A Single-Source Precursor Approach to Self-Supported Nickel-Manganese-Based Catalysts with Improved Stability for Effective Low-Temperature Dry Reforming of Methane

Article abstract of DOI:10.1002/cplu.201600064

Self-supported nickel-manganese-based catalysts were synthesized from heterobimetallic nickel manganese oxalate precursors via a versatile reverse micelle approach. The precursors were subjected to thermal degradation (400 °C) in the presence of synthetic air to form respective metal oxides, which were treated under hydrogen (500 °C) to form Ni₂MnO₄-O₂-H₂, Ni₆MnO₈-O₂-H₂ and NiO-O₂-H₂. Similarly, the precursors were also treated directly under hydrogen at the same temperature to form Ni₂MnO₄-H₂ and Ni₆MnO₈-H₂. The catalysts were extensively investigated by PXRD, SEM, TEM, XPS and BET analyses. The resulting catalysts were applied for dry reforming of methane (DRM) and exhibit better stability and resistance to coking than coprecipitated catalysts. Further, we show that addition of manganese, which is not an active catalyst for DRM alone, to nickel has a significant promotion effect on both the activity and stability of DRM catalysts, and a Ni/Mn ratio lower than 6:1 enables optimized activity for this system.

Full text of DOI:10.1002/cplu.201600064

DOI: 10.1002/cplu.201600064
Full Papers
A Single-Source Precursor Approach to Self-Supported
Nickel–Manganese-Based Catalysts with Improved Stability
for Effective Low-Temperature Dry Reforming of Methane
Prashanth W. Menezes+,[a] Arindam Indra,[a] Patrick Littlewood+,[b] Caren Gçbel,[a]
Reinhard Schomäcker,*[b] and Matthias Driess*[a]
Self-supported nickel–manganese-based catalysts were synthe-
sized from heterobimetallic nickel manganese oxalate precur-
sors via a versatile reverse micelle approach. The precursors
were subjected to thermal degradation (4008C) in the pres-
ence of synthetic air to form respective metal oxides, which
were treated under hydrogen (5008C) to form Ni2MnO4–O2–H2,
Ni6MnO8–O2–H2 and NiO–O2–H2. Similarly, the precursors were
also treated directly under hydrogen at the same temperature
to form Ni2MnO4–H2 and Ni6MnO8–H2. The catalysts were ex-
tensively investigated by PXRD, SEM, TEM, XPS and BET analy-
ses. The resulting catalysts were applied for dry reforming of
methane (DRM) and exhibit better stability and resistance to
coking than coprecipitated catalysts. Further, we show that ad-
dition of manganese, which is not an active catalyst for DRM
alone, to nickel has a significant promotion effect on both the
activity and stability of DRM catalysts, and a Ni/Mn ratio lower
than 6:1 enables optimized activity for this system.
Introduction
Sustainable use of global resources, especially conversion of
unwanted or hazardous materials to industrially important
products, is of special interest. In this respect, the conversion
of greenhouse gases such as CO2 into useful chemicals or fuels
is an indispensable course of action.[1–6] Dry reforming of meth-
ane (DRM) with carbon dioxide (CH4+CO2!2CO+2H2) mediat-
ed by a heterogeneous catalyst is an alternative approach to
convert greenhouse gases into synthesis gas mixtures
(CO+H2), which has been used as the raw material in various
well-established industrial processes such as the production of
methanol and the Fischer–Tropsch synthesis.[7,8] The DRM reac-
tion is endothermic and therefore relatively high temperature
is required.[9–11] However, whilst most research is conducted at
temperatures higher than 7008C, benefits exist for lower-tem-
perature DRM, for example in coupled reactor concepts.[3] Pre-
cious-metal-based catalysts (Ru, Rh, Pt, Ir) have been used as
efficient DRM catalysts for a long time due to high activities
and resistance to carbon deposition, but they are considered
to be too expensive for DRM.[12–15] Moreover, cheaper nickel-
based catalysts have also been studied in the last three de-
cades. However, fast deactivation of the Ni catalyst through
coking (carbon deposition) is a major problem for industrial
application.[10,16–19] This problem is exacerbated at low temper-
atures, although comparatively little research has been con-
ducted to address this directly. Therefore, the design of a more
stable Ni-based catalyst system is of significant importance for
utilization in industry.
Several Ni catalysts for DRM have been explored to minimize
deactivation by coking.[20–23] Solid solutions of NiO–MgO were
observed as promising catalysts where the alkaline-earth oxide
acts as the promoter for DRM.[24–29] Solid-supported Ni catalysts
were studied extensively to find out the role of the support. In-
2À
troduction of Ni into the perovskite structure A2+B4+O3
(where A and B represent the 12-coordinate and six-coordinate
metal cations) is one of the alternative approaches to stabilize
the system.[30–33] Control over the reaction temperature to
enable maximum activity with stability is also required, as
carbon deposition can occur via different routes depending on
the temperature. In the literature, the Ni-based catalysts have
been reported to be active enough at relatively low tempera-
ture.[34–37] At 5508C, close to thermodynamic equilibrium with
38% conversion of CH4 was achieved with Ni supported on
MgO along with 0.1% Pt doping.[36,37] Further lowering of the
reaction temperature to 4008C has also been reported involv-
ing Ni sites supported on mesoporous La2O3–ZrO2.[35] Stabiliza-
tion of Ni0 in an oxide matrix was found to be operative at low
temperature. Recently, a highly dispersed and anticoking Ni–
La2O3/SiO2 catalyst as well as surface area yolk–shell Ni@Ni em-
bedded in SiO have been reported with higher activity and en-
hanced stability.[35b–d] Ternary metal oxides have been used fre-
quently to form the active catalyst in the highly reducing envi-
[a] Dr. P. W. Menezes,+ Dr. A. Indra, Dr. C. Gçbel, Prof. Dr. M. Driess
Metalorganics and Inorganic Materials
Department of Chemistry, Technische Universität Berlin
Strasse des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
[b] P. Littlewood,+ Prof. Dr. R. Schomäcker
Reaction Engineering
Department of Chemistry, Technische Universität Berlin
Strasse des 17. Juni 124, Sekr. TC8, 10623 Berlin (Germany)
E-mail: shomaecker@tu-berlin.de
[+] These authors contributed equally to this work
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cplu.201600064.
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ronment of DRM. Silica-supported nickel manganese oxides
were used by Thomas et al. to form solid solutions of NiO–
MnO.38a It has been shown that addition of MnO to supported
Ni DRM catalysts improves stability, at the cost of activity, by
partially decorating the active Ni surface and also promoting
CO2 adsorption, both of which suppress deactivation resulting
from carbon deposition on the catalyst surface.[38b]
in the amount of nickel relative to the amount of manganese.
The resulting NiMn oxalates and Ni oxalates were first subject-
ed to thermal degradation in the presence of oxygen to form
mixed nickel–manganese (Ni2MnO4 and Ni6MnO8) and nickel
oxides (NiO) which were subsequently treated in H2 to form
mixed phases of Ni/NiO/MnO and Ni/NiO. These systems were
found to be efficient DRM catalysts as well as stable at the re-
action temperature for several hours. Increasing the Mn/Ni
ratio in turn greatly increases the rate of the reaction providing
higher stability to the system. This shows that a higher
amount of Mn is required in the NiMn oxide system to carry
out the reaction effectively with enhanced stability. On the
other hand, thermal treatment of NiMn oxalates in H2 also
leads to the formation of catalysts that are efficient for the
DRM reaction but with lower stability. Here we describe a new
synthetic approach to produce very stable self-supported
NiMn oxide mixed-phase catalysts through a SSP route which
enable efficient DRM at considerably low temperatures.
Because high temperature favours CH4 and CO2 conversion
and can delay coke formation, the DRM reaction at low tem-
perature (below 6008C) has been rarely carried out. This is
mainly due to the fact that at lower temperature the process
thermodynamically favours coke formation and this could
result in deactivation of the catalyst or damage to the reactor
in a longer run. Therefore, it is essential to design catalysts
that can show not only superior activity but also long-term sta-
bility at considerably lower temperatures if the DRM reaction is
to be opened up to process integration concepts to address
issues such as the high energy input required.
Herein, we present the facile formation of a stable self-sup-
ported Ni–MnO (or mixed) catalysts for effective low-tempera-
ture DRM, starting from well-defined heterobimetallic nickel
manganese oxalates as single-source precursors (SSPs). The
benefit of using the SSP approach has been demonstrated pre-
viously by our group.[39–44] This route not only includes the
low-temperature synthesis of a designed material but also ena-
bles definite control over its composition with maximum dis-
persion of the elements on the atomic level as well as control
over the oxidation states of the metals through the molecular
architecture of the precursor and the applied reaction condi-
tions. In addition, the mixing at the molecular level assures
product homogeneity. Furthermore, the anticipated degrada-
tion of such SSPs can lead to unusual and desirable structural
features in the final products such as high surface area, low
density, and the formation of metastable phases with the pos-
sibility of tuning their electronic properties. This method has
already been well established for amorphous conducting
oxides that have shown long-term stability along with high op-
toelectronic performance.[39,40]
Results and Discussion
First, the nickel manganese and nickel oxalates were synthe-
sized via an inverse-micellar route through microemulsions
containing cetyltrimethylammonium bromide (CTAB) as a sur-
factant, 1-hexanol as co-surfactant and hexane as the lipophilic
phase and mixed with an aqueous solution containing Ni2+
,
Mn2+ and oxalate ions. Ni0.85Mn0.15C2O4·2H2O was precipitated
when a 1:1 ratio of Ni2+:Mn2+ was used in solution, whereas
Ni0.66Mn0.34C2O4·2H2O was generated by changing the Ni:Mn
ratio to 1:2. In the absence of Mn2+, NiC2O4·2H2O was pro-
duced. The structure of oxalates comprises of one-dimensional
chains with each manganese/nickel atom being coordinated to
two bidentate oxalate ligands and two water molecules. Thus
the formed oxalate SSPs were degraded in oxygen to produce
nickel manganese oxide phase (see Experimental Section). Sub-
sequently, the treatment of the nickel manganese oxides in H2
produced Ni2MnO4–O2–H2 and Ni6MnO8–O2–H2 and the thermal
treatment of the same SSPs directly in H2 furnished Ni2MnO4–
H2 and Ni6MnO8–H2, respectively (Scheme 1). Similarly, NiO–O2–
H2 was produced from nickel oxalate by heating in air followed
The mixed nickel–manganese and nickel oxalate SSPs can be
synthesized by the microemulsion approach through variation
Scheme 1. Synthetic routes to the DRM catalysts from oxalate single-source precursors (SSPs).
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by treatment with hydrogen. All SSPs were characterized by
powder X-ray diffraction (PXRD) and infrared spectroscopy (IR).
The respective data are in good agreement with those report-
ed for such metal oxalates (Figures S1–S4, Table S1). The rod-
like morphology of NiMn oxalates and cubical microparticles of
Ni oxalate were detected by transmission electron microscopy
(TEM) and scanning electron microscopy (SEM) as shown in
Figures S5 and S6, respectively. The presence of corresponding
elements and the Ni/Mn ratio were determined by energy-dis-
persive X-ray analysis (EDX; Figure S7). The Ni/Mn ratio was
also confirmed by inductively coupled plasma atomic absorp-
tion spectroscopy (ICP-AES, Table S2).
Destruction of the morphology of the oxides after heating in
H2 is evident by SEM (Figure S14). TEM images also showed
new morphological growth in Ni2MnO4–O2–H2 (Figure 2) and
Ni6MnO8–O2–H2 (Figure S16). The reflections of the selected
area diffraction pattern (SAED) of Ni2MnO4–O2–H2 (Figure 2)
The precursor morphology was changed when the SSPs
were heated in air to form the respective mixed oxides (Figur-
es S8 and S9). As shown by EDX and ICP analyses, the Ni/Mn
ratio of the resulting oxides was invariable relative to the
nature of the oxalate SSP (Figure S10 and Table S3). Also, the
formation of monophasic Ni2MnO4, Ni6MnO8 and NiO from the
respective precursors was also confirmed by PXRD analysis
(Figures S11–13). Interestingly, the structure of Ni6MnO8 is con-
sidered to be the rock-salt structure where 6/8 of octahedral
sites are occupied by Ni2+ atoms and 1/8 by Mn4+ atoms, and
by vacancies (alternative (111) planes). Ni2MnO4 belongs to the
spinel type (AB2O4) structures where the Mn4+ ions occupy the
tetrahedral (B) sites and the octahedral (A) sites are preferred
by Ni2+/3+. However, NiO adopts a rock-salt structure. In addi-
tion, the PXRD patterns indicated that post-treatment of mix-
tures of NiMn oxides in H2 furnished mixed-phase materials
(Figure 1). A mixture of elemental Ni, NiO and MnO was detect-
ed in Ni2MnO4–O2–H2 and Ni6MnO8–O2–H2 as well as in the re-
spective catalysts, which were only treated in H2, and which
did not have observably different crystallite sizes according to
the diffraction patterns, whereas only Ni and NiO were present
as components in NiO–O2–H2.
Figure 2. TEM image (top, scale bar 50 nm) and the electron diffraction pat-
terns (bottom) of Ni2MnO4–O2–H2 showing that the platelet-type particles
consisted of pure crystal phases of metallic Ni, MnO and NixMn1ÀxO.
clearly displayed the presence of Ni, MnO and Ni0.75
Mn0.25O.[45,46] EDX analysis at different areas also indicated dif-
ferent Ni/Mn ratios (Figure S15). Interestingly, in the case of
Ni6MnO8–O2–H2, spherical particles were observed (Figure S16)
and the size of the particles varied to a large extent. The SAED
pattern of these spherical particles clearly exhibited the rings
corresponding to Ni and NiO.[45] In addition to particles of
spherical morphology, plate-like particles were also observed
containing only MnO as conferred by Fast Fourier Transform
(FFT) of the lattices (Figure S17). A high amount of Ni was de-
tected by EDX in the spherical particles, whereas the plates
contain Mn-rich phase (Figure S18). Strikingly, in both cases
(Ni2MnO4–O2–H2 and Ni6MnO8–O2–H2), the nickel particles were
always found to be supported on the surface of MnO or NiO
or mixed NiMn oxide to generate a self-supported system. Fur-
ther, it was confirmed that this type of self-supported system
provided additional stability to the system during DRM reac-
tion. Temperature-programmed reduction (TPR) studies also
showed essentially full reduction (ꢀ93%) of precursor materi-
als to both Ni2MnO4–O2–H2 and Ni6MnO8–O2–H2 via a major
step related to reduction of the manganese oxide at 3558C for
Ni6MnO8 and 3458C for Ni2MnO4; two shoulders also appear at
lower temperatures of 180 and 2408C for Ni6MnO8, and 150
and 2138C for Ni2MnO4 which correspond to the reduction to
NiMnO3 and nickel (Figure S38). It is likely that any NiO detect-
Figure 1. PXRD and Miller indices (hkl) of a) Ni2MnO4–O2–H2, b) Ni2MnO4–H2,
c) Ni6MnO8–O2–H2, d) Ni6MnO8–H2 and e) NiO–O2–H2. The dotted lines repre-
sent the crystal phases of MnO (green, JCPDS 71-1177), Ni (red, JCPDS 4-
850) and NiO (blue, JCPDS 47-1049).
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ed by surface techniques after reduction is the result of reoxi-
dation of metallic Ni upon contact with air at room tempera-
ture during transfer to the analysis chamber.
material with a small particle size.[53–55] Similarly, the corre-
sponding descriptions of XPS analysis for the catalysts
Ni2MnO4–H2, Ni6MnO8–O2–H2, Ni6MnO8–H2 and NiO–O2–H2 are
given in Figures S25–S28 (see the Supporting Information).
BET surface area measurements were performed for
Ni2MnO4–O2–H2, Ni2MnO4–H2, Ni6MnO8–O2–H2, Ni6MnO8–H2 and
nickel manganese oxalates and of the respective oxides which
are shown in Figure S29. As expected, high-temperature treat-
ment reduces the surface area of all catalysts. Oxidation of cat-
alyst precursors to obtain Ni6MnO8 and Ni2MnO4 yielded mate-
rials with the highest surface areas, but subsequent reduction
at 5008C to form Ni2MnO4–O2–H2 and Ni6MnO8–O2–H2 caused
a severe decrease in surface area, through destruction of the
particle morphology as seen from SEM results. Ni2MnO4–H2
and Ni6MnO8–H2 which did not undergo oxidation showed sig-
nificantly higher surface areas than Ni2MnO4–O2–H2 and
Ni6MnO8–O2–H2.
For Ni2MnO4–H2 and Ni6MnO8–H2, when NiMn oxalates were
directly heated in an atmosphere of H2, dropletlike morpholo-
gy with wide range of sizes was observed by TEM (Figur-
es S19–S22). The SAED pattern indicated the presence of Ni,
NiO, MnO along with a NixMn1ÀxO phase.[45,46] It was certain by
comparing Ni2MnO4–O2–H2, Ni2MnO4–H2, Ni6MnO8–O2–H2 and
Ni6MnO8–H2 that only Ni6MnO8–O2–H2 did not contain any
NixMn1ÀxO oxide phase. In contrast to other catalysts, NiO–O2–
H2 showed thick blocks and comprises Ni and NiO as detected
by SAED (Figures S23 and S24). The reflections observed in the
SAED pattern for all phases from catalysts are presented in
Table S4.
X-ray photoelectron spectroscopic (XPS) studies of all cata-
lysts have been performed. For Ni2MnO4–O2–H2, as shown in
Figure 3, the two spin-orbit split 2p3/2 and 2p1/2 peaks of Mn 2p
were very close to those of the 2p3/2 and 2p1/2 states of MnO
(2p3/2 ꢁ642.2 eV and 2p1/2 ꢁ653.8 eV).[47–49] The XPS core level
To understand the possible advantages of the reverse-mi-
celle synthesis route, synthesis of a standard Ni2MnO4 catalyst
derived by the coprecipitation method was considered. How-
ever, it was not possible to control the ratio for Ni2MnOx (form-
ing a catalyst with a Ni/Mn ratio of ca. 6:1) for comparison
with Ni2MnO4–O2–H2. Therefore instead of Ni2MnO4, Ni6MnO8
catalyst was synthesized by coprecipitation (similar to
Ni6MnO8–O2–H2) and was then reduced under similar condi-
tions. PXRD analysis of this coprecipitated reduced standard
catalyst confirmed the presence of Ni and MnO phases (Fig-
ure S31). Furthermore, a closer look at TEM analysis revealed
platelike and irregular-type particles (Figures S32–S34) consist-
ing of Ni, NiO, MnO with NixMn1ÀxO phase, and the material
has a similar BET surface area to Ni6MnO8–O2–H2 of 3.69 m2 gÀ1.
The Ni2MnO4–O2-H2, Ni2MnO4–H2, Ni6MnO8–O2–H2, Ni6MnO8–
H2 and NiO–O2–H2 catalysts have been tested for DRM and
their activities (shown as conversion with time on stream at
several temperatures) are presented in Figure 4. Conversions
were by necessity (due to thermodynamic limits) lower than
those of catalysts in the literature which had been tested at
higher temperatures, but our values were comparable to those
of catalysts tested at similar temperatures.[35–36,56] It should be
mentioned here that the gas hourly space velocity (GHSV) for
NiO–O2–H2 used as a standard in our experiments was
2.5 times lower than that of all other catalysts. NiO–O2–H2,
nickel from nickel oxide nanoparticles, thus did not show high
activity or stability. It deactivated faster than all manganese-
containing Ni2MnO4–O2–H2, Ni2MnO4–H2, Ni6MnO8–O2–H2 and
Ni6MnO8–H2 and exhibited at best around half of the rate of re-
action. The behavior as shown in the plot for all other catalysts
was also similar to that of NiO–O2–H2, with a small gradual in-
crease in activity up to roughly 10 h on stream. Catalytic activi-
ty was not improved with the presence of a greater amount of
Ni. In fact, Ni2MnO4–O2–H2 produced from Ni2MnO4 showed
the highest methane conversion followed by Ni2MnO4–H2. This
is striking, since addition of MnO (which is itself inactive for
DRM) to Ni catalysts has previously been shown to improve
only the catalytic stability whilst lowering activity, whereas this
result clearly indicates that an optimum Ni/Mn ratio lower than
6:1 exists in terms of activity. To highlight this, Figure 5 shows
Figure 3. High-resolution XPS spectra of Ni2MnO4–O2–H2 (see text for de-
scription).
spectrum of Ni 2p3/2 (middle) exhibited a peak at 854.5 eV that
could be attributed to that of NiO.[50–52] PXRD and TEM analysis
revealed the presence of elemental nickel (Ni0) in the phases;
however, no peak at 852.9 eV for metallic Ni could be detected
in the XPS spectrum due to the existence of close binding en-
ergies of Ni2+/Ni0 and also because of the possible oxidation
of the surface layer of the catalysts. The O 1s spectrum was de-
convoluted into two peaks (O1 and O2). The first peak (O1) at
529.9 eV can be attributed to the oxygen atoms of the metal
oxide. The peak (O2) at 531.5 eV corresponds to either oxygen
in OH groups, indicating that the surface of the material is hy-
droxylated due to surface hydroxides or the substitution of
oxygen atoms at the surface by hydroxyl groups, and the
number of defect sites with low oxygen coordination in the
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ty.[20] However, it should be noted that the number of different
catalyst compositions studied means that these results are
qualitative and activity is likely to also be strongly affected by
other factors, such as total nickel surface area and particle mor-
phology; this must be studied further to generalise these con-
clusions to other synthetic approaches.
The catalytic stability (for Ni2MnO4–O2–H2, Ni2MnO4–H2,
Ni6MnO8–O2–H2, Ni6MnO8–H2) was significantly improved by the
incorporation of Mn. The Ni particles generated in the H2
stream were dispersed in the MnO support, which helps inhibit
Ni particle sintering at the relatively high temperature of DRM.
This is also reflected in the rate of the reaction. This is an inter-
esting result, since the catalytically active species are nickel-
based, and manganese and its oxides were currently not con-
sidered to be involved as catalysts for the DRM reaction. It is
possible that a large amount of the active sites on the surface
of the catalysts are rendered inactive within the first hour on
stream and that the activity of higher-manganese-containing
catalysts is apparently better stabilized. This would be attribut-
able to a smaller nickel to manganese ratio on the catalyst sur-
face, either preventing nanoparticle sintering or deactivating
carbon formation through maintaining a smaller ensemble
size.[57] Post-reaction TPO experiments (Figure S35) showed
a much higher rate of coking for the more active Ni2MnO4–O2–
H2 (0.25 mgC mgcatÀ1 hÀ1) than for the catalyst with a higher
amount of nickel (0.028 mgC mgcatÀ1 hÀ1, Ni6MnO8–O2–H2), indi-
cating that carbon deposition is not the primary cause of long-
term deactivation but occurs to a greater extent on the more-
active surface. The effect of available surface area on the rate
of carbon formation for the catalysts investigated was not
clear due to the intimate situation of Ni and MnO in the self-
supported system, and the variance in overall particle size, and
there was no correlation between carbon formation and over-
all particle size from TEM results (Figure S36).
Figure 4. Activity and stability tests of Ni2MnO4–O2–H2, Ni2MnO4–H2,
Ni6MnO8–O2–H2, Ni6MnO8–H2 and NiO–O2–H2 for the DRM reaction. CH4/CO2/
À1
À1
He=1:1:8, GHSV=90000 LhÀ1 kgcat (36000 LhÀ1 kgcat for NiO-O2-H2,
18000 LhÀ1 kgcat for MnO), atmospheric pressure. Equilibrium conversions
À1
for 500, 525 and 5508C: 0.28, 0.36 and 0.45, respectively. Carbon dioxide
conversions are presented in Figure S37.
The stability of the Ni2MnO4–O2–H2 and Ni6MnO8–O2–H2 was
also compared with those of catalysts in which the precursors
were directly reduced under H2 atmosphere (Ni2MnO4–H2,
Ni6MnO8–H2). As expected, the initial activities of Ni2MnO4–H2
and Ni6MnO8–H2 were equal to or higher than those of
Ni2MnO4–O2–H2 and Ni6MnO8–O2–H2, because the calcination
step inevitably causes some particle sintering. This is reflected
in the higher BET surface areas of Ni2MnO4–H2 and Ni6MnO8–
H2. However, the long-term stability of such catalysts was
poorer at higher temperature, presumably due to the lack of
prior stabilization via calcination. The process of oxidation–re-
duction forms a self-supported system of nickel and manga-
nese oxide particles which have already developed tempera-
ture-stable surface areas. In comparison, those only treated in
hydrogen have higher initial surface areas but are highly sus-
ceptible to rapid deactivation by this loss of surface area, and
as such undergo higher rates of deactivation at the start of the
reaction; this can be seen more clearly by comparing the rela-
tive rates of similar catalysts at the beginning and end of the
reaction. The chemical potentials for various sintering process-
es are affected by the reaction conditions; for example, signifi-
cant coke deposits are likely to affect the resulting nickel parti-
cle size and nickel–support interactions. However, all catalysts
Figure 5. Activity tests with Ni2MnO4–O2–H2 and Ni6MnO8–O2–H2 for the
DRM reaction at 5258C. 10 mg catalyst used, CH4/CO2/He=1:1:8,
GHSV=180000 LhÀ1 kgcatÀ1, atmospheric pressure.
the activity test results for 10 mg of Ni2MnO4–O2–H2 and
Ni6MnO8–O2–H2 at 5258C. The catalyst with a higher Mn/Ni
ratio was not only more active, but also exhibited a higher H2/
CO ratio. It was also clear that direct degradation of the oxa-
late SSP Ni0.66Mn0.34C2O4·2H2O in H2 (Ni2MnO4–H2) leads to sam-
ples that show lower activity than Ni2MnO4–O2–H2 that had
been prepared by NiMn oxide formation and subsequent H2
treatment. This difference is more prominent in the cases of
Ni6MnO8–O2–H2 and Ni6MnO8–H2, respectively. The results may
also be explained by results from TEM studies due to the varia-
ble size and morphology in Ni2MnO4–H2 and Ni6MnO8–H2. The
fact that the catalytic activity of Ni2MnO4–O2–H2 is better than
that of Ni6MnO8–O2–H2 can be explained by the presence of
a lower amount of Ni to generate a more diluted system,
a higher BET surface area and a direct effect of Mn on the ki-
netics of the DRM reaction through promoting CO2 adsorptivi-
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were deactivated gradually with time on stream. TEM images
of post-reaction catalysts showed large carbon deposits (Fig-
ure S36) in both nanotubes and encapsulated graphite form,
although no strong correlation could be found between the
amount of carbon and the rate of deactivation across all cata-
lysts. Therefore the main cause of deactivation was not attrib-
uted to the carbonization around the active metal centers.
Ni6MnO8–O2–H2 was also compared to a co-precipitated
standard catalyst of the same composition and showed superi-
or stability under all conditions tested as shown in Figure 6.
Ni6MnO8–O2–H2 formed solid carbon at a significantly lower
Figure 7. Results of stability testing (under aggressive conditions for carbon
formation) for Ni6MnO8–O2–H2 and a co-precipitated standard catalyst, with
the overall rate of carbon formation for both tests (inset). T=5258C, CH4/
CO2/He=1:1:3, GHSV=90000 LhÀ1 kgcatÀ, atmospheric pressure.
rials.[31,38a] In order to develop materials intended for applica-
tion, the approach of lowering the Ni/Mn ratio and using new
synthesis techniques has clear benefits, but application of
a thermally stable support or other means of preventing deac-
tivation through thermal degradation is required to maintain
the higher initial activity gained through particle morphology
or lack of calcination.[10,62]
Figure 6. Activity tests with Ni6MnO8–O2–H2 and a co-precipitated standard
catalyst with the same composition. CH4/CO2/He=1:1:8,
GHSV=90000 LhÀ1 kgcatÀ1, atmospheric pressure. Equilibrium conversions
for 500, 525 and 5508C:0.28, 0.36 and 0.45, respectively.
Conclusion
In conclusion, self-supported active catalysts for low-tempera-
ture DRM reaction have been synthesized from heterobimetal-
lic NiMn oxalates as SSPs. The new catalysts showed increased
stability and high resistance to coke formation in comparison
to coprecipitated catalyst. All catalysts exhibited slight deacti-
vation behavior above 5258C, although this was not attributa-
ble to surface carbon formation. However, it is clear from the
screening results that the addition of manganese has a signifi-
cant promotion effect on the DRM catalysts, in terms of stabili-
ty, activity, and H2/CO ratio. Considering that manganese alone
is not an active DRM catalyst, we could now demonstrate that
a Ni/Mn ratio lower than 6:1 leads to optimized activity for
self-supported NiMn catalysts prepared by these methods.
rate than the standard catalyst, which also rapidly deactivated
under less aggressive conditions for carbon formation. This
again indicates that coking may not be the primary cause of
deactivation, although catalysts with higher initial activities
appear to form carbon at a higher rate. The residual activity of
Ni6MnO8–O2–H2 was consistently higher than that of the stan-
dard catalyst shown in Figure 7.
The higher activity and increased stability of Ni2MnO4–O2–H2
can be attributed to the platelets, which consist of Ni, MnO
and NixMnyO, followed by Ni2MnO4–H2, which has droplet-type
particles of similar composition. It appears that the presence of
a mixed nickel–manganese (NixMnyO) phase as well as the opti-
mum Ni/Mn ratio is essential for effective low-temperature
DRM. On the other hand, Ni6MnO8–O2–H2, prepared by the SSP
approach, shows significantly superior activity and stability rel-
ative to the co-precipitated catalyst. This finding can be as-
cribed to the presence of spherical particles in Ni6MnO8–O2–H2
which exclusively contain a Ni-rich (Ni and NiO) phase support-
ed by Mn-rich phases.
Experimental Section
Synthesis of heterobimetallic nickel manganese and nickel
oxalate precursors
For Ni0.85Mn0.15C2O4·2H2O, three micro-emulsions containing cetyl-
trimethylammonium bromide (CTAB, 2.0 g) as a surfactant, 1-hexa-
nol (20 mL) as a co-surfactant, and hexane (35 mL) as the lipophilic
phase were prepared separately with an aqueous solution of 0.1m
nickel acetate, 0.1m manganese acetate and 0.1m ammonium oxa-
late. All three micro-emulsions were mixed slowly and stirred over-
night at room temperature. The green precipitate then obtained
was centrifuged, washed with a 1:1 mixture of chloroform and
In comparison to known catalysts, the stabilities of the
NiMnO catalysts tested herein are not high, as can be seen
from Figures 4 and 7.[58–61] Thermophysical stability appears to
be the critical factor although the rate of carbon deposition is
too high to solve the problems facing low-temperature DRM.
This is potentially due to the high nickel contents of the mate-
ChemPlusChem 2016, 81, 370 – 377
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375
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Full Papers
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 Š). 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
·
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Manuscript received: February 11, 2016
Accepted Article published: February 15, 2016
Final Article published: February 25, 2016
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