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methanol (200 mL) and subsequently dried at 608C for 12 h. Simi-
larly, a mixture of 0.1m nickel acetate, 0.05m manganese acetate
and 0.1m ammonium oxalate produced Ni0.66Mn0.34C2O4·2H2O. For
NiC2O4·2H2O, only 0.1m nickel acetate and 0.1m ammonium oxa-
late were used.
amount of the sample powder was placed on a TEM grid (carbon
film on 300 mesh Cu grid, Plano GmbH, Wetzlar, Germany). The mi-
crostructure (morphology, particle size, phase composition, crystal-
linity) of the samples was studied with a FEI Tecnai G2 20 S-TWIN
transmission electron microscope (FEI Company, Eindhoven, Neth-
erlands) equipped with a LaB6 source at 200 kV acceleration volt-
age. EDX analysis was carried out with an EDAX r-TEM SUTW De-
tector (Si (Li) detector). Images were recorded with a GATAN MS794
P CCD camera. Both SEM and TEM experiments were carried out at
the Zentrum für Elektronenmikroskopie (ZELMI) of the TU Berlin.
The XPS measurements were performed using a Kratos Axis Ultra
X-ray photoelectron spectrometer (Karatos Analytical Ltd., Man-
chester, UK) using an Al-Ka monochromatic radiation source
(1486.7 eV) with 908 takeoff angle (normal to analyzer). The
vacuum pressure in the analyzing chamber was maintained at 2
10À9 Torr. The XPS spectra were collected for O 1s, Mn 2p and Ni 2p
levels with pass energy 20 eV and step 0.1 eV. The binding energies
were calibrated relative to C 1s peak energy position as 285.0 eV.
Data analyses were performed using Casa XPS (Casa Software Ltd.)
and Vision data processing program (Kratos Analytical Ltd.). BET
surface area measurements were carried out using a five-point N2
adsorption analysis on a Micromeritics Gemini with VacPrep 061. It
was not possible to avoid contact of samples with air at room tem-
perature during transfer to analysis. Temperature -programmed re-
duction (TPR) conditions were chosen according to Monti–Baiker
criteria, with approximately 20 mg of catalyst and 30 mLminÀ1
flow of roughly 15% hydrogen in nitrogen, with a heating rate of
Synthesis of nickel manganese and nickel oxides
All oxalate precursors were heated to 4008C at a rate of 28CminÀ1
in dry synthetic air (20% O2, 80% N2), kept at 4008C for 8 h in
a tubular furnace and then cooled down to ambient temperature
to form Ni2MnO4, Ni6MnO8 and NiO oxide phases from the
Ni0.66Mn0.34C2O4·2H2O, Ni0.85Mn0.15C2O4·2H2O and NiC2O4·2H2O, re-
spectively.
Synthesis of catalysts
As-obtained Ni2MnO4, Ni6MnO8, and NiO were heated in a pure hy-
drogen flow (100% H2) of 15 mLminÀ1 for 158C minÀ1 to 5008C
and held for 1 hour to form Ni2MnO4–O2–H2, Ni6MnO8–O2–H2 and
NiO–O2–H2, The oxalate precursors Ni0.66Mn0.34C2O4·2H2O and
Ni0.85Mn0.15C2O4·2H2O form Ni2MnO4–H2 and Ni6MnO8–H2, respec-
tively.
Synthesis of the standard coprecipitated catalyst
108C minÀ1
.
0.1m manganese acetate and 0.1m nickel acetate was fist dis-
solved in water and an aqueous solution of 0.1m ammonium oxa-
late was then added to form Ni0.85Mn0.15C2O4·2H2O. This co-precipi-
tated oxalate precursor was heated at 4008C for 8 h in a tubular
furnace and cooled down to room temperature to form Ni6MnO8
which was subsequently reduced in a pure hydrogen flow (100%
H2) of 15 mLminÀ1 by heating at 158C minÀ1 to 5008C and holding
for 1 hour.
Catalytic Testing for DRM
All catalytic tests were carried out in a quartz fixed-bed tubular re-
actor, heated by an external electric furnace with a temperature
probe in the catalyst bed. The catalyst was diluted with quartz
sand or silica balls for post-reaction samples, with a total bed
volume of 1.25 mL. Before each experiment, the catalyst was re-
duced in situ with a pure hydrogen flow of 15 NmLminÀ1 by heat-
ing at 158CminÀ1 to 500 C and holding for 1 hour. The reactor
feed consisted of methane, carbon dioxide and helium (as diluent)
in a ratio of 1:1:8 with a total flow of 30 mLminÀ1. All experiments
were carried out at atmospheric pressure. The product gas was an-
alyzed using a gas chromatograph (Agilent 7890A) equipped with
a thermal conductivity detector (TCD) and flame ionization detec-
tor (FID). After reaction, temperature-programmed oxidation (TPO)
was performed on the catalysts to quantify coke deposits. The re-
Catalyst Characterization
Phase identification of the samples was conducted using PXRD on
a Bruker AXS D8 advanced automatic diffractometer equipped
with a position-sensitive detector (PSD) and a curved germani-
um(111) primary monochromator. The radiation used was Cu-Ka
(l=1.5418 ). The XRD profiles recorded were in the range of 58<
2q<808 and the diffraction pattern fitting was carried out using
the program WinxPow. The chemical composition of the precursors
and oxides was confirmed by ICP-AES on a Thermo Jarrell Ash
Trace Scan analyzer. The samples were dissolved in acid solutions
(aqua regia) and the results of three independent measurements
were averaged which showed good agreement with the chemical
formulae. The quantification was also estimated by elemental anal-
yses that were performed on a Flash EA 112 Thermo Finnigan ele-
mental analyzer. The different vibrational modes of the precursor
were studied using a BIORAD FTS 6000 FTIR spectrometer under
attenuated total reflection (ATR) conditions. The data were record-
ed in the range of 400–4000 cmÀ1 with an average of 32 scans and
at 4 cmÀ1 resolution. SEM was used to evaluate the size and mor-
phology of the particles and EDX analyses were used to semiquan-
titatively determine the nickel and manganese present on the
sample surfaces. The samples were placed on a silicon wafer and
the measurements were carried on a LEO DSM 982 microscope in-
tegrated with EDX (EDAX, Appollo XPP). Data handling and analysis
were carried out with the software package EDAX. The microstruc-
ture of the samples was investigated by TEM analysis. A small
actor was heated at
a
rate of 58C minÀ1 to 8008C under
a 60 mLminÀ1 flow of synthetic air, and the outlet gas was ana-
lyzed using a quadruple mass spectrometer (InProcess Instruments
GAM 200).
Acknowledgements
Financial support by the DFG (Cluster of Excellence UniCat, ad-
ministered by the TU Berlin) is gratefully acknowledged.
Keywords: carbon dioxide
hydrogen · metal oxides · methane conversion
·
heterogeneous catalysis
·
[2] L. Shi, G. Yang, K. Tao, Y. Yoneyama, Y. Tan, N. Tsubaki, Acc. Chem. Res.
ChemPlusChem 2016, 81, 370 – 377
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