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ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth StreetN.W., Washington, DC 20036Published by American Chemical Society. Copyright © American Chemical Society.However, no copyright claim is made to original U.S. Government works, or worksproduced by employees of any Commonwealth realm Crown government in the courseof their duties.PerspectiveCore-shell catalysts of metal nanoparticlecore and metal-organic-framework shellPan Hu, Joseph V Morabito, and Chia-Kuang (Frank) TsungACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs5012662 • Publication Date (Web): 07 Oct 2014Downloaded from http://pubs.acs.org on October 10, 2014Just Accepted“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are postedonline prior to technical editing, formatting for publication and author proofing. 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ACS cannot be held responsible for errorsor consequences arising from the use of information contained in these “Just Accepted” manuscripts.Core-shell catalysts of metal nanoparticle core and metal-organic-framework shell Pan Hu, Joseph V. Morabito, and Chia-Kuang Tsung* Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, USA ABSTRACT: Encapsulating well-defined nanoparticle catalysts into porous materials to form a core-shell nanostructure can enhance the durability, selectivity, or reactivity of the catalysts, and even provide additional functionalities to the catalysts. Using metal-organic frameworks (MOFs) as the encapsulating porous materials has drawn great interest recent-ly because MOFs, as a class of crystalline nanoporous materials, have well-defined pore structures and unique chemical properties. Also, the structures and properties of MOFs are tunable. In this perspective review, we examine recent pro-gress in the development of synthetic methods for metal@MOF core-shell nanostructures as catalysts. Potential direc-tions in the field are also discussed. Keywords: metal nanocrystal, nanoporous material, metal-organic frameworks, core-shell structure, catalysis. 1. Introduction Heterogeneous catalysis is at the center of many indus-trial processes such as oil refining, chemical manufactur-ing, pollution treatment, and energy conversion.1,2 Cata-lysts change the pathways of a chemical reaction, lower-ing the activation energy and accelerating the reaction rate. Transition metal particles with sizes in the nanoscale have a high percentage of undercoordinated surface at-oms and thus can catalyze different reactions. Recent progress in nanotechnology and colloidal chemistry ena-bles new approaches to rationally tune the catalytic prop-erties of metal nanoparticles by providing routes to pre-cisely engineer their structures, including the size, shape, and chemical composition.3 A new approach to enhance the performance of a catalyst is to fabricate the nanopar-ticle catalysts into a core-shell architecture. This type of core-shell nanostructure consists of inner core nanoparti-cles encapsulated by porous materials. The porous shell materials ensure the accessibility of reactant molecules to the active metal surface and can increase the durability of the catalysts, introduce size selectivity towards different molecules, tune the diffusion rate of the molecules, ma-nipulate the orientation and configuration of the surface molecules, or enrich the reactants on the catalyst surfac-es.4 Various types of metal@porous material core-shell nanostructures have been synthesized with shell materi-als of silica,5,6 carbon,7-10 metal oxides,11-16 and polymers.17 As emerging functional materials with modifiable proper-ties, core-shell nanoparticles also find use in many other fields, such as biomedicine and plasmonics. 18-37 Core-shell catalysts can be categorized based on their structural characteristics. Some of the frequently reported core-shell structures are illustrated in Scheme 1: (a) one core coated by a shell; (b) multiple cores encapsulated in a matrix particle; (c) “yolk-shell” or “bell” structures, con-sisting of a core encapsulated in a hollow shell with a void in between.12,17,38-48 Somorjai and co-workers reported the synthesis of Pt@mesoporous SiO2 core-shell structures, and the nanocatalyst exhibits excellent thermal stability in high temperature reactions.49 Matsumura, Dai and co-workers reported that Pt and Au nanoclusters encapsulat-ed into a porous carbon shell show activity in catalytic hydrogenation and reduction.8,10 Metal nanoparticles in-corporated in oxides, such as Au@ZrO2,50 Au@TiO2,51 Ag@CeO2,52 and Pd@CeO2,53 have also been used as core-shell catalysts. The success in enhancing the catalytic per-formances of these core-shell nanostructures has inspired scientists to search for more porous materials as the shell materials. Scheme 1. Various types of core-shell structures. (a) single core-shell structure; (b) multiple cores in one shell structure; (c) yolk-shell structure. 2. Metal@MOF core-shell nanostructures Metal-organic frameworks (MOFs), also known as po-rous coordination polymers (PCPs), are a class of crystal-line nanoporous materials with well-defined pore struc-tures and tunable chemical properties.54-64 MOFs consist of two main components: bridging organic linkers and inorganic secondary building units (SBUs) of metal ions or oxo-clusters. The organic linkers are ditopic or poly-topic organic ligands that can bind to metal-containing SBUs to generate crystalline framework structures with open porosity. The compositions and topologies of MOFs can be vastly varied with over 20,000 different MOFs be-ing reported in the past decades. Compared to pure inor-ganic nanoporous materials such as aluminosilicate zeo-lites, MOFs have several unique properties such as the