No abstract

No abstract

The catalytic function and activity of manganese oxides ͑MnOx: Mn 2 O 3 , Mn 3 O 4 , Mn 5 O 8 , and MnOOH͒ to the electrochemical reduction of O 2 in 0.10 M KOH aqueous solution have been investigated by cyclic voltammetry at MnOx/Nafion-modified gold electrodes. Two successive reduction current peaks were observed at Nafion-modified electrodes in the cyclic voltammograms, i p,1 for a two-electron reduction of O 2 to hydrogen peroxide (HO 2 Ϫ ) and i p,2 for a two-electron reduction of HO 2Ϫ to OH Ϫ . The peak current heights of i p,1 and i p,2 changed greatly depending on the kind of MnOx species incorporated into the MnOx/Nafionmodified gold electrodes; i p,1 increased and i p,2 decreased. On the assumption that HO 2 Ϫ produced in the first reduction step is chemically decomposed into O 2 and OH Ϫ with a catalytic action of MnOx and that this regenerated O 2 is reduced again in the same first reduction step, we evaluated the catalytic activity of MnOx using the values of i p,1 and i p,2 . MnOOH provided the highest catalytic activity to the electrochemical reduction of O 2. This result was supported by another experiment by using a chemical method where catalytic decomposition of HO 2 Ϫ with MnOx was estimated by measuring the O 2 concentration directly with a commercial oxygen sensor.

Gold nanoparticles with different morphologies (nanocrystallites, perfect nanospheres, plumbs, and nanoaggregates) have been electrodeposited on different substrates, namely, glassy carbon (GC), highly oriented pyrolytic graphite (HOPG), and Au(111) single-crystalline substrates. Au particles with particle size ranging from a few nanometers to a few micrometers have been prepared. The morphology of the electrodeposited Au particles was largely dependent on the nature of the substrate as well as the composition of the electrodeposition bath. For instance, the inclusion of iodide ions during electrodeposition was found to enhance two-dimensional (2D) growth of the Au nanoparticles, and particles with a relatively small particle size down to 10nm were obtained. The inclusion of L-cysteine (as an additive) during the electrodeposition of the Au nanoparticles resulted in a significant influence on the morphology (and the particle size of the Au particles), which strongly depends on the nature of the substrate. Au nanoparticles with crystalline geometry were prepared on the Au(111) substrates in the presence of L-cysteine, while under the same experimental conditions Au aggregates of size up to 300nm were electrodeposited on the GC substrates. Au particles with a perfect spherical shape were electrodeposited on the HOPG electrodes. X-ray diffraction measurements of the electrodeposited Au particles revealed significantly different crystallinity of the Au particles and in turn different ratios of the single-crystalline domains constituting the Au particles. The cyclic voltammetric response toward the oxygen reduction reaction at the different Au nanoparticles showed a versatile behavior ranging from a quasi-reversible two-electron reaction to an irreversible overall four-electron reaction in O2 -saturated 0.5M KOH solution, demonstrating the entirely different electrocatalytic activity of the thus-prepared Au nanoparticles on different substrates.

No abstract