Chemistry Reference
In-Depth Information
surface properties giving results which are in good agreement with those
obtained by periodic calculations [85, 86]. Moreover, the positive point
charges at the interface were replaced by effective core potentials (ECP)
corresponding to Mg
2
þ
to account for the finite size of the cations and to
avoid spurious charge polarization.
The O atoms of the MgO surface that interact directly with the glycerol
or FAME molecule (all from the first layer) were described with the basis
set 6-31
þ
G(d) and Mg atoms with 6-31G(d). For the rest of the oxide
atoms in the cluster the basis set 6-31G was used. The 6-31G (d,p) basis
set was used for the molecular orbitals of Gly and FAME. The adsorption
energy of Gly or FAME (E
ads
) was evaluated according to the following
total energy difference: E
ads
=
E
(molecule-MgO cluster)
E
(MgO cluster)
E
(molecule)
;
where ''molecule'' is either Gly or FAME. Negative values indicate exo-
thermic adsorption.
On the other hand, the atomic net charges (q) were calculated
following the natural bond orbital (NBO) scheme [87], which gives real-
istic values for the charge partitioning. For all the systems the total
charge was zero. Also, Dq
(atom)
was defined for an atom of Gly or FAME as
the atom charge difference between adsorbed and free molecule states.
All the calculations were performed using the Gaussian-03 program
package.
3.3.3 Glycerol adsorption on MgO. First-principles density-functional
calculations were performed for the free glycerol molecule and for the
adsorption of glycerol on representative terrace, edge, and Mg- and O-
corner sites of MgO. The DFT calculations for the optimized geometrical
structure of
the
free Gly molecule
resulted in the
following
intramolecular
interatomic distances (d): C-C (d
(C-C)
E
1.53 Å), C-H
(d
(C-H)
E
0.97 Å). For the Gly
adsorption on MgO, different initial geometries of the glycerol molecule
were evaluated depending on the orientation of the hydroxyl groups
toward the MgO surface. Results presented in Table 3 show the optimized
geometrical structures obtained for Gly adsorption through one, two or
1.10 Å), C-O (d
(C-O)
E
1.42 Å) and O-H (d
(O-H)
E
Table 3 Adsorption energies (E
ads
) and bond distances (d) for Gly adsorption on terrace,
edge and O-apical corner sites of MgO (100).
a
Final structure
nOH(m) n m E
ads
(eV) d
ð
H
O
s
Þ
(Å) d
ð
O
Mg
s
Þ
(Å) d
(O-H)
(Å)
1 Terrace (L
=
5) 1OH(1) 1 1
0.65 1.594 3.228 1.008
2 Terrace (L
=
5) 2OH(1,2) 2 1,2
0.92 1.588 2.252 1.019
3 Terrace (L
=
5) 3OH 3 1,2,3
1.48 1.785 2.293 0.996
4 Edge (L
=
4) 1OH(2) 1 2
1.63 1.050 1.988 1.478
5 Edge (L
=
4) 2OH(1,2) 2 1,2
1.85 1.035 2.082 1.494
6 Edge (L
=
4) 3OH 3 1,2,3
1.62 1.588 2.104 1.013
7 O-corner (L
=
3) 1OH(1) 1 1
0.89 1.519 2.802 1.045
8 O-corner (L
=
3) 2OH(1,2) 2 1,2
1.55 1.524 2.127 1.037
9 O-corner (L
=
3) 3OH 3 1,2,3
0.85 0.985 2.187 1.676
a
Cluster sites as in Fig. 16; n and m: number and position of OH groups interacting with the
surface, respectively.
Entry Cluster
