Protocol for the Nanocasting Method: Preparation of Ordered

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A Protocol for the Nanocasting Method: Preparation of Ordered Mesoporous Metal Oxides Xiaohui Deng, Kun Chen, and Harun Tüysüz Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02645 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 4, 2016

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Chemistry of Materials

A Protocol for the Nanocasting Method: Preparation of Ordered Mesoporous Metal Oxides Xiaohui Deng, Kun Chen and Harun Tüysüz* Max-Planck Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany ABSTRACT: Ordered mesoporous transition metal oxides have attracted considerable research attention due to their unique properties and wide applications. The preparation of these materials has been reported in the literature using soft and hard templating pathways. Compared with soft templating, hard templating - namely nanocasting – is advantageous for synthesizing rigid meso-structures with high crystallinity and has already been applied to numerous transition metal oxides such as Co3O4, NiO, Fe2O3 and Mn3O4. However, nanocasting is often complicated by the multiple steps involved: first, the preparation of ordered mesoporous silica as the hard template, then filtration of the metal precursor into the pores, and finally formation of the metal oxide and removal of the hard template. In this paper, we provide a complete protocol that covers the preparation of most widely used ordered mesoporous silica templates (MCM-41, KIT-6, SBA-15) and the nanocasting process for obtaining ordered mesoporous metal oxides, with emphasizing cobalt oxide as an example. Characterization of the products is presented and the factors that can potentially affect the process are discussed.

Introduction Since the discovery of ordered mesoporous silica in the 1990s initiated research interest in mesoporous materials, efforts investigating ordered mesoporous transition metal oxides have steadily increased due to the unique physicochemical properties and numerous potential applications.1-3 With advantages such as high surface area, open pore system, controllable pore size and morphology, and good thermal/chemical stability of the rigid framework over conventional bulk and nanoparticulate counterparts, ordered mesoporous transition metal oxides have presented themselves as great candidates as materials for a wide range of applications including catalysis, adsorption, sensor, lithium ion batteries, supercapacitors and drug delivery.4-10 In the past two decades, great progress has been made in controlling and optimizing the morphology and stability of mesoporous transition metal oxides. Among these methods, the synthetic strategy can be generally categorized into two major pathways, namely soft-templating and hard-templating (nanocasting).4, 11, 12 Soft templating is based on the use of surfactants or co-block polymers and can be divided into cooperative self-assembly, true liquid crystal templating and evaporation-induced self-assembly (EISA).13-23 These methods have been well discussed in the literature and comprehensive reviews can be found accordingly.4, 5, 24, 25 On the other hand, the hard templating pathway is closely related to the casting process on the macroscopic scale: a rigid mold with voids in the desired shape (morphology, surface curvature) is first filled with materials or precursors.26, 27 During processing, the materials are solidified and a negative replica which replicates the void structure can be obtained after removing the mold. Thus, the fabrication of the mold plays an im-

portant role in the hard templating pathway. A series of methods, such as soft templating, bioinspired templating, and colloidal crystal templating make it possible to diversify the templates, which leads to variations in the morphology and functionality of the replica. Apart from soft templating, in which mesoporous silica is often produced, the porosity of material replicated using a colloidal crystal template is mainly on a sub-micrometer length scale.28, 29. Bioinspired templating totally depends on the scaffolds of the biological tissues, which can range from the macroscale to nanoscale.30, 31 Based on the term ‘casting’, nanocasting is then used to describe the process if the scale (void size/wall thickness etc.) is minimized to nanometers11. Both concepts are shown in Scheme I where the casting of a key and replication of an ordered mesoporous replica are illustrated. Similar to the casting process, in a typical nanocasting preparation for ordered mesoporous metal oxides, first a template with ordered mesostructure, which can be regarded as a nanosized ‘mold’, is prepared. Then the template is infiltrated with precursors of desired metal oxides and the pores are partially or fully filled after the evaporation of solvent. Afterwards the conversion of precursor to solid is conducted (most likely by thermal treatment) and the removal of the template results in the desired product.27, 32

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perfectly adjusted to 12.42 Hard templating has also been employed to synthesize shape controlled mesoporous nanoparticles.43-45 By varying the synthetic conditions, for instance with high infiltration amounts of precursor through several impregnation cycles, the shape of silica can be further retained to a certain extent in the final product.46, 47

Scheme 1. Schematic illustration for the casting process of a key with a mold (a) and nanocasting using ordered mesoporous materials as the hard template (b). Ordered mesoporous silica is most commonly used as the hard template for synthesizing ordered mesoporous metal oxides owing to the highly ordered nanoscale structures and very uniform pore size distribution.4 By the virtue of the developments of silica chemistry, mesostructured silica can be easily prepared on gram scale with great reproducibility.33-35 More importantly, the pore symmetry, pore size, and wall thickness of ordered mesoporous silica can be easily tuned by adjusting the experimental conditions (e.g., hydrothermal conditions, surfactant species, structural directing agents, etc.). Thus, by varying the hard template utilized, one could obtain metal oxides with various mesoporous structure.36-38 Furthermore, due to the presence of surface silanol (Si-OH) groups, the interaction between metal precursors and silica can be reinforced for better replication. This offers silica an obvious advantage as hard template over ordered mesoporous carbon (e.g. CMK-3) since poor surface wetting and solution infiltration in the case of carbon materials could limit the uniform dispersion of metal precursors and the overall quality of the final replica.5, 13, 39, 40 Although, the pretreatment of the carbon surface with oxidizing reagent to create functional groups could overcome this issue, the retention of the mesostructure during the template removal stage is another issue when ordered mesoporous carbon is used5, 41. High temperature calcination under oxygen containing atmosphere is necessary for carbon combustion. However, it is quite possible that the formed crystallites of metal oxides would sinter and grow under such conditions, which can lead to less ordering of product. In the case of silica, the hard template remains thermally stable and chemically inert to transition metal species under relatively high temperatures. (For instance, in the nanocasting synthesis of cobalt oxide using nitrate as the precursor, cobalt silicate was formed under calcination temperatures higher than ~ 780 ˚C). Such properties could confine the growth of metal oxides in the pore channels. Since silica has to be removed using HF or NaOH solutions, it is worth mentioning that oxides (ZnO, MgO, Al2O3) that react with either solution are not easy to be prepared using silica as the hard template. For instance, the preparation of mesoporous ZnO thin film from silica as the hard template can only be achieved when the pH of KOH leaching solution is

Compared with nanocasting, soft templating is of lower cost and can be conducted in a shorter time scale since the preparation of the hard template for nanocasting can already take up to a few days. However, since soft-templating involves complicated sol-gel chemistry, the mesostructure of the final product can be quite sensitive to the experimental conditions (temperature, pH, humidity) and the condensation process of the metal precursors has to be well controlled to avoid disordered products. The resulting material from soft templating often contains amorphous or semi-crystalline phases while in hard templating, the replicated metal oxides can be of good crystallinity thanks to the thermal stability of the silica template. The rigid mesoporous framework also makes various post-treatment methods possible for synthesizing e.g. oxides with low oxidation states, sulfides, and nitrides.48-52 Another advantage of nanocasting is the facile tuning of composition in making mixed oxides since the metal contents in the final product can be easily controlled by the amounts of corresponding precursors; meanwhile the textural parameters can be kept identical owing to the presence of the hard template53-55. On the other hand, in soft templating the variation of precursors could cause considerable change to the system and resulting mesostructure. Moreover, the interactions between surfactant and different precursors can play another factor in the metal ions ‘up-take’ and make it more complicated for controlling the oxide composition.13 Our group has been investigating the preparation of ordered mesoporous transition metal oxides through hard templating pathways, with a focus on cobalt oxides as they demonstrate promising activity in various catalytic reactions such as CO and water oxidation6, 37, 45, 48, 49, 53, 56-63. Here we describe an accessible end-to-end protocol for the preparation of highly ordered mesoporous oxides, including the synthesis of widely employed ordered mesoporous silica (MCM-41, KIT-6 and SBA-15) and the subsequent nanocasting steps to obtain the templated transition metal oxide. The main emphasis is given to Co3O4 while other metal oxides are covered as well. General factors that can affect the process are also discussed. It has to be mentioned that this protocol is only focusing on powder samples from nanocasting pathway. Other research efforts on nanocast mesoporous monolith and thin films are beyond the scope of this manuscript and therefore not discussed.42, 64-69


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Preparation of ordered mesoporous silica as hard template for nanocasting Synthesis of MCM-41 One of the milestones in this field was the discovery of MCM-41 (Mobil Composition of Matter No.41) where the researchers managed to produce hexagonally ordered mesoporous aluminosilicate with pores larger than 2 nm3. The pore size was also tuned in the original work by varying the reaction reagents and conditions. In a typical MCM-41 synthesis, an aqueous solution containing inorganic silica precursors is added to a homogeneous solution which contains surfactant. Once TEOS is added, the interaction of the silicate ions oligomers with surfactant results in rod-like micelles and the packing of the rod-like silica coated micelles produced the hexagonal phase. Finally, the hydrolysis and polymerization of TEOS occurs to form the silica walls over time.70 After precipitation, the solid can be collected by filtration and washed with deionized water. Finally the surfactant is removed through calcination. However, the synthetic pathway towards MCM-41 has been reported to be quite flexible and numerous efforts have been focused on the effects of various surfactants, silica sources and also reaction conditions such as hydrothermal conditions, pH value of the solution and template removal methods. The time scale for the total synthesis also varies greatly from a few hours to 3 days.71 Here, we present in detail a straightforward method for the preparation of high-quality pure-silica MCM-41 adapted from Grün et al. The soft-templating process can be accomplished in 3 hours at room temperature.72 After calcination, the resulting material is thoroughly characterized using small angle X-ray scattering (SAXS), N2 sorption, and electron microscopy. Reagents •

Ammonia hydroxide solution (NH3·H2O; ACS reagent, 28.0-30.0% NH3 basis; Sigma-Aldrich, cat. no. 221228)

•

Hexadecyltrimethylammonium bromide (CTAB; ≥ 98%; Sigma-Aldrich, cat. no. H5882)

•

Tetraethyl orthosilicate (TEOS; reagent grade, 98%; Sigma-Aldrich, cat. no. 131903)

•

Deionized water

Equipment •

Magnetic stirrer (Details for stir bar: eggshaped, Diameter 19 mm, Length 41 mm; VWR International, cat. no. 58949-210)

•

Polypropylene bottle (500 ml; VWR International, cat. no. 414004-126)

•

Vacuum filtration (Vacuum pump, laboratory funnel and filter paper (Schleicher&Schuell MicroScience 595 Filter Paper Circles))

•

Oven (with ventilation system)

•

Ceramic crucible for calcination

Procedure (Timing 1-2 d) 1.

91.5 mL NH3·H2O is mixed with 146 mL deionized water to form a homogeneous solution. Caution: NH3·H2O can cause severe burns to skin. Handle with suitable gloves. Also be aware that ammonia gas can easily evolve from ammonia solution and inhalation or exposure to eyes can cause severe discomfort. Handle under fume hood only.

2.

Add 1.0 gram of CTAB into the solution from step 1 and stir vigorously until a homogeneous solution is obtained. Note: The complete dissolution of CTAB takes a substantial amount of time without any further measures under room temperature. Placing the mixture at 35 ~ 40 ˚C or in a box oven at higher temperatures (70 ˚C) can greatly accelerate the process. However, due to the release of ammonia gas at elevated temperatures, the mixture should be transferred to a polypropylene (PP) container which can be tightly closed during heating. In our experiment, the CTAB was completely dissolved after the solution was heated at 70 ˚C for 30 minutes.

3.

When CTAB is completely dissolved, cool the solution down to room temperature and place it on a stirring plate with a stirring speed of 500 rpm. Make sure the magnetic stirrer is of adequate size so that the liquid can be efficiently mixed. Then, 5.0 mL of TEOS is added to the surfactant solution in one injection. Precipitation occurs very quickly and this indicates the hydrolysis of TEOS.

4.

After 2 hours of stirring, the resulting precipitate can be collected using vacuum filtration. The pore size of the filter paper should be sufficiently large for collecting the precipitate and at the same time allowing the quick passage of the liquid. Caution: due to the presence of ammonium hydroxide, the liquid after filtration is hazardous (refer to caution notes in step 1). Conduct filtration under fume hood only. Then wash the solid with copious amounts of distilled water until neutralized and dry the as-synthesized materials at 90 ˚C overnight.

5.

After the solid is completely dried, crush it into fine powders and transfer it into a crucible for calcination. In detail, the temperature is first ramped to 180 ˚C in 30 min and is held at this temperature for 2 h. Afterwards, the temperature is increased to 550 ˚C in 4 h and it is held at 550 ˚C for another 6 h. Caution: the combustion of CTAB generates an unpleasant odor. Thus, the calcination should be conducted in an oven with sufficient ventilation with ability to guide air flow to an open space. The injection of air to the oven is also crucial for the complete removal of CTAB. After calcination, the white MCM-41 powder (~ 1.3 g) can be obtained. If the product is slightly yellowish color, it indicates the incomplete removal of surfactant due to insufficient air flow. The powder can be recalcined at 550 °C for another 4 hours.



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Anticipated results

•

Deionized water

Synthesized MCM-41 particles are of spherical shape and the diameter is in the range of 400 ~ 600 nm. The ordered structure of MCM-41 was then checked with small angle X-ray diffraction and the pattern is shown in Figure S1a. Distinct peaks which are assigned to the (100), (110) and (200) reflections can be observed. This can be correlated with the p6mm hexagonal symmetry72. The porosity and textural parameters were examined using N2 sorption and the isotherm is plotted in Figure S1b. The reversible capillary condensation is present and the pore size distribution as determined from BJH algorithm is centered at 2 nm (inset). However, it has been pointed out that the BJH method underestimates the pore size of MCM-41 by ca. 1 nm. Thus nonlocal density function theory (NLDFT) should be applied to perform more accurate pore size determination73, 74. The Brunauer-Emmett-Teller (BET) surface area and pore volume is determined to be 1240 m2/g and 0.92 cm3/g, respectively. The TEM image (Figure S1c) further reveals that the highly ordered mesoporous structure as a hexagonal array of uniform pore channels can be clearly observed.

•

1-butanol (99%; Alfa Aesar, cat. no. L13171)

•

Tetraethyl orthosilicate (TEOS; reagent grade, 98%; Sigma-Aldrich, cat. no. 131903)

Equipment •

Magnetic stirrer and hot plate with thermal couple (Details for stir bar: Polygon without pivot ring, diameter 7 mm, length 20 mm)

•

Polypropylene bottle (1000 mL, VWR International, cat. no. 414004-127)

•

Oil bath

•

Round bottom flask 1000 mL

•

Vacuum filtration (Vacuum pump, laboratory funnel and filter paper (Schleicher&Schuell MicroScience 595 Filter Paper Circles))

•


•

Ceramic crucible

Procedure (Timing 3-4 d)

Synthesis of KIT-6 Similar to MCM-41, SBA-15 (Santa Barbara Amorphous type materials - 15) shows the same two-dimensional hexagonal order with p6mm symmetry.34 The main difference concerning the mesostructure is that the pore size of SBA15 is significantly larger than MCM-41, as is the wall thickness between pore channels. This significantly increases the hydrothermal stability of SBA-15. SBA-15 is most commonly synthesized using amphiphilic block copolymers (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)(PEO-PPO-PEO)) as the structure-directing agent and TEOS as the silica source, respectively. Differing from SBA-15, KIT-6 (Korea Advanced Institute of Science and Technology - 6) has Ia3d symmetry with two interpenetrating bi-continuous mesopore systems which are easily accessible.35 PEO-PPO-PEO is also used as the block copolymer but the addition of butanol as another structuredirecting agent is decisive for the formation of the cubic phase. Due to similarities in the processes for the preparation of SBA-15 and KIT-6, in the following section only the synthesis of KIT-6 is illustrated and presented in detail. The reagent amounts concerning the synthesis of SBA-15 are also given at the end of this section.

Reagents •

Pluronic 123 (P123, Poly(ethylene glycol)poly(propylene glycol)-poly(ethylene glycol); EO:PO:EO = 20:70:20, average Mn ~ 5800; Sigma-Aldrich, cat. no. 435465)

•

Hydrochloric acid (HCl; 37-38%; J.T Baker, cat. no. 5800)

The preparation of KIT-6 is based on the original work published by Freddy Kleitz and Ryong Ryoo.35 Modifications are made based on the amounts of reactants used. 1.

Dissolve 13.5 g of P123 in 487.5 g deionized water until a homogeneous solution is obtained. Note: P123 has a glass transition temperature of 39 ˚C and is p astelike under room temperature, which makes it difficult to precisely weigh P123 (Figure 1a). In practice, one can place a closed container of P123 in an oven with the temperature set to higher value (e.g., 70 ˚C for 20 min). After P123 changes to a viscous fluid (Figure 1b) one can use lab pipettes to take the appropriate amount.

2.

Add 26.1 g concentrated HCl to the solution and apply vigorous stirring until P123 is completely dissolved. Cool the resulting solution down to room temperature and adequate stirring needs to be applied from this step on. Caution: Hydrochloric acid is a toxic substance and extremely hazardous and must be used with care. Avoid skin contact, eye contact and inhalation of HCl gas. Handle in fume hood only.

3.

Place the solution in an oil bath with the temperature set to 35 ˚C. After the solution temperature reaches the setting point, add 13.5 g (16.7 mL based on volume) of 1-butanol.

4.

After 1 h of stirring, 29.0 g (30.9 mL based on volume) TEOS is quickly added into the solution. The mixture is then stirred at 35 ˚C for 24 h. Note: the temperature of 35 ˚C is crucial for achieving high quality KIT-6, thus should be accurately controlled. The precipitate forms within 3 hours under vigorous stirring.


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Chemistry of Materials accordingly. For the synthesis of KIT-6 with an aging temperature higher than 70 ˚C, transfer the mixture from step 4 to a polypropylene (PP) container (1 L), seal it tightly and place it in the oil/water bath or heating oven. Alternatively, a PP bottle with a volume of 1 L can also be used as the reaction vessel starting from step 1 and that can simplify the overall process. Caution: Due to the formation of ethanol during the hydrolysis of TEOS, the pressure inside the container can be significantly increased at high temperatures. Thus, check the sealing of the container regularly and avoid solvent loss due to air leakage. For the synthesis with an aging temperature higher than 100 ˚C, Teflonlined stainless steel autoclaves should be used. The hydrothermal treatment takes 24 h.

Figure 1. Photographs of substances and experimental setup for the preparation of KIT-6. a) P123 in glass vessel at room temperature. b) P123 after being placed at 70 ˚C for 20 min. At this stage, P123 is melted and can be handled with lab pipette. c) Mixture obtained after P123 is added into the solution of water and HCl. One can observe undissolved P123 at the bottom of round flask prior to stirring. d) Homogeneous solution after P123 is completely dissolved. e) Photograph of oil bath setup. Thermal couple is inserted on the right side of round flask. Temperature is set at 35 ˚C and rotation speed is 550 rpm. (not suitable for preparing KIT-6 with aging temperature higher than 70 ˚C). f) Precipitation of silica/surfactant composite after hydrothermal treatment. g) Vacuum filtration setup for collecting the precipitate from f. h) Dried composite materials in crucible from f and i) final product (KIT-6) after calcination.

6.

After hydrothermal treatment, the white solid is precipitated on the bottom of the reaction vessel and can be collected using vacuum filtration. Caution: in the case of high aging temperatures, boiling of the liquid after the reaction can occur when opening the container. Special care should be taken and the bottle should be cooled down to a lower temperature before filtration. The solid white product is then dried overnight at 100 ˚C for 24 h.

7.

Crush the solid product into fine powders and transfer them into a crucible for calcination (In the original work the surfactant was first removed by extraction in an ethanol-HCl mixture before calcination).35 The calcination procedure and caution notes (sufficient ventilation) are the same as those described for MCM-41.

Synthesis of SBA-15 For the synthesis of hexagonally ordered mesoporous SBA-15, the detailed modifications to the KIT-6 synthesis are listed in the following: Variation corresponding to step: 1. The mass of P123 and water used are 27.8 g, 504 g respectively. 2. The amount of HCl (37%) used is 15.5 g. 3. No butanol shall be added. 4. 60.0 g (63.8 mL based on volume) TEOS is added into solution under vigorous stirring. 5. Step 5, 6 and 7 are identical to that of KIT-6 preparation. Anticipated results

5.

After step 4, cease the stirring and take out the magnetic stirrer. Hydrothermal treatment needs to be conducted to complete the condensation in the silica region. Note: due to the temperature-dependent hydrophilicity of the PEO block under acidic conditions, textural parameters can be effectively tailored by varying the aging temperature during the hydrothermal treatment. Thus different measures should be taken

The shape of synthesized SBA-15 and KIT-6 are irregular and the particles can be few micrometers in one dimension. However, it can be tuned by varying the synthesis conditions. For instance, the SBA-15 silica spheres can be prepared with the assistance of 1,3,5-trimethylbenzene and KCl.75 The mesostructure of KIT-6 and SBA-15 aged at 100 ˚C were carefully characterized (Figure S2 and S3). As seen from the small angle XRD pattern, SBA-15-100 shows wellresolved peaks that can be assigned to the (100), (110) and



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(200) reflections. This can be associated with p6mm hexagonal symmetry. On the other hand, the pattern of KIT6-100 shows significant (211), (220) and (420) peaks, which are typical for cubic ordered mesostructures with Ia3d space group.45 Both materials show type IV isotherms with a pronounced capillary condensation step, indicating highly ordered mesoporosity. The textural parameters in both materials are rather similar, with a BET surface area of ~ 700 m2/g, pore size of ~ 8 nm and pore volume of ~ 1 cm3/g. The temperature effect on the textural parameters of KIT-6 and SBA-15 are further summarized in Table 1 and 2, where it is clear that the pore volume and pore size increase gradually as the aging temperature is elevated. The structure of SBA-15 and KIT-6 were then investigated with TEM. Microscopic images of SBA-15 show highly-ordered hexagonal arrays of mesopores. Clearly, the pore size and wall thickness as estimated from the images are significantly larger than those of MCM-41. The TEM image of KIT-6 also shows high quality ordering although the 2Dprojection of the mesostructure can be different in various crystal directions. Detailed studies on the characterization, pore structure and adsorption behavior of KIT-6 that has been aged at various temperatures can be found elsewhere.61, 76, 77 Table 1. Structural parameters of calcined KIT-6 samples prepared at different temperatures of hydrothermal treatment.78 Note: pore sizes shown in this table are determined from BJH method. For more accurate pore size analysis, NLDFT method is recommended.77 sample

Aging Temperature (˚C)

BET Surface Area (m2/g)

Total Pore Volume (cm3/g)

Pore size (nm)

KIT-6-RT

RT

461

0.390

3.7

KIT-6-40

40

547

0.505

3.9

KIT-6-60

60

726

0.735

4.8

KIT-6-100

100

694

0.950

7.9

KIT-6-115

115

684

1.199

9.9

KIT-6-135

135

558

1.305

11.4

Table 2. Structural parameters of calcined SBA-15 samples prepared at different temperatures of hydrothermal treatment.78 sample

Aging Temperature (˚C)

BET Surface Area (m2/g)

Total Pore Volume (cm3/g)

Pore size (nm)

SBA-15-40

40

627

0.597

5.3

SBA-15-60

60

637

0.622

5.7

SBA-15-80

80

657

0.693

6.3

SBA-15-100

100

727

0.930

7.9

SBA-15-120

Page 6 of 16 120

723

1.057

9.3

Preparation of ordered mesoporous Co3O4 by nanocasting The preparation of ordered mesoporous transition metal oxides by hard templating have been well summarized in reviews. In general, the solid-liquid method, ‘two-solvent’ method, (incipient) wetness impregnation-calcination method,combustion method and combination of ‘twosolvent’ -melt impregnation method have been developed to load the hard template with metal precursors79-83. In this protocol, we mainly focus on the wetness impregnation-calcination method since it has been intensively studied in our group and widely applied to prepare (mixed) metal oxides without involving complicated reaction conditions. The solid-liquid method, ‘two-solvent’ method, and incipient wetness impregnation method are also described briefly. In wetness impregnation, an excess amount of solvent compared with the total pore volume of mesoporous silica is often used. During the solvent evaporation process, the precursor is expected to migrate into the pore systems under the driving force from capillary motion. After calcination, the pore volume previously occupied by the precursor will shrink dramatically. As previously reported by our group and the research group of Schüth, the low percentage of pore filling can result in a discontinuous or less ordered mesostructured. Hence, multiple cycles of impregnation/calcination is in general necessary for preparing high-quality ordered mesoporous metal oxides through the nanocasting pathway. A pore filling percentage of 15% by the metal oxide is sufficient to form high quality replicas with good material yield.36, 45, 56 Briefly, in a two times impregnation synthesis, mesoporous silica is first filled with the appropriate amount of precursor (based on the total pore filling by the final oxide). Then, calcination at lower temperatures is conducted to form the oxide and meanwhile ‘make room’ for the remaining impregnation cycle. In the second impregnation, slightly lower or the same amount of precursor is applied. After the second impregnation is completed, calcination at high temperature results in the oxide/silica composite. By removing the silica template, ordered mesoporous oxide can be obtained. In this section, the nanocasting process for obtaining ordered mesoporous Co3O4 using two times impregnation is described in detail. 500 mg of KIT-6 aged at 100 ˚C (with a pore volume of 1 cm3/g) is used as the hard template. Calculations concerning the amounts of precursor and solvent used in impregnation are demonstrated and factors that can cause variations to the mesostructure of final product are discussed. Recipes for synthesizing other ordered mesoporous metal oxides including chromium oxide, nickel oxide, ferrihydrite, manganese oxide and cobalt-based mixed spinel oxides are covered as well in the end.


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Chemistry of Materials nervous system upon repeated or prolonged exposure. Avoid skin contact, eye contact, and ingestion. Handle with care. Weigh 500 mg KIT-6-100 in vessel No.1 and place a magnetic stirring bar of appropriate size inside it.

Reagents •

Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O; ACS reagent, ≥ 98%; SigmaAldrich, cat. no. 239267)

•

Ethanol (absolute, ≥ 99.8%; Sigma-Aldrich, cat. no. 34923) Denatured ethanol can also be used for impregnation purpose.

•

Sodium hydroxide (NaOH; Reagent, pellet; VWR Chemicals, cat. no. 470302)

•

Deionized water

Equipment •

Glass or plastic vessel

•

Polypropylene bottle (60 ml, VWR international, cat. no. 414004-123)

•

Magnetic stirrer

•


•

Laboratory centrifuge or vacuum filtration.

Procedure (Timing 3-4 d) The first step of the nanocasting process is to calculate the loading amount of precursor required based on the particular occupation of total pore volume of the template. In a typical transition metal oxide replication, ~ 15% pore volume of silica template is filled with metal oxide after calcination. The calculation is given below. Calculation: The total pore volume of 500 mg KIT-6-100 = × m = 0.5 × 1 ⁄ = 0.5 The pore volume that will be occupied by cobalt oxide = × 15% = 0.075 The weight of cobalt oxide as final product = × = 0.075 × 6.11 ⁄ = 0.458 where denotes the true density of Co3O4 (m.w.: 240.9 g/mol). If Co(NO3)2·6H2O (m.w.: 291.03 g/mol) is used as precursor, based on the cobalt atoms amount in the molecules, the total amount of metal precursor = × 3 × 0.458 = × 3 × 291.03 ⁄"# = 1.67 240.9 /"# 1.

2.

Use a lab pipette to inject ~ 3.7 mL of Co(NO3)2·6H2O stock solution into vessel 1.

3.

Close vessel 1 and place it on a stirring plate at room temperature. Apply vigorous stirring to make sure that the silica and precursor solution can be well mixed. Ultrasonication for a short time period (30 s) before stirring can facilitate the mixing.

4.

After 1 h of stirring, open vessel 1 and place it at 50 ˚C until the ethanol is completely evaporate. An overnight drying at 50 C° is sufficient to obtain a dried sample (for large scale synthesis, the ethanol evaporation can be carried out at 60-65 °C). Note: the evaporation process is critical for the preparation. Due to the gradual evaporation of ethanol and precipitation of the silica hard template, it is difficult to achieve homogeneous distribution of precursor in the pores of the silica. In practice it is common to find a dark pink layer covering the top of the composite material after drying, which indicates a relatively higher content of cobalt precursor in the local area of the silica hard template. This could lead to the presence of disordered or bulk particles in the final product after calcination. However, the distribution of metal precursor could be improved dramatically by using an impregnation vessel with a large inner diameter and wide opening. This way the silica can be better spread and the evaporation of ethanol can be facilitated. For a solvent volume of 3 mL, a glass vessel with inner diameter of 2.3 cm and opening diameter of 1.7 cm was used in our study. Beakers should be used if the nanocasting process is further scaled up (e.g., impregnating solvent volume of 20 mL). One can also further mix the wet-composite (not completely dried) during the drying process by manual stirring so the rest of the solvent can diffuse into the silica at the bottom of the vessel. The use of rotary evaporator after impregnation can also be helpful to ensure a uniform evaporation of solvent from the powders.

Dissolve 1.67 g Co(NO3)2·6H2O in 6 mL ethanol to make a stock solution for impregnation. The molar concentration of Co(NO3)2·6H2O is ~ 0.8 M. Caution: Cobalt (II) nitrate hexahydrate is a toxic substance that can cause damage to the lungs, gastrointestinal tract, upper respiratory tract, and central



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it has been shown that in the case of metal nitrate precursors, the geometry of the container used for calcination is decisive for the quality of the mesoporous metal oxides. When a container with wide opening (Petri-dish in their study) was used, nearly amorphous isolated Fe2O3 nanoparticles were obtained. This was later attributed to the inhibition of the transport of ionic species due to the rapid escape of water at the early stage of calcination. On the other hand, when a container with narrow opening or a quasi-sealed container was used, the oxide crystal directly grew inside the aqueous precursor solution, giving highly ordered particles with large size.

Figure 2. Photographs of substances and experimental setup for the preparation of ordered mesoporous Co3O4 using the nanocasting pathway. a) Glass vessels containing mesoporous silica (left), Co(NO3)2·6H2O (middle) and magnetic stirrer (right). b) Glass vessels containing mesoporous silica (left) and ethanol solution of Co(NO3)2·6H2O (right). c) Impregnation of silica hard template by injecting the stock solution. d) Mesoporous silica impregnated by cobalt precursor (after grinding). e, f) Container effect on the outcome of the impregnation process. As shown, the top layer from the smaller vessel is much darker, indicating inhomogeneous precursor loading in the hard template. The inner and opening diameters of the vessels are 1.8 cm/1.3 cm (left) and 2.3 cm/1.7 cm respectively. g) Cobalt oxide/silica composite obtained after first calcination step. h) Second impregnation step to load product from g with cobalt precursor. i) Crucible containing dried material from h. j) Leaching of sample after second calcination step with 2 M NaOH solution. Changing the solution with a fresh one after overnight leaching is not shown in this figure.

5.

Grind the pink solid after drying with a spatula to obtain a fine and homogenous powder. Then transfer the powder from vessel 1 into the crucible for calcination. The temperature is ramped to 200 ˚C in 30 min and is held at 200 ˚C for 4 h. At this step, Co(NO3)2·6H2O decomposes and forms black Co3O4. Caution: the decomposition of metal nitrates generates nitrogen oxides as toxic gases. Conduct the calcination in oven with sufficient ventilation. Note: the container effect in nanocasting synthesis of mesoporous metal oxides has been discussed before84. Briefly,

6.

Transfer the product from step 5 into vessel No.2 and inject the rest of the stock solution. Repeat the impregnation process from step 3 to step 4: Close vessel 2 and place it on a stirring place at room temperature. After 1 h of stirring, open vessel 2 and place it at 50 ˚C until ethanol is completely evaporated. Caution: after calcination in step 5, the color of the pink solid turns to black, indicating the decomposition of the cobalt precursor and formation of cobalt oxide (Co3O4). Co3O4 is a toxic substance which is suspected of causing cancer. It is harmful if swallowed and may cause allergy or breathing difficulties if inhaled or upon skin contact. Moreover, due to the presence of silica and porous structure, the solid here and obtained later on through the preparation is considered to be fluffy even under weak air flow. Use of a mask is mandatory and the materials should be handled with particular care.

7.

Transfer the dried solid from step 6 into a crucible for calcination. The temperature first ramps to 200 ˚C in 30 min and is held at this temperature for 4 h. Afterwards, the temperature increases to 500 ˚C in 2 h and is held at 500 ˚C for 6 h. This step results in highly crystalline Co3O4.

8.

Transfer the product from step 7 into a polypropylene (PP) bottle that can be tightly closed. Leach the silica hard template using 2 M NaOH aqueous solution. The solution is prepared by dissolving 8 g of NaOH into 100 mL deionized water. Caution: Sodium hydroxide (NaOH) and its solution (especially at high concentrations) are hazardous substances that can cause corrosive burns and pain upon skin and eye contact. Handle with care. Normal glassware made of borosilicate should be avoided during this step due to their instability under highly alkaline conditions. Calculation: The reaction between SiO2 and NaOH follows the chemical equation: $%&' ( 2)*&+ → )*' $%& ( +' & Thus the minimum required volume (V) of 2 M NaOH solution can be calculated. However, in order to make sure that the silica in the composite material can be efficiently removed, the


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Chemistry of Materials leaching process is often done in two separate steps where high temperature and excess volume of NaOH solution are applied. Specifically, 10 mL of 2 M NaOH is first added into the PP bottle. Then the bottle is closed and placed in an oven at 70 ˚C overnight. Regular shaking at the beginning can be beneficial to the process. Afterwards, the solution is decanted and fresh 10 mL NaOH solution is added for the second time. Then the mixture is left at 90 ˚C overnight. The remaining silica as measured by EDX is lower than 1% (atomic ratio).

9.

After step 8, the black solid, which is the ordered mesoporous Co3O4, is collected by centrifugation. The centrifuge programs used are as following: 6000 rpm, 20 min →8000 rpm, 20 min →9000 rpm, 30 min →10000 rpm, 30 min. Note: the final rotation speed should be at least 10000 rpm to guarantee a proper yield. After each step, the liquid residual is replaced with deionized water. The solids are re-dispersed by shaking the centrifuge tubes. The product is then dried at 50 ˚C. As an alternative, vacuum filtration can also be used to collect the product. However, this may take a long period of time due to pore blockage in the filter paper caused by particles. Since the particle size of the ordered mesoporous cobalt oxide synthesized from two-times impregnation are in the range of few hundred nanometers, it is recommended to use filter paper with relatively large pore size.

The described two-times impregnation-calcination pathway can be further applied to synthesize mesoporous cobalt oxide on a gram scale. The final pore filling percentage of the silica hard template and the molar concentration of cobalt precursor solution remain the same. In the case of nanocasting using 5.0 g KIT-6-100 as hard template, 16.7 g Co(NO3)2·6H2O is first dissolved in 70 mL ethanol to make a stock solution. Due to the larger amount of materials applied, the geometry and size of the container for impregnation should be chosen with caution, as this could dramatically affect the evaporation of ethanol and the quality of composite materials obtained (refer to step 4). Since ~ 40 mL cobalt nitrate solution is applied for one impregnation cycle in this case, a beaker with an inner diameter of ~ 6.4 cm and a magnetic stirrer bar with the length of ~ 3.9 cm are used for impregnation. It is worth mentioning that the borosilicate beaker can withstand a calcination temperature of lower than 300 ˚C, so the composite after drying can be directly transferred to the oven for calcination at 200 ˚C for 4 h. After the second impregnation, the material needs to be transferred to a crucible for the final calcination at 500 ˚C. Eventually, the silica is removed by leaching with 100 mL of 2 M NaOH solution in two steps and the mesoporous cobalt oxides can be collected. The expected yield of Co3O4 is ~ 4.5g which is very close to amount of the produced product. Apart from the wetness impregnation method, other methods such as solid-liquid method, ‘two solvent’ method and incipient wetness impregnation method are also

employed to load mesoporous silica with metal precursors. These methods are briefly descried below. Solid-liquid method (Figure 3 a,b) was first reported by Yue et al and the mixing can be simply conducted by grinding the mixed powders of mesoporous silica and metal hydrates as precursors81. During the calcination process, metal nitrates with low melting points (in cases of Co, Ni, Ce and Cr) turn into the liquid phase before decomposition and move into the mesopores of silica hard template. Eventually, the decomposition takes place inside the pore systems and is followed by the formation of ordered mesoporous oxides. The omission of solvent and evaporation process is however two-sided: on the one hand, the process is significantly simplified and the timing can be greatly shortened; on the other hand, the diffusion of metal nitrates into the mesopores can be insufficient for forming highly ordered materials. That also explains why a slow ramping rate of 1 ˚C/min was applied for calcination in the original work. In addition, formation of bulk (nonporous) particles cannot be avoided with this process. The grinding parameters and the crucible geometry could further introduce uncertainties in the reproduction of materials. In a ‘two-solvent’ method (Figure 3 c-g), mesoporous silica is first dispersed in dry hexane by stirring and an aqueous solution of metal precursor is then added to the dispersion. The precursor is then ‘pushed into’ the mesopores by hexane. In order to maximize the impregnated quantity and prevent the growth outside the hard template, the precursor solution is often at high concentrations and the applied volume is equal to the pore volume of silica. This method has been widely applied to the synthesis of ordered mesoporous Mn-based oxides85-87. Incipient wetness impregnation is a common method to prepare supported heterogeneous catalysts. Ordered mesoporous oxides can also be prepared by using mesoporous silica as template and utilizing a high loading of precursors. The impregnation process (Figure 3 h,i) is generally conducted by dropping a precursor solution with high concentration into the dried silica template. It is further mixed by manually pressing with a spatula. It should be noticed that due to the silica pore volume being slowly filled, the precursor solution should be added in multiple steps with gradually decreasing amounts. Sufficient mixing (5 ~ 10 min) is mandatory between each injection step. In our practice where the solution with a total volume of 500 µL is used for impregnation, a work flow of injection (200 µL) – mixing – injection (150 µL) – mixing – injection (100 µL) – mixing – injection (50 µL) - mixing is conducted. After a homogeneous pink powder is obtained, the mixture is dried in an oven at 50 °C and transferred to the crucible for calcination. The incipient wetness impregnation method has been intensively employed to synthesize ordered mesoporous carbon materials following the nanocasting pathway; however this is beyond the scope of this protocol. One of the main limitations of this method in comparison with wetness impregnation is the difficultly with the scale up.



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between p/p0 = 0.45 and p/p0 = 0.8 can be seen, indicating mesoporous structure. In terms of textural parameters, Co3O4-100 (the number indicates hydrothermal temperature of KIT-6) shows a BET surface area of 105 m2/g and a pore volume of 0.2 cm3/g. A single pore size centered at 3-4 nm is shown in the pore size distribution and this value is equal to the wall thickness of silica hard template. The structure was further surveyed using electron microscopy. TEM and SEM images both show highly ordered mesostructures with two pore systems of Co3O4 penetrating each other and forming a dense structure61. As previously mentioned, the pore size and interconnectivity between the two pore channels of KIT-6 can be easily tuned by varying the temperature of the hydrothermal treatment. Such factors can directly affect the mesostructure of the Co3O4 replica. For instance, when using KIT-6 aged at 35 ˚C as the hard template instead, due to the interconnectivity between pore channels being insufficient for simultaneous growth of oxides, the replica often results in uncoupled sub-framework where only one pore channel is filled37, 88. Due to the other unfilled pore channel, a bimodal pore size distribution is observed. Higher surface area can also be expected. A systematic study conducted by our group can be referred to for more details61.

Figure 3. Photographs of loading ordered mesoporous silica with Co(NO3)2·6H2O by using solid-liquid method (a,b), ‘twosolvent’ method (c-g) and incipient wetness impregnation method (h,i). a) 300 mg of KIT-6-100 and 582 mg (2 mmol) of Co(NO3)2·6H2O placed in mortar. b) Powder obtained after grinding the mixture for 10 min. c) 2 g of KIT-6-100 dispersed in 40 mL hexane. Stirring speed is 400 rpm. d) After 3 h of stirring, 2 mL of 4 M Co(NO3)2·6H2O aqueous solution is added into the dispersion dropwise. e) The mixture is stirred overnight. This step should be conducted in a closed container due to the evaporation of hexane. Transparent beaker is used here only for clarity. f) The silica-cobalt nitrate composite in hexane after overnight mixing. g) Product obtained after vacuum filtration. The material is then dried at 50 ˚C, ground and calcined. Caution: Hexane is a hazardous substance. Exposure to hexane can affect and cause damage to the nervous systems. Handle and conduct experiment only in fume hood. h) 500 mg of KIT-6-100 in glass vessel (inner diameter ~ 1.8 cm). i) Manual mixing with spatula after injection of certain amount of 4 M Co(NO3)2·6H2O aqueous solution. An overall volume of 500 µL is used for impregnation and the injection is divided into 4 steps (200 µL, 150 µL, 100 µL and 50 µL).

MCM-41 and SBA-15-100 were also used as hard templates to synthesize ordered mesoporous Co3O4. The amount of precursor was modified according to the total pore volume of each template. As can be seen from the TEM images (Figure 5), both samples show hexagonally ordered mesostructures. Because the pores of MCM-41 are significantly narrower than that of SBA-15-100, the width of parallel nanowires is much smaller. The 2 nm pore size of MCM-41 further complicates the impregnation process as a small amount of bulk materials is present in the final product. Furthermore, the ordering of the Co3O4 replica from MCM-41 is not as good due to the low interconnectivity between neighboring pore channels.

Anticipated results The small angle XRD pattern of nanocast cobalt oxide using KIT-6-100 as the hard template through wetness impregnation is shown in Figure 4. Distinctive (211) and (220) reflections, which are characteristic of a cubically ordered structure, can be observed35. The N2 sorption isotherm was further taken and a clear hysteresis loop

Figure 4. a) Small angle XRD pattern, b) N2 sorption isotherm and pore size distribution, c) TEM image and d) SEM image of ordered mesoporous Co3O4 nanocast from KIT-6100.


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Figure 5. TEM images of mesoporous Co3O4 nanocast from SBA-15-100 (a) and MCM-41 (b).

extremely toxic! Avoid any contact, ingestion and inhalation, handle in designated area). A single impregnation cycle is sufficient to obtained ordered Cr2O3 and it is conducted at room temperature for 3 h. The water is then evaporated at 70 ˚C overnight. Final calcination at 400 ˚C for 6 h (ramp rate 2 ˚C/min) is sufficient to obtain crystalline Cr2O3. The leaching condition remains unchanged. The morphology and domain size of mesoporous Cr2O3 can also be tuned by multiple impregnation cycles with less precursor amounts. Details can be found elsewhere.45, 60

Other Metal Oxides Apart from Co3O4, other highly ordered mesoporous oxides such as NiO, ferrihydrite, Cr2O3, Mn oxide and Cobased mixed oxides have been synthesized and investigated.60, 63, 85, 86, 88 All the materials can be prepared through the pathways as described in last section. However, the quantities of precursors, calcination procedure and in some cases the impregnation cycle are varied.

NiO Nickel nitrate hexahydrate (Ni(NO3)2·6H2O; crystallized, ≥ 97.0% (KT); Sigma-Aldrich, cat. no. 72253) is used as the precursor and a total amount of 1.94 g is dissolved in ~ 7 mL ethanol to make a stock solution for impregnation (based on 0.5 g KIT-6-100 as the hard template, pore volume 1 cm3/g, Caution: Ni(NO3)2·6H2O is toxic. Avoid inhalation, skin contact, eye contact and ingestion, handle with care). The impregnation is conducted in two steps by using half of the stock solution for each cycle. The calcination and leaching process are same as that for the synthesis of cobalt oxide. Ferrihydrite Iron nitrate nonahydrate (Fe(NO3)3·9H2O; ACS reagent, ≥ 98%; Sigma-Aldrich, cat. no. 216828) is used as the precursor and a total amount of 3.23 g is dissolved in 8 mL ethanol to make a stock solution (based on 0.5 g KIT-6-100 as the hard template, pore volume 1 cm3/g. Caution: Fe(NO3)3·9H2O is irritating to the eyes, respiratory system and skin. It may cause fire when in contact with combustible materials. Handle with care). Two times impregnation is necessary to obtain ordered mesostructure and final calcination is conducted at 200 ˚C for 6 h to avoid formation of hematite phase. It should be addressed that due to the hygroscopic nature of Fe(NO3)3·9H2O, the composite materials after impregnation should be sufficiently dried. The leaching is conducted with 2 M NaOH solution.63 Cr2O3 Chromium oxide (CrO3; ACS reagent, ≥ 98%; SigmaAldrich, cat. no. 236470) is used as the precursor and a total amount of 2 g is dissolved in 20 mL water to make the solution for impregnation (1 M; based on 2 g KIT-6-100 as the hard template, pore volume 1 cm3/g. Caution: CrO3 is

Manganese Oxide Mesoporous crystalline manganese oxides can be prepared with manganese nitrate hexahydrate (Mn(NO3)2·6H2O) as the precursor. The impregnation is often done with the ‘two-solvent’ method where a saturated aqueous solution of Mn(NO3)2·6H2O is added dropwise into the suspension of silica in hexane. The solution volume is equal to the pore volume of mesoporous silica. The mixture is then collected by filtration and dried under room temperature until a completely dried powder is obtained. By varying the calcination temperature, various crystalline phases of Mn oxide can be obtained: Calcination at 400 ˚C for 3 h leads to formation of β-MnO2 while Mn2O3 is obtained when the temperature increases to 600 ˚C. Reduction of Mn2O3 to Mn3O4 can be performed at 280 ˚C for 3 h under 5 %H2/95% Ar atmosphere85, 86. Cobalt-Based Mixed Metal Oxides Co3O4 is of spinel structure with cobalt atoms occupying tetrahedral and octahedral centers. Cobalt spinel can be substituted by other transition metal elements and this allows the preparation of mixed metal oxides with same textural parameters but various chemical and physical properties.53, 57, 58 Cu(Fe) incorporated cobalt oxides have been synthesized by mixing the nitrate precursors with desired ratios, keeping the total molar concentration of the precursor solution the same (0.8 M). Impregnation is done with two steps and the intermediate calcination is conducted at 250 ˚C for 4 h. Final calcination is then conducted at 550 ˚C for 6 h after a plateau at 250 ˚C for 6 h. The silica template is then removed with 2 M NaOH aqueous solution. The detailed quantities of metal precursor amounts with respect to atomic ratios are given in Supporting information (Table S1, S2). It should be noticed that in the case of Co/Cu (2/1), a small amount of CuO phase is easy to form due to the local accumulation of copper precursors. Due to the hygroscopic nature of Fe(NO3)3·9H2O, special attention should be paid in the weighing process. The atomic ratio in the final oxides should be thoroughly checked using elemental analysis.53, 58



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Conclusion and Perspective In summary, the preparation of MCM-41, KIT-6 and SBA-15 are presented in detail and the nanocasting process of preparing ordered mesoporous metal oxides, focusing on cobalt oxide, are illustrated step-by-step. Due to the wide applicability of nanocasting, this protocol can be further modified to synthesize other transition metal oxides and mixed oxides. Key parameters such as the loading quantity of precursors, the container/crucible geometry and the impregnation method can affect the nanocasting process. The morphology and pore system are facile to control by varying the hard templates being used. Although transition metal oxides have been widely applied in catalysis and energy related research areas, the efforts to explore the relationship between pore structure and performance will be appreciated. For instance, when using KIT-6-100 and SBA-15-100 as the hard template, the cobalt oxide replicas show the same external surface area but different pore symmetry.56 Such factors could play important roles in the catalyst performance and physical properties. Also, since the material composition in nanocasting can be controlled without dramatically varying the textural parameters, this allows researchers to minimize the influence from factors such as catalyst preparation method, porosity and exposed surface area as they are closely related to catalytic activities. In this way, the identification of true active site can be achieved by only engineering the material composition. Such approach has been employed in CO oxidation reaction where cobalt based oxides as catalysts are investigated.89 By varying the transition metal atoms on different coordination sites, it was found out that the oxidation of octahedrally coordinated Co2+ species show unexpectedly high activity. In another aspect, with the advantages of high surface area, rigid framework and open pore systems which can facilitate the mass transfer, ordered mesoporous oxides can also act as support materials for applications. It has been well demonstrated by electron microscopy studies that the growth and aggregation of Cu nanoparticles can be greatly inhibited by SBA-15 when catalyzing the methanol synthesis25. As the promotion effect between metal and oxide is being investigated more frequently, the introduction of this class of materials as support can be of great benefit to better accommodate and/or separate the active sites and prevent particle agglomeration/growth under harsh reaction conditions.90, 91 However, this has to be based on the situation that the majority of nanoparticles are anchored on the inner wall of the pores channels.

ACKNOWLEDGMENT This work was supported by MAXNET Energy research consortium the Max Planck Society and the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinshaft (DFG) and Fonds der Chemischen Industrie (FCI). We thank Prof. Candace Chan (ASU) for fruitful discussion and proof read.

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Supporting Information Available: Characterization of MCM-41, KIT-6 and SBA-15 silica hard templates (SAXS, N2 sorption, TEM). Detailed calculation for preparation of the mixed metal oxides. This material is available free of charge via the Internet at pubs.acs.org. REFERENCES (1) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. The Preparation of Alkyltriinethylaininonium–Kaneinite Complexes and Their Conversion to Microporous Materials. Bull. Chem. Soc. Jpn. 1990, 63, 988-992. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates. J. Am. Chem. Soc. 1992, 114, 1083410843. (3) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism. Nature 1992, 359, 710712. (4) Gu, D.; Schüth, F. Synthesis of Non-Siliceous Mesoporous Oxides. Chem. Soc. Rev. 2014, 43, 313-344. (5) Ren, Y.; Ma, Z.; Bruce, P. G. Ordered Mesoporous Metal Oxides: Synthesis and Applications. Chem. Soc. Rev. 2012, 41, 4909-4927. (6) Tüysüz, H.; Schüth, F., Ordered Mesoporous Materials as Catalysts. Adv. Catal. 2012, 55, 127-239. (7) Davis, M. E. Ordered Porous Materials for Emerging Applications. Nature 2002, 417, 813-821. (8) Taguchi, A.; Schüth, F. Ordered Mesoporous Materials in Catalysis. Micropor. Mesopor. Mat. 2005, 77, 1-45. (9) Wagner, T.; Haffer, S.; Weinberger, C.; Klaus, D.; Tiemann, M. Mesoporous Materials as Gas Sensors. Chem. Soc. Rev. 2013, 42, 4036-4053. (10) Polarz, S.; Antonietti, M. Porous Materials via Nanocasting Procedures: Innovative Materials and Learning About Soft-Matter Organization. Chem. Commun. 2002, 25932604. (11) Schüth, F. Endo‐and Exotemplating to Create High‐ Surface‐Area Inorganic Materials. Angew. Chem. Int. Ed. 2003, 42, 3604-3622. (12) Wan, Y.; Zhao, D. On the Controllable Soft-Templating Approach to Mesoporous Silicates. Chem. Rev. 2007, 107, 28212860. (13) Petkovich, N. D.; Stein, A. Controlling Macro-and Mesostructures with Hierarchical Porosity through Combined Hard and Soft Templating. Chem. Soc. Rev. 2013, 42, 3721-3739. (14) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. EvaporationInduced Self-Assembly: Nanostructures Made Easy. Adv. Mater. 1999, 11, 579-585. (15) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Generalized Syntheses of Large-Pore Mesoporous Metal Oxides with Semicrystalline Frameworks. Nature 1998, 396, 152-155. (16) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schüth, F.; Stucky, G. D. Generalized Synthesis of Periodic Surfactant/Inorganic Composite Materials. Nature 1994, 368, 317-321. (17) Antonelli, D. M.; Ying, J. Y. Synthesis of Hexagonally Packed Mesoporous TiO2 by a Modified Sol – Gel Method. Angew. Chem. Int. Ed. 1995, 34, 2014-2017. (18) Li, Y.; Luo, W.; Qin, N.; Dong, J.; Wei, J.; Li, W.; Feng, S.; Chen, J.; Xu, J.; Elzatahry, A. A.; Es-Saheb, M. H.; Deng, Y.; Zhao, D. Highly Ordered Mesoporous Tungsten Oxides with a Large


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(91) Yang, X.; Kattel, S.; Senanayake, S. D.; Boscoboinik, J. A.; Nie, X.; Graciani, J.; Rodriguez, J. A.; Liu, P.; Stacchiola, D. J.; Chen, J. G. Low Pressure CO2 Hydrogenation to Methanol over

Gold Nanoparticles Activated on a CeOx/TiO2 Interface. J. Am. Chem. Soc. 2015, 137, 10104-10107.



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Protocol for the Nanocasting Method: Preparation of Ordered

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