As the world's supply of fossil fuels continues to diminish, there is an increasing need for the development of alternative energy sources. Fuel cell technology, coupled with renewable, biomass-derived fuel stocks such as methanol and ethanol, is one such energy source that will find increasing use in both the near- and long-term future. The present generation of catalysts for fuel cells is characterized by limited efficiency and durability, and as such there is a great need to improve upon and develop new catalytic materials. Nanoporous solids, also known as inverse opals, are potentially useful materials, due in part to their relatively regular structure and physical integrity. It is the purpose of this work to initiate and investigation of nanoporous catalytic materials.We have developed nanoporous solids made from the electrodeposition of platinum and palladium metal around a silica nanosphere template. We used these materials to probe and interpret the catalysis of selected alcohols using cyclic voltammetry and chronoamperometry measurements. Initially we examined the electrocatalytic oxidation of methanol and ethanol, under acidic and basic conditions, using Pt and Pd nanoporous solid electrodes. Our results were then compared to planar Pt and Pd substrates, prepared using the same electrodeposition process as the nanoporous materials. The current density enhancement seen for the Pt nanoporous solids was more pronounced under basic conditions than acidic conditions. It was also determined that the nanoporous Pd was more catalytically active for ethanol than for methanol, with the nanoporous Pt producing higher catalytic efficiency for methanol. The enhancement that was observed with the nanoporous solids over their planar counterparts could be the result of either geometric factors or the catalyst surface morphology. The effect of morphology of the nanoporous Pt substrates was examined by studying the electrocatalytic oxidation of 1,2-propanediol under basic conditions. Our work was performed in alkaline media because, for this reactant, electrocatalytic oxidation is more efficient under these conditions. Our data for 1,2-propanediol point to metal morphology as the primary explanation for the enhanced current density relative to the planar solid electrode observed for the reaction with the nanoporous Pt.Our work with 1,2-propanediol proved to be informative, and we continued to examine the geometric vs. morphological enhancement issue by examining the electro-catalytic oxidation of 1,3-propanediol and four butanediols at nanoporous Pt and planar solid Pt electrodes under basic conditions. These reactants were chosen based on the positions of the hydroxyl moieties, so that the role of functional group position could be studied. The electro-catalysis of the aforementioned diols was studied both in terms of their reaction mechanism(s) and their kinetics. Our data indicate that the dominant factor in mediating the electrocatalytic oxidation pathway is the proximity of the hydroxyl groups on the diol and that there is a difference in the morphology of the nanoporous Pt and the planar solid Pt. This work leads to the conclusion that it is the electrode morphology rather than the geometric considerations that appears to be the dominant factor in the enhancement observed for the nanoporous solids and that this enhancement also depends upon the reactants used.
Read
