SUPPORTED OXIDE NH3-SCR CATALYSTS WITH DUAL SITE SURFACE SPECIES AND SYNTHESIS PROCESSES

Abstract

A process for preparing a catalyst material, includes the steps of: (a) providing a support material having surface hydroxyl (OH) groups, wherein the support material is ceria (CeO.sub.2), zirconia (ZrO.sub.2) or a combination of thereof; (b) reacting the support material having surface hydroxyl (OH) groups of step (a) with a precursor containing two transition metal atoms, each chosen independently from the group consisting of: W, Mo, Cr, Ta, Nb, V, Mn; (c) calcining the product obtained in step (b) in order to provide a catalyst material showing dual site surface species containing pairs of transition metal atoms derived from the precursor that are present in oxide form on the support material. Additionally, a catalyst material is obtained by the process set out above, and the catalyst material is used as an ammonia selective catalytic reduction (NH.sub.3-SCR) catalyst for nitrogen oxides (NOx) reduction.

Claims

1. A process for preparing a catalyst material, comprising the steps of: (a) providing a support material having surface hydroxyl (OH) groups, wherein the support material is ceria (CeO.sub.2), zirconia (ZrO.sub.2) or a combination of thereof; (b) reacting the support material having surface hydroxyl (OH) groups of step (a) with a precursor containing two transition metal atoms, each chosen independently from the group consisting of: W, Mo, Cr, Ta, Nb, V, Mn; (c) calcining the product obtained in step (b) in order to provide a catalyst material showing dual site surface species containing pairs of transition metal atoms derived from the precursor that are present in oxide form on the support material.

2. The process according to claim 1, wherein the precursor has the two transition metal atoms directly bonded to one another or linked through one or more oxygen atoms.

3. The process according to claim 1, wherein the transition metal atoms of the precursor are identical.

4. The process according to claim 1, wherein the transition metal atoms of the precursor are bonded to oxygen atoms, to nitrogen atoms and/or to one or more of the following types of groups, each of which may be substituted or unsubstituted: alkyl, aryl, alkoxy, phenoxy.

5. The process according to claim 1, wherein the precursor is selected from the group consisting of: [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O); [.sup.tBuO.sub.3W═O].sub.2(μ-O); (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3.

6. The process according to claim 1, wherein the support material is a ceria (CeO.sub.2) or ceria-zirconia (CeO.sub.2—ZrO.sub.2) support.

7. The process according to claim 1, wherein the support material contains at least 0.3 mmol and at most 2.0 mmol OH groups/g of the support material.

8. The process according to claim 1, wherein the temperature in calcining step (c) is at least 300° C., the duration of the calcining step being least 1 hour.

9. The process according to claim 1, wherein the temperature in calcining step (c) is at most 700° C., and/or the duration of the calcining step is at most 30 hours.

10. The process according to claim 1, wherein the compound obtained in step (b) has at least 0.1 wt % and at most 5.0 wt %, of transition metal atoms derived from the precursor, in elemental analysis of the compound obtained in step (b).

11. The process according to claim 1, wherein the compound obtained after calcining step (c) has at least 0.1 wt % and at most 5.0 wt %, of transition metal atoms derived from the precursor, in elemental analysis of the compound obtained after calcining step (c).

12. A catalyst material as may be obtained by the process according to claim 1.

13. The catalyst material according to claim 11 having at least 0.1 wt % and at most 5.0 wt %, of transition metal atoms derived from the precursor, as measured by elemental analysis.

14. A method comprising applying the catalyst material according to claim 12 as an ammonia selective catalytic reduction (NH.sub.3-SCR) catalyst for nitrogen oxides (NOx) reduction.

15. A compound of formula: [.sup.tBuO.sub.3W═O].sub.2(μ-O).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0036] FIG. 1a shows a schematic representations of metal dispersion in catalysts synthesized by an SOMC approach (b,c,d,e) compared to nano-particle dispersion by conventional synthesis (a).

[0037] FIG. 1b shows illustrative and non-limiting examples of reactions of dimeric precursors with ceria and the formation of supported bimolecular species.

[0038] FIG. 2 shows NO.sub.x conversion as functions of the temperature for the catalyst prepared from dimeric [(.sup.tBuCH.sub.2).sub.3W(═O)].sub.2μ-O (solid triangles), monomeric (.sup.tBuCH.sub.2).sub.3W(≡C.sup.tBu)) tungsten precursors (solid circles) and classical impregnation of (NH.sub.4).sub.10H.sub.2(W.sub.2O.sub.7).sub.6 (solid squares).

[0039] FIG. 3a shows NO.sub.x conversion as a function of the temperature using different amounts (15, 25 and 35 mg) of the catalyst WOx/CeO.sub.2 prepared from dimeric [(.sub.tBuCH.sub.2).sub.3W(═O)].sub.2(μ-O)) precursor.

[0040] FIG. 3b shows NOx conversion as function of temperature over WOx/CeO.sub.2 prepared from dimeric (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3 precursor.

[0041] FIG. 3c shows NO.sub.x conversion as a function of the temperature using the catalyst prepared from dimeric [(.sup.tBuCH.sub.2).sub.3W(═O)].sub.2(μ-O) precursor.

[0042] FIG. 3d shows NO.sub.x conversion as a function of the temperature using the catalyst prepared from dimeric [.sup.tBuO.sub.3W(═O)].sub.2(μ-O) precursor.

[0043] FIG. 4a shows dimeric structures in solution (W—O—W) and

[0044] FIG. 4b monomeric structures in solution (M-OEt).sub.2.

[0045] FIG. 5a shows a synthesis of W(≡C.sup.tBu)(CH.sub.2tBu).sub.3.

[0046] FIG. 5b shows a synthesis of [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O), where μ-O (or μ.sup.2-O) indicates that the oxygen atom is bridging and shared between two tungsten atoms.

[0047] FIG. 6a shows a synthesis of [.sup.tBuO.sub.3W(O)].sub.2(μ-O), where μ-O (or μ.sup.2-O) indicates that the oxygen atom is bridging and shared between two tungsten atoms.

[0048] FIG. 6b shows a synthesis of .sup.tBuO.sub.3W≡W.sup.tBuO.sub.3.

[0049] FIG. 7a shows a surface organometallic reaction of [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O) with surface hydroxides of CeO.sub.2 dehydroxylated at 200° C.

[0050] FIG. 7b shows a DRIFT spectrum of a) ceria dehydroxylated at 200° C., and b) after grafting of [(tBuCH.sub.2).sub.3W═O].sub.2(μ-O).

[0051] FIG. 8 shows .sup.1H MAS (left) and .sup.13C (right), NMR spectra of [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O)/CeO.sub.2-200.

[0052] FIG. 9 shows a preparation of the catalyst {W.sub.2O.sub.x}.sub.a/CeO.sub.2-200 through calcination of the intermediate material. Although the W—O—W structure is confirmed the surrounding structure is not fully defined at this time and, without wishing to be bound by any possible interpretation, the schematic diagram shows a current hypothesis. The distribution of, in particular, O or OH that can bond to W, may not be as shown.

[0053] FIG. 10 shows DRIFT spectra of a) ceria dehydroxylated at 200° C., b) ceria after grafting of [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O) and c) after calcination of the resulted sample at 500° C. under dry air during 16 h.

[0054] FIG. 11 shows DRIFT spectra of ceria dehydroxylated at 200° C. (A), catalyst prepared from dimeric [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O) (B), and catalyst obtained from monomeric W(≡C.sup.tBu)(CH.sub.2.sup.tBu).sub.3 precursor (C).

[0055] FIG. 12 shows diffuse-reflectance Uv-Vis spectra of the WO.sub.x/CeO.sub.2 prepared from dimeric and monomeric precursor (A), UV-Vis DRS spectrum and edge energy of WO.sub.x/CeO.sub.2 prepared from monomeric (B) and dimeric (C) precursors.

[0056] FIG. 13 shows surface organometallic reaction of [.sup.tBuO.sub.3WO].sub.2(μ-O) with surface hydroxides of CeO.sub.2 dehydroxylated at 200° C.

[0057] FIG. 14 shows DRIFT spectra of a) ceria dehydroxylated at 200° C. and b) the material after grafting of [.sup.tBuO.sub.3WO].sub.2(μ-O).

[0058] FIG. 15 shows .sup.13C CP MAS S NMR spectra of [.sup.tBuO.sub.3WO].sub.2(μ-O)/CeO.sub.2-200.

[0059] FIG. 16 shows a preparation of the catalyst {W.sub.2O.sub.x}.sub.b/CeO.sub.2-200 through calcination of the intermediate material [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O)/CeO.sub.2-200. As observed for FIG. 9, the full details of the environment around the W atoms are not yet fully elucidated and the schematic diagram of the final product should be considered to be a non-binding current hypothesis.

[0060] FIG. 17 shows DRIFT spectra of a) ceria dehydroxylated at 200° C., b) the intermediate material after grafting of [.sup.tBuO.sub.3WO].sub.2(μ-O), c) after the calcination of the intermediate pre-catalyst (W.sub.2O.sub.x).sub.b/CeO.sub.2-200.

[0061] FIG. 18 shows a surface organometallic reaction of (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3 with surface hydroxides of CeO.sub.2 dehydroxylated at 200° C.

[0062] FIG. 19 shows DRIFT spectra of a) ceria dehydroxylated at 200° C. and b) the intermediate material after grafting of (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3 on ceria dehydroxylated at 200° C.

[0063] FIG. 20 shows a .sup.13C CPMAS S NMR spectrum of (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3/CeO.sub.2-200.

[0064] FIG. 21 shows a preparation of the catalyst {W.sub.2O.sub.x}c/CeO.sub.2-200 through calcination of the intermediate [(.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3/CeO.sub.2-200. As observed for FIG. 9, the full details of the environment around the W atoms are not yet fully elucidated and the schematic diagram of the final product should be considered to be a non-binding current hypothesis.

[0065] FIG. 22 shows DRIFT spectra of a) ceria dehydroxylated at 200° C., b) the intermediate material after grafting of [(.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3/CeO.sub.2-200, c) after the calcination of the intermediate pre-catalyst.

DETAILED DESCRIPTION OF THE INVENTION

[0066] Among advantageous features of the present invention are: [0067] a process of grafting (chemical reactions between precursors and surface) rather than impregnating; [0068] grafted metals with atomic scale dispersion and a dual site surface structure, obtained from precursors with two transition metal atoms, are obtained rather than nano-particles; [0069] a support which is thermally pre-treated (dehydroxylation), resulting in a desired anchoring point (OH), and grafting yields well-dispersed surface species, thereby preventing sintering of the active metal center.

[0070] In the present invention, new NH.sub.3-SCR catalysts with suitable combinations of a metal selected from transition metal groups and a support material selected from CeO.sub.2, ZrO.sub.2 or their mixtures such as CeO.sub.2—ZrO.sub.2 are disclosed. These catalysts are prepared by new SOMC procedures using various organometallic metal precursors.

[0071] Conventional oxide catalysts normally consist of large metal particles supported on oxides mixed oxides. The active sites are ill-defined. The catalysts disclosed in the present invention may provide nearly 100% atomic scale dispersion of metal. Such highly dispersed metal sites do not only simply give higher density of active sites but also change the catalytic mechanism of NH.sub.3-SCR, in which NH.sub.3 adsorbed on metal sites can actively react with NOx adsorbed on surface of support. In other words, in the new catalysts interaction between the metal and the support material is promoted, thus enhancing the catalytic performance.

[0072] FIG. 1a shows a schematic of metal dispersion in catalysts; conventional methods in prior arts produce mixtures of these species, where a large portion is in the form of nanoparticles (no quantitative estimation of isolated species). Catalysts reported in the prior art have a common problem of low NOx conversion in NH.sub.3-SCR reactions.

[0073] In preferred embodiments of the present invention, metal precursors having two transition metal atoms, the so-called dimeric precursors, react with surface hydroxyl groups of CeO.sub.2 through bonding to two oxygens, forming dual site surface species, an illsutrative and non-limiting example for which is shown schematically for WOx/CeO.sub.2 catalysts in FIG. 1b.

[0074] The catalyst obtained according to the invention using the dimeric precursors ([.sup.tBuO.sub.3W(═O)].sub.2(μ-O)) is clearly higher than the catalytic activity of their counterpart prepared from the monomeric Schrock-type tungsten neopentyl/neopentylidyne complex W(≡C.sup.tBu)(CH.sub.2.sup.tBu).sub.3 (Bu=butyl). This catalyst in turn showed higher catalytic activity than the catalyst prepared by classical impregnation using (NH.sub.4).sub.10H.sub.2(W.sub.2O.sub.7).sub.6 (FIG. 2).

[0075] The catalytic performances of the dimeric catalyst for the NOx reduction are carried using various amounts of the catalyst prepared from prepared dimeric [(.sup.tBuCH.sub.2).sub.3W(═O)].sub.2(82 -O)) precursor (35 to 25 and 15 mg). FIG. 3 shows that performance of samples does not change much when the sample weight decreases by more than half, indicating their high activity.

[0076] Preferred metal precursors in the present invention show a well-defined dimeric structure, and the dimeric structure remains in solution, due to the special sigma bond between W—O—W (FIG. 4a); whereas normal metal precursors consist of only a dative coordination between the O and W (FIG. 4b), which dissociate into monomeric species in a solvent.

[0077] Appropriate support materials, in the form of ceria (CeO.sub.2) and/or zirconia (ZrO.sub.2), can be obtained from commercial suppliers. For example, ceria can be obtained from suppliers such as SOLVAY and typically has a specific surface area of about 250 m.sup.2/g.

[0078] In an advantageous embodiment to provide a certain controlled concentration of OH groups on the support material, in order to provide the material in step (a), of the process of the invention, hydration of the oxide support material (as received in a typical commercial sample) may be carried out in a first instance using moisture, followed by dihydroxylation through heating under reduced pressure. The concentration of OH groups is notably influenced by the temperature of the treatment. In a generally appropriate process for treating a ceria (CeO.sub.2) support material, a pressure of about 10.sup.−5 mbar, at a temperature of 200° C. for typically 16 h constitute advantageous treatment conditions. The concentration of OH groups on the support material can for example be determined by chemical titration through reaction with Al(.sup.iBu).sub.3—the latter reacts quantitatively with surface hydroxyl groups releasing one equivalent of isobutane per OH.

[0079] Preferred support materials in the present invention are ceria (CeO.sub.2) or ceria-zirconia (CeO.sub.2—ZrO.sub.2) supports. Concerning the mixed ceria-zirconia (CeO.sub.2—ZrO.sub.2) support, the amount of ZrO.sub.2 can be in the range 20-80 wt %, preferably between 30-60 wt %. A higher content of ZrO.sub.2 may in practice decrease the concentration of OH groups. CeO.sub.2 and CeO.sub.2—ZrO.sub.2 are not known in the prior art as good support materials for SCR catalysts - these materials normally have lower specific surface area (SSA) than SiO.sub.2.

[0080] In grafting step (b) of the invention, the support material having a controlled concentration of hydroxyl groups (OH) is reacted with a grafting reagent.

[0081] Concerning the functionalization (grafting) stage, generally appropriate solvents include polar solvents and non-polar solvents, such as in particular hydrocarbon solvents. Specific example of solvents include: pentane, hexane, heptane, toluene, xylenes, and mesitylene. In terms of reaction conditions for grating, temperatures may range from room temperature up to reflux conditions and the reaction time may appropriately be from 1 hour to 60 hours.

[0082] Concerning the activation (calcination) process, the activation process may be carried out at temperatures from 200° C.-700° C., preferably between 300° C. and 500° C. Calcination may appropriately be carried out in an oxygen-containing atmosphere, such as dry air.

[0083] In preferred embodiments of the invention, the process is carried out such that the compound obtained in step (b) has at least 0.1 wt % and at most 5.0 wt %, preferably at least 0.5 wt % and at most 2.0 wt %, of transition metal element derived from the precursor as may be determined in elemental analysis of the compound obtained in step (b).

[0084] In preferred embodiments of the invention, the process is carried out such that the compound obtained after calcining step (c) has at least 0.1 wt % and at most 5.0 wt %, preferably at least 0.5 wt % and at most 2.0 wt %, of transition metal element derived from the precursor, in elemental analysis of the compound obtained after calcining step (c).

[0085] Transition metal atoms derived from the precursors used in the present invention, and which may be identified in elemental analysis of the compound obtained after step (b) or (c) of the process of the present invention, may be chosen from the group consisting of: W, Mo, Cr, Ta, Nb, V, Mn. Metals of Group 6 (W, Mo, Cr) and group 5 (Ta, Nb, V) are preferred, and tungsten (W) is particularly preferred.

[0086] In the process for preparing a catalyst material of the present invention, the support material having surface hydroxyl (OH) groups is reacted with a precursor containing two transition metal atoms, each chosen independently from the group consisting of: W, Mo, Cr, Ta, Nb, V, Mn.

[0087] In preferred embodiments, the precursor has the two transition metal atoms directly bonded to one another or linked through one or more oxygen atoms. Further, in preferred embodiments, the transition metal atoms of the precursor are identical.

[0088] In preferred embodiments, the transition metal atoms of the precursor are bonded to oxygen atoms, to nitrogen atoms and/or to one or more of the following types of groups, each of which may be substituted or unsubstituted: alkyl, aryl, alkoxy, phenoxy. Nitrogen atoms to one or both of the transition metal atoms of the precursor may be bonded as amines or imines, or as heterocylic or heteroaromatic nitrogen atoms. The aryl groups include not only homoaromatic and heteroaromatic groups, but anionic aromatic species such as cyclopentadienyl.

[0089] In particularly preferred embodiments, the precursor is selected from the group consisting of: [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O); [.sup.tBuO.sub.3W═O].sub.2(μ-O); (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3.

[0090] Following Table 1 lists different dimeric precursors that were synthesized following procedures reported in literature. It is considered by the present inventors that these dimeric precursors can be used for preparation of SCR catalysts with improved performance following SOMC procedures described in the present invention.

TABLE-US-00001 TABLE 1 Further reported dimeric precursors that may be used in SOMC procedures for preparation of catalysts with dual site surface structures Group VI (Cr) [00001] Group VII (Mn) [00002] [00003] [00004]

TABLE-US-00002 TABLE 2 Further reported precursors and preparation procedures that may be used to synthesize dimeric precursors that may be used in the SOMC procedures for preparation of catalysts with dual site surface structures Group V Starting material(s) [00005] [00006] Synthesis of dimeric precursor [00007]

[0091] In preferred embodiments of the present invention, Group 5 or Group 6 metals are used, which are not known as good active sites for NH.sub.3-SCR when incorporated into zeolite materials. Although metals from these groups may have been used as NH.sub.3-SCR catalysts in single form such as V.sub.2O.sub.5, it was not expected that they would show high NH.sub.3-SCR performance when dispersed over other oxides as support materials. It is therefore considered by the present inventors that it was not easy to predict that the proposed combinations of the metals and support materials in the present invention would lead to significantly improved NH.sub.3-SCR performance, or that atomic scale dispersion of metals over oxides would significantly improve NH.sub.3-SCR performance.

[0092] Catalyst materials of the present invention can interact with gas reactants in a catalytic process. In certain embodiments the catalyst materials may be applied to an inert substrate such as a metal plate, corrugated metal plate, or honeycomb. Alternatively, the catalyst material may be combined with other solids such as fillers and binders in order to provide an extrudable paste that may be transformed into a porous structure such as a honeycomb.

[0093] A catalytic converter based on catalyst materials of the present invention may appropriately include the catalyst material disposed on a supporting element such that passages are made available for the passage of exhaust gases, and the supported catalyst material may appropriately be housed in a metal casing. The metal casing is generally connected with one or more inlets such as pipes for transferring exhaust gases towards the catalyst material.

[0094] In order to function in NH.sub.3-SCR catalysis, the catalytic converter is appropriately connected with a source of ammonia in order for the latter to come into contact with exhaust gas. The ammonia can be provided as anhydrous ammonia, aqueous ammonia, urea, ammonium carbonate, ammonium formate, or ammonium carbamate. In some embodiments, an ammonia storage tank is used to contain the ammonia source.

[0095] An SCR system can be integrated into various systems that require NOx reduction. Applications include engine systems of a passenger vehicle, truck, utility boiler, industrial boiler, solid waste boiler, ship, locomotive, tunnel boring machine, submarine, construction equipment, gas turbine, power plant, airplane, lawnmower, or chainsaw. Catalytic reduction of NOx using catalyst materials according to the present invention is therefore of general interest in situations where fossil fuels are used for power generation, not just for transportation but also in power generation devices, and domestic appliances using fossil fuels.

[0096] 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.

EXPERIMENTTAL SECTION—EXAMPLES

[0097] 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.

Examples of Preparation of Dimeric Metal Precursors

Synthesis of Dimeric [(.SUP.t.BuCH.SUB.2.).SUB.3.W═O].SUB.2.(μ-O)

[0098] Synthesis was carried out through 3 steps (cf. FIGS. 5a and 5b).

[0099] The molecular precursor W(≡CtBu)(CH.sub.2.sup.tBu).sub.3 (FIG. 5a) was prepared by modification of the reported synthesis (Clark, D. N.; Schrock, R. R.; Journal of the American Chemical Society 1978, 100, 6774-6776; Schrock, R. R.; Sancho, J.; Pederson, S. F.; Inorganic Syntheses 1989, 26, 44-51; Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.; Rocklage, S. M.; Pedersen, S. F.; Organometallics 1982, 1, 1645-1651).

[0100] First W(OAr).sub.3Cl.sub.3 (Ar=2,6 diisopropyl benzyl) was prepared by addition of 2,6-diisopropyl phenol to WCl.sub.6 in toluene. After washing of the excess propofol with pentane? the product is collected in black microcrystalline form. A 1.6 M solution of Mg(CH.sub.2.sup.tBu)Cl in ether (43 ml, 68.8 mmol) was added dropwise to a solution of W(OAr).sub.3Cl.sub.3 (9.3 g, 11.3 mmol) in 100 ml of ether at 0° C. The ether was removed under vacuum and the remaining solid was extracted three times with 50 ml of pentane. All volatile products were then removed under vacuum and the remaining oily product was sublimed at 80° C. and 10.sup.−5 mbar giving 3.2 g (60%) of yellow solid with characteristics in NMR: 1H NMR (C.sub.6D.sub.6, 300 MHz): δ 1.56 (9 H, s, ≡CC(CH.sub.3).sub.3), 1.15 (27 H, s, CH.sub.2C(CH.sub.3).sub.3), 0.97 (6 H, s, CH.sub.2C(CH.sub.3).sub.3), .sup.2J(HW)=9.7 Hz). .sup.13C{.sup.1H} NMR (C.sub.6D.sub.6, 75.5 MHz): δ 316.2 (≡CC(CH.sub.3).sub.3, .sup.1J(CW)=230 Hz), 103.4 (CH.sub.2C(CH.sub.3).sub.3), .sup.1J(CW)=90 Hz), 52.8 ((≡CC(CH.sub.3).sub.3), 34.5 (CH.sub.2C(CH.sub.3).sub.3), 34.4 (CH.sub.2C(CH.sub.3).sub.3), 32.4 (≡CC(CH.sub.3).sub.3).

Synthesis of [(.SUP.t.BuCH.SUB.2.).SUB.3.W═O].SUB.2.(μ-O) (FIG. 5b)

[0101] W(≡C.sup.tBu)(CH.sub.2.sup.tBu).sub.3 (3 g, 6.4 mmol) was dissolved in 20 ml of THF and 10 ml of water was added in one portion with rapid stirring. After an hour the white precipitate was filtered off and dried under vacuum to give 2.4 g (90%) of the desired complex with NMR characteristics:

1H NMR (C.sub.6D.sub.6, 300 MHz): δ 1.98 (12 H, s, CH.sub.2C(CH.sub.3).sub.3), 1.31 (54 H, s, CH.sub.2C(CH.sub.3).sub.3).
.sup.13C{1H} NMR (C.sub.6D.sub.6, 75.5 MHz): δ 91.9 (CH.sub.2C(CH.sub.3).sub.3), 1J(CW)=93 Hz), 35.4 (CH.sub.2C(CH.sub.3).sub.3), 32.9 (CH.sub.2C(CH.sub.3).sub.3).

Synthesis of Dimeric Precursor [.SUP.t.BuO.SUB.3.W═O].SUB.2.(μ-O)

[0102] This synthesis was carried out through two steps (FIG. 6).

Synthesis of [Cl.SUB.3.W═O].SUB.2.(μ-O)

[0103] A dichloromethane solution of (Me.sub.3Si).sub.2O (6.15 g, 12.6 mmol in 30 mL of CH.sub.2Cl.sub.2 was added dropwise to a suspension of WCl.sub.6 (10.0 g, 25.2 mmol) in dichloromethane (60 mL) at 0° C. over a period of 30 min. The mixture was stirred for a further 60 min. The supernatant liquor was then removed by filtration and the solid was recovered and dried. Yield 7.52 g, 95%.

Synthesis of [.SUP.t.BuO.SUB.3.W═O].SUB.2.(μ-O)

[0104] A solution of LiOCMe.sub.3 (5.45 g, 0.068 mol) dissolved in 60 mL of THF was added dropwise to a solution of [Cl.sub.3W═O].sub.2(μ-O), (7.0 g, 0.011 mol) dissolved in 20 mL of THF at 0° C. The reaction mixture was warmed to room temperature and stirred for 0.5 h and the THF was removed in vacuo. Then the product was extracted by toluene and crystallized at −30° C., to yield colourless crystals (75%) with NMR characteristics:

1H NMR (C.sub.6D.sub.6, 300 MHz): δ 1.98 (54 H, s, OC(CH.sub.3).sub.3).
.sup.13C{.sup.1H} NMR (C.sub.6D.sub.6, 75.5 MHz): δ 81.9 (OC(CH.sub.3).sub.3), 33 (OC(CH.sub.3).sub.3).
Elemental analysis for W, C and H theoretical (experimental) was: 43 (42), 33.7 (34.1), 6.3 (7) respectively.

Synthesis of Dimeric Precursor (.SUP.t.BuO).SUB.3.W≡W(.SUP.t.BuO).SUB.3

Synthesis of Monomeric Intermediate

[0105] The (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3 dimeric tungsten precursor is obtained in two steps (FIG. 6b). Corresponding suntheses have been provided in the literature: Chisholm, M. H.; Extine, M. W.; J. Am. Chem. Soc. 1975, 97, 5625; Chisholm, M. H.; Gallucci, J. C.; Hollandsworth, C. B.; Polyhedron 2006, 25, 827.

W.SUB.2.(NMe.SUB.2.).SUB.6

[0106] To a slurry of WCl.sub.4 (4.0 g, 0.0122 mol) in 20 ml of ether, a solution of LiNMe.sub.2, (2.5 g, 0.05 mol) in 40 ml of tetrahydrofuran (THF) is added dropwise at 0° C. When the lithium amide addition is complete, the reaction mixture is stirred at 0° C. for 3 h. After that the solution is allowed to warm to room temperature, stirred during ca. 20 h and then refluxed for 2 h. The yellow solution of W.sub.2(NMe.sub.2).sub.6 is filtered through Celite and the solvent was removed under vacuum. The W.sub.2(NMe.sub.2).sub.6 is extracted from the a yellow-brown residue using hexane. For further purification, the product is sublimed and recrystallized to yield bright yellow crystals of W.sub.2(NMe.sub.2).sub.6. 3.4 ppm (C.sub.6D.sub.6, 25° C.). .sup.1H NMR (C.sub.6D.sub.6, 300 MHz): δ 3.25 (36 H, s, N(CH.sub.3).sub.2).

(.SUP.t.BuO).SUB.3.W≡W(.SUP.t.BuO).SUB.3

[0107] W.sub.2(NMe.sub.2).sub.6 (1 g) is dissolved in 20 ml of toluene and 7 ml of .sup.tBuOH in 5 ml of hexane are added dropwise. After that the reaction mixture is stirred during 6 h at room temperature. The solvent and the volatiles are removed under vacuum. The residue is dissolved in hexane, then crystalized at −30 ° C. to yield red crystals 0.5 mg (30%) with NMR characteristics: .sup.1H NMR (C.sub.6D.sub.6, 300 MHz): δ 1.26 (36 H, s, C(CH.sub.3).sub.2).

Example 1- Preparation of Catalyst W.SUB.2.O.SUB.x./CeO.SUB.2-200 .Using [(.SUP.t.BuCH.SUB.2.).SUB.3.W═O].SUB.2.(μ-O)

Preparation of CeO.SUB.2

[0108] In pre-treatment of the support material, ceria (CeO.sub.2), Ceria Actalys HAS-5 Actalys 922 from Solvay (Rare Earth La Rochelle), CeO.sub.2-(200) (ceria with specific surface area of 210±11 m.sup.2 g.sup.−1-), was calcined for 16 h at 500° C. under a flow of dry air, and evacuated under vacuum at high temperature. After moisture, re-hydratation under inert atmosphere the ceria was partially dehydroxylated at 200° C. under high vacuum (10.sup.−5 Torr) for 15 h to give a yellow solid having a specific surface area of 200±9 m.sup.2.Math.g.sup.−1.

[0109] To achieve the grafting and the functionalization of surface hydroxides under optimum conditions, it is desirable to know their amount. Among the reliable quantification methods is chemical titration by reacting them using Al(.sup.iBu).sub.3. This latter is known to react quantitatively with surface hydroxyl groups releasing one equivalent of isobutane per OH. The quantification of isobutane by GC shows that Al(.sup.iBu).sub.3 reacts with OH groups of ceria giving 0.7 mmol OH/g.

Synthesis of Dimeric Precursor

[0110] Dimeric precursors were synthesized as set out in the above synthesis examples preceding Example 1.

Grafting of [(.SUP.t.BuCH.SUB.2.).SUB.3.W═O].SUB.2.(μ-O) Onto CeO.SUB.2-200

[0111] The grafting was carried out in a double Schlenk tube. The molecular precursor [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O) and the ceria partially dehydroxylated at 200° C. were mixed in toluene for 4 h. The solid [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O)/CeO.sub.2-200 was washed three times with toluene. The resulting yellow powder was dried under vacuum (10.sup.−5 Torr) (FIG. 7a).

[0112] The reaction is monitored by DRIFT spectroscopy (FIG. 7b). After the grafting reaction and the removal of the excess complex, the bands between 3400 and 3700 cm.sup.−1 attributed to different vibration mode of v(CeO—H) at 3747 cm.sup.−1 completely disappeared. New bands in the 3100-2850 cm.sup.−1 range and between 1620-1400 cm.sup.−1 are observed, these peaks are characteristic of aliphatic v (C—H) and δ(C—H) vibrations of the chemisorbed ligands on surface. These data confirm the chemical reaction between surface hydroxyl groups of ceria with [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O) tungsten precursor.

[0113] The .sup.1H solid state NMR spectrum (FIG. 8) shows a main resonance at 1.1 ppm due to the methyl groups with a shoulder around 2 ppm consistent with the presence of —CH.sub.2C(CH.sub.3).sub.3. The .sup.13C CPMAS NMR spectrum shows the presence of three well defined peaks at 98, 37 and 34 ppm corresponding —CH.sub.2C(CH.sub.3).sub.3, —CH.sub.2C(CH.sub.3).sub.3 and —CH.sub.2C(CH.sub.3).sub.3 respectively.

Activation to Obtain Catalyst W.SUB.2.O.SUB.x./CeO.SUB.2

[0114] The intermediate material [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O)/CeO.sub.2-200 was then calcined in a glass reactor under continuous flow of dry air at 500° C. for 16 h. The recovered material (W.sub.2O.sub.x).sub.a/CeO.sub.2-200 prior to a catalytic test was characterized. The calcination gives a greyish powder referred to as (W.sub.2O.sub.x).sub.a/CeO.sub.2-200.

[0115] The infrared spectrum of FIG. 10 shows a disappearance of the v(C—H) and δ(C—H) bands, indicating the total decomposition of the organic fragments. Moreover, new bands in the region of OH stretching vibration are observed between 3400 and 3700 cm.sup.−1 attributable to v(CeO—H) and at 3490 cm.sup.−1 assignable to v(O—H).

[0116] The BET surface area of the resulted material measured by N.sub.2 adsorption is ca. 186±9 m.sup.2/g closely approximated the one found for the neat ceria calcined under the same condition, which was ca. 207±10 m.sup.2/g. This would mean that the crystal structure is preserved and the grafting as well as the calcination process induce no particle sintering.

[0117] The catalysts prepared from dimeric precursor [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O) was analyzed by DRIFT and compared to the catalysts prepared from monomeric W(≡C.sup.tBu)(CH.sub.2.sup.tBu).sub.3 precursor. The IR spectrum of calcined catalysts (FIG. 11 C) showed in addition to the absorption bands of hydroxyl groups, supplementary peaks at 1006, 900 cm.sup.−1, attributed to v(W-O.sub.t) and v(W-O.sub.s—W), respectively where t and s correspond to different oxygen positions (terminal, shared) (Micropor Mesopor Mat 132, 103-111). This is even more remarkable given that the comparison with the IR of the catalysts prepared using monomeric precursor (FIG. 11 B) showed only one signal ca. 1010 cm.sup.−1 attributed to the W-Ot as expected since only isolated species are formed in this material. It is interesting to note that these IR band are not observed for the support (FIG. 11).

[0118] A satisfactory understanding of the overall dispersion of the tungsten adspecies was provided by UV-Vis-DRS analysis. It has been largely used to elucidate the structure of supported WO.sub.x as well as mixed oxides containing W.

[0119] More specifically, it has been demonstrated that the UV-vis DRS edge energy of the ligand to metal charge transfer (LMCT) transitions, Eg (eV) bears a linear relationship to the number of bridging W—O—W bonds for WO.sub.x coordinated structure.

[0120] The presence of strong absorbing material can entail and cause distortions of the DRS spectra and affect the consistency of the E.sub.g value. Unfortunately this is the case in the present work where the LMCT transitions of the W (VI) cations and the support CeO.sub.2 overlap. However, it was demonstrated that this effect can be mitigated either by dispersing the sample in a transparent matrix such as MgO, SiO.sub.2, and Al.sub.2O.sub.3 (J. Phys. Chem. B, 104, 6, 2000), or by considering the support as a baseline reference, or quite simply, by additional data manipulation. The data processing includes UV-Vis DRS spectrum deconvolution into multiple signals or by extraction the DRS spectrum of tungsten oxide species by subtracting the absorption bands of the support. For this study prior to the data UV-Vis data acquisition, the samples were diluted in BaSO.sub.4. From the UV-Vis spectra showed in FIG. 12A, a band at 239 nm attributable to the charge-transfer transitions between oxygen and isolated orthotungstate W(VI) is present and may also overlap with the bands of ceria due to Ce.sup.3+, O.sup.−2 and Ce.sup.4+ O.sup.−2 charge transfers. At 346 nm, the feature is most likely due to LMCT bands of bridging W-O-W as found for the K.sub.2W.sub.2O.sub.7 linear chain. This was confirmed by the band edge energies, E.sub.g determination (3.8 ev) close to the one found for the isolated tetrahedral centers in Na.sub.2WO.sub.4 compound (Eg=4.7 eV), and the difference is attributed to the structure distortion as reported in J. Phys. Chem. C, Vol. 111, No. 41, 2007 15.

[0121] The band edge energies E.sub.g of the catalysts prepared from dimeric precursors showed Eg even lower (2.6 eV), close to that found for dimeric O.sub.3W—O—WO.sub.3 structures cf. J. Phys. Chem. C, 111, 41, 2007.

Example 2: Preparation of Catalyst W.SUB.2.Ox/CeO.SUB.2 .From [.SUP.t.BuO.SUB.3.WO].SUB.2.(μ-O)/CeO.SUB.2

Preparation of CeO.SUB.2

[0122] CeO.sub.2 was prepared as in Example 1 above.

Synthesis of Precursor

[0123] Precursors were synthesized as set out in the above synthesis examples preceding Example 1.

Grafting

[0124] A mixture of [.sup.tBuO.sub.3WO].sub.2(μ-O) (600 mg, 0.70 mmol) and CeO.sub.2 (1 g) in toluene (10 mL) was stirred at 25° C. for 4 h. After filtration, the solid [.sup.tBuO.sub.3WO].sub.2(μ-O)/CeO.sub.2-200 was washed three times with toluene. The resulting yellow powder was dried under vacuum (10.sup.−5 Torr), as shown in FIG. 13;

[0125] Characterization of [.sup.tBuO.sub.3WO].sub.2(μ-O)/CeO.sub.2-200 by DRIFT

[0126] The resulted precatalyst was characterized and the DRIFT analysis showed bands between 3400 and 3700 cm.sup.−1 attributed to different vibration mode of v(CeO—H) completely disappeared. New bands in the 3100-2850 cm.sup.−1 range and between 1620-1400 cm.sup.−1 are observed, these peaks are characteristic of aliphatic v(C—H) and δ(C—H) vibrations of the chemisorbed ligands on surface. These data confirmed the chemical reaction between surface hydroxyl groups of ceria with tungsten tert-butoxide precursor by protonolysis and formation of tert-butanol.

Characterization of [.SUP.t.BuO.SUB.3.WO].SUB.2.(μ-O)/CeO.SUB.2-200 .by Solid State NMR

[0127] .sup.13C CP MAS NMR data show the presence of tungsten tert-butoxy fragments, as reflected by the .sup.13C peaks at 26 and 74 ppm, attributable to the methylene groups and quaternary carbon atoms respectively (FIG. 15).

Characterization of [.SUP.t.BuO.SUB.3.WO].SUB.2.(μ-O)/CeO.SUB.2-200 .by N.SUB.2 .Physisorption

[0128] The BET surface area analysis shows a moderate diminution of the surface to 147 m.sup.2/g from the pristine material (240 m.sup.2/g).

Step 3: Activation to Obtain Catalyst (W.SUB.2.O.SUB.x.).SUB.b./CeO.SUB.2-200

[0129] The material [(.sup.tBuCH.sub.2).sub.3W═O].sub.2(μ-O).sub.b/CeO.sub.2-200 is then calcined using glass reactor under continuous flow of dry air at 500° C. for 16 h. The recovered material (W.sub.2O.sub.x).sub.b/CeO.sub.2-200 prior to catalytic test was characterized. The calcination gives a yellowish powders referred to as (W.sub.2O.sub.x).sub.b/CeO.sub.2-200 (FIG. 16).

[0130] The infrared spectrum of FIG. 17 shows a disappearance of the v(C—H) and δ(C—H) bands, indicating the total decomposition of the organic fragments. Moreover, new bands in the region of OH stretching vibration are observed between 3400 and 3700 cm.sup.−1 attributable to v(CeO—H) and at 3490 cm.sup.−1 assignable to v(W—O—H).

Example 3 Preparation of Catalyst W.SUB.2.Ox/CeO.SUB.2 .From (.SUP.t.BuO).SUB.3.W≡W(.SUP.t.BuO).SUB.3./CeO.SUB.2

Preparation of CeO.SUB.2

[0131] CeO.sub.2 was prepared as in Example 1 above.

Synthesis of Precursor

[0132] Precursors were synthesized as set out in the above synthesis examples preceding Example 1.

Grafting

[0133] The grafting reaction of (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3 on CeO.sub.2-200 was performed in glove box. A mixture of a desired amount of (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3 and CeO.sub.2(200) (3 g) in toluene (20 ml) was mixed at 25° C. for 4 h. After filtration, the solid (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3/CeO.sub.2 was washed three times with 10 ml of toluene and 10 ml of pentane. The resulting powder was dried under vacuum (10.sup.−5 Torr) (FIG. 18).

Characterization (.SUP.t.BuO).SUB.3.W≡W(.SUP.t.BuO).SUB.3./CeO.SUB.2-200 .by DRIFT

[0134] The grafting reaction of (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3 on xeria to form (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3/CeO.sub.2-200 is monitored by DRIFT spectroscopy. After the grafting reaction and the removal of the excess complex, the bands between 3400 and 3700 cm.sup.−1 attributed to different vibration mode of v(CeO—H) at 3747 cm.sup.−1 decreased in intensity. New bands in the 3100-2850 cm.sup.−1 range and between 1620-1400 cm.sup.−1 are observed, these peaks are characteristic of aliphatic v(C—H) and δ(C—H) vibrations of the chemisorbed ligands on surface. These data confirm the chemical reaction between surface hydroxyl groups of ceria with tungsten precursor.

Characterization of (.SUP.t.BuO).SUB.3.W≡W(.SUP.t.BuO).SUB.3./CeO.SUB.2-200 .by Solid State NMR

[0135] .sup.13C CP MAS solid state NMR data show the presence of tungsten tert-butoxy fragments, as reflected by the .sup.13C peaks at 30 and 79 ppm, attributable to the methylene groups and quaternary carbon atoms respectively (FIG. 20).

Characterization of (.SUP.t.BuO).SUB.3.W≡W(.SUP.t.BuO).SUB.3./CeO.SUB.2-200 .by Elemental Analysis

[0136] Mass balance measurement carried out on this material of (.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3/CeO.sub.2-200 showed the presence of 3.71 wt % 2.41 wt % of W and C respectively (C/W=10). This strongly suggests that the structure of the tungsten dimeric tret-butoxide fragments are bipodal dimeric species on the surface of the ceria.

Characterization of (.SUP.t.BuO).SUB.3.W≡W(.SUP.t.BuO).SUB.3./CeO.SUB.2-200 .by N.SUB.2 .Physisorption

[0137] The BET surface area analysis shows a moderate diminution of the surface to 157 m.sup.2/g from the pristine material (240 m.sup.2/g), but still high enough, compared to the catalysts reported in the literature. In addition, an hysteresis loop can still observed, indicating the presence of the mesopores.

Activation to Obtain Catalyst (W.SUB.2.O.SUB.x.).SUB.c./CeO.SUB.2-200

[0138] The material [(.sup.tBuO).sub.3W≡W(.sup.tBuO).sub.3/CeO.sub.2-200 is calcined using glass reactor under continuous flow of dry air at 500° C. for 16 h. The recovered material (W.sub.2O.sub.x).sub.c/CeO.sub.2-200 prior to catalytic test was characterized. The calcination gives a yelewish powders referred to as (W.sub.2O.sub.x).sub.c/CeO.sub.2-200

[0139] The infrared spectrum of FIG. 22 shows a disappearance of the v(C—H) and δ(C—H) bands, indicating the total decomposition of the organic fragments. Moreover, new bands in the region of OH stretching vibration are observed between 3400 and 3700 cm.sup.−1 attributable to v(CeO—H) and at 3490 cm.sup.−1 assignable to v(W—O—H).

Catalytic Activity Test Conditions

[0140] Pellet samples of approximate 33 mg were prepared under 1 ton pressure and put into a quartz reactor (diameter 4.5 mm). A mixture of gas consisting of NO 300 ppm, NH.sub.3, 350 ppm, O.sub.2 10%, H.sub.2O, 3%, CO.sub.2 10%, He (balance), was sent through a catalytic bed at the rate of 300 mL/min. The reactor was heated from room temperature to 600° C. with a heating rate of 10° C./ min. The system was kept at 600° C. for 10 min before cooling down to room temperature. Gas composition at the outlet was monitored during the heating up and cooling down by a combination of FTIR, MS and chemiluminiscence.