Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4 Nanorods

Article abstract of DOI:10.1002/anie.201510201

The electronic and structural properties of vanadium-containing phases govern the formation of isolated active sites at the surface of these catalysts for selective alkane oxidation. This concept is not restricted to vanadium oxide. The deliberate use of hydrothermal techniques can turn the typical combustion catalyst manganese oxide into a selective catalyst for oxidative propane dehydrogenation. Nanostructured, crystalline MnWO₄ serves as the support that stabilizes a defect-rich MnO_x surface phase. Oxygen defects can be reversibly replenished and depleted at the reaction temperature. Terminating MnO_x zigzag chains on the (010) crystal planes are suspected to bear structurally site-isolated oxygen defects that account for the unexpectedly good performance of the catalyst in propane activation.

Full text of DOI:10.1002/anie.201510201

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International Edition: DOI: 10.1002/anie.201510201
German Edition: DOI: 10.1002/ange.201510201
Heterogeneous Catalysis
Selective Alkane Oxidation by Manganese Oxide: Site Isolation of
MnOx Chains at the Surface of MnWO4 Nanorods
Xuan Li, Thomas Lunkenbein, Verena Pfeifer, Mateusz Jastak, Pia Kjaer Nielsen,
Frank Girgsdies, Axel Knop-Gericke, Frank Rosowski, Robert Schlçgl, and Annette Trunschke*
Abstract: The electronic and structural properties of vana-
dium-containing phases govern the formation of isolated active
sites at the surface of these catalysts for selective alkane
oxidation. This concept is not restricted to vanadium oxide.
The deliberate use of hydrothermal techniques can turn the
typical combustion catalyst manganese oxide into a selective
catalyst for oxidative propane dehydrogenation. Nanostruc-
tured, crystalline MnWO4 serves as the support that stabilizes
a defect-rich MnOx surface phase. Oxygen defects can be
reversibly replenished and depleted at the reaction temperature.
Terminating MnOx zigzag chains on the (010) crystal planes
are suspected to bear structurally site-isolated oxygen defects
that account for the unexpectedly good performance of the
catalyst in propane activation.
charge transfer between bulk and surface. This is reflected in
the gas-phase-dependent response of the work function, the
electron affinity, and the surface potential barrier, which was
not observed for the less-selective bulk V2O5.[4i]
Herein, we conceptually confirm that the selectivity of
other unselective oxides, such as manganese oxide, is also
tunable by applying an extended site-isolation approach. We
present the first example of a vanadium-free analogue that
accomplishes the efficient activation of propane by establish-
ing a two-dimensional manganese oxide layer in form of
MnOx chains at the surface of phase-pure, rod-shaped,
nanostructured MnWO4 (Figure 1; see also the Supporting
Information, Figure S1). The catalyst was prepared by hydro-
thermal synthesis. A previously reported synthesis proce-
dure,[5] was modified in the current work (as described in the
Supporting Information).
The phase purity of the synthesis product was confirmed
by Rietveld refinement of the powder X-ray diffractogram
(Figure S2) by anisotropic fitting. Transmission electron
microscopy (TEM) imaging reveals typical rod-shaped nano-
particles with diameters from 13 to 51 nm (Figure 1a,b,
Figure S3), which is in agreement with the XRD analysis
(Table S1). Fast Fourier transform (FFT) analysis of the
bright-field TEM images of several particles (Figure S4)
indicates the preferential growth of the rods along the [001]
direction, which is in contrast to a previous report.[5]
Furthermore, a power spectrum (Figure 1b, inset) reveals
elongated spots, in particular for the (011) plane, indicating
a defective structure. Inverse fast Fourier transformation
(IFFT) of the 011 spots (Figure S5) indicates the presence of
planar defects within the lattice. From the basal area of two
condensed nanorods (Figure 1c), surface terminations that
include the (010), (110), and (100) planes can be distin-
guished. Atomic-resolution high-angle annular dark field/
scanning transmission electron microscopy (HAADF-STEM)
images (Figure 1d and e) viewed along the [001] axis indicate
the presence of two kinds of atomic dumbbells, which can be
distinguished by their different contrasts. In HAADF-STEM,
the contrast is due to Rutherford scattering, which is
approximately proportional to Z2. Therefore, the dumbbells
can be attributed to W2O8 (high contrast) and Mn2Oy (low
contrast) dimers. In the schematic representation of the
MnWO4 crystal structure (Figure 1 f), the W2O8 and Mn2Oy
dimers are shown as orange and white edge-sharing octahe-
dra, respectively.
T
he prospective changes in the raw-material basis of the
chemical industry to alternative feedstocks bear new scientific
challenges. This concerns, in particular, the area of oxidation
catalysis where small saturated hydrocarbon molecules are
desired building blocks in value-added processes.[1] The
À
activation of inert C H bonds in alkanes requires highly
active catalysts. Often, high activity entails low selectivity
owing to the overoxidation of more reactive intermediates
and desired products to CO and CO2.[2] Vanadium oxide is the
material that has been most widely studied for the selective
oxidation of hydrocarbons and oxygenates.[3] Surface-sensi-
tive in situ experiments indicate that some well-known
selective catalysts that consist of crystalline vanadium-con-
taining phases are terminated by two-dimensional vanadium
oxide layers.[4] These layers deviate significantly from the bulk
crystal structure in terms of their composition and the
vanadium oxidation state. The layers account for dynamic
[*] X. Li, Dr. T. Lunkenbein, V. Pfeifer, M. Jastak, P. K. Nielsen,
Dr. F. Girgsdies, Dr. A. Knop-Gericke, Prof. Dr. R. Schlçgl,
Dr. A. Trunschke
Department of Inorganic Chemistry
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4–6, 14195 Berlin (Germany)
E-mail: trunschke@fhi-berlin.mpg.de
X. Li, Dr. F. Rosowski
BasCat—UniCat BASF Joint Lab
Technische Universität Berlin
Sekretariat EW K 01, Hardenbergstrasse 36, 10623 Berlin (Germany)
Dr. F. Rosowski
BASF SE, Process Research and Chemical Engineering
Heterogeneous Catalysis
Atomic-resolution HAADF-STEM images of the surface
structure of the (010) plane viewed along the [001] direction
illustrate the preferential surface exposure of Mn ions as
unimers or dimers (Figure 2). The images indicate a slight out-
Carl-Bosch-Strasse 38, 67056 Ludwigshafen (Germany)
Supporting information for this article can be found under http://dx.
doi.org/10.1002/anie.201510201.
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Figure 2. Surface termination of the b plane viewed along the growth
direction [001] by FFT-filtered atomic-resolution STEM images.
a) HAADF, and b) inverted HAADF image. Mn green, W violet. The
micrographs correspond to the magnified images of Figure 1d. The
original images are given in the Supporting Information (Figure S6).
Figure 1. (S)TEM analysis of the MnWO4 powder catalyst. a) Overview
TEM image of the MnWO4 nanorods. b) HRTEM image of one nano-
rod particle viewed along the [100] direction. The inset shows the
power spectra recorded on either side of the defect. c) HAADF-STEM
image of condensed [001] oriented MnWO4 nanorods. The surface
termination was identified by phase analysis of the corresponding
atomic-resolution HAADF-STEM images. d),e) Atomic-resolution
HAADF-STEM images of (c). f) Schematic representation of the crystal
structure of MnWO4 viewed along the [001] direction. Mn white, O red,
W green. The original images are shown in the Supporting Information
(Figure S1, 6c).
Figure 3. Depth profile of the elemental composition of MnWO4 nano-
rods at two different depths in terms of the inelastic mean free path
(IMFP) of electrons measured by synchrotron-based NAP-XPS at
T=3008C applying a total pressure of 0.25 mbar O2 and He at flows of
2 and 2.2 mLminÀ1, respectively.
In summary, an enrichment of the surface of MnWO4
nanorods with manganese in the phase-pure, highly crystal-
line material was indicated by photoelectron spectroscopy.
The specific surface termination is also reflected in the
Raman spectrum. In line with these integral methods, locally
resolved atomic-resolution HAADF-STEM images reveal
partial Mn surface termination of the (010) planes (Figure 2).
Therefore, the Mn enrichment observed by integral methods
could be primarily due to an increased occurrence of Mn-
terminated (010) planes in the nanostructured material
(Figure 2).
The specific surface/nanostructure of MnWO4 is gener-
ated under hydrothermal conditions. In situ Raman spectra
taken during the synthesis (Figure 4) provide important
information on the phase formation of MnWO4. The
MnWO4 nanorods develop at around 1258C when a mixture
of aqueous solutions of Na2WO4 and Mn(NO3)2 is heated in
an autoclave (Figure S10). The transformation of an inter-
mediate (910 cmÀ1) into crystalline MnWO4 is indicated by
of-center shift for some Mn ions relative to their bulk
crystallographic position.
The Raman spectrum of nanostructured MnWO4 agrees
well with the spectrum of crystalline MnWO4 (Figure S7).[6]
However, two additional, previously unreported bands
appear at 615 and 665 cmÀ1. As the phase purity and high
crystallinity of the nanostructured material had been con-
firmed by TEM (Figure 1 and Figure S8) and XRD (Fig-
ure S2), these two bands were attributed to the MnOx clusters
at the surface of the nanorods (Figure S9), which had been
visualized by STEM (Figure 2).[7]
Synchrotron-based near-ambient-pressure X-ray photo-
electron spectroscopy (NAP-XPS) displays a molar Mn/W
ratio of 1.9 (Figure 3), which thus reveals an enrichment of
Mn in the outermost surface (inelastic mean free path
(IMFP) = 0.6 nm).
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Figure 4. In situ Raman spectra (280–1000 cmÀ1) recorded during the
hydrothermal synthesis of MnWO4 nanorods. Bands denoted with
asterisks belong to the sapphire window of the Raman probe.
Figure 5. Catalytic performance of nanostructured MnWO4 (T=4008C,
W/F=1.8–0.9 gsmLÀ1) in comparison to VOx/SiO2 (T=400–4208C,
W/F=1.8 gsmLÀ1) and various manganese oxides (T=330–3958C,
W/F=1.8 gsmLÀ1) in the oxidative dehydrogenation of propane in
a C3H8/O2/N2 feed (10:5:85). The selectivity to propene is shown as
a function of propane conversion. Other carbon-containing products
are mainly CO and CO2.
the appearance of bands at 884, 325, 397, 510, 544, 672, and
698 cmÀ1. The strongest band at 884 cmÀ1 has been assigned to
[6]
=
the stretching mode of W O in distorted WO6 octahedra.
Interestingly, two new bands at 618 and 657 cmÀ1, which were
assigned to surface MnOx clusters,[7] increase in intensity with
time once a synthesis temperature of 1808C has been reached.
MnWO4 crystallizes in a monoclinic structure (wolframite-
type, ICSD-67906) in which WO6 clusters form zigzag chains
by sharing edges along the [001] axis (Figure S9). In basic
medium under hydrothermal conditions (pH 9), WOx clusters
on the surface of the (010) planes could be dissolved by
Table 1: Active-site density and reactivity of the catalysts in the oxidative
dehydrogenation of propane at T=4008C and W/F=1.8 gsmLÀ1 in
a C3H8/O2/N2 feed (10:5:85).
[b]
Catalyst
MnWO4
nanorods MnWO4
Acid-washed VOx/SiO2
Mn2O3
[a]
rC3H8
8.69
2.46
–
0.27
1.0
550
[”10À9 molmÀ2 sÀ1
]
nucleophilic attack of OHÀ ions at the bridging W O W
À À
bonds [Eq. (1)], leaving behind leached (010) surfaces
composed of MnOx zigzag chains. Subsequently, the bridging
Active-site
8.3[c]/3[d]
10.2[e]
33
density [nmÀ2
]
oxygen anions at the (001) surface may condense with the
TOF [”10-3 sÀ1
]
0.63/1.7
–
0.16
2À
dissolved WO4
monomers (shoulder at 926 cmÀ1 in
Figure 4). Hence, the zigzag chains will propagate in one
[a] Treatment described in the Supporting Information. [b] VOx sup-
ported on modified SBA-15 with a surface vanadium density of 1 Vatom/
nm2. [c] Assuming that only MnOx species on (010) surface planes are
active. [d] Assuming that only oxygen defect sites on the surface are
active sites. [e] Mn density on the (100) plane.
À
À
dimension by forming new O W O bridging bonds by edge
sharing. Such a dissolution–recrystallization process may
result in the observed anisotropic crystal growth along the
[001] axis, resulting in the rod-shaped morphology of the
MnWO4 particles with a MnOx enriched surface.
2À
MnWO4 þ 2 OHÀ Ð MnðOHÞ2 þ WO4
ð1Þ
chains on the (010) planes by W2O8 units (Figure 2) appa-
rently account for the improved performance. The propane
conversion can be changed by varying the contact time at
4008C (Figure 5). The comparatively low temperature was
chosen to avoid the influence of homogeneous gas-phase
reactions, which normally contribute to non-negligible con-
version at elevated reaction temperatures above 4508C. The
catalytic performance of the MnWO4 nanorods reached
a steady state after 70 hours on stream and showed no signs
of deactivation within 108 hours (Figure S11). Importantly,
the Mn-terminated MnWO4 nanorods exhibit a much higher
apparent turnover frequency (TOF) than silica-supported
vanadium oxide species (Table 1) when all Mn atoms at the
(010) surfaces and all V atoms are considered to be active
sites. However, in reality, the number of active sites at the
surface of the catalysts might be much lower.
The nanostructured MnWO4 material was studied as
a catalyst in the oxidative dehydrogenation of propane.
Conventional crystalline bulk MnWO4 is rather inactive in the
reaction, which is reflected in no measurable conversion at
4508C. Nanostructuring, however, as achieved by hydro-
thermal synthesis in the present work, can turn this material
into an active catalyst with a similar performance to silica-
supported vanadium oxide (Figure 5). Moreover, the unique
surface structure of the MnWO4 nanorods leads to superior
selectivity to the desired product propylene compared with
bulk manganese oxide. Manganese oxide is very active
(Table 1), but not very selective and transforms propane
almost completely into carbon oxides. The electronic modi-
fication and/or the geometric separation of the MnOx zigzag
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The Mn 2p XPS spectra (Figure S12) indicate that Mn is
structured metal oxides.[16] The identification of the highly
predominantly present in the +2 oxidation state at the surface
and within the subsurface. Tiny differences were observed in
spectra recorded under different reaction atmospheres (Fig-
ure S13); however, the interpretation of the Mn 2p spectra is
not straightforward owing to less distinct variations in the
binding energies of compounds containing Mn in different
oxidation states, elaborate multiplet splitting, and the appear-
active surface MnOx units on the MnWO4 nanorods improves
our understanding of the promotional effect of Mn in many
catalytic systems. The design of selective oxidation catalysts
will benefit from this knowledge, which will also enable the
improvement of current manganese-doped catalytic systems.
ance of satellites.[8] In contrast, near-edge X-ray absorption Experimental Section
The hydrothermal synthesis and the thermal activation procedures of
fine structure (NEXAFS) spectroscopy is sensitive enough to
detect the surface oxidation state of manganese oxides[9] and
changes in the coordination environment of the Mn ions.[10]
The measured line shape of the Mn L2,3 edge (Figure S14)
agrees well with those found for large single crystals of
MnWO4.[11] The spectra confirm the predominance of Mn in
the +2 oxidation state both in oxygen and reaction atmos-
phere. Nevertheless, a small increase in the intensity ratio of
the peaks at 640.0 and 641.4 eV was observed during the
reaction (Table S2). With increasing oxidation state or, in
other words, with increasing coordination of the Mn ions by
oxygen atoms, the intensity in the Mn L2,3 edge spectrum is
shifted towards higher energies.[9] Hence, the changes in the
intensity ratio of the peaks at 640.0 and 641.4 eV indicate that
the concentration of oxygen defects is higher in the reaction
feed than under oxygen atmosphere, suggesting that the
oxygen defects have a substantial impact on the catalyst
performance. The oxygen defects were quantified by two
temperature-programmed oxidation (TPO) and reduction
(TPR) cycles (Figure S15). Before the first TPO run, the
catalyst was heated in argon atmosphere at 4008C for two
hours. Approximately three oxygen atoms per nm2 could be
replenished after this treatment (Table S3). This result under-
lines the notion[12] that only a fraction of the surface atoms is
catalytically active, which, however, holds great challenges in
terms of the identification of active sites. The very similar
hydrogen-consumption profiles of the two TPR runs indicate
that approximately 5% of the surface oxygen atoms were
reversibly removed by reacting with H2 (Table S3).
MnWO4 are described in in the Supporting Information. In situ
Raman measurements were performed with a Kaiser Optics Raman
spectrometer RXN1 equipped with a fiber-optic probe head at a laser
wavelength of 785 nm. Additional experimental details of the
characterization techniques and catalytic tests are summarized in
the Supporting Information.
Acknowledgements
This work was conducted in the framework of the BasCat
collaboration between BASF SE, TU Berlin, FHI, and the
cluster of excellence “Unified Concepts in Catalysis”
(UniCat, www.unicat.tu-berlin.de). X.L. acknowledges the
Berlin International Graduate School of Natural Sciences and
Engineering (BIG NSE) for financial support. We thank
Maike Hashagen, Jasmin Allan, Achim Klein-Hoffmann, Dr.
Olaf Timpe, and Caroline Dessal for technical assistance. We
thank the HZB staff for their continuous support of the
electron spectroscopy activities of the FHI at BESSY II.
Keywords: heterogeneous catalysis · hydrothermal synthesis ·
in situ Raman spectroscopy · manganese tungstate ·
oxidative dehydrogenation
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Angew. Chem. 2016, 128, 4161–4165
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pristine MnWO4 nanorods, the catalytic activity decreased
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solution (Table S4). This further corroborates our argument
that surface MnOx chains are the active sites of the nano-
structured MnWO4 catalyst in the oxidative dehydrogenation
of propane.
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Received: November 2, 2015
Published online: February 23, 2016
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