CO2 Mineralization by MgO Nanocubes in Nanometric Water Films

Water films formed by the adhesion and condensation of air moisture on minerals can trigger the formation of secondary minerals of great importance to nature and technology. Magnesium carbonate growth on Mg-bearing minerals is not only of great interest for CO2 capture under enhanced weathering scenarios but is also a prime system for advancing key ideas on mineral formation under nanoconfinement. To help advance ideas on water film-mediated CO2 capture, we tracked the growth of amorphous magnesium carbonate (AMC) on MgO nanocubes exposed to moist CO2 gas. AMC was identified by its characteristic vibrational spectral signature and by its lack of long-range structure by X-ray diffraction. We find that AMC (MgCO3·2.3–2.5H2O) grew in sub-monolayer (ML) to 4 ML thick water films, with formation rates and yields scaling with humidity. AMC growth was however slowed down as AMC nanocoatings blocked water films access to the reactive MgO core. Films could however be partially dissolved by exposure to thicker water films, driving AMC growth for several more hours until nanocoatings blocked the reactions again. These findings shed new light on a potentially important bottleneck for the efficient mineralization of CO2 using MgO-bearing products. Notably, this study shows how variations in the air humidity affect CO2 capture by controlling water film coverages on reactive minerals. This process is also of great interest in the study of mineral growth in nanometrically thick water films.


Thermodynamic calculations of equilibrium pH
1. MgCO3 equilibrium in pure water (close system).

Note: H2CO3* represents CO2(aq) + H2O
Proton condition (electroneutrality): Assume that all Mg 2+ that become dissolved must equal in concentration the sum of the dissolved carbonic species, that is: Replace in the proton condition: Thus we get: Using MATLAB to solve the equation ( 4), we find an equilibrium pH of ~10.888.

MgCO3 equilibrium in CO2-rich water (opened system).
pCO2 = 2.6 kPa, logCO2 = -1.7 Reactions: Assuming that hydrolysis of Mg 2+ is negligible.The pH of the system is controlled by carbonate speciation.Because CO2 is in equilibrium with the system, it is convenient to write dissociation of carbonic acid instead of protonation: It is easier by summing up these equations into this equilibrium: The proton condition in this case is still similar as in close system: Note: [H3O + ] can be simplified as H + .
However, the assumption can be made to simplify the equation by taking in account that pH is controlled not only speciation of soluble carbonates from the solid, but also from CO2(g).The proton condition can be reduced by assuming negligible [H + ] and [OH -] from water dissociation.Because the system is in equilibrium with CO2(g), the reaction (3) and ( 5) govern pH of the system.

Figure
Figure S2.(a-c) N2(g) adsorption/desorption isotherm results showing (a) raw data, (b) t-plots and (c)BJH analyses.These revealed specific surface area on par with particle sizes, and microporosity of 26 mL/g in Pe5 but of only 3.3 mL/g Pe10.We assign this microporosity to interparticle voids, given their comparable distributions of values in both Pe5 and Pe10 nanocube assemblages.

Figure S3 .
Figure S3.Periclase nanocube morphology and size.Electron microscopy images of (a-d) Pe5 and (fi) Pe10, and (e, j) corresponding schematic representations.Scanning Electron Microscopy (a, f) and Transmission Electron Microscopy revealed (b-c) Pe5 nanocubes clustered as nanobars in hexagonal casings, which are relicts of the synthetic brucite from which they were produced.Arrows in (c) highlight preferential arrangement of the ~8-nm wide Pe5 nanocubes into nanobars in a fashion aligning with previous work 21, 22 .(g-h) Pe10 nanocubes were monodispersed.(d, i) High Resolution Transmission Electron Microscopy revealed diffraction fringes expected from the crystallographic structure of periclase.

Figure S4 .
Figure S4.Microgravimetrically-measured water loadings by exposing a flow of water vapor from 0.9 to 92 % RH at 10 % RH intervals, each with a reaction time of 1 h.The right-hand side of is a size-scaled schematic representation of the total equivalent water films thickness in relation periclase nanocube size.One water monolayer (ML) corresponds to 12 H2O/nm 2 .

Figure S7 .
Figure S7.FTIR spectra of OH stretching regions of (a-c) humidity-and (d,e) temperature-dependent carbonation on (a,c-e) Pe5 and (b) Pe10 over 20-h period.Broad (a-b) and narrow (c) OH stretches show that water films grew but no discrete OH band of brucite (3701 cm -1 ) appeared during carbonation.Additionally, surface OH group (3765 cm -1 ) of periclase were more resilient on carbonation at high temperature (d,e), while they were rapidly consumed at room temperature at all humidity ranges (c).Arrows indicate the growth (↑) and disappearance (↓) of OH-bearing species.

Figure S11 .
Figure S11.Example of XPS in the C1s region of Pe5 and Pe10 samples reacted in 2.6 kPa CO2 with 90 % RH for 20 h.

Figure
Figure S14.(a) Post-hydration of growth AMC from Pe5 under 2.6 kPa CO2 and 50%RH.Top: raw C-O stretches of AMC appeared as doublet by the reaction but became a singlet under CO2-free humid atmosphere of 90% RH.Inset: OH stretching region.Bottom: difference spectra of the C-O stretches show the rise of singlet ~1450 cm -1 (b) Water vapor adsorption experiment on synthetic AMC was conducted under CO2-free atmosphere with varied humidity from 0-90% RH.Top: raw C-O stretches changes their shape and intensity upon hydration.Inset: OH streching region.Bottom: difference spectra of the C-O stretches.

Figure S15 .
Figure S15.Chemical speciation diagram of the Mg 2+ -CO3 2--H2O system at 25 o C. Diagrams were generated with the program SPANA/HYDRA/MEDUSA (https://github.com/ignasi-p/eq-diagr-kth)using the equilibrium constants of Table S4.The system is generally similar to the diagram in Fig S13b in the pH > 8 region, where solid carbonate is in equilibrium with water.The equilibrium pH in this case is also determined at pH 10.8.

Figure S16 .
Figure S16.Time-resolved Gaussian component band intensities of the C-O stretching region of carbonate (a,b,d,e) and of the water bending region (c,f) in Pe5 (a-c) and Pe10 (d-f) during reactions with 2.6 kPa CO2 and 90% RH at 25 °C.

Figure S17 .
Figure S17.Time-resolved Gaussian component band intensities of the C-O stretching region of carbonate (a,b) and of the water bending region (c) in Pe5 during reactions 2.6 kPa CO2 with 90% RH at 30-80 °C.

Figure S18 .
Figure S18.Solubility test on the AMC nanocoatings.FTIR spectra of time-resolved growth of the CO stretches n3tot (a, b) and OH stretches (d, e) and their corresponding band intensities (c, f) of periclasePe5 reacted first under a stream of 20% RH and 2.6 kPa CO2 for 19 h (bottom spectra), followed by continue reaction under a stream of 90% RH with 2.6 kPa CO2 or 0 kPa CO2 over the next 16 h (top spectra).Intensity profiles of (c) CO stretches show continuous production of AMC from pre-grown AMC under CO2-rich 4 ML water films, while exposing this MgO@AMC to CO2-free 4 ML water films only promote hydration effect on carbonate species of AMC.Intensity profiles of (f) OH stretching bands show no appearance of distinct OH group belong to brucite (3701 cm -1 ) under both reaction conditions, while OH bands of water films (3400) grew thicker under CO2-rich 4 ML water films.

TABLE S4 Thermodynamics constants for Mg 2+ -CO3 2--H2O system (closed system)
All reaction constants (logK) were collectively derived from these references: NIST database 17 from SPANA's default database, work of Plummer and co-workers,