The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.

Magnesium hydride is cheap and contains 7.7 wt % hydrogen, making it one of the most attractive hydrogen storage materials. However, thermodynamics dictate that hydrogen desorption from bulk magnesium hydride only takes place at or above 300 degrees C, which is a major impediment for practical application. A few results in the literature, related to disordered materials and very thin layers, indicate that lower desorption temperatures are possible. We systematically investigated the effect of crystal grain size on the thermodynamic stability of magnesium and magnesium hydride, using ab initio Hartree-Fock and density functional theory calculations. Also, the stepwise desorption of hydrogen was followed in detail. As expected, both magnesium and magnesium hydride become less stable with decreasing cluster size, notably for clusters smaller than 20 magnesium atoms. However, magnesium hydride destabilizes more strongly than magnesium. As a result, the hydrogen desorption energy decreases significantly when the crystal grain size becomes smaller than approximately 1.3 nm. For instance, an MgH2 crystallite size of 0.9 nm corresponds to a desorption temperature of only 200 degrees C. This predicted decrease of the hydrogen desorption temperature is an important step toward the application of Mg as a hydrogen storage material.

Abstract.A survey is presented of the storage capacities of a large number of different adsorbents for hydrogen at 77 K and 1 bar. Results are evaluated to examine the feasibility and perspectives of transportable and reversible storage systems based on physisorption of hydrogen on adsorbents. It is concluded that microporous adsorbents, e.g. zeolites and activated carbons, display appreciable sorption capacities. Based on their micropore volume (∼ 1 ml/g) carbonbased sorbents display the largest adsorption, viz. 238 ml (STP)/g, at the prevailing conditions. Optimization of sorbent and adsorption conditions is expected to lead to adsorption of ∼ 560 ml (STP)/g, close to targets set for mobile applications. In the last two decades there has been an increasing interest in the development of (transportable) reversible systems for hydrogen storage with a high capacity, which is critical to the large-scale application of hydrogen fuel cells, in particular for mobile applications [1]. Up to now focus has mostly been on liquid-hydrogen and metal-hydride systems, which both have low energy efficiencies [2]. A higher energy efficiency is attainable with systems in which hydrogen is concentrated by physical adsorption above 70 K using a suitable adsorbent [3][4][5]. Such an absorbent should be safe, light and cheap and of course have a high adsorption capacity. In order to obtain a suitable driving range for automotive applications the United States Department of Energy (DOE) target has been set to 6.5 wt %, which equals 720 ml (STP)/g adsorbent . Schwarz and co-workers [6-8] studied the applicability of molecularly engineered activated carbons and came up with promising results. Much excitement has arisen on recent reports on the use of carbon nanofibers [9] and carbon nanotubes [10,11] In this paper we present a survey of the storage capacity for hydrogen at 77 K and 1 bar of a large number of different types of adsorbents -silicas, aluminas, zeolites, graphite, activated carbons and carbon nanofibers -in a wide range of specific surface areas and of different textures, in order to give further direction to our research on the development of a suitable storage system. PACS