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S41467-020-18567-6.Pdf ARTICLE https://doi.org/10.1038/s41467-020-18567-6 OPEN In situ tuning of electronic structure of catalysts using controllable hydrogen spillover for enhanced selectivity ✉ Mi Xiong1,2, Zhe Gao 1 , Peng Zhao 1, Guofu Wang1, Wenjun Yan1, Shuangfeng Xing1,2, Pengfei Wang1, ✉ Jingyuan Ma3, Zheng Jiang 3, Xingchen Liu 1, Jiping Ma4, Jie Xu 4 & Yong Qin 1,2 1234567890():,; In situ tuning of the electronic structure of active sites is a long-standing challenge. Herein, we propose a strategy by controlling the hydrogen spillover distance to in situ tune the electronic structure. The strategy is demonstrated to be feasible with the assistance of CoOx/ Al2O3/Pt catalysts prepared by atomic layer deposition in which CoOx and Pt nanoparticles are separated by hollow Al2O3 nanotubes. The strength of hydrogen spillover from Pt to CoOx can be precisely tailored by varying the Al2O3 thickness. Using CoOx/Al2O3 catalyzed styrene epoxidation as an example, the CoOx/Al2O3/Pt with 7 nm Al2O3 layer exhibits greatly enhanced selectivity (from 74.3% to 94.8%) when H2 is added. The enhanced selectivity is attributed to the introduction of controllable hydrogen spillover, resulting in the reduction of CoOx during the reaction. Our method is also effective for the epoxidation of styrene deri- vatives. We anticipate this method is a general strategy for other reactions. 1 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 030001 Taiyuan, China. 2 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, 100049 Beijing, China. 3 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 201204 Shanghai, China. 4 State Key Laboratory of Catalysis, Dalian National Laboratory for ✉ Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 116023 Dalian, China. email: [email protected]; [email protected] NATURE COMMUNICATIONS | (2020) 11:4773 | https://doi.org/10.1038/s41467-020-18567-6 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-18567-6 fi ighly ef cient and selective catalysts are important for the numbers of ALD Al2O3). For comparison, two reference catalysts industrial production of commodity chemicals, as well as (CoOx/50Al2O3 and 50Al2O3/Pt) were also synthesized by a Hfi 1 ne chemicals and pharmaceuticals . The development of similar method. – new heterogeneous catalysts for carrying out multipath reactions Figure 1a c present the structure diagrams of CoOx/50Al2O3, with high selectivity to achieve high-yield production of target CoOx/50Al2O3/Pt, and CoOx/100Al2O3/Pt. Figure 1d–f show chemicals is a longstanding challenge2,3. The final product dis- transmission electron microscopy (TEM) images of the three tribution, i.e., the selectivity of a reaction over a given catalyst, is catalysts. TEM images of other catalysts, including CoOx/yAl2O3/ determined by the relative activation barrier among different Pt with different cycles of ALD Al2O3 (35, 65, 80, and 300) and reaction paths, which in nature, primarily depend on the elec- the reference catalyst (50Al2O3/Pt), are shown in Supplementary 4–6 tronic structure of the catalytic active sites . However, under Fig. 2. For all TEM images, Al2O3 hollow structures can be clearly reaction conditions, the electronic structures of active sites are observed. The CoOx/yAl2O3/Pt with different Al2O3 cycles (35, easily affected by temperature, atmosphere, and absorbed 50, 65, 80,100, and 300) possess varied Al2O3 thicknesses from 5, 7,8 species . It is difficult to control the active sites to maintain the 7, 9, 11, 14, to 41 nm. Regardless of Al2O3 thicknesses, the outer optimal electronic structure during the reaction, which favors CoOx and the inner Pt nanoparticles have similar average producing more target products. Some researchers have devoted diameters (Supplementary Figs. 3 and 4), which is consistent with substantial effort to studying how to control and tune the elec- X-ray diffraction (XRD) result (Supplementary Fig. 7 and tronic structures of the active sites responsible for activity and Supplementary Table 1). A high-resolution TEM (HRTEM) 9,10 selectivity under real reaction conditions . In contrast to image of CoOx/50Al2O3/Pt is shown in Supplementary Fig. 5. The numerous studies on identifying catalytically active sites under high-angle annular dark field scanning transmission electron 11–14 real-time reaction conditions , limited methods have been microscopy (HAADF-STEM) images (Fig. 1h, i) of CoOx/ reported for controlling and tuning the electronic structure of 50Al2O3/Pt and CoOx/100Al2O3/Pt clearly show that Pt nano- fi active sites during a reaction. particles are con ned in Al2O3 nanotubes. Energy-dispersive X- In general, the hydrogen migration from the metal particles to ray spectroscopy (EDS) mappings of the catalysts show that the the support, i.e., hydrogen spillover, is a well-known phenom- distributions of Co, O, Al, and Pt (Fig. 1j and l) are consistent enon in heterogeneous catalysis and is involved in many with the positions of the CoOx,Al2O3, and Pt layers. Further, the 15–24 fi important reactions . The ef ciency and spatial extent of distributions of Co, Al, and Pt in CoOx/50Al2O3/Pt were revealed hydrogen spillover strongly depend on the types of supports25,26. clearly by STEM image and EDX mapping (Fig. 2a–d) of a cross- For instance, a well-defined model system demonstrated that on sectional specimen, prepared by focused ion beam (FIB) milling the nonreducible alumina support, hydrogen spillover is limited along the vertical direction of the Al2O3 nanotubes. The line- to short distances, with the hydrogen flux decreasing over dis- scanning profile (Fig. 2e) of cross-sectional composition shows 25 tance to create a concentration gradient . In-depth under- that the signal of Co species was not detected in the Al2O3 standing of hydrogen spillover can not only help to explain nanotubes, which clearly demonstrates the separated structure of experimental phenomena but also aid the design of advanced CoOx/50Al2O3/Pt. The Al peak at ~4.5 nm is ascribed to signal of catalysts with enhanced catalytic performances27–35. Being powder (from the FIB ion milling) remained in the space nearby inspired by these researches, we posit that there is an opportunity the outer surface of the sample (Supplementary Fig. 6). The Co for an approach to modulating the electronic structure of active and Pt contents in the catalysts, measured by inductively coupled sites through regulating hydrogen spillover strength for enhanced plasma-atomic emission spectrometry (ICP-AES), are shown in catalytic performance. Supplementary Table 2. N2 sorption experiments (Supplementary In this work, using the CoOx catalyzed epoxidation reaction of Fig. 8 and Supplementary Table 3) demonstrate that all the styrene as an example, we propose an approach to tune the catalysts possess similar average pore diameter, while their electronic structure of cobalt species during the reaction by Brunauer–Emmett–Teller (BET) surface areas and pore volumes the introduction of controllable hydrogen spillover, to enhance increase with the decrease of Al2O3 thicknesses. the selectivity of styrene oxide (SO). To demonstrate the strategy, X-ray photoelectron spectroscopy (XPS) results (Fig. 3a) reveal 3+ 2+ a CoOx/Al2O3/Pt catalyst is prepared by a facile and general the coexistence of Co and Co . Compared to the spinel 36–38 template-assisted atomic layer deposition (ALD) method ,in Co3O4, the main 2p peaks of CoOx/50Al2O3, CoOx/50Al2O3/Pt, which CoOx and Pt nanoparticles are attached on the outer and and CoOx/100Al2O3/Pt shift to higher binding energy (the 2p3/2 inner surfaces of Al2O3 nanotubes, respectively. The strength of peaks shift from 779.5 to 780.0 eV and the 2p1/2 peaks shift from hydrogen spillover from Pt to CoOx can be precisely tailored by 794.7 to 795.9 eV), and the satellite peaks appear (785.6 eV (2p3/2 varying the thickness of the Al2O3 layer. The CoOx/Al2O3/Pt sat) and 802.3 eV (2p3/2 sat)), indicating that the as-prepared 2+ 3+ 39,40 catalyst with 7 nm-thick Al2O3 layer exhibits greatly enhanced CoOx nanoparticles consist of both Co and Co species . selectivity (from 74.3% to 94.8%) when H2 atmosphere is intro- Hydrogen temperature-programmed reduction (H2-TPR) was duced. Detailed analyses reveal that the cobalt species under the also employed (Fig. 3b). Compared to CoOx/50Al2O3, the peak oxidation condition are reduced to a lower oxidation state by intensity of CoOx/50Al2O3/Pt centered at 622 °C decreases fi introducing the controllable hydrogen spillover, leading to a signi cantly, which is because the CoOx species can be higher SO selectivity. additionally reduced by the spilled active hydrogen from Pt nanoparticles. The peak intensity of CoOx/100Al2O3/Pt centered at 626 °C is remained, possibly because the Al2O3 layer of 100 Results ALD cycles (14 nm) is too thick and the hydrogen spillover effect Synthesis and characterization of the catalysts . CoOx/yAl2O3/Pt is greatly weakened. The differences in reducibility of Co fl catalysts with a separating Al2O3 layer were obtained using car- (Supplementary Table 4) demonstrate that the ux of spilled fi bon nanocoils (CNCs) as sacri cial templates (Supplementary hydrogen species on the nonreducible Al2O3 support decreases Fig. 1). First, Pt nanoparticles and an Al2O3 layer were deposited with increasing distance. onto CNCs by Pt ALD and Al2O3 ALD, respectively. Subse- quently, the CNC templates were removed by calcination under Catalytic performance fi ambient atmosphere. Finally, CoOx nanoparticles were deposited . The epoxidation of ole ns is an impor- by CoOx ALD, producing CoOx/yAl2O3/Pt (where y is the cycle tant chemical reaction since epoxides are key intermediates in 2 NATURE COMMUNICATIONS | (2020) 11:4773 | https://doi.org/10.1038/s41467-020-18567-6 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-18567-6 ARTICLE abc CoOx/50Al2O3 CoOx/50Al2O3/Pt CoOx/100Al2O3/Pt defAl2O3 Al O 2 3 Al O (14 nm) (7 nm) 2 3 Pt (7 nm) CoO x CoO x Pt CoOx ghi jklCo OAlCo OAl Pt Co O Al Pt Fig.
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