explore the fact that certain inorganic oxides (e.g. WO3, TiO2, ZrO2 and CeO2) and polyoxometallates of molybdenum or tungsten tend to influence supported metal centers in ways other than simple dispersion over electrode area. Among useful characteristics of metal oxides and related systems are the following: they can generate –OH groups at low potentials that induce oxidation of passivating CO adsorbates (e.g. on Pt); they can potentially break C-H bonds (e.g. by hydrogen tungsten oxide bronzes); and they can possibly weaken C-O bonds during dimethyl ether oxidation (e.g. through changes of the electronic properties of Pt or Pd as additional component). Also nanostructured gold in combination with platinum has been demonstrated to enhance the overall electrocatalytic performance during oxidation of such small organic molecules as methanol and dimethyl ether. We pursue a concept of utilization of functionalized titanium dioxide, tungsten oxide and/or zirconium oxide matrices (by admixing them with polyoxometallate-modified gold nanoparticles) for supporting and activating noble metal nanoparticles (Pt, Pt-Ru, Pd, Pt-Pd) toward electrooxidation of dimethyl ether in comaprison to methanol. Remarkable increases of the respective electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account improvement of overall conductivity (due to the presence of nanostructured gold) at the electrocatalytic interface (formed by metal oxide support), as well as and possibility of specific Pt-metal oxide or Pt-Au electronic interactions and existence of active hydroxyl groups on transition metal oxo species.in the vicinity of catalytic Pt sites. Further, the capping layers of Keggin-type phosphomolybdates, which are known to undergo fast stepwise multi-electron redox processes as well as to activate Pt-based electrocatalysts, may also contribute to the overall enhancement effect.