Chemistry Reference
In-Depth Information
5
MgO-873
MgO-773
MgO-673
strong
medium
weak
300
400
500
600
700
Temperature (K)
Fig. 5 TPD profiles of CO
2
on MgO-x samples. CO
2
adsorption at 298 K, 10 K/min
heating rate.
OH groups. Then, the observed loss of unidentate carbonate formation
centers (low coordination surface O
2
ions) when the calcination
temperature is increased can be ascribed to both the elimination of
surface defects and the enhancement of surface dehydroxylation.
A measure of the number and strength distribution of basic sites
on MgO-x samples was obtained by TPD of CO
2
preadsorbed at room
temperature. The CO
2
desorption rate as a function of desorption
temperature is presented in Fig. 5. The total base site densities of
desorbed CO
2
(n
b
, mmol/m
2
) were measured by integration of TPD curves
in Fig. 5 and are reported in Table 1. It is observed that n
b
decreased
with calcination temperature, from 4.58 mmol/m
2
(MgO-673) to
3.13 mmol/m
2
(MgO-873), thereby confirming a solid surface transfor-
mation that goes beyond the mere coalescence of the pore structure.
Based on the previous IR characterization data, the TPD profiles of
Fig. 5 were deconvoluted in three desorption peaks: a low temperature
peak at 390 K, assigned to bicarbonates formed on surface OH groups, a
middle-temperature peak at 440 K attributed to bidentate carbonates
desorbed from Mg
2
รพ
-O
2
pairs, and a high-temperature peak at 550 K
resulting from unidentate carbonates released from low-coordination
O
2
anions. By integrating these three CO
2
TPD peaks we determined the
density of strongly basic low coordination (O
3c
and O
4c
) anions identified
as n
O
in Table 1, medium strength Mg
5c
-O
5c
pair sites, n
Mg-O
, and weak
OH
groups, n
OH
. Results in Table 1 show that n
O
and n
OH
decreased while
n
Mg-O
increased with the MgO calcination temperature. In Fig. 4 we plotted
the n
O
/n
Mg-O
ratio as a function of calcination temperature; it is observed
