62
conditions, meaning that their coverage is potentially dependent. The reaction order
with respect to H
+
that is found to be 3/2 supports this mechanism.
The exchange current density is mainly reported on Pt and Pt alloys catalysts. As
there are two Tafel regions, two exchange current densities have been reported. The
extrapolation from the lower slope gave the value close to 10
−10
A cm
−2
, and from the
higher, the value close to 10
−6
A cm
−2
.
The oxygen reduction reaction has a significant structure sensitivity. The results
with Pt single crystals show that the effect of crystallographic orientation is aniondependent. Sulfates and phosphates having a threefold symmetry are strongly
adsorbed on the (111)-oriented surfaces having the same symmetry. Hydration sheet
around perchlorates and fluorides makes them minimally or nonadsorbed, while the
rest of the halides, chlorides, bromides, and iodides are strongly adsorbed having
one or none hydration water. The rate of oxygen reduction generally increases as the
surface area decreases. This effect is, in case of Pt, explained by a variation of the
different crystal planes exposed to the electrolyte as a function of the particle size
(Table 6.1).
It has been demonstrated that the decrease of Pt particle size reduces the true
d-electron number through the more favorable hybridization of the 5d state with the
empty states above the Fermi level, increasing, as a consequence, the energy of
adsorbed oxygen species.
Platinum and palladium are metals known to possess the best catalytic activity
for oxygen reduction in acid solutions in processes of interest, among which the
most attention is focused to their use in fuel cell technology. Due to Pt and Pd cost
and supply limitations, it is required to reduce their loading in the catalyst.
Generally, there are two possibilities to reduce the amount of single-metal and
even enhance the catalyst activity. The first widely adopted approach to do it is to
increase the surface area of the catalyst by dispersing nano-sized Pt or Pd particles
on the proper support, and the second one is to use bi- or tri-metallic alloys containing Pt or Pd with other metals such as Ni, Co, Fe, and Cr. It is now generally accepted
that the size and distribution of the catalyst particles are affected by the physical
structure of the support (porosity and surface area) and the nature of the catalyst
precursor.
Catalyst support has to offer a highly developed surface combined with excellent
electronic conductivity and corrosion resistance. Generally, carbon has been used as
a support in fuel cell systems since it allows one to decrease Pt loading from ca. 4
to 0.2 mg cm
−2
[15]. However, thermodynamically favorable and kinetically slow
Table 6.1 Comparison of the
ORR activity on Pt single
crystals in 0.05 mol dm
−3
H 2 SO 4 , at t = 60 °C [14]
11
j k at 0.90 V
(μA cm
−2
real )
j k at 0.85 V
(μA cm
−2
real )
Pt(111)
80
650
Pt(100)
480
1450
Pt(110)
1900
5400
6 Important Electrocatalytic Reactions
conditions, meaning that their coverage is potentially dependent. The reaction order
with respect to H
+
that is found to be 3/2 supports this mechanism.
The exchange current density is mainly reported on Pt and Pt alloys catalysts. As
there are two Tafel regions, two exchange current densities have been reported. The
extrapolation from the lower slope gave the value close to 10
−10
A cm
−2
, and from the
higher, the value close to 10
−6
A cm
−2
.
The oxygen reduction reaction has a significant structure sensitivity. The results
with Pt single crystals show that the effect of crystallographic orientation is aniondependent. Sulfates and phosphates having a threefold symmetry are strongly
adsorbed on the (111)-oriented surfaces having the same symmetry. Hydration sheet
around perchlorates and fluorides makes them minimally or nonadsorbed, while the
rest of the halides, chlorides, bromides, and iodides are strongly adsorbed having
one or none hydration water. The rate of oxygen reduction generally increases as the
surface area decreases. This effect is, in case of Pt, explained by a variation of the
different crystal planes exposed to the electrolyte as a function of the particle size
(Table 6.1).
It has been demonstrated that the decrease of Pt particle size reduces the true
d-electron number through the more favorable hybridization of the 5d state with the
empty states above the Fermi level, increasing, as a consequence, the energy of
adsorbed oxygen species.
Platinum and palladium are metals known to possess the best catalytic activity
for oxygen reduction in acid solutions in processes of interest, among which the
most attention is focused to their use in fuel cell technology. Due to Pt and Pd cost
and supply limitations, it is required to reduce their loading in the catalyst.
Generally, there are two possibilities to reduce the amount of single-metal and
even enhance the catalyst activity. The first widely adopted approach to do it is to
increase the surface area of the catalyst by dispersing nano-sized Pt or Pd particles
on the proper support, and the second one is to use bi- or tri-metallic alloys containing Pt or Pd with other metals such as Ni, Co, Fe, and Cr. It is now generally accepted
that the size and distribution of the catalyst particles are affected by the physical
structure of the support (porosity and surface area) and the nature of the catalyst
precursor.
Catalyst support has to offer a highly developed surface combined with excellent
electronic conductivity and corrosion resistance. Generally, carbon has been used as
a support in fuel cell systems since it allows one to decrease Pt loading from ca. 4
to 0.2 mg cm
−2
[15]. However, thermodynamically favorable and kinetically slow
Table 6.1 Comparison of the
ORR activity on Pt single
crystals in 0.05 mol dm
−3
H 2 SO 4 , at t = 60 °C [14]
11
j k at 0.90 V
(μA cm
−2
real )
j k at 0.85 V
(μA cm
−2
real )
Pt(111)
80
650
Pt(100)
480
1450
Pt(110)
1900
5400
6 Important Electrocatalytic Reactions