Metal nanoparticles have drawn great attention in heterogeneous catalysis One challenge is that they are easily deactivated by migration-coalescence during the catalysis process because of their high surface energy With the rapid development of nanoscience, encapsulating metal nanoparticles in nanoshells or nanopores becomes one of the most promising strategies to overcome the stability issue of the metal nanoparticles Besides, the activity and selectivity could be simultaneously enhanced by taking advantage of the synergy between the metal nanoparticles and the encapsulating materials as well as the molecular sieving property of the encapsulating materials In this review, we provide a comprehensive summary of the recent progress in the synthesis and catalytic properties of the encapsulated metal nanoparticles This review begins with an introduction to the synthetic strategies for encapsulating metal nanoparticles with different architectures developed to date, including their encapsulation in nanoshells of inorganic oxides and carbon, porous materials (zeolites, metal-organic frameworks, and covalent organic frameworks), and organic capsules (dendrimers and organic cages) The advantages of the encapsulated metal nanoparticles are then discussed, such as enhanced stability and recyclability, improved selectivity, strong metal-support interactions, and the capability of enabling tandem catalysis, followed by the introduction of some representative applications of the encapsulated metal nanoparticles in thermo-, photo-, and electrocatalysis At the end of this review, we discuss the remaining challenges associated with the encapsulated metal nanoparticles and provide our perspectives on the future development of the field

Encapsulated Metal Nanoparticles for Catalysis.

Nanostructured materials of diverse architecture are ubiquitous in industrial catalysis. They offer exciting prospects to tackle various sustainability challenges faced by society. Since the introduction of the concept a century ago, researchers aspire to control the chemical identity, local environment and electronic properties of active sites on catalytic surfaces to optimize their reactivity in given applications. Nowadays, numerous strategies exist to tailor these characteristics with varying levels of atomic precision. Making headway relies upon the existence of analytical approaches able to resolve relevant structural features and remains challenging due to the inherent complexity even of the simplest heterogeneous catalysts, and to dynamic effects often occurring under reaction conditions. Computational methods play a complementary and ever-increasing role in pushing forward the design. Here, we examine how nanoscale engineering can enhance the selectivity and stability of catalysts. We highlight breakthroughs towards their commercialization and identify directions to guide future research and innovation. This Review summarizes how distinct approaches to nanostructural engineering enhance features that determine the selectivity and stability of catalysts.

Nanoscale engineering of catalytic materials for sustainable technologies.

The simultaneous achievement of both high ammonia yield and Faradaic efficiency in electrochemical nitrogen reduction is a long-sought-after goal. However, due to the strong competing hydrogen evolution and extremely low solubility of N2 in aqueous systems, thermodynamic modulation at the catalyst level is insufficient, leaving the current performance still far from practical application. Here, we rationally control the diffusion of the reactants to obtain suppressed proton supply and greatly enhanced nitrogen flux using proton-filtering covalent organic frameworks, forcing a highly selective and active nitrogen reduction. In this proof-of-concept system, we achieved a high performance in the electrochemical ammonia synthesis (ammonia yield rate 287.2 ± 10.0 μg h−1 
 $${\rm{mg}}_{\rm{cat.}}^{-1}$$
 , Faradaic efficiency 54.5 ± 1.1%) using a traditional carbon-based catalyst. The proposed strategy successfully optimizes the mass transfer that greatly facilitates nitrogen reduction, providing powerful guidelines for achieving green ammonia production at a more practical level. The simultaneous achievement of both high ammonia yield and Faradaic efficiency in electrochemical nitrogen reduction is a challenging goal. Now, the diffusion of reactants to the catalyst surface is controlled using a covalent organic framework, which results in high-performance electrochemical ammonia synthesis.

Proton-filtering covalent organic frameworks with superior nitrogen penetration flux promote ambient ammonia synthesis

Encapsulation of metal nanocatalysts by support-derived materials is well known as a classical strong metal-support interaction (SMSI) effect that occurs almost exclusively with active oxide supports and often blocks metal-catalyzed surface reactions. In the present work this classical SMSI process has been surprisingly observed between metal nanoparticles, e.g., Ni, Fe, Co, and Ru, and inert hexagonal boron nitride (h-BN) nanosheets. We find that weak oxidizing gases such as CO2 and H2O induce the encapsulation of nickel (Ni) nanoparticles by ultrathin boron oxide (BOx) overlayers derived from the h-BN support (Ni@BOx/h-BN) during the dry reforming of methane (DRM) reaction. In-situ surface characterization and theory calculations reveal that surface B-O and B-OH sites in the formed BOx encapsulation overlayers work synergistically with surface Ni sites to promote the DRM process rather than blocking the surface reactions.

Reaction-Induced Strong Metal–Support Interactions between Metals and Inert Boron Nitride Nanosheets

ConspectusThe fine design and regulation of catalysts play critical roles in the development of catalysis. The microenvironment, which gives rise to unique spatial structures and electronic propert...

Microenvironment Modulation in Metal–Organic Framework-Based Catalysis

Effective catalysts stabilize specific transition states and control the transport of species to and from catalytically active sites. Enzymes show these traits thanks to their diverse amino acid functional groups encapsulating metal centres, but are limited in the reaction conditions in which they can operate. Realizing a catalyst with this kinetic and transport control that can be used under demanding industrial conditions is challenging. Here, we show a modular approach for the systematic synthesis of polymer–nanocrystal hybrids, where palladium nanocrystals are encapsulated within tunable microporous polymer layers. The polymer chemistry and morphology control the catalytic performance of the metal sites, affecting the transition state for CO oxidation and controlling the transport of CO2 away from the active site. This approach can be applied to other polymer–nanocrystal compositions and catalytic applications, and is therefore expected to have an impact in many areas of catalysis. Encapsulation is an effective strategy to tune metal-catalysed reactions, although its potential has not been fully explored. Here, design principles and advanced understanding of the reactivity of different polymer-encapsulated Pd nanocrystals are provided using CO oxidation as a benchmark reaction.

Maria Vargas

Papers

Transition state and product diffusion control by polymer–nanocrystal hybrid catalysts