Electrocatalytic oxidation of small organic molecules in acid medium: Enhancement of activity of noble metal nanoparticles and their alloys by supporting or modifying them with metal oxides

Abstract

Different approaches to enhancement of electrocatalytic activity of noble metal nanoparticles during oxidation of small organic molecules (namely potential fuels for low-temperature fuel cells such as methanol, ethanol and formic acid) are described. A physical approach to the increase of activity of catalytic nanoparticles (e.g. platinum or palladium) involves nanostructuring to obtain highly dispersed systems of high surface area. Recently, the feasibility of enhancing activity of noble metal systems through the formation of bimetallic (e.g. PtRu, PtSn, and PdAu) or even more complex (e.g. PtRuW, PtRuSn) alloys has been demonstrated. In addition to possible changes in the electronic properties of alloys, specific interactions between metals as well as chemical reactivity of the added components have been postulated. We address and emphasize here the possibility of utilization of noble metal and alloyed nanoparticles supported on robust but reactive high surface area metal oxides (e.g. WO3, MoO3, TiO2, ZrO2, V2O5, and CeO2) in oxidative electrocatalysis. This paper concerns the way in which certain inorganic oxides and oxo species can act effectively as supports for noble metal nanoparticles or their alloys during electrocatalytic oxidation of hydrogen and representative organic fuels. Among important issues are possible changes in the morphology and dispersion, as well as specific interactions leading to the improved chemisorptive and catalytic properties in addition to the feasibility of long time operation of the discussed systems.

The Use of C-MnO2 as Hybrid Precursor Support for a Pt/C-MnxO1+x Catalyst with Enhanced Activity for the Methanol Oxidation Reaction (MOR)

“…Mn possesses a wide range of oxidation states and such oxo-manganese species are generally strong chemical oxidants. Due to the possibility of the Mn 4+ /Mn 3+ redox couple and the presence of labile oxygen, MnO2 shows high promoting and anti-poisoning activities for alcohol electro-oxidation [7]. The main reasons for effectiveness of MnO2 as a promoter are attributed to its surface area, tunnel structure and crystal phase.…”

“…In particular, high surface area metal oxides used as supports or matrices, are capable of physically separating metal particles (to diminish their tendency to undergo degradation by agglomeration) and of interacting mutually with them, thus affecting their chemisorptive and catalytic properties. Oxides are often thought as insulating or semi-conducting materials but certain non-stoichiometric oxides existing in various valance states exhibit conductivity not much lower than that of metals and possess appreciable catalytic activity [7]. In electro-catalysis, the reactions occur at the interface, so surface reactivity is very important.…”

“…A common approach to enhance the activity of Pt involves well dispersed nanoparticle structures, avoiding agglomeration and increasing utilization [7]. Further optimization of Pt-based electro-catalysts has been achieved through the formation of bi-metallic alloys such as PtCo and PtNi (for cathodic ORR) and PtRu (for anodic MOR) [8,9].…”

“…Some metal oxides are more hydrous than the others. The hydrous behavior, which varies from oxide to oxide, favors proton mobility and affects overall reactivity [7].…”

“…This would lead to a reduction of the total Pt loading on the electrode. For this purpose, reaction rates need to be enhanced (i.e., the overvoltage needs to be decreased) by modifying the catalyst composition or structure, to produce a more active electro-catalytic material [7].…”