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13 Metal Cluster Catalysis
13.3 Catalysis of Pt Clusters
Platinum-based heterogeneous catalysts are critical to many important commercial
and industrial chemical processes. One of the aims in heterogeneous catalysis is to
gain a better understanding of the catalytic behaviors of the supported metal clusters
hence to optimize the efficiency and selectivity of industrial catalysts. However, there
still exists rare direct experimental proof that model catalysts consisted of such small
clusters indeed reveal such variations as size-dependence in the catalytic activity.
Promising investigations toward such a capability have been attained by noting the
size-dependent adsorption of small molecules on gas-phase clusters [112–114]. In
particular, extensive results shed light on the sound catalysis of platinum clusters on
various reactions [115–129], among which the catalytic oxidation of CO is known
as one of the most important catalytic reactions. It was notable that, although the
conversion of CO and O 2 into CO 2 in the gas phase is thermodynamically allowed and
has a free enthalpy of −280 kJ/mol [130], the activation energy for the dissociation of
O 2 has to be overcome, which calls on necessary catalysts to initiate such reactions.
Previous investigations have shown that, on highly coordinated Pt(111) singlecrystal surfaces CO can be oxidized with the transfer of oxygen atoms produced
during the dissociation process of O 2 molecules on the platinum surface and hence
no additional activation for the actual oxidation step is needed [131–133]. Note that
this mechanism is sensitive to the temperature and dependent on the character of the
reactive sites in such systems [130, 134, 135]. Further studies on Pt(100) and Pt(110)
surfaces revealed a complicated modification of the overall CO oxidation ascribed
to the adsorption-induced changes of the surface structure, leading to oscillations
of the steady-state rate of catalytic CO oxidation on clean Pt surfaces [136]. The
strong dependence on surface structure of Pt is well interpreted through the studies
on a size dependence of free Pt n clusters [130, 137]. Figure 13.7 presents such
an investigation in Heiz group with a focus on the catalytic oxidation of CO on
monodispersed platinum clusters. The very small clusters consisting only of a few
atoms show pronounced size effects in their catalytic behavior, due to the changing
coordination number in different geometric structures and/or the altered electronic
structures as a function of cluster size [130].
In detail, to obtain identical conditions for the study of the catalytic reactivity of
the different Pt clusters, they first exposed the prepared model catalysts by a calibrated molecular beam doser at 90 K to an average of 20
18 O 2 molecules per Pt
atom, which indicated saturation coverage on the clusters; and then they exposed
the system to the same amount of
12 C
16 O to exclude possible influence on the reactivity by different ratio of the reactant molecules [130, 131, 134, 135, 138]. In the
temperature-programmed reaction (TPR) experiment, they detected the isotopically
labelled CO 2 molecules which are catalytically produced on the cluster surfaces.
As results, the catalytic action was given by integrating the TPR signal of the CO 2
molecules and normalizing to the number of Pt clusters. Figure 13.7A shows the
TPR spectra for the CO oxidation on supported Pt n (8 ≤ n ≤ 20) clusters, where each
cluster size reveals different oxidation temperatures (peaks labelled with α, β 1 , and
13 Metal Cluster Catalysis
13.3 Catalysis of Pt Clusters
Platinum-based heterogeneous catalysts are critical to many important commercial
and industrial chemical processes. One of the aims in heterogeneous catalysis is to
gain a better understanding of the catalytic behaviors of the supported metal clusters
hence to optimize the efficiency and selectivity of industrial catalysts. However, there
still exists rare direct experimental proof that model catalysts consisted of such small
clusters indeed reveal such variations as size-dependence in the catalytic activity.
Promising investigations toward such a capability have been attained by noting the
size-dependent adsorption of small molecules on gas-phase clusters [112–114]. In
particular, extensive results shed light on the sound catalysis of platinum clusters on
various reactions [115–129], among which the catalytic oxidation of CO is known
as one of the most important catalytic reactions. It was notable that, although the
conversion of CO and O 2 into CO 2 in the gas phase is thermodynamically allowed and
has a free enthalpy of −280 kJ/mol [130], the activation energy for the dissociation of
O 2 has to be overcome, which calls on necessary catalysts to initiate such reactions.
Previous investigations have shown that, on highly coordinated Pt(111) singlecrystal surfaces CO can be oxidized with the transfer of oxygen atoms produced
during the dissociation process of O 2 molecules on the platinum surface and hence
no additional activation for the actual oxidation step is needed [131–133]. Note that
this mechanism is sensitive to the temperature and dependent on the character of the
reactive sites in such systems [130, 134, 135]. Further studies on Pt(100) and Pt(110)
surfaces revealed a complicated modification of the overall CO oxidation ascribed
to the adsorption-induced changes of the surface structure, leading to oscillations
of the steady-state rate of catalytic CO oxidation on clean Pt surfaces [136]. The
strong dependence on surface structure of Pt is well interpreted through the studies
on a size dependence of free Pt n clusters [130, 137]. Figure 13.7 presents such
an investigation in Heiz group with a focus on the catalytic oxidation of CO on
monodispersed platinum clusters. The very small clusters consisting only of a few
atoms show pronounced size effects in their catalytic behavior, due to the changing
coordination number in different geometric structures and/or the altered electronic
structures as a function of cluster size [130].
In detail, to obtain identical conditions for the study of the catalytic reactivity of
the different Pt clusters, they first exposed the prepared model catalysts by a calibrated molecular beam doser at 90 K to an average of 20
18 O 2 molecules per Pt
atom, which indicated saturation coverage on the clusters; and then they exposed
the system to the same amount of
12 C
16 O to exclude possible influence on the reactivity by different ratio of the reactant molecules [130, 131, 134, 135, 138]. In the
temperature-programmed reaction (TPR) experiment, they detected the isotopically
labelled CO 2 molecules which are catalytically produced on the cluster surfaces.
As results, the catalytic action was given by integrating the TPR signal of the CO 2
molecules and normalizing to the number of Pt clusters. Figure 13.7A shows the
TPR spectra for the CO oxidation on supported Pt n (8 ≤ n ≤ 20) clusters, where each
cluster size reveals different oxidation temperatures (peaks labelled with α, β 1 , and