NiCu bimetallic catalysts derived from layered double hydroxides for hydroconversion of n-heptane

Article abstract of DOI:10.1016/j.cclet.2021.08.120

Supported NiCu bimetallic catalysts have been produced in-situ on commercial Al₂O₃ by using layered double hydroxides as precursors. The resulting catalysts show a uniform Ni and Cu distribution, thus providing good activity and selectivity in the reforming reaction of n-heptane. The catalytic performance has been found to depend on the Cu/Ni ratio, revealing the synergic catalysis between homogeneously dispersed Ni and Cu sites. The good catalysis of NiCu bimetallic catalysts makes it possible to partly or even completely replace Pt with NiCu bimetallic catalysts.

Full text of DOI:10.1016/j.cclet.2021.08.120

Edited by Editor Name: Mrs J Wang
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NiCu bimetallic catalysts derived from layered double hydroxides for
hydroconversion of n-heptane
Yanru Zhu , Minghuan Yang , Zhen Zhang , Zhe An , Jian Zhang ,
Xin Shu , Jing He
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DOI:
Reference:
S1001-8417(21)00702-6
https://doi.org/10.1016/j.cclet.2021.08.120
CCLET 6702
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Chinese Chemical Letters
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Accepted date:
9 May 2021
29 July 2021
29 August 2021
Please cite this article as: Yanru Zhu , Minghuan Yang , Zhen Zhang , Zhe An , Jian Zhang ,
Xin Shu , Jing He , NiCu bimetallic catalysts derived from layered double hydroxides for hydroconver-
sion of n-heptane, Chinese Chemical Letters (2021), doi: https://doi.org/10.1016/j.cclet.2021.08.120
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Communication
NiCu bimetallic catalysts derived from layered double hydroxides for
hydroconversion of n-heptane
Yanru Zhu1, Minghuan Yang1, Zhen Zhang1, Zhe An1, Jian Zhang1, Xin Shu1, Jing He1,
1State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of
Chemical Technology, Beijing 100029, China
Edited by Editor Name: Mrs J Wang
*Corresponding author. Prof. Jing He, Beijing University of Chemical Technology, Beijing
100029, China, E-mail: jinghe@263.net.cn, hejing@mail.buct.edu.cn
Graphical abstract
NiCu bimetallic catalyst with uniform NiCu dispersion has been demonstrated to provide higher activity than Pt catalyst while with similar selectivity in the
reforming reaction of n-heptane. The synergies between homogeneously dispersed Ni and Cu sites account for the enhancement of activity and selectivity.
ARTICLE INFO
ABSTRACT
Article history:
Received 9 May 2021
Received in revised form 29 July 2021
Accepted 23 August 2021
Available online xxx
Supported NiCu bimetallic catalysts have been produced in-situ on commercial Al2O3 by using
layered double hydroxides as precursors. The resulting catalysts show a uniform Ni and Cu
distribution, thus providing good activity and selectivity in the reforming reaction of n-heptane.
The catalytic performance has been found to depend on the Cu/Ni ratio, revealing the synergic
catalysis between homogeneously dispersed Ni and Cu sites. The good catalysis of NiCu
———

2
bimetallic catalysts makes it possible to partly or even completely replace Pt with NiCu
bimetallic catalysts.
Keywords:
n-Heptane reforming
NiCu bimetallic catalysts
Synergic catalysis
Uniform distribution
Layered double hydroxides
Supported Pt is a common catalyst for the catalytic reforming
reaction [1-4]. Considerable efforts have recently been dedicated
to the development of low-cost and highly efficient non-platinum
reforming catalysts, and great progress has been made with
tungsten carbide [5], Mo-based catalysts such as Mo2C [6],
MoOxCy [7] and MoOx [8-10], as well as supported Ni [11-13]
and Co [13] catalysts. The great challenges with non-platinum
reforming catalysts originate from substantial hydrogenolysis and
cracking reactions [14], which result in low isomerization
selectivity. Increasing the dispersion of supported metals has
been found to contribute to isomerization selectivity [15]. For
example, Ni-La/HY catalyst exhibits higher isomerization
selectivity than Ni/HY in the hydroconversion of n-C8 because
the metal dispersion has been improved by rare earth addition
[15]. Yet the reforming performance of non-noble metal catalyst
has not been reported to reach that of Pt catalyst. The bimetallic
catalyst, in which small amount of platinum is used together with
non-noble metal components such as Ni [16,17] or Cu [18], is till
now a good choice to obtain good activity and selectivity while
reduce the cost of catalysts. For example, with proper Pt
proportions, the Ni-Pt bimetallic catalysts afford higher activity
and are more selective to di-branched alkanes than Pt
monometallic catalyst in the hydroconversion of n-C6 reaction
[17]. The complete replacement of platinum with transition
metals in the paraffin reforming process remains a great
challenge.
calcination and reduction process as for NiCux-ZnAl-
LDO@Al2O3.
The powder X-ray diffraction (XRD) patterns pattern for each
NiCux-ZnAl-LDHs@Al2O3 sample (Fig. 1A and Fig. S1 in
Supporting information) clearly illustrates the [003], [006] and
[012] reflections characteristic of the structure of hydrotalcites
[25], indicating the formation of LDHs on the γ-Al2O3 support.
No other crystalline phases were detected in addition to γ-Al2O3
support and LDHs. Flake-shaped NiCuZnAl-LDHs crystallites
can be observed from the SEM image (Fig. 1B) as growing in
interlaced direction on the surface of Al2O3. As can be seen from
the cross section of NiCuZnAl-LDHs@Al2O3 (Fig. 1C), LDHs
phase grows on the Al2O3 surface in a thickness of about 292 nm,
with the edges of LDH slabs tightly adhered to Al2O3 spheres.
The growths of LDHs and the corresponding LDO results in the
specific surface area increasing from 187 m2/g (for γ-Al2O3) to
218 m2/g (for NiCu1.19-ZnAl-LDHs@Al2O3) or 212 m2/g (for
NiCu1.19-ZnAl-LDO@Al2O3), probably because the LDHs or LDO
crystallites raise the surface roughness. The calcination and
reduction cause no visible destruction of the LDH morphology or
change in surface area on Al2O3 surface, and the resulting
NiCu1.19-ZnAl-LDO@Al2O3 well retains the interlaced
arrangement of flake-shaped slabs (Fig. 1D).
In this work, the feasibility to partly or even completely
replace Pt with NiCu bimetallic catalyst has been demonstrated,
with Ni and Cu in a highly homogeneous dispersion by using Ni
and Cu containing layered double hydroxides as precursors.
Layered double hydroxides (LDHs),
a
hydrotalcite-like
compound, have been demonstrated to be excellent precursors for
supported metal catalysts [19-23]. This work reports the
homogeneously dispersed Ni and Cu centers, which affords good
activity and selectivity in the reforming reaction of n-heptane.
The NiCu bimetallic catalyst even provides higher activity than
Pt catalyst while with comparable selectivity.
The Ni and Cu containing ZnAl-LDHs precursors were
prepared in situ on γ-Al2O3 support according to the reported
method [24]. The Cu/Ni molar ratios were controlled as 0.61,
1.19, 2.11 and 3.17 as determined by inductively coupled plasma
optic emission spectrometer (ICP-OES) analysis (Table S1 in
Supporting information). The Ni and Cu containing ZnAl-LDHs
precursors were activated in the reactor by first calcining in the
air at 550 C and then reducing with H2 flow at 500 C. The
resulting sample was denoted NiCux-ZnAl-LDO@Al2O3 (x
represents the Cu/Ni molar ratio). The reaction was initiated by
introducing n-C7 into the reactor, and the products were analysed
on-line with a gas chromatograph. For comparison, the supported
NiCu bimetallic catalyst with a Cu/Ni molar ratio of 1.14 was
also prepared by incipient wetness impregnation method
(denoted NiCu1.14/ZnAl-LDO@Al2O3), followed by the same
Fig. 1. (A) XRD patterns of (a) -Al2O3, (b) NiCu1.19-ZnAl-LDHs@Al2O3
with a Cu/Ni molar ratio of 1.19; (B) SEM images of the surface; (C) the
cross section of the NiCu1.19-ZnAl-LDHs@Al2O3; and (D) SEM images of
NiCu1.19-ZnAl-LDO@Al2O3 obtained from NiCu1.19-ZnAl-LDHs@Al2O3.
o
o
The transmission electron microscopy (TEM) image (Fig. 2A)
shows that the metal nanoparticles are homogeneously dispersed
in NiCu1.19-ZnAl-LDO@Al2O3. The particles are in a narrow size
distribution from 1.9 nm to 4.2 nm with the maximum of around
3.0 nm, and the lattice spacing of the exposed NiCu (111) facet is
0.206 nm in the high-resolution TEM (HRTEM) image (inset in
Fig. 2A). The high angle annular dark-field scanning

transmission electron microscopy (HAADF-STEM) image (Fig.
2B) indicates a uniform distribution of the metal nanoparticles,
just as observed from the TEM image. The STEM-energy
dispersive X-ray (EDX) elemental mapping (insets in Fig. 2B)
displays that the Ni and Cu elements are distributed
homogeneously and almost in the same position. The
observations from HRTEM image and EDX mapping illustrate a
NiCu alloy structure, which is consistent with an earlier report
[26]. As a comparison, the metal particles in the NiCu1.14/ZnAl-
LDO@Al2O3 are diverse in morphology (Fig. 2C), with a broader
size distribution from 1.7 nm to 6.0 nm, as observed in TEM
image. Separated Ni and Cu particles with both (111) facet
exposed are observed in HRTEM image (inset in Fig. 2C). The
HAADF-STEM image provides a more obvious difference in the
metal distribution between NiCu1.14/ZnAl-LDO@Al2O3 and
NiCu1.19-ZnAl-LDO@Al2O3). The metal particles are observed to
be in a continuous and more chaotic distribution in the HAADF-
STEM image of NiCu1.14/ZnAl-LDO@Al2O3, with visible
aggregation (Fig. 2D). The EDX image (insets in Fig. 2D) exhibit
a discrepant Ni and Cu element distribution. In the XPS spectra
(Fig. 3), the binding energies of Ni0 and Cu0 [27,28] are
measured as 852.7 and 932.8 eV for NiCu1.19-ZnAl-LDO@Al2O3
(Fig. 3A), 0.2 and 0.3 eV higher than that for NiCu1.14/ZnAl-
LDO@Al2O3 (Fig. 3B), consistent with better NiCu dispersion in
NiCu1.19-ZnAl-LDO@Al2O3. The adjacent binding energy at
855.8-856.9 eV in the Ni 2p3/2 XPS spectra originates from a
high-spin divalent state of Ni2+ in the sample [27]. When nickel is
alloyed with copper, changes in the electronic properties of
nickel result in displacement of BEs to lower values depending
on the copper content.
Fig. 3. Ni (A) and Cu (B) 2p XPS spectra for (a) NiCu1.14/ZnAl-
LDO@Al2O3, (b) NiCu1.19-ZnAl-LDO@Al2O3, and (c) 0.17%Pt/ NiCu1.19
-
ZnAl-LDO@Al2O3.
In the hydro-conversion of heptane (n-C7) (Table 1), NiCu1.19
-
ZnAl-LDO@Al2O3 as the catalyst (Table 1, entry 1) shows
visibly higher n-C7 conversion, higher specific activity, higher i-
C7 selectivity, and lower C1-C4 selectivity than NiCu1.14/ZnAl-
LDO@Al2O3 (Table 1, entry 2). The n-C7 conversion over
NiCu1.19-ZnAl-LDO@Al2O3 is even higher than over Pt/ZnAl-
LDO@Al2O3 with a 0.32% Pt loading (Table 1, entry 3), while
with almost the same selectivity. The better apparent activity and
isomerization selectivity of NiCu1.19-ZnAl-LDO@Al2O3 is
supposed to owe to the well-alloyed structure with NiCu uniform
distribution. As revealed by the XPS results, the in situ
exsolution of NiCu from LDHs layers provides stronger metal-
supports interactions that leads to Ni and Cu centers more
electron-deficient, which well accounts for the higher activity of
NiCu1.19-ZnAl-LDO@Al2O3 than NiCu1.14/ZnAl-LDO@Al2O3.
According to previous observation [29], more highly are metal
centers dispersed, more is C-C cleavage inhibited. So the better
selectivity of NiCu1.19-ZnAl-LDO@Al2O3 than NiCu1.14/ZnAl-
LDO@Al2O3 could also be well explained. The use of Pt in 0.17%
loading together with NiCu1.19 bimetallic catalyst promotes both
of n-C7 conversion and i-C7 selectivity, while largely reduces the
C1-C4 selectivity (Table 1, entry 6). The specific activity on
0.17%Pt/NiCu1.19-ZnAl-LDO@Al2O3 is 268% higher than that on
0.32%Pt/ZnAl-LDO@Al2O3. The significant increase in specific
activity (normalized by moles of Pt) can be attributed to the
contribution of NiCu. The ICP results and TEM/STEM images
show that the Pt loading makes no visible impact on components
of Ni or Cu and the homogeneous distribution of NiCu elements
(Table S1 and Fig. S2 in Supporting information). The NiCu
particles retain narrow distribution sized from 3.8 nm to 6.3 nm.
In the XPS spectra (Fig. 3), a visible increase in the BEs of Ni
2p3/2 and Cu 2p3/2 is observed with Pt loaded, illuminating the
electron transfer from NiCu to Pt atoms. Therefore, the
promotion of catalysis observed with 0.17%Pt/NiCu1.19-ZnAl-
LDO@Al2O3 could originate from the synergies of NiCu and Pt
centers. Similar synergy effect of the metal active sites was
previously observed for the Ir-Pt/Al2O3 catalyst prepared by
organometallic grafting method in the ring opening reaction of
methyl-cyclopentane [30].
Fig. 2. TEM image (A) and HAADF-STEM image (B) of catalyst NiCu1.19
-
ZnAl-LDO@Al2O3; TEM image (C) and HAADF-STEM image (D) of
catalyst NiCu1.14/ZnAl-LDO@Al2O3. Insets in (A) and (C) are the HRTEM
images and the particle size frequency distribution histograms. Insets in (B)
and (D) are the elemental mapping images of Ni and Cu for the
corresponding sample, Scale bar: 20 nm.
Table 1
a
Catalytic results for the hydroconversion of n-C7 .

Selectivity (%)
i-C7
Conversion
(%)
Specific activity
(mol mol−1 h−1)c
Entry
Catalystb
Toluene
5.0
C1-C4
37.5
40.0
36.0
33.4
31.3
26.3
27.2
29.2
31.1
NiCu1.19-ZnAl-LDO@Al2O3
NiCu1.17/ZnAl-LDO@Al2O3
0.32%Pt/ZnAl-LDO@Al2O3
33.8
24.9
22.2
44.6
39.7
42.6
65.7
85.0
27.0
44.4
28.8
32.9
1
20.4
24.6
4.1
2
536.3
2340.0
1950.0
1972.9
3028.2
3900.0
1504.3
35.0
5.7
3
21.6
3.7
0.15%Pt/Ni-ZnAl-LDO@Al2O3
4
31.5
4.9
0.16%Pt/NiCu0.61-ZnAl-LDO@Al2O3
0.17%Pt/NiCu1.19-ZnAl-LDO@Al2O3
0.17%Pt/NiCu2.11-ZnAl-LDO@Al2O3
0.17%Pt/NiCu3.17-ZnAl-LDO@Al2O3
0.14%Pt/Cu1.21%-ZnAl-LDO@Al2O3
0.16%Pt/Cu3.91%-ZnAl-LDO@Al2O3
5
38.9
6.2
6
37.4
8.0
7
28.0
16.6
6.1
8
39.3
Fig. 4. Ni (A, C) and Cu (B, D) 2p XPS spectra for (a) Ni-ZnAl-
LDO@Al2O3, (a’) Cu-ZnAl-LDO@Al2O3, (b) NiCu0.61-ZnAl-LDO@Al2O3,
(c) NiCu1.19-ZnAl-LDO@Al2O3, (d) NiCu2.11-ZnAl-LDO@Al2O3 and (e)
9
2145.0
51.2
6.3
22.9
10
NiCu3.17-ZnAl-LDO@Al2O3 without (A, B) and with (C, D) Pt loaded.
a Reaction conditions: 500 oC, 0.7 MPa, H2/n-C7 = 6, WHSV = 4 h−1.
b The loadings of Ni and Cu are shown in Table S1.
In the hydro-conversion of n-C7, in comparison with Ni
catalyst without Cu (Table 1, entry 4), the introduction of Cu in a
Cu/Ni ratio of 0.61 decreases the n-C7 conversion while improves
the isomerization selectivity visibly (Table 1, entry 5). With the
Cu/Ni ratio increasing, n-C7 conversion, specific activity, and
toluene selectivity all increase, while the isomerization selectivity
shows a first increasing and then decreasing change (Table 1,
entries 5 to 8). The dependence of catalytic activity and
selectivity on the Cu/Ni ratio of NiCu bimetallic catalyst
indicates the synergic catalysis between highly homogeneous Ni
and Cu sites [31]. The introduction of Cu in a Cu/Ni ratio of 0.61
increases the electronic density of Ni site, accounting for the
activity reduction on Pt/NiCu0.61-ZnAl-LDO@Al2O3 in
comparison to Pt/Ni-ZnAl-LDO@Al2O3. The activity
enhancement with increasing Cu/Ni ratio is well consistent with
the successive decreases in the electronic density of both Cu and
Ni sites.
The introduction of Cu inhibits the hydrogenolysis and/or
cracking reactions, giving a reduced C1-C4 selectivity (Table 1,
entries 4 to 8). But a visible inhibition of C1-C4 selectivity needs
a higher Cu loading when no Ni is present (Table 1, entries 9 and
10). The toluene selectivity increases with Cu/Ni ratio (Table 1,
entries 5 to 8), but not with the amount of single Cu (Table 1,
entries 9 and 10), indicating the role of synergic catalysis
between Cu and Ni sites in the improvement of toluene
selectivity. When a small amount of Cu added, the XPS show
that the binding energy of Ni shifts toward a low energy of 0.3
eV, and the binding energy of Cu increases 0.3 eV (Fig. 4). The
changes of electronic structure of the metals result in a slight
decrease in catalytic activity. Further increase the ratio of Cu/Ni,
the binding energy of Ni and Cu increases successively. The
activities of catalysts show an increasing trend and the selectivity
c The specific activities representative moles of n-heptane conversion
on per mol metal per hour; specific activities are normalized by moles
of (Ni + Cu) for NiCu-containing samples (without Pt), and are
normalized by moles of Pt for Pt-containing samples.
To better understand why the reforming catalysis of Ni and Cu
even better than supported Pt, the Cu/Ni molar ratios (Table S1)
have been varied in this work. As shown in the XPS spectra (Fig.
4), in comparison with Ni-ZnAl-LDO@Al2O3, Cu introduction
decreases the binding energy (BE) of Ni 2p3/2 (Fig. 4A) slightly,
and meanwhile the BE of Cu0 in NiCu0.61-ZnAl-LDO@Al2O3
rises in comparison to Cu-ZnAl-LDO@Al2O3 (Fig. 4B),
consistent with the electron transfer from Cu to Ni. With
increasing Cu/Ni ratio, both of Ni0 and Cu0 BEs successively
increases. But the reason for the increase in the BEs needs more
investigations. Similar trends are observed on Pt-containing
samples (Figs. 4C and D).

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NiCu1.19-ZnAl-LDO@Al2O3 and 288 oC for NiCu2.11-ZnAl-
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S4 in Supporting information). So it can be deduced that the Cu
sites promote isomerization selectivity by inhibiting
hydrogenolysis reaction. The synergies of Ni and Cu sites on
alloy phase is mainly responsible for the higher isomerization
selectivity observed with NiCu1.19-ZnAl-LDO@Al2O3 and
NiCu2.11-ZnAl-LDO@Al2O3 than with other catalysts.
In summary, NiCu bimetallic catalyst with uniform NiCu
dispersion has been demonstrated to provide higher activity than
Pt catalyst while with similar selectivity in the reforming reaction
of n-heptane. The synergies between homogeneously dispersed
Ni and Cu sites account for the enhancement of activity and
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mmc1.doc
Acknowledgments
Financial supports from National Nature Science Foundation
of China (NSFC: 91634120, 21521005), the National Key
Research
and
Development
Program
of
China
(2017YFA0206804) and the Fundamental Research Funds for the
Central Universities (XK1802-6) are gratefully acknowledged.
Declaration of Competing Interest
The authors declare no competing financial interest.
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