catalytic activity of a nanocluster by designed incorporation (doping) of an impurity, demonstrated here by the catalyzed oxidation of CO on Au 3 Sr/MgO(F 5c ). The
LDOS spectra projected onto the oxygen molecule, the Sr atom, and the Au 3 part of
the metal cluster, are displayed in Figures 17.6e, 17.6f, and 17.6g, respectively.
Doping by a single impurity atom changes significantly the bonding and activation
of O 2 compared to the pure gold tetramer, Au 4 , case; the bonding of O 2 is mainly
to the strontium atom of the Au 3 Sr cluster (Figure 17.7h), and it is characterized by
a substantially higher adsorption energy (1.94 eV compared to 0.18 eV for the configuration shown in Figure 17.7g) and a significant activation of the OaO bond that
is reflected in an increased bond length of 1.37 A ˚ . This activation is due to occupation of the spin-down 2p
à ? oxygen orbital (compare Figures 17.6e and 17.6c
where, in the latter that corresponds to the pure cluster, this state contributes to
the peak just above E F ), resulting in a superoxo-like state of the adsorbate. Bonding
of the oxygen molecule to Au 3 Sr/MgO(F 5c ) occurs via resonances formed between
the Sr states in the energy intervals 5–6 eV and 0–1 eV below E F , with the spin-up
1p k and 2p
à ? states, as well as with the spin-down 1p ? and 2p
à ? orbitals, of the
adsorbed activated O 2 .
Changing catalytic selectivity atom-by-atom: polymerization of acetylene on palladium
nanocatalysts Another important factor in catalysis is the selectivity of a catalytic reaction. So far, however, information on the atom-by-atom evolution of this
astonishing catalytic selectivity is still lacking. In this example we illustrate such
a size-dependent selectivity with the polymerization of acetylene on palladium
nanocatalysts [49]. This reaction over supported Pd particles reveals a direct correspondence between reactivities observed on model systems and the behavior of industrial catalysts under working conditions [68]. In ultra high vacuum (UHV) [69]
as well as under high pressure, large palladium particles of typically thousands of
atoms show an increased selectivity for the formation of benzene with increasing
particle size [68]. In contrast, small palladium particles of typically hundreds of
atoms are less selective for the cyclotrimerization and catalyze butadiene and butene as additional products [68].
The TPR spectra of the different products of the polymerization of acetylene
on small supported, monodispersed palladium clusters are shown in Figure 17.8.
Striking atom-by-atom size-dependent reactivities and selectivities are observed.
Only the three reaction products C 6 H 6 , C 4 H 8 , and C 4 H 6 are detected. Remarkably,
no C 3 H n , C 5 H n and C 8 H n are formed, indicating the absence of CaC bond scission, as already observed on Pd single crystals [70] and Pd particles [68]. Up to Pd 3 ,
only benzene is catalyzed, reflecting a high selectivity for the cyclotrimerization of
acetylene. Pd n (4 a n a 6) clusters reveal a second reaction channel by catalyzing
in addition the formation of C 4 H 6 , which desorbs at around 300 K. The third reaction product, C 4 H 8 , desorbing at a rather low temperature of 200 K, is clearly
observed for Pd 8 . For this cluster size the abundance of the three reaction products
is similar. For even larger clusters (13 a n a 30) the formation of C 6 H 6 increases
with cluster size, whereas the conversion of acetylene to C 4 H 8 reaches a maximum
for Pd 20 . Note that Pd 30 selectively suppresses the formation of C 4 H 6 . (The peak in
17 Nanocatalysis
574
LDOS spectra projected onto the oxygen molecule, the Sr atom, and the Au 3 part of
the metal cluster, are displayed in Figures 17.6e, 17.6f, and 17.6g, respectively.
Doping by a single impurity atom changes significantly the bonding and activation
of O 2 compared to the pure gold tetramer, Au 4 , case; the bonding of O 2 is mainly
to the strontium atom of the Au 3 Sr cluster (Figure 17.7h), and it is characterized by
a substantially higher adsorption energy (1.94 eV compared to 0.18 eV for the configuration shown in Figure 17.7g) and a significant activation of the OaO bond that
is reflected in an increased bond length of 1.37 A ˚ . This activation is due to occupation of the spin-down 2p
à ? oxygen orbital (compare Figures 17.6e and 17.6c
where, in the latter that corresponds to the pure cluster, this state contributes to
the peak just above E F ), resulting in a superoxo-like state of the adsorbate. Bonding
of the oxygen molecule to Au 3 Sr/MgO(F 5c ) occurs via resonances formed between
the Sr states in the energy intervals 5–6 eV and 0–1 eV below E F , with the spin-up
1p k and 2p
à ? states, as well as with the spin-down 1p ? and 2p
à ? orbitals, of the
adsorbed activated O 2 .
Changing catalytic selectivity atom-by-atom: polymerization of acetylene on palladium
nanocatalysts Another important factor in catalysis is the selectivity of a catalytic reaction. So far, however, information on the atom-by-atom evolution of this
astonishing catalytic selectivity is still lacking. In this example we illustrate such
a size-dependent selectivity with the polymerization of acetylene on palladium
nanocatalysts [49]. This reaction over supported Pd particles reveals a direct correspondence between reactivities observed on model systems and the behavior of industrial catalysts under working conditions [68]. In ultra high vacuum (UHV) [69]
as well as under high pressure, large palladium particles of typically thousands of
atoms show an increased selectivity for the formation of benzene with increasing
particle size [68]. In contrast, small palladium particles of typically hundreds of
atoms are less selective for the cyclotrimerization and catalyze butadiene and butene as additional products [68].
The TPR spectra of the different products of the polymerization of acetylene
on small supported, monodispersed palladium clusters are shown in Figure 17.8.
Striking atom-by-atom size-dependent reactivities and selectivities are observed.
Only the three reaction products C 6 H 6 , C 4 H 8 , and C 4 H 6 are detected. Remarkably,
no C 3 H n , C 5 H n and C 8 H n are formed, indicating the absence of CaC bond scission, as already observed on Pd single crystals [70] and Pd particles [68]. Up to Pd 3 ,
only benzene is catalyzed, reflecting a high selectivity for the cyclotrimerization of
acetylene. Pd n (4 a n a 6) clusters reveal a second reaction channel by catalyzing
in addition the formation of C 4 H 6 , which desorbs at around 300 K. The third reaction product, C 4 H 8 , desorbing at a rather low temperature of 200 K, is clearly
observed for Pd 8 . For this cluster size the abundance of the three reaction products
is similar. For even larger clusters (13 a n a 30) the formation of C 6 H 6 increases
with cluster size, whereas the conversion of acetylene to C 4 H 8 reaches a maximum
for Pd 20 . Note that Pd 30 selectively suppresses the formation of C 4 H 6 . (The peak in
17 Nanocatalysis
574