1 Ligand-Free Sub-Nanometer Metal Clusters in Catalysis
31
clusters since, in contrast to alkynes, the strength of the coordination bond with
thiols decreases as the Au particle decreases in size. The isolated Au atoms are
not the true catalyst of the homocoupling but just precursors that evolve during
reaction to few-atom Au clusters, which are indeed the catalytically active species
(see Fig. 1.28).
1.3.4 Hydrogenation Reactions
The hydrogenation of unsaturated bonds is perhaps one of the older transformations
with catalytic metals. Typically performed with finely divided metal powders and
modernly with metal nanoparticles, the hydrogenation is widely accepted to proceed
by H 2 dissociation and spillover of the H atoms on the metal and/or support surface.
For this reason, the use of supported single metal atoms or clusters was not considered
until recently. Indeed, it has been found that not only H 2 dissociation [40] but also
H 2 formation [43] occurs very efficiently over supported isolated metal atoms and
clusters [12, 19, 22, 74]. Two representative reaction examples for catalytic clusters
follow, although much more can be found in the literature [28, 68].
1.3.4.1 CO 2 Methanation
The hydrogenation of CO 2 to methane (Sabatier reaction) is one of the oldest metalcatalyzed hydrogenation reactions, which is now revisited due to the urgent need
of converting CO 2 to useful chemicals and, concomitantly, alleviating its global
warming effect [23]. For that, it is convenient to design metal catalytic systems that
operate at low temperatures (< 250 °C). It has been recently reported that Pt
0
2 clusters
inside a MOF catalyze the reaction at temperatures below 150 °C, and comparison
between isolated Pt atoms inside the same MOF and reference catalysts shows that
the Pt cluster outperforms the rest of materials tested under the low-temperature
conditions, including the industrial catalyst Ru–Al 2 O 3 (Fig. 1.29) [45]. The lack of
activity of Pt
2+ discards these sites as catalytic active species and indicates that Pt
0
2
is the catalytic sites for the hydrogenation of CO 2 .
1.3.4.2 Olefin Hydrogenation
Olefin hydrogenation is another typical example of a metal-catalyzed reaction with
industrial application [73]. The Pt
0
2 clusters supported in the MOF also efficiently
catalyze the hydrogenation of ethylene under industrial reaction conditions, in flow, at
much lower temperature (60 °C) than current industrial processes with nanoparticles
(200–400 °C) and with a sustained TOF of 250 h
−1 (Fig. 1.30) [45]. Moreover, other
< C 6 alkenes such as propylene, 1, 3–butadiene and 1–hexene, among others reacted
similarly well. Also, when isomerically pure E–3–hexene was hydrogenated with
31
clusters since, in contrast to alkynes, the strength of the coordination bond with
thiols decreases as the Au particle decreases in size. The isolated Au atoms are
not the true catalyst of the homocoupling but just precursors that evolve during
reaction to few-atom Au clusters, which are indeed the catalytically active species
(see Fig. 1.28).
1.3.4 Hydrogenation Reactions
The hydrogenation of unsaturated bonds is perhaps one of the older transformations
with catalytic metals. Typically performed with finely divided metal powders and
modernly with metal nanoparticles, the hydrogenation is widely accepted to proceed
by H 2 dissociation and spillover of the H atoms on the metal and/or support surface.
For this reason, the use of supported single metal atoms or clusters was not considered
until recently. Indeed, it has been found that not only H 2 dissociation [40] but also
H 2 formation [43] occurs very efficiently over supported isolated metal atoms and
clusters [12, 19, 22, 74]. Two representative reaction examples for catalytic clusters
follow, although much more can be found in the literature [28, 68].
1.3.4.1 CO 2 Methanation
The hydrogenation of CO 2 to methane (Sabatier reaction) is one of the oldest metalcatalyzed hydrogenation reactions, which is now revisited due to the urgent need
of converting CO 2 to useful chemicals and, concomitantly, alleviating its global
warming effect [23]. For that, it is convenient to design metal catalytic systems that
operate at low temperatures (< 250 °C). It has been recently reported that Pt
0
2 clusters
inside a MOF catalyze the reaction at temperatures below 150 °C, and comparison
between isolated Pt atoms inside the same MOF and reference catalysts shows that
the Pt cluster outperforms the rest of materials tested under the low-temperature
conditions, including the industrial catalyst Ru–Al 2 O 3 (Fig. 1.29) [45]. The lack of
activity of Pt
2+ discards these sites as catalytic active species and indicates that Pt
0
2
is the catalytic sites for the hydrogenation of CO 2 .
1.3.4.2 Olefin Hydrogenation
Olefin hydrogenation is another typical example of a metal-catalyzed reaction with
industrial application [73]. The Pt
0
2 clusters supported in the MOF also efficiently
catalyze the hydrogenation of ethylene under industrial reaction conditions, in flow, at
much lower temperature (60 °C) than current industrial processes with nanoparticles
(200–400 °C) and with a sustained TOF of 250 h
−1 (Fig. 1.30) [45]. Moreover, other
< C 6 alkenes such as propylene, 1, 3–butadiene and 1–hexene, among others reacted
similarly well. Also, when isomerically pure E–3–hexene was hydrogenated with