218
unchanged until the dispersion amount is 20–35%; however, this might be different,
when different supports used [72, 76]. However, the catalytic activity was decreased
as the amount of Pd dispersion increased. Furthermore, as the number of active
metal particles decreases, the electronic properties of metal particles were influenced. Perfect explanation can be provided through the chemisorption of highly
electron–rich diene molecules. If less number of active metal particles available, the
selectivity towards 1,3-butadiene hydrogenation for the butene will be decreased. It
was also observed that the metal atoms with high coordination number were
characteristic of larger particles and whilst small particles possess crystals with
atoms of low co-ordination number. Furthermore, the active nanoparticle size can
also play a significant role in heterogeneous catalysis due to the unique properties at
nanoscale. The utilization of transition metals with a 1–20 nm nanoparticles size in
catalysis is vital as they mimic the activation of metal surface which lead to high
selectivity and efficacy to the desired reaction. These small-sized nanoparticles can
also be called as the clusters derived from metal atoms, and are usually stabilized by
ligands, surfactants, and polymers/dendrimers protecting their surfaces with varying
sizes (ranging from 10 to 100 nm). However, the most active nanoclusters are only
few hundreds of atoms with one or a few nanometres in diameter. It is reported that
the sizes of the noble-metal structures and specific crystal facets, such as Pd, Pt, Au
and Ni [19, 62, 66–68, 73–75, 77–85] significantly influence on their chemisorption
properties and hence, catalytic performance. Hence, special nanostructures with
well-defined shapes and uniform sizes are highly needed to control their performance
for the hydrogenation reaction. According to Piccolo et al. [86], the conversion rates
of butadiene-to-butenes on the fresh model catalysts can be related as follows:
Au(111) < Pd-Au(111) < Pd-Au(110) < Pd(111). Unlike on Pd(111), the butane
production rate was very low and the butenes selectivity reached ~100% on
Pd-Au(111). It should also be noted that the diffusion of hydrogen toward bulk of
the Pd-based single-crystals seems to play a key role and the surface hybrid
formation accounts for the Pd-Au surface activation. Michalak et al. [39] reported
that the catalysts having ensembles of 1.8 and 0.9 nm in size of Pt enhanced 20 and
30% of total hydrogenation of 1,3-butadiene to n-butane when compared to 6.7 and
4.6 nm size of Pt. It can be concluded that the small-sized nanoparticles effected
conversion rate. As shown in Fig. 7, the larger size (4.6 and 6.7 nm) Pt nanoparticles
favour insertion of H-atom at the terminal site of carbon atom, which is similar to
those observed for bulk Pt materials. In case of smaller sized (0.9 and 1.8 nm) Pt
nanoparticles, the insertion H-atom occurred at two places, which includes at low
coordination sites as well as at terminal carbon site. Moreover, Lucci et al. reported
that isolated Pt atoms were responsible for the deactivation of catalyst, thereby less
selectivity towards butene formation. This was due to the preferential hydrogen
activation and unable to break the C-C bond [87]. In contrast, other researchers
reported that the 1,3-butadiene hydrogenation reaction is in fact independent of the
metal particle size [66, 67]. In particular, a study was focused on demonstrating the
hydrogenation of 1,3-butadiene was independent on particle size (in spite of being
structure sensitive) for the case of Pd/Al 2 O 3 catalyst. The catalytic activity of
different-sized Pd nanoparticles was correlated with the Pd(110) and Pd(111)
P. R. Selvakannan et al.
unchanged until the dispersion amount is 20–35%; however, this might be different,
when different supports used [72, 76]. However, the catalytic activity was decreased
as the amount of Pd dispersion increased. Furthermore, as the number of active
metal particles decreases, the electronic properties of metal particles were influenced. Perfect explanation can be provided through the chemisorption of highly
electron–rich diene molecules. If less number of active metal particles available, the
selectivity towards 1,3-butadiene hydrogenation for the butene will be decreased. It
was also observed that the metal atoms with high coordination number were
characteristic of larger particles and whilst small particles possess crystals with
atoms of low co-ordination number. Furthermore, the active nanoparticle size can
also play a significant role in heterogeneous catalysis due to the unique properties at
nanoscale. The utilization of transition metals with a 1–20 nm nanoparticles size in
catalysis is vital as they mimic the activation of metal surface which lead to high
selectivity and efficacy to the desired reaction. These small-sized nanoparticles can
also be called as the clusters derived from metal atoms, and are usually stabilized by
ligands, surfactants, and polymers/dendrimers protecting their surfaces with varying
sizes (ranging from 10 to 100 nm). However, the most active nanoclusters are only
few hundreds of atoms with one or a few nanometres in diameter. It is reported that
the sizes of the noble-metal structures and specific crystal facets, such as Pd, Pt, Au
and Ni [19, 62, 66–68, 73–75, 77–85] significantly influence on their chemisorption
properties and hence, catalytic performance. Hence, special nanostructures with
well-defined shapes and uniform sizes are highly needed to control their performance
for the hydrogenation reaction. According to Piccolo et al. [86], the conversion rates
of butadiene-to-butenes on the fresh model catalysts can be related as follows:
Au(111) < Pd-Au(111) < Pd-Au(110) < Pd(111). Unlike on Pd(111), the butane
production rate was very low and the butenes selectivity reached ~100% on
Pd-Au(111). It should also be noted that the diffusion of hydrogen toward bulk of
the Pd-based single-crystals seems to play a key role and the surface hybrid
formation accounts for the Pd-Au surface activation. Michalak et al. [39] reported
that the catalysts having ensembles of 1.8 and 0.9 nm in size of Pt enhanced 20 and
30% of total hydrogenation of 1,3-butadiene to n-butane when compared to 6.7 and
4.6 nm size of Pt. It can be concluded that the small-sized nanoparticles effected
conversion rate. As shown in Fig. 7, the larger size (4.6 and 6.7 nm) Pt nanoparticles
favour insertion of H-atom at the terminal site of carbon atom, which is similar to
those observed for bulk Pt materials. In case of smaller sized (0.9 and 1.8 nm) Pt
nanoparticles, the insertion H-atom occurred at two places, which includes at low
coordination sites as well as at terminal carbon site. Moreover, Lucci et al. reported
that isolated Pt atoms were responsible for the deactivation of catalyst, thereby less
selectivity towards butene formation. This was due to the preferential hydrogen
activation and unable to break the C-C bond [87]. In contrast, other researchers
reported that the 1,3-butadiene hydrogenation reaction is in fact independent of the
metal particle size [66, 67]. In particular, a study was focused on demonstrating the
hydrogenation of 1,3-butadiene was independent on particle size (in spite of being
structure sensitive) for the case of Pd/Al 2 O 3 catalyst. The catalytic activity of
different-sized Pd nanoparticles was correlated with the Pd(110) and Pd(111)
P. R. Selvakannan et al.