Chemistry Reference
In-Depth Information
TABLE 9.3
Simulated EPR Parameters of High-Spin Fe
3
+
Sites in Fe-MCM-41
Iron Sites
g
-Values
E/D
D
/cm
−
1
I. Framework Iron
(a) Fe
3+
in tetrahedral
coordination
g
= 4.3
0.33
0.3
0.3
0.013
(b) Single Fe
3+
site
g
x
=
g
y
= 2.003
g
z
= 1.99
II. Extra-Framework Iron
(a) Iron clusters
0.033
0.6
g
= 2.45
0.02
0.42
(b) Fe
3+
on the outer
surface of the pore
g
= 2.07
Source:
Konovalova, T.A.,
J. Phys. Chem. B
, 107, 1006, 2003.
g
= 4.3
(a)
g
=2
(b)
(c)
1500
3000
4500
Magnetic field (gauss)
FIGURE 9.13
X-band EPR spectra of Fe(III)-MCM-41: (a) activated at 360°C and measured at 77 K,
(b) after adsorption of 7′-apo-7′,7′-dicyano-β-carotene (77 K), and (c) after irradiation at 365 nm for 2 min.
(From Konovalova, T.A.,
J. Phys. Chem. B
, 107, 1006, 2003. With permission.)
9.13) showed that the adsorption of the carotenoid results in a decrease of the broad
g
=
2.0 signal,
4.3 does not change signii cantly.
The X-band measurements cannot identify which one of the iron sites can react with the
carotenoid. Only the 95 GHz measurements (Figure 9.14) were able to demonstrate that adsorp-
tion of carotenoid results in a signii cant decrease of the
g
while the intensity of the Fe
3+
signal at
g
=
=
2.07 signal and moderate decrease of
the
g
1.999 is almost
unaffected. The results show that the extra-framework Fe
3+
ions located on the surface of the pore
are primarily responsible for carotenoid oxidation. Probably, these sites are more accessible for
bulky organic molecules than the framework iron within silica walls.
=
2.45 signal, while the intensity of the narrow line with
g
x
=
g
y
=
2.003,
g
z
=
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