Atomistic Simulation of Surface Selectivity on Carbonate Formation at Calcium and Magnesium Oxide Surfaces

Abstract

We report atom-level simulations of the surface selectivity and the resulting surface phase diagrams for the {100}, {110}, {111}, and {310} surfaces of CaO and MgO as a function of varying CO 2 and H 2 O partial pressures. This work extends the traditional approach based on ab initio calculations, which can be time-consuming and costly for large systems, by using semiempirical atomistic simulations. The advantage of this approach is that very large numbers of calculations can be performed, thereby allowing a more effective search of the configurational space. The resulting free energies are used to generate the surface phase diagrams. The results indicate that the {100} surfaces of MgO and CaO show different dominant phases at atmospheric concentrations of gaseous water and carbon dioxide. The CaO surface forms a carbonated phase, whereas the MgO surface contains associatively adsorbed water. On the other hand, both {111} surfaces show the dominance of surface hydroxylation, effectively forming a layer of mineral hydroxide, with carbonation only observed at very high carbon dioxide concentrations. Finally, the {310} surfaces show enhanced reactivity with carbonate, most likely a result of the steps on the surfaces. In general, we predict that the minimum CO 2 partial pressure needed to carbonate these surfaces can be controlled by the water partial pressure. The effect of temperature is also considered, and the results show how the number of surface adsorbates decreases as temperature increases.

Calcium-looping for post-combustion CO2 capture. On the adverse effect of sorbent regeneration under CO2

“…This is also in agreement with previously reported UPS and XPS measurements [58] on H 2 O adsorption on faceted (100) and (111) crystals indicating a faster and more intense adsorption of H 2 O as compared to CO 2[35]. Interestingly, atomistic simulations show that, at high temperature, only the (111) surface remains active with an intensified affinity for adsorption of H 2 O as compared to CO[57]. In accordance with ab initio modeling predictions[54], adsorption of either H 2 O or CO 2 is not favorable in the rest of surfaces at high temperature[57].…”

“…Interestingly, atomistic simulations show that, at high temperature, only the (111) surface remains active with an intensified affinity for adsorption of H 2 O as compared to CO[57]. In accordance with ab initio modeling predictions[54], adsorption of either H 2 O or CO 2 is not favorable in the rest of surfaces at high temperature[57]. These studies suggest that, in the absence of H 2 O, CO 2 adsorption will take place in the (111) surfaces whereas increasing H 2 O partial pressures will hinder CO 2 adsorption.…”

The Calcium-Looping technology for CO2 capture: On the important roles of energy integration and sorbent behavior

“…33,89,90,103 It has been proposed that a progressive growth of the regenerated crystal structure along preferential surfaces, which are more stable but less favorable for CaCO 3 nucleation, could play a role on the loss of multicyclic CaO conversion. [104][105][106] On the other hand, empirical results seem to indicate that decarbonation in CO 2 is a complex process involving a two-stage reaction mechanism, which consists of the endothermic chemical decomposition of Thermograms corresponding to the different experiments above reviewed are shown in Figure 8. The higher conversion of the CaO precalcined in air in the first cycle, as observed in Figure 7, is due to its high reactivity in the fast carbonation phase, and its drastic drop in conversion can be associated to the high susceptibility of the soft CaO skeleton resulting from precalcination in air to sintering.…”

Surface chemistry of carbon dioxide revisited

“…Other work [5], comparing binding at Fe 2 O 3 , γ-Al 2 O 3 and TiO 2 nanoparticle surfaces, also showed that thin water films effectively decreased CO 2 adsorption rates, as water competed for the same CO 2 binding sites. Allen et al [200] also report an ab initio-derived phase diagram for the (100), In another study [193], FTIR measurements of iron oxyhydroxide nanoparticles showed that the actual dispositions of surface OH groups, and their resulting hydrogen bond strengths and populations, impact the speciation of the resulting carbonate/bicarbonate species. This was demonstrated for the case of rows of singly-coordinated (-OH) groups on the (110) face of goethite in contrast to the (100) face of lepidocrocite ( Figure 52).…”