Atomically Dispersed Ni/α-MoC Catalyst for Hydrogen Production from Methanol/Water

Article abstract of DOI:10.1021/jacs.0c10776

Methanol-water reforming is a promising solution for H2 production/transportation in stationary and mobile hydrogen applications. Developing inexpensive catalysts with sufficiently high activity, selectivity, and stability remains challenging. In this paper, nickel-supported over face-centered cubic (fcc) phase α-MoC has been discovered to exhibit extraordinary hydrogen production activity in the aqueous-phase methanol reforming reaction. Under optimized condition, the hydrogen production rate of 2% Ni/α-MoC is about 6 times higher than that of conventional noble metal 2% Pt/Al2O3 catalyst. We demonstrate that Ni is atomically dispersed over α-MoC via carbon bridge bonds, forming a Ni1-Cx motif on the carbide surface. Such Ni1-Cx motifs can effectively stabilize the isolated Ni1 sites over the α-MoC substrate, rendering maximized active site density and high structural stability. In addition, the synergy between Ni1-Cx motif and α-MoC produces an active interfacial structure for water dissociation, methanol activation, and successive reforming processes with compatible activity.

Full text of DOI:10.1021/jacs.0c10776

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Atomically Dispersed Ni/α-MoC Catalyst for Hydrogen Production
from Methanol/Water
Lili Lin,● Qiaolin Yu,● Mi Peng,● Aowen Li,● Siyu Yao, Shuheng Tian, Xi Liu, Ang Li, Zheng Jiang,
Rui Gao,* Xiaodong Han, Yong-wang Li, Xiao-dong Wen, Wu Zhou,* and Ding Ma*
Cite This: J. Am. Chem. Soc. 2021, 143, 309−317
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ABSTRACT: Methanol−water reforming is a promising solution for
H2 production/transportation in stationary and mobile hydrogen
applications. Developing inexpensive catalysts with sufficiently high
activity, selectivity, and stability remains challenging. In this paper,
nickel-supported over face-centered cubic (fcc) phase α-MoC has
been discovered to exhibit extraordinary hydrogen production activity
in the aqueous-phase methanol reforming reaction. Under optimized
condition, the hydrogen production rate of 2% Ni/α-MoC is about 6
times higher than that of conventional noble metal 2% Pt/Al2O3
catalyst. We demonstrate that Ni is atomically dispersed over α-MoC
via carbon bridge bonds, forming a Ni1−Cx motif on the carbide
surface. Such Ni1−Cx motifs can effectively stabilize the isolated Ni1
sites over the α-MoC substrate, rendering maximized active site
density and high structural stability. In addition, the synergy between Ni1−Cx motif and α-MoC produces an active interfacial
structure for water dissociation, methanol activation, and successive reforming processes with compatible activity.
MoC, is highly active for this reaction at low temperatures. The
efficient C−H and O−H activation ability and the highly
effective interfacial water gas shift activity of Pt/α-MoC render
to the catalyst an excellent hydrogen evolution rate of 18 000
h−1.10 Despite the excellent activity, the high cost and limited
reserve of noble metals are still the big obstacles for large-scale
applications. Therefore, developing efficient non-noble-metal
catalytic systems capable of working in an aqueous phase with
high reactivity is highly desirable but challenging.
Herein, a novel Ni/α-MoC catalyst is reported to effectively
catalyze the methanol reforming reaction with high activity and
selectivity in aqueous solution below 240 °C. Nickel is
atomically dispersed over an α-MoC surface, creating isolated
Ni sites anchored over α-MoC host via carbon bridge bonds,
forming Ni1−Cx motifs with ultrahigh site density (up to 2 wt
% loading of Ni). The 2% Ni/α-MoC catalyst shows a high
hydrogen evolution activity of 171 μmolH2/gcat/s (ATOF =
1805 h−1) at 240 °C, which is around 6 times higher than that
of the conventional 2% Pt/Al2O3 and the most active non-
noble-metal catalyst reported so far.13 The high activity is
INTRODUCTION
■
Hydrogen transportation, distribution, and storage at different
scales in an efficient and reliable way remains technically and
economically challenging,1−3 which hinders the extensive
utilization of polymer electrolyte membrane fuel cells
(PEMFCs) in stationary and mobile applications.4,5 It has
been proposed that hydrogen stored in a chemical form and
released in situ on demand through efficient catalytic processes
provides a promising solution for hydrogen storage and
application.6−8 Compared with other liquid organic (such as
quinoline and carbazole, etc.) and inorganic (such as NH3 and
borane, etc.) hydrogen carriers, methanol, an important
inexpensive bulk chemical, is able to react with water and
release 3-fold H2 (CH3OH + H2OCO2 + 3H2) under
relatively mild condition, achieving a high H2 gravimetric
density of 18.8% by weight. The well-balanced high H2 storage
capacity and mild H2 release condition make methanol one of
the most attractive liquid forms of H2 carrier candidates.9,10
Methanol reforming in the aqueous phase can remove the
tedious vaporization units from the hydrogen production
device, thus improving economic efficiency. It can also take
advantage of intensified water gas shift reaction (WGSR) to
effectively suppress the selectivity toward byproduct CO.11−13
The pioneer works have demonstrated that hydrogen can be
generated through aqueous-phase methanol reforming over
platinum- and ruthenium-based noble metal catalysts.9,13,14
Recently, we proposed that a new noble metal catalyst, Pt/α-
Received: October 12, 2020
Published: December 28, 2020
https://dx.doi.org/10.1021/jacs.0c10776
J. Am. Chem. Soc. 2021, 143, 309−317
© 2020 American Chemical Society
309

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Figure 1. Structure characterization of the as-prepared Ni/α-MoC catalysts. (a) XRD profiles of the Ni/α-MoC with different Ni loadings. (b) In
situ XPS of 0.5%, 2%, and 7% Ni/α-MoC catalysts. The Ni 2p, C 1s, and Mo 3d regions are presented. (c) Ni K edge XANES spectra of 0.5%, 2%,
3%, 5%, and 7% Ni/α-MoC catalysts along with Ni foil and Ni oxide. The enlarged XANES white-line and pre-edge region are shown in the right
panel. (d) Ni K edge EXAFS spectra and fittings of 0.5%, 2%, 3%, 5%, and 7% Ni/α-MoC catalysts.
attributed to the synergy effect of the Ni1−Cx motif and α-
MoC host with highly effective water dissociation capability.
possibly an indication of charge transfer from Ni to α-MoC
and suggests that the electronic structure of the Ni species is
modulated by α-MoC.19 With increasing Ni loading, the shift
of the Ni binding energy reduces to 0.3 eV at 7% Ni/α-MoC
(Figure 1b, inset), due to less average charge being transferred
to the substrate and the emergence of more metallic character
of the Ni species. This is in good agreement with our
theoretical calculations that isolated Ni atoms transfer more
charges to the α-MoC substrate per Ni atom than those in Ni
particles (Figure S1).
RESULTS AND DISCUSSION
■
α-MoC was prepared from MoO3 through successive NH3
ammonization and CH4/H2 reduction treatments.10,15 Ni
precursors were deposited over the as-synthesized molybde-
num carbide support using a wetness impregnation method
under the protection of Ar. After the carburization activation at
590 °C in the flow of a CH4−H2 mixture, the as-prepared Ni/
α-MoC catalysts were obtained. The exact Ni loadings were
determined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES, Table S1). The X-ray diffraction
(XRD) profiles of the Ni/α-MoC catalysts show only the
diffractions of α-MoC (Figure 1a). Even when the loading of
Ni was increased to 7 wt %, no diffraction peaks associated
with bulk Ni crystal were observed, indicating that the Ni
species are highly dispersed over the substrate with the
crystalline domains below the detection limit of XRD.16,17
The Ni 2p X-ray photoelectron spectroscopy (XPS) results
reveal that overall the Ni species in those samples are closer to
metallic Ni. However, a positive shift of the Ni 2p binding
energy was observed over 2% Ni/α-MoC, which is 0.6 eV
higher than that of metallic Ni(0)18 as shown in Figure 1b
(853.3 vs 852.7 eV). Although the final state effects may affect
the binding energy shift in XPS, the α-MoC support used here
is a conductive material, and therefore, the final state effect
(screening) is less important compared to the nonconductive
substrate. Therefore, it is highly possible that the blue shift is
The charge transfer and interaction between Ni and α-MoC
was further confirmed by the X-ray absorption fine structure
spectroscopy (XAFS) experiments. The Ni K edge (8333 eV)
X-ray absorption near-edge structure (XANES) spectra of all
the Ni/α-MoC catalysts show stronger white-line intensity and
slightly blue-shifted edge position as compared to Ni foil
(Figure 1c; notice that they are different from that of NiO),
demonstrating that the supported Ni is close to the metallic Ni
but under an electron-deficient state (namely, Niδ+).20,21
A
lower white-line intensity was observed over the 7% Ni/α-
MoC catalyst, as compared to that of 0.5% Ni/α-MoC,
suggesting the recurrence of the metal character.22
We further investigated the local chemical environment of
the Ni species by analysis of the extended X-ray absorption fine
structure (EXAFS). Significantly, no Ni−Ni first-shell scatter-
ing was resolved in the EXAFS profiles of the 0.5% and 2% Ni/
α-MoC catalysts (Figure 1d), indicating that nearly 100% of
the Ni species are isolated Ni atoms over α-MoC in these two
catalysts. For the 0.5% and 2% Ni/α-MoC catalysts, the major
310
https://dx.doi.org/10.1021/jacs.0c10776
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Figure 2. Electron microscopy characterization of the 2% Ni/α-MoC catalyst. (a) Low-magnification STEM-HAADF image. High-magnification
(b) STEM-HAADF and (c) STEM-BF images. (d) STEM-HAADF image and the corresponding EDS elemental maps of (e) Ni and (f) Mo of 2%
Ni/α-MoC. Ni is homogeneously distributed within the sample. (g) STEM-HAADF image and (h) the corresponding EELS Ni map. Ni single
atoms are indicated by the green circles. An occasional Ni cluster is indicated by the blue circle. (i) Overlay of the distribution of Ni on α-MoC. (j)
The enlarged Ni elemental map and the corresponding pixel index of the area marked by the green dashed square in panel h. The lateral size of
each pixel is 0.15 nm. (k) The EELS spectra extracted from the pixels with the corresponding colors in panel j, demonstrating that Ni signals are
localized in the central pixel as expected for an atomically dispersed Ni atom. (l) Comparison of the sum spectrum from 33 Ni single atoms and the
reference spectra of metallic Ni and NiO.
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a
Table 1. Catalytic Performance of Aqueous Methanol Reforming over Nickel-Based Catalysts
b
c
entry
1
2
catalysts
temp (°C)
mass-specific activity μmolH2/g/s
ATOF
selectivity (CO) %
2% Ni/α-MoC
2% Pt/Al2O3
2% Ni/MoO3
2% Ni/MoO2
2% Ni/TiO2
2% Ni/SiO2
Ni(acac)2
240
240
240
240
240
240
240
280
340
250
171
30.7
4.94
8.35
5.8
1805
1077
33
54
43
0.7
0.4
d
3
4
5
6
7
11.5
15.7
16.6
26.8
11.6
28.9
29.0
1.0
7.8
49
9.2
e
f
8
e
NiAl−Au (SRM)
54.3
128.7
109
f
9
NiAl-Au (SRM)
e
g
10
Cu/ZnO/Al2O3 (SRM)
a
Reaction conditions: n(CH3OH)/n(H2O) = 1:1, 50 mL of total volume of liquid, 100 mg of activated catalysts; 2 MPa of N2 (10% Ar as internal
b
standard). All the conversions are controlled below 15%. Detailed experimental parameters are shown in the Supporting Information. Catalytic
performance is calculated as the hydrogen production per gram of catalyst per second. The unit is μmolH2/gcat/s. ATOF means average turnover
frequency number; the unit is molH2/molmetal/h. The molar amount of metal is the amount of loaded metal determined by ICP-AES. The
hydrogen production activity of 2% Pt/Al2O3 is in good agreement with the reported value in literature (ref 13). The reaction data was collected
c
d
e
f
g
from refs 27 and 30. SRM: steam reforming of methanol. Ref 27. Refs 28 and 30.
neighbors of Ni are C and Mo (Table S2, Figure S2). The Ni−
C bond length is around 2.01 Å, while the Ni−Mo
coordination shells below 2.70 Å and above 2.85 Å are
attributed to the Mo atoms located at the subsurface of C
atoms (see models I and II in Figure S3). The presence of Ni−
C coordination over the 0.5% and 2% Ni/α-MoC catalysts
indicates that the isolated Ni atoms are anchored over the α-
MoC surface with carbon as bridges (termed as the Ni1−Cx
motif) (see possible configurations in Figure S3). As Ni tends
to form nickel carbide in a carbon-rich environment,23,24 the
strong bonding between Ni and carbon can be expected. The
density functional theory (DFT) calculations also confirm that
the Ni atoms connecting with α-MoC through carbon bridges
are energetically favorable (see Table S3). When the loading of
Ni is sufficiently low (in this case, no more than 2%), the
strong nickel−carbon bonding confers to the Ni/α-MoC
catalyst a unique conformation, with the isolated Ni species
anchoring over α-MoC via the Ni1−Cx motifs. This unique
arrangement of Ni1 species over molybdenum carbide has not
been reported before and is different from that of Pt/α-
MoC,10,19 in which Pt is atomically dispersed over the α-MoC
substrate with only the Pt−Mo coordination. When the Ni
loading is increased to 3% and 5%, the Ni−Ni scattering
emerges with coordination numbers (CNNi−Ni) of 0.9 and 3.4,
respectively, suggesting the formation of Ni clusters or even
small nanoparticles. As these Ni−Ni coordination numbers are
still small, it is reasonable to attribute them to the occasionally
observed small Ni particles in the scanning transmission
electron microscopy (STEM) images (see discussions below),
while the majority of the Ni species in the 3% and 5%Ni/α-
MoC are still atomically dispersed Ni1−Cx motifs.
The elemental mapping results (Figure 2, parts e and f)
indicate that Ni is homogeneously distributed on the surface of
α-MoC without obvious aggregation. Similar homogeneous Ni
distribution was also observed in the 0.5% Ni/α-MoC sample
as shown in Figure S4.
Since Ni has a lower atomic number than Mo, it is hard to
unambiguously visualize Ni atoms on the α-MoC substrate in
the Z-contrast HAADF images.20 Instead, we performed
aberration-corrected STEM imaging with simultaneous
atomic-resolution electron energy-loss spectroscopy (EELS)
mapping. As the L2,3 peaks of Ni do not overlap with the core-
loss peaks of Mo and C in EELS,25 it gives us the opportunity
to “see” the light transition metal atoms on the heavy substrate
and even analyze their electronic structure via EELS fine
structures. As shown in Figure 2h, the majority of the Ni
loading is atomically dispersed on the α-MoC substrate in the
2% Ni/α-MoC sample, with some small subnanometer clusters
observed very occasionally. Parts j and k of Figure 2 illustrate
the atomic-scale EELS mapping results from one of the
isolated Ni atoms. As shown by the extracted spectra in Figure
2k, Ni L signals are clearly observed at the central pixel (red),
while the signal almost vanishes at the surrounding pixels 0.15
nm apart. This indicates that the Ni signal is highly localized
within a single mapping pixel, as in the form of an atomically
dispersed Ni atom (Figure S5). In order to analyze the valence
state of these isolated Ni atoms, we turned to the fine structure
of Ni L edge EELS. The L3/L2 ratio in EELS is a commonly
used parameter to characterize the valence state of transition
metals.26 As shown in Figure 2l and Figure S6, the sum
spectrum from 33 isolated Ni1 atoms in 2% Ni/α-MoC
matches better to the reference spectrum of metallic Ni than
NiO, indicating that the atomically dispersed Ni species are
closer to the metallic state, consistent with the XAFS result.
When the Ni loading is increased to 5% and 7%, in addition to
the Ni single atoms, we start to observe aggregates of Ni
clusters and the formation of Ni nanoparticles that are 2−5 nm
in size (Figure S7).
We further carried out scanning transmission electron
microscopy (STEM) experiments to analyze the structure of
the 2% Ni/α-MoC catalyst in real space at the atomic scale. A
nanoporous structure is clearly demonstrated by the large-scale
Z-contrast STEM image (Figure 2a). The high-magnification
high-angle annular dark-field (HAADF) image (Figure 2b) and
the simultaneously acquired bright-field (BF) image (Figure
2c) show that the porous structure consists of aggregates of α-
MoC particles with 5−10 nm size. To identify the spatial
distribution of Ni species on the α-MoC, we performed Z-
contrast STEM-HAADF imaging with simultaneous character-
istic X-ray energy-dispersive spectroscopy (EDS) mapping.
We evaluated the catalytic performances of a few transition
metal/α-MoC catalysts in the aqueous-phase reforming of
methanol (APRM) reaction for hydrogen production.
Compared with the bare supports, only the introduction of
nickel exhibited significant activity promotion among the
tested 3d metals, with 1% Ni/α-MoC showing a more than 5
312
https://dx.doi.org/10.1021/jacs.0c10776
J. Am. Chem. Soc. 2021, 143, 309−317