22
Z. Zhao and P. K. Shen
2.1.1 Starting Step of Oxygen Reduction
There are two theories about the initial step of oxygen reduction being the key for
the reaction [11]. The adsorption mode of oxygen molecules is divided into two
kinds of reaction pathways, dissociative and associative. The reaction mechanism of
dissociative is O 2 double bond. First, the O–O bond is broken into a free O atom, and
the O atom is adsorbed onto the surface of the catalyst metal, and hydrogen protons
and electrons are obtained by the reaction to further react to form water.
O 2 + ∗ → O 2 ∗
(2.11)
O 2 ∗ + (H
+
+ e
−
) → HO 2 ∗
(2.12)
HO 2 ∗ + (H
+
+ e
−
) → H 2 O + O∗
(2.13)
O ∗ + (H
+
+ e
−
) → HO∗
(2.14)
HO ∗ + (H
+
+ e
−
) → H 2 O + ∗
(2.15)
The reaction mechanism of the oxygen molecules combination is that the adsorbed
oxygen molecules with proton first produce of HO 2 * and then the HO 2 * further
reduces to form water (i.e., as shown in Eqs. 2.11–2.13). From Eqs. 2.11 to 2.13,
we know the process that oxygen molecule adsorbed on the catalyst surface first
obtains an electron and a hydrogen atom to form HO 2 *, and then continue to obtain
an electron and a hydrogen atom to form H 2 O + O* . This step produces H 2 O 2 and
free oxygen atom adsorbed on the surface of the catalyst. The free oxygen atom
obtains an electron and a hydrogen atom to form HO* , and HO* continues to reduce
to form water (as in Eqs. 2.14 and 2.15). The latter two steps reaction are the same
as the combined reaction, and the difference between the two is whether the front
reaction is obtained by adsorption reduction or oxygen molecules separated into free
oxygen atoms directly.
Figure 2.8 shows the opposite reaction process by theoretical calculations. At
a voltage below 0.78 V, H 2 O separates into OH or O, and above 0.78 V while
H 2 O will be dissociated. At this potential, H 2 O is dissociated while electrons/H are
transferred to oxygen atoms adsorbed on the surface of the electrode. OH activation
leads to a slower process of oxygen reduction. This is also the reason why the oxygen
reduction potential or the dissociation initiation potential of H 2 O is high. This process
is consistent with the results of a large number of experiments. H 2 O/OH
− can be
formed only when oxygen is adsorbed on the platinum electrode and there is nothing
with strong adsorption on the surface of the electrode. Conversely, when there is
a strong anion or hydrogen adsorption on the surface of the electrode, the amount
of generated hydrogen peroxide will increase as the coverage of adsorbed chemical
Z. Zhao and P. K. Shen
2.1.1 Starting Step of Oxygen Reduction
There are two theories about the initial step of oxygen reduction being the key for
the reaction [11]. The adsorption mode of oxygen molecules is divided into two
kinds of reaction pathways, dissociative and associative. The reaction mechanism of
dissociative is O 2 double bond. First, the O–O bond is broken into a free O atom, and
the O atom is adsorbed onto the surface of the catalyst metal, and hydrogen protons
and electrons are obtained by the reaction to further react to form water.
O 2 + ∗ → O 2 ∗
(2.11)
O 2 ∗ + (H
+
+ e
−
) → HO 2 ∗
(2.12)
HO 2 ∗ + (H
+
+ e
−
) → H 2 O + O∗
(2.13)
O ∗ + (H
+
+ e
−
) → HO∗
(2.14)
HO ∗ + (H
+
+ e
−
) → H 2 O + ∗
(2.15)
The reaction mechanism of the oxygen molecules combination is that the adsorbed
oxygen molecules with proton first produce of HO 2 * and then the HO 2 * further
reduces to form water (i.e., as shown in Eqs. 2.11–2.13). From Eqs. 2.11 to 2.13,
we know the process that oxygen molecule adsorbed on the catalyst surface first
obtains an electron and a hydrogen atom to form HO 2 *, and then continue to obtain
an electron and a hydrogen atom to form H 2 O + O* . This step produces H 2 O 2 and
free oxygen atom adsorbed on the surface of the catalyst. The free oxygen atom
obtains an electron and a hydrogen atom to form HO* , and HO* continues to reduce
to form water (as in Eqs. 2.14 and 2.15). The latter two steps reaction are the same
as the combined reaction, and the difference between the two is whether the front
reaction is obtained by adsorption reduction or oxygen molecules separated into free
oxygen atoms directly.
Figure 2.8 shows the opposite reaction process by theoretical calculations. At
a voltage below 0.78 V, H 2 O separates into OH or O, and above 0.78 V while
H 2 O will be dissociated. At this potential, H 2 O is dissociated while electrons/H are
transferred to oxygen atoms adsorbed on the surface of the electrode. OH activation
leads to a slower process of oxygen reduction. This is also the reason why the oxygen
reduction potential or the dissociation initiation potential of H 2 O is high. This process
is consistent with the results of a large number of experiments. H 2 O/OH
− can be
formed only when oxygen is adsorbed on the platinum electrode and there is nothing
with strong adsorption on the surface of the electrode. Conversely, when there is
a strong anion or hydrogen adsorption on the surface of the electrode, the amount
of generated hydrogen peroxide will increase as the coverage of adsorbed chemical