Preparation Method of Co3O4 Nanoparticles Using Ordered

Article pubs.acs.org/JPCC

Preparation Method of Co3O4 Nanoparticles Using Ordered Mesoporous Carbons as a Template and Their Application for Fischer−Tropsch Synthesis Geunjae Kwak,† Jongkook Hwang,‡ Joo-Young Cheon,† Min Hee Woo,† Ki-Won Jun,† Jinwoo Lee,*,‡ and Kyoung-Su Ha*,† †

Research Center for Green Catalysis, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Sinseongno 19, Yuseong, Daejeon 305-600, Republic of Korea ‡ Advanced Functional Nanomaterial Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea S Supporting Information *

ABSTRACT: Co3O4 nanoparticles (NPs) with a narrow particle size distribution were fabricated via a facile and novel method using mesoporous carbon materials (CMK-3, MSU-F-C) as sacrificial templates. The particle size distribution of the Co3O4 NPs varied depending on the pore size of the templates. Synthesis of the NPs with concurrent complete removal of the templates was achieved at 593 K, which is lower than the temperatures utilized in previous reports. It was verified that the carbon template was decomposed by catalytic oxidation with cobalt and NOx species generated by thermal decomposition of the cobalt nitrate precursor in air. The prepared NPs, and particularly the Co3O4 NPs synthesized from CMK-3, acted as excellent catalysts for the Fischer−Tropsch synthesis (FTS). The high catalytic performance was associated with the optimum particle size (6−10 nm) of the nanoparticles for FTS and enhanced reducibility.

1. INTRODUCTION Ordered mesoporous carbons (OMCs), synthesized via both hard and soft template methods,1−5 have garnered increasing attention in recent years due to their high surface area, tunable pore size, and interconnected pores. These features make OMCs potentially applicable as catalyst supports, in electrochemical capacitors, and hydrogen storage and separation, thereby fuelling the research initiative.6−10 OMCs appended with various functional groups and having peculiar pore structures have been prepared and utilized as supports or nests for the growth of NPs in order to add various functionalities.11−16 Generally, OMC-supported NPs are prepared by incorporating a desired precursor of the NPs into the uniform mesopores of OMCs; subsequent decomposition, nucleation, and growth of the NPs occur within the pore. This technique of fabricating nanostructures using OMCs as templates may offer the advantages of size and shape control of the nanostructure, as well as control of the extent of NPloading into the OMCs, and of the occupied site of the nanostructures within the OMCs.17 It is recognized that the size of the NPs used in catalysis exerts a significant effect on the catalytic performance, and small particles in the size range of 3−15 nm are more active and stable in catalytic reactions;18,19 this desired size range may be achieved by the confined growth of NPs within the mesopores of OMCs. Thus, the fabrication of NPs from OMC templates can provide a simple strategy for the design of NPs that are best suited to particular applications. © 2013 American Chemical Society

The use of templates containing nanopores in a defined dimension and the self-assembly method are the two conventional approaches for fabricating pure NPs with narrow particle size distributions. In the template approach, NPs can be synthesized via infiltration of an appropriate precursor into the template pore, followed by nucleation and growth. This approach requires the removal of scaffolds using corrosive media or thermal treatment, which may alter the intrinsic properties of the NPs due to surface damage or sintering of the NPs at high temperature. In the self-assembly approach, amphiphilic surfactants are used for fabricating monodispersed NPs with a specific size and shape, and these molecules determine the initial conformation of the grown NPs. The surfactants can be adsorbed on a specific site or facet of the grown NPs and then induce growth along the direction normal to the adsorbed facet, resulting in the growth of NPs with an unexpected shape. Although this approach facilitates variations in the shape and growth direction of NPs (facet-controlled) for further applications,20,21 the catalytically active sites of the NPs may be enshrouded or blocked by the surfactant, with consequent degradation of the catalytic activity of the NPs. Moreover, the self-assembly approach may also require the removal of surfactants as sacrificial templates in order to Received: October 28, 2012 Revised: December 30, 2012 Published: January 7, 2013 1773

dx.doi.org/10.1021/jp3106698 | J. Phys. Chem. C 2013, 117, 1773−1779

The Journal of Physical Chemistry C

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microscope equipped with a TECNAI G20 instrument for obtaining images of the prepared OMCs and NPs. The thermal decomposition behavior of the OMCs and impregnated cobalt precursors (0.2 M Co(NO3)2·6H2O ethanol solution of 20 mL and 0.2 M Co(CH3COO)2·4H2O ethanol solution of 20 mL) in the presence of the mesoporous carbon was evaluated via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), where the samples were heated from 30 to 800 °C at a ramping rate of 10 °C/min under controlled air flow at a flow rate of 100 mL/min. Temperature-programmed reduction (TPR) was performed to determine the reducibility of the as-synthesized Co3O 4 NPs. Prior to the TPR experiments, the samples were pretreated by heating to 350 °C in a He flow and maintained at this temperature for 2 h to remove the adsorbed water and other contaminants, followed by cooling to 50 °C. The reducing gas containing 5% H2 balanced with He was passed through the samples at a flow rate of 30 mL/min while heating to 900 °C at a rate of 10 °C/min; samples were kept at that temperature for 0.5 h. The effluent gas was passed over a molecular sieve trap to remove the generated water and was analyzed by means of GC equipped with a thermal conductivity detector (TCD). Powder X-ray diffraction (XRD) patterns were obtained at room temperature in the 2θ range from 10° to 80° using a Rigaku diffractometer with Cu Kα radiation to identify the crystalline phases of the cobalt oxides. The average NP size was calculated from the most intense peak by using Scherrer’s equation. Prior to XRD analysis of the NPs that had been used in the FTS, the NPs were washed with cyclohexane to remove deposited hydrocarbons and other contaminants. The chemical compositions of the as-prepared Co3O4 NPs and those used in the FTS were characterized by means of X-ray photoelectron spectroscopy (XPS); the spectra were collected using an ESCALAB MK-II spectrometer with a standard Al Kα source (1486.6 eV) and a working pressure of less than 10−7 Pa. The binding energy was corrected using the C 1s (284.4 eV) line as a reference. Catalytic Testing. The FTS was carried out in a down-flow fixed bed reactor. The reactor consisted of a stainless steel tube with an internal diameter of 10 mm, heated by an electronic furnace. The NP catalysts (ca. 0.15 g) were physically mixed with 0.75 g of α-alumina powder to minimize the temperature gradient; loaded into the center of the reactor; reduced in situ with a 5 vol % H2/He flow with heating from ambient temperature to 400 °C, and held at 400 °C for 12 h. The reactor was subsequently cooled to 100 °C, and pressurized syngas (H2/CO/CO2/Ar = 57.3/28.4/9.3/5.0 mole ratio) was introduced into the reactor, where the space velocity, reaction temperature, and pressure were 2000 mLsyn/(gcat. h), 220 °C, and 2.0 MPa, respectively. Ar was used as an internal standard for GC analysis of CO and the products. A hot trap at 150 °C and a cold trap at 20 °C were used to collect wax and liquid products, respectively. The effluent gas from the reactor was analyzed via online gas chromatography (Younglin Acme 6000 GC), employing a GS-GASPRO capillary column connected to a flame ionization detector (FID) for analysis of the hydrocarbons and using a Porapak Q/molecular sieve packed column connected to a TCD. The conversion and selectivity toward the hydrocarbons were calculated based on CO consumption, and the selectivity was calculated on a carbon mole basis (details of calculations are provided in the Supporting Information).

enhance the catalytic performance. In brief, both techniques require an additional step for removing the template/surfactant, which is a stringent and sophisticated process. Thus, more effective and facile techniques for NP synthesis that do not obscure the catalytic performance and in which the intrinsic properties are preserved by preventing surface damage and sintering of NPs during template removal must be developed. The versatility of cobalt oxide (Co3O4) has led to its use in applications such as gas sensors,22 electrochromic devices,23 Liion battery electrodes,24 and heterogeneous catalysts25 for the production of clean automotive fuels. The high catalytic performance and low deactivation rate in the synthesis of long chain hydrocarbons have made cobalt catalysts derived from the oxide phase ideal for use in Fischer−Tropsch synthesis (FTS). We report herein a novel method for preparing pure Co 3O 4 NPs with narrow particle size distributions, using highly soluble and low-cost cobalt precursors that are readily decomposed, on the basis of an in situ template approach without an additional template removal step. In this method, the nucleation and growth of NPs within the pores of OMCs was concurrent with the complete removal of carbon templates through thermal treatment in air at a lower temperature than the conventional thermal decomposition temperatures of pure OMCs in air. Co3O4 NPs with various particle size distributions were fabricated using two types of OMC (CMK-3 and MSU-F-C) as templates. The promising catalytic applicability of the cobalt NPs fabricated herein was demonstrated in the FTS. The prepared NPs were also characterized via various techniques, and their physicochemical properties were correlated with the corresponding catalytic activity.

2. EXPERIMENTAL SECTION Synthesis of Co3O4 NPs. The OMCs, CMK-3 and MSU-FC, were synthesized according to reported procedures.9,26 The CMK-3 carbon was prepared using mesoporous silica SBA-15 as a template and furfuryl alcohol as a carbon source. MSU-F-C was synthesized in a similar manner using the mesocellular silica, MSU-F-silica, as a template. The synthesized OMCs (CMK-3 and MSU-F-C) were pretreated with 1 M nitric acid solution for 2 h to introduce hydrophilic oxygenated functional groups onto the surface of the OMCs in order to facilitate penetration of the cobalt precursors into the mesopores. Following this step, 1 g of the functionalized OMCs was impregnated with a cobalt precursor solution (0.2 M) of 20 mL (1.235 g of Co(NO3)2·6H2O dissolved in absolute ethanol) using the incipient wetness impregnation method, followed by physical mixing under atmospheric conditions for 2 h. The mixture was subsequently dried in a rotary vacuum evaporator at 80 °C for 2 h. After predrying, the cobalt impregnated OMCs were again dried in air at 110 °C for 12 h and then calcined at 300 °C for 5 h at a ramping rate of 1 °C/ min under an air flow. Characterization. The BET surface area, pore volume, and pore size distribution of the OMCs were estimated from nitrogen adsorption−desorption experiments performed at 77 K using a constant-volume adsorption apparatus (Micromeritics Tristar II 3020). The specific surface area of the samples was calculated according to the Brunauer−Emmett−Teller (BET) method, and the pore size distributions were calculated from the desorption branch of the nitrogen isotherms using the Barrett−Joyner−Halenda (BJH) model. Transmission electron microscopy (TEM) was performed with a JEOL JEM-2100 1774



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3. RESULTS AND DISCUSSION OMC materials with different pore sizes were used herein as templates for the synthesis of Co3O4 NPs. Typical TEM images of CMK-3 and MSU-F-C (used as templates) are shown in the middle row of Figure 1. CMK-3 has a hexagonally ordered

Figure 2. TGA profiles of CMK-3 impregnated with (a) cobalt nitrate, (b) cobalt acetate, and (c) cobalt chloride precursors; (d) unmodified CMK-3 carbon template.

of NP loading in the pores. Furthermore, the carboxylic functionalities on the exposed surface of OMCs pretreated with acid solution enhance the percolation of the cobalt precursor solution into the pores of the OMCs by 3D capillary effects. The top row of Figure 1 shows typical TEM images of the Co3O4 NPs, which were generated from CMK-3 (left) and MSU-F-C (right) templates, after removal of the templates by calcination in air. The respective particle sizes of the Co3O4 NPs obtained from the CMK-3 and MSU-F-C templates (denoted as Co-CMK-3 and Co-MSU-F-C) were ca. 6−10 and 10−16 nm, with a narrow particle size distribution (Figure S2). The particle size of Co-CMK-3 was somewhat increased relative to the pore size of the template, which may be attributed to sintering of small particles during calcination, but there was no evidence of significant changes. Lattice spacings of the (311) and (220) planes were observed for the Co-CMK-3 and Co-MSU-F-C samples, representative of the Co3O4 crystallographic planes (see HRTEM images of Figure S3); these results are consistent with X-ray diffraction (XRD) results (Figure S4). Cobalt oxide NPs could also be generated within the mesopores of the OMCs, as shown in the bottom row of Figure 1. Thermal annealing of the impregnated and dried samples using an inert gas atmosphere instead of oxygen-excess conditions resulted in most of the CoO NPs being evenly distributed into the mesopores without any structural damage of the OMCs. The CoO composition of the synthesized cobalt oxide NPs might be attributed to the oxygen deficient atmosphere. The similarity of the shapes and the size distributions of the NPs generated with and without the templates suggests that the cobalt precursor is initially infiltrated into the pores and decomposed to cobalt oxide, and nucleation and growth of the NPs subsequently occurred within the pores. Removal of the carbon templates seems to be preceded by the formation of the NPs within the mesopores. The detailed catalytic performances and physicochemical properties of the OMCs-supported CoO NPs are not included in this paper but will be separately published elsewhere. The cobalt nitrate hexahydrate (CoN) precursor used herein is the salt most commonly used in the preparation of supported cobalt catalysts for FTS, due to its ability to be fully converted into the corresponding oxide and almost unlimited solubility in aqueous media resulting in higher loading in the support. With

Figure 1. TEM images of OMCs (CMK-3 and MSU-F-C) used as templates (middle row), pure Co3O4 NPs synthesized by one-pot method using OMC templates (top row), and Co3O4 NPs within the mesopores of OMCs, synthesized under oxygen-deficient conditions (bottom row).

structure with rodlike walls; in particular, highly one-dimensional ordered channels are arrayed along the rods, which is the reverse hexagonal structure of ordered SBA-15 (left image in the middle row of Figure 1). The pore size determined from the BJH calculation and the BET surface area of CMK-3 were ∼4 nm and 1496 m2/g, respectively (Figure S1, Supporting Information). MSU-F-C carbon has bimodal pores consisting of large cellular pores (>20 nm) and small pores (∼6 nm), which are generated by the partial coating of furfuryl alcohol as a carbon precursor on the main cellular pores and the dissolution of the silicate walls of the MUS-F silica template (right image in the middle row of Figure 2). This bimodal pore structure was also confirmed based on the pore size distributions calculated from the N2 isotherm using the BJH method (Figure S1). MSU-F-C has a three-dimensionally isotropic open pore structure in all directions and a high BET surface area (1084 m2/g). The ordered mesopores of these OMCs with interconnected pore structures facilitate diffusion of the NP precursor solution into the OMCs during the impregnation step, and the extremely large surface area provides a high extent 1775



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cobalt-catalyzed reduction of NOx using carbon as a reducing agent was observed at temperatures ranging from 200 to 300 °C, resulting in the conversion of NOx and carbon to N2 and CO2 as respective reaction products.35 The reported temperature of complete NO x reduction is similar to the decomposition temperature the carbon template of CoN/ CMK-3. That is, the occurrence of decomposition of CMK-3 at the lower temperature is attributed to cobalt-catalyzed oxidation of carbon with NOx gas generated by the dissociation of CoN. During the period of the weight loss of CoN/CMK-3, it was confirmed that the endothermic event was also consistent with the rapid loss of weight from DSC result (Figure S5). The TG profiles of three CMK-3 samples in the absence of NOx molecules were acquired by physically mixing the respective samples with conventional cobalt metal powder and two types of cobalt oxide powder (CoO, Co3O4), followed by thermal analysis under air flow, as shown in Figure 3. The

elevation of the temperature, the CoN precursor undergoes a stepwise partial dehydration to generate cobalt nitrate monohydrate as an intermediate at 50 °C. Subsequent decomposition with evolution of NOx species yields the oxide product, Co3O4, which is completed at 280 °C in a single endothermic step in air.27−29 However, CoN dispersed on a support shows a markedly altered decomposition behavior compared with the unsupported CoN. The complete decomposition of CoN dispersed on oxidic supports (i.e., SiO2, Al2O3, ZrO2, CeO2, TiO2) is generally retarded until high temperature is achieved due to strong interactions between the precursor and support materials during preparation. In particular, the decomposition of CoN dispersed on γ-alumina occurs at a significantly elevated temperature of around 600 °C.30 Figure 2a shows the thermogravimetric (TG) profiles of CoN impregnated on CMK-3 (CoN/CMK-3) under thermal treatment in air. CoN/CMK-3 exhibited thermal decomposition behavior similar to that of unsupported CoN during the calcination process in flowing air, despite the presence of the carbon support. The TG profile shows a rapid increase in the weight loss to 80% above 325 °C, which is higher than the theoretical weight loss corresponding to decomposition of the nitrate moiety of CoN. This result indicates that combustion of the carbon support occurred concurrently with the decomposition of CoN at elevated temperature. The rapid weight loss above 240 °C was attributed to both the loss of residual components (NOx, H2O) dissociated from CoN and complete combustion of the carbon material used as a sacrificial template. The weight of the residual after thermal treatment beyond 325 °C corresponds to the conversion product Co3O4 obtained by the decomposition of CoN. Interestingly, the oxidation onset of pure graphitic carbon in air typically occurs at ca. 530−580 °C.31 Figure 2d shows that pure CMK-3 used herein after acidpretreatment began to decompose at 530 °C, which is in agreement with the reported results. The coincident removal of CMK-3 with the pyrolysis of CoN at the relatively low temperature of 320 °C was thus attributed to catalytic oxidation of the carbon support facilitated by the cobalt species in the pores of CMK-3 and the redox-reaction of the carbon with residual NOx gas molecules generated from dissociation of the nitrate moiety. Under oxidative conditions, cobalt species have been known to induce precombustion of raw carbon at a lower temperature than that of pure carbon in the absence of these species.32,33 However, cobalt-activated oxidation of graphitic carbon generally occurs at temperatures of around 377 °C, which is still higher than the decomposition temperature of CoN-impregnated CMK-3 (320 °C) and which is rather similar to the complete oxidation temperature of the carbon materials containing cobalt acetate (CoAc/CMK-3) and cobalt chloride (CoCl/CMK-3) precursors, respectively (Figure 2b,c). The unusual stepwise decomposition of CoAc/ CMK-3 (Figure 2b) is attributed to the exothermic decomposition of cobalt acetate, which is known to proceed at 220 °C.34 The rapid weight loss of CoAc/CMK-3 in the range of 250−330 °C appears to originate from facilitated carbon oxidation derived from the highly exothermic decomposition of cobalt acetate. In the case of CoN/CMK-3, the lowered decomposition temperature is attributed to reaction of the carbon template with NOx gas derived from decomposition of the nitrate component. Based on research on the reduction of NOx pollutant emission in the combustion of automobile fuels,

Figure 3. TGA profiles of CMK-3 species physically mixed with CoO (black dotted line), Co3O4 (red dotted line), and metallic cobalt (blue dotted line).

onset of decomposition of the samples mixed with the respective powders occurred in almost the same temperature region as that of pure CMK-3 carbon, despite the presence of the cobalt species. This clearly suggests that the oxidation of the carbon template of CoN/CMK-3 was not catalyzed simply by cobalt species produced as final products after air calcination of CoN. It was also confirmed that the weight loss corresponding to decomposition of the carbon template was not observed after thermal treatment at 400 °C for 4 h under a 100 mL/min flow of NO (5%) and He (balance) gas in the absence of catalysts. Although the chemical state of cobalt participating in the catalytic oxidation of CMK-3 was not elucidated, it was confirmed that cobalt species and NOx gas were both operative in the accelerated decomposition of carbon, resulting in the decreased removal temperature of the sacrificial carbon template. The Co 2p3/2 and O 1s XPS spectra of the as-synthesized Co-CMK-3, Co-MSU-F-C, and alumina-supported Co3O4 (Co/Al2O3) species are presented in Figure 4. Co/Al2O3, with a cobalt loading of 20 wt %, was fabricated by a simple impregnation method and was used for comparative purposes. Figure 4a shows the Co 2p3/2 XPS spectra of the samples, each of which exhibits a peak corresponding to Co3O4. The asymmetrical Co 2p3/2 peak of each sample could be decomposed into two components at binding energies of 1776



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Figure 5. TPR profiles of Co-CMK-3 and Co-MSU-F-C. Figure 4. (a) Co 2p3/2 and (b) O 1s XPS spectra of Co-CMK-3, CoMSU-F-C, and Co/Al2O3.

second reduction peak of Co-MSU-F-C was located at a slightly higher temperature region than that observed for Co-CMK-3, which was attributed to the particle size difference. The mean particle size of Co-CMK-3 was smaller than that of Co-MSU-FC, indicating that Co-CMK-3 has higher reducibility than CoMSU-F-C. Quantitative analysis of TPR runs also shows that the amount of hydrogen consumption (24.03 mmol H2/gcat) of Co-CMK-3 was larger than that (23.3 mmol H2/gcat) of CoMSU-F-C, and the respective extents of cobalt reduction obtained from Co-CMK-3 and Co-MSU-F-C were 72.3% and 70.1%. In comparison, the TPR profile of Co/Al2O3 (Figure S6) shows a significant shift of the reduction peaks to the higher temperature region within the range of 300−560 °C due to strong interaction between metallic cobalt and the alumina support. A high temperature peak (T > 700 °C) also appeared in the TPR profile of Co/Al2O3 due to the formation of irreducible cobalt aluminate compounds. Thus, the Co3O4 NPs prepared from OMC templates exhibited excellent reducibility compared with the conventional Co/Al2O3. The catalytic activity of the prepared Co-CMK-3, Co-MSUF-C, and Co/Al2O3 species was evaluated in the Fischer− Tropsch reaction under the following conditions: 220 °C, 2.0 MPa, and H2/CO = 2. The catalytic performance was evaluated in terms of CO conversion, catalytic time yield (CTY), and hydrocarbon selectivities for Co-CMK-3, Co-MSU-F-C, and Co@Al2O3; the data are summarized in Table 1. The variations of CO conversion and hydrocarbon selectivities for respective

779.8 and 782.2 eV, which were assigned to Co3+ and Co2+, respectively.36−38 The surface cobalt composition of the samples could be obtained from the peaks by quantitative analysis. The Co3+/Co2+ ratios of Co-CMK-3 and Co-MSU-FC were 2.45 and 2.43, respectively, which were much lower than the ratio of 3.07 obtained for Co/Al2O3. This difference indicates that there is a larger amount of surface oxygen vacancies on the Co3O4 NPs synthesized from the OMC templates relative to those on Co/Al2O3. This deficiency of oxygen on the surface of the former might contribute to the oxidation reaction of the carbon template with the surface oxygen during the template removal process, with consequent enhancement of the reducibility of the Co species and the catalytic performance. Figure 4b shows the O 1s spectra of all of the evaluated samples. The asymmetric O 1s spectra were fitted using the two component peaks centered at 529.3 and 531.3 eV, which were ascribed to the surface lattice oxygen species and the surface adsorbed oxygen species (O2− or O−), respectively. As expected, the amount of surface adsorbed oxygen on the Co-CMK-3 and Co-MSU-F-C species was significantly lower than that of Co/Al2O3, indicating that the most abundant component of the former two species is the surface lattice oxygen with respective ratios of 94.1% and 94.2% for Co-CMK-3 and Co-MSU-F-C. This result indicates that the Co3O4 NPs synthesized from the OMCs have a peculiar surface chemical composition, giving rise to the oxygen-deficient surface structure. It has been reported that an abundance of oxygen vacancies may promote the reducibility and activity of metal oxide catalytic materials.39 Figure 5 shows typical TPR profiles of Co-CMK-3 and CoMSU-F-C, demonstrating their reduction characteristics. Two reduction steps are observed in the TPR profiles of the cobalt oxide NPs in the peak temperature ranges of 233−235 and 260−450 °C; the first reduction peak in the lower temperature region is assigned to the reduction of Co3O4 to CoO, and the second reduction peak at higher temperature is attributed to the sequential reduction of CoO to metallic cobalt.25,40 In the case of unsupported Co3O4 NPs, the reduction temperature shows a tendency to shift to lower temperature as the particle size decreases.41 Consistent with this reported behavior, the

Table 1. Catalytic Activities of Cobalt Catalysts Synthesized by Using OMCs and Conventional Alumina Supported Cobalt Catalysts at 220 °Ca hydrocarbon selectivity (%)

catalysts

CO conv (%)

FT activity (10−5 molCO gCo−1 s−1)

CH4

C2−C4

C5+

Co-CMK-3 Co-MSU-F-C Co/Al2O3

91.0 64.5 43.2

3.20 2.27 1.32

10.5 15.7 10.8

11.5 11.3 9.9

78.0 73.0 79.2

GHSV: 2000 mLsyn/(gcat. h), P: 2.0 MPa, T: 220 °C, H2/CO ratio: 2.0, feed composition: H2/CO/CO2/Ar = 57.3/28.4/9.3/5.0 (mol %). a

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that this method may be applicable to other systems consisting other metal nitrate precursors, such as Ni(NO3)2 and Fe(NO3)3, and carbon species.

catalysts with the time on stream are shown in Figure S7. The activity of the respective catalysts remained almost constant for 50 h on stream. As shown in Table 1, Co-CMK-3 exhibited the highest catalytic performance (FT activity of 3.22 × 10−5 molCO g−1Co s−1 and C5+ selectivity of 78.0%). This high activity of the Co-CMK-3 catalyst was thought to arise from the high reducibility of the cobalt particles and an average particle size close to the optimum Co particle size for FTS. Based on previous studies,31,42,43 the optimum size of Co NPs for FTS, determined on the basis of FTS activity tests as a function of the particle size, is ca. 6−8 nm, where the cobalt particle size creates optimal domains of active sites. Thus, maximum efficiency during FTS could be achieved with Co-CMK-3 consisting of cobalt particles with an average size of 6−10 nm, relative to the other catalysts at similar cobalt metal loading. The Co-MSU-F-C catalyst exhibited comparatively lower FT activity of 2.02 × 10−5 molCO g−1Co s−1 versus that of Co-CMK3, which was influenced by the effect of particle size on the FTS efficiency. The catalytic performance of the conventional Co/ Al2O3 catalyst was remarkably lower than that of Co-CMK-3 and Co-MSU-F-C, which was apparently attributable to low reducibility due to the strong interaction of the Co surface species with the alumina support. The difference in activity between the NP catalysts synthesized from OMCs and the conventional Co/Al2O3 catalyst can be rationalized based on the degree of reducibility, whereas the difference between the FTS activity of Co-CMK-3 and Co-MSU-F-C can be accounted for based on the difference in particle size, given that the particle size is closely related to the exposed surface area and the number of active sites. Consequently, the cobalt NP catalysts prepared by the currently proposed method exhibited increased activity and hydrocarbon selectivity during FTS relative to conventional catalysts, and Co-CMK-3 demonstrated particularly remarkable catalytic performance. The used catalysts were washed with cyclohexane and compared with the freshly prepared counterparts based on TEM analysis (Figure S8). The particle size distribution of both of the used catalysts was comparable with that of the freshly prepared counterparts. However, considering the fact that shrinkage of the Co3O4 NPs occurred during reduction to the metal phase, this similarity implies that the particle size increased somewhat during FTS due to sintering between small particles.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed calculation of catalytic performance parameters (CO consumption and hydrocarbon selectivity); surface areas and pore size distributions of OMCs determined by BET and BJH methods; particle size distributions; HR-TEM images; XRD data of the synthesized Co-CMK-3 and Co-MSU-F-C species; TPR profile of Co/Al2O3 catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fi[email protected] (K.H.); [email protected] (J.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) under “Energy E fficiency & Resources Programs” (Project No . 2010201010008A) of Ministry of Knowledge Economy, Republic of Korea. This work was further financially supported by the KRICT OASIS project from Korea Research Institute of Chemical Technology.

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REFERENCES

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4. CONCLUSIONS In summary, a facile template-directed strategy for the fabrication of Co3O4 NPs based on impregnation of carbon (CMK-3 and MSU-F-C) templates with CoN solution was presented herein. During calcination of the CoN-impregnated carbon species in air, the decomposition of impregnated-CoN to cobalt oxide was confirmed to concurrently induce the synthesis of Co3O4 NPs as well as combustion of the carbon template at lower temperature than conventional techniques. Thus, this facile preparation synthesis technique furnished pure Co3O4 NPs with narrow particle distributions depending on the pore size of the OMCs at a relatively lower temperature without the requirement for an additional step for the removal of the sacrificial template. It was also confirmed that the coincident (with Co3O4 NP generation) removal of the carbon template was induced by the catalyzed redox reaction between the carbon template and NOx gas molecules in the presence of cobalt species, and the synthesized NPs exhibited high activity and hydrocarbon selectivity during FTS. Although cobalt was introduced onto the carbon species as a prototype, it is thought 1778



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Preparation Method of Co3O4 Nanoparticles Using Ordered

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