Photocatalytic
splitting of water by using oxide semiconductors is one of the promising schemes
to produce hydrogen from water. Among various metal oxides, NaTaO3 was
reported to be one of the most efficient photocatalysts for water decomposition
without cocatalyst. The higher potential of the conduction band of NaTaO3 could lead to be more effective to reduce the water to produce H2
since the tantalates possess conduction bands of Ta5d orbital which are more
negative position than titanate (Ti3d) or niobate (Nb4d) [
Kudo and Kato
synthesized NaTaO3 by means of solid-state reaction at high
temperature (1420 K) for a long reaction time (10 hours), which resulted in the
localized segregation of component and loss of stoichiometry due to evaporation
of the constituent components [
In this report, a
spray pyrolysis was proposed as a way of avoiding problems of solid-state
reaction method. The spray pyrolysis process is an aerosol process in which one
droplet of precursor solution converts into one particle in short residence
time at lower processing temperature than the solid-state reaction process. Because
of short residence time and low temperature, the spray pyrolysis method
produces powders of high phase-purity and uniformity in composition. Particles
produced by the spray pyrolysis are usually spherical and aggregation-free [
The quantum yield
(QY) has been used to compare the intrinsic activity of photocatalyst. The
quantum yield includes the effect of loss of photons scattered or reflected by
the photocatalyst itself and absorbed by some impurities or reactants. The
number of photons absorbed by photocatalyst has been determined by two methods.
One is using the light-flux meter such as a 1815-C (Newport) or a power meter (Advantest, Si
diode, TQ8210 or Molectron Power Max 5200). The other is a chemical actinometry
method. Potassium ferrioxalate method developed by Parker et al. [
Therefore, in this report, a new definition of photocatalytic activity was proposed for the purpose of comparing photocatalysts from the reactor design point of view.
To synthesize the
NaTaO3, a spray pyrolysis system which consists of an ultrasonic
atomizer, vertical furnace reactor, and particle collector was employed (see Figure
Schematic diagram of spray pyrolysis experimental apparatus.
Ta(OC2H5)5 (Aldrich, purity 99.98%) and NaNO3 (Junsei, purity 99.5%) were
used as precursors. For the preparation of 0.125 M colloid solution, Ta(OC2H5)5 was dissolved in distilled water containing excess nitric acid, and it was
stirred vigorously by magnetic stirrer and heated to
Synthesized powders were identified by X-ray diffraction (Rigaku, D/MAX-RB), and the surface morphology and average particle size of the powders were observed by field-emission SEM (Philips, 533M). The dispersion of NiO on surface of powder was observed by field-emission TEM (FEI, Technai G2 F3 S-Twin). The surface area was determined by BET measurement (Micromeritics, ASAP 2000). A diffuse reflection spectrum was obtained by using a UV-vis-NIR spectrometer (Jasco, Ubest V-570) and was converted from reflection to absorbance by the Kubelka-Munk method.
Hydrogen production
reactions were carried out in a closed circulation system as shown in Figure
Schematic diagram of experimental apparatus for the measurement of photoactivity for water splitting.
The moles of photons
absorbed by photocatalyst were measured by a chemical actinometry employing
potassium ferrioxalate (K3Fe(C2O4)3
The XRD pattern of
NaTaO3 powders synthesized by spray pyrolysis is shown in Figure
XRD pattern of NaTaO3 powder synthesized by spray pyrolysis (mean residence time = 1.80 seconds, spray pyrolysis temperature = 1173 K, precursor preparation temperature = 333 K, precursor concentration = 0.125 M, carrier gas (air) flow rate = 5 L/min, Na/Ta = 1.00).
SEM images of NaTaO3 powders synthesized by spray pyrolysis with variation of precursor preparation temperature: (a) 298 K, (b) 313 K, (c) 333 K, (d) 353 K (mean residence time = 2.25 seconds, spray pyrolysis temperature = 1173 K, precursor concentration = 0.125 M, carrier gas (air) flow rate = 4 L/min, Na/Ta = 1.00).
XRD patterns of the
peak at
SEM
images and XRD patterns of NaTaO3 powders with variation of spray
pyrolysis temperature are shown in Figures
SEM images of NaTaO3 powders synthesized by spray pyrolysis with variation of spray pyrolysis temperature: (a) 973 K, (b) 1073 K, (c) 1173 K, (d) 1273 K (mean residence time = 1.80 seconds, precursor preparation temperature = 333 K, precursor concentration = 0.125 M, carrier gas (air) flow rate = 5 L/min, Na/Ta = 1.00).
XRD patterns of the
peak at 2
Diffuse reflectance spectra of the NaTaO3 powder synthesized by spray pyrolysis (mean residence time = 1.80 seconds, spray pyrolysis temperature = 1173 K, precursor preparation temperature = 333 K, precursor concentration = 0.125 M, carrier gas (air) flow rate = 5 L/min, Na/Ta = 1.00).
Physical properties
such as BET surface area, pore volume, and pore size of NaTaO3 were summarized
in Table
Physical properties of naked sodium tantalates synthesized by spray pyrolysis (NaTaO3 powder synthesized by spray pyrolysis; mean residence time = 1.80 seconds; spray pyrolysis temperature = 1173 K; precursor preparation temperature = 333 K; precursor concentration = 0.125 M; carrier gas (air) flow rate = 5 L/min; Na/Ta = 1.00).
Spray pyrolysis | |
---|---|
Band gap (eV) | 4.0 |
BET surface area (m2/g) | 5.8 |
Pore volume (cm3/g) | 0.019 |
Average pore size (nm) | 8.4 |
NiO was loaded on
NaTaO3 synthesized by spray pyrolysis at 1173 K to enhance the photocatalytic
activity for water splitting. Figure
TEM images of NiO(0.05 wt%)/NaTaO3 synthesized by spray pyrolysis; (b) is high magnification of circled location in the (a).
Table
Effect of NiO loading on photocatalytic activities for water splitting in pure water over NaTaO3 (NaTaO3 powder synthesized by spray pyrolysis; mean residence time = 1.80 seconds; spray pyrolysis temperature = 1173 K; precursor preparation temperature = 333 K; precursor concentration = 0.125 M; carrier gas (air) flow rate = 5 L/min; Na/Ta = 1.00; reaction conditions: amount of distilled water, 500 ml; photocatalyst concentration = 1.0 gL−1).
Preparation method | NiO (0.05 wt%) loading | Rate of gas evolution ( | |
---|---|---|---|
H2 | O2 | ||
Spray pyrolysis | X | 1.76 | –- |
O | 222 | 110 |
Cumulative amount of gas evolution with irradiation time in pure water over NiO(0.05 wt%)/NaTaO3. (NaTaO3 powder synthesized by spray pyrolysis, mean residence time = 1.80 seconds, spray pyrolysis temperature = 1173 K, precursor preparation temperature = 333 K, precursor concentration = 0.125 M, carrier gas (air) flow rate = 5 L/min, Na/Ta = 1.00. Reaction conditions: photocatalyst type, NiO(0.05 wt%)/NaTaO3, amount of distilled water, 500 mL, photocatalyst concentration = 1.0 gL−1).
In order to determine
the optimum loading of photocatalyst, effect of photocatalyst loading (NiO (0.2 wt%)/NaTaO3) on the rate of H2 evolution and quantum
yield was tested, and results are shown in Figure
Determination of optimum amount of photocatalyst loading.
The quantum yield (QY), which is defined by (
In order to design an optimum photocatalytic reactor system, consumption of photocatalyst, electric power, volume of slurry reactor, and duration of reaction should be minimized, if catalysts with high-quantum yield were available.
To
evaluate photocatalysts for the purpose of optimal design of photocatalytic
reactor, the apparent photocatalyst activity (APA) is proposed as
This definition is more useful for designing a photocatalytic
reactor than the quantum yield. The calculated values of the apparent photocatalytic
activity from literature were summarized in Table
Quantum yield (QY) obtained by (
Photocatalyst | Reactor | Reactant volume (L) | H2 ( | Actinometer [light used] | Apparent photocatalytic activity | Ref. |
---|---|---|---|---|---|---|
Lamp | Catalyst (g) | QY (%) | ||||
NiO/TiO2 | Pyrex cylinder | 0.3 | 600 | Potassium ferrioxalate [–] |
Lee et al. (2001) [ | |
700 W High pressure Hg lamp | 0.2 | |||||
NiOx/RbTaWO6 | Quartz reaction cell | 0.07 | 69.7 | Spectra radiometer [313 nm] |
Ikeda et al. (2004) [ | |
1 kW superpressure Hg lamp | 0.3 | |||||
Pt/TiO2-A1 and TiO2-R2 | Inner irradiation type reactor | 0.5 | 220 | Thermopile power meter [350 nm] |
Abe et al. (2005) [ | |
400 W high pressure Hg lamp | 0.4 | |||||
NiOx/Ba2In2O5/ In2O3:Cr | Inner irradiation quartz cell | 0.5 | 29.3 | −[320 nm] |
Wang et al. (2005) [ | |
400 W high pressure Hg lamp | 0.37 | |||||
Sr2Nb2O7 | Inner irradiation quartz cell | 0.35 | 402 | Ferrioxalate Fe(C2O4)33− [<300 nm] |
Kim et al. (1999) [ | |
450 W high pressure Hg lamp | 1 | |||||
Pt/(AgIn)0.22 Zn1.56S2 | Side window Pyrex cell | 0.3 | 944 | Photodiode [420 nm] |
Tsuji et al. (2004) [ | |
300 W Xe lamp | 0.3 | |||||
NiOy/ In0.9Ni0.1TaO4 | Pyrex glass cell | 0.25 | 16.6 | −[402 nm] |
Zou et al. (2001) [ | |
300 W Xe lamp | 0.5 | |||||
NiO/NaTaO3 | Inner irradiation quartz cell | 0.35 | 2180 | Ammonium ferrioxalate [270 nm] |
Kato and Kudo (1999) [ | |
400 W high pressure Hg lamp | 1 | |||||
NiO/NaTaO3:La | Inner irradiation quartz cell | 0.39 | 15000 | Ammonium ferrioxalate [270 nm] |
Kudo and Kato (2000) [ | |
400 W high pressure Hg lamp | 1 |
Quantum yield (QY) obtained by (
Photocatalyst | Precursor preparation temperature (K) | Mean residence time (sec) | Reactant volume (L) | H2 ( |
Apparent photocatalytic activity
|
---|---|---|---|---|---|
Spray pyrolysis temperature (K) | Sacrificial agent (methanol 20 vol%) | Catalyst (g) | QY (%) | ||
NiO(0.05 wt%)/ NaTaO3 | 333 | 1.80 | 0.5 | 462.5 | |
1173 | X | 0.5 | |||
NaTaO3 | 333 | 1.80 | 0.5 | 0.27 | |
973 | X | 0.5 | |||
NaTaO3 | 333 | 1.80 | 0.5 | 1.25 | |
1073 | X | 0.5 | |||
NaTaO3 | 333 | 1.80 | 0.5 | 3.31 | |
1173 | X | 0.5 | |||
NaTaO3 | 333 | 1.80 | 0.5 | 0.81 | |
1273 | X | 0.5 | |||
NaTaO3 | 298 | 2.25 | 0.3 | 107.5 | |
1173 | O | 0.12 | |||
NaTaO3 | 313 | 2.25 | 0.3 | 300.0 | |
1173 | O | 0.12 | |||
NaTaO3 | 333 | 2.25 | 0.3 | 237.5 | |
1173 | O | 0.12 | |||
NaTaO3 | 353 | 2.25 | 0.3 | 2150.0 | |
1173 | O | 0.12 | |||
NiO(0.2 wt%)/ NaTaO3 | 353 | 2.25 | 0.3 | 1625.0 | |
1173 | O | 0.06 | |||
NiO(0.2 wt%)/ NaTaO3 | 353 | 2.25 | 0.3 | 3150.0 | |
1173 | O | 0.12 | |||
NiO(0.2 wt%)/ NaTaO3 | 353 | 2.25 | 0.3 | 3637.5 | |
1173 | O | 0.18 | |||
NiO(0.2 wt%)/ NaTaO3 | 353 | 2.25 | 0.3 | 3212.5 | |
1173 | O | 0.30 |
Figure
Evaluation of photocatalyst performance based on quantum yield and apparent photocatalytic activity.
Figure
Comparison of apparent photocatalytic activity of current work with that of published results.
However, the purpose of this paper is not to claim that we prepared superior photocatalyst, but to claim that both quantum yield and apparent photocatalytic activity are equally important when comparing the performance of photocatalyst for water splitting.
(1) NaTaO3 photocatalyst was synthesized by means of spray pyrolysis in relatively mild
experimental conditions:
(2) To enhance the photocatalytic activity for water splitting,
(3) Anew definition of photocatalytic activity was proposed from the reactor design point of view and named as apparent photocatalytic activity (APA), which is defined by the rate of hydrogen production divided by amount of catalyst, volume of reactant, duration of reaction, and power of lamp irradiated.
(4) Quantum yield is an important criterion to screen photocatalyst. However, the high-quantum yield is not always preferable for designing an economically feasible photocatalytic reactor system. Small size reactor, small amount of photocatalyst, short reaction time, and low consumption of power are also important for economically competitive production of hydrogen. Therefore, it is strongly recommended to report the apparent photocatalytic activity along with quantum yield when improvement of photocatalytic activity is to be reported.
(5) In this report, only the case of slurry reactor was discussed. If the type of photocatalytic reactor is a flat-surface reactor, the apparent photocatalytic activity would be reported in terms of unit surface area in place of the volume of reactor, because the goal of reactor design is to maximize the hydrogen evolution per unit surface area.