6.6 Hydrogen Evolution Mechanism
91
In general, a typical HER process on a metal electrode involves the following
steps: (i) a discharge reaction, i.e., a Volmer step (H
+
+ e
−
→ H ad ), or the discharge
reaction of H 3 O
+ ions formed by the dissociation of water (H 3 O
+
+ e
−
→ H ad +
H 2 O); followed by (ii) either a recombination reaction, i.e., a Tafel step (H ad + H ad
→ H 2 ) and/or an electrochemical desorption reaction, i.e., Heyrovsky reaction (H
+
+ H ad + e
−
→ H 2 ; or H 2 O + H ad + e
−
→ H 2 + OH
− ), where H ad refers to an
adsorbed H atom [10]. Among others, the HER investigations of individual Al atoms
with water has been reported using laser induced fluorescence (LIF) suggesting that
the major product appears to be ‘AlOH + H’ with fragmentation of the HAlOH
molecule [89]. Also, the reactivity of Al clusters with multiple water molecules was
studied and it was demonstrated that the first step reaction is the generation of a
HAl n OH(H 2 O) x species in which the additional water molecules play a catalytic
role [41, 90].
Recently, a further insight was presented on the HER investigation of aluminum
clusters with water and methanol/isopropanol mixture reactants. Although aluminum
clusters were found to undertake an etching effect and an addition reaction in the presence of methanol-only and isopropanol-only respectively, products with hydrogen
released were observed interestingly in the reactions with both “water + methanol”
and “isoproanol + water” systems. The use of bireactants (Fig. 6.9) toward Al clusters enables a comparison of their reactivity and gas-phase interaction. Comparing
with alcohols, water dominates the competitive reaction with Al clusters and the
O–H bond in water is readily activated to form aluminum hydroxide cluster products
in the room-temperature fast-flow tube apparatus. The transition states to produce
hydrogen refer to combination of the adsorbed H atoms, which is akin to a Tafel
step in general HER mechanism. Furthermore, water was found to contribute to
Fig. 6.9 Reaction of H 2 O and CH 3 OH (1:1 molar ratio) with Al clusters: a the original Al n
−
spectrum before the reaction, b the spectrum showing the products after simultaneous exposure to
the reactants. c A sketch showing the competition and interaction between H 2 O and CH 3 OH when
reacting with Al n
−
91
In general, a typical HER process on a metal electrode involves the following
steps: (i) a discharge reaction, i.e., a Volmer step (H
+
+ e
−
→ H ad ), or the discharge
reaction of H 3 O
+ ions formed by the dissociation of water (H 3 O
+
+ e
−
→ H ad +
H 2 O); followed by (ii) either a recombination reaction, i.e., a Tafel step (H ad + H ad
→ H 2 ) and/or an electrochemical desorption reaction, i.e., Heyrovsky reaction (H
+
+ H ad + e
−
→ H 2 ; or H 2 O + H ad + e
−
→ H 2 + OH
− ), where H ad refers to an
adsorbed H atom [10]. Among others, the HER investigations of individual Al atoms
with water has been reported using laser induced fluorescence (LIF) suggesting that
the major product appears to be ‘AlOH + H’ with fragmentation of the HAlOH
molecule [89]. Also, the reactivity of Al clusters with multiple water molecules was
studied and it was demonstrated that the first step reaction is the generation of a
HAl n OH(H 2 O) x species in which the additional water molecules play a catalytic
role [41, 90].
Recently, a further insight was presented on the HER investigation of aluminum
clusters with water and methanol/isopropanol mixture reactants. Although aluminum
clusters were found to undertake an etching effect and an addition reaction in the presence of methanol-only and isopropanol-only respectively, products with hydrogen
released were observed interestingly in the reactions with both “water + methanol”
and “isoproanol + water” systems. The use of bireactants (Fig. 6.9) toward Al clusters enables a comparison of their reactivity and gas-phase interaction. Comparing
with alcohols, water dominates the competitive reaction with Al clusters and the
O–H bond in water is readily activated to form aluminum hydroxide cluster products
in the room-temperature fast-flow tube apparatus. The transition states to produce
hydrogen refer to combination of the adsorbed H atoms, which is akin to a Tafel
step in general HER mechanism. Furthermore, water was found to contribute to
Fig. 6.9 Reaction of H 2 O and CH 3 OH (1:1 molar ratio) with Al clusters: a the original Al n
−
spectrum before the reaction, b the spectrum showing the products after simultaneous exposure to
the reactants. c A sketch showing the competition and interaction between H 2 O and CH 3 OH when
reacting with Al n
−