308
J. A. Delgado and C. Godard
larger NPs displayed constant values. The observed behaviour below 11 nm was
attributed to be “geometric” in nature due to the size-sensitive formation of a Pd–
C x phase [38]. Such carbon deposits can block Pd surface sites, thus affecting the
activity. In a complementary study, the same authors reported that the decrease in
PdNPs size from 8 to 2 nm resulted in a decrease in activity, and increase in selectivity
to the ethylene product [39]. This time, the carbon deposits, important for smaller
NPs, were responsible for site isolation thus improving the selectivity by a geometric
effect.
Regarding the shape effect in catalysis, Kiwi-Minsker et al. reported the selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) employing PVP-stabilized Pd
nanocubes (6 and 18 nm), cuboctahedrons (5.5 nm) and octahedrons (31 nm) [40].
Founded on the observed reactivities, two types of active sites were proposed for this
reaction: those on the planes where the semi-hydrogenation of MBY to 2-methyl-3buten-2-ol (MBE) takes place and those at the edges which are responsible for overhydrogenation (Fig. 10.2). According to this criteria, the alkene product is adsorbed
more strongly at low coordination sites, thus resulting in over-hydrogenation. Differently, a similar study carried out on acetylene revealed that the shape of NPs does
not influence the selectivity, while the activity decreased in the order Pd octahedrons >
Pd cuboctahedrons > Pd cubes [25]. Considering the divergent observations regarding the
size and shape effects, it can only be concluded that these effects cannot be generalized. In addition, due to the intimate relationship between size and shape, studies in
this area need to consider their interplay.
Decreasing the particle size to the limit of the nanoscale, we can find firstly small
metal clusters and ultimately single atoms. One of the recent focuses in research on
heterogeneous catalysis is the use of single-atom catalysts (SACs) [13, 41]. In the
recent literature, examples of Pd [14, 42–44], Ni [45], Au [46], Cu [47] and Ru [48]
Fig. 10.2 Effect of the
NPs shape in the
semi-hydrogenation of
2-methyl-3-butyn-2-ol
(MBY). Reproduced with
permission from Ref. [15].
Copyright 2017 Royal
Society of Chemistry
HO
HO
H 2
Pd NPs
MBE
MBY
Activity and
selectivity:
>
>
Fraction of
surface atoms
(110-111):
>
>
CUB
OCT
COT
Pd (100)
Pd (111)
Pd (edge)
J. A. Delgado and C. Godard
larger NPs displayed constant values. The observed behaviour below 11 nm was
attributed to be “geometric” in nature due to the size-sensitive formation of a Pd–
C x phase [38]. Such carbon deposits can block Pd surface sites, thus affecting the
activity. In a complementary study, the same authors reported that the decrease in
PdNPs size from 8 to 2 nm resulted in a decrease in activity, and increase in selectivity
to the ethylene product [39]. This time, the carbon deposits, important for smaller
NPs, were responsible for site isolation thus improving the selectivity by a geometric
effect.
Regarding the shape effect in catalysis, Kiwi-Minsker et al. reported the selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) employing PVP-stabilized Pd
nanocubes (6 and 18 nm), cuboctahedrons (5.5 nm) and octahedrons (31 nm) [40].
Founded on the observed reactivities, two types of active sites were proposed for this
reaction: those on the planes where the semi-hydrogenation of MBY to 2-methyl-3buten-2-ol (MBE) takes place and those at the edges which are responsible for overhydrogenation (Fig. 10.2). According to this criteria, the alkene product is adsorbed
more strongly at low coordination sites, thus resulting in over-hydrogenation. Differently, a similar study carried out on acetylene revealed that the shape of NPs does
not influence the selectivity, while the activity decreased in the order Pd octahedrons >
Pd cuboctahedrons > Pd cubes [25]. Considering the divergent observations regarding the
size and shape effects, it can only be concluded that these effects cannot be generalized. In addition, due to the intimate relationship between size and shape, studies in
this area need to consider their interplay.
Decreasing the particle size to the limit of the nanoscale, we can find firstly small
metal clusters and ultimately single atoms. One of the recent focuses in research on
heterogeneous catalysis is the use of single-atom catalysts (SACs) [13, 41]. In the
recent literature, examples of Pd [14, 42–44], Ni [45], Au [46], Cu [47] and Ru [48]
Fig. 10.2 Effect of the
NPs shape in the
semi-hydrogenation of
2-methyl-3-butyn-2-ol
(MBY). Reproduced with
permission from Ref. [15].
Copyright 2017 Royal
Society of Chemistry
HO
HO
H 2
Pd NPs
MBE
MBY
Activity and
selectivity:
>
>
Fraction of
surface atoms
(110-111):
>
>
CUB
OCT
COT
Pd (100)
Pd (111)
Pd (edge)