A "competitive occupancy" strategy toward Co-N4 single-atom catalysts embedded in 2D TiN/rGO sheets for highly efficient and stable aromatic nitroreduction

Article abstract of DOI:10.1039/c9ta13615k

Single-atom catalysts (SACs) have been promising in various catalytic fields, but the controllable synthesis of SACs with high stability remains challenging. Here, we show a robust strategy toward the fabrication of highly efficient and stable SACs based

Full text of DOI:10.1039/c9ta13615k

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DOI: 10.1039/C9TA13615K
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A “competitive occupancy” strategy toward Co-N4 single-atom
catalysts embedded in 2D TiN/rGO sheets for high-efficient and
stable aromatic nitroreduction
Received 00th January 20xx,
Accepted 00th January 20xx
Ying Gu,a Aiping Wu,b Lei Wang,b Dongxu Wang,b Haijing Yan,b Peng Yu,b Ying Xie,b Chungui Tian,*b
Fanfei Sunc and Honggang Fu*ab
DOI: 10.1039/x0xx00000x
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The single-atom catalysts (SACs) have been promising in catalytic fields, but the controllable synthesis of SACs with high
stability remain challenging. Here, we show a robust strategy toward the highly-efficient and stable SACs based on
“competitive occupancy” of two metal (M) species on graphite oxide (GO). The abundant M1 (Ti) species predominantly
occupy more groups of GO, thus leaving a tiny of groups at the gaps of M1 species to combine with M2 (Co2+ etc.) species,
and consequently, guaranteeing the formation of Co-N4 SACs embedded in 2D TiN/rGO sheets during the nitridation. Also,
r
the TiN can act as a “spacer” to prevent Co-N4 from aggregation, thus improving the stability of the catalyst. The Co-N4/TiN-
rGO exhibits outstanding catalytic performance for fast conversion of high-concentrated aromatic nitro compounds (0.3~2
mM) into amino compounds and excellent re-cycled ability, being at the forefront of reported catalysts. In contrast, the
TiN/rGO has no obvious activity, and Co4N-rGO prepared without competition of Ti shows poor activity and stability, which
indicate vital role of Co-N4 and TiN for remarkable catalytic ability of Co-N4/TiN-rGO. The reaction mechanism is also
proposed based on the theoretical calculation. The strategy is indicative to design Ni (Fe, Cr, Cu)-based SACs .
Introduction
high-efficient catalysis.9-13 The noble metal (Pt, Pd) SACs are
promising for hydrogenation,14 formic acid oxidation15 etc.
The liquid-phase catalysis (oxidation of alcohols and benzene,
coupling reactions, and reduction of nitroarenes etc.) plays a
vital role for producing fine chemical products.1-3 Typically, the
reduction of nitro compounds to corresponding amines is one
of the important processes in the fine and bulk chemical
industry. The high-efficient and low-cost catalysts are necessary
to promote reaction and increase the selectivity. Precious
metals (Pt, Pd etc.) are most effective catalysts due to its special
d-electron structure, but suffer from high cost and low
abundance.4,5 The fact impels intensive attentions on
decreasing the usage of Pt (Pd) or replacing them by non-noble
metals, which can be realized by tuning the shape, size and
designing like-Pt catalysts (WN, Mo2N, Mo2C etc.).6-8 Eventually,
the decrease of size to single atom can change of electronic
state of catalysts, thus tuning the adsorption/activation
capacity of the reactants on single atom catalysts (SACs) for
Especially, transition metal-based SACs (TM-SACs) have
received extensive attention due to their low cost and specific
characteristics benefited for the application in OER,16 H2O2
synthesis17 and CO2 reduction.18
The SACs should be stabilized on supports through M-N (O, S
etc.) bonds due to the high surface energy.19-21 Its activity can
be governed by the kind of metal elements and also the types
of anion sites. The combination of TMs with N (P, C) can tune
the metal d-band structure and change density of states near
the Fermi level, thus regular the catalytic ability.22,23 In this
regard, the single-atom TMs (Fe, Co and Ni) stabilized by N sites
on carbons can activate hydrogen (oxygen etc.) benefiting from
the coordinatively unsaturated metal sites. Generally, the
catalysts are obtained by the pyrolysis of the carbon source
combined with metal ions. The use of excessive metals can
result in the formation of large particles, which should be
etched by acid and thus bring potential environmental
pollution.24-28 Moreover, some particles protected by carbons
can not be completely etched. The particles are potential active
sites, thus can affect the understanding of the activity origin.29,30
Besides, the TM-N SACs embedded into carbon can be formed
by pyrolysis of metal-organic framework (MOFs) precursor
under NH3 atmosphere, followed by acid leaching,31,32 or be
prepared by introducing easily vaporized Zn species into
MOFs.33 The strategy is effective but is effected by the yield and
cost of MOFs to some content.34,35 The easy synthesis of SACs
a. Key Laboratory of Superlight Materials and Surface Technology of Ministry of
Education, College of Materials Science and Chemical Engineering, Harbin
Engineering University, Harbin 150001, P. R. China. E-mail: fuhg@hlju.edu.cn,
fuhg@vip.sina.com;
b. Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education
of the People’s Republic of China, Heilongjiang University. Harbin 150080, China
chunguitianhq@163.com, tianchungui@hlju.edu.cn.
c. Shanghai Synchrotron Radiation Facility (SSRF) Shanghai Institute of Applied
Physics, Chinese Academy of Sciences, Shanghai 201204, China;
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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with good activity and stability is desirable for the practical sequence, which was remained at 28oC water bath for 20 h
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application.
under magnetic stirring. Finally, 80 mL of dDiOsIt:il1l0e.d10w39a/tCe9rTAa1n3d61250K
Based on the analyses on the preparation methods of most mL of 30% H2O2 aqueous solution were added into the reaction
SACs, the suitable path for the preparation of the SACs should system, followed by washing with 2 mM HNO3 aqueous
be able to easily control the metal content and dispersion on solution. Graphene oxides were obtained after centrifuging
supports during the “precursor” step and to avoid the (12000 rpm), washing with alcohol and drying.
subsequent formation of large particles. The separation and
The preparation of M-N/TiN-rGO
stabilization of SACs by a “physical spacer”, rather than only M-
0.1 g of graphene oxide (GO) were dispersed into 10 mL of
N (S, O) bond, is especially desirable to hinder the aggregation
alcohol followed by a sonication for 1 h. Then, 30 mL of
of SACs under the harsh catalytic conditions. The GO with
cyclohexane was added to the dispersion. A certain amount of
abundant functional groups, a well-known support for SACs, can
Cobalt nitrate and TBT was dropwised into GO dispersion and
combine with various metal precursors to give oxides (TiO2 or
stirred for 12 h at room temperature. After that, the dispersion
CoO etc.).36,37 So, it is proposed that one metal species having
was centrifuged and washed three times with cyclohexane to
stronger interaction with GO predominantly occupy more
remove the remained TBT and cobalt nitrate in solvent. Next,
groups on GO, so as to control the amount and dispersion of the
the solids were re-dispersed into 45 mL cyclohexane and then
weaker one on GO. Here, we showed that the high positive
transferred to a 100 mL autoclave and kept in an electric oven
charge of Ti species (TBT, titanium (IV) butoxide) can
at 180oC for 5 h. The product was collected after centrifugation,
predominately combine with GO compared to Co2+ (Fe3+, Ni2+
then washed with ethanol for several times, and dried in oven
etc) species. Thus, most of the groups on GO can be
at 40oC. Finally, the solids were calcinated at NH3 atmosphere
predominantly occupied by Ti species. Only trace Co2+ can
with a heating rate of 3oC min-1 to 800oC and maintained at this
temperature for 2 h (flow rate is 40 sccm). As obtained black
solids were named as Co-N4/TiN-rGO. In order to study the
influence of TBT on the “competitive occupancy” process, the
combine with the groups remained at the gaps of Ti species on
GO. The "competitor” and “spacer” roles of Ti species makes the
formation and stabilization of Co-N4 SACs embedded in 2D
TiN/rGO sheets by the nitridation without additional acid-
amounts of TBT were tuned from 0 mL to 6 mL. The samples
treatment.(Scheme 1) As a result, the Co-N4/TiN-rGO have
obtained in the absence of Ti species was named as Co4N-rGO.
shown excellent activity and stability by taking the reduction of
The samples and corresponding preparation conditions were
aromatic nitro compounds (ANs) into amine as a proof-of-
listed in Table S1. The procedures for the synthesis of Fe-N/TiN-
concept. The catalyst can fast convert the various ANs with
rGO, Ni-N/TiN-rGO, Cr-N/TiN-rGO and Cu-N/TiN-rGO
high-concentration (up to 2 mM) at low temperature. The
nanostructures were similar to that of Co-N/TiN-rGO excepting
conversion rate of 4-NP to 4-AP on Co-N4/TiN-rGO (0.098 s-1) is
the use of the corresponding metal salts instead of cobalt salt.
much higher than on Co4N/rGO (0.037 s-1) which is prepared
without “competitive occupancy” of Ti species. The catalyst Characterizations
have shown superior stability with no loss of activity after 15
Wide-angle X-ray diffraction (XRD) was recorded in the 2θ
times of reuses. The activity and stability is at the forefront of
range of 10-80° on a Bruker D8 Advance X-ray diffractometer
the reported catalysts. The strategy is also indicative to design
with CuKα (λ = 1.5418 Å) radiation (40 kV, 40 mA).
Thermogravimetric (TG) analysis was obtained on a SDT Q600
instrument with a constant flow of air. The scanning electron
Ni (Fe, Cr, Cu)-based SACs by the competitive occupancy of
corresponding metal salts with TBT.
microscopy (SEM) mappings were obtained on a scanning
electron microscope (Hitachi S-4800) with an acceleration
voltage of 5 kV. The transmission electron microscopy (TEM)
Experimental Section
Materials and chemicals
and high-resolution TEM (HRTEM) tests were performed using
a JEM-F200 electron microscope (JEOL, Japan) with an
acceleration voltage of 200 kV. Carbon-coated copper grids
were used as sample holders. X-ray photoelectron spectroscopy
(XPS) analysis was carried out on a VG ESCALABMK II with a Mg
Kα achromatic X-ray source. The content of Co and Ti was
determined with a inductively coupled plasma-optical emission
spectrometry (ICP-OES, PerkinElmer Optima 7000DV analyzer).
Ultraviolet-visible (UV-vis) spectra were recorded using a
spectrophotometer (UV-1800B). Raman spectra was collected
by using a Jobin Yvon HR 800 micro-Raman spectrometer with
a 457.9 nm laser as excitation source. The XAFS at Co K-edge
was recorded at a transmission mode by using ion chambers at
beam line BL14W1 of the Shanghai Synchrotron Radiation
Facility (SSRF) of China with a Si (111) double-crystal
Cobalt nitrate (Co(NO3)2•6H2O), iron nitrate(Fe(NO3)3•9H2O),
nickel
nitrate
(Ni(NO3)2•6H2O),
chromium
nitrate
(Cr(NO3)3•9H2O), cupric nitrate (Cu(NO3)2•3H2O), titanium (IV)
butoxide (Ti(OBu)4), cyclohexane and ethanol (C2H6O, 99.0%)
were purchased from Aladdin Co., China. P-nitrophenol
(C6H5NO3) and Sodium borohydride (NaBH4) was purchased
from Sinopharm Chemical Reagent Co., Ltd. All chemical
reagents were used as received without further purification.
Distilled water was obtained from an analytical laboratory.
Preparation of graphene oxide (GO)
Graphene oxide (GO) was synthesized from natural graphite
powder by a modified Hummers method. In detail, 2 g of
expanded graphite was added into 50 mL of 98 wt% H2SO4 in a
250 mL beaker under magnetic stirring. Then, 2 g of sodium
nitrate and 6 g of KMnO4 was added slowly to the dispersion in
2
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be directly reflected by the colors of the supernatant after the
DOI: 10.1039/C9TA13615K
adsorption of Co precursor (10 mg) on GO in the presence of
monochromator. The photon energy was calibrated with Co
standard foil.
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different amount of TBT (0-6 mL) (Figure S1). Without adding
TBT, the supernatant have no distinct color, indicating the
complete combination of Co2+ with GO. The addition of 1.5 mL
TBT can make a pale pink color of the supernatant, implying the
little combination of Co2+ with GO. The presence of 3 mL (or 6
mL) of TBT leads to deeper color of supernatant close to the
original ones, indicating the combination of trace Co2+ with GO.
The TG analyses (Figure S2 and Table S2) show that the amount
of Ti on the final Co-N/TiN-rGO increased from 12.8% to 26.6%
and 27.1% as the addition of 1.5 mL, 3 mL and 6 mL of TBT.
Obviously, the amount of Ti species on GO increases with the
increase of TBT amount. The adsorption amount is close to
saturation as the addition of 3 mL of TBT. The inductively
coupled plasma optical emission spectrometry (ICP-OES)
analysis have revealed 0.428 wt%, 0.136 wt% and 0.08 wt% Co
amount, corresponding to the use of 1.5 mL, 3 mL and 6 mL TBT
during the “competitive occupancy” process. The “competitive
occupancy” is also existence between the TBT and other metal
ions (Ni, Fe and Cr etc.).
The Catalytic Reduction of 4-Nitrophenol by NaBH4 on different
catalyst.
In a typical procedure, 5.0 mg of catalyst was dispersed into
2 mL distilled water via sonication. 3 mL (3 mM) of a 4-NP
aqueous solution was mixed with 25 mL of distilled water in a
weighing bottle (40 mm × 70 mm). The bottle was immersed in
a thermostatic water bath and then NaBH4 (8 mg) was added
into the dispersion. The concentration of 4-NP after dilution is
0.3 mM. The conversion of 4-NP to 4-AP was initiated by adding
catalyst (5 mg) to the deep-yellow dispersion. The reaction
progress was timely monitored by UV-vis spectrophotometer.
In reused test, the catalyst was separated via centrifugation and
washing with doubly distilled water. After drying at 60oC, the
catalyst was re-used for the next cycling reaction. The catalytic
activity of Co-N4/TiN-rGO was tested at different temperatures
(5-45oC) according to above procedure. In addition, the catalytic
activity of Co-N4/TiN-rGO was evaluated by using higher
concentrations of p-nitrophenol (1-2 mM). For comparison, the
control experiments were also carried out under similar
conditions by using rGO, TiN/rGO and Co4N-rGO as the catalyst
at 25oC. Furthermore, the catalytic reduction of other
nitroarenes were also performed followed the same procedures
at 25oC.
Theoretical calculation
The theoretical calculations are performed within the
framework of density functional theory (DFT) embedded in
CASTEP code. The exchange-correlation energy is treated with
generalized gradient approximation (GGA), using the Perdew-
Wang parametrization. The electronic wave functions at each k-
point are expanded in terms of a plane-wave basis set, and an
energy cutoff of 380 eV is employed. The ultrasoft
pseudopotential (USP) is used, which allows the calculations to
be performed with the lowest possible energy cutoff for the
plane-wave basis set. The whole optimization procedure is
repeated until the average force on the atoms is less than 0.01
eV/Å and the energy change less than 5.0×10-6 eV/atom. The
sampling over the Brillouin zone is treated by a (2×2×2)
Monkhorst-Pack mesh. The thickness of vacuum was 15 Å to
make sure that there was no superficial interaction between
different layers.
Scheme 1. Schematic process of “competitive occupancy” for the
synthesis of Co-N4/TiN-rGO.
Above results show that the amount of Co species on GO can
be controlled by “competitive occupancy” of Ti species. An
appropriate amount of Co can combine with GO by using 3 mL
TBT. So, the sample prepared in the case (named Co-N4/TiN-
rGO, ESI) was characterized in detail to verify the effectiveness
of present strategy toward stable Co-N SACs. The precursor
from hydrothermal treatment shown a thin two-dimensional
layer structure (Figure S3). The XRD patterns of Co-N4/TiN-rGO
sample (Figure 1a) show a peak at around 26o, indexing to (002)
reflection of the hexagonal graphite. The peaks located at 36o,
43o, 62o are indexed to TiN (111), (200), (220) diffractions
(PDF#38-1420), respectively. No diffraction peaks of cobalt
Results and discussion
Material preparation and characterizations
We have proposed a “competitive occupancy” strategy
toward the conventional synthesis and stabilization of SACs. In
the route, the metal species (M1) having stronger interaction
with GO can act the "competitor” and “spacer” of weaker one
(M2). We have demonstrated the idea by taking the Ti species
(TBT, titanium (IV) butoxide, M1) and Co species (Co2+, M2) as a
proof-of-concept. The Co(NO3)2 and TBT are combined with GO
by competitive occupancy. The role of TBT as "competitor” can
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