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Important limitations in the application of light metal hydrides for hydrogen storage are slow kinetics and poor reversibility. To alleviate these problems doping and ball-milling are commonly applied, for NaAlH 4 leading to particle sizes down to 150 nm. By wet-chemical synthesis we have prepared carbon nanofiber-supported NaAlH 4 with discrete particle size ranges of 1-10 microm, 19-30 nm, and 2-10 nm. The hydrogen desorption temperatures and activation energies decreased from 186 degrees C and 116 kJ.mol (-1) for the largest particles to 70 degrees C and 58 kJ.mol (-1) for the smallest particles. In addition, decreasing particle sizes lowered the pressures needed for reloading. This reported size-performance correlation for NaAlH 4 may guide hydrogen storage research for a wide range of nanostructured light (metal) hydrides.

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Hydrogen is regarded as a suitable energy carrier for sustainable energy schemes. However, the reversible storage of hydrogen is still a major challenge, especially for mobile applications. Several storage media have been considered, for example, clathrate structures, [1] metal-organic frameworks, [2] and lithium nitride/amide [3,4] as well as physisorption on carbon or zeolites, [5][6][7] and alanates. [8] Sodium alanate (NaAlH 4 ) is promising because of its high reversible hydrogen storage capacity and optimal thermodynamic stability for reversible hydrogen storage at medium temperatures. Nevertheless, kinetic barriers restrict hydrogen desorption rates. Furthermore, reloading of undoped NaAlH 4 is also slow and not possible under practical conditions. [9,10] It has been found that Ti additives improve the kinetics of hydrogen absorption and desorption, but high pressures (P > 100 bar) and long times are still needed to reload depleted sodium alanate. [10][11][12][13] Further improvement of the kinetics requires new strategies and methods. A possible strategy is to decrease the particle size to the nanometer range, for which it is known that physicochemical properties of such particles may deviate considerably from the bulk properties. [13][14][15][16][17] By using nanosized sodium alanate, the phase segregation to micrometersized NaH and Al during hydrogen extraction from these materials will be prevented, [18,19] which might lead to enhanced rates of hydrogen desorption and absorption.Therefore we prepared, for the proof of the principle, nanosized NaAlH 4 particles supported on a surface-oxidized carbon nanofiber support (CNF ox ) and investigated their hydrogen desorption and absorption properties in relation to the structural properties of the materials. The NaAlH 4 (9 wt %) supported on carbon nanofibers was obtained by impregnation and drying techniques (see the Experimental Section for details) and is referred to here as NaAlH 4 /CNF ox .

Base catalysis is of importance for organic synthesis in general and fine chemicals manufacture in particular. Activated hydrotalcites have recently received a great deal of attention as solid base catalysts; however, no systematic work on the nature of their active sites has been published up till now. In this work two different methods have been applied to activate Mg-Al hydrotalcites to obtain Brønsted-base catalysts for liquid-phase condensation reactions. Activation via thermal treatment followed by rehydration (HT-reh) resulted in irregularly stacked platelets ( approximately 60 nm), whereas the sample activated via aqueous ion-exchange (HT-exc) preserved its original hexagonal hydrotalcite platelets ( approximately 100 nm). The specific activity for the self-condensation of acetone of HT-reh was over 10 times that of HT-exc. The enthalpy of CO2 adsorption on the activated hydrotalcites determined with calorimetry to gain insight into the strength of the basic sites showed very similar values. IR spectra of adsorbed CDCl3 as probe molecule on the differently activated samples revealed large differences in adsorbed amounts, but again the strength of the basic sites appeared to be the same. These results point to steric hindrance for the substrate molecules as the main factor determining differences in catalytic activity. The high accessibility of Brønsted-base sites in HT-reh is proposed to involve a distorted edge structure of the platelets. The edge structure of exchanged samples could be distorted too, either by exchange under reflux conditions or under ultrasonic treatment. In line with the proposed model, the distorted exchanged samples displayed a much higher catalytic activity than HT-exc.

A series of alumina-supported gold catalysts was investigated for the CO-free production of hydrogen by partial oxidation of methanol. The addition of alkaline-earth metal oxide promoters resulted in a significant improvement of the catalytic performance. The methanol conversion was ca. 85 % with all studied catalyst materials, however, the selectivity for hydrogen increased from 15 % to 51 % when going from the unpromoted to a BaO-promoted catalyst. The formation of the undesired byproducts CO, methane, and dimethyl ether was considerably reduced as well. The observed trend in catalyst performance follows the trend in increasing basicity of the studied promoter elements, indicating a chemical effect of the promoter material. Superior catalytic performance, in terms of H(2) and CO selectivity, was obtained with a Au/La(2)O(3) catalyst. At 300 degrees C the hydrogen selectivity reached 80 % with only 2 % CO formation, and the catalyst displayed a stable performance over at least 24 h on-stream. Furthermore, the formation of CO was found to be independent of the oxygen concentration in the feed. The commercial lanthanum oxide used in this study had a low specific surface area, which led to the formation of relative large gold particles. Therefore, the catalytic activity could be enhanced by decreasing the gold particle size through deposition on lanthanum oxide supported on high-surface-area alumina.

The industrially important deperoxidation reaction of cyclohexyl hydroperoxide was combined with the epoxidation of cyclohexene over a series of mesoporous titanium silicates. The process was found to proceed with high selectivity, forming cyclohexanol, cyclohexanone, and epoxy-cyclohexane. The deperoxidation and epoxidation reactions were found to compete. However, by changing the surface hydrophobicity of the catalysts or the applied olefin/peroxide ratio, the overall mechanism could be directed in favor of the epoxidation. In this way, the combined selectivity toward valuable alicyclic oxygenates from cyclohexane oxidation based on peroxide conversion could be increased up to 170%. The catalysts where found to be stable from recycling and filtration experiments.

Olefin epoxidation with cyclohexyl hydroperoxide offers great perspective in increasing the yield from industrial cyclohexane oxidation and the production of epoxides in an apolar medium. Two competing hydroperoxide conversion routes, namely direct epoxidation and thermal decomposition, were identified. The formation of radicals seemed to play a role in both mechanisms. However, olefin epoxidation was found to solely take place at the catalyst. Allylic oxidation of cyclohexene occurs under reaction conditions primarily by molecular oxygen and only constitutes a minor route. The presence of molecular oxygen was found to increase the overall yield of the process by solvent oxidation yielding new cyclohexyl hydroperoxide. Hydrolysis and isomerization of the epoxide were found to be negligible reactions, although the epoxide gets converted at higher concentrations, presumably by the radical initiated polymerization. UV-Vis spectroscopy provided proof for the formation of titanium-hydroperoxide species as the active catalytic site in the direct epoxidation reaction.