LaNixFe1- xO3 (0 ≤ x ≤1) as photothermal catalysts for hydrocarbon fuels production from CO2 and H2O

Article abstract of DOI:10.1016/j.jphotochem.2019.03.045

LaNi_xFe_{1- x}O₃ perovskite compounds (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were successfully synthesized by a sol-gel combustion method. The crystal structure, morphology, BET surface area, oxygen vacancies, band gap and catalytic properties of the catalyst were characterized in detail. The results showed that LaNi_0.4Fe_0.6O₃ compound exhibits the best photothermal catalytic performance. Under the same catalytic conditions (350 ℃ + Vis-light), CH₄ and CH₃OH yields are about 3.5 and 4.0 times, 1.8 and 2.1 times of that of LaFeO₃ and LaNiO₃. It was found that all the solid solutions possesses better catalytic properties than the pure end compounds. The doping of Ni lead to a significant modification with the quantity of oxygen vacancies and band gaps. These findings may further broaden the materials scope for photothermal conversion of CO₂ and H₂O.

Full text of DOI:10.1016/j.jphotochem.2019.03.045

JournalofPhotochemistry&PhotobiologyA:Chemistry377(2019)182–189
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Research paper
LaNixFe1-xO3 (0 ≤ x ≤1) as photothermal catalysts for hydrocarbon fuels
production from CO2 and H2O
⁎⁎
Dongmei Zhenga, Guohui Weia, Lijuan Xua, Qiangsheng Guob, Jianfeng Huc, Na Shab,
,
⁎
Zhe Zhaoa,d,
a School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China
b School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China
c School of Materials Scinece and Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, China
d Department of Materials Science and Engineering, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden
A R T I C L E I N F O
A B S T R A C T
Keywords:
LaNixFe1-xO3 perovskite compounds (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were successfully synthesized by a sol-gel
combustion method. The crystal structure, morphology, BET surface area, oxygen vacancies, band gap and
catalytic properties of the catalyst were characterized in detail. The results showed that LaNi0.4Fe0.6O3 com-
pound exhibits the best photothermal catalytic performance. Under the same catalytic conditions (350 ℃ + Vis-
light), CH4 and CH3OH yields are about 3.5 and 4.0 times, 1.8 and 2.1 times of that of LaFeO3 and LaNiO3. It was
found that all the solid solutions possesses better catalytic properties than the pure end compounds. The doping
of Ni lead to a significant modification with the quantity of oxygen vacancies and band gaps. These findings may
further broaden the materials scope for photothermal conversion of CO2 and H2O.
LaNiO3
LaFeO3
Photothermal catalysis
CO2 reduction
CO2+2H2 OVis CH4+2O2
(1)
1. Introduction
TiO2 [14], Zn/TiLDH [15], Ru/NaTaO3 [16], ZO2@Cu-Zn-Al-LDH
[17], SnNb2O6 [18] and so on have been studied extensively owing to
its semiconductor material. But its band gap requires the use of ultra-
violet energy to initiate the reactions, as its photocatalytic in the ex-
istence of some limitations. Further research of hydrocarbon fuels on
thermal reactions with unique selectivity and high efficiency is still
required. The simultaneous thermochemical reaction of CO2 and H2O
with the oxygen deficient in material at a relatively low temperature
can convert CO2 into CH4 efficiently. And the problem of catalytic
carbon deposition has been solved. However, We combine photo and
thermal processes to develop a new method called photothermal cata-
lytic process, which unite the advantages of the photochemical and
thermochemical catalytic processes in the form of CO2 reduction to CO,
C and O2 while H2O reduction to H2 and O2 in the same system [19,20]
with a high-efficiency reaction rate.
Recent years, the energy crisis and increasing anthropogenic emis-
sions of Greenhouse gases are widely recognized as two of the primary
causes of global environmental problems [1–6]. Carbon dioxide, one of
the most severe environmental issues of recent years, has been identi-
fied as a potential contributor to ozone layer depletion in the strato-
sphere and a relatively strong greenhouse gas [7]. Thus, it is extremely
important to transform CO2 into hydrocarbon fuel and then provided an
alternative possibility for sustainable and clean energy development
because it partly fulfills energy demands and would help to reduce at-
mospheric CO2 [8,9]. Since Halmann discovered the photoelec-
trochemical reduction of CO2 into organic compounds in 1978 and
Hiroshi and co-workers reported the photocatalysts reduction of CO2
into organic compounds over suspending semiconductor particles in
water [10], growing interest in the development of semiconductor
photocatalysts. It’s not high enough for practical use that the generation
of CO and H2 from CO2 mostly [11]. Subsequently, a possible avenue
for sustainable development is to use photocatalysts for the conversion
of CO2 into methane with the help of solar energy. A process involving
only water and carbon dioxide is very promising as it can form a useful
carbon cycle Eq. (1) [12,13].
In our earlier work, mesoporous WO3 [20], the CH4 evolution per-
formance only reaches 21.42 μmol g−1 under thermal conditions, which
is less than that under visible-light irradiation and thermal conditions
(25.77 μmol g−1), the activity of m-WO3 in the photocatalytic conver-
sion of CO2 with H2O is not high enough for practical use. Furthermore,
⁎ Corresponding author at: School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China.
⁎⁎ Corresponding author.
E-mail addresses: shana@sit.edu.cn (N. Sha), zhezhao@kth.se (Z. Zhao).
https://doi.org/10.1016/j.jphotochem.2019.03.045
Received 16 November 2018; Received in revised form 7 March 2019; Accepted 27 March 2019
Availableonline29March2019
1010-6030/©2019ElsevierB.V.Allrightsreserved.

D. Zheng, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry377(2019)182–189
Xu et al has exploited LaCoxFe1-xO3, nanoparticle which LaCo0.6Fe0.4O3
has the best CH4 evolution performance reaches 437.28 μmol g−1 with
showing better photocatalytic activity under a photothermal condition
[21].
2.2. Characterization of sample
The synthesized samples were characterized using X-ray diffraction
(PAN alytical X’pert), their crystalline phases were determined by Cu Ka
radiation (λ =0.154 nm) at 40 kV and 40 mA. The XRD patterns were
Perovskite-type oxides, with ABO3 formula, which have better
photocatalytic activity are ascribed to the corner-shared BO6 octahe-
dron network that facilitates electron transfer [22,23]. Further, the
large atoms at the A-site of ABO3 are responsible for the stabilization
and more active behavior of the perovskite structure [24–26]. There-
fore, advantages of them are applied to catalysis [27,28] and magne-
toresistance devices [29,30]. Simultaneously, the valency and vacancy
can be dominated due to their significant redox properties [31]. How-
ever, La-based catalysts could be the promising catalysts with excellent
performance and low cost. In addition, LaFeO3 as a function material
plays a role in catalytic activity, which drawn great attention for its
promising applicability in the utilization of solar energy and environ-
ment remediation [32–35]. Yet, there are still many problems in the
utilization of photocatalysts, such as poor light absorbance and high
recombination of photogenerated electron-holes pairs when band gap
of 2.1 eV. Thus, various methods have been developed to overcome
those problems and improve the photocatalytic activity [36,37]. Xu
et al had exploited LaCo0.6Fe0.4O3 nanoparticle for the best photo-
catalytic reduction of CO2, it absorbs in the visible region owing to
possible variations in its band gap energy 1.68 eV. As we all know, the
efficiency of the utilization of solar light is related to band gap of ma-
terial, which also affect the photocatalytic activity [38]. It indicates
that narrow band gap can largely improve the visible light absorptive
quality and effectively restrain the recombination of photo generated
electron holes pairs to exhibit a higher photocatalytic activity. Addition
of photocatalysts have shown to prolong the time of recombination
electrons-holes and then improve the lifetime of electron [39,40]. In
this research, we can adjust the recombination of electrons-holes of
LaFeO3 by substituting partially Fe for Ni at B-site in our experiment.
In this paper, we focus on investigating the catalytic activity of
dopant concentration, which photothermal reduction of CO2 with H2O
vapor to CH4. At last, the influence of the nickel content for the per-
ovskite microstructure on photothermal activity was revealed. It shows
that an appropriate range of Ni-doping can facilitate the catalytic pro-
cess. This discovery may further expand the materials scope of photo-
thermal conversion and provide innovative insights into the catalysis of
carbon dioxide reduction.
record in the range between 20°and 80° at the scan rate of 10°min−1
.
The morphology and sizes of the products were investigated by using a
scanning electron microscopy (SEM, JSM-7000 F). BET specific surface
areas of the samples were performed by using a Micrometrics USA
ASAP2020 instrument with adsorption-desorption of N2 at 77 K, sam-
ples were degassed 2 h at 150℃. UV–vis diffuse reflectance spectra of
the samples were measured in the range of 200–800 nm using SHIM-
ADZU UV-3600 using spectral purity BaSO4 as the reference. X-ray
photoelectron spectroscopy (XPS) was characterized by an ESCALAB
250 instrument with Al Kɑ (hv = 1486.6 eV) source at a residual gas
pressure of below 10−10 Pa. Mott-Schottky curves were measured using
a three-electrode system on the CHI604E electrochemistry workstation
2.3. Catalytic measurements
The photothermal catalytic reduction of CO2 was carried out with
100 mg of the sample powdered, which was uniformly placed at the
bottom of a Pyrex glass cell. The whole catalysis reaction temperature
was set at 350℃ with visible light irradiation on (photothermal). The
temperature control was realized by a heating jacket surrounded the
quartz reactor. And the light source used was a 300 W Xe lamp
equipped with a UV- light filter (λ > 420 nm). The light illumination
intensity is measured using CEL-NP2000-2 Full Spectral Optical Power
Meter. And the measured optical power density value is 145 mW/cm2.
Vacuumed the reactor with the vacuum up to empty the air inside the
reactor and then CO2 (99.999 wt %) passed through the reactor at a
flow rate of 27 mL/min for 5 min. The reaction was in a gas tight
system, distilled water (0.3 mL) was injected into the system when the
temperature above 120℃. In a period of insulation for 5 h, took samples
per hour and qualitatively analyzed by GS-Tek (Echromtek A90) and
quantitatively analyzed by gas chromatograph equipped with a hy-
drogen flame detector (FID). The quantification of CH4 yield was based
on the external standard and the use of calibration curve. method.
3. Results and discussion
3.1. XRD analysis
2. Experimental
The crystal structure and phase purity of the LaNixFe1-xO3 (x = 0,
0.2, 0.4, 0.6, 0.8, 1.0) were characterized by XRD and the results were
shown in Fig. 1. According to the characteristic peaks of the perovskite
LaFeO3 and LaNiO3, all the synthesized materials are with perovskite
structure and no impurity phases can be identified from the XRD
spectra. Furthermore, with partial substitution for Fe by Ni, we can see
that the peak intensity of samples decreases and half peak width gra-
dually widened. In addition, a clear crystal structure transition from
orthorhombic Pnma (PDF #88-0641) to trigonal R-3c (PDF #88-0633)
can be observed when x reached 0.6. Besides, the crystallite size de-
termined by the Scherrer method is in the range of 22–35 nm (see
Table1) and so no significant effect of x value on crystallite size can be
defined. The results indicate change in unit cell parameters (a–c), This
may be attributed to the difference in ionic radii of deferent cations
causing crystal lattice contraction due to distortion of Fe/NiO6 octa-
hedra or/and by mixed valence state of Ni in order to neutralize the
charge imbalance.
2.1. Synthesis of catalyst
La(NO3)3·6H2O, Fe(NO3)3·9H2O, Ni(NO3)3·6H2O and citric acid
were procured from Shanghai Titan chemicals Co, Ltd. These chemicals
as raw material were directly used to compound series LaNixFe1-xO3
mixed-oxide perovskites, which was without any further purification
treatment. In a typical experiment, the catalyst in accordance with the
formula LaNixFe1-xO3 (with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were
synthesized by the citric acid sol-gel method. The metal nitrate of
12.99 g La(NO3)3 6H2O of 0.03 mol and 12.12 g Fe(NO3)3 9H2O of
0.03 mol were added to the deionized water of 60 mL, and the molar
ratio of nitrate to acid was 1:1.2. Adding 13.83 g anhydrous citric acid
of 0.072 mol to the mixed solution, stirring and dissolving until a
clarification solution is formed, heating the resulting solution in a water
bath at 80℃ until the sol is formed, and then moving to the oven for
overnight drying at 120℃ until the sol was completely dried, then
sintered at 700℃ for 7 h, and then the perovskite-type LaFeO3 nano-
particles were obtained. According to the above synthesis method, a
series of other doped LaNi0.2Fe0.8O3, LaNi0.4Fe0.6O3, LaNi0.6Fe0.4O3,
LaNi0.8Fe0.2O3 and LaNiO3 nanoparticles were synthesized by adjusting
the amount of metal nitrate of Ni(NO3)3 6H2O, the raw material of Ni in
position B, by the corresponding doping ratio.
3.2. BET surface area and pore structure analysis
N2-sorption isotherms of LaNixFe1-xO3 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0)
materials correspond to type-IV (see Fig. S2 SI), according to the IUPAC
classification. However, the porous LaNixFe1-xO3 exhibits a type-II
183

D. Zheng, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry377(2019)182–189
Table 2
The surface properties of the catalysts were examined by BET surface area.
x
Surface area (BET)(m2/g)
Pore size(nm)
Vp/cm3 g−1
0.0
0.2
0.4
0.6
0.8
1.0
9.44
7.28
9.84
9.60
5.43
4.16
21
35
25
22
35
31
0.050
0.060
0.062
0.054
0.048
0.032
As shown in Fig. 3a) and b), the binding energies of both Ni and Fe
show evidence of the coexistence of the least two oxidation states (+2
and +3), are consistent with the results of literature [41]. The Fe 2p
peak fitting of the LaNixFe1-xO3 catalyst were performed for three
components, Fe2+, Fe3+ and the shake-up satellites reported by Liu
[42]. For Fe 2p3/2, binding energy of 709.99 and 713.61 eV can be
attributed to Fe2+ and Fe3+ respectively. Analogously, the character-
istic binding energy peaks of for Ni 2p can be assigned to Ni 2p3/2
(Fig. 3a)). Binding energy of 854 and 856 eV [43] can be attributed to
Ni2+ and Ni3+ respectively. Moreover, the presence of doublets char-
acteristic of Ni 2p (Ni2+and Ni3+) and Fe 2p (Fe2+ and Fe3+) (see
Table 3) were common for all the studied compounds, thus substantial
amount of oxygen vacancy can be expected. That the total valence of
cation is much less than 6. Simultaneously, the oxygen stoichiometry
(with a formal charge of −2) would change from LaNixFe1-xO3 to La-
Fig. 1. XRD spectra of LaNixFe1-xO3 photocatalysts with different Ni-con-
centration synthesized at 700 ℃.
isotherm with a type-IV(a) hysteresis loop at high relative pressure (P/
P0) range of 0.9–1.0, indicating the presence of macropores. Moreover,
the specific surface area and the pore properties of the LaNixFe1-xO3
catalysts, with the different x values were listed in Table 2. All the
synthesized catalyst materials possessed a specific surface area of 4-
10 m2/g and it is rational to regard them as similar porous materials in
the aspect of pore structure. The measured pore size and pore volume
changed with x value, but no consistent tendency can be defined among
the different materials. However, the difference in BET surface area can
be a factor influencing the final photothermal catalytic performance,
we will discuss this potential effect latter together with the catalytic
performance of different compounds.
NixFe1-xO3-&
. Moreover, perovskite was ABO3, the unit cell was
changed, non-stoichiometry phenomenonin perovskites becomes very
common, arising from oxygen deficiency especially. As shown in
Table 3, it is possible to estimate the concentration of +2 ions for both
Ni and Fe, but it is difficult to be used as a precise estimation due to the
potential artifacts in data fitting. To further elucidate the information of
the oxygen vacancy in various compounds, the O 1s spectra were
analyzed carefully. All O 1s spectra shown in Fig. 3c) can be separated
into three components with characteristic peaks at 528.37, 530.65 and
532.19 eV respectively. According to the previous results [44], they can
be attributed to lattice, surface and adsorbed oxygen respectively. The
presence of the surface adsorption oxygen (such as O−, and O2−) in
XPS spectra is generally associated with the surface oxygen vacancies in
perovskite oxides. Thus, the relative concentration of surface oxygen
vacancies can be roughly estimated according to the relative intensity
ratio (RIR) of O 1s (adsorption oxygen) /O 1s (lattice oxygen) (see
Table 4). The calculated RIR values for the LaNi0.4Fe0.6O3 sample were
1.69, which indicates the maximum quantity of oxygen vacancies [45].
Besides, the photothermal catalytic activities for CO2 conversion with
H2O vapor to CH4 over LaNixFe1-xO3 catalysts showed that the catalytic
activity toward this reaction depends on Oads/Olattice ratios. (see
Table 4). It revealed that oxygen vacancies in oxide semiconductor
photocatalysis play an important role in the photocatalysis. We tenta-
tively hypothesize that oxygen vacancy can preferably adsorb the
oxygen atom in CO2 and H2O molecule and thus enhance the asym-
metry in their molecule structure and thus promote their disassociation
and further reduced to hydrocarbon fuels, follow the DFT, the results
suggest that the order of LaBO3(B = Fe, Mn, Co, Ni)oxygen vacancy
3.3. SEM analysis
The morphological features of the LaNixFe1-xO3 nanoparticles were
characterized by SEM analysis, and the corresponding SEM results were
shown in Fig. 2. As a result of Sol-gel combustion synthesis, it can be
seen that the samples are all porous materials. Particle size expressed by
these SEM images was rather similar between different compounds,
which was also demonstrated by the previous XRD crystallite size
analysis. However, with the increase of x value, the synthesized powder
presented denser microstructure, which may affect the CO2 gas trans-
port during the photothermal catalysis process and hence potentially
affect the catalytic results.
3.4. XPS analysis
As shown in Fig. 3, the chemical state and surface species of the
LaNixFe1-xO3 catalyst were studied by XPS analysis. The survey spectra
(Fig. 3d)) indicate that all of the peaks were ascribed to La, Fe, Ni, O
and C elements and no peaks of other impurity elements were observed.
Table 1
The XRD analysis results.
Samples(x)
Crystal structure
2-Theta(o)
d(121) (nm)
Unit cell a, b, c(nm)
Crystallite size(nm)
0.0
0.2
0.4
32.2394
32.4055
32.5968
0.2774
5.556, 5.560, 7.855
5.521, 5.548, 7.809
5.484, 5.514, 7.780
Unit cell a = b, c(nm)
5.496, 13.381
31
32
22
OrthorhombicPnma
0.2761
0.2745
d(110) (nm)
0.2732
0.6
0.8
1.0
32.7458
32.7015
32.9597
28
35
27
Trigonal R-3c
0.2736
5.472, 13.172
0.2715
5.448, 13.187
184

D. Zheng, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry377(2019)182–189
Fig. 2. SEM images of LaNixFe1-xO3 powders synthesized by sol-gel combustion method at 700 ℃ for 2 h. a) x = 0, b) x = 0.2, c) x = 0.4, d) x = 0.6, e) x = 0.8, f) x
=1.
formation energies is Fe > Mn > Co > Ni(where the larger implies
more difficult to form a vacancy), meanwhile, first-principles calcula-
tions revealed the oxygen vacancies play an important role in adsorbing
oxygen by increasing the adsorption energy of H2O and CO2 in LaFeO3
and LaNiO3 [46–50].
properties of the LaNixFe1-xO3. The UV–vis diffuse reflectance spectra
(DRS) are shown in Fig. 4a). It was interesting that only LaFeO3 showed
clear light absorption edge around 600 nm. All other compounds
showed strong light absorption through the wavelength range of
200–800 nm, indicating that LaNixFe1-xO3 nanoparticles can respond to
visible light for photocatalytic reaction.
In order to estimate the optical band gap (Eg) of the LaNixFe1-xO3
perovskties, we used the following equation to fit the Kubelka Munk
function Eq. (2):
3.5. UV–vis DRS analysis
To elucidate the influence of surface oxygen vacancies on the optical
Fig. 3. The XPS spectra of a) Ni 2p, b) Fe 2p, c) O 1s and d) survey spectra of LaNixFe1-xO3 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) prepared at 700 ℃ for 2 h.
185

D. Zheng, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry377(2019)182–189
Table 3
very precise. Combined with the band gap Eg obtained by Kubelka
Munk function, it is very convenient to deduce the edge of VB (EVB). As
show in Fig. 5a), the typical p-type semiconductor characteristics of
LaNi0.4Fe0.6O3 was revealed by observing the positive slope of the MS
plot. (MS plots of other compounds were provided in SI). By calcu-
lating, the ECB of LaNixFe1-xO3 nanoparticles with x value changing
from 0 to 1 were about −0.313, −0.277, −0.250, −0.255, −0.268
and −0.273 V respectively and the EVB of them were 1.717 V, 1.353,
1.31, 1.395, 1.502, and 1.527 V respectively. To get the possible cata-
lytic reactions [Eqs. (3–5)] to thermodynamically proceed auto-
XPS binding energies relative to the perovskites LaNixFe1-xO3.
Ni2P(eV)
Fe2p(eV)
Catalyst(x)
Ni 2p3/2 (%)
Fe 2p3/2 (%) Fe2p1/2 (%)
0
Ni(3+
Ni(2+
)
)
710.03(66.22%) 723.51(66.50%)
711.85(33.78%) 725.53(33.50%)
710.07(63.83%) 723.52(66.38%)
712.66(36.17%) 725.42(33.62%)
709.73(63.12%) 723.33(65.72%)
711.15(36.88%) 724.80(34.28%)
709.94(69.21%) 723.01(70.75%)
712.90(30.79%) 725.03(29.25%)
710.09(70.91%) 723.59(69.41%)
712.79(29.09%) 725.50(30.59%)
0.2
0.4
0.6
0.8
1.0
853.78(64.77%)
855.10(35.23%)
854.26(64.56%)
855.40(35.44%)
853.29(68.74%)
854.40(31.26%)
854.16(68.83%)
855.78(31.17%)
853.11(72.37%)
854.72(27.63%)
matically, the EVB should be always more positive than Eo(H2O/H+
)
and ECB should be more negative than Eo(CO2/CH4) and Eo(CO2/
CH3OH). As described above, it is rational to expect CH4 to be the
photothermal catalysis product. However, due to the lack of very pre-
cise estimation of the ECB from the flat band approximation, it is not
very definitive to exclude the possible formation of CH3OH, which was
actually observed in the final products in this study.
Fe(2+) Fe(2+
Fe(3+) Fe(3+
)
)
Table 4
Except for the reduction of Eg by forming a solid solution between
LaNiO3 and LaFeO3, it is also noteworthy to point out that mixed B-site
with Ni and Fe will only introduce minor change in ECB and rather
tremendous modification with EVB. As we already know that non-metal
doping mostly introduce occupied orbitals above the energy of the
valence band thus narrowing the band gap, it is rational to attribute the
reduced Eg in LaNixFe1-xO3 (x = 0.2, 0.4, 0.6 and 0.8) to the extra
XPS O1 s analysis of the LaNixFe1-xO3 perovskites.
O 1 s(eV)
x = 0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
x = 1.0
Lattice oxygen
Surface oxygen
Absorbed oxygen
Surface/Lattice
54.28
37.04
8.68
43.38
50.33
6.29
35.60
60.04
4.36
39.83
59.47
0.7
39.98
55.83
4.19
45.48
50.62
3.90
0.68
1.16
1.69
1.49
1.40
1.11
oxygen vacancy resulted from the presence of Ni2+ and Fe2+
.
(1 R)2
1
H2O
O2 + 2H+ + 2e (Eoox = 0.82V vs. NHE )
F (R) =
2R
(3)
(4)
(5)
2
(2)
CO2 + 8e + 8H+
CO2 + 6e + 6H+
CH4 + 2H2O(Eroed
=
0.24V vs.NHE)
Which determines the extent of photon absorption of a semi-
conductor from its reflectance (R). Fig. 4b) represents Tauc plot (F(R)
*E)1/n vs. photon energy hv (eV) where n is 2 for indirect transition and
n is 0.5 for direct transition. In our case, only n = 2 can lead to a
reasonable fitting for the DRS spectra (see Fig. 4b)). All LaNixFe1-xO3
compounds were indirect semiconductor.
CH3OH + H2O(Eroed
=
0.38V NHE)
vs.
3.7. Photothermal catalytic performance
As indicated in Fig. 4b), all the solid solution compounds presented
lower Eg than the two end compound, LaFeO3 and LaNiO3. Eg reached
the lowest value of 1.56 eV when x = 0.4. Lower Eg will promise more
efficient visible light absorption and thus potentially to be beneficial for
the final photocatalytic performance. For detailed possible mechanism
of this reduced band gap by changing x value will be discussed later.
Fig. 6a) and b) shows the amount of CH4 and CH3OH generation
with the photothermal catalytic activity at different Ni-doping levels,
which was evaluated under photothermal conditions at 350 °C and vis-
light irradiation for 6 h. In Fig. 6a) and b), we can find out that the
catalytic activity increased greatly with Ni doping in B-site compares
with LaFeO3 perovskite. After 6 h for the reaction, the methane yield at
different
x
values was in the order of 0.4 > 0.6 > 0.2
3.6. Band structure analysis
> 0.8 > 1.0 > 0, as well as the methanol yield. The accumulated
yield of methane and methanol under photothermal conditions after 6 h
were 134.98, 374.92, 471.39, 422.69, 295.08, 259.20 μmol g−1 and
3.85, 11.91, 15.50, 13.75, 19.11, 7.46 μmol g−1 respectively (see
Table 5). Among them, LaNi0.4Fe0.6O3 was clearly the best material for
both CH4 and CH3OH generation. It was about 3.5 times and 4.0 times
of those yields provided by LaFeO3 and LaNiO3. Look at all the solid
solution compounds LaNixFe1-xO3 (x = 0.2, 0.4, 0.6 and 0.8), the cat-
alytic production of them were always higher than pure by LaFeO3 and
As we all know, the electronic band gap energy of material is critical
for its photocatalytic activity. Additionally, it is also important for
samples to study the positions of conduction band (CB) and valence
band (VB) in order to understand how the different hydrocarbon fuels
will be formed by the photothermal catalysis. In most common sense,
CB edges (ECB)can be approximated by the flat band which can be
measured by Mott–Schottky plot, even such an approximation is not
Fig. 4. UV–vis diffuse reflectance spectra of LaNixFe1-xO3 nanoparticle in different Ni rate a), plots of (F(R)*E)0.5 versus photon energy of LaNixFe1-xO3 sample b).
186

D. Zheng, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry377(2019)182–189
Fig. 5. Mott-Schottky plots of LaNi0.4Fe0.6O3 nanoparticle a). A schematic illustration of the band structure of LaNixFe1-xO3 nanoparticle b).
Fig. 6. CH4 generation over the under visible light irradiation and heating condition at 350 °C a) and CH3OH generation over the under visible light irradiation and
heating condition at 350 °C b).
Table 5
Photothermal catalytic activity and physical properties of the LaNixFe1-xO3 perovskites.
Sample(x)
The first hour yield (μmol g−1
)
yield after 6 h (μmol g−1
)
Total yield (%)
CH4+CH3OH
Selectivity (%)
CH4
product
CH4
CH3OH
CH4
CH3OH
CH3OH
0
89.06
2.06
7.45
10.10
8.86
5.85
4.44
134.98
374.92
471.39
422.69
295.08
259.20
3.85
0.25
0.68
0.86
0.77
0.54
0.47
97.23
96.92
96.82
96.85
97.00
97.20
2.73
3.80
3.18
3.15
3.00
2.80
0.2
0.4
0.6
0.8
1.0
225.78
291.47
243.72
162.92
142.22
11.91
15.50
13.75
19.11
7.46
LaNiO3. The existence of mixed transition metal ions at B-site is bene-
ficial for the photothermal catalysis of CO2 and H2O to produce hy-
drocarbon fuels.
To be noticed, there is no metal co-catalysts were involved in this
study. To be compared with other reported results (see Fig. 7, experi-
mental conditions details in SI table S1) where similar photothermal
catalysis was investigated, the studied non-metal perovskite oxides
showed great promise for further improvement if the detailed catalysis
mechanism can be elucidated in the future.
3.8. Mechanism of photocatalytic reduction of CO2
In order to understand the reaction process, a possible catalytic
mechanism of LaNixFe1-xO3 catalysts for the reduction of CO2 to CH4
and CH3OH were shown in Fig. 8. The first is of the perovskite LaNixFe1-
xO3 surface, creating electron–hole pairs [Eq. (6)], that surface can
adsorb CO2 and H2O molecules as well. H2O is also photoexcitation [Eq.
(7)]. The adsorption of CO2 on LaNixFe1-xO3 suggest that these inter-
actions are not strong enough with stoichiometric, defect-free surfaces
to sufficiently lower the barrier and enable transfer of conduction-band
electrons. The interactions are much stronger at oxygen vacancy sites
Fig. 7. Catalytic results of co-catalysts in other report.
187

D. Zheng, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry377(2019)182–189
Fig. 8. Schematic diagram of the combined photo- and thermal-catalytic reduction of CO2 with H2O vapor to CH4 in one system over LaNixFe1-xO3 perovskites.
where a foreign oxygen atom can fill the vacancy. That may lead to the
dissociation of CO2 upon electron transfer and subsequent release of CO
[Eq. (8)]. Furthermore, H2O adsorbed on surfaces could spontaneously
dissociate into an H atom and an OH group [Eq. (9)]. The adsorption
not only spatially enables the transfer of photoexcited electrons to re-
action species, but also alters the form of active species to lower the
photon energy requirement for reactions. Notably, the CO intermediate
may also undergo further reduction. Finally, the activated CO species
would transform into CH4 [Eq. (4)] through reacting with the formed
atomic hydrogen on the surface via a photogenerated electron-induced
multistep reduction process involving electron and proton transfer, C–O
bond breaking, and C–H bond formation. However, the VB edge of all
LaNixFe1−xO3 compounds is not negative enough for the production of
CH3OH [Eq. (5)] through photocatalytic reduction process. So we have
to point out that the production of amount of CH3OH was small in our
study. Notably, surface Vo plays a very important role in the activation
and dissociation of CO2 molecule on the surface of LaNixFe1-xO3. In
summary, the photothermal catalytic activity of LaFeO3 was enhanced
by Ni-doping, which adjusted the band structure and reduced the band
gap. These findings may be of importance for a variety of applications.
Photocatalytic:
morphology and surface area of the sample have also changed. By
contrast, the CH4 and CH3OH generation rate of LaNi0.4Fe0.6O3 for the
reduction of CO2 with H2O, under the same photothermal reaction
conditions, were enhanced 3.5 and 4.0 times, 1.8 and 2.1 times better
than LaFeO3 and LaNiO3. According to these results, we believe that the
coexistence of Ni and Fe on B-site can promote the photothermal cat-
alysis of CO2 and H2O. The working mechanism can be the enhanced
light adsorption and the extra oxygen vacancies. The more detailed
mechanism for this doping effect need more investigation to elucidate.
The LaNixFe1-xO3 composites can work as highly active and stable
visible-light-driven photocatalysts. This research will lead to new ef-
forts in exploring more effective and inexpensive visible-light-driven
photocatalysts.
Acknowledgements
The authors highly acknowledge the financial support of the
National Key R&D Program of China (Nos. 2017YFB1103500
2017YFB1103502).
&
Appendix A. Supplementary data
LaNixFe1 xO3 + hv
e
+ h+
(6)
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2019.03.
045.
2H2 O+ 4h+
4H+ + O2
(7)
Thermochemical:
CO2 + V
CO + O
(8)
(9)
o
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