Surfactant-Free Synthesis of Crystalline Mesoporous Metal Oxides by a Seeds/ NaCl-Mediated Growth Strategy

Abstract

Transitional metal oxides (TMOs) with ultra-high specific surface areas (SSAs), large pore volume, and tailored exposed facets appeal to significant interests in heterogeneous catalysis. Nevertheless, synthesizing the metal oxides with all the above features is challenging. Herein, the so-called seeds/NaCl-mediated growth method is successfully developed based on a bottom-up route. First, the (Brunauer-Emmett-Teller) BET SSAs of TMOs prepared with this method are significantly higher, where the BET SSAs of CeO₂ , SnO₂ , Nb₂ O₅ , Fe₃ O₄ , Mn₃ O₄ , Mg(OH)₂ , and ZrO₂ reached 187, 275, 518, 212, 147, 186, and 332 m² g^-1 , respectively. Second, these TMOs exhibit unique mesoporous structures, generated mainly by the aggregation of rod-like or other aspherical primary nanoparticles. More importantly, no environmental-unfriendly organic surfactants or expensive metal alkoxides are involved in this method. Therefore, the entire synthesis protocol fully fitted the "green synthesis" definition, and the corresponding TMOs prepares displayed excellent catalytic performance.

Keywords: mechanochemistry; rod-like nanoparticles; salt template; seeds-mediated growth method; transitional metal oxides.

Figure 1

The process for building the structure of Ce(OH)₃ seeds on NaCl. a) Scheme of Seeds/NaCl‐mediated growth method for porous TMOs synthesis. b) SEM image of Ce(OH)₃‐CeCl₃‐NaCl after the second ball‐milling. c) SEM image of Ce(OH)₃‐CeCl₃‐NaCl under a higher magnification. d) The EDS mappings of a selected cubic NaCl nanoparticle.

Figure 2

The evidence for the seeds/NaCl‐mediated growth method. a) the XRD patterns of CeO₂ seeds before and after growth. b) N₂ adsorption curve of CeO₂‐2Na. c) the pore size distribution of CeO₂‐2Na. d,e) TEM images of CeO₂‐seeds with different magnifications. f,g) TEM images of CeO₂‐2Na with different magnifications. h) HRTEM image of CeO₂‐2Na.

Figure 3

The texture of porous metal oxides by this seeds/NaCl‐mediated method. a) N₂ adsorption curves of prepared porous TMOs, b) TEM images of Mn₃O₄‐2Na, c) TEM images of Mg(OH)₂‐1Na, d) TEM images of Nb₂O₅‐1Na.

Figure 4

The characterization of hybrid NiO‐Co₃O₄‐seed. a) TEM image of the NiO‐Co₃O₄‐seed. b) the XRD diffraction of NiO‐Co₃O₄‐seed. c–e) Cs‐corrected HRTEM images of NiO‐Co₃O₄‐seed. f) HAADF image and EDS elemental mappings of NiO‐Co₃O₄‐seed.

Figure 5

The catalytic performance of NiO‐Co₃O₄‐seed. a) Light‐off curves of CH₄ combustion over NiO‐Co₃O₄‐seed, Co₃O₄‐NaCl, and NiO‐Co₃O₄‐P under the WHSV of 40,000 ml g⁻¹ s⁻¹. b) the CH₄ catalytic combustion activation energy of NiO‐Co₃O₄‐seed, and Co₃O₄‐NaCl. c) N₂ adsorption curves of NiO‐Co₃O₄‐seed, NiO‐Co₃O₄‐P, and Co₃O₄‐NaCl. d) the CH₄‐TPR curves of NiO‐Co₃O₄‐seed, NiO‐Co₃O₄‐P, and Co₃O₄‐NaCl. e) O 1s XPS spectra of NiO‐Co₃O₄‐seed, NiO‐Co₃O₄‐P, and Co₃O₄‐NaCl. f) Co 2p XPS spectra of NiO‐Co₃O₄‐seed, NiO‐Co₃O₄‐P, and Co₃O₄‐NaCl. g) Time‐dependent mass spectra of C¹⁶O¹⁶O, C¹⁶O¹⁸O, and C¹⁸O¹⁸O species during the ¹⁸O₂ isotope labeling experiment over NiO‐Co₃O₄‐seed at 280 °C.