Laser-induced luminescence of model Fe/Al2O3 and Cr/Al2O3 catalysts

Article abstract of DOI:10.1134/S0023158408020183

The laser-induced luminescence of Cr³⁺ impurity ions in model Fe/Al₂O₃ and Cr/Al₂O₃ catalysts with different calcination temperatures was studied. It was found that an additional luminescence band at 770 nm appeared in the luminescence spectra of low-temperature samples as a result of the interaction of octahedrally coordinated Cr³⁺ ions with Fe³⁺ impurity ions. In the θ-Al₂O₃ phase with a concentration of Cr ³⁺ ions higher than 0.1 wt %, the interaction of the Cr ³⁺-Cr³⁺ ion pairs in the immediate surroundings resulted in the appearance of N_θ lines due to the splitting of R _θ lines. The differences of these lines from the N _α lines of α-Al₂O₃ were related to the individuality of the crystal lattice of the θ phase and the coordination of Cr³⁺ impurity ions in the immediate surroundings, which is different from that in the α phase. Based on the laser-induced luminescence spectroscopic data, it was found that regions with a local Cr ³⁺ concentration higher than the average Cr³⁺ concentration in the bulk of a catalyst by one order of magnitude were formed in the α-Al₂O₃-Fe₂O₃ system with the bulk Fe and Cr concentrations of 2.5 and 0.04 wt %, respectively, which was calcined at 1220°C, as a result of the diffusion of chromium and iron ions.

Full text of DOI:10.1134/S0023158408020183

ISSN 0023-1584, Kinetics and Catalysis, 2008, Vol. 49, No. 2, pp. 291–298. © MAIK “Nauka /Interperiodica” (Russia), 2008.
Original Russian Text © V.N. Snytnikov, V.O. Stoyanovskii, T.V. Larina, O.P. Krivoruchko, V.A. Ushakov, V.N. Parmon, 2008, published in Kinetika i Kataliz, 2008, Vol. 49, No. 2,
pp. 307–314.
Laser-Induced Luminescence
of Model Fe/Al2O3 and Cr/Al2O3 Catalysts
V. N. Snytnikov, V. O. Stoyanovskii, T. V. Larina, O. P. Krivoruchko,
V. A. Ushakov, and V. N. Parmon
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
e-mail: snyt@catalysis.nsk.su
Received November 27, 2006
Abstract—The laser-induced luminescence of Cr3+ impurity ions in model Fe/Al2O3 and Cr/Al2O3 catalysts
with different calcination temperatures was studied. It was found that an additional luminescence band at
770 nm appeared in the luminescence spectra of low-temperature samples as a result of the interaction of octa-
hedrally coordinated Cr3+ ions with Fe3+ impurity ions. In the θ-Al2O3 phase with a concentration of Cr3+ ions
higher than 0.1 wt %, the interaction of the Cr3+–Cr3+ ion pairs in the immediate surroundings resulted in the
appearance of Nθ lines due to the splitting of Rθ lines. The differences of these lines from the Nα lines of
α-Al2O3 were related to the individuality of the crystal lattice of the θ phase and the coordination of Cr3+ impu-
rity ions in the immediate surroundings, which is different from that in the α phase. Based on the laser-induced
luminescence spectroscopic data, it was found that regions with a local Cr3+ concentration higher than the aver-
age Cr3+ concentration in the bulk of a catalyst by one order of magnitude were formed in the α-Al2O3–Fe2O3
system with the bulk Fe and Cr concentrations of 2.5 and 0.04 wt %, respectively, which was calcined at
1220°C, as a result of the diffusion of chromium and iron ions.
DOI: 10.1134/S0023158408020183
INTRODUCTION
The aim of this work was to study the pair interac-
tion of Fe–Cr and Cr–Cr centers in model Fe/Al2O3 and
Cr/Al2O3 catalysts using laser-induced luminescence at
transition metal concentrations of no higher than
2.5 wt %.
Many catalysts for the dehydrogenation of light
alkanes have been developed based on Al2O3 as a sup-
port and the oxides of transition metals (such as Fe and
Cr) as active components [1, 2]. The properties of the
active centers of these systems depend on the interac-
tion of transition metal ions with Al2O3 and, probably,
impurities, which inevitably occurred in real samples.
Laser-induced luminescence is one of a few optical
techniques for the detection of low impurity concentra-
tions and the bulk states of ions in heterogeneous sys-
tems [3]. Snytnikov et al. [4] studied the luminescence
of a number of catalysts and Al2O3 as a support with
trace amounts of chromium, iron, manganese, etc. It
was found that the phase and elemental compositions of
a thin surface layer, as well as the temperature of this
layer under conditions of real pressures and gas atmo-
spheres, can be determined from the laser-induced
luminescence spectra using the characteristic lumines-
cence lines of Cr3+ ions for α- and θ-alumina phases.
The appearance of the octahedrally coordinated Cr3+
impurity ions of corundum in metastable alumina
forms was detected using luminescence excited by UV
radiation from an ArF laser with a wavelength of
EXPERIMENTAL
The pseudoboehmite powder, which was prepared
by spraying a suspension in a drier at an outlet temper-
ature of 110°C, was mixed with a specified amount of
powdered Cr(NO3)3 · 9H2O or Fe(NO3)3 · 9H2O of ana-
lytical grade in a laboratory mixer; then, the contents
were wetted with distilled water acidified to pH 2.0. In
this case, pastes with a 55 wt % moisture content were
obtained; rings with the diameters douter = 15 mm and
dinner = 4 mm and 15 mm in height were formed from
these pastes. These rings were dried at room tempera-
ture for 72 h and then calcined in a muffle furnace at
750, 970, and 1220°C for 8 h. The study was performed
with powder samples prepared by crushing the rings. In
addition, a ruby single crystal (Cr3+ in α-Al2O3) was
used for comparing absorption spectra [4].
The laser-induced luminescence experiments were
193 nm with a sensitivity of no worse than 10–7 wt %. performed on a spectroscopic bench at the Boreskov
Snytnikov et al. [5] found the single-photon mode of Institute of Catalysis, Siberian Branch of the Russian
the action of pulsed radiation from an ArF laser on the Academy of Sciences. This bench and the experimental
samples of Al2O3 for luminescence.
procedure were described elsewhere [3, 4]. The spectra
291

292
SNYTNIKOV et al.
Table 1. Characteristics of the test samples of Al2O3
Sample
Temperature*, °C
Phase composition of samples according to XRD data
no.
composition
1
2
3
4
5
6
7
8
9
Al2O3
750
A mixture of γ-Al2O3 and χ-Al2O3 (no more than 15%)
Lattice parameter a = 7.920 Å
0.1% Cr/Al2O3
0.5% Cr/Al2O3
2.5% Cr/Al2O3
0.1% Fe/Al2O3
0.5% Fe/Al2O3
2.5% Fe/Al2O3
Al2O3
970
970
970
A mixture of δ-Al2O3 and trace α-Al2O3 (H43 = 5**)
A mixture of δ-Al2O3 and trace α-Al2O3 (H43 = 8)
0.1% Cr/Al2O3
10 0.5% Cr/Al2O3
A mixture of δ-Al2O3 and trace α-Al2O3–α-Cr2O3 solid solution based
on the α-Al2O3 structure
11 2.5% Cr/Al2O3
970
970
Diffraction pattern from θ-Al2O3;
The α-Al2O3 phase was not detected
Diffraction pattern from δ-Al2O3;
12 0.1% Fe/Al2O3
The α-Al2O3 phase was not detected
A mixture of δ-Al2O3 and trace α-Al2O3 (H43 = 10)
A mixture of δ-Al2O3 and trace α-Al2O3 (H43 = 90)
δ-Al2O3 (d/n024 = 1.7319 Å)
13 0.5% Fe/Al2O3
14 2.5% Fe/Al2O3
15 Al2O3
970
970
1220
1220
16 0.1% Cr/Al2O3
α-Al2O3–α-Cr2O3 solid solution based on the α-Al2O3 structure
(a wide range of solutions with averaged d/n024 = 1.7362 Å)
17 0.5% Cr/Al2O3
18 2.5% Cr/Al2O3
19 0.1% Fe/Al2O3
20 0.5% Fe/Al2O3
21 2.5% Fe/Al2O3
1220
1220
1220
1220
1220
α-Al2O3–α-Cr2O3 solid solution based on the α-Al2O3 structure
(d/n024 = 1.7393 Å)
α-Al2O3–α-Cr2O3 solid solution based on the α-Al2O3 structure
(d/n024 = 1.7411 Å)
α-Al2O3–Fe2O3 solid solution based on the α-Al2O3 structure
(a wide range of solutions with averaged d/n024 = 1.7405 Å)
α-Al2O3–Fe2O3 solid solution based on the α-Al2O3 structure
(a wide range of solutions with averaged d/n024 = 1.7380 Å)
α-Al2O3–Fe2O3 solid solution based on the α-Al2O3 structure
(a wide range of solutions with averaged d/n024 = 1.7436 Å)
*The time of sample calcination at the specified temperature was 5 h.
**Peak intensity.
were excited using an ArF laser (λ = 193 nm) with the
pulse duration τp = 15 ns and a radiation power density
of no higher than 0.1 MW/cm2 at the sample surface
(the heating of an absorbing layer was insignificant at
this power density) [5]. The laser-induced lumines-
cence spectrum was measured with an MDR-12 high-
aperture monochromator (LOMO, Russia) and an
LN/CCD-1100PF/UV spectroscopic chamber (Prince-
ton Instruments, United States) with the spectral sensi-
tivity range 180–900 nm.
RESULTS
Table 1 summarizes the characteristics (composi-
tion, calcination temperature, and XRD data) of the test
samples.
According to X-ray fluorescence spectrometric
data, the starting hydroxides, which were used for pre-
paring alumina, contained the following impurities
(wt %): Si, 0.57; Fe, 0.05; Cr, 0.01; Mn, 0.01; Na,
<0.01. The concentration of other impurities was no
higher than 0.001 wt %. In the samples doped with Fe
to a concentration of 2.5 wt %, increased Cr and Mn
impurity concentrations of 0.04 and 0.005 wt %,
respectively, were detected (Table 2).
XRD analysis allowed us to detect an α-Al2O3 phase
in the bulk of the samples with a sensitivity of a few
wt %. Changes in the oxide lattice of Al2O3 upon dop-
ing with Cr and Fe and the formation of solid solutions
like Cr2O3–Al2O3 and Fe2O3–Al2O3 based on the
α-Al2O3 structure were also monitored using XRD
The analysis of samples for impurities was per-
formed on a SPRUT-001 X-ray fluorescence spectrom-
etry analyzer (Russia). The X-ray diffraction (XRD)
analysis of samples was performed on URD-63 and
HZG-4 diffractometers (Germany) using CuKα radia-
tion. The electronic diffuse reflectance spectra were
measured on a UV–VIS 4501 instrument (Shimadzu,
Japan).
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LASER-INDUCED LUMINESCENCE OF MODEL Fe/Al2O3 AND Cr/Al2O3 CATALYSTS
293
Intensity, Kubelka–Munk units
analysis. The sets of interplanar spacings in δ- and θ-
Cr6s+
I
II
Al2O3 phases are similar; because of this, the sensitivity
of XRD analysis in the quantitative determination of
either of these individual phases was no higher than
20% (Table 1) [6].
36000–41000
27000
(‡)
101
d–d Cr3s+
4
21700
d–d Cr3s+
11
The diffuse reflectance UV–VIS spectra of electron
transitions allowed us to determine the concentration
and coordination of the main doping elements. Figure 1
shows the electronic diffuse reflectance spectra of sam-
ples 4, 11, and 18 with a Cr content of 2.5 wt %; the
absorption spectrum of a ruby single crystal; and the
diffuse reflectance spectra of samples 7, 14, and 21 with
a Fe content of 2.5 wt % with various calcination tem-
peratures.
16600
Cr3+
Cr3+
100
17800
18
14600
10–1
10–2
Ruby
24700
Cr3+
38400
Cr3+
The laser-induced luminescence observed in the test
samples was due to the radiative transitions of photoex-
cited octahedrally coordinated Cr3+ ions in the lattice of
Al2O3. As the concentration of chromium in the sam-
ples was increased up to 0.5 wt %, the luminescence
intensity at R lines, which are characteristic of Cr3+ in
α- and θ-Al2O3 phases, and a diffuse luminescence
band, which is characteristic of low-temperature Al2O3
phases, increased. As the doping impurity concentra-
tion was further increased, the luminescence intensity
decreased. This is consistent with the well-known con-
centration quenching of the Rα luminescence lines of
Cr3+ in corundum at a concentration of higher than
0.15 wt % [7]. The introduction of Fe3+ ions into the lat-
tice ofAl2O3 in the iron doping of the samples with con-
centrations to 2.5 wt % resulted in the luminescence
quenching of Cr3+ centers.
II
I
4T1(4P) 6A1
101
100
7
14
21
6A1
4E(4D)
4T2(4D) 6A1
(b)
4E; 4A1(4G) 6A1
(4T1 + 4T1)
(6A1 + 6A1)
4T2(4G)
6A1
10–1
Fe3+/Al2O3, Oh
α-Fe2O3
1. Samples with a Calcination Temperature of 750°C
10000
20000
30000
ν, cm–1
40000
50000
According to XRD data, samples 1–7 (Table 1),
which were calcined at 750°C, were mixtures of
γ-Al2O3, which is characteristic of the pseudoboehmite
series with the lattice parameter a = 7.920 Å, and χ-
Al2O3 (the concentration of the latter phase was no
higher than 15%). The presence of other crystallized
phases was not detected.
Fig. 1. Electronic diffuse reflectance spectra of (a) samples 4,
11, and 18 containing 2.5% Cr and (b) samples 7, 14, and
21 containing 2.5% Fe and (a) the absorption spectrum of a
ruby single crystal (see Table 1 for sample numbers and cal-
cination temperatures): (I) region of d–d transitions and
(II) region of (a) Cr –O and (b) Fe(III)–O charge-transfer
bands.
6+
The electronic diffuse reflectance spectra of chro-
mium-doped samples (Fig. 1, sample 4) exhibited
intense lines at 27000 cm–1. According to published
data [2, 8], these lines correspond to Cr6+ surface sites
intensities began to reach saturation at a 0.5 wt % chro-
mium content of the sample.
(Cr6s+ ) with an O2–
Cr6+ ligand–metal charge-
The iron-doped samples (Table 1, sample 7) exhib-
ited intense LMCT bands due to ligand–metal (Fe3+)
charge transfer at wavelengths longer than 30000 cm–1.
In addition, lines due to 4T2(4G)
at 14000 cm–1 and (4E; 4A1)
22300 cm–1 and a line at 18500 cm–1, which corre-
sponds to the (4T1 + 4T1)
transition (EPT) for α-Fe2O3, were observed [9]. The
transfer (LMCT) band at 37000 cm–1. In addition to
these lines, lower intensity lines at 16600 and
21700 cm–1, which are ascribed to d–d transitions in
6A1 d–d transitions
6A1 d−d transitions at
Cr3+ surface sites (Cr3s + ), and weak lines at 14600,
17800, and 24700 cm–1 due to d–d transitions in octa-
hedrally coordinated Cr3+ ions in the lattice of Al2O3
can also be seen. The intensity of the above bands
increased with chromium content. The exception is
(6A1 + 6A1) electron pair
provided by lines at 27000 and 37000 cm–1, whose samples with an Fe content of 0.1 wt % exhibited lines
KINETICS AND CATALYSIS Vol. 49 No. 2 2008

294
Luminescence intensity, arb. units
SNYTNIKOV et al.
α
9000
8000
7000
6000
5000
4000
3000
2000
1000
H43
α
(a)
α
α
α
3
14
13
12
1
2
4
11
10
9
3000
2500
2000
1500
1000
500
(b)
4T2(4G) 6A1,
Fe3+/Al2O3, Oh
8
5
1
15 20 25 30 35 40 45 50 55 60 65
2θ, deg
6
Fig. 3. X-ray diffraction patterns of samples 8–14 calcined
at 970°C over the angle range 2θ = 20°–55°. α refers to lines
that correspond to α-Al O (see Table 1 for sample numbers
2
3
and calcination temperatures).
7
660
680
700
720
740
760
780
λ, nm
traces. Only samples 11 (2.5% Cr/Al2O3) and 12 (0.1%
Fe/Al2O3) consisted of a θ-Al2O3 phase without the
detectable presence of α-Al2O3. From a comparison of
H43 peak intensities, it follows that samples 13 and 14
containing 0.5 and 2.5 wt % Fe, respectively, were
characterized by the highest α-Al2O3 contents
(Table 1). It is believed that an increase in the concen-
tration of iron facilitated the formation of α-Al2O3.
As compared with the electronic diffuse reflectance
spectra of samples 1–7 (calcination temperature of
750°C), the spectra of samples 8–14 (calcination tem-
perature of 750°C) were characterized by lower absor-
bance over the entire spectral range with the retention
of characteristic lines (Fig. 1, spectra 11 and 14).
The laser-induced luminescence spectra of sam-
ples 8–14 (Fig. 4) exhibited the Rα lines of α-Al2O3 at
693.5 and 692.0 nm (marked as Rα in Fig. 4) against the
background of a diffuse band characteristic of low-tem-
perature Al2O3 phases except for samples 11 (2.5%
Cr/Al2O3) and 12 (0.1% Fe/Al2O3), which did not con-
tain the α phase according to XRD data. Intense Rθ
luminescence lines at 682.6 and 685.9 nm, which sug-
gested the presence of θ-Al2O3 in the samples, were
detected in all of the above samples. In this sample
series, the intensities of Rα and Rθ lines were near the
threshold of sensitivity at the used excitation level and
Fig. 2. Luminescence spectra of γ-Al O samples doped with
2
3
(a) Cr and (b) Fe. Sample temperature, 77 K (see Table 1 for
sample numbers and composition).
at 27000 and 37000 cm–1 from Crs6+ ions and weak
bands from Cr3+ in the wavelength regions 16500–
18000 and 21000–23000 cm–1.
The laser-induced luminescence spectra of sample 1
with an initial Cr concentration of 0.01 wt % and chro-
mium-doped samples 2–4 exhibited a broad lumines-
cence band with a band head at 680 nm (Fig. 2), which
corresponds to the broadened Rθ lines of a θ-Al2O3
phase [4]. The luminescence intensity of the chro-
mium-doped samples decreased towards long wave-
lengths up to λ ≈ 1000 nm. In samples 5–7, which were
doped with Fe3+ ions, an increase in the concentration
of iron resulted in the luminescence quenching of Cr3+
primarily at a band at 680–730 nm and, consequently,
in the formation of a luminescence maximum at
770 nm.
2. Samples with a Calcination Temperature of 970°C
According to XRD data (Fig. 3), samples 8–14 CCD detector exposure only for sample 14 (2.5%
(Table 1), which were calcined at 970°C, were mixtures Fe/Al2O3) with the highest α-phase content (according
of a predominant δ-Al2O3 phase and α-Al2O3 phase to XRD data).
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LASER-INDUCED LUMINESCENCE OF MODEL Fe/Al2O3 AND Cr/Al2O3 CATALYSTS
295
Luminescence intensity, arb. units
In the series of samples 8–11, both the intensity of
a diffuse band and the intensities of Rθ lines increased
as the chromium content of the samples was increased
up to 0.1 wt % followed by a decrease in the lumines-
cence intensity as the Cr content of the samples was
further increased to 2.5 wt %. As the Fe concentration
was increased from 0.05 to 2.5 wt % in the series of
samples 8 and 12–14, the quenching of Rθ lumines-
cence lines occurred.
45100
(‡)
40100
Nθ
35100
30100
Rθ
25100
20100
15100
10100
5100
11
In addition to the well-known Rα and Rθ lines, the
luminescence spectra of samples 9–11 with Cr contents
higher than 0.1 wt % exhibited a number of new lines
in the region 688.5–711.0 nm (Fig. 4). An intense line
at 696.6 nm was detected in sample 11 (2.5% Cr/Al2O3)
with a maximum Cr3+ content; the presence of α-Al2O3
in this sample was not detected using the above tech-
niques.
Rα
10
9
8
100
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
(b)
3. Samples with a Calcination Temperature of 1220°C
According to XRD data, α-Al2O3 was formed in
sample 15 (i.e., Al2O3 with initial background Cr and
Fe concentrations) at a calcination temperature of
1220°C. In samples 16–21, solid solutions based on the
α-Al2O3 were formed: Al2O3–Cr2O3 (samples 16–18)
and Al2O3–Fe2O3 (samples 19–21). Upon the introduc-
tion of Cr3+ or Fe3+ ions into the lattice of Al2O3, the
replacement of the Al3+ ion by bulkier cations resulted
in an increase in interatomic distances; because of this,
the positions of XRD lines shifted toward increased lat-
tice parameters. Thus, for a pure α-Cr2O3 phase, the
Rθ
Rα
13
14
8
12
position of the 024 line corresponds to d/n024
=
675 680 685 690 695 700 705 710 715 720
1.8152 Å, and d/n024 = 1.8406 Å for α-Fe2O3. The line
positions for the samples of an α-Al2O3 phase with Cr
and Fe concentrations to 0.5 wt % are inconsistent with
the value of d/n024 = 1.7400 Å given in an XRD data-
base [10]. This can be indirect evidence for the incom-
plete formation of an α-alumina phase at low concen-
trations of dopant ions.
λ, nm
Fig. 4. Luminescence spectra of samples 8–14 doped with
(a) Cr and (b) Fe and calcined at 970°C. Sample tempera-
ture, 77 K (see Table 1 for sample numbers and composi-
tion).
The electronic diffuse reflectance spectra of chro- bands at 17 800 and 24 700 cm–1 were prominent; they
corresponded to the Cr3+ sites formed from impurities.
mium-doped samples are indicative of the almost com-
plete disappearance of absorption bands at 27000 and
The luminescence spectra of chromium-doped
Al2O3 samples 15–21 exhibited intense Rα lines due to
Cr3+ and a band due to vibronic transitions at 705–
715 nm, which is characteristic of ruby (Fig. 5, sam-
ple 19) [11]. The concentration quenching of lumines-
cence at Rα lines came into play at chromium concen-
trations higher than 0.1 wt %. Moreover, the laser-
induced luminescence spectra of samples 16–19 at
77 K usually exhibited R lines due to Mn4+ impurity
ions, whose concentration was 0.01 wt % (Table 2) [4]
and the R-doublet splitting was ∆ν = 80 cm–1.
37000 cm–1 due to surface Crs6+ ions. The Cr3+ sites,
which are characterized by bands at 14600, 17800, and
24700 cm–1, were predominant in the samples. It is
likely that a small amount of surface Cr3s + sites with
characteristic bands at 16600 and 21700 cm–1 occurred
(Fig. 1, sample 18). Upon the addition of iron, the
Fe(III)–O and Fe–O LMCT bands at 38600 and
40000 cm–1, respectively; the Fe3+–Fe3+ (4T1 + 4T1)
(6A1 + 6A1) EPT bands at 18500 cm–1 for α-Fe2O3; and
an intense narrow line at 22300 cm–1 due to the
In addition to the above lines, Nα luminescence lines
were detected in samples 16–18 at chromium concen-
(4E; 4A1)
6A1 transition in Fe3+ were observed. The
intensity of the above bands increased with iron content trations higher than 0.1 wt % (Fig. 6). Vink and
of the samples. In the samples containing 0.1 wt % Fe, DeBruin [12] related these lines to the splitting of chro-
KINETICS AND CATALYSIS Vol. 49 No. 2 2008

296
Luminescence intensity, arb. units
SNYTNIKOV et al.
Luminescence intensity, arb. units
105
104
103
102
N
α
12100
10100
8100
6100
4100
2100
100
Rα Cr3+
3NN, 2-2
4NN, 1-1
18
17
R
α
4NN, 1-2
4NN, 1-3
3NN, 2-3
16
21
680 685 690 695 700 705 710 715 720 725
λ, nm
77 K
R Mn4+
Fig. 6. Luminescence spectra of α-Al O samples (T =
calc
2
3
1220°C) doped with (16–18) Cr and (21) Fe. Sample tem-
293 K
perature, 77 K (see Table 1 for sample composition).
lower Fe contents, in which the above lines were not
detected.
DISCUSSION
660
670
680
690
700
710
720
λ, nm
The radiative transitions of the photoexcited d3 elec-
trons of octahedrally coordinated Cr3+ impurity ions in
the most ordered α- and θ-alumina phases were respon-
sible for luminescence R lines. The Rα and Rθ line posi-
tions coincide with the position found previously [3, 4]
for samples with trace concentrations of Cr3+ impurity
ions in aluminum oxide. A broad diffuse luminescence
band with a band head corresponding to the broadened
Rθ lines of a θ phase and the absence of intense lines
from the spectra of γ-, χ-, and δ-Al2O3 were due to the
strong disordering of the crystal lattices of the above
phases. The presence of hydroxyl groups (OH–) in the
first coordination sphere of the Cr3+ impurity ion can
also have a certain effect.
Fig. 5. Laser-induced luminescence of sample 19 (0.1%
Fe/Al O ; 1220°C) at 293 and 77 K.
2
3
mium Rα lines due to the pair interaction of Cr3+–Cr3+
ions from the immediate surroundings. We detected the
quenching of Nα lines at Cr concentrations in the sam-
ples higher than 0.5 wt %; this quenching was due to a
concentration effect common to Rα and Nα lines.
Luminescence quenching at the Rα lines of α-Al2O3
occurred as the Fe content was increased from 0.05 to
2.5 wt %. Nevertheless, we detected the Nα lines of Cr3+
ions in the luminescence spectrum of sample 21 (2.5%
Fe/Al2O3), unlike the spectra of samples 19 and 20 with
The R-line luminescence intensity (ICr) at 685.9–
693.5 nm in samples 1–4 was related to the chromium
content by the expression ICr ~ [Cr]0.5. As the Fe content
of the samples was increased from 0.05 to 2.5 wt %, the
character of Cr3+ luminescence quenching depended on
Table 2. Concentrations of impurities in the test samples ac- the luminescence wavelength. Thus, the intensity of
cording to X-ray fluorescence spectrometry
Cr3+ R lines at 682–695 nm depended on the Fe concen-
tration in accordance with the law ICr(682–695) ~
Sample
Impurity concentration, wt %
[Fe]−0.75. At the same time, this relationship was close
to ICr(770) ~ exp(–[Fe]) at 770 nm. It is most likely that
no.*
composition
Cr
Mn
Fe
the high efficiency of Cr3+ luminescence quenching in
15 Al2O3
0.01
0.11
0.02
0.04
0.01
0.01
0.01
0.05
0.05
0.05
0.15
2.30
the region of R lines upon doping with iron was related
16 0.1% Cr/Al2O3
19 0.1% Fe/Al2O3
21 2.5% Fe/Al2O3
to the 4T2(4G)
6A1 transition at 14350 cm–1 for octa-
hedrally coordinated Fe3+ ions in Al2O3. Thus, the
observed broad luminescence band with a maximum at
770–800 nm in these samples was a consequence of a
* Corresponds to the sample number in Table 1.
KINETICS AND CATALYSIS Vol. 49 No. 2 2008

LASER-INDUCED LUMINESCENCE OF MODEL Fe/Al2O3 AND Cr/Al2O3 CATALYSTS
297
considerable interaction between photoexcited Cr3+ and
Fe3+ impurity ions. This is additional evidence that it is
of importance to take into account the interaction of
impurity ions even at their trace concentrations.
luminescence intensity of Rα lines as the concentration
of Cr was increased up to 0.1 wt %. The intensity of Nα
and Rα lines decreased at a Cr content of the samples
higher than 0.5 wt %. The intensity ratio between Nα
and Rα luminescence lines was found proportional to
the concentration of chromium ions in the samples:
IN /IR ~ [Cr]. This allowed us to determine the
local concentration of emitting Cr3+ ions from this ratio
even in the presence Fe3+ ions, which quench the above
lines.
At a calcination temperature of 970°C, the forma-
tion of a δ-Al2O3 phase with α-Al2O3 traces occurred in
the majority of samples (8–10, 13, and 14). In the series
of samples 8–11, the intensities of both a diffuse lumi-
nescence band and Rθ lines increased as the concentra-
tion of chromium was increased up to 0.1 wt % fol-
lowed by a decrease in the intensity of luminescence as
the Cr content of the samples was increased to 2.5 wt %.
In the series of samples 8 and 12–14, an increase in the
Fe content from 0.05 to 2.5 wt % resulted in a decrease
in the Rθ-line luminescence intensity by more than
three orders of magnitude. Nevertheless, the concentra-
tion of an α-Al2O3 phase in the samples with iron ions
added, which was calculated from the luminescence
intensity of Rα lines using a given relationship for α-
Al2O3 [3], was consistent with XRD data.
In all of the samples with a calcination temperature
of 970°C, intense luminescence lines were detected at
682.6 and 685.9 nm, which suggest the presence of a θ-
Al2O3 phase in the samples (this phase was not detected
by XRD analysis). This discrepancy in conclusions
drawn based on the use of various techniques can be
due to difficulties in the XRD detection of the θ phase
against the background of the δ phase because of the
similar sets of interplanar spacings in these phases. At
the same time, the θ phase can easily be detected by an
intense characteristic doublet of narrow Rθ lines in
luminescence spectra. Nevertheless, the question of
qualitative differences (line width and position, decay
times, etc.) between the luminescence spectra of the δ
and θ phases is still an open question. This problem can
be solved with the availability of a reference sample of
the pure δ-Al2O3 phase.
1, α
2, α
We used this opportunity to calculate the local con-
centration of Cr3+ in sample 21 (2.5% Fe/Al2O3;
1220°C) with a high Fe content. The Cr3+ concentration
calculated from the IN /IR ratio measured in laser-
1, α
2, α
induced luminescence spectra was found equal to
0.7 wt %. At the same time, according to X-ray fluores-
cence analysis, the average chromium content of this
sample was equal to 0.04 wt % (Table 2). This consid-
erable increase in the laser-induced luminescence
intensity of the sample under discussion can be
explained by the ejection of Cr3+ ions by Fe3+ ions from
nanosized domains in α-Al2O3, which were formed at
high calcination temperatures (1220°C) in the
α-Al2O3–Fe2O3 system, to the boundaries of these nan-
odomains with the formation of a region with a high
(0.7 wt %) local concentration of chromium. This con-
centration is higher than the average bulk concentration
of Cr3+ by one order of magnitude. This ejection of
impurities to the phase boundary microdomains has
been repeatedly observed in cerium oxide [14] and alu-
minum oxide [15].
In a ruby single crystal at 293 K, the characteristic
R1-line halfwidth was ~11 cm–1 and it decreased to
The luminescence spectra of the test samples with
calcination temperatures of 970 and 1220°C (Figs. 4–6)
exhibited the well-known Rα, Rθ, and Nα lines; in this
case, the Nα lines were due to the pair interaction of
Cr3+–Cr3+ ions. However, new luminescence lines,
which were marked as Nθ, were detected in the region
688.5–711.0 nm for samples with a calcination temper-
ature of 970°C and a Cr content of higher than 0.1 wt %
(Fig. 4). The intensity of the most intense Nθ lines at
696.7 and 701.4 nm increased with the Cr content of the
0.12–0.30 cm–1 upon cooling the single crystal to 77 K
[13]. In this case, the residual R-line halfwidth prima-
rily depended on the occurrence of nonuniform stresses
in a crystal and crystal lattice defects. In sample 15
(Al2O3) at 293 K, the characteristic R1-line halfwidth
was ~23 cm–1; this line broadened to ~28 cm–1 as the
concentration of Cr3+ was increased. The observed
broadening of R1 and other lines in the test samples of
samples in accordance with a quadratic law (more rap- model catalysts can also be due to crystal lattice defects
idly than the intensities of Rθ lines). This allowed us to
assume that the Nθ lines are due to the pair interaction
of Cr3+ ions in the θ-Al2O3 phase analogously to Nθ
lines in corundum. We failed to find published experi-
mental or theoretical data on the splitting of Cr3+ Rθ
lines in a pure phase of θ-Al2O3 because of the pair
interaction of Cr3+–Cr3+ ions in the immediate sur-
roundings.
and, probably, an increase in the number of interacting
pairs of chromium ions at Cr3+ concentrations of higher
than 0.5 wt %. The change in the ratio between the line
intensities IR and IR from 1.77 (T = 293 K) for a ruby
1
2
single crystal and α-Al2O3 samples with small Cr and
Fe additives to 1.90 (T = 293 K) as the chromium con-
centration in sample 15 was increased to 2.5 wt % sug-
gests an increase in the number of crystal lattice
The samples with a calcination temperature of
1220°C were characterized by a linear increase in the defects.
KINETICS AND CATALYSIS Vol. 49 No. 2 2008

298
SNYTNIKOV et al.
CONCLUSIONS
Studies of a commercial chromium-containing dehy-
drogenation catalyst (OAO Katalizator), whose compo-
sition is more complex than that of the model test sam-
ples, are currently in progress.
The above study of aluminum oxides doped with
chromium and iron supported the assumption that laser-
induced luminescence spectra can provide valuable
information on the internal structure of catalysts. Thus,
the interaction of Cr3+ and Fe3+ impurity ions in low-
temperature γ-, χ-, and δ-Al2O3 phases can result in the
appearance of an additional luminescence band due to
Cr3+ ions at 770 nm. At a concentration of Cr3+ ions
higher than 0.1 wt % in samples containing a θ-Al2O3
phase, Nθ lines due to the splitting of Rα lines on the
interaction of Cr3+–Cr3+ ion pairs in the immediate sur-
roundings were observed. Differences between the Nα
lines and the well-known Nα lines in corundum are due
to the individuality of the crystal lattice of the θ-Al2O3
phase and the coordination of Cr3+ impurity ions in the
immediate surroundings other than that in the α-Al2O3
phase.
ACKNOWLEDGMENTS
This work was supported in part by the “Support to
Leading Scientific Schools” program (grant no. NSh-
1484-2003.3). We are grateful to G.M. Zhidomirov for
critical comments and helpful discussions.
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KINETICS AND CATALYSIS Vol. 49 No. 2 2008