Iron modified titanium-hafnium binary oxides as catalysts in total oxidation of ethyl acetate

Article abstract of DOI:10.1016/j.catcom.2016.03.014

Multicomponent iron-titanium-hafnium oxide materials with different compositions were prepared by combination of homogeneous precipitation with urea and incipient wetness impregnation techniques and tested as catalysts for ethyl acetate oxidation as representative VOCs. Nitrogen physisorption, XRD, Raman, UV-Vis, XPS, M?ssbauer spectroscopy and TPR analyses reveal co-existence of substituted Fe_xTi_{1 - x}O₂ oxide, finely dispersed iron oxide species with supper paramagnetic behavior and well crystallized α-Fe₂O₃ particles, which relative part depends on hafnium content in titania lattice. The effect of phase composition on the catalytic behavior of these materials in ethyl acetate oxidation was discussed.

Full text of DOI:10.1016/j.catcom.2016.03.014

Catalysis Communications 81 (2016) 14–19
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Iron modified titanium–hafnium binary oxides as catalysts in total
oxidation of ethyl acetate
a
b
c
d
Tanya Tsoncheva a, , Radostina Ivanova , Jiří Henych , Nikolay Velinov , Martin Kormunda ,
⁎
Momtchil Dimitrov a, Daniela Paneva c, Michaela Slušná b,e, Ivan Mitov c, Vaclav Štengl b
Institute of Organic Chemistry with Centre of Phytochemistry, BAS, Bulgaria
Materials Chemistry Department, Institute of Inorganic Chemistry AS CR v.v.i., 25068 Řež, Czech Republic
a
b
c
Institute of Catalysis, BAS, Sofia, Bulgaria
d
Faculty of Sciences, University of Jan Evangelista Purkyne, Ceske Mladeze 8, 400 96 Usti nad Labem, Czech Republic
Faculty of Environment, University of Jan Evangelista Purkyne, Kralova Vysina 7, 400 96 Usti nad Labem, Czech Republic
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Multicomponent iron–titanium–hafnium oxide materials with different compositions were prepared by combi-
nation of homogeneous precipitation with urea and incipient wetness impregnation techniques and tested as
catalysts for ethyl acetate oxidation as representative VOCs. Nitrogen physisorption, XRD, Raman, UV–Vis, XPS,
Mössbauer spectroscopy and TPR analyses reveal co-existence of substituted FexTi1 − xO2 oxide, finely dispersed
iron oxide species with supper paramagnetic behavior and well crystallized α-Fe2O3 particles, which relative part
depends on hafnium content in titania lattice. The effect of phase composition on the catalytic behavior of these
materials in ethyl acetate oxidation was discussed.
Received 6 January 2016
Received in revised form 10 March 2016
Accepted 24 March 2016
Available online 17 April 2016
Keywords:
Titania–hafnia binary oxides
Iron modifications
Support effect
© 2016 Elsevier B.V. All rights reserved.
Ethyl acetate oxydation
1. Introduction
The aim of current investigation is to demonstrate the possibility to
control the state of supported on titania–hafnia binary oxides iron spe-
The knowledge of the specific effects within the multi-component
nanostructured metal oxides is prerequisite for the optimization of
their properties. Recently, titanium oxide has received much attention
due to its superior optical, electrical, mechanical and catalytic properties
combined with non-toxicity and cost effectiveness [1]. The introduction
of dopant into TiO2 lattice may significantly affect the electronic band
edges or introduce impurity states in the band gap [2]. Formation of
FexTi1 − xO2 [3,4], mixture of FexTi1 − xO2 and superparamagnetic he-
matite particles [5] or mixture of FexTi1 − xO2 and pseudo brookite
Fe2TiO5 phases [6] were registered after TiO2 doping with iron. Segrega-
tion of α-Fe2O3 phase was reported with the increasing of iron content
up to 10% in [7], while Bonamali et al. [8] did not observed its formation
even at 50 wt% Fe. However, to the best of our knowledge, there are only
few reports on hafnium-doped TiO2. Using density functional theory,
Lezhong et al. [9] reported that Hf incorporation in TiO2 leads to
narrower band gap, but no experimental evidence has been still report-
ed. No data for the multi-component Ti–Hf–Fe oxide system are still
available.
cies by simple variation of the support composition. Pioneer investiga-
tions on the catalytic behavior of these materials in total oxidation of
ethyl acetate as representative VOCs are carried out.
2. Experimental
2.1. Materials
Hafnium-doped titania samples were prepared by homogeneous
hydrolysis of aqueous solution of TiOSO4 and HfOSO4 with urea as a pre-
cipitation agent according to the procedure described in [10–12]. Typi-
cally, 100 g of TiOSO4 were dissolved in 1 L hot water acidified with
10 ml 98% H2SO4. After dilution in 4 L distillated water, HfOSO4 was
added for the preparation of binary materials. The pH of the initial solu-
tion of TiOSO4 and HfOSO4 was 2–4. Then, the solution was mixed with
400 g urea and the mixture was heated at 373 K for 6 h. During the
heating, the urea started to decompose and the pH of the solution in-
creased gradually. At the end of the precipitation procedure the pH of
the solution became neutral or slightly alkaline pH (7–8). Iron modifica-
tions (12 wt%Fe) were obtained by incipient wetness impregnation of
thus obtained composites using 0.2 M aqueous solution of Fe
(NO3)3·9H2O. The impregnated samples were dried at room tempera-
ture for 24 h and then, treated in air at 773 K for 2 h for precursor
⁎
Corresponding author at: Institute of Organic Chemistry with Centre of
Phytochemistry, BAS, Sofia, Ak.G. Bontchev str.bl.9, Bulgaria
E-mail address: tsoncheva@orgchm.bas.bg (T. Tsoncheva).
http://dx.doi.org/10.1016/j.catcom.2016.03.014
1566-7367/© 2016 Elsevier B.V. All rights reserved.

T. Tsoncheva et al. / Catalysis Communications 81 (2016) 14–19
15
Table 1
Samples composition, specific surface area (SBET), total pore volume (Vt) and specific activity (SA) for TiO2, binary TiO2-HfO2 oxides and their iron modifications.
Sample
Support composition, wt%
Fe, wt%
SBET
Vt,
SA,
m2 g−1
cm3g−1
%m−2 g ∗ 102
Ti
Hf
0.0
O
Hf/Ti + Hf
TiO2
HfTi(0.8)
HfTi(1.8)
HfTi(9.9)
60.0
59.6
59.2
55.6
53.4
43.5
40.0
60.0
59.6
59.2
55.6
53.4
43.5
40.0
40.0
39.9
39.7
38.3
37.3
33.3
31.8
40.0
39.9
39.7
38.3
37.3
33.3
31.8
0.23
0.22
0.23
0.22
0.24
0.32
0.24
0.15
0.15
0.14
0.15
0.16
0.18
0.17
319
300
326
307
306
488
463
131
119
115
112
114
118
145
15
11
10
13
11
7
0.5
1.1
6.1
0.8
1.8
9.9
14.9
34.8
41.3
HfTi(14.9)
HfTi(34.8)
HfTi(41.3)
Fe/TiO2
Fe/HfTi(0.8)
Fe/HfTi(1.8)
Fe/HfTi(9.9)
Fe/HfTi(14.9)
Fe/HfTi(34.8)
Fe/HfTi(41.3)
9.3
23.2
28.2
0.0
7
12
12
12
12
12
12
12
45
34
54
48
47
46
37
0.5
1.1
6.1
9.3
0.8
1.8
9.9
14.9
34.8
41.3
23.2
28.2
decomposition. The samples were denoted as HfTi(x) and Fe/HfTi(x) for
the parent oxides and their iron modifications, respectively, where x is
Hf/Hf + Ti ratio (wt%) and data for their composition are listed in
Table 1.
absorption in the 600–500 cm−1 range for all Hf doped materials
could be carefully assigned to the presence of TiHfO4 structures. The Ti
2p core level spectra (Fig. 2) represent highly resolved peaks at binding
energy of 458.5 eV and split about 5.7 eV which is assigned to Ti\\O
bonds in TiO2 [16]. The second component at BE of 456.9 eV represents
titanium ions in Ti2O3. Note the almost linear increase in the Ti3+ con-
tent with the increase of Hf amount which could be due to homoge-
neous incorporation of Hf in titania with the formation of defects
(Table 3, Fig. S3). The Hf 4f spectra for all materials (Fig. S4) represent
peaks at BE of 19 and 17 eV, which is assigned to Hf4+ ions. For all Hf
doped titania samples the calculated Hf/Hf + Ti ratio overcomes the ex-
pected nominal one (Table 3) indicating high degree of exposure of Hf
ions at the surface. On the base of this observation partial segregation
of finely dispersed HfO2 particles over the titania ones could not be ex-
cluded. The O1s core level spectra (Fig. S5) could be assumed as super-
position of peaks assigned to oxygen in various states, such as H2O
molecules, surface OH\\groups as well as to oxygen anions in Ti–O
and Hf–O structures. The observed higher oxygen content than the the-
oretic one (Table 3) indicates high concentration of hydroxyl groups
which are usually related to oxygen vacancies [17].
2.2. Characterization and catalytic tests
The surface area of the samples was determined from nitrogen
physisorption isotherms using a Coulter SA3100 instrument. Elemental
analysis was performed by a MiniPal 4.0 energy-dispersive X-ray fluores-
cence spectrometer. Powder X-ray diffraction patterns were collected on
a Bruker D8 Advance diffractometer with Cu Kα radiation. The UV–Vis
spectra were recorded on a Jasco V-650 UV–Vis spectrophotometer.
Raman spectra were acquired with a Thermo Fischer Scientific DXR
Raman microscope. The X-ray photoelectron spectroscopy analyses
were measured in high-vacuum chamber equipped with SPECS Xray
XR-50 and SPECS PHOIBOS 100 Hemispheric Analyzer. The Mössbauer
spectra were obtained at room and liquid nitrogen temperature with a
Wissel electromechanical spectrometer. The TPR/TG (temperature-pro-
grammed reduction/thermo-gravimetric) analyses were performed on a
Setaram TG92 instrument in a flow of 50 vol% H2 in Ar (100 cm min−1
)
and heating rate of 5 K min−1
.
The ethyl acetate (EA) oxidation was tested under temperature pro-
grammed regime in a flow type apparatus (1.21 mol% EA in air,
WHSV − 100 h−1) equipped with GC for analyses. The selectivity of
the obtained products was calculated as Si = Yi/X ∗ 100, where Yi was
the yield of (i) product and X was the EA conversion.
Table 2
XRD data for TiO2, various TiO2–HfO2 binary oxides and their iron modifications.
Sample
Phase
D, nm
Strain
a, Å
c, Å
e ∗ 103, a.u.
3. Results and discussion
TiO2
TiO2 (anatase)
TiO2 (anatase)
TiO2 (anatase)
TiO2 (anatase)
TiO2 (anatase)
TiO2 (anatase)
α-Fe2O3 (hematite)
TiO2 (anatase)
α-Fe2O3 (hematite)
TiO2 (anatase)
α-Fe2O3 (hematite)
TiO2 (anatase)
α-Fe2O3 (hematite)
TiO2 (anatase)
α-Fe2O3 (hematite)
TiO2 (anatase)
α-Fe2O3 (hematite)
TiO2 (anatase)
4.3
5.0
4.4
5.1
5.6
4.4
4.4
5.6
4.5
5.9
5.4
5.5
5.3
6.1
4.5
8.4
5.8
9.1
21.6
6.067
6.179
8.299
7.080
6.338
8.273
8.273
6.629
9.042
6.416
2.709
6.787
8.140
5.932
9.049
4.363
7.586
3.223
2.848
3.793
3.802
3.801
3.804
3.806
3.794
5.041
3.795
5.016
3.795
5.006
3.799
5.043
3.804
5.032
3.801
5.043
3.796
5.052
9.518
9.522
9.529
9.545
9.556
9.509
13.821
9.487
13.813
9.491
13.854
9.508
13.865
9.505
13.872
9.522
13.872
9.532
13.763
3.1. Characterization of TiO2–HfO2 binary oxide supports
HfTi(0.8)
HfTi(1.8)
HfTi(9.9)
HfTi(14.9)
Fe/TiO2
XRD pattern of pure titania (not shown) exhibits all reflections typ-
ical of anatase phase (JCPDS 21-1272). The observed increase in the lat-
tice parameters after doping with Hf (Table 2) indicates incorporation of
Hf4+ ions into the titania lattice. The increase of Hf content above 15%
provokes the formation of amorphous phase. These structural changes
are also confirmed by the observed increase in the BET surface area
(Table 1) and with the decrease in the intensity of the main Eg mode
at 149 cm−1 in the Raman spectra (Fig. 1a) [13]. The strong absorption
feature in the UV–Vis spectrum of pure TiO2 (Fig. S1) at ca. 350 nm is
due to d–d electronic transition between Ti4+ ion and O2− ligand in an-
atase. No significant changes in the band gap are observed after titania
doping with hafnia and this is in contrast with the theoretic calculations
reported in [10]. The broad absorption band in the 900–400 cm−1 re-
gion in FTIR spectrum of titania (Fig. S2) could be assigned to Ti–O–Ti
bending vibrations [14]. In accordance with [15] the increased
Fe/HfTi(0.8)
Fe/HfTi(1.8)
Fe/HfTi(9.9)
Fe/HfTi(14.9)
Fe/HfTi(34.8)
Fe/HfTi(41.3)
α-Fe2O3 (hematite)
TiO2 (anatase) S.G.: I41/amd (141) — tetragonal.
α-Fe2O3 (hematite) S.G.: R-3cH (167) — trigonal.

16
T. Tsoncheva et al. / Catalysis Communications 81 (2016) 14–19
Fig. 1. Raman spectra of TiO2 and TiO2–HfO2 binary oxides (a) and UV–Vis spectra (b) of their iron modifications.
3.2. Characterization of iron modified TiO2–HfO2 binary oxides
assigned to superposition of bands originated from iron ions in small
oligonuclear FexOy clusters and bulk Fe2O3 particles.
Nitrogen physisorption measurements demonstrate that the high
temperature treatment during the modification procedure strongly de-
creases the specific surface area, which for all iron modifications varies
in the 110–145 m2 g−1 range (Table 1). XRD patterns indicate presence
of anatase phase with particle size of about 5–9 nm (Table 2). Reflec-
tions, typical of hematite phase (JCPDS 33-0664) with average crystal-
lite size of 4–22 nm, are also detected. The observed decrease in the
unit cell parameters for all iron modifications could be attributed to
the substitution of Ti4+ ions by smaller Fe3+ ions in anatase lattice.
The incorporation of Fe3+ ions is also confirmed by the red shift of the
absorption band in the UV–Vis spectra (Fig. 1b) [8]. In agreement with
XRD data, the increased absorption in the 300–600 nm region can be
Room temperature Mössbauer spectra of all iron modifications are
well fitted with sextet and doublet components (Fig. S6). The parame-
ters of sextet component (Table 4) are characteristic for α-Fe2O3 with
average crystallite size above 13.5 nm. In accordance with [18] the
Db1 component in the spectra with QS of about 1.21–1.28 mm/s could
be assigned to FexTi1 − xO2. The second doublet component with QS of
0.70–0.78 mm/s can be assigned to smaller α-Fe2O3 particles with
superparamagnetic behavior. This assumption is confirmed by the
Mössbauer spectra, recorded under the temperature of liquid nitrogen
(LNT, Fig. S6, Table 4), where an increase in the relative part of Sx com-
ponent at the expense of a decrease in the Db2 component is detected.
The preservation of a large portion of doublet part in the LNT spectra
Fig. 2. Ti 2p core level spectra of selected samples: (a) TiO2; (b) HfTi(1.8); (c) HfTi(14.9); (d) HfTi(34.8).

T. Tsoncheva et al. / Catalysis Communications 81 (2016) 14–19
17
Table 3
reveals existence of hematite-like particles about 4 nm. Thus, we cannot
fully exclude partial contribution of the shell of these particles [19] in
the Db1 component which renders difficult precise quantitative analy-
ses on the base of relative part of Db1 and Db2. However analyzing
the changes in the relative part of Db and Sx components in the spectra
with the variations in the support composition (Table 4), one can as-
sume that the incorporation of small amount of Hf4+ in titania pro-
motes the formation of substituted FexTi1 − xO2 and finely dispersed
hematite like particles. We can speculate that these hematite particles
form on the interface layer of the substituted FexTi1 − xO2 ones. More-
over, the increase in the degree of occupation of Ti4+ positions by
Hf4+ in binary oxides results in segregation of larger portion of bulk he-
matite phase. Taking into account XPS data (Table 3), where a linear de-
pendence between Ti3+ and Hf was observed, we assume that Fe3+ ions
substitute predominantly Ti4+, but not Ti3+ ions into the solid lattice.
The TPR profiles of iron modifications (Fig. 3) represent one signifi-
cant TPR effect centered at 620–646 K and second, less pronounced
one, above 700 K, which according to [20] could be assigned to step-
wise reduction of hematite to Fe. The shift of the reduction peaks to
lower temperature for all Hf modified materials as compared to pure
TiO2, most pronounced for Fe/HfTi(1.8), indicates presence of finely dis-
persed Fe2O3 particles. The increase in the high temperature TPR effect
for the samples with high Hf content confirms the increase in the por-
tion of larger hematite particles. Note that the overall reduction degree
for all materials is 42–65% (Table S1). In accordance with Mössbauer
data (Table 4), the observed increase in the reduction degree with Hf
content increase (Table S1) indicates that these hardly reducible iron
ions are mainly included into substituted FexTi1 − xO2 structures.
Surface composition (at.%) of selected modifications on the base of XPS analyses (in
brackets wt%).
Sample
C
Hf
O
Ti
Ti3+/Ti3+
Ti4+,%
+
Hf/Ti + Hf O/Ti +
Hf
TiO2
HfTi(1.8)
12.38
0
65.01 22.61 11.39
2.87
10.23 0.28* 65.72 23.77 14.67
1.17(4.4)
2.73
HfTi(14.9) 10.38 1.50 66.39 21.73 15.67
HfTi(34.8) 20.21 3.73 58.57 17.49 21.88
6.45(25.7) 2.85
17.58(44.3) 2.76
Table 4
Mössbauer parametra of iron modifications of TiO2 and TiO2–HfO2 binary oxides.
Sample
Components IS,
mm/s
QS,
mm/s
Heff
T
,
FWHM,
mm/s
G,
%
Fe/TiO2
Sx-α-Fe2O3 0.37
Db1
Db2
Sx-α-Fe2O3 0.35
Db1
Db2
Sx-α-Fe2O3 0.37
Db1
Db2
Sx-α-Fe2O3 0.37
Db1
Db2
Sx-α-Fe2O3 0.45
Db1
Db2
Sx-α-Fe2O3 0.36
Db1
Db2
Sx-α-Fe2O3 0.38
Db1
Db2
Sx-α-Fe2O3 0.39
Db1
Db2
−0.24
1.27
0.73
−0.18
1.26
0.73
−0.24
1.26
0.73
−0.24
1.24
0.71
0.38
1.24
0.72
−0.22
1.21
0.70
−0.20
1.28
0.76
−0.22
1.26
51.5
–
–
51.3
–
–
51.2
–
–
51.1
–
–
53.5
–
–
51.2
–
–
51.1
–
–
51.0
–
–
0.36
0.49
0.46
0.45
0.48
0.46
0.34
0.50
0.46
0.35
0.50
0.46
0.40
0.48
0.41
0.37
0.49
0.42
0.38
0.45
0.43
0.37
0.50
0.41
13
41
46
6
43
51
9
44
47
13
44
43
15
48
37
20
44
37
28
31
41
45
31
24
0.31
0.34
Fe/HfTi(0.8)
Fe/HfTi(1.8)
Fe/HfTi(9.9)
0.30
0.34
0.30
0.33
0.31
0.34
Fe/HfTi(9.9)
(LNT)
0.41
0.44
Fe/HfTi(14.9)
Fe/HfTi(34.8)
Fe/HfTi(41.3)
3.3. Catalytic properties
0.31
0.34
In Fig. S7 are presented temperature profiles of ethyl acetate oxida-
tion on various Ti–Hf supports and their iron modifications. In order to
simplify the comparison between various materials, the temperature
dependences only for selected samples are presented in Fig. 4a and
the temperature at which 50% conversion (T50) is achieved for all mate-
rials are shown in Fig. 4b. Acetaldehyde (AA), ethanol (Et), ethene (C2)
and acetic acid (AcAc) were found as by-products and their distribution
0.33
0.36
0.34
0.36
0.78
Isomer shift (IS), quadruple splitting (QS), the effective internal magnetic field (Heff), the
line widths (FWHM), and the relative weight (G).
Fig. 3. TPR–TG (a) and TPR–DTG (b) profiles of iron modifications of TiO2 and TiO2–HfO2.

18
T. Tsoncheva et al. / Catalysis Communications 81 (2016) 14–19
Fig. 4. Temperature dependencies of ethyl acetate conversion for selected catalysts (a) and effect of Hf content on the catalytic activity of various binaryoxides and their iron modifications,
represented as the temperature at which 50% conversion is achieved (b).
Fig. 5. Selectivity to CO2 and various by-products (acetaldehyde (AA),ethanol (Et), acetic acid (AcAc) and ethylene (C2)) for TiO2 and different TiO2–HfO2 oxides (left) and their iron
modifications (right) at 45% conversion.

T. Tsoncheva et al. / Catalysis Communications 81 (2016) 14–19
19
at 45% conversion is compared in Fig. 5. Pure titania and its hafnia mod-
ifications represent about 80% conversion above 650 K (Fig. S7). Catalyt-
ic activity decrease combined with an increase in the AcAc and C2
selectivity is observed after titania doping with small amount of Hf
(Figs. 4 and 5). The observed slight increase in the catalytic activity of
the samples with higher Hf content could be assign to the increase in
the specific surface area of the samples (Tables 1 and 2). In order to pre-
cise the discussion and to exclude the role of texture parameters on the
catalytic activity, the specific catalytic activity per unit surface area (SA)
was calculated (Table 1). For all binary oxides a decrease in SA as com-
pared to pure TiO2 is observed. However, no clear relation between the
changes in SA with Hf increase in the samples is observed. This could be
due to lack of homogeneity in the catalytic sites in binary materials. On
the base of XPS analyses, we can assume that this could be due to the
formation of a mixture of various metal oxide phases, such as TiO2,
HfO2 and TiO2–HfO2, which proportion varies with Hf/Ti ratio. It
seems also that the facilitated formation of Ti3+ defects after Hf incor-
poration decreases the number of active Ti4+–Ti3+ redox pairs.
project is acknowledged. This work was also supported by Large Infra-
structures for Research, Experimental Development and Innovation -
NanoEnviCz – Nanomaterials and Nanotechnologies for Environmental
Protection and Sustainable Future No. LM2015073. The authors thank
to Z. Hájková for Raman measurements.
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The modification of TiO2 and TiO2–HfO2 composites with iron
strongly increases both catalytic activity (Fig. 4, Fig. S7) and CO2 selec-
tivity (Fig. 5). As compared to the commercial copper catalyst (Fig. S7)
and the data reported in the literature (Table S2) these materials repre-
sent significantly high catalytic activity. The observed about 4–5-fold in-
crease in the SA after the modification of Ti–Hf oxides with iron clearly
indicate its decisive participation in the ethyl acetate oxidation. Note the
highest catalytic activity which was registered for Fe/HfTi(1.8), where a
big portion of particles with superparamagnetic behavior was observed
by Mössbauer analysis (Table 4) and also absence of any simple relation
between the iron phase composition and the specific activity of the
samples. These features urge the authors to assume different contribu-
tions of various iron species in the ethyl acetate oxidation. Taking into
account that the ethyl acetate oxidation realizes by Mars van Krevelen
mechanism [21], where the release of oxygen from the metal oxide lat-
tice is of primary importance, and also on the base of TPR analysis, we
assume the facilitated effect of the formation of finely dispersed hema-
tite particles. The combined Mössbauer and TPR data clearly demon-
strate that the increase in the dispersion of hematite particles is
promoted by the formation of hardly reducible and probably low active,
FexTi1 − xO2 layer. The proportion of various iron species, and the relat-
ed catalytic properties, could be simply regulated by Hf incorporation in
titania lattice. In order to precise the role of various species in this com-
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The state of iron species supported on titania–hafnia binary oxides
could be easily controlled by support composition. Small amount of Hf
in titania facilitates the formation of substituted FexTi1 − xO2 and finely
dispersed hematite particles, while the increase of hafnium content pro-
motes segregation of larger hematite particles. The catalytic activity in
ethyl acetate oxidation is in a complex relation with sample composi-
tion. The best catalytic activity is achieved for iron modification of tita-
nia doped with 1.8% Hf, where the presence of relatively large portion
of easily reducible hematite particles is observed.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2016.03.014.
Acknowledgments
Financial support of project DFNI E02/2/2014, and Joint research
project between ASCR and BAS and Bulgarian-Italian BAS-CNR bilateral