Postsynthesis Modulation of the Catalytic Interface inside a Hollow

Dec 15, 2016 - (10-13) Most of them have been synthesized by hollowing a nanoparticle template with preincorporated catalytic species inside.(14-16) H...
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Postsynthesis Modulation of the Catalytic Interface inside a Hollow Nanoreactor: Exploitation of the Bidirectional Behavior of MixedValent Mn3O4 Phase in the Galvanic Replacement Reaction Dong-Gyu Lee,†,‡ Soo Min Kim,‡ Sun Mi Kim,§ Si Woo Lee,§,⊥ Jeong Young Park,§,⊥ Kwangjin An,*,∥ and In Su Lee*,†,‡ †

National Creative Research Initiative Center for Nanospace-confined Chemical Reactions and ‡Department of Chemistry, Pohang University of Science and Technology (POSTECH), Gyeongbuk 790-784, Korea § Center for Nanomaterials and Chemical Reactions Institute for Basic Science, Daejeon 34141, Korea ⊥ Graduate School of EEWS, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea ∥ School of Energy and Chemical engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea S Supporting Information *

ABSTRACT: This paper proposes a postsynthesis strategy to modulate the synergy of the catalyst/support interface of the preformed hollow nanoreactor by replacing preloaded support material inside the cavity, which therefore enhances the variability of the nanoreactor approach. This strategy exploits the newly explored bidirectional behavior of an Mn3O4 nanoparticle of mixed-valent state, which either sequentially or simultaneously templates reduction of noble-metal ions and oxidation of Ce3+ ions during galvanic replacement. The application of this process to the modification of the preformed hollow silica nanoreactor led to the replacement of the catalyst-immobilizing Mn3O4 layer inside the cavity with CeO2 while preserving the tiny size and well-dispersed state of supported catalysts and, as a result, allowing for the introduction of a synergistic noble-metal/CeO2 interface exerting the highly improved catalytic performance in CO oxidation reaction.

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INTRODUCTION Hollow silica nanospheres are receiving increasing attention as the most suited framework for application in nanoreactors1−5 that can selectively and sustainably catalyze the transformation of organic molecules or template the surfactant-free syntheses of metal nanocrystals with controlled morphologies.6−9 Functionalization of the interior space selectively and differentially according to the catalytic species is the most important and challenging factor in developing hollow nanoreactors.10−13 Most of them have been synthesized by hollowing a nanoparticle template with preincorporated catalytic species inside.14−16 However, alternative attempts were made very recently using the “post-decoration approach”, involving the site-specific decoration of a preformed cavity with catalytic nanocrystals, which potentially allows for a platform-based fabrication process that facilitates adjusting the catalytic parameters according to prespecified applications.17−19 For instance, we recently suggested the postsynthetic functionalization of a hollow nanoreactor by depositing catalyst nanocrystals on the Mn3O4-coated interior surface of the preformed hollow and porous silica nanoshell.20,21 In the course of our continued efforts to enhance the variability of this platform-based approach, current research intends to diversify and modulate the synergy at the catalyst−support interface by replacing the catalyst-supporting layer inside the cavity with more active support materials. © XXXX American Chemical Society

In heterogeneous catalysis, the metal−oxide interface plays an important role, because the interfacial effect alters catalytic performance.22,23 When group VIII metals such as Rh, Pt, Pd, and Ir were supported on certain oxides such as TiO2, Nb2O5, Ta2O5, and CeO2, strong metal−support interactions (SMSIs) appeared by charge transfer at the interfaces.24 Rational design of nanocatalysts composed of transition metals on reducible oxides enables activity enhancement, because of SMSI. Among oxide supports, CeO2 is considered an excellent support for the preparation of automotive catalysts owing to its distinct oxygen storage ability as well as noble metal stabilizing effect.25 Recently, several reports demonstrated that Pt-supported CeO2 enhanced CO oxidation rates, compared with other oxides supported by Pt, owing to the synergistic effect between Pt and CeO2.26,27 With these insights, the introduction of CeO2 as a catalyst support was envisioned to be able to significantly enhance the catalytic activity of the supported noble metals in redox-involving reactions such as the water−gas shift reaction and oxidation of CO and various organic molecules, which therefore help to further expand the utility of the hollow nanoreactor. Received: September 25, 2016 Revised: November 9, 2016

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DOI: 10.1021/acs.chemmater.6b04097 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

nanoparticles were synthesized through a reported thermal decomposition method.33 The surfactant-free MnFe2O4 and Fe3O4 nanoparticles were then prepared through the same solid-state transformation procedure that was applied for the synthesis of sf-Mn3O4. BaMn2O4 (Alfa Aesar) was used as purchased without any purification. The control experiments for the galvanic replacement with Ce3+ were carried out with a procedure similar to that for the synthesis of Mn3O4@CeO2, except for the use of the Mn3O4-dissolved solution, surfactant-free MnFe2O4, Fe3O4 nanoparticles, and BaMn2O4 instead of sf-Mn3O4. Dual Galvanic Replacement Reaction on the sf-Mn3O4 with Ce3+ and M2+ (M = Pt, Pd, Ir, Rh). For the dual replacement reaction with Ce3+ and Pt2+, 1 mg of sf-Mn3O4 was immersed in 1 mL of an aqueous solution containing Ce(NO3)3·6H2O (250 mg/mL) and Na2PtCl4 (5 mg/mL) and stirred at 90 °C for different reaction periods (10 min and 0.5, 1, 2, and 6 h). The resulting nanoparticles of the Mn3O4@CeO2/Pts were retrieved by centrifugation and purified by repeating the dispersion in an aqueous suspension and centrifugation three times. The dual galvanic replacement reactions with Ce3+ and Pd2+, Ir3+, and Rh3+ were carried out in a mixture solution of Ce(NO3)3·6H2O (250 mg/mL) with Na2PdCl4, IrCl3, and RhCl3 for 6, 15, and 4 h, respectively. Control experiments for the dual galvanic replacement reaction were performed by using surfactant-free MnFe2O4, Fe3O4, and BaMn2O4 nanoparticles instead of sf-Mn3O4. Dual Galvanic Replacement in a Sequential Manner. The Mn3 O4 /Pts nanoparticles were prepared beforehand through modifying the previously reported procedure which included the reaction of the 1 mg of sf-Mn3O4 nanoparticles in 1 mL of an aqueous solution of Na2PtCl4·xH2O (5 mg/mL) at 70 °C for 3 h.34 The CeO2 deposition was then carried out by immersing 3 mg of the Mn3O4/Pts in a 1 mL of aqueous solution of Ce(NO3)3·6H2O (500 mg/mL) at 70 °C. Preparation of the HMON@h-SiO2 Nanoparticles. MnO nanoparticles and MnO@Mn3O4@SiO2 nanoparticles were prepared by following the reported method.24,34 The SiO2 encapsulation reaction time is continued for 4 h. The HMON@h-SiO2 was prepared through modifying the previously reported procedure.23 A total of 1 mg/mL of MnO@Mn3O4@SiO2 nanoparticles was immersed in a 0.5 M NH2OH solution and stirred at room temperature for 15 h. The resulting HMON@h-SiO2 nanoparticles were isolated from the reaction dispersion by the centrifugation and then purified by repeating the dispersion in water and the centrifugation. Galvanic Replacement on the Mn3O4 Layer of the HMON@hSiO2. Prior to the reaction, aqueous solutions of the HMON@h-SiO2 and Ce(NO3)3·6H2O (500 mg/mL) solution were degassed, respectively, by N2 bubbling each for 0.5 h. A total of 3 mg of HMON@h-SiO2 was immersed into 1 mL of a Ce(NO3)3·6H2O (500 mg/mL) aqueous solution, whose pH was adjusted to 2.2 by the addition of diluted HNO3 solution, at 90 °C, and stirred for 8 h under the N2 environment. The resulted HMON/CeO2@h-SiO2 nanoparticles were collected by the centrifugation and purified by repeating the dispersion in an aqueous suspension and the centrifugation three times. The control experiment without N2 protection was carried out under same conditions except air atmosphere. Dual Galvanic Replacement Reaction on the Mn3O4 Layer of the HMON@h-SiO2 Simultaneously with Ce3+ and M2+ (M = Pt, Pd, Ir, Rh). All reactant solutions were degassed before the reaction by the N2 bubbling for 0.5 h, and reactions were all performed under the N2 atmosphere. For the dual replacement reaction with Ce3+ and Pt2+, 1 mg of HMON@h-SiO2 nanoparticles was immersed into 1 mL of an aqueous mixture solution containing Ce(NO3)3·6H2O (50 mg/mL) and Na2PtCl4 (1 mg/mL), whose pH was adjusted to 2.2 beforehand by adding the diluted HCl solution, and stirred at 90 °C for 15 h. The resulting (CeO2/Pts)@h-SiO2 nanoparticles were retrieved by centrifugation and purified by repeating the dispersion in an aqueous suspension and centrifugation three times. The dual galvanic replacement reactions with Ce3+ and Pd2+, Ir3+, and Rh3+ were carried out in a mixture solution of Ce(NO3)3·6H2O (250 mg/mL) with Na2PdCl4, IrCl3, and RhCl3 (1 mg/mL) at 90 °C for 2 h, 70 °C for 2 h, and 90 °C for 15 h, respectively.

This study found that the mixed-valent Mn3O4 phase could exert bidirectional performance in templating the galvanic replacement reaction, which induces both the reduction of noble metal ions and oxidation of low-valent Ce3+ ions. This bidirectional function of the Mn3O4 was exploited to fabricate noble metal/CeO2 nanocomposites, in which a high density of noble metal nanocrystals were supported on the hollow CeO2 nanoparticle,25,28−31 by transforming Mn3O4 nanoparticles through either sequential or simultaneous reactions with noble metal and Ce3+ ions. The resulting hollow nanoreactor exhibited highly improved performance in catalyzing the CO oxidation reaction, which is attributed to the synergistic catalyst/support interaction at the newly created Pt/CeO2 interface (Scheme 1). Scheme 1. Post-Synthesis Modification Protocol of the Hollow Silica Nanoreactor

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EXPERIMENTAL SECTION

General Consideration. Any reagent including MnCl2·4H2O (Kanto), sodium oleate (TCI), 1-octadecene (Aldrich), Igepal CO-520 (Aldrich), tetraethyl orthosilicate (Acros), NH4OH (Samchun chem.), hydroxylamine solution (Wako, 50%), NaOH (Samchun chem.), HNO3 (Samchun chem), HCl (Samchun chem.), BaMn2O4 (Alfa Aesar), Na2PtCl4·xH2O (Strem), Na2PdCl4·3H2O (Strem), RhCl3· xH2O (Strem), and IrCl3·xH2O (Strem) was used as purchased without any purification. Ce(NO3)3·6H2O was used as purchased after drying under vacuum for 2 days. The analyses using transmission electron microscopy (TEM) and electron diffraction spectroscopy (EDS) were performed with a JEOL JEM-2100. EDS mapping was performed using a JEOL JEM-ARM200F with Cs corrected FE-TEM at Gumi Electronics and Information Technology Research Institute. The contents of the metal elements in the nanoparticle were measured by inductive coupling plasma atomic emission spectrometry (ICPAES; Shimadzu, JP/ICPS-7500). X-ray diffraction patterns were obtained using an X-ray diffractometer (18 kW) (Rigaku MAX-2500, Japan). Preparation of sf-Mn3O4. MnO nanoparticles and MnO@ Mn3O4@SiO2 nanoparticles were prepared by following the reported method.21,32 The MnO@Mn3O4 nanoparticles encapsulated by the SiO2 shell were thermally transformed into Mn3O4 by annealing under air at 500 °C for 5 h. The annealed solids were dispersed in a degassed NaOH solution (3.0 M, pH 14.0) and stirred at room temperature under the N2 atmosphere for 15 h to remove the silica shell. The resulting solids, sf-Mn3O4, were collected by centrifugation and washed with water by repeating dispersion in an aqueous suspension and centrifugation three times. Galvanic Replacement Reaction of sf-Mn3O4 Nanoparticles in a Ce3+ Solution. A total of 3 mg of sf-Mn3O4 nanoparticles was immersed in 1 mL of an aqueous solution of Ce(NO3)3·6H2O (500 mg/mL) and stirred at 70 °C for different reaction times (10 min and 0.5, 1, 2, and 4 h). The resulting Mn3O4@CeO2 nanoparticles were isolated by centrifugation and purified by repeating dispersion in an aqueous suspension and centrifugation three times. For the control reaction at a different temperature, the reaction time is differentiated each time (90 °C, 10 min, 0.5 h, and 1 h, and 50 °C, 1, 2, 4, and 8 h). Control Experiments for the Galvanic Replacement with Ce3+ Ions. For the preparation of Mn3O4-dissolved solution, 15 mg of sf-Mn3O4 were dispersed in 50 μL of HCl solution at room temperature for overnight. Oleic acid stabilized MnFe2O4 and Fe3O4 B

DOI: 10.1021/acs.chemmater.6b04097 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Dual Galvanic Replacement Reaction on the Mn3O4 Layer of the HMON@h-SiO2 in a Sequential Manner: Replacement of the Catalyst-Immobilizing Mn3O4 Layer of the HMON/Ms@hSiO2 Nanoreactor with CeO2. The (HMON/Ms)@h-SiO2 (M = Pt, Pd, Ir, Rh) nanoreactors were prepared by following the reported method.17 In the procedure to prepare HMON/Pts@h-SiO2, 1 mg of HMON@h-SiO2 was immersed into a 1 mL of Na2PtCl4·xH2O (1 mg/mL) aqueous solution, whose pH was adjusted to 2.4 by the addition of diluted HCl solution, at 70 °C, and stirred for 2 h. The resulted HMON/Pts@h-SiO2 nanoparticles were collected by the centrifugation and purified by repeating the dispersion in an aqueous suspension and the centrifugation three times. The preparation of the HMON/Pds@h-SiO2, HMON/Irs@h-SiO2, and HMON/Rhs@hSiO2 were conducted by immersing 1 mg of HMON@h-SiO2 in Na2PdCl4·3H2O (1 mL, 1.5 mg/mL, 90 °C), IrCl3·xH2O (1 mL, 1 mg/mL, 70 °C), and RhCl3·xH2O (1 mL, 1 mg/mL, 70 °C) solutions, at pH 2.4, and stirring for 6, 1, and 2 h, respectively. The galvanic replacement reaction on the Mn3O4 layer of the (HMON/Ms)@hSiO2 was performed through a similar procedure which was applied for the reaction the HMON@h-SiO2. A total of 3 mg of (HMON/Ms)@ h-SiO2 was immersed into 1 mL of an aqueous solution of Ce(NO3)3· 6H2O (500 mg/mL), whose pH was adjusted to 2.2 by the addition of diluted HNO3 solution, and stirred at 90 °C for 15 h. The resulted (CeO2/Ms)@h-SiO2 nanoparticles were collected by the centrifugation and purified by repeating the dispersion in an aqueous suspension and the centrifugation three times. Catalytic CO Oxidation over (Mn 3O4/CeO2/Pts)@h-SiO2 Catalysts. Catalytic CO oxidation was carried out in a batch reactor. In detail, a ceramic heater was placed in the reaction chamber to heat the catalysts which were prepared by drop-casting of colloidal core@ shell nanospheres on a silica substrate. The average thickness of the core@shell nanosphere layers was 1.8 μm, which was confirmed by scanning electron microscopy (SEM) (Figure S10). The chamber was evacuated down to 5 × 10−8 Torr by a turbo pump. 40 Torr CO, 100 Torr O2, and 620 Torr He were inserted into the chamber sequentially, and the heater was heated to desired reaction temperatures. The gases were circulated through the reaction channel by a recirculation pump, and a gas chromatography equipped with a thermal conductive detector (TCD) analyzed the gaseous products. The reaction rates are presented as turnover frequencies (TOFs) which were determined in units of product CO2 molecules produced per Pt metal surface site per second. The number of metal sites is calculated by geometrical considerations based on SEM measurements of the surface area of the nanospheres. In this study, the reaction results were obtained in a kinetically controlled regime, because the reaction was relatively slow and the conversion of the reaction was lower than 10%.35

Figure 1. (a) TEM and HRTEM (inset) images, (b) XRD patterns, and (c) ICP−AES-determined Mn and Ce contents of the sf-Mn3O4 and samples isolated during its reaction with Ce(NO3)3·6H2O at different time periods.

currently, the Mn3O4 dissolved, became porous, and finally crumbled at 4 h into a collapsed aggregate of CeO2 NCs (Figure 1a). The replacement of the Mn3O4 by CeO2 in a 1:1.5 ratio was also evident from XRD (Figure 1b) and inductively coupled plasma atomic emission spectroscopy (ICP−AES) (Figure 1c) of solids sampled at different reaction times. By taking the reported standard reduction potentials of Ce3+(aq)/ CeO2(s) (1.33 V) and Mn3O4(s)/Mn2+(aq) (1.82 V) pairs, the above observed process can be understood as a redox reaction between Mn3O4 solid and Ce3+ ions in solution, which goes in the reverse direction to what usually occurs in metallic systems.37 The experiments at higher temperature displayed more rapid CeO2 deposition most likely due to more positive potential between redox pairs (Figure S2). When additional control reactions were carried out in a Mn3O4-dissolved HCl solution or with other metal oxide, such as BaMn2O4, MnFe2O4, and Fe3O4, instead of Mn3O4, only that with the BaMn2O4 resulted in the similar CeO2 deposition, which implies that solid surface including reducible Mn3+ is responsible for the galvanic replacement with Ce3+ ions (Figure S3). More interestingly, when the sf-Mn3O4 was immersed in a mixture solution of Na2PtCl4·xH2O and Ce(NO3)3·6H2O at 90 °C and pH 2.2, the sf-Mn3O4 was monitored to be replaced continuously from the shell by a mixed deposit of reduced Pt and oxidized CeO2 nanogranules (Figure 2a,b, Figure S4). TEM images of the resulting nanocomposite, Mn3O4@CeO2/ Pts, showed a core@shell-type superstructure in which tightly agglomerated Pt and CeO2 nanogranules form a shell around the partially dissolved Mn3O4 core (Figure 2a,b). This process can be explained by two spontaneous galvanic reactions, each of which involves positive redox couples: (i) Ce3+(aq)/CeO2(s) and Mn3O4(s)/Mn2+(aq) and (ii) Mn3O4(s)/Mn3+ and PtCl42− (aq)/Pt(s), which occurred simultaneously on the sf-Mn3O4 template (Figure 2a,b,d, Scheme 1). Among control experiments, by excluding the sf-Mn3O4 or by immersing BaMn2O4, MnFe2O4, or Fe3O4, instead of the sf-Mn3O4, only the reaction with Mn(II)Fe2O4 yielded deposition of Pt nanogranules at the surface; in contrast, the reaction with BaMn(III)2O4 yielded only the deposition of CeO2 (Figure S5). On the other hand, when the sf−Mn3O4 spheres were preornamented with 1.7

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RESULTS AND DISCUSSION Bidirectional Behavior of a Mn3O4 Nanoparticle during Galvanic Replacement. This study began by investigating whether Mn3O4, which can reduce noble metal ions,20,34,36 could also be involved in any oxidative reaction with Ce3+ ions by reversing the redox direction. When a Ce(NO3)3· 6H2O solution was mixed with an aqueous suspension of 20 (±3)-nm surfactant-free Mn3O4 nanoparticles (sf-Mn3O4) at 70 °C, the initial brown color gradually faded and the suspension turned opaque milky-white within 4 h. Transmission electron microscopy (TEM), high resolution TEM (HR-TEM), and X-ray diffraction (XRD) of the resulting solids revealed the transformation of the sf-Mn3O4 sphere to tiny CeO2 nanogranules of 3.3 (±0.7)-nm size that aggromerated into clusters of 13 (±1)-nm overall diameter (Figure 1). Polygonal CeO2 NCs began to be deposited dispersedly on the Mn3O4 surface in as little as 10 min, and additional deposition between 30 min and 1 h led to the evolution of planar aggregates of CeO2 nanogranules from small patches to full surface coverage around the sf-Mn3O4 (Figure S1). ConC

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Figure 3. TEM, HRTEM (inset), and STEM-EDS element mapping images of (a) HMON@h-SiO2, (b) CeO2@h-SiO2, and (c) (CeO2/ Pt)@h-SiO2, which were obtained from the immersion of the HMON@h-SiO2 in Ce(NO3)3·6H2O and Na2PtCl4·xH2O mixture solutions, repectively, and (d) (HMON/Pts)@h-SiO2, and (e) (CeO2/Pts)@h-SiO2, which resulted from the reaction of the (HMON/Pts)@h-SiO2 in a Ce(NO3)3·6H2O solution. Histograms of (d) and (e) show the size distributions of supported Pt nanocrystals.

Figure 2. TEM, HRTEM, and EDS line-profiling image samples isolated after immersing the sf-Mn3O4 in a mixture solution of Na2PtCl4·xH2O and Ce(NO3)3·6H2O for (a) 0.5 h and (b) 6 h and (c) after immersing the Mn3O4/Pt in a Ce(NO3)3·6H2O solution for 4 h. (d) ICP-AES-determined Mn, Pt, and Ce contents of samples obtained during the reaction of the sf-Mn3O4 in a mixture solution of Na2PtCl4·xH2O and Ce(NO3)3·6H2O.

solutions of Ce(NO3)3 with Na2PdCl4·3H2O, RhCl3·xH2O, and IrCl3·xH2O, respectively (Figure 4).

(±0.3)-nm Pt NCs in a Na2PtCl4 solution, then isolated Mn3O4/Pts were reimmersed in a Ce(NO3)3 solution at 90 °C; for examining the possible sequential deposition process, the replacement of Mn 3 O 4 in the Mn 3 O 4 /Pts with CeO 2 proceeded slowly but continuously and produced CeO2@Pts nanocomposite in which tiny 1.6 (±0.2)-nm Pt NCs were dispersed near the shells of spherical clusters of CeO2 nanogranules (Figure 2c, Figure S6). Galvanic Replacement of the Mn3O4 Layer inside the Cavity of the Preformed Hollow Nanoreactor: Incorporation of Noble-Metal/CeO2 Interfaces. The next study exploited the validated bidirectional behavior of the Mn3O4 template in the galvanic replacement process to incorporate a into preformed hollow silica nanospheres (Scheme 1). A hollow silica nanosphere, HMON@h-SiO2, with the Mn3O4 thin layer-coated interior surface was immersed in Ce(NO3)3· 6H2O solution at 90 °C and pH 2.2 to induce the galvanic replacement reaction. Initial attempts revealed that reaction in air caused formation of CeO2 patches on the silica that obstruct the Ce3+ ions from reaching the Mn3O4 layer inside (Figure S7),38 whereas under N2, the interior cavity surface, which had initially been coated by the Mn3O4 layer, gradually became covered with CeO2 NCs over a period of 2 h; this process generated a core@shell-type nanosphere, CeO2@h-SiO2, which is filled with heavily clustered CeO2 nanogranules of 2.7 (±0.5)-nm average size (Figure 3a,b). The Mn3O4 layer of the HMON@h-SiO2 also served a dual templating function in a mixture solution of Na2PtCl4·xH2O and Ce(NO3)3·6H2O, thus leading to simultaneous growth of Pt and CeO2 on the inner cavity surface. TEM and STEM elemental mapping images of the resultant (CeO2/Pt)@h-SiO2 revealed the conversion of the initial Mn3O4 layer mostly into a 8.9 (±0.9)-nm-thick shell composed of a 1:0.6 mixture of agglomerated Pt and CeO2 nanogranules, which represents successful introduction of the desired Pt/CeO2 interface into the hollow nanosphere (Figure 3c). This dual replacement process was also used to introduce Pd, Rh, and Ir with CeO2 on the Mn3O4 layer of HMON@hSiO2 to generate (CeO2/Ms)@h-SiO2 (M = Pd, Rh, Ir) nanocomposites, by treating the HMON@h-SiO2 with mixture

Figure 4. TEM, HRTEM, and STEM-EDS element mapping images of (a) (CeO2/Pd)@h-SiO2, (b) (CeO2/Ir)@h-SiO2, and (c) (CeO2/ Rh)@h-SiO2, which were prepared by treating HMON@h-SiO2 in mixture of solutions of Ce(NO3)3 with Na2PdCl4, IrCl3, and RhCl3, respectively.

Next, we evaluated the possibility of sequentially functionalizing the interior of the HMON@h-SiO2 step by step to connect the individual galvanic replacement steps for Pt and CeO2 depositions, because this process may improve the precision and control of the catalyst/support interface. For this purpose, (HMON/Pts)@h-SiO2, in which 1.2 (±0.3)-nm Pt NCs are densely and dispersedly loaded on the Mn3O4 thin layer at the cavity surface, was synthesized and then immersed in a Ce(NO3)3·6H2O solution at 90 °C and pH 2.2 (Figure 3d). The galvanic replacement between the Mn3O4 layer of the (HMON/Pts)@h-SiO2 and Ce3+ ions preceded relatively slowly but steadily over time (Figure 5). Therefore, after 15 h of reaction time, ca. 35.5% of Mn3O4 had been replaced by CeO2 nanogranules that had grown inward to partially fill the cavity; the result was (CeO2/Pts)@h-SiO2 nanospheres with CeO2/Pt interfaces inside the cavity (Figure 3e). It was remarkable that the immobilized Pt NCs maintained their tiny size of 1.3 (±0.3) nm and well-dispersed state while their support was changed from Mn3O4 to CeO2. Moreover, by applying a similar procedure, Mn3O4 layers in a series of hollow D

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Evaluation of the Effectiveness of the (CeO2/Pts)@hSiO2 in Catalyzing CO Oxidation Reaction. To verify the effectiveness of these hollow nanoreactors, which were modified through the postsynthesis modulation protocol, we made a comparative examination of the efficiencies at which (CeO2/Pts)@h-SiO2 and (HMON/Pts)@h-SiO2 catalyzed CO oxidation. We found that these nanoreactors were catalytically reactive, due to the facile diffusion of reactant and product molecules through the porous SiO2 shell.39 In addition, catalytic activity of Pt in this oxidation is greatly increased due to strong metal−support interaction when metallic Pt contacts oxide supports due to strong metal− support interaction.26,40−42 Depending on reaction time, three different catalysts [(Mn3O4/Pts)@h-SiO2, (Mn3O4/CeO2/ Pts)@h-SiO2, and (CeO2/Pts)@h-SiO2] were produced by the controlled galvanic replacement that provided distinct interfaces: Pt−Mn3O4, Pt−Mn3O4/CeO2, and Pt-CeO2. CO oxidation reactions were conducted in a batch reaction system under 40 Torr CO, 100 Torr O2, and 620 Torr He at 463−503 K.43−45 The number of Pt sites was calculated by geometrical considerations based on scanning electron microscopy (SEM) measurements of the surface area of the nanospheres, and turnover frequencies (TOFs) were determined in units of product CO2 molecules produced per Pt metal surface site per second. TOF values of different kinds of Pt catalysts which were either single crystal,43,46 cluster,47 nanoparticle,23,39,43,47 or core@shell structures39 were reported in many works in the literature (see Supporting Information Table S1). The reported TOF of the Pt(111) surface at 100 Torr O2 and 40 Torr CO was ∼3 sites−1·s−1 at 513 K.43,46 This value is a little lower than those of Pt nanoparticles encapsulated by tetradecyltrimethylammonium bromide (TTAB) within a factor of 2, because the higher number of kinks and edge sites at Pt nanoparticle surfaces induced higher activity enhancement compared to those of the Pt single crystal surface.43 Comparing to the reported TTAB-capped Pt nanoparticles (TOF = 3.0 sites−1·s−1 at 493 K),43 (Mn3O4/Pts)@h-SiO2 had a relatively higher TOF value (4.16 sites−1·s−1 at 493 K) due to the support effect by Mn3O4 (Figure 7a). When CeO2 species were added into

Figure 5. Time course TEM and HRTEM (inset) images with EDS data (inset) of (a) the (HMON/Pts)@h-SiO2 and samples isolated during its reaction with Ce(NO3)3 under N2 atmosphere at 90 °C for (b) 10 min, (c) 0.5 h, (d) 1 h, (e) 2 h, and (f) 15 h.

nanoreactors (HMON/Ms)@h-SiO2 (M = Pd, Rh, Ir) could be all successfully replaced with the CeO2 support without damaging the immobilized metal NCs; this process allowed fabrication of various hollow nanoreactors of (CeO2/Ms)@hSiO2 (M = Pd, Rh, Ir) that bear CeO2-supported noble-metal catalyst interfaces in the protected interior (Figure 6).

Figure 7. Catalytic activity result of (CeO2/Pts)@h-SiO2, (Mn3O4/ CeO2/Pts)@h-SiO2, and (Mn3O4/Pts)@h-SiO2 for CO oxidation reactions at different temperatures: (a) TOFs and (b) Arrhenius plots.

the hollow nanospheres, the TOF of (CeO2/Pts)@h-SiO2 was further increased to 7.75 sites−1·s−1 at 493 K. In the previous works of CO oxidation over Pt/SiO2 and Pt/CeO2 catalysts in 40 Torr CO and 100 Torr O2, similar trends were found, although the reactions were carried out in a plug-flow reactor. When 2.5 nm sized Pt nanoparticles were supported on either mesoporous SiO2 or CeO2, the TOFs were 0.6 sites−1·s−1 at 513 K and 0.5 sites−1·s−1 at 473 K, respectively. Considering the temperature for the calculation of TOFs, the Pt/CeO2 catalyst

Figure 6. TEM, HRTEM (inset), and STEM-EDS element mapping images of (a) (CeO2/Pds)@h-SiO2, (b) (CeO2/Irs)@h-SiO2, and (c) (CeO2/Rh)@h-SiO2, through connecting the individual galvanic replacement steps for noble metal such as Pt, Pd, or Ir and CeO2 deposition. E

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Chemistry of Materials Notes

showed a much higher CO oxidation rate than the Pt/SiO2, due to the SMSI effect.23 The calculated activation energy, Ea [kcal/ mol], was 21.3 for (Mn3O4/Pts)@h-SiO2, 20.6 for (Mn3O4/ CeO 2 /Pts)@h-SiO 2 , and 19.4 for (CeO 2 /Pts)@h-SiO 2 (Figure 7b). These results clearly show that the Pt−CeO2 interface increased CO oxidation more than did Pt−Mn3O4 interfaces. The Pt−Mn3O4/CeO2 interface provided by (Mn3O4/CeO2/Pts)@h-SiO2 had an intermediate effect on CO oxidation (TOF = 5.25 sites−1·s−1 at 493 K). These overall results are consistent with previous reports that active oxygens provided by the oxide lattice at the Pt−oxide interface can accelerate CO oxidation and that this effect is much stronger at a Pt−CeO2 interface than at a Pt−Mn3O4 interface.23,26 Our results demonstrate that Pt/Mn3O4 in the hollow silica nanoreactors could be effectively transformed to Pt/CeO2, which increased the catalytic efficiency of CO oxidation. The outer hollow silica wall provided both a protected layer during the replacement reaction for catalyst preparation and a versatile channel for reactants and products during catalytic reaction. When the (CeO2/Pt)@h-SiO2 was reanalyzed after the CO oxidation nanoparticles by using TEM, the any significant change was observed in the morphology of CeO2 coating on the interior cavity surface. And, CeO2-immobilized Pt nanocrystals were also found to maintain their well-dispersed state during the CO oxidation reaction with a just slight increase in their sizes from 1.3 ± 0.3 nm to 1.9 ± 0.3 (Figure S9).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF-2016R1A3B1907559) (I.S.L.) and (2.150640.01) (K.A.). J.Y.P. acknowledges the support by IBS-R004-G4. We acknowledge Dr. Jongwon Kim at Chungbuk National University for help with the discussion about appling standard reduction potential to our phenomena.

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CONCLUSION In summary, we report discovery of the bidirectional templating capability of Mn3O4 in the galvanic replacement reaction and exploit this ability to develop a postsynthesis method to control the nature of the catalytic interface in a hollow nanoreactor. Using this method we fabricated hollow silica nanoreactors that bear a range of CeO2-immobilized noble-metal catalysts, by replacing the preloaded Mn3O4 layer without damaging the immobilized catalysts. This postsynthesis approach may help the evolution of platform-based fabrication of hollow nanoreactors, in which the size, composition, and dispersion of the cataytic NCs and their interfacial properties can be adjusted according to the prespecified applications. The support− exchange process in the presence of core metal nanoparticles resulted in increased activity in the catalytic CO oxidation reaction by creating another oxide support inside the hollow nanoreactor. This strategy of galvanic replacement reaction opens new possibilities to increase catalytic activity by controlling synergy at the catalyst/support interface.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04097.

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REFERENCES

Experimental details and additional analysis data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (I.S.L.). *E-mail: [email protected] (K.A.). ORCID

In Su Lee: 0000-0002-2588-1444 F

DOI: 10.1021/acs.chemmater.6b04097 Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.chemmater.6b04097 Chem. Mater. XXXX, XXX, XXX−XXX