Abstract:

Heterogeneous catalysts are usually synthesized by the conventional wet-chemistry methods, including wet-impregnation, ion exchange, and deposition-precipitation. With the development of catalyst synthesis, great progress has been made in many industrially important catalytic processes. However, these catalytic materials often have very complex structures along with poor uniformity of active sites. Such heterogeneity of active site structures significantly decreases catalytic performance, especially in terms of selectivity, and hinders atomic-level understanding of structure-activity relationships. Moreover, loss of exposed active components by sintering or leaching under harsh reaction conditions causes considerable catalyst deactivation. It is desirable to develop a facile method to tune catalyst active site structures, as well as their local chemical environments on the atomic level, thereby facilitating reaction mechanisms understanding and rational design of catalysts with high stability.

Atomic layer deposition (ALD), a gas-phase technique for thin film growth, has emerged as an alternative method to synthesize heterogeneous catalysts. Like chemical vapor deposition (CVD), ALD relies on a sequence of molecular-level, self-limiting surface reactions between the vapors of precursor molecules and a substrate. This unique character makes it possible to deposit various catalytic materials uniformly on a high-surface-area support with nearly atomic precision. By tuning the number, sequence, and types of ALD cycles, bottom-up precise construction of catalytic architectures on a support can be achieved.

In this review, we focus on the design and synthesis of supported metal catalysts using ALD. We first review strategies developed to precisely tailor the size, composition, and structures of metal nanoparticles (NPs) using ALD. Catalytic performances of these ALD metal catalysts are also discussed and compared to conventional catalysts. We highlight synthetic strategies for synthesis of metal single-atom catalysts and bottom-up precise synthesis of dimeric metal catalysts. Their impact on catalysis is discussed. We demonstrate that metal oxide ALD on metal NPs can enhance catalytic activity, selectivity, and especially stability. In particular, we show that site-selective blocking of metal NPs with an oxide overcoat improves selectivity and contributes to an understanding of the distinct functionalities of the low-and high-coordination sites in catalytic reactions on the atomic level. Next, we discuss an effective method to construct bifunctional catalysts via precisely-controlled addition of a secondary functionality using ALD. Finally, we summarize the advantages of ALD for the advanced design and synthesis of catalysts and discuss the challenges and opportunities of scaling up ALD catalyst synthesis for practical applications.

Key words: Atomic layer deposition, Supported metal catalyst, "Bottom-up" synthesis, Single-atom catalyst, Dimeric metal catalysts, Metal-oxide interfaces, Confinement effect