Isolation and functional charecteristics of xylanases and cellulases of thermoascus aurantiacus

Abstract

Highly-dispersed platinized-carbon hydrogen anodes are developed for utilization in alkaline and acidic media hydrogen/air fuel cells by tailor-making both their macro and microstructures. The present hydrogen anodes are found to sustain intermittent load currents up to 800 mA/cm² in 6M KOH at 60°C, up to 1200 mA/cm² in 2.5M HSO at 40°C, and up to 800 mA/cm² in 7M HPO at 60°C. Load currents up to 350 mA/cm² and 800 mA/cm² can be sustained on these hydrogen anodes in 6M KOH and 2.5M HSO electrolytes for about 100 h and 200 h respectively, with little deterioration.

Physicochemical characterization of the electrode materials by photoelectron spectroscopy, X-ray absorption spectroscopy, chemisorption, X-ray diffraction, electron microscopy, and electron spin resonance reveals the presence of oxidic impurities of platinum, which retard its catalytic activity. On heat treatment in vacuum, platinized-carbon shows the absence of oxidic impurities of platinum and the presence of a higher concentration of active platinum in relation to platinized-carbon without such treatment. The occlusion of oxidic impurities of platinum by heating the catalytic substrate in vacuum is found to enhance its activity. This treatment, however, results in the sintering of the platinum crystallites and consequently a decrease in the available surface area of active platinum.

The present electrochemical studies have demonstrated that carbon electrodes with platinum-group bimetal catalysts can indeed exhibit higher activities than those for the individual components. The optimal activity, however, depends on the nature as well as the concentration of the individual metals. Of the three systems studied, Pt-Ru on carbon with composition 4 wt.% Pt + 6 wt.% Ru showed the highest activity.

In Chapter II, it had been shown that for the single-component Pt/carbon, the activity was enhanced by vacuum heat treatment, which led to the decomposition and consequent removal of the oxides of Pt. In the case of the bimetal Pt-Ru/carbon electrodes, high activities are observed without any heat treatment; in fact, vacuum heat treatment causes a drop in the electrochemical activity. It has been possible to understand this in the light of the XPS, XAS, and ESR measurements, which reveal that the presence of Ru suppresses the formation of any oxide of Pt and all the Pt is present in the active state. For Pt/carbon electrodes, this could be achieved only by heating in vacuum. The drop in the activity of Pt-Ru/carbon electrodes on heat treatment of the Pt-Ru/carbon is due to sintering and consequent loss of active surface area.

Phase diagrams, X-ray, and electron diffraction studies on unsupported Pt-Ru indicate that these form a complete solid solution. It appears that for the supported catalysts too, only Pt-Ru forms a “solid solution” and consequently shows the maximum synergistic effect. The higher activity of the Pt-Ru/carbon compared to Pt-Pd/carbon and Pd-Ru/carbon or the single-component Pt/carbon is not only due to the suppression of the formation of oxides of Pt but also due to the synergistic effect of Ru. Electron diffraction and microscopy clearly show that, unlike the other catalysts, which consist of dispersed crystallites, Pt-Ru/carbon consists of fine particles which appear amorphous.

The electrochemical studies conducted to examine the feasibility of coconut-shell carbon as a support for platinum and platinum-group bimetal catalysts in acid electrolytes show that the overall performance of Pt/carbon electrodes is comparable with the best Pt/carbon electrodes reported in the literature. The bimetal/carbon electrodes are found to exhibit higher activities than the Pt/carbon electrodes both in alkali and acid electrolytes. In 6M KOH, at 30°C, 4 wt.% Pt + 6 wt.% Pd/carbon electrodes, i.e., type-21 electrodes, can sustain current loads as high as 2 A/cm² with a polarization of 425 mV. The 4 wt.% Pt + 6 wt.% Ru/carbon electrodes, i.e., type-17 electrodes, can withstand load currents of 1.6 A/cm² and 1.3 A/cm² in 2.5M HSO and 7M HPO electrolytes respectively at 60°C with a polarization of about 600 mV.

The study demonstrates that carbon electrodes with (4 wt.% Pt + 6 wt.% Ru) catalyst do possess reasonably high catalytic activity for indirect oxidation of ammonia. The lower activity of these electrodes for the indirect oxidation of ammonia in relation to their performance as hydrogen anodes is due to the mass-transfer polarization effects arising from the presence of nitrogen in the former. The catalytic activities of these electrodes for direct oxidation of ammonia are, however, poor.