Heterogeneous Catalysts. Группа авторов
final contemporary example is the use of cleverly designed Au25‐loaded BaLa4Ti4O15 water splitting photocatalyst with the cluster‐based active sites protected by chromium oxide shell for enhanced activity and stability [77]. The chromium oxide shell is impermeable to O2 but permeable to H+, thus allowing the distinction of active sites for the evolutions of H2 (by photoelectron reduction) and O2 (by photohole oxidation). This resulted in a 19‐fold improvement in performance and excellent longevity of the catalyst due to the prevention of gold cluster sintering.
Mixed metal clusters offer the advantages of synergistic action and cooperativity by two different metals [53]; a recent highly comprehensive review covers the advances in the use of heterometallic cluster‐based catalysts [78]. As an example to illustrate that “each atom counts,” a series of chemically synthesized metal cluster catalysts were made using a family of clusters differing by an atom or two from each other – Ru6Sn, Ru5Pt, Ru5PtGe, and Ru5PtSn [36]. The obtained materials were tested (Figure 5.7) for the single‐step conversion of dimethyl terephthalate (DMT) into cyclohexanedimethanol (CHDM). The Ru5PtSn‐based catalyst demonstrated superior activity, as evidenced by the highest conversion of the tested catalysts, as well as excellent selectivity toward the desired product CHDM. The only substantial by‐product is dimethyl hexahydroterephthalate (DMHT), which is an intermediate of the hydrogenation pathway. Further optimization of the catalyst design and catalytic test conditions could result in even better performance. For a detailed and critical update on specific examples of a wide range of cluster‐based catalysts and their performance in various catalytic processes, curious readers are referred to a monumental recent review by Liu and Corma [79].
Figure 5.7 Bar chart comparing the activity and selectivity of the Ru5PtSn catalyst with those of bi‐ and trimetallic analogues for the hydrogenation of dimethyl terephthalate.
Source: Hungria et al. 2006 [36]. Reproduced with permission of John Wiley & Sons.
5.5 Conclusion
This chapter has covered atomically precise metal clusters made using physical (UHV) and chemical approaches. Catalysts containing cluster‐like species as active sites with respect to their size regime (e.g. sub‐3 nm) can be made using conventional methods, but in this case, it is rare that atomic precision is reliably achieved as a range of cluster sizes can be formed since such methods lack precise control over particle size. Deposition–precipitation using heated solutions containing urea, which gradually decomposes, releasing ammonia that slowly increases the pH, seems to be promising method for making supported metal clusters [80].
One interesting area of making cluster‐based catalysts is focused on fabrication of clusters in porous materials, such as zeolites [81] and metal–organic framework materials [82] (see Chapters 7 and 8). The idea is that growth of the clusters (starting from simple mono‐atomic precursors) is confined by the size of the cavities available. Although the chemistry of clusters in zeolites is well developed with an additional bonus of clusters interacting with H+ in forming metal cluster‐proton adducts acting as “collapsed bifunctional sites” capable of both acid and redox catalysis [81], true atomic precision of such species can be hard to achieve. Recently, the [Cu3(μ‐O)3]2+ cluster in zeolites attracted significant attention owing to the interesting catalytic chemistry in the conversion of methane to methanol [83, 84]. Yet, there are studies that demonstrate that the presence of larger CuOx particles (up to 3 nm) in such catalysts could also be active [85, 86].
Clusters supported within zeolites can undergo dynamic particle changes during catalysis, as was demonstrated by detailed high‐resolution electron microscopy study of Pt clusters in zeolite MCM‐22 [87]. In fact, it may be questionable if zeolite frameworks can truly limit cluster growth since it was demonstrated that the formation of truly large NPs within zeolite crystals does damage zeolite frameworks. Interestingly, such damage coincided with increased catalytic performance, which was interpreted as improvement of the mass transport‐limited reaction (by overcoming the narrow pores of the zeolite framework) [88]. Finally, clusters assembled in metal–organic framework materials could potentially become new high‐intensity research area due to molecular‐based nature of the framework complementing atomic precision of metal clusters [82]. There is evidence of promising high stability of such catalysts [82]. Ultimately, the control over cluster assembly with the help of functional groups built within a framework could be achieved with clever design of the framework.
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