In situ imaging of amorphous intermediates during brucite carbonation in supercritical CO2

Progress in understanding crystallization pathways depends on the ability to unravel relationships between intermediates and final crystalline products at the nanoscale, which is a particular challenge at elevated pressure and temperature. Here we exploit a high-pressure atomic force microscope to directly visualize brucite carbonation in water-bearing supercritical carbon dioxide (scCO(2)) at 90 bar and 50 degrees C. On introduction of water-saturated scCO(2), in situ visualization revealed initial dissolution followed by nanoparticle nucleation consistent with amorphous magnesium carbonate (AMC) on the surface. This is followed by growth of nesquehonite (MgCO3 center dot 3H(2)O) crystallites. In situ imaging provided direct evidence that the AMC intermediate acts as a seed for crystallization of nesquehonite. In situ infrared and thermogravimetric-mass spectrometry indicate that the stoichiometry of AMC is MgCO3 center dot xH(2)O (x = 0.5-1.0), while its structure is indicated to be hydromagnesite-like according to density functional theory and X-ray pair distribution function analysis. Our findings thus provide insight for understanding the stability, lifetime and role of amorphous intermediates in natural and synthetic systems. Non-classical crystallization may proceed through formation of intermediate phases, but it is not known whether these are linked to the final crystallization. Here, using an atomic force microscope at 90 bar, brucite carbonation is directly observed, with an amorphous intermediate acting as the seed for crystalline nesquehonite.

Automated summary

Understanding crystallization pathways at nanoscale under high pressure and temperature is challenging. This study used high-pressure atomic force microscopy to directly observe brucite carbonation in water-bearing supercritical carbon dioxide, revealing that the amorphous magnesium carbonate acts as a seed for the crystallization of nesquehonite. Additionally, in situ infrared and thermogravimetric-mass spectrometry analyses indicated the stoichiometry of the amorphous intermediate.