straint) a rather low energy barrier DE b (LHt) ¼ 0.1 eV occurring at d(CO 1 ) A 2.0 A ˚
is determined for the LHt oxidation channel with the end product of a weakly adsorbed carbon dioxide molecule (@0.2 eV). The total exothermicity of the reaction
MgO(F 5c )/Au 8 þ O 2 (g) þ CO(g) ! MgO(F 5c )/Au 8 O þ CO 2 (a) is @4.8 eV and it is
@5 eV when the CO 2 product is desorbed, here (g) and (a) denote gas phase and
adsorbed molecules, respectively. This LHt low-temperature oxidation mechanism
was found by these studies (with similar energetics) for reactions on the gold
cluster deposited on either a defect-free MgO(100) surface or one containing an
F 5c , and both are expected to be relatively insensitive to the Au n cluster size, correlating with the experimental results. As aforementioned, the higher temperature
channel is strongly enhanced for Au 8 supported on a defect-rich MgO(100) support
(compare the CO 2 signals at @240 K in Figures 17.5A-d,e). This trend correlates
with the simulations of a LH-periphery (LHp) reaction mechanism, starting with
the CO adsorbed on the top-facet of the Au 8 cluster and the peroxo O 2
à molecule
bonded to the periphery of the interfacial layer of the cluster (Figure 17.5B-f/g;
initial state), where the distance between the C atom and the oxygen marked O 1 is
d(CO 1 ) ¼ 4.49 A ˚ ). Indeed, mapping of the potential energy surface along the CaO 1
reaction coordinate revealed, for MgO(F 5c )/Au 8 , a rather broad reaction barrier
DE b (LHp) @ 0.5 eV at d(CO 1 ) @ 2.0 A ˚ ) (Figure 17.5B-g), while for the defect-free
substrate a significantly higher barrier was found, DE b (LHp) @ 0.8 eV; the reaction
product is shown in Figure 17.5B-h for the MgO(F 5c )/Au 8 catalyst. Another reaction path was found theoretically, starting from the optimal configuration of O 2
Ã
adsorbed on the MgO(F 5c )/Au 8 catalyst. A gas phase CO molecule brought to the
vicinity of the peroxo molecule reacts spontaneously (without an energy barrier)
forming a carbon dioxide molecule weakly bound to the catalyst. This Eley–Rideal
(ER)-type reaction mechanism corresponds to a low temperature generation of
(weakly adsorbed) CO 2 through direct reaction of gaseous CO with a preadsorbed
peroxo O 2
à molecule. This reaction channel can occur even at 90 K, i.e. during the
initial dosing stage in the experiment.
The catalytic behavior can be further changed by cluster size and impurity doping. To understand the origin of this size-dependent and element-specific behavior
the atomic structure and the electronic spectra of these model catalysts were
studied by extensive first-principles simulations, and the optimized structures of
Au 8 (with two relevant isomers), Au 4 , and Au 3 Sr adsorbed on MgO(F 5c ) are shown
in Figure 17.7, before (a to d) and after (e to h) O 2 adsorption. As shown above
these clusters bind to the MgO(F 5c ) surface quite strongly (calculated binding energy of 2.65 eV to 4.06 eV), and their binding is significantly enhanced (typically by
about 2 eV) due to the F center defects. These high binding energies and the ‘‘anchoring’’ effect by the F centers correlate well with the observed thermal stability of
the supported clusters in the TPR experiments. Another important finding pertains to the charge state of the adsorbed clusters: in particular, our calculations
predict that the interaction with the surface is accompanied by charge transfer [14]
of 0.5 e, 0.3 e, and 0.3 e to the adsorbed Au 8 , Au 4 and Au 3 Sr clusters, respectively.
Binding and activation of molecular O 2 by the model catalyst are necessary elementary steps in the CO oxidation process [65, 66], and thus understanding the
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 571
is determined for the LHt oxidation channel with the end product of a weakly adsorbed carbon dioxide molecule (@0.2 eV). The total exothermicity of the reaction
MgO(F 5c )/Au 8 þ O 2 (g) þ CO(g) ! MgO(F 5c )/Au 8 O þ CO 2 (a) is @4.8 eV and it is
@5 eV when the CO 2 product is desorbed, here (g) and (a) denote gas phase and
adsorbed molecules, respectively. This LHt low-temperature oxidation mechanism
was found by these studies (with similar energetics) for reactions on the gold
cluster deposited on either a defect-free MgO(100) surface or one containing an
F 5c , and both are expected to be relatively insensitive to the Au n cluster size, correlating with the experimental results. As aforementioned, the higher temperature
channel is strongly enhanced for Au 8 supported on a defect-rich MgO(100) support
(compare the CO 2 signals at @240 K in Figures 17.5A-d,e). This trend correlates
with the simulations of a LH-periphery (LHp) reaction mechanism, starting with
the CO adsorbed on the top-facet of the Au 8 cluster and the peroxo O 2
à molecule
bonded to the periphery of the interfacial layer of the cluster (Figure 17.5B-f/g;
initial state), where the distance between the C atom and the oxygen marked O 1 is
d(CO 1 ) ¼ 4.49 A ˚ ). Indeed, mapping of the potential energy surface along the CaO 1
reaction coordinate revealed, for MgO(F 5c )/Au 8 , a rather broad reaction barrier
DE b (LHp) @ 0.5 eV at d(CO 1 ) @ 2.0 A ˚ ) (Figure 17.5B-g), while for the defect-free
substrate a significantly higher barrier was found, DE b (LHp) @ 0.8 eV; the reaction
product is shown in Figure 17.5B-h for the MgO(F 5c )/Au 8 catalyst. Another reaction path was found theoretically, starting from the optimal configuration of O 2
Ã
adsorbed on the MgO(F 5c )/Au 8 catalyst. A gas phase CO molecule brought to the
vicinity of the peroxo molecule reacts spontaneously (without an energy barrier)
forming a carbon dioxide molecule weakly bound to the catalyst. This Eley–Rideal
(ER)-type reaction mechanism corresponds to a low temperature generation of
(weakly adsorbed) CO 2 through direct reaction of gaseous CO with a preadsorbed
peroxo O 2
à molecule. This reaction channel can occur even at 90 K, i.e. during the
initial dosing stage in the experiment.
The catalytic behavior can be further changed by cluster size and impurity doping. To understand the origin of this size-dependent and element-specific behavior
the atomic structure and the electronic spectra of these model catalysts were
studied by extensive first-principles simulations, and the optimized structures of
Au 8 (with two relevant isomers), Au 4 , and Au 3 Sr adsorbed on MgO(F 5c ) are shown
in Figure 17.7, before (a to d) and after (e to h) O 2 adsorption. As shown above
these clusters bind to the MgO(F 5c ) surface quite strongly (calculated binding energy of 2.65 eV to 4.06 eV), and their binding is significantly enhanced (typically by
about 2 eV) due to the F center defects. These high binding energies and the ‘‘anchoring’’ effect by the F centers correlate well with the observed thermal stability of
the supported clusters in the TPR experiments. Another important finding pertains to the charge state of the adsorbed clusters: in particular, our calculations
predict that the interaction with the surface is accompanied by charge transfer [14]
of 0.5 e, 0.3 e, and 0.3 e to the adsorbed Au 8 , Au 4 and Au 3 Sr clusters, respectively.
Binding and activation of molecular O 2 by the model catalyst are necessary elementary steps in the CO oxidation process [65, 66], and thus understanding the
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 571