Environmental Engineering Reference
In-Depth Information
table 12.2
Hydrogen production using natao
3
as catalyst synthesized by several methods
Compound
Cocatalyst (wt.%)
Hydrogen (μ·h
−1
)
Oxygen (μ·h
−1
)
Sm : NaTaO
3
None
140
65
La : NaTaO
3
None
160
80
La : NaTaO
3
RuO
2
0.2
660
377
La : NaTaO
3
RuO
2
0.6
3,246
1,623
La : NaTaO
3
RuO
2
1.0
4,108
2,160
La : NaTaO
3
RuO
2
1.0
11,300 (after 3 h of irradiation)
10,050 (after 3 h of irradiation)
table 12.3 Kinetic parameters obtained from the photocatalytic degradation
of methylene blue under uv light and using natao
3
perovskite-type compounds
Semiconductor
Sol-gel synthesis
degradation (%)
k
(min
−1
)
t
1/2
(min)
NaTaO
3
SG-600
95
0.0078
89
La : NaTaO
3
SG-600
66
0.0084
83
Sm : NaTaO
3
SG-600
91
0.0106
65
Therefore, RuO
2
enhances the activity of La : NaTaO
3
samples for hydrogen production because it traps the electrons and
avoids easy recombination of the electron-hole pair during the reaction.
On the other hand, NaTaO
3
perovskite-type compounds have also been tested as a photocatalyst for degradation of organic
dyes [12]. When NaTaO
3
compounds were used as a photocatalyst for methylene blue degradation, excellent activity was
observed. In this case, sample doped with Sm showed the best half-life for this reaction (Table 12.3).
In comparison with TiO
2
, NaTaO
3
perovskite-type compounds are more effective for methylene blue degradation [12].
12.1.2 Pyrochlore-type Structure: Sm
2
mtao
7
(m = fe, in, ga), and bi
2
mta0
7
(m = fe, in, ga)
Several investigations have been conducted that focus on pyrochlore-type compounds with the formula A2BB′O7 where A and
B sites can be substituted by metal ions in order to develop photocatalysts with a narrow band gap. This is because it has been
reported that some metal oxides containing cations with d
10
electronic configuration such as Ga
3+
, In
3+
, Ge
4+
, Sn
4+
, and Sb
5+
are
attractive for the synthesis of pyrochlore-type compounds [24-27]. Furthermore, the conduction band of d
10
metal oxides pres-
ents larger dispersion than that of d
0
transition metals, which allows high mobility of the photoexcited electrons [28]. Therefore,
semiconductor oxides combining cations with 4f-d
10
-d
0
electronic configurations are considered interesting for water splitting
reaction and organic dye degradation. In particular, pyrochlore-type structure compounds prepared by means of a solid state
reaction have shown excellent activity in photoinduced processes. This activity is due to the distortion of their octahedral, which
causes an increase in the concentration of holes, thus having an important effect on the charge mobility.
Particularly in our group, novel pyrochlore-type compounds, for example, Sm
2
MTaO
7
(M = Fe, In, Ga), and Bi
2
MTa0
7
(M = Fe, In, Ga), have been synthesized via the sol-gel method with attractive characteristics as potential photocatalyst materials
[14-19]. The prepared compounds had improved photophysical properties and demonstrated excellent results for the inactiva-
tion of microorganisms as well as for organic dye degradation and hydrogen production.
Sm
2
GaTaO
7
was prepared for the first time using this process, and its structural features are presented in Figure 12.5, which
indicate that it is a compound with monoclinic structure and consisting of irregular Ga/Ta octahedra linked at the corners and
interconnected into a hexagonal tungsten bronze (HTB)-type network forming two dimensional (2d) HTB blocks, similar to
the one reported for Sm
2
FeTaO
7
[14]. Therefore, Sm
2
GaTaO
7
can be considered as a pyrochlore-related compound.
Sm
2
GaTaO
7
was evaluated as a photocatalyst for the water splitting reaction, where it was found that it is able to produce
hydrogen without loading the cocatalyst; however, when RuO
2
was added, hydrogen evolution was enhanced (Fig. 12.6). The
material 0.2RuO
2
/Sm
2
GaTaO
7
showed the highest hydrogen evolution; however, when the amount of RuO
2
increased the
hydrogen evolution decreased considerably and it was even less than for Sm
2
GaTaO
7
. These results revealed that RuO
2
as a
cocatalyst provides efficient active sites for hydrogen evolution; however, an excess of RuO
2
has a negative effect on the reaction.
It is assumed that the overload of RuO
2
limited the light absorption by Sm
2
GaTaO
7
, reducing the generation of electron-hole
pairs and also acting as recombination centers.
The results revealed that the crystal structure and the constitutive elements play an important role in the photocatalytic activity and
the presence of RuO
2
is necessary because it acts as an effective cocatalyst to enhance hydrogen evolution activity of Sm
2
GaTaO
7
[15].
On the other hand, Bi
2
MTaO
7
pyrochlore-type compounds have been tested as photocatalysis for organic dye degradation
[16-19]. According to the results, activity is higher in samples prepared via sol-gel than in samples prepared via the solid state.
