92
6 Cooperative Active-Sites Mechanism
the activation of the O–H bond in alcohols when reacting with Al cluster anions.
These investigations help improve the understanding of the HER mechanism and
indicate potential application for hydrogen generation [10]. Subsequent studies have
demonstrated how partial atomic charges and bonding orbitals [93], and the doping
of heteroatoms could affect the HER processes on clusters [38, 94–99], shedding
light on an alternative Eley–Rideal and Langmuir–Hinshelwood mechanisms in the
presence of two OH-group molecules [100].
Based on these HER investigations of −OH group molecules on Al clusters,
the complementary-active-sites (CAS) mechanism [7] has been well established to
explain the size-selective reactivity of metal clusters with polar molecules [5, 9, 29,
40, 42, 101, 102]. Recently, the reactions of vanadium clusters with water find a
prominent hydrogen evolution reaction (HER) of single H 2 O molecule for V n≥3
+ but
no HER products were observed in the same condition for n = 1, 2. DFT-calculation
results reveal that the wagging vibration of −OH group results in readily formed
V–O–V intermediate states which allow the terminal hydrogen to interact with an
adsorbed hydrogen atom giving rise to H 2 release. The presence of three vanadium
atoms decreases the energy barrier of the rate-determine step transition state, resulting
in effective H 2 production from a single water molecule. This mechanism is essentially different from the aformentioned reactivity of water with aluminum clusters by
dissociative chemisorption of at least two water molecules at multiple surface sites
followed by a Tafel step of recombination of the two adsorbed H atoms.
6.7 Summary
The understanding of size-selective metal reactivity towards polar molecules in gas
phase shed light on complementary active sites. This established mechanism (by
assigning one metal atom acts as a Lewis acid and a second Al atom acts as a Lewis
base) well explained the contradiction for size-selectivity of Al n
− reacting with water,
alcohols, thiols, and some other molecular systems. It provides an insight into the
origin of hydrogen evolution reactions. It is expected this mechanism will induce
further understanding metal–organic reactivity and formation, as well as potential
application in catalysis and industrial production. Recently Behrens et al. [103]
reported a comprehensive experimental and theoretical analysis of the active site
structure of a heterogeneous catalyst that is of crucial importance in industry to
produce methanol. The active site of such catalyst can be thought of as the ensemble
of atoms that directly catalyzes a reaction. Knowledge of the composition of the
active site is significant for understanding the properties of catalysts [104, 105],
and helping probing more challenging catalysts the interactions between the metals
and the oxide supports are highly synergistic and sensitive to the environmental and
reaction conditions [106, 107].
6 Cooperative Active-Sites Mechanism
the activation of the O–H bond in alcohols when reacting with Al cluster anions.
These investigations help improve the understanding of the HER mechanism and
indicate potential application for hydrogen generation [10]. Subsequent studies have
demonstrated how partial atomic charges and bonding orbitals [93], and the doping
of heteroatoms could affect the HER processes on clusters [38, 94–99], shedding
light on an alternative Eley–Rideal and Langmuir–Hinshelwood mechanisms in the
presence of two OH-group molecules [100].
Based on these HER investigations of −OH group molecules on Al clusters,
the complementary-active-sites (CAS) mechanism [7] has been well established to
explain the size-selective reactivity of metal clusters with polar molecules [5, 9, 29,
40, 42, 101, 102]. Recently, the reactions of vanadium clusters with water find a
prominent hydrogen evolution reaction (HER) of single H 2 O molecule for V n≥3
+ but
no HER products were observed in the same condition for n = 1, 2. DFT-calculation
results reveal that the wagging vibration of −OH group results in readily formed
V–O–V intermediate states which allow the terminal hydrogen to interact with an
adsorbed hydrogen atom giving rise to H 2 release. The presence of three vanadium
atoms decreases the energy barrier of the rate-determine step transition state, resulting
in effective H 2 production from a single water molecule. This mechanism is essentially different from the aformentioned reactivity of water with aluminum clusters by
dissociative chemisorption of at least two water molecules at multiple surface sites
followed by a Tafel step of recombination of the two adsorbed H atoms.
6.7 Summary
The understanding of size-selective metal reactivity towards polar molecules in gas
phase shed light on complementary active sites. This established mechanism (by
assigning one metal atom acts as a Lewis acid and a second Al atom acts as a Lewis
base) well explained the contradiction for size-selectivity of Al n
− reacting with water,
alcohols, thiols, and some other molecular systems. It provides an insight into the
origin of hydrogen evolution reactions. It is expected this mechanism will induce
further understanding metal–organic reactivity and formation, as well as potential
application in catalysis and industrial production. Recently Behrens et al. [103]
reported a comprehensive experimental and theoretical analysis of the active site
structure of a heterogeneous catalyst that is of crucial importance in industry to
produce methanol. The active site of such catalyst can be thought of as the ensemble
of atoms that directly catalyzes a reaction. Knowledge of the composition of the
active site is significant for understanding the properties of catalysts [104, 105],
and helping probing more challenging catalysts the interactions between the metals
and the oxide supports are highly synergistic and sensitive to the environmental and
reaction conditions [106, 107].