Atomic Lego Catalysts Synthesized by Atomic Layer Deposition

ConspectusThe catalytic performance of heterogeneous catalysts in terms of activity, selectivity, and stability depends greatly on their structure, involving characteristics such as particle size, composition, and morphology. Achieving atomic-level-precise control of these structures is essential to understanding structure–activity relations and to achieving selective catalysis at high activity. However, conventional methods of catalyst synthesis including impregnation, ion exchange, and precipitation often lack precise control and produce inhomogeneous active-site structures, thus leading to poor catalytic performance in many cases.Atomic layer deposition (ALD) as an alternative synthetic method from the gas phase has recently drawn great attention. Its unique character of alternative self-limiting molecular surface reactions enables ALD to deposit catalytic materials on high-surface-area powder substrates with not only high uniformity and but also high precision. Importantly, with the aid of selective deposition, it appears to be possible to construct catalytic nanostructures on the surface of powder substrates atom-by-atom from the bottom up, analogous to Lego building blocks. These well-defined ALD Lego catalysts hold promise for deepening the fundamental understanding of structure–activity relationships and practical applications.In this Account, I discuss the following aspects of the recent developments of ALD Lego catalysts in our laboratory: (i) the synthesis of single-atom catalysts (SACs) by anchoring atoms separately on substrates using metal ALD, wherein the types of nucleation sites and the deposition temperatures are adjusted to avoid metal atom aggregation; (ii) the atom-controlled synthesis of subnanometer cluster catalysts by the selective deposition of additional metal atoms on the host metal atoms of SACs; (iii) the synthesis of bimetallic nanoparticle (NP) catalysts by the selective deposition of additional metal atoms on host metal NPs, where the size, composition, and structure of metal NPs are precisely tuned by varying the number and sequence of metal ALD cycles; and (iv) the oxide decoration of metal NPs by applying oxide ALD on metal catalysts, where oxide blocks can be constructed on the surface of metal NPs precisely from atomically dispersed species to oxide clusters, and to continuous oxide overlayers. Such controlled oxide decoration using ALD enables the optimization of metal/oxide interfaces and balancing the trade-off between the stability and accessibility of metal NPs for high catalytic performance. In some cases, we show that ALD oxide blocks might nucleate preferentially at the low- or high-coordination sites of metal NPs to tune the catalyst selectivity. In brief, the above Lego catalysts manifest the advances of ALD in catalyst synthesis and greatly facilitate the atomic-level understanding of structure–activity relationships in a number of catalytic reactions. Finally, the rapid growth of ALD Lego catalyst families might bring about opportunities in industrial applications in the near future.