Chemistry Reference
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g
2
tetr
= 4.3
g
1
tetr
= 9.0
g
oct
= 2.0
(a)
(b)
0
2000
4000
6000
8000
Magnetic field (gauss)
FIGURE 9.12
X-band EPR spectra of Fe(III)-MCM-41 activated at (a) 260°C and (b) 360°C, and measured
at 77 K. (From Konovalova, T.A.,
J. Phys. Chem. B
, 107, 1006, 2003. With permission.)
et al. 1998). The X-band spectrum of Fe-MCM-41 also exhibits a broad (~2000 G) signal with
g
≈ 2.
A
g
2.0 signal in zeolites is commonly assigned to extra-framework Fe
3+
ions (Goldfarb et al.
1994, Kosslick et al. 1998). Figure 9.12b shows that activation of Fe-MCM-41 at higher tempera-
ture diminishes the
g
=
=
4.3 framework iron signal, and signii cantly increases the extra-framework
iron signal at
g
2.0. This is consistent with the observation that tetrahedral coordination of the
framework Fe
3+
ions is not very stable (Kosslick et al. 1998).
To obtain additional information regarding the different types of Fe
3+
sites in Fe-MCM-41 EPR
measurements at higher microwave frequencies were carried out. It was found that the
g
=
4.3 signal
is not observed at 94.3 GHz and higher frequencies. This might be due to excessive broadening by
frequency-dependent relaxation mechanisms. It is also possible that with frequency increase the
electron Zeeman interaction becomes comparable to
D
resulting in inhomogeneous line broaden-
ing. In contrast, the shape of the
g
=
2.0 signal is better determined at higher frequencies.
At 94-287 GHz (Konovalova et al. 2003) the
g
=
2.0 line is resolved into two broad peaks and
an intense narrow signal. To determine
g
-values and the ZFS parameters
D
and
E
for different Fe
3+
signals, spectral simulations were performed using powder matrix diagonalization approach which
is important for high-spin iron systems (Yang and Gaffney 1987, Gaffney et al. 1993). Simulations
were carried out using a Gaussian lineshape and varied isotropic linewidth,
E/D
ratio and
g
-values.
The parameters obtained at higher frequencies were used for spectral simulations at lower frequen-
cies. Simulated parameters are given in Table 9.3. It was demonstrated (Konovalova et al. 2003) that
high-frequency/high-i eld EPR is a promising technique to increase spectral resolution for proper
assignment of different Fe
3+
sites, which cannot be resolved by the X-band experiments. The broad
unresolved EPR line at 9 GHz in the
g
=
2 region is due to overlapping signals from Fe
3+
sites with
different zero-i eld parameters. The peak with
g
=
=
2.45 is assigned to aggregated Fe
3+
. The signal
2.07 can be attributed to Fe
3+
coordinated to oxygen atoms on the surface of the pore.
A narrow line with
g
x
=
with
g
=
0.3 was attributed to a single Fe
3+
site.
Figure 9.13 compares X-band EPR spectra of Fe-MCM-41 before (a) and after (b) and (c) carote-
noid adsorption. The sample with incorporated Car exhibits a signal with
g
g
y
=
2.003,
g
z
=
1.999, and
E/D
=
0.0002, char-
acteristic of carotenoid radical cation prior to irradiation (Figure 9.13b). Irradiation of the samples
at 365 nm (77 K) increases the Car
•+
signal intensity (Figure 9.13c). The X-band experiments (Figure
=
2.0028
±
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