Department of Chemical Engineering, University of California, Davis, California 95616, USA.
Faraday Discuss. 2018 Sep 3;208(0):9-33. doi: 10.1039/c8fd00076j.
Molecular metal complexes on supports have drawn wide attention as catalysts offering new properties and opportunities for precise synthesis to make uniform catalytic species that can be understood in depth. Here we highlight advances in research with catalysts that are a step more complex than those incorporating single, isolated metal atoms on supports. These more complex catalysts consist of supported noble metal clusters and supported metal oxide clusters, and our emphasis is placed on some of the simplest and best-defined of these catalysts, made by precise synthesis, usually with organometallic precursors. Characterization of these catalysts by spectroscopic, microscopic, and theoretical methods is leading to rapid progress in fundamental understanding of catalyst structure and function, and to expansion of this class of materials. The simplest supported metal clusters incorporate two metal atoms each-they are pair-site catalysts. These and clusters containing several metal atoms have reactivities determined by the metal nuclearity, the ligands on the metal, and the supports, which themselves are ligands. Metal oxide clusters are also included in the discussion presented here, with Zr6O8 clusters that are nodes in metal-organic frameworks being among those that are understood the best. The surface and catalytic chemistries of these metal oxide clusters are distinct from those of bulk zirconia. A challenge in using any supported cluster catalysts is associated with their possible sintering, and recent research shows how metal nanoparticles can be encapsulated in sheaths with well-defined porous structures-zeolites-that make them highly resistant to sintering.
负载型金属分子配合物作为催化剂备受关注，它们提供了新的性质和精确合成的机会，可用于制造均匀的催化物种，从而能够深入理解这些物种。在此，我们重点介绍了比负载单原子金属更为复杂的催化剂的研究进展。这些更复杂的催化剂由负载型贵金属簇和负载型金属氧化物簇组成，我们的重点是其中一些最简单、定义最明确的催化剂，这些催化剂通常是通过精确合成制备的，使用有机金属前体。通过光谱、微观和理论方法对这些催化剂进行表征，正在快速推进对催化剂结构和功能的基础理解，并扩展了这类材料的应用。最简单的负载型金属簇每个簇含有两个金属原子——它们是双位催化剂。这些含有几个金属原子的簇的反应性由金属核、金属上的配体以及本身也是配体的载体决定。这里呈现的讨论也包括金属氧化物簇，其中最好理解的是作为金属有机骨架节点的 Zr6O8 簇。这些金属氧化物簇的表面和催化化学性质与体相氧化锆不同。使用任何负载型簇催化剂的一个挑战都与它们可能的烧结有关，最近的研究表明，如何可以将金属纳米颗粒封装在具有明确定义的多孔结构（沸石）的护套中，从而使其具有很强的抗烧结性。
