that AlEt 3 species are bound to the NP surface in the vicinity of active sites, and thus
an increase in the AlEt 3 concentration hinders the access of substrate molecules.
Indeed, the catalytic activity decrease at higher Al/Ir ratios (Table 16) and free AlEt 3
was found in the [(cod)Ir(μ-O 2 C 8 H 15 )] 2 + AlEt 3 catalytic solution for Al/Ir ! 3.
However, the results of this study also showed that the stabilisers may be
non-charged aluminium alkyl carboxylates, AlEt 2 (O 2 C 8 H 15 ) or Al–O–Al containing
alkylalumoxanes generated by hydrolysis of AlEt 3 (Fig. 3).
Similarly, Finke and coworkers employed a polyoxometalate-containing organometallic precursor, [Bu 4 N] 5 Na 3 [(1,5-COD)IrÁP 2 W 15 Nb 3 O 62 ], for the preparation of
Ir NPs [88]. This precursor was reduced with H 2 in propylene carbonate as solvent,
yielding Ir(0) n clusters that are highly sensitive of synthesis conditions. Isolable and
redispersable nanoclusters with a monodisperse size of 2.1(0.3) nm were obtained at
22
C and 1.2 mM precursor concentration (Table 17). However, non-redispersable
and polydisperse NPs of 5(2) nm in size were generated at 60
C and 0.24 mM
concentration. It was found that under the latter conditions, a (1,5-COD)Ir(solvent) 2
+
species is generated, which undergoes a fast and uncontrolled reduction, and thus
leads to the formation of low-quality Ir(0) n clusters. As expected, these Ir NPs
exhibit low activity in cyclohexene hydrogenation in comparison with that shown
by the monodisperse nanoclusters (Table 17).
Table 16 Hydrogenation of cyclohexene by Ziegler-type iridium nanoparticles
Al/Ir ratio
Stability at 200
C
NP size (nm)
TTO
TOF (h
À1
)
2
No
1.4 Æ 0.7
180,000 after 52 h
3,500
3
Yes
1.5 Æ 0.5
155,000 after 144 h
1,100
4
Yes
1.7 Æ 0.4
100,000 after 150 h
700
Reaction conditions: cyclohexene/Ir ¼ 1,375, Ir (0.02 mM), cyclohexene (1.65 M), solvent
(cyclohexane), H 2 (2.76 bar), 22
C
TTO total turnover
Fig. 3 Representation of
possible stabilisers on the
surface of Ir(0) NPs
generated from [(cod)Ir
(μ-O 2 C 8 H 15 )] 2 + AlEt 3
Iridium Nanoparticles for Hydrogenation Reactions
417
an increase in the AlEt 3 concentration hinders the access of substrate molecules.
Indeed, the catalytic activity decrease at higher Al/Ir ratios (Table 16) and free AlEt 3
was found in the [(cod)Ir(μ-O 2 C 8 H 15 )] 2 + AlEt 3 catalytic solution for Al/Ir ! 3.
However, the results of this study also showed that the stabilisers may be
non-charged aluminium alkyl carboxylates, AlEt 2 (O 2 C 8 H 15 ) or Al–O–Al containing
alkylalumoxanes generated by hydrolysis of AlEt 3 (Fig. 3).
Similarly, Finke and coworkers employed a polyoxometalate-containing organometallic precursor, [Bu 4 N] 5 Na 3 [(1,5-COD)IrÁP 2 W 15 Nb 3 O 62 ], for the preparation of
Ir NPs [88]. This precursor was reduced with H 2 in propylene carbonate as solvent,
yielding Ir(0) n clusters that are highly sensitive of synthesis conditions. Isolable and
redispersable nanoclusters with a monodisperse size of 2.1(0.3) nm were obtained at
22
C and 1.2 mM precursor concentration (Table 17). However, non-redispersable
and polydisperse NPs of 5(2) nm in size were generated at 60
C and 0.24 mM
concentration. It was found that under the latter conditions, a (1,5-COD)Ir(solvent) 2
+
species is generated, which undergoes a fast and uncontrolled reduction, and thus
leads to the formation of low-quality Ir(0) n clusters. As expected, these Ir NPs
exhibit low activity in cyclohexene hydrogenation in comparison with that shown
by the monodisperse nanoclusters (Table 17).
Table 16 Hydrogenation of cyclohexene by Ziegler-type iridium nanoparticles
Al/Ir ratio
Stability at 200
C
NP size (nm)
TTO
TOF (h
À1
)
2
No
1.4 Æ 0.7
180,000 after 52 h
3,500
3
Yes
1.5 Æ 0.5
155,000 after 144 h
1,100
4
Yes
1.7 Æ 0.4
100,000 after 150 h
700
Reaction conditions: cyclohexene/Ir ¼ 1,375, Ir (0.02 mM), cyclohexene (1.65 M), solvent
(cyclohexane), H 2 (2.76 bar), 22
C
TTO total turnover
Fig. 3 Representation of
possible stabilisers on the
surface of Ir(0) NPs
generated from [(cod)Ir
(μ-O 2 C 8 H 15 )] 2 + AlEt 3
Iridium Nanoparticles for Hydrogenation Reactions
417