Colloidal Solution Combustion Synthesis: Toward Mass Production of

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Colloidal Solution Combustion Synthesis: Towards Mass Production of Crystalline Uniform Mesoporous CeO Catalyst with Tunable Porosity 2

Albert A. Voskanyan, Kwong-Yu Chan, and Chi-Ying Vanessa Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00505 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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Chemistry of Materials

Colloidal Solution Combustion Synthesis: Towards Mass Production of Crystalline Uniform Mesoporous CeO2 Catalyst with Tunable Porosity Albert A. Voskanyan, Kwong-Yu Chan,* Chi-Ying Vanessa Li The Department of Chemistry, The University of Hong Kong, Pokfulam, Hong Kong

ABSTRACT: Mesoporous metal oxides with uniform porosity are of considerable interest. Their economical production at a large scale in an efficient manner, however, still remains a challenging task for commercialization. In this work, we demonstrate for the first time a scalable, economic, energy and time efficient method for the synthesis of crystalline mesoporous CeO2 catalyst with tailored porosity, by utilizing colloidal SiO2 as template. The size and amount of colloidal particles can tune the porosity of the CeO2 nanostructure as well as alter the heat transfer and heat balance of combustion. As-prepared CeO2 possesses uniform 22 nm pores, 0.6 ml/g pore volume, which is the highest pore volume for CeO2 reported. The obtained mesoporous CeO2 catalyst exhibited excellent activity for soot and carbon monoxide oxidation. In principle this method can be applied to synthesize different high porosity crystalline oxides and mesoporous CuO was also successfully prepared, thus demonstrating its generality.

INTRODUCTION Mesoporous metal oxides are commonly synthesized via soft or hard templates.1,2 Soft templates, such as surfactants or block copolymers, decompose prematurely during high temperature treatment, leading to a collapse of the mesoporous framework before complete crystallization.3-5 Since prolonged heat treatment cannot be applied, only amorphous or semi-crystalline metal oxides are produced.6 Crystalline metal oxides can be prepared by impregnating inorganic precursors into hard templates of mesoporous silica or carbon which can sustain relatively high temperatures.7-10 However, the yield of nanocasting is low since the volume of the final oxide is only a few percent of the total mesopore volume filled by precursors.11 Mesoporous hard templates (e.g. SBA-15, KIT-6, CMK-3) are expensive and additional capital costs are needed. Limitations for mass production of mesoporous non-siliceous oxides, from soft or hard templates, have been comprehensively reviewed.12 Ordered porous materials can also prepared by colloidal crystal templating where polystyrene (PS), poly-(methyl methacrylate) (PMMA) beads or silica microspheres are used as templates.13-25 In this method, precursors infiltrate into the interstitial spaces of three-dimensional (3D) close-packed colloidal arrays without disrupting their periodicity. After subsequent solidification of the precursors and colloidal template removal, porous inverse replicas with periodic 3D scaffolds known as inverse opals are obtained.26,27 The pore size of the produced material depends on the diameter of colloidal spheres used and 3D open macroporous structures with various diameters (50-500 nm) are obtained by this method. The synthesis of mesoporous inorganic oxides is challenging due to the difficulty of fabricating small polymeric colloids. However, mesoporous carbons with large mesopores were successfully synthesized by colloidal imprinting (13-24 nm) and colloidal templating (10-40 nm) methods using silica spheres.28,29 Despite these recent advances in this field, it is still a great challenge to synthesize crystalline mesoporous metal oxides with a uniform and tunable porous structure at a large scale in a facile and cost effective way.

Recently, solution combustion synthesis (SCS) has become a popular method to mass produce nanosize crystalline metal oxides in an energy and time efficient manner.30-40 High temperature required for crystal nucleation is achieved by the self-generated heat. After ignition, the conversion of precursors into a product takes less than a minute. Rapid cooling (typically a few seconds) does not provide sufficient time for extended crystal growth, leading to nanoscale crystals. Despite many advantages of SCS, commercialization is discouraged by the poor control of porous structure in the metal oxides produced.41-48 Here, we report for the first time, a simple scalable production of crystalline uniform mesoporous metal oxide by combustion of a colloidal solution. Distinguished from conventional SCS, the method is denoted as colloidal solution combustion synthesis (CSCS). Colloids play a significant role beyond that of a template. Colloidal particles significantly moderate combustion and reaction is confined in nanodomains between colloids. CSCS can be generally applied to synthesize various metal oxides using different hard colloidal particles. Ceria (CeO2) has wide applications in catalysis, solid oxide fuel cells, lithium ion batteries, oxygen sensors, solar cells and biotechnology.49-54 It is selected here to demonstrate the CSCS method. The synthesis of mesoporous CeO2 via CSCS is schematically illustrated in Figure 1 and details are given in the experimental section. Briefly, colloidal SiO2 particles are added to an aqueous solution of Ce(NO3)3 and glycine (CH2NH2COOH). Glycine is used as a fuel. It is low cost and its zwitterionic character leads to high coordination with nitrates. Upon heating and water evaporation, a gel is formed between SiO2 colloids. The CH2NH2COOH/Ce(NO3)3 gel ignites at 150 °C with a rapid increase in temperature and evolution of gases leading to a formation of crystalline CeO2 nanoparticles between the colloids. To obtain high void-fraction mesoporous CeO2, SiO2 was removed by alkaline etching. Simple and scalable CSCS involves low cost precursors. Mass production is promised by minimum capital requirements, in contrast to other templating methods.

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Figure 1. Schematic of the CSCS method for synthesizing crystalline mesoporous CeO2 with tailored porosity: (a) colloidal solution (b) gel (c) combustion confined by colloids (d) CeO2-SiO2 nanocomposite (e) highly mesoporous CeO2 after removal of colloidal particles. Compared to hard templating method using ordered mesoporous silicas (SBA-15, KIT-6) and carbons (CMK-3), CeO2 synthesized by CSCS can have larger pore sizes. Importantly, avoiding the use of organic solvent or surfactant, CSCS provides a green chemistry route to scalable synthesis of functional nanomaterials. In addition to synthesizing highly porous metal oxides, the CeO2-SiO2 nanocomposite in Figure 1(d) is an alternative product which can have unique functional properties such as abrasives used in chemical-mechanical polishing.55,56 EXPERIMENTAL SECTION 99.0%), Chemicals. Cerium nitrate (Ce(NO3)3·6H2O, (Cu(NO3)2·2H2O, 99.0%), glycine (CH2NH2COOH, 98.0%), LUDOX TMA colloidal SiO2 (34 wt% of ~20-22 nm SiO2 suspension in water, ρ = 1.23 g ml-1) were purchased from SigmaAldrich and used without further purification. Ultrapure water (18.2 MΩ cm) was used to prepare all the solutions. Commercial CeO2 (99 %, 5 micron) from Sigma Aldrich was used for comparison. Synthesis of Mesoporous CeO2. The synthesis of mesoporous CeO2 nanostructures is outlined below. For each experiment, 2 g of Ce(NO3)3·6H2O, 0.4 g of CH2NH2COOH were dissolved in 5 ml of water and varying amounts of colloidal SiO2 were added. The samples synthesized are denoted as ceria-0, ceria-1, ceria-2, and so on, according to increasing amount of colloids added, as listed in Table 1. The solution was transferred into the glass beaker (volume 100 ml) and was heated at 150 oC on a hot plate. After several minutes, combustion occurred with a rapid increase in temperature due to the exothermic reaction between the Ce(NO3)3 and CH2NH2COOH. The resulting powder was naturally cooled down to room temperature in few minutes. It is then immersed in 2 M NaOH at 80 oC for 4 h. The samples were subsequently washed with water and ethanol three times and dried at 120 oC to obtain pure mesoporous CeO2. For the synthesis of mesoporous CuO, 2 g of Cu(NO3)2·2H2O, 0.438 g of CH2NH2COOH were dissolved in 5 ml of water and 1 ml of colloidal SiO2 solution was added. After combustion CuO-SiO2 composite was obtained and SiO2 was removed as similarly described above for CSCS synthesis of CeO2. Table 1. Amount of SiO2 colloids added for preparation of different CeO2 samples. Sample

Volume of colloidal SiO2 added (ml)

Volume of Ce(NO)3 CH2NH2COOH aq. Solution (ml)

ceria-0

0

5

ceria-1

0.2

5

ceria-2

0.5

5

ceria-3

1

5

ceria-4

1.2

5

Characterization. K-type thermocouples with a 0.1 mm diameter probe were used to monitor the reaction temperature over time. The output signals of the thermocouples were passed to a computer using a multichannel data acquisition line (Data Translation Inc.) with a monitoring frequency of 1 kHz using Quick DAC software. The data were results of at least three sets of repeating measurements. The compositions of as-synthesized powders were determined using powder X-ray diffraction (XRD) with CuKα radiation at 40 kV and 40 mA (D8 Advance, Bruker). The powder microstructures were examined by scanning electron microscopy (SEM) (Hitachi S-4800 with an accelerating voltage of 7kV). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies were conducted to characterize the structure and composition of the reaction products (Phillips TECNAI 20 with an accelerating voltage of 200kV). A Micromeritics ASAP 2020 analyzer was used to obtain the Brunauer-Emmet-Teller (BET) surface areas and BarrettJoyner-Halenda (BJH) pore size distributions of oxides using nitrogen as the adsorbent gas at 77K. Oxide powders were degassed at 473K and 10-6 torr overnight under vacuum prior to the analysis. Zeta potential was measured by Zetasizer (Malvern UK). X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra DLD multi-technique system with monochromatic AlKα X-ray source and the sample was scanned 4 times. The charge on ceria samples was corrected by setting the binding energy of the adventitious carbon (C 1s) to 284.8 eV.57 Peak areas were measured after a Shirley background subtraction58 using Gaussian-Lorentzian functions. The photopeaks were analyzed by using CASA XPS software. Differential scanning calorimetry (DSC) curves were recorded using a DSC Q20 instrument with a heating rate of 10 oC/min in N2 atmosphere. The heat of dehydration was calculated from the integration of the area under the peaks. Catalytic Activity Tests. The catalytic activity of CeO2 samples for soot oxidation was performed in a Hiden Analytical fixed-bed micro-reactor system at atmospheric pressure under a gas flow (50 ml min-1) of 4% O2 with He as a carrier gas at a 10 °C min-1 rate. A commercial carbon black powder (purchased from Cabot) was used to model diesel soot. The catalyst (50 mg) and soot (10 mg) were grinded together in an agate mortar for 10 min to ensure a tight contact between them. The outlet gaseous composition was analyzed by a Hiden quadrupole mass spectrometer Qic 20. The catalytic activity of CeO2 samples for CO oxidation was also performed under atmospheric pressure in a Hiden Analytical fixed-bed system. 30 mg of the catalyst was loaded into a quartz microreactor and end-blocked by quartz wool. The reaction mixture consisting of 4% CO and 4% O2 with He as carrier gas was delivered with a total flow rate of 50 ml min-1, and heating at rate of 10 °C min-1 was applied from room temperature up to 500 o C. The reaction temperature measured by a K-type thermocouple inserted into the reactor was varied by a temperature controller. The outlet gaseous composition was analyzed by a Hiden quadrupole mass spectrometer.


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Figure 2. (A) TEM image of initial colloidal SiO2, (B, C) TEM images, and (D) HRTEM image (inset: SAED pattern) of ceria-3. RESULTS AND DISCUSSION Four samples of CeO2 were synthesized by CSCS with incremental amounts of SiO2 added. Table 1 shows the volume of colloidal SiO2 added to 5 ml of CH2NH2COOH/Ce(NO3)3 solution. The resulting samples are denoted as ceria-0, ceria-1, ceria-2, and ceria-3. Figure 2 shows typical transmission electron microscopy (TEM) images of the SiO2 particles and corresponding CSCS synthesized ceria-3. (Additional TEM images at a larger magnification in Figure S1 have the crystalline fringes more clearly revealed). A highly porous CeO2 with uniform spherical pores is clearly visible in Figure 2(B-D). From the TEM images of Figure 2(B-D), the pore size is approximately 20-22 nm. The size and uniformity for ceria-2 and ceria-3 samples is consistent with the sharp peak in the BJH pore size distribution determined by N2 sorption measurements shown in Figure 3A, and is in agreement with the size of the initial colloidal particles shown in Figure 2(A). On the other hand, for colloidal crystal templating when polymers are used as a template, there is a big pore shrinkage after polymer removal, and generally the final pore size of prepared material is 10-50% smaller than the diameter of the original template used.17-19 In contrast to “soft” polymeric crystal templates, SiO2 spheres are much “harder” preventing the shrinkage caused by crystal growth of replica material. The average pore size for ceria-1, however, is smaller than the size of original colloidal template used and a much broader peak centered at around 9 nm can be seen. As shown in Table 1, the volume of colloidal silica used in ceria-1 is 1/25 the volume of precursor solution, compared to 1/10 in ceria-2. Hence the fraction of 22 nm pores will be much less compared to ceria-2 and ceria-3 samples. During template removal, some of these pores may have collapsed or shrunk under the high solid fraction. Besides, the surface charge of SiO2 spheres is dictated by the cerium nitrate

concentration. If the amount of colloids is small the surface charge will be highly positive creating strong repulsive forces and preventing close-packing of the spheres. TEM and SEM images for other CeO2 samples are shown in Figures S2 and S3. A broad range of pore size in ceria-1 is also evident in Figure S2B. The surface area, pore volume, average pore size, crystal size, and maximum temperature during CSCS for the series of samples are listed in Table 2. As shown in Figure 4, the total pore volume and specific surface area of the CeO2 increase monotonically with addition of SiO2.

Figure 3. (A) BJH pore size distribution plots (B) N2 sorption isotherms vertically shifted for clarity. (C) Temperature-time profiles (D), XRD patterns for different CeO2 samples.



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Ceria-3 has a very large pore volume of 0.6 ml/g, in contrast to 0.06 ml/g for ceria-0 synthesized by conventional SCS. The porosity estimated from pore volume of 0.6 ml/g and CeO2 density (7.28 g/ml) is 81% (calculations shown in SI). This high value of porosity is also observed in porous materials synthesized from other colloidal crystal templates.27,59 A small part of the porosity also comes from micropores formed between CeO2 nanoparticles due to the gas evolution during combustion making the mesopores interconnected. The measured micropore volume is 0.075 ml/g, which is equivalent to 12.5% porosity. The presence of micropores corresponds to the steep rise in the N2 isotherm (Figure 3B) at low relative pressure (90%) of common aerogels which are open frameworks formed by networked nanoparticles. Figure 2(B, C) also shows open porous structure formed by connected nanoparticles, features which are similar to those of aerogels. The ceria-3, however, have uniform spherical cavities that are partially ordered, as opposed to unorganized and irregular pores in aerogels. Table 2. Textural parameters of produced different CeO2 samples. Sample

ceria-0

ceria-1

ceria-2

ceria-3

ceria-4

Vol. of SiO2 solution added (ml)

0

0.2

0.5

1

1.2

No ignition

Tmax (oC)

510

379

315

295

SBET (m2 g-1)

13.7

39.3

62.8

81.7

BJH Pore volume (ml g-1)

0.06

0.23

0.41

0.6

Average pore size (nm)

9.6

13.6

21.7

22

XRD particle size (nm)

12.1

7.6

5

4.1

Ceria-3 has the highest pore volume, compared to other literature values of hard template synthesized CeO2 as shown in Table 3. Close examination of the TEM image (Figure 2(C) and HRTEM image Figure 2(D)) reveals that the pore walls are about 5 nm thick. The walls of ceria-3 nanostructure follow the curvature of the template spheres demonstrating a strong interaction of cerium nitrate with silica spheres before combustion. The corresponding selected area electron diffraction (SAED) in the inset of Figure 2(D) displays a ring pattern. The measured d spacing values are in good agreement with the face-centered cubic CeO2 structure.

Figure 4. Surface area and pore volume of different CeO2 prepared by CSCS versus amount of SiO2 colloids added (inset: maximum combustion temperature versus amount of SiO2 colloids added). Specific conditions of the CSCS help to avoid coagulation of colloids, which is necessary for forming a uniform and highly

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porous structure. The colloids are stabilized by surface charges, according to the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory.60 Surface charge of colloids should be adequate to provide repulsion and prevent coagulation, but not too excessive in affecting close-packing.22 The presence of deprotonated surface silanol groups is confirmed by measured zetapotential of -25.8 mV. After addition into Ce(NO3)3 and CH2NH2COOH mixture, the zeta potential becomes positive (12.9 mV) due to adsorption of Ce3+ cations. The charge reversal of SiO2 surface maintains electrostatic repulsion and prevents coagulation, as similarly reported for trivalent cations adsorbed onto SiO2.61 Glycine molecules are also positively charged in the acidic solution of pH 4.5 and when adsorbed further contribute to repulsion by charge as well as steric hindrance. The uniformity of cavities (Fig. 2 and Fig. 3A) demonstrates the absence of colloid coagulation and no collapse of pores during SiO2 removal, except for ceria-1. In contrast, though conventional SCS have the same chemistry, the heat transfer, gas evolution and nucleation and crystal growth occur in an unorganized manner without the regulation of a colloidal matrix. Typical temperaturetime profiles for CSCS of the different CeO2 samples are shown in Figure 3C. The maximum temperature decreases from 510 oC for ceria-0 to 295 oC for ceria-3, as shown in inset of Fig. 4. Further increase of SiO2 to 1.2 ml suppresses temperature rise and combustion does not take place. The presence of SiO2 in CSCS promotes the formation of small nanocrystals due to the confinement of combustion and the alteration of temperature profile. At the same time, dispersed rigid spherical colloids create physical barriers that prevent contact and agglomeration of nanocrystals. Table 3. Comparison of textural properties for CeO2 synthesized by CSCS and hard templating method. Sample

Template

Pore size

Pore volume

SBET

(nm)

(ml g-1)

(m2 g-1)

Ref.

ceria-3

Colloidal SiO2

22

0.6

81.7

This work

CeO2

MCM-48

3.5

0.24

198

62

CeO2

KIT-6

6.7

0.18

112

63

CeO2

CMK-3

5; 35

0.42

148

64

CeO2

KIT-6

3.2

-

152

65

CeO2

SBA-15

3.8

0.23

101.3

66

CeO2

PS

90; 14.8

0.23

101.1

67

CeO2

PMMA

189

0.41

47.6

68

CeO2

PMMA

10.2; 260

0.2

75.3

69

X-ray diffraction (XRD) patterns of the CeO2 samples in Figure 3D have well defined peaks that can be indexed to the facecentered cubic phase of CeO2 (Fm3m, JCPDS, file No. 34-0394). The peaks broaden successively from ceria-0 to ceria-3, with each additional amount of SiO2, indicating a corresponding decrease in size of CeO2 nanocrystals as shown in Figure 5A and Table 1. The average crystalline size of ceria-3 sample calculated from Scherrer equation using (111) peak was found to be ~ 4.1 nm, in good agreement with the TEM observations. Close examination of the XRD in Figure 3(D) reveals a gradual downshift of Ce (111) peaks around 28.5° from ceria-0 to ceria-3, and also shown in Figure 5(A) with larger magnification. This shift corresponds to a gradual increase in the lattice constant of CeO2 as shown in Figure 5(B) and could be caused by oxygen vacancies and associated increase in Ce3+. To analyze Ce species in the CeO2 samples, X-ray photoelectron spectra (XPS) of the CSCS samples are compared in Figure S4 together with CeO2


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synthesized by conventional SCS (ceria-0) and a commercial CeO2.

Figure 5. (A) Comparison of broadening of XRD peaks for different CeO2 samples. (B) Lattice constant of different CeO2 samples prepared by CSCS versus amount of SiO2 colloids added. From the 3d energy of different Ce species, the relative concentration of Ce3+ can be calculated from the deconvoluted curves shown in Figure S5. The calculated concentration of Ce3+ in ceria-3 nanostructures is 34.9% of all Ce species, while it is 6.1% in ceria-0. The calculations are shown in SI and the computed relative Ce3+ concentrations are tabulated in Table S1. The O1s region of the XPS spectrum is also analyzed and shown in Figure S6. Two main peaks can be resolved and assigned to lattice O2- and chemisorbed OH-. The relative concentrations of these species are determined and tabulated in Table S1. OHconcentration is highest on ceria-3 surface. An additional peak in the O1s region appears only for ceria-3 and can be assigned to adsorbed water molecules.

Figure 6. DSC curves with calculated heats of dehydration for different CeO2 samples. The enhanced presence of water in ceria-3 is consistent with differential scanning calorimetry (DSC) results of different CeO2 samples as compared in Figure 6. The endothermic peak before 100 °C corresponds to energy required to remove adsorbed water. As shown in Figure S7, the heat of dehydration can be directly correlated to surface area of CeO2. The strongest hydration is observed for ceria-3, which also has the highest specific surface area. The calculated direct band gap energy for ceria-3 from UV/Vis absorption spectra is red shifted (Ed = 2.93 eV) compared with the bulk CeO2 (Ed = 3.15 eV), which is resulted from the high concentration of defects (Figure S8).70 The catalytic activity of ceria-3 was tested for soot and CO oxidation and results are illustrated in Figure 7. Diesel soot oxidation is selected for its important role in vehicle emission control to minimize carbon particulates59,71-75 and CO oxidation is

another monitored environmental pollutant.76-79 Mesoporous ceria-3 catalyst shows excellent activity, reaching complete soot oxidation at 350 oC which is 200 oC lower than that for commercial CeO2 (Figure 7A). These results compared favorably with other literature reports for soot combustion. For example, a similar temperature of ~350 oC was recorded on CeO2 rice-ball nanostructures,72 whereas a 3D ordered macroporous CeO2 synthesized by colloidal crystal templating, reported by Zhang et al.,73 showed 90% diesel soot oxidation at around 400 oC. Shape dependent activity was also investigated showing that nanocubes and nanorods of ceria are more active than the polycrystalline octahedral particles, achieving 50% conversion between 400-420 o 75 C. Here, ceria-3 also exhibits good activity for CO oxidation, achieving 100% CO conversion at 300 oC as shown in Figure 7(B). This compares well with the best literature results on a bare CeO2 catalyst. The enhanced activity of ceria-3 can be attributed to a number of favorable properties including high surface area, high porosity, small crystal size, and increased Ce3+ concentration on the oxide surface, all resulting from combustion synthesis of an optimized colloidal solution.

Figure 7. Catalytic soot (A) and CO oxidation (B) results for different CeO2 catalysts. With the presence of a colloid, SCS is significantly modified in several fundamental aspects. Results for CSCS synthesized CeO2 demonstrate 6 times increase in surface area, 10 times increase in pore volume, a narrow pore size distribution, partially ordered porosity, 3 times smaller particle size, 5.5 increase in Ce3+ relative concentration, and 200 °C decrease in temperature required for completion oxidation of soot. These properties are also significantly better than conventional mesoporous CeO2 synthesized in small scale. The tunability of CSCS is well illustrated by the control and correlation of surface area, pore volume, and pore size by colloid addition. CSCS can be generally applied to synthesize metal oxides with similar properties and results of CSCS synthesized CuO are shown in Figure 8 with uniform pores (~20 nm), high pore volume (0.27 ml/g) and high surface area (74.5 m2/g).



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Notes

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The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors acknowledge financial support from clean energy and University Development Fund (UDF) for Initiative for Clean Energy and Environment (ICEE). The authors thank Mr. Frankie Chan of Electron Microscopy Unit at The University of Hong Kong (HKU) for assistance in TEM characterization. Mr. Nick Ho for XPS characterization at Hong Kong University of Science and Technology (HKUST). Also, Dr. Yee-Yan Tay of School of Materials Science and Engineering at Nanyang Technological University (NTU) for the discussion on XPS results. A. A. Voskanyan gratefully acknowledges support from a Hong Kong research Grant Council (RGC) for Postgraduate Scholarship.

REFERENCES Figure 8. BJH pore size distribution curve and TEM image (inset: SAED) of CuO synthesized via CSCS. The integration of colloidal crystal templating with solution combustion synthesis provides exciting opportunities to large scale bottom-up synthesis of porous nanostructures with structural controllability. CSCS can be further extended to synthesize various metal oxides (binary, ternary), metals, metal sulfides, carbides with a uniform mesoporous structure, as well as bicontinuous nanocomposites with embedded colloidal particles, as illustrated in configuration (d) of Figure 1. Embedded colloidal particles can be metals such as Ag, Au, Pt, or metal oxides such as ZnO, Fe3O4, TiO2 yielding a large range of functional composites with designed uniform and well organized structure. Moreover, colloidal polymeric templates such as PS and PMMA should be also explored by this method. CONCLUSIONS In this work, we developed a new, efficient, scalable method for the synthesis of crystalline uniform mesoporous CeO2 catalyst via a CSCS. The catalyst has high surface area, high pore volume and showed excellent catalytic activity for soot and CO oxidation. The synthesis is fast, energy efficient, and low costs in raw materials and equipment. The addition of a colloid limits combustion to take place in confined and uniformly distributed nanospace, leading to a highly uniform porous structure. The structure of the oxide can be tuned by the amount and size of colloids added (pore diameter, pore wall thickness, surface area, and pore volume). Because of the large scale commercial availability of starting materials, CSCS has the essential features for economic massproduction of high quality porous nanocrystalline materials for various industrial applications. Large scale production of CeO2 has been demonstrated by 0.5 kg mesoporous ceria-3 catalyst produced within five hours.

ASSOCIATED CONTENT Supporting Information Additional TEM and SEM images of different ceria samples: XPS results and calculations for ceria samples: graph showing correlation between heat of dehydration and surface area; UV/vis absorption spectra of ceria-3 catalyst. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * Email: [email protected].

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