A gradient reduction strategy to produce defects-rich nano-twin Cu particles for targeting activation of carbon-carbon or carbon-oxygen in furfural conversion

Article abstract of DOI:10.1016/j.jcat.2020.05.027

Complexity of chemical linkages (C-C/C-H/C-O, C=C/C=O, or C-O-H/C-O-C) in biomass-derived molecules makes the selective activation of targeted bonds much more challenging, expecting well-defined catalysts and definite catalytically-active sites. This work demonstrates an effective gradient reduction strategy to control the definite structure of catalytically-active sites, affording defects-rich nano-twin Cu particles. This strategy just involves the reduction (calcination under H₂) of Cu^II-containing layered double hydroxides (LDHs) simply with controlling the reduction gradient (interval time) of Cu^II species in two chemical micro-environments (Cu^II-O-Cu^II and Cu^II-O-M^II/III/IV (M ≠ Cu)) in the brucite-like layer of LDHs. The nano-twin Cu particles efficiently promote the target activation of C-O and C=C in the conversion of furfural to cyclopentanone (CPO). With ~100% furfural conversion, the defects-rich nano-twin Cu particles afford a CPO selectivity of 92%, 50% higher than regular spherical Cu particles. The multi-stepped defect sites, originating from the planar defects, play a decisive role in promoting the CPO selectivity by facilitating the hydro-deoxygenation to C-O of 4-hydroxycyclopentenone (HCP) and hydrogenation to C=C of HCP or cyclopentenone.

Full text of DOI:10.1016/j.jcat.2020.05.027

Journal Pre-proofs
A Gradient Reduction Strategy to Produce Defects-Rich Nano-Twin Cu Parti-
cles for Targeting Activation of Carbon-Carbon or Carbon-Oxygen in Furfu-
ral Conversion
Yanru Zhu, Jian Zhang, Xiaodan Ma, Zhe An, Shaowei Guo, Xin Shu,
Hongyan Song, Xu Xiang, Jing He
PII:
S0021-9517(20)30205-0
https://doi.org/10.1016/j.jcat.2020.05.027
YJCAT 13757
DOI:
Reference:
To appear in:
Journal of Catalysis
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Revised Date:
Accepted Date:
21 March 2020
13 May 2020
18 May 2020
Please cite this article as: Y. Zhu, J. Zhang, X. Ma, Z. An, S. Guo, X. Shu, H. Song, X. Xiang, J. He, A Gradient
Reduction Strategy to Produce Defects-Rich Nano-Twin Cu Particles for Targeting Activation of Carbon-Carbon
or Carbon-Oxygen in Furfural Conversion, Journal of Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.
2020.05.027
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A Gradient Reduction Strategy to Produce Defects-Rich Nano-Twin Cu
Particles for Targeting Activation of Carbon-Carbon or Carbon-Oxygen in
Furfural Conversion
Yanru Zhu, Jian Zhang, Xiaodan Ma, Zhe An, Shaowei Guo, Xin Shu, Hongyan Song, Xu Xiang,
and Jing He*
State Key Laboratory of Chemical Resource Engineering& Beijing Advanced Innovation Center for
Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, China
*corresponding author. E-mail: jinghe@263.net.cn or hejing@mail.buct.edu.cn.
Postal address: Box 98, North Third Ring Road 15, Chaoyang District, Beijing, 100029, China
- 1 -

Abstract
Complexity of chemical linkages (C-C/C-H/C-O, C=C/C=O, or C-O-H/C-O-C) in biomass-
derived molecules makes the selective activation of targeted bonds much more challenging, expecting
well-defined catalysts and definite catalytically-active sites. This work demonstrates an effective
gradient reduction strategy to control the definite structure of catalytically-active sites, affording
defects-rich nano-twin Cu particles. This strategy just involves the reduction (calcination under H2)
of CuII-containing layered double hydroxides (LDHs) simply with controlling the reduction gradient
(interval time) of CuII species in two chemical micro-environments (CuII-O-CuII and CuII-O-MII/III/IV
(M  Cu)) in the brucite-like layer of LDHs. The nano-twin Cu particles efficiently promote the target
activation of C-O and C=C in the conversion of furfural to cyclopentanone (CPO). With ~ 100 %
furfural conversion, the defects-rich nano-twin Cu particles afford a CPO selectivity of 92 %, 50 %
higher than regular spherical Cu particles. The multi-stepped defect sites, originating from the planar
defects, play a decisive role in promoting the CPO selectivity by facilitating the hydro-deoxygenation
to C-O of 4-hydroxycyclopentenone (HCP) and hydrogenation to C=C of HCP or cyclopentenone.
Keywords: gradient reduction strategy, nano-twin Cu particles, layered double hydroxides (LDHs),
defects sites, furfural-to-cyclopentanone
- 2 -

1. Introduction
Cu is an earth-abundant and inexpensive 3d transition metal, and Cu or Cu-based nanoparticles
perform well as catalyst in the selective activation of oxygen-containing chemical bonds, leading to
hydrogenation, dehydrogenation, hydrogenolysis, or reforming of C-O and/or C=O under specific
conditions [1-4]. Cu-based nanomaterials have thus been widely used in the catalytic conversion of
biomass [5] and a variety of industrially promising or important reactions, including synthesis of
alcohols from syngas [6] or CO2 [7], steam reforming of methanol [8], oxidation of CO [9], water-
gas shift reaction [10], selective hydrogenation of carbonyl compounds [11], and dehydrogenation of
alcohols [12]. The conversion of oxygenates has been found to strongly depend on the structure of
catalytically-active sites, and the activation of carbon-oxygen bonds is also structure-sensitive [13].
Grain boundaries (GBs), twin boundaries (TBs), and stacking faults are typical planar defects for
metallic Cu, interrupting the periodicity of the atom arrangement and providing new possibilities in
catalytic reactions with their unique electronic states [13]. Grain boundaries have been explored as
high-active catalytic sites for CO2 reduction [14] or CO reduction [15], which promotes the selectivity
of CO or formic acid. Twin boundaries-rich Cu nanoparticles show enhanced selectivity in the
electrochemical reduction of CO to C2+ products [16]. Atomic steps on the Cu surface, induced by
planar defects and lattice strain of three-dimensionally ordered macroporous Cu-Fe catalysts, serve
as high-activity oxygenation sites in the synthesis of higher alcohols from syngas [17]. Stepped Cu
surface, created by the termination of a planar defect, is described as high energy sites for methanol
synthesis over Cu/ZnO/Al2O3 industrial catalysts [7]. A dramatic increase in the activity and
selectivity to H2 and CO in the catalytic electroreduction of CO2 has been observed by controlling the
surface atomic coordination of model spherical Cu particles with decreasing Cu particle size [18].
Recently, artificially constructing defects on the surface of Cu particle to improve catalysis is growing
more and more attractive [19]. GBs-rich Cu particles have been prepared by depositing Cu onto a
film of aligned carbon nanotubes through e-beam evaporation [15]. Huang et al. reported fivefold
twinned star decahedron Cu showing abundant surface defects at TBs terminations [20]. The twinned
- 3 -

structure has been achieved by controlling the growth rate of Cu with weak reduction agent and low
reduction temperature [20]. Three-dimensionally freestanding nano-twinned Cu structures, showing
coherent twin boundaries, have been directly printed by a localized pulsed electro-deposition strategy
[21]. High density of twin defects have been produced with Pt-Cu hierarchical quasi great
dodecahedrons by directed growth of multiple branches on icosahedral seeds [22]. The fabrication of
specific defect structures have made great achievements with bulk Cu nanoparticles. However, fine
manipulation for the definite structures or surface defects of supported Cu nanoparticles remains as a
grand challenge.
Here, this work reports a gradient reduction strategy to control the definite defect structure of
supported Cu particles by producing nano-twin Cu particles from the topological transformation of
CuII-containing layered double hydroxides (LDHs). LDHs, a well-known anionic clay, consists of
positively charged brucite-like layer and exchangeable interlayer anions [23,24]. The brucite-like
layer accommodates a wide range of divalent (MII) and high-valence (MIII or MIV) metal cations with
the metal cations surrounded approximately octahedrally by hydroxide ions [23]. The octahedral units
form infinite layers by edge-sharing [23], with MII-O-MII or MII-O-MIII/IV linkage allowed while
MIII/IV-O-MIII/IV linkage forbidden [25]. In virtue of the structural characteristics of brucite-like layer,
LDHs have many potential applications in heterogeneous catalysis, functioning as catalyst, support,
or catalyst precursor [26,27]. By calcination of reducible-metal containing LDHs under a H2
atmosphere [28], supported metal nanoparticles can be produced, which is one of the significant
applications of LDHs. Supported Cu has been produced from Cu-containing LDHs and the interfaces
of Cu-support have been controlled [29]. Supported Ni has also been produced by the reduction of
NiAl-LDHs [30,31], and interlayer anions (carbonate or nitrate) have been revealed to influence the
exposure plane of Ni particles [30]. This work utilizes the difference in the chemical environments
of divalent Cu cations (CuII-O-CuII and CuII-O-MII/III/IV (M  Cu)) to control the production of Cu0
from CuII in CuII-containing LDHs. By tailoring the temperature-programmed rate in the reduction
of CuII-containing LDHs (CuIIZnIIAlIIIZrIV-LDHs here), the reduction of CuII-O-CuII is segregated
- 4 -

kinetically from that of CuII-O-ZnII/CuII-O-AlIII/CuII-O-ZrIV, which is defined as “gradient reduction”
in this work, and generates the defects-rich nano-twin Cu particles supported on Zn(Al)(Zr)O.
The catalytic conversion of biomass and biomass-derived platform molecules has attracted more
and more attention for the urgent demand of sustainable and clean production of fuels and chemicals
[32-34]. Amongst the platform molecules, furfural is one of the most promising compounds in
sustainable production of value-added C4/C5 chemicals or derived biofuels [35]. However, just like
other biomass-derived molecules, the chemical linkages (C-C/C-H/C-O, C=C/C=O, and C-O-H/C-
O-C) in furfural are so diversified that the selective activation and directed conversion of targeted
chemical bonds are of great challenge [36]. And that, the transformation of furfural to value-added or
fine chemicals usually involve multiple successive steps [37], including hydrogenation of C=C and/or
C=O [38,39], hydrogenolysis of C-O [40,41], hydro-deoxygenation [42], and/or rearrangement [43].
Each step could have competitive side-reactions, and even a change in the step sequence could lead
to diverse products. Hydrogenation rearrangement of furfural to cyclopentanone (CPO) has been
particularly attractive since it offers a clean and sustainable alternative to produce CPO [44], a very
useful compound under heavy demand [45] but still relying exclusively on fossil-based feedstock
[46,47]. According to previous report [48], the furfural-to-CPO reaction starts with the hydrogenation
of C=O in furfural on metal sites, the resulting furfuryl alcohol (FA) is subsequently converted to 4-
hydroxy-2-cyclopentenone (HCP) on acidic sites via a Piancatelli rearrangement, and finally the C-
OH in HCP carrys on selective hydro-deoxygenation followed by hydrogenation to C=C on metal
sites to produce CPO. However, the process could be accompanied by side-reactions in each step
(Scheme 1), such as hydrogenation to C=C in furfural or FA, C-OH breakage (hydrogenolysis) of
FA, and hydrogenation to C=O in HCP or its derivative [35]. Conventional reforming and/or
hydrogenation catalysts, widely applied in petrochemical industry, have once been applied to
transform biomass-derived to fuels [49,50]. The development of well-defined catalysts [51,52] and
definition of active sites [53] are supposed to help promoting the furfural-to-CPO transformation, as
realized in other reactions of both model [53] and real [7] systems. Here, with the defects-rich nano-
- 5 -

twin Cu particles as catalyst, furfural has been efficiently conversed to CPO in high selectivity. An
effective breakage of C-O and targeted activation to C=C on well-defined defects-rich Cu particles
are demonstrated.
Scheme 1. Proposed reaction pathway in hydrogenation of furfural.
2. Experimental
2.1. Materials
Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Al(NO3)3·9H2O, Na2CO3, and NaOH were all of analytical
purity and purchased from Sinopharm. ZrO(NO3)2·6H2O (99.99%) was purchased from Aladdin.
Furfuryl alcohol (98.0 %), cyclopentanone (99.0 %), cyclopentanol (99.0 %), and tetrahydrofurfuryl
alcohol (97.0 %) were purchased from Alfa-Aesar. Furfural (99.0 %) was purchased from Acros
Organics. 2-cyclopentenone (98.0 %), 1-hexanol (99.0 %), and 4-hydroxy-2-cyclopentenone (98.0 %)
were purchased from Innochem. All chemicals were used as received without purification.
2.2. Preparation
The Cu, Zn, Al, Zr-containing layered double hydroxides (denoted as CuIIZnIIAlIIIZrIV-LDHs)
with Cu/Zn/Al/Zr molar ratio of 2/7/2/1 (M2+/(M3++M4+) = 3, typical component of LDHs) was
synthesized by a pH-constant co-precipitation method. Typically, Cu(NO3)2·3H2O (12 mmol),
Zn(NO3)2·6H2O (42 mmol), Al(NO3)3·9H2O (12 mmol), and ZrO(NO3)2·6H2O (6 mmol) were
dissolved together in 200 mL of deionized water, which was referred to as solution A. Solution B was
2−
a mixture of NaOH (120 mmol) and Na2CO3 (31.5 mmol) ([OH−] = 1.6[M2+ + M3+ + M4+] and [CO3 ]
= 2[M3+ + M4+]) in 200 mL of deionized water. Solution A and B were simultaneously added drop-
wise to a four-necked flask containing 200 mL of deionized water under constant pH (9.0). After
- 6 -

aging at 65 oC for 12 h, the solid was filtrated, washed thoroughly with deionized water till the filtrate
is neutral, and dried at 80 oC overnight, to give CuIIZnIIAlIIIZrIV-LDHs. Similar procedure was used
to prepare ZnIIAlIIIZrIV-LDHs (Zn/Al/Zr molar ratio of 9/2/1) sample in the absence of
Cu(NO3)2·3H2O. CuIIZnIIAlIIIZrIV-LDHs was reduced under H2 atmosphere at 450 oC (to insure the
LDHs completely turn into layered double oxide (Zn(Al)(Zr)O) supported Cu) for 2 h in a tube-
o
o
furnace with the temperature programmed from ambient to 450 C in 2 C min-1. After the tube-
furnace was cooled to ambient, 10 % H2/N2 was introduced into the furnace for 2 h. The reduced
sample, denoted Cu0-Zn(Al)(Zr)O-2, was used for catalytic reaction and characterization immediately.
CuIIZnIIAlIIIZrIV-LDHs was reduced under H2 atmosphere at 450 oC for 2 h in a tube-furnace with the
temperature programmed from ambient to 450 oC in 10 oC min-1. After the tube-furnace was cooled
to ambient, 10 % H2/N2 was introduced into the furnace for 2 h. The reduced sample, denoted Cu0-
Zn(Al)(Zr)O-10, was used for catalytic reaction and characterization immediately. ZnAlZr-LDHs
was calcinated at 450 oC for 2 h under H2 atmosphere with the temperature programmed from ambient
to 450 oC in 2 oC min-1, to give Zn(Al)(Zr)O.
2.3. Characterizations
Quantification of Cu was performed using a Shimadzu ICPS-75000 inductively coupled plasma
optical emission spectrometer (ICP-OES). Before the measurements, 30 mg of sample was dissolved
in 5 mL of nitric acid at ambient. The solution was transferred to a 10-mL volumetric flask, and
diluted with deionized water to volume. Powder X-ray diffraction (XRD) patterns were recorded with
a Shimadzu XRD-6000 diffractometer (Cu Kα radiation, λ = 0.154 nm) in the 2θ ranges of 3–80 o
o
with a scan speed of 5 min-1. Transmission electron microscope (TEM) images were taken with
JEOL 2100 high resolution transmission electron microscope operating at an accelerating voltage of
200 kV. The sample for TEM measurements was dispersed in ethanol under ultrasonic conditions
before depositing on Cu micro grids, and covered by a carbon coating of several nanometers to
prevent from charging the sample. The size distribution of Cu was based on at least 300 particles in
total counted in varied regions. High-angle annular dark-field scanning (HAADF-STEM) images
- 7 -

were taken on a JEM-ARM 200F electron microscope capable of sub-angstrom resolution. H2-
temperature programmed reduction (H2-TPR) was carried out on a Micrometric ChemiSorb 2750
chemisorption system with a thermal conductivity detector (TCD). About 100 mg of sample was
loaded in a U type quartz reactor. The TPR was carried out with a heating ramp rate of 2 oC min-1 or
10 oC min-1 in a stream of 10 % H2 in Ar from 50 oC to 600 oC, with a total flow rate of 40 mL min-1.
The samples were dispersed onto a Au grid coated with a thin holey carbon film. The dispersion of
Cu was determined by N2O oxidation followed by H2 titration using the procedure described by
previously reports [54,55]. Typically, about 50 mg of reduced Cu-containing sample was loaded in a
U-type quartz reactor and pre-treated online at 350 °C for 1 h under 10 % H2/Ar mixture (40 mL
min-1). Afterward, the sample was cooled to 70 °C in a flow of Ar (40 mL min-1). Then pulse N2O
was introduced into the carrier gas until N2O peak reached saturation to completely oxidize the
surface metallic Cu. The sample was then cooled to room temperature and followed by a H2-TPR.
H2-TPR was carried out with a heating ramp rate of 10 °C min-1 in 10 % H2/Ar mixture to 400 °C,
with a total flow rate of 40 mL min-1. The amount of consumed H2 was recorded by TCD. It was
assumed that one hydrogen molecule reduced one surface Cu2O to Cu. The Cu dispersion is defined
as the fraction of Cu atoms exposed at the surface, and was calculated as follows:
Nsurface
Ntotal
amount of consumed H2
DCu


100%
Ntotal (by ICP)
Quasi-in-situ X-ray photoelectron spectra (XPS) were recorded on a Thermo Fisher Scientific
ESCALAB-250 instrument (USA) with an excitation source of Al Ka radiation. A C1s peak at 284.8
eV was used as a calibration peak. In addition to a binding energy scan range from 0 to 1200 eV for
the identification of all detectable elements, detailed scans for chemical state identification and
quantification were recorded. Quasi-in-situ Cu K-edge X-ray absorption spectra (XAS) were recorded
on the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF) operated at 2.2 GeV
and 200 mA. A Si (111) double-crystal monochromator was used to reduce the harmonic component
of the monochrome beam. All the samples were measured in the transmission mode. Before
measurements, the as-prepared Cu-containing sample was pretreated carefully to ensure that the
- 8 -

reduced sample was not exposed to air. Specimens were made by directly applying the fine powders
to Scotch tape inside a glove box kept under vacuum package, sealed by the valve bag. The IFEFFIT
software was used to analyze the extended X-ray fine structure (EXAFS) data at the Cu K-edge. For
temperature-programmed desorption of ammonia (NH3-TPD) test, about 100 mg of reduced sample
was first activate at 400 °C for 1 h in 10 % H2/Ar, followed by purging with high purity Ar (50 mL
min-1) for 1 h at 410 °C. Afterward, as the temperature decreased to 50 °C, NH3 was injected until
saturation. NH3-TPD was carried out with a heating ramp rate of 10 °C min-1 to a sample temperature
of 500 °C. The calibration curve for the NH3 amount as a function of the peak area was done in
advance and the amount of acid sites was calculated according to the area of desorption peaks. In situ
Fourier transform infrared (FT-IR) spectra for CO or CPEO adsorption, and in situ FT-IR spectra for
the hydrogenation of HCP were recorded on Nicolet 380 model (Thermo Electron Corporation)
equipped with an MCT-A detector cooled by liquid nitrogen. KBr beam-splitter was used to obtain
spectra in the range of 4000-650 cm-1. The spectrum was collected at a spectrum resolution of 2 cm-
1
and an accumulation of 64 scans. About 15 mg of the sample was pressed into a disk for each
experiment. Before measurements, the reduced sample was pretreated in situ by a flowing of H2 (20
mL min-1) at 400 oC for 1 h. For IR measurement of CO adsorption, the reduced sample was cooled
down to 30 °C in Ar. Then the sample was exposed to CO flow (20 mL min-1) at 30 oC for 1 h followed
by purging with Ar to remove gaseous CO for 15 min. The FTIR spectra were recorded. The resulting
spectra have deducted the background at 30 oC. For IR measurement of CPEO adsorption, the reduced
sample was cooled down to 50 °C in Ar. Then, pure CPEO was introduced by flowing Ar for 30 min
at 50 °C. The FT-IR spectra were recorded intervals of Ar flushing. The resulting spectra have
deducted the absorption of gaseous CPEO and the background. For IR measurement of HCP
hydrogenation, the reduced sample was cooled down to 100 °C in Ar. Then, pure CPEO was
introduced by vacuum depression vaporization, and purged in Ar to remove the gaseous and weakly
adsorbed HCP for 30 min. With the introducing of H2 flow (20 mL min-1), the spectra of the
hydrogenation reaction process were recorded intervals of H2 flow at 100 °C. The resulting spectra
- 9 -

1
13
have deducted the absorption of gaseous HCP and the background. H and C nuclear magnetic
resonance (NMR) spectra were recorded on a Bruker AV-400 (400 MHz) spectrometer. Chemical
shifts were reported in parts per million (ppm). High resolution mass spectra (HRMS) were obtained
on a XEVO-G2QTOF spectrometer and were reported as m/z (relative intensity).
2.4. Catalytic tests
General procedure for the liquid phase hydrogenation conversion of furfural in a autoclave
reactor: Typically, furfural (5.2 mmol, 0.5 g), catalyst (25 mg) and ultrapure water (10 mL) were
placed into a stainless-steel autoclave reactor with a magnetic stirrer, and then the reactor was sealed
and flushed with pure hydrogen for four times. Subsequently, the reactor was charged with pure
hydrogen to 2.5 MPa. After that, the reactor was heated to 160 °C, and the pressure was increaced to
4.0 MPa. The reaction was initiated by stirring at a speed of 800 rpm. During the reaction, the
hydrogen pressure was kept constant by supplying hydrogen continuously from gas cylinder through
the pressure regulator to compensate for the consumed hydrogen in the reactor. After the reaction,
the reactor was rapidly cooled to room temperature and then depressurized. The organic products
were extracted by dichloromethane from the liquid and then analyzed by gas chromatograph
(Shimadzu GC-2014C) equipped with a inno-WAX capillary column (30.0 m × 250 μm × 0.25 μm).
The product was purified via silica column chromatography (eluted with ethyl acetate and acetonitrile)
for NMR and MS measurements. The amounts of products were determined based on GC data using
the internal standard method (n-hexanol as the internal standard). The conversion of furfural (%) and
the selectivity of main products (%) were calculated by
mol of furfural
Conversion  (1-
)100%
mol of furfural loaded initially
and
mol of product
Selectivity of product 
100%
mol of furfuralconverted
.
Procedure for the conversion of furfural in a continuous-flow reactor: The conversion of furfural
to CPO was also carried out in a vertical fixed-bed reactor. The catalyst was pelletized and sieved to
20–40 mesh size. Then, 0.5 g of the reduced sample was loaded in a stainless steel tubular reactor
- 10 -

with an inner diameter of 10 mm and a length of 300 mm. The reaction was operated at 160 °C and
2 MPa H2, with a H2 flow of 50 mL min−1. The feedstock comprising a diluted aqueous solution of
furfural (5 wt%) was continuously introduced into the reactor by an HPLC pump. The products were
obtained when the reaction reached the steady state. The liquid and gas products were cooled and
collected in a gas-liquid separator immersed in an ice-water trap. The liquid products were analyzed
by the same procedures as the liquid phase hydrogenation conversion in a autoclave reactor above.
3. Results and discussion
3.1. Nano-twin Cu particles produced by gradient reduction strategy
In this work, Zn, Al, and Zr-containing mixed metal oxides (Zn(Al)(Zr)O) supported Cu
particles was produced from CuIIZnIIAlIIIZrIV-LDHs. CuIIZnIIAlIII-LDHs is a classical precursor [56]
for producing Zn(Al)O supported Cu, and ZrIV-doping is to provide suitable acidity [57] in the
catalytic reaction. In the X-ray diffraction (XRD) pattern of CuIIZnIIAlIIIZrIV-LDHs (Fig. S1), the
(003), (006), (009), (015), (018), (110), and (113) reflections typical of hydrotalcite structure are
clearly observed at 2θ = 11.8, 23.5, 34.6, 39.1, 46.6, 60.1, and 61.3 . No other crystallite phases are
o
resolved. Then CuIIZnIIAlIIIZrIV-LDHs was reduced with H2 under 450 C. In the reduction, the
o
o
temperature was programmed from room temperature to 450 C in 2 C min-1, affording Cu0-
Zn(Al)(Zr)O-2. The Cu content was determined to be 11.2 wt% and the specific surface area was 86.5
m2/g for Cu0-Zn(Al)(Zr)O-2 (Table S1). In the XRD pattern of Cu0-Zn(Al)(Zr)O-2, the reflections
characteristic of Cu0 or ZnO are clearly observed, while no reflections of LDHs exists (Fig. S1). The
observations indicate CuIIZnIIAlIII-LDHs has been completely transformed to mixed metal oxides
supported Cu0 species (Cu0-Zn(Al)(Zr)O-2) in the thermal treatment and reduction with H2.
According to TEM image (Fig. 1a), Cu particles in Cu0-Zn(Al)(Zr)O-2 are well-dispersed on the
surface of Zn(Al)(Zr)O with a maximum size distribution of around 4.6 nm. The Cu dispersion was
17.7 % for Cu0-Zn(Al)(Zr)O-2. In the high-angle annular dark-field scanning transmission electron
microscopic (HAADF-STEM) images, each Cu particle for Cu0-Zn(Al)(Zr)O-2 is irregular and
- 11 -

consists of at least two partially overlapped Cu ensembles, exhibiting a visible nano-twined structure
(Fig. 1b and 1c). Focusing on one Cu nano-twin particle, diversified planar defects are observed,
including dislocations, stacking faults, and twin boundaries (Fig. 1d). The plannar defectes result in
surface kinks (green lines) at the termination of twin boundaries (Fig. 1e), constituting multi-stepped
surface defects. Visible ZnO species, showing 100 plane, are observed surrounding the Cu nano-twin
particles (Fig. S2), and the presence of disordered Cu0-ZnOx interfaces are also observed. The growth
orientation of 111 plane for Cu nano-twin particle exactly matches the growth orientation for ZnO
species, indicative of the induction or promotion of ZnO (100) to Cu nano-twin particles. No CuZn
alloy has been resolved. The role of ZnO on nano-twinned structure of Cu particles deserves an in-
depth research that is under investigation in our laboratory. For comparison, Cu0-Zn(Al)(Zr)O-10 was
o
prepared by calcination of CuIIZnIIAlIIIZrIV-LDHs in H2 under 450 C with the temperature
programmed rate raised to 10 oC min-1. Cu0-Zn(Al)(Zr)O-10 possesses a specific surface area of 88.7
m2/g, a Cu dispersion of 18.4 % (Table S1), and a Cu particle size of around 4.6 nm (Fig. S3), all
similar to Cu0-Zn(Al)(Zr)O-2. While in the HAADF-STEM image of Cu0-Zn(Al)(Zr)O-10 (Fig. S4),
only regular surface defects are observed, resulting from the regular spherical fcc particles or Wulff
polyhedra. However, based on the STEM images only, it is hard to distinguish whether the observed
moire patterns are defects in nano-twin or the projections from the overlapped individual crystals.
More convinced investigations will be carried out in the following sections.
Extended X-ray absorption fine structure (EXAFS) spectra, CO adsorbed Fourier-transformed
infrared (FT-IR) spectra, and XPS spectra were applied to further reveal the local coordination
environment, surface defects, and chemical states of nano-twin Cu particles. According to EXAFS
results (Fig. 1f and Table S2), the Cu-Cu coordination number (7.8) for Cu0-Zn(Al)(Zr)O-2 is lower
than that for Cu0-Zn(Al)(Zr)O-10 (8.5), and the Cu-Cu Debye-Waller factor (0.011) for Cu0-
Zn(Al)(Zr)O-2 is higher than that for Cu0-Zn(Al)(Zr)O-10 (0.009), suggesting a more severely
distorted Cu environment associated with abundant Cu defects for nano-twin Cu particles. In the FT-
IR spectra with CO adsorded (Fig. 1g), both Cu0-Zn(Al)(Zr)O-2 and Cu0-Zn(Al)(Zr)O-10 present the
- 12 -

bands at 2106 and 2093 cm-1, which can be assigned [58] to the CO adsorption on Cu defect sites
(such as step or edge sites) and Cu terrace sites of low index planes. According to the relative area of
absorption bands, Cu defect sites are estimated to account for 56 % for Cu0-Zn(Al)(Zr)O-2, and only
21 % for Cu0-Zn(Al)(Zr)O-10. The nano-twinned Cu particles in Cu0-Zn(Al)(Zr)O-2 provide more
surface defect sites than regular spherical nanoparticles in Cu0-Zn(Al)(Zr)O-10, which supports the
observations in STEM images. In quasi-in situ Cu 2p3/2 XPS spectra (Fig. 1h and 1i), the binding
energy around 932 eV, corresponding to the Cu0 and/or CuI state, [59] is observed for both Cu0-
Zn(Al)(Zr)O-2 and Cu0-Zn(Al)(Zr)O-10. But in the Cu LMM X-ray excited Auger electron spectra
(XAES), only the kinetic energy at 918.9 eV, corresponding to the Cu0 state [60] is observed. So it is
approved that Cu is mainly present in metallic state for both Cu0-Zn(Al)(Zr)O-2 and Cu0-
Zn(Al)(Zr)O-10.
- 13 -

Fig. 1. (a) TEM image of Cu0-Zn(Al)(Zr)O-2 with particle size distribution (inset); (b-e) HAADF-
STEM images for Cu nano-twin particles in Cu0-Zn(Al)(Zr)O-2 with coloured intensity map (c),
displaying polytype planar defects (d) and highlighted stepped surfaces (e); (f) Fourier-transform of
the k3-weighted EXAFS spectra at the Cu K-edge; (g) FT-IR spectra for CO adsorbed at 303 K
followed by Ar purge for 15 min; (h) Cu 2p3/2 XPS, and (i) Cu LMM XAES spectra.
As discussed above, the two designed temperature-programmed rates during the reduction are
the key factors for the distinctive structures. To reveal the origin of nano-twin Cu particles, H2-TPR
measurements of CuIIZnIIAlIIIZrIV-LDHs were performed with a temperature-programmed rate of 2
or 10 oC min-1. Besides, the reduction of ZnAlZr-LDHs (without Cu) to produce Zn(Al)(Zr)O was
introduced as a control. As observed in Fig. S5, no visible H2 consumptions is observed for ZnAlZr-
LDHs in the range of 100-400 oC. For the reduction of CuIIZnIIAlIIIZrIV-LDHs, two H2 consumptions
are observed in either case, which can be attributed to the reduction of CuII-O-CuII and CuII-O-M
(here M = Zn, Al, or Zr) species in CuIIZnIIAlIIIZrIV-LDHs, according to the previous report [61]. No
big differences have been revealed except that the reduction temperaure of either CuII-O-CuII or CuII-
O-M species with a heating rate of 2 oC min-1 is about 20 oC lower than that with a heating rate of 10
oC min-1. The reduction kinetics changes with varied temperature-programmed rate, and the integral
areas from the reduction of CuII are incomparable, while the amounts of H2 consumption should be
constant. So the H2-TPR profiles have been transformed to H2 consumption as a function of time (Fig.
2a). Theoretically, the integral area for heating rate of 2 oC min-1 is similar to that for heating rate of
10 oC min-1 in the H2-TPR profiles. Interestingly, the time interval between the reduction of CuII-O-
CuII and CuII-O-M species is about 40 min for a heating rate of 2 oC min-1, while only 6 min for 10
oC min-1. That is, in a slower temperature-programmed rate (2 oC min-1 here), after CuII-O-CuII specie
has gone through the conversion to Cu0 and a formation of primary Cu ensembles for about 40 min,
CuII-O-M species has just started to be reduced. So the Cu0 species from CuII-O-M has the chance to
grow onto the primary Cu ensembles already from CuII-O-CuII but with another growth orientation,
- 14 -

thus affording nano-twin Cu particles. We call such a reduction process as “gradient reduction”. The
gradient reduction of CuII and the formation of nano-twin Cu particles have been directly visualized
from in situ HR-TEM images (Fig. 2b-f). In the in situ calcination of CuIIZnIIAlIIIZrIV-LDHs under
H2 from ambient temperature in 2 oC min-1, irregular clusters are observed to emerge at 150 oC (Fig.
2b). With temperature held at 150 oC, the cluster gradually grows, affording a ensemble (1) with size
of ~ 3 nm and a interplanar spacing of 0.209 nm attributed to Cu (111) (Fig. 2c and 2d). In about 150
s, another ensemble (2) appears and grows at upper right of Cu ensemble (1) (Fig. 2e), and finally
forming a nano-twined Cu particle with two partially overlapped Cu ensembles (Fig. 2f). The in situ
HR-TEM results provide strong evidence for the origin of nano-twin Cu particles by a reduction of
CuII-O-M specie after the redution of CuII-O-CuII specie, rather than the aggregation of two individual
Cu particles. So defects-rich nano-twin Cu particles could be conveniently prepared from Cu-
containing LDHs by a gradient reduction process of CuII in different chemical micro-environments.
This gradient reduction strategy, originally proposed here, could also be extended to developing
diverse defects-rich supported nano-metallic materials. However, due to the existence of the energy
from the electron beam in TEM, the reduction of CuIIZnIIAlIIIZrIV-LDHs occurs in a lower
temperature under the conditions of in situ TEM experiment than that under practical condition. The
evidence from in situ TEM experiment could not powerfully explain the difference in reduction
behavior between two heating rates (2 and 10 oC min-1). This deserves an in-depth investigation using
more suitable in situ characterization techniques, and further study is being carried out in our
laboratory.
- 15 -

Figure 2. (a) H2-TPR profiles as a function of time for CuIIZnIIAlIIIZrIV-LDHs with a temperature
programmed rate of 2 or 10 oC min-1. Sequential HR-TEM images (b-f) from the same area in the in
situ calcination of CuIIZnIIAlIIIZrIV-LDHs under H2 at 150 oC. A 5 nm scale bar in b-f.
3.2. Catalytic performance of defects-rich nano-twin Cu particles
The Zn(Al)(Zr)O supported Cu particles were applied as the catalysts for the aqueous phase
o
hydrogenation of furfural in autoclave reactor under 2.5 MPa initial H2 pressure and 160 C. The
qualitative identification of each product are based on the results from 1H and 13C nuclear magnetic
resonance (NMR) spectra and mass spectra (MS) (see details in the Supporting Information,
Appendix). Regular Cu particles (Fig. 3a left) exhibit more sluggish kinetics of furfural-to-CPO
conversion than nano-twin Cu particles (Fig. 3b left). FA, HCP, and CPEO as intermediates are
detected in either case. Over regular spherical Cu particle, a CPO selectivity of 61.5 % with furfural
conversion of 96.0 % in 3 h has been achieved, with the selectivity to HCP and CPEO of up to 11.5 %
and 20.1 %. With the reaction time prolonged to 4 h, neither HCP nor CPEO is transformed
completely, and meanwhile the over-hydrogenation of C=O in CPO (to CPL) is observed visibly.
Over defects-rich nano-twin Cu particles, a selectivity of 92.3 % to CPO with an almost complete
conversion of furfural in 3 h has been afforded (Fig. 3b). With the reaction time prolonged to 4 h,
- 16 -

both HCP and CPEO have been transformed completely. The results indicate the defect sites on the
surface of nano-twin particles are highly efficient for the selective hydro-deoxygenation of C-OH in
HCP and the selective hydrogenation to C=C in CPEO.
The catalytic conversion of HCP (Fig. 3, right), one of the major intermediates in the conversion
of furfural to CPO, confirms the superior catalysis of nano-twin Cu particles to that of regular
spherical Cu particles. In the selective hydro-deoxygenation of HCP, nano-twin Cu particles afford a
complete conversion of furfural within 90 min. The conversion of CPEO is accomplished in 2.5 h,
and the yield of CPO approaches 100 %. No visible excessive hydrogenation product (CPL) has been
detected. On regular spherical Cu particles, however, neither HCP nor CPEO has been complete
transformed in 2.5 h, while excessive hydrogenation from CPO to CPL is clearly observed.
Fig. 3. Reaction progress profiles of liquid phase hydrogenation under 2.5 MPa initial H2 pressure of
o
o
furfural at 160 C (left) and HCP at 100 C (right) over (a) regular spherical Cu particle in in Cu0-
Zn(Al)(Zr)O-10 and (b) nano-twin Cu particle in in Cu0-Zn(Al)(Zr)O-2.
- 17 -

3.3. Defect sites dependent catalytic reaction mechanism
To further confirm the structure-function relationship discovered above, the as-prepared Cu0-
Zn(Al)(Zr)O-2 was re-treated by calcination in N2 at 450 oC for 2 h followed by holding in H2 at 450
oC for 2 h (denoted Cu0-Zn(Al)(Zr)O-2-R). The defects-rich Cu nano-twin particles in Cu0-
Zn(Al)(Zr)O-2 are transformed to regular spherical Cu particles in Cu0-Zn(Al)(Zr)O-2-R (Fig. S6).
Based on the FT-IR results of CO adsorption, the percentage of Cu defect sites for Cu0-Zn(Al)(Zr)O-
2-R has been decreased to 31 %, just at a similar level to that for Cu0-Zn(Al)(Zr)O-10 (Fig. S7). By
testing the intrinsic catalysis in a continuous-flow system, Cu0-Zn(Al)(Zr)O-2 affords a CPO
formation rate of 77 mmol/mol(surface Cu)/s, while only 40 mmol/mol(surface Cu)/s for Cu0-Zn(Al)(Zr)O-
2-R and 24 mmol/mol(surface Cu)/s for Cu0-Zn(Al)(Zr)O-10 in same case (Fig. 4). With the decrease in
Cu defect sites, CPO formation rate accordingly decreases. It is clear that surface Cu defects is one
important factor to efficiently promote the selectivity of CPO.
Fig. 4. (a) Variation of CPO formation rate with the percentage of Cu defect sites at < 20% furfural
conversion, under 2 MPa H2 pressure at 160 oC in a fixed-bed reactor.
The in situ FT-IR spectra for HCP hydrogenation were further recorded (Fig. 5) to elucidate the
role of surface defects of nano-twin Cu particles in the selective hydrogenation of C-OH, C=O, and
C=C bonds. The absorption bands assigned to C=O at 1710 and 1672 cm−1, C=C at 1584 cm−1 on
- 18 -

defects-rich nano-twin Cu particles and 1579 cm−1 on regular spherical Cu particles, C-C at 1194
cm−1, C-O adsorbed with O terminal in linear at 1107 cm−1, and C-O adsorbed with O terminal in
bridged mode at 1052 cm−1 are observed. On defects-rich Cu particles, the bands assigned to C-O in
O-terminal adsorption gradually decrease in intensity and eventually disappear with on-stream time
under hyrogenation conditions, indicative of an efficient cleavage of C-O linkage, which is not visibly
observed on regular spherical Cu particles. On defects-rich Cu particles, a new band at 1273 cm-1,
assigned to η2-type adsorbed C-O [62,63], emerges and gradually increases in intensity with the
vanishment of O-terminal adsorption. This transformation of adsorbed C-O (from O-terminal
adsorption to simultaneous adsorption of C and O) benefits the breakage of C-O, well accounting for
more efficient hydro-deoxygenation to C-O in HCP on defects-rich Cu than on regular spherical Cu.
The C=C absorption decreases rapidly in intensity till disappears whilst leaving the C=O unchanged
on defects-rich Cu particles. Visible weakening of both C=O and C=C absorption, whilst none
disappearing, has been observed on regular spherical Cu particles. This illuminates a highly selective
hydrogenation to C=C, rather than to C=O, on defects-rich Cu surface. To clarify the activation to
C=C in HCP or CPEO, the FT-IR spectra with CPEO adsorbed are recorded. Since no FT-IR spectra
of CPEO adsorption has been reported previously, the FT-IR spectrum of CPEO in gaseous state has
first been recorded (Fig. S8). Adsorbed on defects-rich Cu surface, the absorption band of C=C is
observed at same position as that in gaseous state (Fig. 5a). With Ar purging, the intensity of ν (C=C)
fleetly decreases in intensity and eventually disappears on defects-rich Cu, indicative of the activation
of C=C and subsequent conversion to C-C (Fig. 5a). But such adsorption and activation to C=C in
CPEO are not observed on regular spherical Cu particles (Fig. 5b). The FT-IR results therefore
suggest that the distinctive defects on nano-twin Cu surface play a decisive role in promoting the
CPO selectivity by facilitating the selective hydro-deoxygenation of C-O in HCP and selective
hydrogenation to C=C in HCP and CPEO.
- 19 -

Fig. 5. Operando FT-IR spectra of HCP hydrogenation process (left) at 100 °C recorded at intervals
of H2 flow (0, 1, 2, 5, and 10 min), and FT-IR spectra of CPEO adsorption (right) at 50 °C recorded
at intervals of Ar flushing (0, 2, 5, and 30 min) for (a) nano-twin Cu particles and (b) regular Cu
particles. The absorption bands at 1650-1750, 1580 ± 10, 1273, 1194, 1107, and 1052 cm−1, according
to previous reports [62,63], are ascribed to the ν (C=O), ν (C=C), η2 type adsorbed C-O, ν (C-C) of
methylene, monodentate and bidentate adsorbed C-O group in HCP or CPEO.
On Zn(Al)(Zr)O (without Cu) with a specific surface area of 97.2 m2/g (Table S1), furfural can
barely be converted. Using FA as catalyzed substrate on Zn(Al)(Zr)O, a selectivity of 100 % to HCP
is obtained with a FA conversion of 40 %, under either N2 or H2 atmosphere. No CPEO, CPO, or
CPL is detected. That means, it is the metallic Cu that is responsible for the hydrogenation of furfural-
to-FA and HCP-to-CPO, while Zn(Al)(Zr)O for the conversion of FA to HCP. The conversion of FA
to HCP involves the rearrangement of furan ring to five-membered carbon ring, which is reported as
- 20 -

referring to an acid-catalyzed Piancatelli rearrangement [43]. According to the results from
temperature programmed desorption of ammonia (NH3-TPD) (Fig. S9), the distribution of acidic
strength and the number or density of acid sites for Cu0-Zn(Al)(Zr)O-2 and Cu0-Zn(Al)(Zr)O-10 are
similar, well accounting for their similarity in the catalysis for FA conversion.
4. Conclusions
In summary, this work first reports an effective gradient reduction strategy to control the definite
structure of Cu sites by producing defects-rich nano-twin Cu particles. This strategy just involves the
reduction of Cu-containing LDHs simply with controlling the reduction gradient of CuII species under
two chemical micro-environments, exhibiting two-step nucleation-growth processes. The nano-twin
Cu particles efficiently promote the target activation of C-O and C=C in the conversion of furfural to
CPO. The defects-rich Cu particles afford a CPO selectivity of 92 % with ~ 100% furfural conversion,
50 % higher than regular spherical Cu particles. The multi-stepped surface defect sites, originating
from the planar defects, play a decisive role in promoting the CPO selectivity by facilitating the
hydro-deoxygenation of C-O and hydrogenation to C=C in 4-hydroxycyclopentenone intermidate.
The strategy reported here can be extended to development of surface defects-rich supported metal
catalysts for a broad spectrum of green chemical processes.
Acknowledgements
Financial supports from NSFC of China (21521005, 91634120), National Key R&D Program of
China (2017YFA0206804) and the Fundamental Research Funds for the Central Universities
(ZY1936 and XK1802-6) are gratefully acknowledged. We also thank Jun Luo and Wei Xi (Tianjin
University of Technology) for their help in the in situ HR-TEM measurement and discussion.
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A Gradient Reduction Strategy to Produce Defects-Rich Nano-Twin Cu
Particles for Targeting Activation of Carbon-Carbon or Carbon-Oxygen in
Furfural Conversion
Yanru Zhu, Jian Zhang, Xiaodan Ma, Zhe An, Shaowei Guo, Xin Shu, Hongyan Song, Xu Xiang,
and Jing He*
An effective and convenient “Gradient Reduction” strategy is originally proposed to produce
defects-rich nano-twin Cu0 particles from CuII-containing LDHs. Based on a unique perspective onto
the bonding feature in brucite-like layer of LDHs, kinetically segregated reduction of CuII-O-CuII
from that of CuII-O-M has been revealed. The decisive role of defect Cu sites in the selective
activation of target bonds in furfural-to-cyclopentanone reaction is demonstrated.
- 26 -

> An effective and convenient “gradient reduction” strategy to construct surface defects by producing
nano-twin Cu particles is presented.
> Utilizing the bonding feature and chemical micro-environments in brucite-like layer of LDHs, a
kinetically segregated reduction of CuII-O-CuII from that of CuII-O-M is revealed.
> The particles efficiently promote the target activation of C-O and C=C in the conversion of furfural
to cyclopentanone.
> The defect sites on nano-twin Cu play a decisive role in promoting the cyclopentanone selectivity.
- 27 -

A Gradient Reduction Strategy to Produce Defects-Rich Nano-Twin Cu
Particles for Targeting Activation of Carbon-Carbon or Carbon-Oxygen in
Furfural Conversion
Yanru Zhu, Jian Zhang, Xiaodan Ma, Zhe An, Shaowei Guo, Xin Shu, Hongyan Song, Xu Xiang, and Jing He*
The authors declare no competing financial interest.
- 28 -