Two-Step Carbothermal Welding To Access Atomically Dispersed Pd1

Jun 12, 2019 - Herein, we report a novel carbothermal welding strategy to prepare atomically dispersed Pd sites anchored on a three-dimensional (3D) Z...
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Two-Step Carbothermal Welding To Access Atomically Dispersed Pd1 on Three-Dimensional Zirconia Nanonet for Direct Indole Synthesis

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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,# Yadong Li,‡ and Yuen Wu*,† †

Department 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 ‡ Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China § Beijing 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 ∥ Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China ⊥ National Synchrotron Radiation Laboratory, Shanghai Institute of Applied Physics, Shanghai 201800, P. R. China # National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

= 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 three-dimensional (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 an abundant nanoscale contact surface and exhibit a rigid, stable, and porous macrostructure assembled by small nanoscale domains. This specific structure would greatly promote molecular accessibility and catalytic activity and exhibit strong resistance to the external stimuli such as adsorbents/solvents and high temperature.18 To date, several effective approaches including oriented attachment and cold welding have been realized in the bottom-up construction of nanoscale supports, such as metals and sulfides/carbide, into well-defined 3D porous structures.19−21 However, a rational assembly of nanoscale oxides by a universal method to an ordered and porous macrosuperstructure has rarely been 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 a singlephase high-entropy 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

ABSTRACT: Herein, we report a novel carbothermal welding strategy to prepare atomically dispersed Pd sites anchored on a three-dimensional (3D) ZrO2 nanonet (Pd1@ZrO2) via two-step pyrolysis, 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(μ3OH)4]12+ nodes of UiO-66-NH2 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 are gasified and the ZrO2 nanoclusters are 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 sinteringresistant Pd1@ZrO2 catalyst via atom trapping. This Pd1@ ZrO2 nanonet can act as a semi-homogeneous catalyst to boost the direct synthesis of indole through hydrogenation and intramolecular condensation processes, with an excellent turnover frequency (1109.2 h−1) and 94% selectivity.

T

he search for efficient and cost-effective heterogeneous single site catalysts (SSCs) loaded on proper supports is significant in many fields of energy and pharmaceutical industry.1−9 Various support materials, including nitrogendoped 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 © 2019 American Chemical Society

Received: March 27, 2019 Published: June 12, 2019 10590

DOI: 10.1021/jacs.9b03182 J. Am. Chem. Soc. 2019, 141, 10590−10594

Communication

Journal of the American Chemical Society

obtained Pd1@ZrO2 retained its initial shape, but its surface became netlike and fluctuant (Figures 1D and S7). Correspondingly, the color of the samples evolved from faint yellow to black and finally to orange (Figure S8). The intrinsic morphology and spatial distribution of PdCl2@ UiO-66-NH2, Pd1@NC/ZrO2, and Pd1@ZrO2 were recorded by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) for which they were well-fitted because of the alleviator electron beam damage versus bright field.24 HAADF-STEM (Figure 2A) and energy-dispersive X-

defects is greatly desired. 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, a highly porous macrostructure 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 Figure 1A. We choose a Zr6-based UiO-66-

Figure 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 HAADF-STEM, and (L) FT-EXAFS of Pd1@ZrO2.

ray spectroscopy (EDS) (Figure 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 were observed (Figure 2C). Extended X-ray absorption fine structure (EXAFS) Fourier transform (FT) curves of PdCl2@ UiO-66-NH2 exhibit only one prominent peak at about 1.84 Å, associated with the Pd−Cl scattering path (Figure 2D). Afterward, this mixture was pyrolyzed at 900 °C (Ar, 3 h). A uniform distribution of C, N, O, Zr and Pd were recorded by HAADF-STEM (Figure 2E), EDS (Figure 2F), and line-scan (Figure S4). Notably, the specific signal of chlorine decreased, suggesting the Pd−Cl was broken. Aberration-corrected HAADF-STEM images (Figure 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 (Figure 2H). After air-etching, Pd was still uniformly atomically dispersed onto the surfaces of the rigid and porous ZrO2 nanonet (Figures 2I−K and S9). Since the atomic number between Pd and Zr is similar, it is not easy to distinguish the Pd single atoms on the ZrO2 nanonet. FT-EXAFS 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 (Figure 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 (Figures S10 and S11). The disappearance of D and G peaks in Raman spectra (Figure S13) and an obviously increased Zr−O peak (522 cm−1, Figure

Figure 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.

NH2, which features of [Zr6(μ3-O)4(μ3-OH)4]12+ nodes linked to 12 carboxylates of 2-amino-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 ultrasmall ZrO2 nodes will aggregate with each other along the carbon skeletons which derive 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. The initial UiO-66-NH2 and PdCl2@UiO-66-NH2 both exhibited a narrow size distribution (300−400 nm, Figure S1) and similar crystal pattern (Figure S2). Then, the PdCl2@UiO66-NH2 (Figure 1B) was pyrolyzed at 900 °C (Ar, 3 h) in a home-built tube furnace (Figure S3). No obvious Pd nanoparticles were detected after thermal treatment (Figures 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 (Figure S5). Switching Ar to air at the optimal temperature 600 °C (Figure S6), the 10591

DOI: 10.1021/jacs.9b03182 J. Am. Chem. Soc. 2019, 141, 10590−10594

Communication

Journal of the American Chemical Society

resonance (EPR) was used to detect the unpaired electrons in samples. A signal at a g value of 2.003 appeared after the removal of carbon (Figure 4A), suggesting that abundant

S14) in FT-IR spectra 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 Figure 3A. The UiO-66-NH2 with dangling −NH2

Figure 3. (A) Illustration of the interface transformation from Pd−N/ C to Pd−O. (B) TGA and (C) XRD patterns.

could absorb Pd2+ uniformly within its cavities (namely Pd2+ ions “pool”) via coordination interaction.25,26 After first-step pyrolysis (900 °C, Ar), the NC can simultaneously reduce and anchor the Pd sites, forming Pd1−N/C interface. Since, 600 °C is below the boiling point of Pd (bp 2980 °C, mp 1554 °C), it is unlikely that Pd species would be evaporated away from the substrate during air-etching. The pyrolysis of PdCl2@UiO-66NH2 was tracked by thermogravimetric analysis (TGA) (Figure 3B). The typical weight loss at 200−900 °C 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 (Figure 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 Figure 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 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 first form small PdO crystals and be evaporated away above the 750 °C, 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 the ZrO2 nanonet, electron paramagnetic

Figure 4. (A) EPR of Pd1@NC/ZrO2 (red), Pd1@ZrO2 (blue), and U-ZrO2 (violet). HRTEM images of (B) Pd1@NC/ZrO2 and (E) Pd1@ZrO2. 3D atom-overlapping Gaussian-function fitting mappings of i, ii, iii, and iv in (B) and (E) correspond to C, D, F, and G, respectively. (H) Conversion of 2-(2-nitrophenyl)acetaldehyde for Pd1@NC (red), Pd1@ZrO2 (blue), and U-ZrO2 (violet). (I) Chemoselectivity and TOFs.

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 Figure 4B, ultrasmall t-ZrO2 (d111 = 0.296 nm) was embedded in the NC matrix. The 3D atomoverlapping Gaussian-function fitting mapping (i and ii, extracted from the square in Figure 4B) displays the selected area containing two materials with distinct crystallinity. These two components are node-derived ZrO2 nanocrystallites (Figure 4C) and linker-derived amorphous NC (Figure 4D). Figure 4E presents the HRTEM image of as-synthesized Pd1@ ZrO2, which clearly reveals that the ZrO2 nanonet was composed of a mixture phase of t-ZrO2 (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 Figure 4E), helping to add the defect concentration.29 Correspondingly, 3D atom-overlapping Gaussianfunction fitting mappings of iii and iv are shown in Figure 4F and G, respectively. It was considered that a carbon metabolism mechanism plays a key role in the formation of ZrO2 ordered-welding and the generation of a 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 10592

DOI: 10.1021/jacs.9b03182 J. Am. Chem. Soc. 2019, 141, 10590−10594

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Journal of the American Chemical Society Author Contributions

enough O* cannot prevented the aggregation of supported Pd precursor under high temperature (Figure S16). The support of SSC influences not only their thermal stability but also their catalytic performances. Here, we first used Pd1@NC (Figures S17−S20) and Pd1@ZrO2 to examine the catalytic performance for one-pot synthesis of indole, which is a vital structural motif in biological systems and pharmaceutical industries.30 The catalytic behaviors (Figure 4H and I) of three catalysts [Pd1@NC, Pd1@ZrO2, and pure UiO-66-derived ZrO2 (denoted as U-ZrO2)] were studied toward 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 2,3-dihydroindole was not observed under the condition due to the stability of the indole ring. The stability of the Pd1@NC and Pd1@ZrO2 was conducted by a recycling test (Figure S21). No obvious change was observed for the activity and selectivity throughout 10 cycles. The EXAFS spectrum (Figure S24) and AC HAADF-STEM image (Figure 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 efficiently expose the surface-bound Pd sites (Figure S26), losing carbon skeletons. In summary, we have reported a novel carbothermal welding strategy to prepare atomically dispersed Pd sites anchored on a ZrO2 nanonet (Pd1@ZrO2) via two-step pyrolysis. Direct synthesis of indole testing shows that Pd1@ZrO2 (94% and 1109.2 h−1) with the Pd−O interface delivers superior performance compared with Pd1@NC (46% and 294.4 h−1) with Pd−N/C moieties. Our findings shed light on the rational design of coordination supports for SSC in industrial applications.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b03182. Detailed experimental procedures; SEM and TEM images; XRD data; XANES data (PDF)

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Y.Z., H.Z., and W.C. contributed equally to this work.

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Yue Lin: 0000-0001-5333-511X Zheng Jiang: 0000-0002-0132-0319 Xun Hong: 0000-0003-2784-2868 Shiqiang Wei: 0000-0002-2052-1132 Yadong Li: 0000-0003-1544-1127 Yuen Wu: 0000-0001-9524-2843 10593

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DOI: 10.1021/jacs.9b03182 J. Am. Chem. Soc. 2019, 141, 10590−10594