Single-atom molybdenum immobilized on photoactive carbon nitride as efficient photocatalysts for ambient nitrogen fixation in pure water

Article abstract of DOI:10.1039/c9ta06653e

A series of Mo single-atom catalysts were prepared by calcining a low-cost primary material of urea with various amounts of Na₂MoO₄·2H₂O. Isolated Mo centers are immobilized on in situ formed polymeric carbon nitride via coordinating with two N donors to form two-coordinated MoN₂ species. The low-coordinated Mo centers can serve as active sites for N₂ chemisorption and activation, achieving high photocatalytic activity for NH₃ evolution with a rate of 50.9 μmol g_cat^-1 h^-1 in pure water. In the presence of ethanol as the electron scavenger, the NH₃ evolution rate can reach 830 μmol g_cat^-1 h^-1, and the catalyst shows a quantum efficiency of 0.70% at 400 nm. This is the first single-atom catalyst that can drive photocatalytic N₂ fixation in pure water with comparable performance to recent reports for photocatalytic N₂ reduction. Experimental investigations and density functional theory calculations demonstrate that the coordinatively unsaturated metal center in the single-atom catalysts can strongly adsorb N₂via an end-on configuration to elongate the NN bond from 1.11 ? to 1.15 ?, and thus photoexcited electrons can transfer to the weakened NN bond for efficient nitrogen fixation under ambient conditions. These findings provide new insight for solar-driven N₂ fixation by atomically dispersing low-coordination metal centers on photoactive supports.

Full text of DOI:10.1039/c9ta06653e

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DOI: 10.1039/C9TA06653E
ARTICLE
Single-Atom Molybdenum Immobilized on Photoactive Carbon
Nitride as Efficient Photocatalysts for Ambient Nitrogen Fixation
in Pure Water
Received 00th January 20xx,
Accepted 00th January 20xx
Xiang-Wei Guo,‡a Shuang-Ming Chen,‡b Hong-Juan Wang,‡a Zhi-Ming Zhang,*a Hong Lin,a Li Songb
and Tong-Bu Lu*
DOI: 10.1039/x0xx00000x
A series of Mo single-atom catalysts were prepared by calcining low-cost primary material of urea with various amounts of
Na2MoO42H2O. Isolated Mo centers are immobilized on in-situ formed polymeric carbon nitride via coordinating with two
N donors to form two-coordinated MoN2 species. The low-coordinated Mo centers can serve as active sites for N2
−1
chemisorption and activation, achieving high photocatalytic activity for NH3 evolution with a rate of 50.9 μmol gcat h−1 in
pure water. In the presence of ethanol as electron scavenger, the NH3 evolution rate can reach to 830 μmol gcat−1 h−1, and
the catalyst shows a quantum efficiency of 0.70% at 400 nm. This is the first single-atom catalyst that can drive
photocatalytic N2 fixation in pure water with comparable performance to recent reports for photocatalytic N2 reduction.
Experimental investigations and density functional theory calculations demonstrate that coordinatively unsaturated metal
center in the single-atom catalysts can strongly adsorb N2 via an end-on configuration to elongate the N≡N bond from
1.11 Å to 1.15 Å, thus photoexcited electrons can transfer to the weakened N≡N bond for efficient nitrogen fixation under
ambient conditions. These findings provide new insight for solar-driven N2 fixation by atomically dispersing low-
coordination metal centers on photoactive supports.
Photocatalysis may provide a promising way by utilizing
solar energy for N2 fixation, which can supply photoexcited
electrons to activate N≡N bond for the NH3 synthesis (N2 +
Introduction
Ammonia synthesis from abundant N2 in Earth’s atmosphere
represents a crucial and challenging chemical transformation
to sustain all living organisms, as well as to generate potential
carbon-free condensed fuels as the liquefiable feature of
NH3.1-3 Up to date, industrial N2 fixation through classical
Haber-Bosch process and the biological ammonia synthesis are
recognized as two most important and efficient strategies.1 For
Haber-Bosch process, this industrial N2 fixation has to be
conducted under rigorous reaction conditions (15−25 MPa,
573−823 K), consuming 1-3% of global energy supply,4-6 and
releasing a huge amount of greenhouse gas.7,8 Its energy-
intensive issue has motivated many scientists to explore
alternative strategies for less-energy ammonia synthesis under
ambient conditions, especially for solar-driven N2 fixation as its
highly attractive and great potential to sustainable energy
solutions.9,10
3H2O → 2NH3 + 3/2O2).11-18 However, photocatalytic NH3
synthesis is still largely hampered as the poor binding between
N2 and photocatalysts, resulting in inefficient electron transfer
from catalyst to the lowest unoccupied molecular orbital
(LUMO) of N2. As a result, the prerequisite for N2 fixation is to
construct electron-rich CUMCs for chemisorption of N2 to build
a bridge between N2 and semiconductor. Recently, subtly
designed defects in photocatalysts to create CUMCs with low-
valence state has attracted wide attention.19-24 In this field,
numerous pioneering works, for instance,
a series of
photocatalysts BiOBr,20 Bi5O7Br,21 Layered-Double-Hydroxide22
and Mo-doped W18O4923 with CUMCs, have been performed to
promote the chemisorption and activation of N2. This progress
guides scientists to devote to introducing ultra low-
coordinated metal centers into photocatalysts to boost NH3
evolution.
Recently, single-atom catalysts (SACs) with extremely
exposed metal centers are widely concerned in heterogeneous
catalysis as their unique catalytic activity and maximum of
atom efficiency.25-36 For N2 fixation, first-principles calculations
have proposed SACs as ideal catalysts for solar-driven N2
fixation.37 However, single atom (SA) photocatalysts were
rarely explored. Presently, only atomically dispersed Cu and Fe
anchored on polymeric carbon nitride (PCN) were constructed
for photocatalytic N2 reduction with ethanol as electron
a. Institute for New Energy Materials
Materials Science and Engineering, Tianjin University of Technology, Tianjin
300384, P. R. China. E-mail: zmzhang@email.tjut.edu.cn;
lutongbu@mail.sysu.edu.cn.
& Low Carbon Technologies, School of
b. National Synchrotron Radiation Laboratory, CAS Center for Excellence in
Nanoscience, University of Science and Technology of China, Hefei, Anhui 230029,
P. R. China.
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
‡ The authors contributed equally to this work.
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before and (d) after recycling tests, isolated Mo centers are marked
scavengers, affording the NH3 evolution rate of 186 and 300
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−1
μmol gcat h−1, respectively.38,39 As well known, Mo element
with red circles.
DOI: 10.1039/C9TA06653E
has played critical role for N2 fixation in the most abundant
nitrogenise,40 and scientists have made great efforts to explore
Mo-based catalysts for artificial synthesis of NH3 under less-
energy conditions.23,41-42 Anchoring ultra low-coordinated SA
Mo on photoactive support, that can utilize solar energy to
replace the hydrolysis of ATP in organisms, may represent a
possible alternative to nitrogenise for ambient N2 fixation.
Nevertheless, this remains an urgent and great challenge. Up
to date, SA photocatalysts for N2 fixation in pure water has yet
to be achieved.43
Based on above consideration, we achieved a facile and large-
scale synthesis of Mo-based SA photocatalysts (Mo-PCN) with low-
coordinated MoN2 species as active sites for N2 fixation. The two-
coordinated Mo centers in Mo-PCN can effectively adsorb N2 and
transfer enriched photoexcited electrons to the weakened N≡N
bond, achieving efficient NH3 production with a rate of 50.9 μmol
gcat h−1 in pure water, and as high as 830 μmol gcat h−1 with
ethanol as an electron scavenger. To our knowledge, this is the first
SAC for photocatalytic N2 fixation in pure water, which exhibits
comparable performance to recent reports for photocatalytic N2
reduction (Table S1).
−1
−1
Figure 2. (a) XANES and (b) FT-EXAFS curves of Mo-PCN and
references at Mo Kedge. (c) FT-EXAFS fitting curves of 3-Mo-
PCN at Mo Kedge. (d) N 1s XPS spectra of PCN and 3-Mo-PCN.
nanoparticles can be observed in Mo-PCN (Figure 1a, S2-S4).
Elemental mapping analysis reveals the homogeneous
distribution of Mo, C and N elements over the SACs (Figure 1b,
S5 and S6). These findings are consistent with the powder X-
ray diffraction (PXRD) results, where only the humps of PCN
were observed (Figure S7). Further, infrared spectroscopy (IR)
spectra also show no obvious change of PCN after loading SA
Mo centers (Figure S8). In Mo-PCN, SA Mo centers can be
obviously discerned by atomic-resolution high angle annular
dark-field scanning TEM (HAADF-STEM) imaging, where Mo
SAs were confirmed by individual bright dots circled with red
circles (Figure 1c and 1d, Figure S9 and S10). More than 99.5%
Mo species on PCN show the size less than 0.2 nm (Figure S11),
demonstrating the SA dispersion of Mo centers, and the Mo
contents were determined by inductively coupled plasma mass
spectrometry (ICP-MS) to be 0.109, 0.231 and 0.346 wt% in 1-,
2- and 3-Mo-PCN, respectively (Figure S12).
Results and discussion
A series of Mo-PCN SACs (1-, 2- and 3-Mo-PCN) were facilely
prepared by calcining a low-cost mixture of urea with various
amounts of Na2MoO42H2O at 450 °C (Figure S1). The urea can in
situ transform to photoactive PCN,44-46 which was used to stabilize
SA Mo centers. High resolution transmission electron microscopy
(HRTEM) image demonstrates that Mo-PCN shows similar
morphology with that of pure PCN, and no Mo-based
To further determine the local structure of Mo-PCN,
extended X-ray absorption fine structure (EXAFS) and X-ray
absorption near-edge structure (XANES) were performed. In
the XANES curves (Figure 2a and S13), the absorption edge
energy of these SACs all between those of Mo foil and MoO2
references, revealing the positive low oxidation state of Mo SA
center. As shown in Figure 2b, only one obvious FT peak was
observed at ca. 1.2 Å in the FT-EXAFS curves of 3-Mo-PCN,
which was mainly attributed to the scattering of Mo-N
coordination. Meanwhile, no obvious Mo-Mo scattering peaks
were observed, confirming the atomic dispersion of Mo
centers on PCN, further supporting the HAADF-STEM
observation (Figure 1c). Quantitative EXAFS fitting reveals that
the Mo center was individually anchored on PCN via two Mo-N
coordination bonds (Figure 2c, S14 and S15, Table S3).
Additionally, one O2 molecule was considered to be adsorbed
Figure 1. (a) HRTEM and (b) elemental mapping images of 3-Mo-
PCN. Atomic-resolution HAADF-STEM images of 3-Mo-PCN (c)
2 | J. Name., 2012, 00, 1-3
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on Mo-PCN (Table S2). Further, with increasing the Mo content, the experimental turnover number (TON) of 276ViteowwAratircdlesOnMlinoe
DOI: 10.1039/C9TA06653E
the intensities of peaks A in N and C K-edge spectra (at 397.1
centers, and the catalyst shows a quantum efficiency of 0.70% at
400 nm. Figure 3b gave the relative mass activity on per gram of
Mo. Clearly, these SACs exhibit almost a constant mass activity of ca.
14.7 mmol gMo h−1 in pure water, implying that each isolated Mo
−1
center is competent for catalytic N2-to-NH3 conversion. These
results confirm the reliability of the NH3 production, and these SACs
with only 0.109-0.346 wt% Mo contents exhibit comparable activity
to recent reports for photocatalytic N2 reduction (Table S1). Also,
−1
the NH3 evolution rate can reach to as high as 239 mmol gMo h−1
with ethanol as an electron scavenger. In the photocatalytic process,
negligible H2 or N2H4 can be detected in the photocatalytic products
(Figure S20), indicating a high selectivity for NH3 formation in the
aqueous phase. Further, the 15N-labelling experiment was carefully
1
performed, and the product was identified by H NMR spectra. As
shown in Figure S21, the doublet pattern with coupling constant of
+
JN-H = 72 Hz, corresponding to 15NH4 in the acidic solution, can be
obviously observed, revealing that the produced NH3 indeed derives
from N2 fixation.23,47 This result was reaffirmed by the control
experiments, where no NH4+ can be detected under Ar, with PCN as
Figure 3. (a) Time course of NH3 evolution in the photofixation of N2
in pure water under 300 Xenon lamp illumination. (b)
W
Photocatalytic NH3 evolution rate of SACs with various Mo contents the catalyst or in the dark (Figure S22 and Table S4). During N2
normalized to Mo. (c) Photocatalytic oxygen evolution in the 3-Mo-
PCN-containing photocatalytic system. (d) Recycling tests for 3-Mo-
PCN with N2 in pure water under 300 W Xe lamp illumination.
fixation, O2 evolution was detected with a yield close to 3/4 of NH3
at the initial reaction stages (Figure 3c). This is a strong evidence
that water molecule can act as the source of electron for NH3
formation. 3-Mo-PCN also displays excellent stability with
a
constant NH3 formation rate after three cyclic tests (Figure 3d).
Further, 3-Mo-PCN retains its pristine morphology, and no
aggregation of SA Mo centers can be observed after three
successive catalytic cycles (Figure 1d, S23 and S24), demonstrating
outstanding durability of Mo-PCN SACs. Based on above
experiments, the origin of N atoms from the decomposition of PCN
can be excluded, confirming that N2, H2O and light are all necessary
for the NH3 formation.
and 286.6 eV, respectively) decrease obviously as the
reduction in number of π* bonds (Figure S15). This result
indicates that introducing Mo centers can locally alter the
electronic structure of PCN matrix.34 X-ray photoelectron
spectroscopy (XPS) characterization was also carried out to
examine the electronic structure change of PCN after Mo
loading (Figure 2d and S16). After loading SA Mo, the peak at
398.6 eV assigned to sp2 hybridized aromatic N in C=N–C shifts
to higher–energy at a certain extent, while no obvious change
was observed towards the peaks of tertiary N in N–(C)3, the N
in aromatic cycles bonded to three carbon atoms and π
excitations at 399.7, 401.1 and 404.6 eV, respectively (Figure
2d).34,47 These observations demonstrate the coordination of N
in C=N–C with SA Mo centers, which can reduce the electron
density of N atom via the formation of Mo-N bonds, being
consistent with the result of EXAFS analysis (Figure S17).
The catalytic activity for N2 fixation of Mo-PCN was evaluated in
double distilled water with different pH values under 300 W Xenon
lamp irradiation, indicating that these SACs are most active in pH 5
water (Figure S18). During a typical process, SAC was dispersed in
N2-saturated water, which can directly serve as the source of
electron and proton for NH3 production. Upon the irradiation, NH3
formed immediately, while the bare PCN was inactive (Figure 3a).
The activity of the SACs exhibits an approximately linear growth
with increasing the Mo contents, and total NH3 generation outputs
in pure water for 1-, 2- and 3-Mo-PCN within 12 h are 190.7, 402.8
and 611.2 μmol gcat−1, respectively (Figure 3a, S19). The NH3 yield
was double checked by the ion chromatograph (IC), which shows
comparable results to those determined by Nessler’s reagent (Table
S3). In the presence of ethanol as an electron scavenger, the NH3
Figure 4. (a) Time-resolved fluorescence kinetics at 298 K. (b) LT-
FTIR spectra after N2 adsorption at 80 K. (c) Transient photocurrent
responses of PCN and 3-Mo-PCN in N2 and Ar atmosphere. (d)
Charge density difference of Mo-PCN with adsorbed N2, the yellow
and cyan isosurfaces represent charge depletion and charge
-1
evolution rate of 3-Mo-PCN was as high as 830 μmol gcat h-1, with
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accumulation, respectively (carbon, grey; nitrogen, blue;
molybdenum, purple).
there is an identical process that the adsorbed N2 binds one
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*
proton coupled with an electron traDnOsfI:e1r0.1t0o39/fCo9rTmA06N625H3E
intermediate. Further, three protons are consecutively added
to the distal N atom to release one NH3 molecule for the distal
process. While for the alternating process, the protons were
alternately added to two N atoms in a N2 molecule, releasing
the first NH3 molecule until the fifth proton was adsorbed. As
shown in Figure S27, the rate determining step for both distal
and alternating processes is the transformation of N2 to N2H*,
which is an endothermic process requiring an energy barrier of
0.81 eV. These results imply that two reaction pathways can
simultaneously exist in the photocatalytic N2 fixation process.
Steady and time-resolved fluorescence were performed to verify
the transfer of photoexcited electron from PCN to Mo SA.
Compared with bare PCN, the photoluminescence intensity of
a
broad PL band around 450 nm for Mo-PCN becomes much lower
with increasing amounts of Mo loading (Figure S25). Time-resolved
fluorescence kinetics measurements also reveal that the average
radiative lifetimes reduce obviously in an order of PCN (10.62 ns) >
1-Mo-PCN (6.63 ns) > 2-Mo-PCN (5.82 ns) > 3-Mo-PCN (5.06 ns)
(Figure 4a), indicating efficient transfer of photoexcited electron
from PCN to Mo center to enhance the separation efficiency of
photoinduced electron-hole pairs.
To determine the status of absorbed N2 on Mo SA centers, Low-
temperature Fourier transform infrared (LT-FTIR) spectroscopy and
temperature-programmed desorption (TPD) measurements were
further performed. After N2 adsorption, the LT-FTIR spectra of 3-
Conclusions
In conclusion, we have developed a facile and large-scale
synthetic strategy to construct Mo-PCN SA photocatalysts for
N2 reduction to NH3, through calcining cheap urea with various
Mo-PCN shows
a
broad peak at 2197 cm−1 (Figure 4b),
amounts
of
Na2MoO42H2O.
Various
investigations
corresponding to adsorbed N2 molecules on Mo centers with an
end-on configuration.7 In the TPD experiment, the desorption peak
of chemisorbed N2 was only obviously observed on Mo-PCN SAC
(Figure S26), indicating the chemical adsorption of N2 on SA Mo
center in the SAC. In addition, the interfacial charge kinetics for PCN
and 3-Mo-PCN were detected to investigate the electron transfer
between SA Mo and absorbed N2 via the transient photocurrent
response experiments (Figure 4c). In this process, the photocurrent
generated by transfer of photoexcited electrons to the electrode in
N2 atmosphere is lower than that obtained under Ar, due to the
competitive interfacial electron transfer to chemisorbed N2
molecules.19,22 Moreover, compared with PCN, 3-Mo-PCN shows a
significant reduced photocurrent, confirming favorable energetic
electron transfer to the N≡N triple bond of chemisorbed N2 via the
SA center.
demonstrate for the first time that the resulting two-
coordinated Mo center can strongly adsorb N2 via an end-on
configuration to elongate the N≡N bond from 1.11 to 1.15 Å to
weaken the N≡N bond. Further, the CUMCs in Mo-PCN SACs
can act as a bridge to accept photoexcited electrons from PCN,
and donate the energetic electrons to N≡N triple bond to
achieve efficient N2 reduction at ambient conditions. The
photocatalytic NH3 production was first achieved with SACs
using water as a reducing agent, and the NH3 evolution rate
can reach as high as 14.7 mmol gMo−1 h−1 (50.9 μmol gcat−1 h−1).
This work provides new insight for solar-driven nitrogen
fixation by atomically dispersing low-coordination transition
metal centers on photoactive supports.
Density functional theory (DFT) calculations were conducted
to better understand the catalytic process for N2 reduction
(Figure S27). It should be noted that the side-on mechanism is
not considered here, as its configuration on SA Mo is not
stable. Also, the LT-FTIR experiments confirm the end-on
configuration of the absorbed N2 molecules (Figure 4b). For
the end-on configuration, DFT calculations demonstrate that
only physical adsorption exists between pure PCN and N2
molecule. The distance between adsorbed N2 and bare PCN is
approximately 3.30 Å, and the N≡N bond length is the same as
that of isolated N2 molecule. For Mo-PCN, the Mo center can
bind N2 and H2O molecule with an adsorption energy of -1.31
eV and -0.391 eV, respectively, implying a stronger chemical
adsorption between Mo center and N2 molecule. When N2
molecule approaches Mo center, a Mo-N coordination bond
was formed with a distance of 1.90 Å (Figure S28). Meanwhile,
the N···N distance was elongated from 1.11 Å to 1.15 Å,
indicating the N≡N bond is weaken via the formation of Mo-
N≡N. For the photocatalytic N2 reduction, there are two
reaction mechanisms for the end-on configuration, the distal
(Mn–N≡N → Mn+3≡N + NH3 → Mn + NH3) and alternating (Mn–
N≡N → Mn–NH=NH → Mn–NH2–NH2→ Mn + 2NH3) mechanisms
(Figure S29).38-41 In both distal and alternating mechanisms,
Experimental
Materials
Na2MoO4·2H2O (AR. ≥ 99.0%), urea (AR. ≥ 99.0%) and ethanol (AR.
≥99.7%) were purchased from Tianjin Fuchen Chemical Reagent Co.
Ltd. (Tianjin, China). Hydrochloric Acid (HCl ~37%) was purchased
from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).
Potassium sodium tartrate tetrahydrate (GR. 99.5%) and
Ammonium chloride-15N (10 atom%, ≥ 98.5%) were purchased from
Shanghai Aladdin Bio-Chem Technology Co. LTD. (Shanghai, China).
Nessler’s reagent was purchased from Beijing Innochem Science &
Technology Co. LTD. (Beijing, China). DMSO-d6 was purchased from
Cambridge Isotope Laboratories, Inc. (Andover, MA, USA). 15N2 ( >99
atom%) was purchased from China Dalian Delin Gas Packing co., Ltd.
(Dalian, China). Double distilled water was used in all the
experiments.
Characterization
XRD patterns were collected using a Rigaku Smartlab 9 KW
diffractometer. UV-Vis absorbance spectra were recorded on a
PemkinElmer Lambda 750 UV/VIS/NIR spectrometer. FT-IR was
recored on a PemkinElmer Frontier Mid-IR FTIR spectrometer.
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ICP-MS were recorded on a Agilent 7800. TEM and EDS The concentration of ammonia was measured by a colorimetric
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characterizations were achieved using JEOL JEM-2100, FEI method with Nessler’s reagent.50-53 AftDerOI:t1h0e.103p9h/oCt9oTcAa0t6a6ly5t3iEc
Talos F200X and FEI Titan Cubed Themis G2 300 with a probe reaction, the suspension was centrifuged, and the solution was
corrector. XPS measurements were performed with a Thermal filtered through a 220 nm membrane filter. To make sure the
scientific ESCALAB250Xi system with the Al Kα radiation as the absorbance for each solution is within the linear range of the
X-ray source. The amount of NH4+ in the solution was analysed calibration curve, the filtrate after the photocatalytic reaction was
by ion chromatography (DX-120, DIONEX) and Nessler’s diluted five times with double distilled water. Then, 100 µL of
reagent. Low-temperature Fourier transform infrared potassium sodium tartrate solution was added to 5 mL of the above
spectroscopy (LT-FTIR) spectra were performed using a Bruker solution. After blending, 150 µL of Nessler's reagent was added to
IFS 66v Fourier-transform spectrometer equipped with a this solution, which was left to stand for 30 min for full color
+
mercury−cadmium−tellurium (MCT) detector at BL01B in NSRL processing (Figure S31). The concentration of NH4 was determined
in Hefei, China. Mo K-edge XAFS spectra were collected at the using an UV-vis spectrophotometer at 420 nm. Calibration curves
Shanghai Synchrotron Radiation Facility (BL14W1, SSRF). The for NH3 assay using Nessler’s reagent (R2 = 0.9997) showed highly
+
X-ray was mono-chromatized by a double-crystal Si (311) linear responses with the concentration of NH4 in the
monochromator for SSRF. The acquired EXAFS data were concentration range of 0-2000 µg/L (Figure S32a).
processed according to the standard procedures using the
The concentration of ammonia was also measured by ion
WinXAS3.1 program.48 Theoretical amplitudes and phase-shift chromatography method. Ion chromatography for a series of
functions were further calculated with the FEFF8.2.49
reference solutions with suitable NH4Cl concentrations (0.50, 1.0,
1.5, and 2.0 µg/mL) were acquired and a calibration curve (R2
=
Synthesis of Polymeric Carbon Nitride (PCN)
0.9996)) was plotted (Figure S32b). The filtrate after the
photocatalytic reaction was acidized with 0.1 M HCl and then
measured by ion chromatography method.
The ammonia production rates with different detection methods
was shown in Table S2, which supplied similar results with the
detected NH3 concentration matching closely.
PCN was synthesized by calcining urea at 450 ℃for 3 h (ramp
rate, 5 ℃/min) in a crucible with a cap. The capped crucible
was placed in the muffle furnace under the air atmosphere.
After cooling to room temperature, the yellow product was
washed with double distilled water and ethanol, and
subsequently dried at 80 ℃ for 12 h.
Determination of Apparent Quantum Efficiency
In this process, LED light (λ = 400 nm, 100 mW·cm-2, irradiation area
0.8 cm2) was utilized as a monochromatic light source. The
quantum efficiency at 400 nm was calculated on the basis of the
following equation: quantum efficiency = 100% × (number of
generated ammonia × 3 / number of incident photons at 25 °C). The
quantum efficiency of 3-Mo-PCN at 400 nm was determined to be
0.7% in the presence of ethanol as electron scavenger.
Synthesis of Mo-PCN SACs
4 g urea was dissolved in 16 mL double distilled water, followed by
the addition of 1, 2 or 3 mg of Na2MoO42H2O into this aqueous
solution. The resulting mixture was stirred for 3 h at room
temperature to ensure the uniform dispersion. Then, the water was
removed by rotary evaporator. The resulting white product was
calcined at 450 ℃ (ramp rate, 5 ℃/min) for 3 h in a muffle furnace.
The obtained yellow product was washed with 1 M HCl, double
distilled water and ethanol, and subsequently dried at 80 ℃ for 12
h. The yellow products were named as 1-, 2- and 3-Mo-PCN,
respectively. Also, this facile experimental procedure can be easily
enlarged by changing the dosage of urea/Na2MoO42H2O. As a
result, 20 g urea with 5 mg, 10 mg and 15 mg of Na2MoO42H2O can
be used to synthesize 1-, 2- and 3-Mo-PCN, respectively, limited by
the space of muffle furnace in the lab.
Isotope-labeled Experiments
An isotopic labeling experiment used 15N2 as the feeding gas to
clarify the nitrogen source of ammonia. After the photocatalytic
reaction, the obtained filtrate was then mixed with 20% DMSO-d6.
NMR measurements were then performed on a Bruker III HD 400
MHz system. The standard solution of NH4Cl with 15N enrichment of
1
10% was further detected by HNMR to confirm the generation of
15NH4+ during the photocatalytic process.54-56
Photocatalytic Reaction
N2-Temperature-Programmed Desorption (TPD) Measurements
3 mg of photocatalyst was added to 6 mL double distilled water
within a quartz reactor, and dispersed well by ultrasonication (60
min). The pH value of the photocatalytic system was adjusted with
0.1 M HCl. The reactor was equipped with a recycled water to
maintain the room temperature. High-purity N2 was bubbled into
the solution in dark with stirring for 60 min to obtain a N2-saturated
aqueous suspension. Then, the reactor was irradiated using the 300
W Xe lamp (PLS-SXE300D) for 12 h.
TPD measurements were performed to understand N2 adsorption
on the surface of PCN and 3-Mo-PCN. Typically, the sample was
placed in a glass tube, and was pretreated by He gas flow at 300 °C
for 2 h, and then cooled down to 50 °C. The adsorption of N2 was
conducted in a 99.999% N2 gas fow for 1 h at 50 °C. After purging
with He gas for 1 h to remove the residual N2, the sample was
heated from 50 °C to 400 °C at a rate of 10 °C min-1. The TPD signal
was recorded using a thermal conductivity detector (TCD). The N2-
TPD measurements were performed on a chemisorption apparatus
(Micromeritics AutoChem II 2920).
+
The Methods for NH4 Detection
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ARTICLE
Journal Name
8
9
C. J. M. V. Ham, M. T. M. Koper and D. G. H. VHieewttAertricslechOenliinde,
Chem. Soc. Rev., 2014, 43, 5183-5191. DOI: 10.1039/C9TA06653E
A. J. Medford and M. C. Hatzell, ACS Catal., 2017, 7, 2624-
2643.
Computational Details
The first-principles calculations were performed by using the Vienna
ab initio simulation package (VASP) based on the spin-polarized
density functional theory.57,58 The generalized gradient
approximation (GGA) of Perdew-Burke-Ernzerh (PBE) was adopted
for the exchange-correlation function.59 And the electron-ion
interaction was described by the projected augmented wave (PAW)
method.60 The electronic wave functions were expanded by a plane
wave basis set with an energy cutoff of 450 eV. The van der Waals
interactions were described using DFT-D3 method with Becke-
Jonson damping.61 A 2 × 2 × 1 supercell PCN contains 32 N atoms
and 24 C atoms was selected as the slab model. In order to avoid
the interaction between two layers, the vacuum layers were set to
be 15 Å. During the calculations, the Brillouin zones were sampled
by a 3 × 3 × 1 grid centered at the gamma (Γ) point, all the atoms
were fully relaxed until the total energy variation was smaller than
10-5 eV, and the force on each atom was less than 0.02 eV/ Å. The
Gibbs free energy change (∆G) in the simulated pathway was
calculated as follows:
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∆G = ∆E + ∆EZPE – T∆S
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Where, ∆E is the adsorption energy difference obtained directly
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computed from the vibrational frequencies, T is the temperature
(298.15 K), and ∆S is the entropy difference between the adsorbed
state and gas state. As the vibrational entropy of the adsorbed state
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are taken from the NIST database.62
24 L. Tao, M. Qiao, R. Jin, Y. Li, Z. Xiao, Y. Wang, N. Zhang, C. Xie,
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Conflicts of interest
There are no conflicts to declare.
26 H. Tao, C. Choi, L. Ding, Z. Jiang, Z. Han, M. Jia, Q. Fan, Y. Gao,
H. Wang, A. W. Robertson, S. Hong, Y. Jung, S. Liu and Z. Sun,
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27 J. Wang, Z. Li, Y. Wu and Y. Li, Adv. Mater., 2018, 30,
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28 W. Chen, J. Pei, C. He, J. Wan, H. Ren, Y. Zhu, Y. Wang, J.
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Acknowledgements
This work was supported by National Key R&D Program of
China (2017YFA0700104), the National Natural Science
Foundation of China (21722104, 21671032, 21331007), and
Natural Science Foundation of Tianjin City of China
(18JCJQJC47700, 17JCQNJC05100).
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