The fundamental physical properties of nanocrystals, such as their electronic band structure and optical activity, can be drastically different from those of the corresponding bulk materials, mainly due to their large surface-to-volume ratio. Among variety of properties, variation (lowering) of melting temperature of nanoparticles is a very important issue as it decreases the functional range of the solid phase. In this talk we discuss mechanism of melting of nanoclusters from a perspective of numerical modeling and relate the computational results to experimental observations. Conventionally, when studying melting of nanoparticles, the particles and the host medium are both in direct contact with a temperature source. In such conditions, melting of the nanoparticles starts at their surfaces, at reduced temperatures [1,2]. The typical dependence of the melting temperature on the size of nanoparticles is presented in the Figure 1. It shows that generally for particles built from smaller number of atoms melting is observed at lower temperature. However, the microscopic mechanism of melting may be affected by many factors. In particular, the irradiation with laser light can selectively and homogeneously excite the metallic nanoparticles without direct heating of the particle environment. The time of the laser pulse can be very short (fs) and conse-quently the melting may happen in non-equilibrium conditions. Also, the initial structural changes may lead to intermediate non-homogenous structures. Therefore, the microscopic mechanism of melting may be complex, and depending not only on the size of particles but also on theirs initial structures, rate of heating and the local temperature (kinetic energy of particles) distribution. We will show how the above factors affect the mechanism of melting of nanoparticles. The numerical simulation results will be confronted with the experimental observations which show that, although the as-prepared nanorods are defect-free, point and planar defects are present in gold nanorods after laser irradiation. The defects are mainly (multiple) twins and stacking faults (planar defects). They are the precursors that drive the convertion of nanorods (110) facets into the more stable (100) and (111) facets and hence minimize their surface energy. These observations suggest that short-laser pulsed photothermal melting begins with the creation of defects inside the nanorods followed by surface reconstruction and diffusion [3].

Properties of methane adsorbed in confined geometries are interesting from both fundamental and practical points of view. At ambient temperatures adsorption at the supercritical conditions is usually studied, as it is interesting from the point of view of methane storage. At the same time, the analysis of low temperature adsorption properties is essential for understanding the mechanism of adsorption, as a function of temperature and the pore size. In this work we study phase diagram of methane confined in carbon slit pores of the width between 1nm and 4nm. We analyze the mechanism of the layering transition(s) and capillary condensation at subcritical conditions, for temperatures between 80 K and 180 K, () then we study the capacity of storage up to the room temperature. The mechanism of the layers formation is strongly temperature dependent, and changes from a sharp adsorption at low temperature to more continuous one at higher temperatures. The size of pore defines the number of layers adsorbed: 1 nm pore allows adsorbing 2 layers of methane molecules. Capillary condensation is observed in 4 nm pores.

One of potential perspectives for clean fueling of cars is the use of hydrogen-powered fuel cells. A major challenge in the massif implementation of hy-drogen-fuelled vehicles still consist in designing hydrogen storage systems that are reversible, light, cheap and able of delivering a driving range of few hundreds of kilometers. Between the possible hydrogen adsorbents carbon porous structures with locally slit-shaped pore geometry remain the most promising ones.From the analysis of the computer simulations of adsorption properties of the new structures, a rich spectrum of relations between structural characteristics of OCFs and ensuing hydrogen adsorption (structure-function relations) emerges: (i) storage capacities higher than in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets, which creates additional surface areas (with respect to infinite graphene sheets), carried mainly by edge sites; ii) for OCFs with a ratio of in-plane to edge sites approximately equal 1 and surface areas of 3800-6500 m2/g, we found at 77 K record maximum excess adsorption of 75-85 g H2/kg C, and record storage capacity of 100-260 g H2/kg C (at 100 bar); (iii) additional increase of hydrogen uptake could potentially be achieved by chemical substitution and/or intercalation of OCF structures, in order to increase the energy of adsorption. In consequence we conclude that OCF structures, if synthesized, will allow the hydrogen uptake at the level required for vehicular applications.

Hydrogen adsorption in slit shaped pores built up from truncated graphene fragments has been simulated using Grand Canonical Monte Carlo technique and the influence of pore wall edges on hydrogen storage by physisorption has been analyzed. We show that due to the additional gas adsorption at the pore edges the adsorbed gravimetric amount significantly increases (by a factor of two) with respect to models of pores with infinite graphene walls. The contribution of the edges' adsorption to the total hydrogen uptake is independent of the pore wall shape but it depends on its surface. We also show that the maximum of the excess adsorption shifts towards higher pressures when the edge contribution increases. This information can be used to characterize experimentally structures of porous adsorbents and complement pore size distribution analysis usually performed with gases others than hydrogen. We suggest that porous carbons built from polycyclic hydrocarbons can achieve storage performances required for practical applications

his paper demonstrates that nanospace engineering of KOH activated carbon is possible by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. High specific surface areas, porosities, sub-nanometer (<1 nm) and supra-nanometer (1-5 nm) pore volumes are quantitatively controlled by a combination of KOH concentration and activation temperature. The process typically leads to a bimodal pore size distribution, with a large, approximately constant number of sub-nanometer pores and a variable number of supra-nanometer pores. We show how to control the number of supra-nanometer pores in a manner not achieved previously by chemical activation. The chemical mechanism underlying this control is studied by following the evolution of elemental composition, specific surface area, porosity, and pore size distribution during KOH activation and preceding H3PO4 activation. The oxygen, nitrogen, and hydrogen contents decrease during successive activation steps, creating a nanoporous carbon network with a porosity and surface area controllable for various applications, including gas storage. The formation of tunable sub-nanometer and supra-nanometer pores is validated by sub-critical nitrogen adsorption. Surface functional groups of KOH activated carbon are studied by microscopic infrared spectroscopy.

n recent years, a great emphasis has been placed on replacing fossil fu- els with clean, renewable energy for use in vehicles. One potential solution is the use of hydrogen gas as a fuel source to power a fuel cell. For ve- hicular use, the US Department of En- ergy (DOE) has identified major chal- lenges to implementing a hydrogen- powered which include design hydro- gen storage systems capable of deliv- ering a driving range of hundreds of kilometers. Mechanism of hydrogen adsorption in carbon porous structures is a fundamental problem for these applications. It can be shown that it is not possible to increase hydrogen storage capacity only by modification of slit geometry without simultaneous increase of the specific surface. So, we have introduced structures with higher surfaces and analyzed their adsorption properties. These new models of hypothetical structures represent ordered carbon structure with low density architecture required for effective application of porous carbons for mobile storage. We call them Open Carbon Frameworks (OCF). Theoretically they may have the specific surfaces exceeding 6000 m 2 /g

In recent years, a great emphasis has been placed on replacing fossil fuels with clean, renewable energy for use in vehicles. One potential solution is the use of hydrogen gas as a fuel source to power a fuel cell. For vehicular use, the US Department of Energy (DOE) has identified major challenges to implementing a hydrogen-powered which include design hydrogen storage systems capable of delivering a driving range of hundreds of kilometers. Mechanism of hydrogen adsorption in carbon porous structures is a fundamental problem for these applications.