A General Strategy for Fabricating Isolated Single Metal Atomic Sites

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A General Strategy for Fabricating Isolated Single Metal Atomic Sites Catalysts in Y Zeolite Yiwei Liu, Zhi Li, Qiuying Yu, Yanfei Chen, Ziwei Chai, Guofeng Zhao, Shoujie Liu, WengChon Cheong, Yuan Pan, Qinghua Zhang, Lin Gu, Lirong Zheng, Yu Wang, Yong Lu, Dingsheng Wang, Chen Chen, Qing Peng, Yunqi Liu, Limin Liu, Jiesheng Chen, and Yadong Li J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 25, 2019

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Journal of the American Chemical Society

A General Strategy for Fabricating Isolated Single Metal Atomic Sites Catalysts in Y Zeolite Yiwei Liu†∆, Zhi Li†∆, Qiuying Yu‡, Yanfei Chen§, Ziwei Chai∥, Guofeng Zhao⊥, Shoujie Liu#, WengChon Cheong†, Yuan Pan†, Qinghua Zhang¶, Lin Gu¶, Lirong Zheng£, Yu WangΘ, Yong Lu⊥, Dingsheng Wang†, Chen Chen†, Qing Peng†, Yunqi Liu§, Limin Liu∥, Jiesheng Chen*‡, and Yadong Li*† † Department

of Chemistry, Tsinghua University, Beijing 100084, China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China §State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China University of Petroleum, Qingdao, Shandong 266580, China ∥Beijing Computational Science Research Center, Beijing 100193, China ⊥ School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China # College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, China ¶ Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China £ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China Θ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China ‡

ABSTRACT: Exploring high-performance zeolite-supported metal catalysts is of great significance. Herein, we develop a strategy for fabricating isolated single metal atomic sites catalysts in Y zeolite (M-ISAS@Y, M=Pt, Pd, Ru, Rh, Co, Ni, Cu) by in-situ separating and confining metal-ethanediamine complex into β-cages during crystallization process followed by thermal treatment. The M-ISAS are stabilized by skeletal oxygens of Y zeolite, and the crystallinity, porosity, and large surface area are well inherited in M-ISAS@Y. As a demonstration, acidic Pt-ISAS@Y is used for n-hexane isomerization involving consecutive catalytic dehydrogenation/hydrogenation on Pt-ISAS and isomerization on Brønsted acid sites. The TOF value of Pt-ISAS reaches 727 h-1, five times over Pt nanoparticles (~3.5 nm), with the total isomer selectivity over 98%. This strategy provides a convenient route to fabricate promising zeolite-based M-ISAS catalysts for industrial applications.

INTRODUCTION Zeolite supported metal catalysts have been widely used in industrial reactions, such as hydrogenation/dehydrogenation, hydroalkylation, hydrocracking, isomerization, and catalytic reforming.1-6 However, in these commercial catalysts, the metal nanoparticles (NPs) with diameters in the range of 1.5 to 4 nm will either block the micropores (< 2 nm) of zeolite skeleton or are just attached on the external surface, both resulting in unsatisfactory catalytic activities. In theory, isolated single metal atomic sites (M-ISAS) catalysts have maximum efficiency of metal atoms and minimum volume occupancy.7-9 Very recently, zeolite supported metal catalysts have made considerable progress in methane oxidation,10-13 NOx reduction,14, 15 water-gas shift reaction,16 etc.17-23 but the ISAS and NPs are usually symbiotic under conventional incipient wet impregnation (IWI) conditions, in which NPs are

commonly catalytically inactive. Hence, developing new synthetic methodology to realize homogeneous distribution of ISAS in zeolite crystals would be a promising protocol to obtain high-performance zeolite supported metal catalysts but remains a great challenge. Herein, we develop a general strategy for fabricating MISAS within Y zeolite (M-ISAS@Y, M=Pt, Pd, Ru, Rh, Co, Ni, Cu) by first in-situ separating and confining metalethanediamine (M-EDA) complex into the β-cages of Y zeolite during crystallization process, followed with thermal reduction to anchor metal atoms within zeolites. Comparing with conventional IWI method that the precursors commonly aggregate on the shallow layers of zeolite crystals owing to the microporous diffusing limitation, the in-situ synthesis strategy ensures uniform dispersion of the precursors in the whole bulk, and more importantly, the M-EDA is monodispersed in each

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β-cage which effectively avoid agglomeration. As a representative, isolated platinum atoms are firmly stabilized by skeletal oxygens in the plane of six-membered rings determined by aberration-corrected scanning transmission electron microscopy, extended X-ray absorption fine structure, and density functional theory calculations. These single atomic sites are accessible through both β-cages and super cages without obstruction, which contributed to the excellent catalytic activity in the probe reaction of ethane dehydrogenation. More importantly, with the combination of catalytic dehydrogenation/hydrogenation on Pt-ISAS and isomerization on Brønsted acidity sites of zeolite, acidulated Pt-ISAS@Y (Pt-ISAS@HY) is used for isomerization of nhexane and affords turnover frequency (TOF) values up to 717 h-1 at ambient H2 pressure with isomer selectivity over 98%, which is about 5 times over the corresponding Pt nanopaticles (3.5 nm) loaded on Y zeolite (Pt NPs@HY), and superior to the reported Pt-based zeolite catalysts.24-26 Scheme 1. A schematic illustration of the in-situ separation and confinement of platinum precursor in β-cage followed by thermal treatments. Figure 1. (a) PXRD of Y zeolite, Pt-EDA@NaY, and PtISAS@NaY respectively. (b) TEM image and pore structure (inset), (c) elements mapping, and (d) AC-HAADF-STEM image of Pt-ISAS@NaY.

RESULTS AND DISCUSSION Synthesis and Characterization of M-ISAS@Y. MISAS@Y zeolite catalysts were synthesized by in-situ confining M-EDA precursors into the β-cages of Y zeolites and reduced by flowing H2 to form isolated single metal atomic sites. This strategy is highly effective in yielding various high quality zeolite-based M-ISAS catalysts, including Pt, Pd, Ru, Rh, Co, Ni, and Cu. Pt-ISAS@NaY was chosen as an example to elucidate the synthetic process (Scheme 1). Typically, PtCl2 and excess ethylenediamine (EDA) were mixed in distilled water and stirred until completely dissolved, in which EDA acted as both ligand to adjust the size of the precursors and protective agent to prevent metal precipitation in the intense alkaline hydrothermal conditions for Y zeolite crystallization. Then, NaOH, NaAlO2, SiO2, and directing agent were introduced into the aqueous solution reaching the final molar composition of 3.36 Na2O: 1.0 Al2O3: 8.4 SiO2: 250 H2O: 0.015 PtCl2: 2.0 EDA, and the final pH value of the starting gel was over 14. After crystallization at 100 °C for 12 hours without stir, the obtained products were washed with distilled water adequately, followed by calcination in static air and flowing H2 successively.

Powder X-ray diffraction (PXRD) was used to determine the crystalline structure of synthesized Pt-containing zeolites. As shown in Figure 1a, the diffraction peaks of Pt-containing zeolites before and after calcination both fitted well with the standard FAU zeolite (NaY), without detectable peaks corresponding to Pt or oxide crystal structures, indicating the absence of Pt nano-particles in the as-synthesized samples. It is noteworthy that the crystallinity of Y zeolite was not affected by the addition of Pt-EDA, as confirmed by transmission electron microscopy (TEM) which shows the distinct channels of Y zeolite (Figure 1b, S1, S2). In addition, the porosity of Pt-ISAS@NaY zeolite showed no obvious variation comparing to Y zeolite with a Brunauer-EmmettTeller (BET) surface area of about 652 m2/g (Figure S3). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) images revealed that Si, Al and Pt elements were homogeneously distributed over the entire Y zeolite and the Si/Al ratio was about 2.5:1 (Figure 1c and S4). Pt single atoms were clearly observed as bright dots from aberration-corrected HAADF-STEM image (AC-HAADFSTEM, Figure 1d, highlighted by red circles). The Pt content was about 0.22 wt% determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Extended X-ray absorption fine structure (EXAFS) furthermore confirmed the atomic dispersion of platinum. As shown in Figure 2b, only one notable peak contributed by Pt-O scattering paths was examined in the Fourier-transformed (FT) k3-weighted EXAFS spectrum while the Pt-Pt peak of Pt foil standards was not detected in Pt-ISAS@NaY, demonstrating isolated Pt atoms were formed and stabilized by the oxygen atoms in the zeolites after removing the EDA ligands. The Xray absorption near-edge structure (XANES) curve (Figure 2a)


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of Pt-ISAS@NaY zeolite showed white-line peak between those of Pt foil and PtO2, suggesting the Pt atoms carry positive charges (Ptδ+) rather than Pt0. We investigated further the CO adsorption behaviour of Pt-ISAS@Y using to provide additional information about the dispersion of Pt species. Only a weak band appeared at 2116 cm-1 was observed in Fouriertransform infrared (FTIR) spectroscopy (Figure S5), which can be ascribed to CO linearly adsorbed on Ptδ+. And the dispersion of Pt was nearly 100% calculated from CO pulse chemisorption (Table S1) indicating completely accessible Pt sites.27,28 Combining the evidences above, we can concluded that isolated single Pt atomic sites were successfully fabricated in Y zeolite. We deduced that the pore size and structure of Y zeolite make a significant contribution to the high dispersion of Pt-EDA precursors, because each nano-sized β-cage can only ensconce one Pt-EDA precursor and constrain it by the small windows.

Figure 2. (a) XANES of Pt L3-edge for Pt-ISAS@NaY with reference materials Pt foil and PtO2. (b) Fourier transforms of k3weighted Pt L3-edge EXAFS experimental data for Pt foil, PtISAS@NaY, and PtO2. (c) EXAFS fitting curves of PtISAS@NaY at R space. (d) EXAFS fitting curves of PtISAS@NaY at k space. (e) The calculated stable location site of Pt-ISAS in Y zeolite cage.

Density functional theory (DFT) calculations and EXAFS fitting (Figure 2c and 2d) were further carried out to synergistically obtain the quantitative structural configuration of Pt-ISAS in Y zeolite skeleton. A stable location site in the plane of the six member rings of the zeolite framework for isolated Pt atoms was determined (Figure 2e) as the most stable configuration. In this structure, Pt was coordinated to two oxygen atoms from Al-O-Si bridges with the Pt-O

bonding distance of about 2.01 Å (Table S2). Noteworthy, the Pt sites were accessible from both β-cages and super cages of Y zeolite (Figure 2e), which was benefit for the catalytic interaction between Pt-ISAS and reaction substrates. We also found that the presence of aluminum was very important for the stability of Pt-ISAS. According to the DFT calculation results, the interaction between Pt atoms and O atoms from Al-O-Si bridges were much stronger than that from Si-O-Si bridges, which well explained the successful fabrication of ISAS in our work but more tend to form nano-clusters in purely siliceous zeolites in the previously reported works.29-31

Figure 3. TEM images, elements mapping, AC-HAADF-STEM images, and EXAFS experimental datas for metal foil, MISAS@NaY, and relative metal oxide, (a) Pd-ISAS@NaY, (b) Ru-ISAS@NaY, (c) Rh-ISAS@NaY, (d) Co-ISAS@NaY, (e) NiISAS@NaY, (f) Cu-ISAS@NaY.

To explore the scope of this in-situ confinement strategy for ISAS fabrication in Y zeolite, platinum was replaced by other metals. We indeed found that various M-ISAS@Y (M = Ru, Rh, Pd, Co, Ni, Cu) can be successfully synthesized by similar synthetic procedures. Elemental mappings (Figure 3a2-f2) indicated the homogeneous dispersion of metal elements in MISAS@NaY. For novel metals Pd, Ru, and Rh, distinguishable bright spots on the low-contrast zeolites supports can be observed from AC-HAADF-STEM images (Figure 3a3-c3).



No metal-metal coordination was detected in the EXAFS (Figure 3a4-c4), confirming the sole presence of atomically dispersed metals in M-ISAS@Y. Whereas, for light metals Co, Ni, and Cu, in spite of their contrast with zeolite supports are not so clear that only few spots can be observed (Figure 3d3f3), we can confirm their atomic dispersions by the EXAFS analysis (Figure 3d4-f4). TEM images (Figure 3a1-f1) and PXRD patterns (Figure S6) of M-ISAS@NaY demonstrated the high crystallinity of M-ISAS@Y and the XANES curves (Figure S7-S12) indicated the positive charge state of all metal single atoms in Y zeolite. Especially, for Cu and Co, owing to their two potential different oxidation states, we carefully compared the near-edge absorption energy of ISAS and corresponding oxides. The near-edge absorption energy of CuISAS was between that of Cu2O and CuO (Figure S10) suggesting a positive charge of between 1+ and 2+. Whereas, for Co-ISAS, the oxidation state was plausibly close to 2+ because of its similar near-edge absorption energy with CoO (Figure S11). The metal content was determined by ICP-OES and listed in Table S3. Dehydrogenation and isomerization test of Pt-ISAS@Y zeolite. The catalytic performance of M-ISAS@NaY was firstly evaluated using ethane dehydrogenation as a probe reaction. Albeit all of the noble metals of group VIIIB are active in the nonoxidative dehydrogenation of alkanes, until now, Pt is the only one utilized in industrial production because of its superior activation of paraffinic C-H bonds and low activity to C-C bonds cleavage.32 As shown in Figure 4a, Pt-ISAS@NaY indeed showed superior ethane dehydrogenation activity with a conversion of 27%, whereas the conversion was 18% for Pt NPs@NaY (the average size of Pt NPs is 3.5 nm, Figure S13, the Pt dispersion was determined by CO chemsorption, Figure S5, Table S1), and the selectivity of Pt-ISAS was also slightly higher than that of Pt NPs (Figure S14). In addition, we found that the catalytic activity of Pt NPs@NaY decreased about 20% within 30 minutes (Figure 4b), which can be attributed to the surface coverage of active Pt sites originated from coke formation, in accordance with the previous study of Pt NPs catalysts for ethane dehydrogenation.33 In contrast, the catalytic activity of Pt-ISAS@NaY showed less diminution with more subtle colour change (Figure S15), indicating the suppressed coke formation. We characterized the recovered Pt-ISAS@NaY catalyst. The crystalline structure of zeolite was well retained and no obvious diffraction peaks of bulk Pt were observed in PXRD (Figure S16). However, although most of the Pt species still existed as single atoms, a small portion of Pt was found aggregating into NPs or nanoclusters under such a high temperature (Figure S17). Inspired by the meliorative dehydrogenation activity of PtISAS, we further speculated that the acidulated Pt-ISAS@Y (denoted as Pt-ISAS@HY) might be a promising bifunctional catalyst for naphtha reforming, which is an important industrial process to make high octane gasoline, traditionally run over Pt NPs loaded onto an acidic support.34-38 The structural integrity and the atomic dispersion of platinum of Pt-ISAS@HY are verified by PXRD (Figure S18), TEM, elements mapping, AC-HAADF-STEM (Figure S19), and EXAFS (Figure S20). n-Hexane isomerization is used as the model reaction to study naphtha reforming (Figure S21), in which Pt catalyzes hydrogenation and dehydrogenation

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reactions and the acidic support provides the active sites needed for olefin isomerization.39-42 As shown in Figure 4c, the total isomerization activities of different Pt species loaded on Y zeolites are displayed as turnover frequency (TOF), which is based on the platinum content in Y zeolites (moles n-hexane converted on per mole Pt site per hour). Pt-ISAS@HY exhibited an high TOF value of 727 h-1 at 340 oC and ambient H2 pressure, and the overall isomer selectivity is as high as 98.1%. In contrast, the total activity of Pt NPs@HY (based on the total surface area of platinum calculated by geometrical methods and TEM analysis of average nanoparticle size of about 3.5 nm, Figure S12) was only 141 h-1, about 20 percent of corresponding Pt-ISAS catalyst under the similar loading content and the same reaction conditions. However, if either the Pt species or acid sites are removed from the zeolite skeleton, the resulted HY and Pt-ISAS@NaY both exhibited extremely poor performance, which confirmed the synergetic catalysis of Pt-ISAS and Brønsted acid sites. In addition, both TOF and the mass activity of Pt-ISAS@HY are superior to the reported Pt-based catalysts (Table S4).

Figure 4. (a) The conversion of ethane dehydrogenation use PtISAS@NaY and Pt NPs@NaY as catalysts respectively. (b) Catalyst duration test of ethane dehydrogenation over PtISAS@NaY and Pt NPs@NaY. (Gas flow rate: C2H6 10ml/min, N2 40ml/min) (c) Total TOF and isomer selectivities of PtISAS@HY, Pt NPs@HY, and Pt-ISAS@NaY at different reaction temperature and ambient pressure respectively. (d) Mass activity


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(in terms of total moles of n-hexane converted per gram of catalyst per second) for Pt-ISAS@HY, Pt NPs@HY, PtISAS@NaY, HY, and NaY respectively. (e) The representative cooperative catalysis process of n-hexane isomerization to isohexane by Pt-ISAS and the Brønsted acidity of HY.

Density Functional Theory (DFT) Calculations. To shed light on the origin of significant performance improvement of Pt-ISAS than Pt NPs, DFT calculations were conducted to understand the reaction mechanism. Typically, there are five steps in the whole catalysis process of n-hexane isomerization (Figure 4e). In the olefin isomerization process (Step Ⅱ to Step Ⅳ), the catalytic Brønsted acid sites were identical in both Pt-ISAS@HY and Pt NPs@HY, hence n-hexane dehydrogenation in the first step and the hydrogenation of isomerized olefin in the last step dominated by Pt species were deeply studied. As can see from the reaction profiles (Figure 5), n-hexane was adsorbed by Pt-ISAS and Pt NPs with adsorption energy of 1.38 eV and 1.49 eV respectively. The cleavage of the first C-H bond on Pt-ISAS is spontaneous during the adsorption process (IS1). Then the second C-H bond cleaves (IS-1 to IMS-1) with the energy barrier of 0.6 eV. In contrast, two C-H bonds cleave simultaneously (IS-1’ to IMS-1’) on the surface of Pt NPs with an energy barrier of 0.81 eV, higher than that on Pt-ISAS. In addition, the energy needed for olefin desorption from Pt NPs was as high as 2.57 eV (IMS-1’ to FS-1’), nearly 1 eV higher than that for the desorption from Pt-ISAS (IMS-1 to FS-1), which was much less favorable for the subsequent isomerization on Brønsted acid sites. Likewise, the hydrogenation of isomerized olefin on Pt-ISAS also go through two steps to generate isohexane (IS-2 to IMS-2 to IMS-3) with energy barriers of 0.34 eV and 1.12 eV, and finally the product desorbed from Pt-ISAS (IMS-3 to FS-2) with an energy barrier of 0.32 eV. However, although the hydrogenation energy barrier (IS-2’ to IMS-2’) on Pt NPs was only 0.44 eV, the as-formed isohexane needed 1.44 eV energy input to desorb from the surface of Pt NPs (IMS-2’ to FS-2’), which makes the hydrogenation cycle on Pt NPs much more difficult than that on Pt-ISAS. Moreover, the strongly adsorbed products would cover the active sites and serve as precursors for hydrogenolysis and coke formation,43 which was consistent with the experimental results that Pt-ISAS demonstrated better performance than Pt NPs in both activity and selectivity.

Figure 5. Reaction pathway and corresponding energy of PtISAS@HY and Pt NPs@HY for the dehydrogenation of n-hexane,

and hydrogenation of isomerized olefin. Red line refers to the reaction on Pt-ISAS, blue line refers to the reaction on Pt NPs.

CONCLUSION In summary, a general strategy is developed to anchor isolated metal atomic sites (M=Pt, Pd, Ru, Rh, Co, Ni, and Cu) in Y zeolite. In this approach, the high dispersion of metal precursors effectively avoids the agglomeration that often occurs in traditional incipient wet impregnation methodology. This strategy would be extended to other type zeolites with similar pore structures. The higher activity and selectivity of Pt-ISAS@HY catalyst in isomerization of n-hexane reveal the enormous potential of zeolite-based ISAS catalysts as promising candidates for petroleum industrial applications. Catalysis research based on these zeolite supported M-ISAS catalysts is in full swing and relevant results will be reported in the near future. EXPERIMENTAL SECTION Syntheses of Directing Agent Solution. NaOH, NaAlO2, and H2O were mixed and stirred until the solution is clear. Then 40 wt% colloidal silica was dropwise added to the solution, followed by a continuous stirring for 4 hours. Then the mixture was aged for 3 days under room temperature. The final molar composition of the directing agent is 18.4 Na2O: 1.0 Al2O3: 18.5 SiO2: 366 H2O. Syntheses of M-ISAS@NaY. PtCl2 and excess ethylenediamine (EDA) were mixed in distilled water and stirred until the solution is clear, then NaOH, NaAlO2, SiO2, and directing agent were added to the aqueous solution successively. The final molar composition of the starting gel is 3.36 Na2O: 1.0 Al2O3: 8.4 SiO2: 250 H2O: 0.015 PtCl2: 2.0 EDA, after a continuous stirring for 4 hours and aged for 4 hours at room temperture. The reaction mixture was then transferred into a Teflon-lined stainless steel autoclave and the crystallization was conducted in a conventional oven at 100 °C for 12 hours. The as-synthesized solid products were centrifuged, washed with water and ethanol for several times to remove excess Pt-EDA complex, and then dried at 80 °C in the vacuum oven overnight. Then the obtained Pt-EDA@NaY was calcined in static air at 400 °C for 2 hours and reduced in flowing H2 (5%) at 200 °C for 1 hours to prepare PtISAS@NaY. Pd, Ru, Cu, Co, Ni-ISAS@NaY were synthesized by similar synthetic procedures with the same molar composition of the starting gel with Pt-ISAS@NaY. The reduction temperatures are 150 °C for Pd, 200 °C for Ru, 300 °C for Cu, 300 °C for Ni, 400 °C for Co, respectively. RhISAS@NaY was synthesized by similar synthetic procedures with a molar composition of the starting gel of 3.36 Na2O: 1.0 Al2O3: 8.4 SiO2: 250 H2O: 0.005 RhCl3: 1.0 EDA. The reduction temperature is 150 °C. Syntheses of M-ISAS@HY. 1g M-EDA@NaY was suspended in 1.5 M NH4Cl aqueous solution under continuous stirring at 80 oC for 6 hours. This process is repeated for three times. Then the solid products were centrifuged, washed with water and dried at 80 °C in the vacuum oven overnight. Then the obtained NH4+ form M-EDA@Y zeolite is calcined in static air at 400 °C for 2 hours and reduced in flowing H2 (5%) at specific temperature for 1 hour to prepare M-ISAS@HY.



Ethane dehydrogenation. Ethane dehydrogenation is performed using a fixed glass tube reactor loaded with 0.20 g catalyst. The reaction feed consisted of ethane in nitrogen with gas flow rate maintained at 10 mL/min and 40 mL/min respectively. The reactants and products were analyzed using an on-line gas chromatograph (SP 6890, Shandong Lunan Inc.) with a flame ionization detector (FID). Catalytic n-hexane isomerization. n-Hexane isomerization is performed utilizing a tubular fixed catalyst bed reactor at ambient pressure. Briefly, 1 g catalyst was loaded in a stainless steel reactor. The remaining space in the reactor tube was filled with quartz sand. Before the reactor system was heated to the target temperature, the catalyst was pretreated under the gas flow of H2 12.5 ml/min for 2 hours. Then, the gas flow was changed to n-hexane 0.025 ml/min and H2 12.5 ml/min with a heating rate of 2 K/min. Quantitative analysis of product composition was accomplished with a Varian GC3800 gas chromatograph (GC) with a flame ionization detector (FID).

ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures, DFT calculations methods, and additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Author Contributions ∆Y.

L. and Z. L. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by China Ministry of Science and Technology under Contract of 2016YFA (0202801), the National Natural Science Foundation of China (21521091, 21390393, U1463202, 21890383), 111 Project (B16028), China Postdoctoral Science Foundation (2017M620737), and Key Laboratory of Polyoxometalate Science of Ministry of Education. The computation supports from Tianhe-2JK computing time award at the Beijing Computational Science Research Center (CSRC) and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501. LL is supported by NSFC (Nos. 51572016, 51861130360 and U1530401), Science Challenge Project (TZ2018004) and the Fundamental Research Funds for the Central Universities, and Newton Advanced Fellowship under grant number NAFR1180242.

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Table of Contents (TOC)


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Scheme 1. A schematic illustration of the in-situ separation and confinement of platinum precursor in β-cage followed by thermal treatments.



Figure 1. (a) PXRD of Y zeolite, Pt-EDA@NaY, and Pt-ISAS@NaY respectively. (b) TEM image and pore structure (inset), (c) elements mapping, and (d) AC-HAADF-STEM image of Pt-ISAS@NaY.


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Figure 2. (a) XANES of Pt L3-edge for Pt-ISAS@NaY with reference materials Pt foil and PtO2. (b) Fourier transforms of k3-weighted Pt L3-edge EXAFS experimental data for Pt foil, Pt-ISAS@NaY, and PtO2. (c) EXAFS fitting curves of Pt-ISAS@NaY at R space. (d) EXAFS fitting curves of Pt-ISAS@NaY at k space. (e) The calculated stable location site of Pt-ISAS in Y zeolite cage.



Figure 3. TEM images, elements mapping, AC-HAADF-STEM images, and EXAFS experimental datas for metal foil, M-ISAS@NaY, and relative metal oxide, (a) Pd-ISAS@NaY, (b) Ru-ISAS@NaY, (c) Rh-ISAS@NaY, (d) Co-ISAS@NaY, (e) Ni-ISAS@NaY, (f) Cu-ISAS@NaY.


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Figure 4. (a) The conversion of ethane dehydrogenation use Pt-ISAS@NaY and Pt NPs@NaY as catalysts respectively. (b) Catalyst duration test of ethane dehydrogenation over Pt-ISAS@NaY and Pt NPs@NaY. (Gas flow rate: C2H6 10ml/min, N2 40ml/min) (c) Total TOF and isomer selectivities of Pt-ISAS@HY, Pt NPs@HY, and Pt-ISAS@NaY at different reac-tion temperature and ambient pressure respectively. (d) Mass activity (in terms of total moles of n-hexane converted per gram of catalyst per second) for Pt-ISAS@HY, Pt NPs@HY, Pt-ISAS@NaY, HY, and NaY respectively. (e) The representative cooperative catalysis process of n-hexane isomerization to isohexane by Pt-ISAS and the Brønsted acidity of HY.



Figure 5. Reaction pathway and corresponding energy of Pt-ISAS@HY and Pt NPs@HY for the dehydrogenation of n-hexane, and hydrogenation of isomerized olefin. Red line refers to the reaction on PtISAS, blue line refers to the reaction on Pt NPs.


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A General Strategy for Fabricating Isolated Single Metal Atomic Sites

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