Development and Properties of Materials as Disproportionation Catalysts for Sulphur Dioxide

Abstract

Solar radiation can only be concentrated for power production during day time with clear sky when direct radiation is available but energy is needed 24 hours per day. Therefore, it is important to integrate a thermal energy storage (TES) into concentrating solar power (CSP) systems. In CSP the state-of-the-art TES is molten salt. Its storage capacity is limited by ist heat capacity. To increase the storage capacity, a sulphur-based thermochemical cycle is introduced, where the energy is stored in the chemical bonds of sulphur. Sulphur as Energy storage material has many advantages over molten salts. It has more than 20 times the storage capacity. In addition, sulphur is stable under ambient conditions which allows for long-term storage at low cost. The solid sulphur cycle concludes three steps. In the first step, the heat from a CSP plant is utilised to decompose sulphuric acid at temperatures ranging from 650 to 850°C, resulting in the formation of sulphur dioxide (SO2). Subsequently, the SO2 is mixed with water and reacts in a disproportionation reaction, resulting in the generation of elemental sulphur and sulphuric acid. In the last step, sulphur can be burned in air thereby undergoing an exothermic reaction. The heat that is produced can be utilised in the generation of electricity in a gas-turbine. The product of this step is SO2 which is then used to produce sulphuric acid in a wellestablished process in the industry. The current work is focussing on the second step, the optimization of SO2 disproportionation. Previous studies by Wong et al. on SO2 disproportionation showed that this reaction has slow reaction kinetics. For this reason, a variety of common heterogeneous and homogeneous catalysts were tested. Dissolved iodide showed the best catalyst activity up to this point. However, for industrial applications the reaction rate of the SO2 disproportionation catalysed by iodide is still too slow as enormous reactor volumes would be required to process SO2 streams for generating sulphur at relevant scale. The goal of this work is to discover the relevant properties the catalyst requires to support the SO2 disproportionation reaction more efficiently. In the first step, further catalyst screening will be done. It is assumed that because of the similar redox potentials of iodine/iodide and sulphur dioxide/sulphur, the redox potential may be a relevant property of the SO2 disproportionation catalyst. In the context of a heterogeneous catalyst reaction, the Claus reaction has a certain degree of similarity to the SO2 disproportionation. A literature survey on this reaction revealed that basic sites are required for the adsorption of SO2 on the catalyst. It can therefore be assumed that this might also hold for SO2 disproportionation. These potentially relevant properties were considered to select candidates for the catalyst screening process. For metal oxides the electron-donating power is given by the optical basicity. To ensure optimal comparability and accessibility of the data pool, the initial candidates considered were metal oxides. Quantum Espresso will be used to understand this reaction, providing data like Surface energy, adsorption/desorption energy and activation energy.