intermediate, which promotes the selective, outer-sphere hydrogen transfer to the
carbonyl group and explains the high activity/selectivity observed with this catalyst.
6.2.6 Ir NPs Supported on Al 2 O 3
Another metal oxide support extensively used for MNP immobilisation is alumina or
Al 2 O 3 , which can be found in various forms including alpha (α-Al 2 O 3 ) and gamma
(γ-Al 2 O 3 ). In addition to a high chemical and hydrothermal stability, its high surface
area and acid-base properties make it a useful support for heterogeneous catalysis. In
2010, Finke et al. carried out kinetic and mechanistic studies about the formation of
Ir NPs on γ-Al 2 O 3 (Ir(0) n /γ-Al 2 O 3 ) from an oxide-supported metal complex, based
on [Ir(1,5-COD)Cl] 2 on γ-Al 2 O 3 (Ir(1,5-COD)Cl/γ-Al 2 O 3 ) [118]. They found that
that the NP formation followed a two-step mechanism based on a slow continuous
nucleation and a fast autocatalytic surface growth. In fact, Finke et al. used the
hydrogenation of cyclohexene to cyclohexane as a reporter reaction, being able to
monitor the formation of Ir NPs (2.9 Æ 0.4 nm) during the synthesis (acetone, 22
C,
3 bar H 2 and 1.700 equiv. of cyclohexene). The resulting catalysts, Ir(0)n/γ-Al2O,
showed a higher activity and longer lifetime than previous catalysts reported until the
date based on Ir NPs supported on alumina. In the same year, Piccolo et al.
investigated the decomposition of Ir(acac) 3 (acac ¼ acetylacetonate) impregnated
on silica-alumina (40 wt% silica, ASA) by TEM, in situ XRD and
thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS)
[119]. Here, the heating treatments under oxidative (O 2 ), reductive (H 2 ) and inert
conditions (Ar) were compared. After the calcination under air of Ir(acac) 3 on ASA,
large and not very well-distributed IrO 2 NPs (8 Æ 4 nm) were formed, the acac
ligands having been burnt to CO 2 and H 2 O. However, under reductive conditions
small Ir NPs (1.4 Æ 0.2 nm) are formed, while the acac ligand is decomposed
(hydrogenolysis) and hydrogenated to alkanes and water. Lastly, thermal treatment
under Ar was not able to completely remove acac ligands from the MNP surface. In a
consecutive work, the same group employed Ir/ASA for the tetralin hydroconversion
in the presence of H 2 S as an active, robust and thiotolerant catalyst [120].
The behaviour of Ir NPs supported on alumina and silica in the enantioselective
hydrogenation of ethyl pyruvate was studied by Bachiller-Baeza et al. in 2013
[121]. More specifically, they generated Ir NPs on silica and alumina by two
different synthetic methods (incipient wetness impregnation and flame spray pyrolysis) and tested these catalysts in the ethyl pyruvate hydrogenation using
cinchonidine as chiral modifier. Although these catalysts showed good catalytic
activities, they only exhibit moderate enantioselectivities to (R)-ethyl lactate [enantiomeric excess: 26% (Ir/SiO 2 ) and 15% (Ir/Al 2 O 3 ); Fig. 17]. However, interesting
differences depending on the preparation method and the support were found. It was
observed that the choice of support is more important than the preparation method,
since the metal-support interactions strongly influence the surface structures of the Ir
NPs (i.e. size, morphology and electron density). According to this, the best catalyst
turned out to be Ir/SiO 2 prepared by the wetness impregnation method.
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L. M. Martínez-Prieto et al.
carbonyl group and explains the high activity/selectivity observed with this catalyst.
6.2.6 Ir NPs Supported on Al 2 O 3
Another metal oxide support extensively used for MNP immobilisation is alumina or
Al 2 O 3 , which can be found in various forms including alpha (α-Al 2 O 3 ) and gamma
(γ-Al 2 O 3 ). In addition to a high chemical and hydrothermal stability, its high surface
area and acid-base properties make it a useful support for heterogeneous catalysis. In
2010, Finke et al. carried out kinetic and mechanistic studies about the formation of
Ir NPs on γ-Al 2 O 3 (Ir(0) n /γ-Al 2 O 3 ) from an oxide-supported metal complex, based
on [Ir(1,5-COD)Cl] 2 on γ-Al 2 O 3 (Ir(1,5-COD)Cl/γ-Al 2 O 3 ) [118]. They found that
that the NP formation followed a two-step mechanism based on a slow continuous
nucleation and a fast autocatalytic surface growth. In fact, Finke et al. used the
hydrogenation of cyclohexene to cyclohexane as a reporter reaction, being able to
monitor the formation of Ir NPs (2.9 Æ 0.4 nm) during the synthesis (acetone, 22
C,
3 bar H 2 and 1.700 equiv. of cyclohexene). The resulting catalysts, Ir(0)n/γ-Al2O,
showed a higher activity and longer lifetime than previous catalysts reported until the
date based on Ir NPs supported on alumina. In the same year, Piccolo et al.
investigated the decomposition of Ir(acac) 3 (acac ¼ acetylacetonate) impregnated
on silica-alumina (40 wt% silica, ASA) by TEM, in situ XRD and
thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS)
[119]. Here, the heating treatments under oxidative (O 2 ), reductive (H 2 ) and inert
conditions (Ar) were compared. After the calcination under air of Ir(acac) 3 on ASA,
large and not very well-distributed IrO 2 NPs (8 Æ 4 nm) were formed, the acac
ligands having been burnt to CO 2 and H 2 O. However, under reductive conditions
small Ir NPs (1.4 Æ 0.2 nm) are formed, while the acac ligand is decomposed
(hydrogenolysis) and hydrogenated to alkanes and water. Lastly, thermal treatment
under Ar was not able to completely remove acac ligands from the MNP surface. In a
consecutive work, the same group employed Ir/ASA for the tetralin hydroconversion
in the presence of H 2 S as an active, robust and thiotolerant catalyst [120].
The behaviour of Ir NPs supported on alumina and silica in the enantioselective
hydrogenation of ethyl pyruvate was studied by Bachiller-Baeza et al. in 2013
[121]. More specifically, they generated Ir NPs on silica and alumina by two
different synthetic methods (incipient wetness impregnation and flame spray pyrolysis) and tested these catalysts in the ethyl pyruvate hydrogenation using
cinchonidine as chiral modifier. Although these catalysts showed good catalytic
activities, they only exhibit moderate enantioselectivities to (R)-ethyl lactate [enantiomeric excess: 26% (Ir/SiO 2 ) and 15% (Ir/Al 2 O 3 ); Fig. 17]. However, interesting
differences depending on the preparation method and the support were found. It was
observed that the choice of support is more important than the preparation method,
since the metal-support interactions strongly influence the surface structures of the Ir
NPs (i.e. size, morphology and electron density). According to this, the best catalyst
turned out to be Ir/SiO 2 prepared by the wetness impregnation method.
434
L. M. Martínez-Prieto et al.