WO2023089351A1

WO2023089351A1 - Nh3-scr catalysts synthesized by surface organometallic chemistry process with multiple grafting steps

Info

Publication number: WO2023089351A1
Application number: PCT/IB2021/000829
Authority: WO
Inventors: Phuc Hai NGUYEN; Cherif Larabi; Kai Chung Szeto; Mostafa Taoufik
Original assignee: Toyota Motor Europe NV SA; Centre National de la Recherche Scientifique CNRS; Ecole Superieure de Chimie Physique Electronique de Lyon; Universite Claude Bernard Lyon 1
Current assignee: Toyota Motor Europe NV SA; Centre National de la Recherche Scientifique CNRS; Ecole Superieure de Chimie Physique Electronique de Lyon; Universite Claude Bernard Lyon 1
Priority date: 2021-11-22
Filing date: 2021-11-22
Publication date: 2023-05-25
Anticipated expiration: 2024-05-22
Also published as: EP4436715A1

Abstract

The present invention relates to 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 (CeO₂), zirconia (ZrO₂) or a combination thereof; (b1) reacting the support material having surface hydroxyl (OH) groups of step (a) with a precursor compound containing a metal element M, wherein M is from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W) or M is manganese (Mn); (b2) removing organic parts of the precursor compound grafted in step (b1) by calcination to provide a calcined support material bearing metal element M; (c1) rehydration of the calcined support material obtained in step (b2); (c2) adjustment of the surface hydroxyl (OH) group level of the rehydrated calcined support material obtained in step (c1); wherein after the execution of step (c2), the process is started again at step (b1) and at least process steps (b1) and (b2) are repeated. In preferred embodiments of the invention, the sequence of steps (b1) > (b2) > (c1) > (c2) is carried out repeatedly so that overall at least process steps (b1) and (b2) are repeated at least 2 times and at most 10 times in total, preferably at least 3 times and at most 5 times. In a second aspect, the present invention relates to a catalyst material as may be obtained by the process set out above. In a third aspect, the present invention relates to the use of the catalyst material set out above as an ammonia selective catalytic reduction (NH₃-SCR) catalyst for nitrogen oxides (NOx) reduction.

Description

NH3-SCR CATALYSTS SYNTHESIZED BY SURFACE ORGANOMETALLIC CHEMISTRY PROCESS WITH MULTIPLE GRAFTING STEPS

Field of the Invention

[0001] The present invention relates to the synthesis of ammonia selective catalytic reduction (NH3-SCR) catalysts for nitrogen oxides (NOx) reduction.

Background Art

[0002] Toxic NOx gases (NO, NO2, N2O) included in exhaust gases from fossil-fuel-powered vehicles or stationary sources such as power plants are required to be converted to N2 before being released to the environment. This is normally done by using different types of NOx reduction catalysts such as three-way catalysts (TWO), NOx storage reduction (NSR), or selective catalytic reduction (SCR) using ammonia as external reducing agent (NH3-SCR).

[0003] Metal oxides such as V₂O₅ are known to be good NH3-SCR catalysts. It has been suggested that the catalytic activity is achieved by the complementary features of acidity and reducibility of the surface species. Briefly, NH3 is adsorbed on a Brpnsted acid site (V⁵⁺-OH) followed by N-H activation through the adjacent V=O surface groups through a redox cycle (V⁵⁺=O/V⁴⁺-OH). The resulting surface complex reacts with gaseous or weakly adsorbed NO through Langmuir-Hinshelwood and Eley-Rideal mechanisms, respectively, to form NH2NO intermediate species which undergo decomposition into N2 and H2O. An alternate mechanism (amide-nitrosamide) involving the adsorption of NH3 over Lewis acid sites has also been proposed. Furthermore, under realistic conditions, particularly when a peroxidation catalytic convertor is placed upstream of the SCR catalytic convertor, this gives rise to formation of nitrogen dioxide which favors the SCR reaction known as fast-SCR. Indeed NO2 allows fast re-oxidation of the reduced species. However, the optimal NO2/NO ratio is one, and the presence of excess NO2 is also reduced through slower reaction leading to a lower total SCR reaction rate. Metal oxide catalysts such as V₂O₅ are developed mostly by synthesis routes such as impregnation, which normally produce nanoparticles of metal dispersed on a support. The problem of such catalysts is the low performance, such as low NOx conversion and/or low N2 selectivity.

[0004] Prior art catalysts have often used Cu, Fe, which are well recognized as good active sites for NH3-SCR when incorporated into zeolite materials. As regards support materials, prior art has often used SiC>2, which has high specific surface area, and may be expected to improve SCR performance by increasing the quantity of active sites.

US 9,283,548 B2 discloses catalysts of the type: MA I CeC>2 (M = Fe, Cu; A = K, Na), the synthesis route being impregnation, with chelating agents such as EDTA, DTPA being used.

J. Phys. Chem. B 2006, 110, 9593 - 9600 [Tian 2006] discloses catalysts of the type: VOx I AO2 (A = Ce, Si, Z), the synthesis route being impregnation. Applications include propane oxidative dehydrogenation (ODH). Dispersion and physisorption of the vanadium oxo-isopropoxide is achieved, rather than chemisorption.

J. Phys. Chem. B 1999, 103, 6015 - 6024 [Burcham 1999] discloses catalysts of the type: Nb20s I SiC>2, AI2O3, ZrC>2, TiCh, the synthesis route being impregnation. The reference discusses surface species of isolated Nb, characterized by vibrational spectroscopy. The preparation is carried out in water, and the metal is deposited on the surface, rather than being grafted by protonolysis.

J. Phys. Chem. C 2011, 115, 25368-25378 [Wu 2011] discloses catalysts of the type: VOx I CeO2, SiO2, ZrO2, the synthesis route being impregnation. Iso-propanol is used as a solvent, not leading to grafting of the precursor on the surface, but instead only dispersion and physisorption of the vanadium oxo- isopropoxide.

Appl. Catal. B 62, 2006, 369 [Chmielarz 2006] describes catalysts of the type: Fe or Cu/SiCh (3 different forms). It is widely known that Cu and Fe show good NH3-SCR performance when zeolites are used (ion-exchange synthesis). The catalyst materials were used for deNOx by NH3-SCR. Synthesis was carried out by molecular designed dispersion (MDD) using precursors Fe(acac)s, Cu(acac) (acac = acetylacetonate).

Science 2007, 317, 1056-1060 [Avenier 2007] describes cleavage of dinitrogen on isolated silica surface-supported tantalum(III) and tantalum(V) hydride centers [(ESi-O)2Taⁿⁱ-H] and [(ESi-O)2Ta^v-H3].

EP 2 985 077 Al describes SiC^-supported molybdenum or tungsten complexes, such as trialkyltungsten or molybdenum oxo complexes, their preparation and use in olefin metathesis.

Summary of the Invention

[0005] In order to address the problems associated with prior art products and processes in the field of ammonia selective catalytic reduction (NH3-SCR) catalysts for nitrogen oxides (NOx) reduction, the processes and products of the present invention have been developed.

[0006] The Surface Organometallic Chemistry (SOMC) approach is capable of modifying the surface of support materials by grafting organometallic precursors, i.e. forming chemical bonds between precursors and surface hydroxyl groups, and thus preserving the local structure of the grafted material to minimize the formation of diversified species on the surface of support materials that are normally created through conventional synthesis methods. This methodology can be used to synthesize metal oxide catalysts supported with different metals. A typical SOMC procedure to synthesize materials consists of 3 steps as follows: • Step 1: Preparation, example: o Support materials:

■ calcination

■ hydratation

■ dehydroxylation to generate controlled concentrations of hydroxyl groups o Metal precursors:

■ Synthesis (for those that are not readily available)

• Step 2: Grafting o Allow metal precursors to react with surface hydroxyl groups of the support material in a solution, for example toluene, typically at room temperature (~ 25 °C) o Washing and drying

• Step 3: Activation o Remove remaining organic ligands, typically by calcination at around 500 °C or higher for 16 hours under air flow

[0007] The present invention discloses the development of new oxide NH3-SCR catalysts with improved NOx reduction performance by using new SOMC procedures.

[0008] Thus, in a first aspect, the present invention relates to 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 (CeO2), zirconia (ZrO2) or a combination thereof;

(bl) reacting the support material having surface hydroxyl (OH) groups of step (a) with a precursor compound containing a metal element M, wherein M is from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W) or M is manganese (Mn); (b2) removing organic parts of the precursor compound grafted in step (bl) by calcination to provide a calcined support material bearing metal element M;

(cl) rehydration of the calcined support material obtained in step (b2);

(c2) adjustment of the surface hydroxyl (OH) group level of the rehydrated calcined support material obtained in step (cl); wherein after the execution of step (c2), the process is started again at step (bl) and at least process steps (bl) and (b2) are repeated.

[0009] In preferred embodiments of the invention, the sequence of steps (bl) > (b2) > (cl) > (c2) is carried out repeatedly so that overall at least process steps (bl) and (b2) are repeated at least 2 times and at most 10 times in total, preferably at least 3 times and at most 5 times. Three to five separate steps of grafting (bl) and calcination (b2) may thus appropriately be carried out.

[0010] In a second aspect, the present invention relates to a catalyst material as may be obtained by the process set out above.

[0011] In a third aspect, the present invention relates to the use of the catalyst material set out above as an ammonia selective catalytic reduction (NH3-SCR) catalyst for nitrogen oxides (NOx) reduction.

Brief description of the Figures

[0012] Figure 1 shows a schematic illustrative example of a multiplestep SOMC procedure to prepare a well-defined polymeric species: 1) preparation of the starting material (support and precursor); 2) grafting; 3) (intermediate) calcination; 4) rehydration and dehydroxylation; 5) grafting; 6) calcination; 7) rehydration dehydroxylation; 8) grafting; 9) (final) calcination. Route (a) leads to formation of 1-D polymer while route (b) leads to formation of 2-D polymer. Figure 2 shows results obtained by NbOx/CeCh catalysts prepared by multiple step SOMC procedures (to the same metal loading of Nb 1.2 wt.%/CeO2), demonstrating higher NH3-SCR performance (NOx conversion) at high temperatures.

Figure 3 shows the NH3-SCR performance of NbOx/CeCh-ZrCh catalysts prepared by multiple step SOMC procedures (to the same metal loading of Nb 1 wt.%).

Figure 4 shows the ad-species (or "ads species', which refers to adsorbed species on a surface) obtained via classical impregnation a) isolated, b) dimeric, c) 1-D polymeric and d) 2-D polymeric.

Figure 5 shows results obtained by NbOx/CeO2 catalysts prepared by normal (1 step) SOMC procedure (limited at 1.8 wt% Nb) compared to a catalyst (with 3.5 wt% Nb) prepared by a multiple step SOMC procedure (twice).

Figure 6 shows the structure of the adsorbed surface species prepared by classical impregnation ID (linear or zig-zag type structures), 2D (sheetlike structure) and 3D (crystalline or amorphous and random structures).

Figure 7 shows

and ¹³C NMR spectra of A)

and ¹³C NMR solution spectra of the [Nb(0Et)s]2 precursor, B)

and ¹³C solid state NMR MAS spectra of the resulting material from the grafting of 1 wt% of [Nb(0Et)s]2 on CeO₂-2oo-

Figure 8 shows textural properties analyses of the CeO₂-2oo and the resulting material from the grafting of 1 wt% of [Nb(0Et)s]2 in one step : A) N2 adsorption-desorption isotherm and BJH pore size distribution B) the values of BET and the pore volume.

Figure 9 shows DRIFT spectra of a) ceria dehydroxylation at 200 °C (heating rate: 5 °C/min from 20 - 200 °C), b) after grafting of [Nb(OEt)s]2, c) after calcination of [Nb(OEt)₅]2@CeO₂-2oo- Figure 10 shows Transmission Electron Microscopy (TEM) micrographs of synthesized material (calcined [Nb(OEt)s]2@CeO₂-2oo) (A) and energy-dispersive X-ray spectroscopy (EDXS) analysis of Nb (B).

Figure 11 shows DRIFT spectra of first cycle : a) ceria dehydroxylation at 200 °C , b) after grafting of [Nb(OEt)₅]2, first cycle (0.5 wt% of Nb), c) after calcination of [Nb(OEt)s]2@CeO₂-2oo; second cycle : d) after rehydration and dehydroxylation at 200 °C, e) after grafting of [Nb(OEt)₅]2, second cycle (0.5 wt% of Nb), e) after calcination and the final catalysts (1 wt% of Nb).

Figure 12 shows textural properties analyses of the CeO₂-2oo and the resulting material from the grafting of 1 wt% of [Nb(OEt)s]2 in two steps (0.5 wt% + 0.5 wt%) : A) N2 adsorption-desorption isotherm and BJH pore size distribution B) the values of BET and the pore volume.

Figure 13 shows DRIFT spectra of the catalyst as well as the intermediates resulting from the synthesis through five successive grafting cycles: spectra of dehydroxylated process (a, d, g, j, m), spectra of the grafting steps (b,e,h,k,n) and spectra of the calcined materials (c, f, i, I, 0).

Figure 14 shows textural properties analyses of the CeO₂-2oo and the resulting material from the grafting of 1 wt% of [Nb(OEt)s]2 in five steps : A) N2 adsorption-desorption isotherms B) the values of BET surface area with the Nb content.

Figure 15 shows high-resolution transmission electron microscopy (HRTEM) micrographs of the synthesized material through five successive grafting steps containing 1 wt% of Nb.

Figure 16 shows powder X-Ray diffraction of the ceria and the materials functionalized by 1 wt% of Nb through one, two and five steps.

Figure 17 shows diffuse-reflectance Uv-Vis spectra of the NbOx/CeCh with various preparation methods. Figure 18 shows DRIFT spectra of a) ceria zirconia dehydroxylation at 200 °C, b) after grafting of [Nb(0Et)s]2, c) after calcination of [Nb(OEt)5]2@Ceo.5Zro.s02-2oo-

Figure 19 shows textural properties analyses of the Ceo.sZro.sC^oo and the resulting material from the grafting of 1 wt% of [Nb(0Et)s]2 in one step.

Figure 20 shows diffuse-reflectance Uv-Vis spectra of Ceo.5Zro.5O2 and NbOxZCeo.5Zro.5O2.

Figure 21 shows DRIFT spectra of first cycle : a) ceria zirconia dehydroxylation at 200 °C , b) after grafting of [Nb(OEt)₅]2, first cycle (0.5 wt% of Nb), c) after calcination of [Nb(OEt)5]2@Ceo.5Zro.s02-2oo; second cycle : d) after rehydration and dehydroxylation at 200 °C, e) after grafting of [Nb(OEt)₅]2, second cycle (0.5 wt% of Nb), e) after calcination and the final catalsts (1 wt% of Nb).

Figure 22 shows textural properties analyses of Zro.5Ceo.5O2.200 and the resulting material from the grafting of 1 wt% of [Nb(OEt)s]2 in two steps (0.5 wt% + 0.5 wt%).

Figure 23 shows diffuse-reflectance Uv-Vis spectra of the NbO_x- Ceo.5Zro.5O2 with various preparation methods.

Figure 24 shows DRIFT spectra of the catalyst as well as the intermediates that resulted from the synthesis through five successive grafting cycles. A) Spectra of dehydroxylated process, B) spectra of the grafting steps and C) spectra of the calcined materials.

Figure 25 shows textural properties analyses of the Ceo.5Zro.5O2.200 and the resulting material from the grafting of 1 wt% of [Nb(OEt)s]2 in five steps ; N2 adsorption-desorption isotherms and the values of BET surface area with the Nb content.

Figure 26 shows siffuse-reflectance Uv-Vis spectra of the NbO_x- Ceo.5Zro.5O2 with various preparation methods. Detailed description of the invention

[0013] In the present invention, multistep grafting is carried out, i.e. with multiple functionalization steps under controlled conditions in order to obtain final catalysts.

[0014] It is considered that the new synthesis procedures described in the present invention may lead to at least two special effects:

- increased metal loading in the final catalysts;

- controllable (special) structures of metal surface species, typically polymeric structures as represented in Figure 1.

[0015] Even for a fixed total metal loading, the multiple grafting procedure can improve catalytic performance.

[0016] It is considered that the NH3-SCR mechanism depends on the catalytic behavior and the role of the monomeric as well as the polymeric species in the SCR process. It is widely accepted that the NH3-SCR de-NOx reaction takes place via the Eley-Rideal mechanism, involving both the redox sites and the acidity of the catalysts. Briefly, the adsorption as well as the activation of NH3 occur preferentially on the acid sites located at the interface between the metal and the support. The dissociation of the N-H bond of the adsorbed NH3 involving a hydrogen transfer, results in a new and more reactive ammonium ion. This latter species subsequently reacts with NO, from the gas phase or weakly adsorbed, yielding the nitrosamide (NH2NO) intermediate that undergoes fast decomposition into N₂ and H₂O. Then, the catalytic cycle is complete when the oxygen refills the surface vacancies created by the oxygen consumption and the regeneration of the oxo species. It was suggested through a combined experimental results and density functional theory (DFT) calculations, that the polymeric vanadyl-based NH3-SCR mechanism is more favourable. \Faraday Discuss., 2013, 162, 233-245; Journal of Catalysis 255 (2008) 197-205; ACS Catal. 2015, 5, 5787-5793; Sci Adv. 2018,4,1-8] [0017] Consequently, the more important is the adsorption and the activation capacities, the more important will be the NHs-SCR-catalytic activity; this is the distinctive characteristic of the catalyst developed by design, through surface organometallic chemistry.

[0018] The conventional preparation approach, based on the excess loading of the metal, results in a large number of species with complex structure including isolated and dimensional polymeric species ID, 2D or 3D as marked in Figure 4, with uncontrolled degree of polymerization. In the view of their number and their accessibility the M-O-M moieties can be maximized in the bidimensional structure (species (d) in Figure 4).

[0019] Appropriate support materials in the form of ceria (CeCh) and/or zirconia (ZrCh) 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 m²/g.

[0020] 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 dehydroxylation 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 (CeO2) support material, a pressure of at most around 10'⁵ mbar, at a temperature of at least 200 °C, for example for a typical treatment time of 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 AI('Bu)3 - the latter reacts quantitatively with surface hydroxyl groups releasing one equivalent of isobutane per OH group. [0021] In a generally appropriate manner, when carrying out the process of the present invention, the support material provided in step (a) may contain at least 0.3 mmol and at most 2.0 mmol OH groups/g of the support material, preferably at least 0.5 mmol and at most 1.3 mmol OH groups/g of the support material.

[0022] In the present invention, the term "dehydroxylation" may be used to refer to a process of adjusting the level of OH groups on the support material as described above, starting from a hydrated support material. This adjustment of the level of OH groups may also be referred to as "partial dehydroxylation" or "controlled dehydroxylation".

[0023] Preferred support materials in the present invention are ceria (CeO2) or ceria-zirconia (CeO2 - ZrO2) supports. Concerning the mixed ceriazirconia (CeO2 - ZrO2) support, the amount of ZrO2 can be in the range 20-80 wt%, preferably between 30-60 wt%. A higher content of ZrC>2 may in practice decrease the concentration of OH groups. CeO2 and CeO2-ZrO2 are not particularly known in the prior art as good support materials for SCR catalysts - these materials normally have lower specific surface area (SSA) than SiO2.

[0024] In a generally appropriate manner, when carrying out the process of the present invention, the temperature in calcining step (b2) may be at most 700°C, and/or the duration of the calcining step at most 30 hours. Furthermore, the temperature in calcining step (b2) may appropriately be at least 300°C, preferably at least 400°C, and the duration of the calcining step may be least 1 hour, preferably at least 8 hours.

[0025] Rehydration in step (cl) of the calcined material obtained in step (b2) may be carried out by addition of vapour pressure of water, i.e. normal atmospheric levels of gas phase water at room temperature. In other embodiments, a sample of the calcined material obtained in step (b2) may be heated, for example at temperature of up to 100 °C, and for a time of several hours such as 6 h, in the presence of the moisture. [0026] In a generally appropriate manner, when carrying out the process of the present invention, step (c2), the adjustment of the surface hydroxyl (OH) group level, may be carried out under (high) vacuum conditions, with a pressure of less than 10'⁴ mbar. Furthermore, the temperature during step (c2) may appropriately be at least 120°C, preferably at least 170°C, and the duration at least 1 hour, preferably at least 10 hours.

[0027] Concerning the functionalization (grafting) stage (bl), generally appropriate solvents include apolar 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 grafting, temperatures may range from room temperature up to reflux conditions and the reaction time may appropriately be from 1 hour to 60 hours.

[0028] Preparation of appropriate precursor compounds may also be found in patent applications WO 2020/245620 and WO 2020/245621.

[0029] 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.

[0030] In preferred embodiments of the invention, the process is carried out such that the metal element M is incorporated in a total amount of at most 15 wt%, with respect to the total mass of the catalyst material after the last (b2) step. A level of loading of metal element M, with respect to the total mass of the catalyst material after the last calcining (b2) step, of at least 7.0 wt% and at most 10.0 wt%, in some cases (particularly with four or more grafting steps) above 10.0 wt% is particularly envisaged for tungsten (W) as metal element M. A level of loading of metal element M, with respect to the total mass of the catalyst material after the last calcining (b2) step, of at least 0.5 wt% and at most 6.0 wt%, preferably at least 1.0 wt% and at most 5.0 wt%, is particularly envisaged for niobium (Nb) as metal element M. [0031] In preferred embodiments of the invention, the process is carried out such the metal element M is incorporated in an amount of at least 0.1 wt% and at most 5.0 wt% at each (bl) step, with respect to the total mass of the catalyst material after the last (b2) step.

[0032] In preferred embodiments of the invention, the sequence of steps (bl) > (b2) > (cl) > (c2) is carried out repeatedly so that overall at least process steps (bl) and (b2) are repeated at least 2 times and at most 10 times in total, preferably at least 3 times and at most 5 times.

[0033] In preferred embodiments of the invention, the precursor compound to be used in grafting step (bl) is:

(Pl) a compound containing at least one alkoxy or phenoxy group bound though its oxygen atom to a metal element from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W);

(P2) a compound containing at least one hydrocarbon group bound though a carbon atom to a metal element from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W); or

(P3) a compound containing at least one hydrocarbon group bound though a carbon atom to a metal element which is manganese (Mn).

[0034] In preferred embodiments of the invention, the compound (Pl) containing at least one alkoxy or phenoxy group bound though its oxygen atom to a metal element from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W) is at least one compound selected from the group consisting of: [Nb(OEt)₅]2; Nb(OAr)₅ where Ar is the 1,3,5-trimethylphenyl (CH₃)₃C6H2- group; [W=O(OEt)4]2; [V(=O)(OEt)₃]₂; [V(=O)(O'Pr)₃]; and [Ta(OEt)₅]2. More preferably, the compound (Pl) is [Nb(OEt)s]2 or [W=O(OEt)4]2-

[0035] In other embodiments of the invention, the compound (P2) containing at least one hydrocarbon group bound though a carbon atom to a metal element from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W) is at least one compound selected from the group consisting of: WEC^tBu(CH₂ ^tBu)3; and MofO Mesityh.

[0036] In other embodiments of the invention, the precursor compound to be used in grafting step (bl) shows two transition metal atoms directly bonded to one another or linked through one or more oxygen atoms. Preferably, the two thus linked transition metal atoms of the precursor are identical. The two transition metal atoms of the precursor may be 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. Precursors to be used in such embodiments may be selected from the group consisting of: [(^tBuCH2)3W=O]2(p-O); [^tBuO3W=O]2(p-O); (^tBuO)₃W=W(^tBuO)₃.

[0037] In particularly preferred embodiments, the Group 5 metal niobium (Nb) or the Group 6 element tungsten (W) is provided on a ceria or ceria-zirconia support.

[0038] 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.

[0039] 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. [0040] In order to function in NH₃-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.

[0041] 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.

[0042] 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

[0043] 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. 1} Synthesis of Nb/CeO2 catalysts

[0044] A surface organometallic approach was applied, since it allows a better control of the surface species. The grafting steps were performed in a double-Schlenck, to highlight the effect of the grafting cycles on the structure and the NH3-SCR catalytic activity. Three catalyst were prepared containing the same amount of the metal ca. 1 wt% were prepared by one step (grafting 1 wt%), two steps (grafting (0.5 wt% at each cycle) and five steps (grafting 0.2 wt% each cycle).

1A: One grafting cycle

[0045] The grafting reaction was performed as follow: A mixture of [Nb(OEt)5]2 (68,7 mg, 0.11 mmol) and CeO₂ -200 (2 g) in pentane (10 mL) was stirred at 25 °C for 2 h. After filtration, the obtained yellowish powder [Nb(OEt)5]2@CeO₂-2oo was dried under vacuum (10^-5 Torr). Prior to calcination the material was characterized.

Scheme I: The reaction grafting of [Nb(OEt)₅]₂ on CeO_2.200

[0046] ¹H MAS NMR (ppm, 500 MHz): δ 4.8 (OCH₂CH₃), 1.3 (OCH₂CH₃) ¹³C CP MAS NMR (ppm, 200 MHz): δ 68.5 (terminal OCH2CH3), 64.6 (bridging OCH2CH3, or physisorbed ethanol), 18.3 (terminal OCH2CH3), 16.5 (bridging OCH2CH3, or physisorbed ethanol) by comparison with the ^XH NMR of the starting precursors in CeDg (Figure I).

[0047] The surface area of the materials evaluated using ASAP2020 micrometrics revealed the presence of hysteresis loop due to the capillary condensation while desorbing the N₂ (Figure 8) - this suggests the presence of mesopores. The value of the surface area decreased from 180 m².g ¹ measured for neat support (CeO₂-2oo) to 170 m².g ¹ and the pore volume from 0.25 cm³/g to 0.19 cm³/g. In general, taking into the account the uncertainties in the quantification (ca. 5 %), these are not significant deviations.

[0048] Elemental analysis performed on the sample revealed the presence of %Nb = 0.99 %_wt, which is in agreement with the expected amount.

[0049] The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to Ce-OH (Figure 9 a) reacted selectively with niobium complex [Nb(OEt)s]2- In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ region respectively are found (Figure 9 b). After calcination at 500 °C under a flow of dry air, the intensities of v(C-H) and b(C-H) stretching bands disappeared, indicating the total decomposition of the organic part of the supported complex.

[0050] TEM studies performed on the material after thermal treatment (calcination under a flow of dry air at 500 °C (Figure 10) showed that the sample is constituted by a stacking of crystallites of about 10 nm in thickness. Energy-dispersive X-ray spectroscopy (EDXS) indicated that the niobium is well distributed on ceria surface. The distance between Nb atoms was estimated to be ca. 4 nm, just an indication as here 3D support is being evaluated, using a 2D image. IB: Two grafting cycles

[0051] The catalyst with the same amount of Nb (1 wt%) was prepared through two successive grafting steps as follows:

1^st cycle : Grafting of [Nb(0Et)s]2 on CeCh (0.5 wt% of Nb [Nb(OEt)₅]2@CeO2). A mixture of [Nb(OEt)₅]2 (42.8 mg, 0.07 mmol) and 2.5 g CeO^oo in pentane (10 mL) was stirred at 25 °C for 2 h. After filtration, the obtained solid [Nb(OEt)₅]2@CeO₂-2oo was dried under vacuum (10“⁵ Torr).

2^nd cycle : The material [Nb(OEt)s]2@CeO2, obtained in the first cycle was calcined using a glass reactor under continuous flow of dry air at 500 °C for 16 h. The material recoveredwas characterized. To create an additional anchoring site, the calcined material {Nb0_x}o.5-CeO₂ was rehydrated by addition of vapor pressure of water. Afterwards, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C, the excess as well as the physisorbed water were removed. At this point, a material which is ready to be grafted was obtained, thus it is impregnated again with a pentane solution of [Nb(OEt)₅]2 - a solution of 34.1 mg of [Nb(OEt)s]2 (0.05 mmol) in 20 ml of pentane to provide 2 g of {Nb0_x}o.5-CeO₂. The solid was filtered, dried and calcined, resulting in {NbO_x}i-CeO2.

[0052] The catalyst as well as the intermediates were characterized. The MAS solid state NMR revealed the presence of the organic fragments (ethoxy ligands) on the materials.

[0053] ^XH MAS NMR (ppm, 500 MHz): 5 4.8 (OC/ CH₃), 1.3 (OCH₂C/^) ¹³C CP MAS NMR (ppm, 200 MHz): 5 68.5 (terminal OCH2CH3), 64.6 (bridging OCH2CH3, or physisorbed ethanol), 18.3 (terminal OCH₂CH₃), 16.5 (bridging OCH2CH3, or physisorbed ethanol).

[0054] Elemental analysis performed on the sample revealed the presence of 0.98 wt%, which is close to the expected value of ca. 1 wt%.

[0055] The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to Ce-OH reacted selectively with niobium complex [Nb(0Et)s]2. In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ region respectively are found. After calcination, the DRIFT analyses revealed the complete disappearance of CH group of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to hydroxyl group (Nb-OH, and Ce-OH). The same DRIFT observations and comments apply to the 2^nd cycle (Figure 11).

[0056] The surface area of the catalyst indicated a slight decrease from 180 m².g^-1 to 165 m²/g after the first grafting and this downward trend of the surface area was confirmed, it was found to be ca. 145 m².g ¹ after the second grafting cycle (Figure 12 A). The same goes for the evolution of the pore volumes that decreased from 0.23 to 0.18 cm³/g (Figure 12 B).

1C: Five grafting cycles

[0057] The catalyst with the same amount of Nb (1 wt%) was prepared through five successive graftings as follows:

1^st cycle : Grafting of [Nb(OEt)₅] on CeCh (0.2 wt% of Nb [Nb(OEt)s]2@CeO2). A mixture of [Nb(OEt)s]2 (55 mg, 0.08 mmol) and 8 g CeO₂2oo in pentane (30 mL) was stirred at 25 °C for 2 h. After filtration, the obtained solid [Nb(OEt)₅]2@CeO₂-2oo was dried under vacuum (10“⁵ Torr).

2^nd cycle : The material [Nb(OEt)₅]2@CeO2, obtained in the first cycle was calcined using glass reactor under continuous flow of dry air at 500 °C for 16 h. The material recoveredwas characterized. To create additional anchoring site, the calcined material {Nb0_x}o.2-CeO₂ was rehydrated by addition of vapor pressure of water. Afterwards, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C, the excess as well as the physisorbed water were removed. At this point, a material which is ready to be grafted was obtained, thus it was impregnated again with a pentane solution of [Nb(OEt)s]2 - a solution of 44.8 mg of [Nb(OEt)s]2 (0.07 mmol) in 30 ml of pentane to provide 6.5 g of {Nb0_x}o.2-CeO₂. The solid was filtered, dried and calcined, resulting in {Nb0_x}o.4-CeO₂.

3^rd cycle : The material {Nb0_x}o.4-CeO₂ obtained was rehydrated by addition of vapor pressure of water. Afterwards, the material was dehydroxylated under high vacuum (IO^-5 Torr) at 200 °C for 16 h. The resulting material was furthermore impregnated with a pentane solution of [Nb(OEt)s]2 - a solution of 41.4 mg of [Nb(OEt)5]2 (0.07 mmol) in 30 ml of pentane to provide 6 g of {Nb0_x}o.4-CeO₂. The solid was filtered, dried and calcined, resulting in {NbO_x}o.6-CeO₂.

4^th cycle : The material {Nb0_x}o.6-CeO₂ obtained was rehydrated by addition of vapor pressure of water. Afterward, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C for 16 h. The resulting material was furthermore impregnated with a pentane solution of [Nb(OEt)s]2 - a solution of 35.9 mg of [Nb(OEt)₅]2 (0.06 mmol) in 30 ml of pentane to provide 5.2 g of {Nb0_x}o.6-CeO₂. The solid was filtered, dried and calcined, resulting in {NbO_x}o.8’CeO₂.

5^th cycle : The material {Nb0_x}o.8-CeO₂ obtained was rehydrated by addition of vapor pressure of water. Afterwards, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C for 16 h. The resulting material was furthermore impregnated with a pentane solution of [Nb(OEt)s]2 - a solution of 29 mg of [Nb(OEt)s]2 (0.04 mmol) in 30 ml of pentane to 4.2 g of {Nb0_x}o.8-CeO₂. The solid was filtered, dried and calcined, resulting in {NbO_x}i- CeO2.

[0058] The catalyst as well as the intermediates were characterized by: DRIFT, XRD, TEM-EDXS, NMR, UV-Vis and BET. The MAS solid state NMR revealed the presence of the organic fragments (ethoxy ligands) on the materials.

[0059] ¹H MAS NMR (ppm, 500 MHz): 5 4.8 (OCH2CH₃), 1.3 (OCH₂CH3) ¹³C CP MAS NMR (ppm, 200 MHz): 5 68.5 (terminal OCH2CH3), 64.6 (bridging OCH2CH3, or physisorbed ethanol), 18.3 (terminal OCH₂CH₃), 16.5 (bridging OCH2CH3, or physisorbed ethanol).

[0060] The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to Ce-OH reacted selectively with niobium complex [Nb(OEt)₅]2, suggested by the decrease in their intensities after grafting. In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ region respectively are found, in spite of the low amount of the grafted complex (0.2 wt%). After calcination, the DRIFT analyses revealed the complete disappearance of CH group of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to hydroxyl group (Nb-OH, and Ce-OH). The same DRIFT observations and comments apply to the other cycles (2^nd, 3^rd, 4^th and 5^th) (Figure 13). Briefly, the intensities of the signals characteristic of v(OH) decreased after grafting, this phenomenon was accompanied by the appearance of new band characteristic of v(C-H) and b(C-H). The bands ascribed to the organic fragments vanish upon calcination. Finally, the rehydration and dehydroxylation bring the system back to the first step.

[0061] The surface is decreased continuously with the rise of the number of grafting cycles, from 180 for ceria to the surprisingly small value ca. 27 m².g^-1 for the sample resulting from the last (fifth) cycle (Figure 14 B). It is worth noting that these values were replicated and confirmed by the BET equipment. According to the adsorption isotherms of the materials containing 0.8 (after fourth functionalization) and 1 wt% (after 5 successive graftings) of Nb, became non-porous. This might be due the changes in the crystalline structure upon the various thermal treatments (Figure 14 A).

[0062] Elemental analysis performed on the sample revealed the presence of %Nb = 1.00 %wt, perfectly fitted with the expected amount and comparable to the amount of the other materials prepared by one and two grafting cycles. [0063] High-resolution transmission electron microcopy of a selected crystal material after the grafting of 1 wt% of Nb catalyst provided us access to the crystalline structure. Indeed, the crystal planes and their direction are observed. However, some defects on the surface of the crystal and the changes in the planes direction are noticeable, this is due presumably to the presence of another crystalline structure (NbO_x). It seems that this strategy of preparation (several grafting cycles) resulted in the formation of polymeric type structure.

[0064] X-ray diffraction analyses were performed on the materials prepared through one, two and five steps of grafting. The results revealed that the crystalline structure (cubic fluorite) is preserved for all samples. From the diffraction pattern, the mean size of microcrystals could be evaluated, since it is related to diffraction's peak broadening by Scherrer's equation. The average crystal sizes found are summarized in Table 1, and indicated that the ceria particles have the tendency to agglomerate with the repeating cycles and result in the decrease of the surface area.

Table 1 : Average particle size of ceria samples calcined at various temperatures, estimated measured using Scherrer’s equation

a⁾The average size of the particles was calculated using the following equation (Scherrer's equation): where:

T - size of the particles (A)

A - X-Ray wavelength (A).

0 - Bragg angle.

H - full width at half maximum (FWHM) of the measured line.

H' - full width at half maximum (FWHM) of the instrument's response. b^>The surface area is calculated assuming that the particles have a perfect spherical shape, S = 60000/pxd where: p- Specific gravity of ceria (7.215 g.cm^-3) d- Particle diameter (A°).

[0065] A satisfactory understanding of the overall dispersion of the niobium adspecies was provided by UV-Vis-DRS analysis. The UV-vis DRS spectra are highlighted in Figure 17, this shows i) a band at 259 nm attributable to the charge-transfer transitions between oxygen and niobium (IV) in tetrahedral coordination of the monomeric species. It may also overlap with the bands of ceria due to Ce³⁺ O'² and Ce⁴⁺ O'² charge transfers, at 346 nm more presumably due to octahedral Nb (V) monomeric species. The baseline shifts observed for the spectra of the material prepared through 5 grafting steps could be due the presence of a crystalline phase of Nb20s.

[0066] One may first of all note the convergence of the results from XRD and UV-Visible spectra. 2:Svnthesis of Nb/CeO^-ZrO? Cen gZrn catalysts

[0067] A surface organometallic approach was applied to functionalize ceria zirconia with Nb through one, two and five steps.

2A: One grafting cycle fl wt% of N )

[0068] The grafting reaction was performed as follows: A mixture of [Nb(OEt)₅]2 (68.7 mg, 0.11 mmol) and CeO2-ZrO2 -200 (2 g) in pentane (10 mL) was stirred at 25 °C for 2 h.

[0069] After filtration, the obtained yellowish powder [Nb(OEt)₅]2@CeO₂-2oo was dried under vacuum (IO^-5 Torr). Prior to calcination, the material was characterized.

Scheme II: The reaction grafting of [Nb(OEt)₅]₂ on CeO_2.200 CeO₂-ZrO_2.200

[0070] The material recovered during the intermediate steps was characterized by DRIFTS. The spectra highlighted in Figure 18 indicated that the dehydroxylated ceria zirconia (spectrum a) showed the presence of peaks that can be assigned to isolated, bridged and either coordinated to Ce or Zr. After grafting of [Nb(OEt)₅]2 the intensity of the peak attributed to the OH stretching vibration decreased dramatically. New bands in the 3100-2850 cm ¹ range, as well as 1620-1400 cm ¹ appeared, peaks characteristic of aliphatic v(C-H) and b(C-H) vibrations of the chemisorbed ligands on surface (spectrum b). The calcination of the sample (spectrum c) withdrew the organic part of the material.

[0071] The surface area of the materials evaluated using ASAP2020 micrometrics revealed the presence of small hysteresis loop due to the capillary condensation while desorbing the N2, this suggesting the presence of mesopores. Compared to ceria the porosity of the ceria zirconia is lower. The value of the surface area decreased from 97 m².g ¹ measured for neat support to 82 m².g this suggesting that the support structure is preserved after grafting and different thermal treatments.

[0072] Elemental analysis performed on this sample showed as expected the presence of 0.98 wt% of Nb.

[0073] The UV-vis DRS spectrum of the Nb functionalized ceria zirconia showed the presence of the isolated Nb oxo sites in a tetrahedral geometry due to the absorbance observed between 230 and 280 nm. It is to be noted that these absorbance bands may also overlap with the bands of ceria due to Ce³⁺ O'² and Ce⁴⁺ O'² charge transfers.

2B: Two grafting cycles

[0074] The catalyst with the same amount of Nb (1 wt%) was prepared through two successive graftings as follows:

1^st cycle : Grafting of [Nb(OEt)₅]2 on CeC>2 (0.5 wt% of Nb [Nb(OEt)5]2@Ceo.5Zro.s02). A mixture of [Nb(OEt)s]2 (85.6 mg, 0.13 mmol) and 5 g Ceo.5Zro.5O2 in pentane (10 mL) was stirred at 25 °C for 2 h. After filtration, the obtained solid [Nb(OEt)5]2@Ceo.5Zro.s02-2oo was dried under vacuum (10“⁵ Torr).

2^nd cycle : The material [Nb(OEt)5]2@Ceo.5Zro.s02, obtained in the first cycle was calcined using glass reactor under continuous flow of dry air at

500 °C for 16 h. The material recovered{NbO_x}o.5-Ceo.5Zr₀.502 was rehydrated by addition of vapor pressure of water. Afterwards, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C, and the excess as well as the physisorbed water were removed. At this point, a material which is ready to be grafted was obtained, and thus it was impregnated again with a pentane solution of [Nb(OEt)₅]2 - a solution of 68.2 mg of [Nb(OEt)₅]2 (0.11 mmol) in 20 ml of pentane to provide 4 g of {NbOxj-o.5-Ceo.5Zro.5O2. The solid was filtered, dried and calcined, resulting in {NbOxj-1-Ceo.5Zro.5O2.

[0075] The catalyst as well as the intermediates were characterized by DRIFTS (Figure 21). The spectra highlighted in Figure 21 indicated that the dehydroxylated ceria zirconia (spectrum a) showed the presence of peaks that can be assigned to isolated, bridged and either coordinated to Ce or Zr. After grafting of [Nb(OEt)₅]2 the intensities of the peaks attributed to the OH stretching decreased and they were not totally consumed (spectrum a). New bands in the 3100-2850 cm ¹ range, as well as 1620-1400 cm ¹ appeared. These peaks are characteristic of aliphatic v(C-H) and b(C-H) vibrations of ethoxy ligands (spectrum b). The calcination of the sample (spectrum c) withdrew the organic part from the material. The rehydration and di hydroxylation of the samples led to the regeneration of the M-OH (M can be Ce, Zr or Nb). The same observation as before can be found. Moreover, the M-OH appeared to react with the [Nb(OEt)₅]2 complex in a second batch (spectrum e).

[0076] The surface area of the resulted material was measured, and the results are shown in Figure 22. The surface area decreased from 97 m².g^-1 measured for neat support (Ceo.5Zro.5O2) to 58 m².g ¹ after grafting the complex through two steps. It seems that the structure of the support is modified, and the changes are not only the result of the functionalization but most likely due the thermal treatments.

[0077] The UV absorbance spectra of the ceria zirconia, the materials containing 0.5 wt% and 1 wt% of Nb are depicted in Figure 23. A band at 259 nm attributable to the charge-transfer transitions between oxygen and niobium can be observed. However, this overlaps with the bands of ceria due to Ce³⁺ O'² and Ce⁴⁺ O'² charge transfers.

2C: Five grafting cycles

[0078] The catalyst with the same amount of Nb (1 wt%) on ceria zirconia was prepared through five successive graftings as follows:

1^st cycle : Grafting of [Nb(OEt)s] on Ceo.5Zro.5O2 (0.2 wt% of Nb [Nb(OEt)₅]2@Ceo.5Zr₀.502). A mixture of [Nb(OEt)₅]2 (55 mg, 0.08 mmol) and 8 g CeO₂-2oo in pentane (40 mL) was stirred at 25 °C for 2 h. After filtration, the obtained solid [Nb(OEt)₅]2@ Ceo.5Zr₀.502-2oo was dried under vacuum (10“⁵ Torr).

2^nd cycle : The material [Nb(OEt)₅]2@Ceo.5Zr₀.502-2oo, obtained in the first cycle was calcined using glass reactor under continuous flow of dry air at 500 °C for 16 h, yielding {NbO_x}o.2-Ceo.5Zr₀.502-2oo- Afterward, this was rehydrated by addition of vapor pressure of water and was dehydroxylated under high vacuum (10 ^s Torr) at 200 °C, the excess as well as the physisorbed water were removed. The resulting material was impregnated again with a pentane solution of [Nb(OEt)₅]2 - a solution of 44.8 mg of [Nb(OEt)₅]2 (0.07 mmol) in 30 ml of pentane to provide 6.5 g of {NbO_x}o.2-Ceo.5Zr₀.502-2oo- The solid was filtered, dried and calcined, resulting in {NbO_x}o.4-Ceo.5Zro.502-2oo-

3^rd cycle : The material {NbO_x}o.4-Ceo.5Zr₀.502-2oo obtained was rehydrated by addition of vapor pressure of water. Afterwards, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C for 16 h. The resulting material was furthermore impregnated with a pentane solution of [Nb(OEt)s]2 - a solution of 41.4 mg of [Nb(OEt)s]2 (0.07 mmol) in 30 ml of pentane to provide 6 g of {NbO_x}o.4-Ceo.5Zro.502-2oo- The solid was filtered, dried and calcined, resulting in {NbO_x}o.6-Ceo.5Zr₀.502-2oo- 4^th cycle : The material {NbO_x}o.6-Ceo.5Zr₀.502-2oo obtained was rehydrated by addition of vapor pressure of water. Afterward, the material was dehydroxylated under high vacuum (IO^-5 Torr) at 200 °C for 16 h. The resulting material was furthermore impregnated with a pentane solution of [Nb(OEt)s]2 - a solution of 35.9 mg of [Nb(OEt)₅]2 (0.06 mmol) in 30 ml of pentane to provide 5.2 g of {NbO_x}o.6-Ceo.5Zro.502-2oo- The solid was filtered, dried and calcined, resulting in {NbO_x}o.8-Ceo.5Zr₀.502-2oo-

5^th cycle : The material {NbO_x}o.8-Ceo.5Zro.502-2oo obtained was rehydrated by addition of vapor pressure of water. Afterward, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C for 16 h. The resulting material was furthermore impregnated with a pentane solution of [Nb(OEt)s]2 - a solution of 29 mg of [Nb(OEt)₅]2 (0.04 mmol) in 30 ml of pentane to provide 4.2 g of {NbO_x}o.8-Ceo.5Zro.502-2oo- The solid was filtered, dried and calcined, resulting in {NbO_x}i-Ceo.5Zr_0.502-2oo-

[0079] The materials recovered during each step for the process of five successive graftings (grafting, calcination and dehydroxylation) were monitored by means of DRIFT spectroscopy. The resulted spectra are depicted in Figure 24, the spectra are presented in a simplified manner, indeed the spectra of the same processes (grafting calcination and di hydroxylation are combined together.

[0080] The DRIFT analyses spectra (Figure 24A), showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to surface hydroxyl groups (terminal and bridging OH) reacted with niobium complex [Nb(OEt)₅]2. Bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ regions respectively are found in the spectra of the grafted materials (Figure 24A). After calcination, the DRIFT analyses revealed the complete disappearance of CH group of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to hydroxyl group. [0081] Elemental analysis performed on the sample revealed the presence of %Nb = 1.01 %wt, which fitted perfectly with the expected amount and was comparable to the amount found for the other samples.

[0082] The surface area decreased dramatically from 98 to 34 m².g^-1 with the increase of the Nb content and the number of the grafting cycles (Figure 25). As observed for the ceria, ceria zirconia also loses a considerable amount of porosity upon thermal treatment.

[0083] The UV absorbance spectra of the ceria zirconia, the materials containing 0.6, 0.8 and 1 wt% of Nb are depicted in Figure 26. A band at 259 nm attributable to the charge-transfer transitions between oxygen and niobium can be observed. However, this overlaps with the bands of ceria due to Ce³⁺ O'² and Ce⁴⁺ O'² charge transfers. However, there is not any evidence of the presence of crystalline Nb20s.

3) Niobium based catalysts supported on ceria with higher levels of Nb loading

[0084] In practical examples, the maximum Nb loading in NbOx/CeCh prepared by a normal SOMC procedure is about 1.8 (Nb wt.%); by a multiple grafting procedure (2 loops), a catalyst containing 3.5 Nb wt.% could be prepared, resulting in improved NOx activity (cf. Figure 5).

3A) Grafting of rNbfOEt l? on CeCb (TNbfOEt 'b@CeO )

[0085] A mixture of [Nb(OEt)s]2 (1 g, 1-6 mmol) and 6 g CeO2-(200) in toluene (30 mL) was stirred at 25 °C for 12 h. After filtration, the obtained solid [Nb(OEt)s]2@CeO₂-2oo was washed three times with toluene in order to extract the unreacted complex. The resulting yellow powder was dried under vacuum (10“⁵ Torr). ^XH MAS NMR (ppm, 500 MHz): 6 4.8 (OC//₂CH₃), 1.3

NMR (ppm, 200 MHz): 6 68.5 (terminal OCH₂CH₃), 64.6 (bridging OCH₂CH₃), 18.3 (terminal OCH₂CH₃), 16.5 (bridging OCH₂CH₃). Elemental analysis %Nb = 1.82 %wt %C = 1.66°/o_wt C/Nb = 8.5 (th 8). The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm-1) corresponding to Ce-OH reacted selectively with niobium complex [Nb(0Et)s]2- In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ region respectively are found.

3B) Synthesis of < NbO Ti-CeO?

[0086] In order to obtain the Nb active species, the material [Nb(OEt)₅]₂@CeO₂ was calcined using glass reactor under continuous flow of dry air at 500 °C for 16 h. The material recovered prior to catalytic testing was characterized. The DRIFT analyses showed the complete disappearance of CH group of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to hydroxyl group (Nb-OH, and Ce-OH). The surface area of the catalyst indicated a slight decrease of the surface area to 185 m²/g after calcination in comparison to the neat ceria dehydroxylated at 200 °C (220 m²/g). The XRD showed that the fluorite crystalline type structure was preserved upon thermal treatment. The microscopic observation of the samples showed that samples were constituted of a stacking of crystallites of about 10 nm. In addition, this technique highlights that niobium metal was well distributed on ceria surface.

3C) Excess loading of NbO on ceria ■fNbOr>?-CeO? (two successive graftings)

[0087] In addition to create additional anchoring site, the material {NbO_x}i-CeO₂ obtained in (3C) above was rehydrated by addition of vapor pressure of water. The sample was heated at 100 °C for 6 h in the presence of the moisture. Afterward, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C, and the excess as well as the physisorbed water were removed. At this point, a material which is ready to be grafted was obtained, thus it is impregnated again with a toluene solution of [Nb(0Et)s]2- A solution of 1 g of [Nb(0Et)s]2 (1-6 mmol) in 20 ml of toluene was added to 4 g mg of {NbO_x}i-CeO₂. The solid was filtered and washed 3 times with 10 ml toluene and then 3 times with 10 ml pentane to remove the unreacted complex. ^XH MAS NMR (ppm, 500 MHz): 5 4.8 (OC/ CH₃), 1.3 (OCH₂C/ ) ¹³C CP MAS NMR (ppm, 200 MHz): 5 68.5 (terminal OCH₂CH₃), 64.6 (bridging OCH₂CH₃), 18.3 (terminal OCH₂CH₃), 16.5 (bridging OCH₂CH₃). Elemental analysis %Nb = 3.0. The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to Ce-OH reacted selectively with niobium complex [Nb(OEt)s]2- In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ region respectively are found. The yellowish material was calcined using glass reactor under continuous flow of dry air at 500 °C for 16 h. The material recovered prior to catalytic testing was characterized. The DRIFT analyses showed the complete disappearance of CH group of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to hydroxyl group (Nb-OH, and Ce-OH). The surface area of the catalyst indicated a slight decrease of the surface area to 145 m²/g after calcination the material was noted as {NbO_x}₂-CeO₂.

3D) Excess loading of NbO on ceria <NbO_jr' -CeO? (three successive graftings)

[0088] The material {NbO_x}₂-CeO₂ obtained in (3C) above was rehydrated by addition of vapor pressure of water. The sample was heated at 100 °C for 6 h in the presence of the moisture. Afterwards, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C. The obtained material was then impregnated again with a toluene solution of [Nb(OEt)s]₂. A solution of 700 mg of [Nb(0Et)s]2 (1.1 mmol) in 20 ml of toluene was added to 2 g mg of {NbOx -CeC . The solid was filtered and washed 5 times with 5 ml toluene and then 4 times with 5 ml pentane to remove the unreacted complex. ^XH MAS NMR spectrum (ppm, 500 MHz) of the resulting material showed signals at 1.3 ppm, 1.5 ppm respectively attributed to aliphatic O-CH₂C/^ of terminal and bridged ethoxy ligands. Signals were observed at 5 and 8.5 ppm (broad and less intense) ascribed to the O-C/ 2CH3 of terminal and bridged ethoxy ligands (by comparison with the ^XH NMR of the starting precursors in CeDg). Moreover, ¹³C CP MAS NMR data highlighted signals at 19 ppm with shoulder at 18 ppm assigned to methyl group of OCH2CH3, terminal and bridged ethoxy ligands. Peaks at 64 and 68 ppm were assigned to OCH2CH3, terminal and bridged ethoxy fragments correspondingly. As a conclusion, it seems that after grafting the [Nb(OEt)s]2 preserves the dimer structures. Elemental analysis %Nb = 4.3 wt%. The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to Ce-OH reacted selectively with niobium complex [Nb(OEt)₅]2. In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ region respectively are found. The yellowish material was calcined using glass reactor under continuous flow of dry air at 500 °C for 16 h. The material recovered prior to catalytic testing was characterized. The DRIFT analyses showed the complete disappearance of CH group of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to hydroxyl groups (Nb-OH, and Ce-OH). The surface area of the catalyst indicated a slight decrease of the surface area to 145 m²/g after calcination the material was noted as {NbO_x}3-CeO2. 4) Tungsten based catalysts supported on ceria

4A) Grafting of [W=O(OEt)4]2 on CeO₂ ([W=O(OEt)4]2 @CeO₂ )

[0089] A mixture of [W=O(OEt)4]2 (0.625 g, 1 mmol) and 6 g CeO₂.(200) in toluene (30 mL) was stirred at 25 °C for 12 h. After filtration, the obtained solid [W=O(OEt)4]2-CeO2 was washed three times with toluene in order to extract the unreacted complex and then with pentane to remove toluene. The resulting yellow powder was dried under vacuum (IO^-5 Torr).

[0090] ¹H MAS NMR (ppm, 500 MHz): 5 4.8 (OCH₂CH₃), 1.3 (OCH₂CH₃) ¹³C CP MAS NMR (ppm, 200 MHz): 5 68.5 (terminal OCH2CH3), 64.6 (bridging OCH2CH3), 18.3 (terminal OCH₂CH₃), 16.5 (bridging OCH₂CH₃). Elemental analysis %W= 4.1 Wt%, %C = 1.2%_wt, C/W = 4.5 (th 6). The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to Ce-OH reacted selectively with tungsten complex [Nb(0Et)s]2- In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ regions respectively are found.

4B) Synthesis of { WOx}1-CeO2

[0091] In order to obtain the Nb active species, the material [W=O(OEt)₄]2@CeO2 was calcined using glass reactor under continuous flow of dry air at 500 °C for 16 h. The material recovered prior to catalytic testing was characterized. The DRIFT analyses showed the complete disappearance of CH groups of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to a hydroxyl group (W-OH, and Ce-OH). The surface area of the catalyst indicated a decrease of the surface area to 145 m²/g after calcination in comparison to the neat ceria dehydroxylated at 200 °C (220 m²/g). 40 Excess loading of WO, on ceria

(two successive graftings)

[0092] The material {W0_x}i-CeO₂ obtained in (4B) above was rehydrated by addition of vapor pressure of water. The sample was heated at 100 °C for 6 h in the presence of the moisture. Afterward, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C. At this point, a material which is ready to be grafted was obtained, thus it is impregnated again with a toluene solution of [W=O(OEt)4]2- A solution of 417 mg of [W=O(OEt)4]2 (0.55 mmol) in 20 ml of toluene was added to 4 g mg of {W0_x}i-CeO₂. The solid was filtered and washed 3 times with 10 ml toluene and then 3 times with 10 ml pentane to remove the unreacted complex. ^XH MAS NMR (ppm, 500 MHz): 5 4.8 (OC/ CH₃), 1.3 (OCH₂C/^) ¹³C CP MAS NMR (ppm, 200 MHz): 5 68.5 (terminal OCH2CH3), 64.6 (bridging OCH2CH3), 18.3 (terminal OCH2CH3), 16.5 (bridging OCH2CH3). Elemental analysis W = 7.6 wt%. The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to Ce-OH reacted selectively with tungsten oxo complex [W=O(OEt)₄]2- In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ region respectively were found. The yellowish material was calcined using glass reactor under continuous flow of dry air at 500 °C for 16 h. The material recovered prior to catalytic testing was characterized. The DRIFT analyses showed the complete disappearance of CH group of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to hydroxyl groups (W-OH, and Ce-OH). The surface area analysis of the final catalyst indicated a decrease of the surface area to 115 m²/g after calcination the material was noted as {WO_x>2-CeO2. 4D^ Excess loading of WO, on ceria CeO? (three successive graftings^

[0093] The material {WOx -CeCh obtained in (4C) above was rehydrated by addition of vapor pressure of water. The sample was heated at 100 °C for 6 h in the presence of the moisture. Afterward, the material was dehydroxylated under high vacuum (IO ⁵ Torr) at 200 °C. The obtained material is then impregnated again with a toluene solution of [W=O(OEt)4]2- A solution of 167 mg of [W=O(OEt)4]2 (0.22 mmol) in 20 ml of toluene was added to 2 g mg of {WOx ’CeCh. The solid was filtered and washed 5 times with 5 ml toluene and then 4 times with 5 ml pentane to remove the unreacted complex. The ^XH MAS NMR spectrum (ppm, 500 MHz) of the resulting material showed signals at 1.3 ppm, 1.5 ppm respectively attributed to aliphatic O-CH2C/ 3 of terminal and bridged ethoxy ligands. Signals at 5 and 8.5 ppm (broad and less intense) were ascribed to the O-C/ 2CH3 of terminal and bridged ethoxy ligands (by comparison with the ^XH NMR of the starting precursors in CgDg). Moreover, ¹³C CP MAS NMR data highlighted signals at 19 ppm with a shoulder at 18 ppm assigned to the methyl group of OCH2CH3, in terminal and bridged ethoxy ligands. Peaks at 64 and 68 ppm were assigned to OCH2CH3, to terminal and bridged ethoxy fragments correspondingly. As a conclusion, it seems that after grafting the [W=O(OEt)4]2 preserves the dimer structures. Elemental analysis indicated %W = 9.8 wt%. The DRIFT analyses showed that the bands at higher wavenumbers (v(OH) = 3400-3700 cm ¹) corresponding to Ce-OH reacted selectively with tungsten complex [W=O(OEt)₄]2- In addition, bands characteristic of v(C-H) and b(C-H) in the 2850-3050 and 1110-1470 cm ¹ region respectively are found. The yellowish material was calcined using glass reactor under continuous flow of dry air at 500 °C for 16 h. The material recovered prior to catalytic testing was characterized. The DRIFT analyses showed the complete disappearance of CH group of the ethoxy moieties and the appearance of new signals around 3690 cm ¹ attributed to hydroxyl groups (W-OH, and Ce-OH). The surface area of the catalyst measured was ca. 90 m²/g the catalyst was noted as {WO_x}3-CeO2.

Claims

36 Claims

1. Process for preparing a catalyst material, comprising the steps of:

(bl) reacting the support material having surface hydroxyl (OH) groups of step (a) with a precursor compound containing a metal element M, wherein M is from Group 5 (V, Nb, Ta) or Group 6 (Cr, Mo, W) or M is manganese (Mn);

(b2) removing organic parts of the precursor compound grafted in step (bl) by calcination to provide a calcined support material bearing metal element M;

(cl) rehydration of the calcined support material obtained in step (b2);

2. Process according to claim 1, wherein the sequence of steps (bl) > (b2) > (cl) > (c2) is carried out repeatedly so that overall at least process steps (bl) and (b2) are repeated at least 2 times and at most 10 times in total, preferably at least 3 times and at most 5 times.

3. Process according to claim 1 or 2, wherein the metal element M is incorporated in a total amount of at most 15 wt%, with respect to the total mass of the catalyst material after the last (b2) step.

4. Process according to any of claims 1 to 3, wherein the metal element M is incorporated in an amount of at least 0.1 wt% and at most 5.0 wt% at each 37

(bl) step, with respect to the total mass of the catalyst material after the last (b2) step.

5. Process according to any of claims 1 to 4, wherein the precursor compound is:

6. Process according to claim 5, wherein the compound (Pl) is at least one compound selected from the group consisting of: [Nb(OEt)₅]2; [W=O(OEt)₄]2; Nb(OAr)₅ where Ar is the 1,3,5-trimethylphenyl (CH₃)₃C6H2- group; [V(=O)(OEt)₃]₂; [V(=O)(OⁱPr)₃]; and [Ta(OEt)₅]₂.

7. Process according to claim 5, wherein the compound (Pl) is [Nb(OEt)₅]2 or [W=O(OEt)₄]₂.

8. Catalyst material as may be obtained by the process according to any of claims 1 to 7.

9. Use of the catalyst material according to claim 8 as an ammonia selective catalytic reduction (NH₃-SCR) catalyst for nitrogen oxides (NOx) reduction.