98
7 The Reactions with Monoxides for Pollution Removal
the O 2 molecule and hence catalysts are generally used to overcome a large energy
barrier. Finding more efficient and selective catalyst materials with lower operating
temperatures for the oxidation of CO is beneficial. Recent years researchers have
found that iron oxide clusters are important alternative species to effect the oxidation
of CO at low temperature without the activation of strong O–O bonds [21]. Iron
oxides are very practical in pollution abatement applications as they are abundant
and inexpensive, and hence attracted reasonable research interest [23–35].
7.2 Transition Metal Clusters React with CO
Probing the unique size-dependent properties of small metal clusters and metal oxide
clusters, CO-chemisorbed metal clusters, and especially their reactivity and catalysis
towards CO has attracted extensive interest in this field [36–61].
7.2.1 Cobalt Clusters React with CO
Cobalt metal is a useful catalyst in many industrial processes, especially in C 1 chemistry or Fisher-Tropsch synthesis which refers to a collection of chemical reactions
that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons
[62, 63]. It was also noted that cobalt is the best metal catalyst for the conversion of
methane into large hydrocarbon molecules [43]. It is of potential importance to study
the reactivity of cobalt clusters, so as to provide insights into the micro-processes
which occur at the catalytic surface of cobalt metal, for instance, what kind or size of
cobalt particles are expected to display greater catalytic activity [64, 65]. The reactivity of anionic cobalt clusters with CO has been studied by Kapiloff and Ervin [66],
who noted the sequential addition of CO to the cobalt cluster anions leading to saturated species Co 2 (CO) 7
− , Co 3 (CO) 10
− , Co 4 (CO) 12
− , Co 5 (CO) 13
− , and Co 6 (CO) 15
−
for which skeletal structures were proposed in accordance with electron-counting
rules, as shown in Figs. 7.1 and 7.2. Different numbers of CO molecules add to the
clusters but do not break the C-O bonds, and almost no fragmentation of the metal
cores. The DFT calculations showed that there are small CO-adsorption energies
which could be partially removed by collisions with the buffer gas before reaction or
cluster fragmentation. It is worth mentioning that, comparing with smaller clusters,
the larger clusters produced greater fragmentation versus addition products.
An investigation by Guo et al. [43] reported the reactions of mass-selected clusters
Co n
+ (n = 2–8) with CO in the gas phase using a selected ion drift tube affixed with
a laser vaporization source (SIDT-LV) operated under well-defined thermal conditions. All reactions for “CO + Co n
+ (n = 2–8)” were found to be association reactions
although their absolute rate constants displayed a strong dependence on cluster size.
Among these cobalt clusters, Co 4
+ and Co 5
+ display a higher reactivity toward the
7 The Reactions with Monoxides for Pollution Removal
the O 2 molecule and hence catalysts are generally used to overcome a large energy
barrier. Finding more efficient and selective catalyst materials with lower operating
temperatures for the oxidation of CO is beneficial. Recent years researchers have
found that iron oxide clusters are important alternative species to effect the oxidation
of CO at low temperature without the activation of strong O–O bonds [21]. Iron
oxides are very practical in pollution abatement applications as they are abundant
and inexpensive, and hence attracted reasonable research interest [23–35].
7.2 Transition Metal Clusters React with CO
Probing the unique size-dependent properties of small metal clusters and metal oxide
clusters, CO-chemisorbed metal clusters, and especially their reactivity and catalysis
towards CO has attracted extensive interest in this field [36–61].
7.2.1 Cobalt Clusters React with CO
Cobalt metal is a useful catalyst in many industrial processes, especially in C 1 chemistry or Fisher-Tropsch synthesis which refers to a collection of chemical reactions
that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons
[62, 63]. It was also noted that cobalt is the best metal catalyst for the conversion of
methane into large hydrocarbon molecules [43]. It is of potential importance to study
the reactivity of cobalt clusters, so as to provide insights into the micro-processes
which occur at the catalytic surface of cobalt metal, for instance, what kind or size of
cobalt particles are expected to display greater catalytic activity [64, 65]. The reactivity of anionic cobalt clusters with CO has been studied by Kapiloff and Ervin [66],
who noted the sequential addition of CO to the cobalt cluster anions leading to saturated species Co 2 (CO) 7
− , Co 3 (CO) 10
− , Co 4 (CO) 12
− , Co 5 (CO) 13
− , and Co 6 (CO) 15
−
for which skeletal structures were proposed in accordance with electron-counting
rules, as shown in Figs. 7.1 and 7.2. Different numbers of CO molecules add to the
clusters but do not break the C-O bonds, and almost no fragmentation of the metal
cores. The DFT calculations showed that there are small CO-adsorption energies
which could be partially removed by collisions with the buffer gas before reaction or
cluster fragmentation. It is worth mentioning that, comparing with smaller clusters,
the larger clusters produced greater fragmentation versus addition products.
An investigation by Guo et al. [43] reported the reactions of mass-selected clusters
Co n
+ (n = 2–8) with CO in the gas phase using a selected ion drift tube affixed with
a laser vaporization source (SIDT-LV) operated under well-defined thermal conditions. All reactions for “CO + Co n
+ (n = 2–8)” were found to be association reactions
although their absolute rate constants displayed a strong dependence on cluster size.
Among these cobalt clusters, Co 4
+ and Co 5
+ display a higher reactivity toward the