Effect of H2/CO2 mixture gas treatment temperature on the activity of LaNiO3 catalyst for hydrogen production from formaldehyde aqueous solution under visible light

Article abstract of DOI:10.1016/j.jallcom.2009.09.104

H₂/CO₂ mixture gas treatment at moderate temperature is found to be effective in improving the activity of LaNiO₃ catalyst for hydrogen production from formaldehyde aqueous solution under visible light. The LaNiO₃, as a precursor, was treated at various temperatures (573-973 K) for 4 h under continuous flow mixture gas with a molar ratio of H₂/CO₂ = 4:1. Among all the resultant photocatalyst, the LaNiO₃ treated at 773 K (Ni/LaNiO_3-δ-La₂O₂CO₃), exhibited highest photocatalytic activity when exposed to visible-light irradiation and was further characterized by XRD, SEM, TEM, UV-diffuse reflectance spectroscopy. The results indicated that Ni/LaNiO_3-δ-La₂O₂CO₃ photocatalyst had narrow bandgap and can efficiently transfer photogenerated electrons on the surface and thus reduce H⁺ to hydrogen.

Full text of DOI:10.1016/j.jallcom.2009.09.104

Journal of Alloys and Compounds 489 (2010) L13–L16
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Effect of H2/CO2 mixture gas treatment temperature on the activity of
LaNiO3 catalyst for hydrogen production from formaldehyde aqueous
solution under visible light
Lishan Jia∗, Juanjuan Li, Weiping Fang
Department of Chemical Engineering and Biochemical Engineering and Department of Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, Fujian, China
a r t i c l e i n f o
a b s t r a c t
Article history:
H2/CO2 mixture gas treatment at moderate temperature is found to be effective in improving the activity
of LaNiO3 catalyst for hydrogen production from formaldehyde aqueous solution under visible light. The
LaNiO3, as a precursor, was treated at various temperatures (573–973 K) for 4 h under continuous flow
mixture gas with a molar ratio of H2/CO2 = 4:1. Among all the resultant photocatalyst, the LaNiO3 treated
at 773 K (Ni/LaNiO3−ı–La2O2CO3), exhibited highest photocatalytic activity when exposed to visible-
light irradiation and was further characterized by XRD, SEM, TEM, UV-diffuse reflectance spectroscopy.
The results indicated that Ni/LaNiO3−ı–La2O2CO3 photocatalyst had narrow bandgap and can efficiently
transfer photogenerated electrons on the surface and thus reduce H+ to hydrogen.
Received 29 June 2009
Received in revised form
15 September 2009
Accepted 15 September 2009
Available online 25 September 2009
Keywords:
Visible light
Hydrogen
© 2009 Elsevier B.V. All rights reserved.
Ni/LaNiO3−ı–La2O2CO3
Photocatalyst
H2/CO2 mixture gas
1. Introduction
As reported previously [14], the LaNiO3 could be rapidly mod-
ified at moderate temperature under a stoichiometric mixture of
Hydrogen, an attractive clean and renewable energy source,
is considered as an ideal fuel for the future. Our current energy
infrastructure is dominated by fossil fuel use, which leads to green-
house gas emissions. If we can synthesize a catalyst to produce
hydrogen from water by harvesting sunlight, it could be a key to
solve these problems. However, most of developed photocatalysts
capable of splitting water can work only in the ultraviolet region
[1–7]. To effectively utilize the visible light that accounts for 43%
of the solar spectrum [8], it is therefore necessary to develop a
visible-light responsive photocatalyst, and substantial efforts have
been made in recent years to explore such a material. The photo-
catalytic activity can be enhanced by loading metal oxide and/or
metal on various semiconductors as cocatalyst [9–11]. For exam-
ple, Wu et al. [12] reported that NiO–Ni–InTaO4 prepared by an
innovative sol–gel method exhibited high photocatalytic activity
for water splitting under visible-light irradiation. Domen research
group [13] synthesized Rh/Cr2O3-loaded (Ga1−xZnx)(N1−xOx) cat-
alyst with high catalytic activity for overall water splitting under
visible-light irradiation, and found that both Rh and Cr2O3 are indis-
pensable for achieving photocatalytic overall water splitting in this
(Ga1−xZnx)(N1−xOx)-based system.
CH4 and CO2. The resultant Ni–La2O2CO3 catalyst showed high
activity and stability for the CO2 reforming of methane reaction,
which can be attributed to the high dispersion of metal Ni. Based on
the above reports, we think whether it is possible to synthesize a
catalyst containing metallic nickel, nickel oxide and oxycarbonate
species. Therefore, the LaNiO3, as a precursor, was treated at
various temperatures for 4 h under continuous flow mixture gas
with a molar ratio of H2/CO2 = 4:1. In the present work, the effect
of H2/CO2 mixture gas treatment temperature on the activity
of LaNiO3 for hydrogen production from formaldehyde aqueous
solution under visible light is investigated by characterization
analysis.
2. Experimental
2.1. Catalyst synthesis
The precursor LaNiO3 was prepared by sol–gel method [15]. La(NO3)3·nH2O
and Ni(NO3)3·6H2O were weighed in equimolar amounts and dissolved in distilled
water, then citric acid was added into the mixed solution under vigorous stirring
with a citric acid/total cations molar ratio of 1/2. The excess water was slowly
removed under infrared lamp irradiation until a viscous liquid was produced. Sub-
sequently, the prolonged heating formed a crispy and bubbly precursor with black
colour [16]. The obtained samples were manually ground with an agate mortar,
and then they were calcined at 673 K for 2 h, and at 1073 K for 4 h to form LaNiO3
perovskite phase.
∗
Corresponding author. Tel.: +86 592 2188283; fax: +86 592 2184822.
E-mail address: Jials@xmu.edu.cn (L. Jia).
450 mg of the LaNiO3 powder was packed into the reactor and heated to the
specified temperature (573–973 K) at a rate of 283 K min−1, and then maintained at
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.09.104

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L. Jia et al. / Journal of Alloys and Compounds 489 (2010) L13–L16
that temperature for 4 h in a flow of H2/CO2 mixture (20 ml min−1). After that, the
sample was cooled to room temperature under mixture flow.
2.2. Catalyst characterization
X-ray diffraction (XRD) patterns were obtained on a Panalytical X-pert spathic
powder diffractometer with Cu-Ka radiation source. The accelerating voltage and
the applied current were 40 kV and 30 mA, respectively.
Hydrogen-temperature-programmed reduction (TPR) was performed with a
quartz tube fixed-bed micro-reactor. Prior to the TPR measurement, each 50 mg
catalyst sample was pretreated in Argon at 573 K for 30 min. After being cooled to
303 K, the sample was exposed to flow (30 ml min−1) of 5 vol.% H2/Ar and ramped
to 933 K at a heating rate of 283 K min−1
.
SEM images of the samples were obtained on a LEO1530 scanning electron
microscope. Transmission electron microscopy (TEM) was performed on a Phillips
Analytical FEI Tecnai 30 electron microscope operated at an acceleration voltage of
300 kV.
Ultraviolet–visible (UV–vis) diffuse reflection spectroscopy of the photocata-
lysts was recorded using a Varian Cary 5000 UV–vis spectrophotometer. The spectra
were collected in the 200–800 nm range at room temperature using BaSO4 as a
reference.
Fig. 1. XRD analysis of (a) LaNiO3 before treatment and after treatment at various
temperatures: (b) 573 K; (c) 673 K; (d) 773 K; (e) 873 K; (f) 973 K. (ꢀ) LaNiO3, (ꢂ)
LaNiO2.7, (ꢀ) NiO, (+) Ni and (ꢁ) La2O2CO3.
2.3. Photocatalytic splitting of water
The photocatalytic reaction was evaluated in a self-made quartz inner irra-
diation type reaction vessel. The catalyst (0.1 g) was suspended in aqueous
formaldehyde solution (H2O 175 mL, HCHO 25 mL) by a magnetic stirrer. Prior to
irradiation, the solution was continuously bubbled with N2 at a rate of 60 ml min−1
for 30 min, and then the gas content was checked by GC to confirm that no oxygen
was present. 2 mol/L NaNO2 solution introduced into the water jacket as an inter-
nal circulation cooling medium to eliminate UV-light (cut-off ꢀ < 400 nm) [17,18].
Irradiation was conducted using a 125 W xenon lamp and the reaction temperature
was kept at 323 K.
The gas evolved was gathered and analysed by GC (TCD, molecular sieve 5 Å
column and Ar carrier). Blank reactions were carried out without catalyst under
visible-light irradiation and no production of hydrogen was observed.
3. Results and discussion
3.1. XRD analysis
Fig. 2. TPR curves of the LaNiO3. Weight: 50 mg; gas flow rate: 5% H2/Ar,
Fig. 1 shows the powder X-ray diffraction (XRD) patterns of
samples with different treatment temperatures, along with LaNiO3
data for comparison. The X-ray diffraction analysis exhibits that
at 573 K, the structure of perovskite is preserved and the main
phase detected was LaNiO2.7; at 673 K, the predominant phases are
LaNiO2.7 and nickel oxide; it begins to change at around 773 K; at
this temperature, the main phase detected was lanthanum oxycar-
bonate La2O2CO3 (JCPDS 01-084-1963). This is because the LaNiO3
treated at 773 K under H2/CO2 mixture gas, was reduced into La2O3
and La2O3 further reacted with CO2 to form La2O2CO3. Diffractive
peaks indexed to metal Ni can also be found at 2ꢁ = 44.349◦, indicat-
ing that the Ni ions are effectively reduced into metal Ni. Besides,
there was a slight indication of the formation of the LaNiO2.7
phase with a line at 2ꢁ = 32.828◦ and 67.9288◦. At 873 and 973 K,
LaNiO3 was completely destroyed, the only phases detected being
La2O2CO3 and metallic nickel Ni. The sample obtained at 773 K was
designated as Ni/LaNiO3−ı–La2O2CO3. The details of samples are
summarized in Table 1.
30 ml min−1
.
3.2. TPR analysis
Analysis of the reduction profiles of LaNiO3 (Fig. 2) shows two
peak formations, which correspond to different Ni intermediate
species. We can observe that the first reduction event takes place
at low temperatures between 513 and 653 K. This peak was split
into two peaks, one at 594 K and the other at 627 K, which is prob-
ably due to the similar reduction properties of Ni3+ and Ni2+ in the
LaNiO3 matrix. The first peak at 594 K corresponds to the forma-
tion of the LaNiO2.7 without total destruction of the perovskite-type
structure, tolerating a significant oxygen deficiency phase, and the
second peak at 627 K is due to the partially reduction of Ni3+ to Ni2+
.
The highest temperature peak appears centred at around 761 K, cor-
responding to Ni0 and La2O3. Based on these results, the reduction
Table 1
X-ray diffraction analysis and bandgap of different samples.
Main phase (calcined)
Treatment temperature (K)
Main phase (after treated)
Bandgap (eV)
LaNiO3
LaNiO3
LaNiO3
LaNiO3
LaNiO3
LaNiO3
Untreated
573
673
773
873
LaNiO3
LaNiO2.7
LaNiO2.7, NiO
La2O2CO3, LaNiO2.7, Ni
La2O2CO3, Ni
La2O2CO3, Ni
–
2.73
2.52
2.42
2.87
2.90
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L. Jia et al. / Journal of Alloys and Compounds 489 (2010) L13–L16
L15
dropped. No O2 evolution occurs during the reaction due to the irre-
versible consumption of formaldehyde by photogenerated holes in
the valence band of the photocatalyst [19].
3.4. SEM measurement
Fig. 4 shows SEM images of the prepared samples, along with
LaNiO3 data for comparison. When treated at 573–773 K, the par-
ticle became more uniform. The 773 K treated sample displays
regularly shaped particles. However, treatment at 873 and 973 K
showed similar feature that the grains became irregular and some
aggregated together.
3.5. TEM and DRS measurement
Fig. 5(a) displays the TEM images of the LaNiO3 treated at 773 K.
There are some dotted dark spots less than 9 nm, which should
be the reduced Ni species and some light small crystals that was
possibly the LaNiO3−ı species. The Ni and LaNiO3−ı particles are
highly dispersed in the La2O2CO3, which made it possible for the
reactant easily gets in contact with the catalytic active site [20].
The bandgap for the Ni/LaNiO3−ı–La2O2CO3 is evaluated by the
extrapolation method, which is shown in Fig. 5(b). The positions of
the bandgap were determined by the interception of the straight
Fig. 3. Initial rates of H2 evolution in formaldehyde aqueous solution over LaNiO3
after treatment at (a) 573 K, (b) 673 K, (c) 773 K, (d) 873 K and (e) 973 K.
steps are written as follows:
LaNiO3 + H2 → LaNiO2.7 + NiO + H2
(between 513 and 653 K)
line fitted through the low-energy side of the curve [F(R )·hꢃ]2
LaNiO2.7 + H2 → La2O3 + Ni + H2
∞
versus hꢃ, where F(R ) is the Kubelka–Munk function and hꢃ
∞
(between 683 and 803 K)
is the energy of the incident photon [21,22]. The sample shows
a bandgap-narrowing at roughly 2.42 eV, which can effectively
absorb visible light. However, the sample LaNiO3 calcined at 1073 K
showed no bandgap, indicating the rhombohedral LaNiO3 does not
display any optical absorption [23]. The energy values of different
samples are summarized in Table 1.
3.3. Effect of treatment temperature on photocatalytic activity
The photocatalytic activity of LaNiO3 for hydrogen production
from formaldehyde aqueous solution was negligible. However, the
treatment of LaNiO3 with continuous H2/CO2 mixture flow at dif-
ferent temperatures resulted in clearly observable H2 evolution,
which were shown in Fig. 3. The rates of H2 evolution varied with
treatment temperature to a maximum at 773 K with activities of
36 ␮mol h−1 g−1, beyond which the activity of the sample abruptly
3.6. Stability of photocatalytic activity on Ni/LaNiO3−ı–La2O2CO3
Fig. 6 shows the time course of H2 evolution from an aque-
ous formaldehyde solution under visible light (ꢀ > 420 nm) using
Fig. 4. SEM images of (a) LaNiO3, (b–f) LaNiO3 samples treated at (b) 573 K, (c) 673 K, (d) 773 K (e) 873 K and (f) 973 K.

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L. Jia et al. / Journal of Alloys and Compounds 489 (2010) L13–L16
sample Ni/LaNiO3−ı–La2O2CO3. The rate of H2 evolution in the
second and subsequent runs with intermittent evacuation was
also unchanged, and the reaction continued to proceed steadily.
The experimental results demonstrate that the catalyst has a
good reproducibility and can be used repeatedly. Therefore,
Ni/LaNiO3−ı–La2O2CO3 is a stable catalyst for photocatalytic H2
evolution under visible-light irradiation.
4. Conclusions
The catalytic activity for overall water splitting under visible
light was found to be improved by LaNiO3 treated at moder-
ate temperature under a flow of H2/CO2 mixture. The degree of
activity improvement was dependent on the treatment temper-
ature. The results showed that LaNiO3 treated at 773 K, had the
highest photocatalytic activity for overall water splitting. XRD mea-
surements revealed that the sample prepared at 773 K contains a
mixture of LaNiO2.7 and metallic nickel Ni and La2O2CO3. The opti-
cal absorption edge of sample shifted to the low-energy region,
as proven in DRS. TEM results showed that Ni and LaNiO2.7 par-
ticles dispersed on La2O2CO3 as a cocatalyst. All these indicated
the catalyst should be an active photocatalyst under visible-light
irradiation.
Acknowledgments
This work was supported by the Key Laboratory for Chemical
Biology of Fujian Province and State Key Laboratory for Physical
Chemistry of Solid Surface of Xiamen University. We thank the
Analysis and Testing Centre of Xiamen University for the analysis
and observation work.
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