# Preparation ofNaTaO3 by Spray Pyrolysis and  Evaluation of Apparent Photocatalytic Activity for  Hydrogen Production from Water

Authors: Hyun Woo Kang; Eun-Jung Kim; Seung Bin Park
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

NaTaO3 photocatalyst was prepared by spray pyrolysis process and tested as photocatalyst for water splitting under UV light. Precursor solution was prepared from NaNO3 and Ta(OC2H5)5 in nitric acid solution and spray-pyrolyzed in air at between 973 and 1273 K. Considerable enhancement of photocatalytic activity was achieved by loading 0.05∼0.2 wt% of NiO on the surface of NaTaO3. The NiO loading was more effective on the NaTaO3 synthesized by spray pyrolysis in comparison with that synthesized by solid-state reaction. The quantum yield (QY) of NiO/NaTaO3 photocatalyst was measured by chemical actinometry using potassium ferrioxalate and compared with the apparent photocatalytic activities (APA) which would be more useful for the purpose of photocatalytic reactor design than the quantum yield. The apparent photocatalytic activity (APA) was defined by the rate of hydrogen production divided by weight of catalyst, volume of reactant mixture, duration of irradiation, and power of UV lamp. The validity of the apparent photocatalytic activity (APA) was discussed based on our results and reported activities of NaTaO3 photocatalyst loaded with or without NiO.


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## Body

## 1. Introduction

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) [1–3].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 [2, 3].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 [4, 5].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. [6, 7] has
been a standard chemical actinometer and used over a broad band of wavelength. However,
the exact evaluation of quantum yield for the heterogeneous reaction has not
been easy because of the difficulty of measuring the intrinsic number of
photons absorbed by photocatalyst. In addition, the quantum yield has been strongly
dependent upon the determination methods as well as experimental conditions. It
has been frequently encountered that the values of quantum yield have been
directly related only to the rate of H2 evolution, although the
experimental conditions such as the amount of catalyst, reactor type, and power
of the lamp are quite different.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.


## 2. Experiment

### 2.1. Synthesis of photocatalyst

To synthesize the
NaTaO3, a spray pyrolysis system which consists of an ultrasonic
atomizer, vertical furnace reactor, and particle collector was employed (see Figure 1). A commercial ultrasonic atomizer has four oscillators (Dong-Lim Eng.)
with atomizing frequency of 1.7 MHz. The droplets generated by atomizer were
transported into vertical furnace reactor by carrier gas (air) at the flow rate
of 4~5 L/min and passed through a 0.84 m long and 30 mm i.d. quartz tube. The reactor temperature was
controlled at between 973 K and 1273 K. The prepared particles were collected
by a thimble filter (28 mm × 100 mm, Toyo Roshi Kaisha Ltd.).Figure 1
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 313~353 K or use without
heating at 298 K. When the precursor solution became clear or colloid without
any precipitates, NaNO3 was added and stirred for dissolution. NiO as
cocatalyst was loaded by an impregnation method from aqueous solution of Ni(NO3)2⋅6H2O (Aldrich).
NaTaO3 was added to the solution and stirred by magnetic stirrer. Then,
water was removed by using a rotary vacuum pump. Finally, the NiO/NaTaO3 powder was calcined at 540 K for 3 hours under air.


### 2.2. Characterization of photocatalyst

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.


### 2.3. Photocatalytic activity measurement

Hydrogen production
reactions were carried out in a closed circulation system as shown in Figure2.
The system consisted of reactor, reservoir, and detector (gas chromatograph). The
reactor was a donut shape but it had only one Pyrex outer tube without inner quartz
tube so that the wall of UV lamp was in direct contact with reactant mixture. Removal
of the quartz inner tube reduced the construction cost of this type of slurry
reactor. Inner diameter and length of the outer tube were 34 mm and 320 mm,
respectively. UV-C lamp of 15 W (SANKYO, Japan) was used as an illuminating
source. The wave length of this lamp was predominantly 254 nm (4.9 eV). The
photocatalyst powder of 0.06~0.5 g was dispersed in 300~500 mL of distilled
pure water or with methanol in the reservoir and stirred with a magnetic
stirrer. Before turning on the light, the system was deaerated with N2 gas for an hour and kept at atmospheric pressure. The amount of gas evolved was
determined online with gas chromatograph equipped with TCD detector.Figure 2
Schematic diagram of
experimental apparatus for the measurement of photoactivity for water splitting.


### 2.4. Measurement of quantum yield

The moles of photons
absorbed by photocatalyst were measured by a chemical actinometry employing
potassium ferrioxalate (K3Fe(C2O4)3⋅3H2O)
synthesized from K2C2O4⋅H2O
and FeCl3 to calculate quantum yield. Solution of 0.006 M K3Fe(C2O4)3⋅3H2O in
0.06 M H2SO4 was prepared and poured into reaction system for
irradiation. After 0.1 M K3Fe(CN)6 was added to sample
irradiated, the absorbance was measured by UV-vis spectrophotometer (AIS, DT
1000CE). This work was carried out in a dark room. Finally, the light intensity
of an examined light source was determined by[8] (1)I(einsteins/min)=AV2V3εdφλtV1, where A is absorbance, d is length of absorption path, ε is
extinction coefficient, φλ is QY
at wavelength of light used, V1 is volume of sample,
V2 is volume of
actinometer irradiated, V3 is volume of diluted solution of
V1,
and t is irradiated time.


## 2.1. Synthesis of photocatalyst

To synthesize the
NaTaO3, a spray pyrolysis system which consists of an ultrasonic
atomizer, vertical furnace reactor, and particle collector was employed (see Figure 1). A commercial ultrasonic atomizer has four oscillators (Dong-Lim Eng.)
with atomizing frequency of 1.7 MHz. The droplets generated by atomizer were
transported into vertical furnace reactor by carrier gas (air) at the flow rate
of 4~5 L/min and passed through a 0.84 m long and 30 mm i.d. quartz tube. The reactor temperature was
controlled at between 973 K and 1273 K. The prepared particles were collected
by a thimble filter (28 mm × 100 mm, Toyo Roshi Kaisha Ltd.).Figure 1
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 313~353 K or use without
heating at 298 K. When the precursor solution became clear or colloid without
any precipitates, NaNO3 was added and stirred for dissolution. NiO as
cocatalyst was loaded by an impregnation method from aqueous solution of Ni(NO3)2⋅6H2O (Aldrich).
NaTaO3 was added to the solution and stirred by magnetic stirrer. Then,
water was removed by using a rotary vacuum pump. Finally, the NiO/NaTaO3 powder was calcined at 540 K for 3 hours under air.


## 2.2. Characterization of photocatalyst

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.


## 2.3. Photocatalytic activity measurement

Hydrogen production
reactions were carried out in a closed circulation system as shown in Figure2.
The system consisted of reactor, reservoir, and detector (gas chromatograph). The
reactor was a donut shape but it had only one Pyrex outer tube without inner quartz
tube so that the wall of UV lamp was in direct contact with reactant mixture. Removal
of the quartz inner tube reduced the construction cost of this type of slurry
reactor. Inner diameter and length of the outer tube were 34 mm and 320 mm,
respectively. UV-C lamp of 15 W (SANKYO, Japan) was used as an illuminating
source. The wave length of this lamp was predominantly 254 nm (4.9 eV). The
photocatalyst powder of 0.06~0.5 g was dispersed in 300~500 mL of distilled
pure water or with methanol in the reservoir and stirred with a magnetic
stirrer. Before turning on the light, the system was deaerated with N2 gas for an hour and kept at atmospheric pressure. The amount of gas evolved was
determined online with gas chromatograph equipped with TCD detector.Figure 2
Schematic diagram of
experimental apparatus for the measurement of photoactivity for water splitting.


## 2.4. Measurement of quantum yield

The moles of photons
absorbed by photocatalyst were measured by a chemical actinometry employing
potassium ferrioxalate (K3Fe(C2O4)3⋅3H2O)
synthesized from K2C2O4⋅H2O
and FeCl3 to calculate quantum yield. Solution of 0.006 M K3Fe(C2O4)3⋅3H2O in
0.06 M H2SO4 was prepared and poured into reaction system for
irradiation. After 0.1 M K3Fe(CN)6 was added to sample
irradiated, the absorbance was measured by UV-vis spectrophotometer (AIS, DT
1000CE). This work was carried out in a dark room. Finally, the light intensity
of an examined light source was determined by[8] (1)I(einsteins/min)=AV2V3εdφλtV1, where A is absorbance, d is length of absorption path, ε is
extinction coefficient, φλ is QY
at wavelength of light used, V1 is volume of sample,
V2 is volume of
actinometer irradiated, V3 is volume of diluted solution of
V1,
and t is irradiated time.


## 3. Results and Discussion

The XRD pattern of
NaTaO3 powders synthesized by spray pyrolysis is shown in Figure 3.
The main peak appeared at 2θ = 22.8°. The
XRD pattern confirmed that the homogeneous NaTaO3 crystal of high crystallinity was synthesized. SEM images of
NaTaO3 were shown in Figure 4. The prepared powders were generally
spherical. However, some hollow and shell-like particles are present together. To
optimize the preparation condition, the effect of the precursor preparation
temperature on the crystal characteristics was tested. In Figure 5, the optimum
precursor preparation temperature was 353 K, and most of samples were prepared
at this temperature.Figure 3
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).
(a)(b)(c)(d)Figure 5
XRD patterns of the
peak at2θ=22.8∘ for NaTaO3 powder synthesized at several precursor
preparation temperatures.SEM
images and XRD patterns of NaTaO3 powders with variation of spray
pyrolysis temperature are shown in Figures 6 and 7, respectively. Based
on these two results, the optimum spray pyrolysis temperature was found to be
1173 K. According to the diffuse reflectance spectra of NaTaO3 powder in Figure 8, the bandgap of NaTaO3 was estimated to be 4.0 eV.
It was determined from the onset of the absorption using the following equation: (2)Eg(eV)=1240λg(nm).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).
(a)(b)(c)(d)Figure 7
XRD patterns of the
peak at 2θ = 22.8° for NaTaO3 powder synthesized at several spray
pyrolysis temperatures.Figure 8
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 1. The BET surface area was larger by ten times than that of particles
synthesized by conventional solid-state reaction.Table 1
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 pyrolysisBand gap (eV)4.0BET surface
area (m2/g)5.8Pore volume
(cm3/g)0.019Average pore
size (nm)8.4NiO was loaded on
NaTaO3 synthesized by spray pyrolysis at 1173 K to enhance the photocatalytic
activity for water splitting. Figure 9 is TEM image of NiO-loaded NaTaO3 synthesized by spray pyrolysis. Figure 9(b) is a magnified image of the circle
in Figure 9(a). In Figure 9(a), NiO
particles (10 nm) were highly dispersed on the surface of NaTaO3 synthesized by spray pyrolysis.TEM images of
NiO(0.05 wt%)/NaTaO3 synthesized by spray pyrolysis; (b) is high
magnification of circled location in the (a).
(a)(b)Table2 shows the
photocatalytic activities for water splitting in pure water over NaTaO3 with and without NiO. The rate of hydrogen evolution of naked sodium tantalate
photocatalysts prepared by spray pyrolysis was 1.76μmoL/hr while that of NaTaO3 loaded with NiO was dramatically increased to 222μmoL/hr. The activity enhancement
factor by NiO/NaTaO3 synthesized in the present work was much larger
than that of NiO/NaTaO3 synthesized by means of the conventional
solid-state reaction [2, 3]. Figure 10 shows that the cumulative amount of gas
evolution increases almost constantly with irradiation time and the ratio of
hydrogen and oxygen is maintained at 2.Table 2
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 methodNiO (0.05 wt%) loadingRate of gas evolution (μmol/hr)H2O2Spray pyrolysisX1.76–-O222110Figure 10
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 11. The optimum loading was
found to be around 0.4 g/L.Figure 11
Determination of
optimum amount of photocatalyst loading.The quantum yield (QY), which is defined by (3),
has been used to evaluate the activity of photocatalyst over a broad band of
wavelength: (3)QY=molesofproductformedmolesofphotonsabsorbed=2×molesofH2molesofphotonsabsorbedbyphotocatalyst.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(4)APA=molesofH2evolution(X)(Y)(Z)(W),where X=weightofcatalyst,Y=volumeofreactant,Z=durationofreaction and W=poweroflamp.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 Table3, along with the reported values
of quantum yield. For comparison, the values of APA and quantum yield of the
current work were summarized in Table 4. It should be noted that the reactors, types
of lamps, and measurement methods of photon intensity were different in those tables.Table 3
Quantum yield (QY) obtained by (1) and
apparent photocatalytic activity (APA) obtained by (2) of several
semiconductor photocatalysts for water splitting.
PhotocatalystReactorReactant volume (L)H2 (μmol/hr)Actinometer [light used]Apparent photocatalytic activityRef.LampCatalyst (g)QY (%)(μmole/g⋅L⋅hr⋅W)NiO/TiO2Pyrex
cylinder0.3600Potassium
ferrioxalate
[–]14.29Lee et al. (2001) [9]700 W High
pressure Hg lamp0.212.0NiOx/RbTaWO6Quartz
reaction cell0.0769.7Spectra
radiometer
[313 nm]3.32Ikeda et al. (2004) [10]1 kW
superpressure Hg lamp0.31.5Pt/TiO2-A1
and
TiO2-R2Inner
irradiation type reactor0.5220Thermopile
power meter
[350 nm]2.75Abe et al. (2005) [11]400 W high
pressure Hg lamp0.44.0NiOx/Ba2In2O5/ In2O3:CrInner
irradiation quartz cell0.529.3−[320 nm]0.40Wang et al. (2005) [12]400 W high
pressure Hg lamp0.374.2Sr2Nb2O7Inner
irradiation quartz cell0.35402Ferrioxalate Fe(C2O4)33− [<300 nm]2.55Kim et al. (1999) [13]450 W high
pressure Hg lamp123Pt/(AgIn)0.22 Zn1.56S2Side window
Pyrex cell0.3944Photodiode [420 nm]34.96Tsuji et al. (2004) [14]300 W Xe lamp0.320NiOy/ In0.9Ni0.1TaO4Pyrex glass
cell0.2516.6−[402 nm]0.44Zou et al. (2001) [15]300 W Xe lamp0.50.67NiO/NaTaO3Inner
irradiation quartz cell0.352180Ammonium
ferrioxalate [270 nm]15.57Kato and Kudo (1999) [2]400 W high
pressure Hg lamp128NiO/NaTaO3:LaInner
irradiation quartz cell0.3915000Ammonium
ferrioxalate
[270 nm]96.15Kudo and Kato (2000) [16]400 W high
pressure Hg lamp150Table 4
Quantum yield (QY) obtained by (1) and
apparent photocatalytic activity (APA) obtained by (2) of photocatalysts in
the present study for water splitting (reactor, Pyrex cylinder; lamp, 15 W UV-C
lamp; actinometer, potassium ferrioxalate).
PhotocatalystPrecursor preparation
temperature (K)Mean
residence time (sec)Reactant volume (L)H2 (μmol/hr)Apparent photocatalytic activity(μmole/g⋅L⋅hr⋅W)Spray pyrolysis temperature (K)Sacrificial agent (methanol 20 vol%)Catalyst (g)QY (%)NiO(0.05 wt%)/ NaTaO33331.800.5462.5123.31173X0.56.5NaTaO33331.800.50.270.07973X0.53.8×10−3NaTaO33331.800.51.250.331073X0.51.8×10−2NaTaO33331.800.53.310.881173X0.54.6×10−2NaTaO33331.800.50.810.221273X0.51.1×10−2NaTaO32982.250.3107.5199.11173O0.121.5NaTaO33132.250.3300.0555.61173O0.124.2NaTaO33332.250.3237.5439.81173O0.123.3NaTaO33532.250.32150.03981.51173O0.1230.1NiO(0.2 wt%)/ NaTaO33532.250.31625.06018.51173O0.0622.8NiO(0.2 wt%)/ NaTaO33532.250.33150.05833.31173O0.1244.1NiO(0.2 wt%)/ NaTaO33532.250.33637.54490.71173O0.1850.9NiO(0.2 wt%)/ NaTaO33532.250.33212.52379.61173O0.3045.0Figure12 is a plot of APA versus quantum yield for data
summarized in Tables 3 and 4. It is clear that the high-quantum yield is
not always preferable criterion from the reactor design point of view. For example, the rate of H2 evolution
reported by Abe et al. [11] is 220μmoL/hr and that of present study by using NaTaO3 is 237.5μmoL/hr, and the quantum yields are also very
similar to each other. However, the APA for the catalyst of Abe et al. [11] is
2.7μmoL/g⋅L⋅hr⋅W
whereas that of the present work is 439.8μmoL/g⋅L⋅hr⋅W, although the rate of hydrogen evolution is very similar.Figure 12
Evaluation of
photocatalyst performance based on quantum yield and apparent photocatalytic
activity.Figure13 is a plot of apparent
photocatalytic activity of current work and published results. The APA of
current work is almost equal or slightly better than the highest APA reported
by Kudo and Kato [16] although the quantum yield of the current work is lower
than that of Kudo and Kato’s result.Figure 13
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.


## 4. Conclusion

(1) NaTaO3 photocatalyst was synthesized by means of spray pyrolysis in relatively mild
experimental conditions: 973~1273 K of reaction temperature and 1.80~2.25
seconds of residence time. The reaction temperature and duration of reaction
are 1420 K and 10 hours, respectively, when NaTaO3
was prepared by the conventional solid-state
reaction(2) To enhance the photocatalytic activity for water splitting,0.05~0.2 wt% of NiO
was loaded on the surface of NaTaO3 powder synthesized by spray
pyrolysis, which resulted in a considerable increase of photocatalytic activity
in comparison with that of NaTaO3 powder synthesized by means of
conventional solid-state reaction.(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.



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*Source: 519643-2008-06-04.xml*