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Highly conductive boron-doped polycrystalline diamond thin films (p 10 11 cm) were prepared via microwave plasma chemical vapor deposition (CVD). The electrochemical behavior for oxygen reduction was examined in 0.1 M KOH using linear sweep voltammetry Oxygen reduction was found to be highly inhibited, the cathodic voltammetric peak being observed at --1.2 V vs. Ag/AgCl, compared with the standard potential for the two-electron reduction of oxygen (02 + H2O + 2e = HO; + ow, E°' = -0.234 V vs. Ag/AgCl at pH 13). This demonstrates that, even in the presence of dissolved oxygen, diamond retains a relatively wide potential window; which could be advantageous in certain types of analytical applications. Possible interpretations for the high overpotential for oxygen reduction include a lack of adsorption sites for oxygen and/or reduced intermediates, a low density of states or a potential drop within a thin (='2 nm) surface layer; all of which have also been proposed for highly ordered pyrolytic graphite. The experimental data were fitted using digital simulation, which showed that the reduction peak appearing at ca. -1.2 V vs. Ag/AgCl is predominantly due to the reduction of oxygen to peroxide. Rotating disk electrode measurements were also consistent with an overall two-electron process. Experiments involving the addition of superoxide dismutase also supported this conclusion. The oxygen reduction reaction is proposed to occur on the sp3 carbon component of the surface, with a very small contribution from sp2 carbon impurities at smaller overpotentials.

In developing advanced fuel cells and other electrochemical reactors, it is desirable to combine the advantages of solid polymer electrolytes with the enhanced catalytic activity associated with temperatures above 100~ This will require polymer electrolytes which retain high ionic conductivity at temperatures above the boiling point of water. One possibility is to equilibrate standard perfluorosulfonic acid polymer electrolytes such as Nafion TM, with a high boiling point Bronsted base such as phosphoric acid. The Nafion/H3P04 electrolyte has been evaluated with respect to water content, ionic conductivity and transport of oxygen, and methanol vapor. The results show that at elevated temperatures reasonably high conductivity (>0.05 ~Q-1 cm-1) can be obtained. Methanol permeability is shown to be proportional to the methanol vapor activity and thus decreases with increasing temperature for a given methanol partial pressure. Comparisons and distinctions between this electrolyte and pure phosphoric acid are also considered.

The photocatalytic decomposition efficiency of gas-phase acetaldehyde was studied using a titanium dioxide thin film under weak UV illumination. Acetic acid and carbon dioxide were detected as the main reaction products. It was found that the apparent quantum yields (QY) for acetaldehyde degradation are determined by the normalized absorbed photon number (I norm /s -1 ), a parameter that is defined as the ratio of the number of absorbed photons to the number of adsorbed acetaldehyde molecules. This result is similar to that for 2-propanol degradation reported previously (Ohko, Y.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. A 1997, 101, 8057). However, although the QY values for 2-propanol degradation reached a constant value (ca. 28%) for very low relative light intensity (in the I norm region less than 10 -4 s -1 ), those for acetaldehyde degradation continued to increase with decreasing I norm and reached 180% for an initial concentration of 1000 ppmv, at an I norm value of 3 × 10 -5 s -1 . This discrepancy is due to the existence of radical chain reactions for the latter reaction. Compared to the maximum QY yield for 2-propanol decomposition (28%), which involves no chain-type reactions, the maximum QY for acetaldehyde conversion to acetic acid (∼150%) implies a radical chain-type process with a chain length of approximately five.

The remote bleaching of a dye, methylene blue, by UV-irradiated TiO 2 was studied in the gas phase. A TiO 2 -coated glass plate was faced to a glass plate coated with methylene blue, separated by a small gap (12.5 to 500 µm), and the TiO 2 coating was irradiated with UV light from the back. As a result, methylene blue was bleached in the gas phase containing oxygen, although it was inhibited by ethanol vapor. The remote bleaching is not a simple reduction of methylene blue to its leuco form; methylene blue is probably oxygenated or decomposed. The bleaching process may be caused by active oxygen species that are generated at the TiO 2 surface and transported to the methylene blue layer.