9.3 Palladium Clusters Catalyse Cross-Coupling Reactions
147
to the graphene support suggested that a unique interfacial interaction was occurring
between Pd nanoclusters and defected graphene.
The remarkable activity coupled with minimal leaching raised two questions: (1)
what is the microscopic reaction mechanism that makes these supports so active? (2)
Can one use this information to optimize the catalysis and identify additional active
supports?
To answer some of these questions, Yuan et al. [81]. Recently undertook a series
of synergistic studies combining kinetic experiments on various palladium-based
catalysts and coupled it to first-principles theory to determine the activation energy
in each step of the catalytic cycle of Suzuki reaction for both free and supported Pd n
species [34]. The studies examined the effects of the support, active-site structure, Pd
cluster size, and the electron rich ligands. To understand the effect of particle size and
the particle-support interactions on the catalytic activity, Pd nanoparticles supported
on graphene oxide were synthesized by three different methods: (i) impregnation
of Pd precursor with graphene oxide followed by hydrazine and microwave-heating
co-reduction (CO(MW)), [77, 82] (ii) strong electrostatic adsorption (SEA) [83] of
Pd precursor on graphene oxide followed by hydrogen reduction (SEA-H) in tube
furnace, and (iii) SEA followed by solvent-free microwave heating reduction (SEAMW). In the first CO(MW) and third SEA-MW methods, microwave irradiation was
used to induce vacancy defect sites that can strongly anchor Pd nanoparticles [84].
In comparison, the second method (i.e., SEA-H method) immobilizes the cationic
Pd precursors to deprotonated oxygen functional groups and results in uniform Pd
nanoparticles, but no vacancy defects are formed. TEM images of the resulting catalysts, as shown in Fig. 9.2, confirmed that the microwave irradiation changes the
metal-support interaction.
Further, they evaluated the three supported catalysts and a commercially available
supported catalyst along with two homogenous Pd-based catalysts. For each catalyst
system, the activation energy and TOF were determined for the model Suzuki reaction using 4-bromobenzoic acid and phenyl boronic acid as reagent. The findings are
shown in Fig. 9.2d. The Pd/G CO and Pd/G SEA-H catalysts possess similar particle
size distributions (avg. ~2 nm), allowing a size independent comparison between
catalysts of different metal-support interactions. The activation energy of the Pd/G
SEA-H was four times larger than that of Pd/G CO, and the TOF is 8,000 h
−1 versus
114,000 h
−1 respectively. This demonstrated that the catalytic activity is highly sensitive to the catalyst-support interaction. The activation energy of Pd/G CO catalyst was
also found to be significantly lower than that of the homogenous catalysts Pd/PPh 3
and Pd/CyJP. The activation energy of Pd/G SEA-MW was about three times lower
than that of Pd/G SEA-H. It was proposed that the reduction in activation energy was
caused by the microwave heating and induced defect sites. The activation energy of
Pd/G SEA-MW was slightly higher than that of the Pd/G CO catalyst which was most
likely due to the larger nanoparticle size reducing the ratio of lower coordination Pd
active sites. In fact, the activation energy of the Pd/G SEA-H was found to be similar
to the activation energy of commercially available Pd/C catalyst, which is believed to
catalyze the cross-coupling reaction via a leaching induced homogenous mechanism
147
to the graphene support suggested that a unique interfacial interaction was occurring
between Pd nanoclusters and defected graphene.
The remarkable activity coupled with minimal leaching raised two questions: (1)
what is the microscopic reaction mechanism that makes these supports so active? (2)
Can one use this information to optimize the catalysis and identify additional active
supports?
To answer some of these questions, Yuan et al. [81]. Recently undertook a series
of synergistic studies combining kinetic experiments on various palladium-based
catalysts and coupled it to first-principles theory to determine the activation energy
in each step of the catalytic cycle of Suzuki reaction for both free and supported Pd n
species [34]. The studies examined the effects of the support, active-site structure, Pd
cluster size, and the electron rich ligands. To understand the effect of particle size and
the particle-support interactions on the catalytic activity, Pd nanoparticles supported
on graphene oxide were synthesized by three different methods: (i) impregnation
of Pd precursor with graphene oxide followed by hydrazine and microwave-heating
co-reduction (CO(MW)), [77, 82] (ii) strong electrostatic adsorption (SEA) [83] of
Pd precursor on graphene oxide followed by hydrogen reduction (SEA-H) in tube
furnace, and (iii) SEA followed by solvent-free microwave heating reduction (SEAMW). In the first CO(MW) and third SEA-MW methods, microwave irradiation was
used to induce vacancy defect sites that can strongly anchor Pd nanoparticles [84].
In comparison, the second method (i.e., SEA-H method) immobilizes the cationic
Pd precursors to deprotonated oxygen functional groups and results in uniform Pd
nanoparticles, but no vacancy defects are formed. TEM images of the resulting catalysts, as shown in Fig. 9.2, confirmed that the microwave irradiation changes the
metal-support interaction.
Further, they evaluated the three supported catalysts and a commercially available
supported catalyst along with two homogenous Pd-based catalysts. For each catalyst
system, the activation energy and TOF were determined for the model Suzuki reaction using 4-bromobenzoic acid and phenyl boronic acid as reagent. The findings are
shown in Fig. 9.2d. The Pd/G CO and Pd/G SEA-H catalysts possess similar particle
size distributions (avg. ~2 nm), allowing a size independent comparison between
catalysts of different metal-support interactions. The activation energy of the Pd/G
SEA-H was four times larger than that of Pd/G CO, and the TOF is 8,000 h
−1 versus
114,000 h
−1 respectively. This demonstrated that the catalytic activity is highly sensitive to the catalyst-support interaction. The activation energy of Pd/G CO catalyst was
also found to be significantly lower than that of the homogenous catalysts Pd/PPh 3
and Pd/CyJP. The activation energy of Pd/G SEA-MW was about three times lower
than that of Pd/G SEA-H. It was proposed that the reduction in activation energy was
caused by the microwave heating and induced defect sites. The activation energy of
Pd/G SEA-MW was slightly higher than that of the Pd/G CO catalyst which was most
likely due to the larger nanoparticle size reducing the ratio of lower coordination Pd
active sites. In fact, the activation energy of the Pd/G SEA-H was found to be similar
to the activation energy of commercially available Pd/C catalyst, which is believed to
catalyze the cross-coupling reaction via a leaching induced homogenous mechanism