Molybdenum cell performance

Transition metal carbides, such as tungsten carbide and its alloys, tantalum carbide, titanium carbide, and molybdenum carbide (Cowling et al, 1970,1971 Voorhies et al., 1972 Scholl et ah, 1992,1994 Borup et al., 2007), have been studied as catalysts for electrochemical reactions. However, it has been found that these transition metal carbides are unstable under high potentials and in acid solution, and this limits their application as PEM fuel cell catalysts (Borup et al., 2007). Transition metal nitrides have been studied as electrochemical catalysts in PEM fuel cell environments, and Zhong et al. (2006) showed that molybdenum nitride supported on carbon powder resulted in a cell performance of about 0.3 V at 0.2 A cm, and the catalyst was stable for 60 h of cell operation. However, the long-term performance durability is still questionable. 

A criterion for the suitability of a spectroscopy cell for investigations of working catalysts can be formulated as follows the activity or selectivity data and activation energy values have to be in agreement with the catalytic performance data measured with a conventional fixed-bed reactor. Table 1 is a comparison of the conversion and selectivity values characterizing an alumina-supported molybdenum-vanadium oxide catalyst during propane ODH obtained with a conventional fixed-bed reactor and with a spectroscopic cell that fulfills this requirement (Banares and Khatib, 2004). Similar considerations have also been reported earlier for other methods, such as X-ray diffraction (Clausen et al., 1991). 

Molybdenum is essential to the formation and activity of assimila-tory nitrate reductases. Cells must assimilate molybdate from the environment, metabolize molybdenum in some manner to form active molybdenum cofactor, and then incorporate it into a large molecular weight protein so that it can perform a reversible redox reaction with nitrate. Investigations on the aqueous Mo (III) model systems for nitrate reduction and the coordination of molybdate by naturally produced phenolates will hopefully lead to an understanding of the complex process of molybdenum acquisitions by and molybdenum function in nitrate reductases. 

Electron-transfer (ET) reactions play a central role in all biological systems ranging from energy conversion processes (e.g., photosynthesis and respiration) to the wide diversity of chemical transformations catalyzed by different enzymes (1). In the former, cascades of electron transport take place in the cells where multicentered macromolecules are found, often residing in membranes. The active centers of these proteins often contain transition metal ions [e.g., iron, molybdenum, manganese, and copper ions] or cofactors as nicotinamide adenine dinucleotide (NAD) and flavins. The question of evolutionary selection of specific structural elements in proteins performing ET processes is still a topic of considerable interest and discussion. Moreover, one key question is whether such stmctural elements are simply of physical nature (e.g., separation distance between redox partners) or of chemical nature (i.e., providing ET pathways that may enhance or reduce reaction rates). 

At temperatures below about 125°C, CO adsorption on platinum is very strong. Even few ppms in the H2 stream cause substantial performance losses on the anode. Therefore, the use of platinum alone is not viable for HOR in the presence of CO in low temperature fuel cells. Thus, platinum-ruthenium, platinum-molybdenum and platinum-tin are being used as anode electrocatalysts for hydrogen oxidation in the presence of CO because they tolerate low ppms of CO without excessive polarization losses. Timgsten carbide (WC) also shows high CO-tolerance [38,44]. 

Performance characteristics of sealed lithium—molybdenum disulphide cells are compared with those sealed cylindrieal sintered plate nickel-cadmium batteries in Table 9.15. 

The performance characteristics of lithium molybdenum sulphide cells are shown in Table 9.16. 

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