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

Single-walled carbon nanotubes (SWNT) were reported to have record high hydrogen storage capacities at room temperature, indicating an interaction between hydrogen and carbon matrix that is stronger than known before. Here we present a study of the interaction of hydrogen with activated charcoal, carbon nanofibers, and SWNT that disproves these earlier reports. The hydrogen storage capacity of these materials correlates with the surface area of the material, the activated charcoal having the largest. The SWNT appear to have a relatively low accessible surface area due to bundling of the tubes; the hydrogen does not enter the voids between the tubes in the bundles. Pressure ± temperature curves were used to estimate the interaction potential, which was found to be 580 AE 60 K. Hydrogen gas was adsorbed in amounts up to 2 wt % only at low temperatures. Molecular rotations observed with neutron scattering indicate that molecular hydrogen is present, and no significant difference was found between the hydrogen molecules adsorbed in the different investigated materials. Results from density functional calculations show molecular hydrogen bonding to an aromatic CÀC bond that is present in the materials investigated. The claims of high storage capacities of SWNT related to their characteristic morphology are unjustified.

This review article illustrates possible aspects for molecularweight enlargement and immobilisation of transition-metal complexes that have been developed for bridging the gap between homogeneous and heterogeneous catalysis. The recycling of homogeneous catalysts can be performed using different types of supports, such as dendrimers, hyperbranched polymers, nanostructured materials or stabilized nanoparticles in combination with suitable filtration methods

The cover picture shows the concept of operating a homogeneous catalyst under continuous operation conditions: The recycling of molecular-weight-enlarged transition-metal complexes by suitable filtration methods and equipment in order to combine the advantages of both classical homogeneous and heterogeneous catalysis. Various aspects of molecular-weight enlargement and immobilization of catalysts, and recent developments, are reported in the Microreview by D. Vogt et al. on p. 4011 ff.

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