Resolving kinetics and dynamics of a catalytic reaction inside a fixed bed reactor by combined kinetic and spectroscopic profiling

Article abstract of DOI:10.1039/c2cy20489d

The oxidative dehydrogenation of ethane to ethylene was studied using a MoO₃ based catalyst supported on γ-alumina spheres. The measurement of species and temperature profiles through a fixed bed reactor shows for the first time the reaction pathways inside the catalyst bed directly. Oxidative dehydrogenation of ethane to ethylene and water occurs on the redox sites of MoO₃ only in the presence of gas phase oxygen. Further oxidation of the product ethylene to carbon dioxide occurs as a subsequent reaction step by lattice oxygen of MoO₃. Deep oxidation of ethylene to CO₂ is the only existing reaction in the absence of gas phase oxygen reducing MoO₃ to MoO₂. Oxidation of CO and C ₂H₆ by lattice oxygen does not occur. The reduction of the catalyst can be followed by in situ fiber Raman spectroscopy as a function of the oxygen partial pressure. The in situ Raman measurements are complemented by ex situ micro-Raman spectroscopy and X-ray diffraction. The combined measurement of kinetic and spectroscopic reactor profiles presents a novel approach in in situ catalysis research to establish catalyst structure-function relationships under technically relevant conditions of temperature and pressure. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20489d

Catalysis
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Resolving kinetics and dynamics of a catalytic reaction
inside a fixed bed reactor by combined kinetic and
spectroscopic profiling
Cite this: Catal. Sci. Technol.,
2013, 3, 169
Michael Geske, Oliver Korup and Raimund Horn*
The oxidative dehydrogenation of ethane to ethylene was studied using a MoO3 based catalyst supported
on g-alumina spheres. The measurement of species and temperature profiles through a fixed bed reactor
shows for the first time the reaction pathways inside the catalyst bed directly. Oxidative dehydrogenation
of ethane to ethylene and water occurs on the redox sites of MoO3 only in the presence of gas phase
oxygen. Further oxidation of the product ethylene to carbon dioxide occurs as a subsequent reaction step
by lattice oxygen of MoO3. Deep oxidation of ethylene to CO2 is the only existing reaction in the absence
of gas phase oxygen reducing MoO3 to MoO2. Oxidation of CO and C2H6 by lattice oxygen does not occur.
The reduction of the catalyst can be followed by in situ fiber Raman spectroscopy as a function of the
oxygen partial pressure. The in situ Raman measurements are complemented by ex situ micro-Raman
spectroscopy and X-ray diffraction. The combined measurement of kinetic and spectroscopic reactor profiles
presents a novel approach in in situ catalysis research to establish catalyst structure–function relationships
under technically relevant conditions of temperature and pressure.
Received 12th July 2012,
Accepted 16th August 2012
DOI: 10.1039/c2cy20489d
www.rsc.org/catalysis
the catalyst through the reactor wall in a backscattering geometry9
or shine through the entire reactor in transmission geometry.7,8
1 Introduction
During the last decades the spectroscopic characterization of
working catalysts has received more and more attention
because it was observed that many catalysts adapt dynamically
to the reaction conditions e.g. by changing their particle
structure1 or bulk and surface composition.2 Due to physical
constraints of many spectroscopic methods, the usage of one
and the same reactor for kinetic measurements and spectro-
scopic characterization of the working catalyst is rather the
exception than the rule. As reviewed by Meunier,3 most spectro-
scopic cells are designed for optimal spectroscopic performance
but are kinetically often ill defined. Only a few methods such as
UV/Vis,4,5 Raman,5 EPR6 and X-ray absorption spectroscopy5,7
have been successfully used to measure kinetic and spectro-
scopic data simultaneously in kinetically well defined fixed bed
tubular reactors. In the field of IR spectroscopy deficiencies of
standard cells, e.g. temperature gradients, are partly solved by
optimizing the cell and flow geometry.8
Some of these methods even provide spatially resolved spectro-
scopic data illustrating how the catalyst adapts dynamically to the
varying chemical potential along the flow direction of the reactor.7,9
Resolving spatial gradients in heterogeneous catalytic reactors
becomes in fact more and more important as reviewed by Urakawa
and Baiker.10 However, whereas spectroscopic profiling has been
demonstrated for fixed bed reactors,7,9 simultaneous kinetic data
have never been measured. On the other hand, whenever spatially
resolved kinetic data were reported, e.g. in honeycomb catalysts11 or
powdered catalysts12 corresponding spectroscopic data were miss-
ing. In the present work we combine both information for the first
time by conducting spatially resolved kinetic and spectroscopic
measurements in a fixed bed tubular reactor simultaneously.
To demonstrate the feasibility of this new method, the
oxidative dehydrogenation (ODH) of ethane to ethylene
(eqn (1)) on g-Al2O3 supported MoO3 was chosen as test reaction.
1
2
C2H6 + O2 - C2H4 + H2O
(1)
All in situ spectroscopic methods listed above employ electro-
magnetic radiation of different frequency which can either be
guided into the reactor by means of optical fibers,4 interact with
DrH1 (298.15 K) = À104.88 kJ molÀ1
Ethane ODH lends itself as a test reaction because of the
well known redox properties of this relatively simple catalyst13
Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin,
Germany. E-mail: horn_r@fhi-berlin.mpg.de; Tel: +49 30 84134585
c
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and the superior Raman activity of MoO3.14 Beyond that, ethane
ODH is interesting from a practical point of view for producing
valuable ethylene from cheap and abundant ethane.15
The advantage of resolving spatial profiles in a fixed bed
reactor is that transient processes are translated into spatial
gradients which can be conveniently measured at steady state.
In contrast to most in situ studies, the spectroscopic technique
is here adapted to a kinetically well defined reactor instead of
using a cell optimized for spectroscopy at the expense of an ill
defined flow and mixing pattern. Spectroscopic profiling
reveals how the catalyst adapts to the changing chemical
potential along the flow coordinate. Quantifying temperature
and spectroscopic profiles along the central symmetry line of
the reactor has the advantage that no heat losses occur, which
is a severe problem if windows are used for optical access.8 As
such, spatial reactor profiles are a powerful add-on to classic
kinetic studies,16 pulse experiments17 or isotope transients18,19
which have all been applied to study ODH reactions on transi-
tion metal oxide catalysts.
Fig. 1 Experimental setup for simultaneous measurement of kinetic and
spectroscopic profiles through a fixed bed tubular reactor. (Left) Picture of the
catalyst bed of g-alumina spheres loaded with nominal 50 wt% MoO3. (Right)
Schematic and zoom of the probe geometry.
distributor and as a guide for the fused silica sampling capillary
(OD 700 mm, ID 520 mm) running through the catalyst bed. A small
fraction of the reacting gas mixture was sampled through an
orifice (25 mm) in the wall of the sampling capillary and transferred
to a mass spectrometer (QME 200, Balzers) for analysis. A thermo-
couple (OD 350 mm) for measuring the gas phase temperature was
inserted into the capillary with its tip aligned with the sampling
orifice. The capillary-thermocouple assembly could be translated
up and down through the catalyst bed with mm resolution by
means of a stepper motor to measure spatial profiles of all major
species and the gas temperature. The overall spatial resolution of
the sampling probe is mainly governed by the intake region of the
sampling orifice and the scan velocity. For the present work it
amounts to about 100 mm. For spatially resolved Raman spectro-
scopy the thermocouple was exchanged with the optical fiber
sensor and also aligned with the sampling orifice.
The optical setup comprising the sensor and the coupling optics
for the incoming laser and the emitted Raman/Rayleigh light is
shown in Fig. 2. The optical sensor consists of a beveled fiber with an
outer diameter of 140 mm positioned inside a light guiding capillary
with an inner and outer diameter of 150 mm and 363 mm respectively
(both Polymicro Technologies). The laser light was coupled to the
central fiber using a 10Â objective and was emitted at the beveled
end of the fiber as a sharp ring illuminating the catalyst. Raman and
Rayleigh scattered light from the catalyst was then collected by the
likewise beveled end of the light guiding capillary. At the plane exit of
the light guiding capillary the emitted light was collimated by a
concave mirror, passed through a long range edge filter (RazorEdge
647.1 nm, SEMROCK) to remove Rayleigh scattered light and was
focussed onto a fiber bundle transferring the light into a spectro-
meter for analysis. Each Raman spectrum was accumulated for 300 s
at two wavelength ranges, 180–650 cmÀ1 and 650–1050 cmÀ1
respectively. A high resolution grating (1800 lines cmÀ1) was used
to obtain high quality Raman spectra from the catalyst.
In summary, measuring spatially resolved kinetic and spec-
troscopic profiles along the centerline of a fixed bed tubular
reactor is a novel approach to resolve the dynamics of a catalyst
under working conditions.
2 Materials and methods
g-Alumina spheres from Sasol (1 mm diameter, BET surface area:
157 m2 gÀ1) were used as support and ammonium heptamolybdate
tetrahydrate (AHM, (NH4)6Mo7O24Á4H2O, 99.0%) from Merck as
molybdenum precursor. Gases were supplied by Westfalen: argon
(99.999%), ethane (99.95%) and oxygen (99.999%).
2.1 Catalyst preparation
A 50 wt% g-alumina supported molybdenum oxide catalyst was
prepared by incipient wetness impregnation. The required mass
of AHM was dissolved in deionized water at a pH of 5.2. In total
20 impregnation steps were conducted and the solvent was
evaporated between each step at 120 1C for 2 h. After the
impregnation the catalyst was dried at 120 1C overnight and then
calcined at 540 1C in static air for 6 h. Catalyst coating reduced the
available surface area from 157 m2 gÀ1 of the pure support to
56 m2 gÀ1. Prior to the reactor measurements the catalyst was
activated in a flow of oxygen (100 ml minÀ1) at 540 1C for 1 h.
2.2 Experimental setup
A detailed description of the reactor setup capable of measuring
species, temperatures and spectroscopic profiles through a fixed
bed tubular reactor can be found in an earlier publication by the
authors.20 In short the reactor consists of a fused silica tube with
an inner diameter of 18 mm and a wall thickness of 10 mm in
which a 3 cm high bed of catalyst spheres is positioned. A picture
and a drawing of the experimental setup are presented in Fig. 1.
The catalyst bed was positioned on top of a cylindrical piece
of ceramics, termed front heat shield in Fig. 1, which was gas
2.3 Catalyst characterization
tightly fitted in the reactor tube by wrapping it with a ceramic The surface area of the catalyst was determined by nitrogen BET
mate (Interam 3 M). The front heat shield also served as a flow measurements at 77 K with an Autosorb-6B (Quantachrome).
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Fig. 2 (A) Coupling scheme for fiber Raman spectroscopy. The sensor tip (C) is
inserted in the sampling capillary of the spatial profile reactor20 and aligned with
the sampling orifice. (B) Magnified view showing the end of the light guiding
capillary bringing back Rayleigh and Raman light scattered by the catalyst.
(C) Beveled fiber tip inside a light guiding capillary for irradiation and collection
of scattered light perpendicular to the fiber axis.
Fig. 3 Species and temperature profiles for ethane ODH measured through a
MoOx/g-Al2O3 sphere bed at p = 1 bar. Point a marks the maximum C2H4
concentration, Point b marks the position of complete O2 conversion. GP denotes
the gas phase before and after the catalyst bed and FHS the ceramic front heat
shield.
Phase composition was analyzed by X-ray diffraction (STOE
STADI-P and Autosampler) over a 2y range from 101 to 1101
using the Ka line of copper. Raman measurements were con-
ducted using the 647.1 nm line of an Ar–Kr laser (Series 2018,
Spectra Physics) and the first stage of a triple spectrometer
(TriVista TR 557, S&I GmbH) equipped with a CCD camera
(Spec10: 100BR, Princton Instruments). The CCD camera was
operated at À120 1C to minimize thermal noise. With the
1800 cmÀ1 grating in the spectrometer a resolution of about
0.4 cmÀ1 was reached. For ex situ analysis the spheres were analyzed
under a Raman microscope (Olympus, BX51WI) equipped with a
10Â objective. The laser power under the microscope was restricted
to about 1 mW, avoiding beam damage to the sample as found by
Tinnemans et al.21 For comparison, about 25 mW laser power was
used at the exit of the fiber Raman sensor inside the profile reactor
without damaging the sample because the laser light leaves the
beveled fiber tip unfocused. The illuminated area is estimated
the alumina support. Adsorbed ethane is totally oxidized on these
OH groups without the possibility of desorption of intermediate
products. If acidic OH groups of the alumina support were
exposed, CO2 would appear readily at the beginning of the catalyst
bed. As shown in Fig. 4 this is not the case because the high
nominal MoO3 loading corresponds to about four times a mono-
layer covering the alumina sites completely (one monolayer cover-
age equals about 5 Mo units nmÀ2).23 Ethylene in contrast is
formed on the redox sites of the catalyst.24 The same authors claim
that further oxidation of ethylene occurs also on molybdenum
redox sites. This reaction sequence is directly shown here for the
first time on a catalyst under reaction conditions. Fig. 4 presents a
close-up view of the C2H4 and CO2 profiles in the catalyst bed.
A linear regression of the initial slopes from the first two
mm of the catalyst bed reveals that the primary product
ethylene is formed with a non-zero slope, whereas the initial
to be 30 times larger than that below the microscope (Afiber
1 mm2 vs. Amicroscope E 0.03 mm2).
E
3 Results and discussion
3.1 Kinetic measurement
Fig. 3 shows the gas species and catalyst temperature profiles
measured through the MoOx/g-Al2O3 catalyst bed under ethane
ODH conditions on the left side and a photograph of the catalyst
bed on the right side. In the following, the catalyst composition
is loosely denoted MoOx, taking into account that the MoO3
present before reaction is partly reduced under reaction condi-
tions. Oxygen is consumed within the first 19 mm of the bed,
marked as point b in Fig. 3. The main reaction products in the
oxidation zone are H2O, C2H4, CO and CO2. H2 formation is
observed when gas phase oxygen is largely consumed.
According to Heracleous et al.,22 CO2 is the major product (S >
70%) of ethane direct oxidation on strongly acidic OH-groups of
Fig. 4 Initial slopes of C2H4 and CO2 profiles characteristic for a primary (>0) and
secondary (B0) oxidation product respectively.
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formation rate of CO2 from deep oxidation of ethane is close to
zero (o20% of the ethylene value), marking CO2 as secondary
product. Both maximum slopes of ethylene and CO2 in contrast
are nearly 10 times larger than the initial CO2 formation rate
showing a much faster production of CO2 from C2H4 than from
direct ethane oxidation. However, ethane is readily oxidized to
CO as indicated in Fig. 3 by the high initial CO formation at the
entrance of the catalyst bed. Ethylene formation is clearly a
catalytic process because nearly no C2H4 is formed in the
catalytically inactive front heat shield (FHS). The formation of
ethylene by oxidative dehydrogenation of ethane with gas phase
oxygen and the combustion of ethylene by lattice phase oxygen
leads to a maximum in the ethylene profile at about 15 mm,
marked as a in Fig. 3. At point a the oxygen partial pressure in
the gas stream has declined from initially 63 mbar to about
7 mbar. CO2 production continues even beyond the point of
complete oxygen conversion b, indicating the participation of
lattice oxygen as oxidizing agent. Interestingly the only carbon
containing species with a finite consumption rate after point b
is ethylene. This can be taken as evidence that lattice oxygen is
responsible for deep oxidation of C2H4 to CO2, whereas the
oxidation of CO and C2H6 by lattice oxygen is not detectable.
A calculation shows that when the experiment was interrupted,
reduction of the last 10 mm of the catalyst bed from MoO3 to
MoO2, the only reduced molybdenum phase detected by XRD and
Raman spectroscopy, would have offered additional 6.5 mmol
lattice oxygen taken as O2. Dividing this value by the CO2 formation
rate after full O2 consumption (B0.01 mmol minÀ1) and including
also H2O formation (B0.005 mmol minÀ1) shows that complete
reduction of the bed would have required about seven more hours
time on stream. Therefore, the catalyst downstream point B was not
yet in steady state. Indeed it was visually observed that the spheres
at the exit became darker with longer time on stream (MoO3 is
colorless, MoO2 dark violet). H2 formation increases with increas-
ing degree of reduction in the absence of gas phase O2.
Fig. 5 Combination of high and low wave number scans of a Raman spectrum
from the entrance region at 2.4 mm. The dotted line represents the normalized
background of the fused silica fiber Raman sensor used.
pressure (position 2.4 mm) together with a fused silica back-
ground which results from the fiber optic Raman sensor itself.
For further analysis only the High Wave Number (HWN) part
of the spectrum is used because the n OQMo (993 cmÀ1) and
the n Mo–O–Mo (821 cmÀ1) vibrations are the most intense and
characteristic vibrations for MoO3. The Raman shifts and
assignments are taken from Seguin et al.28
Fig. 6 shows the corrected Raman spectra in dependence of
the sampling position. The procedure of how the Raman
spectra of the species were extracted from the raw data and
how the spectra were corrected for the local brightness of the
catalyst bed is explained in Section 3.3. Without a quantitative
analysis, which is beyond the scope of this paper, there is a
clear correlation between the oxygen partial pressure indicated
in blue in Fig. 6 and the oxidation state of the catalyst. With
decreasing oxygen partial pressure the MoO3 crystallites
become partly reduced and more defect rich.
As already discussed by Ressler et al.27 the transformation
from MoO3 to MoO2 proceeds through a collapse of individual
3.2 In situ Raman profile measurements
The visually observed zoning of the catalyst bed was analyzed by
spatially resolved in situ Raman spectroscopy using the novel
fiber optic sensor shown in Fig. 2. Unfortunately the sensitivity
of the Raman fiber sensor is not yet high enough to evaluate
and follow the signals of all relevant vibrations such as that of
Mo–O–Al, which was reported to be the catalytic active site.25
Nevertheless it was possible to follow the reduction of
crystalline MoO3, which exhibits a much higher Raman scatter-
ing cross section than all the other species present.26 Even
though the trioxide might not be the active site for the catalytic
conversion, it can be deduced from the species profiles that its
lattice oxygen is responsible for ethylene oxidation to CO2. The
high mobility of lattice oxygen at the local temperatures in the
reactor is in good agreement with the findings by Ressler et al.27
At the entrance of the bed the catalyst spheres are yellow/
grey and turn violet/black after about 19 mm, where the gas
phase O2 partial pressure approaches zero (cf. Fig. 3). Fig. 5
shows exemplarily one Raman spectrum measured at the
entrance region of the catalyst bed at 56 mbar O2 partial
Fig. 6 In situ HWN Raman spectra along the catalyst bed. MoO3 Raman signals
vanish at the position where gas phase oxygen is almost fully consumed.
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MoO3 layers. The reducing domain size of the trioxide goes
along with a decrease of the Raman signal of MoO3 until all
MoO3 signals disappear at around 19 mm where the catalyst is
completely reduced to MoO2. This decline can be resolved by
our fiber optic Raman probe, which has a spatial resolution of
about 1 mm as shown in a previous work by the authors.29
3.3 Raman signal deconvolution
The collected Raman spectra are a superposition of three
contributions: (i) the Raman spectrum of the catalyst, (ii) a
constant fused silica Raman background generated inside the
fiber and transmitted to the capillary by cross talk without
leaving the fiber tip and (iii) a variable fused silica Raman
background resulting from Raman scattered light generated
inside the excitation fiber, emitted at the beveled fiber tip and
recollected by the light guiding capillary.
The constant fraction of the fused silica Raman background
was measured by placing the Raman sensor outside the reactor
in air. The fused silica Raman background measured at a
certain position inside the catalyst bed was generally higher
than the fused silica Raman background measured outside the
reactor due to the additional variable portion of the back-
ground signal which is a measure of the darkness and self-
absorption of the catalyst bed. To correct the catalyst Raman
spectra for the latter, the fused silica Raman background
measured outside the reactor was multiplied with a factor F
to match the fused silica Raman background measured inside
the catalyst bed (cf. Fig. 5 green spectrum vs. blue spectrum).
The obtained background spectrum was subtracted from the
local Raman spectrum inside the catalyst bed and the resulting
difference spectrum was divided by (F À 1) to correct for the
self-absorption of the catalyst bed (eqn (2)).
Fig. 7 Photographs (top) and Raman spectra (bottom) of MoOx/g-Al2O3 catalyst
spheres corresponding to different positions in the catalyst bed. (A) Entrance
region at the inlet O2 partial pressure of 63 mbar, (B) from about 19 mm where
gas phase O2 is nearly fully consumed and (C) from the exit region. * denotes
characteristic Raman bands of MoO3 and # Raman bands of MoO2. Unlabeled
bands are listed in the text.
A transformation of MoO3 to MoO2 is clearly visible in the
oxygen depleted outlet. Characteristic Raman shifts for MoO3
are:28 n OQMo: 993 cmÀ1, n OMo2: 821 cmÀ1, n OMo3: 666 cmÀ1
+
472 cmÀ1, d OQMo: 378 cmÀ1, d OMo3: 339 cmÀ1, d OQMo:
292 cmÀ1 + 285 cmÀ1, d OQMo2: 246 cmÀ1 + 218 cmÀ1
199 cmÀ1 and a deformation mode at 159 cmÀ1. Raman shifts
+
for MoO2 (744 cmÀ1, 586 cmÀ1, 571 cmÀ1, 497 cmÀ1, 469 cmÀ1
,
460 cmÀ1, 366 cmÀ1, 351 cmÀ1, 230 cmÀ1, 209 cmÀ1 and
203 cmÀ1) are not assigned to specific vibration modes, but are
Ssample,brightness corrected = Ssample/(F À 1)
(2)
All changes in the brightness corrected Raman spectra, such
as those shown in Fig. 6, are real and due to changes in the
catalyst structure and composition.
3.4 Ex situ sphere bed analysis
The advantages of ex situ analysis are the often higher sensi-
tivity and lower setup complexity. Especially for materials
maintaining their bulk structure after reaction, ex situ investi-
gations are still important. This was shown by the work of
Ledoux et al. concerning carbon modified MoO3 in the n-butane
dehydrogenation using XRD analysis for the determination of
the phase composition.30
Also in the present work, ex situ catalyst characterization was
performed to cross check the results from the in situ measure-
ments. The reaction was interrupted after 5 h time on stream.
Catalyst spheres from three different regions of the catalyst bed
were extracted; bright yellow/grey spheres from the inlet region
(region A), grey spheres from about 19 mm downstream (region B)
and dark spheres from the end of the catalyst bed (region C). Ex
situ Raman spectra of these spheres measured under a Raman
microscope are presented in Fig. 7.
Fig. 8 XRD diffractograms of ‘as prepared’ spheres and spheres after reaction
taken from different positions inside the catalyst bed (symbols measured data,
solid orange line 10 point FFT fit). (A) Spheres from inlet region showing only
MoO3 reflexes. (B) Spheres from the oxygen depleted zone at about 19 mm
showing a phase mixture of MoO2 and MoO3. (C) Spheres from the end of the
catalyst bed showing predominantly MoO2 reflexes.
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representative for this phase and described by Camacho et al.31 in a rapid consecutive reaction. With decreasing gas phase
The dominant presence of the signals from the trioxide phase oxygen partial pressure, MoO3 becomes gradually reduced to
results from the much lower Raman cross section of the dioxide MoO2 such that the catalyst presents actually a phase mixture of
and strong self-absorption.32 Nevertheless, the coexistence of the MoO3 and MoO2 and possible suboxide phases that were not
trioxide and the dioxide phase at the position of full oxygen detected by methods used in this work. Interestingly, the lattice
conversion is clearly visible in Fig. 7B.
oxygen does hardly attack ethane or carbon monoxide but rather
To confirm the Raman assignments, XRD diffractograms oxidizes selectively the desired product ethylene to carbon
were measured on spheres from regions A, B and C. The results dioxide and water. At very low oxygen partial pressures in the
are shown in Fig. 8 together with a diffractogram of the ‘as gas phase, hydrogen appears as another product. The reduction
prepared’ spheres. The main reflexes were assigned according of MoO3 goes to completion and basically phase pure MoO2 is
to Regalbuto et al.33 and Kumari et al.34
formed upon full conversion of gas phase oxygen. From a
methodical point of view it was shown that spatially resolved
Raman spectroscopy could be coupled with the spatial kinetic
data, revealing the intimate interplay between reaction kinetics
and catalyst dynamics. The combination of kinetic and spectro-
scopic reactor profiles takes in situ catalysis research to a new
level and motivates further improvements and the extension to
other spectroscopic methods.
4 Conclusion
The present work demonstrates for the first time combined
kinetic and spectroscopic reactor profiles measured with high
resolution through a fixed bed tubular reactor under in situ
conditions. Oxidative dehydrogenation of ethane to ethylene on
a nominally MoO3/g-Al2O3 catalyst was chosen as a test reac-
tion. Already a qualitative analysis of the obtained kinetic and
spectroscopic data provides detailed insight into the kinetics of
the reaction and the dynamics of the catalyst which are closely
coupled. Ex situ catalyst characterization by micro-Raman
spectroscopy and X-ray diffraction complements the in situ
data. Fig. 9 summarizes the mechanistic picture that was
derived schematically. In the entrance section of the catalyst
bed where the gas phase oxygen partial pressure is still high
(region A), MoOx is present as MoO3 which functions as a
catalyst for oxidative dehydrogenation of ethane to ethylene
and water. Also by reaction with gas phase oxygen, CO is formed
Acknowledgements
¨
First and foremost the authors thank Prof. Dr. Robert Schlogl
for supporting this work with scientific input and funds.
Additionally we thank Achim Klein–Hoffmann for shaping the
tips of the Raman sensor, Frank Girgsdies for XRD analysis and
Giesela Lorenz for the BET service. We are also very grateful to
the German Research Foundation for funding the Emmy
Noether Junior Research Group ‘High Temperature Catalysis’.
Finally we acknowledge funding by the Federal Ministry of
Education and Research (BMBF) within the framework of the
Cluster of Excellence ‘Unifying Concepts in Catalysis’.
References
1 P. L. Hansen, J. B. Wagner, S. Helveg, J. R. Rostrup-Nielsen,
B. S. Clausen and H. Topsoe, Sience, 2002, 295, 2053–2055.
¨
2 A. C. Sanfiz, T. W. Hansen, D. Teschner, P. Schnorch, F. Girgsdies,
¨
A. Trunschke, R. Schlogl, M. H. Looi and S. B. A. Hamid, J. Phys.
Chem. C, 2010, 114, 1912–1921.
3 F. C. Meunier, Chem. Soc. Rev., 2010, 39, 4602–4614.
4 R. L. Puurunen, B. G. Beheydt and B. M. Weckhuysen, J. Catal., 2001,
204, 253–257.
5 U. Bentrup, Chem. Soc. Rev., 2010, 39, 4718–4730.
6 A. Brueckner, Chem. Soc. Rev., 2010, 39, 4673–4684.
7 J.-D. Grunwaldt, S. Hannemann, C. G. Schroer and A. Baiker, J. Phys.
Chem. B, 2006, 110, 8674–8680.
8 Y. Yang, R. S. Disselkamp, J. Szanyi, C. H. F. Peden, C. T. Campbell
and J. G. Goodwin, Rev. Sci. Instrum., 2006, 77, 094104.
9 M. G. O’Brien, A. M. Beale, S. D. M. Jacques, M. D. Michiel and
B. M. Weckhuysen, ChemCatChem, 2009, 1, 99–102.
10 A. Urakawa and A. Baiker, Top. Catal., 2009, 52, 1312–1322.
11 J. Sa, D. L. A. Fernandes, F. Aiouache, A. Goguet, C. Hardacre,
D. Lundie, W. Naeem, W. P. Partridge and C. Stere, Analyst, 2010,
135, 2260–2272.
12 J. Touitou, K. Morgan, R. Burch, C. Hardacre and A. Goguet, Catal.
Sci. Technol., 2012, 2, 1811–1813.
13 T. Ressler, R. E. Jentoft, J. Wienold, M. M. Gu¨nter and O. Timpe,
J. Phys. Chem. B, 2000, 104, 6360–6370.
14 P. A. Spevack and N. S. McIntyre, J. Phys. Chem., 1992, 96,
9029–9035.
15 F. Cavani and F. Trifiro, Catal. Today, 1995, 24, 307–313.
16 F. Klose, M. Joshi, C. Hamel and A. Seidel-Morgenstern, Appl. Catal.,
A, 2004, 260, 101–110.
Fig. 9 Mechanistic picture of the ODH of ethane over a MoO3 catalyst: The
conversion to ethylene occurs exclusively in the presence of gas phase oxygen (A
and B). Further oxidation of only ethylene, and not ethane or CO, using lattice
oxygen (labeled with *) to CO2 is associated with the reduction of the catalyst to
MoO2 (B and C).
17 J. Haber and E. Lalik, Catal. Today, 1997, 33, 119–137.
c
174 Catal. Sci. Technol., 2013, 3, 169--175
This journal is The Royal Society of Chemistry 2013

View Article Online
Catalysis Science & Technology
Paper
18 K. Chen, E. Iglesia and A. T. Bell, J. Phys. Chem. B, 2001, 105, 26 D. S. Zingg, L. E. Makovsky, R. E. Tischer, F. R. Brown and
646–653. D. M. Hercules, J. Phys. Chem., 1980, 84, 2898–2906.
19 K. Chen, A. Khodakov, J. Yang, A. T. Bell and E. Iglesia, J. Catal., 27 T. Ressler, J. Wienold, R. E. Jentoft and T. Neisius, J. Catal., 2002,
1999, 186, 325–333. 210, 67–83.
20 R. Horn, O. Korup, M. Geske, U. Zavyalova, I. Oprea and R. Schlogl, 28 L. Seguin, M. Figlarz, R. Cavagnat and J.-C. Lassegues, Spectrochim.
Rev. Sci. Instrum., 2010, 81, 064102. Acta, Part A, 1995, 51, 1323–1344.
21 S. J. Tinnemans, M. H. F. Kox, M. W. Sletering, T. A. Nijhuis, 29 O. Korup, S. Mavlyankariev, M. Geske, C. F. Goldsmith and R. Horn,
¨
`
T. Visser and B. M. Weckhuysen, Phys. Chem. Chem. Phys., 2006, 8,
2413–2420.
22 E. Heracleous and A. A. Lemonidou, Catal. Today, 2006, 112, 23–27.
Chem. Eng. Process., 2011, 50, 998–1009.
30 M. J. Ledoux, F. Meunier, B. Heinrich, C. Pham-Huu, M. E. Harlin
and A. O. I. Krause, Appl. Catal., A, 1999, 181, 157–170.
´
´
23 E. Heracleous, A. F. Le, I. A. Vasalos and A. A. Lemonidou, Catal. 31 M. A. Camacho-Lopez, L. Escobar-Alarcon, M. Picquart, R. Arroyo,
´
G. Cordoba and E. Haro-Poniatowski, Opt. Mater., 2011, 33, 480–484.
Lett., 2003, 88, 47–53.
24 E. Heracleous, A. A. Lemonidou and J. A. Lercher, Appl. Catal., A, 32 M. Dieterle and G. Mestl, Phys. Chem. Chem. Phys., 2002, 4, 822–826.
2004, 264, 73–80. 33 J. R. Regalbuto and J. W. Ha, Catal. Lett., 1994, 29, 189–207.
25 A. Christodoulakis, E. Heracleous, A. A. Lemonidou and 34 L. Kumari, Y.-R. Ma, C.-C. Tsai, Y.-W. Lin, S. Y. Wu, K.-W. Cheng and
S. Boghosian, J. Catal., 2006, 242, 16–25.
Y. Lios, Nanotechnology, 2007, 18, 115717.
c
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