Basic catalysis on MgO: generation, characterization and catalytic properties of active sites - Catalysis - page 10

Fig. 3 Scheme of a stepped MgO (1 0 0) surface with O Lc and Mg Lc ions in different
positions (L: coordination number). Terrace sites: O 5c ,Mg 5c ; edge sites: O 4c ,Mg 4c ; corner
sites: O 3c ,Mg 3c .

2.8

2.6

2.4

2.4

2.0

2.2

2.0 1.6

1.8

1.2

1.6

0.8

1.4

0.4

650 700 750 800 850 900

Temperature (K)

Fig. 4 Strong/medium-strength base site and U.C./B.C. ratios as a function of calcination
temperature. (B.C.: bidentate carbonate; U.C.: unidentate carbonate).

containing a high concentration of low-coordination O 2 sites located
on defects of the crystalline solid surface. Then, the increase of the
calcination temperature would remove OH groups and also surface solid
defects creating a smoother and thermodynamically more stable struc-
ture, as suggested by the XRD data of Table 1. This interpretation is
consistent with the results reported by Morterra et al. [54] on MgAl 2 O 4
and by Evans and Whateley [61] on MgO. These authors investigated by
IR of CO 2 the role of surface hydroxylation on the generation of strong
basic sites and concluded that the strong basicity, responsible for
unidentate carbonate formation, is promoted by the presence of surface