324
J. A. Delgado and C. Godard
effects induced by the stabilisers (AOT < PVA ≈ PVP). Regarding the alkene selectivity, a certain pattern was correlated to the particles size but not to the stabilizer.
Interestingly, in view of the detrimental effect of treating the catalysts with ozone/H 2
(for the removal of the surface stabilizer), the authors suggested a positive role of the
stabilizer. However, other studies recommend the removal of the stabilizer prior to
the reaction to maximize the available active sites. For instance, a UV–ozone (UVO)
treatment was employed for the surface cleaning of PVP-stabilized Pd nanocubes
supported on carbon [132]. This time, the stabilizer removal resulted in a four-fold
increase of the activity during the semi-hydrogenation of acetylene.
Therefore, although the stabilizer can provide improvements of the catalyst performance, its selection is a critical part of the catalyst design and should consider the
target alkyne. For instance, in liquid phase, it is generally assumed that a certain
mobility of the capping agent is beneficial for the reagents to access the metal surface.
In this case, a common strategy to improve the alkene selectivity has been the use of
nitrogen-containing stabilizers. In contrast, gas–solid reactions appear highly sensitive to the presence of organic residues at the metal surface, thus involving the use
surface cleaning procedures prior to catalysis (sometimes just calcination or thermal
treatments). For gas phase applications, stabilizers provided only with easy decomposable moieties (e.g. based on C, H, O, and N) are recommended, thus avoiding the
possible blockage of active sites.
10.5 Other Parameters Affecting the Selectivity
10.5.1 Subsurface H and C
The occurrence of over-hydrogenation issues during the semi-hydrogenation of
alkynes has been commonly linked with the existence of surface or subsurface
hydrides [133]. The low activation barrier for H 2 dissociation combined with the
small size of the hydrogen atoms makes them ready to easily populate either the
surface or the subsurface of palladium catalysts [134].
Analogous versions of carbon (surface and subsurface) have been also identified
as a modifier of the performance of catalysts and particularly relevant for those used
in the semi-hydrogenation of acetylene. According to DFT calculations, subsurface
carbon displays an improved thermodynamic factor on Pd surface, which justifies the
generally observed positive impact on catalysis [134, 135]. Regarding its localization
within the NPs, Kiwi-Minsker et al. reported the favoured formation of carbides on
step sites of small particles [37]. Depending on the reaction conditions, the formation
of oligomeric hydrocarbons (C 8+ , coke) are commonly observed during the semihydrogenation of acetylene. Some authors have observed an improvement of the
alkene selectivity with the formation of such a carbon deposits on specific sites of
the metal surface [23].
J. A. Delgado and C. Godard
effects induced by the stabilisers (AOT < PVA ≈ PVP). Regarding the alkene selectivity, a certain pattern was correlated to the particles size but not to the stabilizer.
Interestingly, in view of the detrimental effect of treating the catalysts with ozone/H 2
(for the removal of the surface stabilizer), the authors suggested a positive role of the
stabilizer. However, other studies recommend the removal of the stabilizer prior to
the reaction to maximize the available active sites. For instance, a UV–ozone (UVO)
treatment was employed for the surface cleaning of PVP-stabilized Pd nanocubes
supported on carbon [132]. This time, the stabilizer removal resulted in a four-fold
increase of the activity during the semi-hydrogenation of acetylene.
Therefore, although the stabilizer can provide improvements of the catalyst performance, its selection is a critical part of the catalyst design and should consider the
target alkyne. For instance, in liquid phase, it is generally assumed that a certain
mobility of the capping agent is beneficial for the reagents to access the metal surface.
In this case, a common strategy to improve the alkene selectivity has been the use of
nitrogen-containing stabilizers. In contrast, gas–solid reactions appear highly sensitive to the presence of organic residues at the metal surface, thus involving the use
surface cleaning procedures prior to catalysis (sometimes just calcination or thermal
treatments). For gas phase applications, stabilizers provided only with easy decomposable moieties (e.g. based on C, H, O, and N) are recommended, thus avoiding the
possible blockage of active sites.
10.5 Other Parameters Affecting the Selectivity
10.5.1 Subsurface H and C
The occurrence of over-hydrogenation issues during the semi-hydrogenation of
alkynes has been commonly linked with the existence of surface or subsurface
hydrides [133]. The low activation barrier for H 2 dissociation combined with the
small size of the hydrogen atoms makes them ready to easily populate either the
surface or the subsurface of palladium catalysts [134].
Analogous versions of carbon (surface and subsurface) have been also identified
as a modifier of the performance of catalysts and particularly relevant for those used
in the semi-hydrogenation of acetylene. According to DFT calculations, subsurface
carbon displays an improved thermodynamic factor on Pd surface, which justifies the
generally observed positive impact on catalysis [134, 135]. Regarding its localization
within the NPs, Kiwi-Minsker et al. reported the favoured formation of carbides on
step sites of small particles [37]. Depending on the reaction conditions, the formation
of oligomeric hydrocarbons (C 8+ , coke) are commonly observed during the semihydrogenation of acetylene. Some authors have observed an improvement of the
alkene selectivity with the formation of such a carbon deposits on specific sites of
the metal surface [23].