Electrochemical Nanoreactors of Mixed Metal-Oxide-Supported Noble- Metal-Nanoparticles for Efficient Oxidation of Dimethyl Ether and Ethanol

Abstract

Noble metal-based (e.g. Pt, Pd, PtRu or PtSn) nanoparticles have been recognized as the most active catalytic systems towards oxidation of small organic molecules (ethanol, dimethyl ether) at low and moderate temperatures. Although all of them can be ideally oxidized to carbon dioxide, their actual electrooxidation mechanisms can significantly differ and may result in distinct electrocatalytic efficiencies and final reaction products. There is a need to develop functionalized electrocatalytic systems exhibiting specific reactivity and capability to induce the slow reaction steps at ambient conditions. Platinum based nanoparticles are readily poisoned by the strongly adsorbed CO-type intermediate species requiring fairly high overpotentials for their removal. For example, to enhance activity of Pt-based catalysts toward the ethanol oxidation, additional metal or metal oxide nanostructures (rhodium, iridium, tin or molybdenum and tungsten oxides) will be intentionally introduced to the electrocatalytic interface. We are also going to demonstrate that catalytic activity of platinum-based nanoparticles can be significantly enhanced through their interfacial modification with ultra-thin monolayer-type films of mixed-metal-oxo-species of titanium, cerium, zirconium or molybdenum and tungsten in their simple and polyoxometallate forms Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account possibility of specific interactions of noble metals with transition metal oxide species as well as existence of active hydroxyl groups in the vicinity of catalytic noble metal sites. It is noteworthy that certain metal and mixed-metal oxides can generate –OH groups at low potentials: they induce oxidation of passivating CO adsorbates (e.g. on Pt) or breaking C-H bonds (e.g. during oxidation of methanol or dimethyl ether). When combined with dispersed Rh or Ir, immobilized in the organized manner in the vicinity Pt sites, they tend to weaken C-C bonds during ethanol oxidation. We will also demonstrate that deposition of Pt and bimetallic PtRu nanoparticles onto nanostructured zirconia support results in the enhancement of their catalytic activities toward electrooxidation of dimethyl ether, another possible alternate fuel. Once more, it is reasonable to expect that the abilities of zirconia to facilitate proton mobility and to donate active hydroxyl groups at the electrocatalytic interface in acid media are responsible for the observed enhanced catalytic effects. Specific interactions with platinum and ruthenium components are also postulated.