sites, O 4c and O 3c , respectively, the Pd atom binds slightly more strongly with an
energy of 1.2–1.5 eV, in addition, the atom is more reactive. However, on both O 3c
and O 4c sites the third C 2 H 2 molecule is only weakly bound or even not bound to
the Pd(C 4 H 4 ) surface complex, with the binding energy less than the activation
energy of the formation of C 6 H 6 . Thus, Pd atoms adsorbed on O nc sites cannot
explain the observed activity. This is consistent with the results of the study on the
adsorption properties of CO on Pd 1 /MgO [48], where the experimental results
were rationalized in terms of Pd atoms, which are stabilized on oxygen vacancies
(F centers) in neutral or charged states, F or F
þ , respectively. The interaction of
a Pd atom with the F center is much stronger, 3.4 eV, which makes these centers
good candidates for Pd binding. On F
þ centers binding energies of about 2 eV
have been computed [48]. The presence of trapped electrons at the defect site results in a more efficient activation of the supported Pd atom. In fact, the complex
(C 4 H 4 )(C 2 H 2 )/Pd 1 /F 5c , Figure 17.4a, shows a large distortion and a strong interFig. 17.4. (a) Temperature programmed
reaction spectra of C 6 H 6 formed on Ag, Pd,
and Rh atoms deposited on defect-rich MgO
thin films grown on Mo(100) surfaces. As
comparison the same experiment was
performed on a clean defect-rich MgO film.
The calculated transition state of the cyclotrimerization reaction on Pd atoms adsorbed
on an F center of the MgO(100) surface is also
shown. (b) TPR spectra of CO 2 from a defectrich MgO thin film and deposited Pd atoms
after exposure to O 2 and CO (lower and upper
spectrum) as well as from deposited Pd atoms
after exposure to CO and O 2 . Calculated transition states of the two observed mechanisms
(I) and (II) are also depicted.
17 Nanocatalysis
564
energy of 1.2–1.5 eV, in addition, the atom is more reactive. However, on both O 3c
and O 4c sites the third C 2 H 2 molecule is only weakly bound or even not bound to
the Pd(C 4 H 4 ) surface complex, with the binding energy less than the activation
energy of the formation of C 6 H 6 . Thus, Pd atoms adsorbed on O nc sites cannot
explain the observed activity. This is consistent with the results of the study on the
adsorption properties of CO on Pd 1 /MgO [48], where the experimental results
were rationalized in terms of Pd atoms, which are stabilized on oxygen vacancies
(F centers) in neutral or charged states, F or F
þ , respectively. The interaction of
a Pd atom with the F center is much stronger, 3.4 eV, which makes these centers
good candidates for Pd binding. On F
þ centers binding energies of about 2 eV
have been computed [48]. The presence of trapped electrons at the defect site results in a more efficient activation of the supported Pd atom. In fact, the complex
(C 4 H 4 )(C 2 H 2 )/Pd 1 /F 5c , Figure 17.4a, shows a large distortion and a strong interFig. 17.4. (a) Temperature programmed
reaction spectra of C 6 H 6 formed on Ag, Pd,
and Rh atoms deposited on defect-rich MgO
thin films grown on Mo(100) surfaces. As
comparison the same experiment was
performed on a clean defect-rich MgO film.
The calculated transition state of the cyclotrimerization reaction on Pd atoms adsorbed
on an F center of the MgO(100) surface is also
shown. (b) TPR spectra of CO 2 from a defectrich MgO thin film and deposited Pd atoms
after exposure to O 2 and CO (lower and upper
spectrum) as well as from deposited Pd atoms
after exposure to CO and O 2 . Calculated transition states of the two observed mechanisms
(I) and (II) are also depicted.
17 Nanocatalysis
564