Atomic-Scale Observation of the Metal–Promoter Interaction in Rh-Based Syngas-Upgrading Catalysts

Article abstract of DOI:10.1002/anie.201902750

The direct conversion of syngas to ethanol, typically using promoted Rh catalysts, is a cornerstone reaction in CO₂ utilization and hydrogen storage technologies. A rational catalyst development requires a detailed structural understanding of the activated catalyst and the role of promoters in driving chemoselectivity. Herein, we report a comprehensive atomic-scale study of metal–promoter interactions in silica-supported Rh, Rh–Mn, and Rh–Mn–Fe catalysts by aberration-corrected (AC) TEM. While the catalytic reaction leads to the formation of a Rh carbide phase in the Rh–Mn/SiO₂ catalyst, the addition of Fe results in the formation of bimetallic Rh–Fe alloys, which further improves the selectivity and prevents the carbide formation. In all promoted catalysts, Mn is present as an oxide decorating the metal particles. Based on the atomic insight obtained, structural and electronic modifications induced by promoters are revealed and a basis for refined theoretical models is provided.

Full text of DOI:10.1002/anie.201902750

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Accepted Article
Title: Atomic-Scale Observation of the Metal-Promoter Interaction in
Rh-Based Syngas Upgrading Catalysts
Authors: Xing Huang, Detre Teschner, Maria Dimitrakopoulou, Alexey
Fedorov, Benjamin Frank, Ralph Kraehnert, Frank Rosowski,
Harry Kaiser, Stephan Schunk, Christiane Kuretschka, Robert
Schlögl, Marc Willinger, and Annette Trunschke
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902750
Angew. Chem. 10.1002/ange.201902750
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10.1002/anie.201902750
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Atomic-Scale Observation of the Metal-Promoter Interaction in
Rh-Based Syngas Upgrading Catalysts
Xing Huang,* Detre Teschner, Maria Dimitrakopoulou, Alexey Fedorov, Benjamin Frank, Ralph
Kraehnert, Frank Rosowski, Harry Kaiser, Stephan Schunk, Christiane Kuretschka, Robert Schlögl,
Marc-Georg Willinger* and Annette Trunschke*
Abstract: The direct conversion of syngas to ethanol is a
cornerstone reaction in evolving technologies of CO2 utilization
and hydrogen storage, which is typically performed using
promoted Rh catalysts. A rational catalyst development requires
a detailed structural understanding of the activated catalyst and
in particular, the specific roles that promoters play in driving the
chemoselectivity of this process. Herein, we report for the first
time a comprehensive and comparative atomic-scale study of
metal-promoter interaction in silica-supported Rh, Rh-Mn and Rh-
Mn-Fe catalysts by aberration-corrected transmission electron
microscopy (AC-TEM). We uncover that while the catalytic
reaction leads to the formation of a Rh carbide phase in the Rh-
Mn/SiO2 catalyst, the addition of Fe results in the formation of
bimetallic Rh-Fe alloys. The latter further improves the selectivity
and prevents the carbide formation. In all promoted catalysts, the
Mn is present as oxide decorating the metal particles. Based on
the atomic insight presented in this work, structural and electronic
modifications induced by promoters are revealed and a basis for
refined theoretical models is provided.
improves both the specific activity and the selectivity to ethanol.[1b,
2-3]
The effect of promoters has been investigated extensively
experimentally[4] and by DFT calculations,[5] however, our present
understanding of the underlying metal-promoter interaction
remains very limited. It has widely been proposed that an intimate
contact between the metal and a promoter is essential for
inducing the so-called strong metal-promoter interaction (SMPI)
that enhances the catalytic performance;[2, 6] that said, the atomic
nature of this interaction is not clear.
Structural complexity of promoted Rh-based catalysts makes
the mutual interaction between the components challenging to
resolve by spectroscopic tools at the atomic scale. The difficulties
are primarily related to the low catalyst loading and minute
concentrations of promoters.[4a] Studying such catalysts using
electron microscopy is plugged by issues with a high beam
sensitivity of the system as well as the charging effects due to the
non-conductive silica support.[7] The current lack of atomic-scale
structural and compositional information, and in particular, the
mechanism by which the promoters influence the catalytic
function of the active Rh, is still under debate[1a, 2, 5b, 7] and begs
for more experimental and theoretical work. In the last decade,
advances in high-resolution TEM yielded an unprecedented level
of insight into the local atomic arrangement of the complex
catalysts.[8] Herein, we apply AC-TEM to investigate promoted Rh
catalysts and systematically study the interaction between the
metal and the added promoters as well as their evolution under
the reaction conditions.
The direct conversion of syngas (CO+H2) to ethanol and higher
alcohols (C2+) is an attractive alternative to the conventional
processes of ethylene hydration or biomass fermentation.[1]
Heterogeneous Rh-based catalysts are among the best systems
for this process.[2] While Rh alone provides poor selectivity to
ethanol and other C2+ oxygenates due to the competing
methanation reaction, the addition of promoters such as Mn or Fe
[a]
[b]
Dr. X. Huang, Dr. D. Teschner, Prof. Robert Schlögl
Department Heterogeneous Reactions, Max Planck Institute for
Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an
der Ruhr, Germany
E-mail: xinghuang0214@mail.ipc.ac.cn
Dr. X. Huang, Dr. D. Teschner, Dimitrakopoulou, Dr. MProf. Robert
Schlögl, Dr. M. G. Willinger, Dr. A. Trunschke
Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-
Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
E-mail: marc.willinger@scopem.ethz.ch
Figure 1. Selectivity and conversion in syngas upgrading over (a) Rh/SiO2, (b)
Rh-Mn/SiO2, and (c) Rh-Mn-Fe/SiO2 (space-time yield of 0.80 mmol ml-1 min-1
after 800 h TOS). Reaction conditions: 260°C, 54 bar, 60% H2, 20% CO, 20%
N2, (a) 11667 h-1, (b) 5833 h-1, (c) 3889 h-1. Others: C4+ alcohols and alkanes as
well as alkenes and various oxygenates.
E-mail: trunschke@fhi-berlin.mpg.de
[c]
[d]
Dr. B. Frank, Dr. R. Kraehnert, Dr. F. Rosowski
BasCat - UniCat BASF Joint Lab, Hardenbergstraße 36, 10623
Berlin, Germany
Dr. F. Rosowski, Dr. C. Kuretschka
Three frequently studied catalysts, i.e. un-promoted Rh on
silica, Mn promoted Rh on silica, and Mn-Fe co-promoted Rh on
silica were prepared using a co-impregnation method with
loadings of, respectively, 2.5 wt% Rh, 1.5 wt% Mn, and 0.5 wt%
Fe. The samples were catalytically tested and investigated by
TEM after calcination, reductive activation and catalytic reaction.
This strategy allowed us to follow reaction induced modifications
and the evolution of the metal-promoter interaction at three
relevant catalyst states, as reported in details below.
BASF SE, Process Research and Chemical Engineering,
Heterogeneous Catalysis, Carl-Bosch-Straße 38, 67056,
Ludwigshafen, Germany
Dr. H. Kaiser, Dr. S. Schunk
hte GmbH, Kurpfalzring 104, 69123 Heidelberg, Germany
Dr. A. Fedorov
[e]
[f]
Department of Mechanical and Process Engineering, ETH Zürich,
Leonhardstrasse 21, 8092 Zürich, Switzerland
Supporting information for this article is given via a link at the
document.((Please delete this text if not appropriate))
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10.1002/anie.201902750
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resolution image of such structure). We note that these structures
are very sensitive to the electron beam and require low-dose
imaging to avoid beam-induced structural modification and
reduction.
The size of crystalline nanoparticles in the calcined Rh-
Mn/SiO2 (Figure 2c) and Rh-Mn-Fe/SiO2 (Figure 2e) catalysts was
comparable to that in calcined Rh/SiO2. The observed lattice
fringes are in agreement with the structure of Rh2O3 (Figure 2d),
although local EDX analysis reveals the simultaneous presence
of the promoter signal (Figure S4, Figure S5b, EDX 1). Notably,
more raft-like structures were observed in the promoted catalysts
(see arrows in Figures 2c,e). A representative HAADF-STEM
image of such a raft-like aggregate in the calcined Rh-Mn-Fe/SiO2
catalyst is shown in Figure 2f. The corresponding FFT shows a
weak and broad ring that can be indexed to the (110) reflection of
Rh2O3 (inset in Figure 2b). EDX analysis of such structures
reveals the simultaneous presence of Rh, Mn, and Fe. Although
the distribution of Mn, Fe and Rh can be inhomogeneous within
individual aggregates (Figure S5c-l), the close proximity of
promoters and Rh indicates that the raft-like structures comprise
a complex mixture of oxides. XPS analysis (Figure S2) shows that
the 3d peak of Rh in both, the calcined Rh-Mn/SiO2 and Rh-Mn-
Fe/SiO2 catalysts is slightly shifted to higher binding energies
(309.2-309.3 eV) compared to the Rh peak in the calcined
Rh/SiO2 (308.9 eV). This may be due to a partial incorporation of
promoters into the Rh2O3 matrix and formation of complex
RhxMnyFezOn oxides. The raft-like structures are highly sensitive
to the electron beam. Extended irradiation leads to the formation
of metallic Rh nanoparticles that are surrounded by a layer of
lower contrast which, according to EDX, consists of a binary Mn-
Fe oxide (Figures S4a,b). Although in this case, the core/shell
configuration was induced by electron beam reduction, this
transformation may serve as a model for the chemical state that
eventually evolves under reducing reaction conditions.
To conclude, we can propose a structural model for the
calcined Rh-Mn-Fe/SiO2 catalyst (Figure S5m) according to which
i) the promoters are in an intimate contact with the rhodium oxide,
and ii) complex mixed oxides containing MnxFeyOz doped Rh2O3
(RhxMnyFezOn) exist in the catalyst.
Next, we characterized reduced and spent catalysts. HAADF-
STEM micrographs recorded after the H2 reduction or long-term
catalytic testing reveal crystalline metallic nanoparticles (Figures
3, S3c,d, and S6-S8). The particle size distribution is similar in all
catalysts with a maximum close to 2 nm. However, some larger
particles with sizes up to 20-40 nm appear in the spent catalysts
(Figures S9,10). Since those were absent in the freshly reduced
catalysts (Figures S7-8), we assume that they form by sintering
Figure 2. HAADF-STEM images of calcined (a,b) Rh/SiO2, (c,d) Rh-Mn/SiO2
and (e-f) Rh-Mn-Fe/SiO2; Insets of (b,d,f) show respectively the FFTs of the
corresponding nanostructures. Arrows mark the raft-like structure present in
the calcined catalyst.
Catalytic testing reveals that the selectivity to ethanol
increases from Rh/SiO2 to Rh-Mn/SiO2 and to Rh-Mn-Fe/SiO2
(Figure 1). Mn promoter enhances both activity and selectivity,
while addition of Fe as a second promoter increases the yield
further due to a distinct increase in ethanol selectivity at
comparable conversion. However, the Mn-Fe-promoted catalyst
is less active compared to Rh-Mn/SiO2 catalyst (Figure S1). Since
9]
our catalytic data is consistent with previous reports,[2, the
synthesized catalysts can be considered as representative and
suitable model catalysts for the investigation of the metal-
promoter interaction and associated synergistic effects.
The morphologies and structures of the freshly calcined
catalysts were first characterized by high annular dark-field
scanning TEM (HAADF-STEM). The HAADF-STEM image of the
promoter-free Rh/SiO2 reveals mostly crystalline Rh2O3
nanoparticles that are highly dispersed and homogeneously
distributed over the SiO2 surface (Figure 2a). Lattice fringes with
d-spacings of 2.56 Å and 2.25 Å are consistent with (110) and (2-
13) facets of the corundum-like Rh2O3 (Figure 2b).[10] The
corresponding fast Fourier transformation (FFT) shows a weak
and broad ring that can be assigned to the (110) reflection of
Rh2O3 (inset in Figure 2b). The presence of Rh2O3 was also
confirmed by X-ray photoelectron spectroscopy (XPS, Figure S2).
Besides the crystalline nanoparticles, survey of the material also
reveals less abundant poorly crystalline particles of a raft-like
morphology (see arrows in Figure 2a and Figure S3b for a high-
during the catalytic reaction.
A detailed analysis of the
microstructures is shown in Figures 3, S3c,d, S6-S8. The lattice
fringes with a d-spacing of 2.23-2.25 Å correspond to the (111)
planes of metallic Rh in Rh/SiO2 (Figure S3d) and Rh-Mn/SiO2
(Figure S6a). In contrast, the intensity profiles taken from the
region indicated in Figures 3a,c reveal that the (111) lattice d-
spacing of metal particles in Rh-Mn-Fe/SiO2 is slightly decreased
after both reduction and catalytic testing. This could be a result of
incorporation of promoter elements into the Rh during the
reduction.[11] Indeed, the formation of Rh-Fe alloy was suggested
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the electronic structure of Rh is modified, which can be caused by
the alloying with Fe. In the Rh 3d XPS spectrum of the reduced
and spent Rh-Mn/SiO2 catalysts, the impact of Mn on the
electronic structure of Rh is barely noticeable (Figure S12a).
Furthermore, the amount of Fe in the Rh-Fe alloy particles was
quantified by performing compositional EDX analysis on many
individual particles (Figure S13). The statistical analysis indicates
a Fe/Rh mass ratio of 0.1 (atomic ratio Fe/Rh=0.18) in the
reduced and 0.14 (atomic ratio Fe/Rh=0.26) in the used Rh-Mn-
Fe/SiO2 catalyst, respectively. However, according to the
inductively coupled plasma optical emission spectrometry (ICP-
OES), the overall Fe/Rh ratio in the catalyst should be about 0.2
(atomic ratio Fe/Rh=0.36). Thus, not all of Fe in the catalysts was
forming an alloy with Rh during reduction and catalytic reaction. It
is possible that some of the Fe is present as highly dispersed
oxidic or sub-oxidic species after H2 reduction and/or catalytic
reaction.
We used electron energy-loss spectroscopy (EELS) to
provide further evidence for the formation of the bimetallic Rh-Fe
alloys. Figure 3f shows the EELS spectra of Fe L2,3 excitation
edges from calcined, reduced, and spent Rh-Mn-Fe/SiO2
catalysts, respectively. Although the Fe L2,3-edges appear weak
due to the low Fe content, the chemical position and shape of the
spectra change with the pre-treatment conditions of the catalyst.
Comparison of Fe L2,3 edges clearly shows that both, hydrogen
treatment and catalytic reaction, lead to reduction of the iron
oxide. This is manifested by the shift of the Fe L3 ionization edge
(defined at half maximum of the peak) from ca. 709 eV in case of
the calcined sample to ca. 707.8 eV for both, the reduced and
used catalyst. In fact, the preservation of the metallic state in the
catalysts even after exposure to ambient conditions indicates
formation of a Rh-Fe alloy, which stabilizes Fe in the metallic
state.[13] The bimetallic Rh-Fe alloy was further characterized by
STEM-EELS mapping (Figure S14). Note that the temperature of
H2 reduction and catalytic reaction was much lower than typically
required for the reduction of Fe oxide. Its reduction can be
understood by the hydrogen spill-over: once Rh2O3 gets reduced
by H2 (or H2+CO) to metallic Rh, the latter serves as a catalyst for
H2 dissociation and thus facilitates the reduction of iron oxide. In
contrast, the reducibility of Mn oxide is lower, reflected by the
more negative reduction potential. Hence, the Mn is likely existing
as oxide in the reduced and the spent catalysts. This may explain
why there is no Rh-Mn alloy formed in the Mn promoted Rh
catalysts. The oxidic nature of Mn is further confirmed by XPS
(Figure S15), although the oxidation state is not conclusive due to
the lack of chemical shifts for the various Mn oxides.[14]
So far, we have revealed that after the catalytic reaction
sintering of nanoparticles occurs in both Mn, and Mn-Fe promoted
catalysts. In the bi-promoted catalyst, the nanoparticles (including
the sintered particles) have been systematically characterized
and confidently demonstrated to be the Rh-Fe alloy, as discussed
above. In Mn promoted catalyst, however, the morphology of
sintered particles is different in comparison to that of metallic Rh,
suggesting a different phase. To reveal their composition, EELS
and EDX characterizations were performed. Figure 4a shows an
EELS spectrum collected from the dotted line across a sintered
particle in the used Rh-Mn/SiO2 catalyst. For comparison, the
EELS spectrum of a Rh-Fe alloy particle in the spent Rh-Mn-
Fe/SiO2 catalyst is also shown (Figure 4b). The O K edge present
Figure 3. Structural and compositional characterization of (a,b) reduced and
(c,d) spent Rh-Mn-Fe/SiO2 catalysts. (a,c) HAADF-STEM images; insets show
fast Fourier transformation (FFT) and intensity profiles taken from the assigned
regions of interest (ROI); (b,d) HAADF-STEM images and corresponding EDX
mappings; (e) Schematic model hypothesized for Rh-Mn-Fe/SiO2 after
reduction and catalytic reaction; (f) STEM-EELS spectra at the Fe L-edge of
calcined, reduced and used Rh-Mn-Fe/SiO2. To prevent air contact, the
reduced sample was transferred from a glove box into TEM using a vacuum
transfer (VT) holder.
in Fe modified Rh catalysts (Rh-Fe/TiO2)[12] during the syngas
conversion, however, without a direct evidence for such phase.
In order to confirm the alloy formation, we analyzed the atomic
composition by STEM-EDX elemental mapping. Figures 3b,d
show HAADF-STEM images and corresponding EDX maps of Rh,
Fe and Mn obtained from the H2 reduced and spent catalysts.
Additional EDX mapping analysis on different sized particles in
the spent Rh-Mn-Fe/SiO2 catalyst are presented in Figure S11a.
Elemental mapping confirms that Rh and Fe are always
distributed over the same region, thus strongly suggesting that Rh
and Fe form a bimetallic alloy. In addition, EDX-line scans show
that the signal intensities due to Rh and Fe are almost
synchronized, which indicates
a homogeneous bulk alloy
structure rather than a surface alloy (Figure S11b).
Contrary, the Mn signal is distributed around the Rh-Fe
particles, and thus is not involved in the alloy formation. The
distribution of Mn is similar in the case of Rh-Mn/SiO2 (Figures 3d
and S6b,e) and is independent of the presence of the Fe
promoter. In agreement with analyses of lattice spacing and
elemental mapping, a minor binding energy shift of the Rh 3d
peak to a higher energy in the XPS spectrum of the reduced and
the spent Rh-Mn-Fe/SiO2 catalyst (Figure S12b,c) suggests that
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RhCx exists, we measured the carbon K edge of the small
nanoparticles by EELS-line scan (Figure S18). The summarized
EELS spectra (Figure S18b,d) unambiguously reveal the
presence of the carbon signal that could be ascribed to the
carbidic structure (Figure S18e). The complementary study based
on XPS reveals a slight change in the electronic structure of Rh
in the spent Rh-Mn/SiO2 catalyst (Figure S19), which could be
related to the formation of RhCx.
In conclusion, we have performed a detailed electron
microscopy study of silica-supported Rh, Rh-Mn, and Rh-Mn-Fe
catalysts of different states (calcined, reduced, spent), which
provides not only the atomic-scale insight into the structural and
chemical states of metal and promoters, but also insights about
the metal-promoter interaction under reaction conditions. Our
characterization reveals that in unpromoted calcined Rh catalyst,
Rh exists as a crystalline oxide phase. Addition of promoters,
either Mn alone or Mn and Fe together, leads to formation of
complex oxides in which metal and promoters are in close
proximity. In the best performing catalyst, i.e., Rh-Mn-Fe/SiO2, the
metal and the promoters undergo a series of structural and
chemical transformations. Due to the higher reducibility of Fe
oxide compared to Mnoxide, the hydrogen spill-over from Rh only
leads to a reduction of oxidic Fe to the metal state, followed by
alloying with Rh. The alloying process is quite efficient, leading to
incorporation of about 50% of Fe in Rh after reduction in hydrogen
for 1 hour. After a long catalytic reaction (about 1 month), more
Figure 4. Comparison of STEM-EELS spectra recorded from sintered
particles of postreaction (a) Rh-Mn@SiO2 and (b) Rh-Mn-Fe@SiO2 catalysts.
O signal is from the SiO2 support.
in both spectra is due to the silica support underneath the
particles. The Mn L edge is recorded from the surrounding MnOx
species. The strong Fe L edge (Figure 4b) shown in the spent Rh-
Mn-Fe/SiO2 catalyst again demonstrates the formation of a Rh-Fe
alloy. The most notable difference of two spectra lies in the carbon
K edge that is detected from the sintered particle of Mn promoted
catalyst, while there is no carbon signal present on the Rh-Fe
alloy particle of the bi-promoted catalyst. Further analysis reveals
that the fine-structure of carbon K edge doesn’t fit to either the
finger-print of amorphous carbon or to the one of graphitic
carbon.[15] Instead, it is rather similar to that of carbides.[16] EELS
and EDX maps clearly reveals a similar distribution of C and Rh
signals over the entire particle (Figure S16), unambiguously
evidencing the formation of RhCx. We have repeated this
measurement on different sintered particles and they all show the
same result: the formation of RhCx exclusively occurs in the spent
Rh-Mn/SiO2 rather than in the spent Rh-Mn-Fe/SiO2 catalysts
(Figure S17). To the best of our knowledge, this is the first
evidence of RhCx formed in syngas upgrading. Thus, the Rh-Fe
alloy shows higher resistance against the carbide formation,
possibly due to the lower CO disassociation rate of this catalyst
compared to the Rh-Mn/SiO2 (Figure S1). The modified electronic
properties of Rh in the Rh-Fe alloy may also lead to weakened
absorption of carbonaceous species and an increased energy
barrier for carbon diffusion,[17] thus preventing the carbide
formation.
than 70% of Fe is found in the bulk alloy form, instead of the
5c, 5d]
surface alloy that was proposed in calculations.[2,
TEM
characterization of the spent Rh-Mn catalyst also reveals the
formation of Rh carbide in the catalytic CO hydrogenation
process, which is however not detected in the Rh catalyst that
contains both Mn and Fe. Hence, an additional role of Fe in
preventing metal-catalyzed coking is uncovered in the process of
syngas conversion over Rh-based catalysts.
Overall, analysis of Mn- and Mn-Fe-promoted, silica-
supported Rh catalysts by TEM confirms the description of the
active phase as a perforated core-shell structure on the support
material, and reveals details of metal-promoter interaction at the
atomic level. Our results provide
a guideline for further
development of DFT models of such catalysts for elucidation of
the reaction mechanisms. Improved understanding of relations
between catalyst structure and reaction induced modifications
reported here provides a basis for a rational design of improved
alcohol synthesis catalysts.
Acknowledgements
This work was conducted in the framework of the BasCat
collaboration between BASF SE, Technical University Berlin,
Fritz-Haber-Institut der Max-Planck-Gesellschaft, and the cluster
of excellence “Unified Concepts in Catalysis” (UniCat
www.unicat.tu-berlin.de).
Having identified Rh carbide in sintered particles in Rh-
Mn/SiO2 catalyst, we assessed if a similar phenomenon occurs
on the smaller particles. However, the structural characterization
of smaller particles by STEM provides no clear evidence for a
RhCx phase (Figure S6). On the other hand, it may be the case
that some carbon atoms absorb in the sub-surfaces of the
nanoparticles (Figure S6f), and due to the low thickness of the
sub-surfaces and low carbon content, the nanoparticles appear
as a bulk metallic Rh. In order to examine whether the sub-surface
Keywords: atomic-scale imaging • metal-promoter interaction •
syngas-to-ethanol conversion • Rh-based catalysts •
heterogeneous catalysis
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10.1002/anie.201902750
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
Atomic insights about the metal-
promoter interaction and the roles of
promoters (Mn, Fe) in Rh-based
catalysts for the syngas conversion to
ethanol are provided based on a
careful aberration-corrected high-
resolution transmission electron
microscopy study.
Xing Huang,* Detre Teschner, Maria
Dimitrakopoulou, Alexey Fedorov,
Benjamin Frank, Ralph Kraehnert, Frank
Rosowski, Harry Kaiser, Stephan
Schunk, Christiane Kuretschka, Robert
Schlögl, Marc-Georg Willinger* and
Annette Trunschke*
Page No. – Page No.
Atomic-Scale Observation of the
Metal-Promoter Interaction in Rh-
Based Syngas Upgrading Catalysts
This article is protected by copyright. All rights reserved.