Thermodynamic and kinetic properties of potassium alanate (KAlH 4 ) are investigated. Its pressure-compositionisotherm measurement exhibits two plateaus for hydrogen absorption/desorption in KAlH 4 , with gravimetric hydrogen densities of 1.2 ( 0.1 and 2.6 ( 0.2 mass% and reaction enthalpies of 81 and 70 kJ • mol -1 H 2 , respectively. However, the nonisothermal decomposition of KAlH 4 occurs through three endothermic events at temperatures of 294, 311, and 347 °C with the release of hydrogen. Whereas the high temperature event is clearly attributed to K 3 AlH 6 decomposition, the low temperature events occur by two reactions, denoting the existence of an intermediate phase during KAlH 4 decomposition. FTIR measurements suggest that this intermediate phase is a K y AlH x compound (y g 1, x g 4) with a high coordination about the aluminum. TiCl 3 -doped KAlH 4 also exhibits three decomposition events, but with significant reduction of desorption temperatures (∼50 °C) as well as activation energies that is attributed to particle size reduction and creation of charged vacancies.

No abstract

No abstract

No abstract

No abstract

The solid-state reaction between LiNH2 and LiH potentially offers an effective route for hydrogen storage if it can be tailored to meet all the requirements for practical applications. To date, there still exists large uncertainty on the mechanism of the reaction--whether it is mediated by a transient NH3 or directly between LiNH2 and LiH. In an effort to clarify this issue and improve the reactivity, the effects of selected nitrides were investigated here by temperature-programmed desorption, X-ray diffraction, in-situ infrared analysis, and hydrogen titration. The results show that the reaction of LiNH2 with LiH below 300 degrees C is a heterogeneous solid-state reaction controlled by Li+ diffusion from LiH to LiNH2 across the interface. At the LiNH2/LiH interface, an ammonium ion Li2NH2+ and a penta-coordinated nitrogen Li2NH3 could be the intermediate states leading to the production of hydrogen and the formation of lithium imide. In addition, it is identified that BN is an efficient "catalyst" that improves Li+ diffusion and hence the kinetics of the reaction between LiNH2 and LiH. Hydrogen is fully released within 7 h at 200 degrees C with BN addition, rather than several days without the modification.

No abstract

Xanthine dehydrogenase (XDH) from the bacterium Rhodobacter capsulatus catalyzes the hydroxylation of xanthine to uric acid with NAD(+) as the electron acceptor. R. capsulatus XDH forms an (alphabeta)(2) heterotetramer and is highly homologous to homodimeric eukaryotic XDHs. The crystal structures of bovine XDH and R. capsulatus XDH showed that the two proteins have highly similar folds; however, R.capsulatus XDH is at least 5 times more active than bovine XDH and, unlike mammalian XDH, does not undergo the conversion to the oxidase form. Here we demonstrate electrocatalytic activity of the recombinant enzyme, expressed in Escherichia coli, while immobilized on an edge plane pyrolytic graphite working electrode. Furthermore, we have determined all redox potentials of the four cofactors (Mo(VI/V), Mo(V/IV), FAD/FADH, FADH/FADH(2) and two distinct [2Fe-2S](2+/+) clusters) using a combination of potentiometric and voltammetric methods. A novel feature identified in catalytic voltammetry of XDH concerns the potential for the onset of catalysis (ca. 400 mV), which is at least 600 mV more positive than that of the highest potential cofactor. This unusual observation is explained on the basis of a pterin-associated oxidative switch during voltammetry that precedes catalysis.