Tsukuda et al. reported a size-controlled synthesis of 15, 30 or 60 atoms Ir
clusters within a OH-terminated poly(amidoamine) (PAMAM) dendrimer of generation 6 (Ir x :G6; x ¼ 15, 30 or 60) [127]. After deposition of Ir x :G6 in SiO 2 , these Ir
catalysts selective hydrogenated the –NO 2 group of 2-nitrobenzaldehyde into
2-aminobenzaldehyde and anthranil under mild conditions (30
C, 1 bar of H 2 ). A
clear size dependency in the selectivity of the reaction was observed; the smaller the
iridium cluster is, the more selective the catalyst is towards the formation of
2-aminobenzaldehyde (Table 20).
As we have previously mentioned, an efficient way to improve the selectivity of
supported MNPs in the hydrogenation of substrates with different functional groups
is the use of a second metal. In 2013, Tomishige and coworkers reported silicasupported Ir NPs decorated with ReO x (Ir-ReO x /SiO 2 ) as active catalyst for the
selective hydrogenation of unsaturated aldehydes towards the corresponding alcohols [128]. The catalysis was performed in water at low temperature (30
C) and
moderate H 2 pressure (8 bar). Here, the Ir atoms, in cooperation with the basic oxide
species, heterolytically dissociate H 2 into H
δ+ and H
δ– (Fig. 22a). This heterolytic H 2
cleavage explains the high activity and selectivity exhibited by this catalyst in the
gram-scale hydrogenation of crotonaldehyde (92% conversion and 90% selectivity;
initial TOF ¼ 12,600 h
À1 ). This synergy between Ir NPs and ReOx species significantly increased the selectivity, keeping the characteristic high activity of noble
metals. Another interesting synergistic effect was observed by Liao et al. on bimetallic PdIr NPs supported on mesoporous silica nanoparticles (PdIr/MSN) prepared
by an impregnation-hydrogenation method [129]. They observed that the incorporation of Ir on Pd NPs significantly enhances the activity and selectivity of the
catalyst in the hydrogenation of nitrobenzene to aniline. Here, there is an evident
promoting effect, since bimetallic NPs with a molar Ir/Pd ratio of 0.1 (PdIr0.1/MSN)
present an activity up to 28 and 8 times higher than those containing monometallic
Ir/MSN and Pd/MSN, respectively (Fig. 22b). The promotional effect of Ir was
explained by two factors: (1) an increase in the number of surface-active sites,
consequence of a small NP size, and (2) a modification of the electronic properties
of the MNP surface, which intensifies the electronic interactions. Liang et al. also
observed a significant promotion effect of Ir in the selective hydrogenation of phenol
to cyclohexanone over bimetallic PtIr NPs supported on hollow mesoporous silica
spheres (HMSs) [130]. They generated the bimetallic PtIr on HMSs with different
metal molar ratios (Ir/Pt: 0.5, 0.2 or 0.1) following a simple impregnation-reduction
method. All catalysts showed similar selectivities towards cyclohexanone (80–85%),
but different conversions were obtained depending on the amount of Ir incorporated.
The most active catalyst was the one with a Ir/Pt molar ratio of 0.1 (PtIr0.1), with
60% of conversion (8.6 times higher than the monometallic Pt/HMS; Fig. 22c, left)
and good levels of stability (4 recycle times; Fig. 22c, right). Here, the positive effect
of the Ir is attributed to the enhancement of the metal dispersion (higher number of
active sites) and the modulation of the NP electronic state (influence the MNP
catalytic behaviour). Another example about a synergistic effect was reported by
Ding et al. [131], in which ultrasmall silica-supported bimetallic NPs (Pd-Ir/SiO 2 ,
Pd-Pt/SiO 2 , Pd-Au/SiO 2 , Pt-Ir/SiO 2 and Ru-Pt/SiO2) were active in the selective
Iridium Nanoparticles for Hydrogenation Reactions
439
clusters within a OH-terminated poly(amidoamine) (PAMAM) dendrimer of generation 6 (Ir x :G6; x ¼ 15, 30 or 60) [127]. After deposition of Ir x :G6 in SiO 2 , these Ir
catalysts selective hydrogenated the –NO 2 group of 2-nitrobenzaldehyde into
2-aminobenzaldehyde and anthranil under mild conditions (30
C, 1 bar of H 2 ). A
clear size dependency in the selectivity of the reaction was observed; the smaller the
iridium cluster is, the more selective the catalyst is towards the formation of
2-aminobenzaldehyde (Table 20).
As we have previously mentioned, an efficient way to improve the selectivity of
supported MNPs in the hydrogenation of substrates with different functional groups
is the use of a second metal. In 2013, Tomishige and coworkers reported silicasupported Ir NPs decorated with ReO x (Ir-ReO x /SiO 2 ) as active catalyst for the
selective hydrogenation of unsaturated aldehydes towards the corresponding alcohols [128]. The catalysis was performed in water at low temperature (30
C) and
moderate H 2 pressure (8 bar). Here, the Ir atoms, in cooperation with the basic oxide
species, heterolytically dissociate H 2 into H
δ+ and H
δ– (Fig. 22a). This heterolytic H 2
cleavage explains the high activity and selectivity exhibited by this catalyst in the
gram-scale hydrogenation of crotonaldehyde (92% conversion and 90% selectivity;
initial TOF ¼ 12,600 h
À1 ). This synergy between Ir NPs and ReOx species significantly increased the selectivity, keeping the characteristic high activity of noble
metals. Another interesting synergistic effect was observed by Liao et al. on bimetallic PdIr NPs supported on mesoporous silica nanoparticles (PdIr/MSN) prepared
by an impregnation-hydrogenation method [129]. They observed that the incorporation of Ir on Pd NPs significantly enhances the activity and selectivity of the
catalyst in the hydrogenation of nitrobenzene to aniline. Here, there is an evident
promoting effect, since bimetallic NPs with a molar Ir/Pd ratio of 0.1 (PdIr0.1/MSN)
present an activity up to 28 and 8 times higher than those containing monometallic
Ir/MSN and Pd/MSN, respectively (Fig. 22b). The promotional effect of Ir was
explained by two factors: (1) an increase in the number of surface-active sites,
consequence of a small NP size, and (2) a modification of the electronic properties
of the MNP surface, which intensifies the electronic interactions. Liang et al. also
observed a significant promotion effect of Ir in the selective hydrogenation of phenol
to cyclohexanone over bimetallic PtIr NPs supported on hollow mesoporous silica
spheres (HMSs) [130]. They generated the bimetallic PtIr on HMSs with different
metal molar ratios (Ir/Pt: 0.5, 0.2 or 0.1) following a simple impregnation-reduction
method. All catalysts showed similar selectivities towards cyclohexanone (80–85%),
but different conversions were obtained depending on the amount of Ir incorporated.
The most active catalyst was the one with a Ir/Pt molar ratio of 0.1 (PtIr0.1), with
60% of conversion (8.6 times higher than the monometallic Pt/HMS; Fig. 22c, left)
and good levels of stability (4 recycle times; Fig. 22c, right). Here, the positive effect
of the Ir is attributed to the enhancement of the metal dispersion (higher number of
active sites) and the modulation of the NP electronic state (influence the MNP
catalytic behaviour). Another example about a synergistic effect was reported by
Ding et al. [131], in which ultrasmall silica-supported bimetallic NPs (Pd-Ir/SiO 2 ,
Pd-Pt/SiO 2 , Pd-Au/SiO 2 , Pt-Ir/SiO 2 and Ru-Pt/SiO2) were active in the selective
Iridium Nanoparticles for Hydrogenation Reactions
439