Remarkable Carbon Dioxide Hydrogenation to Ethanol on a Palladium/Iron Oxide Single-Atom Catalyst

Article abstract of DOI:10.1002/cctc.201800362

The hydrogenation of CO₂ into value-added chemicals is one of the most investigated methods to reduce CO₂ emissions in the atmosphere and thereby contributes to a sustainable chemical industry. Whereas the catalytic hydrogenation of CO₂ into methanol and synthetic hydrocarbons is well established, the effective and selective transformation of CO₂ into higher alcohols is still challenging. Here, we show that Pd single atoms anchored on the surface of Fe₃O₄ are very active for the hydrogenation of CO₂ to ethanol at 300 °C, even at atmospheric pressure. By comparing various Pd/MO_x catalysts, we conclude that the metal–oxide interface has a strong influence on catalytic behavior.

Full text of DOI:10.1002/cctc.201800362

Accepted Article
Title: Remarkable CO2 hydrogenation to ethanol on Pd/Fe3O4 single-
atom catalyst
Authors: Francisco J Caparros, Lluis Soler, Marta D Rossell,
Inmaculada Angurell, Laurent Piccolo, Oriol Rossell, and Jordi
Llorca
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201800362
Link to VoR: http://dx.doi.org/10.1002/cctc.201800362
A Journal of
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10.1002/cctc.201800362
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Remarkable CO2 hydrogenation to ethanol on Pd/Fe3O4 single-
atom catalyst
Francisco J. Caparrós,[a] Lluís Soler,[b] Marta D. Rossell,[c] Inmaculada Angurell,*[a] Laurent Piccolo,[d]
Oriol Rossell,[a] and Jordi Llorca*[b]
Abstract: CO2 hydrogenation into value-added chemicals is one of
interest in recent years and it is mainly performed on oxide-
supported Cu, Pd and Au catalysts, being Cu/ZnO/Al2O3 the
catalyst used in industry.[3] However, limited success has been
achieved in the selective CO2 hydrogenation to ethanol and
higher alcohols, which are preferable in many industrial
processes. Additionally, ethanol holds potential as a hydrogen
carrier in fuel cell technologies.[4] The direct synthesis of ethanol
and higher alcohols from CO2 and H2 in heterogeneous catalysis
is usually regarded as a combination of the RWGS reaction and
the subsequent reaction of syngas using a catalyst incorporating
C-C coupling and CO hydrogenation functionalities.[5] Therefore,
catalysts combining FT synthesis and methanol synthesis
capabilities have been investigated.[6] Alternatively, supported
catalysts based on Rh[7] or Pt,[8] as well as CoMoS,[9] have also
shown ability to convert syngas into ethanol. Recently, small Au
nanoparticles supported on TiO2 capable of activating
simultaneously H2 and CO2,[10] as well as bimetallic Pd-Cu
nanoparticles supported on TiO2 active for C-C coupling,[11] have
shown high selectivity toward ethanol in the hydrogenation of
CO2 in batch operation mode. Motivated by recent advances in
single-atom catalysis (SAC),[12] here we report on a Pd/Fe3O4
catalyst containing Pd single atoms which exhibits outstanding
selectivity and yield toward ethanol in CO2 hydrogenation under
mild conditions.
the most investigated methods to reduce CO2 emissions into the
atmosphere and thereby contribute to
a sustainable chemical
industry. While the catalytic hydrogenation of CO2 into methanol and
synthetic hydrocarbons is well established, the effective and
selective transformation of CO2 into higher alcohols is still
challenging. Here we show that Pd single atoms anchored on the
surface of Fe3O4 are very active for the hydrogenation of CO2 into
ethanol at 300 °C, even at atmospheric pressure. By comparing
various Pd/MOx catalysts we conclude that the metal/oxide interface
has a strong influence on the catalytic behavior.
The conversion of carbon dioxide by hydrogen not only mitigates
its emission into the Earth’s atmosphere but also produces
commodity chemicals that can be used either as fuels or as
precursors in many industrial chemical processes.[1] The
catalytic transformation of CO2 is often carried out at elevated
pressure and moderate temperature in the range of 200−400 °C.
The CO2 hydrogenation to CO through the reverse water gas
shift (RWGS) reaction occurs on various precious metals (Pt, Pd,
Rh and Au) and non-precious metals (Cu, Fe and Ni) supported
on inorganic oxides.[2] In addition to the classical Fischer-
Tropsch (FT) process over Co- and Fe-based catalysts for the
synthesis of long-chain hydrocarbons, the CO2 hydrogenation to
CH4 (Sabatier reaction) is nowadays widely studied in the
framework of the power-to-gas technologies to produce
synthetic natural gas and it is usually performed on Ni, Ru and
Rh catalysts supported on various oxides.[2] Besides, the CO2
hydrogenation to CH3OH (methanol) has gained tremendous
[a]
F.J. Caparrós, Dr. I. Angurell, Prof. O. Rossell
Departament de Química Inorgànica i Orgànica, Secció de Química
Inorgànica
Figure 1. Representative aberration-corrected HAADF-STEM images of
0.1Pd/Fe3O4 showing Pd single atoms (a) and 0.4Pd/Fe3O4 showing Pd
clusters of 1.2±0.2 nm (b). Representative TEM image of 3Pd/Fe3O4
containing Pd nanoparticles of 4.3±0.4 nm (c). Pd nanoparticles and single
metal atoms are enclosed in circles and squares, respectively.
Universitat de Barcelona
Martí i Franquès, 1-11, 08028 Barcelona, Spain
E-mail: inmaculada.angurell@qi.ub.es
Dr. L. Soler, Prof. J. Llorca
[b]
Institute of Energy Technologies, Department of Chemical
Engineering and Barcelona Research Center in Multiscale Science
and Engineering.
Universitat Politècnica de Catalunya
EEBE, Eduard Maristany 16, 08019 Barcelona, Spain
E-mail: jordi.llorca@upc.edu
Three Pd/Fe3O4 catalysts were prepared containing 0.11,
0.38 and 3.0 wt% Pd as measured by inductively coupled
plasma (ICP) spectroscopy. They are referred to as 0.1Pd/Fe3O4,
0.4Pd/Fe3O4 and 3Pd/Fe3O4, respectively. The different Pd
loading values were selected according to ref.[13] to obtain
catalysts containing different Pd particle sizes. In particular,
0.1Pd/Fe3O4 contains exclusively Pd single atoms (Figure 1a
and S1), 0.4Pd/Fe3O4 contains small Pd clusters of 1.2±0.2 nm
(Figure 1b), and 3Pd/Fe3O4 contains Pd nanoparticles of 4.3±0.4
nm (Figure 1c).
[c]
[d]
Dr. M.D. Rossell
Electron Microscopy Center, EMPA
Swiss Federal Laboratories for Materials Science and Technology
Überlandstrasse 129, 8600 Dübendorf, Switzerland
Dr. L. Piccolo
Univ Lyon, Université Claude Bernard—Lyon 1, CNRS
IRCELYON—UMR 5256, Villeurbanne Cedex, France
Supporting information for this article is given via a link at the end of
the document.
The series of Pd/Fe3O4 samples was first investigated as
prepared for the hydrogenation of CO2 at 300-400 °C and
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10.1002/cctc.201800362
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COMMUNICATION
atmospheric pressure under dynamic conditions in a packed bed
reactor with a feed gas composition of 10% CO2 + 40% H2 +
50% N2 and a weight hourly space velocity (F/W) of 60 L gcat
catalyst for C-C coupling merits to be highlighted since no
methane or methanol were observed in any case.
−1
The selectivity values of the catalysts with higher Pd loading,
0.4Pd/Fe3O4 and 3Pd/Fe3O4, were similar to that of 0.1Pd/Fe3O4
at high temperatures (350 and 400 °C), except that CH4 was
also formed over 3Pd/Fe3O4 at 400 °C. Also, the CO2 conversion
was higher at increasing Pd loading (entries 5-6 and 8-9).
However, at 300 °C both samples exhibited a poor selectivity
toward ethanol, with activities of only 51 and 6 mmolEtOH gPd-1 h-1
(entries 4 and 7). The difference in ethanol yield is even more
pronounced on the basis of the calculated Pd surface area: 279,
h−1. The conversion of CO2, selectivity values of all the products
present, and activity values normalized on the basis of Pd
weight or on the basis of calculated Pd surface area are
compiled in Table 1, entries 1-9. Remarkably, Pd/Fe3O4
catalysts showed, in addition to CO, an outstanding selectivity
towards the formation of alcohols, particularly ethanol. For the
0.1Pd/Fe3O4 catalyst, the selectivity towards ethanol was 97.5%
-1
at 300 °C and the activity was 413 mmolEtOH gPd h-1 (entry 1).
This result is among the highest catalytic activities ever reported
for the synthesis of ethanol from CO2 hydrogenation.[10,11] At
increasing temperature the selectivity towards ethanol
decreased at the expense of CO, but propanol and traces of
butanol were produced, in particular at 400 °C (entries 2 and 3).
This was accompanied by an increase in CO2 conversion. The
high selectivity toward ethanol and the total absence of
hydrocarbon production on 0.1Pd/Fe3O4 is remarkable. Taking
into account that a blank run over the bare Fe3O4 support
yielded only CO at high temperatures (entries 10-12), it is likely
that the first step in the reaction is the transformation of CO2 into
CO over the Fe3O4 support through the RWGS reaction, and
then Pd assists in the C-C coupling. This scheme is reinforced
by the fact that higher partial pressures of CO at high
temperature favour the formation of propanol, possibly through a
CO insertion-type mechanism.[7c] This reaction mechanism was
supported by studying the CO hydrogenation over 0.1Pd/Fe3O4
under the same reaction conditions (Table 2). The catalyst was
100 % selective to ethanol at 300 °C (entry 1) and at higher
temperatures the yield toward propanol (accompanied by traces
of butanol) increased progressively (entries 2-3). Moreover, at
high temperature ethane was also formed. The capacity of the
-2
17 and 2 mmolEtOH mPd h-1 for 0.1Pd/Fe3O4, 0.4Pd/Fe3O4 and
3Pd/Fe3O4, respectively. There is a clear relationship between
ethanol yield and Pd particle size, which is directly correlated
with the metal exposed; the smaller the Pd particle size the
higher the ethanol yield, being the ethanol selectivity boosted by
the presence of Pd single atoms.
Figure 2a shows the yield of products other than CO of
0.1Pd/Fe3O4 over time on stream at different temperatures. It is
seen that the catalyst suffers deactivation, being the decay
particularly severe at 400 °C. At this temperature, the drop in
ethanol and propanol yields occurs simultaneously to the
appearance of methane and ethane. To gain insight into the
processes associated to the deactivation phenomenon,
additional experiments were carried out and quenched at
different conditions, as indicated in Figure 2a. The
corresponding aberration-corrected HAADF-STEM images are
reported in Figure 2b-d. The sample operating at 300 °C (Figure
2b) shows exclusively Pd single atoms, which reinforces the
observation that these species are responsible for the selective
formation of
Table 1. Initial catalytic performances of Pd/Fe3O4 catalysts at 1 bar, CO2:H2=1:4 (molar) and F/W=60 L gcat−1 h−1
.
Selectivity (%)
#
catalyst
T (ºC)
300
350
400
300
350
400
300
350
400
300
350
400

CO2 (%)
CO
-
CH4
MeOH
EtOH
97.5
16.0
17.8
13.9
14.6
1.3
PrOH
-
mmolEtOH·gPd-1 h-1
mmolEtOH·mPd-2 h-1
1
0.1Pd/Fe3O4
0.3
2.4
3.9
1.0
2.8
5.6
1.2
3.6
8.8
-
-
2.5
413
459
846
51
144
26
6
279
310
571
17
47
9
2
83.3
77.5
84.9
81.1
97.4
85.1
80.1
94.6
-
-
-
0.7
4.7
1.1
4.2
1.3
3.1
4.7
1.2
-
3
-
-
4
0.4Pd/Fe3O4
3Pd/Fe3O4
Fe3O4
-
-
5
-
-
6
-
-
7
-
-
11.8
15.1
1.3
2
8
-
0.1
24
5
8
9
2.8
-
-
-
-
2
10
11
12
-
-
-
-
-
-
0.4
1.1
100
100
-
-
-
-
-
-
-
-
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10.1002/cctc.201800362
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COMMUNICATION
reaction conditions for CO2 hydrogenation. The sample was
inactive at 300 °C and at higher temperature the main product
was CO, with only minor amounts of ethanol (selectivity of only
2.4-5.1% at 350-400 °C).
Table 2. Initial catalytic performance of 0.1Pd/Fe3O4 catalyst at 1 bar,
CO:H2=1:4 (molar) and F/W=60 L gcat−1 h−1
.
Selectivity (%)
#
T
(ºC)
CO2
(%)
C2
MeOH
EtOH
PrOH
mmolEtOH
·gPd-1 h-1
1
2
3
300
350
400
0.2
0.4
1.5
-
-
-
-
100
-
244
12.6
9.6
79.3
71.5
8.1
18.9
379
1317
Since the selective hydrogenation of CO2 into ethanol is
favoured at low temperature over 0.1Pd/Fe3O4 and pressure has
a positive effect on CO2 hydrogenation reactions, this catalyst
was tested at 250-300 °C and 3-30 bar total pressure with a
stoichiometric CO2:H2 ratio for alcohol formation of 1:3 (Table 3).
An increase of pressure resulted in higher CO2 conversion rates
and the maximum selectivity toward ethanol was obtained at
Figure 2. (a) Yield of products other than CO obtained on 0.1Pd/Fe3O4 over
−1
time on stream at 1 bar, CO2:H2=1:4 (molar) and F/W=60 L gcat h−1. (b-d)
Representative aberration-corrected HAADF-STEM images recorded at
different stages, as indicated in (a). Pd nanoparticles and single metal atoms
are enclosed in circles and squares, respectively.
-1
250 °C, being the highest ethanol activity 440 mmolEtOH gPd h-1
at 250 °C and 30 bar (entry 5). It is remarkable that under these
conditions the only products of the reaction are ethanol (98.0 %
selectivity) and methanol (2.0 % selectivity). The increase of
temperature from 250 to 300 °C had a positive effect on CO2
conversion and propanol selectivity at 3 and 30 bar, but the
selectivity toward ethanol decreased considerably at the
expense of CO (entries 4 and 6). A blank run was conducted
over the bare Fe3O4 support at 3 bar and 250-300 °C, and no
CO2 transformation was recorded.
ethanol. The sample just reaching 400 °C (Figure 2c and S2)
contains both Pd single atoms and Pd nanoparticles of about
1.1±0.3 nm. These Pd nanoparticles are likely produced at 300-
350 °C and are responsible for the partial deactivation of the
catalyst. In contrast, the sample deactivated strongly at 400 °C
(Figure 2d and S3) only contains Pd nanoparticles of 2.2±1.2
nm; they are no longer selective to the production of alcohols
and methanation is the main reaction occurring over this sample.
These observations are also consistent with the fact that the
ethanol selectivity recorded over 0.4Pd/Fe3O4 and 3Pd/Fe3O4 at
300 °C are much lower than that obtained over 0.1Pd/Fe3O4
(13.9 and 11.8 vs. 97.5 %, respectively; entries 4, 8 and 1 in
Table 1), but similar at 350-400 °C (16.0 and 17.8 %, entries 2-3
Table 3. Initial catalytic performance of 0.1Pd/Fe3O4 catalyst at 250-300 °C
and 1-30 bar under CO2:H2=1:3 (molar) and F/W=60 L gcat−1 h−1
.
Selectivity (%)
#
P
(bar)
T
(ºC)
CO2
(%)
CO
MeOH
EtOH
PrOH
mmolEtOH
·gPd-1 h-1
in Table 1). Hence, the 0.1Pd/Fe3O4 catalyst suffers
a
1
2
3
4
5
6
1
250
300
250
300
250
300
0.1
0.3
0.3
0.8
1.4
2.9
-
17.4
2.6
82.6
97.4
79.6
25.5
98.0
46.1
-
101
413
291
235
440
429
progressive loss of ethanol selectivity due to agglomeration of
Pd single atoms into small Pd nanoparticles at increasing
reaction temperature, whereas 0.4Pd/Fe3O4 and 3Pd/Fe3O4
initially contain Pd nanoparticles and are less selective to
ethanol at low temperature. Recently, the sintering of Pd atoms
supported on a model Fe3O4 (001) surface under a CO
atmosphere has been observed by STM.[14] Taking into account
that CO is an intermediate in the CO2 hydrogenation on
Pd/Fe3O4, the rapid deactivation observed could be related to
the mobility of Pd atoms under a CO-rich atmosphere. As an
additional proof of the role of Pd single atoms in the formation of
ethanol, in a separate experiment the 0.1Pd/Fe3O4 catalyst was
heated under N2 at 400 °C to transform the Pd single atoms into
nanoparticles (Figure S4) and then tested under the same
-
-
3
-
20.4
2.1
-
71.0
-
1.4
-
30[a]
2.0
46.8
5.0
2.1
[a] F/W=8 L gcat−1 h−1
.
The results reported so far demonstrate the potential of Pd
single atoms supported on Fe3O4 for the selective hydrogenation
of CO2 to ethanol. But are Pd single atoms self-sufficient for the
production of ethanol? We prepared a 0.1Pd/Al2O3 catalyst
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10.1002/cctc.201800362
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containing Pd single atoms following the same procedure as for
the preparation of 0.1Pd/Fe3O4 and tested it under the same
reaction conditions. The results are reported in Table 4 and it is
clearly seen that the sample is quite active in the transformation
of CO2 to CO but poorly selective to the formation of ethanol.
Since bare Fe3O4 is active in the RWGS reaction, we can
conclude that a particular interaction is established between Pd
single atoms and Fe3O4 to construct specific active sites for C-C
coupling, similarly to the specific architectures discussed in
Fe3O4-based multifunctional catalysts for FT synthesis.[15] This
could be related to the abundant vacancies and interstitial sites
observed in Fe3O4 in response to reducing environments.[16] In a
similar way, we also prepared 0.1Pd/ZrO2 and 0.1Pd/CeO2,
which appeared inactive in the hydrogenation of CO2, even at 30
bar.
Acknowledgements
This work has been funded by projects MINECO/FEDER
ENE2015-63969-R and GC 2017 SGR 128. JL is a Serra Húnter
Fellow and is grateful to ICREA Academia program. LP thanks R.
Checa and E. Leclerc for their technical help with the high-
pressure catalytic bench. Access to the TEM facilities at IBM
Research-Zꢀrich (Switzerland) under the IBM/Empa Master
Joint Development Agreement is gratefully acknowledged.
Keywords: carbon dioxide • heterogeneous catalysis • ethanol •
palladium • single-atom catalysis
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#
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(ºC)
CO2
(%)
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CH4
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·gPd-1 h-1
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Experimental Section
Three Pd/Fe3O4 catalysts were prepared following the preparation
method described in detail in the Supplementary Information. Briefly,
magnetite nanoparticles were first functionalized with dopPPh2 (N-(3,4-
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dihydroxyphenethyl)-4-(diphenylphosphino)benzamide)
linker
by
sonication in methanol for 2 h. Then Pd was loaded from K2[PdCl4] to
Fe3O4dopPPh2 in water, followed by reduction with NaBH4. The resulting
nanoparticles were extracted with a magnet, washed thoroughly with
water and acetone and dried under reduced pressure. The samples were
characterized by ICP-OES, BET and Aberration-corrected HAADF-STEM.
Catalytic tests at 1-30 bar total pressure were carried out in fixed bed
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reactors placed inside tube furnaces.
A full description of the
methodology is given in the Supporting Information.
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346, 1215–1218.
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Pd single atoms anchored on the
surface of Fe3O4 are very active for
the hydrogenation of CO2 into ethanol
Francisco J. Caparrós, Lluís Soler,
Marta D. Rossell, Inmaculada Angurell,*
Laurent Piccolo, Oriol Rossell, Jordi
Llorca*
Page No. – Page No.
Remarkable CO2 hydrogenation to
ethanol on Pd/Fe3O4 single-atom
catalyst
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