(353e) Reaction Equilibrium of Carbonation of Transition Metal Oxides and Hydroxides: a Practical Method to Screen Possible Reactions for CO2 Capture

Current technologies for capturing CO2 include solvent-based systems such as Selexol, Rectisol, and alkanolamine-based materials such as ethanolamine. There is a general consensus that these materials are not cost effective and are not suitable for high temperature processes, such as post-combustion flue gas, having temperatures > 450 K. Therefore, new materials are needed that can capture and release CO2 reversibly with acceptable operating costs at relatively high temperature. Accordingly, solid sorbent materials have been proposed for capture of CO2 through a reversible chemical transformation. [1] There are very many candidate materials for solid sorbents, most of which result in the formation of a carbonate product. We have used first principles density functional theory (DFT) to calculate the structural, electronic, and thermodynamic properties of transition metal (Mn, Fe, Co, Ni, Cu, Zn, and Cd) oxides, hydroxides, and carbonate solids. We present a method for constructing van't Hoff plots of CO2 absorption/desorption based on reaction free energies with and without finite temperature effect. We compare our calculations, both with and without phonon contributions, to experimental data where possible. We have found that addition of finite-temperature corrections does not greatly improve the reaction thermodynamics of carbonate formation from oxides and hydroxides compared with uncorrected (0 K) DFT energies. In contrast, the calculated and experimental slopes of the van't Hoff plots are in better accord when finite temperature corrections are included. Phonon calculations are computationally expensive, thus we propose that the reaction thermodynamics can be adequately described in the temperature range of interests using DFT energies of the solids and the statistical mechanical properties of the gases by adding some uncertainty bounds to the reaction enthalpies. This approach can be used reliably to facilitate the screening of large numbers of possible reactions at acceptable computational cost. When promising materials are found, detailed phonon free energy calculations can be performed to obtain more accurate reaction equilibrium data.

[1] Ranjani V. Siriwardane and Robert W. Stevens, Jr. Novel Regenerable Magnesium Hydroxide Sorbents for CO2 Capture at Warm Gas Temperatures. Ind. Eng. Chem. Res., 2009, 48(4), 2135?2141.