Mechanochemical Synthesis of High Entropy Oxide Materials under

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Mechanochemical Synthesis of High Entropy Oxide Materials under Ambient Conditions: Dispersion of Catalysts via Entropy Maximization Hao Chen,†,‡,§ Wenwen Lin,† Zihao Zhang,† Kecheng Jie,§ David R. Mullins,‡ Xiahan Sang,∥ Shi-Ze Yang,∥ Charl J. Jafta,‡ Craig A. Bridges,‡ Xiaobing Hu,# Raymond R. Unocic,∥ Jie Fu,*,† Pengfei Zhang,*,‡,⊥ and Sheng Dai*,‡,§ †

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ‡ Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996, United States ∥ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ⊥ School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, China # Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: The solid-solution metal oxide (NiMgCuZnCo)O is the first known high-entropy (HE) metal oxide synthesized, forming a poster child of the emerging high-entropy oxide materials, which is derived from high-temperature synthesis methodologies (>900 °C). In this work, we report the mechanochemical synthesis of this known HE metal oxide (NiMgCuZnCo)O under ambient conditions. The advantage of this approach was further demonstrated by the introduction of up to 5 wt % noble metal into (NiMgCuZnCo)O, as single atoms or nanoclusters, which showed good stability at high temperature and produced a high catalytic activity in the hydrogenation of atmospheric CO2 to CO. The latter work demonstrated the unique advantage of using HE materials to disperse catalysis centers. “Mechanochemistry” has attracted increasing attention1−4 since it was first reported in 1993 for the synthesis of zeolite A5 because it can be used to react solids quickly, quantitatively, and with little-to-no solvent use.6,7 Mechanochemical synthesis (MS) has shown promising performance in the rapid roomtemperature synthesis of porous metal−organic frameworks (MOF),8−12 covalent organic frameworks (COF),13 and porous organic polymers.14 Beyond porous materials synthesis, ball milling has been used to promote catalysis of C−H bond functionalization/amination15,16 and in the generation of Baylis−Hillman products.17 More relevant to our work, MS has been used in the synthesis of perovskite materials (ABO3), which would normally require 800−1200 °C.6,18,19 The local heating produced by mechanochemistry (accelerating diffusion by mechanical energy and frictional heating) leads to a near instantaneous quenching of induced atomic diffusion that enables the preparation of metastable phases, which has © 2019 American Chemical Society

inspired us to study the solid-state synthesis of entropystabilized mixed metal oxides. Rost et al.20 reported the preparation of configurationally disordered and entropy-stabilized mixed metal oxides, this new class of materials showed a significantly increased hightemperature stability because the random distribution of the multicomponent metals on the cation sublattice produces a homogeneously dispersed metal oxide solid solution.21 Because the entropic contributions to Gibbs free energy are temperature dependent and disorder of the multicomponent cation site at high temperature leads to a more negative free energy contribution, this enables the stabilization of unusual cation Received: March 9, 2019 Accepted: May 17, 2019 Published: May 22, 2019 83

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work,21 the Pt-based high-entropy material was synthesized by codeposition with heating at 900 °C and the Pt loading was 0.3 wt %, which was used for CO oxidation. Our original conjecture is based on the possibility to significantly lower the Pt loading through our catalysts. in this work, we found that without post-heat treatment, the only room temperature ball-milling can produce the similar quality of HEO materials. We first investigate this approach on the known HE phase (NiMgCuZnCo)O, to demonstrate that a pure entropystabilized phase can be obtained at ambient temperature. The commercial oxide powders (rock salt (RS) NiO, MgO, and CoO, tenorite (T) CuO, and wurtzite (W) ZnO) were ground together increments from 0.5 to 2.0 h. X-ray diffraction (XRD) patterns showing the phase evolution vs milling time are depicted in Figure 1a. The patterns were typically multiphasic up to 1.5 h of milling, with the expected rock salt and tenorite crystal structures observed. The presence of primarily rock salt and tenorite phases after 1 h of reaction, is similar to the result of reaction at 800 °C in the hightemperature process,20 indicating a commonality in the reaction pathway. The five metal oxides were completely converted to a single phase after only 2 h of ball milling with no remaining precursor peaks: NiO(RS) + MgO(RS) + CoO(RS) + CuO(T) + ZnO(W) = (NiMgCuZnCo)O(RS).20 Five peaks attributed to the (111), (200), (220), (311), and (222) planes of the cubic Fm3̅ m entropy-stabilized rock salt oxide (NiMgCuZnCo)O were observed.24 In prior literature, the pure entropy-stabilized phase was only obtained at a calcination temperature above 850 °C.20,25 In contrast, our synthesis occurred with an ambient temperature of the grinding reactor lower than 70 °C and with this temperature developed by the milling process itself. Then, we examined 2 and 5 wt % loading of the precious metals Pt and Ru with an equimolar ratio of the precursor M2+ oxides. These precious metals were chosen because of their extensive use in industrial catalysis.26−28 There is a small peak (44.4°) just next to (200) peak of HEO as shown in Figure 1a. This peak may be the bimetallic oxides alloys (CuMgOx, CoMgOx, CuCoOx, NiCoOx) as reported.29−32 After addition of PtO2 or RuO2 into the precursors, mechanochemical grinding was performed and a single phase of rock salt HE oxide was observed. XRD confirmed that no diffraction peaks attributed to Pt or Ru metal or binary metal oxides were present in the 2 h milled samples33,34 as shown in Figure 1b. This indicated that Pt and Ru form subnanometer or even single atom dopants. Moreover, to ensure that no large

species in the rock salt structure, such as Zn2+. The entropystabilized rock salt material, containing equimolar contributions of Ni, Mg, Cu, Zn, and Co oxides and denoted as J14 has already attracted much attention.21,22 Yao et al.23 first reported a general route for alloying up to eight dissimilar elements into single-phase solid-solution nanoparticles by thermally shocking precursor metal salt mixtures loaded onto carbon supports. Although these entropy-stabilized mixed metal oxides and nonoxides can be achieved independent of the synthesis method, very high temperatures (900−1300 °C) or long reaction times (e.g., >48 h) have typically been used to crystallize HE materials.20 Thus, the natural focus in this field has been to look for materials in which configurational disorder on a metal site produces stability at high temperatures. This has drawn research away from lower-temperature synthetic approaches to metastable materials and has excluded a potentially wide range of materials that cannot be prepared at high temperatures but exhibit beneficial HE properties. Our strategy for the room-temperature, rapid, and solventfree synthesis of entropy-stabilized metal oxides and halides is illustrated schematically in Scheme 1. From this strategy, we Scheme 1. Mechanochemical Synthesis of Pt/Ru(NiMgCuZnCo)O Entropy-Stabilized Metal Oxide Solid Solution

find three outcomes enabling for high-entropy research: (1) relying only on mechanical energy and frictional heating, mechanochemical grinding enables the crystallization of single phase (NiMgCuZnCo)O without additional thermal treatment; (2) bulk noble metal oxides can be dispersed on the nanometer scale onto, or even on the single atom scale into, the rock salt lattice of [Pt/Ru-(NiMgCuZnCo)O] by solidstate grinding assisted by HE stabilization. In our published

Figure 1. X-ray diffraction patterns for (a) NiMgCuZnCoOx synthesized by ball milling with different times; (b) 2 and 5 wt % Pt/RuNiMgCuZnCoOx synthesized by ball milling with 2 h; and (c) 2 and 5 wt % Pt/Ru-NiMgCuZnCoOx synthesized by ball milling with 2 h and 500 °C treatment for 2 h. 84

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Figure 2. High-angle annular dark-field scanning transmission electron microscopy images for 5 wt % Pt-(NiMgCuZnCo)O after 500 °C treatment with 2 h. Highly dispersed Pt particles in the size range of 2−3 nm on entropy-stabilized metal oxide particles are shown in panels a and b, while panels c and d show atomically dispersed Pt single atoms. (e) Elemental mapping for 5 wt % Pt-(NiMgCuZnCo)O after 500 °C treatment for 2 h.

Figure 3. EXAFS spectra of 2 and 5 wt % Pt-(NiMgCuZnCo)O synthesized by ball milling for 2 h, followed by a 2 h heat treatment at 500 °C.

species were highly dispersed in the materials. The STEM-EDS analysis in Figure 2e, as well as the HRTEM-EDS analysis in Figure S5, for Ni, Mg, Cu, Zn, and Co in 5 wt % Pt-500 indicated that the transition metals were not clustered or segregated, confirming the formation of a uniformly dispersed solid solution. As shown in Figure 3, the EXAFS spectrum showed that the Pt existed as single atoms and nanoparticles, which is in accordance with the TEM results. EXAFS results have been reported for MgxNixCoxCuxZnxO.24 The cation− cation peak positions reported for that sample are similar to the Pt-cation peak position we report here, as might be expected if Pt replaced the cations in the rock salt structure. Reducing CO2 by H2 into valuable chemicals and fuels using heterogeneous catalysis is the key to the utilization of excess atmospheric CO2. The high-temperature stability of supported metal catalysts is very important in all real-world catalysis processes,26,27,36 especially for well-dispersed precious-metalbased catalysts.37−43 Because of their relatively low Tammann temperature (the point at which supported metal crystallites

amorphous particles were present in samples with PtO2/RuO2 incorporated at the precursor stage, the entropy-stabilized samples were heated at 500 °C in air for 2 h (Figure 1c). No peaks relating to PtO2 or RuO2 were observed after the heat treatment. As well known, precious metals, such as redox cycle Pt−PtO2, tend to sinter at high temperature because of their high surface energy, leading to a significant loss of catalytic activity.35 Further characterization of (NiMgCuZnCo)O milled for 2 h with dispersed 5 wt % Pt was conducted using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The results after treatment at 500 °C are shown in Figure 2, for samples 5 wt % Pt-500. STEM images show that Pt was dispersed both as small nanoparticles (Figure 2b) with typical sizes in the range of 2−3 nm and as single atoms (Figure 2c and d); no larger sintered Pt particles were observed after treatment under 500 °C in O2. Other highresolution pictures obtained by HAADF-STEM for 5 wt % Pt500 are shown in Figures S3−7, which also prove that Pt 85

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Figure 4. (a) CO2 hydrogenation activity of 2 wt % Pt-500, 5 wt % Pt-500, 2 wt % Ru-500, and 5 wt % Ru-500 under 500 °C reaction temperature, (b) CO2 hydrogenation stability at 500 °C over 5 wt % Pt-500, and (c) STEM result of 5 wt % Pt-500 after hydrogenation of CO2 at 500 °C for 2 h.

intrinsic properties of HEO (intrinsic high-temperature stabilities through entropy maximization) and introduction of up to 5 wt % noble metals (Ru and Pt) as single atoms or nanoclusters highly dispersed into (NiMgCuZnCo)O. This entropy-stabilized metal oxide solid solution acted as an excellent support to stabilize highly dispersed even single atomically dispersed Pt or Ru, at temperatures up to 500 °C. We, then, added the CO2 hydrogenation using 5 wt % Ptloaded on low entropy metal oxides, synthesized by same synthetic conditions, and the results are shown in Figure S12. It was found that the catalytic activity both for CO2 conversion and CO yield was lower than that 5 wt % Pt loaded on high entropy material. The most important is that the catalytic activity decreased as the temperature increased, indicating that the Pt was not protected and began to aggregate at 500 °C when Pt loaded on low-entropy metal oxide supports. From the results of XRD and STEM for after hydrogenation of CO2 at 500 °C for 2 h, we can clearly see that the state of Pt (both for single atoms and highly dispersed Pt particles) does not change any more after catalysis. In this work, we show that low-temperature mechanochemical synthesis can be used to convert a high-temperature process into a readily scalable solid-state synthesis [(NiMgCuZnCo)O and Pt/Ru-(NiMgCuZnCo)O]. Contrary to typical high-temperature syntheses reported in the literature for entropy-stabilized compounds, this approach combines the effects of particle size reduction, mechanical collision, instant frictional heating, local strain, and defect formation to induce pure phase formation and crystallization. It is intriguing to consider that the reaction pathway for the milling may bear some relationship to the high temperature reaction process, as suggested by the time-dependent milling data for (NiMgCuZnCo)O, such that the lower-temperature mecha-

develop liquidlike properties and exhibit an enhanced ability to migrate), sintering occurs rapidly, leading to the minimization of the surface area accessible for catalysis and thereby reduction of catalytic activities. Here, we find that a lowtemperature mechanochemically synthesized HE oxides provide a solution for developing stable catalysts. As shown by XRD, TEM, EXAFS, and XPS, we have produced welldispersed Pt as single atoms in the (NiMgCuZnCo)O lattice with a small portion formed into typically 2−3 nm particles after a 500 °C heat treatment for 2 h. Heating at high temperatures under hydrogen had little detrimental effect on the catalytic activity. As shown in Figure 4a, under catalytic testing the 2 wt % Ru500 and 2 wt % Pt-500 catalysts provided 33.9% and 36.6% yields of CO with CO2 conversions of 40.1% and 43.4%, respectively. For 5 wt % Ru-500 and 5 wt % Pt-500, the yields of CO were increased to 45.7% and 46.1% with CO 2 conversions of 45.4% and 47.8%, respectively. Few other gaseous products were detected, and the CO selectivities over these four catalysts were all above 95%. To further test the high-temperature activity and stability, we measured the CO2 hydrogenation at 500 °C of 5 wt % Pt500 for 12 h to test for loss of catalytic activity (Figure 4b). It was observed that the CO2 conversion, CO yield, and selectivity were nearly unchanged during the 12 h at high temperature, indicating high-temperature stability for the 5 wt % Pt-500 catalyst. Furthermore, as shown in Figure S11a−d, the CO2 conversions for all the catalysts increased with temperature from 300 to 500 °C. Remarkably, the 5 wt % Pt(NiMgCuZnCo)O still showed better CO2 conversion and slightly lower CO yield even after calcination at 700 °C (44.9 % yield of CO at a 500 °C reaction temperature). The advantages of our catalysts in this work were utilization of the 86

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21706228) and the Zhejiang Provincial Natural Science Foundation of China (No. LR17B060002).

nochemical approach could provide an alternate route to the synthesis of metastable intermediates. In addition, this set of conditions is ideal for the development of catalysts with nanosized support particles containing well dispersed catalytically active species. It is interesting to consider the role that entropy can play in the synthesis and stability of these materials. Molecular diffusion in the solid or crystalline state (∼10−15 m2 s−1) is typically six orders of magnitude less than in solution. The high entropic value caused by the close proximity of more than five metal species will contribute even at lower temperatures to a preferred Gibbs free energy, assisting in the homogeneous distribution of cation species in the lattice during milling.

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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/acsmaterialslett.9b00064.

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REFERENCES

Synthesis and characterization of materials, calculation details, XRD result, HAADF-STEM and SEM-EDS images, EXAFS, and CO2 hydrogenation over other related catalysts (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hao Chen: 0000-0002-6658-4198 Kecheng Jie: 0000-0001-8146-4875 David R. Mullins: 0000-0003-3495-7188 Xiahan Sang: 0000-0002-2861-6814 Shi-Ze Yang: 0000-0002-0421-006X Charl J. Jafta: 0000-0002-9773-6799 Craig A. Bridges: 0000-0002-3543-463X Xiaobing Hu: 0000-0002-9233-8118 Raymond R. Unocic: 0000-0002-1777-8228 Jie Fu: 0000-0002-3652-7715 Pengfei Zhang: 0000-0001-7559-7348 Sheng Dai: 0000-0002-8046-3931 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The catalysis synthesis and testing (H. C., P. Z., S. D.) were supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. Structural characterization (C. J. and C. B.) was supported by the U.S. DOE, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Electron microscopy work was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The Microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. J. F. was supported by the National Natural Science Foundation of China (No. 21436007, 87

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