Catalytic Conversion of CO2 to Value-Added Products under Mild Conditions

Article abstract of DOI:10.1002/cctc.201801346

The catalytic synthesis of multi-carbon alcohols (MCA, C_nH_2n+1OH, n≥3) and higher hydrocarbons from CO₂ and H₂ under low or even ambient temperature are realized for the first time over a prepared bimetallic catalyst composed of nanoparticles of Pt and Ru supported on Fe₃O₄ (Ru?Pt/Fe₃O₄). At 40 °C, the selectivity for alcohols, MCA, and higher hydrocarbons reached 77.1 %, 4.5 %, and 19.5 %, respectively, while that for methane was only 3.4 % (carbon based). As revealed by isotope tracer experiments using O¹⁸ labeled water, in the hydrogenation of CO₂ over Ru?Pt/Fe₃O₄, MCA could form by catalytic hydrolysis of alkyl, a novel reaction pathway enabling the formation of MCA at low temperature, which is different from the previously reported one based on CO insertion at high temperature. It was discovered that in Ru?Pt/Fe₃O₄, both Ru and Pt nanoparticles played catalytic roles in the reduction of CO₂ to CH_x species and the carbon-carbon coupling reaction to form alkyl, while the catalytic hydrolysis of formed long-chain alkyl occurred on Pt nanoparticles.

Full text of DOI:10.1002/cctc.201801346

Accepted Article
Title: Catalytic conversion of CO2 to value-added products under mild
conditions
Authors: Yuan Wang, Yulv Yu, and Jin Huang
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10.1002/cctc.201801346
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Catalytic conversion of CO2 to value-added products under mild
conditions
Yulv Yu, Jin Huang, Yuan Wang*
Abstract: The catalytic synthesis of multi-carbon alcohols (MCA,
with H2 to produce gasoline-range hydrocarbons with
selectivity of 78.6% at 340 °C [19]. Chen and co-workers
reported composite catalyst CuZnFeK, over which
a
CnH2n+1OH, n≥3) and higher hydrocarbons from CO2 and H2 under
low or even ambient temperature are realized for the first time over a
prepred bimetallic catalyst composed of nanoparticles of Pt and Ru
supported on Fe3O4 (Ru-Pt/Fe3O4). At 40 °C, the selectivity for
alcohols, MCA, and higher hydrocarbons reached 77.1%, 4.5%, and
19.5%, respectively, while that for methane was only 3.4% (carbon
based). As revealed by isotope tracer experiments using O18 labeled
water, in the hydrogenation of CO2 over Ru-Pt/Fe3O4, MCA could
form by catalytic hydrolysis of alkyl, a novel reaction pathway
enabling the formation of MCA at low temperature, which is different
from the previously reported one based on CO insertion at high
temperature. It was discovered that in Ru-Pt/Fe3O4, both Ru and Pt
nanoparticles played catalytic roles in the reduction of CO2 to CHx
species and the carbon-carbon coupling reaction to form alkyl, while
the catalytic hydrolysis of formed long-chain alkyl occurred on Pt
nanoparticles.
a
hydrogenation of CO2 under 6 MPa at 300 °C could produce
alcohols and hydrocarbons with mass selectivities of 36.6 % and
+
56.4%, respectively, and the mass ratio of C2 alcohols to C1
alcohol was 6.76 [20]. We also noticed that Han and coworkers
reported an interesting PtRu/Fe2O3 catalyst, prepared by
reactions of a mixture of H2PtCl6, RuCl3, and Fe(NO3)3 with
NaCO3, followed by reducing the obtained precipitate in
hydrogen at 400 °C, which could catalyze hydrogenation of CO2
+
at 200°C in DMI (1,3-dimethyl-2-imidazolidinone) to form C2
alcohols with a selectivity of 36.0% (carbon based) [21]. Han et
al. further found that at 200 °C, over a Pt/Co3O4 catalyst
dispersed in a mixture of water and DMI, C1-C4 alcohols could
be synthesized by hydrogenation of CO2 with a selectivity of
33.1% (carbon based) [22]. Xiao et al. reported that over a
cobalt catalyst (CoAlOx) at 140 °C, ethanol could be produced
with a selectivity as high as 92%, and a small amount of
methanol, propanol, and butanol were also detected in the
products [23]. However, to the best of our knowledge, catalysts
which can realize the conversion of CO2 and H2 to multi-carbon
products at ambient temperature have so far been scarce.
CO2 is a cheap, nontoxic, and abundant carbon source. The
rising concentration of CO2 in the atmosphere has been causing
serious problems such as the greenhouse effect and ocean
acidification. Therefore, the conversion of surplus CO2 to
hydrocarbon or oxygenated hydrocarbon is a significant subject
[1-13]. The development of new catalytic systems capable of
realizing the conversion of CO2 to multi-carbon products
(hydrocarbons or alcohols) at low or even ambient temperature
is an attractive subject because such systems will not only
provide high value products by consuming CO2, but also avoid
over-emission of CO2 in the conversion process. MCA with 3-8
carbon atoms are not only liquid fuels but also widely used as
fine chemicals or solvents in producing pharmaceuticals,
polymers, surfactants, detergents, paints, and printing inks [14-
16]. The conversion of CO2 and H2 to multi-carbon compounds
(CCMC) should be an ideal CO2 conversion route, and H2 can
be manufactured in a large scale from renewable energy
sources, including solar energy, hydropower and biomass.
We have been engaging in the development of iron oxide or
tin oxide supported metal nanocluster catalysts with metal
nanoclusters stabilized with ethylene glycol and simple ions as
building blocks for a long time [24-27]. In this paper, we report a
novel heterogeneous catalyst composed of nanoparticles of Ru
and Pt supported on Fe3O4 (Ru-Pt/Fe3O4), which could catalyze
the conversion of CO2 and H2 to C1-C6 linear alcohols and C1-
C6 light hydrocarbons at 40-60 °C, and C1-C8 alcohols and C1-
C8 hydrocarbons at 80 °C.
The Ru-Pt/Fe3O4 catalyst was prepared by depositing Pt
nanoclusters stabilized with ethylene glycol and simple ions [28]
and Ru oxide nanoparticles [29] on
a Fe3O4 support,
consecutively, followed by reducing the obtained solid
intermediate with 2.8 MPa of hydrogen in water at 150 °C (see
the Experimental Section for details). The Pt and Ru contents of
the prepared catalyst were measured by inductively coupled
plasma atomic emission spectrometer (ICP-AES) to be 1.8 %
and 3.6 %, respectively.
Pioneering efforts have been made to create catalytic
systems for the conversion of CO2 to multi-carbon compounds.
It was reported that higher hydrocarbons could be synthesized
from CO2 and H2 at high temperature over some heterogeneous
catalysts. Song reported a Fe-Co bimetallic catalyst which could
catalyze the reaction of CO2 with H2 to produce higher
hydrocarbons with a selectivity of 69% and a small amount of
MCA at 300 °C [17]. Recently, Ge and Sun prepared an efficient
catalyst Na-Fe3O4/HZSM-5 which can catalyze hydrogenation of
CO2 to produce hydrocarbons containing 78% of gasoline-range
(C5-C11) ones at a CO2 conversion of 22% at 320 °C [18]. Sun
et al. reported that a bifunctional catalyst composed of indium
oxide (In2O3) and zeolites could catalyze the reaction of CO2
Figure 1. TEM image of Ru-Pt/Fe3O4 (a) and metal particle size
distribution histogram (b).
Dr. Y. L. Yu, J. Huang, Prof. Y. Wang
Beijing National Laboratory for Molecular Sciences, State Key Lab
for Structural Chemistry of Unstable and Stable Specie
College of Chemistry and Molecular Engineering, Peking University
Beijing 100871 (China)
E-mail: wangy@pku.edu.cn
Supporting information for this article is given via a link at the end of the document.
As shown in the transmission electron microscope (TEM)
image of Ru-Pt/Fe3O4 (Figure 1), metal nanoparticles dispersed
on Fe3O4 had an average diameter of 3.7 nm, with a standard
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10.1002/cctc.201801346
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deviation of 0.9 nm, and aggregates of metal nanoparticles with
sizes of ∼10 nm also existed in the catalyst.
The catalytic properties for CCMC over Ru-Pt/Fe3O4 under
different temperatures are listed in Table 1. It was surprise to us
that at 40 °C, Ru-Pt/Fe3O4 could catalyze CCMC to produce
alcohols (C1-C6) and higher hydrocarbons (C2-C6). The
selectivity for alcohols reached as high as 77.1%, while that for
methane was only 3.4% (carbon based). In the products of the
reaction at 60 °C, the selectivity for alcohols was 41.2%, and
that for MCA was 9.1%, respectively. Moreover, the selectivity
for higher hydrocarbons was 44.2%, and that for C2+ compounds
reached 61.9%. When the reaction was performed at 80 °C, C1-
C8 alcohols and C1-C8 hydrocarbons were produced at a CO2
conversion of 11.4%, and selectivity for MCA and higher
hydrocarbons were 6.2% and 55.5%, respectively. At 130 °C,
Ru-Pt/Fe3O4 could catalyze CCMC to produce alcohols (C1-C8)
and higher hydrocarbons (C2-C8), but the selectivity for methane
reached 60.1%, which could be ascribed to the relative high H
termination reaction rate at high temperature. CO was not
detected in the gaseous mixture recovered from the reaction
system, suggesting that the concentration of CO in the mixture
was less than the detection limit of the gas chromatography (∼60
ppm). The distributions of produced alcohols under different
temperatures are shown in Figure 5. It should be mentioned that
over a PtRu/Fe2O3 catalyst prepared according to the previously
reported method [21], only methanol and ethanol were detected
in the hydrogenation of CO2 at 130 °C (Supporting Information
Table S1).
X-ray photoelectron spectroscopy (XPS) measurement on
Ru-Pt/Fe3O4 (Figure 2) revealed that the binding energies of Ru
3p3/2 and Pt 4f7/2 levels were 462.6 eV and 71.2 eV, respectively,
indicating that Ru and Pt were in metallic states.
The X-ray diffraction (XRD) pattern of Ru-Pt/Fe3O4
(Supporting Information Fig S1), exhibited diffraction peaks at
18.3, 30.1, 35.4, 37.1, 43.1, 53.4, 57.0, and 62.5° corresponding
to the diffraction signals of (111), (220), (311), (222), (400),
(422), (511), and (440) crystal planes of the Fe3O4 support,
respectively. The diffraction peaks for the supported metal
particles were too weak to analyze due to the strong diffraction
signals of the support.
Figure 2. XPS of platinum (a) and ruthenium (b) in Ru-Pt/Fe3O4.
Pt and Ru nanoparticles with the inter-fringe distances of
0.227 and 0.206 nm, respectively, were observed in the HRTEM
images of as-prepared Ru-Pt/Fe3O4 (Figure 3). EDX mapping
measurements on Ru-Pt/Fe3O4 revealed that some Pt
nanoparticles contacted Ru nanoparticles in this catalyst (Figure
4b-d). Based on the aforementioned characterization results, we
can conclude that the prepared bi-metallic catalyst Ru-Pt/Fe3O4
is composed of nanoparticles of Pt and Ru deposited on Fe3O4.
Figure 4. HAADF-STEM image of Ru-Pt/Fe3O4 (a), EDX elemental
mapping of platinum (b) and ruthenium (c) in the same area, and
merged image of b and c (d).
The stability of Ru-Pt/Fe3O4 in the hydrogenation of CO2
was tested at 80 ºC by catalyst recycling test. Over the fresh
catalyst, the conversion of CO2 in 6 days was 11.5%, and the
selectivity for higher hydrocarbons and alcohols were 55.5% and
14.9%, respectively. Over the recovered catalysts, the
conversion of CO2 in 6 days (10.3%) decreased by 10%, and the
selectivity for higher hydrocarbons and alcohols (53.5 % and
12.5%, respectively ) declined slightly, implying a good stability
of the catalyst under these conditions.
Figure 3. HRTEM image of Ru-Pt/Fe3O4.
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Table 1. Catalytic performance over Ru-Pt/Fe3O4 in CCMC under
different temperatures.
1.6, respectively, while those in propanol and butanol produced
in non-labeled water were less than 0.1 (Supporting Information
Figure S2). In the mass spectrum of carbon dioxide collected
from the isotope tracer experiment reaction system, the ion
peaks with m/z values of 44, 46, and 48, corresponding to
[16O=C=O16]·+, [18O=C=O16]·+, and [18O=C=O18]·+, respectively,
were clearly observed (Supporting Information Figure S3). The
relative intensities of the ion peaks for [16O=C=O16]·+,
[18O=C=O16]·+, and [18O=C=O18]·+ were 65, 100, and 54,
respectively. The transfer of O18 from H2O18 to CO2 could be
attributed to the water gas shift reaction (CO + H2O = CO2 + H2).
If MCA in this experiment formed via the CO insertion pathway
exclusively, the ratio of O18 to O16 in MCA would be the same as
that of CO18 to CO16 derived from the hydrogenation of carbon
dioxide in the reaction system, and the isotope ratio of O18 to O16
in the produced MCA would be no more than 0.9 as calculated
from the aforementioned relative peak intensities [(54+100×0.5)/
(65+100×0.5)]. However, the isotope ratio of O18 to O16 in
propanol (1.4) and butanol (1.6) formed in O18 labeled water was
much higher than 0.9, indicating that MCA could form by the
hydrolysis of alkyl over Ru-Pt/Fe3O4. The new reaction pathway
for the formation of MCA over Ru-Pt/Fe3O4 is shown in Figure 6.
T (ºC)
Sel (%)a
Conv (%)a
+
CH4
3.4
RHb
19.5
44.2
55.5
36.3
Alcohol
77.1
41.2
14.9
3.6
Alcohol (C2
)
MCAc
4.5
40
60
24.8
17.7
9.5
2.0
2.1
14.6
29.6
60.1
9.1
80
6.2
11.5
15.6
130
2.2
0.9
Reaction conditions: water, 30 mL; initial pressure, 3.0 MPa (H2/CO2=3:1);
Mass of catalyst in reactions at 40, 60, 80 ºC was 0.50 g, and that in reaction
at 130 ºC was 0.15 g. Time for reaction at 40, 60, and 80 ºC was 6 days, and
that for reaction at 130 ºC was 0.33 days. [a] Selectivity (Sel) and conversion
(Conv) (%) calculations were based on the moles of carbon. [b] RH
represents higher hydrocarbons (C2+). [c] MCA produced at 40 and 60 ºC
were C3-C6 alcohols, while those produced at 80 and 130 ºC were C3-C8
alcohols.
Figure 6. Reaction pathway for MCA formation based on alkyl
hydrolysis over Ru-Pt/Fe3O4.
Hydrogenation of CO2 over Ru-Pt/Fe3O4 in O18 labeled
water was also conducted at 130 ºC that is close to the reaction
temperature (140 ºC) in previous reports [22]. After 8 hours of
reaction, the ratios of O18 to O16 in the produced butanol and
pentanol were 1.7 and 1.4, respectively, which were much larger
than that in CO2 (0.8) collected from the reaction system
(Supporting Information Figure S4, 5). This result indicated that
multi-carbon alcohols could also form by catalytic hydrolysis of
alkyl over Ru-Pt/Fe3O4 at 130 ºC.
In the mass spectrum of water collected from the isotope
tracer experiment reaction system, the ion peaks with m/z
values of 18 and 20 corresponded to [H2O16]·+ and [H2O18]·+,
respectively. The isotope ratio of O18 to O16 in water declined to
2.1 after reaction, which should be mainly derived from the
formation of H2O16 in the hydrogenation of CO2 (Supporting
Information Figure S6). The isotope ratio of O18 to O16 in
propanol (1.4) and butanol (1.6) formed in O18 labeled water was
lower than that in water after reaction (2.1), implying that a part
of O in MCA came from CO2 or Fe3O4.
Figure 5. Distribution of the produced alcohols over Ru-Pt/Fe3O4 by
CCMC at 40 °C (a), 60 °C (b), 80 °C (c) and 130 °C (d).
It was reported that CO insertion into a metal-alkyl bond,
followed by reduction of the formed intermediates with H2 over
some catalysts at high temperatures, could lead to the formation
+
In order to further understand the formation mechanism of
MCA through the reactions of CO2, H2, and H2O over Ru-
Pt/Fe3O4, control experiments were carried out. Ru-Pt/Fe3O4,
propene, CO2, and water were charged in an autoclave which
was kept at 130 °C for 8 h to verify if propanol could form by
hydrolysis of propene over the Ru-Pt/Fe3O4 catalyst (see the
Experimental Section for details). Propanol was not detected in
the reaction mixture after the treatment, implying that over the
present catalyst MCA formed by the hydrolysis of alkyl, rather
than alkene.
of C2 alcohols in the conversion of CO2 [20, 22, 30], and the
alkyl could form through the hydrogenation of CO followed by
carbon-carbon coupling [20]. However, it was found that MCA
could form by hydrolysis of alkyl over the present catalyst in our
isotope tracer experiments, which was different from the
previously reported CO insertion pathway. When CCMC over
Ru-Pt/Fe3O4 was conducted in O18 labeled water (5 mL, purity:
97%, isotope tracer experiment) at 80 °C (See the experimental
section), the intensity of ion peak corresponding to [CH2O18H]+
with a m/z value of 33 in the mass spectrum of formed MCA
dramatically enhanced. The isotope ratio of O18 to O16 in the
propanol and butanol produced in O18 labeled water was 1.4 and
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Moreover, a Pt//Fe3O4 catalyst containing 1.8% of Pt and a
Ru/Fe3O4 catalyst containing 3.6 % of Ru were prepared by the
methods reported in this work in the absence of Ru and Pt,
respectively. Hydrogenation of CO2 was performed over
Pt/Fe3O4 and Ru/Fe3O4 in 3 mL of O18 labeled water at 80 °C
for 6 days, respectively. After the experiments, the ratios of O18
to O16 in carbon dioxide collected from the reaction systems of
Pt/Fe3O4 and Ru/Fe3O4 were measured to be 1.2 and 1.1,
respectively (Supporting Information, Figure S7, 8). The transfer
of O18 from H2O18 to CO2 in H2 over Pt/Fe3O4 or Ru/Fe3O4
indicated that either Pt or Ru nanoparticles supported on Fe3O4
could catalyze the dissociation of water to form active hydroxyl
[31].
conditions over a prepared bimetallic catalyst Ru-Pt/Fe3O4
composed of nanoparticles of Pt and Ru on a Fe3O4 support.
MCA (C3-C6) and higher hydrocarbons (C2-C6) were produced
through the reactions over the catalyst even at 40 °C. A new
mechanism of the catalytic conversion of CO2 to MCA, i.e. the
hydrolysis of formed alkyl, was revealed. It was discovered that
Ru nanoparticles played an important role in the catalytic
hydrogenation of CO2 to form CHX species, while Fe3O4
supported Pt nanoparticles could catalyze the formation and
hydrolysis of long-chain alkyl. Insight into the formation
mechanism of MCA over Ru-Pt/Fe3O4 provided valuable
information for further development of efficient catalysts for the
conversion of CO2 and H2 to MCA and higher hydrocarbons at
mild conditions.
Higher hydrocarbons (C2-C8) were detected in the products
of hydrogenation of CO2 over Ru/Fe3O4 in 30 mL of water at 80
°C (Supporting Information, Table S1), while MCA was not
detected. This result revealed that Fe3O4 supported Ru
nanoparticles could catalyze carbon-carbon coupling reaction to
form long-chain alkyl, but could not catalyze the coupling of
hydroxyl with long-chain alkyl to form MCA. On the other hand,
MCA (C3-C4) and higher hydrocarbons (C2-C5) were detected in
the products of hydrogenation of CO2 over Pt/Fe3O4
(Supporting Information, Table S1), implying that Fe3O4
supported Pt nanoparticles could catalyze not only the carbon-
carbon coupling reaction to form long-chain alkyl, but also the
coupling of hydroxyl with long-chain alkyl to form MCA.
Acknowledgements
The authors express their thanks for the support from the NSFC
(grants 21573010 and 21821004), and the Chinese Ministry of
Science and Technology (2016YFE0118700).
In the hydrogenation of CO2 over Ru/Fe3O4 at 80 °C, the
average conversion rate of CO2 was 50 mmolCO2/molRu/h, and
the selectivity for methane was as high as 89.2%. This result
revealed that Fe3O4 supported Ru nanoparticles could catalyze
the hydrogenation of CO2 to form CHX* (* represents specie
adsorbed on catalysts), and the formed CHX* tended to react
with H* to form methane over the catalyst. However, in the
hydrogenation of CO2 over Pt/Fe3O4 under the same reaction
conditions, the average conversion rate of CO2 was only 6.3
mmolCO2/molPt/h, and the selectivity for methane was 10.6%
(Supporting Information, Table S1), implying that Fe3O4
supported Pt nanoparticles were not as efficient as Ru/Fe3O4 in
catalyzing the reduction of CO2 with H2 to form CHX*, but the
formed CH* over the catalyst tended to couple with each other to
generate long-chain alkyl. The formed long-chain alkyl could
terminate with H* or OH* over Fe3O4 supported Pt nanoparticles
to form higher hydrocarbons or MCA.
Keywords: carbon dioxide
• multi-carbon compounds •
hydrolysis • mild conditions • heterogeneous catalysis
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