Engineering of Ruthenium–Iron Oxide Colloidal Heterostructures: Improved Yields in CO2 Hydrogenation to Hydrocarbons

Article abstract of DOI:10.1002/anie.201910579

Catalytic CO₂ reduction to fuels and chemicals is a major pursuit in reducing greenhouse gas emissions. One approach utilizes the reverse water-gas shift reaction, followed by Fischer–Tropsch synthesis, and iron is a well-known candidate for this process. Some attempts have been made to modify and improve its reactivity, but resulted in limited success. Now, using ruthenium–iron oxide colloidal heterodimers, close contact between the two phases promotes the reduction of iron oxide via a proximal hydrogen spillover effect, leading to the formation of ruthenium–iron core–shell structures active for the reaction at significantly lower temperatures than in bare iron catalysts. Furthermore, by engineering the iron oxide shell thickness, a fourfold increase in hydrocarbon yield is achieved compared to the heterodimers. This work shows how rational design of colloidal heterostructures can result in materials with significantly improved catalytic performance in CO₂ conversion processes.

Full text of DOI:10.1002/anie.201910579

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International Edition: DOI: 10.1002/anie.201910579
German Edition: DOI: 10.1002/ange.201910579
Heterogeneous Catalysis
Engineering of Ruthenium–Iron Oxide Colloidal Heterostructures:
Improved Yields in CO Hydrogenation to Hydrocarbons
2
Aisulu Aitbekova, Emmett D. Goodman, Liheng Wu, Alexey Boubnov, Adam S. Hoffman,
Arda Genc, Huikai Cheng, Lee Casalena, Simon R. Bare, and Matteo Cargnello*
Abstract: Catalytic CO reduction to fuels and chemicals is
catalysts should be active for both RWGS and FT synthesis.
Among many catalytic materials that have been investigated
for this process, iron is a promising candidate because of its
activity for both steps of the reaction and cheap price. It has
been demonstrated that replacing CO with CO2 in the
reaction feed does not lead to significant changes in product
distribution over iron catalysts, suggesting that the metal is
2
a major pursuit in reducing greenhouse gas emissions. One
approach utilizes the reverse water-gas shift reaction, followed
by Fischer–Tropsch synthesis, and iron is a well-known
candidate for this process. Some attempts have been made to
modify and improve its reactivity, but resulted in limited
success. Now, using ruthenium–iron oxide colloidal hetero-
dimers, close contact between the two phases promotes the
reduction of iron oxide via a proximal hydrogen spillover
effect, leading to the formation of ruthenium–iron core–shell
structures active for the reaction at significantly lower temper-
atures than in bare iron catalysts. Furthermore, by engineering
the iron oxide shell thickness, a fourfold increase in hydro-
carbon yield is achieved compared to the heterodimers. This
work shows how rational design of colloidal heterostructures
can result in materials with significantly improved catalytic
[2]
effective for both the FT and CO -FT synthesis. Further-
2
more, recent studies have demonstrated that potassium-
doped iron is effective for converting CO directly to light
2
[
3]
olefins. Unlike iron, other metals that are used for the FT
synthesis, such as cobalt and ruthenium, predominantly
perform CO methanation. For this reason, iron has been
[
2]
2
the subject of many investigations aimed at improving its
performance for CO hydrogenation. In order to improve the
2
activity of iron-based catalysts, various metal promoters and
alloys of iron with different metals have been studied.
Particular attention has been paid to alloying iron with
ruthenium owing to two main reasons: first, ruthenium is the
most active FT metal, thus its addition is expected to increase
conversion in iron-based systems; second, alloying iron with
ruthenium results in a change in the electronic d-band
performance in CO conversion processes.
2
Introduction
CO hydrogenation to hydrocarbons has received signifi-
2
cant attention in the scientific community owing to the
possibility of reducing emissions of this greenhouse gas while
producing fuels and chemicals. A proposed process to achieve
this result is a modified Fischer–Tropsch (FT) synthesis; in
this, the reverse water-gas shift (RWGS) reaction first
[4]
structure that can give rise to distinct catalytic properties.
The synthesis of promoted iron catalysts is usually
performed via impregnation of a support using two metal
salt precursors. However, such a preparation technique does
not guarantee exclusive formation of uniform alloy nano-
particles. Instead, these bimetallic catalysts likely contain iron
and ruthenium phases that are present both as alloys and
converts CO into CO intermediate, which is further con-
2
verted into hydrocarbons by the FT process (CO -FT syn-
2
[
1]
thesis). Instead of performing the two steps separately, it is
often more convenient to incorporate them into a single
reactor. However, there are several challenges associated
with direct CO2 conversion into hydrocarbons, including
[4,5]
separate nanoparticles of the individual metals. It has been
reported that reduction of alumina-impregnated ruthenium
and iron salts leads to formation of both Fe/Ru alloys and
[1]
[6]
thermodynamic limitations and catalyst poisoning. Effective
segregated iron nanoparticles. As a result, it is likely that the
metals are present in various phases and oxidation states and
that this heterogeneity can help explain the reported varia-
bility in the catalytic activity regarding the effects of
ruthenium addition on FT activity of iron-based catalysts.
For example, some studies show that the addition of Ru
increases CO conversion and shifts product distribution from
[
*] A. Aitbekova, E. D. Goodman, Dr. L. Wu, Prof. M. Cargnello
Department of Chemical Engineering and SUNCAT Center for
Interface Science and Catalysis, Stanford University
443 Via Ortega, Stanford, CA, 94305 (USA)
E-mail: mcargnello@stanford.edu
[
4,5d,e]
Dr. L. Wu, Dr. A. Boubnov, Dr. A. S. Hoffman, Dr. S. R. Bare
Stanford Synchrotron Radiation Lightsource
SLAC National Accelerator Laboratory
olefins towards heavier hydrocarbons,
while others claim
that Ru addition does not have significant effects on activity
[5c]
and selectivity.
CO -FT processes over ruthenium–iron
2
2575 Sand Hill Rd., Menlo Park, CA, 94025 (USA)
catalysts have been studied to a significantly lesser extent;
however, previous reports on this topic also presents con-
flicting results. Similar to the case of FT synthesis, while some
reports show that Ru shifts the product distribution from
Dr. A. Genc, Dr. H. Cheng, Dr. L. Casalena
Thermo Fisher Scientific
5350 NE Dawson Creek Dr., Hillsboro, OR, 97124 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201910579.
[
7]
shorter to longer chain hydrocarbons, others do not observe
[
8]
significant effects of Ru promotion. In this contribution, we
&
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investigate the effect of ruthenium on iron-based systems for
with a lower contrast iron oxide nanoparticle. Very few
the CO -FT process by utilizing well-defined ruthenium–iron
isolated ruthenium or iron oxide nanoparticles were ob-
served. Lattice fringe analysis of the iron oxide phase
identified distances between two adjacent planes to be
0.294 nm and 0.485 nm (Supporting Information, Figure S1),
which can be assigned to (220) and (111) planes of inverse
2
oxide colloidal heterodimers and core–shell structures. We
demonstrate that the close contact between the two phases
promotes the reduction of iron oxide via a proximal hydrogen
spillover effect, enabling the formation of the active iron/iron
carbide phase for the reaction at significantly lower temper-
atures than in iron oxide catalysts. We also observed the
transformation of heterodimers into core–shell structures
owing to the encapsulation of ruthenium by iron oxide upon
reductive pretreatment. Furthermore, by engineering core–
shell structures with a thin iron oxide shell, we achieved
a fourfold increase in hydrocarbon yield compared to the
heterodimers.
[
10a]
spinel structured Fe O , respectively
Therefore, the iron
oxide in the as-synthesized heterodimers is being present as
3
4
.
[10a,c]
Fe O , as ascertained by previous studies.
It is hypothe-
3
4
sized that the formation of heterodimers is obtained through
deposition of metallic iron first, which then promotes further
decomposition of the iron precursor and its oxidation to iron
[
11]
oxide material grown epitaxially. The heterodimers were
then deposited onto g-Al O support, and the organic ligands
2
3
[
12]
removed using a fast calcination treatment. The nominal
weight loading in terms of total metal content (Ru + Fe) was
set to 1 wt% with respect to the support. The heterodimer
structure was maintained after deposition and calcination, as
demonstrated by high-angle annular dark field scanning
transmission electron microscopy (HAADF-STEM) imaging
and energy-dispersive X-ray scattering (EDS) maps of the
catalyst after calcination (Figure 1c–f). These images confirm
that the heterodimer nature of the materials is maintained
after the calcination process to remove ligands used during
the synthesis. Furthermore, X-ray absorption spectroscopy
(XAS) characterization of the supported sample after the fast
calcination at 7008C showed that the Ru was partially
oxidized (as evidenced by a small Ru-O scattering path in
the EXAFS) and the Fe O4 was oxidized into g-Fe O
Results and Discussion
The heterodimers were synthesized using a seed-mediated
colloidal approach. This method leads to structures where
ruthenium and iron oxide nanoparticles are located in direct
contact with each other by way of shared crystallographic
facets. Ru nanoparticle seeds (4.8 nm average particle size)
were obtained through thermal decomposition of Ru (CO)
in oleylamine using a previously reported procedure. Iron
oxide nanoparticles (13.1 nm average particle size) were then
grown onto the Ru seeds by addition of Fe(CO) and its
3
[
12
9]
5
subsequent decomposition at 3008C in a mixture of 1-
octadecene, oleic acid, and oleylamine, optimizing previously
3
2
3
[
10]
reported synthesis procedures. A detailed synthesis proce-
dure is given in the Supporting Information. Representative
transmission electron microscopy (TEM) images (Figure 1a)
demonstrate the formation of the heterodimers, with each
higher contrast spherical Ru nanoparticle in direct contact
(Supporting Information, Table S1, Figure S2).
The catalytic activity of the heterodimer catalyst was
compared to various control catalysts to determine how the
interaction of ruthenium and iron oxide phases affects their
properties. The three control samples were Ru/Al O , Fe O /
2
3
2
3
Figure 1. a),b) Representative TEM image of as-synthesized Ru/Fe O heterodimers and corresponding particle size distributions. c) Representa-
3
4
tive HAADF-STEM image and d)–f) EDS maps of the Ru/Fe O /Al O catalyst after calcination.
2
3
2
3
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Al O and a physical mixture of these two. The nominal
weight loading in terms of total metal content was 1 wt% for
all control samples. The ruthenium nanoparticles for the Ru/
and their physical mixture resulted in an intermediate
selectivity between the two. However, among all the catalysts
tested, only the heterodimer Ru/Fe O /Al O sample pro-
2
3
2
3
2
3
Al O3 sample were from the same batch as the seeds
employed for the synthesis of the heterodimers. Iron oxide
nanoparticles were synthesized through thermal decomposi-
duced hydrocarbons and methanol. The fact that the physical
mixture only produced CO and CH , while the Ru/Fe O /
2
4
2
3
Al O heterodimer catalyst formed hydrocarbons (Figure 2a)
2
3
tion of Fe(acac) in a mixture of benzyl ether and oleylamine
demonstrates the importance of proximity between rutheni-
um and iron oxide phases. We set out to investigate the reason
for the observed differences in the catalytic activity between
Ru/Fe O /Al O and Fe O /Al O .
3
[13]
(
Supporting Information, Figure S3). The control samples
were also subjected to the same fast calcination treatment to
remove organic ligands. X-ray absorption near-edge spectra
2
3
2
3
2
3
2
3
(
XANES) characterization was performed on the supported
Characterization of iron catalysts with ex situ techniques
provides only limited information, as Fe is known to become
oxidized by oxygen when exposed to air precluding accurate
iron oxide sample after the fast calcination (Supporting
Information, Figure S2). The slightly lower overall intensity
of the g-Fe O standard was due to the fact that the spectrum
[
5b,14]
characterization of the active phase.
Therefore, we
2
3
was collected at a different facility. This difference, however,
did not affect the identification of the iron state in the catalyst
showing that the iron was being present exclusively as g-
Fe O .
studied our materials with in situ XAS to monitor changes
in the oxidation state of iron and ruthenium in the Ru/Fe O /
2
3
Al O heterodimer and Fe O /Al O catalysts. In situ Ru K-
2
3
2
3
2
3
edge XANES and EXAFS data of the Ru/Fe O /Al O
3
2
3
2
3
2
In a first catalytic test, all catalysts were reduced at 3008C
sample after the reductive pretreatment in a flow of hydrogen
show that the partially oxidized ruthenium completely
reduces to metallic state (Supporting Information, Figure S4,
Table S1). Figure 3a,b and the Supporting Information,
Tables S2, S3 report results and quality of linear combination
fitting (LCF) during the reductive pretreatment using rele-
vant standards (Supporting Information, Figures S5). Fig-
ure 3c,d shows corresponding in situ Fe K-edge XANES of
the Ru/Fe O /Al O and Fe O /Al O samples. Representa-
in pure H and tested for CO hydrogenation at 6 bar and
2
2
3
008C in a 3:1 mixture of H /CO . Catalytic measurements
2 2
were performed under differential conditions with CO2
conversion of less than 1% by adjusting the gas hourly space
velocity (GHSV). The catalytic results are reported in
Figure 2. Equations used to calculate selectivity can be found
in the Supporting Information. In this low conversion regime,
all catalysts produced carbon monoxide and methane as the
2
3
2
3
2
3
2
3
[3]
dominant products, in agreement with previous works. The
Fe O /Al O catalyst produced almost exclusively CO, the
tive LCF data and fits are presented in the Supporting
Information, Figures S6, S7. The initial iron phase after the
2
3
2
3
Ru/Al O catalyst formed CH with about 70% selectivity,
fast calcination for both catalysts is g-Fe O3 (Supporting
2
3
4
2
Information, Figure S2). However, the temperature-depen-
dent evolution of the Fe oxidation state varies dramatically
between the two samples. Heated in pure H , both samples
2
first reduce from g-Fe O to Fe O , although this reduction
2
3
3
4
occurs at a significantly lower temperature for the heterodi-
mer sample compared to the Fe O /Al O catalyst. For the
2
3
2
3
heterodimer sample, this first reduction starts at 508C and is
complete by 1008C, while the Fe O /Al O sample starts
2
3
2
3
reducing at 1008C and finishes at about 2308C. A more
important difference between the two samples becomes
evident after this first phase transformation. In the Ru/
Fe O /Al O heterodimer catalyst, the iron phase completely
2
3
2
3
transitions to metallic iron at about 3008C. Instead, the Fe O /
2
3
Al O slowly transitions from Fe O to FeO, before metallic
2
3
3
4
iron starts to form. This final reduction is not complete until
008C. In other words, in the Ru/Fe O /Al O catalyst,
5
2
3
2
3
complete reduction of Fe O3 happens at a temperature
2
2
008C lower than the Fe O alone.
2 3
There is no consensus regarding the nature of active sites
of CO -FT/FT iron catalysts. However, at least three phases of
2
iron have been identified as possible active sites under
reaction conditions: several states of iron oxides and iron
[14]
carbides, and metallic iron. There is general agreement that
under reaction conditions these various phases co-exist and
[
14]
their relative concentrations undergo dynamic changes.
Among them, iron carbides are thought to be crucial for the
[
15]
formation of hydrocarbons, while iron oxides have been
[
3a]
Figure 2. a) CO conversion (black &
) and CO selectivity (green !);
b) hydrocarbon and methanol distributions.
identified as being active for WGS/RWGS reactions. Thus,
since the pretreatment of the Fe O /Al O catalyst was
2
2
3
2
3
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Figure 3. a),b) Linear combination fitting of the in situ Fe K-edge XANES spectra showing the relative fraction of different Fe phases found in the
heterodimer and Fe O /Al O catalysts, respectively; c),d) In situ Fe K-edge XANES spectra used in the linear combination fitting for the
2
3
2
3
heterodimer and Fe O /Al O catalysts, respectively.
2
3
2
3
performed at 3008C, and at this temperature the catalyst is
comprised mostly FeO phase, then this is an indication that
the FeO phase is responsible for the observed RWGS
reactivity of this catalyst. To confirm that the formation of
metallic iron is a prerequisite for hydrocarbon synthesis, we
reduced the Fe O /Al O sample at 5508C and measured its
The profound differences in the catalytic behavior of all
samples reduced at 3008C are, therefore, related to the
reduction of the iron phase in the heterodimers at lower
temperatures, driven by the proximity of iron oxide to the
metallic Ru nanoparticles in the sample. It is well-established
that noble metals promote the reduction of transition-metal
2
3
2
3
[
16]
catalytic activity again. After this high-temperature reduc-
tion, the sample indeed produced hydrocarbons and methanol
with a product distribution similar to that of the heterodimers
reduced at 3008C (Supporting Information, Figure S8). Sim-
ilar catalytic selectivity of the heterodimers reduced at 3008C
and the iron oxide sample reduced at 5508C suggest that the
role of ruthenium is to promote the reduction of iron oxide to
the active iron phase. However, TEM characterization of the
post-catalysis sample showed that there was severe sintering
of the nanoparticles with an increase in their average size
from 6.0 nm to 15.1 nm as a result of the high-temperature
reduction treatment (Supporting Information, Figure S9).
The activity of the catalyst after 5508C reduction decreased
compared to the heterodimer sample reduced at 3008C, when
normalized by mass of total metal. However, GHSV was
adjusted to maintain similar conversion levels (< 1%) for
both catalysts to accurately compare selectivity data (Sup-
porting Information, Figure S8). We realize that mass normal-
ization is not a fair normalization technique given the nature
of our materials, however, we did not investigate differences
in intrinsic activity in detail owing to challenges associated
with comparing activity of bimetallic and monometallic
oxides via hydrogen spillover effects. Activation barriers
for hydrogen activation on metals are usually lower compared
[
16]
to metal oxides. When metals activate and split molecular
hydrogen, hydrogen atoms spill onto the oxide phase through
hydroxyl groups present on its surface, promoting the
reduction of the transition metal to a lower oxidation state
and/or to the metallic phase. The hydrogen spillover effect
depends critically on the metal/metal oxide pair combination,
[
16]
and on the distance between the two phases. On alumina,
which is the support used in our study, the critical distance was
[16]
found to be approximately 15 nm. Therefore, we attribute
the improved catalytic performance of the heterodimers to
a greater extent of reduction of the iron oxide, promoted by
the proximity of the metallic ruthenium phase via hydrogen
spillover. Despite a plethora of studies on ruthenium–iron
oxide systems for CO -FT/FT synthesis, promotion of iron
2
oxide reduction by ruthenium and detailed evolution of iron
oxide phases upon reduction have not been reported before.
This effect cannot be activated in the physical mixture
because of the far distance between ruthenium and iron
oxide phases (micrometers) or in the sample with only iron
oxide phase because of the absence of metallic ruthenium that
can activate hydrogen.
[4]
systems nicely described in previous studies.
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Further HAADF-STEM characterization of the Ru/
Fe O /Al O heterodimer sample after catalysis demonstrated
Supporting Information, Table S1. The best fit model to the
EXAFS data confirms that the fresh catalyst comprises both
metallic Ru and Ru oxide, and the coordination numbers are
consistent with the Ru being present as nanoparticles.
However, after reduction, the RuꢀO scattering path at
2
3
2
3
an unexpected structural transformation of the catalyst. Here,
we observed the presence of a core–shell structure, with
a high-contrast core surrounded by a lower-contrast shell
material of the nanoparticles supported on alumina (Fig-
ure 4). EDS-mapping suggested that the core material is
mostly composed of Ru, encapsulated by Fe and C, whereas
the shell mostly of Fe and O. These maps suggest that
ruthenium was encapsulated by the iron phase during the
reduction and/or catalytic reaction. EDS line scans (Support-
ing Information, Figure S10) and EDS tomography (Support-
ing Information, Videos S1, S2) of the fresh and the post-
catalysis heterodimer sample further demonstrate the core–
shell nature of the nanoparticles in the spent catalyst.
2.01 ꢀ disappeared, indicating that the sample completely
reduced to metallic Ru. At the same time, while the reduced
and the post-catalysis samples mostly consist of one dominant
contribution owing to RuꢀRu at 2.66 ꢀ, the two samples have
non-negligible contributions at 2.58 ꢀ (Supporting Informa-
tion, Figures S12–S14, Table S1). Previous studies identified
[
18]
that this distance corresponds to the RuꢀFe scattering path.
Thus, the fact that we see appearance of RuꢀFe contribution
in the reduced and the post-catalysis samples is consistent
with Ru encapsulation by Fe, in agreement with the EDS
characterization. The EDS maps in Figure 4 suggest that
while the core mostly comprised of metallic Ru, the presence
of the RuꢀFe contribution in the EXAFS modelling suggests
formation of alloying of Ru surface layers with Fe. While
XANES at the Ru edge does not show significant differences
between the reduced catalyst and metallic Ru, the EXAFS
analysis clearly indicates the appearance of the RuꢀFe
contribution after reduction of the catalyst (Supporting
Information, Figures S12–S14, Table S1). We believe that
the interaction between Ru and Fe is only pronounced in
EXAFS because it is a fraction of surface Ru atoms that
would interact with Fe, and the XANES spectrum at the Ru
K-edge is relatively insensitive to charge transfer (s!p). In
a Ru nanoparticle of an average size of 4.8 nm, the fraction of
surface atoms is about 16%. This value is consistent with the
obtained RuꢀFe coordination numbers (Supporting Informa-
tion, Table S1). EXAFS fitting at the Fe edge, however, did
not show FeꢀRu contribution in the reduced sample probably
owing to the even smaller fraction of Fe interacting with Ru
(ca. 3%) in the core–shell structure.
Figure 4. a) Representative HAADF-STEM image and b)to f) energy
dispersive X-ray spectroscopy maps of b) C, c) O, d) superposition of
Al, Fe, and Ru, e) Fe, and f) Ru in the post-catalysis heterodimer
sample.
We also observed similar core–shell structures in the pure
iron oxide sample after catalysis (Supporting Information,
Figure S9). Lattice fringe analysis clearly demonstrates that
the shells are crystalline, with lattice constants corresponding
to either g-Fe O3 or Fe O4 (Supporting Information, Fig-
To understand whether the encapsulation occurs during
the reductive pretreatment or during reaction, we reduced the
heterodimers at 3008C and characterized the sample using
TEM. These micrographs indicate the presence of thin
overlayers of an iron phase, in this case iron oxide (Support-
ing Information, Figure S11). This observed behavior is
consistent with a strong metal–support interaction effects
between a reducible oxide and Group 8 noble metals, where
the reduced oxide phase covers the surface of a supported
2
3
ure S9). The presence of an iron oxide shell evidenced by
(S)TEM and EDS in the spent catalysts is likely the result of
oxidation following exposure to air rather than a realistic
representation of the catalyst structure under reaction con-
ditions. Indeed, in situ Fe K-edge XANES showed that the
heterodimer sample predominantly consisted of metallic iron
and iron carbide phases with no observable contribution from
g-Fe O3 (Supporting Information, Figure S15). However,
2
[
17]
metal as a result of more favorable surface energy. Upon
during the in situ experiments we observed that upon cool
down in the reaction mixture, the contribution owing to
reduction of surface g-Fe O to FeO , the latter migrates on
2
3
x
the ruthenium metal surface, leading to encapsulation of the
ruthenium. Such transformation of ruthenium–iron oxide
heterodimers into core–shell structures has not been reported
before.
metallic iron decreased, while that of g-Fe O3 increased
2
(Supporting Information, Figure S15). Thus, we surmise that
the active phase of the catalyst consists mainly of metallic iron
and iron carbide phases which, upon exposure to air, oxidize
and form an iron oxide shell. This result is easily understood
given the susceptibility of iron to oxidation. EDS maps
(Figure 4) are consistent with this claim since we found a non-
negligible carbon signal arising from the core of the nano-
structures (Figure 4c). The overall characterization by com-
We further studied the Ru/Fe O /Al O sample by mod-
2
3
2
3
eling in situ Ru K-edge EXAFS data of the fresh, reduced,
and post-catalysis samples. The EXAFS data are shown in the
Supporting Information, Figures S12, S13 and the numerical
results of the modeling for each sample are presented in the
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bining XAS and (S)TEM provides an idea of the challenges
electronic interactions between the Ru core and iron shell
associated in determining active phases and the need to
combine in situ tools for a full description of catalyst
structure.
become stronger and, thus, affect the catalytic selectivity of
the CO hydrogenation products. The catalytic activity and
2
selectivity of the two samples was also compared at higher
Our experiments suggest that once ruthenium promotes
the reduction of iron oxide and a core–shell structure is
formed, the catalytic activity of the heterodimers is similar to
that of pure iron oxide. This result can be explained by the
presence of a relatively thick shell of iron (4.3 nm on average)
that dominates the activity. However, electronic promotion of
the iron phase can be obtained if the iron oxide shell thickness
is decreased. Based on this knowledge, we prepared Ru/FeOx
core–shell nanoparticles with an average shell thickness of
CO conversion (Supporting Information, Figure S18).
2
Finally, we demonstrate that the benefits of phase
proximity in the heterodimer structures are not limited to
ruthenium, and provide a general framework for the develop-
ment of more efficient heterogeneous catalysts using colloidal
nanomaterial design. To achieve this goal, we synthesized Pd/
FeO heterodimers in a straightforward extension of the Ru/
x
FeO synthesis process. These nanoparticles were deposited
x
onto alumina and activated by reduction at 3008C as
1
.2 nm directly from colloidal synthesis (Figure 5a) and
previously described for the ruthenium counterpart, and their
deposited them onto the same Al O3 support as for the
performance for CO -FTexamined under the same conditions
2
2
heterodimers. EDS line scans confirmed the presence of iron
in the shell and ruthenium in the core (Supporting Informa-
tion, Figure S16). The catalyst was reduced in pure hydrogen
at 3008C and catalytic activity was tested under differential
conditions. Interestingly, the core–shell nanoparticles with
much thinner shell made significantly more hydrocarbons
compared to the heterodimers at similar conversion, demon-
strating a twofold and fourfold increase in the selectivity to
hydrocarbons and the hydrocarbon yield, respectively, com-
pared to the heterodimers (Figure 5b). TEM analysis of the
post-catalysis sample showed no sintering and no changes of
the supported core–shell structures (Supporting Information,
(Supporting Information, Figure S19). The catalyst produced
hydrocarbons from CO -FT, thus supporting the promotion
2
mechanism of reduction of iron oxide by noble metals via
hydrogen spillover. However, Pd/FeO /Al O had higher
x
2
3
selectivity to methanol probably due to either alloying of Pd
and Fe metals or partially exposed Pd surface. TEM
characterization of the fresh and post-catalysis samples
(Supporting Information, Figure S19) suggests Pd encapsula-
tion within Fe, consistent with our ruthenium–iron oxide
results.
Figure S17). We hypothesize that because of the thin shell, Conclusion
In conclusion, we demonstrated a synergistic effect in
ruthenium–iron oxide heterodimer catalysts where promo-
tion of the iron phase for CO hydrogenation to hydrocarbons
2
is realized through hydrogen activation and spillover from the
proximal ruthenium phase to the iron component. Addition-
ally, we observed the transformation of heterodimers into
ruthenium–iron core–shell structures owing to the encapsu-
lation of ruthenium by iron oxide upon reductive pretreat-
ment. Thanks to this knowledge, we realized core–shell
structures with a fourfold increase in the hydrocarbon yield
compared to the initial heterodimers, thus designing an
efficient catalyst for CO conversion to hydrocarbons. This
2
work is the first example of the synthesis and application of
ruthenium–iron oxide heterodimers and core–shell structures
for CO -FT. The well-defined structure of our starting
2
materials enabled us to study the cooperative mechanism of
ruthenium promotion and its implication on the catalytic
performance.
Acknowledgements
We gratefully acknowledge support from the Stanford
Precourt Institute for Energy through a seed grant. M.C.
acknowledges additional support from the School of Engi-
neering at Stanford University and from a Terman Faculty
Fellowship. A.A. acknowledges support from a Stanford
Graduate Fellowship (SGF) and an EDGE fellowship. E.D.G.
acknowledges support from the National Science Foundation
Graduate Research Fellowship under Grant DGE-1656518.
Figure 5. a) Representative TEM image of core–shell Ru-FeO nano-
x
particles deposited on the Al O support and b) hydrocarbon and
2
3
methanol distributions and CO selectivity over the Ru/Fe O hetero-
2
3
dimers and Ru-FeO core–shell nanoparticles reduced at 3008C.
x
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Angewandte
Research Articles
Chemie
Part of this work was performed at the Stanford Nano Shared
Facilities (SNSF), supported by the National Science Foun-
dation under award ECCS-1542152. Part of this work was also
performed at Stanford Synchrotron Radiation Lightsource
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Manuscript received: August 19, 2019
Accepted manuscript online: September 23, 2019
Version of record online: && &&
,
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&
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Angewandte
Research Articles
Chemie
Research Articles
Heterogeneous Catalysis
A. Aitbekova, E. D. Goodman, L. Wu,
A. Boubnov, A. S. Hoffman, A. Genc,
H. Cheng, L. Casalena, S. R. Bare,
M. Cargnello*
&&&& — &&&&
Engineering of Ruthenium–Iron Oxide
Colloidal Heterostructures: Improved
Ruthenium promotes reduction of iron
oxide in ruthenium–iron oxide hetero-
dimers by a proximal hydrogen spillover
effect, leading to the formation of ruthe-
nium–iron core–shell structures active for
CO hydrogenation to hydrocarbons.
2
Yields in CO Hydrogenation to
Tuning the shell thickness leads to a four-
fold increase in hydrocarbon yield.
2
Hydrocarbons
Angew. Chem. Int. Ed. 2019, 58, 2 – 9
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
&&&&
These are not the final page numbers!