MESOPOROUS MATERIALS

Abstract

The invention relates to the field of mesoporous materials and in particular to mesoporous rare earth oxides and a method of their synthesis.

Claims

1. A method of making a mesoporous rare earth oxide material, the method comprising; at a reduced pressure, contacting a mesoporous template with a precursor solution that comprises a rare earth metal salt or neutral complex of a rare earth metal, so as to impregnate the mesoporous template; calcining the impregnated mesoporous template to form a rare earth oxide material in situ; and removing the mesoporous template from the calcined material.

2. A method according to claim 1, comprising contacting the mesoporous template with a non-aqueous precursor solution so as to impregnate the mesoporous template

3. A method according to claim 2, wherein the precursor solution comprises ethanol as a solvent.

4. A method according to any preceding claim, wherein the precursor solution comprises cerium.

5. A method according to any preceding claim, wherein the precursor solution is a saturated solution, optionally comprising more than 20 wt % of precursor salt.

6. A method according to any preceding claim, comprising calcining at more than one temperature.

7. A method according to claim 6, comprising calcination at a first temperature for a first period of time and at a higher second temperature for a second period of time.

8. A method according to any preceding claim, comprising chemically removing the mesoporous template, by treating with a chemical capable of eroding or digesting the template.

9. A method according to claim 8, wherein the mesoporous template is a silicate material, the method comprising treatment with aqueous alkali metal hydroxide at room temperature.

10. A method according to claim 9, wherein the aqueous alkali metal hydroxide is NaOH(aq) and has a concentration of between around 1-3 M.

11. A method according to any preceding claim, wherein the method comprises making a mixed metal oxide material, comprising atoms/ion of a rare earth metal and one or more further metals, using a precursor solution comprising a mixture of a rare earth metal salt and/or neutral metal complex of the rare earth and one or more further metal salts and/or neutral metal complexes of the further metals, in solution.

12. A method according to any preceding claim, comprising exposing the mesoporous template to the reduced pressure and subsequently contacting the precursor solution therewith.

13. A method according to any preceding claim, wherein the reduced pressure is between around 10.sup.2-10.sup.11 atm, or 10.sup.5-10.sup.8 atm.

14. A method according to any preceding claim, wherein the pressure is increased whilst the mesoporous template and the precursor solution are in contact, subsequent to contacting the mesoporous template with the precursor solution at a reduced pressure.

15. A method according to any preceding claim, comprising drying the impregnated mesoporous template, to remove a portion or substantially all of the solvent, by gently heating at a temperature between around 75-125 C.

16. A method according to any preceding claim, comprising method may comprise repeating one or more, or a sequence of steps.

17. A method according to claim 16, comprising contacting the mesoporous template with a precursor solution at a reduced pressure and, optionally, washing and/or drying the impregnated mesoporous template, on more than one occasion.

18. A method according to claim 16 or 17, comprising repeating a treatment to remove the mesoporous template.

19. A method according to any one of claims 16-18, comprising calcining the impregnated mesoporous template on more than one occasion.

20. A method according to claim 19, wherein calcination is conducted between successive impregnation steps.

21. A mesoporous rare earth oxide material obtained or obtainable by a method in accordance with any preceding claim.

22. A mesoporous rare earth oxide material wherein the mesopores are defined by crystalline rare earth metal oxide material and/or aligned crystallites of rare earth oxide material.

23. A mesoporous rare earth oxide material according to claim 22, wherein the order is evident as high angle reflections in powder XRD patterns taken from the material; and peaks in small angle X-ray scattering data and/or digital diffraction patterns obtained from regions of TEM images.

24. A mesoporous rare earth oxide material, characterised by one or a combination of; a specific pore volume greater than 50% or 60% of the theoretical value; high angle reflections in powder XRD patterns taken from the material; and peaks in small angle X-ray scattering (SAXS) data and/or digital diffraction patterns obtained from regions of TEM images; powder XRD patterns with peak widths indicative of average crystallite sizes in the range of at least 20 nm, or between around 20-40 nm; as fit to the Scherrer equation; mesopores defined by crystalline rare earth metal oxide material and/or aligned crystallites of rare earth oxide material.

25. A mesoporous rare earth oxide material of any one of claims 21-24, wherein the material is a ceria or doped ceria.

26. The use of a mesoporous rare earth oxide material according to any one of claims 21-25 to catalyse an oxidation process and/or a reduction process.

27. The use according to claim 26, wherein the mesoporous rare earth oxide material is used as a catalyst in a process which comprises both oxidation and reduction of species contacting the catalyst.

28. The use according to claim 26 or 27, as a redox catalyst in an exhaust stream, as a two way catalyst or as a three way catalyst.

29. The use of a mesoporous rare earth oxide material according to any one of claims 21-25 as an electrode, such as a fuel cell electrode and/or an electrolyte material of a fuel cell.

30. The use according to claim 29, in an intermediate temperature solid oxide fuel cell.

31. The use of a mesoporous rare earth oxide material according to any one of claims 21-25 in a photovoltaic device.

32. The use according to claim 31, wherein the mesoporous rare earth oxide material is used as a solid support for a photovoltaic material, such as a photoactive dye.

33. The use according to claim 31 or 32, as a bulk heterojunction.

34. An article comprising a mesoporous rare earth metal oxide material in accordance with any one of claims 21-25.

35. An article according to claim 34, comprising a redox catalyst.

36. An article according to claim 35, wherein the article comprises a cartridge for use in an exhaust stream, such as a vehicle exhaust or a power generator exhaust.

37. An article according to claim 35 or 36, wherein the article is or comprises a catalytic converter for a vehicle.

38. An article according to claim 34 or 35, wherein the article is a fuel cell electrode.

39. An article according to claim 34 or 35, wherein the article is a fuel cell electrolyte structure.

40. An article according to claim 34 or 35, comprising both a fuel cell electrode and an electrolyte structure.

41. An article according to claim 30, wherein the mesoporous rare earth metal oxide material of the electrode and electrolyte structure is of the same or similar composition and/or density.

42. An article according to claim 34 or 35, wherein the article is a fuel cell, in which one or more electrodes or an electrolyte structure comprises a mesoporous rare earth oxide material in accordance with any one of claims 21-25.

43. An article according to claim 34, wherein the article is a photovoltaic device.

44. An article according to claim 43, wherein the photovoltaic device is a bulk heterojunction.

45. An article according to claim 44, wherein the heterojunction comprises a said mesoporous rare earth oxide material impregnated with a photoactive dye.

46. A solar cell, comprising a bulk heterojunction according to claim 45.

Description

DESCRIPTION OF THE DRAWINGS

[0138] Example embodiments will now be described with reference to the following drawings in which:

[0139] FIG. 1 is a schematic diagram of apparatus used to perform vacuum impregnation of mesoporous templates;

[0140] FIG. 2. shows powder XRD patterns of (a) Ceria-S; (b) CGO-S; (c) Ceria-K; (d) CGO-K; (e) Ceria-X reference material. An explanation of the sample nomenclature is set out below.

[0141] FIG. 3 shows: (a) Physisorption isotherms and (b) pore size distributions for the SBA-15 template and the products obtained using it. In (b), the lines in bold are from the desorption isotherm and lighter lines from the adsorption isotherm.

[0142] FIG. 4 shows Small Angle X-ray Scattering patterns for (a) SBA-15; (b) Ceria-S; (c) CGO-S. Miller indices related to the hexagonal pore structure are indicated.

[0143] FIG. 5 shows TEM images of the SBA-15 template showing: (a) the hexagonal arrangement of cylindrical pores viewed in the [001] zone axis with DDP inset; and (b) several particles with their pore structure viewed primarily in the [100] direction.

[0144] FIG. 6 shows TEM images of Ceria-S. (a) Particles showing widespread mesoporous material including particles viewed along the [100] and [001] zone axes of the hexagonal pore structure. (b) High resolution image of mesoporous material. Bridges between the rods are circled and the interpore and interplanar distances are indicated. (c) DDP of image B showing the 111 spot of ceria. (d) Enlargement of centre of the DDP from C showing spots related to the pore structure. (e) Reverse Fourier Transform of spots in D showing the pore structure in real space.

[0145] FIG. 7 shows TEM images of CGO-S. (a) Particles showing widespread mesoporous structure (arrowed). (b) High resolution image of mesoporous material. Bridges between the rods are circled. (c) DDP of image (b) showing the 111 spot of CGO.

[0146] FIG. 8 shows (a) Physisorption isotherms and (b) pore size distributions for the KIT-6 template and the products obtained using it. In (b), lines in bold are from the desorption isotherm and lighter lines from the adsorption isotherm.

[0147] FIG. 9 shows Small Angle X-ray Scattering patterns for (a) KIT-6; (b) Ceria-K; (c) CGO-K. Miller indices related to the cubic pore structure are indicated.

[0148] FIG. 10 shows TEM images of the KIT-6 template showing: (a) the extent of the pore structure with a DDP (inset) of a region (circled) of pores viewed in the [210] zone axis; and (b) a high resolution image of the cubic pore structure.

[0149] FIG. 11 shows TEM images of Ceria-K. (a) Particles showing widespread mesoporous material including particles viewed along the [311] zone axis of the cubic pore structure. (b) High resolution image of mesoporous material viewed along the zone axis of the pore structure. (c) DDP of image (b) showing the full pattern of ceria viewed along the [110] crystallographic zone axis.

[0150] FIG. 12 shows TEM images of CGO-S. (a) Particles showing widespread mesoporous structure (arrowed). (b) High resolution image of mesoporous material showing the pore structure and crystal planes. (c) DDP of image (b) showing the full pattern of CGO viewed along the [110] crystallographic zone axis.

[0151] FIG. 13 shows TPR spectra of (a) nanoparticulate ceria reference; (b) CGO-K; (c) Ceria-K; (d) Ceria-S and (e) CGO-S

[0152] FIG. 14 is a schematic diagram of a dye-sensitised solar cell;

[0153] FIG. 15 shows photoluminescence (PL) results for standard and reduced Ceria-K and silica and titanium oxide reference materials.

[0154] FIG. 16 (a)-(f) show TEM images from in-situ heating of CGO-K. The series of images was taken as the temperature was increased from room temperature to 1000 CC. FIG. 14(g) shows a TEM image of the sample after the heating experiment. The digital diffraction patterns (DDPs) are shown adjacent to the TEM images, acquired from the particle circled in (a).

[0155] FIG. 17 shows plots comparing the effect of thermal treatment on (a) specific surface area and (b) pore volume for various mesoporous ceria-based materials with specific surface area and pore volume losses shown for the CGO-S product. The substances plotted are as follows: 1 (-.diamond-solid.-) CGO-S; 2 (-.circle-solid.-) ceria synthesised using P123 non-ionic surfactant as described in M. Lundberg et al, Microporous and Mesoporous Materials, 54 (2002) 97; 3 (-.circle-solid.-) ionic templated ceria (heated for 4 h) as described in J. A. Wang et al, Chemistry of Materials 14 (2002) 4676; 4 (-.circle-solid.-) ionic templated ceria (heated for 2 h) as described in A Trovarelli et al Journal of Catalysis 178 (1998) 299; and 5 (-.diamond-solid.-) ionic templated ceria (heated for 2 h) as described in L. Pino et al, Materials Technology 20 (2005) 18.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0156] Disclosed herein is a new method for impregnating/infiltrating a mesoporous template under vacuum (reduced pressure), to prepare mesoporous rare earth oxide materials. The novel vacuum impregnation (VI) methods of the present invention reduces the reliance on capillary action for the incorporation of the precursor salt solution into the mesoporous template. The method has yielded better results than the traditional incipient wetness impregnation technique (IWIT) and is both simpler and quicker to perform.

Experimental

[0157] Mesoporous silica templates SBA-15 and KIT-6 were synthesised following procedures available in the literature, as described by Zhao et al, Science 279 (1998) 548 and Kleitz et al, Chemical Communications (2003) 2136. and In a typical synthesis of SBA-15, 2 g of the non-ionic surfactant, Pluronic P123 (EO.sub.20PO.sub.70EO.sub.20; where EO.sub.n is poly(ethylene oxide) and PO.sub.n is poly(propylene oxide), was added to 15 cm.sup.3 deionized water and 60 cm.sup.3 of 2M HCl and stirred at 40 C. for 8 h. The Pluronic surfactant was obtained from Sigma-Aldrich. Pluronic is a trademark of BASF.

[0158] 4.25 g tetraethylorthosilicate (TEOS 99%; obtained from Fluka) was added and stirred for 24 h at the same temperature. This mixture was hydrothermally treated at 100 C. for 24 h in a Teflon container. Teflon is a trademark of the DuPont Chemical Company. The resulting white solid was filtered, washed and dried. The surfactant was removed by calcining at 500 C.

[0159] In a typical synthesis of KIT-6, 6 g of Pluronic P123 was added to 180 cm.sup.3 deionized water and 50 cm.sup.3 of 2M HCl and stirred at 35 C. for 6 h. 6 g of n-butanol (Sigma, 99%) was added and stirred for 1 h. 12.48 g TEOS was added and the mixture was stirred at the same temperature for 24 h followed by hydrothermal treatment as above. The resulting white solid was filtered, washed and dried. The surfactant was removed as above.

[0160] The precursor salt solutions used to make the mesoporous rare earth oxide materials described below were prepared by dissolving in 0.5 cm.sup.3 of ethanol either 1 g of the metal nitrate, e.g. Ce(NO.sub.3).sub.3.6H.sub.2O; (Acros, 99.5%) to prepare mesoporous CeO.sub.2 or 1 g of a mixture of nitrates, e.g. a 9:1 molar ratio of the Ce(NO.sub.3).sub.3.6H.sub.2O and Gd(NO.sub.3).sub.3.6H.sub.2O (Alfa Aesar, 99.9%) to prepare the Ce.sub.0.9Gd.sub.0.1O.sub.1.9 (CGO).

[0161] Apparatus for performing vacuum impregnation is shown schematically in FIG. 1. The VI method used is as follows:

[0162] Initially, all taps were closed. A particle trap was placed in line with the vacuum pump to prevent contamination of the rotary pump used to apply the vacuum.

[0163] In a typical VI experiment a volume of template (0.1-2.0 g depending on the experiment) was placed into the test tube. The apparatus was then assembled as per FIG. 1.

[0164] The rotary pump was switched on and tap T2 was opened slowly to ensure no template was sucked down the vacuum line (due to the light and voluminous nature of the template). The test tube was tapped lightly to help remove air pockets during the early stage of the evacuation process. The template was then left to evacuate for 4-6 h.

[0165] A precursor solution consisting of the impregnating salt (or other precursor material), was stirred for at least 6 h at 30-60 C. to ensure it was a homogeneous mixture. The solutions used in this work were ethanol saturated with Ce(NO.sub.3).6H.sub.2O, or ethanol with replacement of 10 mol % of the cerium salt by Gd(NO.sub.3).6H.sub.2O.

[0166] The volume of precursor solution used in the impregnation step exceeded the volume required to impregnate all of the pores in the template in order to ensure the vacuum was not compromised during impregnation by ingress of air through the dropping funnel. The solution was placed in the dropping funnel and left for any air bubbles to dissipate.

[0167] T2 was closed and the pump was switched off.

[0168] T1 was opened slowly at first to allow the solution to completely cover the template and then, once sufficient volume of solution was admitted to completely impregnate the template, T1 was closed.

[0169] Accordingly, at this stage, and at the reduced pressure, the mesoporous template was contacted with the precursor solution.

[0170] T2, and then T3 were opened to return the system to atmospheric pressure and complete the impregnation. This could be observed by rapid infiltration of the precursor solution into the template.

[0171] Excess solution was then decanted before the sample was dried at 95-105 C. in air in an oven.

[0172] The sample was calcined at 400 and 600 C. in air for 5 h at each temperature (ramp rate 1 C. min.sup.1) using a tube furnace.

[0173] The silica template was dissolved by stirring with approximately 20 cm.sup.3 of 1-2 M NaOH. Centrifugation was used to recover the product from the solution. This step was repeated three times. The product was then dried in an oven at 105 C. overnight.

[0174] For the materials used for the photovoltaic work, a vacuum desiccator was used instead of a test tube and a water pump was used instead of a rotary pump. The precursor solution was added directly into the air inlet. After the product was dried at 95-105 C. in air in an oven, the sample was calcined at 400 C. in air for 5 h (ramp rate 1 C. min.sup.1), and then impregnated a second time. After this the sample was calcined at 400 and 600 C. in air for 5 h at each temperature (ramp rate 1 C. min.sup.1), before the silica template was dissolved by stirring with approximately 20 cm.sup.3 of 1-2 M NaOH. Buchner filtration was used to recover the sample from the solution. The sample was sucked dry. This step was repeated three times.

[0175] Products are referred to below by their composition (Ceria or CGO) and with a suffix to represent the mesoporous template used to prepare them (-S for SBA-15 and -K for KIT-6).

[0176] For comparison, a nanoparticulate but non-mesoporous ceria (which is referred to below as Ceria-X) was produced without the use of a template, by calcining cerium citrate (prepared from Ce(NO.sub.3).sub.3.6H.sub.2O and citric acid (Alfa Aesar, 99.5%).

[0177] A Micrometrics ASAP 2020 instrument operating at 77 K was used to obtain Brunauer-Emmett-Teller (BET) Nitrogen adsorption/desorption isotherms, Specific Surface Areas (SSAs) and Barret-Joyner-Halenda (BJH) pore-size distributions (PSD) of all products.

[0178] Powder X-Ray Diffraction (XRD) data was collected using a Philips PW 1710 diffractometer with Cu K.sub.a radiation (=1.54 ). Scan rates in a typical experiment were 1 min.sup.1 over a range of 2=10-80. Peak-width analysis was performed by fitting a Gaussian curve to the raw data and applying the Sherrer equation in order to obtain estimates of average crystallite size (see Klug, H.; Alexander, L. in: X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, John Wiley, New York, 1974, p. 618).

[0179] Small-angle X-Ray Scattering (SAXS) patterns were collected using a Hecus X-ray Systems Generation 1 instrument incorporating a modified compact Kratky camera with slit focussing and a PSD. Samples were run using a SpiCap attachment and with Cu K.sub.a radiation at 40 kV and 30 mA. Data were analysed using FindGraph peak-fitting software.

[0180] Transmission Electron Microscopy (TEM) images were recorded using a JEOL JEM 2011 instrument fitted with a LaB.sub.6 filament and operating at 200 kV. Semi-quantitative elemental analysis by Energy-dispersive X-ray Spectroscopy (EDS) was performed using the Oxford Instruments X-Ray analysis ISIS 300 detector mounted on the TEM instrument. The DigitalMicrograph 3.4.4 graphics suite (Gatan) was used to analyse the TEM images and to obtain Digital Diffraction Patterns (DDPs) from the images by fast Fourier transform. Unless stated, no TEM images were manipulated using inverse-FFT functions. Elemental analysis on bulk samples was performed by Inductively Coupled PlasmaMass Spectrometry (ICP-MS) using an Agilent 7500 instrument.

[0181] Temperature-programmed reduction (TPR) experiments were collected using custom-built TPR equipment coupled to a quadrupole mass spectrometer system. The sample was heated from ambient to 800 C. at 5 C./min under a flow of 5% H.sub.2 in Ar.

[0182] Temperature-programmed desorption (TPD) experiments were performed in the same way but under a flow of pure Ar. 50 mg of sample was used in each experiment, gases were passed through water and oxygen filters prior to use and flow rates were 45 cm.sup.3 min.sup.1. TPR experiments were run under identical mass spectrometer and other settings to allow direct comparison of the spectra.

[0183] Results and Discussion

[0184] Powder XRD

[0185] Powder XRD patterns for all four productsCeria-S, Ceria-K, CGO-S and CGO-Kas well as the nanoparticulate CeO.sub.2 (Ceria-X) are shown in FIG. 2. All patterns could be indexed to the cubic Fluorite structure (Fm3m with a5.41 ) expected for pure CeO.sub.2 and for the CGO and there was no evidence of any impurity phases. Peak broadening is seen in the patterns for all materials which is indicative of small crystallites. Using the Scherrer approach, estimates of average crystallite size were found to be in the range 23.5-34.2 nm for the four products and 32.9 nm for the Ceria-X reference. The data are presented in Table 1 and are discussed below. As mentioned above, the assumptions upon which the Scherrer equation is basedespecially regarding the particles as being close to spherical in shapemay not be applicable to the present materials. Accordingly, whilst these average crystallite sizes may be compared with one another, they may not in all cases be quantitatively representative of actual average crystallite sizes in a sample.

TABLE-US-00001 TABLE 1 Summary of structural data for the four products: average crystallite size (D) from XRD line broadening; SSA and specific pore volume (V.sub.p) from gas sorption experiments; pore sizes (d.sub.p) from maxima in the BJH pore size distribution plots; pore spacings from DDPs of TEM images (d.sub.TEM) and the SAXS patterns (d.sub.SAXS). D SSA V.sub.p d.sub.p d.sub.TEM d.sub.SAXS (nm) (m.sup.2g.sup.1) (cm.sup.3g.sup.1) (nm) (nm) (nm) SBA-15 800-890 1.0-1.1 5.9-7.3 7.5-8.6 9.3 Ceria-S 34.2 85.7 0.29 2.4-3.0, 8.9-9.7 9.6-13.8 CGO-S 23.5 108.6 0.32 2.5-3.0, 8.7-9.8 9.2 11-17 KIT-6 840-990 1.2-1.4 6.4-7.2 8.9-10.5 9.6 Ceria-K 24.3 114.7 0.35 2.2-3.0, 8.5-9.4 ~8 CGO-K 22.5 137.5 0.38 2.1-2.7 9.1-9.2 8.8 ~8

[0186] Comparison experiments were conducted on a mesoporous ceria obtained using a conventional IWIT method. The IWIT mesoporous ceria had a SSA of 129 m.sup.2/g (i.e. comparable to the SSA of the Ceria-K and CGO-K materials listed in table 1), a d.sub.p value of 4-10 nm (i.e. less defined pore sizes than for Ceria-K and CGO-K) and a V.sub.p, of 0.25 cm.sup.3/g (i.e. significantly smaller than the V.sub.p of Ceria-K and CGO-K).

[0187] Chemical Composition

[0188] Using the EDS method in the TEM the Ceria products were confirmed to contain Ce and O and the CGO products Ce, O and Gd. The only impurity was Si, undoubtedly remaining from the templates, which was detected in all samples at levels of 4.5, 6.2, 6.0 and 6.3 mole % for Ceria-S, Ceria-K, CGO-S and CGO-K, respectively. EDS mapping showed no local concentrations of Si. Rather, it appeared to be distributed evenly throughout each sample, at least at the effective resolution of the instrument used (5-10 nm). Molar Ce:Gd ratios were measured to be 11:1 and 10:1 for CGO-S and CGO-K. Analysis by ICP-MS gave values of 2 to 4 mol % Si content for these samples.

[0189] Structures of SBA-15, Ceria-S and CGO-S

[0190] Gas Physisorption

[0191] Gas adsorption-desorption isotherms, and the pore size distributions (PSDs) obtained for these, are given in FIG. 3 for the SBA-15 template and the Ceria-S and CGO-S products. The isotherms for all three materials are Type IV with Type H3 hysteresis which is typical of mesoporous materials where capillary condensation occurs in the mesopores. Values for SSA, specific pore volume and pore size obtained from the physisorption experiments are summarised in Table 1. For SBA-15, these relate to several batches of material and were in agreement with the literature. Considering that the densities of ceria and CGO are around 2.72 times those of silica, the SSAs and specific pore volumes of the Ceria-S and CGO-S products indicate that very porous products had been achieved.

[0192] The SBA-15 showed narrow peaks in the PSD at 5.9-7.3 nm (determined from the adsorption and desorption branches) indicating a good quality template. The two products each showed a sharp peak at around 2.7 nm and a broader peak centred at around 12 nm for Ceria-S and around 14 nm for CGO-S, confirming that the products were largely mesoporous.

[0193] The broad peak for both materials at around 30 nm can be assigned as interparticle porosity, and so not related to the mesopore structure. The peak at around 2.7 nm can be assigned as the pores formed in the product after removal of the walls of the template.

[0194] Taking the more reliable value of interpore spacing for SBA-15 (from SAXS) of 9.3 nm and subtracting the SBA-15 pore diameter, 5.9-7.3 nm, one arrives at a value of 2.0-3.4 nm for wall thickness in the template, which is consistent with the size of the small pores detected in the products (2.4-3.0 and 2.5-3.0 nm). This is evidence that templating was successful and that the product had taken on the inverse (or negative) structure of the template.

[0195] There are three possible explanations for the broad peaks centred on 12 and 14 nm. Firstly, bundles of loose nanorods existing outside the ordered mesopore structure may give rise to a broad peak at around this pore size. Secondly, in the mesopore product structure itself, the edges of the mesopores are accessible as long slots (in the [100] direction in FIG. 6a, for example), unlike in the SBA-15 template. Physisorption through these openings would be expected to give pore sizes above 2.7 nm because of their high aspect ratio. Finally, short missing sections of nanorod in the mesoporous product would leave voids whose diameter would be the sum of two pore and one nanorod diameters, about 14-15 nm (for CGO-S, values from Table 1).

[0196] Small Angle Xray Scattering (SAXS)

[0197] In the SAXS patterns presented in FIG. 4, diffraction peaks for SBA-15 at Miller indices of 100, 110 and 200 of the mesopore structure are clearly visible superimposed as shoulders on the large undiffracted instrumental peak centred at 2=0. This confirms the ordered nature of the mesoporous structure.

[0198] For CGO-S small peaks were evident at positions which matched those of the template. These were more evident on subtracting the background (FIG. 4c). The d.sub.100 spacings of the pore structures are compared in Table 1 for the SBA-related materials. This demonstrates that the CGO-S had been successfully templated by the SBA-15. Although the mesoporous rare earth oxide material has an inverse structure in which voids within the mesoporous template are filled, and the pores are formed where the template material has been removed, the size and symmetry of the repeat unit of the mesoporous structure can be expected to be the same for both the mesoporous rare earth oxide material and the mesoporous template from which it has been formed.

[0199] The absence of clear SAXS peaks for Ceria-S is discussed further below.

[0200] Electron Microscopy

[0201] The extensive and widespread ordered pore structure of the SBA-15 template is clearly seen in the images presented in FIG. 5 and by the DDP of the circled area which shows the hexagonal arrangement of mesopores when viewing along the [001] zone axis. In FIG. 5b the pores are viewed along the [100] zone axis and are seen to be gently curved, which is a known characteristic of SBA-15.

[0202] This curved structure is replicated in the image of the SBA-15-templated product, Ceria-S, in FIG. 6a. The large agglomeration (1 m) in the image consists largely or entirely of mesoporous particles and there are examples of both the and [001] orientations. It should be kept in mind that some mesoporous particles may not appear in the image, if there is overlap with other particles and/or if their pore structure is not aligned with the electron beam of the TEM. The interpore spacing of 8.9-9.7 nm was measured directly from the images and also from DDPs. This is consistent with the corresponding spacing for the SBA-15 mesoporous template, the value of which was obtained by TEM and SAXS (see Table 1).

[0203] The high resolution image in FIG. 6b shows several important features. First, the imaged area of the sample consists of cylindrical nanorod structures of uniform diameter separated by narrower mesopores. This is the inverse of the SBA-15 structure, and again demonstrates that the pores of the template have been impregnated with material and that the walls of the mesoporous silica template have subsequently been removed to leave mesopores between the nanorods.

[0204] Importantly, the two circled regions of FIG. 6b highlight two examples of nanoscale bridges which interconnect the nanorods and so hold the structure together. This is indicative of complete or substantially complete impregnation of the mesoporous template during synthesis. These bridges are thought to improve mechanical and/or thermal stability of the mesoporous rare earth oxide materials.

[0205] The extensive impregnation which has been possible in use of the method of the present invention may also give rise to larger mesoporous crystallites. Indeed, the area imaged is essentially a single crystal of mesoporous rare earth oxide material. The crystal lattice planes are visible in the image and are seen to remain parallel across the structure and between nanorods. Therefore, the bridges between nanorods must play an important role in achieving this long-range alignment of the lattice during crystallisation and grain growth.

[0206] The DDP in FIG. 6c confirms the single crystalline nature of the imaged sample as it contains only one pair of spots which are consistent with the 111 planes of the ceria lattice. However, these spots are in fact extended into short arcsof 18 in this casewhich indicates that the lattice planes change direction gradually across the image while remaining essentially parallel (as is seen on careful study of the images themselves). This strained crystal structure is an interesting and general feature of the mesoporous products prepared in this study.

[0207] FIG. 7 shows very similar features for the CGO-S material. The yield of mesoporous material was very high and was observed throughout the sample. Some of the particles which have their pore structures aligned with the TEM beam are indicated in the image. Interpore spacings obtained from images and DDPs agreed very well with SAXS data for this sample and were consistent with the interpore spacings obtained for SBA-15 (Table 1). The parallel nanorodsseparated by narrow pores and interconnected by small bridgesare seen in the high resolution image in FIG. 7b. The long-range alignment of the crystal lattice, between nanorods and across the bridges, is also evident. In the DDP of the image in FIG. 7c the spots are consistent with the 111 interplanar distance of the CGO lattice. The continuous angular variation over 28 in the position of the 111 spot indicates againas was seen for Ceria-Sa gentle change of direction of the lattice planes across the sample.

[0208] The TEM images show some variation in the width of the nanorods which define the mesopores of these materials. The resulting small variations in interpore distance and pore dimensions observed for Ceria-S may have caused scattering and line-broadening in the SAXS experiment and so may explain the absence of peaks for Ceria-S. This hypothesis is supported by a comparison with data obtained from the CGO-S material, in which the variations were generally smaller. The CGO-S material did give rise to peaks in the SAXS pattern. Doping ceria with Gd is known to aid sintering in CGO and so this may have aided the filling of the pores of the template during crystallisation and grain growth of the CGO, resulting in the slightly higher pore volume observed, and more geometrically well-defined nanorods, than in Ceria-S.

[0209] Structures of KIT-6, Ceria-K and CGO-K

[0210] Gas Physisorption

[0211] The gas adsorption-desorption isotherms and the PSD plots derived from them are presented for KIT-6and for the Ceria-K and CGO-K made using itin FIG. 8. The isotherms for all three materials are again Type IV with Type H3 hysteresis, which is typical of mesoporous materials. SSA and pore volume values were obtained from the physisorption data and are displayed in Table 1.

[0212] For KIT-6, these values refer to a number of batches of material. As for the SBA-16, SSAs and pore volumes were very high, as expected from the literature. For the two products, SSAs and pore volumes were all significantly higher than for the corresponding materials prepared using the SBA-15 template. This may be at least partly because of SSA and pore volume being higher for KIT-6 than for SBA-15. However, it may also be a consequence of KIT-6 having a three-dimensional, rather than a one-dimensional, pore structure like SBA-15, and so facilitating precursor impregnation.

[0213] The PSD plots show KIT-6 to have a single narrow peak around 7 nm. Ceria-K and CGO-K showed sharp peaks around 2.5 nm then a poorly-defined broad feature centred on about 8 nm (clearer for CGO-K) and finally a broad peak around 25-30 nm, which can be assigned as interparticle porosity.

[0214] As above, the peaks at around 2.5 nm verify the presence of mesopores in the mesoporous rare earth oxide products, which remain after the mesoporous template has been removed.

[0215] The poorly-defined peaks around 8 nm can be explainedas aboveby interparticle porosity, the effect of short missing sections of nanorod giving rise to relatively wide pores, or to adsorption through letter-box shaped pores/openings in the structure.

[0216] It should be noted that for all products, the mesoporous structures of the resulting rare earth oxides are the negative of the mesoporous template. Accordingly, the pores are not simply spherical or cylindrical pores, but rather are complex shapes which surround the rare earth oxide nanorods and are interconnected between themselves. Hence, interpretation of the physisorption results in terms of precise pore shape and dimensions, using standard models (which assume simple pore shapes) can be difficult for these materials.

[0217] SAXS

[0218] In FIG. 9, the SAXS pattern for KIT-6 shows one very clear peak corresponding to the 211 planes of the pore structure, a shoulder for 220 and broad features for two sets of higher index planes. Again, the ceria product gave rise to a smooth curve with no resolvable peaks while the CGO-K exhibited the 211 peak quite clearly along with a broad feature around 28=2 (which, as above, can be more clearly seen after background subtraction). The pore spacing value, d.sub.220, for the mesoporous rare earth oxide product CGO-K is seen to be 10% lower than for the template, KIT-6, in Table 1. This indicates some contraction of the mesopore structure of the product during calcination or template removal.

[0219] Electron Microscopy

[0220] TEM images of KIT-6 showed large particles, some larger than 1 m, that contained arrays of ordered mesopores across their entirety. FIG. 10a shows such a particle which may be an agglomeration of several smaller ones. The pores are seen to be uniform along their length and parallel to each other. The inset DDP was taken from the circled area of the image and shows spots which can be indexed to the [211] zone axis of the cubic KIT-6 pore structure. FIG. 10b is a higher resolution image showing a region of another particle. The orientation of the pores is seen to change across the image, indicating the presence of microdomains in the pore structure.

[0221] TEM images of the Ceria-K and CGO-K products obtained using the KIT-6 template are presented in FIGS. 11 and 12.

[0222] The image in FIG. 11a confirms that a very high yield of mesoporous particles was obtained and examples are indicated in the image. Two of these have pore structures that are well enough aligned to the TEM beam to allow them to be indexed, and they are both observed down the [311] zone axis of the (inverse/negative) KIT-6 pore structure.

[0223] FIG. 11b shows a very clear high resolution image of the mesoporous structure of Ceria-K. The concentrations of ceria material between the pores are observed as dark, roughly circular features. These are not simple nanorods as in SBA-15, since the direction of the wormholes in the KIT-6 template changes through the structure. They are better considered to be caused by the overlapin the direction normal to the plane of the imageof nodes or junctions between intertwined (non-linear) nanorods in the inverse KIT-6 structure, giving rise to dark contrast in the image.

[0224] These features are clearly ordered in a hexagonal arrangement with angles between planes measured at 60. This indicates that the pore structure is viewed here along its [111] zone axis. In addition, the planes of the crystal lattice are also clearly seen.

[0225] The DDP in FIG. 11c is taken from the whole image and shows a complete pattern which can be indexed to the Fluorite structure of ceria viewed along the direction. Importantly, this DDP demonstrates that the crystal structure of Ceria-K was aligned across the material, it being essentially a porous single crystal, and that the diffraction spots are in fact converted to short arcs by the gentle variation in direction of the lattice planes across the sample. This same phenomenon was discussed above for Ceria-S and CGO-S.

[0226] TEM images showed that the CGO-K material had also been successfully prepared with widespread mesoporous structure. This is seen in FIG. 12a where particles with aligned pore structures are identified. At high resolution, the ordered pore structure is seen to consist of essentially single crystalline CGO. FIG. 12b shows such a region of the sample. The crystal lattice planes are clearly visible and this image was used to generate the DDP in FIG. 12c which shows a complete diffraction pattern consistent with CGO viewed along the [110] zone axis. Again, the bending of the lattice planes gives rise to the arcs seen in the DDP.

[0227] Behaviour and Properties of Mesoporous Rare Earth Oxide Materials

[0228] Reduction Behaviour

[0229] TPR spectra were obtained in flowing dilute Hydrogen by recording the water signal (m/q=18) as a function of temperature for the Ceria-X reference material and for all four products. The five spectra are presented together in FIG. 13. The peaks are grouped and labelled as T.sub.1 to T.sub.4, in order of increasing temperature. The reference material exhibited a very small peak at around 100 C. (T.sub.1) which can be attributed to the desorption of physisorbed water from the ceria surface. The only other feature is a very large peak at 745 C. (T.sub.4) which is attributed to reduction of relatively unreactive sample oxygen species by the hydrogen, usually assigned as bulk or lattice oxygen. Comparing first this spectrum with those of the four mesoporous products taken together, a number of important and general differences are seen.

[0230] The most important change is the occurrence of one large new peak at about 520 C. (T.sub.3). A second smaller new peak (T.sub.2) also appears at around 430 C. as a shoulder on T.sub.3. Because of the size of the (T.sub.2+T.sub.3) feature and its appearance at intermediate temperatures, it is attributed to the reduction of a large amount of reactive oxygen in the mesoporous materials. Furthermore, the fact that the T.sub.4 peaks are much smaller than for the reference sample indicates that the amount of relatively unreactive oxygen is much smaller. Together, these changes mark a significant shift towards active, easily available oxygen in the mesoporous samples.

[0231] The T.sub.1 peaks are broader and much larger than for the Ceria-X reference. These properties can be explained by the combination of up to three effects. (1) the high SSAs of the mesoporous materials allow them to accommodate a large amount of surface water and the mesopore network may delay its desorption in the transient TPR experiment to above 100 C. (2) Ceria-based materials are known to be hygroscopic which would increase further the amount of water on the surfaces and may delay it's desorption to temperatures above 100 C. (3) The presence of highly reactive peroxide and superoxide species on the surface of high surface area ceria has been reported. The reduction of these at around 150 C. may contribute to the T.sub.1 peaks in the mesoporous materials.

TABLE-US-00002 TABLE 2 Positions of peaks in TPR spectra ( C.) T.sub.1 T.sub.2 T.sub.3 T.sub.4 Ceria-X ~100 745 Ceria-S 125 514 712 Ceria-K 154 523 696 CGO-S 94 435 525 664 CGO-K 156 450 519 665

[0232] The TPR peak positions for all samples given in Table 2 allow a comparison of these values for all of the materials tested. Peak T.sub.1 was discussed above and peak T.sub.2 is a minor shoulder whose position is hard to determine accurately. T.sub.3 shows little variation which suggests that the corresponding reduction reactions are not sensitive to composition or to the mesopore structure.

[0233] T.sub.4 is lower for all mesoporous materials than for Ceria-X, implying that the pore structure facilitates the reduction of the bulk material. In addition, oxygen ion diffusion/conductivity in CGO is known to be greater than for undoped ceria, which is likely to favour the reduction kinetics in CGO. It is relevant, then, that T.sub.4 is lower for CGO samples than for the corresponding ceria samples.

[0234] The marked increase in reducibility of all four mesoporous products compared to a high SSA ceria is of great interest in relation to their applications as reduction and/or oxidation catalysts.

[0235] Photovoltaic (PV) Studies

[0236] PV studies were conducted to investigate ordered mesoporous ceria as a potential bulk heterojunction material for dye-sensitised solar cells.

[0237] FIG. 14 shows a schematic diagram of a dye-sensitised solar cell. Dye-sensitised solar cells are based on an organic semiconductor dye that is sandwiched between two conducting electrodes, at least one of which is transparent to light (typically indium tin oxide; ITO). The dye absorbs photons (suggested by the zig-zag lines passing through the ITO layer). This results in the formation of excitons 1. Excitons are bound states of energyan electron and an electron holethat are a means of transporting energy without transporting charge. To create a current the exciton binding energy must be overcome to separate the exciton into its constituent charges. The electron 2 and the electron hole 3 must then be captured in a process called exciton quenching within the exciton lifetime before recombination occurs. Recombination is shown schematically at 5. If recombination occurs then the charge will be lost and the energy cannot contribute to the cell potential, decreasing the efficiency of the solar cell. The excitons travel through the polymer by a random walk process, and in a typical lifetime they will recombine within 20 nm. As it is difficult to synthesise a cell with a 20 nm thick dye layer that can absorb all of the incident solar radiation, a porous inorganic semiconductor (commonly TiO.sub.2) is often placed in the cell which, when blended with the dye, is called a bulk heterojunction 6. The purpose of the bulk heterojunction is to accept the electrons from the dye and transport them to the cathode, as shown schematically at 4. An electron reaches the bulk heterojunction 6 and is transported towards the cathode.

[0238] The PV studies were was conducted using Ceria-K. Two samples of Ceria-K were used to construct a dye-sensitised solar cell, comprising a bulk heterojunction formed from the mesoporous ceria and the dye, as described generally above.

[0239] One sample was reduced in 5% H.sub.2/Ar at 450 C. for 1 h (Ceria, reduced) before the PV studies to increase the concentration of cerium(III) ([Ce.sup.3+]), the other sample examined was untreated (Ceria, unreduced).

[0240] Two parameters were examined: the photoluminescence quenching half-life, , and the quenching efficiency, which measures the efficiency of the material to separate the exciton charges to prevent exciton annihilation. Photoluminescence (PL) spectroscopy is an effective technique for determining the quenching efficiency of excitons in a dye-sensitised solar cell. The sample is excited with a laser pulse, at a known frequency, creating excitons; electron-hole pairs. Some exitons recombine causing photoemission some time after the initial excitation (PL). These emitted photons are detected as a function of time, using a specially-designed detector with a temporal resolution of about 1 ps. Comparison of the energies of the laser pulse and the PL emission provides the quenching efficiency. An efficient bulk heterojunction would allow a large proportion of electrons and holes to be collected (so generating useful electrical power) before recombination, so reducing the amount of PL. Analysis of the decay of emitted PL light intensity with time gives the half-life, , of the PL of the material.

[0241] Preliminary photoluminescence experiments showed that Ceria-K quenched the excitons produced in the organic dye rapidly (FIG. 15) and much faster than titania, which is widely considered for this application (Macaira et al, Renew. Sustain. Energy Rev. 27 334 (2013)). Results are summarised in Table 3. This meant that, compared to reference materials, the excitons were easily extracted from the dye before recombination occurred. The reduced ceria quenched the excitons faster than the standard (unreduced) Ceria-K sample. This may be due to the increased [Ce.sup.3+] which would be expected to lead to an increased electronic conductivity. The quenching efficiency of both materials was high (90-93%), however.

TABLE-US-00003 TABLE 3 Quenching half-life Quenching Sample (ps) efficiency (%) Reference silica 334 Reference titania 126 Ceria-K 34 90 Reduced Ceria-K 23 93

[0242] The PL experiment showed that the ordered mesoporous matrix had a great affinity for exciton quenching even though its properties had not been rigorously developed for solar cells.

[0243] As effective exciton quenching occurs if the excitons can be removed from the dye before they have time to recombine, a small distance between the dye and semiconductor of 5-20 nm is ideal. In this case, the mesoporous ceria is the semiconductor and has a half-pore width of around 1.5-2 nm. This means that these materials have the potential to increase dye-sensitised solar cell efficiencies.

[0244] Moreover, by selection of the mesoporous template used to form the materials, mesopore morphology may be tailored for the particular application, and so the material could be designed for use with a particular dye, for example. Alternatively or in addition, the oxide composition itself may be matched for a particular application, for example to optimise electron transfer from the dye.

[0245] As discussed above, the method provides for preparation of mesoporous rare earth oxide materials with greater order in the pore walls, so decreasing grain boundary resistances and improving the electrical properties, to the benefit of these photovoltaic and electrode applications.

[0246] To improve the organic dye impregnation (itself a viscous liquid when in solution because of its high Mr), and the contact with the semiconductor, vacuum impregnation could be used again.

[0247] The inventors note that ceria has an additional benefit that it absorbs ultra-violet (UV) radiation, and would therefore provide additional protection for the organic dye against UV degradation.

[0248] Thermal Resistance Studies

[0249] Materials for use in applications such as heterogeneous catalysis and fuel cells are required to be thermally stable at their operating temperatures. CGO-K was studied by heating the material to 1000 C. inside a TEM in vacuo over a period of 4 h.

[0250] CGO-S was studied by calcining the material to 500 C. and 650 C. for 48 h and 72 h, respectively, in air. These samples were then characterised using TEM and nitrogen physisorption.

[0251] In-Situ TEM Heating of CGO-K

[0252] The results for the in-situ heating experiments are presented in FIG. 16. A large agglomerate was selected for observation based on its composition of ordered mesporous particles and nanoparticulate material. The sample was heated to 1000 C. inside the TEM over a period of about 4 h. After a short period at 1000 C. the carbon grid failed.

[0253] FIG. 16 presents an area of the agglomeration which features a 150150 nm particle, presented in the [111] zone axis of the mesopore structure, that was selected for observation during the experiment (circled in FIG. 16(a)).

[0254] Surrounding this particle were other mesoporous particles as indicated in the image. Upon heating, it is shown that either different mesoporous particles became visible or the particles visible at room temperature rotated to present a different pore axis. The primary feature, of which DDPs were taken at each temperature, can be seen to pass from the [111] zone axis in FIG. 16(a) into a misaligned [111] orientation in FIG. 16(b). In FIGS. 16(c) and (d), all but two of the primary spots disappeared due to this misalignment. In FIG. 16(e), new spots appeared that were consistent with the (303) reflection. In FIG. 16(f), the particle is again in the [111] orientation.

[0255] It was confirmed that the orientation of particles within the sample had not altered in FIG. 16(f), by taking reference points in the image and comparing them to the previous images in the series. The lattice constants extracted from the DDP reflections did not decrease, within experimental variation, when increasing the temperature up to 1000 C.

[0256] The ordered mesoporous materials showed high thermal stability at temperatures coinciding with the lower edge of the reduction peak (determined by TP studies to be approximately 500 C.), at which these materials could be used for redox applications such as in catalysis and SOFCs. When heated above this temperature for prolonged periods there was some evidence of sintering. However, heating in-situ in the TEM showed that over periods of ca. 4 h the materials could be heated to temperatures of up to 1000 C., and cooled down again to room temperature, without loss of structure.

[0257] It is possible that the mesoporous crystals had already grown sufficiently large that their equilibrium melting temperature made them stable at intermediate temperatures.

[0258] In contrast, the nanoparticles also present in the samples have a lower equilibrium melting temperature.

[0259] There is also some evidence from scattering experiments that the non-mesoporous by-products have been more affected by the thermal treatment than the mesoporous rare earth oxide materials.

[0260] It may be that the sintering of the by-products would cause the mass of the mesoporous crystals to rise (by Otswald ripening) and consequently equilibrium melting temperature of the sample.

[0261] Unlike all of the mesoporous ceria materials previously reported, ordered mesoporous CGO studied here (CGO-S) showed a plateau in measured SSA (see Thermal Stability below) up to approximately 500 C., across which the specific surface area decreased only by a small amount.

[0262] Thermal Stability

[0263] Previous literature reports on the thermal stability of mesoporous ceria have shown a steady decrease in the specific surface area with increasing temperature (FIG. 17(a)).

[0264] Note that the cause for the increase in the specific surface area in the report from Wang et al, Chemistry of Materials 14 (2002) 4676 was due to incomplete removal of the template at 200 C. At 400 C. the template had been fully removed causing the pores to become unblocked, increasing the specific surface area.

[0265] Unlike all of the mesoporous ceria materials previously reported, ordered mesoporous CGO (sample CGO-S described above) appeared to have a plateau up to approximately 500 C. where the specific surface area decreased only a small amount.

[0266] Comparing the pore volume data for CGO-S with the ceria synthesised by Lundberg et al., Microporous and Mesoporous Materials 54 (2002) 97 using P123 in cooperative self-assembly (FIG. 17(b)) it can be seen that initially there is much less pore volume lost in CGO-S. It should also be noted that CGOSV.sub.2 was heated for between 12-24 times longer than the other samples presented in FIG. 17.

[0267] These initial physisorption results are consistent with the proposed annealing/sintering mechanism of the pore walls, described herein. The remaining samples, with high concentrations of nanoparticles, were more affected by lower temperatures than the ordered mesoporous material prepared in accordance with the invention, and the ordered mesoporous material appears to be capable of maintaining a high pore volume even after being subjected to high temperatures for extended periods of time.

CONCLUSIONS

[0268] In summary therefore, four mesoporous rare earth oxide materials, prepared in accordance with the present invention have been characterised in detail using powder XRD, TEM, gas physisorption, SAXS and TPR studies.

[0269] These data suggest that ordered mesoporous materials have been produced, which represent a significant improvement on established methods for preparing such materials, such as incipient wetness impregnation, as discussed above. All of the compositions and templates examined have been produced on multiple occasions at high yields, showing that the methods described herein are reproducible (Table 1).

[0270] The pore volumes of the products were determined to be high and the pore size and spacings related well to the templates from which the materials were synthesised. TEM studies confirmed that the samples had a 3D structure, this being the negative of the original template.

[0271] The materials were not only produced in high yields and high yields of mesoporous material, but also displayed relatively large regions of single crystal morphology within the pore walls.

[0272] The dimensions of the mesopore structures were successfully obtained from TEM images.

[0273] All of the mesoporous materials prepared showed dramatically increased reducibility in TPR experiments compared to a high SSA nanoparticulate ceria reference. This is very promising for their potential applications in oxidation catalysts and in SOFC components.

[0274] The thermal stability of the mesoporous rare earth oxide materials has also been demonstrated.

[0275] The photoluminescence behaviour of these materials is also promising in terms of their photovoltaic applications.