Two-step Carbothermal Welding to Access Atomically Dispersed Pd1

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Two-step Carbothermal Welding to Access Atomically Dispersed Pd1 on 3D Zirconia Nanonet for Direct Indole Synthesis yafei zhao, Huang Zhou, Wenxing Chen, Yujing Tong, Chao Zhao, Yue Lin, Zheng Jiang, Qingwei zhang, Zhenggang Xue, Weng-Chon Cheong, Benjin Jin, Fangyao Zhou, Wenyu Wang, Min Chen, Xun Hong, Juncai Dong, Shiqiang Wei, Yuen Wu, and Yadong Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03182 • Publication Date (Web): 12 Jun 2019 Downloaded from http://pubs.acs.org on June 12, 2019

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Two-step Carbothermal Welding to Access Atomically Dispersed Pd1 on 3D Zirconia Nanonet for Direct Indole Synthesis Yafei Zhao,1,† Huang Zhou,1,† Wenxing Chen,3,† Yujing Tong,1 Chao Zhao,1 Yue Lin,1 Zheng Jiang,5 Qingwei Zhang,1 Zhenggang Xue,1 Weng-Chon Cheong,2 Benjin Jin,1 Fangyao Zhou,1 Wenyu Wang,1 Min Chen,1 Xun Hong,1 Juncai Dong,4 Shiqiang Wei,6 Yadong Li,2 Yuen Wu1,* 1Department

of Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), University of Science and Technology of China, Hefei 230026, China; 2Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China; 3Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China; 4Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China; 5National Synchrotron Radiation Laboratory, Shanghai Institute of Applied Physics, Shanghai 201800, P. R. China; 6National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China. Supporting Information Placeholder ABSTRACT: Herein, we reported a novel carbothermal welding strategy to prepare atomically dispersed Pd sites anchored on 3D ZrO2 nanonet (Pd1@ZrO2) via two-step prolysis, which were evolved from isolated Pd sites anchored on linker-derived nitrogen-doped carbon (Pd1@NC/ZrO2). First, the NH2-H2BDC linkers and Zr6-based [Zr6(μ3-O)4(μ3-OH)4]12+ nodes of UiO-66NH2 were transformed into amorphous N-doped carbon skeletons(NC) and ZrO2 nanoclusters under an argon atmosphere, respectively. The NC supports can simultaneously reduce and anchor the Pd sites, forming isolated Pd1-N/C sites. Then, switching the argon to air, the carbonaceous skeletons will be gasified and the ZrO2 nanoclusters were welded into a rigid and porous nanonet. Moreover, the reductive carbon will result in abundant oxygen (O*) defects, which could help to capture the migratory Pd1 species, leaving a sintering-resistant Pd1@ZrO2 catalysts via atom trapping. This Pd1@ZrO2 nanonet can act as a semihomogeneous catalyst to boost the direct synthesis of indole through hydrogenation and intramolecular condensation processes, with an excellent TOF (1109.2 h-1) and 94% selectivity.

The search for efficient and cost-effective heterogeneous single site catalysts (SSC) loaded on proper support is significant in many fields of energy and pharmaceutical industry.1-9 Various support materials, including nitrogen-doped carbon (NC) and defect-rich oxides/sulfides/nitrides, have been employed to prevent atom aggregation to access SSC.10-14 Metallic oxides (e.g.,TiO2, CeO2) can expose oxygen-vacancies to trap atomic atoms over isolated M1-O (M=metal) interfaces, which ensures them frequently-used supports.1,7 Besides, the final catalytic performance of SSC is highly dependent on the interaction between isolated atoms and oxide supports.15 Moreover, the adsorption of reactants and desorption of products are closely related to supports’ size and their 3D nanostructures.16 Downsizing the oxide supports is an effective strategy to increase the surface area-to-volume, which is benefit for the exposure of active sites and contact area. However, the nanoparticles usually cause the problems of poor thermal and chemical stability.17 Hence, an ideal support should contain abundant nanoscale contact surface and exhibit rigid, stable, and porous macro-

structure assembled by small nanoscale-domains. This specific structure would greatly promote molecular accessibility and catalytic activity and exhibit strong resistant to the external stimulates such as adsorbents/solvents, high temperature.18 To date, several effective approaches including oriented attachment and cold welding have been realized the bottom-up construction of nanoscale supports, such as metals, sulfides/carbide, into welldefined 3D porous structures.19-21 However, a rational assembling of nanoscale oxides by a universal method to an ordered and porous macro-superstructure has been rarely studied. Recently, Hu et al. observed that the carbon metabolism enables efficient increase surface defect within the carbon support and alloys multiple metallic elements into single-phase highentropy-alloy.22 it inspires us that the carbon matrix may serve as an effective tool to assist the directed attachments of oxide nanoparticles and modulate their ordered assembly. Meanwhile, the search for a new strategy to simultaneously control their morphology, spatial distribution, and surface defects is greatly desired. Proverbially, metal-organic frameworks (MOFs) composed of nodes (metal ions/clusters) and organic linkers are a class of porous crystalline materials.23 Pyrolysis of MOFs can be used as a vital route to nodes-residing composite materials.9 The linker-derived NC can act as a fence to restrain the node's collision and coalescence, forming a thermodynamically stable NC and oxide (metal) coexistence system. If we can selectively remove NC and synchronously assemble the remaining oxide nodes in situ, highly porous macro-structure may be fabricated. In addition, carbon metabolism could create abundant oxygen (O*) defects on the surface of oxides, which would help to capture the mobile atoms and fabricate a sintering-resistant SSC. A mechanism illustrating the carbothermal welding process is summarized in Fig. 1A. We choose a Zr6-based UiO-66-NH2, which features of [Zr6(μ3-O)4(μ3-OH)4]12+ nodes linked to 12 carboxylates of 2-amion-terephthalate ligands, as the host to encapsulate the PdCl2 within its pores. The pyrolysis process includes two steps: (1) Under the protection of Ar, the ultra-small ZrO2 nodes will aggregate with each other along the carbon skeletons which derived from the carbonization of linkers; (2) Switching Ar to air, the linker-derived NC would be selectively etched and the remained ZrO2 nodes formed an ordered and porous nanonet.


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Fig. 1 (A) Schematic illustration of synthetic route of Pd1@ZrO2. TEM images of (B) PdCl2@UiO-66-NH2, (C) Pd1@NC/ZrO2 and (D) Pd1@ZrO2. The initial UiO-66-NH2 and PdCl2@UiO-66-NH2 both exhibited a narrow size distribution (300-400 nm, Fig. S1) and similar crystal pattern (Fig. S2). Then, the PdCl2@UiO-66-NH2 (Fig. 1B) was pyrolyzed at 900 °C (Ar, 3 h) in a home-built tube furnace (Fig. S3). No obviously Pd nanoparticles were detected after thermal treatment (Fig. 1C and S4). The single-mind PdCl2/NH2 coordination configuration may efficiently prevent the formation of Pd-Pd bonds at high temperature. By contrast, without the help of dangling -NH2 groups, the support cannot provide effective N defects sites to stable isolated Pd atoms (Fig. S5). Switching Ar to air at the optimal temperature 600 °C (Fig. S6), the obtained Pd1@ZrO2 retained its initial shape, but its surface became net-like and fluctuant (Fig. 1D and Fig. S7). Correspondingly, the color of the samples was evolved from faint yellow to black and finally to salmon (Fig.S8). The intrinsic morphology and spatial distribution of the PdCl2@UiO-66-NH2, Pd1@NC/ZrO2 and Pd1@ZrO2 were recorded by the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) for which they were well-fitted because of the alleviator electron beam damage versus bright field.24 HAADF-STEM (Fig. 2A) and Energy-dispersive X-ray spectroscopy (EDS) (Fig. 2B) suggest a homogeneous distribution of Pd precursor on the whole skeletons of PdCl2@UiO-66-NH2. The PdCl2@UiO-66-NH2 retained its initial structural details after adsorption and no Pd nanoparticles was observed (Fig. 2C). Extended X-ray absorption fine structure (EXAFS) Fourier transform (FT) curves of PdCl2@UiO-66-NH2 exhibits only one prominent peak at about 1.84 Å, associated with the Pd-Cl scattering path (Fig. 2D). Afterward, this mixture was pyrolyzed at 900 oC (Ar, 3 h). A uniform distribution of C, N, O, Zr and Pd were recorded by HAADF-STEM (Fig. 2E), EDS (Fig. 2F), and line-scan (Fig. S4). Notably, the specific signal of chlorine decreased, suggesting the Pd-Cl was broken. Aberrationcorrected HAADF-STEM images (Fig. 2G) reveal that Pd2+ was reduced to isolated Pd sites embedded on NC skeletons. The main peak at 1.43 Å can be attributed to Pd-N/C (Fig. 2H). After airetching, Pd was still uniformly atomically dispersed onto the surfaces of rigid and porous ZrO2 nanonet (Fig. 2I-2K and Fig.

Fig. 2 (A) HAADF-STEM, (B) EDS, (C) AC HAADF-STEM and (D) FT-EXAFS of PdCl2@UiO-66-NH2. (E) HAADF-STEM, (F) EDS, (G) AC HAADF-STEM and (H) FT-EXAFS of Pd1@NC/ZrO2. (I) HAADF-STEM, (J) EDS, (K) AC HAADFSTEM and (L) FT-EXAFS of Pd1@ZrO2. S9). Since the similar atomic number between Pd and Zr, it is not easy to distinguish the Pd single atoms on the ZrO2 nanonet. FTEXAFS was further performed to elucidate the structure of residual Pd species at the atomic level. A new peak assigned to Pd-O at 1.48 Å (Fig. 2L) appears for Pd1@ZrO2 and the Pd-N/C peak at 1.43 Å disappears, verifying that the coordination interface was transformed from Pd-N/C to Pd-O.6 The change of the absorption threshold reveals a transformation in the spatial coordination of the Pd species (Fig. S10-S11). The vanish of D and G peaks in Raman spectra (Fig. S13) and an obviously increased Zr-O peak (522 cm-1, Fig. S14) at FT-IR further indicate that NC was etched by air. Base on the above results, a proposed mechanism illustrating the interface transformation from Pd-N/C to Pd-O is depicted in Fig. 3A. The UiO-66-NH2 with dangling -NH2 could absorb Pd2+ uniformly within its cavities (namely Pd2+ ions “pool”) via coordination interaction.25-26 After first-step pyrolysis (900 oC, Ar), the NC can simultaneously reduce and anchor the Pd sites, forming Pd1-N/C interface. Since, 600 oC is below the boiling point of Pd (bp 2980 oC, mp 1554 oC), it is unlikely that Pd species would be evaporated away from the substrate during airetching. The pyrolysis of PdCl2@UiO-66-NH2 was tracked by thermogravimetric analysis (TGA) (Fig. 3B). The typical weight loss at 200-900 oC was attributed to linker thermal degradation. The linker-derived NC (28.75%) was quickly removed within 180 s. Zr-based nodes were synchronously welded with each other, generating porous ZrO2 nanonet (71.25%). The as-prepared Pd1@NC/ZrO2 exhibited a typical tetragonal ZrO2 (t-ZrO2) phase in XRD patterns (Fig. 3C). The porous Pd1@ZrO2 was mainly constructed by a small amount of t-ZrO2 and a large proportion of monoclinic ZrO2 (m-ZrO2) nanocrystals. Interestingly, the reductant NC substrate would change the normal phase evolution along with the temperature. Different from the normal phase evolution from t-ZrO2 to m-ZrO2,27 the critical size t-ZrO2 still dominate in the annealed MOF-derived ZrO2 composite under the protection of NC skeletons. After the removal of NC, a phase evolution from t-ZrO2 to m-ZrO2 could be observed. The first-step pyrolysis is strictly related to the formation of well-defined and ordered ZrO2 nanonet. As shown in the control experiments in Fig. S15, the pyrolysis process including the one-step air-pyrolysis strategy (Route 2) without the pretreatment in Ar only result in the irregular and sintered large ZrO2 nanoparticles. Moreover, the NC



Fig. 3 (A) Illustration of the interface transformation from Pd-N/C to Pd-O. (B) TGA and (C) XRD patterns. will generate abundant defects on the surface of substrates, which can firmly capture the nonvolatile Pd species. Without the strong trapping effect by defects, the adsorbed Pd salts will firstly form small PdO crystals and be evaporated away above the 750 oC, leaving a pure ZrO2 substrate. Additionally, the NC template also play a dominant role in directly guiding the welding of ZrO2 nodes with each other along the carbon skeletons, inheriting its initial octahedron features. To further understand the migration phenomenon of Pd from NC supports to ZrO2 nanonet, electron paramagnetic resonance (EPR) was used to detect the unpaired electrons in samples. A signal at g value of 2.003 appeared after the removal of carbon (Fig. 4A), suggesting abundant oxygen vacancies emerged on the surface of [email protected] Then, high resolution TEM (HRTEM) was performed to investigate the surface architecture of Pd1@NC/ZrO2 and Pd1@ZrO2. As shown in Fig. 4B, ultra-small t-ZrO2 (d111=0.296 nm) were embedded in the NC matrix. 3D atom-overlapping gaussian-function fitting mapping (i and ii, extracted from the square in Fig. 4B) displayed the selected area contain two materials with distinct crystallinity. These two components are node-derived ZrO2 nanocrystallites (Fig. 4C) and linker-derived amorphous NC (Fig. 4D). Fig. 4E presents the HRTEM image of as-synthesized Pd1@ZrO2, which clearly reveals that ZrO2 nanonet was composed of a mixture phase of tZrO2 (d111=0.296 nm) and m-ZrO2 (d-111=0.312 nm). Furthermore, a clear grain boundary was generated in the adjacent two highly crystallized domains during grain coarsening (iii and iV, extracted from the square in Fig. 4E), helping to add the defect concentration.29 Correspondingly, 3D atom-overlapping gaussianfunction fitting mappings of iii and iV were showed in Fig. 4F and Fig. 4G, respectively. It was considered a carbon metabolism mechanism play a key role in the formation of ZrO2 orderedwelding and the generation of defective ZrO2 nanonet. The carbon metabolism involving the release of CO/CO2 gas can be proposed as [C+O→CO(CO2)↑+O*], where O* is assigned to surface oxygen defects. This carbon metabolism would greatly enhance the concentration of oxygen defects, which may help to capture and stabilize the atomic Pd species. By contrast, commercial ZrO2 without enough O* cannot prevented the aggregation of supported Pd precursor under high temperature (Fig. S16). The support of SSC influences not only their thermal stability but also their catalytic performances. Here, we first used Pd1@NC (Fig. S17-S20) and Pd1@ZrO2 to examine the catalytic performance for one-pot synthesis of indole, which is a vital

Fig. 4 (A) EPR of Pd1@NC/ZrO2 (red), Pd1@ZrO2 (blue), and UZrO2 (violet). HRTEM images of (B) Pd1@NC/ZrO2 and (E) Pd1@ZrO2. 3D atom-overlapping gaussian-function fitting mapping of i, ii, iii, and iV in (B) and (E) were corresponding to Fig. C, D, F, and G, respectively. (H) Conversion of 2-(2nitrophenyl)acetaldehyde for Pd1@NC (red), Pd1@ZrO2 (blue), and U-ZrO2 (Violet). (I) Chemoselectivity and TOFs. structural motif in biological systems and pharmaceutical industries.30 The catalytic behaviours (Fig. 4H and 4I) of three catalysts [Pd1@NC, Pd1@ZrO2, and pure UiO-66-derived ZrO2 (denoted as U-ZrO2)] were studied towards the hydrogenation of 2-(2-nitrophenyl)acetaldehyde. Promisingly, both Pd1@NC, Pd1@ZrO2 achieved a high conversion: 95% and 60%, in 60 min, respectively. But, a chemoselectivity of 94% with a high TOF of 1109.2 h-1 of 1a was obtained by using the Pd1@ZrO2, distinctly higher than that of the Pd1@NC (46%, 294.4 h-1). The U-ZrO2 support showed inert activity to this hydrogenation process, suggesting that the catalytic performance is strongly dependent on the metal sites. The activity trend Pd-O>Pd-N/C in the performed catalytic tests indicate the preferential NC supports may become poisoning factors or physical barriers that detrimental to catalyst performance. Importantly, further reduction into 2,3dihydroindole was not observed under the condition due to the stability of the indole ring. The stability of the Pd1@NC and Pd1@ZrO2 were conducted by a recycling test (Fig. S21). No obvious change was observed for the activity and selectivity throughout 10 cycles. The EXAFS spectrum (Fig. S24) and AC HAADF-STEM image (Fig. S25) all display that Pd atoms are still atomically dispersed after reaction, demonstrating the stability of single Pd sites on the Pd1@ZrO2. In addition, a notable decrease of specific surface area may be efficienty expose the surface-bound Pd sites (Fig. S26), losing carbon skeletons. In summary, we have reported a novel carbothermal welding strategy to prepare atomically dispersed Pd sites anchored onZrO2 nanonet (Pd1@ZrO2) via two-step prolysis. Direct synthesis of indole testing shows that Pd1@ZrO2 (94% and 1109.2 h-1) with the Pd-O interface delivers superior performance than Pd1@NC (46% and 294.4 h-1) with Pd-N/C moieties. Our findings shed light on the rational design of the coordination supports for SSC in industrial applications.


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ASSOCIATED CONTENT Supporting Information Detailed experimental sections; figures; tables. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors: *[email protected]

Author Contributions †These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Key R&D Program of China 2017YFA (0208300), and the National Natural Science Foundation of China (21671180). We thank the BL14W1 station for XAFS measurement in shanghai Synchrotron Radiation Laboratory (SSRL) for help in characterizations.

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Two-step Carbothermal Welding to Access Atomically Dispersed Pd1

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