Facile Synthesis of Highly Porous Metal Oxides by Mechanochemical

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Facile Synthesis of Highly Porous Metal Oxides by Mechanochemical Nanocasting Weiming Xiao, Shize Yang, Pengfei Zhang, Peipei Li, Peiwen Wu, Meijun Li, Nanqing Chen, Kecheng Jie, Caili Huang, Ning Zhang, and Sheng Dai Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05405 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 14, 2018

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Chemistry of Materials

Facile Synthesis of Highly Porous Metal Oxides by Mechanoch Mechanochemical echanochemical Nanocasting Weiming Xiao,†,‡ Shize Yang,§,‡ Pengfei Zhang,+,ᴦ,* Peipei Li,+ Peiwen Wu,+ Meijun Li,# Nanqing Chen,# Kecheng Jie,# Caili Huang,+ Ning Zhang,†,* Sheng Dai+,#,* † Institute of Applied Chemistry, College of Chemistry, Nanchang University Nanchang, Jiangxi 330031, P. R. China + Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA § Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA

ᴦ School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China # Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA ABSTRACT: Metal oxides with high porosity usually exhibit better performance in many applications, as compared with the corresponding bulk materials. Template-assisted method is generally employed to prepare porous metal oxides. However, the template-assisted method is commonly operated in wet conditions, which requires solvents, soluble metal oxide precursors and a long time for drying. To overwhelm those drawbacks of the wet procedure, a mechanochemical nanocasting method is developed in current work. Inspired by solid-state synthesis, this strategy proceeds without solvents, and the ball milling process can enable pore replicated in a shorter time (60 min). By this method, a series of highly porous metal oxides were obtained, with several cases approach the corresponding surface area records (e.g., ZrO2: 293 m2 g-1, Fe2O3: 163 m2 g-1, CeO2: 211 m2 g-1, CuOx-CeOy catalyst: 237 m2 g-1, CuOx-CoOy-CeOz catalyst: 202 m2 g-1). Abundant nanopores with clear lattice fringes in metal oxide products were witnessed by scanning transmission electron Microscopy (STEM) in high angle annular dark field (HAADF). By combination of mechanochemical synthesis and nanocasting, current technology provides a general and simple pathway to porous metal oxides.

INTRODUCTION Embedding porosity into metal oxides can significantly advance their performance in many applications such as catalysis, sensors, microelectronics, photovoltaics and biomedicines. It is understandable since larger active surfaces, faster mass transfer and higher storage volume can be expected in porous metal oxides.1-6 The porous metal oxides are usually prepared by template-assisted processes, which proceed via either soft- or hard-templating methods.7-11 For the soft-templating method, self-assembly between metal precursors and block copolymers results in ordered organicinorganic mesophases,11-13 followed by calcination to form crystalline metal oxides at the same time removing organic templates. The soft-templating approachwith membrane intermediates aging from several days to one month, is timeconsuming, meanwhile this technology is limited to porous metal oxides with low crystalline temperature (e.g., 0.1) together with clear hysteresis loops (Figure 3b and 3c), typical features of mesoporous materials. What’s attractive is the high specific surface areas achieved by CuOxCeOy-MN and CuOx-CoOy-CeOz-MN (237 and 203 m2 g-1, respectively) (Table 1). Current work also tried to prepare porous perovskite LaMnO3. Although only amorphous phase was obtained, the sample even after calcinated at 700 oC offered a high surface area of 162 m2 g-1 (Figure S8).

Considering the mechanochemical nanocasting procedure was operated in the steel stainless reactor with stainless-steel ball, the Fe contents in the model metal oxides were analyzed. In this work, the Fe contents in the ZrO2-MN-600, La2O3-MN and CuOx-CeOy-MN were examined, and their weight percents were 0.028%, 0.0067% and 0.22%, respectively. It suggests that the abrasion of steel can be neglected in this procedure. In addition, the CO oxidation performance of CuOx-CeOy-MN and CuOx-CoOy-CeOz-MN was investigated. As illustrated in Figure 3d, the two hybrids are active for CO oxidation. The T100 values (Temperature for 100% CO conversion) are 200 and 160 oC, respectively, which are comparable to many of copper-ceria catalysts. 42-45 Due to the facile synthesis procedure, CuOx-CeOy-MN and CuOx-CoOy-CeOz-MN may have the potential applications in the removal of toxic CO.

Figure 4. STEM-HAADF and STEM-BF images of Fe2O3 (a-d) and CeO2 (e-f).

Figure 3. (a) XRD patterns of CuOx-CeOy-MN and CuOx-CoOyCeOz-MN, (b) and (c) N2 adsorption-desorption isotherms (77 K) and the pore size distributions of CuOx-CeOy-MN and CuOxCoOy-CeOz-MN (the isotherm of CuOx-CeOy-MN is offset along the y axis by 30 cm3 g-1 for clarity), (d) CO conversion over CuOx-CeOy-MN and CuOx-CoOy-CeOz-MN at different temperature (reaction condition: catalyst 20 mg, space velocity 30,000 mL (h g cat)−1, 1 vol% CO balance in dry air).

The porous morphologies of metal oxide materials were observed by scanning transmission electron Microscopy (STEM) in high angle annular dark field (HAADF) or bright field (BF). As shown in Figure 4, Fe2O3-MN is composed of irregular particle aggregates in the range of several hundred nanometers (Figure 4a). Rich porosity created by the removal of silica sphere is observed in amplified images (Figure 4c4d). The apparent pores locate in ~4-20 nm, which match well with the PSD by N2 adsorption isotherm. Moreover, the Fe2O3-MN sample has a well crystalline structure with clear lattice fringes (012), as shown in the high-resolution images (Figure 4b). Moreover, interstitial porosity from both bumps

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Chemistry of Materials and nanoparticles are seen in CeO2-MN, with the formation of hierarchical pores (Figure 4e-4f). Meanwhile, structural details of the hybrid metal oxide CuOx-CoOy-CeOz-MN were studied by STEM-HAADF images (Figure 5). A sponge-like nanoarchitecture with a number of accessible pores was observed in the sample (Figure 5a). Previous literatures already confirmed the synergetic interaction of copper, cobalt and cerium species in CuOx-CoOy-CeOz catalyst, while the dispersion or structure of those elements in atomic scale has not been clarified yet.40 High-resolution STEM-HAADF images illustrate many aggregated nanoparticles with clear crystalline structures (Figure 5b-5d). Two classes of sub-10 nm particles are carefully recognized by their lattice fringes, which could be attributed to CeO2 and Cudoped Co3O4. Subsequent element mapping in a 48*48 nm zone also supports this observation, because cerium and cobalt species locate in different domains (Figure 5e-5h). Current STEM and XRD results suggest that the CuOx-CoOy-CeOzMN catalyst is primarily composed of sub-10 nm CeO2 and Cudoped Co3O4 particle aggregates.

ly prepared. Importantly, a well dispersion of active metal species in the hybrids even at sub-nano scale can be guaranteed by this strategy, thus offering an exceptional catalytic performance in CO oxidation. It is expected that a number of advanced porous materials will be prepared by this mechanochemical nanocasting method in the near future.

ASSOCIATED CONTENT Supporting Information. The N2 adsorption-desorption isotherms of Com-SiO2, ZrO2-MN, Fe2O3-MN, CeO2, La2O3, CoOx, bulk-CuOx, CuO-MN and LaOx-MnOx; SEM image of Com-SiO2; XRD patterns of CeO2, La2O3, CoOx, bulk-CuOx, CuO-MN, LaOxMnOx and ZrO2/Com-SiO2 hybrid; STEM-HAADF images of LaOx-MnOx; XPS spectra of ZrO2-MN-500 are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

* [email protected] (N. Zhang); [email protected] (P. Zhang); [email protected] (S. Dai). Author Contributions ‡ These authors contributed equally to this work.

ACKNOWLEDGMENT P. F. Zhang and S. Dai was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy. W.M.X. and N.Z. thank the National Natural Science Foundation of China (No. 21062013 and 21663016) and the China Scholarship Council. The electron microscopy (S.Z.Y.) was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division and through a user proposal at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, which is a U.S. Department of Energy Office of Science User Facility. P. F. Zhang acknowledges Shanghai Pujiang Program (Grant No. 17PJ1403500), Thousand Talent Program and National Natural Science Foundation of China (Grant No. 21776174) for the partial support.

REFERENCES

Figure 5. STEM-HAADF images of CuOx-CoOy-CeOz-MN (a-d), and the element mapping signals: f) Ce_M, g) Co_L, h) O_K.

CONCLUSIONS In summary, an efficient, general and simple route to directing high porosity into metal oxide has been explored based on the mechanochemical nanocasting. The method does not need solvents and can shorten the pore filling process from 1-3 days to 60 min. Especially, current strategy inherently ignores the solubility issue of metal precursors, which significantly expands the library of resources available for porous metal oxides. By this method, many metal oxides and their hybrid catalysts with high surface areas were successful-

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