Field of the Invention

The present invention relates to a new modified titania containing Ti ³⁺ and a modified surface and process for its synthesis.

Background Art

Titania (Ti0 ₂ ) is widely used in photo-catalysis. However, pure titania shows issues due, inter alia, to its large band gap. To improve the photo- catalytic applications of titania, in practice titania is commonly used in one of three modified forms, as are schematically illustrated in Figure 1, these being:

1) Mixed oxides (Fig. 1A);

2) Ti ³⁺ -doped TΊO2 (Fig. IB): Ti ³⁺ is created in bulk, by one of the following methods 2.i) introducing foreign element(s) or their oxide(s) (transition metal, metalloids, or anions) into the titania crystals, 2.ii) mixing titania with other oxide(s), or 2.iii) a "self-doping" process;

3) Surface-modified titania (Fig. 1C): foreign element(s), mostly noble metals, are added on the surface.

A certain number of general problems are associated with the types of materials schematically illustrated in Figs. 1A, IB and 1C and their synthesis procedures:

a) Material A: In mixed oxides, with no Ti ³⁺ , there are no substantial changes in the band gap of Ti0 ₂ , and so the photocatalytic activity of Ti0 ₂ remains limited to (high energy) irradiation;

b) Material B: With doped Ti0 ₂ , Ti(III) is created and dispersed in bulk, but only part of the Ti ³⁺ (mostly that at the surface) contributes to the overall performance of photo-catalysis because the irradiation penetration depth is limited to a thin layer on the surface. Consequently, the effectiveness of using dopants is low, which may lead to a high cost of Ti ³⁺ -doped Ti0 ₂ , especially in case the doping materials are expensive such as Pt, Au.

c) Material C: with surface-modified Ti0 ₂ with addition of metal, the lack of Ti ³⁺ gives rise to the problem mentioned in a) above. Furthermore, a high loading of metal may hinder irradiation from interacting with Ti0 ₂ , which prevents maximum performance in photo-catalysis of the Ti0 ₂ . In addition, if the metal particles are too large (> 2 nm), lower photocatalytic activity is reported because the metal particles can act as electron-hole recombination centers, thus lowering photo-catalytic performance of the Ti0 ₂ .

d) Concerning synthesis methods/procedures, it may be noted that (1) only one feature, either Ti(III)-doping or a modified surface, is achieved, and (2) if both properties are desired, multiple steps are required. For example, loading metal on the surface of a material of type B. However, such second synthesis steps may have negative effects on the results of the first step.

In what follows, the state of the art as regards titania-based catalysts that contain Ti ³⁺ or have a modified surface will be discussed with reference to the following documents:

Non- Patent Literature (NPL) Documents:

NPL reference 1: Onfroy et al., Applied Catalysis A: Genera! 298 (2006) 80-87, "Acidity of titania-supported tungsten or niobium oxide catalysts: correlation with catalytic activity"

NPL reference 2: Znad et al., International Journal of Photoenergy, Vol

2012, Article 10 548158, 9 pages, Znad et al, "Ta/Ti0 ₂ - and Nb/Ti0 ₂ -mixed oxides as efficient solar photocatalysts: preparation, characterization, and photocatalytic activity" NPL reference 3: Joshi et al., App Catalysis A General, 2009, 357: 26-33, "Visible light induced photoreduction of methyl orange by N-doped mesoporous titania"

NPL reference 4: Maruska et al., Sol Energy Mater, 1979, 1: 237-247, "Transition-metal dopants for extending the response of titanate photoelectrolysis anodes"

NPL reference 5: Zuo et al., Journal of American Chemical Society, 2010, 132, 11856-11857, "Self-doped Ti ³⁺ enhanced photo-catalyst for hydrogen production under visible light"

NPL reference 6: Qi et al., Applied Catalysis B: Environmental, 160-161 (2014) 621-628, "Enhanced photocatalytic performance of Ti0 ₂ based on synergistic effect of Ti ³⁺ self-doping and slow light effect"

NPL reference 7: Jiang et al., Nanoscale, 2015, 7, 5035-5045, "Thin carbon layer coated Ti ³⁺ -Ti0 ₂ nanocrystallites for visible-light driven photocatalysis"

NPL reference 8: Zuo et al., Angew. Chern. Int Ed., 2012, 51, 6223 - 6226, "Active Facets on Titanium(III)-Doped Ti0 ₂ : An Effective Strategy to Improve the Visible-Light Photocatalytic Activity"

NPL reference 9: Sasan et al., Nanoscale, 2015, 7, 13369, "Self-doped Ti ³⁺ -Ti0 ₂ as a photocatalyst for the reduction of C0 ₂ into a hydrocarbon fuel under visible light irradiation"

NPL reference 10: Ren et al., Scientific Reports, 5:10714, "Controllable Synthesis and Tunable Photocatalytic Properties of Ti ³⁺ -doped Ti0 ₂ "

NPL reference 11: Chen et al., Advanced Functional Materials, 2015, 001: 10.1 002/adfm.201502978, "Ti ³⁺ Self-Doped Dark Rutile Ti0 ₂ Ultrafine Nanorods with Durable High-Rate Capability for Lithium-Ion Batteries"

NPL reference 12: Liu et al., Materials Letters, 162 (2016) 138-141, "One-step synthesis of Ti ³⁺ doped Ti0 ₂ single anatase crystals with enhanced photocatalytic activity towards degradation of methylene blue" NPL reference 13: Valentine Rupa et al., Cata!. Lett., 2009,132: 259-267, "Titania and Noble Metals Deposited Titania Catalysts in the Photodegradation of Tartrazine"

NPL reference 14: Soignier et al., Organometallics, Vol. 25, No.7, 2006, "Tantalum hydrides supported on MCM-41 mesoporous silica: activation of methane and thermal evolution of the tantalum-methyl species"

NPL reference 15: Avenier et al., Science, 317, 1056 (2007), "Dinitrogen Dissociation on an Isolated Surface Tantalum Atom"

NPL reference 16: Liu et al., Chem. Mater. 2011, 23, 5282-5286, "Cu(II) oxide amorphous nanoclusters grafted Ti ³⁺ self-doped Ti0 ₂ : an efficient visible light photocatalyst"

Patent Documents:

Patent Document 1: US 4 544 649

Patent Document 2: EP 2 985 077 Al

In NPL reference 1, mixed oxides of the type NbOx/Ti0 ₂ and WOx/Ti0 ₂ are prepared (a material of type A according to the classification above). Synthesis is carried out by wetness impregnation. For WOx/Ti0 ₂ , the support was impregnated with an aqueous solution of ammonium meta-tungstate ((NH ₄ ) ₆ H ₂ WI ₂ 04O) then dried and calcined. For NbOx/Ti0 ₂ , the support was impregnated with a mixture of 7 wt. % of niobium (V) oxalate and 93 wt. % of oxalic acid diluted in the required amount of water, dried and calcined.

In NPL reference 2, Ta/Ti0 ₂ - and Nb/Ti0 ₂ -mixed oxide photo-catalysts were prepared by a simple impregnation method. Thus, tantalum oxide Ta ₂ 0 ₅ or niobium oxide Nb ₂ 0 ₅ were suspended in ethanol under stirring in the presence of Ti0 ₂ powder and the suspension was sonicated at room temperature, followed by drying, grinding and calcination. Patent Document 1 discloses catalysts comprising an oxide of tantalum supported on titania. A precursor of tantalum oxide may be deposited by impregnation of titania, the exemplified precursor being Ta^HsO^.

In NPL reference 3, a templating method was used to prepare N-doped ΊPO2 (a material of type B according to the classification above. Specifically, N- doped mesoporous titania was synthesized using the biopolymer chitosan as a template and also as a nitrogen source along with ammonium hydroxide. Three different types of N-doped mesoporous titania were synthesized by varying the composition of chitosan and titania precursor (titanium isopropoxide).

In NPL reference 4, photoelectrolysis of water with doped ^" PO2 and SrTi0 ₃ electrodes is reported. Dopants used are transition metals V, Cr, Mn, Fe, Co, Ni, as well as Al. The dopants were added to the starting Ti0 ₂ or SrTi0 ₃ powder in the form of their oxides.

In NPL reference 5, a "self-doped" Ti ³⁺ enhanced photo-catalyst is prepared by combustion of an ethanol solution of titanium(IV) isopropoxide and 2-ethylimidazole at 500°C in air, followed by annealing, giving a blue powder. It is postulated that during the combustion, the imidazole will react with oxygen and form CO, C0 ₂ , NO, N0 ₂ , etc. The Ti(IV) could be reduced to Ti(III) by the reducing gas (CO and NO).

In NPL reference 6, a multi-step process with templates is proposed. In order to prepare ordered Ti0 ₂ inverse opals, a forced impregnation method was used in an infiltration process. Polystyrene (PS) colloidal crystal templates were soaked completely in an absolute methanol bath and then immersed in tetrabutyl titanate. After vacuum impregnation, the coated templates were removed from the tetrabutyl titanate bath and were dried and hydrolyzed in air at room temperature overnight. Then, the samples obtained were heated to remove the templates, giving Ti0 ₂ with an inverse opal structure. By adjusting the diameters of the template PS spheres, titania inverse opals with different sizes could be obtained. In NPL reference 7, carbon-Ti0 ₂ composites were synthesized by a solvothermal method and a subsequent thermal treatment. The titania precursor tetrabutyl titanate was added into ethanol and stirred, followed by oleic acid and oleylamine. Heating in an autoclave at 180°C for 24 h was followed by a collection of the resulting products by centrifugation, further washing and drying, following by high temperature heating (500°C to 900°C) to obtain carbon-TiC>2 composites. The oleic acid is considered to be directly pyrolysed onto T1O2 and then Ti ⁴⁺ partly reduced to Ti ³⁺ on the surface of the Ti0 ₂ by carbothemal reduction for the carbon-encapsulated structure.

In NPL references 8 and 9, heating in an autoclave at 220°C, with titanium powder and hydrochloric acid, is used to produce Ti(III)-doped T1O2.

In NPL reference 10, to prepare Ti ³⁺ -doped ΊPO2, a two-step hydrothermal synthesis procedure was implemented. First, a titanium (IV) bis(ammonium lactato) dihydroxide solution was dispersed in glucose solution with stirring. The solution obtained was then transferred to an autoclave for a hydrothermal reaction at 170°C for 8 hours. Then the products were washed by deionized water and ethanol, filtered and calcined at 500°C for 3 hours, giving dried Ti0 ₂ powders. Sodium borohydride in water was mixed with the TΊO2 powder for hydrothermal reactions in an autoclave at 180°C for 16 hours.

In NPL reference 11, Ti ³⁺ -doped TΊO2 was prepared by reaction of Mg powder in isopropanol with TiCI ₃ and heating in an autoclave at 180°C. The resulting precipitate was cooled and washed with ethanol, then dispersed in HCI, stirred, washed, dried at 60°C, and calcined in air 500°C for 20 minutes.

In NPL reference 12, Ti ³⁺ -doped Ti0 ₂ was prepared by a hydrothermal method using Ti nanopowder, HF and HCI.

In NPL reference 13, nanoparticles of Ti0 ₂ were synthesized by a sol-gel technique and photo-deposition of about 1% noble metal on Ti0 ₂ was carried out (M/TΊO2, M = Ag, Au, and Pt). In NPL reference 14, a surface organometallic chemistry (SOMC) method is used to bind Ta to a silica surface bearing free hydroxyl groups. A Ta(=CH ^t Bu)(CH2 ^t Bu)3 complex is made to react with the OH groups of a MCM- 41 mesoporous silica dehydroxylated at 500 °C to form the monosiloxy surface species [(!SiOJTa^CHteuXQ Bu^], with elimination of 1 molar equivalent per atom of Ta of neopentane:

Further surface modification by heating with hydrogen was studied. The same types of Ta-surface-modified silica species are studied in NPL reference 15 with respect to their ability to induce dinitrogen dissociation.

Patent Document 2 describes Si0 ₂ -supported molybdenum or tungsten complexes, such as trialkyltungsten or molybdenum oxo complexes, their preparation and use in olefin metathesis.

Summary of the Invention

In order to address the problems associated with prior art products and processes in the field of photocatalysis, the present invention proposes a new modified titania and a process for its preparation.

In one aspect, the present invention thus relates to a process for preparing a modified titania material, comprising the steps of:

(a) providing a titania (Ti0 ₂ )-based support; (b) reacting the titania (Ti0 ₂ )-based support with an organometallic compound of a metallic element M from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W) of the Periodic Table.

In another aspect, the present invention thus relates to a modified titania material as may be obtained by the above-mentioned process of the invention.

In another aspect, the present invention relates to a modified titania material having surface Ti-O-M bonds wherein the metal atom M is an element from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W) of the Periodic Table, and contains alkyl, aryl, alkenyl or alkynyl groups bound to the metal atom M through single, double or triple metal-carbon bonds.

In the surface-modified titania and titania-containing materials of the present invention, the existence of Ti ³⁺ has been demonstrated by electronic paramagnetic resonance (EPR). Apart from photocatalysis applications, the new materials of the invention may find use in dinitrogen cleavage, or in photosensors.

It is considered by the inventors that advantages of the proposed materials and the synthesis procedures include one or more of the following:

1. The material possesses both Ti ³⁺ doping and surface modification, which is expected to further improve photocata lytic activity;

2. Ti(III) is located on a thin layer of the surface of the materials, thus at least partially solving problems a and c mentioned above;

3. In general, low amount of added metal is required, thus at least partially solving problems a and c mentioned above;

4. The preparation procedure is simple - the material can be synthesized by a simple, one-step synthesis procedure based on surface organometallic chemistry, helping to solve problem d mentioned above. Brief description of the Figures

Figure 1 shows schematic representations of three known general types of modified T1O2 materials (A, B, C) as well as the modified TΊO2 of the present invention.

Figure 2 provides an illustrative and non-limiting example of a surface organometallic (SOMC) process according to the present invention to synthesize Ta-bearing T1O2 material.

Figure 3a shows the electronic paramagnetic resonance (EPR) spectrum of a Ta-bearing T1O2 material synthesized by SOMC. Figure 3b shows in an illustrative and non-limiting schematic way one possible mechanism of Ti ³⁺ creation: mobile oxygen from the support may insert into one of the Ta- ^l Bu bonds.

Figure 4 shows infrared (IR) spectra between 4000 and 1000 cm ¹ of the starting titania support (curve a) and what is obtained after [Ta(CH2 ^t Bu)3(=CH ^t Bu)] grafting to yield the material referred to as Ta/Ti02 (curve b), as shown in Example 1 hereinunder. The disappearance of the signal of OH groups and appearance of the signal of CH groups is evidence of successful grafting.

Figure 5 shows electronic paramagnetic resonance (EPR) spectra of the Ti0 ₂ support (Figure 5a) and for the Ta(V) supported on T1O2 (Figure 5b). An increase of Ti ³⁺ on Ta/Ti02 (0.18%) vs. Ti02 (0.065%) is observed.

Figure 6 shows IR spectra between 4000 and 1000 cm ¹ of a starting material Ti02-Si02 support (lower curve, a) and after [Ta(CH ^t Bu)3(=CH ^t Bu)] grafting Ta/Ti0 ₂ -Si0 ₂ (upper curve, b) as shown in Example 2 hereinunder. The disappearance of the signal of OH groups and appearance of the signal of CH groups is evidence of successful grafting.

Figure 7 shows 1H NMR spectra (Figure 7a) and cross-polarization (CP) ¹³ C NMR spectra (Figure 7b) of Ta/Ti0 ₂ -Si0 ₂ . The appearance of H and C signals from CH groups is evidence of successful grafting. Figure 8 shows EPR spectra of the TiO2-SiO2(30 %) starting material (Figure 8a) and for the Ta(V)/TiO2-SiC>2(30 %) (Figure 8b). An increase of Ti ³⁺ on Ta/Ti0 ₂ -Si02 vs. Ti0 ₂ -Si02 (0%) is observed.

Figure 9 shows IR spectra of the TiO2-SiO2(30 %) starting material (lower curve) and the spectrum after [W(CH2 ^t Bu)3(ºC ^t Bu)] grafting (upper curve).

Figure 10 shows the EPR Spectrum for the W(VI) supported on T1O2- SiO ₂ (30 %). An increase of Ti ³⁺ on W/Ti0 ₂ -Si0 ₂ (0.2 wt% in metal) vs. Ti0 ₂ - Si0 ₂ (0 wt% in metal) is observed.

Figure 11 shows a hv - ((F(R)*hv) ² ) Curve of TΊO2 ((lower curve) and TiO2-SiO2(TiO2=30 at%) (upper curve).

Figure 12 shows X-ray diffraction (XRD) patterns for Ti02-Si0 ₂ (TiO ₂ =30 %at) and T1O2-500· Based on this data, estimated sizes of crystallized phases are as follows:

Figure 13 shows EPR signals for Ta/Ti0 ₂ -Si0 ₂ (Ti0 ₂ = 30 at%) changing with UV on/off. It is considered that the observed properties here show that the material prepared could be used in photosensing applications.

Detailed description of the invention

The titania-based support provided as a starting material in step (a) of the present invention can advantageously have a specific surface area (B.E.T.) chosen from 200 to 500 m ² /g, more particularly from 250 to 450 m ² /g. The specific surface area (B.E.T.) is measured according to the standard ISO 9277 (1995). The support physically can be a powder, an extrudate or a range of catalytic shapes. The final compound is sufficiently stable to allow moulding or pelletisation of the final catalyst; during this stage a binder may be added.

In order to comply with this preferred requirement, the titania-based support is preferably subjected to a so-called "activation" treatment which can advantageously comprise a thermal (or dehydration) treatment. The said activation treatment makes it possible to remove the water contained in the titania and/or mixed-oxide precursor, and also partially the hydroxyl groups, thus allowing some residual hydroxyl groups and a specific porous structure to remain. The choice of the titania precursor will preferably impact the conditions of the activation treatment, e.g. the temperature and the pressure. For example, the activation treatment can be carried out under a current of air or another gas, particularly an inert gas, e.g. nitrogen, as well as under reduced pressure (from low vacuum to ultra-high vacuum, preferably under high vacuum), at a temperature chosen from 50 to 1000°C, preferably from 100 to 900°C.

According to another embodiment of the present invention, the synthesis of the supported metal complex 1 is favored when the support is subjected to an activation treatment as defined above at a temperature higher than 350°C, e.g. chosen from 400 to 1000°C.

In the process of the present invention, the titania-based support provided as a starting material in step (a) of the present invention can advantageously be a titania (TiChj-based support which contains, as a molar percentage of all atoms other than oxygen (O), more than 50% of Ti, preferably more than 65% of Ti, still more preferably more than 90% of Ti, and most preferably more than 99% of Ti. It also possible however for atoms other than oxygen and titanium to be present in the support, and the present inventors have in particular studied mixed titania-silica supports, where there may be more moles of titanium (Ti) than silicon (Si), or the other way round. Advantageous embodiments thus also include titania (Ti02)-based supports which contain silica (Si0 ₂ ), the molar percentage of S1O2 in the support being at least 10% and at most 80%. In a preferred type of support, based on a combination of titania and silica, the molar percentage of Si0 ₂ in the support is at least 60% and at most 80%, with respect to the sum of the moles of Si02 + ΊPO2, more preferably at least 65% and at most 75%, and most preferably about 70%. In another embodiment of the support, the molar percentage of Ti0 ₂ in the support is at least 60% and at most 80%, with respect to the sum of the moles of S1O2 + Ti0 ₂ , more preferably at least 65% and at most 75%, and most preferably about 70%.

In the process for preparing a modified titania material of the invention, the above-described titania (Ti0 ₂ )-based support is reacted with an organometallic compound of a metallic element M from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W) of the Periodic Table.

Very generally, this surface grafting reaction may be expected to be performed at a temperature of between -20°C to 150°C. An applicable reaction duration is expected to be between 5 minutes and 12 hours. Aprotic solvents, both polar and apolar, may be used. Under certain circumstances, such as when grafting organometallic compounds having apolar groups and not polar groups, apolar solvents such as hydrocarbon solvents may be generally appropriate, for example linear or cyclic alkanes, notably C5 to CIO, such as pentane, for example.

In preferred processes of the invention, the metallic element M element from Group 5 or Group 6 of the Periodic Table is tantalum (Ta) or tungsten (W).

In preferred processes of the invention, the organometallic compound contains alkyl, aryl, alkenyl or alkynyl groups bound to the metal atom through single, double or triple metal-carbon bonds. Two examples of preferred organometallic compounds in the invention are Ta(=CH- ^t Bu)(CH ₂ - ^t Bu) ₃ and W(ºC- ^t Bu)(CH ₂ - ^t Bu) ₃ .

Figure 2 provides an illustrative and non-limiting example of a surface organometallic (SOMC) process according to the present invention to synthesize Ta-bearing Ti0 ₂ material.

Without wishing to be bound by any specific theory, the inventors postulate that, as shown in Figure 3b in an illustrative and non-limiting schematic way, one possible mechanism of Ti ³⁺ creation would be for mobile oxygen from the support to insert into one of the Ta^Bu moieties. It is not known exactly where oxygen atoms may be inserted, and whether inter a/iaTa- C-OH units and/or Ta-O-C units are produced.

Within the practice of the present invention, it may be envisaged to combine any features or embodiments which have hereinabove been separately set out and indicated to be advantageous, preferable, appropriate or otherwise generally applicable in the practice of the invention. The present description should be considered to include all such combinations of features or embodiments described herein unless such combinations are said herein to be mutually exclusive or are clearly understood in context to be mutually exclusive.

Experimental section - Examples

The following experimental section illustrates experimentally the practice of the present invention, but the scope of the invention is not to be considered to be limited to the specific examples that follow.

Titania was prepared by calcination of commercial Degussa P25 Ti0 ₂ at 500 °C under air-flow (15h) and dehydroxylation at 500 °C under vacuum (16 h) and kept under inert conditions. The ensuing material, Ti0 _2-5 oo was obtained. The reaction of Ti0 _2-5 oo with the Ta(V) organometallic complex Ta(=CH ⁱ Bu)(CH ₂ ⁱ Bu)3 was studied by reacting a self-standing pellet of the material (ca. 20 mg) for the IR study and loose powders (ca. 200 mg) for the other studies to vapour pressure of the tantalum complex or solutions of the tantalum complexes.

Example 1 - Synthesis of Ta/Ti0 ₂

The following table summarizes the synthesis procedure:

Without wishing to be bound by any particular theoretical interpretation, the treatment under dynamic vacuum here is thought to result in elimination of water from the surface and the reduction of the number of surface hydroxyl groups, with the formation of Si-O-Si bonds. The treatment thus corresponds to a way to control the total number of surface hydroxyls, eventually leading to isolated surface SiOH. However, it is not generally intended to completely eliminate surface -OH groups since these are useful to enable organometallic species such as Ta-containing organometallic species to react with and become bound to the surface. "Dehydroxylation" mentioned above is therefore not generally to be interpreted as complete removal of hydroxy groups, but instead a reduction of the quantity thereof on the surface.

Figure 4 shows infrared (IR) spectra between 4000 and 1000 cm ¹ of the starting titania support (curve a) and what is obtained after [Ta(CH2 ^t Bu)3(=CH ^t Bu)] grafting to yield the material referred to as Ta/Ti0 ₂ (curve b), as shown in Example 1 hereinunder. The disappearance of the signal of OH groups and appearance of the signal of CH groups is evidence of successful grafting.

Figure 5 shows electronic paramagnetic resonance (EPR) spectra of the Ti0 ₂ support (Figure 5a) and for the Ta(V) supported on Ti0 ₂ (Figure 5b). An increase of Ti ³⁺ on Ta/Ti0 ₂ (0.18%) vs. Ti0 ₂ (0.065%) is observed.

Example 2 - Synthesis of Ta/Ti0 _2~ Si0 ₂ The following table summarizes the synthesis procedure:

Figure 6 shows IR spectra between 4000 and 1000 cm ¹ of a starting material Ti0 ₂ -Si0 ₂ support (lower curve, a) and after [Ta(CH ^t Bu)3(=CH ^t Bu)] grafting Ta/Ti0 ₂ -Si0 ₂ (upper curve, b). The disappearance of the signal of OH groups and appearance of the signal of CH groups is evidence of successful grafting.

Figure 7 shows 1H NMR spectra (Figure 7a) and cross-polarization (CP) ¹³ C NMR spectra (Figure 7b) of Ta/Ti0 ₂ -Si0 ₂ . The appearance of H and C signals from CH groups is evidence of successful grafting.

Figure 8 shows EPR spectra of the TiO ₂ -SiO ₂ (30 %) starting material (Figure 8a) and for the Ta(V)/TiO ₂ -SiO ₂ (30 %) (Figure 8b). An increase of Ti ³⁺ on Ta/ Ti0 ₂ -Si0 ₂ vs. Ti0 ₂ -Si0 ₂ (0%) is observed.

Example 3 - Synthesis of W/Ti02~Si02

The following table summarizes the synthesis procedure:

Figures 9 and 10 provide characterization of this support grafted with tungsten species.