Capturing Greenhouse Gases with Sorosilicates

Abstract:

Silicates which are abundant on earth are made up of SiO4 ions of charge -4, bonded to positive ions. Sorosilicates are a sub-category of silicates consisting of pairs of SiO4 units joined by a shared oxygen to create Si2O7 ions of charge -6. A good example is thortveitite which includes scandium as the positive ion (Sc2Si2O7).

When highly charged negative ions like SiO4 or Si2O7 are hydrated they react with surrounding water molecules to generate hydroxyl (OH-) ions which mineralize carbon dioxide to carbonates [1].

(SiO4----) + H2O + CO2 => (HSiO4---) + (HCO3-)

Unlike sulfates or phosphates, silicates do not readily dissolve in water to create hydrated ions. An alternative option is to grind the bulk silicate material down to a fine powder to enhance CO2 mineralization [2]. In this paper interaction of CO2 and other GHGs with nanoparticles (NPs) of sorosilicates is investigated to clarify the process at the microscopic level.

The research is carried out with first principle calculations using DFT method with B3LYP functional and Pople type basis sets. Atomic models consist of NPs of Sc2Si2O7 with molecules of CO2, methane (CH4) and nitrous oxide (N2O) on the surface. Two major constituents of air, nitrogen (N2) and oxygen (O2) are also included for comparison.

CO2 is attracted to the Sc sites on the NP surface with an interaction energy (IE) of 0.6 eV. N2O is attracted to the same sites with an IE of 0.5 eV. O2 and N2 located near the Sc sites have IEs of 0.3 eV and 0.4 eV, respectively. CH4 which is oriented with its hydrogens facing oxygen sites on the surface has an IE of 0.1 eV.

Results suggest that sorosilicate particles might be useful to capture CO2 and N2O in air, but not CH4.

[1] H. Gokturk, "Carbon mineralization using phosphate and silicate ions," AGU Fall Meeting, 2013
[2] The Olivine Foundation, www.smartstones.nl