Highly Dispersed Metal Carbide on ZIF-Derived Pyridinic-N-Doped Carbon for CO2 Enrichment and Selective Hydrogenation

Article abstract of DOI:10.1002/cssc.201800016

Catalytic conversion of CO₂ into chemicals is a critical issue for energy and environmental research. Among such reactions, converting CO₂ into CO has been regarded as a significant foundation to generate a liquid fuels and chemicals

Full text of DOI:10.1002/cssc.201800016

Accepted Article
Title: Highly dispersed metal carbide over ZIF-derived pyridinic-N
doped carbon for CO2 enrichment and selective hydrogenation
Authors: Yunhua Li, Xiaohu Cai, Sijing Chen, Hua Zhang, Hongliang
Zhang, Jinqing Hong, Binghui Chen, Dong-Hau Kuo, and
Wenju Wang
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Highly dispersed metal carbide over ZIF-derived pyridinic-N
doped carbon for CO2 enrichment and selective hydrogenation
Yunhua Li,*[a] Xiaohu Cai,[a] Sijing Chen,[a] Hua Zhang,[a] Hongliang Zhang,[a] Jinqing Hong,[a] Binghui
Chen,[a] Dong-Hau Kuo,[b] Wenju Wang† [c]
Abstract: CO2 catalytic conversion to chemicals is a critical issue for
energy and environmental. Among them, converting CO2 to CO has
been regarded as a significant foundation to generate a series of
liquid fuels and chemicals in a large scale. In this work, M/ZIF-8-C
(M= Ni, Fe, Co and Cu) with highly dispersed metal carbide was
prepared for CO2 selective hydrogenation. Under the same
conditions of metal loadings, CO2 hydrogenation activity to CO
follows the order: Ni/ZIF-8-C≈Fe/ZIF-8-C>Co/ZIF-8-C>Cu/ZIF-8-C.
These catalysts are composed of carbide or metal supported on
pyridinic N structure. ZIF-8-derived pyridinic nitrogen and carbide
favor CO2 adsorption while dispersed Ni or Fe carbide and metal
species serve as an active site for CO2 hydrogenation. Extraordinary
catalytic performance of Ni supported catalyst results from high
dispersion of metal and exposure of carbide. Based on high-
sensitivity low-energy ion scattering (HS-LEIS) and line scan results,
density functional theory (DFT) was used to understand reaction
mechanism of CO2 selective hydrogenation over Ni/ZIF-8-C. The
result presents that product CO derives mainly from the direct
cleavage of C-O bond in CO2* rather than decomposition of COOH*.
CO* desorption energy on Ni/ZIF-8-C is lower than that for further
hydrogenation and dissociation. Comparison Ni/ZIF-8-C with ZIF-8-C
indicates that synergy effects of highly dispersed metal or carbide
and weak CO adsorption result in high CO selectivity for CO2
hydrogenation.
greenhouse effect and energy conversion through a thermal
catalytic hydrogenation, photocatalysis or electrocatalysis
process.[1,2] Valuable chemicals from CO2 conversion include
formic acid,[3] methanol,[4] hydrocarbons[5] and carbon
monoxide.[6, 7] Since thermocatalytic CO2 conversion via CO can
produce a bunch of liquid fuels and chemicals massively through
existing technologies, such as methanol and Fischer-Tropsch
synthesis,[8-11] CO2 selective hydrogenation to CO has recently
received great attentions.
At present, catalysts for this reaction mainly involve supported
Rh, Pt, and Ru[12-14] and Cu, Co, Ni.[15-18] The base metal
catalysts are favored due to a relatively low cost. For example,
Gonçalves et al. reported that highly dispersed nickel supported
on silica synthesized by magnetron sputtering deposition
method shows high activity and selectivity for CO synthesis.[11]
Additionally, highly dispersed Cu on β-Mo2C also demonstrated
the superior activity for CO2 hydrogenation to CO. [16]
As we all known, heterogeneous catalytic reaction process
involves diffusion, adsorption and surface reaction of reactants
and desorption and diffusion of products. However, a large
number of studies on CO2 catalytic conversion have generally
focused on designing and synthesizing active sites for CO2
hydrogenation. By comparison, reactant adsorption/product
desorption in heterogeneous catalytic process has been
considered scarcely. Adsorption of reactants on the catalyst
surface is a prerequisite to further catalytic reaction. Using the
adsorbents, metal carbide or pyridinic-N doped carbon, which
can capture and enrich reactants, is beneficial to the more
Introduction
occurrence of
subsequent
catalytic
conversion.
For
hydrogenation, CO2 enrichment on catalysts also can overcome
its low solubility in aqueous solution to some extent when CO2 is
converted.[19] On the other hand, to obtain high yield, the
generated product should be apt to desorb in time from the
surface of catalysts to avoid further reactions. That is to say,
desorption energy of CO* is relatively lower than that of other
reactions,[20] while in contrast, hydrogenation activity is moderate
in this case. The d-band center shift tremendously influences the
energy for dissociative adsorption of adsorbents.[21] If the surface
atoms undergo compressive strain force, the d-orbital overlap
increases and d-band center drops. Thus, surface atom can
weakly bind with adsorbents, even resulting in them release. To
achieve compressive strain force between metals, certain
dissimilar metal with the larger radii is required to introduce into
parent metal. Based on these principles, designed catalyst for
CO2 hydrogenation to CO might be composed of dissimilar metal
and alkaline component.
Efficient utility of carbon dioxide, as a cheap and renewable
carbon resource, has become an important topic for the
[a]
Prof. Y. Li, X. Cai, S. Chen, Dr. H. Zhang, Prof. H. Zhang, J. Hong,
Prof. B. Chen
Department of Chemical and Biochemical Engineering, College of
Chemistry and Chemical Engineering
Xiamen University
Xiamen 361005, PR China
E-mail: yunhuali@xmu.edu.cn (Y. Li)
Prof. D-H Kuo
Department of Materials Science and Engineering
National Taiwan University of Science and Technology
No. 43, Sec. 4, Keelung Road, Taipei 10607, Taiwan
Prof. W. Wang
[b]
[c]
School of Energy and Power Engineering
Nanjing University of Science and Technology
Nanjing 210094, China
E-mail: E-mail address: wangwenju1982@163.com
The catalyst performances in the reversible water gas shift
(RWGS) reaction are tremendously affected by the particle size
of metal. Large metallic particles or aggregates generally might
decrease the selectivity of RWGS and contribute to produce
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201800016
ChemSusChem
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[12]
more CH4.
Metal-organic frameworks (MOFs), constituted of
peaks for Ni/ZIF-8-C at 42.8o, 49.8o and 73.1o are assigned to
(111), (200) and (220) reflections of cubic Ni3ZnC0.7 (JCPDF#28-
0713),[30] whereas Fe/ZIF-8-C and Co/ZIF-8-C display the
diffraction peaks corresponding to (111) of Fe (JCPDF#06-
0696) , (031) of Fe3C (JCPDF#35-0772), (111) and (200) facets
of Co (JCPDF#15-0806) and (103) of Co3C (JCPDF#26-0450),
respectively. These data indicate that M/ZIF-8-C (M=Ni, Fe and
Co) catalysts are composed of Ni3ZnC0.7, Fe and Fe3C, Co and
Co3C, respectively, as well as ZnO crystal phase. In contrast,
Cu/ZIF-8-C has only diffraction peaks ascribed to Cu
(JCPDF#04-0836).
the metal ions or clusters and organic ligands,[22, 23] provide the
versatile and abundant choices to design and construct the
nanostructures. It has been proved that MOFs are effective
templates or precursors for the synthesis of porous transition
metal materials. For adsorption, zeolitic imidazolate framework-8
(ZIF-8) treated by ammonia or ethylene diamine has showed
high CO2 adsorption capacity due to the more amounts of
basicity.[24-26] For catalytic hydrogenation, Hu et al. synthesized
Fe2O3 supported ZIF-8 or MIL-53 (Al) for CO2 hydrogenation to
hydrocarbons.[5] Maina et al. prepared the TiO2 and Cu−TiO2
doped ZIF-8 materials to efficiently photocatalyze CO2
conversion to methanol and CO.[27] In the meanwhile, these
studies also confirm that base metal can be efficiently dispersed
in MOF-derived catalysts from conventional preparation
methods.[28]
In this work, trigged by CO2 adsorption capacity and
hydrogenation performance of MOF based material, we
synthesized transitional metal (Fe, Co, Ni and Cu) supported
alkaline ZIF-8 catalysts for CO2 hydrogenation. Results show
that Ni/ZIF-8-C shows high catalytic performance for CO2
hydrogenation to CO due to highly dispersion of metal and
carbide. We mainly focus on the nature of high catalytic
performance of Ni/ZIF-8-C. Study indicates that pyridinic
nitrogen and metal carbide can capture and enrich CO2 while at
the same time the later can also serve as an active site for the
CO2 hydrogenation. Based on the characterizations, DFT
calculation was conducted to understand reaction mechanism of
CO2 selective hydrogenation to CO over Ni3ZnC0.7 of Ni/ZIF-8-C.
Results showed that in comparison with CO* hydrogenation and
dissociation, the lower desorption energy accounts for high CO
selectivity. The synergistic effect of alkaline ZIF-8 carrier,
transitional metal carbide dispersed and weak CO adsorption
brings about the outstanding selectivity for CO2 hydrogenation to
CO. The successful design and synthesis of multifunctional
catalysts are helpful to CO2 catalytic conversion in a large scale.
Figure 1. XRD patterns of M/ZIF-8-C catalysts.
Table 1 presents structure and composition of the supported
samples. As given in Table 1, the base metal and Zn loadings
are about 7.5 wt.% and 24.0 wt.% in all as-prepared M/ZIF-8-C
catalysts, respectively, which means that all the catalysts
possess the similar metal contents.
Table 1. Physiochemical characterization result of M/ZIF-8-C catalysts
Results and Discussion
catalyst
Metal
Zn
Particle size
[a]/nm
C/N[b]
loading/%
content/%
/mole ratio
Physiochemical characterizations
Ni/ZIF-8-C
Fe/ZIF-8-C
Co/ZIF-8-C
Cu/ZIF-8-C
7.38
7.60
6.85
7.61
23.94
24.62
25.31
24.15
10.50
13.10
17.70
19.80
2.09
2.06
2.16
2.02
Figure S1 shows that XRD result of the as-obtained ZIF-8, which
has the similar characteristic peaks to simulated pure phase.[29]
After calcination treatment, diffraction peaks for ZIF-8-C shift
towards higher 2θ values in comparison with ZIF-8, indicating
that calcination at high temperature leads to ZIF-8 lattice
shrinking under the conditions of maintaining the crystal
structure. SEM (Figure S2) also presents that the morphology of
the as-obtained ZIF-8-C through pyrolysis process inherits the
structure of ZIF-8 to some extent. By comparison, there are
numerous small particles distributed homogeneously on M/ZIF-
8-C.
[a] The data are estimated by the Scherrer’s equation from XRD results
[b] C/N ratio comes from element analysis
To depict the alkaline properties on catalyst surfaces, CO2-
TPD of M/ZIF-8-C and ZIF-8-C catalysts were presented in
Figure 2a. The peak around 110 ºC originates from the weak
alkaline site while that in the range from 450 ºC to 750 ºC,
corresponds to the strong one.[31] The ZIF-8-C possesses the
highest peak of CO2 desorption among all the samples. This was
because alkaline sites are associated with the N–H species in
the ZIF-8,[24, 32] which may partially decompose due to catalysis
of base metal in calcination process. Element analysis also
After impregnating the solution of base metal salt, all the as-
prepared samples demonstrate characteristic peaks of 31.7o,
34.4o, 36.2o relative to (100), (002), (101) of ZnO phase
(JCPDF#89-0510)[30] in Figure 1. Additionally, the characteristic
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shows that the molar ratio of C/N is about 2.1 in M/ZIF-8-C
(Table 1), a little higher than the imidazole-derived carbon to
nitrogen ratio 2. It is known that chemisorption and activation of
reactants are prerequisite to catalytic reaction. The addition of
ammonia in synthesis process contributes to the increment of
the alkalinity of catalysts, rendering adsorption and conversion
of reactant CO2 easier. Moreover, with the addition of base
metal, metal carbide, corresponding to desorption peak at 375–
627 ºC,[33] is formed dramatically accompanied by decreasing in
the N-H species. Therefore, adsorption of CO2 over M/ZIF-8-C
partially derives from the existence of metal carbide.[34]
The N 1s XPS spectra of all the catalysts (Figure 3) can be
deconvoluted into three peaks at 398.50, 399.08 and 400.99 eV,
assigned to pyridinic, pyrollic and graphitic N species or N
interacting with metal.[35] In comparison with XPS spectra of N1s
in ZIF-8-C, relative decreasing in pyridinic-N of M/ZIF-8-C leads
to the lower alkalinity sites.[24] This is consistent with CO2-TPD
results. XPS of Ni 2p in Figure S3 shows the coexistence of Ni0
and Ni2+ in Ni/ZIF-8-C.[36]
a
b
Figure 3. XPS spectra of ZIF-8-C and M/ZIF-8-C sample of N 1s.
High-resolution transition electron microscopy (HRTEM)
microphotographs of Ni/ZIF-8-C (Figure 4) were used to further
investigate the size and ingredient of nanocrystalline Ni3ZnC0.7
.
The histogram of the nanoparticles shows that the nanoparticle
size is about 9.64 nm (Figure 4a). Line scan results show that
the abundant elements of the nanoparticle are Ni and Zn (Figure
4b) and contents of them increase simultaneously. Additionally,
there is consensus on the change of carbon and nitrogen
Figure 2. (a) CO2-TPD curves of ZIF-8-C and M/ZIF-8-C catalysts; (b) Raman
spectrum of ZIF-8-C and M/ZIF-8-C catalysts.
The Raman spectrum was conducted to understand the
existence of carbon and nitrogen (Figure 2b). The typical D
bands at 1320 cm−1 and G bands at 1570 cm−1 on all the
catalysts are associated with the sp3 defect sites and bond
stretching of all sp2-bonded pairs, including C–C and N–C,[35]
respectively. The ID/IG of ZIF-8-C, Ni/ZIF-8-C, Fe/ZIF-8-C,
Co/ZIF-8-C and Cu/ZIF-8-C are 0.823, 0.804, 0.793, 0.778 and
0.761, respectively. Compared with ZIF-8-C, the lower ratio of
the peak intensity for M/ZIF-8-C is due to decreasing in defects
from N doping and further graphitization in calcination process.
contents regardless on the surface or surrounding of Ni3ZnC0.7
.
The interplanar spacing of 0.21 nm is corresponding to (111)
lattice planes of Ni3ZnC0.7 on Ni/ZIF-8-C (Figure 4c, d). Energy
dispersive spectroscopy (EDS) element mapping data (Figure
S4) for Ni/ZIF-8-C further demonstrate that loadings of Ni and Zn
vary simultaneously as well as C, O and N element. These
results confirm again the existence of Ni3ZnC0.7 over Ni/ZIF-8-C,
consistent with the XRD tests. Notably, Ni3ZnC0.7 is not highly
crystalline in TEM microphotographs though obvious XRD
diffraction peaks are observed for Ni/ZIF-8-C. The more harsh
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calcination temperature (for example 650°C for 2h) was required
to test the effect of the highly crystalline of Ni3ZnC0.7 on catalytic
performance. By comparison, catalytic performance of Ni/ZIF-8-
C with the highly crystalline is the lower than Ni/ZIF-8-C in this
work (Figure S5). This might be ascribed to not only
agglomeration of particles and the formation of Ni metal phase,
but also further decrement of surface area due to the loss of
carbon and nitrogen element. In addition, uncalcined Ni/ZIF-8-C
without Ni3ZnC0.7 crystalline phase also has the lower catalytic
results than Ni/ZIF-8-C (Figure S5). Therefore, appropriate
crystalline benefits high dispersion of Ni3ZnC0.7.
result implies that it is necessary to the existence of base metals
on the surface of catalysts. Among all supported catalysts,
Ni/ZIF-8-C and Fe/ZIF-8-C show high activity towards CO2
hydrogenation to CO and CO2 conversion are 43.81% and 43.82%
at 420 ºC accompanied with CO selectivity of 97.37% and
97.22%, respectively. Cu/ZIF-8-C shows the lower activity than
other catalysts and CO2 conversion is only 24.66% at 420 ºC.
The turnover frequencies (TOF) of CO2 about Ni/ZIF-8-C,
Fe/ZIF-8-C, Co/ZIF-8-C and Cu/ZIF-8-C are 88.58 μmol*gNi-1*s-1,
-
84.48 μmol*gFe-1*s-1, 60.60 μmol*gCo-1*s-1 and 47.48 μmol*gCu
1*s-1, respectively. To clarify the efficiency of ZIF-8-C based
catalysts, further comparison with literatures were carried out in
Table 2. Obviously, M/ZIF-8-C illustrates higher catalytic results
than most of other work under studied reaction conditions. The
long-run experiment shows that the catalytic performance of the
Ni/ZIF-8-C catalyst remains stable with respect to both CO2
conversion and product selectivity in a stability test for 36 h
(Figure 7). XRD result shows the diffraction intensity of the spent
catalysts is slightly higher than the fresh ones (Figure S6). In
addition, TG and DTG analysis of the ZIF-8-C and Ni/ZIF-8-C in
nitrogen atmosphere were conducted to understand the stability
of the MOFs (Figure S7). From Figure S7, the weight loss occurs
mainly from 600 ºC to 900 ºC . In contrast, it is negligible in the
range of 320 ºC and 420 ºC , at which CO2 hydrogenation was
evaluated.
200
80
60
40
20
0
9.64 3.41 nm
C
N
O
Zn
Ni
a
b
150
100
50
50nm
0
5
10
15
20
Particle size(nm)
0
0
10
20
30
40
50
Position(nm)
c
d
Ni3ZnC0.7
d(111)
a
Ni3ZnC0.7
d(111)
Ni3ZnC0.7
d(111)
Figure 4. (a) TEM image of Ni/ZIF-8-C, (b) TEM-EDS line scan on Ni3ZnC0.7
and (c, d) HRTEM images of Ni/ZIF-8-C.
To further clarify the catalyst structure, high-sensitivity low-
energy ion scattering (HS-LEIS) was conducted. LEIS only
probes the outermost atomic layer.[37] If there is only a carbon
layer on metal particles, then no Ni and Zn signal will be
observed. HS-LEIS analysis (Figure 5) shows that metal
elements present along with carbon and nitrogen on the outer
surface layer or below the surface layer 2.0 nm for Ni/ZIF-8-C.
Combining the above analysis, we believe that Ni3ZnC0.7 is
supported rather than wrapped by N-doped carbon.
b
Catalytic performances
The activity and selectivity of M/ZIF-8-C toward hydrogenation of
CO2 were evaluated in the temperature range between 320 ºC
and 420 ºC and under the conditions of CO2/H2 (1:4) at 101.32
KPa. For comparison, ZIF-8-C and ZnO/C were also used to
catalyze CO2 conversion (Figure 6). Over all the catalysts, CO2
conversion generally increases with the increase of temperature,
indicating that high temperature is favorable to conversion of
CO2. By comparison with metal supported catalysts, CO2
conversion on individual ZIF-8-C or ZnO/C is negligible. This
Figure 5. HS-LEIS spectra (a) Ni/ZIF-8-C at 3 keV 4He+, (b) Ni/ZIF-8-C at 5
keV 20Ne +.
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Table 2. Comparison of CO2 conversion and CO yield of M/ZIF-8-C with
literatures for hydrogenation CO2 to CO.
Ni is dispersed in the form of Ni3ZnC0.7, in which Ni has two
types of valences, Ni0 and Ni2+. This implies that Ni in Ni3ZnC0.7
is highly dispersed in the form of similar single atom in catalysts
despite 9.64 nm particle size of Ni3ZnC0.7. In turn, base metal in
Fe/ZIF-8-C and Co/ZIF-8-C reduced exsits in the form of
individual carbide and metal crystal particle despite the smaller
particle size of Fe/ZIF-8-C. Obviously, the disbenifit of this
existence form brings about the relatively lower dispersion of
active sites than Ni/ZIF-8-C. Moreover, the number of exposed
carbide from CO2-TPD results after reduction is also in
accordance with the order: Ni/ZIF-8-C>Fe/ZIF-8-C>Co/ZIF-8-
C>Cu/ZIF-8-C. Combining with catalytic performances, we
believe that hydrogenation activity of Ni/ZIF-8-C and Fe/ZIF-8-C
result from higher dispersion of metal species and the more
exposure of carbide.
No
Catalyst
Reaction conditions
XCO2/
%
YCO
/%
Ref.
-1
1
2
3
4
Au@UIO-
67
0.2 g, GHSV: 12000 mL h-1 gcat
CO2:H2 = 1:3, 2.0 MPa, 408 °C
;
30.2
34.0
12.4
42.5
28.6
19.7
10.2
34.0
38
39
40
11
-1
Ni/Ce-Zr-O
0.5 g, GHSV: 6000 mL h-1 gcat
CO2:H2 = 1:1, 1.0 Mpa, 550 °C
-1
NiO/CeO2
1.0 g, GHSV: 14400 mL h-1 gcat
CO2:H2 = 1:1, 1.0 Mpa, 450 °C
Ni-IMP
/nSiO2
12 mg, GHSV: 250000 mL h-1
gcat-1CO2:H2:Ar = 1:4:45, 1.0 Mpa,
420 °C
-1
5
6
7
BaCexZr0.8
−xY0.16
Zn0.04O3
0.5 g, GHSV: 2400 mL h-1 gcat
CO2:H2 = 1:1, 1.0 Mpa, 600 °C
36.3
20.9
21.0
34.8
20.7
20.8
41
42
12
-1
K-Pt
/mullite
0.1 g, GHSV: 30000 mL h-1 gcat
CO2:H2:Ar = 45:45:10, 0.1 Mpa,
425 °C
-1
Pt/TiO2
0.5 g, GHSV: 6000 mL h-1 gcat
CO2:H2:Ar = 45:45:10, 0.1 Mpa,
400 °C
-1
8
9
Ni/ZIF-8-C
Fe/ZIF-8-C
0.2 g, GHSV: 15000 mL h-1 gcat
;
;
43.8
43.8
42.7
42.6
This
work
CO2:H2 = 1:4, 0.1 MPa; 420 °C
0.2 g, GHSV: 15000 mL h-1 gcat
-1
This
CO2:H2 = 1:4, 0.1 MPa; 420 °C
work
a
Figure 7. CO2 conversion and CO and CH4 selectivity of Ni/ZIF-8-C with time-
on-stream. Reaction conditions: 0.2 g of catalyst, 380 ºC, CO2/H2 = 1:4, GHSV
=15000 mL/(g.h).
Reaction mechanism of CO2 hydrogenation
It’s generally accepted that CO2 methanation occurs over the
traditional Ni supported catalysts.[43, 44] However, a large amount
of CO product was obtained for CO2 hydrogenation over Ni/ZIF-
8-C. CO2 hydrogenation to CO can proceed via the
decomposition of carboxyl intermediate (COOH*) from CO2
hydrogenation into CO* and OH*. Alternatively, CO2* can
dissociate into CO* and O* via the direct C−O bond cleavage
pathway. The obtained CO* is consumed through three ways. It
is either hydrogenated to COH* or CHO*, or desorbed into CO
gas, or further dissociated to C* and O*. CO selectivity is
primarily determined by the relative rate of these three
processes. When the rate of CO* desorption is faster than the
others, a large amount of CO product is generated for CO2
hydrogenation. Conversely, when CO* is strongly bound,
hydrogenation or dissociation likely occur than desorption.[20, 45,
46] Thus, the catalyst is likely to show lower CO selectivity.
b
Figure 6. (a) CO2 conversion and (b) CO, CH4 selectivity of M/ZIF-8-C.
Reaction conditions: 0.2 g of catalyst, 320 ºC-420 ºC, CO2/H2 = 1:4, GHSV
=15000 mL/(g.h).
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Figure 8. Reaction coordinate of CO2 hydrogenation
To understand the essence of high CO selectivity, DFT
calculations were conducted for CO2 selective hydrogenation
reactions over Ni3ZnC0.7 supported N-modified carbon (Figure 8).
Direct CO2* dissociation to CO* has lower activation energy than
COOH* decomposition. Product CO mainly comes from the
direct cleavage of C-O band in CO2*. Activation energy of CO*
hydrogenation to COH* or CHO* is 1.82 or 1.45 eV for direct
hydrogenation and that of CO* dissociation to C* and O* is 2.68
eV, respectively, while that for desorption of CO* is 1.36 eV. By
comparison, energy required for desorption CO* is lower than
hydrogenation and decomposition, indicating that CO* is facile to
desorb from the surface rather than further conversion. Better
routes to yield CO are as follows: adsorbed CO2* firstly
dissociates into CO* and O*, which is further hydrogenated via
OH* to H2O*. After that, CO* releases from the surface of
catalysts into gas phase CO. This accounts for high CO
selectivity over Ni/ZIF-8-C. As the radius of Ni atom is smaller
than Zn, with the Zn addition, the surface atoms undergo
compressive strain force in Ni3ZnC0.7 and the d-orbital overlap
increases.[21] This weakens the adsorption strength of CO* with
Ni3ZnC0.7. In situ CO FTIR result also confirmed that the peak
position of the linear-bonded CO over Ni/ZIF-8-C has higher
wavenumber than the traditional CO adsorption, about 2100 cm-
Conclusions
Combining CO2 adsorption properties of ZIF-8 and uniformly
dispersed metal and carbide, M/ZIF-8-C was prepared from
precursor solution of base metal impregnating calcined ZIF-8.
Under the conditions of the similar metal loadings, Ni/ZIF-8-C
and Fe/ZIF-8-C present higher catalytic activity for CO2
hydrogenation to CO than Co/ZIF-8-C and Cu/ZIF-8-C.
Dispersed Ni or Fe species in catalysts catalyzes hydrogenation
while pyridinic nitrogen and carbide interacts as an adsorbent for
CO2 enrichment and activation. DFT calculation shows that
generated CO is mainly from direct CO2* dissociation. Since
CO* generated is apt to desorb from the surface of Ni/ZIF-8-C in
comparison with CO* further reactions, high CO selectivity is
obtained. Comparison of catalytic performances over Ni/ZIF-8-C
with ZIF-8-C in the absence of nickel shows that synergistic
effect among dispersed metal carbide and weak CO adsorption
accounts for high CO selectivity from CO2 hydrogenation. These
results will benefit to design and synthesize various catalysts
involving CO2 conversion and utility to chemicals, such as
hydrocarbons and cyclic carbonate.
1
(Figure S8). This result indicates that the CO–catalyst bond is
elongated and the intensity of CO adsorption is relatively weak
on Ni/ZIF-8-C.
Experimental Section
Herein, the activation of the adsorbed CO2 occurs on N-doped
carbon or metal carbide along with H2 activation over carbide
species surrounded by N-doped carbon. Consequently,
hydrogen activated by metal carbide reacts with dissociated
oxygen atom from CO2 to convert into H2O* and CO*. Finally,
generated CO* desorbs from the surface of catalysts into CO in
gas phase. Thus, high CO selectivity is obtained from CO2
hydrogenation over Ni/ZIF-8-C.
Calculation Details
Density functional theory with empirical dispersion corrections
(DFT-D) calculations was executed to evaluate reaction
mechanism of CO2 selective hydrogenation on the periodic
Ni3ZnC0.7/C5N1 surface (see Calculation section in Supporting
Information for detail). The specific calculation details were given
in literature.[47] For the surface structure, two atomic layers of
Ni3ZnC0.7 were cleaved at the 2D C5N1 plane. By the calculation
of adsorption energy, Ni3ZnC0.7/C5N1 features are clarified. The
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10.1002/cssc.201800016
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linear synchronous transit (LST) calculation was achieved
according to literature.[48]
catalysts were reduced at 450 ºC for 2 h with H2. The carbon
dioxide temperature-programmed desorption (CO2-TPD) was
conducted on a Micromeritics Autochem II 2920 instrument.
Prior to the measurement, the sample was prereduced with 50
vol.% H2/N2 at 450 ºC for 2 h. When the sample was cooled to
50 ºC, the gas flow was switched to CO2 (99.99%, purity) for
adsorption. After flushed with argon, the sample was heated up
for CO2 desorption. Raman spectrum measurements were
carried out using a Raman spectrometer (Model 2000, Renishaw,
UK). Prior to the measurement, the sample was prereduced with
H2 (99.999%, purity) at 450 ºC for 2 h and exposed in the 2 vol.%
O2/N2 for 1 h. CO Fourier transform infrared spectroscopy (CO-
FTIR) of the catalysts was in situ collected in a reactor cell
placed in a Bruker Vertex 70 FTIR spectrometer.
Synthesis of ZIF-8 and ZIF-8-C
ZIF-8 was synthesized by the impregnation method according to
the literature.[29] Typically, 5.2520 g zinc acetate dihydrate was
first dissolved in 24.0 mL deionized water and 3.2850 g 2-
methyl-1H-Imidazole was dissolved in 48.5 mL ammonia
solution (25 wt.%). Then, zinc acetate solution was added
dropwise into 2-methyl-1H-Imidazole one. The as-obtained
sample was vigorously mixed at room temperature for 4 h before
centrifugation and wash with deionized water. Subsequently, the
samples were dried at 65 ºC in air overnight. The resulted
sample is denoted as ZIF-8. Then it was calcined at 450 ºC for 1
h under N2 at a heating rate of 5 ºC/min and 550 ºC at a heating
rate of 2 ºC/min for another 1 h. The resulted sample is ZIF-8-C.
Catalytic reactions
The CO2 hydrogenation performances over M/ZIF-8-C catalysts
were evaluated with a fixed-bed reactor under atmospheric
pressure. Typically, 0.2 g catalyst was loaded into a quartz
reactor (i.d.13 mm). Prior to the catalytic testing, the catalyst
was pretreated at 450 ºC for 2 h with H2. In catalytic reaction
process, the feed gas containing 20 vol.% CO2, 80% vol.% H2
Preparation of M/ZIF-8-C and ZnO/C
A series of 10% M/ZIF-8-C catalysts were prepared by ethanol
solution of Ni(acac)2 (Fe(acac)3, Co(acac)3 or Cu(acac)2)
impregnating ZIF-8-C. Typically, 0.7224
g Ni(acac)2 was
dissolved in 150.0 mL ethanol to form a dark green solution at
60 ºC. Subsequently, 1.5000 g ZIF-8-C was dissolved into the
above solution under continuous agitation. After stirred for 16 h
under air atmosphere, the products were collected by rotary
evaporation at 60 ºC. Finally, the obtained claybank solid, 10%
Ni/ZIF-8-C precursor was dried at 60 ºC in air overnight, after
that, the sample was calcined at 450 ºC for 1 h and 550 ºC for
another 1 h under inert atmosphere (N2) to obtain Ni/ZIF-8-C.
The ZnO/C catalyst was prepared through calcination the ZIF-8
at 450 ºC for 1 h under air atmosphere.
was introduced with
a flow rate of 50 mL/min. K-type
thermocouples at the central axis and the outside of the reactor
were tested and controlled the reaction temperature,
respectively. The effluent gas products were analyzed by an on-
line gas chromatograph (GC9010) with two columns, Porapak Q
and Molecular Sieve 5Å and a thermal detector. The turnover
frequency of CO2 (TOF, defined as moles of CO2 converted per
g of M in the catalyst per second) was calculated on the basis of
CO2 conversion.
Acknowledgements
Catalyst characterization
This work was supported by National Natural Science
Foundation of China (Grant No. 21476188, 21106118).
Analysis of metal element in the catalysts was performed on
inductive couple plasma (ICP, Agilent ICP-MS 4500–300).
Carbon and nitrogen element analysis was carried out with an
elemental analyzer (Vario ELIII, Elementar). X-ray diffraction
Conflicts of interest
(XRD) patterns were recorded by
a Rigaku Ultima IV
There are no conflicts to declare
diffractometer with Cu Ka radiation at 35 kV, 15 mA at a scan
rate of 10º/min. Prior to the measurement, the sample was
prereduced with H2 (99.999%, purity) at 450 ºC for 2 h and
exposed in the 2 vol.% O2/N2 for 1 h. Scanning electron
microscope (SEM) images of catalysts were acquired with a
Hitachi S-4800 FE-SEM. High-resolution transition electron
microscopy (HRTEM) images were collected using a TECNAI
F30 (300 kV) instrument. Energy dispersive spectroscopy (EDS)
line-scan profile result and elemental mapping were also
obtained by using a TECNAI F30 electron microscope. X-ray
photoelectron spectroscopy (XPS) tests were obtained by the
PHI Quantum 2000 Scanning ESCA Microprobe equipment.
High sensitivity-low energy ion scattering (HS-LEIS) tests of
the samples were conducted on an Ion-TOF Qtac100 low-
energy ions scattering analyzer with a 20Ne+ beam energy of 5
keV and a 4He+ beam energy of 3 keV. Prior to the test, the
Keywords: carbide • zeolitic imidazolate framework-8 •
adsorption • CO2 selective hydrogenation • DFT
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Yunhua Li,*[a] Xiaohu Cai,[a] Sijing Chen,[a] Hua Zhang,[a] Hongliang Zhang,[a] Jinqing Hong,[a] Binghui
Chen,[a] Dong-Hau Kuo,[b] Wenju Wang†[c]
Title
Highly dispersed metal carbide over ZIF-derived pyridinic-N
doped carbon for CO2 enrichment and selective hydrogenation
[a]
Prof. Y. Li, X. Cai, S. Chen, Dr. H. Zhang, Prof. H. Zhang, J. Hong,
Prof. B. Chen
Department of Chemical and Biochemical Engineering, College of
Chemistry and Chemical Engineering
Xiamen University
Xiamen 361005, PR China
E-mail: yunhuali@xmu.edu.cn (Y. Li)
Prof. D-H Kuo
Department of Materials Science and Engineering
National Taiwan University of Science and Technology
No. 43, Sec. 4, Keelung Road, Taipei 10607, Taiwan
Prof. W. Wang
[b]
[c]
School of Energy and Power Engineering
Nanjing University of Science and Technology
Nanjing 210094, China
E-mail: E-mail address: wangwenju1982@163.com
Supporting information for this article is given via a link at the end of
the document.
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