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, Nb2O5, Fe3O4, Mn3O4, 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.


Synthesis of mesoporous CeO2-2Na
In the solid-state nucleation step (I), a mixture of 1 g NaCl and 2 mmol CeCl3 was added to a 50 ml stainless steel ball mill jar, which contained four large stainless-steel balls (diameter: 1.2 cm) and twelve small stainless-steel balls (diameter: 0.4 cm).The ball-milling jar was then sealed, transferred to a threedimensional high-speed vibrating ball mill machine (MSK-SFM-3), and ran for 0.5 h.Subsequently, 2 mmol of non-stoichiometric NaOH solid was added to the above mixture, and the grinding was continued for another 0.5 h.In the solution-growth step, the product after the second ball milling was transferred to a round-bottomed flask and quickly dissolved in 50 ml of deionized water.After 5 min of sonication, 0.5 mol/L NaOH solution was slowly dropped into the solution under vigorous magnetic stirring until the pH became 9.The stirring of the solution was continued for 30 min.After the precipitate was washed three times by centrifugation, the sample was dried at 80℃ for 24 h.The final CeO2-2Na was produced by calcined at 300℃ for 1 hour.The resulting CeO2 sample based on the amount of NaOH was named as CeO2-2Na, where the CeO2-xNa indicated that x mmol NaOH was used in the ball-milling step.The synthesis steps of Mn3O4, Co3O4, Mg(OH)2, and Fe3O4 were the same as those of Mn3O4.

Synthesis of mesoporous SnO2
A mixture of 1 g NaCl and 2 mmol SnCl2 was together added to a 50 ml stainless steel ball milling jar with the same milling parameters as above.The ball-milling jar was then sealed, transferred to the MSK-SFM-3 machine, and ball-milled for 0.5 h.After the first grinding, 1 mmol of non-stoichiometric NaOH solid was added to the above mixture, and then ball milling was continued for another 0.5 h.In the solution growth period, the above ball-milled mixture was quickly dispersed in 20 ml of ethanol.Then the 50 ml deionized H2O was slowly added into the above solution to induce the hydrolysis of SnCl2.To fully hydrolyze SnCl2, the above solution was then stirred at room temperature for 1 h.After the precipitate was washed three times by centrifugation, the final SnO2-1Na was obtained after drying and calcination at 300 o C for 2 h.The synthetic steps of ZrO2-2Na, Nb2O5-1Na were the same as SnO2-1Na.

Synthesis of porous NiO-Co3O4-seed
Firstly, a mixture of 1 g NaCl and 1 mmol NiCl2 was added to the ball milling reactor and the milling parameters were kept unchanged.Different from the above growth process, 2 mmol of stoichiometric NaOH was added to fully participate in NiCl2.In the solution growth period, the above NiO-NaCl mixture was quickly dispersed into 100 ml CoCl2 solution (0.03 mol/L) and sonicated for 5 min.Then, the 0.5 mol/L NaOH solution was dropped into the above solution under strong magnetic stirring until the PH of the solution was 9.The stirring of the solution was continued for 30 min.After the precipitate was washed

Figure S1 .
Figure S1.(a) XRD patterns of the intermediate products of ball-milling NaCl and CeCl3.(b) the magnified XRD patterns of NaCl-CeCl3 and Ce(OH)3/CeCl3/NaCl.(c) XRD patterns of the mixture by ball milling CeCl3 and alkali metal salts (NaCl and KCl).

Figure S2 .
Figure S2.The SEM images of CeCl3-NaCl after the first ball-milling.

Table S1 .
Comparison of the BET SSAs of TMOs with literature.