How to control selectivity in alkane oxidation?

Article abstract of DOI:10.1039/C8SC04641G

The well-defined particle morphology of crystalline MnWO₄ catalysts investigated in the present study facilitates obtaining insight into the origin of selectivity limitations in alkane oxidation. Hydrothermal synthesis at variable pH values granted access to a series of phase-pure MnWO₄ catalysts with particles ranging from cube-like (aspect ratio 1.5) to rod- or needle-like (aspect ratio 6.8) shapes. Kinetic studies reveal a strong dependence of the propane consumption rate on the particle shape. The true origin of the structure sensitivity was unraveled by comprehensive bulk and surface analysis using nitrogen adsorption, XRD, SEM, ADF-STEM, STEM-EELS, XPS, multi-laser excitation Raman and DRIFT/operando FTIR spectroscopies, temperature-programmed oxidation (TPO), in situ NEXAFS, and DFT calculations. The active phase is composed of a thin manganese oxy-hydroxide layer formed on the surface of crystalline MnWO_4. The differences in catalytic performance within the series clearly illustrate that the structural motif as the most popular descriptor in oxidation catalysis is not essential, since all MnWO₄ catalysts in the series under study exhibit the same bulk crystal structure and bulk chemical composition and are phase pure and homogenous. The variable particle shape serves as a proxy that reflects the formation of varying abundance of redox active Mn²⁺/Mn³⁺ surface sites, which correlates with catalytic activity. Operando FTIR spectroscopy directly confirms the formation of Mn-OH surface species by abstraction of hydrogen atoms from the propane molecule on nucleophilic oxygen atoms and suggests that active site regeneration occurs via oxidative dehydrogenation of Mn-OH species indicating a single-site nature of the active sites that does not allow four-electron reduction of molecular oxygen. Instead, intermediates are created that cause side reactions and lower the selectivity. The findings highlight fundamental design criteria that may be applied to advance the development of new alkane oxidation catalysts with improved selectivity.

Full text of DOI:10.1039/C8SC04641G

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How to control selectivity in alkane oxidation?†
a
Xuan Li,ab Detre Teschner,ac Verena Streibel,a Thomas Lunkenbein,
Cite this: DOI: 10.1039/c8sc04641g
Liudmyla Masliuk,a Teng Fu,a Yuanqing Wang,ab Travis Jones,
Friedrich Seitz,a
a
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Frank Girgsdies,a Frank Rosowski,bd Robert Schloglac and Annette Trunschke
¨
a
*
The well-defined particle morphology of crystalline MnWO4 catalysts investigated in the present study
facilitates obtaining insight into the origin of selectivity limitations in alkane oxidation. Hydrothermal
synthesis at variable pH values granted access to a series of phase-pure MnWO4 catalysts with particles
ranging from cube-like (aspect ratio 1.5) to rod- or needle-like (aspect ratio 6.8) shapes. Kinetic studies
reveal a strong dependence of the propane consumption rate on the particle shape. The true origin of
the structure sensitivity was unraveled by comprehensive bulk and surface analysis using nitrogen
adsorption, XRD, SEM, ADF-STEM, STEM-EELS, XPS, multi-laser excitation Raman and DRIFT/operando
FTIR spectroscopies, temperature-programmed oxidation (TPO), in situ NEXAFS, and DFT calculations.
The active phase is composed of a thin manganese oxy-hydroxide layer formed on the surface of
crystalline MnWO4. The differences in catalytic performance within the series clearly illustrate that the
structural motif as the most popular descriptor in oxidation catalysis is not essential, since all MnWO4
catalysts in the series under study exhibit the same bulk crystal structure and bulk chemical composition
and are phase pure and homogenous. The variable particle shape serves as a proxy that reflects the
formation of varying abundance of redox active Mn2+/Mn3+ surface sites, which correlates with catalytic
activity. Operando FTIR spectroscopy directly confirms the formation of Mn–OH surface species by
abstraction of hydrogen atoms from the propane molecule on nucleophilic oxygen atoms and suggests
that active site regeneration occurs via oxidative dehydrogenation of Mn–OH species indicating a single-
site nature of the active sites that does not allow four-electron reduction of molecular oxygen. Instead,
intermediates are created that cause side reactions and lower the selectivity. The findings highlight
fundamental design criteria that may be applied to advance the development of new alkane oxidation
catalysts with improved selectivity.
Received 18th October 2018
Accepted 20th December 2018
DOI: 10.1039/c8sc04641g
rsc.li/chemical-science
oxidant. The multiplicity of potential reaction pathways initi-
ated by C–H activation on the surface of metal oxides,2 and the
Introduction
simultaneous occurrence of active oxygen species require cata-
lysts that master selectivity predictively.
A quarter of the market share of chemical intermediates and
consumer products is produced by selective oxidation of
hydrocarbon molecules. Taking into account the dimensions of
these applications, selective formation of desired reaction
products is of vital importance in terms of sustainable utiliza-
tion of resources and carbon dioxide mitigation.1 Most
frequently, complex mixed oxides are used as catalysts in
heterogeneous processes applying molecular oxygen as an
Thus research is focused on the identication of descriptors
that provide directions in catalyst development and contribute
to an improved understanding of relationships between reac-
tion mechanisms and the dynamics of the atomistic surface
structure.3,4 Facet sensitivity has been proposed as an important
parameter that controls the establishment of selective single-
site catalysts.5–7 The concept is of fundamental importance in
heterogeneous catalysis and materials science.8–12 The effects of
size, shape and termination of active catalyst particles has been
most frequently discussed in metal-catalyzed reactions.9,11–15
Similarly, an impact of the catalyst particle morphology on
catalytic properties has been found for nano-crystalline metal
oxides.14,16–18 Shape-dependent activities in oxidation reactions
over binary oxides such as MoO3,19,20 MgO,21–23 Co3O4,24,25
CeO2 26–28 and Cu2O,29 and also complex oxides, such as single-
phase MoVTeNb oxide,6,30 have been attributed to the specic
activity of selected crystal planes.
aDepartment of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellscha,
Faradayweg 4-6, 14195 Berlin, Germany. E-mail: trunschke@i-berlin.mpg.de; Tel:
+49 30 8413 4457
bUniCat-BASF Joint Lab, Technische Universitat Berlin, Sekr. EW
K 01,
¨
Hardenbergstraße 36, 10623 Berlin, Germany
cDepartment of Heterogeneous Reactions, Max-Planck-Institut fur Chemische
¨
¨
Energiekonversion, Stistraße 34-36, 45470 Mulheim a. d. Ruhr, Germany
dBASF SE, Process Research and Chemical Engineering, Heterogeneous Catalysis, Carl-
Bosch-Straße 38, 67056 Ludwigshafen, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8sc04641g
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A full understanding of activity–structure relationships over Aldrich) while stirring leading to a mixed solution of pH ¼ 6.7.
oxide catalysts is, however, challenging due to the complexity of Subsequently, the pH of the mixed solution was adjusted to 6.3,
metal oxide surfaces in terms of polarity,31 hydroxyl-hydrate 8.0, 9.1, and 9.9, respectively, by adding appropriate amounts of
cover,32 and defects.33–35 Furthermore, deviations from the 0.1 M HNO3 (64–66%, Sigma Aldrich) or 0.1 M NaOH (98%, Alfa
bulk with respect to elemental composition and oxidation states Aesar), respectively. The mixtures were transferred to the auto-
have been reported.36,37 A thin surface layer detected on the clave and the temperature was raised from 293 K to 453 K at
surface of vanadium oxide-based catalysts accounts for a rate of 5 K minꢀ1. The synthesis temperature was kept at 453 K
a dynamic charge transfer between the bulk and surface, which for 12, 24, and 48 h, respectively. Aer cooling down the gel at
is reected in the response of the work function, electron a rate of 5 K minꢀ1, the products of hydrothermal synthesis
affinity and surface potential barrier to the chemical potential were isolated by centrifugation and washed twice with de-
of the gas phase in analogy to the behavior of gas sensors with ionized water (MilliPore®). In the nal step, the solids were
strong impact on catalytic properties.38–42
dried in a muffle furnace in air at 353 K for 12 h. The products of
In the present work we investigate the actual cause of hydrothermal synthesis were annealed in argon (ow rate: 50
structure sensitivity in oxidation catalysis over metal tungstate mL minꢀ1) at 673 K (heating rate: 5 K minꢀ1) for 2 h using
catalysts by studying the impact of shape and size of primary a rotary tube furnace (XERION ADVANCED HEATING Ofen-
catalyst particles in the oxidative dehydrogenation (ODH) of technik GmbH, Freiberg, Germany) resulting in seven phase-
propane as a relevant case study over a series of highly crystal- pure MnWO4 catalysts referred to as “freshly activated cata-
line, but nano-structured manganese tungstate catalysts. lysts” in the following. The catalysts are called pH x.x@y h,
Compared to previously reported activities of metal molybdates where x.x corresponds to the pH value adjusted before hydro-
MMoO4 (M ¼ Ni, Co, Mn, Mg and Zn),43–47 and tungstates MWO4 thermal synthesis and y corresponds to the hydrothermal
(M ¼ Ni, Co, Zn, Fe and Ce)48,49 in the oxidative dehydrogenation synthesis time at 453 K.
of propane, nano-structured MnWO4 exhibits an exceptionally
high specic reaction rate that has been attributed to active
sites in a well-dened manganese oxide surface layer, which is
Catalyst characterization
formed under the specic conditions of hydrothermal High resolution TEM (HRTEM) and high resolution annular
synthesis.50,51 The result inspired us to prepare a series of phase- dark eld scanning transmission electron microscopy (ADF-
pure MnWO4 catalysts with different particle morphologies by STEM) were performed on a double corrected JEOL JEM-
adopting the hydrothermal synthesis conditions.
Rigorous analysis of bulk and surface structures is per- hexapole aberration correctors for probe and image forming
formed applying variety of complementary techniques lenses, respectively, and a cold eld emission gun (CFEG). The
ARM200F equipped with CEOS CESCOR, and CEOS CETCOR
a
including multi-laser excitation Raman spectroscopy, X-ray acceleration voltage was set to 200 kV. The specimens were
photoelectron spectroscopy (XPS) and electron energy loss prepared by drop deposition from an ethanolic suspension onto
spectroscopy (EELS). In situ X-ray absorption spectroscopy (XAS) lacey-carbon-coated Cu grids. Field emission scanning electron
and operando Fourier transform infrared spectroscopy (FTIRS) microscopy (FESEM) was carried out with a Hitachi S4800
are applied to gain insight into the sequence of reaction steps instrument operating at 5 kV. STEM-EELS measurements were
involved in selective and unselective oxidation.
conducted with a Gatan Quantum Image Filter. The spectra
Owing to the regular, well-dened surface structure of the were dark current and background corrected.
nano-structured MnWO4 catalysts, direct spectroscopic
The surface area of the catalysts was determined using
evidence of propane activation and assistance of oxygen in the a volumetric N2 physisorption apparatus (Autosorb-6-B, Quan-
regeneration of active sites is provided. The ndings unravel the tachrome). The catalysts were treated in a dynamic vacuum at
origin of limited selectivity to valuable products in alkane 573 K for 2 h prior to adsorption of nitrogen at 77 K. The specic
oxidation over this type of catalyst and attach importance to the surface area was calculated according to the BET method in the
multifaceted appearance of the active site isolation concept, pressure range p/p0 ¼ 0.05–0.35 using 11 data points.
which has been postulated more than 50 years ago,52 trans-
ferring this knowledge to the age of big data.53
X-ray diffraction (XRD) measurements were performed in
Bragg–Brentano geometry on a Bruker AXS D8 Advance II theta/
theta diffractometer, using Ni ltered Cu Ka radiation and
a position sensitive LynxEye silicon strip detector. The XRD data
were evaluated by whole powder pattern tting according to the
Rietveld method as implemented in the TOPAS soware
[version 4.2, copyright 1999–2009 Bruker AXS] taking into
account the anisotropic crystallite shape of the catalyst particles
Experimental and computational
details
Synthesis of catalysts
The hydrothermal synthesis of nano-structured MnWO4 was as described in detail before.51
¨
performed in an analytical autoclave HPM-PT-040-Monch (Pre-
mex Reactor GmbH) adopting a method that has been reported a confocal Raman spectroscopy system (S&I Spectroscopy &
previously.54 In the rst step, a 0.2 M aqueous solution of Imaging GmbH) including
monochromator (Princeton
Raman measurements have been carried out using
a
Mn(NO3)2 (Mn(NO3)2$4H2O, 98%, Roth) was added to a 0.2 M Instruments) connected with a liquid nitrogen cooled CCD
aqueous solution of Na2WO4 (Na2WO4$2H2O, 99%, Sigma camera. The Raman spectra were obtained using a variety of
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individual lasers operating at 532 nm, 488 nm, 457 nm, 442 nm
Oxidative dehydrogenation of propane was carried out using
and 355 nm excitation, respectively. A laser power lower than a setup for partial oxidation (Integrated Lab Solutions) with 8
0.50 mW was used to avoid beam damage. In addition, a He–Ne xed bed quartz reactors (6 mm inner diameter) in parallel.
632.8 nm laser (1.5 mW) was applied for excitation using Each reactor was equipped with a thermocouple for measuring
a Horiba Jobin LABRAM instrument equipped with a micro- the temperature inside the catalyst bed containing 100–300 mg
scope (Olympus). A pressed wafer of the catalyst was mounted of catalyst previously pressed and sieved to a particle size of
on the sample holder in air at ambient temperature without 250–355 mm. The catalytic performances were determined at
pretreatment when recording the spectrum.
atmospheric pressure. The reactant feed was composed of C3H8,
Operando FTIRS measurements were carried out using O2, and N2 in a volume ratio of 10 : 5 : 85 at a total ow rate of
a Varian 670 FTIR spectrometer equipped with a MCT detector. 10–20 mL minꢀ1. The gas mixtures were analyzed by using an
The spectra were recorded at a resolution of 2 cmꢀ1 accumu- online gas chromatograph (Agilent 7890). A system of Plot-Q
lating 128 scans. Self-supported wafers (area weight of 28–45 mg and Plot-MoleSieve 5A columns connected to a thermal
cmꢀ2) were transferred into an IR cell that is connected to conductivity detector (TCD) separated the permanent gases CO,
a vacuum line and a gas delivery system. Prior to the reaction, CO2, N2, O2, and CH4. A system of a FFAP and a Plot-Q column
the catalysts were treated at 673 K in a ow of 50 mL minꢀ1 He connected to a ame ionization detector (FID) allowed the
for 2 hours. The oxidative dehydrogenation of propane was separation of C2–C3 hydrocarbons and oxygenates. The freshly
investigated at atmospheric pressure and 673 K using C3H8/O2/ activated catalysts were loaded into the reactor and the
He ¼ 10/5/85 feed at a ow rate of 10 and 5 mL minꢀ1
,
temperature was increased in the feed to reaction temperature
respectively. The product was analyzed using an on-line gas 673 K. Conversion of propane X, selectivity to product j Sj and
chromatograph (Varian Mirco GC 490) equipped with 10 m Pora propane consumption rates r were calculated according to
Plot Q and 1 m COX column modules and with TCD detectors. eqn (1)–(3)
In situ diffuse reectance infrared Fourier transform spec-
n
X
nici
i¼1
3cC H ;inlet
troscopy (DRIFTS) was conducted using an Agilent Cary 680
FTIR spectrometer equipped with a MCT detector at a spectral
resolution of 4 cmꢀ1 and accumulation of 512 scans. An in situ
cell (Harrick Praying MantisTM diffuse reectance attachment
DRP-P72 in combination with a low temperature CHC-CHA
reaction chamber with ZnSe windows) was used. Spectra were
taken at 77 K or 313 K, respectively, aer appropriate pretreat-
ment in the reaction chamber. The amount of the catalyst used
was approximately 50 mg. 50 mL minꢀ1 argon was purged
during heating, and a heating rate of 5 K minꢀ1 was applied.
X-ray photoelectron spectra were recorded at room temper-
ature, using non-monochromatized Al Ka (1486.6 eV) excitation
and a hemispherical analyzer (Phoibos 150, SPECS). The
binding energy scale was calibrated by the standard Au 4f7/2 and
Cu 2p3/2 procedure. Theoretical cross sections were used to
calculate the elemental composition.55
X ¼
Sj ¼
(1)
(2)
3
8
nici
n
X
nici
i¼1
Fc
C3H8;outlet
r ¼
(3)
mcatSBET
where ni is the carbon atom number in the product molecule i,
ci is the concentration of the product i analyzed by gas chro-
matography, c
and c
are the concentrations of
C3H8, inlet
C3H8, outlet
propane in the feed gas at the reactor inlet and outlet, F is the
total ow rate of the inlet gas stream, mcat is the weight of the
catalyst loaded into the reactor, and SBET is the specic surface
area of the catalyst.
Near edge X-ray absorption ne structure (NEXAFS) analysis
was conducted at the near ambient pressure XPS end station of
the ISISS beam line at HZB/BESSY II (Berlin, Germany). Details
of the setup have been published earlier.56 The experiments
DFT calculations
were performed at a total pressure of 0.25 mbar in O2/He or All DFT calculations were performed using the Quantum
C3H8/O2/He mixtures with a total gas ow of 1.2 sccm at ESPRESSO package57 using ultraso pseudopotentials (PP) from
temperatures between 573 K and 673 K. NEXAFS spectra were the Original QE PP library and the exchange and correlation
recorded in total electron yield (TEY) mode. Due to the low potential developed by Perdew, Burke, and Ernzerhof58 with
inelastic mean free path of electrons in solids, electron yield X- Cococcioni's and de Gironcoli's simplied rotationally invariant
ray absorption spectroscopy is more surface sensitive than Hubbard U applied to the Mn d-orbitals.59 Wave functions were
uorescence based techniques.
expanded in a planewave basis set with a kinetic energy (charge
Temperature-programmed oxidation (TPO) was performed density) cutoff of 30 Ry (300 Ry). A k-point mesh equivalent to (6
in a xed-bed quartz reactor using 300 mg of the catalyst. Prior ꢁ 6 ꢁ 1) for the (010) surface unit cell was used. The Mn-rich
to the rst TPO measurement, the catalyst was pretreated at 673 MnWO4 surfaces were modeled with a symmetric 54 atom
K for 2 h in Ar (ow rate: 50 mL minꢀ1, heating rate: 5 K minꢀ1). slab (4 layers of stoichiometric MnWO4 and an additional layer
The TPO measurement was carried out in a mixture 0.24% O2/ of MnO4). The initial ion positions and lattice parameters for
He (ow rate 100 mL minꢀ1), applying a heating rate of 5 MnWO4 under ambient conditions were taken from the crystal
K minꢀ1 and a holding time at 673 K of 60 min. O2 consumption structure analysis.60 The ion positions of the surface atoms were
was monitored with a paramagnetic detector.
allowed to relax while holding the central 24 atoms (2 layer of
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stoichiometric MnWO4) and the lattice parameters xed. Similarly, MnO6 octahedra form zigzag chains along the same
˚
Approximately 20 A of vacuum was used to separate periodic crystallographic axis by sharing corners with WO6 octahedra.
images. The Mn atoms were ordered ferromagnetically in the Along the [100] axis the two types of zigzag chains stack
(010) plane and antiferromagnetically in alternating planes in alternately by edge-sharing.50,51
the direction normal to the surface.
SEM images (Fig. 1) of the as-synthesized MnWO4 nano-
The value of U (3.05 eV) applied to the Mn d-orbitals in this oxides indicate morphological alteration, which is induced by
work was computed for Mn in a (2 ꢁ 2 ꢁ 2) type I antiferro- changes of the synthesis protocol (pH and time). The variations
magnetic supercell of MnO using linear response.59 The calcu- can be expressed by different aspect ratios (AR) of the primary
lation was performed using ion positions and lattice parameters catalyst particles (Table 1). Fig. 1 shows exemplarily catalysts
for MnO taken from the crystallography open database, and an with very different aspect ratios. Cube-like particles are ob-
(8 ꢁ 8 ꢁ 8) k-point mesh was employed with Marzari–Vander- tained at low pH (6.3 and 6.7), while with increasing pH (8.0–
bilt cold smearing with a smearing parameter of 0.001 Ry.61 9.9) and dwell time (12–24 h) the AR increases, which is re-
Subsequent optimization of the experimental lattice parameters ected in a pronounced rod-like morphology. Only for higher
with U ¼ 3.05 eV led to a minor expansion of the lattice, from dwell times (48 h) at the highest pH (9.9) the AR decreases
˚
˚
4.45 A to 4.53 A.
again, which is attributed to dissolution of the nano-rods in the
The Mn rich surface model employed in this work was highly alkaline medium over prolonged time.
developed in accordance with previous TEM ndings, where the
Previous STEM images have shown a defective character of
(010) surface was found to be terminated in an Mn–O–Mn rods with an aspect ratio of 5.1, which can be described as
motif.50 Both sides of the slab were then hydroxylated and the extended bulk defects that propagate to the surface.50 The
ion positions were allowed to relax to their ground states before particle length determined by TEM differed from the particle
computing the OH stretching frequencies by using nite dimension along the h001i direction determined by XRD.51
displacements of the H and O atoms involved in OH bonding. Hence, it was concluded that the rod-like particles are
Note that using only nite displacements of H resulted in composed of a number of crystals along the h001i direction. In-
changes of less than 1 cmꢀ1 in the computed vibrational modes. depth analysis of annular dark-eld scanning transmission
electron microscopy (ADF-STEM) images of the catalyst pH
9.9@12 h with an aspect ratio of 5.1 (Fig. 2A–D) reveals that the
extended defects represent hW3Oyi trimers formed by conden-
Results and discussion
sation of two MnWO4 crystals. The defects propagate to the
[010] surface. The crystals before and aer the trimeric inter-
growth are shied by one W atom along [010] (Fig. 2C). The
high-resolution ADF-STEM image in Fig. 2D displays a surface
terminated with the (010) plane of MnWO4 viewed along [100].
The surface exhibits a partial enrichment of manganese (Mn
atoms highlighted by the blue arrows). In contrast, ADF-STEM
images of MnWO4 pH 6.3@12 h with a low aspect ratio (1.5)
(Fig. 2E and F) reveal the absence of extended defects. For
example, the high resolution ADF-STEM micrograph presented
in Fig. 2F reproduces the boundary region of two condensed
MnWO4 crystals, which do not themselves show irregular
geometric structures.
Polycrystalline MnWO4 catalysts were prepared by hydro-
thermal synthesis followed by thermal treatment in argon at
673 K. Particle shape and size were adjusted through variation
of pH51 and synthesis time. The synthesis parameters and
general characteristics of the different catalysts are summa-
rized in Table 1. The phase-purity of the catalysts was
conrmed by Rietveld renement (ESI, Fig. S1†). All catalysts
are solely composed of the wolframite-type structure, which is
based on a distorted hexagonal closed packing of oxygen
atoms with Mn and W atoms each occupying one fourth of the
octahedral
interstices.62
The
ortho-tungstate
family
MII(W,Mo)VIO4 comprises molybdates and tungstates of biva-
II
˚
lent metals with an ionic radius smaller than 0.77 A (M ¼ Fe,
Mn, Co, Ni, Mg, Zn).54 Distorted WO6 octahedra form zigzag
chains by sharing edges along the [001] crystallographic axis.
The variation in synthesis conditions is not only reected in
different particle morphologies, but also in varying activity and
Table 1 General information about the catalysts prepared by hydrothermal synthesis at different pH and times and activated in Ar at 673 K for 2
hours
Catalyst
Catalyst IDa
pHb
Synthesis timec (hours)
Aspect ratiod (AR)
SBET (m2 gꢀ1
)
pH 6.3@12 h
pH 6.7@12 h
pH 8.0@12 h
pH 9.1@12 h
pH 9.9@12 h
pH 9.9@24 h
pH 9.9@48 h
19 112
19 113
19 114
19 251
19 116
20 111
20 112
6.3
6.7
8.0
9.1
9.9
9.9
9.9
12
12
12
12
12
24
48
1.5
1.7
3.2
3.5
5.1
6.8
5.4
25.9
22.9
22.1
21.0
28.7
21.5
19.2
a
b
c
Required to identify clearly different batches of catalyst synthesis. pH value adjusted before hydrothermal synthesis. Time of hydrothermal
synthesis at 453 K. d Aspect ratio of primary catalyst particles determined by electron microscopy.
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Fig. 1 SEM images of the as-synthesized catalysts pH 6.3@12 h (AR ¼ 1.5), pH 8.0@12 h (AR ¼ 3.2), pH 9.9@12 h (AR ¼ 5.1), pH 9.9@24 h (AR ¼
6.8), and pH 9.9@48 h (AR ¼ 5.4). The scale bar is 500 nm.
Fig. 2 ADF-STEM images of MnWO4 nanoparticles pH 9.9@12 h (AR ¼ 5.1) (A–D) and pH 6.3@12 h (AR ¼ 1.5) (E and F) at different magnifications.
The inset in (B) denotes the Fast Fourier Transform (FFT) of the presented area. (C) shows a magnified FFT filtered image of the region highlighted
by the white box in (B). The white arrows in (C) correspond to the shift direction. The violet color in (C) displays a region of a W trimeric
intergrowth, while the green areas are attributed to W dimers before and after the intergrowth. The inset in (C) represents a magnified inverted
ADF-STEM image of the region highlighted by the violet box in (C). The red arrow points to a W trimer. (D) demonstrates a (010) terminated
surface of a MnWO4 crystal viewed along [100]. The blue arrows highlight surface Mn sites.
selectivity in propane oxidation. The catalysts with a low aspect phase reactions. All catalysts produce exclusively propene, CO
ratio prepared at pH < 9.9 that lack defects show lower catalytic and CO2.
activity compared to the catalysts prepared at pH ¼ 9.9 that are
The activity changes with time on stream (Fig. 3a). All cata-
characterized by a higher abundance of bulk and surface lysts prepared aer adjustment of a starting pH < 9.9 deactivate
defects (Fig. 3). Propane and oxygen conversions were kept gradually. No systematic behavior is observed for the three
below 12% and 30% (oxygen conversion not shown), respec- catalysts synthesized applying different hydrothermal times at
tively, to avoid mass transport limitation and signicant hot- pH ¼ 9.9. The catalyst pH 9.9@12 h initially deactivates, but at t
spots. Furthermore, the maximum reaction temperature was > 20 h the activity increases again with time on stream. In
limited to 673 K to minimize contributions of homogenous gas- contrast, the activity of the catalyst pH 9.9@24 h passes through
a maximum at t ¼ 10 h, while the catalyst prepared for 48 h pH
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Fig. 3 (a) Propane conversions as a function of time-on-stream (TOS) (T ¼ 673 K, W/F ¼ 1.8 g s mLꢀ1, feed: 10% C3H8, 5% O2 and 85% N2), and (b)
selectivity as a function of conversion under steady state conditions (t > 100 h, T ¼ 673 K, W/F ¼ 0.9–2.4 g s mLꢀ1, feed: 10% C3H8, 5% O2 and 85%
N2).
9.9@48 h exhibits increasing activity with time on stream (Fig. 1 and 4) are modied structurally very little by the reaction
without reaching the steady state. For the sake of convenience, atmosphere, which is in agreement with the moderate deacti-
the state aer 100 h time on stream is called steady-state also for vation of the catalyst (Fig. 3a, black data points).
this catalyst. These observations indicate that all catalysts
undergo signicant modications under reaction conditions.
Due to the marked changes in the activity from the initial to
the steady-state, the results of spectroscopic surface character-
The changes are neither caused by destruction of the bulk ization of freshly activated catalysts are only used for the
crystal structure as shown by XRD (Fig. S2†), nor by signicant interpretation of initial catalytic performance measured at t ¼ 0.
changes in the overall morphology (Fig. 4). SEM (Fig. 4, AR ¼ 3.2 Differences in the initial propane conversion over the catalyst
and AR ¼ 5.1) and (S)TEM (Fig. 5) images of the spent rod-like series (Fig. 3a) cannot be related to differences in the specic
MnWO4 with a higher aspect ratio display a partially recon- surface area, because the specic surface area of all catalysts is
structed surface (see arrows in Fig. 4) along the growth direc- similar and varies between 20 and 30 m2 gꢀ1 (Table 1). The
tion, which is not observed in the SEM images of the used cube- consumption rates of propane normalized to the specic
like MnWO4 (Fig. 4, AR ¼ 1.5). High-resolution ADF-STEM surface area and measured at t ¼ 0 are presented in Table 2. The
images in Fig. 5C and D suggest that the surface reconstruc- table also comprises specic reaction rates measured under
tion is independent of the crystallographic surface termination. steady-state conditions (t > 100 h) for comparison. Systematic
Compared to the fresh catalyst (Fig. 2A and 5A), a higher frac- correlations between the aspect ratio and the normalized cata-
tion of low aspect ratio particles is observed in the overview lytic activity were not observed, neither at t ¼ 0 h nor at t > 100 h.
TEM image of the spent catalyst pH 9.9@12 h (Fig. 5B), most The catalysts prepared at the highest pH are generally more
likely caused by splitting of the rods into smaller fractions active compared to the catalysts prepared at lower pH (Fig. 3,
under reaction conditions. This is also supported by anisotropic Table 2), but no clear trend becomes obvious.
Rietveld renement of the XRD patterns of the used catalysts
The selectivity as a function of conversion under steady-state
(Table S1†). The analysis suggests that in particular the longer conditions is presented in Fig. 3b. The selectivity of propene
rods are composed of several coherently scattering domains decreases with increasing propane conversion in agreement
stacked in the crystallographic c direction, and the number of with consecutive reactions of formed propene resulting nally
these domains increases aer the use of the catalyst (Table S1†). in the formation of CO and CO2. The S–X trajectories seem to
In contrast, MnWO4 nano-cubes (pH 6.3@12 h (AR ¼ 1.5)) follow roughly the same trend for all seven catalysts suggesting
Fig. 4 SEM images of the catalysts pH 6.3@12 h (AR ¼ 1.5), pH 8.0@12 h (AR ¼ 3.2), and pH 9.9@12 h (AR ¼ 5.1) after oxidative dehydrogenation of
propane at T ¼ 673 K. The arrows denote reconstructed surfaces. The scale bar is 200 nm.
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Fig.
6
Initial consumption rate of propane (filled symbols) and
consumption rate of propane in the steady-state (open symbols) as
a function of the Mn to W atomic ratio in the near surface region
determined by XPS on the fresh (filled symbols) and used (open
symbols) catalysts (see also Table 2).
Fig. 5 (S)TEM images of pH 9.9@12 h (AR ¼ 5.1) before (A) and after
(B–D) oxidative dehydrogenation of propane at T ¼ 673 K at different
magnifications.
previous publication.51 In short, the basic conditions of the
hydrothermal synthesis provoke preferential leaching of tung-
sten oxide moieties at (010) crystal planes. Re-crystallization
proceeds via condensation of the dissolved [WO4]2ꢀ species
with W–O(H) groups at high-energy (001) basal planes resulting
in an anisotropic growth along the crystallographic h001i
direction and an enrichment of Mn(OH)x species on the surface.
The Mn/W ratio in the bulk is in agreement with the stoichi-
ometry of the compound. The Mn/W ratio near the surface
(Table 2) increases with increasing starting pH, and remains
almost constant with increasing hydrothermal synthesis time at
pH ¼ 9.9 except for the catalyst pH 9.9@48 h. For the latter the
Mn surface concentration decreases again slightly perhaps due
to re-dissolution or increasing aggregation of manganese oxide
clusters due to the extended hydrothermal synthesis time under
basic conditions.
no signicant changes in the reaction network within the series
of catalysts. Extrapolation to zero percent conversion suggests
that parallel reactions of propane leading to deep oxidation
occur to a certain extent. Differences in selectivity at compa-
rable conversion comprise only up to 10% among all catalysts.
In the following discussion, we will therefore concentrate on an
explanation of differences in activity.
The surface properties of the catalysts have been studied by
spectroscopic techniques and temperature-programmed
oxidation. The molar Mn/W ratio determined by photoelec-
tron spectroscopy for all seven catalysts is presented in Table 2
and Fig. 6. The increase in activity at t ¼ 0 with increasing pH
during hydrothermal synthesis can be attributed to an enrich-
ment of Mn in the near surface region (Fig. 6) in agreement with
facet-specic dissolution–recrystallization processes under
hydrothermal conditions that are responsible for the develop-
ment of the rod-like shape of the primary catalyst particles. The
mechanism of particle growth has been discussed in our
The catalytic activity of the nano-structured MnWO4 catalysts
is clearly associated with the surface concentration of manga-
nese as evidenced by the trend in the graphical representation
of the reaction rate at t ¼ 0 as a function of the Mn/W atomic
Table 2 Normalized consumption rates of propane at t ¼ 0 and under steady-state conditions (t > 100 h, T ¼ 673 K, W/F ¼ 1.8 g s mLꢀ1, feed: 10%
C3H8, 5% O2 and 85% N2) in the oxidative dehydrogenation of propane over nano-structured MnWO4 catalysts, oxygen defect densities on the
freshly activated catalysts determined by TPO, and elemental composition near the surface of fresh and used catalysts determined by XPS
Oxygen defect
Aspect
rpropane, initial (t¼0)
Nab/
(Mn + W)
rpropane, steady-state
Catalyst
ratioa (AR)
(10ꢀ9 mol mꢀ2 sꢀ1
)
Mnb/W
densityc ðVOꢂꢂ =nm2Þ
(10ꢀ9 mol mꢀ2 sꢀ1
)
Mnd/W
pH 6.3@12 h
pH 6.7@12 h
pH 8.0@12 h
pH 9.1@12 h
pH 9.9@12 h
pH 9.9@24 h
pH 9.9@48 h
1.5
1.7
3.2
3.5
5.1
6.8
5.4
1.1
2.4
10.0
9.1
9.4
11.4
7.6
1.13
1.20
1.30
1.32
1.30
1.32
1.27
0.11
0.13
0.10
0.079
0.10
0.17
0.056
0.57
0.75
3.81
4
3.06
2.87
0.67
0.36
1.1
2.9
2.8
9.2
1.07
1.13
1.25
1.23
1.26
n.d.
n.d.
7.1
11.3
a
b
Determined by electron microscopy. Determined by X-ray photoelectron spectroscopy (XPS) of the fresh thermally activated catalysts.
c
d
Determined by temperature-programmed oxidation (TPO) of the fresh thermally activated catalyst. Determined by X-ray photoelectron
spectroscopy (XPS) of catalysts used in oxidative dehydrogenation of propane.
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ratio determined by XPS near the surface of the freshly activated
catalysts (lled symbols) (Fig. 6). The trend is conrmed by
taking into account the rates measured under steady-state
conditions and the Mn/W ratios of the used catalysts (open
symbols). The correlation is not linear, which indicates that
clustering effects may play a role with increasing coverage of the
surface by manganese oxide species. Based on the trend, the
rate appears to be zero at a Mn/W ratio of 1, which is in
agreement with the very low activity of the commercial MnWO4
reference sample that was not prepared by hydrothermal
synthesis and not exposed to surface leaching.50
Although the solids were carefully washed aer hydro-
thermal synthesis, all catalysts contain sodium impurities
derived from Na2WO4 used as the tungsten precursor or NaOH
applied to adjust the pH (Table 2). Sodium may act as catalyst
poison; however, no systematic trend in terms of sodium
content, synthesis conditions, activity and selectivity (Table 2,
Fig. 3) is noted.
Fig. 8 Raman spectra of pH 9.9@12 h recorded using different exci-
tation energies. The applied laser wavelength is denoted on the right
side of the spectra.
In agreement with the increased Mn/W ratio on the surface
of the freshly activated catalysts measured by XPS, surface
manganese oxide species that do not belong to the bulk crystal
structure of MnWO4 have been detected by Raman spectroscopy
(Fig. 7 and 8). Fig. 7 presents the Raman spectra of all seven
catalysts recorded using an excitation wavelength of 532 nm.
The spectroscopic patterns are consistent with those of single-
crystalline manganese tungstate reported in the literature,63,64
Multi-laser excitation Raman spectroscopic analysis of the
pH 9.9@12 h catalyst (Fig. 8) reveals a profound inuence of the
excitation energy on the relative intensity of the Raman bands at
620 and 665 cmꢀ1 pointing to a resonance Raman enhancement
effect,67 sensitive to surface Mn–O(H)–Mn species that are
characterized by electronic transitions at lower energy typical of
Mn–O charge transfer bands.68,69 Moreover, this observation
demonstrates that the corresponding surface Mn oxide species
are structurally separated from the bulk. Manganese may be
anchored on the outermost layer by forming bridging Mn–O–W
and Mn–O–Mn bonds as well as hydroxyl groups. The bands are
clearly visible in the spectrum measured applying an excitation
wavelength of 532 nm. Their intensities with respect to the
fundamental modes of MnWO4 increase further by using the
633 nm laser; however, the signal to noise ratio of the latter
except for two additional peaks centered at 620 and 665 cmꢀ1
.
These two bands were attributed to surface MnOx species.50,65
Because the translational symmetry is broken at the surface,
observation of vibrational modes originating from surface Mn–
O(H)–Mn species is possible by Raman spectroscopy, although
this method is normally a bulk analysis technique. The vibra-
tional frequency of 665 cmꢀ1 actually coincides with the A1g
mode of MnO6 units in the Mn3O4 spinel structure that contains
manganese in a mixed +II/+III oxidation state.66
Fig. 7 Raman spectra recorded using excitation at 532 nm of (a) freshly activated nano-structured MnWO4 catalysts and (b) catalysts after
oxidative dehydrogenation of propane. The spectra were normalized by the maximum intensity followed by background subtraction.
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spectrum is rather low. Therefore, the comparison of all cata-
lysts, which is presented in Fig. 7, was performed using an
excitation wavelength of 532 nm.
The two bands at 620 and 665 cmꢀ1 are still detectable in the
Raman spectra of the used catalysts (Fig. 7b), but their relative
intensities I620/I885 and I665/I885 with respect to the Ag(8) mode of
bulk MnWO4 at 885 cmꢀ1 (an O2–W stretching vibration) are
decreased for catalysts pH 9.9@12 h and pH 9.9@24 h, which is
in accordance with the diminished steady-state activity of these
catalysts (Fig. 3a). In contrast, the relative intensities of the two
bands are increased in the case of used pH 9.9@48 h in
agreement with the activity that increases with time on stream.
However, no direct correlation between these intensity ratios
and catalytic performance was observed suggesting that the
corresponding manganese oxide species are just precursors of
active sites, but not quantitatively relevant in catalysis.
Fig. 10 Ball-and-stick model of the Mn-rich termination of a MnWO4
(010) surface viewed along the h100i axis (left) and the h010i axis
(right). Purple, grey, red and white balls represent Mn, W, O and H
atoms, respectively. The solid black line shows the surface unit cell.
Under ambient conditions the catalysts are covered by
a hydrate-hydroxyl shell as evidenced by DRIFTS of the catalysts
pH 6.3@12 h and pH 9.9@12 h presented as examples for low
and high aspect ratios, respectively, during catalyst pretreat-
ment in Fig. 9. The broad feature in the range between 3700 and
3000 cmꢀ1 corresponds to physisorbed water, which cannot be
removed completely even aer treatment in Ar at 673 K. A sharp
band at 3695 cmꢀ1 due to isolated W–OH groups70 observed
only in the spectra of the catalyst pH 6.3@12 h synthesized at
lower pH (Fig. 9a) disappears at temperatures between 373 K
and 473 K due to dehydroxylation. Weak features at around
3463 and 3408 cmꢀ1 and an intense band at 3366 cmꢀ1 are
visible in the spectra of all catalysts aer partial dehydroxylation
at 673 K. The intensity of the most intense peak at 3366 cmꢀ1
normalized to the area weight of the respective pellet shows
a trend (Fig. S3†) similar to that of the Mn/W ratio determined
by XPS (Fig. 6) strongly supporting the assignment of the band
to hydroxyl groups that coordinate to outermost Mn ions.51
Formation of a well-ordered inter-chain hydrogen-bonding
network might be responsible for the low frequency of the
hydroxyl stretching vibrations.51,71,72 DFT calculations also
suggest the existence of such surface structures. Fig. 10 shows
the computed Mn-rich OH terminated surface. By viewing the
surface along the h100i axis two types of hydrogen can be seen to
be present. Viewing the structure along the h010i direction
reveals that both types are oriented such that they can
participate in the hydrogen-bonding network, though to
different degrees. The difference in hydrogen bonding mani-
˚
fests itself as differences in O/H (O–H) bond lengths, 2.23 A
˚
˚
˚
(0.98 A) and 1.92 A (0.99 A). Similarly, the calculated vibrational
frequency of the OH stretch for these moieties differs slightly,
3518 cmꢀ1 and 3464 cmꢀ1. Both values, however, are in
reasonable agreement with experiment (Fig. 9), suggesting that
such a well-ordered inter-chain hydrogen-bonding network may
explain the observed low frequency OH stretching modes.
In summary, the surface chemistry of the freshly prepared
nano-structured catalysts can be described as follows. The near-
surface region of the formed nanoparticles is more and more
enriched in manganese with increasing OHꢀ concentration in
the synthesis gel at the beginning of the hydrothermal synthesis
of the catalysts. The accumulation of surface Mn species occurs
during ageing in the autoclave resulting in catalysts with varying
surface composition (Fig. 6, Table 2). The terminating Mn
surface chains are composed of hydroxyl-hydrate species
(Fig. 10), such as in Mn(OH)2$2H2O. Inter-chain hydrogen-
bonding is crucial in terms of a stabilization of the Mn termi-
nating layer on (010) surface planes.
Physisorbed water is essentially desorbed and OH groups are
partially dehydroxylated during pre-treatment of the nano-
structured MnWO4 catalysts in Ar at 673 K resulting in
a surface that is enriched in oxygen vacancies, which can be
shown by temperature-programmed oxidation (Fig. S4†). In
Fig. 9 In situ DRIFT spectra of (a) pH 6.3@12 h and (b) pH 9.9@12 h catalysts during thermal pre-treatment in argon.
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these experiments, at rst the catalysts were pretreated in argon local symmetry and will change both line shape and intensity.74
at 673 K for 2 hours followed by TPO. The pretreatment corre- The interpretation of Mn L-edge spectra is therefore not
sponds to the pretreatment before catalysis. All catalysts exhibit straightforward. However, generally, the L3/L2 white-line
a broad O2 consumption band centered at around 653 K indi- intensity ratio and the energy separation between the L3 and
cating an oxygen-decient surface aer pretreatment in the L2 peaks decrease with increasing valence of Mn.75 Moreover,
inert gas that corresponds to the condition of the catalyst the energy of the multiplet center of the Mn L-edge increases
surface at t ¼ 0 in the catalytic test. The surface concentration of with increasing valence. Near edge X-ray absorption ne struc-
oxygen vacancies within the present series of nano-structured ture (NEXAFS) spectra at the Mn L2,3 edge at 573 K in the
MnWO4 catalysts is given in Table 2. The oxygen consumption presence of owing oxygen (Fig. 11a) indicate that the
is much smaller than the theoretical monolayer consumption. predominant oxidation state of Mn in both pH 6.3@12 h and
Therefore, the oxygen defects that react with gas phase oxygen pH 9.9@12 h catalysts is two.73 The applied measurement
are supposed to be localized on the surface. However, no direct mode, total electron yield (TEY), while capturing surface infor-
correlation between oxygen defect density on the surface and mation, has an information depth of several nanometers,
synthesis conditions or the aspect ratio can be observed making it sensitive to the near surface region. Therefore, the
(Fig. S5†). The defect density increases with increasing pH and oxidation state 2+ is in agreement with the bulk oxidation state
decreases again at the highest pH and in particular with of crystalline, stoichiometric MnWO4.
increasing time due to defect healing processes. Generally, the
Comparison of the spectra in Fig. 11a, in particular the
more active catalysts exhibit in most of the cases a higher difference between the spectra of the two catalysts (Fig. 11b),
oxygen defect concentration aer activation, but there is no reveals, however, tiny differences that may indicate variations
clear correlation between activity and defect density (Fig. S5†). in the surface structure of the catalyst pH 6.3@12 h and
Two catalysts that exhibit very different aspect ratios and catalyst pH 9.9@12 h. Because the spectrum of the catalyst pH
activity (catalysts pH 6.3@12 h and pH 9.9@12 h) were studied 6.3@12 h exhibits more intensity at 639.6 eV, the L3/L2 edge
by in situ X-ray absorption spectroscopy (Fig. 11). X-ray intensity ratio of the latter is increased (Table S2†). Conse-
absorption at the Mn L edge is dominated by transition into quently, in the presence of oxygen the surface of the less active
Mn 3d states and, hence, characteristic of the Mn oxidation catalyst pH 6.3@12 h appears to be less oxidized compared to
state and coordination.73 The Mn L-edge splits into two multi- that of the catalyst pH 9.9@12 h that exhibits a higher propane
plets L3 at 638–642 eV and L2 at 650–655 eV due to spin–orbit consumption rate. The assignment is in agreement with the
splitting of the Mn 2p3/2 and 2p1/2 core levels. The oxidation spectral changes observed by comparing the spectra of the
state of manganese has a large inuence on the peak position, catalyst in an oxygen atmosphere and in a propane oxidation
line shape and intensity ratio of L3/L2 lines.74 Furthermore, feed (Fig. 11c and d). In the feed of the reactants the catalyst
coordination by different ligands, which has an impact on the pH 9.9@12 h also exhibits higher intensity at 639.6 eV, i.e.
ligand eld strength and chemical bonding, causes variation in a higher concentration of Mn2+ (or a higher degree of
Fig. 11 NEXAFS spectra measured at the Mn L2,3-edge in total electron yield (TEY) mode and a total pressure of 0.25 mbar in an oxidizing
atmosphere (O2/He ¼ 2/2.2 sccm) of (a) the catalysts pH 6.3@12 h and pH 9.9@12 h at T ¼ 573 K; (b) shows the difference between the two
spectra shown in (a); (c) presents the spectra of the catalyst pH 9.9@12 h in the feed (O2/C3H8/He ¼ 1/2/1.2 sccm) and in an oxidizing atmosphere
(O2/He ¼ 2/2.2 sccm) at T ¼ 653 K; the difference of the spectra shown in (c) is presented in (d).
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reduction), compared to that measured under an oxygen the surface spectrum, a decrease in the L3/L2 white-line ratio is
atmosphere. discernible in the surface spectrum compared to the bulk
These observations support that MnWO4 catalysts are sub- spectrum suggesting the presence of manganese also in an
jected to changes in the valence state under conditions of oxidation state higher than two on the surface of the catalyst.78,79
propane oxidation and the catalyst operates as a redox-type
In summary, the nano-structured MnWO4 catalysts exhibit
catalyst with manganese being the redox active element. Simi- a clear enrichment in manganese on the surface as measured
larly, the oxygen storage capability of Na2WO4/Mn/SiO2 applied quantitatively by XPS and conrmed by Raman spectroscopy.
as a catalyst in the oxidative coupling of methane has been The oxidation state of topmost manganese ions is rather low
attributed to the presence of manganese oxide.76
switching between 2+ and 3+ as suggested by NEXAFS and EELS.
In Scheme 1 the supposed catalytic cycle of the selective
Complementary to NEXAFS, the surface and bulk of the
catalyst pH 9.9@12 h were analyzed by line scans along the pathway in oxidative dehydrogenation of propane over MnWO4
surface layer using electron energy loss spectroscopy (EELS) is presented. Under reaction conditions in the presence of gas
(Fig. 12). The O–K edge spectra of the bulk and surface differ in phase oxygen, the oxygen vacancies (Table 2, Fig. S5†) are
particular with respect to the peak at around 530 eV (Fig. 12a). partially lled, forming Mn–O–Mn sites on the outermost
The peak intensity is diminished in the surface spectrum. surface that represent appropriate adsorption sites for propane
Taking into account the surface enrichment of Mn measured by molecules (Scheme 1, Step 1).
XPS and Raman spectroscopy, the peak at 530 eV in the bulk
The activation of the catalyst in inert gas does not lead to
spectrum might be dominated by contributions of oxygen complete dehydroxylation, as evidenced by DRIFTS (Fig. 9) and
atoms coordinated to tungsten.77 In contrast, the spectroscopic conrmed by FTIR spectroscopy (Fig. 13) applied to analyze one
patterns on the oxygen K-edge in the surface spectrum resem- of the most active catalysts (pH 9.9@12 h) under working
bles the O–K edge spectrum of a mixed Mn2+/Mn3+ oxide,78 conditions. Aer activation at 673 K in inert gas a sharp band
conrming an enrichment of manganese on the surface and due to residual Mn–OH groups at 3373 cmꢀ1 is still present in
suggesting the presence of a Mn2+/Mn3+ redox couple in the FTIR spectrum (Fig. 13a, blue spectrum) indicating that
agreement with NEXAFS. The nding is supported by bulk and dehydroxylation is difficult. Therefore, residual OH groups have
surface spectra on the Mn L-edge. The bulk spectrum corre- been included into the model of the freshly activated catalyst in
sponds mainly to Mn2+
.
78 Despite the low signal-to-noise ratio of Scheme 1 (sites at which Step 1 (propane adsorption) occurs).
The intensity of the band at 3373 cmꢀ1 increases signi-
cantly aer switching from He to the reaction gas feed at 673 K
(Fig. 13a, red spectrum). The catalyst converts 2.6% propane
and produces propene with a selectivity of 40.5% under the
applied conditions in the FTIR cell as measured simultaneously
by gas chromatography. The observed increase in the intensity
of the peak assigned to Mn–O–H stretching vibrations is the
result of establishing a steady state concentration of Mn–OH
species under reaction conditions of the operando experiment
due to abstraction of hydrogen from the C–H bonds of propane
by nucleophilic surface Mn-oxygen species (Scheme 1, Steps 2
and 3) or due to hydroxylation by the reaction product water.
Steps 2 and 3 do not require the presence of oxygen in the
reaction mixture. This could be experimentally veried by
exposing the catalyst in a separate experiment to a 10% C3H8/
90% He ow aer removing all oxygen by ushing the cell with
He for 90 minutes (Fig. 13a, green spectrum). The intensity of
the OH band is increased to the same degree as it is in the
oxygen-containing feed mixture. On switching back to C3H8/O2/
He and thus starting the reaction, no further changes of the OH
band are observed. This implies that the replenishment of OH
groups is faster than dehydroxylation.
It can be shown that formation of OH groups is not simply
caused by reaction of the catalyst surface with the reaction
product water. The selectivity to CO2 and CO in the propane
oxidation experiment in the FTIR cell corresponds to 37.2% and
6.7%, respectively. According to the reaction stoichiometry, the
concentration of formed H2O comprises approximately
0.65 vol%. In a control experiment the catalyst was exposed to
a ow of helium containing an even higher concentration of
steam (1.2 vol% H2O) at the reaction temperature (Fig. S6†). The
Fig. 12 STEM-EELS measurements of the surface (red) and bulk (blue)
of MnWO4 showing (a) the O K- and (b) the Mn L-edges. The squares in
the STEM image of MnWO4 in (c) indicate the region where EELS
measurements were conducted. Red: surface; blue: bulk. The black
scale bare in (c) is 10 nm.
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Scheme 1 Catalytic cycle of the selective pathway in oxidative dehydrogenation of propane to propene on nano-structured MnWO4 catalysts.
The reaction steps as discussed in the text are indicated by numbers in the blue circles.
Fig. 13 Operando FT-IR spectra of the catalyst pH 9.9@12 h at 673 K, gas composition and sequence of spectra as described in the legends from
top to bottom; (a) flow rate 10 mL minꢀ1; (b) flow rate 50 mL minꢀ1
.
intensity of the hydroxyl peak increases only slightly demon- comprise dehydroxylation and reoxidation steps to regenerate
strating that the catalyst can abstract hydrogen more easily from the catalyst. In classical redox mechanisms it is generally
propane than from H2O. Consequently, the increase in peak assumed that the surface is dehydroxylated rst of all and the
intensity under operation is not due to hydroxylation of the oxygen vacancies formed in this process are subsequently
catalyst surface by the formed couple product water, but it is relled by oxidation with gas-phase oxygen. To determine the
indeed due to abstraction of H atoms from the propane mole- sequence of dehydroxylation and reoxidation steps in the
cule by nucleophilic oxygen species associated with Mn on the present catalytic cycle, the time evolution of the OH band
catalyst surface (Scheme 1, Steps 2 and 3).
intensity aer switching from the reaction mixture (Fig. 13b, red
As a result of the homolytic splitting of the C–H bonds in spectrum) to He (Fig. 13b, violet spectrum) was monitored. In
propane the catalyst is more reduced aer desorption of pro- pure He the intensity declines only very slowly over a 30 minute
pene (Scheme 1, Step 4) and the catalytic cycle needs to period indicating that the OH groups formed by reaction of
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propane with the surface of MnWO4 cannot be dehydroxylated undergo changes under reaction conditions as evidenced by
easily in inert gas at 673 K. Only aer switching to a 10%O2/90% electron microscopy (Fig. 5).
He mixture (Fig. 13b, dark blue spectrum) the peak area of the
Photoelectron spectroscopy of fresh and used catalysts
O–H band returns quickly to the area originally measured in He revealed a clear correlation between propane consumption rates
at 673 K (Fig. 13b, blue spectrum). The experiment would be in and the concentration of surface Mn ions in the near-surface
agreement with an oxygen-assisted dehydroxylation mechanism region of the well-dened nano-structured manganese tung-
(Scheme 1, Steps 5 and 6) that can also be interpreted in terms state catalysts (Fig. 6). Therefore, the catalytic activity is attrib-
of an oxidative dehydrogenation of active sites, which involves uted to manganese oxide surface species that have also been
the intermediate formation of electrophilic intermediates, such detected qualitatively by Raman spectroscopy (Fig. 7 and 8) and
as HO2 radicals.80,81 The assumption of intermediate formation EELS (Fig. 12). Decoupling of the electronic structure of these
of HO2 is highly speculative, but it could explain the limited manganese oxide surface species from the bulk has been proven
selectivity of the MnWO4 catalysts to propene and the absence by the observation of resonance effects measured by multi-
of oxygenates in the product mixture. Oxygen-containing reac- wavelength Raman spectroscopy (Fig. 8).
tion intermediates are most likely rapidly over-oxidized in the
presence of such a reactive oxidizing agent.
NEXAFS spectroscopy suggested that the activation of
propane on MnWO4 occurs according to a redox mechanism
that involves a change in the formal oxidation state of Mn. So
far, a Mn3+/Mn2+ redox couple is proposed. NEXAFS spectros-
copy was performed at reaction temperature and in the pres-
General discussion and conclusions
The most popular descriptor concerning the activity and selec- ence of the owing reactants, albeit at reduced pressure. The in
tivity of metal oxides in oxidation catalysis is the structural situ NEXAFS study of two catalysts that differ signicantly in
motif that is a component of crystalline bulk phases, although steady-state conrmed the general trend in the differences
deviations from the regular bulk crystal structure on the surface between the two catalysts in particular with respect to the
of prominent oxidation catalysts have been clearly proven degree of oxidation (Fig. 11). It should be noted at this point
experimentally.82–85 The present series of nano-structured that NEXAFS spectroscopy under operating conditions illus-
manganese tungstate catalysts is an impressive example that trates the experimental challenge that is involved in the detec-
disproves again concepts solely based on ideal crystal structure tion of differences within a surface layer of a thickness of one or
motifs as functional active ensembles in oxidation catalysis, few nanometers in the presence of a major bulk phase
since all MnWO4 catalysts in the series under study exhibit the composed of the same chemical elements.
same bulk crystal structure and bulk chemical composition and
are phase pure and homogenous, but show very different cata- hydrogen atoms are abstracted from the propane molecule by
lytic properties. surface manganese oxygen species under the formation of
Operando infrared spectroscopy clearly conrms that
Only the needle-like anisotropy of the primary catalyst surface hydroxyl groups (Fig. 13a). Furthermore, infrared spec-
particles is developed differently within the sample series; troscopy suggests oxidative dehydrogenation of active sites
however, a direct correlation between the shape and catalytic instead of a classical dehydroxylation-re-oxidation mechanism
properties was not observed. However, we discovered that for the regeneration of the catalyst (Fig. 13b). We postulate that
MnWO4 evolves selectivity when a layer of MnOx is exposed on such a mechanism might include intermediate formation of
the surface, which occurs preferentially on the (010) plane.50 In HO2 radicals, which could be responsible for deep oxidation to
the present case of hydrothermally prepared nano-structured carbon oxides observed as the major side products on this type
manganese tungstate catalysts the unique surface layer is of catalyst. Direct experimental proof of these intermediates is
established under the hydrothermal preparation conditions. challenging and requires further studies. The infrared spec-
Synthesis parameters, such as pH and ageing time, can be used troscopy result still suggests preferential occurrence of Mn2+
/
to control the formation of the surface layer as evidenced by Mn3+ single sites on the surface of the catalyst that do not allow
multi-wavelength Raman spectroscopy (Fig. 8). Consequently, facile four-electron reduction of molecular oxygen other than on
the catalytic performance is governed by the synthesis to assemblies of four redox active metal atoms in one surface
a certain degree (Fig. 3).
cluster.86,87
The present study demonstrates that the formation of a thin
However, the catalysts undergo further surface changes
under reaction conditions of propane oxidation. No bulk layer of active transition metal oxide species on the surface of
structural modications were observed with time on stream. a well-dened crystalline bulk structure may provide a tool for
The catalysts prepared at pH < 9 just deactivate slightly, but tuning catalytic performance in alkane oxidation reactions.
catalysts prepared at pH > 9 show a more complex behavior However, it illustrates at the same time the constraints in the
including the initial activation followed by deactivation or just discovery of catalysts with improved performance by big data
an activation process. The higher complexity in the time-on- analysis or other descriptor-based approaches when just bulk
stream behavior of the group of catalysts prepared at pH > 9 structural parameters are taken into account. We postulate that
is attributed to the basic conditions in the autoclave and pro- efficient catalysts for lower alkane activation might be found
longed synthesis times that lead to more severe dissolution– based on nano-structuring of crystalline materials that have
recrystallization processes compared to the synthesis at lower shown so far no catalytic activity in their macro-structural
pH and shorter times. Extended defects are formed that appearance. Hydrothermal synthesis51,88,89 or atomic layer
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deposition90 might be a promising synthetic tool in this respect. 21 J. S. J. Hargreaves, G. J. Hutchings, R. W. Joyner and
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¨
¨
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T. Lunkenbein, R. Schlogl and A. Trunschke, J. Catal.,
and oxygenates to carbon oxides.
2015, 329, 574–587.
24 L. Hu, Q. Peng and Y. Li, J. Am. Chem. Soc., 2008, 130, 16136–
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25 X. Xie, Y. Li, Z.-Q. Liu, M. Haruta and W. Shen, Nature, 2009,
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Conflicts of interest
There are no conicts to declare.
26 H.-X. Mai, L.-D. Sun, Y.-W. Zhang, R. Si, W. Feng,
H.-P. Zhang, H.-C. Liu and C.-H. Yan, J. Phys. Chem. B,
2005, 109, 24380–24385.
Acknowledgements
The work was conducted in the framework of the BasCat 27 K. Zhou, X. Wang, X. Sun, Q. Peng and Y. Li, J. Catal., 2005,
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zentrum Stuttgart (HLRS) for access to the supercomputer
HazelHen, Maike Hashagen for surface area determination, and
Jasmin Allan for XRD measurements.
W.-X. Li and W. Huang, Angew. Chem., Int. Ed., 2014, 53,
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30 R. K. Grasselli, D. J. Buttrey, P. DeSanto, J. D. Burrington,
C. G. Lugmair, A. F. Volpe Jr and T. Weingand, Catal.
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32 C.-R. Chang, Z.-Q. Huang and J. Li, Wiley Interdiscip. Rev.:
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