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Preassembly Strategy to Single Cu-N3 Sites Inlaid Porous Hollow Carbonitride Spheres for Selective Oxidation of Benzene to Phenol Ting Zhang, Di Zhang, Xinghua Han, Ting Dong, Xinwen Guo, Chunshan Song, Rui Si, Wei Liu, Yuefeng Liu, and Zhongkui Zhao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10703 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018
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Preassembly Strategy to Single Cu-N3 Sites Inlaid Porous Hollow Carbonitride Spheres for Selective Oxidation of Benzene to Phenol Ting Zhang,† Di Zhang,† Xinghua Han,‡ Ting Dong,‡ Xinwen Guo,† Chunshan Song,†,ǁ Rui Si,*,§ Wei Liu,# Yuefeng Liu,*,# and Zhongkui Zhao*,† †
State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R. China. ‡ School of Chemical Engineering and Technology, North University of China, Taiyuan 030051, P. R. China. ǁ
EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering and Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States.
§
Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P.R. China.
#
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China.
Supporting Information ABSTRACT: Developing single-atom catalysts with porous micro-/nano-structures for high active-site accessibility is of great significance but still remains a challenge. Herein, we for the first time report a novel template-free preassembly strategy to fabricate the single Cu atoms mounted porous hollow graphitic carbonitride spheres via thermal polymerization of supramolecular preassemblies composed of melamine-Cu complex and cyanuric acid. Atomically dispersed Cu-N3 moieties were unambiguously confirmed by the spherical aberration correction electron microscopy and extended X-ray absorption fine structure spectroscopy. More importantly, it exhibits outstanding catalytic performance for selective oxidation of benzene to phenol at room temperature, especially shows extremely higher phenol selectivity (90.6 vs. 64.2%) and stability than the supported Cu nanoparticles, originating from the isolated unique Cu-N3 sites in the porous hollow structure. 86% of benzene conversion with 96.7% of unexpectedly high phenol selectivity at 60 oC for 12 h has been achieved, suggesting a great potential for practical application. This work paves a new way to fabricate variety of single-atom catalysts with diverse graphitic carbonitride architectures.
Phenol, as an industrially important chemical, has been mostly produced from benzene by three-step cumene process in industry, which leads to heavy environmental pollution.1 Direct oxidation of benzene to phenol with clean oxidants, such as oxygen or H2O2, have been considered as an ideal process.2 In this regard, graphitic carbon nitride (g-C3N4), also called as graphitic carbonitride, was found to be active for selective oxidation of benzene to phenol as metal-free catalyst under mild conditions.3 However, it meets the barrier of low catalytic activity.4 The iron-doped carbon nitride showed the improved activity, yet unsatisfactory.3,5 Owing to unique electronic properties, remarkable promoted catalytic properties, high atom utilization, single atom catalysts (SACs) have attracted great interest.6-11 Fe-SACs/N-doped carbon materials notably enhanced the activity of benzene oxidation to
phenol, thus opened a new field for investigation.12,13 Furthermore, copper-based materials have been generally considered as efficient selective oxidation catalysts.14-16 Considering the abundant N content and chemical stability of g-C3N4,17 and N atoms provide anchoring sites for stabilizing metal atoms, Cu SACs/g-C3N4 composite is expected to be more efficient for selective oxidation of benzene to phenol. Thermal condensation is a promising approach to prepare SACs. However, metal atoms embedded inside bulk support suffer from disadvantage regarding of the depressed active sites accessibility to reactants.18,19 To deposit single metal atoms on the surface of various supports is considered as an alternative to acquire increased accessibility of SACs,20-26 yet suffers from the barrier of an extreme low metal loading. The further increase in accessibility but with a relatively high loading remains interesting issues. SACs with unique morphologies prepared by pyrolysis using hard templates showed improved accessibility.27-31 However, the hard-template method suffers from complex preparation process, active metal leaching and high cost in the essential de-template process such as hydrothermal alkali/acid treatment or high temperature vaporization. Especially for the heating acid treatment, inappropriate choosing of leaching solution would result in significant loss of active metals.32,33 In spite of extensive efforts, searching a facile template-free approach for preparing the single metallic atoms mounted in nano-/micro- structures is still challenging. It was previously reported that different g-C3N4 nano-/micro-structures were successfully prepared by forming a preorganized supramolecules.34 Therefore, we envisage that the single atoms implanted in g-C3N4 nano-/micro-structures can be prepared by a template-free preassembly method by using metal complex as a buliding block for supramolecular assembly, but it remains unknown. Herein, we put forward, for the first time, a novel template-free preassembly strategy to fabricate single Cu atoms inlaid porous hollow g-C3N4 spheres (HCNS) through thermal polymerization of Cu-containing supramolecular assemblies as-formed by the preassembly of melamine-Cu complex with cyanuric acid
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Scheme 1. Illustration of the synthesis for Cu-SA/HCNS.
(Scheme 1), generating a new and efficient single-atom Cu catalyst (denoted as Cu-SA/HCNS) for selective oxidation of benzene to phenol. Typically, copper nitrate as Cu source first complexes with melamine to produce melamine-Cu complex in DMSO. Then, the cyanuric acid-dissolved DMSO was decanted tardily into the above solution under stirring. The resulting white precipitate was washed with deionized water and then ethanol followed by thermal polymerization under N2 atmosphere at 550 o C to form Cu-SA/HCNS directly. Thanks to both atomically dispersed Cu-N3 species and promoted accessibility by porous hollow structure, Cu-SA/HCNS exhibits outstanding catalytic performance in selective oxidation of benzene to phenol, especially shows much higher phenol selectivity than the supported Cu nanoparticles at room temperature. Importantly, 86% of high benzene conversion with 96.7% of unexpectedly high phenol selectivity at 60 oC for 12 h is achieved, indicating a great potential for practical application. This work not only presents a novel and highly efficient catalyst for benzene selective oxidation to phenol, but also paves a new way for designing and fabricating diverse single-atom catalysts in different graphitic carbonitride nano-/micro-architectures regarding of the other transformations. SEM and TEM images (Figure 1a,b) reveal that the Cu-SA/HCNS catalyst featuers a regular hollow spheral structure
Figure 1. (a) SEM, (b) TEM, (c) HRTEM, (d) HAADF-STEM images of Cu-SA/HCNS.(e) XRD patterns of HCNS matrix and Cu-SA/HCNS.
with ca. 3nm pores (Figure S1). HRTEM image indicates no nanoparticles or clusters to be observed on Cu-SA/HCNS (Figure 1c). The single Cu atoms are identified as bright spots (highlighted by yellow circles) by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 1d). Figure 1e shows two characteristic XRD peaks appearing at 27.4o and 13.0o assigned to g-C3N4 with decreasing intensity for Cu-SA/HCNS in contrast to bare HCNS, indicating the insertion of Cu atom distorts the crystal structure of g-C3N4. It should be noted that, although Cu-SA/HCNS shows a lowering crystallinity, the morphology and main matrix of g-C3N4 aren’t largely influenced. From elemental analysis results, Cu-SA/HCNS shows similar N/C ratio to HCNS (ca. 1.5, Table S1), presenting a further evidence for no obvious change in carbonitride matrix after introducing Cu atoms. From N2 adsorption-desorption measurements, Cu-SA/HCNS has higher specific surface area and
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BJH pore volume than bare HCNS (Figure S1), possibly resulting from the replacement of melamine by melamine-Cu complex in the supramolecular aggregates. The XRD peak of HCNS at 27.4o assigned to interlayer stacking of the conjugated aromatic systems shifts to 26.8o for Cu-SA/HCNS (inset in Figure 1e), indicating an increased carbonitride interlayer distance. Thus the loose carbonnitride structure was obtained (Figure S2). The porous hollow loose structure benefits for promoting accessibility of single Cu atoms to reactants, subsequently endowing Cu-SA/HCNS with promoted catalytic performance. The coordination-unsaturated sites like in-plane N atoms in g-C3N4 is easily formed during thermal polymerization,35-37 which is crucial to forming isolated Cu atoms in HCNS. The XPS peak of Cu-SA/HCNS concerning pyridinic N shows a shift to a lower BE value contrast to that of HCNS (Figure S3 a,b), suggesting Cu atoms coordinate with pyridinic N to form the Cu-Nx moieties.38,39 The melamine-Cu complex (marked as Cu(NO3)2(Mel)) was prepared in DMSO solvent. The weaken strength of triazine ring vibration and redshift of side-chain C-N shtreching confirm the formation of Cu(NO3)2(Mel.) complex via Cu-N bond (Figure S4).40 N1s XPS spectra (Figure 2 a,b) further features a new
Figure 2. (a,b) N 1s XPS spectra of Cu-SA/HCNS and bare HCNS. (c) Normalized Cu K-edge XANES spectra of Cu foil, CuO, Cu2O and Cu-SA/HCNS. (d) The k3-weighted Fourier transform spectra from Cu K edge EXAFS. (e) The corresponding EXAFS fitting curve of Cu-SA/HCNS. (f) The model of Cu-N3 sites in Cu-SA/HCNS: Cu (yellow), C (gray), N (blue), and H (white).
bond (399.0 eV), indicating the formation of Cu-N bond.30 The N atoms in Cu(NO3)2(Mel.) complex strongly coordinates with Cu atoms. Meanwhile, the N atoms from cyanuric acid also may stabilize Cu atoms in thermal polymerization process, resulting in the uniform distribution of the isolated Cu atoms in Cu-SA/HCNS.41 The Cu content determined by ICP-AES analysis is ca. 0.85 wt%. Although Cu 2p XPS spectrum of fresh Cu-SA/HCNS shows no peak regarding of surface Cu species, but a peak at 932.3 eV assigned to Cu+ emerges after being etched for 60 s (Figure S3c). The contents of surface Cu before and after etching are 0.33 at% and 0.64 at%, respectively (Table S2), indicating the well distributed Cu atoms in HCNS matrix.
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To explore the electronic structure and coordination environment of Cu species in Cu-SA/HCNS, we conducted the X-ray absorption fine structure (XAFS) measurements. The X-ray absorption near edge spectroscopy (XANES) of fresh Cu-SA/HCNS exhibits similar edgeshape of both Cu2O and CuO references (Figure 2c), identifying its oxidized Cu+ (1
