32 resultados para Oxidation rate
Resumo:
The initial rate of the photocatalysed oxidation of methylene blue, MB, by dissolved oxygen in solution, ri(MB), is measured for a series of titania on glass samples exhibiting a wide range of activities. The samples used include two different types of commercial self-cleaning glass and a lab-made sol-geltitania film. The activities of these samples are also assessed using a resazurin-based photocatalyst activity indicator ink, i.e. Rz paii, for which the initial rates of the photocatalysed reduction of Rz were measured, ri(Rz). A plot of ri(MB)vs. ri(Rz) reveals a goodstraight line, thereby demonstrating a linear correlation (for TiO2films on glass at least) between the slow (usually hours) photocatalysed oxidation of organic materials, such as MB, and the fast (typically minutes) photocatalysed irreversible reduction of a dye, like Rz, in a paii. The possible use of paii technology for assessing, in a simple, quick and inexpensive manner, photocatalytic films both in the laboratory and in situ is discussed briefly.
Resumo:
Direct alcohol fuel cells (DAFCs) mostly use low molecular weight alcohols such as methanol and ethanol as fuels. However, short-chain alcohol molecules have a relative high membrane crossover rate in DAFCs and a low energy density. Long chain alcohols such as butanol have a higher energy density, as well as a lower membrane crossover rate compared to methanol and ethanol. Although a significant number of studies have been dedicated to low molecular weight alcohols in DAFCs, very few studies are available for longer chain alcohols such as butanol. A significant development in the production of biobutanol and its proposed application as an alternative fuel to gasoline in the past decade makes butanol an interesting candidate fuel for fuel cells. Different butanol isomers were compared in this study on various Pt and PtSn bimetallic catalysts for their electro-oxidation activities in acidic media. Clear distinctive behaviors were observed for each of the different butanol isomers using cyclic voltammetry (CV), indicating a difference in activity and the mechanism of oxidation. The voltammograms of both n-butanol and iso-butanol showed similar characteristic features, indicating a similar reaction mechanism, whereas 2-butanol showed completely different features; for example, it did not show any indication of poisoning. Ter-butanol was found to be inactive for oxidation on Pt. In situ FTIR and CV analysis showed that OHads was essential for the oxidation of primary butanol isomers which only forms at high potentials on Pt. In order to enhance the water oxidation and produce OHads at lower potentials, Pt was modified by the oxophilic metal Sn and the bimetallic PtSn was studied for the oxidation of butanol isomers. A significant enhancement in the oxidation of the 1° butanol isomers was observed on addition of Sn to the Pt, resulting in an oxidation peak at a potential ∼520 mV lower than that found on pure Pt. The higher activity of PtSn was attributed to the bifunctional mechanism on PtSn catalyst. The positive influence of Sn was also confirmed in the PtSn nanoparticle catalyst prepared by the modification of commercial Pt/C nanoparticle and a higher activity was observed for PtSn (3:1) composition. The temperature-dependent data showed that the activation energy for butanol oxidation reaction over PtSn/C is lower than that over Pt/C.
