66
magnitude of the adsorption heat. It appears that the correlation between the electronic work function and the activity of metals for hydrogen evolution is very likely
to be the most reliable.
The platinum group metals are the most active catalysts for the HER. These platinum group metals are generally stable in corrosive environments.
The heat of adsorption of the intermediate remains the most straightforward
parameter on which development of new cathode catalysts can be based. A combination of two metals from the two branches of the volcano curve was expected to
result in enhanced activity [28], which would indicate a direct correlation between
composition and heat of adsorption. But predictions based on the volcano curve do
not show any general validity.
For applications in various processes, different metals, alloys, intermetallic compounds, amorphous alloys, and oxides have been used e.g., Mo-based alloys [29], or
thermally prepared [30], Ni and Mo with addition of small amounts of Re, W, V, or,
as a third component, Co, Cr, Fe [31]. Their activity has been found to be only due
to a large surface area since their Tafel slope [32] for the HER is the same as for
pure Ni.
Some oxides, such as RuO 2 [33] and IrO 2 [34], have high catalytic activity for H 2
evolution and are among the most active materials for this reaction. A Tafel slope of
−40 mV is observed with these surfaces in both acid and alkaline solutions. Although
reduction of RuO 2 by evolving H 2 is thermodynamically possible, it does not occur
[35] probably because proton penetration into RuO 2 cannot be assisted by electric
field since that oxide is a metallic conductor. Oxide electrodes are not poisoned by
traces of metallic impurities in solution. This is primarily not only due to the
extended surface area but also due to a weak chemisorption on wet oxides caused by
the presence of chemisorbed OH groups. So, underpotential deposition of metals is
not observed on RuO 2 .
Adzic et al. developed a method to form a metal monolayer on oxide surfaces
using a cation adsorption on oxides [36], followed by a reduction of adsorbed cation
by a short potential pulse, which reduces only surface cations. Neutral atoms on
oxide surfaces can be galvanically displaced by any more positive cations. We return
to this method in Chap. 8.
Raney Nickel is a very active form of Ni. It is obtained by de-alloying Ni alloyed
with Al or Zn, which is leached away in caustic solution. The dissolution of the
soluble component leaves a porous structure with a very large surface area and particularly active metal sites A. decisive modification of the kinetic pattern is obtained
as Raney Ni is used. However, under long-term cathodic load, the structure of the
electrocatalyst may change with extensive sintering and recrystallization phenomena leading to progressive deactivation.
6 Important Electrocatalytic Reactions
magnitude of the adsorption heat. It appears that the correlation between the electronic work function and the activity of metals for hydrogen evolution is very likely
to be the most reliable.
The platinum group metals are the most active catalysts for the HER. These platinum group metals are generally stable in corrosive environments.
The heat of adsorption of the intermediate remains the most straightforward
parameter on which development of new cathode catalysts can be based. A combination of two metals from the two branches of the volcano curve was expected to
result in enhanced activity [28], which would indicate a direct correlation between
composition and heat of adsorption. But predictions based on the volcano curve do
not show any general validity.
For applications in various processes, different metals, alloys, intermetallic compounds, amorphous alloys, and oxides have been used e.g., Mo-based alloys [29], or
thermally prepared [30], Ni and Mo with addition of small amounts of Re, W, V, or,
as a third component, Co, Cr, Fe [31]. Their activity has been found to be only due
to a large surface area since their Tafel slope [32] for the HER is the same as for
pure Ni.
Some oxides, such as RuO 2 [33] and IrO 2 [34], have high catalytic activity for H 2
evolution and are among the most active materials for this reaction. A Tafel slope of
−40 mV is observed with these surfaces in both acid and alkaline solutions. Although
reduction of RuO 2 by evolving H 2 is thermodynamically possible, it does not occur
[35] probably because proton penetration into RuO 2 cannot be assisted by electric
field since that oxide is a metallic conductor. Oxide electrodes are not poisoned by
traces of metallic impurities in solution. This is primarily not only due to the
extended surface area but also due to a weak chemisorption on wet oxides caused by
the presence of chemisorbed OH groups. So, underpotential deposition of metals is
not observed on RuO 2 .
Adzic et al. developed a method to form a metal monolayer on oxide surfaces
using a cation adsorption on oxides [36], followed by a reduction of adsorbed cation
by a short potential pulse, which reduces only surface cations. Neutral atoms on
oxide surfaces can be galvanically displaced by any more positive cations. We return
to this method in Chap. 8.
Raney Nickel is a very active form of Ni. It is obtained by de-alloying Ni alloyed
with Al or Zn, which is leached away in caustic solution. The dissolution of the
soluble component leaves a porous structure with a very large surface area and particularly active metal sites A. decisive modification of the kinetic pattern is obtained
as Raney Ni is used. However, under long-term cathodic load, the structure of the
electrocatalyst may change with extensive sintering and recrystallization phenomena leading to progressive deactivation.
6 Important Electrocatalytic Reactions