Greatly Improving Electrochemical N2 Reduction over TiO2 Nanoparticles by Iron Doping

Article abstract of DOI:10.1002/anie.201911153

Titanium-based catalysts are needed to achieve electrocatalytic N₂ reduction to NH₃ with a large NH₃ yield and a high Faradaic efficiency (FE). One of the cheapest and most abundant metals on earth, iron, is an effective dopant for greatly improving the nitrogen reduction reaction (NRR) performance of TiO₂ nanoparticles in ambient N₂-to-NH₃ conversion. In 0.5 m LiClO₄, Fe-doped TiO₂ catalyst attains a high FE of 25.6 % and a large NH₃ yield of 25.47 μg h^?1 mg_cat^?1 at ?0.40 V versus a reversible hydrogen electrode. This performance compares favorably to those of all previously reported titanium- and iron-based NRR electrocatalysts in aqueous media. The catalytic mechanism is further probed with theoretical calculations.

Full text of DOI:10.1002/anie.201911153

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Accepted Article
Title: Greatly Improving Electrochemical N2 Reduction over TiO2
Nanoparticle by Fe Doping
Authors: Tongwei Wu, Zhe Xing, Shiyong Mou, Chengbo Li, Yanxia
Qiao, Qian Liu, Xiaojuan Zhu, Yonglan Luo, Xifeng Shi,
Yanning Zhang, and Xuping Sun
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201911153
Angew. Chem. 10.1002/ange.201911153
Link to VoR: http://dx.doi.org/10.1002/anie.201911153
http://dx.doi.org/10.1002/ange.201911153

10.1002/anie.201911153
Angewandte Chemie International Edition
COMMUNICATION
Greatly Improving Electrochemical N2 Reduction over TiO2
Nanoparticle by Fe Doping**
Tongwei Wu,[a] Xiaojuan Zhu,[a,b] Zhe Xing,[a] Shiyong Mou,[a] Chengbo Li,[a] Yanxia Qiao,[a] Qian Liu,[a]
Yonglan Luo,[b] Xifeng Shi,[c] Yanning Zhang,[a] and Xuping Sun*[a]
Abstract: It is highly needed to develop Ti-based catalysts
simultaneously achieving a large NH3 yield with a high Faradic
efficiency (FE) for electrocatalytic N2 reduction to NH3. Here, we
report that Fe, one of the cheapest and most abundant metals on the
earth, acts effectively as a dopant to greatly improve the NRR
performances of TiO2 nanoparticle for ambient N2-to-NH3 conversion.
In 0.5 M LiClO4, such Fe-doped TiO2 catalyst attains a high FE of
far.
As one of the cheapest and most abundant metals on the
earth,[32] Fe also exists in biological nitrogenases for natural N2
fixation.[33] Fe compounds have been widely utilized as catalysts
for artificial N2 fixation in the Haber-Bosch[4] and
electrochemical[34-39] processes. It is thus natural for us to
explore using Fe as a dopant for TiO2, which however has not
been reported before. Herein, we report on our recent
experimental results that Fe-doped TiO2 is superior in
performances for electrocatalytic N2 reduction under ambient
conditions. In 0.5 M LiClO4, such catalyst achieves a high FE of
‒1
25.6 % and a large NH3 yield of 25.47 µg h‒1 mgcat. at ‒0.40 V vs.
reversible hydrogen electrode, comparing favourably to the
behaviours of all reported Ti- and Fe-based NRR electrocatalysts in
aqueous media. The catalytic mechanism is further discussed by
theoretical calculations.
‒1
25.6 % and a large NH3 yield of 25.47 µg h‒1 mgcat. at ‒0.40 V
vs. reversible hydrogen electrode (RHE), outperforming pristine
TiO2 as well as all Ti- and Fe-based NRR electrocatalysts
reported before. Remarkably, it also shows high electrochemical
and structural stability. The NRR mechanism on Fe-doped TiO2
(101) surface is further discussed using density function theory
(DFT) calculations.
As an essential activated nitrogen source, NH3 is extensively
used to manufacture dyes, polymers, fertilizers, and explosives,
and it also serves as carbon-neutral energy carrier with high
energy density.[1‒3] To date, the dominant industrial route for NH3
synthesis is the Haber-Bosch process using N2 and H2 as the
feeding gases, but this process operating at high temperature
and high pressure consumes a large amount of energy and
emits CO2.[4] Electrochemical N2 reduction offers an attractive
alternative in an environmentally benign and sustainable manner,
but the strong N≡N bond makes it inert and thus difficult to take
part in
a
chemical reaction, underlying the need of
electrocatalysts with high activity for the N2 reduction reaction
(NRR).[5‒8]
So far, noble metals perform efficiently for the NRR, but the
scarcity and high cost limit their application in large-scale
production.[8‒11] Research focus has thus been increasingly
shifted to develop noble-metal-free alternatives.[12‒24] TiO2 are
highly adaptable as a semiconductor catalyst due to its long-
term thermodynamic stability, natural abundance, and
nontoxicity.[25] Recent studies have demonstrated that oxygen-
defected TiO2 has good electrocatalytic activity for the NRR,[26,27]
and heteroatoms (B,[28] C,[29] V,[30] and Zr,[31]) are effective
dopants to enhance the NRR performances of TiO2 catalysts.
However, Ti-based catalysts simultaneously achieving a large
NH3 yield with a high Faradic efficiency (FE) are not available so
Figure 1. (a) XRD patterns of pristine TiO2 and Fe-TiO2. TEM images of (b)
pristine TiO2 and (c) Fe-TiO2. (d) HRTEM image taken from Fe-TiO2. (e) STEM
image and the corresponding EDX elemental mapping images of (f) Fe, (g) Ti,
and (h) O for Fe-TiO2.
Figure 1a shows the X-ray diffraction (XRD) patterns for
pristine TiO2 and Fe-TiO2 (see Experimental Section for
preparative details). Clearly, both materials show diffraction
peaks characteristic of anatase TiO2 (JCPDS NO. 97-009-3098)
and the peaks of iron oxides are not obtained for Fe-TiO2.
Moreover, Fe-doped TiO2 exhibits broader diffraction peaks
compared to pristine TiO2, indicating a slightly poor crystal
quality due to Fe ions doping.[40‒43] Figure S1 shows the XRD
patterns of Fe-TiO2 with various RFe values (RFe is the mass ratio
of Fe:Ti). As observed, the peak at 25.3° not only is slightly
shifted toward lower diffraction angle (25.2°), but also the peak
at 37.8° obviously weaken and broaden with increasing RFe
value to 2.8 % (Figure S1), which is attributed to the increase of
TiO2 lattice constants by Fe doping.[31,42] Therefore, it can be
inferred that the Fe ions incorporate into the TiO2 lattice, forming
an iron-titanium solid solution. Transmission electron microscopy
(TEM) images of TiO2 (Figure 1b) and Fe-TiO2 (Figure 1c)
indicate that they both are nanoparticles and Fe doping has no
obvious influence on the particle size. High-resolution TEM
(HRTEM) images taken from TiO2 and Fe-TiO2 nanoparticles
[a]
T. Wu, Z. Xing, S. Mou, C. Li, Y. Qiao, Dr. Q. Liu, X. Zhu, Prof. Y.
Zhang, Prof. X. Sun
Institute of Fundamental and Frontier Sciences, University of
Electronic Science and Technology of China, Chengdu 610054,
Sichuan (China)
E-mail: xpsun@uestc.edu.cn
[b]
[c]
X. Zhu, Y. Luo
Chemical Synthesis and Pollution Control Key Laboratory of
Sichuan Province, College of Chemistry and Chemical
Engineering, China West Normal University, Nanchong 637002,
Sichuan (China)
Dr. X. Shi
College of Chemistry, Chemical Engineering and Materials
Science, Shandong Normal University, Jinan 250014, Shandong
(China)
Supporting information for this article is given via a link at the end
of the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201911153
Angewandte Chemie International Edition
COMMUNICATION
show the lattice fringes with a distance of 0.349 nm and 0.352
nm (Figure S2 and Figure 1d), which corresponds to the (101)
plane of anatase TiO2. This difference is ascribed to that Fe-
doping has a slight effect on the crystalline structure of TiO2.
Thus, it is reasonable to assume that Ti4+ can be replaced by Fe
ions without significant alteration of the crystal structure.[41]
Scanning TEM (STEM) and corresponding energy dispersive X-
ray (EDX) elemental mapping images (Figure 1e-h) further
confirm the uniform distribution of Fe, Ti, and O elements in Fe-
TiO2 nanoparticle.
produced NH3 were spectrophotometrically determined using the
indophenol blue method[48] and possible by-product N2H4 were
confirmed through Watt and Chrisp method.[49] Corresponding
calibration curves are shown in Figure S4 and S5. Time-
dependent current density curves for 2 h from –0.35 to –0.55 V
are presented in Figure 3a and corresponding UV-Vis absorption
spectra of the electrolytes stained with indophenol indicator are
displayed in Figure 3b. Results show that the peak of highest
absorbance intensity is located at –0.40 V. Corresponding FEs
and NH3 yields at different potentials are plotted in Figure 3c.
Both FEs and NH3 yields increase as moving negatively applied
potential until –0.40 V, and then decrease gradually, which is
ascribed to the competitive selectivity toward the hydrogen
evolution reaction (HER).[18,50] The highest FE (25.6%) and
maximum NH3 yield (NH3 yield: 25.47 µg h‒1 mgcat.‒1) are
achieved at ‒0.40 V, outperforming pristine TiO2 shown in Figure
3d (FE: 3.65 %; NH3 yield: 5.36 µg h‒1 mgcat.‒1). It should be
mentioned that this catalyst also compares favourably to the
behaviors of all reported Ti- and Fe-based NRR electrocatalysts
in aqueous electrolytes (Table S1). The NH3 yields are also
determined through ion chromatography analysis (Figure S6).
As observed, both techniques offer comparable values.
Figure 2. (a) XPS survey spectrum of Fe-TiO2. XPS spectra in (b) Fe 2p, (c) Ti
2p, and (d) O 1s regions.
The X-ray photoelectron spectroscopy (XPS) survey spectrum
of Fe-TiO2 confirms the presence of Fe, Ti, and O elements
(Figure 2a).[44] As shown in Figure 2b, the two characteristics
peaks centred at binding energies of 711.4 and 724.72 eV are
presented in Fe 2p spectrum, which are attributed to Fe 2p3/2
and 2p1/2, respectively. After deconvolution, both the peaks can
be resolved into two peaks at 711.78 and 725.08 eV, which are
assigned to Fe3+, and the peaks at 709.08 and 723.32 eV
correspond to Fe2+. XPS spectrum of Ti 2p (Figure 2c) shows
that the typical Ti 2p3/2 peak is observed at 458.59 eV, which
corresponds to Ti4+, and other two Ti 2p1/2 peaks at 463.02 eV
and 464.39 eV are assigned to Ti3+ and Ti4+, respectively.
Compared to pristine TiO2 (Figure S3), the peaks for Ti 2p3/2 and
Ti 2p1/2 of Fe-TiO2 exhibit positive shifts of 0.38 and 0.48 eV,
respectively, which could be ascribed to lattice distortions.[45]
The existence of Ti3+ species indicates that Ti4+ ions obtain
electrons from nearby oxygen vacancies.[44] For O 1s peaks
(Figure 2d), it can be seen that three peaks is deconvoluted, and
the maximum peak appears at about 529.86 eV, which is
attributed to the Ti-O bond; the other two oxygen peaks are
obtained at 531.55 eV and 532.98 eV, which are ascribed to the
surface chemiadsorbed oxygen (O) and hydroxyl groups (OH),
respectively.[44,46,47]
Figure 3. (a) Time-dependent current density curves of Fe-TiO2/CP under
various potentials in N2-saturated 0.5 M LiClO4 for 7200 s. (b) UV-Vis
absorption spectra of the electrolytes stained with indophenol indicator at
various potentials. (c) FEs and NH3 yields over Fe-TiO2/CP at various
potentials. (d) FEs and NH3 yields over Fe-TiO2/CP and TiO2/CP at ‒0.40 V for
2 h. (e) Amount of NH3 over different electrodes at −0.40 V after electrolysis
for 2 h. (f) Cycling test of Fe-TiO2/CP for NRR at –0.40 V.
We also compare the amount of NH3 generated over three
electrodes: Fe-TiO2/CP, TiO2/CP and bare CP are considered.
As shown in Figure 3e, the latter two electrodes yield NH3 with
much smaller amount, implying Fe-TiO2 has superior NRR
activity for NH3 formation. Moreover, we also employed Fe-
TiO2/CP as the working electrode in Ar-saturated electrolyte at
‒0.40 V for 2 h and corresponding UV-Vis absorption spectrum
is presented in Figure S7. Results show that it is very close to
The electrochemical NRR tests were conducted in a typical H-
cell separated by a piece of Nafion membrane in 0.5 M LiClO4
solution under ambient conditions. Fe-TiO2 catalyst was
deposited on carbon paper (Fe-TiO2/CP with a Fe-TiO2 loading
of 0.10 mg cm-2) as the working electrode. All potentials are
recorded with respect to a RHE scale. The concentrations of
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10.1002/anie.201911153
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COMMUNICATION
the blank electrolyte, indicating almost no NH3 production.
Significantly, no N2H4 is detected after 2 h electrolysis (Figure
S8), revealing Fe-TiO2/CP possesses excellent selectivity
toward N2 to NH3 conversion. Electrochemical impedance
spectroscopy (Figure S9) analysis of Fe-TiO2/CP shows that its
charge-transfer resistance is lower than that of TiO2/CP,
suggesting that electron transfer is accelerated as introducing
Fe dopant into TiO2.[51] We also performed NRR electrolysis at –
0.40 V in 0.1 M Na2SO4. Figure S10 compares the NH3 yields
and FEs in two electrolytes, suggesting Fe-TiO2/CP is also
active but with decreased performance metrics in 0.1 M Na2SO4.
In addition, consecutive five recycling tests were considered to
evaluate stability of Fe-TiO2/CP at –0.40 V, which is a critical
factor in practical applications (Figure 3f). Both FEs and NH3
yields show a slight variation, indicating the high stability of Fe-
TiO2/CP during N2 reduction process. XRD analysis after long-
term NRR electrolysis (Figure S11) demonstrates that this
catalyst is no change in the crystalline phase, implying it is
extremely stable during NRR process. TEM image shows that
this catalyst still maintains its nanoparticle morphology after
long-term NRR electrolysis (Figure S12). 15N isotopic labeling
experiment was also performed to confirm the N source
originated from Fe-TiO2/CP through NRR process. Figure S13
displays 1H nuclear magnetic resonance (NMR) spectra. As
observed, only 15NH4+ signals are detected when 15N2 is bubbled
into the cathode.
than that of pristine TiO2 (101). Figure S15 shows the room
temperature electron spin resonance (ESR) spectra of TiO2 and
Fe-TiO2. The Fe-TiO2 shows a definite oxygen vacancy signal
located at g = 2.003, demonstrating a large number of oxygen
vacancies are formed in the wake of Ti3+ centers.[26,53] The
unpaired electrons from V1-decorated Fe-TiO2 present open-
shell singlet configuration (), indicating the absence of Ti3+
(Figure S16a),[52] Given Ti3+ exists in this catalyst, the second
oxygen vacancy is further considered and corresponding three
possible vacancy sites are selected, including V2-1, V2-2, and V2-3
(Figure S14). Calculation results show that V2-1 site is most
preferable with a lower Ef (3.71 eV) and this energy consumption
is comparable with that of V1-decorated pristine TiO2 (101). It
implies that V2-1 site generating is reasonable on V1-decorated
Fe-doped TiO2(101) surface [named as V1+V2-1-decorated Fe-
TiO2 (101)], as shown in Figure 4a. For this case, bi-Ti3+ appears
around V2-1 site (Figure S16b). Based on above results, it is
reasonably concluded that V1 and V2-1 are coexistence on Fe-
doped TiO2 (101) surface.
Subsequently, we calculate the whole NRR process on V1-
and V1+V2-1-decorated Fe-TiO2 (101) surfaces. For V1-decorated
Fe-TiO2 (101) surface, N2 molecule is adsorbed on Fe site by
end-on coordination with a free energy change of 0.07 eV (G =
0.07 eV) and Bader charge analysis indicates that 0.08 e- is
transferred into N2 molecule, suggesting N2 is effectively
activated. Nevertheless, the hydrogenation process of N2 is
terminated at *NH2NH2 intermediate, as shown in Figure S17,
which suggests that NH3 formation is difficult on V1-decorated
Fe-TiO2 (101) surface. Then, N2 adsorption on V1+V2-1
-
decorated Fe-TiO2 (101) surface is calculated and results show
that it prefers to adsorb on V2-1 site between four- and five-
coordination Ti atom sites (named as Ti4c and Ti5c) by end-on
coordination with G = 0.14 eV (Figure 4b and S16b). Bader
charge analysis shows that 0.15 e- is transferred into N2
molecule, which is twice higher than those of V1-decorated
pristine TiO2 (101) and V1-decorated Fe-TiO2 (101) surfaces.
The bond length of N2 is obviously elongated from 1.112 to
1.123 Å, which is higher than those of V1-decorated pristine TiO2
(101) and V1-decorated Fe-TiO2 (101) surfaces whose bond
lengths change from 1.112 to 1.118 Å. We calculate the work
function (WF) on V1-decorated pristine TiO2 (101), V1-decorated
Fe-TiO2 (101) and V1+V2-1-decorated Fe-TiO2 (101) surfaces,
and results show that corresponding values are 5.01, 5.11 and
4.71 eV, respectively. It can be seen that V1+V2-1-decorated Fe-
TiO2 (101) has lowest WF value and thus the back-donation
electron on this surface is most readily transferred to N2
molecule. The first hydrogenation of *N2 to *NNH experiences an
uphill pathway with G = 0.43 eV (Figure 4c). Hydrogenation of
*NNH has two pathways, including *NNH to *NNH2 and *NNH to
*NHNH processes, respectively. For distal pathway, *NNH to
*NNH2 process consumes energy in 0.55 eV, but the
subsequent *NNH2 is hydrogenated to *NHNH2 rather than *N
and *NH3, and thus is termed as the mixed pathway. Our
calculation results show that the mixed pathway, *NNH2 to
*NHNH2 process, is a downhill pathway with G = 1.27 eV. And
then *NHNH2 to *NH2NH2 process is an uphill pathway with G =
0.58 eV. Importantly, *NH2NH2 is quickly hydrogenated to *NH2
and *NH3 with G = 2.65 eV, and NH3 desorptions in the whole
NRR process consume lower energies (0.57~0.59 eV). For the
alternating pathway, *NNH to *NHNH process is an uphill
pathway with G = 0.13 eV, which is lower than that of distal
pathway (*NNH to *NNH2) with G = 0.55 eV. Then, the
Figure 4. Free energy diagram of NRR on V2-1 site at U= –0.40 V and
corresponding atom configurations. Ti, gray; Ti3+ (Ti4c), orange; Ti3+ (Ti5c),
purple; Fe, brown; O, red; N, blue; H, white.
DFT calculations on the active site and corresponding
electronic structure were carried out to gain further insight into
NRR mechanism at an atom scale. XRD and HRTEM indicate
that (101) surface is mainly exposed in Fe-TiO2 system and thus
is applied to catalyze NRR. In general, Fe atom prefers to
substitute five-coordination Ti atom on TiO2 (101) surface and it
is more probable to create oxygen vacancy (V) because of the
charge compensation between the metal dopant and lattice
defect.[52] Our calculations show that the V1 site near the Fe
dopant atom is most probable on Fe-TiO2(101) surface with
oxygen vacancy formation energy of 1.42 eV (Ef = 1.42 eV),
which is lower than that of V1-decorated pristine TiO2 (101)
surface with Ef = 3.70 eV, as shown in Figure S14. It suggests
that Fe-doped TiO2 (101) surface has more oxygen vacancies
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10.1002/anie.201911153
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10.1002/anie.201911153
Angewandte Chemie International Edition
COMMUNICATION
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This article is protected by copyright. All rights reserved.

10.1002/anie.201911153
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Tongwei Wu, Zhe Xing, Shiyong Mou,
Chengbo Li, Yanxia Qiao, Qian Liu,
Xiaojuan Zhu, Yonglan Luo, Xifeng Shi,
Yanning Zhang, and Xuping Sun*
Page No. – Page No.
Greatly Improving Electrochemical N2
Reduction over TiO2 Nanoparticle by
Fe Doping **
Fe doping greatly improves electrochemical N2 reduction over TiO2 nanoparticle for
ambient N2-to-NH3 fixation with excellent selectivity. In 0.5 M LiClO4, this catalyst attains
a high Faradic efficiency of 25.6 % and a large NH3 yield of 25.47 µg h‒1 mgcat.‒1 at ‒0.40
V vs. reversible hydrogen electrode. The catalytic mechanism is discussed and the
synergistic effect of bi-Ti3+ and oxygen vacancy is responsible for the superior activities.
This article is protected by copyright. All rights reserved.