Novel Highly Acidic Mixed-Metal (W, Zr) Catalytic Matrices for Dispersed Noble Metal Nanoparticles: Electrooxidation of Hydrogen and Small Organic Molecules

Abstract

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Pt and bimetallic platinum-ruthenium (PtRu) electrocatalytic nanoparticles over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the oxidation of hydrogen and small organic molecules (formic acid, methanol, ethanol or dimethyl ether) have been enhanced even at decreased loadings of noble metal nanostructures (for hydrogen oxidation) as well as in terms of both increasing the electrocatalytic currents and lowering the onset potentials of organic fuels’ reactions. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized noble metals (Pt, Pd or PtRu) and metal oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the competitive interactions (via the surface hydroxyl groups) with undesirable reaction intermediates (including CO adsorbates). Thus their desorption (“third body effect”) or even oxidative removal (e.g. of CO to CO2) are feasible. Furthermore, during oxidation of ethanol, when rhodium nanoparticles have been dispersed in between WO3 and ZrO2 layers (toward formation of the nanoreactor system), significant electrocatalytic current enhancements are observed. The result can be rationalized in terms of the formation of sub-microstructured nano-reactors in which Rh induces splitting of C-C bonds in C2H5OH molecules toward CHx/CH4 and CO adsorbate species, or even methanol-type intermediates, that could be further electrooxidized at PtRu catalytic centers. Further mechanistic studies along this line are planned.