Basic catalysis on MgO: generation, characterization and catalytic properties of active sites - Catalysis - page 9

O
C
OO
M

MgO-873

O O
C
O
M

O
C

O
M

OH

a

b

c

d

MgO-773

a
b
c
d

MgO-673

a

b
c
d

1800 1600 1400 1200

n ( cm -1 )

Fig. 2 Infrared spectra of CO 2 adsorbed on MgO-x catalysts upon evacuation at in-
creasing temperatures: (a) 298 K, (b) 373 K, (c) 473 K, (d) 573 K.

broad bands for both, unidentate and bidentate carbonate species, sug-
gest the presence of surface sites with different coordination numbers,
i.e., in different chemical environment, that bind CO 2 with a distribution
of basic strength. In contrast, narrower and smoother bands were ob-
tained on more crystalline MgO particles resulting from calcination at
higher temperatures [27, 55, 60].
From the spectra of Fig. 2 we determined the unidentate carbonate/
bidentate carbonate band intensity ratios (U.C./B.C.); the obtained values
for MgO-673, MgO-773 and MgO-873 samples are plotted in Fig. 4. It is
observed that the (U.C./B.C.) intensity ratio on MgO calcined at 673K was
2.5 and then decreased with the calcination temperature. This result may
be interpreted by considering that the decomposition of Mg(OH) 2
at relatively low temperature, i.e. 673 K, generates hydroxylated MgO