287688 Density Functional Theory Calculations of the Interaction of Water with Forsterite (100) Surface

Thursday, November 1, 2012: 1:50 PM
415 (Convention Center )
Valentina Prigiobbe1, Dong-Hee Lim2, Ana Suarez-Negreira2 and Jennifer Wilcox3, (1)Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, TX, (2)Energy Resources Engineering, Stanford University, Stanford, CA, (3)Department of Energy Resources Engineering, Stanford University, Stanford, CA

Density functional theory calculations of the interaction of water with forsterite (100) surface

Valentina Prigiobbe1, Dong-Hee Lim2, Ana Suarez-Negreira2, Jennifer Wilcox2

1Department of Petroleum and Geosystems Engineering, University of Texas at Austin, USA

2Department of Energy Resources Engineering, Stanford University, USA

The interaction of water (H2O) with forsterite (Mg2SiO4), the magnesium-end member of the silicate mineral olivine ((Mg,Fe)2SiO4), is a fundamental mechanism in the weathering process investigated in the geosciences and engineering such as hydration of the upper Earth's mantle, transport of contaminants in the subsurface, and mineral carbonation for carbon dioxide (CO2) storage.

Weathering of Mg2SiO4 in aqueous systems consists of a solid-liquid interfacial reaction where H2O and CO2 react with the mineral. In particular, Mg2SiO4 undergoes hydration (adsorption of H2O molecules) and dissolution (release of Mg2+ cations), which are described by the overall chemical reaction:

Mg2SiO4(s) + 4H+ 2Mg2+(aq) + H4SiO4(aq)

As suitable supersaturated conditions are achieved in solution the carbonation reaction (precipitation of magnesite (MgCO3)) takes place:

Mg2+(aq) + CO2-3(aq) MgCO3(s)

The kinetics of the weathering reaction are controlled by the hydration and the dissolution of Mg2SiO4 and may be catalyzed by organic molecules such as oxalate and citrate (Olsen and Rimstidt, 2008; Krevor and Lackner, 2011; Prigiobbe and Mazzotti, 2011). The interaction of the organic molecule with the Mg2SiO4 surface depends on the atomic and electronic structure of its interface, the bulk structure and composition and the impact of thermodynamic conditions such as temperature and pressure. In case the selection or the design of the best catalyzing organic compound to enhance the dissolution process is required a deep understanding of the interaction is needed.

In this work, we present the results of density functional theory (DFT)-based electronic structure calculations to investigate the interaction of water (H2O) with forsterite (Mg2SiO4) at the most stable stoichiometric (100) surface (Watson et al., 1997).

Periodic ab initio DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) (Kresse and Hafner, 1993) applying the Perdew-Burke-Enzerhoff (PBE) generalized gradient approximation (GGA) with pseudopotential based upon projector-augmented waves (PAW)-type. The morphology and the energetics of the bulk as well as of the gaseous H2O molecule are in reasonable agreement with the experimental data using a cutoff energy for the planewave basis set to 460 eV and a grid of 4x2x4 for the bulk (Figure 1.a). A slab with four unit cells of bulk was created with a vacuum space of 20 to minimize the interaction between periodic images and the stoichiometric (100) surface reacted with H2O. The surface comprises 14 reactive sites of which eight are oxygen atoms, namely O1, O2, O3, four are magnesium atoms, namely, Mg1 and Mg2, and two are silicon atoms, Si (Figure 1.b).

DFT calculations were performed together with bond valence and density of state (DOS) analysis at 0 K to investigate the morphology and the electronic states of the (100) surface upon adsorption of dissociated and molecular H2O. Ab initio thermodynamics was used to extend the DFT calculations to 422 K and 100 bar, to provide predictions of the changes in surface reactivity as a function of thermodynamic variables.

Figure 1. Structure of the stoichiometric (100) Mg2SiO4 surface. (a) Lateral view of the four layer slab and (b) top view of the surface reactive sites. In red are the O atoms, in yellow the Mg atoms, and in blue the Si atoms.

We have investigated H2O adsorption on the (100) Mg2SiO4 surface by placing one molecule vertically to the surface and tilted around its axis with its reactive site (i.e., O or H) 1.9 above the reactive surface sites. The effect of surface coverage has been investigated up to one monolayer of H2O molecules. Preliminary results of the adsorption energy (Eads) of H2O defined as

Eads = EH - ES - EH2O(gas),

where EH is the minimum free energy of the surface with adsorbed molecular or dissociated water and EH2O corresponds to the minimum free energy gaseous water molecule, indicate that the hydration of the (100) Mg2SiO4 is favored. The surface becomes more stable upon the adsorption of molecular H2O under different coverage except in one case when H2O reacts with the Mg site located at Mg2a (Figure 1.b). With the exception of this case, Eads varies between -3.596 eV/H2O mole and -0.851 eV/H2O with an average value of -1.345 eV/H2O, which is in the range of previously calculated Eads by de Leeuw et al. (2000), Stimpfl et al. (2006), and King et al. (2010). The minimum Eads corresponds to the adsorption onto the oxygen atom type O3 located at the O3c indicating that this is the most reactive site as a matter of fact under this condition the normalized dislocation at the O3 atom site is 1.43 times the initial position. The high reactivity of the O3 site was already suggested by molecular dynamics simulations performed by Liu et al. (2009) for the study of the dissolution of forsterite with water and oxalate ions suggesting that the adsorption of water onto this atom was critical for the release into solution of Mg2 atom.

The effect of temperature, pressure, and entropic effect are taken into account through ab initio thermodynamic calculations and the type of adsorption mechanism (i.e, chemisorption and physisorption) at the reactive site and the subsequent change of its electronic structure are determined by analysis of the bond valence and DOS.


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