Platelet structures made up of layered silicates (clays) are traditional and abundant materials with numerous applications, e.g. in pottery, as waste barriers or as inexpensive filler materials. The so-called smectite clays consist of ca. 1 nanometer thick and charged sheets (negative surface charge and a smaller, positive, edge charge), which in the dehydrated state stack like decks of cards by sharing charge-compensating ions. Smectite clays have shown the ability to absorb relatively large amounts of certain molecules, and recently there has been a particular interest related to interactions between clay minerals and CO2, partly because geological structures are being investigated as storage sites for anthropogenic CO2. We have previously shown how a synthetic clay (fluorohectorite) is able to absorb quite large amounts of CO2 under specific temperature and pressure conditions. However, the molecular mechanisms of the process, the extent of CO2 capture as a function of clay charge and structure, as well as the temperature and pressure dependence, has been largely unknown. In this interdisciplinary project, we have addressed several of these questions via extended international collaborations. We have for example demonstrated that fluorohectorite-clay shows a particular ability for CO2-uptake when nickel is used as the stabilizing ion, and that the uptake can be adjusted by changing the surface charge. We have furthermore described how pressure, temperature and the presence of water affect these processes. The maximum CO2-uptake is estimated to more than 0.7 tons per cubic meter of material, and the absorption takes place at much lower CO2-pressure (ca. 10 atm.) when the surface charge is changed from 0.7 to 0.5 pfu (charge per formula unit). There is in addition a hysteresis-effect which secures that the absorbed CO2 is not released until the pressure is lowered and/or the temperature is increased significantly from the conditions during absorption. This type of knowledge is essential for potential use of these materials as CO2-sorbents.
In addition to providing a fundamental understanding of the mechanisms of small-molecule and in particular CO2 capture in layered materials, this project has given us various tools for tuning the degree of capture as well as the temperature of sorption and release. The project has lifted the collaboration between the Norwegian participants (IFE/NTNU) and institutions abroad, and has been essential for giving a PhD-student a solid international experience. An important effect of the project is that synthetic clays must be considered as serious candidates when evaluating the use of CO2 or small-molecule sorbent materials in the near future.
This project aims to contribute to the long-term development of innovative solutions for CO2 capture and retention by synthetic or natural clays, such as for capturing post combustion CO2 from industrial processes and power production. The present applicant group has already demonstrated the potential for use of such environmentally friendly nano-porous clays in this context, and recently we published a study that has attracted considerable interest, where the absorption for a specific type of clay is quantified. However, the molecular mechanisms of the process, the extent of carbon capture as a function of clay charge and structure, as well as the temperature and pressure dependence, still pose open questions. These points will be addressed by performing simulations to assess the molecular interactions associated with incorporation of CO2 in the clay. Further information will be gained by performing scattering (neutron and x-ray) investigations of clay systems as a function of pressure and temperature, complemented by macroscopic investigations. The project is an interdisciplinary and international collaboration involving materials scientists, physicists and chemists, working at universities and research institutes. Training of young researchers is at the heart of the project.