Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article The preparation of Ag/ZIF-8@ZIF-67 core-shell composites as excellent catalyst for degradation of the nitroaromatic compounds Aotian Gu a , Jiuyu Chen a , Qianhong Gao a , Muhammad Musaa Khan a , Peng Wang a , Yan Jiao b , Zongxiang Zhang c , Ying Liu a,b , Yi Yang a,b, ⁎ a Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China b Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Nanjing University of Information Science & Technology, Nanjing 210044, China c Jiangsu Environmental Protection Key Laboratory of Monitoring for Organic Pollutants in Soil, Taizhou Environmental Monitoring Center, Jiangsu, Taizhou 225300, China ARTICLE INFO Keywords: Ag/ZIF-8@ZIF-67 catalyst Nitroaromatic compounds Core-shell ZIFs MNP@MOF composite Superior catalytic activity ABSTRACT The high effective Ag/ZIF-8@ZIF-67 core-shell composite was fabricated by a convenient strategy and applied in the reduction of 4-nitrophenol. The obtained samples were fully characterized by transmission electron mi- crocopy (TEM), element mapping, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and the like. Catalytic experiments showed that the Ag/ZIF-8@ZIF-67 exhibit superior catalytic activity in the reduction of 4-nitrophenol and another two nitroaromatic compounds. The apparent rate constant (K app ) and the conversion frequency (TOF) of Ag/ZIF-8@ZIF-67 0.4 for 4-nitrophenol were 12.22 × 10 -3 s -1 and 5.611 min -1 , respectively, and preserved high catalytic performance after four cycles. The catalytic activity of Ag/ZIF-8@ZIF- 67 was significantly increased compared to Ag/ZIF-8, which was mainly attributed to the high specific surface area of ZIF-8@ZIF-67, the synergy of ZIF-8 core and ZIF-67 shell and the restriction of pores to Ag NPs. 1. Introduction Metal-organic frameworks (MOFs) are the most attractive porous materials having unique properties such as, adjustable size of the pores, exceptional thermal and chemical robustness under different condi- tions, and high specific surface area [1–3]. Because of these unique advantages, MOFs are widely applied in many fields [4–7]. In recent research, MOFs were also considered to be particularly promising supports for metal nanoparticles and in some respects were superior to other porous materials such as zeolites, silica and clay [8–13]. Among the MOF-immobilized metal nanoparticle (MNP) materials, the MNP@ MOF formed by encapsulating MNPs in the pores of MOF is a special type of composite material [14–16]. The confinement of the MOFs gives MNP@MOF high catalytic efficiency while still maintaining high cata- lytic activity after multiple catalytic cycles. In addition, the tunable pores of the MOFs that encapsulate these metal nanoparticles also provide controlled selectivity for MNP@MOF [17]. Although the re- search on MNP@MOF has shown very encouraging results, multi-metal MOFs may be able to further improve the performance of MNP@MOF. Recently, multi-metal MOFs have gradually shown great potential in many applications due to their excellent physical and chemical prop- erties [18–20]. As an important subclass of MOFs, multi-metal ZIFs have also attracted widespread attention. For instance, Pan et al. used the ZIF-8@ZIF-67 framework as a precursor to synthesize a CoP/ NCNHP nanostructure by a pyrolysis-oxidation-phosphorization strategy. The novel hybrid nanostructure still exhibits excellent stability after almost 36 h of water splitting and hardly produces potential decay [21]. After the thermal decomposition of core-shell ZIF-8@ZIF-67, the composite materials with graphitic carbon shell and nitrogen-doped carbon core were first synthesized by Tang et al. The composite mate- rial combined the advantages of these two carbon perfectly, and ex- hibited outstanding electrochemical performance [22]. Li et al. also used a similar ZIF-8@ZIF-67 structure as a template to prepare Zn/Co- ZIF with a controlled hollow structure. Pd@H-Zn/Co-ZIF prepared based on this structure has superior activity and selectivity in semi- hydrogenation of acetylene [23]. A multi-metal ZIFs that reported by Kaur et al. achieved excellent CO 2 and H 2 uptakes by adjusting the Zn/ Co ratio [24]. The synthesis of controllable ZIF-8@ZIF-67 and https://doi.org/10.1016/j.apsusc.2020.146160 Received 23 October 2019; Received in revised form 16 March 2020; Accepted 18 March 2020 ⁎ Corresponding author at: Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China. E-mail address: yangyi@njust.edu.cn (Y. Yang). Applied Surface Science 516 (2020) 146160 Available online 19 March 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved. T