Chemistry Reference
In-Depth Information
0.150
MgO-673
0.125
MgO-773
MgO-873
0.100
0.075
0.050
0.5
1.0
1.5
2.0
2.5
3.0
n
O
, density of strong base sites (
μ
mol/m
2
)
Fig. 15 Initial monoglyceride conversion rate as a function of strong base site density.
Reaction conditions as in Fig. 14.
Fig. 16 Clusters used for modeling the MgO (1 0 0) surface. (a) Perfect terrace site,
Mg
25
O
25
(Mg-ECP)
25
; (b) defective edge site, Mg
22
O
22
(Mg-ECP)
19
; (c) defective O-apical
corner site, Mg
22
O
22
(Mg-ECP)
12
; (d) defective Mg-apical corner site, Mg
23
O
23
(Mg-ECP)
14
.
represent the strongly basic O
4c
(L
=
4) and O
3c
(L
=
3) sites, respectively,
and a Mg-apical corner (Mg
3c;
L
=
3) that models a Lewis acid site.
3.3.2 Computational details. DFT molecular orbital calculations
were carried out using the gradient corrected Becke's three parameters
hybrid exchange functional in combination with the correlation
functional of Lee, Yang and Parr (B3LYP) [84]. The terrace site at the MgO
(100) surface was represented by the Mg
25
O
25
(Mg-ECP)
25
cluster consisting
of two layers (first layer: Mg
9
O
16
;secondlayer:Mg
16
O
9
). For the topological
defects at edges and corners similar modeling was used; a Mg
22
O
22
(Mg-
ECP)
19
cluster was used for modeling the edge topological defect of
MgO due to the intersection of two [100] and [010] oriented planes; a
Mg
22
O
22
(Mg-ECP)
12
cluster was used for modeling the oxygen corner
topological defect of MgO due to the intersection of three [100], [010]
and [001] oriented planes whereas the a similarly generated Mg
23
O
23
(Mg-
ECP)
14
cluster was used for modeling a magnesium apical corner.
To take into account the Madelung field due to the rest of the extended
surface, the cluster was embedded in an array of
2 point charges. This
embedding technique was used previously for the study of both bulk and
