Environmental Engineering Reference
In-Depth Information
exceed the band gap of CuO. The excited electron can either react directly with O
2
for O
2
∙−
formation (reaction 3.14) or reduce
the Cu
2+
lattice to Cu
+
, leading to reactions (3.15)-(3.16) with the formation of O
2
∙−
radicals
(3.14)
CuOe O uO O
cb
(
−
)
+→ +
2
•
−
2
(
)
→
CuO
e
−
CuOCu
(
+
)
(3.15)
cb
•
(3.16)
CuOCuOCuOCu O
(
+
)
+→ +
2
(
2
+
)
−
2
∙
in equation 3.17
equation 3.17 shows that the equilibrium between H
+
and O
2
∙−
leads to the formation of the HO
2
∙
radical. The HO
2
leads to the production of H
2
O
2
in equation 3.18:
)
(3.17)
HO HO
+
+=
2
−
•
(
partialO
•
−
2
2
(3.18)
CuOCu O artialO H uO Cu HO
(
+
)
+
•
(
•
−
)
+ →
+
(
2
+
)
+
2
2
22
or the alternative reaction
(3.19)
2
Cu IOH uIIHO
()
++→+
2
+
2
()
2
2
2
equation 3.18 is analogous to the behavior of Fe
2+
ions (Fenton reaction) in the Haber-Weiss cycle [187]. Indeed, the produc-
tion of H
2
O
2
as expected based on equation 3.19 was evaluated experimentally in both the dark and illuminated culture
medium in the presence and absence of bacterial cells, respectively. Production was observed to be higher than that for the
sample kept in the dark at the beginning of the experiment, but showed similar values after a 4-h reaction. This behavior was
observed both in the presence and in the absence of
E. coli
, although higher levels of H
2
O
2
were measured in the culture
medium containing
E. coli.
Paschoalinoa et al. suggested that the Cu(I) ions on the CuO is the active catalytic species in accor-
dance with equation 3.20:
(3.20)
CuOCu uO
(
1
+
)
→
+
Cu
1
+
vacancy
The oxidative side of the redox cycle would involve the h
+
in equation 3.14 and convert H
2
O into ∙OH, allowing for the hole
transfer and finally the formation of H
2
O
2
in equation 3.22.
hHOHOH
+
+ →+
2
+
(3.21)
•
OH OH HO
+ →
22
(3.22)
•
•
This mechanism for copper toxicity implies the capacity of CuO to generate reactive oxygen species (ROS) involving Cu ions
via Fenton-like reactions through the Haber-Weiss cycle. The inhibition of
E. coli
enzymatic activity is due to Cu ion binding
with the
E. coli
cell walls and thiol proteins [188] since Cu
+
has the ability to bind indiscriminately to cell walls [189]. It is
worth noting that Fenton-like Cu
+
/Cu
2+
reactions can also take place in the absence of irradiation [190, 191]. This reaction
without irradiation can be concisely described as copper-catalyzed Haber-Weiss reactions. Cu(II) is initially reduced by the
reductant X
red
(eq. 3.23), followed by reoxidation with hydrogen peroxide (eq. 3.24), resulting in a net production of the
hydroxyl radical [165].
()
(3.23)
X uII XCu I
red
+ →+
()
ox
(3.24)
Cu IHO uIIOH H
()
+ →+
()
−
+
•
22
3.3.3 Cu
2
O-Based Nanocomposites for h
2
evolution
TiO
2
nanomaterials represent the most important semiconductor catalysts for hydrogen production from splitting water. The principle
of the TiO
2
photocatalytic system for water in Fujishima and Honda's pioneering work is depicted in Figure 3.17 [56]. A photocatalyst
absorbs UV and/or VL irradiation from sunlight or an illuminated light source. The electrons in the VB of the photocatalyst are excited
to the CB, while the holes are left in the VB. This, therefore, creates the negative-electron (e
−
) and positive-hole (h
+
) pairs. This stage
is referred to as the semiconductor's “photoexcited” state. After photoexcitation, the excited electrons and holes separate and migrate
to the surface of the photocatalyst. Here, in the photocatalytic water-splitting reaction, they act as both a reducing agent and an
