The striking atom-by-atom size-dependent selectivity for the polymerization of
acetylene is summarized as follows: C 6 H 6 is catalyzed with a selectivity of 100%
on cluster sizes up to Pd 3 . The selectivity for C 4 H 6 reaches a maximum for Pd 6
(@30%) and the production of C 4 H 8 is most efficient (@40%) for Pd 20À25 . If one
assumes stoichiometric reactions, as indicated in Figure 17.9c, and estimates the
relative number of C 2 H 2 from Figure 17.9a one observes a proportional increase in
acetylene with the number of palladium atoms per cluster up to Pd 13 . Surprisingly,
at this cluster size the surface-to-bulk ratio as well as the coordination number of
Pd in the cluster changes as at this size one Pd atom sits completely in the cluster.
In addition, according to the free stoichiometric chemical reactions, each reaction
requires a minimum number of Pd atoms, which are 3, 4, and 6. The experimental
results are surprisingly consistent, that is C 4 H 6 is formed for Pd n with n b 4 and
C 4 H 8 for cluster sizes with n b 6.
Analyzing the products formed on small size-selected Pd n ð1 a n a 30Þ clusters
deposited on MgO(100) thin films indicates that the surface intermediate C 4 H 4 is
produced efficiently on all cluster sizes. Thus at least two acetylene molecules are
adsorbed in a p-bonded configuration at the initial stage of the reaction [71]. The
observed size-dependent selectivity may then be understood by regarding the influence of the cluster size to steer the reaction either towards the cyclotrimerization
to form C 6 H 6 or towards a direct hydrogen transfer from adsorbed C 2 H 2 to the
C 4 H 4 intermediate to catalyze the formation of C 4 H 6 or C 4 H 8 , respectively. Cyclotrimerization is generally observed when a third acetylene molecule is adsorbed in
a p-bonded configuration, which results in a change from sp-hybridization towards
sp
2 -hybridization [72]. This bonding configuration leads to a weak activation of the
CaH bond, in analogy to ethylene [73]. The hydrogenation of the Pd n (C 4 H 4 ) metallocycle, on the other hand, is favored by the adsorption of di-s/p bonded acetylene to three Pd atoms, effecting a more efficient activation of the CaH bond, in
analogy to ethylene [73].
For Pd atoms adsorbed on defect sites the Pd(C 4 H 4 ) intermediate is formed with
an energy gain of around 2 eV. A third adsorbed C 2 H 2 molecule is purely p-bonded
(Figure 17.4a) and the activated acetylene molecule reacts with the intermediate to
form benzene with a total exothermicity of about 7 eV. The weakly bound C 6 H 6
(0.3 eV) then desorbs at low temperature from the nanocatalyst [71]. A second reaction channel, the formation of butadiene, C 4 H 6 , opens for Pd 4 . This channel reveals highest selectivity for Pd 6 , in this case a third C 2 H 2 molecule can bind in a
di-s/p bond configuration to three Pd atoms. The charge transfer from the substrate to the cluster further enhances the activation of the CaH bonds. For even
larger cluster sizes the adsorption of two di-s/p-bonded C 2 H 2 molecules becomes
possible and opens up the third reaction path, the formation of C 4 H 8 . In our experiments this is clearly observed for Pd 8 . Purely geometrical arguments (possible
adsorption of two di-s/p-bonded C 2 H 2 molecules close to the C 4 H 4 intermediate)
suggest that this third channel is more pronounced for the larger clusters, and
indeed our results show maximal C 4 H 8 formation for cluster sizes of 20–25 Pd
atoms. For the largest clusters of the measured range, e.g. Pd 30 , the increased
number of metal–metal bonds and the concomitant delocalization of the charge
transferred from the substrate to the cluster results in less charge density available
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 577
acetylene is summarized as follows: C 6 H 6 is catalyzed with a selectivity of 100%
on cluster sizes up to Pd 3 . The selectivity for C 4 H 6 reaches a maximum for Pd 6
(@30%) and the production of C 4 H 8 is most efficient (@40%) for Pd 20À25 . If one
assumes stoichiometric reactions, as indicated in Figure 17.9c, and estimates the
relative number of C 2 H 2 from Figure 17.9a one observes a proportional increase in
acetylene with the number of palladium atoms per cluster up to Pd 13 . Surprisingly,
at this cluster size the surface-to-bulk ratio as well as the coordination number of
Pd in the cluster changes as at this size one Pd atom sits completely in the cluster.
In addition, according to the free stoichiometric chemical reactions, each reaction
requires a minimum number of Pd atoms, which are 3, 4, and 6. The experimental
results are surprisingly consistent, that is C 4 H 6 is formed for Pd n with n b 4 and
C 4 H 8 for cluster sizes with n b 6.
Analyzing the products formed on small size-selected Pd n ð1 a n a 30Þ clusters
deposited on MgO(100) thin films indicates that the surface intermediate C 4 H 4 is
produced efficiently on all cluster sizes. Thus at least two acetylene molecules are
adsorbed in a p-bonded configuration at the initial stage of the reaction [71]. The
observed size-dependent selectivity may then be understood by regarding the influence of the cluster size to steer the reaction either towards the cyclotrimerization
to form C 6 H 6 or towards a direct hydrogen transfer from adsorbed C 2 H 2 to the
C 4 H 4 intermediate to catalyze the formation of C 4 H 6 or C 4 H 8 , respectively. Cyclotrimerization is generally observed when a third acetylene molecule is adsorbed in
a p-bonded configuration, which results in a change from sp-hybridization towards
sp
2 -hybridization [72]. This bonding configuration leads to a weak activation of the
CaH bond, in analogy to ethylene [73]. The hydrogenation of the Pd n (C 4 H 4 ) metallocycle, on the other hand, is favored by the adsorption of di-s/p bonded acetylene to three Pd atoms, effecting a more efficient activation of the CaH bond, in
analogy to ethylene [73].
For Pd atoms adsorbed on defect sites the Pd(C 4 H 4 ) intermediate is formed with
an energy gain of around 2 eV. A third adsorbed C 2 H 2 molecule is purely p-bonded
(Figure 17.4a) and the activated acetylene molecule reacts with the intermediate to
form benzene with a total exothermicity of about 7 eV. The weakly bound C 6 H 6
(0.3 eV) then desorbs at low temperature from the nanocatalyst [71]. A second reaction channel, the formation of butadiene, C 4 H 6 , opens for Pd 4 . This channel reveals highest selectivity for Pd 6 , in this case a third C 2 H 2 molecule can bind in a
di-s/p bond configuration to three Pd atoms. The charge transfer from the substrate to the cluster further enhances the activation of the CaH bonds. For even
larger cluster sizes the adsorption of two di-s/p-bonded C 2 H 2 molecules becomes
possible and opens up the third reaction path, the formation of C 4 H 8 . In our experiments this is clearly observed for Pd 8 . Purely geometrical arguments (possible
adsorption of two di-s/p-bonded C 2 H 2 molecules close to the C 4 H 4 intermediate)
suggest that this third channel is more pronounced for the larger clusters, and
indeed our results show maximal C 4 H 8 formation for cluster sizes of 20–25 Pd
atoms. For the largest clusters of the measured range, e.g. Pd 30 , the increased
number of metal–metal bonds and the concomitant delocalization of the charge
transferred from the substrate to the cluster results in less charge density available
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 577