were totally different. While as-synthesised Ir/TiO 2 showed a very low conversion
and poor selectivity towards the corresponding alcohol (26% selectivity and 1%
conversion after 3 h reaction), pre-reduced Ir/TiO 2 greatly enhanced the selective
hydrogenation of the carbonyl group achieving a selectivity towards geraniol of
around ca. 90% at a conversion of 26% (3 h reaction). The enhancement of both
activity and selectivity is probably due to an increase in the ratio of Ir(0)/Ir
δ+ after the
reduction treatment.
In 2016, Hagelin-Weaver, Bowers et al. investigated the effects on metal-support
interactions on the pairwise selective parahydrogen addition over Ir/TiO 2 and the
subsequent hydrogenation of propene [108]. Comparing Ir/TiO 2 with Ir supported in
other non-reducible oxides such as SiO 2 and Al 2 O 3 , they found that after a reduction
treatment of Ir/TiO 2 at 500
C, the pairwise selectivity increases up to 20 times. The
strong metal-support interaction (SMSI) of the partially reduced titania, together
with the presence of flat-shaped Ir NPs and Cl
À ions, is responsible for this increase
in the pairwise selectivity. Toledo-Antonio et al. also studied the influence of metalsupport interactions on Ir NPs over TiO 2 but using this time the hydrogenation of
cyclohexene as model reaction [109]. In particular, they generated Ir NPs on anatase
TiO 2 nanoparticles and anatase TiO 2 nanotubes via wet impregnation of H 2 IrCl 6
followed by a calcination (400
C under air, 4 h) and hydrogenation steps (400
C
under H 2 , 2 h). 1.3 nm half truncated cuboctahedral nanoparticles were generated on
TiO 2 nanoparticles, while full 1.4 nm cuboctahedral particles were obtained on TiO 2
nanotubes. The differences in morphology and metal-support interactions resulted in
different catalytic performances. At 0
C, Ir NPs on TiO 2 nanotubes showed a
slightly higher activity than the truncated ones on TiO 2 nanoparticles (TOFs: 4.7
Vs 3.2 s
À1 ). The lower activity of Ir truncated NPs was explained by their poorer
metallic character due to a different metal-support interaction. Therefore, the TiO 2
crystal planes that interact with Ir NPs strongly influence both the morphology and
the catalytic activity of the resulting catalysts. The same group also generated
bimetallic (IrPt) NPs on TiO 2 nanotubes (NT) via wet impregnation of H 2 IrCl 6
and H 2 PtCl 6 . The catalytic activity of these bimetallic materials was checked in
the cyclohexene disproportionation reaction [110]. In general, the bimetallic catalyst
was more active in cyclohexene disproportionation reaction than the corresponding
monometallic ones. More specifically, under 150
C cyclohexene is hydrogenated to
cyclohexane, whereas above 200
C, cyclohexene is dehydrogenated to benzene
(Fig. 11). However, Ir does not only promote Pt catalysts. In 2014 Li et al. reported
how Ir is also able to promote Au/TiO 2 catalysts for the selective hydrogenation of
cinnamaldehyde [111]. Monometallic Ir/TiO 2 is more active than Au/TiO 2 but much
less selective towards the formation of cinnamyl alcohol (Fig. 12a). The bimetallic
catalysts Au-Ir/TiO 2 , however, showed a hydrogenation rate five times higher than
Au-Ir/TiO 2 but a similar selectivity to the alcohol (84%). Therefore, the incorporation of Ir on Au-Ir/TiO 2 increases the activity while maintaining the selectivity of
Au. This synergic effect was explained by substrate-metal interactions. They proposed that cinnamaldehyde is preferentially adsorbed on bimetallic NPs through the
carbonyl groups, since carbonyl carbon is adsorbed on gold atoms (negatively
428
L. M. Martínez-Prieto et al.
and poor selectivity towards the corresponding alcohol (26% selectivity and 1%
conversion after 3 h reaction), pre-reduced Ir/TiO 2 greatly enhanced the selective
hydrogenation of the carbonyl group achieving a selectivity towards geraniol of
around ca. 90% at a conversion of 26% (3 h reaction). The enhancement of both
activity and selectivity is probably due to an increase in the ratio of Ir(0)/Ir
δ+ after the
reduction treatment.
In 2016, Hagelin-Weaver, Bowers et al. investigated the effects on metal-support
interactions on the pairwise selective parahydrogen addition over Ir/TiO 2 and the
subsequent hydrogenation of propene [108]. Comparing Ir/TiO 2 with Ir supported in
other non-reducible oxides such as SiO 2 and Al 2 O 3 , they found that after a reduction
treatment of Ir/TiO 2 at 500
C, the pairwise selectivity increases up to 20 times. The
strong metal-support interaction (SMSI) of the partially reduced titania, together
with the presence of flat-shaped Ir NPs and Cl
À ions, is responsible for this increase
in the pairwise selectivity. Toledo-Antonio et al. also studied the influence of metalsupport interactions on Ir NPs over TiO 2 but using this time the hydrogenation of
cyclohexene as model reaction [109]. In particular, they generated Ir NPs on anatase
TiO 2 nanoparticles and anatase TiO 2 nanotubes via wet impregnation of H 2 IrCl 6
followed by a calcination (400
C under air, 4 h) and hydrogenation steps (400
C
under H 2 , 2 h). 1.3 nm half truncated cuboctahedral nanoparticles were generated on
TiO 2 nanoparticles, while full 1.4 nm cuboctahedral particles were obtained on TiO 2
nanotubes. The differences in morphology and metal-support interactions resulted in
different catalytic performances. At 0
C, Ir NPs on TiO 2 nanotubes showed a
slightly higher activity than the truncated ones on TiO 2 nanoparticles (TOFs: 4.7
Vs 3.2 s
À1 ). The lower activity of Ir truncated NPs was explained by their poorer
metallic character due to a different metal-support interaction. Therefore, the TiO 2
crystal planes that interact with Ir NPs strongly influence both the morphology and
the catalytic activity of the resulting catalysts. The same group also generated
bimetallic (IrPt) NPs on TiO 2 nanotubes (NT) via wet impregnation of H 2 IrCl 6
and H 2 PtCl 6 . The catalytic activity of these bimetallic materials was checked in
the cyclohexene disproportionation reaction [110]. In general, the bimetallic catalyst
was more active in cyclohexene disproportionation reaction than the corresponding
monometallic ones. More specifically, under 150
C cyclohexene is hydrogenated to
cyclohexane, whereas above 200
C, cyclohexene is dehydrogenated to benzene
(Fig. 11). However, Ir does not only promote Pt catalysts. In 2014 Li et al. reported
how Ir is also able to promote Au/TiO 2 catalysts for the selective hydrogenation of
cinnamaldehyde [111]. Monometallic Ir/TiO 2 is more active than Au/TiO 2 but much
less selective towards the formation of cinnamyl alcohol (Fig. 12a). The bimetallic
catalysts Au-Ir/TiO 2 , however, showed a hydrogenation rate five times higher than
Au-Ir/TiO 2 but a similar selectivity to the alcohol (84%). Therefore, the incorporation of Ir on Au-Ir/TiO 2 increases the activity while maintaining the selectivity of
Au. This synergic effect was explained by substrate-metal interactions. They proposed that cinnamaldehyde is preferentially adsorbed on bimetallic NPs through the
carbonyl groups, since carbonyl carbon is adsorbed on gold atoms (negatively
428
L. M. Martínez-Prieto et al.