Inorganic Chemistry
Article
with temperature, as reported by Pineau and co-workers.14 The
reduction of Fe2O3 to metallic iron proceeds via two-step
Fe2O3 → Fe3O4 → Fe or three-step Fe2O3 → Fe3O4 → FeO
→ Fe mechanisms depending on the temperature and H2O/H2
ratio and could be affected by the formation of metastable FeO
intermediate phases.13−15 The introduction of additives can
lead to the formation of mixed oxides, the reduction
mechanism of which is different from that for simple oxides.
According to the literature studies, the addition of Cr,16 Ce,17
and Al17 to hematite increases the transformation temperature
of Fe2O3 to Fe3O4, whereas the addition of Ru, Os, Ag, Au,18
and Cu17 leads to a decrease in the reduction temperature.
Zhao and co-workers19 found that the doping with Ca
increases the reduction rate of Fe2O3; especially Ca promotes
the phase transition of FeO to metallic iron. Addition of Cu
significantly decreases the reduction temperature of hematite
to magnetite (Fe2O3 → Fe3O4).17
It should be noted that the research on the reduction of
catalysts was mostly carried out using temperature-pro-
grammed reduction by hydrogen (TPR-H2), which does not
show the real state of the catalyst, i.e., the phase composition
and chemistry under reaction conditions. Certainly, this
approach can be successfully used for studying pure iron
oxides when the main routes of the reduction are predictable.
However, the addition of dopants can lead to the formation of
different types of solid solutions with iron oxide, the
interaction of additives, the formation of new simple and
mixed oxides, and metal nanoparticles at various stages of
reduction, which can result in the acceleration or inhibition of
the reduction reactions. In addition, there is a difference in the
reduction by hydrogen or CO: the involvement of CO as a
reducing agent may also produce iron carbide in an excessive
CO environment. In order to obtain more detailed information
about the reduction mechanism, another approach should be
used: the approach based on the application of in situ
techniques that are sensitive to both the chemical state of
cations and the phase compositions. Earlier, such an approach
based on a combined use of TPR-H2, in situ X-ray diffraction,
and X-ray photoelectron spectroscopy was applied for
investigation of reduction of mixed Mn−Zr and Mn−Ga
oxides.20,21
was calcined in air at 450 °C for 1 h and then at 700 °C for 1 h. Three
nanocomposite catalysts with the following compositions were
preparedCu10Fe74Al16, Cu5Fe78Al17, and Fe82Al18where the
indexes correspond to the CuO, Fe2O3, and Al2O3 content (wt %),
respectively. Reference samples (α-Fe2O3 and CuO) were prepared in
the same way. The samples were characterized by XRD, XANES,
TPR-CO, the nitrogen adsorption technique, and transmission
electron microscopy (TEM).
Temperature-programmed reduction was carried out in a 10% CO/
He flow (20 sccm) using a ChemBET Pulsar TPR/TPD analyzer
(QuantaChrome Instruments, USA). Before an experiment, each
sample (about 70 mg) was dried in a He flow at 150 °C for 20 min.
During the experiment, the sample was heated with a constant heating
rate (20 °C/min) from the ambient temperature to 1000 °C. The CO
consumption was measured with a thermal conductivity detector. To
exclude the effect of evolving CO2 on the detector reading, a NaOH
trap was placed between the reactor and the detector. All TPR-CO
profiles were normalized to the sample weight.
The phase composition was studied by XRD using a D8 Advance
diffractometer (Bruker Corp., Germany) equipped with a Lynxeye
linear detector. The diffraction patterns were obtained using the Ni-
filtered Cu Kα radiation (λ = 1.5418 Å) in the θ/2θ configuration.
The phases were identified using the powder diffraction database
PDF-4+. The crystallite size was estimated by the Scherrer formula.24
The quantitative content of phases in the samples was found by the
Rietveld method using the TOPAS program.
An in situ X-ray diffraction study was carried out in a flow of 1%
CO in He using the same D8 Advance diffractometer equipped with a
reactor chamber XRK-900 (Anton Paar GmbH, Austria). The total
flow rate was 100 sccm. XRD patterns were collected stepwise in the
temperature range from 30 to 700 °C with a step of 50 °C; the
heating rate was 12 °C/min. The 2θ range from 21 to 60° was
scanned using a step of 0.05°.
The XANES study was performed at the Structural Materials
Science beamline at the Kurchatov Synchrotron Radiation Source
(National Research Center “Kurchatov Institute”, Moscow, Russia).
The experimental station was described in detail elsewhere.25 The
spectra were obtained at the Cu K-edge in the transmission mode
using a channel-cut Si(111) monochromator. Powder samples were
pressed into thin self-supporting pellets, mounted to a stainless steel
holder and then placed in a custom reaction chamber for in situ
XANES measurements. To obtain an appropriate X-ray absorption,
the samples were diluted with a fine powder of hexagonal BN. The
reduction process was studied in a flow of 5% CO in N2 at
atmospheric pressure in the temperature range from the ambient
temperature to 600 °C by the stepwise manner. Two custom-made
gridded ionization chambers, filled with appropriate N2−Ar mixtures,
were used as detectors. The ionization currents were measured by
Keithley 6487 digital picoamperemeters (Keithley Instruments LLC,
USA). The energy scale was calibrated using the first inflection point
of the Cu K-edge spectra at 8979 eV. The obtained data were
analyzed using the ATHENA software.26
The specific surface area of the catalysts was determined by the
Brunauer−Emmett−Teller (BET) method using nitrogen adsorption
isotherms measured at the liquid nitrogen temperature with an
automatic volumetric adsorption unit ASAP 2400 (Micromeritics
Instrument Corp., USA).
TEM images were obtained using a JEM-2010 microscope (JEOL
Ltd., Japan) at an accelerating voltage of 200 kV and a lattice
resolution of 1.4 Å. Energy dispersive X-ray (EDX) analysis was
carried out using an energy dispersive spectrometer with a Si XFlash
detector (Bruker Corp., Germany) with an energy resolution of 128
eV.
In this work, we use temperature-programmed reduction by
CO (TPR-CO), XRD, and XANES spectroscopy to study in
situ the behavior of CuO, Fe2O3, Al2O3−Fe2O3, and CuO−
Al2O3−Fe2O3 nanocomposite catalysts during the reduction in
CO. XRD provides information about the crystallographic
structure and the phase composition, whereas XANES reveals
the local structure and the oxidation state of cations. CuO−
Al2O3−Fe2O3 has been chosen as the object of study because
catalysts of such a type are actively used in the high-
temperature WGS reaction and in the oxidation of
CO.10,22,23 To elucidate the role of the promoters (Cu and
Al) in the reduction process, we investigated the reduction of
the CuO−Al2O3−Fe2O3 nanocomposite catalysts by CO and
compared the obtained results with the reduction of pure
oxides (CuO and Fe2O3) and the Al2O3−Fe2O3 catalyst.
2. EXPERIMENTAL SECTION
3. RESULTS
The catalysts were prepared as follows: the salts of the precursors
(Fe(NO3)3·9H2O, Al(NO3)3·9H2O, and Cu(NO3)2·3H2O) were
mixed in the required ratios; then, the mixture was heated to give a
homogeneous melt of salt hydrates and was kept at a temperature of
200 °C until water was completely removed; finally, the solid residue
3.1. As-Prepared Samples. Three as-prepared Fe-based
nanocomposites and pure CuO and Fe2O3 have been studied
by XRD, TEM, EDX, and the nitrogen adsorption technique.
The main data are summarized in Table 1. The CuO sample is
B
Inorg. Chem. XXXX, XXX, XXX−XXX