64
An important feature of the theory is the prediction that the position of the d-band
center determines the binding energy of the adsorbate. Subsequent studies have
demonstrated a strong correlation between these two variables for a great number of
systems involving small atoms and molecules interacting with similar metal surfaces. The theory was proven very useful in explaining metal alloy reactivity and the
strain, ligand, and ensemble effects [21]. The identification of ε d as a key factor in
the reactivity of metal surfaces is significant. Note that the d band theory inherently
assumes that surface reactivity is a local property, which differs from another earlier
perspective that bulk properties determine catalytic properties.
The activity for a number of reactions often peaks on a certain metal when correlated with some measure of reactivity across a series of metals, thus displaying a
“volcano” trend. Using extensive density functional theory (DFT) calculations,
Nørskov and co-workers have convincingly demonstrated that this behavior is the
result of key competing processes in a reaction network connected via a common
intermediate [22], just as described by the Sabatier principle. The principle states
that catalytic activity is often maximized when the reactants interact with the catalysts with intermediate strength.
6.2 Hydrogen Evolution Reaction
Hydrogen is a clean fuel that can be produced from renewable sources [23]. Thus, it
can provide sustainable energy in the hydrogen economy plans [24]. Energy generated from renewable sources can produce hydrogen by water electrolysis. The
stored hydrogen can then be used in fuel cells to generate electricity or used as a fuel
[25]. The hydrogen evolution reaction is the best studied and understood electrocatalytic reaction. Development of kinetics of electrode reactions is linked to the
work on HER. Thus, diagnostic criteria derived from Tafel slopes and current,
potential curves for reaction mechanisms determination, are best for HER. Theory
predicts various values for Tafel slopes, which are summarized in Table 6.3.
Table 6.3 Theoretical values of Tafel slopes for the hydrogen evolution reaction at 25 °C
Mechanism Rate-determining step
Tafel slope, b/mV dec
−1
Langmuir adsorption
condition
Temkin adsorption conditions
Θ H → 0
Θ H → 1
Volmer
−120
−40
−60
Heyrovsky
−40
−120
−60
V-T
Volmer
−120
−120
−120
Tafel
−30
∞
−60
V-T
Volmer-Tafel
−120
∞
−180
V-H
Volmer-Heyrovsky
−120
−120
−120
6 Important Electrocatalytic Reactions
An important feature of the theory is the prediction that the position of the d-band
center determines the binding energy of the adsorbate. Subsequent studies have
demonstrated a strong correlation between these two variables for a great number of
systems involving small atoms and molecules interacting with similar metal surfaces. The theory was proven very useful in explaining metal alloy reactivity and the
strain, ligand, and ensemble effects [21]. The identification of ε d as a key factor in
the reactivity of metal surfaces is significant. Note that the d band theory inherently
assumes that surface reactivity is a local property, which differs from another earlier
perspective that bulk properties determine catalytic properties.
The activity for a number of reactions often peaks on a certain metal when correlated with some measure of reactivity across a series of metals, thus displaying a
“volcano” trend. Using extensive density functional theory (DFT) calculations,
Nørskov and co-workers have convincingly demonstrated that this behavior is the
result of key competing processes in a reaction network connected via a common
intermediate [22], just as described by the Sabatier principle. The principle states
that catalytic activity is often maximized when the reactants interact with the catalysts with intermediate strength.
6.2 Hydrogen Evolution Reaction
Hydrogen is a clean fuel that can be produced from renewable sources [23]. Thus, it
can provide sustainable energy in the hydrogen economy plans [24]. Energy generated from renewable sources can produce hydrogen by water electrolysis. The
stored hydrogen can then be used in fuel cells to generate electricity or used as a fuel
[25]. The hydrogen evolution reaction is the best studied and understood electrocatalytic reaction. Development of kinetics of electrode reactions is linked to the
work on HER. Thus, diagnostic criteria derived from Tafel slopes and current,
potential curves for reaction mechanisms determination, are best for HER. Theory
predicts various values for Tafel slopes, which are summarized in Table 6.3.
Table 6.3 Theoretical values of Tafel slopes for the hydrogen evolution reaction at 25 °C
Mechanism Rate-determining step
Tafel slope, b/mV dec
−1
Langmuir adsorption
condition
Temkin adsorption conditions
Θ H → 0
Θ H → 1
Volmer
−120
−40
−60
Heyrovsky
−40
−120
−60
V-T
Volmer
−120
−120
−120
Tafel
−30
∞
−60
V-T
Volmer-Tafel
−120
∞
−180
V-H
Volmer-Heyrovsky
−120
−120
−120
6 Important Electrocatalytic Reactions