Reduction of mixed Mn-Zr oxides: In situ XPS and XRD studies

Article abstract of DOI:10.1039/c5dt01440a

A series of mixed Mn-Zr oxides with different molar ratios Mn/Zr (0.1-9) have been prepared by coprecipitation of manganese and zirconium nitrates and characterized by X-ray diffraction (XRD) and BET methods. It has been found that at concentrations of Mn below 30 at%, the samples are single-phase solid solutions (Mn_xZr_1-xO_2-δ) based on a ZrO₂ structure. X-ray photoelectron spectroscopy (XPS) measurements showed that manganese in these solutions exists mainly in the Mn⁴⁺ state on the surface. An increase in Mn content mostly leads to an increase in the number of Mn cations in the structure of solid solutions; however, a part of the manganese cations form Mn₂O₃ and Mn₃O₄ in the crystalline and amorphous states. The reduction of these oxides with hydrogen was studied by a temperature-programmed reduction technique, in situ XRD, and near ambient pressure XPS in the temperature range from 100 to 650 °C. It was shown that the reduction of the solid solutions Mn_xZr_1-xO_2-δ proceeds via two stages. During the first stage, at temperatures between 100 and 500 °C, the Mn cations incorporated into the solid solutions Mn_xZr_1-xO_2-δ undergo partial reduction. During the second stage, at temperatures between 500 and 700 °C, Mn cations segregate on the surface of the solid solution. In the samples with more than 30 at% Mn, the reduction of manganese oxides was observed: Mn₂O₃ → Mn₃O₄ → MnO.

Full text of DOI:10.1039/c5dt01440a

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Reduction of mixed Mn–Zr oxides: in situ XPS and
XRD studies
Cite this: DOI: 10.1039/c5dt01440a
a,b
a,b
c
c
O. A. Bulavchenko,* Z. S. Vinokurov, T. N. Afonasenko, P. G. Tsyrul’nikov,
a,b
a,b
a,b
S. V. Tsybulya, A. A. Saraev and V. V. Kaichev
A series of mixed Mn–Zr oxides with different molar ratios Mn/Zr (0.1–9) have been prepared by co-
precipitation of manganese and zirconium nitrates and characterized by X-ray diffraction (XRD) and BET
methods. It has been found that at concentrations of Mn below 30 at%, the samples are single-phase
x 2
solid solutions (Mn Zr1−xO2−δ) based on a ZrO structure. X-ray photoelectron spectroscopy (XPS)
4
+
measurements showed that manganese in these solutions exists mainly in the Mn state on the surface.
An increase in Mn content mostly leads to an increase in the number of Mn cations in the structure of
2 3 3 4
solid solutions; however, a part of the manganese cations form Mn O and Mn O in the crystalline and
amorphous states. The reduction of these oxides with hydrogen was studied by a temperature-
programmed reduction technique, in situ XRD, and near ambient pressure XPS in the temperature range from
1
00 to 650 °C. It was shown that the reduction of the solid solutions Mn
stages. During the first stage, at temperatures between 100 and 500 °C, the Mn cations incorporated into
the solid solutions Mn 2−δ undergo partial reduction. During the second stage, at temperatures
between 500 and 700 °C, Mn cations segregate on the surface of the solid solution. In the samples with
more than 30 at% Mn, the reduction of manganese oxides was observed: Mn → Mn → MnO.
x
Zr1−xO2−δ proceeds via two
Received 16th April 2015,
Accepted 22nd July 2015
x
Zr1−xO
DOI: 10.1039/c5dt01440a
www.rsc.org/dalton
2
O
3
3 4
O
reactions is not yet clear. The main reason for this incompre-
hension is due to the complexity of the reduction of Mn–Zn
Introduction
Materials based on zirconium dioxide (ZrO ) demonstrate mixed oxides.
2
unique properties and hence find wide applications in various
industrial fields. For example, their high durability, corrosion formation of the solid solutions Zr1−xMn O ,
x 2
Manganese cations can enter the lattice of ZrO
2
with the
in which
9
,12–15
resistance, and low thermal conductivity allow the application lattice oxygen possesses sufficiently high mobility and hence
1
,2
10,16–18
of ZrO
The high stability and ionic conductivity of ZrO
materials allow their use as electrochemical oxygen sensors, oxygen that is incorporated into disperse MnOx rather than
2
in the production of coatings for various toolware.
high reactivity. On the other hand, some authors
2
-based suppose that the active species in oxidation reactions is mobile
3
–5
solid electrolytes in fuel cells, etc.
In heterogeneous cata- lattice oxygen of the solid solution. Moreover, under reduction
lysis, ZrO is widely used as a support with high thermal conditions, segregation of manganese with the formation of
2
6
–8
resistance.
high catalytic activity in a number of practically important Zr1−xMn
reactions. For instance, recently it has been shown that Mn–Zn One of the main ways to study the redox properties of
mixed oxides can effectively catalyze the gas-phase oxidation of different oxides is a temperature-programmed reduction (TPR)
Moreover, solid solutions based on ZrO exhibit dispersed MnO may occur on the surface of the solid solution
2
x
x 2
O .
9
–11
hydrocarbons or chlorocarbons.
Although there is agree- technique. On the basis of its results, it is possible, for
ment that the catalytic performance of these catalysts is deter- example, to draw some conclusions about the presence of
mined by their redox properties, the exact mechanism of these various forms of manganese oxides. However, TPR is an in-
direct method that allows monitoring only the absorption of
hydrogen rather than the change in the structural character-
istics of catalysts and in the charge state of Mn and Zr cations.
The aim of this work was to obtain detailed information on
various stages of the reduction of Mn–Zn mixed oxides. The
study was carried out using a combination of methods: temp-
a
Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia.
E-mail: isizy@catalysis.ru
b
Boreskov Institute of Catalysis SB RAS, Lavrentieva Ave. 5, 630090 Novosibirsk,
Russia
c
erature-programmed reduction, X-ray diffraction (XRD), and
Institute of Hydrocarbon Processing SB RAS, Neftezavodskaya Str. 54, 644040 Omsk,
Russia
X-ray photoelectron spectroscopy (XPS).
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in the sample up to 85 wt%. Note that the specific surface area
of most of the samples xMn(1 − x)Zr exceeds the specific
surface area of individual oxides of manganese and zirconium
Results and discussion
XRD study of as-prepared samples
The results of the study of freshly synthesized samples indicate that were subjected to the same stages of synthesis.
that the concentration of Mn in the samples xMn(1 − x)Zr sig-
nificantly affects their phase composition, unit cell parameters Mn
Fig. 1 shows the lattice parameter of the solid-solution
as a function of the manganese content x calcu-
y
Zr1−yO
4
of the solid solution and the size of the coherently scattering lated based on the XRD data obtained in our experiments and
9
,10,23
domain (CSD). The main results of the XRD study are pre- the XRD data published elsewhere.
As seen from the
sented in Table 1. At x = 0, the sample xMn(1 − x)Zr is ZrO picture, the lattice parameter in the solid solutions depends
2
with the lattice parameter of 5.115 Å. Although this modifi- on several factors: the content of Mn cations, preparation con-
cation of ZrO cannot be considered strictly cubic, its lattice ditions, temperature, the environment and the duration of
2
parameter was calculated in the cubic approximation because calcination that affects the oxidation state of Mn and the
of the low calcination temperature (650 °C) and because of the
small size of the obtained particles that led to the broadening
of diffraction peaks. In comparison, the lattice parameter of
2
2
2
the metastable cubic phase ZrO is 5.09 Å.
An increase in the Mn content resulted in a monotonic
decrease in the lattice parameter of zirconia, indicating that
Mn cations modify the lattice of ZrO
2
to form a solid solution
3
+
Mn Zr O . Indeed, substitution of smaller cations Mn
y
1−y 2
4
+
(ionic radius 0.66 Å) for bigger cations Zr (ionic radius
0
.79 Å) should lead to a decrease in the lattice parameter of
1
0
the solid solution based on zirconia. The detected decrease
in the size of CSD correlates with an increase in the surface
area (Table 1), i.e. the dispersion of particles Mn
increases. In the samples with the high Mn content (x ≥ 0.5),
there also appears a phase of manganese oxide Mn . A
y 2
Zr1−yO
2 3
O
further increase in the concentration of Mn (x = 0.7) leads to
the formation of an additional phase β-Mn O .
3
4
The multi-phase composition of the samples leads to a non-
monotonic dependence of their specific surface area on the con-
centration of Mn. With an increase in the Mn content in the
x range from 0.0 to 0.4, the specific surface area increases from
2
−1
Fig. 1 Our experimental (circles) and estimated (dotted line) depen-
dence of the lattice parameter of Mn on the Mn content x.
Other symbols show published lattice parameters of Mn
obtained under other conditions: in air at 600 °C for 4 h (diamonds); in
4
3 to 103 m g and then almost does not change until x = 0.6.
y
Zr1−yO
2
The addition of Mn in larger amounts leads to a drop in S to
BET
y
Zr1−yO
2
2
−1
7
9 and 44 m g at x = 0.7 and 0.9, respectively. Reducing the
5
9
10
specific surface area at high concentrations of Mn may be
associated with an increase in the content of manganese oxides hydrogen at 800 °C for 10 h (triangles).
air at 600 °C but for 3 h (stars); in air at 800 °C for 3 h (squares); in
23
Table 1 Characteristics of fresh samples xMn(1 − x)Zr
Lattice parameter
2
−1
Sample
Phase composition
ZrO
wt%
of Mn
y
2
Zr1−yO , Å
CSD, Å
S
BET, m g
0
0
0
0
0
Mn1Zr
2
100
100
100
100
90
10
95
5.115(1)
5.081(1)
5.040(1)
5.019(1)
5.003(1)
250
140
120
120
90
290
90
43
85
94
103
100
.12Mn0.88Zr
.3Mn0.7Zr
.4Mn0.6Zr
.5Mn0.5Zr
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
y
Zr1−y
O
O
O
O
2
2
2
2
y
Zr1−y
y
Zr1−y
y
Zr1−y
2
O
Zr1−y
3
0
.6Mn0.4Zr
.7Mn0.3Zr
y
O
2
2
4.988(1)
4.987(2)
100
80
2
O
Zr1−y
3
5
50
280
70
0
y
O
β-Mn
3
O
O
3
4
30
20
15
45
30
100
370
400
100
480
700
800
Mn
Mn
2
0
1
.9Mn0.1Zr
Mn0Zr
y
Zr1−y
O
2
4.983(5)
44
36
β-Mn
3
O
4
Mn
Mn
2
O
O
3
2
3
—
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nonstoichiometry of the oxide with respect to oxygen. Accord-
9
,10
ing to literature data
(Fig. 1, diamonds, stars), the lattice
parameter decreases with the increasing Mn content almost
linearly (up to x = 0.5). It indicates the formation of a wide
range of solid solutions, as in our case. The dotted line in
Fig. 1, which is drawn through two points x = 0 and x = 0.12,
extrapolates the dependence of the formation of the solid solu-
tion Mn Zr O to y = x. It is evident that in the range from x =
y
1−y 4
0
.0 to x = 0.3, the experimental lattice parameters coincide with
the calculated data, which may indicate complete incorporation
of Mn into the lattice of ZrO . Moreover, even the point at x =
2
0
.4 deviates from the extrapolated data only slightly. For large
values of x, the deviation of the experimentally observed lattice
parameters from the calculated data increases. One of the
factors determining this behavior is the formation of manga-
nese oxide particles on the surface of solid solutions. According
to the XRD data, the phase of Mn O appears already at x = 0.5,
2
3
and its amount increases with the Mn content.
2
Thus, the change in the lattice parameter of the ZrO -based
Fig. 3 TPR profiles of the xMn(1 − x)Zr samples under study.
phase demonstrates that the coprecipitation method actually
2
provides the formation of the solid solutions Mn Zr1−yO .
y
Taking into account the linearity of the initial part of the
obtained dependence, one can assume that all Mn atoms are
incorporated into the lattice of ZrO in the samples with the
2
concentration of Mn up to 30 at%. With a further increase in
the Mn content, the number of Mn cations included in the
y 2
solid solution Mn Zr1−yO also increases; however besides this,
Mn
y 4
Zr1−yO 10–15 nm in size. For 0.5Mn0.5Zr, large particles
of Mn O are also observed. Energy dispersive X-ray (EDX) ana-
2
3
lysis indicates that Mn : Zr ratios are 26 : 74 and 53 : 47 for
.3Mn0.7Zr and 0.5Mn0.5Zr, respectively. These values are
close to the original content of elements.
0
a part of the manganese cations enter the composition of crys-
talline oxides Mn O , Mn O , or an amorphized state.
2
3
3 4
TPR-H
2
study
Transmission electron microscopy (TEM) characterization
The reduction of mixed Mn–Zr oxides was studied by TPR
Fig. 2 shows the TEM images of 0.3Mn0.7Zr and 0.5Mn0.5Zr. under a hydrogen atmosphere. The TPR profiles obtained for
The sample consists of small particles of the solid solution xMn(1 − x)Zr in the range from 30 to 900 °C are shown in
Fig. 3; the positions of the observed peaks and the amounts of
absorbed hydrogen are summarized in Table 2.
Table 2 TPR data for xMn(1 − x)Zr samples
Peak deconvolution
Peak
ratio,
%
Amount of
absorbed H
mmol g−
Total amount
2
of absorbed H ,
mmol H per g
2
2
,
1
Sample
T
max, °C
0
0
Mn1Zr
.12Mn0.88Zr
635
320
100
52
48
69
31
1
0.456
0.389
0.360
1.089
0.480
0.02
0.456
0.755
5
80
340
70
220
0
0
.3Mn0.7Zr
.4Mn0.6Zr
1.569
1.76
5
3
5
290
3
4
5
40
60
73
26
26
14
42
18
8
48
37
7
1.31
0.45
0
0
.5Mn0.5Zr
.7Mn0.3Zr
0.592
0.320
0.952
0.396
0.262
1.524
1.167
0.237
2.26
3.19
50
10
50
250
65
3
Fig. 2 TEM images and EDX analysis of 0.3Mn0.7Zr (a) and 0.5Mn0.5Zr
b) samples.
450
550
(
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The reduction of 0Mn1Zr (i.e., ZrO
2
) appears as one narrow 450 °C can be due to the reduction of the crystalline oxide
peak of the hydrogen absorption with a maximum near Mn O . The shift of these peaks to the higher temperatures
2
3
6
35 °C. Such a dependence is not typical of well-crystallized may be associated with an increase in the size of manganese
ZrO . However in some cases, TPR profiles of ZrO exhibit oxide particles.
intense peaks, whose position and intensity depend on the
2
2
The origin of the high-temperature peak, which is observed
1
4,24,25
phase composition and the synthesis method used.
example, Damyanova et al. observed two peaks at 440 and reduction of cations Mn → Mn in the lattice of zirco-
6
ates and hydroxyl groups. Dravid et al. suggested that the the reduction of zirconia itself, which is indicated by the
partial reduction of ZrO may result from the high mobility of presence of this peak in the TPR profile of the Mn-free sample
For at 550–580 °C, is still debatable. It is usually attributed to the
2
5
3+
2+
1
0,11,26
30 °C and attributed them to the reduction of surface carbon- nia.
However, sometimes this peak is associated with
1
4
29
2
oxygen at elevated temperatures. In our case, heating the 0Mn1Zr. Fig. 3 shows that the high-temperature peak for the
sample 0Mn1Zr in argon to 200 °C resulted in the release of samples xMn(1 − x)Zr shifts to lower temperatures with an
only adsorbed water and CO
temperature. Consequently, the hydrogen absorption peak at 0Mn1Zr) is significantly shifted in comparison with the peak
35 °C (Fig. 3) should be attributed to some transformations for all the Mn-containing samples. For the samples with x = 0
of ZrO itself, for example, to the partial reduction of zirco- and x = 0.12, this shift is 55 °C, while a further increase in the
nium in the surface layers of 0Mn1Zr. Mn content leads to an additional shift of the high-tempera-
2 2
, which decreased rapidly with increase in the Mn content. This peak for “pure” ZrO (sample
6
2
For the samples containing Mn, there are two ranges of ture peak toward lower temperatures by 30 °C.
hydrogen absorption: at 150–500 °C and 500–640 °C, respect-
ively (Fig. 3). On the TPR profiles of the samples 0.12Mn0.88Zr
and 0.3Mn0.7Zr, there is a broad low-temperature peak with a
maximum (Tmax) at 320 and 340 °C, respectively. In the litera-
ture, peaks in the range 200–500 °C are often associated with
the reduction of highly dispersed manganese oxides that do
In situ XRD study
To determine structural transformations that accompany the
reduction of mixed Zr–Mn oxides, we recorded in situ XRD pat-
terns of the samples 0.3Mn0.7Zr and 0.5Mn0.5Zr during their
reduction with hydrogen at temperatures up to 700 °C. For
1
0,26
not manifest themselves by XRD.
0.3Mn0.7Zr, the increase in temperature led to a shift of the
However, this assumption is questionable, because the
presence of an X-ray amorphous phase is usually not proven.
Taking into account that up to 14 wt% of manganese oxides,
peaks of the solid solution Mn , and no other changes
in the diffraction patterns occurred (Fig. 4).
Fig. 5 shows a series of diffraction patterns recorded during
the reduction of 0.5Mn0.5Zr. At room temperature, the sample
contains two phases: the solid solution Mn Zr1−yO4 and
y
Zr1−yO
4
1
4
Mn cations can dissolve in the structure of ZrO2, and that
the samples 0.12Mn0.88Zr and 0.3Mn0.7Zr consist of only the
y
solid solution Mn
attributed to the reduction of Mn cations located in the lattice
of ZrO . For the sample 0.4Mn0.6Zr, the low-temperature peak
has a more complex shape: there is a small shoulder near
20 °C. According to the XRD data (Table 1), the lattice para-
meter of this sample slightly deviates from the value calculated
for the solid solution Mn0.4Zr0.6 (Fig. 1).
y 2
Zr1−yO , the peak at 320–340 °C should be
Mn
2
O
3
. The reduction of this sample leads to a change in the
, which is
lattice parameter of the solid solution Mn
y
Zr1−yO
4
2
indicated by the shift of the corresponding peaks to larger
angles. Besides, the reduction leads to the transformations of
manganese oxides. For example, in the range of 275–325 °C,
relative intensities of the peaks of Mn O gradually decrease,
2
2
3
O
2
On the other hand, this sample does not contain crystalline
manganese oxides. Therefore, the additional peak at 220 °C
can be attributed to the reduction of an amorphous oxide
MnO , which may exist on the surface of Mn Zr O particles
x
y
2
1−y 2
6
in the form of, for example, an epitaxial layer.
At higher concentrations of Mn, the shape of the low-temp-
erature peak becomes even more complicated: there appears
an additional peak at higher temperatures, the intensity of
which increases with an increase in the Mn content. For
example, TPR profiles of the samples with x = 0.5 and 0.7
exhibit three peaks in the low temperature region with the
maxima located at 290, 350, 410 °C and at 250, 365, 450 °C,
respectively. According to the XRD data (Table 1), these
samples contain the crystalline phase Mn
proceeds in two steps: (1) Mn → Mn
2) Mn O → MnO at 350–500 °C.
2
O
O
3 4
3
, whose reduction
at 250–350 °C and
Therefore, it can be
assumed that the hydrogen absorption peaks at 290 and
50 °C can be attributed to the reduction of amorphous MnO
2
O
3
2
7,28
(
3
4
Fig. 4 Series of diffraction patterns (λ = 1.7273 Å) recorded in situ
during the reduction of 0.3Mn0.7Zr with hydrogen in the temperature
2
x
,
while the peaks at 350 and 410 °C and the peaks at 365 and range from 30 to 700 °C.
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Fig. 5 Series of diffraction patterns (λ = 1.0157 Å) recorded in situ
during the reduction of 0.5Mn0.5Zr with hydrogen in the temperature
range from 30 to 700 °C.
y 2
Fig. 7 Differential dependence of the lattice parameter of Mn Zr1−yO
on temperature for 0.3Mn0.7Zr (1) and 0.5Mn0.5Zr (2).
while the peaks of Mn O increase. At 375 °C, there appears
3
4
the phase of MnO. Upon further heating, the relative intensity
of the reflections of MnO increases (Fig. 5). For clarity, Fig. 6
shows diffraction patterns recorded at 275–400 °C in the 2θ and a high-temperature peak starting near 480 °C (Fig. 7). The
range 17–28°.
maximum of the low-temperature peak is located at 290 and
Hence, the XRD data agree well with the TPR results, illus- 350 °C for 0.3Mn0.7Zr and 0.5Mn0.5Zr, respectively. It can be
trating the two-stage reduction of manganese: Mn O
→
seen that the change in the lattice parameter correlates well
with the TPR results shown above (Fig. 3).
2
3
Mn
3 4
O → MnO.
Fig. 7 represents the change in the lattice parameter of
After the reduction at 700 °C, the lattice parameter of
the solid solution Mn Zr O as a function of temperature. It Mn Zr1−yO2 increases from 5.040 to 5.074 Å for 0.3Mn0.7Zr
y
1−y
2
y
can be seen that the lattice parameter changes in two steps: and from 5.003 to 5.053 Å for 0.5Mn0.5Zr; i.e., the lattice para-
there is a wide low-temperature peak between 100 and 550 °C meter of the solid solution approaches the value that is charac-
teristic of “pure” zirconium oxide. This change in the lattice
parameter may result from the change both in the compo-
sition of the solid solution (exit of manganese cations to the
surface, i.e., the segregation of Mn) and in the oxidation state
of Mn cations in the lattice of the solid solution, for example,
3
+
2+ 15,23,29
Mn → Mn .
In the latter case, the number of anion
vacancies changes and may affect the lattice parameter as well.
To elucidate the reasons for the increase in the lattice para-
meter of Mn Zr1−yO2 during its reduction, we conducted an
y
experiment on the oxidation of reduced samples. For this,
samples reduced at 700 °C were calcined in air at 650 °C for
4
h. It was found that after such treatment, the lattice para-
meter of Mn Zr1−yO2 decreased to 5.053 Å and 5.018 Å for
y
0.3Mn0.7Zr and 0.5Mn0.5Zr, respectively. It means that the
lattice parameters did not return to initial values (Table 1),
which most likely indicates a partial exit of manganese cations
from the bulk structure of the solid solution. The following
XPS data confirm this idea. It was interesting to compare bulk
(
XRD and TPR data) and surface (XPS data) behavior of the
solid solution during reduction.
In situ XPS study
Fig. 6 Series of diffraction patterns (λ = 1.0157 Å) recorded in situ
during the reduction of 0.5Mn0.5Zr with hydrogen in the temperature
range from 275 to 400 °C.
The reduction of 0.3Mn0.7Zr in hydrogen was additionally
studied in situ using near ambient pressure XPS. The obtained
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3
4
relative concentrations (atomic ratios) of Zr and Mn in the cesses. The Mn 2p3/2 binding energy of 642.4 eV corresponds
4
+
surface layer of 0.3Mn0.7Zr, as well as the Zr 3d5/2 and Mn 2p3/2 to manganese in the Mn state. For comparison, MnO,
binding energies are shown in Table 3. Fig. 8 shows the Zr 3d Mn , and MnO are characterized by the Mn 2p3/2 binding
and Mn 2p core-level spectra that demonstrate the change in energies in the range of 640.6–641.7,
2
O
3
2
3
5–40
35–38,40,41
641.7–641.9,
respectively. The reduction in
The Zr 3d spectra demonstrate that zirconium, even after the hydrogen at 350 °C and above leads to a shift of the Mn 2p
3
5–37,39–41
the chemical composition of this sample during the reduction. and 641.9–642.6 eV,
4
+
treatment in hydrogen at 620 °C, remains in the state Zr . As spectrum to lower binding energies. Considering the shift of
is well known, the Zr 3d spectrum is a doublet Zr 3d5/2–Zr 3d3/2 the Mn 2p3/2 peak (641.5–641.6 eV) and the presence in the
The integral intensities of its components relate as 3 : 2, spectra of intense “shake up” satellites that are typical of
.
2
+ 35,36,42
and the spin–orbit splitting (the difference between the Zr 3d3/2 Mn ,
one can assume that manganese in the structure
4
+
and Zr 3d5/2 binding energies) is 2.43 eV. In our case, all the of the solid solution Mn Zr O is initially in the state Mn ;
Zr 3d spectra are well described with one doublet with the Zr 3d5/2 however, then it reduces to Mn at 350 °C. These data confirm
binding energy ranging from 182.2 to 182.5 eV (Table 3), our supposition that the low-temperature peaks of hydrogen
y
1−y 2
2
+
4
+
which corresponds to zirconium in the Zr state. The stoichio- absorption with a maximum in the range 320–340 °C
metric oxide ZrO is characterized by the Zr 3d5/2 binding (Fig. 3), which were observed for the samples 0.12Mn0.88Zr,
energy in the range of 182.2–183.3 eV.
The shape and position of the Mn 2p spectra significantly manganese cations in the lattice of Mn Zr O . Note that with
2
3
0–33
0.3Mn0.7Zr, and 0.4Mn0.6Zr, correspond to the reduction of
y
1−y 2
change during the reduction, which certainly indicates a an increase in the reduction temperature, the Zr 3d5/2 peak
change in the chemical state of Mn. The Mn 2p spectrum shifts toward higher binding energies by 0.4 eV. Earlier, a
obtained ex situ in a vacuum at room temperature is a doublet similar shift was observed after annealing a ZrO film in a
2
3
1
Mn 2p3/2–Mn 2p1/2 with the component ratio of 2 : 1 and with vacuum at 600 °C. Most likely, this shift is related to the
the spin–orbit splitting of 11.8 eV. The doublet peaks have an partial removal of oxygen from the lattice of the solid solution.
asymmetrical shape, which results from multielectron pro- Indeed, an increase in the reduction temperature from 350 to
6
1
20 °C leads to an increase in the Zr 3d5/2 binding energy from
82.3 to 182.5 eV and to a decrease in the atomic ratio [O]/[Zr +
Mn] from 2.04 to 1.90. At this, the Mn 2p3/2 binding energy
does not change, which means that reduction indeed leads to
the formation of oxygen vacancies in the structure of the solid
solution Mn Zr O .
Table 3 Atomic ratios of elements in the surface layer of the sample
.3Mn0.7Zr and the Zr 3d5/2 and Mn 2p3/2 binding energies (eV)
observed during reduction
0
y
1−y 2
T, °C
[Mn]/[Zr]
[O]/[Zr + Mn]
Zr 3d5/2
Mn 2p3/2
The atomic ratio [Mn]/[Zr] at temperatures between 30 and
5
00 °C varies in a narrow range of 0.19–0.23, whereas the
a
3
1
3
5
6
0
0.22
0.21
0.19
0.23
0.31
2.22
2.09
2.04
1.97
1.90
182.2
—
182.3
182.4
182.5
642.4
—
641.5
641.5
641.6
further heating to 620 °C leads to a significant increase in the
atomic ratio to 0.31. According to the XPS data, a change in
the atomic ratio [Mn]/[Zr] on the surface indicates that the
reduction at high temperatures leads to the segregation of Mn
on the surface of the solid solution. Consequently, the high-
temperature peaks of hydrogen absorption (Fig. 3) are
determined by the segregation and reduction of manganese
cations.
40
50
00
20
a
Ex situ analysis of the as-prepared sample in a vacuum.
Reduction mechanism
On the basis of these data, we suggested a mechanism for the
reduction of mixed Mn–Zr oxides (Fig. 9). With the example of
0
.3Mn0.7Zr (Mn0.3Zr0.7O ), let us consider how the reduction
2
y 2
of the solid solutions Mn Zr1−yO proceeds. According to the
TPR data (Fig. 3) and the data on the change in the lattice
parameter (Fig. 7), the reduction of the solid solutions pro-
ceeds in two steps. The XPS results indicate that the initial
state of manganese in the solid solution mainly corresponds
4
+
3+
to Mn at the surface. One can assume that the Mn ions are
in the bulk, because for “pure” manganese oxides, Mn is
2 3
O
formed under the same synthesis conditions. In the first stage,
at temperatures of 100–500 °C, manganese cations undergo
2
+
reduction to Mn , whose presence is confirmed by XPS.
However, the presence of the second TPR peak indicates that
not all manganese cations are reduced to Mn in the first
Fig. 8 Normalized Zr 3d and Mn 2p core-level spectra of 0.3Mn0.7Zr
obtained in a vacuum at room temperature (RT) and during reduction in
2
+
H
2
at 350, 500, and 620 °C, respectively.
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Zr. The color of the samples was dark brown. Hereinafter, the
samples are referred to as xMn(1 − x)Zr, where x is the portion
of Mn cations.
The specific surface areas (SBET) of the samples were deter-
mined by the Brunauer–Emmett–Teller method from adsorp-
tion of
2
N at 180 mbar measured at liquid nitrogen
temperature using a Sorpty-1750 apparatus (Carlo Erba). The
obtained data were within a relative error of 4%.
The as-prepared samples were characterized ex situ by a
powder XRD technique using a D8 Advance diffractometer
(
Bruker) equipped with a Lynxeye linear detector. The ex situ
XRD patterns were obtained in the 2θ range from 15° to 65°
with a step of 0.05° using monochromatic Cu K radiation (λ =
.5418 Å).
TEM images were obtained with the use of
α
1
a
JEM-2010 microscope (JEOL, Japan) with a resolution of 1.4 Å.
EDX analysis was carried out using an energy dispersive
spectrometer with a Si (Li) detector and an energy resolution
of 130 eV.
Fig. 9 Mechanism for the reduction and re-oxidation of the solid solu-
tion Mn0.3Zr0.7O .
2
The temperature-programmed reduction in hydrogen
(
2
TPR-H ) was performed with 100 mg of a sample in a quartz
reactor using a flow setup equipped with a thermal conduc-
tivity detector. The reducing mixture (10 vol% of H in Ar) was
2
fed at 40 mL min− . The rate of heating from room tempera-
1
2
stage. The lattice parameter of Mn0.3Zr0.7O in this case varies
because of changes in the oxidation state of manganese
cations in the bulk of the solid solution. In the second stage,
ture to 900 °C was approximately 10 °C min−
1
.
at temperatures of 500–650 °C, manganese cations exit from In situ XRD measurements
the bulk of the solid solution and segregate on its surface
The reduction of the catalysts was studied in situ using syn-
chrotron radiation at the Siberian Synchrotron and Terahertz
Radiation Center (Novosibirsk, Russia). The in situ diffracto-
(Table 3). The lattice parameter at this stage increases to
5
.074 Å because of the decrease in the number of Mn cations
in the oxide. The partial exit of Mn from the bulk of the solid
solution is also confirmed by XRD in the experiments on the
re-oxidation of the pre-reduced sample after which the lattice
parameter does not return to the initial value of 5.003 Å but
becomes equal to 5.053 Å.
At high concentrations of manganese, a part of the Mn
cations in xMn(1 − x)Zr are not incorporated into the structure
of the solid solution and form oxide particles of crystallized
meter was equipped with
a high-temperature reactor
chamber XRK-900 (Anton Paar). A sample was loaded into the
reactor on an open holder, allowing hydrogen to pass through
the sample volume. The chamber was mounted on the diffracto-
meter so that the monochromatic synchrotron radiation
beam was incident on the sample surface at an angle of
approximately 15°. The in situ diffraction patterns were
recorded in the 2θ ranges from 15° to 46° for 0Mn1Zr and
2 3 3 4 x
phases Mn O and Mn O and an amorphized phase MnO .
The reduction of Mn in these states proceeds reversibly via the
0.3Mn0.7Zr and from 31° to 62° for 0.5Mn0.5Zr with an acqui-
sition time of 1 min. The wavelength of synchrotron radiation
was 1.7273 and 1.0157 Å, respectively. The samples were
reduced at atmospheric pressure under a flow of H diluted
2
successive change in the oxidation state of the Mn cations:
2
8
2 2 3 3 4
MnO → Mn O → Mn O → MnO.
−1
with He. The heating rate was approximately 5 °C min ; the
total flow rate was 150 mL min−
1
.
Experimental section
Sample preparation and characterization
In situ XPS measurements
Samples were synthesized by calcination of the corresponding In situ XPS experiments were performed at the ISISS (Innova-
hydroxides, which were prepared by precipitation from a joint tive Station for In Situ Spectroscopy) beamline in the synchro-
solution of nitrates ZrO(NO3)2 and Mn(NO ) . The precipi- tron radiation facility BESSY II (Berlin, Germany). The
3 2
1
9
tation was carried out under constant mixing at 550 rpm via experimental station was described in detail elsewhere. In
gradual addition of an NH OH solution until pH = 10. The short, this station is equipped with an electron energy analyzer
resulting precipitate was filtered, washed with water until pH = PHOIBOS-150 (SPECS Surface Nano Analysis GmbH), a gas
, and dried at 120 °C. The powder was ground in a mortar cell, and a system of electron lenses. The lens system was com-
and calcined at 650 °C in air for 4 h. Using this method, bined with three differential pumping stages that provided
samples were synthesized with different amounts of Mn and UHV conditions in the electron energy analyzer even when the
4
6
9
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pressure in the gas cell was 10 mbar. The high brilliance of the
synchrotron radiation combined with a short travel length of
Acknowledgements
the photoelectrons through a “high-pressure” zone in the gas This work was supported by the Russian Science Foundation
cell allowed us to obtain high-quality core-level spectra under (Research project no. 14-23-00037). The authors thank
flow conditions. In these experiments, the Zr 3d, Mn 2p, O 1s, V. A. Rogov for TPR-H measurements and E. Gerasimov for
2
and C 1s core-level spectra of 0.3Mn0.7Zr were recorded under TEM experiments and gratefully acknowledge A. Yu. Klyushin,
a H
2
flow at 140, 350, 500, and 620 °C. The total pressure of H
2
Dr M. Hävecker, and Dr Axel Knop-Gericke for fruitful discus-
was 0.5 mbar. All the spectra were obtained with a photon sions and their assistance in carrying out in situ XPS experi-
energy of 860 eV. Atomic ratios [Mn]/[Zr] and [O]/[Zr + Mn] ments. The authors also appreciate the support of the staff of
were calculated on the basis of total intensities of the Zr 3d, BESSY-II during the beam time.
Mn 2p, and O 1s spectra normalized to the ring current and
2
0
cross-sections published elsewhere taking into account the
XPS analysis depth.
Because the in situ XPS experiments at room temperature
were heavily hindered with a strong charge effect, the chemical
state of the as-prepared 0.3Mn0.7Zr catalyst was studied using
an X-ray photoelectron spectrometer (SPECS Surface Nano Ana-
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1
2
2
2
2
3
2
1
2
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