Metal dicyanamides as solid adsorbents for CO2/N2 separation and efficient water oxidation catalysts

Abstract

The demand for the energy has been increasing exponentially and it is expected to double by 2050 as a result of population increase in the world. The fact that fossil-based fuels are limited and they releases CO2 gas, which affects the environment negatively. This situation encourages researchers to two important disciplines of science: 1) removal of CO2 from atmosphere and 2) developing alternative sources of energy that are clean and efficient. Low carbon future will be realized by post-combustion CO2 capture with air separation therefore adsorbents are needed for CO2 capture. Since surface volume, adsorption enthalpy, and functional groups inside the pores affect the interactions between CO2 and surface, new porous materials should be designed specifically that possess all of the aforementioned features. In this thesis study, a new metal dicyanamide compound, Co(hmt)(dca)2, with free nitrogen atoms in the pores were synthesized. A high adsorption of CO2 was observed while the material exhibits almost no N2 uptake at room temperature. In the Co(hmt)(dca)2 crystal, the pore opening defined as almost 3.8 Å and kinetic diameters of N2 and CO2 are 3.6 Å and 3.3 Å, respectively. There is a similarity between kinetic diameter of the crystal and N2 resulting in limited diffusion of N2 and it is the origin of the high selectivity obtained in this study. Although metal dicyanamides have widely been studied in different fields it is the first study in the area of CO2 gas storage. The negative effects of the CO2 level can also be overcome by developing alternative sources of energy that are carbon-free. Hydrogen economy, which involves splitting water using light to produces O2 and H2 has received much attention in the recent years since it is carbon-free and it is based on only water and sun light that are of great abundance. Novel catalysts should be developed to overcome one of the most challenging steps of hydrogen economy, water oxidation, which is one of the half reactions of water splitting. In this thesis study, simple metal dicyanamides have been investigated as water oxidation catalysts. A new family of metal dicyanamides with the formula, M(dca)2(DMF)2, (M = Co, Ni, and Fe), was synthesized and characterized. A current density of 1 mA.cm-2 obtained at an overpotential of 580 mV and by Ni doping the value could be decreased down to 513 mV. This is the first study that involves the application of cobalt-dicyanamide systems in electrochemical water oxidation catalysis. Electrocatalytic studies as well as long term (70 h) electrolysis studies show that these materials can efficiently oxidize water and are robust during long courses catalytic processes.