The nanostructure and spectroscopic properties of Upsalite, a mesoporous magnesium carbonate
Abstract
SNAC medium In the previous SNIC projects, SNIC 2015/1-204 and SNIC 2016/1-334, we applied stochastic quenching (SQ) along with molecular dynamics simulations to model the structure of Upsalite. The SQ method has previously shown to accurately describe the potential energy landscape of metallic liquids as well as oxides and carbon like materials. Stochastical quenches of different sizes (120-480 atoms) with MgCO3 were prepared and the ground state energy of each structure was determined by Density Functional theory. The found total energies per atom were in proximity with those found by White et al., who prepared a model of hydrated amorphous magnesium carbonate (MgCO3∙3D2O) by combining a quantum chemistry method and experimental total scattering data and somewhat higher than the total energy of magnesite (MgCO3). Varying the volume of the model structure does however result in collapse of the structures. This implies that there is no local energy minimum on the potential energy surface of Upsalite, i.e. that this structure is thermodynamically unstable at this composition.
Whether Upsalite bulk contains hydrogen is under some dispute. Forsgren et al. studied the Fourier Transform-Infrared spectra of Upsalite and found no vibrations due to H2O bound in the structure, but OH vibrations can clearly be seen in the measured spectra. Furthermore, Cheung et al. measured a relatively high content of hydrogen, 0.28 wt% (12.3 at%) in Upsalite, attributed to surface Mg(HCO3)2, Mg(OH)(HCO3) or adsorbed water . These results raise several questions. Is hydrogen incorporated into the structure of Upsalite as OH-groups or only at the Upsalite pore surfaces. The great surface area of Upsalite enables strong surface interactions with water and a procentual water uptake of about 20-30 wt%. The moist uptake is strongly dependent on the storing conditions (high/low humidity) of the material. In the previous SNIC projects, the hydrogen content in the model was varied within the range of different experimental observations of the H2O/OH content in the material and up to the water content of Nesquehonite. It is seen that Upsalite is destabilized by increasing water content, increasing the heat of formation of Upsalite and corroborating with previous experimental studies.
In addition, recent experimental data show that Upsalite consists not only of amorphous MgCO3 but also of nanometer sized MgO crystallites. To assess these results, further simulations of nano-sized crystals of MgO and amorphous MgCO3 interfaces were performed within SNIC 2016/1-334.
In this small SNIC project we aim to finalize calculations on these MgO/MgCO3 interfaces and compile these results into a first publication on the theoretical structure of Upsalite. In addition we will calculate orbital-projected partial density of states and simulate X-ray absorption spectra and compare this data with X-ray Absorption Spectroscopy measurements performed on MAX IV Lab this autumn.
