Strategies for Stabilizing Atomically Dispersed Metal Catalysts

Abstract

usually consist of ill-defined catalytic sites such that the reaction may undergo different catalytic pathways at different sites with different structural features. [2] The catalytic performance of a typical supported metal catalyst depends on numerous parameters, such as the composition, size, and shape of the metal nanoparticles, the nature of the supports, and the metal-support interactions, making it highly challenging to identify the catalytic site and thus simultaneously achieve high activity and high selectivity. [3] That is to say, conventional heterogeneous metal catalysts are still far away from the requirement of green chemistry, in which the reaction selectivity is crucial. Atomically dispersed metal catalysts have attracted more and more attention as they have maximal atomic utilization as well as uniform coordination environments, thus providing an ideal model for mechanistic investigation. [4] However, isolated metal atoms have extremely high surface energy so that they tend to aggregate to form nanoparticles, especially under harsh preparation or catalysis conditions. [5] During the past decades, several strategies have been developed to stabilize single-site metal catalysts on various supports. As shown in Figure 1, anchoring organometallic catalysts onto the surface of supports through organic linkers or weak interactions (e.g., hydrogen bonds and electrostatic interactions) and encapsulating organometallics in the micropores of the support represent two early approaches to immobilize singleatom metal catalysts and thus enhance their stabilities for catalysis. The metal centers of the catalysts prepared by these two methods do not have direct interactions with supports. The knowledge learned from homogeneous catalysts can be easily migrated into heterogeneous systems. Since there have been already nice review articles on heterogenized organometallic catalysts, [6] these early-studied catalysts are not included in the scope here. Readers should also refer to some excellent reviews in the literature for a more comprehensive view on atomically dispersed metal catalysts. [7] Here, we mainly discuss how to stabilize single metal atoms on supports through direct chemical interactions and focus on the following three different classes of atomically dispersed catalysts:Metal centers in this class of catalysts are co-stabilized by both supports and organic ligands. To prepare this type of catalysts, mononuclear organometallic catalysts with labile ligands are usually used as precursors to allow the direct deposition of catalytic metal atoms onto oxide supports through metal-oxygen interactions. The easy reaction between the labile ligands on the organometallic precursors and the Atomically dispersing metal atoms on supports provides an ideal strategy for maximizing metal utilization for catalysis, which is particularly important for fabricating cost-effective catalysts based on Earth-scarce metals. However, due to the high surface energy and thus instability of single atoms, it remains challenging ...

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