Heterogeneous Catalysts. Группа авторов
ligands bound to the cluster core stabilize clusters, help to solubilize them in an appropriate solvent, and, at the same time, should be blocking reagents from accessing metal core (as space‐filled images in Figure 5.4). Removal of ligands is likely to compromise the stability of the original clusters and could lead to formation of ultrasmall clusters, which could be responsible for the observed activity. A potential solution could be the use of stable, atomically precise nanoclusters with uncoordinated sites at the metal core, of which synthesis was only very recently developed [58]. Finally, the recovery and recycling of valuable catalyst is a major problem of homogeneous catalysis; clusters could be potentially recovered by extraction if the ligand periphery is appropriately modified (charge, functional groups, etc.) [59]. In some cases, clusters could be tethered to the support by flexible molecular linkers, which would form strong bonds with the support and the cluster ligand periphery, keeping the cluster intact and flexible enough to act as homogeneous catalyst; this approach effectively heterogenizes (anchors) homogeneous cluster‐based active site, enabling easy recycling [60].
The use of metal clusters as precursors in fabrication of heterogeneous catalysts was pioneered (among others) by Ichikawa [61], Gates [62, 63], Johnson, and Thomas [60, 64–66]. Chemically synthesized, purified, and fully characterized clusters can be deposited onto various supports (activated carbon, oxides, etc.) using impregnation and adsorption methods. In most cases, researchers have excellent control over the metal weight loading (by using known desired amounts of support and cluster precursor). In such “site‐isolated” catalysts, each cluster will form a single active site at the surface of support. In this way, atomically precise control over composition of the active site could be achieved so that the nature of the cluster chosen as a precursor in catalyst fabrication will fully define the nature of the active site.
There is one important issue pertinent to this approach – activation of the catalysts. Activation of the “as‐deposited” cluster‐based catalysts requires the removal of ligands and expose the metal core surface, as well as to establish a direct chemical interaction between the cluster core and support – effectively grafting the cluster core onto the support. Such ligand removal could be justified in light of the steric protection of the metal core by the ligands (as illustrated in Figure 5.4) that prevent the access of the reagents to the catalytically active cluster metal core. Care must be taken to preserve the composition of the cluster during such activation (as opposed to fragmentation or aggregation of the clusters). Ichikawa suggested oxidation of the supported metal clusters under mild conditions, followed by the reduction of resulting oxide particles [61]. An alternative milder approach was developed by Johnson and Thomas [60, 64–66], where mild heat treatment of the supported metal carbonyl clusters under vacuum resulted in the formation of the naked cluster cores immobilized on the support. This approach is based on the understanding that an equilibrium exists between free CO and CO ligands bound to the cluster core, which could be shifted (according to Le Chatelier's principle) by choice of conditions; and the removal of free gaseous CO by pumping could be quite efficient. By drawing an analogy between homogeneous and heterogeneous catalysis, Gates highlighted the interaction between the metal cluster core and the ligands in the as‐synthesized cluster [62, 63]. In classical homogeneous catalysis, where monometallic complexes are used, both the steric and electronic factors of the ligands can be fine‐tuned to achieve optimal catalytic performance. An example of tuning steric factors in cluster‐based catalysis was mentioned above with respect to the use of clusters with chiral ligands in stereoselective homogeneous catalysis. In the case of heterogeneous catalysts, tweaking the interaction between the metal clusters and the support material presents an opportunity for tuning of the electronic factors via, for example, charge transfer between support and the cluster‐based active site.
Some supports are known to enhance catalytic activity of metal particles, and in particular, reducible supports such as titania (TiO2) and ceria (CeO2) are known to belong to this category and exhibit the so‐called strong metal–support interactions. Other supports, such as silica (SiO2) and boron nitride (BN), are known to be inert – they do not boost catalytic activity of supported particles.
Gold metal clusters on inert supports were shown to exhibit size‐dependent activity in the oxidation of styrene oxidation by air alone (in many other cases peroxides are used as oxidants or initiators) [67]. The authors used phosphine‐stabilized ∼1.5 nm Au clusters (“Au55”) deposited from solution onto inert SiO2 and BN supports and activated by heat treatment under vacuum at 200 °C. The activity in the partial oxidation of styrene (as contrary to the complete oxidation to CO2) was observed only when particle size remained below 2 nm in the activated catalysts. Noteworthy, overloading of support with the same precursor (use of 6 wt% cf. 0.6 wt%) resulted in pronounced aggregation and loss of activity, while catalysts with low loading of sub‐2 nm particles were stable and recyclable (Table 5.1). The control over the metal loading is elegantly simple in this case because the calculated amount of cluster is dissolved in the appropriate solvent (dichloromethane) and stirred with determined amount of support (BN or SiO2), followed by the removal of solvent under vacuum and thermal activation at 200 °C.
Table 5.1 Catalytic results of the partial oxidation of styrene using O2 alone for supported Au55 and comparison catalysts prepared by various techniques.
Source: Reprinted with permission from Turner et al. [67]. Copyright 2008, Springer Nature.
| Catalyst | Preparationa) | Exact loadingb) (wt%) | Mean Au sizec) (nm) | Conversion (%) | Selectivity (%) | ||
|---|---|---|---|---|---|---|---|
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| 0.6‐wt% Au55/BN | Au55 | 0.63 | 1.6 | 19.2 | 82.3 | 14.0 | 3.9 |
| 0.6‐wt% Au55/SiO2 | Au55 | 0.67 | 1.5 | 25.8 | 82.1 | 12.0 | 5.7 |
| 0.6‐wt% Au55/SiO2 | Au55 | 0.67 | — | 21.4 | 69.2 | 23.7 | 7.1 |
| Recycled 1 | |||||||
| 0.6‐wt% Au55/SiO2 |
Au55
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