9.5 Search for Alternate Cheaper Catalysts
159
These results show that the electronic properties of the individual Pd and Ni active
sites in the bimetallic cluster are significantly modified. As seen in Fig. 9.11b–d, the
Ni sites of the bimetallic Pd/Ni clusters donate charge to the Pd sites, with the Pd
sites becoming negatively charged, while the Ni sites become positively charged.
This modifies the charge donating and withdrawing capability of the atoms, which
may then translate to the modification of the activation energies.
In Fig. 9.11a–d, the activation energy for the free and supported 13-atom and
4-atom clusters are shown. For the free 13-atom clusters, Ni 13 is most active for the
oxidative addition step, and the Pd 12 Ni cluster (where the Ni atom is the active site)
is most active in the transmetallation and reductive elimination steps. This supports
the concept that the charge transfer from Ni to Pd results in a Ni site that is activated
towards charge accepting steps such as transmetallation and reductive elimination.
The activation energy then follows Ni < Pd
− (Ni) < Ni
+ (Pd) < Pd for the steps where
the cluster acts as a charge donor. Pd
− (Ni) denotes the Pd site on a bimetallic cluster
where Ni has donated charge to Pd, and Ni
+ (Pd) denotes the Ni site on a bimetallic
cluster where the Ni has donated charge to Pd. In contrast, for the steps where the
cluster acts as a charge acceptor, the activation energy follows a trend as Ni
+ (Pd)
< Pd < Pd
− (Ni) < Ni for transmetallation, and Ni
+ (Pd) < Pd
− (Ni) < Pd < Ni for
reductive elimination. When different metal atoms are contained in the same cluster,
it is possible for different steps in the reaction to be carried out on different active
sites. Such a pathway may only occur when there are adjacent bimetallic surface
atoms, this mechanism will not occur for core-shell bimetallic particles where the
atoms are segregated into shells.
Depositing the bimetallic clusters on graphene results in a further decrease in the
activation energy, as seen in Fig. 9.11b. After deposition, the lowest activation energy
pathway follows a co-catalysis process on different Ni and Pd sites. For all three steps,
the activation energies of the supported Pd 12 Ni cluster are all lower than that of the
supported Pd 13 cluster. Decreasing the cluster size to four atoms further lowers the
activation energy of the catalysis. The activation energy of the free and supported
Ni 2 Pd 2 clusters are shown in Fig. 9.11c, d, and these clusters are found to be even
more active than the 13-atom clusters. These results are consistent with the concept
that the Ni becomes more positively charged in the bimetallic cluster, and hence it
becomes a better charge acceptor, while Pd sites become more negatively charged
and are more active in charge donating steps. By combining the graphene support
that enhances charge transfer with the small size of the cluster and the bimetallic
charge transfer, it was found that the supported Pd 2 Ni 2 cluster is the most active
cluster studied here towards the Suzuki cross-coupling reaction.
9.6 Summary
In this chapter, we have attempted to provide a conceptual framework for understanding the dramatic increase in catalytic activity that can be achieved by optimizing
the electronic interactions between the metal cluster and the conducting support.
159
These results show that the electronic properties of the individual Pd and Ni active
sites in the bimetallic cluster are significantly modified. As seen in Fig. 9.11b–d, the
Ni sites of the bimetallic Pd/Ni clusters donate charge to the Pd sites, with the Pd
sites becoming negatively charged, while the Ni sites become positively charged.
This modifies the charge donating and withdrawing capability of the atoms, which
may then translate to the modification of the activation energies.
In Fig. 9.11a–d, the activation energy for the free and supported 13-atom and
4-atom clusters are shown. For the free 13-atom clusters, Ni 13 is most active for the
oxidative addition step, and the Pd 12 Ni cluster (where the Ni atom is the active site)
is most active in the transmetallation and reductive elimination steps. This supports
the concept that the charge transfer from Ni to Pd results in a Ni site that is activated
towards charge accepting steps such as transmetallation and reductive elimination.
The activation energy then follows Ni < Pd
− (Ni) < Ni
+ (Pd) < Pd for the steps where
the cluster acts as a charge donor. Pd
− (Ni) denotes the Pd site on a bimetallic cluster
where Ni has donated charge to Pd, and Ni
+ (Pd) denotes the Ni site on a bimetallic
cluster where the Ni has donated charge to Pd. In contrast, for the steps where the
cluster acts as a charge acceptor, the activation energy follows a trend as Ni
+ (Pd)
< Pd < Pd
− (Ni) < Ni for transmetallation, and Ni
+ (Pd) < Pd
− (Ni) < Pd < Ni for
reductive elimination. When different metal atoms are contained in the same cluster,
it is possible for different steps in the reaction to be carried out on different active
sites. Such a pathway may only occur when there are adjacent bimetallic surface
atoms, this mechanism will not occur for core-shell bimetallic particles where the
atoms are segregated into shells.
Depositing the bimetallic clusters on graphene results in a further decrease in the
activation energy, as seen in Fig. 9.11b. After deposition, the lowest activation energy
pathway follows a co-catalysis process on different Ni and Pd sites. For all three steps,
the activation energies of the supported Pd 12 Ni cluster are all lower than that of the
supported Pd 13 cluster. Decreasing the cluster size to four atoms further lowers the
activation energy of the catalysis. The activation energy of the free and supported
Ni 2 Pd 2 clusters are shown in Fig. 9.11c, d, and these clusters are found to be even
more active than the 13-atom clusters. These results are consistent with the concept
that the Ni becomes more positively charged in the bimetallic cluster, and hence it
becomes a better charge acceptor, while Pd sites become more negatively charged
and are more active in charge donating steps. By combining the graphene support
that enhances charge transfer with the small size of the cluster and the bimetallic
charge transfer, it was found that the supported Pd 2 Ni 2 cluster is the most active
cluster studied here towards the Suzuki cross-coupling reaction.
9.6 Summary
In this chapter, we have attempted to provide a conceptual framework for understanding the dramatic increase in catalytic activity that can be achieved by optimizing
the electronic interactions between the metal cluster and the conducting support.