Indium oxide as a superior catalyst for methanol synthesis by CO2 hydrogenation

Article abstract of DOI:10.1002/anie.201600943

Methanol synthesis by CO₂ hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficie

Full text of DOI:10.1002/anie.201600943

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201600943
German Edition: DOI: 10.1002/ange.201600943
Methanol Synthesis
Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2
Hydrogenation
Oliver Martin, Antonio J. Martín, Cecilia Mondelli, Sharon Mitchell, Takuya F. Segawa,
Roland Hauert, Charlotte Drouilly, Daniel Curulla-FerrØ, and Javier PØrez-Ramírez*
Abstract: Methanol synthesis by CO2 hydrogenation is
attractive in view of avoiding the environmental implications
associated with the production of the traditional syngas
feedstock and mitigating global warming. However, there still
is a lack of efficient catalysts for such alternative processes.
Herein, we unveil the high activity, 100% selectivity, and
remarkable stability for 1000 h on stream of In2O3 supported
on ZrO2 under industrially relevant conditions. This strongly
contrasts to the benchmark Cu-ZnO-Al2O3 catalyst, which is
unselective and experiences rapid deactivation. In-depth char-
acterization of the In2O3-based materials points towards
a mechanism rooted in the creation and annihilation of
oxygen vacancies as active sites, whose amount can be
modulated in situ by co-feeding CO and boosted through
electronic interactions with the zirconia carrier. These results
constitute a promising basis for the design of a prospective
technology for sustainable methanol production.
gistic structural and electronic effects between its components
hampers the rational optimization of this material.[4a,6]
Among other catalysts studied,[7] only Cu-ZnO-Ga2O3/SiO2
and LaCr0.5Cu0.5O3 displayed improved methanol formation
rates and high selectivities (up to 99.5%), but their scalability
and long-term stability have not been assessed. Recent
experiments on Cu/CeOx/TiO2 model surfaces[8] also showed
promising results, but no attempt has been made to translate
this material into a practically relevant polycrystalline solid.
In our quest for a suitable catalyst, we were intrigued by
the much simpler In2O3 system. This reducible oxide is
commonly used together with SnO2 as a very stable con-
ductive transparent layer in organic light-emitting diodes and
thin-film transistors.[9] Moreover, it has demonstrated high
activity and selectivity in multiple catalytic transformations
involving CO2, including electrochemical conversion into
formic acid,[10] photocatalytic reduction to CO,[11] and meth-
anol steam reforming.[12] Recently, density functional theory
(DFT) studies on CO2 hydrogenation over non-defective[13]
and defective[14] In2O3(110) surfaces suggested that methanol
is the most favorable product and that the reaction follows
a mechanism comprising the cyclic creation and annihilation
of oxygen vacancies. Analysis by impedance and infrared
spectroscopy has revealed an increased conductivity for In2O3
that has been exposed to H2 at mild temperatures,[15]
suggesting that vacancies can be present at conditions
relevant to methanol synthesis. Subsequent testing of a com-
mercial In2O3 sample in CO2 hydrogenation showed reaction
rates comparable to those of Cu-ZnO systems but only
moderate methanol selectivities (up to 55%),[16] in contrast to
the DFT predictions.[14] Based on these premises, we synthe-
sized bulk In2O3 in a controlled manner and showed that its
selectivity to the C1 alcohol can be tuned up to 100% under
a wide range of industrially relevant conditions (T= 473–
573 K, P = 1.0–5.0 MPa, and gas hourly space velocity
(GHSV) = 16000–48000 hÀ1). Moreover, we showed that its
activity can be boosted either through CO co-feeding and/or
the use of a ZrO2 support, which interacts strongly with the
active phase, without lowering the selectivity. This was
correlated with an increase in oxygen vacancies as charac-
terized by spectroscopic and temperature-programmed sorp-
tion methods. In2O3/ZrO2 exhibited excellent stability in
a 1000 h run and could be prepared in an equivalently
performing technical shape, hence paving the way for the
development of a new technology for sustainable methanol
production.
M
ethanol is a key building block in the chemical industry,[1]
with prospects as a sustainable energy carrier if its production
is accomplished from CO2 (captured from large-point emit-
ters) and H2 (retrieved from renewable sources).[2] This
application demands novel catalysts as the ternary Cu-ZnO-
Al2O3 system currently employed for methanol synthesis from
mixed syngas (CO/CO2/H2) exhibits limited activity in CO2
hydrogenation, because of the inhibiting effect of the water
byproduct,[3] low selectivity, owing to its significant activity in
the parasitic reverse water–gas shift (RWGS) reaction,[4] and
insufficient stability, due to water-induced sintering of the
active phase.[5] Furthermore, the intricate network of syner-
[*] O. Martin, Dr. A. J. Martín, Dr. C. Mondelli, Dr. S. Mitchell,
Prof. J. PØrez-Ramírez
ETH Zurich, Department of Chemistry and Applied Biosciences
Institute for Chemical and Bioengineering
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mail: jpr@chem.ethz.ch
Dr. T. F. Segawa
ETH Zurich, Department of Chemistry and Applied Biosciences
Laboratory of Physical Chemistry
Vladimir-Prelog-Weg 2, 8093 Zurich (Switzerland)
Dr. R. Hauert
Empa, Swiss Federal Laboratories for Materials Science and
Technology
Überlandstrasse 129, 8600 Dübendorf (Switzerland)
Dr. C. Drouilly, Dr. D. Curulla-FerrØ
Total Research & Technology Feluy
We assessed the CO2 hydrogenation performance of self-
prepared nanosized In2O3 (11 nm, Table 1) versus the bench-
mark Cu-ZnO-Al2O3 catalyst under typical methanol syn-
Zone Industrielle Feluy C, 7181 Seneffe (Belgium)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600943.
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Table 1: Characterization data for selected catalysts.
time yield (STY) over In2O3 is highly temperature-sensitive
and greatly exceeds that of the copper catalyst above 560 K.
Importantly, In2O3 is only moderately affected by H2O
inhibition, as evidenced by the small drop (20%) in reaction
rate in a H2O co-feeding CO2 hydrogenation experiment at
573 K (molar ratio of H2O/CO2/H2 = 0.3:1:4). In contrast,
a 70% decrease in reaction rate has been reported for Cu-
ZnO-Al2O3 even at 503 K and much larger GHSV values.[18]
These results curtail the performance of the Cu-based
material at conversion levels relevant for industrial applica-
tions.[4a]
Further experiments at the standard reaction temperature
of 573 K revealed a linear increase of the STY upon elevating
the pressure (1.0–5.0 MPa, Figure S1b) and a beneficial
impact for an excess of hydrogen up to a molar feed ratio of
H2/CO2 = 8:1, after which the reaction rate remained constant
(Figure S1c). As no metallic In was detected by powder X-ray
diffraction (XRD) after the reaction (Figure 2a, pattern A),
the latter result supports the hypothesis that oxygen vacancies
on the indium oxide are the active sites for this reaction. To
verify this idea,[14] we studied the reducibility of In2O3 by
temperature-programmed reduction in hydrogen (H2-TPR).
The reduction feature observed at 373–673 K, prior to the
onset of the bulk reduction (Figure 2b), is in agreement with
the existence of such vacancies at the reaction temperatures
applied. Subsequently, the In2O3 surface was investigated by
temperature-programmed desorption (TPD) of CO2 (Fig-
ure 2d). As oxygen-defective In2O3 can be created through
thermal treatment or exposure to reducing agents,[14,15] we
pretreated the samples either in Ar (at T= 573 K) or in
diluted H2 (at T= 473 K). Aside from a peak at approximately
360 K, which was attributed to physisorbed CO2 based on the
analysis of a fully oxidized sample (labeled as O2), two
additional signals with maxima at approximately 650 and
590 K were observed; these were assigned to vacancies that
are thermally induced (Ov1) or formed by H2 (Ov2), respec-
tively.[14]
[a]
[b]
Sample
SBET
dcrystallite
[nm]
Odefect
[%]
À1
[m2 gcat
]
In2O3
In2O3
In2O3
ZrO2
In2O3/ZrO2
In2O3/ZrO2
Ar
113
38
33
110
82
84
11
17
16
–
23
16
24
11
15
14
A[c]
B[c]
fresh
4 h on stream
1000 h on stream
8
10
[a] Determined from the (222) reflection of In2O3 in the XRD patterns.
[b] The fraction of surface oxygen atoms adjacent to a defect was
calculated from the deconvoluted O 1s XPS signal (for a detailed
description, see Figure S2 in the Supporting Information). [c] These
labels refer to the points shown in Figure 1b.
X-ray photoelectron spectroscopy (XPS) of In2O3 pre-
treated in Ar evidenced a component in the O 1s region
related to oxygen atoms next to a defect[11] (Odefect, 23%,
Table 1; see also Figure S2a, 531.7 eV). The similar but less
intense feature for an oxidized sample (19%) supports the
presence of oxygen vacancies as well as other types of defects,
such as sub-coordinated In atoms. The surface nature of the
defects is corroborated by the disappearance of the Odefect
signal upon Ar+ sputtering (Figure S2b). After use in CO2
hydrogenation at 573 K, a lower amount of oxygen defects
(16%) was determined (Table 1, A), which is in line with the
substantial drop in the surface area and the noticeable
sintering of the In2O3 particles (Table 1 and Figure S3). The
latter is induced by H2 as indicated by the significantly
decreased surface area (25 m2 gcatÀ1) after the treatment of
fresh In2O3 in pure H2 at 573 K for one hour. The CO2-TPD
profile of the catalyst after the reaction (Figure 2d) confirmed
contributions from Ov1 and Ov2, and hence, both types of
vacancies could participate in the process. Nevertheless,
additional catalytic experiments point to the predominant
role of thermally induced Ov1 vacancies: 1) In2O3 pretreated
in Ar yielded higher STY values than In2O3 activated in H2
Figure 1. a) Methanol STY and selectivity for CO2 hydrogenation over
bulk In2O3, In2O3/ZrO2 (9 wt.% In), and the benchmark Cu-ZnO-Al2O3
catalyst at various temperatures. b) Methanol STY over bulk In2O3 as
a function of the CO concentration in the feed at 573 K. Reaction
conditions: P=5.0 MPa, H2/CO2 =4:1, and GHSV=16000 hÀ1
.
thesis conditions (Figure 1a). In2O3 showed 100% selectivity
to methanol at all temperatures tested, whereas the ternary
catalyst only reached a maximum of 47% at 473 K owing to
the dominance of the concurrent RWGS reaction.[4a,17] In fact,
the selectivity in CO2 hydrogenation can be tuned by adjust-
ing the GHSV to higher values, at which the slower RWGS
vanishes. For In2O3, this occurs at GHSV> 16000 hÀ1 (see the
Supporting Information, Figure S1a), whereas for Cu cata-
lysts, very high space velocities (6 ” 105 hÀ1)[18] are usually
required, leading to low single pass conversions. The space
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(Figure S1a) or at higher temperatures and lower H2 pres-
sures[19] than those used in our study. Therefore, the proper
adjustment of the reaction conditions is pivotal to achieving
high space time yields and selectivities at reasonable con-
version levels over In2O3.
Based on its strongly reducing properties, CO was co-fed
in the CO2/H2 mixture as a strategy to generate vacancies
in situ. Indeed, treatment of In2O3 in diluted CO at 573 K
unveiled a fast reduction of the surface at the beginning of this
analysis (Figure 2c). XPS characterization of a sample
extracted after 10 min evidenced an improved Odefect signal
(31%) with respect to the catalyst activated in Ar (23%). The
stepwise addition of CO to the gas feed from R = CO/(CO2 +
CO) = 0 (point A) to 80% (point B) at 573 K doubled the
STY (Figure 1b). The Odefect fraction in In2O3 substantially
increased from A to B (Table 1), which is in line with the
performance trend described. Furthermore, electron para-
magnetic resonance (EPR; Figure 2e) spectroscopy revealed
a single signal at g = 1.896 for In2O3 used in CO2 hydro-
genation (A), which is related to delocalized electrons
populating the conduction band that result from the abstrac-
tion of O atoms.[20,21] The additional, broader, and stronger
signal at g ꢀ 2.2 for the sample exposed to the CO-rich feed
(B) may correlate with another type of vacancy involving
unpaired electrons. The relatively low intensity of the EPR
signals reflects the surface character of the vacancies, which is
in accordance with the XPS results. Both techniques corrob-
orate the role of CO as a vacancy generator and booster of the
CO2 hydrogenation activity to counterbalance the efficient
annihilation of vacancies in the presence of CO2, as indicated
by the very stable STY at R = 0–80%. Only at R = 100%
(CO/H2), the reaction rate progressively dropped to 0 within
4 h (point C). No methanol was produced under these
conditions also over a fresh catalyst. In line with the decaying
CO2 signal in the CO treatment (Figure 2c) assigned to the
steady reduction of the bulk, XRD analysis revealed that CO
reduced In2O3 to metallic In in the absence of CO2 (Fig-
ure 2a). This phase thus appears to be unable to catalyze CO2
hydrogenation. As the catalyst remained inactive after
switching the syngas composition back to R = 0%, metallic
In cannot be reoxidized by CO2.
Figure 2. a) XRD patterns, b) H2-TPR profiles, c) CO2 evolution upon
treatment in diluted CO at 573 K, d) CO2-TPD curves, and e) EPR
spectra for In2O3 after different treatments (Ar, O2, H2; A, B, and C
refer to the points depicted in Figure 1b), the fresh ZrO2 carrier, and/
or the supported catalyst after CO2 hydrogenation. The XRD patterns
of samples A and B exhibit reflections of In2O3 only. The latter is
completely transformed into metallic In upon exposure to CO/H2 (C).
The pure ZrO2 carrier did not show any CO2 adsorption in the CO2-
TPD experiment. The observed EPR signals were assigned to conduc-
tion-band electrons (g=1.896), Zr3+ centers (gk =1.960 and
g? =1.977), F centers with trapped single electrons (g=2.003), and
other unpaired electrons (g=2.058 and ꢀ2.2).
Aiming at developing a technically relevant catalyst based
on this promising active phase, In2O3 was supported (9 wt.%
In) onto various carriers to achieve 1) higher dispersion,
2) better resistance against sintering, and possibly also
3) beneficial interactions with the supporting material.
Based on the methanol STY and the In-based reaction rate
in a CO2 hydrogenation experiment (Figure 3a), the use of
ZrO2 was advantageous compared to that of other carriers,
leading to a material that outperformed bulk In2O3 at any
reaction temperature (Figure 1a). Evaluation of ZrO2-sup-
ported catalysts with lower or higher In loadings showed that
9 wt.% In is the optimum for the STY, while the reaction rate
per gram of In increases with a decreased loading (Figure S6).
HRTEM analysis (Figure 3c) visualized intimately mixed
crystals of cubic In2O3 and monoclinic ZrO2 in the optimized
sample. XRD indicated a smaller particle size for supported
In2O3 with respect to the bulk catalyst (Table 1). As the latter
holds also in the case of the poorly performing In2O3/Al2O3
(Figure S4a), and 2) In2O3 calcined at higher temperatures
(723 vs. 573 K) featured a higher density of active sites, as its
activity increased (Figure S4a) in spite of the reduced surface
area (Figure S4b). This was further supported by operando
diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS), which evidenced bands in agreement with ad-
sorbed CO2 bridging two In atoms around the Ov1 site and
hydrogenated intermediates formed thereof and did not
contain signals specific to carbonate species, which would
form upon CO2 activation over Ov2 sites (Figure S5). These
findings are in agreement with the DFT results by Ge et al.[14]
Interestingly, the DRIFTS analysis also revealed the presence
of adsorbed CO species whereas no CO was detected at the
outlet of the cell, corroborating the slow kinetics of the
RWGS reaction under the applied conditions. This undesired
pathway becomes only relevant at lower space velocities[16]
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Remarkably, XPS analysis of In2O3/ZrO2 after CO2
hydrogenation indicated an increase in the Odefect signal
(Table 1). In fact, its area was larger than the sum of the
individual contributions from the carrier and the active phase.
Additional insights were given by EPR spectroscopy. At least
one type of vacancy was found in the fresh support (gk = 1.960
and g? = 1.977, Figure 2e), which is related to electrons
located in the vicinity of zirconium cations changing their
oxidation state from + 4 to + 3.[22] These electrons were
depleted upon deposition of In2O3, suggesting that the
reduced Zr centers abstract O atoms from the active phase,
thereby creating more and/or new kinds of vacancies. In fact,
the EPR spectrum of the supported catalyst contains the same
features as those found for bulk In2O3 samples used in CO2
hydrogenation without (A) and with (B) CO co-feeding, as
well as two additional signals at g = 2.003 and 2.058; the first
one was assigned to vacancies with single trapped electrons in
ZrO2 (F centers).[22] Although ZrO2 itself exhibits no activity
for CO2 hydrogenation under the conditions applied, it
cannot be excluded that the vacancies contained in this
carrier might also be involved in the reaction mechanism over
the supported catalyst.
Aside from this electronic promotion, the ZrO2 carrier
also plays an essential role in preventing the sintering of the
In2O3 phase, as demonstrated by the excellent stability of
In2O3/ZrO2 over 1000 h on stream (Figure 3b). This long-term
test was carried out in CO-free syngas to expose the catalyst
to the maximum amount of H2O. The selectivity remained
unaltered at 99.8% throughout the experiment, and the
activity was only reduced by 8% within the first 400 h on
stream, after which the reaction rate was constant. Likewise,
the In2O3 crystallite size and the catalyst surface area
remained unchanged (Table 1). This excellent performance
was also observed for a technical In2O3/ZrO2 catalyst
obtained by impregnation of extrudates of the carrier. In
striking contrast, the conventional Cu-ZnO-Al2O3 catalyst
underwent substantial loss of activity (44% in 100 h, Fig-
ure 3b), which is due to sintering of the Cu and ZnO phases
(Figure S8). A comparison of the performance of In2O3/ZrO2
with alternative materials studied in the literature is pre-
sented in Table S5.
In conclusion, In2O3 has emerged as a highly efficient
catalytic system for the hydrogenation of CO2 to methanol
featuring 100% selectivity and outstanding activity under
industrially relevant conditions. By confirming the creation of
oxygen vacancies by thermal desorption and their annihila-
tion as the key mechanism during the catalytic cycle, we
devised strategies at the process and catalyst level to boost its
performance. Accordingly, we were able to increase the
amount of active vacancies by adding CO to the gas feed or
using the electronically interacting ZrO2 as a carrier for In2O3.
Remarkably, the supported In2O3/ZrO2 catalyst was stable for
1000 h on stream and could be scaled up to a technical form
showing analogous catalytic properties. Hence, we have
gained fundamental and applied understanding that paves
the way for the exploitation of this novel catalytic system in
other processes beyond methanol synthesis.
Figure 3. a) Methanol STY for different supported catalysts after 4 h on
stream. b) Evolution of the methanol STY with time on stream (TOS)
over In2O3/ZrO2 and Cu-ZnO-Al2O3. c) HRTEM micrograph of the
In2O3/ZrO2 catalyst obtained after 4 h on stream. Reaction conditions:
T=573 K, P=5.0 MPa, H2/CO2 =4:1, and GHSV=16000 hÀ1
.
(Table S4), the catalytic properties of supported In2O3 seem
to be determined by electronic interactions with the carrier
rather than by geometric effects. This is substantiated by the
equally good performance of a material prepared with cubic
instead of monoclinic zirconia and by the formation of
inactive metallic In in fresh samples with other carriers
(Figure S7). The former finding also suggests that the inter-
face between In2O3 and the support might be irrelevant for
CO2 activation.
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activated in Ar at 573 K and 0.5 MPa for 1 h, whereas the Cu-ZnO-
Al2O3 catalyst was pretreated in 5 vol.% H2/Ar under the same
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Acknowledgements
We thank Total Research & Technology Feluy for sponsoring
this research and the Scientific Center for Optical and
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Keywords: CO2 hydrogenation · indium oxide ·
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Received: January 27, 2016
Revised: February 21, 2016
Published online: March 17, 2016
Angew. Chem. Int. Ed. 2016, 55, 6261 –6265
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6265