Well-defined atomically dispersed metal catalysts (or single-atom catalysts) have been widely studied to fundamentally understand their catalytic mechanisms, improve the catalytic efficiency, increase the abundance of active components, enhance the catalyst utilization, and develop cost-effective catalysts to effectively reduce the usage of noble metals. Such single-atom catalysts have relatively higher selectivity and catalytic activity with maximum atom utilization due to their unique characteristics of high metal dispersion and a low-coordination environment. However, freestanding single atoms are thermodynamically unstable, such that during synthesis and catalytic reactions, they inevitably tend to agglomerate to reduce the system energy associated with their large surface areas. Therefore, developing innovative strategies to stabilize single-atom catalysts, including mass-separated soft landing, one-pot pyrolysis, co-precipitation, impregnation, atomic layer deposition, and organometallic complexation, is critically needed. Many types of supporting materials, including polymers, have been commonly used to stabilize single atoms in these fabrication techniques. Herein, we review the stabilization strategies of single-atom catalyst, including different synthesis methods, specific metals and carriers, specific catalytic reactions, and their advantages and disadvantages. In particular, this review focuses on the application of polymers in the synthesis and stabilization of single-atom catalysts, including their functions as carriers for metal single atoms, synthetic templates, encapsulation agents, and protection agents during the fabrication process. The technical challenges that are currently faced by single-atom catalysts are summarized, and perspectives related to future research directions including catalytic mechanisms, enhancement of the catalyst loading content, and large-scale implementation are proposed to realize their practical applications.

Graphical Abstract

Single-atom catalysts are characterized by high metal dispersibility, weak coordination environments, high catalytic activity and selectivity, and the highest atom utilization. However, due to the free energy of the large surface area, individual atoms are usually unstable and are prone to agglomeration during synthesis and catalytic reactions. Therefore, researchers have developed innovative strategies, such as soft sedimentation, one-pot pyrolysis, coprecipitation, impregnation, step reduction, atomic layer precipitation, and organometallic complexation, to stabilize single-atom catalysts in practical applications. This article summarizes the stabilization strategies for single-atom catalysts from the aspects of their synthesis methods, metal and support types, catalytic reaction types, and its advantages and disadvantages. The focus is on the application of polymers in the preparation and stabilization of single-atom catalysts, including metal single-atom carriers, synthetic templates, encapsulation agents, and the role of polymers as protection agents in the manufacturing process. The main feature of polymers and polymer-derived materials is that they usually contain abundant heteroatoms, such as N, that possess lone-pair electrons. These lone-pair electrons can anchor the single metal atom through strong coordination interactions. The coordination environment of the lone-pair electrons can facilitate the formation of single-atom catalysts because they can enlarge the average distance of a single precursor adsorbed on the polymer matrix. Polymers with nitrogen groups are favorable candidates for dispersing active single atoms by weakening the tendency of metal aggregation and redistributing the charge densities around single atoms to enhance the catalytic performance. This review provides a summary and analysis of the current technical challenges faced by single-atom catalysts and future research directions, such as the catalytic mechanism of single-atom catalysts, sufficiently high loading, and large-scale implementation.

中文翻译：

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用于稳定能量存储和转换的单原子催化剂的高级策略

定义明确的原子分散金属催化剂（或单原子催化剂）已被广泛研究，以从根本上了解其催化机理，提高催化效率，增加活性组分的丰度，提高催化剂利用率，并开发具有成本效益的催化剂以有效地减少贵金属的使用。这种单原子催化剂具有较高的金属分散性和低配位环境的独特特性，具有较高的选择性和催化活性，原子利用率最大。然而，独立的单个原子在热力学上是不稳定的，因此在合成和催化反应过程中，它们不可避免地倾向于聚集以降低与其大表面积相关的系统能量。所以，迫切需要开发创新策略来稳定单原子催化剂，包括质量分离软着陆、一锅热解、共沉淀、浸渍、原子层沉积和有机金属络合。在这些制造技术中，许多类型的支撑材料，包括聚合物，通常用于稳定单个原子。在此，我们回顾了单原子催化剂的稳定化策略，包括不同的合成方法、特定的金属和载体、特定的催化反应及其优缺点。特别是，本综述重点介绍了聚合物在单原子催化剂合成和稳定化中的应用，包括它们作为金属单原子载体、合成模板、包封剂、和制造过程中的保护剂。总结了单原子催化剂目前面临的技术挑战，并提出了与未来研究方向相关的展望，包括催化机理、催化剂负载量的提高和大规模实施，以实现其实际应用。

图形概要

单原子催化剂具有金属分散性高、配位环境弱、催化活性和选择性高、原子利用率高等特点。然而，由于大表面积的自由能，单个原子通常不稳定，在合成和催化反应过程中容易发生团聚。因此，研究人员开发了创新策略，如软沉降、一锅热解、共沉淀、浸渍、阶梯还原、原子层沉淀和有机金属络合，以稳定实际应用中的单原子催化剂。本文从单原子催化剂的合成方法、金属和载体类型、催化反应类型及其优缺点等方面总结了单原子催化剂的稳定化策略。重点是聚合物在单原子催化剂制备和稳定化方面的应用，包括金属单原子载体、合成模板、包封剂，以及聚合物在制造过程中作为保护剂的作用。聚合物和聚合物衍生材料的主要特征是它们通常含有丰富的杂原子，例如 N，这些杂原子具有孤对电子。这些孤对电子可以通过强配位相互作用锚定单个金属原子。孤对电子的配位环境可以促进单原子催化剂的形成，因为它们可以扩大吸附在聚合物基质上的单个前体的平均距离。具有氮基团的聚合物是分散活性单原子的有利候选者，通过减弱金属聚集的趋势和重新分布单原子周围的电荷密度以提高催化性能。本综述总结和分析了当前单原子催化剂面临的技术挑战和未来的研究方向，例如单原子催化剂的催化机理、足够高的负载量和大规模实施。