action of the third C 2 H 2 molecule. These results indicate that F and F
þ centers can
act as basic sites on the MgO surface and turn the inactive Pd atom into an active
catalyst. Notice that the supported Pd atoms on defect sites not only activate the
cyclization reaction, but also favor benzene desorption, as shown by the very small
(C 6 H 6 )/Pd 1 /F 5c adsorption energy. The complete reaction path for this specific
nanocatalyst has been calculated [52]. The first barrier of the reaction path is that
for the formation of the intermediate Pd(C 4 H 4 ) and is only 0.48 eV. The formation
of the C 4 H 4 intermediate is thermodynamically favorable by 0.82 eV. On (C 4 H 4 )/
Pd 1 /F 5c the addition of the third acetylene molecule is exothermic by 1.17 eV,
leading to a very stable (C 4 H 4 )(C 2 H 2 )/Pd 1 /F 5c intermediate (Figure 17.4a). To
transform this intermediate into benzene one has to overcome a barrier of 0.98 eV.
The corresponding energy gain is very large, 3.99 eV, and mainly related to the aromaticity of the benzene ring. Once formed, C 6 H 6 is so weakly bound to the supported Pd atom that it immediately desorbs. Thus, the reaction on Pd/F 5c is rate
limited in the last step, the conversion of (C 4 H 4 )(C 2 H 2 ) into C 6 H 6 . This is different
from the Pd(111) surface where the rate determining step for the reaction is benzene desorption. The calculations are consistent with the experimental data. In
fact, on Pd 1 /F 5c the computed barrier of 0.98 eV corresponds to a desorption temperature of about 300 K, as observed experimentally, Figure 17.4a. On Pd(111)
surfaces, the bonding of benzene is estimated to be A1.9 eV. This binding is consistent with a desorption temperature of 500 K as observed for a low coverage of
C 6 H 6 on Pd(111) [53].
The electronic structure of palladium atoms, 4d
10 5s
0 , is unique and may be responsible for this specific catalytic property for the acetylene cyclotrimerization.
This raises the question as to whether other transition metal atoms are also reactive for this reaction. Results are shown for deposited Rh (4d
8 5s
1 ) and Ag (4d
10 5s
1 )
atoms. Ag atoms are almost unreactive (Figure 17.4a), on supported Rh atoms,
however, benzene is formed and desorbs at around 430 K (Figure 17.4a).
For CO oxidation, it was first verified that the clean MgO(100) thin films are
inert; e.g., no
13 C
16 O
8 O was formed in a one-heating-cycle experiment after adsorbing
18 O 2 and
13 C
16 O (Figure 17.4b) or vice versa [54]. When Pd atoms are
trapped on the F 5c s, preadsorption of oxygen and subsequent saturation of CO
leads to the formation of carbon dioxide, with desorption peaks at 260 K and
around 500 K (Figure 17.4b). The existence of two desorption peaks in the TPR
spectrum (Figure 17.4b) suggests the presence of two different reaction mechanisms. Note that preadsorption of
13 CO suppresses the catalytic process as it poisons the nanocatalyst (Figure 17.4b). Ab initio calculations showed that a single Pd
atom binds strongly to the oxygen vacancy (binding energy of 3.31 eV), with a
small amount of charge (0.15 e) transferred to the adsorbed atom. In comparison,
the binding energy of Pd atoms to terrace oxygen sites is only 1.16 eV. The enhanced binding to the F 5c is also reflected in the corresponding bonding lengths of
1.65 and 2.17 A ˚ for MgO(F 5c )aPd and MgOaPd, respectively. Binding of two CO
molecules saturates the MgO(F 5c )aPd system; occupying the MgO(F 5c )aPd system
with three CO molecules leads to spontaneous (barrierless) desorption of one of
the molecules. In the most stable configuration the two CO molecules are not
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 565
þ centers can
act as basic sites on the MgO surface and turn the inactive Pd atom into an active
catalyst. Notice that the supported Pd atoms on defect sites not only activate the
cyclization reaction, but also favor benzene desorption, as shown by the very small
(C 6 H 6 )/Pd 1 /F 5c adsorption energy. The complete reaction path for this specific
nanocatalyst has been calculated [52]. The first barrier of the reaction path is that
for the formation of the intermediate Pd(C 4 H 4 ) and is only 0.48 eV. The formation
of the C 4 H 4 intermediate is thermodynamically favorable by 0.82 eV. On (C 4 H 4 )/
Pd 1 /F 5c the addition of the third acetylene molecule is exothermic by 1.17 eV,
leading to a very stable (C 4 H 4 )(C 2 H 2 )/Pd 1 /F 5c intermediate (Figure 17.4a). To
transform this intermediate into benzene one has to overcome a barrier of 0.98 eV.
The corresponding energy gain is very large, 3.99 eV, and mainly related to the aromaticity of the benzene ring. Once formed, C 6 H 6 is so weakly bound to the supported Pd atom that it immediately desorbs. Thus, the reaction on Pd/F 5c is rate
limited in the last step, the conversion of (C 4 H 4 )(C 2 H 2 ) into C 6 H 6 . This is different
from the Pd(111) surface where the rate determining step for the reaction is benzene desorption. The calculations are consistent with the experimental data. In
fact, on Pd 1 /F 5c the computed barrier of 0.98 eV corresponds to a desorption temperature of about 300 K, as observed experimentally, Figure 17.4a. On Pd(111)
surfaces, the bonding of benzene is estimated to be A1.9 eV. This binding is consistent with a desorption temperature of 500 K as observed for a low coverage of
C 6 H 6 on Pd(111) [53].
The electronic structure of palladium atoms, 4d
10 5s
0 , is unique and may be responsible for this specific catalytic property for the acetylene cyclotrimerization.
This raises the question as to whether other transition metal atoms are also reactive for this reaction. Results are shown for deposited Rh (4d
8 5s
1 ) and Ag (4d
10 5s
1 )
atoms. Ag atoms are almost unreactive (Figure 17.4a), on supported Rh atoms,
however, benzene is formed and desorbs at around 430 K (Figure 17.4a).
For CO oxidation, it was first verified that the clean MgO(100) thin films are
inert; e.g., no
13 C
16 O
8 O was formed in a one-heating-cycle experiment after adsorbing
18 O 2 and
13 C
16 O (Figure 17.4b) or vice versa [54]. When Pd atoms are
trapped on the F 5c s, preadsorption of oxygen and subsequent saturation of CO
leads to the formation of carbon dioxide, with desorption peaks at 260 K and
around 500 K (Figure 17.4b). The existence of two desorption peaks in the TPR
spectrum (Figure 17.4b) suggests the presence of two different reaction mechanisms. Note that preadsorption of
13 CO suppresses the catalytic process as it poisons the nanocatalyst (Figure 17.4b). Ab initio calculations showed that a single Pd
atom binds strongly to the oxygen vacancy (binding energy of 3.31 eV), with a
small amount of charge (0.15 e) transferred to the adsorbed atom. In comparison,
the binding energy of Pd atoms to terrace oxygen sites is only 1.16 eV. The enhanced binding to the F 5c is also reflected in the corresponding bonding lengths of
1.65 and 2.17 A ˚ for MgO(F 5c )aPd and MgOaPd, respectively. Binding of two CO
molecules saturates the MgO(F 5c )aPd system; occupying the MgO(F 5c )aPd system
with three CO molecules leads to spontaneous (barrierless) desorption of one of
the molecules. In the most stable configuration the two CO molecules are not
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 565