the two carbonate species (d) and (e). Structure (e) requires a preformed Au 2 O 2
À
where the molecular axes of Au 2 and O 2 lie parallel to each other. Since this structure of Au 2 O 2
À is 1 eV less stable than the ground state discussed above (the
binding energy of oxygen in this configuration is 0.39 eV vs. the optimal binding
energy of 1.39 eV) it is unlikely to be formed and consequently structure (e) does
not play a relevant role in the catalytic cycle. On the other hand, formation of the
most stable structure (d) by an ER mechanism with CO(g) inserting in the middle
of the OaO bond in Au 2 O 2
À has a barrier of only 0.3 eV, which is easily overcome
under the experimental conditions. The absence of the complex Au 2 C 2 O 4
À in the
experimental mass spectrum, as well as the calculated high barriers for CO diffusion along the AuaAu axis justify the considerations of only ER mechanisms for
the final reaction step Eq. (2c). It is also worth noting that further intermediates
such as Au 2 O
À and Au 2 CO 2
À are not observed experimentally. This leads to only
two possible scenarios for the final reaction step (2c): ER reactions of CO(g) with
structures (c) or (d), and the reaction proceeds to completion releasing two CO 2
molecules. The formation of CO 2 from the reaction between CO(g) and structure
(c) (Figure 17.2C) involves a low barrier of 0.3 eV, resulting in the formation of a
metastable Au 2 CO 2
À complex, where CO 2 is bound to Au 2 via the carbon atom.
However, the large heat of reaction (4.75 eV) from the formation of the first CO 2
molecule is much more than that needed to overcome the binding energy (0.52 eV)
of the remaining CO 2 to Au 2
À , readily facilitating its desorption from the metal
cluster. The second scenario involves two branches [35]. First, thermal dissociation
of CO 2 from the carbonate (d) is endothermic by 1.12 eV but produces highly reactive Au 2 O
À which reacts without barrier with CO(g) to produce CO 2 . Second, ER
reaction of CO(g) with (d) to produce CO 2 involves a modest barrier of 0.5 eV but
readily releases two CO 2 molecules since the remaining Au 2 CO 2
À configuration,
where CO 2 is bound to Au 2 via one of the oxygen atoms, is unstable. This is the
more likely branch to occur in the experiments (100–300 K) described above. Thus,
these studies showed that both experimental and theoretical results prove the existence of the full catalytic cycle.
In another example [42, 43] it was demonstrated that gas-phase platinum cluster
anions, Pt n
À (n ¼ 3–7) efficiently catalyze the oxidation of CO to CO 2 in the presence of N 2 O or O 2 near room temperature in a full thermal catalytic reaction cycle.
At the end of the process the intact cluster is regenerated and each step is exothermic and occurs rapidly at thermal energies. In these experiments the Pt n
À
clusters produced are first thermalized by collisions with a buffer gas. When either
O 2 or N 2 O is introduced downstream in the flow tube, Pt n O
À and Pt n O 2
À ions are
formed in rapid exothermic reactions [44]. With the introduction of CO into the
gas cell the Pt n
À clusters reappear, as observed with mass spectrometry. From
these experiments it was concluded that neutral CO 2 is stoichiometrically formed
on the clusters according to the scheme in Figure 17.3a and that negligible fragmentation occurs for the metal cluster with n b 4. Further experiments revealed
that, at low energies, the cross-section approaches the calculated collision limit,
meaning that the reactions are quite efficient. When Pt n O 2
À ions are selected initially, sequential loss of oxygen atoms is observed, forming two CO 2 products, as
17 Nanocatalysis
560
À
where the molecular axes of Au 2 and O 2 lie parallel to each other. Since this structure of Au 2 O 2
À is 1 eV less stable than the ground state discussed above (the
binding energy of oxygen in this configuration is 0.39 eV vs. the optimal binding
energy of 1.39 eV) it is unlikely to be formed and consequently structure (e) does
not play a relevant role in the catalytic cycle. On the other hand, formation of the
most stable structure (d) by an ER mechanism with CO(g) inserting in the middle
of the OaO bond in Au 2 O 2
À has a barrier of only 0.3 eV, which is easily overcome
under the experimental conditions. The absence of the complex Au 2 C 2 O 4
À in the
experimental mass spectrum, as well as the calculated high barriers for CO diffusion along the AuaAu axis justify the considerations of only ER mechanisms for
the final reaction step Eq. (2c). It is also worth noting that further intermediates
such as Au 2 O
À and Au 2 CO 2
À are not observed experimentally. This leads to only
two possible scenarios for the final reaction step (2c): ER reactions of CO(g) with
structures (c) or (d), and the reaction proceeds to completion releasing two CO 2
molecules. The formation of CO 2 from the reaction between CO(g) and structure
(c) (Figure 17.2C) involves a low barrier of 0.3 eV, resulting in the formation of a
metastable Au 2 CO 2
À complex, where CO 2 is bound to Au 2 via the carbon atom.
However, the large heat of reaction (4.75 eV) from the formation of the first CO 2
molecule is much more than that needed to overcome the binding energy (0.52 eV)
of the remaining CO 2 to Au 2
À , readily facilitating its desorption from the metal
cluster. The second scenario involves two branches [35]. First, thermal dissociation
of CO 2 from the carbonate (d) is endothermic by 1.12 eV but produces highly reactive Au 2 O
À which reacts without barrier with CO(g) to produce CO 2 . Second, ER
reaction of CO(g) with (d) to produce CO 2 involves a modest barrier of 0.5 eV but
readily releases two CO 2 molecules since the remaining Au 2 CO 2
À configuration,
where CO 2 is bound to Au 2 via one of the oxygen atoms, is unstable. This is the
more likely branch to occur in the experiments (100–300 K) described above. Thus,
these studies showed that both experimental and theoretical results prove the existence of the full catalytic cycle.
In another example [42, 43] it was demonstrated that gas-phase platinum cluster
anions, Pt n
À (n ¼ 3–7) efficiently catalyze the oxidation of CO to CO 2 in the presence of N 2 O or O 2 near room temperature in a full thermal catalytic reaction cycle.
At the end of the process the intact cluster is regenerated and each step is exothermic and occurs rapidly at thermal energies. In these experiments the Pt n
À
clusters produced are first thermalized by collisions with a buffer gas. When either
O 2 or N 2 O is introduced downstream in the flow tube, Pt n O
À and Pt n O 2
À ions are
formed in rapid exothermic reactions [44]. With the introduction of CO into the
gas cell the Pt n
À clusters reappear, as observed with mass spectrometry. From
these experiments it was concluded that neutral CO 2 is stoichiometrically formed
on the clusters according to the scheme in Figure 17.3a and that negligible fragmentation occurs for the metal cluster with n b 4. Further experiments revealed
that, at low energies, the cross-section approaches the calculated collision limit,
meaning that the reactions are quite efficient. When Pt n O 2
À ions are selected initially, sequential loss of oxygen atoms is observed, forming two CO 2 products, as
17 Nanocatalysis
560