Stabilizing Co2C with H2O and K promoter for CO2 hydrogenation to C2+ hydrocarbons

Abstract

The decomposition of cobalt carbide (Co2C) to metallic cobalt in CO2 hydrogenation results in a notable drop in the selectivity of valued C2+ products, and the stabilization of Co2C remains a grand challenge. Here, we report an in situ synthesized K-Co2C catalyst, and the selectivity of C2+ hydrocarbons in CO2 hydrogenation achieves 67.3% at 300°C, 3.0 MPa. Experimental and theoretical results elucidate that CoO transforms to Co2C in the reaction, while the stabilization of Co2C is dependent on the reaction atmosphere and the K promoter. During the carburization, the K promoter and H2O jointly assist in the formation of surface C* species via the carboxylate intermediate, while the adsorption of C* on CoO is enhanced by the K promoter. The lifetime of the K-Co2C is further prolonged from 35 hours to over 200 hours by co-feeding H2O. This work provides a fundamental understanding toward the role of H2O in Co2C chemistry, as well as the potential of extending its application in other reactions.

Full text

CHEMISTRY
Stabilizing Co
2
C with H
2
O and K promoter for CO
2
hydrogenation to C
2+
hydrocarbons
Mingrui Wang
1
, Peng Wang
2
, Guanghui Zhang
1
*, Zening Cheng
3
, Mengmeng Zhang
1
,
Yulong Liu
1
, Rongtan Li
4
, Jie Zhu
1
, Jianyang Wang
4
, Kai Bian
1
, Yi Liu
1
, Fanshu Ding
1
,
Thomas P. Senftle
2
*, Xiaowa Nie
1
, Qiang Fu
4
, Chunshan Song
5
*, Xinwen Guo
1
*
The decomposition of cobalt carbide (Co
2
C) to metallic cobalt in CO
2
hydrogenation results in a notable drop in
the selectivity of valued C
2+
products, and the stabilization of Co
2
C remains a grand challenge. Here, we report
an in situ synthesized K-Co
2
C catalyst, and the selectivity of C
2+
hydrocarbons in CO
2
hydrogenation achieves
67.3% at 300°C, 3.0 MPa. Experimental and theoretical results elucidate that CoO transforms to Co
2
C in the re-
action, while the stabilization of Co
2
C is dependent on the reaction atmosphere and the K promoter. During the
carburization, the K promoter and H
2
O jointly assist in the formation of surface C
*
species via the carboxylate
intermediate, while the adsorption of C
*
on CoO is enhanced by the K promoter. The lifetime of the K-Co
2
C is
further prolonged from 35 hours to over 200 hours by co-feeding H
2
O. This work provides a fundamental un-
derstanding toward the role of H
2
O in Co
2
C chemistry, as well as the potential of extending its application in
other reactions.
Copyright © 2023 The
Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
NonCommercial
License 4.0 (CC BY-NC).
INTRODUCTION
The continuously increasing greenhouse gas CO
2
levels originated
from human activities have caused a series of environmental issues,
including sea level rises, heat waves, and ocean acidification (1).
Among the strategies considered, catalytic CO
2
reduction with
green H
2
, which is generated from water splitting by renewable
energy, to valuable C
2+
hydrocarbons (HCs) provides a potential
technology to reduce CO
2
concentration and an alternative nonpe-
troleum-based production route toward light olefins (C
2
~ C
4=
) and
liquid fuels (C
5+
) (2,3). However, the chemical inertness of CO
2
molecules (C═O bond of 750 kJ mol
−1
) and the high energy
barrier of C─C coupling are detrimental to the formation of C
2+
products (4,5). Accordingly, it is highly attractive and challenging
to develop effective catalysts for selective CO
2
conversion to
C
2+
HCs.
Much attention has been paid to using CO Fischer-Tropsch syn-
thesis (CO-FTS) catalysts for CO
2
hydrogenation, and, in general,
CO
2
is initially converted to CO intermediate via a reverse water
gas shift (RWGS) reaction, followed by CO hydrogenation to C
2+
HCs (6). For instance, iron carbide (FeC
x
) exhibits high activity
for CO
x
hydrogenation to olefins in a high-temperature range
(320° to 350°C) (7–9). Cobalt carbide (Co
2
C) is also a promising
catalyst for CO
x
conversion owing to its adequate activation of
C═O bonds and promoting effect on C─C coupling (10).
Notably, it has distinct features in different reaction atmospheres.
As for CO-FTS, nanoprism Co
2
C with preferentially exposed
(101) and (020) facets has been proven to be responsible for the syn-
thesis of C
2
~ C
4=
, while electronic (alkali metal) and structural pro-
moters (Mn and Ce) are instrumental in its morphological control
(11–15). The synthesis of Co
2
C is commonly divided into two steps,
including an initial reduction and carburization in syngas (CO and
H
2
) or CO. Wavelet transform and linear combination fitting results
of in situ x-ray absorption spectroscopy (XAS) indicate that cobalt
oxide (CoO) is first reduced to metallic cobalt (Co
0
), and then car-
burized to Co
2
C (16). Moreover, Co
2
C is commonly used below
260°C and at near atmospheric pressure in CO-FTS to avoid its de-
composition to Co
0
and graphite (17). Despite numerous efforts,
Co
2
C is unstable and still underutilized for CO
2
hydrogenation
(18,19). Compared with CO-FTS, the conversion of equimolar
CO
2
consumes more H
2
, and the presence of excess H
2
and CO
2
accelerates the decomposition of presynthesized Co
2
C (18).
Besides, the activation of inert CO
2
often requires a high tempera-
ture, and it thus puts forward higher requirements for the thermal
stability of Co
2
C. As a result, the Co
2
C catalyst tends to partially de-
compose to Co
0
under CO
2
hydrogenation, which leads to a rapid
side reaction toward CH
4
(20). In recent studies, SiO
2
support in-
teracted with Co
2
C was used for improving the stability, but a recon-
struction to Co
0
occurred while C
1
products dominate under
reaction conditions (21). Co
2
C with different morphologies was
prepared from the ZIF-67 precursor by Zhang et al. (22) for
RWGS reaction, but the atmospheric pressure condition only
enables a reduction of CO
2
to CO and limits the selectivity to C
2+
HCs. The facile synthesis and stabilization of Co
2
C under a C-lean
and H-rich atmosphere at higher temperatures and pressure are
major bottleneck for its catalytic application in CO
2
hydrogenation
to C
2+
HCs.
H
2
O is a common impurity for CO
2
capture from flue gas (23),
and more H
2
O is generated in CO
2
-FTS compared with CO-FTS;
therefore, its impact on the catalyst structure and performance
needs to be pinpointed. As for Fe-based catalysts, H
2
O-induced ox-
idation causes the evolution of FeC
x
to iron oxides (FeO
x
) and a de-
crease of the activity in both CO- and CO
2
-FTS (8,24). H
2
O has also
1
State Key Laboratory of Fine Chemicals, Frontier Science Center for Smart Mate-
rials, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering,
Dalian University of Technology, Dalian 116024, China.
2
Department of Chemical
and Biomolecular Engineering, Rice University, Houston, TX 77005, USA.
3
Zhundong Energy Research Institute, Xinjiang Tianchi Energy Co., Ltd., Changji
831100, China.
4
State Key Laboratory of Catalysis, Dalian National Laboratory for
Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, China.
5
Department of Chemistry, Faculty of Science, The Chinese
University of Hong Kong, Shatin, NT, Hong Kong SAR, China.
*Corresponding author. Email: gzhang@dlut.edu.cn (G.Z.); tsenftle@rice.edu (T.P.S.);
chunshansong@cuhk.edu.hk (C.S.); guoxw@dlut.edu.cn (X.G.)
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 1 of 14

been proven as a poison owing to its competitive adsorption and
oxidation of Co
0
catalyst in CO-FTS (25–27). Thus, the rapid
removal of H
2
O boosts the syngas conversion to C
2+
HCs (28).
However, up to now, the effect of H
2
O on Co
2
C catalyst is still am-
biguous in CO
2
hydrogenation, clarification, and optimization
which is of importance to improving the practical application.
Here, we report the in situ synthesis and stabilization of Co
2
C
with H
2
O from a K-modified Co
3
O
4
precursor for CO
2
hydrogena-
tion. In situ x-ray diffraction (XRD) and simulated results of chem-
ical potential corroborate that the carburization route derived from
CoO dominates under CO
2
hydrogenation conditions, while the re-
action atmosphere in conjunction with the K promoter is important
for the stabilization of Co
2
C. Notably, co-feeding 2 volume % of
H
2
O accelerates the carburization by enhancing the formation of
surface carboxylate, which prefers to further split for following C
permeation. The K promoter also endows the adsorption of C
atoms on the CoO surface, favoring the subsequent carburization.
CO
2
converts to C
2+
HCs via CO
*
intermediate derived from car-
bonate splitting on K-Co
2
C catalyst, and a selectivity to C
2+
HCs
up to 67.3% is achieved at 300°C, 3.0 MPa. Furthermore, H
2
O
(0.5 volume %) is applied to inhibit the decomposition of surface
Co
2
C and markedly prolongs the catalyst lifetime from 35 to over
200 hours. This work reveals a unique promoting role of H
2
O in
Co
2
C-catalyzed CO
2
hydrogenation and provides a mechanistic un-
derstanding of the formation and evolution of Co
2
C. It is expected
to advance the application of Co
2
C in CO
2
hydrogenation, and po-
tentially other reactions.
RESULTS AND DISCUSSION
Synthesis, catalytic performance, and kinetics
As displayed in Fig. 1A, Co
3
O
4
and K-modified Co
3
O
4
precursors
were prepared via a citric acid–induced sol-gel method and subse-
quent incipient wetness impregnation (IWI) using the solution of
K
2
CO
3
(0.98 wt %; details in Materials and Methods). XRD patterns
and thermogravimetric analysis (TGA) results confirm that the
foamy gels were completely decomposed to Co
3
O
4
after the calcina-
tion at 450°C (fig. S1). The transmission electron microscopy
(TEM) and high-resolution TEM (HRTEM) images on Co
3
O
4
and K-Co
3
O
4
illustrate that there is no substantial change in the
average particle size (21 to 24 nm, beyond the sensitive size range
in CO
2
hydrogenation) and morphology of Co
3
O
4
with the K mod-
ification (fig. S2). The particle size of K-Co
3
O
4
exhibits a narrower
distribution owing to the secondary calcination. Energy-dispersive
spectroscopy (EDS) mappings confirm that the K promoter is uni-
formly dispersed across the Co
3
O
4
surface (fig. S3). These precur-
sors were used in CO
2
hydrogenation reaction at 300°C, 3.0 MPa
with a space velocity of 6000 ml g
−1
hour
−1
. We collected the
spent samples, which were tested for 3 hours of CO
2
hydrogenation,
after careful passivation. The characteristic XRD peaks for spent
samples derived from Co
3
O
4
and K-Co
3
O
4
are assigned to the
phases of metallic Co (Co
0
), which consists of face center
cubic–Co and hexagonal closest packed (hcp)–Co, and Co
2
C, re-
spectively (these two spent samples are denoted as Co
0
-t and K-
Co
2
C, respectively) (Fig. 1A). It indicates that these two phases
were in situ generated under the reaction conditions.
Some differences in the catalytic performance were observed on
Co
0
-t and K-Co
2
C. As shown in Fig. 1B, K-Co
2
C offers a selectivity
to C
2+
HCs of 46.5%, whereas it is only 6.0% on Co
0
-t. High-value
products C
2
~ C
4=
and C
5+
occupy 13.4 and 16.5% of K-Co
2
C with
an olefin/paraffin (O/P) ratio of 0.8 and a chain growth factor α of
0.53. In contrast, methane dominates (selectivity as 94.0%) on Co
0
-
t, while ethane is the only C
2+
product. The normalized space-time
yields (STYs) of C
2+
HCs based on catalyst mass and surface area
reach 7.7 mmol g
cat
−1
hour
−1
and 0.31 mmol m
−2
hour
−1
on K-
Co
2
C sample, while those on Co
0
-t are 1.7 mmol g
cat
−1
hour
−1
and 0.08 mmol m
−2
hour
−1
, respectively. We further modulated
the K content and found that only K-Co
2
C (K fraction as 0.98 wt
%) delivered a moderate conversion (38.2%) and high selectivity
and STY to C
2+
HCs, while 0.49% K-Co
3
O
4
and 1.96% K-Co
3
O
4
samples only yield 2.2 and 1.8 mmol g
cat
−1
hour
−1
, respectively
(Fig. 1C and table S1). Further investigations on Na- and Cs-mod-
ified samples show decreased selectivity to C
2+
HCs of 37.0 and
42.8%, while their STYs are 6.4 and 5.9 mmol g
cat
−1
hour
−1
, respec-
tively. In addition, the reaction temperature and pressure on K-
Co
2
C were optimized (table S2). We found that both CO
2
conver-
sion and CH
4
selectivity increase with the temperature (260° to
340°C), indicating that a high-reaction temperature promotes the
CO
2
activation but also boosts the deep hydrogenation to CH
4
.
The optimum yield to C
2+
HCs and those to C
2
~ C
4=
and C
5+
(2.2 and 2.7 mmol g
cat
−1
hour
−1
) were obtained at 300°C (fig. S4).
Tests at various pressures show that the CO
2
conversion and C
2+
HCs selectivity monotonically increase with the reaction pressure,
whereas CO selectivity decreases (fig. S5A). Under pressurized con-
ditions (0.8 to 3.0 MPa), more C
2+
HCs were detected (above
34.5%), and all the spent samples show well-defined reflections of
Co
2
C. In contrast, the sample evaluated at atmospheric pressure (0.1
MPa) consists of CoO and Co
2
C, while CO is the dominant product
(61.8%), revealing that the reaction pressure affects the carburiza-
tion and C─C coupling (fig. S5B). Considering that Co
2
C tends
to decompose to Co
0
at a high temperature (above 260°C) and a
high pressure (above 2 MPa) in CO-FTS (29), this in situ synthe-
sized K-Co
2
C catalyst operated at 300°C and 3.0 MPa extends its
application in catalytic CO
2
conversion.
To inhibit the deep hydrogenation of the C
1
intermediate and
increase the proportion of valuable C
2+
HCs, we further optimized
the H
2
/CO
2
ratio to 2/1. At a comparable conversion (24.2 and
21.6%), Co
0
-t (42,000 ml g
−1
hour
−1
) and K-Co
2
C (4500 ml g
−1
hour
−1
) exhibit the selectivity to C
2+
HCs as 3.3 and 67.3%, respec-
tively (Fig. 1D). The selectivity to C
2
~ C
4=
and C
5+
on K-Co
2
C
reach 31.6 and 28.7% with an O/P ratio of 4.5. The detailed distri-
bution of C
2+
HCs is shown in fig. S6 and table S3. The decreased
space velocity and H
2
/CO
2
ratio favor the carbon chain growth (α of
0.59) and inhibit the rehydrogenation of olefins, leading to a higher
proportion of valuable C
2
~ C
6=
. Kinetic analysis was conducted at
high space velocities to get insights on enhanced C
2+
HCs genera-
tion on K-Co
2
C. The apparent activation energies (E
a
) for CH
4
and
C
2+
HCs generation were estimated as 96.7 ± 4.1 kJ mol
−1
and 50.1
± 1.5 kJ mol
−1
on K-Co
2
C versus 80.4 ± 4.6 kJ mol
−1
and 92.9 ± 4.8
kJ mol
−1
on Co
0
-t, revealing that C
2+
HCs formation is more facile,
whereas CH
4
formation is inhibited on K-Co
2
C (Fig. 1, E and F).
Apparent reaction orders on K-Co
2
C (CO
2
α
1
= 0.48 ± 0.03 and
H
2
β
1
= 0.98 ± 0.05) and Co
0
-t (CO
2
α
2
= 0.88 ± 0.01 and H
2
β
2
= 0.32 ± 0.02) further evidence that the enhanced CO
2
or sup-
pressed H
2
activation profit the C
2+
HCs formation instead of
CH
4
on K-Co
2
C (Fig. 1G) (30). Moreover, compared with K-
Co
2
C, the similar CO
2
reaction order (α
3
= 0.47 ± 0.01) and the ev-
idently increased H
2
reaction order (β
3
= 1.34 ± 0.04) of 1.96% K-
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 2 of 14

Co
3
O
4
(fig. S7) suggest that the excessive K promoter inhibits the H
2
activation, instead resulting in the declined catalytic performance.
We further investigated the specific properties of CO
2
and H
2
ad-
sorption and activation on Co
2
C and Co surfaces using density
functional theory (DFT) calculations. Compared with Co, the
CO
2
adsorption and dissociation energies on the Co
2
C surface de-
creased from −0.08 to −0.26 eV, and −1.19 to −1.57 eV, respectively
(fig. S8A). Thus, CO
2
adsorption and activation are enhanced on
Co
2
C. There is essentially no difference between the molecular ad-
sorption energies of H
2
on the two surfaces (fig. S8B). However, the
dissociation energy of H
2
on the Co surface (−1.22 eV) is
exothermic, whereas the dissociation energy on Co
2
C (0.04 eV) is
energetically less favorable.
Structural characterization
The specific surface areas of Co
0
-t and K-Co
2
C are 20.9 and 24.8 m
2
g
−1
(table S4), and their average particle sizes are 23.2 ± 0.1 and 26.6
± 0.2 nm as estimated from the XRD results, respectively. TEM and
representative HRTEM images show that K-Co
2
C represents a cy-
lindrical morphology. The interplanar distances of 0.200, 0.218, and
0.243 nm match the (210), (020), and (101) planes of Co
2
C, respec-
tively (Fig. 2, A to C) (31). We verified that the K promoter is still
Fig. 1. Catalyst synthesis and catalytic properties. (A) Scheme for the reaction-induced in situ synthesis and XRD patterns of K-Co
2
C and Co
0
-t samples. (B) Catalytic
performance on Co
0
-t and K-Co
2
C at 300°C, 3.0 MPa, space velocity = 6000 ml g
−1
hour
−1
, H
2
/CO
2
= 3. (C) C
2+
HCs space-time yield (STY) and catalytic performance for
adjusted K loadings and alkali metal promoters at the same reaction conditions. (D) Catalytic performance at optimized reaction conditions, Co
0
-t: H
2
/CO
2
= 2, 42,000 ml
g
−1
hour
−1
; K-Co
2
C: H
2
/CO
2
= 2, 4,500 ml g
−1
hour
−1
. (E) Activation energies for CO
2
conversion and C
2+
HCs or CH
4
formation on K-Co
2
C. (F) Activation energies on Co
0
-t
sample. (G) CO
2
and H
2
reaction orders evaluated at 260°C, 3.0 MPa. wt %, weight %; a.u., arbitrary units.
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 3 of 14

uniformly dispersed on the Co
2
C surface (fig. S9), suggesting that
there is no obvious migration or agglomeration of K during the re-
action. As for Co
0
-t, the (002) and (100) planes of hcp-Co were
clearly observed (fig. S10). Previous DFT results by Zhang et al.
(32) suggest that (101) and (020) facets of Co
2
C compete for
CH
x*
coupling, leading to the high selectivity to C
2+
HCs on K-Co
2-
C. Ultraviolet (UV) Raman spectroscopy (Fig. 2D) with a 320-nm
laser was used to probe the information of C species at about 10 nm
in depth without the interference of fluorescence (33). Two Raman
shifts observed on K-Co
2
C of ~1453 and ~ 1606 cm
−1
are ascribed
to the D and G bands of C species in Co
2
C, which represents the A
1g
vibration of disordered graphite and E
2g
vibration of graphitic
carbon, respectively (21,34). The I
D
/I
G
value (calculated using the
deconvoluted peak area) for K-Co
2
C is ~1.38, suggesting the low
disorder degree and surface energy (21). In contrast, no signal
was detected on the metallic Co
0
-t sample.
Considering the carbide is sensitive to air, the surface properties
of spent catalysts were further investigated using quasi in situ x-ray
photoelectron spectroscopy (XPS) without exposure to air. The K
2p features of K
+
species in K
2
CO
3
were observed at 296.8 and
294.2 eV on K-Co
2
C (fig. S11) (35). The binding energies of Co
2p
1/2
and 2p
3/2
in Co
2
C (793.5 and 778.3 eV on K-Co
2
C) and Co
0
(793.6 and 778.4 eV on Co
0
-t) are similar, except for the signals at
798.0 and 783.5 eV associated with the superposition of Co
0
satellite
peaks and Co
2+
2p peaks (Fig. 2E) (36). The Co
2
C species on the
surface of K-Co
2
C was also identified according to the signal of
the C 1s band at 283.1 eV, assigned to the Co─C bond (Fig. 2F).
The characteristic peaks of the C═O group and the C─C bond
originated from the surface adsorbed species appear at 290.0 and
284.9 eV, respectively (22). The surface C/Co molar ratio, which
was estimated on the basis of the deconvoluted results, on K-
Co
2
C (0.27) is much higher than that on Co
0
-t (0.12), suggesting
the enhanced carburization with K modification.
Adsorption properties and reaction mechanism
Temperature programmed desorption (TPD) of CO
2
, CO, and H
2
combined with online mass spectrometry (MS) was performed to
characterize the adsorption properties (fig. S12). Compared with
Co
0
-t, K-Co
2
C shows an enhanced medium-strong adsorption of
CO
2
and CO (300° to 450°C), but a diminished adsorption of H
2
.
The medium-strong adsorbed CO
2
and CO species on K-Co
2
C,
which is more inclined to be activated and involved in the subse-
quent conversion, can create a relatively C-rich and H-lean environ-
ment, inhibiting the deep hydrogenation while promoting the C─C
coupling (37). In situ diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) was carried out to gain insight into the re-
action pathways, and the detailed peak assignments are listed in
table S5. As shown in Fig. 3A, the spectra for CO
2
pre-adsorbed
on K-Co
2
C were collected during the pressurization of CO
2
to 1.2
MPa at 50°C and subsequent heating to 260°C. CO
2
was initially
activated as monodentate carbonate (m-CO
32−
, at 1511, 1437,
1395, and 1278 cm
−1
) and bidentate carbonate (b-CO
32−
, at 1676
and 1625 cm
−1
) on the K-Co
2
C surface (38–41). The features of ad-
sorbed CO (CO
ads
) on Co
2
C (2078 cm
−1
) and Co
0
(2059 cm
−1
) were
detected once the pressure was increased to 0.6 MPa, and that of
bicarbonate (HCO
3
−
, at 1606 cm
−1
) emerges at 200°C, 1.2 MPa,
Fig. 2. Structural characterizations of spent catalysts. (A) TEM images and particle size distribution of K-Co
2
C. (Band C) HRTEM images of K-Co
2
C. (D) Ultraviolet Raman
spectra of K-Co
2
C and Co
0
-t. Quasi in situ x-ray photoelectron spectroscopy (XPS) spectra of K-Co
2
C and Co
0
-t (E) Co 2p spectra and (F) C 1s spectra.
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indicating that the residual H species originated from the pretreat-
ment or the hydroxyl group assists in the surface reaction (38). This
CO
ads
species reveals that, on the K-Co
2
C surface, a small amount of
CO
2
preferentially converted to CO at mild conditions (38). After
switching to H
2
(Fig. 3B and fig. S13A), the features of bidentate
formate (b-HCOO
*
, the stretching and bending vibrations of
C─H band at 2766, 2680, and 1400 cm
−1
, and the vibration of
OCO group at 1591 cm
−1
) and that of generated gaseous CO
(CO
gas
, at 2178 and 2111 cm
−1
) and CH
4
(3015 and 1306 cm
−1
)
and the vibrations of C─H bonds in C
2+
HCs as CH
3
(2956
cm
−1
) and CH
2
(2929 cm
−1
) (the enlarged spectra are shown in
fig. S13B) were observed (39,42). The varying tendency of above
species as determined by the featured peak (table S6) intensity
with time is shown in Fig. 3C. It can be seen that the CO
32−
rapidly reduces starting at 50°C, 1.2 MPa with an increasing inten-
sity of CO
ads
, indicating that CO
32−
initially converts to CO
ads
without the H assistance. Surface HCO
3
−
has gradually accumulat-
ed and reaches a steadystate after 260°C, 1.2 MPa. The introduction
of H
2
results in the reduction of CO
ads
and m-CO
32−
with an in-
crease of CH
x*
from 0 to 30 min and continuous generation of
CO
gas
and HCs. Moreover, the intensity of b-HCOO
*
increases
after H
2
addition but its further conversion is not observed. It is
speculated that CO
ads
derived from CO
32−
is an important interme-
diate for CH
x*
generation, which is further coupled to C
2+
HCs on
K-Co
2
C catalyst, whereas the HCO
3
−
and b-HCOO
*
seem acting as
spectators. By comparison, as for Co
0
-t catalyst (figs. S14 and S15),
the intensities of b-HCOO
*
rapidly decreased after switching to H
2
,
while those of monodentate formate (m-HCOO
*
) and product CH
4
simultaneously increased, indicating that the generation of CH
4
mainly undergoes the b-HCOO
*
–mediated pathway. Moreover,
the signal of CO
ads
also emerged during CO
2
adsorption (200° to
260°C) and quickly vanished after the H
2
introduction. It reveals
that the CO
ads
species is also possibly involved in the CH
4
formation.
In situ synthesis and stabilization of Co
2
C
We resorted to in situ XRD and theoretical calculations to shed light
on the structural evolution of Co-species during the reaction. The
reduction properties were first determined as discerned by the in
situ XRD patterns of as-prepared Co
3
O
4
and K-Co
3
O
4
in H
2
(fig.
S16). We found that K addition increased the complete reduction
temperature from 300° to 360°C, suggesting that, to some extent,
it inhibits the reduction of cobalt oxide toward the Co
0
. Under
the CO
2
hydrogenation atmosphere (Fig. 4A), the Co
3
O
4
modified
with K was first reduced to CoO and then carburized to Co
2
C at
260°C within 2 hours, and the reflection of Co
0
was absent through-
out the test. In combination with the semiquantitative XPS analysis
(Fig. 2, E and F) that the surface C/Co ratio (0.27) on K-Co
2
C is
relatively lower than the standard stoichiometric coefficient in
Co
2
C (0.50), we speculate that, with K modification, the Co
2
C
was in situ generated, followed by its partial decomposition to
Co
0
on the catalytic surface. Regarding that Co
0
species generally
Fig. 3. Reaction mechanism studies. In situ DRIFTS spectra on K-Co
2
C for (A) CO
2
pre-adsorption. (B) Intermediates conversion with switching to H
2
at 260°C, 1.2 MPa. (C)
Evolution of surface species according to the intensity of infrared featured bands.
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 5 of 14

participates in deep hydrogenation to CH
4
, a strategy against this
decomposition is still required. In comparison, the Co
3
O
4
without
K was reduced to CoO and then directly to Co
0
at 260°C within 50
min (fig. S17 and green arrows in Fig. 4B). Ab initio thermodynam-
ics methods were applied to derive the phase diagram and depict the
structural evolution of the catalyst. The chemical potentials for C
(μ
C
) and O (μ
O
) are determined according to the gas-phase compo-
sition and reaction conditions (43). As displayed in Fig. 4B, the
initial reaction condition is located at the red circle (μ
O
=−7.02
eV, μ
C
=−9.69 eV) which is near the computed boundary of the
CoO phase. This μ
O
value is below the Co
3
O
4
phase. Therefore,
once Co
3
O
4
is exposed to the environment, a reduction in the
CoO will occur. During the reaction, μ
O
decreases, while μ
C
increas-
es due to the CO generated from CO
2
reduction. As a result, the final
chemical potentials migrate to the black circle (μ
O
=−7.79 eV, μ
C
=
−8.16 eV), which is inside the domain of Co
2
C, suggesting that its
formation from CoO is thermodynamically favorable under reac-
tion conditions. Considering that the Gibbs free energy change
(ΔG) is more negative for the carburization from CoO than that
from Co
0
(fig. S18), and no bulk Co
0
was observed during the evo-
lution (Fig. 4A), we speculate that the thermodynamic advantages
directly propel CoO into carburization without the reduction to
Co
0
. We then investigated the structural evolution in CO
2
hydroge-
nation for the K-Co
0
sample, which was obtained from the decom-
position of K-Co
2
C. The in situ carburization of metallic cobalt
toward Co
2
C was also observed, exhibiting the gradually increased
C
2+
HCs selectivity and reduced CH
4
selectivity along with the re-
action (fig. S19). The CH
4
dominates in the initial products of K-
Co
0
[time on stream (TOS) = 1 hour] or Co
0
-t samples, suggesting
that the introduction of the K promoter does not substantially in-
fluence the product selectivity. The temperatures for the emergence
of Co
2
C reflections and the disappearance of Co
0
reflections in-
crease to 270° and 330°C, respectively (Fig. 4C), suggesting that
the synthesis of Co
2
C from Co
0
precursor requires a high temper-
ature for the C permeation into the Co lattice. We are thus confident
that the in situ synthesis of Co
2
C in CO
2
hydrogenation starts from
K-modified CoO (blue arrows in Fig. 4B) instead of Co
0
. By com-
parison, the syngas preferentially induces the reduction to Co
0
,
while the following carburization originates from Co
0
(16). The re-
placement of CO with CO
2
results in an increased μ
O
, favoring the
unique CoO route for the synthesis of Co
2
C. Moreover, this oxide-
mediated carburization is different from that of iron catalysts al-
though their carbides have many similarities. Metallic iron is
more advantageous than its oxide for the formation of FeC
x
, and
this key difference is helpful to understand the distinct evolution
and deactivation pathways for cobalt- and iron-based catalysts in
CO
x
conversion (44,45).
Structural stability is a key factor for catalysts. As evidenced by
the in situ XRD patterns in N
2
at 0.8 MPa (fig. S20), K-Co
2
C is ther-
mally stable below 340°C without bulk decomposition. Its stability
in the presence of reactants and products is also important but still
uncertain. Co
2
C is considered a metastable phase under the H-rich
environment, and, from the in situ XRD results of K-Co
2
C in pure
H
2
, it can be clearly seen that the transition of Co
2
C to Co
0
occurs at
290°C (fig. S21). A recent report has proven that CO
2
adsorbs on the
surface as carboxylate (CO
2δ−
) and then splits to C
*
in the presence
of H
2
, enabling the following C permeation (22). We performed the
in situ XRD investigations on K-Co
2
C in pure CO
2
and found that
Fig. 4. Structural evolutions and theory calculations. (A) In situ XRD patterns of K-Co
3
O
4
in CO
2
hydrogenation at 0.8 MPa. (B) Phase diagram of Co-O-C trinary system
derived from the DFT energies of bulk crystals. (C) In situ XRD patterns of K-Co
0
sample in CO
2
hydrogenation at 0.8 MPa; the K-Co
0
sample was synthesized from the
decomposition of K-Co
2
C in H
2
at 340°C, 0.8 MPa. In situ XRD patterns of K-Co
2
C (D) in pure CO
2
at 0.8 MPa, (E) in 25 volume % H
2
O/N
2
at 0.1 MPa. Potential energy profiles.
(F) CO
2
dissociation to CO
*
and O
*
on Co
2
C (101) surface with or without K
2
O.
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 6 of 14

excessive CO
2
also removes the C from Co
2
C at 300°C (Fig. 4D).
Previous results have shown that H
2
O can oxidize hcp-Co to CoO
or cobalt hydroxide but its effect on Co
2
C is unclear (46). We inves-
tigated the evolution of K-Co
2
C in 25 volume % (the theoretical
value of volume fraction and the same as below) H
2
O/N
2
and
found that excessive H
2
O causes the transition of Co
2
C to hcp-Co
in the range of 280° to 310°C (Fig. 4E). There is no obvious oxida-
tion in bulk below 340°C, possibly originated from the fast O
*
removal with K-promoting effect (47). When the H
2
O content
was decreased to 5%, this transition intensively occurred in the tem-
perature region of 330° to 340°C (fig. S22), which is higher than that
with co-feeding 25% H
2
O, implying that the H
2
O concentration
also has an impact on the decomposition of Co
2
C. Inspired by
the above findings, we believed that the single reactant or product
(H
2
O) of high concentrations could destroy the K-Co
2
C but a
unique balance between them in CO
2
hydrogenation presents an
opportunity for its stabilization. We further inquired into the
effect of the K promoter. We prepared a Co
2
C sample without K
(denoted as Co
2
C-t) through a reduction-carburization procedure
from Co
3
O
4
. However, it rapidly decomposed to hcp-Co at 260°C
in CO
2
hydrogenation within 20 min as confirmed by the in situ
XRD results (fig. S23). Moreover, the DFT results on the Co
2
C
(101) surface reveal that with the modification using the K
2
O pro-
moter, the direct splitting of CO
2
to CO is more facile, considering
that the energy barrier and energy of reaction (E
reaction
) reduce from
1.10 to 0.56 eV and −0.36 to −1.04 eV, respectively (Fig. 4F). Since
CO
2
is adverse to the stabilization of Co
2
C whereas CO enhances
the carburization, the promoting effect on CO
2
direct dissociation
here is also important for stabilizing the Co
2
C. Hence, we reveal that
the reaction atmosphere in conjunction with the K promoter is
crucial in the stabilization of Co
2
C.
H
2
O promoted carburization and stabilization
Since H
2
O is a common impurity for CO
2
capture and more H
2
O is
generated in CO
2
-FTS compared with CO-FTS, we further delve
into its influences on the carburization and the catalytic reaction.
We simulated the carburization from K-CoO in 10% CO/N
2
and,
as shown in Fig. 5A, in situ XRD results demonstrate that the trans-
formation of CoO to Co
2
C started at 260°C and finished at 280°C as
evidenced by the vanishing CoO feature at 36.5°. While 2% H
2
O was
co-fed (at 0.1 MPa, Fig. 5B), despite the constant temperature for
the beginning of carburization at 260°C, the transition of CoO to
Co
2
C was accomplished at 270°C. Once the H
2
O content was in-
creased to 5%, the onset and end temperatures for the carburization
shifted to 290° and 300°C, respectively (fig. S24). As stated above, a
moderate amount of H
2
O facilitates the carburization of K-CoO
and reduces the carburizing temperature, while its content is
crucial. Most previous studies reported the negative effect of H
2
O
on the active centers, including the induced oxidation or sintering
of Co
0
nanoparticles (48–50). So far, the promoting effect of H
2
O on
the carburization to Co
2
C has never been reported to the best of our
knowledge.
To obtain more evidence on the catalytic surface, we collected
the quasi in situ XPS spectra in the NAP-XPS system during the
carburization of K-CoO. As shown in the Co 2p spectra (Fig. 5, C
and D), the characteristic peaks at 780.3 and 796.4 eV and their sat-
ellite features at 786.3 and 802.9 eVare attributed to the divalent Co
species in the octahedral site of CoO (51). In 10% CO/N
2
, the fea-
tures of CoO kept almost unchanged, while only a faint signal of
Co─C bond was first detected at 300°C, inferring a slight surface
carburization of K-CoO (Fig. 5C). However, by co-feeding 2%
H
2
O, the peak of Co─C bond (283.0 eV) initially emerged at
250°C, and then gradually developed with the increasing tempera-
ture (Fig. 5D). Meanwhile, the featured Co 2p peaks completely
shifted from CoO (780.0 eV) to Co
2
C (778.1 eV) at 300°C, illustrat-
ing the full carburization of the surface. These results explicitly
verify the promoting effect of H
2
O in the carburization of K-
CoO. Furthermore, for the fresh K-CoO sample, the K 2p
1/2
and
2p
3/2
peaks at 295.2 and 292.7 eV are associated with K
+
species
in K
2
O (52). With the increasing temperature in CO or CO +
H
2
O flow, these characteristic peaks gradually shifted to high
binding energies and lastly reached at 295.7 and 293.0 eV at
300°C, revealing that the surface K
2
O species reacted with the CO
to form carbonate.
To gain insight into this promotion, we contrasted the interac-
tion between CO and pristine CoO surface with and without
H
2
O. As shown in the quasi in situ XPS spectra, the surface CoO
(780.2 eV) was fully reduced to Co
0
(778.5 eV) at 300°C in 10%
CO, while co-feeding 2% H
2
O prevents this reduction (Fig. 6A).
For C 1s spectra (Fig. 6B), a shoulder peak at ~288.0 eV assigned
to surface C─O species, including the possible CO
32−
, HCO
3
−
,
and CO
2δ−
, appears with H
2
O addition. These adsorbed species
were further determined by in situ DRIFTS. The spectra collected
at 1 and 30 min are used for probing the initial and final adsorbed
species, respectively, while the changing processes are recorded in
figs. S25 and S26. As shown in Fig. 6C, for the CoO-t sample in 10%
CO, the initial peaks at 1609, 1509, and 1268 cm
−1
correspond to b-
CO
32−
adsorbed on the CoO surface and those at 1366 and 1318
cm
−1
correspond to polydentate carbonate (p-CO
32−
) species
(40). Along with the surface reduction (fig. S25), some adsorbed
species gradually shift to HCO
3
−
(1604 cm
−1
, H is from surface hy-
droxyl) and m-CO
32−
(1463, 1377 cm
−1
) on Co
0
. While 2% H
2
O
was added, a band at 1438 cm
−1
and the shoulder peak at 1425
cm
−1
, which are attributed to the stretching vibration of the C═O
band and the bending vibration of the C─H band in formyl (HCO
*
)
species, were observed on the CoO surface (53). The b-CO
32−
and
monodentate formate (m-HCOO
*
, which is identified by the
stretching vibrations of the OCO group at 1542 and 1358 cm
−1
,
and the bending vibration of C─H band at 1392 cm
−1
) were also
observed (54). Without K modification, these species mostly
convert to adsorbed m-CO
32−
, which is inactive for carburization.
As for the K-CoO sample, a signal at 1745 cm
−1
associated with ad-
sorbed CO on three- or fourfold hollow sites (55) was both observed
with and without co-feeding H
2
O, suggesting that the K promoter
provides additional sites for enhanced CO adsorption, which is in
accordance with the CO-TPD-MS results (fig. S12B). In 10% CO, b-
HCOO
*
at 1565 and 1364 cm
−1
, and m-CO
32−
are initially adsorbed
and accumulated, while additional HCO
3
−
species (1605 cm
−1
) was
observed at 30 min on the K-CoO surface. Notably, with the addi-
tion of 2% H
2
O, the bands at 1556, 1415, and 1336 cm
−1
, which are
assigned to the CO
2δ−
, were detected (22,56,57). With an extension
of the treatment time, these species have accumulated in 1 to 10 min
and then are gradually consumed along with the surface carburiza-
tion (fig. S26). The final adsorbed species on a fully carburized
surface is HCOO
*
while physically adsorbed H
2
O (1660 cm
−1
)
was also detected. In literature results, compared with HCOO
*
and CO
32−
, CO
2δ−
is easier to split and form C
*
species for carbu-
rization (22). According to the above XPS and infrared (IR) results,
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 7 of 14

we speculate that the promotion originates from the H
2
O-facilitated
formation of surface C─O species, including the HCO
*
or CO
2δ−
species. However, without the K promoter, the HCO
*
converts to
CO
32−
; while only the K promoter and H
2
O jointly act on the
CoO surface, the active CO
2δ−
species can be generated and in-
volved in the subsequent carburization.
We calculated the dissociation of CO
2δ−
(CO
2*
) toward C
*
species on the CoO (200) surface. As demonstrated in fig. S27,
with the presence of K
2
O promoter and H
2
O, the CO
2*
dissociation
energy (reaction I: CO
2*
→CO
*
+ O
*
) declines about 0.7 to 2.07 eV.
Notably, the reaction energies for C
*
formation of reactions II (CO
*
→C
*
+ O
*
) and III (CO
*
+ CO
*
→C
*
+ CO
2*
) reduce to 0.67 and
−0.54 eV, which are lower than those on clean CoO (6.47 and 3.19
eV) or K
2
O-CoO (5.45 and 2.47 eV) surfaces. The results show that
K
2
O and H
2
O jointly promote the dissociation of surface CO
2δ−
to
C
*
species. Besides, after the dissociation of the C─O species to C
*
,
the adsorption of surface C atoms is important for carburization,
and we further investigated this process on CoO. The average
number of electrons transferred between C atoms and the CoO
surface (described by Bader charge) and the average formation en-
ergies of adsorbed C atoms (ΔE
form
) calculated with and without
K
2
O co-adsorption are summarized in table S7. Two or three C
atoms (marked as 2C and 3C) were deposited on the CoO surface
to restore the situation of multicarbon co-adsorption. The Bader
charges are 2.13 (2C) and 2.16 (3C) on the clean CoO surface.
They increase to 2.28 (2C) and 2.22 (3C) in the presence of K
2
O,
indicating that more electrons are transferred from the C atoms
to the CoO surface, and that K
2
O enhances the electronic interac-
tion between C
*
and the catalyst surface. As a result, with the K
2
O
addition, the ΔE
form
decreases from 3.69 (2C) and 3.63 eV (3C) to
3.25 (2C) and 3.20 eV (3C), respectively. This reduction in forma-
tion energies (exceed, 0.4 eV) demonstrates that the adsorption of
C
*
species on the CoO surface is easier with K
2
O promotion, which
favors C accumulation and permeation to form a bulk carbide.
These calculation results agree with the experimental observations
that K
2
O and H
2
O promote carbide formation.
Fig. 5. Facilitating carburization of K-CoO sample with a moderate amount of H
2
O. In situ XRD patterns recorded (A) in 10 volume % CO/N
2
at 0.8 MPa and (B) in 10
volume % CO + 2 volume % H
2
O/N
2
at 0.1 MPa. Quasi in situ XPS spectra in the near-ambient pressure (NAP) XPS system recordedafter reaction at 0.1 MPa (C) in 10 volume
% CO/N
2
and (D) in 10 volume % CO + 2 volume % H
2
O/N
2
.
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 8 of 14

Given that the surface of K-Co
2
C decomposes in the reaction
(Fig. 2F), there is a possibility to exploit the beneficial effect of
co-feeding H
2
O for inhibiting this decomposition and improving
the catalytic stability. We first evaluated the effect of co-fed H
2
O
content on the performance (fig. S28; the data are collected at
TOS = 3 hours, while there is no obvious deactivation) and found
that when it increases to 0.25 and 0.5%, the CO
2
conversion slightly
decreases from 37.9 to 35.6% and 35.3%, respectively. However,
more H
2
O addition exacerbates the activity declination, while the
conversion drops to 24.2% with co-feeding 2.5% H
2
O. While the
added H
2
O content was increased to 25%, the overall CO
2
conver-
sion further dropped to 17.5% and the CH
4
(selectivity as 86.8%)
dominates in the products. The generated H
2
O in the actual reac-
tion or its extra addition gives rise to the competitive adsorption
with other reactants or intermediates and increases the rate of
reverse reaction, which jointly causes a gradually reduced CO
2
con-
version (28). Moreover, a volcanic curve was observed for C
2+
HCs
selectivity and yield with H
2
O content, and at the optimum content
of 0.5%, the C
2+
HCs selectivity increases to 53.1% and its STY of
C
2+
HCs is 8.0 mmol g
−1
hour
−1
. As can be seen from the C
2+
HCs
composition (fig. S29), co-feeding 0.5% H
2
O leads to a more cen-
tralized product distribution (α decreases from 0.53 to 0.48) and an
enhanced generation of light olefins (the C
2
~ C
4=
selectivity in-
creases to 19.9% and O/P ratio increases to 1.0), indicating that
co-feeding 0.5% H
2
O here protects the surface Co
2
C from decom-
position. Moreover, in CO-FTS, H
2
O sometimes promotes the for-
mation of CH
x*
species via an H
2
O-assisted methylidyne
mechanism and increases the C
5+
selectivity (58). However, a slight-
ly decreasing C
5+
selectivity was observed with H
2
O addition here.
It is speculated that the low concentration of H
2
O (0.5% ~ 2.5%)
does not substantially influence the reaction pathway but is
enough to modulate the surface compositions. The promoting
effect of H
2
O on stabilizing the structure of Co
2
C and its catalytic
features is expected to enable its practical application in the CO
2
hydrogenation reaction with high temperature and pressure.
The catalytic stability of K-Co
2
C in CO
2
hydrogenation was ex-
amined, as shown in figs. S30 and S31, we found that the H
2
O
content has an impact on the catalyst deactivation. Within the
Fig. 6. Eect of co-feeding H
2
O on structural properties and catalytic performance. Quasi in situ XPS spectra collected on CoO-t sample after treating in 10 volume %
CO with or without co-feeding 2 volume % H
2
O (A) Co 2p spectra and (B) C 1s spectra. (C) In situ DRIFTS spectra collected at 1 and 30 min on CoO-t and K-CoO samples
under different treated conditions. (D) Catalytic stability test with co-feeding 0.5 volume % H
2
O within 210 hours. Reaction conditions: 300°C, 3.0 MPa, space velocity =
6000 ml g
−1
hour
−1
, H
2
/CO
2
= 3. Quasi in situ XPS spectra of K-Co
2
C sample that was used in CO
2
hydrogenation with co-feeding 0.5 volume % H
2
O (E) Co 2p spectra and
(F) C 1s spectra.
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 9 of 14

initial 35 hours, without co-feeding H
2
O, the CO
2
conversion and
C
2+
HCs selectivity decreased from 38.2 to 31.6% and 46.5 to 38.8%,
respectively. The addition of H
2
O effectively inhibited this activity
decline, and when co-feeding 0.5% or more H
2
O, the catalytic per-
formance basically kept stable in the initial 35 hours. We further
compared the changes in catalytic performance with various H
2
O
contents (fig. S32). The drops of C
2+
HCs STY can be clearly ob-
served when the H
2
O content is 0 or 0.25%. As for the H
2
O
content of 0.5 ~ 2.5%, the ratios of CO
2
conversion collected at 3
and 35 hours (ratio A) and those of C
2+
HCs STY (ratio B) are all
in the range of 96 to 98%, which are higher than those recorded at 0
or 0.25% H
2
O. It reveals that co-feeding H
2
O is beneficial to miti-
gating the deactivation, and when the H
2
O content exceeded 0.5%,
the features of deactivation are quite similar. Furthermore, without
co-feeding H
2
O, the decrease of the activity (31.6 to 24.2%) and se-
lectivity alterations were clearly observed in 35 to 50 hours, while
final CH
4
selectivity increased to 74.7% and the STY of C
2+
HCs
dropped from 7.6 to 2.6 mmol g
−1
hour
−1
within 50 hours on
stream (fig. S33). By comparison, with co-feeding 0.5% H
2
O (Fig.
6D), the K-Co
2
C catalyst showed higher stability throughout a
210-hour test. The final selectivity to C
2+
HCs remained at 49.2%,
exceeding 90% of the initial values (53.0%, at 15 hours), and the STY
of C
2+
HCs still retained 7.2 mmol g
−1
hour
−1
at 210 hours. The
XRD patterns of spent samples (fig. S34) show that co-feeding
25% H
2
O caused the complete decomposition of Co
2
C, which is
in accordance with the in situ XRD results (Fig. 4E). However, no
substantial difference was observed in spent samples with co-
feeding 0.5 or 2.5% H
2
O, indicating that their evolution mainly
occurs on the catalytic surface. The quasi in situ XPS spectra of
the spent K-Co
2
C with 0.5% H
2
O are shown in Fig. 6 (E and F).
The binding energies of Co 2p
1/2
and Co2p
3/2
at 793.5 and 778.3
eV reveal that the catalytic surface is Co
2
C. Notably, the addition
of 0.5% H
2
O enhances the signal of the Co─C bond at 283.1 eV
(C 1s spectra) in comparison to the results of the K-Co
2
C sample
which was used without co-feeding H
2
O (Fig. 2F). The estimated
surface C/Co ratio increased from 0.27 (without H
2
O) to 0.46
with co-feeding 0.5% H
2
O, particularly substantiating that the de-
composition of Co
2
C has been inhibited by adding moderate H
2
O
in the reaction.
In conclusion, we provide an in situ synthesized and stable K-
Co
2
C catalyst for CO
2
hydrogenation which shows outstanding ac-
tivity and selectivity to C
2+
HCs up to 67.3% at 300°C, 3.0 MPa.
Compared with metallic cobalt catalysts, K-Co
2
C is more competi-
tive in accelerating the formation of C
2+
HCs, and a C-rich and H-
lean surface environment suppresses the side reaction toward CH
4
.
This K-Co
2
C catalyst is promising to be combined with the zeolites
or membrane reactors to further optimize the product composition
and alleviate the downstream separation needs for improving its
practical applications (3,59). Adsorbed CO derived from carbonate
splitting is an important intermediate for coupling to C
2+
HCs as
confirmed by in situ DRIFTS. Multispectral studies, including in
situ XRD, quasi in situ XPS, and DFT calculations, reveal that
Co
2
C is directly generated from CoO instead of undergoing the re-
duction to Co
0
. The reactants (H
2
and CO
2
) or the product (H
2
O) of
high concentrations all can cause the decomposition of Co
2
C to
Co
0
, while the delicate balance of the reaction atmosphere and the
K promoter plays vital roles in the stabilization of Co
2
C. Co-feeding
a small amount of H
2
O instead facilitates the carburization and sta-
bilization of Co
2
C. During the carburization, 2% H
2
O and the K
promoter jointly act on the CoO surface and stimulate the forma-
tion of key CO
2δ−
species, followed by its dissociation to active C
*
species. The K promoter also enables the subsequent C
*
adsorption
on the CoO surface, favoring the C accumulation and permeation.
Inspired by this finding, 0.5% H
2
O was co-fed in the feed gas to sta-
bilize the surface structure of K-Co
2
C and it markedly prolongs the
catalytic stability from 35 to over 200 hours. Given that H
2
O is an
inevitable impurity in the flue gas (a key source for CO
2
capture)
and generally considered a poison in CO
2
hydrogenation reaction,
our study highlights the unique promoting effect of H
2
O in feed gas
on Co
2
C catalyst and it is expected to improve its practical applica-
tion in CO
2
conversion. However, considering that the excessive
H
2
O addition is conversely detrimental to Co
2
C, its separation
before the catalytic conversion or the surface hydrophobic or hy-
drophilic modifications to control its content within the ideal
range is still needed. The fundamental understanding of the forma-
tion and stabilization of Co
2
C also provides opportunities for its po-
tential application in other reactions.
MATERIALS AND METHODS
Catalyst preparation
Synthesis of Co
3
O
4
and modied Co
3
O
4
precursor
Co(NO
3
)
2
·6H
2
O was purchased from Aladdin Biochemical Tech-
nology Co., Ltd. Citric acid, Na
2
CO
3
, K
2
CO
3
, and Cs
2
CO
3
were pur-
chased from Bodi Chemical Trade Co., Ltd. Co
3
O
4
precursor was
prepared using a citric acid–induced sol-gel method. Typically,
citric acid and Co(NO
3
)
2
·6H
2
O (molar ratio = 13/20) were dissolved
in ethanol and the mixed solution was aged at 30°C for hydrolysis
and complexation. Then, the ethanol was fully evaporated at 80°C.
The obtained foamy gel was dried at 80°C for 12 hours and further
calcined in air at 450°C for 4 hours to get the Co
3
O
4
precursor.
Alkali metal–modified Co
3
O
4
precursors were synthesized by an
IWI method using an aqueous solution of their carbonates. The
molar amount of Na
2
CO
3
or Cs
2
CO
3
was fixed as 0.025 mmol,
whereas that of K
2
CO
3
was tried as 0.013, 0.025, and 0.050 mmol
(the corresponding mass fractions of K are 0.49, 0.98, and 1.96%).
As an example, 0.0035 g (0.025 mmol) of K
2
CO
3
was dissolved in
0.12 g of H
2
O through sonication, and then the solution was
dropped on 0.2 g of Co
3
O
4
under stirring at room temperature.
The resulting power was dried at 80°C for 12 hours and calcined
in air at 400°C for 2 hours to get the 0.98% K-Co
3
O
4
precursor.
Synthesis of testing samples
We used the in situ XRD reactor chamber to prepare the samples
which were denoted as K-CoO, K-Co
0
, CoO-t, and Co
2
C-t, respec-
tively. The K-CoO sample was prepared from the reduction of
0.98% K-Co
3
O
4
in H
2
at 240°C, 0.8 MPa. The K-Co
0
sample was
obtained from the decomposition of K-Co
2
C in H
2
at 340°C, 0.8
MPa. The CoO-t sample was prepared from the reduction of
Co
3
O
4
precursor in H
2
at 220°C, 0.1 MPa. The Co
2
C-t sample
was prepared from two-step treatments from Co
3
O
4
precursor, in-
cluding the initial reduction at 260°C, and the following carburiza-
tion in CO, as recorded in fig. S23A. Before the ex situ structural
characterizations, the Co
0
-t and K-Co
2
C samples were carefully pas-
sivated in the flow of 1% O
2
/N
2
(20 ml min
−1
) at ~20°C for 1 hour
and then were transferred to the glove box for preservation.
SCIENCE ADVANCES |RESEARCH ARTICLE
Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 10 of 14

Catalyst characterization
XRD patterns were recorded on a Rigaku SmartLab 9-kW diffrac-
tometer with Cu Kα radiation (λ = 1.5406 Å) and with a scanning
rate of 8°/min. In situ XRD measurements were performed in an
XRK 900 reactor chamber and the heating process was controlled
by a TCU 750 temperature control unit. The patterns were collected
for each 10 min until the structure is stable for at least 1 hour at the
given conditions. As for the structural evolution in CO
2
hydrogena-
tion, the sample was directly heated to the target temperature in the
reactive gas at 0.8 MPa. For the structural evolution in other atmo-
spheres (H
2
/CO
2
/CO/H
2
O), the specific conditions were intro-
duced in the corresponding results.
Quasi in situ XPS was performed on a spectrometer equipped
with an Al K x-ray source at 300 W. Before the test, the samples,
including the K-Co
2
C and Co
0
-t (without the passivation), were
carefully transferred from the glove box to the XPS analysis
chamber without the exposure to air. Besides, the carburization pro-
cesses for CoO-t and K-CoO samples were simulated in 10% CO/N
2
or 10% CO + 2% H
2
O/N
2
in the reaction chamber equipped with
the NAP-XPS system. Here, these quasi in situ XPS spectra were re-
corded after reaching the setting temperatures for 20 min. The
thermal couples were placed at the side of the powder sample for
in situ XRD (set temperature, ±30°C) and on the upper surface of
the sample piece for quasi in stu XPS, respectively (fig. S35).
In situ DRIFTS experiments were performed using the Thermo
Scientific Nicolet iS50 spectrometer with a mercury cadmium tellu-
ride detector, and the background spectrum was collected after N
2
purge for at least 1 hour. The flow rate of reactive gas (CO
2
/H
2
/N
2
=
21:63:16) or N
2
is 30 ml min
−1
, whereas that of CO
2
, CO, or H
2
is 10
ml min
−1
. As for mechanism studies on K-Co
2
C and Co
0
-t samples,
the testing temperature is not above 260°C for avoiding the destroy
from CO
2
or H
2
on catalyst structure. As shown in fig. S36, these
samples were in situ synthesized in the IR cell at 300°C, 1.2 MPa
for 3 hours, and then was purged (300°C) and cooled (50°C) in
N
2
, atmospheric pressure before the adsorption of CO
2
and the fol-
lowing switching to H
2
. The co-fed H
2
O at atmospheric pressure for
in situ XRD or DRIFTS and quasi in situ XPS test is stored in a glass
wash bottle, which is placed before the reactor chamber. Its content
is dependent on the controlled temperature of wash bottle. The the-
oretical H
2
O content was calculated according to the following
equation and was used for the expression. The actual H
2
O
content was determined using the NaOH and silica gel as the absor-
bents (fig. S37), while the above results are listed in table S8.
Theoretical ContentH2O¼p
H2O
p0100%
TPD tests were carried out on a Quantachrome ChemBET
Pulsar analyzer, while the desorbed species was detected by the
Pfeiffer GSD-350 online MS. Typically, the samples were loaded
into a quartz tube and flushed with N
2
at 300°C for 1 hour to
remove undesired adsorbates, and then cooled to room tempera-
ture. The samples were first retreated in the reactive gas at 300°C
for 1 hour to remove the external passivation layer, and then
flushed with N
2
for 1 hour. These samples were cooled to 30°C to
adsorb CO
2
or CO in 1 hour, followed by switching to N
2
to purge
for 30 min. Then, the desorption and analysis program were con-
ducted with a heating rate of 10°C min
−1
to 700°C.
TEM and HRTEM images were obtained on a Tecnai F30
HRTEM instrument (FEI Corp) with a voltage of 300 kV. EDS ele-
mental mapping images were obtained on a JEM ARM200F
thermal-field emission microscope equipped with a probe spherical
aberration (Cs) corrector with a voltage of 200 kV. UV Raman
spectra were collected on a homemade triple-stage UV Raman spec-
trometer with a resolution of 2 cm
−1
. The wavelength of UV laser
line from a double-frequency 514-nm laser was set at 320 nm. The
textural properties were determined by Ar absorption-desorption
on a Quantachrome AUTO-SORB-1-MP sorption at 87 K. The
surface area was calculated using the Brunauer-Emmett-Teller
(BET) method. The concentrations of alkali metals and Co were de-
termined by inductively coupled plasma optical emission spectrom-
etry on a PerkinElmer AVIO 500 instrument and the results are
listed in table S9. TGA was conducted on a TGA-SDTA851e ther-
mobalance in the air flow (25 ml min
−1
) with a heating rate of 5°C
min
−1
from 50° to 800°C.
Catalytic activity test
The CO
2
hydrogenation was conducted in a stainless steel fixed-bed
flow reactor with an 8-mm inner diameter. A total of 150 mg of
Co
3
O
4
or alkali metal–modified Co
3
O
4
precursor (20 to 40
meshes) was diluted with 450 mg of quartz sand (20 to 40
meshes), and then loaded into the middle of the reactor. Catalytic
performance was tested in the reactive gas (CO
2
/H
2
/Ar = 1:3:1.5,
space velocity = 6000 ml g
−1
hour
−1
,P= 3.0 MPa unless otherwise
noted) at 260°, 300°, and 340°C. The products were collected at
about 3 hours after reaching the steady state on steam. As shown
in fig. S38, all the products, including the liquid oxygenates and
C
5+
HCs, were heated by an oven and heating belt at 100°C for
full vaporization and accessed into the online chromatography
(Agilent, 7890B) for the analysis by the thermal conductivity detec-
tor and flame ionization detector, while the Ar was used as an in-
ternal standard. CO
2
conversion and CO selectivity were calculated
on a carbon-atom basis according to the following equations
CO2Conv:ð%Þ ¼ nCO2;in nCO2;out
nCO2;in 100%
CO Sel:ð%Þ ¼ nCO;out
nCO2;in nCO2;out 100%
where n
CO
2
,in
and n
CO
2
,out
represent the concertation of CO
2
at the
inlet and outlet. n
CO,out
represents the CO concertation at the outlet.
The total selectivity of alcohols and dimethyl ether is below 1.5%
and therefore was not reported here.
The selectivity to HC C
n
H
m
was calculated for representing the
HCs distribution
CnHmSel:ð%Þ ¼ nCnHm;out
Σ iCiHm;out 100%
where C
n
H
m,out
represents moles of detected individual
HCs product.
SCIENCE ADVANCES |RESEARCH ARTICLE
Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 11 of 14

The STY, the O/P ratio, and the chain growth factor α were cal-
culated as shown below
STYaðmmol g1h1Þ¼CO2Conv:CnHmSel:ð1CO Sel:Þ6000
22:45:5
STYbðmmol m2h1Þ¼STYa
SBET
O=P ratio ¼Sel:ðC2≏C¼
4Þ
Sel:ðC2≏C0
4Þ
lnWn
n¼nlnαþnð1αÞ2
α
where STY
a
and STY
b
represent the STY values normalized based
on catalyst mass and surface area (S
BET
; table S4). W
n
and nrepre-
sent the mass fraction and the carbon number of HCs products. The
carbon balance was in the range of 95 to 105%.
The contents (0.25 to 25%) of H
2
O mentioned in this work rep-
resent those of extra-added H
2
O in the carburization gas or feed gas.
The H
2
O was added using a steel gas wash bottle, which can be pres-
surized to 3.0 MPa, and the setting temperatures are listed in table
S8. For the kinetics tests (activation energy and reaction order), the
conversion was decreased below 10% by varying the space velocity.
Theory computational methods
The basic thermodynamic calculations (ΔG, ΔH, and ΔS) were
carried out by the Aspen Plus V11 software. All spin-polarized
DFT calculations were performed with the Vienna Ab initio Simu-
lation Package. The exchange-correlation energies were calculated
by the generalized gradient approximation approach with the
Perdew-Burke-Ernzerhof functional (60). Core electrons were
frozen and treated with the projector-augmented wave theory
(61). Valence electrons were taken as Co (4s
2
3d
7
), C (2s
2
2p
2
), and
O (2s
2
2p
4
). The plane-wave basis set was truncated at 500 eV for gas
molecules and surface calculations and was increased to 650 eV for
bulk crystals to mitigate the effect from Pulay stress. Gaussian
smearing with 0.1-eV width was applied to oxides and molecules
while first order of Methfessel-Paxton smearing was applied to
metal and carbides (62). Brillouin zones were treated with the Mon-
khorst-Pack k-points mesh (63). The k-points, lattice parameters,
space groups, and magnetics are summarized in table S10. The
van der Waals interaction correction was applied for all calculations
with the DFT-D3 method (64). The climbing image nudged elastic
band method was used to find transition states, combined with the
dimer method. It was verified that each transition state had only one
imaginary vibrational frequency along the reaction coordinate di-
rection. A three-layer slab model was built for the CoO (200)
surface and the bottom two layers were fixed during structural op-
timization. The criteria of force convergence for surface calculations
were set to 0.03 eV/Å on all atoms. The Hubbard U-correction was
applied to oxides for localized d-state electrons (65). A value of 3.3
eV for Co was taken from the work of Wang et al. (66). Bader charge
analysis was performed to investigate the charge transfer of the ad-
sorbate-surface system (67). The adsorption and activation of CO
2
and H
2
were investigated on Co
2
C (101) and (020) surfaces, and the
Co (002) surface. All surface calculations were performed in the
Monkhorst-Pack scheme using k-points of 2 × 2 × 1 with the addi-
tion of a vacuum layer of 15 Å to avoid interactions between repeat-
ing periodic elements. In the structural optimization of Co
2
C
surfaces, the bottom 1/3 Co and C atoms are fixed in their original
atomic positions, while the remaining top 2/3 Co and C atoms to-
gether with the adsorbate are completely relaxed. We constructed
three kinds of CoO (200) surfaces: clean surface, surface containing
only K
2
O, and surface containing both K
2
O and H
2
O, and calculat-
ed the reaction energy of the stepwise decomposition of CO
2δ−
to C
*
species on these three surfaces. The optimized structures involved
are displayed in figs. S39 and S40, respectively.
Ab initio thermodynamics (68) was applied to construct the bulk
phase diagram, locate the reaction conditions on the resulting phase
diagram, and calculate formation energies. The explanation per-
taining to the equilibrium assumptions inherent to this thermody-
namics method is shown in the Supplementary Materials. In the
bulk phase diagram, the phase with lowest formation energy is iden-
tified as the stable phase. The chemical potentials of C and O are
determined by the free energies of the gas species with the assump-
tion that the gas phase is equilibrated with the catalysts. The free
energy of a molecule is calculated according to the following equa-
tions
μO¼GCO2GCO
μC¼GCO μO
Ggas ¼EDFT
gas þZPEgas þΔH0K!533:15K
gas TS533:15K
gas þRTln pgas
pref
where G
CO
2
and G
CO
are the free energy of CO
2
and CO. G
gas
is the
free energy of the gas molecule, EDFT
gas is the DFT energy of the gas
molecule, ZPE
gas
is the zero-point vibrational energy of the gas mol-
ecule, ΔH0K!533:15K
gas is the enthalpy change of the gas molecule from
0 to 533.15 K, Tis set as 533.15 K according to the initial carburi-
zation temperature for K-Co
3
O
4
sample, and S533:15K
gas is the entropy
of the gas molecule at 533.15 K, Ris the gas constant, p
gas
is the
partial pressure of the gas, and p
ref
is the reference pressure,
which is set to 1 bar. The following temperature and partial pres-
sures were used in determining the chemical potential of O and C
in Fig. 4B: red dot (533.15 K, 1.45 bar CO
2
, 1 × 10
−10
bar CO), black
dot (533.15 K, 1.45 bar CO
2
, 1.645 × 10
−3
CO). The pressure of CO
was set to 1 × 10
−10
bar to represent that it does not exist in the
initial reaction condition, while all other partial pressures were mea-
sured from our experiments.
The average formation energy ∆E
form
of each C atom adsorption
on the CoO (200) surface with or without the co-adsorption of K
2
O
is defined with the following equation
ΔEform ¼ ðEtotal Esur zμ0
CÞ=z
where E
total
is the total DFT energy of a surface adsorption structure
with zC atoms, E
sur
is the DFT energy of the clean CoO surface
(either with or without co-adsorbed K
2
O). zis set as 2 and 3,
while μ0
Cis set as −8.16 eV on the basis of the above calculation
result of the chemical potential of carbon.
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 12 of 14

Supplementary Materials
This PDF le includes:
Supplementary Text
Figs. S1 to S40
Tables S1 to S10
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Acknowledgments: We acknowledge the State Key Laboratory of Catalysis, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences for providing the quasi in situ XPS test. We also
thank W. Liu at Dalian Institute of Chemical Physics, Chinese Academy of Sciences and C. Shi at
Dalian University of Technology for helping on EDS and TPD-MS characterization. Funding: This
work was supported by the National Natural Science Foundation of China (22172013 and
22288201), The Major Science and Technology Special Project of Xinjiang Uygur Autonomous
Region (2022A01002-1), The Fundamental Research Funds for the Central Universities
(DUT22LK24, DUT22QN207, and DUT22LAB602), The Liaoning Revitalization Talent Program
(XLYC2008032), The CUHK Research Startup Fund (no. 4930981), The Donors of the American
Chemical Society Petroleum Research Fund (PRF #59759-DNI6), and Tingzhou Youth Program
(2021QN08). Author contributions: Conceptualization: M.W., G.Z., T.P.S., C.S., and X.G.
Methodology: M.W., P.W., M.Z., R.L., and J.Z. Investigation: M.W., P.W., M.Z., R.L., and Yi Liu.
Visualization: M.W.,Z.C., Yulong Liu, K.B., and F.D.Supervision: G.Z., T.P.S., X.N., Q.F., C.S., and X.G.
Writing—original draft: M.W., P.W., R.L., J.W., J.Z., and G.Z. Writing—review and editing: G.Z.,
T.P.S., X.N., Q.F., C.S., and X.G. Competing interests: The authors declare that they have no
competing interests. Data and materials availability: All data needed to evaluate the
conclusions in the paper are present in the paper and/or the Supplementary Materials.
Submitted 27 November 2022
Accepted 11 May 2023
Published 16 June 2023
10.1126/sciadv.adg0167
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Wang et al.,Sci. Adv. 9, eadg0167 (2023) 16 June 2023 14 of 14