Hierarchically Structured Ceria-Silica: Synthesis and Thermal

Article pubs.acs.org/JPCC

Hierarchically Structured Ceria-Silica: Synthesis and Thermal Properties Peter W. Dunne,† Anna M. Carnerup,† Agnieszka Węgrzyn,‡ Stefan Witkowski,‡ and Richard I. Walton*,† †

Department of Chemistry, University of Warwick, Coventry, CV4 7AL, United Kingdom Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland

‡

ABSTRACT: A one-pot method for the synthesis of hierarchically structured ceria-silica composite materials is reported along with the results of their characterization by a variety of physical techniques. Crystallization of ceria in an aqueous mixture of anionic, cationic, and neutral surfactants, namely Pluronic P123 (EO20PO70EO20), CTAB (hexadecyltrimethylammonium bromide), and SDS (sodium dodecyl sulfate) leads to the formation of a suspension of capped ceria nanoparticles. Subsequent addition of tetraethoxysilane followed by aging at 40−80 °C leads to the condensation of silica. After thermal removal of the organic species, the formation of high surface area composites directed by the interaction of the capped nanoparticles and the remaining surfactants is possible. The thermal stability and redox activity of the composite materials have been studied by in situ powder X-ray diffraction, TGA/DSC, transmission electron microscopy, Ce LIII-edge XANES, and temperature-programmed reduction under H2/N2. Encapsulation of the ceria nanoparticles in the templated silica matrix leads to high thermal stability with the nanocrystalline nature of the ceria retained upon heating to 900 °C in air with no annealing evident by in situ thermodiffractometry. Temperature-programmed reduction shows large hydrogen uptake at around 600 °C, corresponding to complete reduction of all Ce(IV) to Ce(III) in the case of a cerium-rich sample (Ce:Si = 5:12). This reduction leads to amorphization of the ceria followed by the collapse of the hierarchical structure with formation of Ce2Si2O7 crystallites embedded in amorphous silica. For a sample of lower cerium content, crystalline Ce6(Si4O13)(SiO4)2 is formed under reductive conditions.

1. INTRODUCTION Ceria, CeO2, is a key material in many modern catalysis applications, ranging from catalytic wet oxidation to fluid catalytic cracking, the water-gas shift reaction, and, perhaps most commonly encountered, three-way catalysis for the removal of harmful gases from automotive emissions.1−7 Ceria may be employed either as a support material for noble metals or as a catalyst in its own right. The importance of ceria in these applications arises from the ease with which cerium in the fluorite structure can cycle between the +4 and +3 oxidation states by oxide migration through the lattice. This gives a high oxygen storage capacity, which allows ceria to act as an oxygen buffer in catalytic systems, releasing or accepting oxygen as required, for example in response to the lean-rich fluctuations found in auto exhausts.8 When confined to the nanoscale, ceria shows an increase in activity in many of the above-mentioned applications. This increased activity has been attributed variously to the increased surface area, the presence of surface superoxide anions and the relative ease of oxygen vacancy formation in nanoparticles versus the bulk material.9−14 Recent work has also shown how ceria nanocrystals may have a striking lattice parameter dependence on crystallite size, which may in part be responsible for their enhanced oxygen storage and redox properties.14−16 These effects have been exploited in catalysis; for example, Carretin et al. have shown that gold deposited on nanocrystal© 2012 American Chemical Society

line ceria exhibits up to double the activity of gold on bulk ceria in CO oxidation.17 A particularly cogent example of the importance of nanoscale ceria to catalytic processes was recently reported by Vaysillov et al., who have shown through computational and experimental methods that oxygen transfer from ceria to supported platinum particles occurs only on nanostructured ceria.18 As a result of this high catalytic reactivity, there has been a great deal of interest in the synthesis of ceria nanoparticles, with a wide variety of preparative methods having been investigated. These include sol−gel and solvothermal routes,19−26 mechanochemical,27 sonochemical, and microwave syntheses,28,29 as well as combustion and pyrolysis methods.30,31 A significant limitation encountered with pure nanocrystalline ceria is its poor thermal stability, particularly under the reducing conditions in which ceria catalysts must operate. At relatively low temperatures in a variety of atmospheres, from air to hydrogen, ceria undergoes significant sintering and particle growth; this results in the loss of surface area and catalytic activity.32,33 Even a small degree of sintering has a large effect on both the crystallite size and the presence of oxide-ion vacancies. A potential means of overcoming the problem of Received: April 4, 2012 Revised: May 10, 2012 Published: May 24, 2012 13435

dx.doi.org/10.1021/jp3031978 | J. Phys. Chem. C 2012, 116, 13435−13445

The Journal of Physical Chemistry C

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number of pure ceria samples were prepared in the presence of individual surfactants (C1−3) and in the ternary surfactant mixture (C4) in order to study the formation mechanism of the composite materials. The molar ratios of reagents used in the preparation of each sample are given in Table 1.

sintering of ceria is to support it on a more thermally stable high surface area material such as silica or alumina. It has been shown that supporting ceria on silica enhances its catalytic activity, which, combined with the stability of silica at high temperatures and the abundance of microporous and mesoporous silicas, makes supporting ceria nanoparticles on mesoporous silica a very promising avenue to explore.34−38 Previous methods of adding metal oxides to mesoporous silica include incipient wetness impregnation,39,40 whereby the desired metal is loaded onto the support by steeping the support in a solution of a metal salt or complex followed by solvent evaporation and calcination to give metal oxide crystallites in and on the silica,35,36 and precipitation deposition, a similar process with an additional precipitation step giving supported metal oxide or hydroxide species prior to calcination.41 Both of these approaches require the mesoporous silica to be preformed and the oxide is added to this silica in a postsynthetic step.42 This strategy leads to little control over the distribution of oxide particles in the host, and they may be unevenly distributed, typically concentrated at the mouths of the porous silica structure.43−45 For example, Strunk et al. deposited ceria onto mesoporous silicas using various Ce(IV) alkoxide complexes and found that the choice of cerium precursor was crucial to obtain materials with an even distribution of ceria nanocrystals, and in some cases clustering of the ceria at pore mouths was inevitable.45 The aim of the work described herein was to investigate the preparation, in a one-pot procedure, of composite ceria-silica materials that may have hierarchical structures and porosity, such that the ceria nanoparticles may be distributed throughout a high surface area silica matrix, isolated from each other, thus bypassing the sintering issues, while maintaining or enhancing the favorable redox properties of ceria itself. By using the large literature on the formation of mesoporous silicas as a guide we were able to select a templating approach to control the even distribution of ceria in the mesoporous solid to yield novel hierarchically structured composite materials.

Table 1. Synthesis Conditions of Samples Studieda millimoles in 100 mL H2O

a

sample

P123

CTAB

SDS

Ce

Si

C1 C2 C3 C4 CS0 CS1 CS2

0.12 0 0 1.2 0.12 0.12 0.12

0 2.1 0 2.1 2.1 2.1 2.1

0 0 3.5 3.5 3.5 3.5 3.5

5 5 5 5 0 1 5

0 0 0 0 12 12 12

See text for explanation of the surfactants and sources of Ce and Si.

2.2. Laboratory Characterization. Powder X-ray diffraction (XRD) patterns were recorded over a 2-θ range of 5−60°, with a step size of 0.02° and a collection time of 2 s per step using a Siemens D5000 diffractometer operating with Cu Kα (average wavelength = 1.5418 Å) radiation. Average particle sizes were calculated using the Scherrer equation from peak broadening obtained by fitting the patterns in Xfit.48 Infrared spectra were recorded on a Perkin-Elmer Spectrum100 diamond ATR-FTIR spectrometer. TG/DSC traces were recorded from room temperature to 1000 °C at a rate of 10 °C/minute under a flow of air on a Mettler-Toledo instrument. In situ XRD studies with temperature were carried out on a Bruker D8 diffractometer (Cu Kα radiation) fitted with an Anton Parr XRK900 reaction chamber and VÅNTEC-1 solid state detector under both air and 5% H2 in N2 atmospheres (in separate experiments). Heating was carried out at a rate of 0.2 °C/second, with scans taken at intervals of 50 °C up to 900 °C, with a dwell time of 5 min prior to each scan. The scan range was 20 to 60° 2θ at a step size of 0.008° and a collection time of 0.3 s/step. TEM images and electron diffraction patterns were obtained on a JEOL 2000FX microscope, fitted with a Gattan Orius 11 megapixel digital camera, operating at 120 kV. Samples were prepared by dispersion in acetone and deposited onto lacey carbon coated copper grids. Elemental analysis for cerium and silicon was performed by MEDAC Ltd. using ICPOES. Nitrogen adsorption/desorption isotherms were recorded on a Micromeritics ASAP 2020 porosimeter where samples were degassed at a pressure of 10 μm Hg at 150 °C for 2 h before measurement. 2.3. Further Characterization. The TPR (temperatureprogrammed reduction) and TPO (temperature-programmed oxidation) of the samples (30 mg) were carried out from room temperature to 1000 °C with a linear heating rate of 5 °C/min. Each cycle including reduction and oxidation was repeated 3 times. Measurements were performed in a fixed-bed flow microreactor. The TPR runs were carried out in a flow of 5 vol. % of H2 diluted in Ar with a total flow rate of gas mixture of 6 mL/min, in TPO runs a mixture of O2 in He (5 vol. %) was used with a flow rate of 6 mL/min. Evolving water was removed from the effluent gas by means of a cold trap. The evolution of hydrogen or oxygen was detected by a microvolume thermal conductivity detector. All experiments were

2. EXPERIMENTAL SECTION 2.1. Synthesis. All chemicals were purchased from SigmaAldrich and used as received. The synthetic method was developed from a synthesis of mesoporous silica reported by Chen et al.46,47 who used a mixture of three surfactants to enable a tunable morphology control. Pluronic P123 (EO20PO70EO20), CTAB (hexadecyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) were dissolved in 100 g of distilled water at 40 °C with vigorous stirring (1000 rpm) in a Nalgene bottle. Ammonium cerium nitrate, (NH4)2Ce(NO3)6, was then added to this mixture and the pH was adjusted to 8.5 using 2 M NaOH. TEOS (tetraethoxysilane) was added as a silica source and the stirring speed reduced to 500 rpm. This mixture was sealed and kept at 40 °C for 24 h before being transferred to an oven at 80 °C for a further 24 h. The resultant solid product was separated by centrifugation from the liquid and washed several times with deionized water and methanol, then dried overnight at 70 °C. These asprepared samples were calcined at 600 °C for 6 h, with a heating rate of 1 °C min−1, and a cooling rate of 5 °C min−1 to remove organic species; these conditions were chosen on the basis of the thermogravimetric analysis results discussed below. Two samples, one with with a low and one with a high ceria content (CS1 and CS2, respectively), were studied in detail, with a pure silica analogue (CS0) for comparison purposes. A 13436



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performed in an automated apparatus supplied by ChemBET PULSAR (Quantachrome). Ce LIII-edge XANES experiments were performed using beamline B18 of the Diamond Light Source, U.K.49 This beamline provides X-ray energies in the range 2.05−35 keV using a fixed-exit, double-crystal Si(111) monochromator, which provides an energy resolution (ΔE/E) of 2 × 10−4. The optics of the beamline include a collimating mirror and a toroidal focusing mirror before and after the monochromator, respectively. The measurements presented here were carried out using the Cr coating of these two optical elements. A pair of harmonic rejection mirrors with a Ni stripe were also used. Under this configuration, the typical flux on the sample is of the order of 5 × 1011 photons s−1 and the size of the beam at that position is approximately 200 μm vertical by 150 μm horizontal. The ceria-silica composite samples were studied undiluted but reference materials CeO2 and CeCl3·7H2O were ground finely with polyethylene powder (∼80% by mass as diluent) and pressed into 13 mm diameter pellets of ∼1 mm thickness under a pressure of 5 tonnes. XANES data were collected in transmission mode with ion chambers before and behind the sample filled with appropriate mixtures of inert gases to optimize sensitivity. The spectra were measured with a step size equivalent to less than 0.5 eV. Data were normalized using the program Athena50 with a linear pre-edge and polynomial postedge background subtracted from the raw ln(It/I0) data.

3. RESULTS AND DISCUSSION 3.1. Synthesis. The ternary surfactant mixture was chosen for synthesis on the basis that it offered a route to a number of different morphologies for mesoporous silica, as previously reported.46,47 This prior work reported the formation of mesoporous silica platelets and hollow mesoporous silica spheres using a cationic−anionic−neutral surfactant mixture, with control over the product morphology achieved by varying the ratio of cationic to anionic surfactants. Given the likelihood of both the ceria precursor and the ceria itself interfering with the self-assembly of the surfactants through chemical interactions, particularly with the ionic surfactants, it was considered that this ternary surfactant mixture may provide a robust and adaptable system which may mitigate the disruption caused by the introduction of a cerium salt to a single surfactant synthesis and may also allow the stabilization of the ceria nanoparticles in solution, aiding in their dispersion throughout the forming mesoporous silica. The mechanism by which the composite materials are formed was investigated in some detail. Initially, the influence of the surfactants on the formation of ceria was examined by preparing ceria nanoparticles by precipitation in the presence of each individual surfactant as well as the ternary surfactant mixture under the same conditions as used in the final synthesis, but without the addition of a silica source. In each case, the precipitate obtained can be identified as ceria by powder XRD, Figure 1, albeit with small particle sizes giving considerable peak broadening. The infrared spectra of these ceria nanoparticles are also shown in Figure 1. These spectra indicate that neither P123 nor CTAB interact significantly with the ceria, as there are no bands due to the organics seen in either of these samples. The ceria prepared in the presence of SDS on the other hand shows the typical C−H stretching (3000−2850 cm−1) and deformation (1466 and 1380 cm−1) bands of the anionic surfactant. The sulfate stretching region

Figure 1. XRD patterns (top) and infrared spectra (bottom) of ceria prepared in the presence of (a) P123, (b) CTAB, (c) SDS, and (d) the ternary mixture. Gray peaks in XRD patterns are due to the aluminum sample holder used . The Miller indices are assigned using the Fm3̅m unit cell expected for CeO2.

between 1300 and 850 cm−1 provides more information: this region shows four distinct bands, with the band at 960 cm−1 arising from the ν1 symmetric sulfate stretch while the triply degenerate ν3 band is split into three distinct bands at 1064, 1165, and 1245 cm−1, indicative of C2v symmetry due to coordination of the sulfate group to the oxide surface.51−53 The sample prepared in the mixed surfactant system shows the same SDS bands and exhibits a number of small extra bands at 1370 and broadening of the envelope around 1100 cm−1, which, based on the spectrum of the pure surfactants, appear to arise from P123, which exhibits bands in these regions. The TG trace of the P123 ceria sample, Figure 2a, shows a largely smooth weight loss with a total weight loss to 1000 °C of 12.64%. A small kink is observed in the trace at approximately 250 °C, suggesting that a small amount of P123 may interact with the ceria nanoparticles and remain attached throughout the washing process. This is also confirmed by the DSC trace, Figure 2b, which has a small exothermic peak at 240 °C, which, while masked, is also present in the DSC trace of free P123. TG analysis of sample C2 prepared with CTAB indicates that the cationic surfactant does not interact with the ceria with only a smooth continuous weight loss observed in this case, with a total weight loss of 7.8%, which could arise from the loss of surface water and/or hydroxide from the transformation of surface Ce(OH)x to CeO2. The TG trace of the sample prepared in the presence of 13437



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Figure 2. TG (top) and DSC (bottom) traces of ceria prepared in the presence of the surfactants (a and b) and the ceria silica composites from the ternary surfactant mixture (c and d).

Figure 3. TEM images of samples prepared at Ce:Si ratios of 0:12 (a), 2.5:12 (b), 5:12 (c), and 7.5:12 (d). Selected area electron diffraction patterns (insets) of the cerium containing samples confirm the presence of ceria nanoparticles in the composites.

SDS shows three distinct weight loss steps centered at 190 °C, 290 and 730 °C. The 8% weight loss at 730 °C may be attributed to the loss of SO2 from sulfate groups that are adsorbed on the ceria surface. Given that the only source of sulfur is bound dodecyl sulfate, with a molar mass of 255 g mol−1, approximately four times that of SO2, this 8% weight

loss corresponds to a total dodecyl sulfate contribution to the mass of 32%, which matches the total observed weight loss. The sample prepared in the ternary surfactant mixture has a TG trace similar to that of the ceria-SDS sample; however the distinction between the first two weight loss steps is less defined, which combined with a large new exotherm in the 13438



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Figure 4. Proposed mechanism of ceria-silica composite formation.

DSC trace centered around 300 °C is consistent with the presence of an amount of P123 in the sample. The 4.6% weight loss at 730 °C due to SO2 corresponds to a DS− contribution of 18.4%, lower than the total observed weight loss of 27%, again indicating the presence of 9 wt % of water and P123. The significant differences in the DSC traces of the ceria prepared in the ternary mixture compared to the SDS sample would seem to confirm that there is some interaction between the surfactants, with the large extra exotherm at 300 °C corresponding to the decomposition temperature of free P123. It may thus be inferred that a mixture of surfactants interacts with the ceria surface, mediated by the surface bound dodecyl sulfate. Interestingly, the TG and DSC traces of the silica containing materials, Figure 2c,d, are quite different to the pure ceria samples. The non-ceria containing sample CS0 shows an early weight loss, which may be ascribed to the removal of water, followed by a smooth continuous weight loss above 200 °C. Given the low isoelectric point of silica54 coupled with the high pH of these syntheses, it is likely that the formed silica is negatively charged, and it may further be suggested that this would lead to the adsorption of positively charged CTA+ moieties on the silica surface. A weak exotherm at 240 °C is also observed, close to the decomposition temperature of free CTAB. Sample CS1 shows two weight loss steps with accompanying exotherms at 320 and 400 °C, most likely due to P123 decomposition. The high ceria sample, CS2, also exhibits an exothermic weight loss event around 300 °C, again, similar to that observed for the ceria sample from the ternary mixture. In both the high and low ceria containing composite materials, the loss of SO2 at high temperatures is not observed. This indicates that during the course of the reaction, the surface bound dodecyl sulfate groups are displaced by the negatively charged silica, which caps the ceria nanoparticles. The negatively charged surface of these silica capped ceria nanoparticles then binds CTA+ groups. The presence of these organic groups on the surface allows further interactions with

the remaining surfactants to occur, which is confirmed by the lack of any significant exothermic decomposition events in a sample prepared without CTAB. These interactions lead to the templating of the ceria-silica composite materials. The mechanism of formation was further elucidated by preparing a series of samples while varying one component of the reaction mixture, such as the ceria concentration, while keeping all other conditions the same. TEM images of the resulting products, shown in Figure 3, revealed that in the absence of ceria hollow silica vesicles are formed by condensation of silica on CTAB-SDS micelles, with mesopores perpendicular to the surface templated by P123. On adding a small amount of ceria precursor to the reaction mixture vesicle formation is disrupted leading to the formation of mesoporous sheet type structures, while further increases in ceria content leads to the formation of compact porous whorls and string like aggregates and, at very high ceria contents, particulate aggregates. A similar structural evolution of the composite materials is observed on increasing the CTAB:SDS ratio in the reaction mixture. This, combined with the observations from the IR spectra and TG/DSC traces of the pure ceria samples, indicates that the introduction of ceria into the reaction reduces the amount of SDS available for templating due to its strong interaction with the ceria nanoparticles, effectively increasing the CTAB:SDS ratio, leading to the generation of a range of morphologies which are largely consistent with those reported previously for the pure silica system.46,47 On the basis of the above discussion, a proposed mechanism for the formation of these ceria silica composite materials is summarized in Figure 4. The XRD patterns of the calcined silica and ceria-silica composite samples, CS0, CS1, and CS2, respectively, are shown in Figure 5a. The pure silica sample prepared in the ternary surfactant mixture shows a broad feature between 15 and 25° 2θ, indicating the presence of amorphous silica. Samples CS1 and CS2 with low and high ceria contents, respectively, both show broad peaks corresponding to ceria. The crystallite 13439



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Table 2. Composition by Weight % and Properties of Calcined Composite CeO2−SiO2 Samples sample

% Ce

% Si

CeO2 size (nm)

CS0 CS1 CS2

18.67 45.91

34.51 20.46

2.7 2.4

BET surface area (m2 g−1)

pore diameters (nm)

400 299 173

3.9, 7.5 3.9, 7.5 5

sample may best be described as a combination of H3 and H4 type loops, indicating the presence of mesopores within the material, confirmed by Barrett, Joyner, and Halenda (BJH) analysis, Figure 5c, which reveals a bimodal pore distribution with pore diameters of 3.9 and 7.5 nm, ascribed to CTAB and P123 templated mesopores, respectively, Table 2.47 Sample CS1, containing a relatively small amount of ceria, exhibits a similar isotherm, although the adsorption at low relative pressure is not as steep, giving an isotherm that more closely resembles a Type II profile, which, coupled with the much less pronounced H4 hysteresis loop would indicate a loss of mesoporosity. The pore distribution of sample CS1, shows a similar bimodal distribution, but with a significant reduction of pores of diameter 3.9 nm. This suggests that CTAB templating is not as important during the synthesis of this sample, which is consistent with the observations made above concerning the synthesis of the material and the interaction of the three surfactants with the ceria particles. In the case of sample CS2, only one adsorption step is observed at high p/p0 with a more typical type II profile and a H3 hysteresis loop, indicative of a more macroporous aggregated plate-like or fibrous material, with a broad peak in the pore distribution profile centered around 5 nm. This general trend of decreasing porosity with increasing ceria content is further evidenced by the single-point BET surface areas of approximately 400, 300, and 173 m2 g−1 calculated for each of the three samples CS0, CS1, and CS2, respectively. Figure 6 shows TEM images of samples CS1 and CS2 at low and high magnification. These images show that the samples have different overall morphologies. The TEM images of the low ceria content sample, CS1, may be interpreted as the presence of ceria nanoparticles embedded within the framework of a disordered wormhole type mesoporous silica. Sample CS2, however, containing five times more ceria, shows a filamentous structure consisting of strings of silica coated ceria nanoparticles arranged into sheet type structures, in agreement with the proposed mechanism and observations from the nitrogen adsorption experiments. 3.2. Thermal Stability. XRD patterns measured in situ heating in static air to 900 °C show very little crystal growth in the composite samples, as indicated by the Scherrer analysis of the peak broadening, shown in Figure 7. The ceria crystallites encapsulated within the silica matrix increase in diameter from 2.7 to 7.2 and from 2.4 to 4.8 nm for samples CS1 and CS2, respectively. This is in stark contrast to a high surface area ceria sample which shows a large increase in crystallite size on heating above 600 °C, reaching ∼60 nm at 900 °C. It is also worth noting that no new Bragg peaks are observed, indicating that crystallization of silica does not occur, nor does any reaction between the ceria and the silica. TEM images of both composite samples after heating in air to 900 °C, Figure 8, show that while the gross morphology is largely retained, the amorphous silica matrix of each sample has fused to some extent. This fusion of the silica hosts is reflected by N2

Figure 5. XRD patterns (a), N2 adsorption−desorption isotherms, closed and open symbols, respectively, (b), and pore size distribution curves (c) of mesoporous silica and ceria-silica composites. The sharp peaks in the XRD pattern of CS0 are due to the ceramic sample holder.

diameters, Table 2, estimated by applying the Scherrer equation are 2.7 and 2.4 nm, respectively, smaller than the value obtained for the ceria sample prepared with three templates in the absence of TEOS, for which a crystallite size of 4.3 nm is estimated. The N2 adsorption−desorption isotherm of sample CS0, Figure 5b, is almost identical to that reported by Yeh et al. for hollow mesoporous silica vesicles,47 described as a combination of Type II and Type IV isotherms, showing a two-step adsorption isotherm with the first uptake between 0.65 and 0.8 p/p0 followed by a second strong adsorption on approaching a relative pressure of 1. The hysteresis loop in this 13440



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Figure 6. TEM images of CS1 (a and b) and CS2 (c and d) at low and high magnification.

Figure 8. TEM images of CS1 (a and b) and CS2 (c and d) at low and high magnification after heating to 900 °C in air.

Figure 7. Crystallite sizes of high surface area ceria, CS1 and CS2 as determined by Scherrer analysis of XRD patterns collected in situ on heating to 900 °C in air.

Figure 9. TPR traces of CS1 and CS2 with a commercial high surface area ceria for comparison.

loaded sample CS1 having a sharp uptake centered at 560 °C on a broad underlying uptake. CS2 also shows a large, sharp, hydrogen uptake peak centered at 600 °C with a shoulder on the low temperature side and a further uptake centered just above 700 °C. Significantly, there is no high temperature bulk reduction of ceria seen in the composite materials. Integration of the TPR profiles shows a hydrogen uptake that would correspond to complete reduction of Ce(IV) to Ce(III) has occurred in sample CS2, whereas around one-half of the cerium in CS1 is reduced, Table 3. The ceria nanocrystals in both samples CS1 and CS2 have similar sizes of 2.7 and 2.4 nm, respectively, suggesting that reduction behavior should be

adsorption isotherms which reveal a change in surface area from 299 to 89 m2 g−1 for CS1, and 173 to 74 m2 g−1 for CS2. It can also be seen from the TEM images, however, that the ceria nanoparticles remain isolated within the silica host. This is particularly evident in the case of sample CS2, which still contains strings of sub-5 nm ceria nanoparticles embedded in the fused amorphous silica, indicating that not only has the ceria crystallite growth been severely limited due to silica capping, but also that migration of the nanoparticles has been prevented. 3.3. Redox Properties. High surface area ceria is wellknown to show a two-step hydrogen uptake in its TPR profile at around 500 and 800 °C, ascribed to the reaction of easily reducible surface oxygen atoms at low temperatures, followed by the removal of bulk oxygen at higher temperatures, respectively.55−57 This is commonly used as a diagnostic test of its redox catalytic properties. The TPR traces of the composite materials, Figure 9, both show a massive hydrogen uptake at relatively low temperatures, with the low cerium

Table 3. Cerium Content of the Two Composite Samples and Results of TPR Studies

a

13441

sample

Ce (mmol g−1)a

H uptake (mmol g−1)

CS1 CS2

1.33 3.27

0.75 3.22

Determined by elemental analysis. dx.doi.org/10.1021/jp3031978 | J. Phys. Chem. C 2012, 116, 13435−13445


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similar; however, this is not the case. Sample CS1 has a much higher silica to ceria ratio of 9.3:1, compared to 2.2:1 for sample CS2, and this higher silica level may hinder reduction by limiting the ease with which the reductive gases can reach the ceria nanoparticles. To confirm the oxidation state of cerium in our materials, the XANES spectra of CS2 before and after reduction were recorded, as shown in Figure 10. The spectrum of CS2 is

Figure 11. Surface plot of XRD patterns measured in situ during the reduction of CS1(a), and the diffraction pattern of CS1 after reduction (b), with tick marks corresponding to Ce6(Si4O13)(SiO4)2.

Figure 10. XANES spectra of CS2 before and after reduction at 900 °C in 5% H2/N2. Inset shows the XANES spectra of HSA ceria and CeCl3·7H2O standards.

identical to that of the standard HSA ceria, showing the typical double peak and pre-edge feature of Ce 4+ in oxide materials,58,59 with a high energy peak due to excitation of an electron from core Ce 2p levels to a valence 5d levels and a lower energy absorption due to the excitation of an electron from the Ce 2p shell in the valence band to a Ce 4f levels. After reduction at 900 °C under a flow of 5% H2/N2 the XANES spectrum closely resembles that of CeCl3·7H2O: the high energy peak disappears and there is a shift to lower energy giving just one sharp absorption, clearly showing that complete reduction to Ce3+ has indeed occurred. (Note that in CeCl3·7H2O, Ce3+ is found directly coordinated to seven oxygens of water molecules,60 so this material is a suitable reference for Ce3+ resulting from reduction of CeO2.) In situ XRD patterns collected during the reduction of CS1 are shown in Figure 11. Heating CS1 under a flow of 5% H2 in N2 causes a decrease in the intensity of the ceria peaks upon reaching 700 °C, followed by an almost entirely featureless pattern at 800 °C. This matches well with the TPR profile which shows the hydrogen uptake between 550 and 700 °C, indicating the reduction of cerium(IV) occurs between these temperatures leading to the observed loss of crystallinity of the ceria phase. Upon further heating, the emergence of a new crystalline phase is observed, which corresponds to the Ce(III) containing hexacerium tetrasilicate bis(silicate), Ce6(Si4O13)(SiO4)2,61 which crystallizes further upon cooling. TEM images of sample CS1 after reduction at 850 and 900 °C are shown in Figure 12. After reduction at 850 °C, the powder diffraction pattern was largely amorphous, however the TEM images reveal a crystalline network branching out through the amorphous silica host. The presence of darker spots, presumably ceria nanoparticles, at the centers of some of these crystals may suggest that the reduction proceeds with the

Figure 12. TEM images of CS1 after reduction at 850 °C (a) and 900 °C (b).

dissolution of cerium from the ceria nanoparticles into the surrounding silica coupled with the crystallization of the hexacerium tetrasilicate bis(silicate) phase. The unusual morphology of these crystalline regions may indicate that growth of the cerium silicate phase is directed to some degree by the host silica matrix, which, prior to reduction possessed a similar disordered branching structure. Further reduction at 900 °C leads to larger, more crystalline domains of Ce6(Si4O13)(SiO4)2 within the amorphous silica. Figure 13(a) shows the XRD patterns of CS2 collected in situ during reduction under 5% H2/N2. No change is observed up to 600 °C. Above this temperature, the characteristic peaks 13442



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is accompanied by the collapse of the silica matrix, resulting in a material composed of crystallites of the dicerium disilicate embedded in amorphous silica. The reduction of ceria nanoparticles deposited amorphous silica has previously been studied by Kep̨ iński et al.,61,63,64 who prepared silica supported ceria materials by impregnation of amorphous silica gels with colloidal CeO2 suspensions, and also by Strunk et al. who deposited CeO2 onto mesoporous silicas from cerium alkoxides, and found irreversible reduction which was ascribed to the formation cerium silicate phases.45 These studies reported that the reduction of ceria nanoparticles supported on silica proceeded by the formation of an amorphous overlayer at temperatures between 700 and 800 °C for 30% CeO2/SiO2. Further reduction to 900 °C gave the hexacerium tetrasilicate bis(silicate) phase as a thin coating on the silica. Increasing the reduction temperature further caused the crystallization of the silica to cristobalite and the formation of Ce2Si2O7. This dicerium disilicate phase was also found to develop after similar treatment of a 72% CeO2 material and its formation from silica supported Rh/CeO2 materials was also reported by Krause et al.65 Attempts to reoxidise these samples in situ after reduction proved unsuccessful, with TPR/TPO cycling studies (Figure 15) indicating that temperatures in excess of 900 °C would be required to achieve any reoxidation. This is in agreement with prior work by both Krause et al.65 and Strunk et al., who both reported difficulties in reoxidising cerium silicate phases formed by the reduction of silica supported Rh/CeO2 and ceria on mesoporous silica, respectively. The extremely small size and consequent high activity of the ceria nanoparticles in intimate contact with the surrounding silica host, coupled with the drive toward these stable cerium silicate phases may serve to explain the low temperatures required for complete reduction of these composite materials; however, the very high temperatures required to reoxidise the samples lead to the formation of large ceria particles, confirmed by XRD patterns and TEM images of samples reoxidized ex situ at 1100 °C. Because of this the subsequent TPR profiles more closely resemble that of bulk ceria, with only a high temperature uptake corresponding to the removal of oxygen from the bulk material

Figure 13. Surface plot of XRD patterns measured in situ during the reduction of CS2 (a), and the diffraction pattern of CS2 after reduction (b) with tick marks corresponding to Ce2Si2O7.

of ceria begin to lose intensity before a completely amorphous pattern is observed at 750 °C. This is followed on heating to 800 °C by the crystallization of the Ce(III) material dicerium disilicate, Ce2Si2O7,62 which crystallizes further upon reduction at higher temperatures. TEM images of CS2 samples isolated at various stages of the reduction process (Figure 14) show that the sample remains essentially the same up to 650 °C, followed by the spreading of the cerium in the mesoporous silica matrix at 750 °C to give an amorphous material which retains the gross morphology of the initial composite material. Reduction at 900 °C causes the crystallization of dicerium disilicate, which

4. CONCLUSIONS We have successfully developed a novel route to composite CeO2−SiO2 materials making use of a three-surfactant templating approach that is able to accommodate the incorporation of metal salts or metal nanocrystallites during the condensation of mesoporous silica. This leads to hierarchically structured materials in which nanocrystalline ceria is dispersed and embedded within high surface area silica that possesses mesoporosity. In this state, the nanocrystalline ceria has exceptional thermal stability in air, with little evidence for crystal growth to 800 °C, in contrast to precipitated nanocrystalline ceria that shows rapid crystal growth above 600 °C. The behavior of the composite materials under reductive gas, conditions used to test catalysis properties, is more complex, with amorphization followed by crystallization of mixed cerium silicate phases. These cerium silicate phases require high temperatures for reoxidation to Ce(IV) to occur, making the nanoceria-silica system unsuitable for harsh catalytic applications. The synthetic route to these hierarchically structured composite materials may prove adaptable to the inclusion of other metal oxide nanoparticles which may benefit

Figure 14. TEM images of CS2 and after reduction at 750 °C (a) and 900 °C (b). 13443



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from the enhanced thermal stability afforded by incorporation into the inert high surface area silica host.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Leverhulme Trust for funding this work and the British Council who provided funds for A.W. to visit the University of Warwick. Some of the equipment used in the materials characterization at the University of Warwick was obtained through the Science City Advanced Materials project “Creating and Characterising Next Generation Advanced Materials” with support from Advantage West Midlands (AWM) and part funded by the European Regional Development Fund (ERDF). We are grateful for the STFC for provision of beamtime at Diamond Light Source, and we thank Dr. Silvia Ramos for her assistance with measuring the XANES data on B18. The TPR and TPO experiments at the Jagiellonian University were carried out with equipment purchased with financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract No. POIG.02.01.0012-023/08).

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Hierarchically Structured Ceria-Silica: Synthesis and Thermal

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