the TPR spectrum of C 4 H 6 at 200 K is part of the fragmentation pattern of C 4 H 8 .)
For Pd 20 the experiments were repeated in the presence of D 2 [49]; D 2 was exposed
prior and after C 2 H 2 . The results clearly indicate that no product containing deuterium is formed. Consequently D 2 is not involved in the polymerization reaction.
However, the presence of D 2 opens a new reaction channel, the hydrogenation of
acetylene. In addition, D 2 blocks the active sites on the palladium clusters for the
polymerization, as the formation of the products is slightly reduced when exposing
D 2 prior to C 2 H 2 . On Pd(111), pre-dosing with H 2 completely suppresses the cyclotrimerization but enhances the hydrogenation of acetylene to form ethylene
[53].
The relative ion intensities of the products in the TPR spectra scale with the
product formation by the model catalysts. By integrating the total area of the TPR
spectra shown in Figure 17.8, the number of catalytically produced benzene, butadiene and butene molecules per cluster is obtained and illustrated in Figure 17.9a.
Figure 17.9b shows the selectivities S for the formation of the products P 1 (C 6 H 6 ),
P 2 (C 4 H 6 ), and P 3 (C 4 H 8 ) for different cluster sizes calculated from, e.g.:
S(P 1 ) ¼ (number of P 1 per cluster/number of
P
P n per cluster) Ã 100 [%]
Fig. 17.9. Reactivity (a) (expressed as the
number of product molecules per cluster)
and selectivity (b) (expressed as the relative
amount in %) of the polymerization of C 2 H 2
on size-selected Pd n (n ¼ 1–30) deposited on
defect-rich MgO thin films. Also shown is the
relative number of reacted C 2 H 2 as a function
of cluster size.
17 Nanocatalysis
576
For Pd 20 the experiments were repeated in the presence of D 2 [49]; D 2 was exposed
prior and after C 2 H 2 . The results clearly indicate that no product containing deuterium is formed. Consequently D 2 is not involved in the polymerization reaction.
However, the presence of D 2 opens a new reaction channel, the hydrogenation of
acetylene. In addition, D 2 blocks the active sites on the palladium clusters for the
polymerization, as the formation of the products is slightly reduced when exposing
D 2 prior to C 2 H 2 . On Pd(111), pre-dosing with H 2 completely suppresses the cyclotrimerization but enhances the hydrogenation of acetylene to form ethylene
[53].
The relative ion intensities of the products in the TPR spectra scale with the
product formation by the model catalysts. By integrating the total area of the TPR
spectra shown in Figure 17.8, the number of catalytically produced benzene, butadiene and butene molecules per cluster is obtained and illustrated in Figure 17.9a.
Figure 17.9b shows the selectivities S for the formation of the products P 1 (C 6 H 6 ),
P 2 (C 4 H 6 ), and P 3 (C 4 H 8 ) for different cluster sizes calculated from, e.g.:
S(P 1 ) ¼ (number of P 1 per cluster/number of
P
P n per cluster) Ã 100 [%]
Fig. 17.9. Reactivity (a) (expressed as the
number of product molecules per cluster)
and selectivity (b) (expressed as the relative
amount in %) of the polymerization of C 2 H 2
on size-selected Pd n (n ¼ 1–30) deposited on
defect-rich MgO thin films. Also shown is the
relative number of reacted C 2 H 2 as a function
of cluster size.
17 Nanocatalysis
576