A nitrogen fixation strategy to synthesize NO: Via the thermally assisted photocatalytic conversion of air

Article abstract of DOI:10.1039/d0ta06747d

Developing a novel strategy for energy-efficient and clean synthesis of NO from distributed sources is highly desirable. Herein, we present a facile and green method to synthesize NO through the thermal-Assisted photocatalytic oxidation of N2 using simulated air as the reactant in a flow reactor. The TiO2/WO3 heterostructured nanorods were constructed and exhibited good activity for NO photosynthesis assisted by heating in the range of 200-350 °C. The yield rate of NO reached 0.16 mmol g-1 h-1 at 300 °C with a quantum efficiency of 0.31% at 365 nm. 15N isotope labeling experiments proved the origin of NO from N2 oxidation. Experimental and theoretical results revealed that the lattice oxygen in the heterostructures participated in the photooxidation reaction of N2, and the electron transfer from WO3 to TiO2 at the interface of the heterojunction under illumination could facilitate the adsorption of N2 and the formation of NO? intermediates and thus enhanced the photocatalytic N2 oxidation performance. Importantly, the solar-driven oxidation generated NO can be directly used for the synthesis of fine chemicals including nitric acid and β-nitrostyrolene. Our work opens a new avenue for nitrogen fixation via solar-driven N2 oxidation from distributed sources of air. This journal is

Full text of DOI:10.1039/d0ta06747d

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DOI: 10.1039/D0TA06747D
Journal Name
ARTICLE
A nitrogen fixation strategy to synthesize NO via the thermally-
assisted photocatalytic conversion of air
Yu Yu , Changhong Wang , Yifu Yu *, Yanmei Huang , Cuibo Liu , Siyu Lu , Bin Zhanga,b,*
a,†
a,†
a,
a
a
c
Received 00th January 20xx,
Accepted 00th January 20xx
Developing a novel strategy for energy-efficient and clean synthesis of NO at distributed sources is highly desirable. Herein,
DOI: 10.1039/x0xx00000x
we present a facile and green way to synthesize NO through the thermal-assisted photocatalytic oxidation of N
simulated air as reactant in a flow reactor. The TiO /WO heterostructured nanorods were constructed and exhibited good
activity for NO photosynthesis assisted with the heat of 200-350 °C. The yield rate of NO reached 0.16 mmol g h at 300 °C
with a quantum efficiency of 0.31 % at 365 nm. 15N isotope labeling experiments proved the origin of NO from N
oxidation.
Experimental and theoretical results revealed that the lattice oxygen in the heterostructures participated in the
photooxidation reaction of N and the electron transfer from WO to TiO at the interface of heterojunction under
illumination could facilitate the adsorption of N and the formation of NO* intermediate and thus enhanced the
photocatalytic N oxidation performance. Importantly, the solar-driven generated NO can be directly used for the synthesis
of fine chemicals including nitric acid and β-nitrostyrolene. Our work opens a new avenue for nitrogen fixation via solar-
driven N oxidation at distributed source of air.
2
using
2
3
-1
-1
2
2
3
2
2
2
2
searching energy-efficient and clean solutions to directly oxidize
1
. Introduction
N in air to NO at distributed sources are urgently required.
Solar light provides an inexhaustible source of globally
available energy, and therefore the photocatalysis conversion
of solar energy into chemical energy has been recognized as a
2
Nitrogen monoxide (NO), as one of the most important active
nitrogen, has been widely used in manufacturing bulk chemical
of nitric acid and various kinds of fine chemicals. Commercial
NO is produced by combining the artificial ammonia synthesis
1
-4
2
0-22
promising technology to address the global energy crisis.
Recently, photocatalytic N oxidation with water to produce
2
(
(
Haber-Bosch process) and the catalytic oxidation of ammonia
Ostwald process).5 Both two highly energy-intensive
nitrate was presented, but, the batch reactor control
accompanying the potential photoreduction of products limited
-9
processes are accompanied with the discharge of huge amounts
of greenhouse gas.1
plants would cause serious security risk and energy waste
during transportation. Considering that N
main ingredients of air, directly bonding N
attractive from the points of energy saving and environmental
protection. But, the direct reaction of N and O is symmetry-
forbidden and thus the reaction rate is negligible even at
2
3, 24
the development.
to NO requires a photocatalyst able to generate holes to oxidize
(E = 1.26 V vs. normal hydrogen electrode) and many
2
Theoretically, solar-driven N oxidation
0-13
Moreover, centralized NO chemical
N
2
1
4
2
and O
to O
2
are the
2
in air is
2
5
semiconductors meet this requirement. Moreover, the
standard enthalpy (∆H) of this reaction (Eq. 1) is 90.3 kJ/mol NO,
indicating it is an endothermic reaction and the equilibrium is
2
2
2
1
unfavorable for NO production at ambient conditions.
N
2(g) + O2(g) ⇌ 2NO(g)
(1)
1
combustion temperatures. Inspired by the lightning-induced
Notably, the reaction entropy (∆S) is 12.4 J/K per mol NO,
nitrogen fixation in nature, plasma-driven oxidation of N
(Birkeland-Eyde process) was proposed to produce NO.
However, the development of this technique is hindered by high
2
with
suggesting that the equilibrium NO concentration would rise
1
5-17
O
2
1
, 25
rapidly with temperature.
Thus, it is reasonable to expect
that a thermal-assisted photocatalyst with suitable band
1
8, 19
energy consumption and low throughput.
Therefore,
2
structure could drive the photooxidation of N to NO with a
continuous reaction system.
Herein, TiO /WO nanorods with Z-scheme heterojunction
2 3
were fabricated and adopted as photocatalyst for NO synthesis
with the assistance of low-temperature heating (200-350 °C,
a. Department of Chemistry, School of Science, Institute of Molecular Plus, Tianjin
University, Tianjin 300072, China. E-mail: yyu@tju.edu.cn; bzhang@tju.edu.cn
Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Frontiers Science
b.
2 3
Scheme 1). The TiO /WO heterostructures showed the ability
Center for Synthetic Biology (Ministry of Education), Tianjin University, Tianjin
for photocatalytic NO synthesis with a yield rate of 0.16 mmol
3
00072, China
-1
-1
15
c.
g h at 300 °C and quantum efficiency of 0.31 % at 365 nm. N
isotope labeling experiments proved that NO originated from N
oxidation in air. The lattice oxygen was confirmed as the active
oxygen species for N photooxidation based on the results of
Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou
50000, China
These authors contributed equally to this work.
4
2
†
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
2
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The powder X-ray diffraction (XRD) patterns were performed on
View Article Online
DOI: 10.1039/D0TA06747D
a Bruker D8 Focus Diffraction System using a Cu Kα source
(λ=0.154178 nm). Scanning electron microscopy (SEM) images
were taken with a Hitachi S-4800 scanning electron microscope.
Transmission electron microscopy (TEM), high-resolution
transmission electron microscopy (HRTEM), and scanning
transmission electron microscope EDS (STEM-EDS) element
mapping images were obtained with a Tecnai G2 F20 system
equipped with EDAX. In situ and ex situ X-ray photoelectron
spectroscopy (XPS) measurements were performed on a
photoelectron spectrometer using Al Kα radiation as the
excitation source (PHI 5000 VersaProbe). All the peaks were
calibrated with C 1s spectrum at a binding energy of 284.8 eV.
UV-Vis absorbance spectra were recorded on a Lambda 750S
UV-vis-NIR spectrometer (Perkin-Elmer) equipped with an
integrating sphere. Photoluminescence (PL) measurements
were carried out on a Jobin-Yvon-Fluorrolog 3-21 fluorescence
photometer. The time-resolved PL spectra were obtained on
the Steady State and Transient State Fluorescence
Spectrometer (FLSP920). Fourier Transform Infrared
Spectroscopy (FTIR) spectroscopy was carried out on a Bruker
Scheme 1 Schematic illustration of the thermal-assisted
photocatalytic synthesis of NO in air.
electron paramagnetic resonance (EPR). The time-resolved
photoluminescence (PL) spectra, in situ X-ray photoelectron
spectrum (XPS) and density functional theory (DFT) calculation
2 3
were performed to probe the high activity origin of TiO /WO
heterostructures. Importantly, the external heating source
could be replaced by enhanced incident light and the solar-
driven generated NO at the flow reactor outlet can serve as the
distributed source for the direct synthesis of nitric acid and β-
nitrostyrolene.
ALPHA-T Fourier transform infrared spectrometer. The N
2
adsorption and desorption isotherms were measured using
Autosorb-iQ2-MP instrument (Quantachrome Company,
America). Ultraviolet photoelectron spectroscopy (UPS)
measurements were carried out with an unfiltered He I (21.2
eV) gas discharge lamp and a total instrumental energy
resolution of 100 meV. The electron spin resonance (ESR) tests
were conducted with an electron paramagnetic resonance A300
spectrometer (Bruker AXS Company, Germany). The ESR
2
. Experimental section
2
.1 Synthesis of materials
Synthesis of WO nanorods. In a typical procedure, WCl (0.15
3
6
g) was dissolved in 30 mL ethanol by stirring for 20 min to form
a yellow solution. Then, the yellow ethanol solution was
transferred to a 50 mL Teflon-lined stainless-steel autoclave and
heated at 180 °C for 24 h. After cooling down naturally to room
temperature, blue precipitates were collected and washed in
deionized water and ethanol three times, respectively, and then
dried in a vacuum oven for 12 h. Finally, the blue precipitates
were annealed at 500 °C for 4 h in air to produce WO
Synthesis of TiO /WO heterostructured nanorods. 0.2 g of the
as-obtained WO nanorods were dispersed in 30 mL
isopropanol under sonication for 0.5 h. Then, 30 μL
diethylenetriamine (DETA) and 0.2 mL titanium (IV)
isopropoxide were successively added under strong stirring to
form a homogenous suspension. The mixed suspension was
transferred to a 50 mL Teflon-lined stainless-steel autoclave and
heated at 180 °C for 10 h. After cooling down naturally to room
temperature, blue precipitates were collected and washed in
deionized water and ethanol three times, respectively, and then
dried in a vacuum oven for 12 h. Finally, the blue precipitates
were annealed at 500 °C for 4 h in air to produce the shallow
-
2
analysis was performed to detect the spin reactive ·O species
adsorbed on the photocatalysts or dissolved in water/methanol
under UV-vis light irradiation by using 5,5-dimethyl-1-pyrroline
N-oxide (DMPO) as a spin trap. The electron paramagnetic
resonance (EPR) spectra were recorded on a Bruker EMX-8
spectrometer by applying an X-band (9.43 GHz, 1.5 mW)
microwave and sweeping magnetic field at RT. The gas
chromatograph-mass spectrometer (GC-MS) was carried out
with TRACE DSQ. The liquid phase product was measured on
Agilent 7890A with a thermal conductivity (TCD) and a flame
ionization detector (FID). Nuclear magnetic resonance (NMR)
spectra were recorded on Varian Mercury Plus 400 instruments
at 400 MHz (1H NMR).
3
nanorods.
2
3
3
2
.3 Thermal-assisted photocatalytic N
The thermal-assisted photocatalytic N
simulated air (N :O = 4:1) was carried out in a homemade
2
oxidation
2
oxidation in the
2
2
yellow TiO heterostructured nanorods.
Synthesis of TiO nanosheets. The TiO nanosheets were
obtained by dissolving WO in the TiO /WO heterostructured
nanorods. 0.2 g of the as-obtained TiO /WO nanorods were
dispersed in 20 mL 5.0 mol L NaOH solution for 12 h under
stirring. Then the white precipitates of TiO nanosheets were
2
/WO
3
equipment (Scheme S1). A cylindrical quartz reactor is
surrounded by heating resistance wire and equipped with a top
quartz window for light irradiation. The simulated solar
illumination was obtained by passing light from a 300 W Xenon
lamp equipped with an AM 1.5 G filter, and the power intensity
2
2
3
2
3
2
3
-1
2
-2
of the incident light was calibrated to be 100 mW cm (1 sun).
Typically, 0.1 g photocatalyst was loaded on the quartz sand
breathable panel in the middle height of the reactor. The
temperature of the photocatalyst was controlled by the buried
collected and washed in deionized water and ethanol three
times, respectively, and dried in a vacuum oven for 12 h.
2
.2 Material Characterizations
2
| J. Name., 2012, 00, 1-3
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thermocouple connecting to a heating source. In a typical reaction solution under stirring. And the reaction solution was
View Article Online
DOI: 10.1039/D0TA06747D
measurement procedure, the photocatalyst in the reactor was composed of 1, 2-dichloroethane solvent and 2 mmol styrene
-1
firstly heated to 400 °C with Ar flow (0.8 L min ) for 2.5 h to reactant. After the reaction, the 25 mL reaction solution was
remove the surface adsorbed impurities. Then, the temperature steamed to remove the solvent for qualitative tests by GC-MS.
decreased to reaction temperature under the protection of Ar And the produced β-nitrostyrolene was further purified for the
gas. Subsequently, the reaction atmosphere switched to air NMR test.
-1
with a flow rate of 0.8 L min . Finally, the NO concentration was
real-time monitored by the NO analyzer (Thermo Fisher 2.5 The measurement of Quantum Efficiency
x
Scientific, Model 42i). Notably, when the thermal assisted
temperature was low (≤ 200 °C), the NO concentration in the
Quantum Efficiency (QE) was tested at 300 °C under the
illumination of 365 nm wavelength light, which was obtained by
product was too low to be directly detected by the NO
x
3
00 W Xenon lamp equipped with 365 nm single-wavelength
analyzer. At this time, a 50 L gas pocket was used to collect and
concentrate the product with a lower flow rate of simulated air
-2
filter and the light intensity was measured to be 17.2 mW·cm .
0
.1 g photocatalyst was added into the quartz reactor and the
-
1
(
0.2 L min ). Then, the gas pocket with concentrated production
irradiation region was a circular area with a diameter of 3.0 cm.
1
5
was detected through the NO
experiments were conducted using the ultrapure
source. For N isotope labeling experiment, 15N
= 4:1) were mixed in the gas pocket and used as a
reactant for photocatalytic
x
analyzer. N isotope labeling
27
The details of the QE calculation were shown as follows:
1
5
N
2
as N-
The number of absorbed photons (Nabsorbed):
1
5
and O
2
-2
2
2
t (s) × P (W · cm ) × s (cm ) × λ (m)
1
5
Nabsorbed =
h (J · s) × c (m · s -1
(4)
(
N :O
2 2
)
1
5
N
2
oxidation at 300 °C and the The QE of N
2
to NO:
2
× nNO (mol) × NA (mol -1
)
production was collected with a gas pocket for further GC-MS
test. In this work, the related calculation of yield rate and yield
were shown as follows:
QE =
× 100%
(5)
Nabsorbed
P: The optical power.
λ: The wavenumber of the incident light.
c: The speed of light.
h: Planck constant.
t: The illumination time
v × 10 -3
t
0
-1
-1
Yield rate of NO (mmol g h ) = ∫ C × t ×
(2)
M3 (NO) × mcat. × T
-
t
v × 10
-1
Yield of NO (mmol g ) = ∫ C × t ×
(3)
0
M(NO) × mcat.
where C is the concentration of NO detected by the NO
analyzer (ppb), t is the instant time (min), M is the molar mass
x
2
.6 Theoretical Simulation
-1
(
g mol ), mcat. is the mass of catalyst (g), T is the reaction
Spin-polarized density functional theory (DFT) computations
were performed as implemented in the plane wave set Vienna
cumulative time (h), and v is the gas flow rate at the inlet of NO
analyzer (0.7 L min ).
x
-1
2
8, 29
ab initio Simulation Package (VASP) code.
Projector
augmented wave (PAW) potentials were used to describe the
interaction between ions and valence electrons. Generalized
gradient approximation (GGA) in the Perdew-Burke-Ernzerhof
2
.4 In situ synthesis of Nitric acid and β-Nitrostyrolene
3
0
In the procedure of nitric acid synthesis, simulated air with the
-1
gas flow rate of 0.4 L min was used as the reactant for
photocatalytic NO synthesis at 300 °C. Then, the outlet gas of
the reactor was directly absorbed by 60 mL 50 mM sodium
hydroxide solution under stirring, in which the generated NO
(
PBE) form was adopted to describe the exchange-correlation
3
1
between electrons. The DFT-D3 method was used for the vdW
corrections. The kinetic-energy cut off was set as 500 eV. An
3
2
−4
energy convergence threshold of 10 eV was set in the self-
consistent field (SCF) iteration. The geometry optimization
within the conjugate gradient method was performed with
forces on each atom less than 0.05 eV/Å. A p (1 × 1) slab model
with four W-O octahedral layers and a p (1 × 3) slab model with
2 2
would react with the excess O and H O to produce nitric acid.
The produced nitric acid reacted with sodium hydroxide to
generate nitrate for further quantification. Colorimetric
methods were applied to determine the concentration of
nitrate.26 Typically, 5 mL of the absorbing solution was
successively added with 0.1 mL 1.0 M HCl and 0.01 mL 0.8 wt%
sulfamic acid solution standing for 10 minutes at room
temperature. The absorption spectrum was tested using an
ultraviolet-visible spectrophotometer and the absorption
intensities at the wavelength of 220 nm and 275 nm were
recorded. The final absorbance value was calculated by the
three Ti-O layers were adopted to simulate the WO
3
(001) and
TiO (101) surfaces, respectively. Vacuum layers larger than 15
2
Å were inserted along the c direction to eliminate the periodic
image interactions. Half of the O atoms in the bottom O layer of
6
+
3
WO slab were removed to balance the W valance state of the
pseudo surface. The bottom W-O and Ti-O layer were fixed
while other layers and the adsorbates were fully relaxed during
structural optimizations. The Brillouin zone was sampled by a k-
point mesh of 4 × 4 × 1. Gibbs free energies for each gaseous
and adsorbed species were calculated according to the
expression:
equation: A = A220nm – 2A275nm. The calibration curve was plotted
using a series of concentrations of NO
-
3
-N from 0 to 2.0 ppm.
And the sodium nitrate applied for plotting calibration curve
was pretreated by drying in the oven at 105-110 °C for 2 h in
advance. In the procedure of β-nitrostyrolene synthesis,
simulated air with the gas flow rate of 0.4 L min was used as
the reactant for photocatalytic NO synthesis at 300 °C. Then, the
outlet gas of the reactor was directly ventilated into 25 mL
G = EDFT + EZPE - TS
(6)
-1
where EDFT and EZPE are the total energy and zero-point energy
calculated with VASP, TS is the entropy contribution at 298.15
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K. The adsorption energy of O
the expression:
2
or N
2
is calculated according to under solvothermal condition followed by thermal annealing to
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DOI: 10.1039/D0TA06747D
produce TiO
WO
where Eadsorbate/slab is the total energy of adsorbate on the nanosheets coated on the WO
surface; Eslab is the total energy of the surface and Eadsorbate is S3). HRTEM image (Fig. 1b) displayed different lattice fringes of
the total energy of free O or N molecule.
WO (001) and TiO (101) planes in core and shell, respectively.
XRD pattern (Fig. 1c) confirmed the existence of WO (JCPDS No.
3-1035) and TiO (JCPDS No. 21-1272) in the TiO /WO
heterostructured nanorods without any peaks of other
heterostructure was designed and synthesized as a impurities. Moreover, the FTIR spectrum of TiO /WO
model thermal-assisted photocatalyst for N oxidation in air.
heterostructured nanorods was tested and shown in Fig. S4. The
2 3
/WO heterostructured nanorods. The surface of
Eads = Eadsorbate/slab -(Eslab + Eadsorbate) (7)
3
nanorods becomes rough (Fig. S2) and a thin layer of TiO
nanorods (inset in Fig. 1b, Fig.
2
3
2
2
3
2
3
4
2
2
3
3
. Results and discussion
TiO
2
/WO
3
2
3
2
-1
peaks located at 3413 and 1622 cm were due to the stretching
and bending vibrations of the surface adsorbed water
−1
molecules. And the peaks located at 585 and 650 cm were
3
3
attributed to the stretching vibrations of Ti-O bond in TiO
2
.
Moreover, the W-O stretching and vibration bands in the region
-1
-1
33
9
60 cm to 800 cm were also detected. No other absorption
peaks appeared, suggesting that there were no nitrogen-
containing impurities adsorbed on the surface of TiO /WO
heterostructures. And the Brunauer-Emmett-Teller (BET)
surface area of TiO /WO heterostructured nanorods was
calculated to be 72.0 m g through the nitrogen adsorption-
desorption isotherm (Fig. S5). For comparison, TiO nanosheets
were obtained through the dissolution removal of WO from
TiO /WO heterostructures by alkali (Fig. S6).
The bandgap energies (E ) of WO nanorods, TiO
and TiO /WO heterostructures were calculated to be 2.52, 3.03
and 2.76 eV, respectively, from the UV-vis absorbance spectra
Fig. 1d and Fig. S7). The valence band (VB) positions of WO
nanorods and TiO nanosheets could be estimated at +2.88 and
2.63 V versus normal hydrogen electrode (NHE) based on the
2
3
3
4
2
3
2
-1
2
3
2
3
g
3
2
nanosheets
2
3
(
3
2
+
3
5
results of ultraviolet photoelectron spectroscopy (Fig. S8). As
a result, the conduction band (CB) positions of WO nanorods
and TiO nanosheets could be deduced as +0.36 and -0.40 V
versus NHE. Combining these data, the corresponding band
structure diagram of WO nanorods and TiO nanosheets could
be depicted as shown in Fig. S9. By using 5,5-dimethyl-1-
2 3
/WO heterostructured nanorods pyrroline N-oxide as the spin trap, ESR test exhibited that TiO2
3
2
3
2
Fig. 1 Characterization of TiO
with a Z-scheme heterojunction. (a) HRTEM image of WO
nanorods (Inset: TEM image). (b) HRTEM image of the TiO /WO
3
2 3
nanosheets and TiO /WO heterostructures could photo-reduce
-
36
-
O
2
into ·O
2
radical, while any signal of ·O
2
could not be
2
3
heterostructured nanorods (Inset: TEM image). (c) XRD patterns observed for WO
WO (+0.36 V) and TiO
3
(Fig. 1e). Considering that the CB potential of
(-0.40 V) was more positive and negative
of WO
UV-Vis absorbance spectra and the Tauc plots (inset) of WO
nanorods and TiO nanosheets. (e) DMPO spin-trapping ESR
spectra recorded for ·O
different samples. (f) Schematic illustration for the electron
transfer in TiO /WO Z-scheme heterojunction.
3
2 3
nanorods and TiO /WO heterostructured nanorods. (d)
3
2
-
than the standard potential of O
2
/·O
2
(-0.33 V vs NHE),
3
respectively, it was reasonable to conclude a Z-scheme
heterojunction mechanism (Fig. 1f) rather than the type II
2
-
2
under UV-Vis light irradiation for
heterojunction mechanism (Fig. S10) in TiO
heterostructure under irradiation. The photogenerated
electrons in CB potential of WO transferred onto TiO and
combined with the photogenerated holes in VB of TiO , and
thus suppressing the recombination of self-photogenerated
2 3
/WO
2
3
3
2
2
First, WO
solvothermal method along with thermal annealing. SEM image
Fig. S1) and TEM image (inset in Fig. 1a) showed their smooth
3
nanorods were synthesized through a simple
3
7
carriers.
TiO /WO
photocatalytic N
(
2
3
heterostructures were applied for thermal-assisted
oxidation using the simulated air (N :O = 4:1)
surface and dozens of nanometer diameter. Typical HRTEM
image (Fig. 1a) displayed the single-crystal structure of
nanorods with the WO
further proved the nanorods as pure WO
Then, WO nanorods reacted with titanium (IV) isopropoxide
2
2
2
as the reactant. A homemade quartz flow reactor surrounded
by heating resistance wire was used and 300 W Xenon lamp
equipped with an AM 1.5 G filter (1 sun) served as the simulated
solar irradiation. As shown in Fig. 2a, no NO was detected when
3
(001) plane. XRD pattern (Fig. 1c)
3
(JCPDS no. 43-1035).
3
4
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no catalysts were used with air reactant and the TiO
heterostructures were used as catalysts with Ar reactant under 0.16 mmol g h . Firstly, the yield rate of NO increased from
2
/WO
3
subsequent study, which the yield rate of NO could reach to
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-1
-1
DOI: 10.1039/D0TA06747D
-1
-1
solar irradiation at 200 °C. Moreover, NO was not detected 0.16 to 0.91 mmol g h as the illumination intensity enhanced
when the TiO /WO heterostructures were used as catalysts from 1 sun to 5 sun (Fig. S11), confirming the great effect of
with air reactant under dark at 200 °C, but we could detect NO illumination intensity on the performance of photocatalytic NO
production when the TiO /WO heterostructures were used as synthesis. Then the 365 nm monochromatic light was chosen as
catalysts with air reactant under solar irradiation at 200 °C. the light source to evaluate the photocatalytic efficiency and
2
3
2
3
2
7
These results suggested that the production of NO derived from the QE of 0.31% was obtained. Moreover, when little water
the photocatalytic reaction vapor was introduced into the reactor, the water molecules
may adsorb on the surface of TiO /WO heterostructures and
inhibit the adsorption of N , thus leading slight decline of NO
2
3
2
3
9
yield rate (Fig. S12). Finally, the NO yield showed no obvious
decay after eight cycles (Fig. 2d). And there was no obvious
decline of the time-dependent yield rate of NO during the
continuou 8 h photocatalytic test (Fig. S13). The morphology
and crystalline structure after long-term tests still maintained
(
Fig. S14), proving the excellent stability of TiO
heterostructures.
To confirm the origin of NO, nitrogen isotope labeling
2 3
/WO
1
5
14
experiments were conducted using the
2 2
N and N as the N-
source, respectively. The gas-phase products were analyzed by
1
5
gas chromatograph-mass spectrometry (GC-MS). NO (m/z =
1
5
3
(
1) can only be detected when
N
2
was used as the reactant
1
5
15
Fig. 2e) and no NO (m/z = 31) was detected in the feed
N
2
gas (Fig. S15), suggesting that NO originated from the oxidation
of N . Furthermore, no nitrous oxides were detected after
replacing air with Ar, further confirming that the source of NO
lied in N oxidation rather than impurities in the reaction system
Fig. S16). Finally, the performance of WO nanorods and the
physically mixed TiO +WO was studied for comparison. As
shown in Fig. 2f, the NO yield rate of TiO /WO heterostructures
was distinctly higher than those of WO nanorods and
mixtures, proving that the Z-scheme heterojunction
2
2
(
3
2
3
2
3
3
Fig
over a TiO
conditions (“N/A” denotes “not detected”). (b) Photocatalytic
yield rate of NO over TiO /WO heterostructures at different
.
2 The thermal-assisted photooxidation performance of N
2
TiO
2
+WO
3
2
/WO photocatalyst. (a) NO yield rate under different
3
2 3
in TiO /WO heterostructure could enhance the performance.
Notably, when the incident light source switched to 10 suns, the
solar-driven surface temperature of photocatalysts increased to
2
3
temperatures. (c) Time-dependent yield rate curves of NO over 200 °C (Fig. S17). At this time, the NO yield rate was even higher
TiO /WO heterostructures at different temperatures. (d) Cycle than that under 1 sun illumination with 300 °C external heating
tests for photocatalytic NO synthesis over TiO /WO
owing to more photogenerated carriers (the bottom in Fig. 2f).
heterostructures (1 hour for a cycle). (e) Mass spectra of GC-MS Thus, the external heating source is expected to be replaced by
analysis in the N
isotope experiment. (f) The yield rate of NO the adoption of a stronger incident light derived from a
2
3
2
3
2
condenser, indicating the promising application potential of this
technique in the future.
To further probe the origin of high performance of TiO /WO
2 3
heterostructures, temperature-programmed desorption (TPD)
over different samples under 1 sun illumination assisted with
external heating of 300 °C (up) and the yield rate of NO over
TiO /WO nanorods under 10 sun illumination without external
2 3
heating (bottom).
of NO, EPR, time-resolved PL spectra, in situ
2 3
of air reactant over TiO /WO heterostructures assisted with
2
00 °C heating. And this discovery was consistent with previous
works about solar-driven nitrogen oxidation by the water to
2
3, 24, 38
produce nitrate/nitrite.
rate of NO over TiO
Then the photocatalytic yield
3
/WO heterostructures at different
2
temperatures were studied. As shown in Fig. 2b, the NO yield
rate improved rapidly with the increase of temperature from
2
2
00 °C to 350 °C. But, the time-dependent yield rate curves (Fig.
c) exhibited a decline during the long-term test at 350 °C. Thus,
we chose 300 °C as the operating temperature for the
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ARTICLE
Journal Name
the interface of TiO
which was consistent with the previously deduced Z-scheme
2
/WO
3
heterostructure under irradiation,
View Article Online
DOI: 10.1039/D0TA06747D
heterojunction mechanism in TiO
f).
The reaction Gibbs free energies (ΔG) of N
2 3
/WO heterostructure (Fig.
1
2
photooxidation
were simulated to unveil the reaction path (Fig. 3e). Based on
the results of the aforementioned in situ and ex situ
3
characterizations, the WO surface was positively charged by
removing two electrons from the pure surface to reflect the
4
1, 42
interfacial charge transfer.
on different samples were calculated (Fig. S19). Pure WO
eV) and interfacial WO in TiO /WO heterostructures (-2.48 eV)
showed more negative adsorption energies than those of pure
First, the N
2
adsorption energies
3
(-1.69
3
2
3
TiO
-0.22 eV). This result revealed that N
proceeded on the surface of WO
heterostructure and interfacial charge transfer facilitated N
2
(-0.37 eV) and interfacial TiO
2
in TiO
2
/WO
3
heterostructure
photooxidation
(
2
3
in the TiO
2
/WO
3
2
adsorption. Then, the N≡N bond was broken and one of the N
atoms would take one surface lattice O atom away to form the
first NO molecule, leaving the other N atom combining with the
lattice O on WO
NO formation over pure WO
Interestingly, the free energy on the interfacial WO
only 1.52 eV. The as-formed NO* easily released with ΔG value
of -0.11 eV and 0.21 eV over pure and interfacial WO surfaces,
respectively, to form the second NO, leaving two surface oxygen
vacancies in WO . Because the O adsorption energy on the
interfacial TiO
interfacial WO
interfacial TiO
absorbed on the interfacial TiO
photoelectrons, and then transferred to interfacial WO
back the surface oxygen vacancies. The interfacial WO
promote the adsorption of N
intermediate compared with pure WO
was facilitated over TiO /WO heterostructures.
To further excavate the application value of this solar-driven
oxidation strategy, the generated NO was directly used to
3
surface. The free energy increase for the first
was as high as 3.59 eV.
surface was
3
Fig. 3 Experimental characterization and theoretical calculation
3
of the photooxidation performance of N
heterostructures. (a) EPR spectra of untested TiO
heterostructures and tested TiO /WO
different atmospheres. (b) Time-resolved PL spectra of different
samples. (c) W 4f and (d) Ti 2p XPS spectra of TiO /WO
heterostructures with and without light irradiation. (e) Reaction
free energy diagram of N oxidation over different samples.
2
over TiO
2
/WO
3
3
2
/WO
3
2
3
heterostructures under
3
2
2
(-1.12 eV) was more negative than that of
(-0.36 eV) (Fig. S20) and the surface of
2
3
3
2
2
was negatively charged (Fig. 1f), O in air were
2
2
and reduced by
3
to fill
could
XPS and DFT calculations were performed (Fig. 3). First, the NO-
TPD profile showed that chemisorbed NO on TiO /WO
3
2
3
2
and the formation of NO*
, and thus the reaction
heterostructures desorbed at 296 °C (Fig. S18), indicating
external heating can not only increase the NO equilibrium
concentration but also promote the desorption of NO. Then, as
shown in Fig. 3a, no EPR signals were detected for untested
3
2
3
N
2
TiO
vacancies in TiO
EPR signals for TiO
photocatalytic test using air as the reactant. Interestingly, when
the TiO /WO heterostructures were tested using pure N as the
2
/WO
3
heterostructures, suggesting no immanent oxygen
/WO heterostructures. There were also no
/WO heterostructures after the
synthesize the value-added products. 0.64 μmol nitrate
generated during 8 h accumulation time when the air flow at
the outlet of the reactor was absorbed by alkaline solution (Fig.
2
3
2
3
4
a and Fig. S21). β-nitrostyrolene played an important role in
2
3
2
the pharmaceutical industry and can be produced by the
reactant, an obvious EPR signal of oxygen vacancies at g factor
of ~2.002 appeared, indicating that the lattice oxygen in the
43, 44
reaction of styrene and NO.
As shown in Fig. 4b, β-
TiO
2
/WO
3
heterostructures participated in the
N
2
1
7
photooxidation reaction. As shown in Fig. 3b, the TiO
2
/WO
3
heterostructures possessed an extended PL lifetime compared
to WO , indicating an enhanced separation efficiency of
3
4
0
photogenerated charge carriers. The intrinsic activity of
photocatalysts was closely related to their electronic structure,
2 3
and thus the electronic structure of TiO /WO heterostructure
was investigated. Under irradiation, the XPS peak of W 4f
shifted to higher binding energy by 0.2 eV (Fig. 3c). On the
contrary, the XPS peak of Ti 2p shifted to lower binding energy
by 0.2 eV compared to that under darkness (Fig. 3d). The in situ
Fig. 4 The direct use of the as-produced NO. Synthesis of (a)
nitric acid and (b) β-nitrostyrolene using photocatalytic
generated NO.
3 2
XPS results indicated the electron transfer from WO to TiO at
6
| J. Name., 2012, 00, 1-3
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
Journal Name
ARTICLE
nitrostyrolene with 100% selectivity (Fig. S22) was obtained 5. C. Dai, Y. Sun, G. Chen, A. C. Fisher and Z. J. Xu, Angew. Chem. Int.
View Article Online
DOI: 10.1039/D0TA06747D
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using Ar gas as the feed gas (Fig. 4 and Fig. S23). This further
demonstrated that the generated NO originated from N
P. Ding, Y. Wu and Y. Xiong, J. Am. Chem. Soc., 2020, DOI:
10.1021/jacs.0c05097.
2
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acid and fine chemicals. Moreover, NO and the nitrate has also
been proved as the raw materials for ammonia synthesis.
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2 3
In summary, TiO /WO nanorods with Z-scheme heterojunction
were successfully synthesized and adopted as thermal-assisted
photocatalyst for solar-driven NO synthesis from air in a flow
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efficiently photo-oxidize N in air into NO with a yield rate of
.16 mmol g h at 300 °C and a quantum efficiency of 0.31 %
2 3
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2
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spectra and in situ XPS, DFT calculation revealed that the
electron transfer at the interface of Z-scheme TiO /WO
heterojunction could not only facilitate the photo-charges
separation but also promote the adsorption of N and the
formation of NO* intermediate, and thus enhanced the
reaction. Moreover, the external heating source could be
replaced by stronger incident light and the solar-driven
generated NO can serve as the distributed source for the direct
synthesis of nitric acid and fine chemical of β-nitrostyrolene,
indicating the promising application potential of this technique.
This work offers a new avenue for energy-efficient and green
synthesis of NO from the air.
1
1
1
1
2
3
3
2
2
3
9. P. Peng, C. Schiappacasse, N. Zhou, M. Addy, Y. Cheng, Y. Zhang,
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Conflicts of interest
There are no conflicts to declare
22. X. Gao, L. An, D. Qu, W. Jiang, Y. Chai, S. Sun, X. Liu and Z. Sun,
Sci. Bull., 2019, 64, 918-925.
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Nat. Commun., 2013, 4, 2249.
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 21701122) and the Natural
Science Foundation of Tianjin City (17JCJQJC44700).
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Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/D0TA06747D
Table of Contents
A thermally-assisted photocatalytic conversion of air as an alternative nitrogen
fixation strategy is reported to synthesize NO, which can be directly used for the
synthesis of fine chemicals including nitric acid and β-nitrostyrolene.