10 Progress in the Selective Semi-hydrogenation of Alkynes …
327
the availability of hydrides at both the surface and the subsurface, and restricts the
formation of Pd ensembles at the surface [134, 145]. Other reports addressed the
poisoning effect of CO in the reduction of active sites and therefore the activity
as a function of the reaction conditions (T and P) [9]. Additional care must be
taken, when considering the use of CO in bimetallic formulations due to deep metal
reorganization. For instance, Lopez et al. investigated the effect of CO on PdM alloys
(M = Cu, Ag, Au, Zn, Ga, Sn, Pb and Bi) by DFT calculations and concluded that
none of the studied cases evidenced satisfactory resistance against metal segregation
under high CO pressures [9]. Other reports have addressed also the evaluation of
alkali, sulphur and amine compounds as modifiers in this reaction [144].
10.5.4 Effect of the Support
Crespo-Quesada et al. thoroughly discussed the role of the support at meso/microlevel
in the selective hydrogenation of alkynes [10]. Although the main function of the
carrier is to assure the metal dispersion and prevent the metal sintering during catalysis, depending on the synthetic strategy, it might highly influence properties such
as the metal distribution, particle size, which in turn would affect the catalytic
performance [146].
Furthermore, textural or electronic properties of the support have an impact on
important phenomena such as the diffusion of reaction molecules from the media to
the active sites or charge transfer processes which concerns the adsorption modes
at the metal surface. Under special conditions, the support could even modify the
structure of the active phase. For instance, depending on the reducibility of the
support, thermal treatments might induce the migration of reduced species towards
the metal surface, giving rise to what is known as strong metal-support interactions
(SMSI). Such a phenomenon might result in site blocking or the generation of new
bimetallic phases [144, 147].
The presence of specific surface functionalities within the support can be induced
by means of pre-treatment steps. For example, treating carbon-based supports with
strong acids (e.g. HNO 3 , HCl), bases (e.g. NaOH) or oxidizing agents (e.g. H 2 O 2 ,
ozone), can cause the formation of oxygenated functionalities, which in turn might
influence the dispersion or the reactivity of the metallic phase [148]. The following
paragraphs address the effect of the support on the reactivity of nanocatalysts prepared
by colloidal techniques in the semi-hydrogenation of alkynes.
In relation with the SMSI, Semagina et al. studied the effect of performing reductive thermal treatments on a Pd/ZnO/SMFs (sintered metal fibres) catalyst prior the
semi-hydrogenation of 2-methyl-3-buten-2-ol (MBE) [149]. Superior alkene selectivity was observed for the reduced catalyst under hydrogen at 773 K (95% at full
conversion) when compared to a Pd/Al 2 O 3 catalyst. This observation was attributed
to the formation of a bimetallic PdZn phase during the high-temperature reductive treatment with enhanced thermodynamic selectivity. More recently, Wang et al.
reported the preparation of a Pd-In/In 2 O 3 catalyst by an impregnation method [150].
327
the availability of hydrides at both the surface and the subsurface, and restricts the
formation of Pd ensembles at the surface [134, 145]. Other reports addressed the
poisoning effect of CO in the reduction of active sites and therefore the activity
as a function of the reaction conditions (T and P) [9]. Additional care must be
taken, when considering the use of CO in bimetallic formulations due to deep metal
reorganization. For instance, Lopez et al. investigated the effect of CO on PdM alloys
(M = Cu, Ag, Au, Zn, Ga, Sn, Pb and Bi) by DFT calculations and concluded that
none of the studied cases evidenced satisfactory resistance against metal segregation
under high CO pressures [9]. Other reports have addressed also the evaluation of
alkali, sulphur and amine compounds as modifiers in this reaction [144].
10.5.4 Effect of the Support
Crespo-Quesada et al. thoroughly discussed the role of the support at meso/microlevel
in the selective hydrogenation of alkynes [10]. Although the main function of the
carrier is to assure the metal dispersion and prevent the metal sintering during catalysis, depending on the synthetic strategy, it might highly influence properties such
as the metal distribution, particle size, which in turn would affect the catalytic
performance [146].
Furthermore, textural or electronic properties of the support have an impact on
important phenomena such as the diffusion of reaction molecules from the media to
the active sites or charge transfer processes which concerns the adsorption modes
at the metal surface. Under special conditions, the support could even modify the
structure of the active phase. For instance, depending on the reducibility of the
support, thermal treatments might induce the migration of reduced species towards
the metal surface, giving rise to what is known as strong metal-support interactions
(SMSI). Such a phenomenon might result in site blocking or the generation of new
bimetallic phases [144, 147].
The presence of specific surface functionalities within the support can be induced
by means of pre-treatment steps. For example, treating carbon-based supports with
strong acids (e.g. HNO 3 , HCl), bases (e.g. NaOH) or oxidizing agents (e.g. H 2 O 2 ,
ozone), can cause the formation of oxygenated functionalities, which in turn might
influence the dispersion or the reactivity of the metallic phase [148]. The following
paragraphs address the effect of the support on the reactivity of nanocatalysts prepared
by colloidal techniques in the semi-hydrogenation of alkynes.
In relation with the SMSI, Semagina et al. studied the effect of performing reductive thermal treatments on a Pd/ZnO/SMFs (sintered metal fibres) catalyst prior the
semi-hydrogenation of 2-methyl-3-buten-2-ol (MBE) [149]. Superior alkene selectivity was observed for the reduced catalyst under hydrogen at 773 K (95% at full
conversion) when compared to a Pd/Al 2 O 3 catalyst. This observation was attributed
to the formation of a bimetallic PdZn phase during the high-temperature reductive treatment with enhanced thermodynamic selectivity. More recently, Wang et al.
reported the preparation of a Pd-In/In 2 O 3 catalyst by an impregnation method [150].