Crystal structure prediction and ammonia dynamics in strontium ammine complex

In parallel with the rapid technological and economic developments around the world the life standarts get higher and the world population increases and various structural changes that come with them are rapidly increasing the world energy requirement. On the other hand, the reserves of fossil fuels that are presented to us by nature and which are a source of energy that we have been using exessively in the last century are in rapid depletion. Besides the limited amount of these reserves, fossil fuels produce the greenhouse gases as a result of burning the fuel which causes very serious problems for our world. Because of these reasons, it is obvious that alternative energy sources are needed to be developed irrespective of how much fossil fuel reserves remain. Hydrogen which is the most abundant element in the planet is a promising alternative energy source that it has significant properties and a great potential as an energy carrier. It has high energy density and is a relatively clean energy source compared to fossil fuels. However, there are some difficulties in using hydrogen as an energy source; the hydrogen production and storage at high efficiency are big challenges. The main goal when storing hydrogen is to achieve the highest possible volumetric density in storage. The second important criterion is the reversibility of hydrogen in absorption and desorption. In order to provide all these desired properties in the best way, many studies have been conducted on storage techniques of hydrogen. These storage techniques can be examined in three main categories as gas, liquid, solid phase. Gas phase storage is generally carried out in high pressure vessels with a pressure of 20 MPa. Depending on the weight and type of the tank, 1 – 7 wt % hydrogen can be stored. The new generation of lightweight composite cylinders can withstand pressures up to 80 MPa and at this pressure hydrogen can be stored up to a volume of 36 kg-m3. It is an economical solution compared to other methods, but due to the fact that the energy density of hydrogen is very low, the amount of stored hydrogen in mass is limited. Furthermore, about 20 % of the fuel energy is spent to compress the hydrogen during storage. Liquid storage is made in super insulated double-walled cryogenic vessels that minimize heat transfer and boiling. In comparison to gas storage it is relatively light method, but since cryogenic temperature is required for this storage to be made, the energy required for liquefaction is very high. Solid storage can be done with the help of metal hydrides, carbon nanotubes, metal organic frameworks (MOF) and metal amines. In storage with metal hydrides, hydrogen is chemically in interaction with metal atoms and upon the thermal treatment hydrogen releases. Storage with metal hydrides; reliable, requires little space and less energy to refill compare to gas and liquid storage. Despite these advantages, this method also has some disadvantages such as high energy requirement, heavy weight and high cost in releasing the fuel. All reversible hydrides operating at normal temperature and atmospheric pressure contain transition metals, so that the gravimetric storage capacity of hydrogen is less than 3 %. Carbon nanotubes are tubular transformed graphite sheets with dimensions in microns. Nanotubes can be produced in single-wall or multi-wall forms, and there are also nanotubes formed with various additives such as alkali metals (Li, K). Hydrogen storage capacities of carbon nanotubes vary according to the type of nanotube (single-walled, multi-walled), whether tubes are close or open, the measurements of the tube (tube diameter, length etc.) and the activity of the tube surfaces. Some of the carbon nanotubes obtained in the studies have shown to have a very high hydrogen capacity. However, very low temperatures and high pressures are required to achieve this high capacity. Moreover, the cost of nanotubes is high and the technologies and facilities available today are not sufficient to produce high amounts of carbon nanotubes. Metal organic frameworks (MOFs, Metal Organic Frameworks) are chemical compounds with high porosity, containing a metal ion or ions that coordinated with an organic molecule. This porous system provides a high surface area for hydrogen absorption. Although 4.5 wt % hydrogen can be stored at cryogenic temperatures, the hydrogen storage capacity at room temperature is about 1 wt %. Metal amine compounds have an easier absorption and release kinetics than metal hydrides. Although metal amines are known compounds for about a century, hydrogen storage with these structures is a new concept. Their general formula is M(NH3)nXm where M is a metal cation (Sr, Mg, Ca, Cr, Zn ...), and X is an anion such as Cl, SO4. Molecular ammonia contains 17.8 wt % of hydrogen and it can be used as a direct fuel in internal combustion engines or in solid oxide fuel cells or it can be catalyzed to hydrogen and nitrogen molecules below 650 K with high efficiency. Although pure ammonia carries a high amount of hydrogen, it is not safe to use it alone due to its toxicity. Therefore, storing ammonia in the form of metal ammines also solves the transportation of ammonia. The great advantage of this method is that the absorption and release of ammonia are reversible, fast and easy to control. In this study, metal amine compounds of Sr(NH3)nCl2 (for n=8, 6, 4, 2, 1) were investigated. Strontium metal ammine complex carry great potential as an energy carrier. In the literature, it has been stated that this compound can store hydrogen up to 8.21 wt % at a temperature of 273 K and pressure of 1 bar so that the ammonia release can start at suitable temperatures which are not so high for practical use. Moreover, SrCl2 can absorb eight ammonia molecules per unit formula, which theoretically corresponds to a volumetric density of 642 kg - (NH3)m-3 at room temperature. Considering this great potential, it was aimed to determine the lowest energy structures of strontium ammine compounds and analyze their ammonia dynamics in the recent study. At first, crystal structure prediction was performed to find the unknown crystal structures using CASPESA method which is a crystal structure prediction program developed by our research group. The resulting structures from CASPESA were further optimized with the DFT. The best DFT structures were applied to phonon calculations to select the stable structures. Then, the selected structures have been employed in the nudged elastic band calculations to reveal the ammonia dynamics.