H2-scr method with a catalyst comprising palladium / titanium dioxide obtainable by impregnation of a colloidal suspension

Abstract

The invention relates to a method for the catalytic reduction of nitrogen oxides (NOx) in the presence of hydrogen (H2) by bringing a gaseous feedstock comprising nitrogen oxides into contact with a catalyst at a temperature of between 15°C and 600°C, at a HSV of between 10 000 h-1 and 150 000 h-1, the H2 / NOx molar ratio being between 2:1 and 100:1, the catalyst comprising palladium at a content of between 0.01% and 8% by weight of elemental palladium relative to the total weight of the catalyst, and a support comprising titanium dioxide, the catalyst being obtainable by a preparation method comprising a colloidal-route palladium impregnation step.

Description

[0001] CATALYTIC REDUCTION PROCESS OF NITROGEN OXIDES (NOx) IN THE PRESENCE OF HYDROGEN

[0002] TECHNICAL FIELD

[0003] The present invention relates to the field of nitrogen oxide (NOx) emission treatment systems by catalytic reduction in the presence of hydrogen.

[0004] PREVIOUS TECHNIQUE

[0005] Nitrogen oxide (NOx) emissions from combustion are a major concern for society. Increasingly stringent standards are being implemented by government agencies to limit the impact of combustion emissions on the environment and human health. Selective catalytic reduction, known by the acronym "SCR" (for "Selective Catalytic Reduction"), is emerging as an effective technology for eliminating nitrogen oxides in oxygen-rich exhaust gases for both industrial and mobile applications, such as diesel engines, hydrogen engines, and, more generally, lean-burn engines. Selective catalytic reduction is achieved using a reducing agent, typically ammonia, and can therefore be referred to as NH3-SCR.This technology achieves good performance in denitrification (DeNOx) through the use of highly active and selective catalysts. However, the use of ammonia is often questioned due to its hazardous nature and toxicity. Instead, aqueous urea is used as a reducing agent (known as AdBlue® in Europe). Aqueous urea decomposes into ammonia and carbon dioxide upstream and / or at the catalyst, thus reducing NOx. However, when gas temperatures are below 180-200°C, this system cannot effectively treat NOx emissions. Indeed, the decomposition of the aqueous urea solution, which requires temperatures above 180°C, limits the efficiency of NH3-SCR. Furthermore, DeNOx efficiency is generally not maximized before 220-250°C, which represents a limitation for a number of processes.This is why much research has focused on new technologies based on alternative reducing agents, particularly those using hydrogen (H2-SCR). Indeed, the catalysts used in such applications are more efficient at low temperatures, for example between 100°C and 250°C, where gaseous hydrogen can react selectively with NOx on the catalyst surfaces. However, there are known problems with H2-SCR catalysts, such as their rather limited efficiency range for effective NOx conversion and / or the high formation of undesirable byproducts such as nitrous oxide (N2O). In fact, the conversion window in existing H2-SCR processes is quite narrow and generally concentrated at low temperatures (<250°C).However, the operation of current combustion engines (diesel, gasoline, H2) results in significant exhaust temperature variations, and H2-SCR technology alone is not currently sufficient to treat NOx. It must be combined with an NH3-SCR system. To become an effective solution on its own, the operating range would need to be extended to higher temperatures (for example, up to 450-500°C).

[0006] Furthermore, existing H2-SCR processes can generate high levels of N2O, with N2O yields reaching between 20 and 40% under certain conditions. Since N2O is a potent greenhouse gas, heavily regulated in most global automotive markets, its formation by an H2-SCR catalyst must be minimized before the H2-SCR catalyst composition is selected for NOx control.

[0007] Finally, at higher temperatures, gaseous hydrogen reacts preferentially with oxygen, which exists in large quantities in the exhaust gases of most compression-ignition engines.

[0008] One objective of the present invention is to propose a hydrogen denitrification (DeNOx) process in the presence of a specific catalyst, enabling the following effects to be obtained simultaneously:

[0009] - an improvement in conversion efficiency at low temperature, i.e. less than or equal to 200°C, preferably less than 180°C, and even more preferably less than 150°C;

[0010] - an improvement in selectivity by reducing the formation of undesirable secondary compounds, particularly N2O and / or NO2;

[0011] - a greater range of conversion in terms of temperature, including up to temperatures reaching 300°C to 400°C.

[0012] The Applicant has surprisingly discovered that a palladium-based catalyst prepared by a specific process, e.g., colloidal, possibly including platinum, gold, and / or tungsten-based dopants on a titanium dioxide support, fulfills all the aforementioned objectives. Furthermore, the catalyst used in such a process exhibits high hydrothermal stability, enabling its use under the harsh conditions of an automotive exhaust system.

[0013] OBJECTS OF THE INVENTION The present invention relates to a process for the catalytic reduction of nitrogen oxides (NOx) in the presence of hydrogen (H2) by contacting a gaseous feed comprising nitrogen oxides and a catalyst, at a temperature between 15°C and 600°C, at a VVH of between 10,000 h -1 and 150,000 hours -1, the H2 / NOX molar ratio being between 2:1 and 100:1 , said catalyst comprising palladium, at a content of between 0.01 and 8% by weight of element palladium relative to the total weight of the catalyst, and a support comprising titanium dioxide, said catalyst being obtained by a preparation process comprising at least the following steps: a) a colloidal suspension of palladium oxide or palladium hydroxide is prepared in aqueous phase; b) said suspension is impregnated onto the support comprising titanium dioxide; c) the catalyst precursor obtained in step b) is dried at a temperature below 250°C; d) the catalyst precursor obtained in step c) is calcined at a temperature between 250°C and 900°C.

[0014] According to one or more embodiments of the invention, the gaseous charge comprises between 10 ppm and 3000 ppm molar of NOx relative to the total weight of the gaseous charge.

[0015] According to one or more embodiments of the invention, said catalyst further comprises at least one dopant selected from platinum, gold and / or tungsten, taken alone or in mixture.

[0016] According to one or more embodiments of the invention, when said catalyst includes platinum as a dopant, the platinum content is between 0.01 and 10% by weight relative to the total weight of the catalyst.

[0017] According to one or more embodiments of the invention, the molar ratio between palladium and platinum is between 0.1 and 10 mol / mol.

[0018] According to one or more embodiments of the invention, when said catalyst includes gold as a dopant, the gold content is between 0.01 and 10% by weight relative to the total weight of the catalyst.

[0019] According to one or more embodiments of the invention, the molar ratio between palladium and gold is between 0.1 and 10 mol / mol. According to one or more embodiments of the invention, when the dopant is gold, the gold is added before step a) of the preparation process, by dry impregnation, which consists of contacting the support with a solution containing at least one gold precursor.

[0020] According to one or more embodiments of the invention, said catalyst comprises platinum and tungsten as dopants.

[0021] According to one or more embodiments of the invention, the tungsten content is between 0.5 and 30% by weight of tungsten element relative to the total weight of the catalyst.

[0022] According to one or more embodiments of the invention, the specific surface area of ​​the support is between 10 m 2 / g and 300 m 2 / g.

[0023] According to one or more embodiments of the invention, the catalyst is shaped by deposition as a coating on a honeycomb structure or a plate structure, or is shaped as an extrudate containing up to 100% of said catalyst.

[0024] According to one or more embodiments of the invention, the honeycomb structure is formed of parallel channels open at both ends or comprises porous filtering walls for which the adjacent parallel channels are alternately blocked on either side of the channels.

[0025] According to one or more embodiments of the invention, the quantity of catalyst deposited on said structure is between 50 and 240 g / L for filtering structures and between 80 and 320 g / L for structures with open channels.

[0026] LIST OF FIGURES

[0027] Figure 1 represents the conversion of NOx [C] as a function of the reduction temperature [T]. The curves marked by squares, triangles, diamonds, crosses and circles correspond respectively to the tests carried out with catalysts A, B, C, D and E synthesized according to example 1, example 2, example 3, example 4 and example 5.

[0028] Figure 2 shows the concentration of N2O [Conc] as a function of the reduction temperature [T]. The curves marked by squares, triangles, diamonds, crosses, and circles correspond respectively to the tests carried out with catalysts A, B, C, D, and E synthesized according to Example 1, Example 2, Example 3, Example 4, and Example 5. DESCRIPTION OF THE PROCEDURES

[0029] Other features and advantages of the process according to the invention will become apparent from the following description of non-limiting examples of implementations, with reference to the figures attached and described below.

[0030] 1. Definitions

[0031] In the sense of the present invention, the different embodiments presented can be used alone or in combination with each other, without limitation of combination.

[0032] In the context of the present invention, different parameter ranges for a given step, such as pressure ranges and temperature ranges, can be used alone or in combination. For example, in the context of the present invention, a preferred range of pressure values ​​can be combined with a preferred range of temperature values.

[0033] In the following text, the expressions "between ... and ..." and "between ... and ..." are equivalent and mean that the limit values ​​of the interval are included within the described range of values. If this were not the case and the limit values ​​were not included within the described range, this clarification will be provided by the present invention.

[0034] In this description, the term "include" is synonymous with (means the same as) "include" and "contain," and is inclusive or open-ended, not excluding other elements not mentioned. It is understood that the term "include" includes the exclusive and closed term "consist."

[0035] According to the present invention, pressures are absolute pressures, also noted as abs., and are given in absolute MPa (or abs. MPa), unless otherwise indicated.

[0036] In the following text, chemical element groups are given according to the CAS classification (CRC Handbook of Chemistry and Physics, publisher CRC Press, editor-in-chief DR Lide, 81st edition, 2000-2001). For example, group VIII (or VI I IB) according to the CAS classification corresponds to the metals in columns 8, 9, and 10 according to the new IIIPAC classification, and group VIB to the metals in column 6.

[0037] The metal content is measured by X-ray fluorescence.

[0038] The BET specific surface area is measured by nitrogen physisorption. The BET specific surface area is measured by nitrogen physisorption according to ASTM D3663-03 as described in Rouquerol F.; Rouquerol J.; Singh K. "Adsorption by Powders & Porous Soils: Principle, methodology and applications", Academic Press, 1999.

[0039] The total pore volume is measured by mercury porosimetry according to ASTM D4284-92 with a wetting angle of 140°, for example using an Autopore® III model device from Microméritics®.

[0040] In this application, ppm values ​​are molar ppm values ​​(unless otherwise defined).

[0041] Gas Hourly Space Velocity (GHSV) refers to the volumetric flow rate of the gas feed at the reactor inlet in m³ / h divided by the catalyst volume in m³ 3 content in the reactor.

[0042] We hear "nitrogen oxides" or "NOx" referring to nitrogen oxides such as NO and NO2.

[0043] 2. Catalyst

[0044] The catalyst used in the hydrogen denitrification (DeNoX) process (H2- SCR) comprises at least palladium, and possibly at least one dopant, on a support comprising titanium dioxide (TiU2).

[0045] The palladium content is advantageously between 0.01 and 8% by weight of palladium element relative to the total weight of the catalyst, preferably between 0.05 and 8% by weight, and even more preferably between 0.1 and 5% by weight.

[0046] In one embodiment according to the invention, said catalyst comprises at least one dopant selected from platinum, gold and / or tungsten, taken alone or in mixture.

[0047] In one embodiment according to the invention, the catalyst comprises platinum as a dopant. In this embodiment, the platinum content is advantageously between 0.01 and 10% by weight of platinum element relative to the total weight of the catalyst, preferably between 0.05 and 8% by weight, and even more preferably between 0.1 and 5% by weight.

[0048] Advantageously, the molar ratio between palladium and platinum is between 0.1 and 10 mol / mol, preferably between 0.2 and 5 mol / mol, and even more preferably between 0.5 and 3 mol / mol.

[0049] In another embodiment of the invention, the catalyst comprises gold as a dopant. In this embodiment, the gold content is advantageously between 0.01 and 10% by weight of the elemental gold relative to the total weight of the catalyst, preferably between 0.05 and 8% by weight, and even more preferably between 0.1 and 5% by weight. Advantageously, the molar ratio of palladium to gold is between 0.1 and 10 mol / mol, preferably between 0.2 and 5 mol / mol, and even more preferably between 0.5 and 3 mol / mol.

[0050] In one particular embodiment, the catalyst comprises platinum and tungsten as dopants. In this embodiment, the tungsten content is advantageously between 0.5 and 30% by weight relative to the total weight of the catalyst, preferably between 1 and 25% by weight. Advantageously, the molar ratio of palladium to tungsten is between 0.001 and 0.3 mol / mol, preferably between 0.005 and 0.05 mol / mol, and even more preferably between 0.006 and 0.07 mol / mol.

[0051] The specific surface area of ​​the catalyst is generally between 10 m² / g and 300 m² / g, preferably between 10 m² / g and 150 m² / g, more preferably between 30 m² / g and 120 m² / g, and even more preferably between 40 and 80 m². 2 / g.

[0052] The pore volume of the catalyst is generally between 0.2 ml / g and 1.1 ml / g, preferably between 0.3 ml / g and 1 ml / g.

[0053] The catalyst support comprises at least titanium dioxide (TiCh). Preferably, the catalyst support is made of titanium dioxide.

[0054] Preferably, TiCh is in its anatase and rutile forms, with the rutile:anatase mass ratio preferably between 95:5 and 50:50.

[0055] The specific surface area of ​​the substrate is generally between 10 m² / g and 300 m² / g, preferably between 10 m² / g and 150 m² / g, more preferably between 30 m² / g and 120 m² / g, and even more preferably between 40 and 80 m² 2 / g.

[0056] The porosity volume of the support is generally between 0.2 ml / g and 1.1 ml / g, preferably between 0.3 ml / g and 1 ml / g.

[0057] 3. Process for preparing the catalyst by colloidal route

[0058] Palladium is deposited by colloidal method and the dopant(s) by dry impregnation method.

[0059] In particular, the catalyst preparation process includes at least the following steps: a) a colloidal suspension of palladium oxide or palladium hydroxide is prepared in aqueous phase; b) said suspension is impregnated onto the support comprising titanium dioxide; c) the catalyst precursor obtained in step b) is dried at a temperature below 250°C; d) the catalyst precursor obtained in step c) is calcined at a temperature between 250°C and 900°C.

[0060] The various stages of the process according to the invention are explained in detail below. colloidal oxide of

[0061] Colloidal suspension is generally obtained by hydrolysis of the palladium cation in aqueous medium, leading to the formation of suspended palladium oxide or hydroxide particles. Preferably, the colloidal suspension is obtained from an aqueous solution comprising at least one palladium precursor salt and an aqueous solution of alkali or alkaline earth hydroxide.

[0062] The aqueous solution of alkali or alkaline earth hydroxide is generally selected from the group consisting of aqueous solutions of sodium hydroxide and aqueous solutions of magnesium hydroxide. Preferably, the aqueous solution is an aqueous solution of sodium hydroxide.

[0063] Typically, an aqueous solution containing at least one palladium precursor salt [also referred to here as solution (II)] is supplied to a suitable apparatus, followed by an aqueous solution containing at least one alkali or alkaline earth hydroxide [also referred to here as solution (I)]. Alternatively, solutions (I) and (II) can be added to the apparatus simultaneously. Preferably, aqueous solution (II) is added first, followed by aqueous solution (I).

[0064] The precursor salt of palladium is generally selected from the group consisting of palladium chloride, palladium nitrate, and palladium sulfate. Palladium nitrate is the preferred precursor salt.

[0065] The colloidal suspension generally remains in the device for a residence time of between 0.5 and 20 hours.

[0066] The concentrations of solutions (I) and (II) are generally chosen to obtain a pH of the colloidal suspension between 1.0 and 3.5. Thus, the pH of the colloidal suspension can be modified during this residence time by adding quantities of acid or base compatible with the stability of the colloidal suspension.

[0067] In general, the preparation temperature is between 5°C and 40°C and preferably between 15°C and 35°C.

[0068] The palladium concentration is preferably between 5 and 150 millimoles per liter (mmol / L), more preferably between 8 and 80 millimoles per liter. b) Deposition of the colloidal suspension prepared in step a) by impregnation

[0069] The colloidal suspension prepared in step a) is then impregnated onto the support.

[0070] The support may optionally undergo a series of treatments prior to the impregnation step, such as calcination or hydration. The support may also already contain one or more metallic elements before the impregnation of the colloidal suspension. Metallic elements may also be introduced into the colloidal suspension. These metallic elements may be introduced either by conventional techniques or by using the process according to the present invention.

[0071] The colloidal suspension is preferably poured onto the support. Preferably, the volume of the colloidal suspension impregnated onto the support is between 0.9 and 1.1 times the porous volume of the support. This process can be carried out either discontinuously, i.e., the preparation step of the colloidal suspension precedes the impregnation step on the support and the bulk of the colloidal suspension is sent in one go to the impregnation step, or continuously, i.e., the product obtained in step a) is sent continuously after adjusting the residence time of the colloidal suspension in step b).

[0072] For example, a continuous process can be cited where solutions (I) and (II) are poured simultaneously into a container which pours continuously into an area including the support to be impregnated.

[0073] After impregnation, the impregnated substrate is generally cured in a wet state for 0.5 to 40 hours, preferably for 1 to 30 hours, and even more preferably for 1 to 24 hours. Longer curing times are not excluded, but do not necessarily provide any improvement. c) Drying of the catalyst precursor obtained in step b) The catalyst precursor obtained at the end of step b) is dried to remove all or part of the water introduced during impregnation, at a temperature below 250°C, preferably between 70°C and 200°C. The drying time is generally between 0.5 and 20 hours. Longer drying times are not excluded, but do not necessarily provide any improvement.

[0074] Drying is generally carried out under hydrocarbon combustion air, preferably methane, or under heated air containing between 0 and 80 grams of water per kilogram of combustion air, an oxygen content between 5% and 25% by volume, and a carbon dioxide content between 0% and 10% by volume. d) Calcination of the dried catalyst precursor obtained in step c)

[0075] After drying, the catalyst precursor obtained at the end of step c) is calcined under air, preferably combustion air, and more preferably methane combustion air, containing between 40 and 80 grams of water per kg of air, an oxygen content of between 5% and 15% by volume, and a CO2 content of between 4% and 10% by volume. The calcination temperature is generally between 250°C and 900°C, preferably between approximately 300°C and approximately 500°C. The calcination time is generally between 0.5 hours and 5 hours. The volumetric flow rate per hour (VVH) is generally between 150 and 3000 liters of combustion air per hour per liter of catalyst, preferably between 300 and 1500 liters of combustion air per hour per liter of catalyst.

[0076] Addition of doping agent(s)

[0077] According to one variant, the catalyst may contain one or more dopants as a promoting metal, in particular gold, platinum, and / or tungsten. The promoting metal(s) may be introduced onto the support before or during step a), or after steps b), c), or d). The deposition of the dopant(s) onto the support may advantageously be carried out by any method known to those skilled in the art, preferably by dry impregnation.

[0078] Preferably, when the dopant is platinum or tungsten, said dopant is added at the end of step d) of the preparation process, preferably by dry impregnation, which consists of bringing the catalyst obtained at the end of step d) into contact with a solution, containing at least one precursor of platinum, or tungsten, the volume of the solution of which is advantageously between 0.9 and 1.1 times the porous volume of the catalyst precursor to be impregnated.

[0079] When a platinum precursor is introduced into aqueous solution, a mineral or organic platinum precursor is advantageously used. In mineral form, the platinum precursor can be selected from hexachloroplatinic acid, hexahydroxyplatinic acid, platinum dihydroxytetramine, or platinum diaminonitrite. Preferably, the precursor salt of platinum is hexachloroplatinic acid. In organic form, the platinum precursor can be selected from organometallic complexes such as platinum bisacetylacetonate.

[0080] The tungsten precursor that can be used is well known to those skilled in the art. For example, tungsten sources include oxides and hydroxides, tungstic acids and their salts, particularly ammonium salts such as ammonium tungstate and ammonium metatungstate, phosphotungstic acid and its salts, and possibly silicotungstic acid (H4SiWi2O4O) and its salts. Tungsten sources can also be heteropolycompounds of the Keggin, lacunar Keggin, substituted Keggin, and Dawson types, for example. Ammonium oxides and salts such as ammonium metatungstate or heteropolyanions of the Keggin, lacunar Keggin, or substituted Keggin type are preferred.

[0081] In one embodiment of the invention, the catalyst comprises palladium, platinum, and tungsten. In this embodiment, the platinum precursor and the tungsten precursor are preferably supplied in two separate impregnation stages, preferably two dry impregnation stages. More preferably, the tungsten precursor is supplied before the palladium impregnation stage, and the platinum precursor is supplied after the palladium impregnation stage.

[0082] Following the step(s) of impregnation of the platinum and / or tungsten precursor(s), an optional maturation step, a drying step, and possibly a calcination step are carried out under the same operating conditions as steps b), c) and d) mentioned above.

[0083] In the embodiment in which the dopant is gold, the gold precursor is preferably added before step a) of the preparation process, preferably by dry impregnation, which consists of bringing the support into contact with a solution, containing at least one gold precursor, the volume of the solution of which is advantageously between 0.25 and 1.5 times the porous volume of the support to be impregnated.

[0084] The gold precursor used has an oxidation state greater than 0 and is soluble in aqueous solution. The salt of the gold precursor can be, for example, a halide. It can preferably be chosen from the group consisting of gold chlorides, such as gold trichloride, tetrachloroauric acid, or sodium or potassium tetrachloraurate. Preferably, the precursor used is tetrachloroauric acid.

[0085] Following the gold precursor impregnation step, an optional maturation step, a drying step, and optionally a calcination step are carried out under the same operating conditions as steps b), c), and d) mentioned above. Then, steps a) to d) of the preparation process as described above are carried out.

[0086] 4. Catalyst shaping

[0087] The catalyst used in the denitrification (DeNOx) process according to the invention is advantageously formed by deposition as a coating (or "washcoat") on a honeycomb structure, primarily for mobile applications, or on a plate structure, particularly for stationary industrial applications. The invention can also be formed into extruded or granular products.

[0088] The honeycomb structure is formed of parallel channels open at both ends (flow-through) or has porous filter walls, in which case adjacent parallel channels are alternately blocked on either side to force the gas flow through the wall (wall-flow monolith). This coated honeycomb structure constitutes a catalytic block. The structure can be composed of cordierite, silicon carbide (SiC), aluminum titanate (AlTi), alpha alumina, mullite, or any other material with a porosity between 30 and 70%. The structure can be made of sheet metal, stainless steel containing chromium and aluminum, or FeCrAl steel.

[0089] The amount of catalyst deposited on said structure is between 50 and 240 g / L for filter structures and between 50 and 320 g / L for structures with open channels.

[0090] The coating itself (the "washcoat") comprises the catalyst, advantageously combined with a binder such as cerine, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a cerine-zirconia mixed oxide, tungsten oxide, or spinel. This coating is advantageously applied to the structure by a washcoating method, which consists of dipping the monolith into a slurry of catalyst powder according to the invention in a solvent, preferably water, and potentially binders, metal oxides, stabilizers, or other promoters. This dipping step can be repeated until the desired amount of coating is achieved. In some cases, the slurry can also be sprayed into the monolith. Once the coating has been applied, the monolith is calcined at a temperature of 300 to 600°C, usually for 1 to 10 hours.

[0091] The said structure can be coated with one or more coatings. The coating including the catalyst is advantageously associated with, i.e. covers or is covered by, another coating having adsorption or reduction capacities of pollutants in particular NOx and / or promoting the oxidation of pollutants, in particular carbon monoxide (CO) and hydrocarbons (HC).

[0092] Another possibility is to provide the catalyst in the form of an extrudate, a bead, or any other shape known to those skilled in the art. In this case, the resulting structure can contain up to 100% catalyst.

[0093] The catalyst support used in the process according to the invention can advantageously be shaped by any technique known to those skilled in the art. Shaping can advantageously be carried out, for example, by extrusion, pelletizing, the oil-drop coagulation method, rotary plate granulation, or any other method well known to those skilled in the art. The supports thus obtained can be in various shapes and sizes. Advantageously, the various constituents of the support or catalyst can be shaped by mixing to form a paste and then extruding the resulting paste, or by mixing powders and then pelletizing, or by any other known process for agglomerating a powder containing alumina. The supports thus obtained can be in various shapes and sizes. Preferably, shaping is carried out by mixing and extrusion.

[0094] The catalyst supports according to the invention are generally in the form of cylindrical or multilobed extrudates, such as bilobed, trilobed, or multilobed, with a straight or twisted shape, but may also be manufactured and used in the form of crushed powders, tablets, rings, beads, and / or wheels. Preferably, the catalyst supports according to the invention are in the form of spheres or extrudates. Advantageously, the support is in the form of extrudates with a diameter between 0.5 and 8 mm, and more particularly between 0.7 and 3 mm. The shapes may be cylindrical (which may or may not be hollow) and / or twisted cylindrical and / or multilobed (2, 3, 4, or 5 lobes, for example) and / or rings. The multilobed shape is advantageously preferred.

[0095] The structure coated with the catalyst or catalyst support is advantageously integrated into the exhaust line of an industrial process or internal combustion engine. An oxidation catalyst, whose function is to oxidize volatile organic compounds (VOCs), and a filter to remove particulate matter from the exhaust gases can be placed either upstream or downstream of the structure. In some cases, to treat the NH3 produced by the catalyst, the catalyst can be combined with a NOx treatment catalyst of the NH3-SCR type or with an NH3 removal catalyst (Ammonia Slip Catalyst, according to Anglo-Saxon terminology), or even both.

[0096] 5. H2-SCR Process

[0097] The catalytic reduction process of nitrogen oxides (NOx), using the catalyst as mentioned above, includes a step of contacting the catalyst with nitrogen oxides (NOx), preferably at a temperature between 15°C and 600°C, preferably between 30°C and 500°C, and more preferably between 40°C and 400°C. The WH (GHSV) involved is preferably between 10,000 h' 1 and 150,000 h' 1 , preferably between 20,000 and 80,000 h' 1 , the H2 / NOx molar ratio being between 2:1 and 100:1, preferably between 5:1 and 40:1.

[0098] According to one or more embodiments, the gaseous load to be treated comprises between 10 ppm and 3000 ppm molar of NOx relative to the total number of moles of the gaseous load, preferably between 50 and 800 ppm, and even more preferably between 70 and 300 ppm.

[0099] The said gaseous charge may further comprise between 2% and 12% molar oxygen (O2), between 0 ppm and 500 ppm molar CO, between 0 and 30% molar H2O, and a sulfur content in the form of SO2 less than 200 ppm, preferably less than 150 ppm molar, relative to the total number of moles of said gaseous charge.

[0100] The amount of catalyst is adjusted by a person skilled in the art according to the amount of NOx present in the gas to be treated.

[0101] The process is advantageously implemented in the presence of the catalyst shaped as described above.

[0102] EXAMPLES

[0103] The invention is illustrated by the following examples, which are in no way limiting. The specific surface area of ​​the titanium dioxide support (Aldrich™ P25) is 55 m². 2 / g and the total pore volume is 0.75 ml / g.

[0104] Example 1: Conventional dry impregnation 1% wt Pd [non-compliant]

[0105] The Pd / TiO2 catalyst was prepared by dry impregnation of a titanium dioxide (Aldrich™ P25) support in powder form with an aqueous solution of palladium nitrate Pd(NOs)2. The resulting powder was then dried at 100°C for 12 hours and subsequently calcined in air at 500°C with a ramp rate of 5°C / min under 1 Lh / g of air for 2 hours. This yielded catalyst A. The Pd content, analyzed by X-ray fluorescence, was 1 wt% relative to the total weight of the support.

[0106] Example 2: Impregnation with 1 wt% Pd colloidal solution [conformal]

[0107] A colloidal suspension of palladium oxide is prepared under stirring at 25°C by diluting 2.84 g of a palladium nitrate solution Pd(NOa)2 containing 8.5% wt of palladium with about 45 m of demineralized water, then adding about 10 ml of a sodium hydroxide solution to reach a pH of 2.4. This solution is then impregnated onto the support of titanium dioxide (Aldrich™ P25) in powder form.

[0108] The resulting catalyst precursor is then dried in air at 100°C for 12 hours, and subsequently calcined for 2 hours at 500°C with a ramp rate of 5°C / min under 1 Lh / g of air. This yields catalyst B. The Pd content, analyzed by X-ray fluorescence, is 1 wt% relative to the total weight of the support.

[0109] Example 3: Conventional dry impregnation of 1 wt% Pd then 1 wt% Pt [non-compliant] The Pd / TiO2 catalyst was prepared by dry impregnation of a titanium dioxide support (Aldrich™ P25) in powder form with an aqueous solution of palladium nitrate Pd(NOa)2. The resulting catalyst precursor was then dried at 100°C for 12 hours and subsequently calcined in air at 500°C with a ramp of 5°C / min under 1 Lh / g of air for 2 hours. A C1 solid was obtained. The Pd content, analyzed by X-ray fluorescence, was 1 wt% relative to the total weight of the support.

[0110] The solid C1 prepared above is then dry-impregnated with a solution containing the precursor of Pt (H2PtCl.6H 2OThe resulting solid is dried in an oven at 100°C for 12 hours and then calcined in a tubular kiln at 450°C for 2 hours with a ramp rate of 5°C / min under 1 Lh / g of air. Catalyst C is obtained. The Pt content, analyzed by X-ray fluorescence, is 1 wt% relative to the total weight of the catalyst. (colloidal solution + 1 wt% Pt)

[0111] A colloidal suspension of Pd oxide is prepared under stirring at 25°C by diluting 2.84 g of a palladium nitrate solution Pd(NOa)2 containing 8.5 wt% palladium with approximately 45 mL of demineralized water, followed by the addition of approximately 10 mL of sodium hydroxide solution to achieve a pH of 2.4. This solution is then impregnated onto a titanium dioxide (Aldrich™ P25) support in powder form. The resulting catalyst precursor is dried in air at 100°C for 12 hours and then calcined for 2 hours at 500°C with a ramp rate of 5°C / min under 1 Lh / g of air. The resulting solid is D1. The Pd content, as analyzed by X-ray fluorescence, is 1 wt% relative to the total weight of the catalyst. The solid D1 prepared above is then dry-impregnated with a solution containing the Pt precursor (H2PtCl.6H2O). The resulting solid is oven-dried at 100°C for 12 hours and then calcined in a tubular kiln at 450°C for 2 hours with a ramp of 5°C / min under 1 Lh / g of air.Catalyst D is obtained. The Pt content analyzed by X-ray fluorescence is 1% by weight relative to the total weight of the catalyst. colloidal solution, then 1% dry

[0112] First, the TiU2 support (Aldrich™ P25) in powder form is dry-impregnated with a solution containing a mixture of the W precursor ((NFUjeHLW^O^)) and oxalic acid (molar ratio of W precursor / oxalic acid = 0.2). The resulting mixture is then statically matured in a saturator (water desiccator) for 1 hour and subsequently air-dried at 100°C for 16 hours. This yields a catalyst precursor E1. The W content, analyzed by X-ray fluorescence, is 10% by weight relative to the total weight of the catalyst.

[0113] Next, a colloidal suspension of Pd oxide is prepared under stirring at 25°C by diluting 2.84 g of a palladium nitrate solution Pd(NOa)2 containing 8.5 wt% palladium with about 45 ml of demineralized water, then adding about 10 ml of a sodium hydroxide solution to reach a pH of 2.4. This solution is then impregnated onto the catalyst precursor E1 prepared above.

[0114] The resulting catalyst precursor is dried in air at 100°C for 12 hours, then calcined for 2 hours at 500°C with a ramp rate of 5°C / min under 1 Lh / g of air. The Pd content, analyzed by X-ray fluorescence, is 1% by weight relative to the total weight of the catalyst. A solid E2 is obtained.

[0115] The solid E2 prepared above is then dry-impregnated with a solution containing the Pt precursor (H2PtCl6.6H2O). The resulting catalyst precursor is oven-dried at 120°C for 16 hours and then calcined in a tube kiln at 450°C for 2 hours with a ramp rate of 5°C / min under 1 Lh / g of air. Catalyst E is obtained. The Pt content, analyzed by X-ray fluorescence, is 1% by weight relative to the total weight of the catalyst.

[0116] For the catalytic tests, 200 mg of catalysts A to E in powder form is placed in a quartz reactor. The reactor is fed with 150 L / h of a gas mixture having the following molar compositions: 400 ppm NO, 4000 ppm H2, 10% O2, 15% H2O qpc N2. (qpc = quantity to compensate).

[0117] A Fourier transform infrared spectroscopy (FTIR) analyzer allows the measurement of the concentration of NO, NO2, NH3, and N2O species at the reactor outlet. NOx conversions are calculated as follows:

[0118] NOX conversion = (NOX input - NOX output) / NOX input

[0119] In these formulas, the input and output indices indicate the content before and after catalytic reduction, respectively.

[0120] The results of NOx conversion are shown in Figure 1. The curves marked by squares, triangles, diamonds, crosses and circles correspond respectively to the tests carried out with catalysts A, B, C, D and E synthesized according to example 1, example 2, example 3, example 4 and example 5.

[0121] It is observed that catalysts B, D, and E synthesized according to the invention offer an optimized ignition temperature and improved low-temperature efficiency (T<170°C) compared to catalyst A synthesized by dry impregnation. Furthermore, the temperature range of effectiveness of catalysts D and E is broader than that of catalysts A and C synthesized according to the prior art.

[0122] N2O emissions are shown in Figure 2. The curves marked by squares, triangles, diamonds, crosses, and circles correspond respectively to the tests carried out with catalysts A, B, C, D, and E synthesized according to Examples 1, 2, 3, 4, and 5. Catalyst B, according to the invention (Pd colloidal impregnation), offers improved selectivity with lower N2O emissions compared to catalyst A, which does not conform to the invention (Pd dry impregnation). Similarly, catalysts D and E, according to the invention, offer improved selectivity with lower N2O emissions compared to catalyst C, which does not conform to the invention. A summary of the catalyst performance is given in Table 1 below:

[0123] Table 1 It appears that catalysts B, D and E according to the invention make it possible to convert NOx over a wide temperature range, in particular for temperatures below 170°C, with very good selectivity.

Claims

DEMANDS 1. Process for the catalytic reduction of nitrogen oxides in the presence of hydrogen by contacting a gaseous feed comprising nitrogen oxides and a catalyst, at a temperature between 15°C and 600°C, at a VVH between 10,000 h' 1 and 150,000 h' 1, the H2 / NOX molar ratio being between 2:1 and 100:1 , said catalyst comprising palladium, at a content of between 0.01 and 8% by weight of element palladium relative to the total weight of the catalyst, and a support comprising titanium dioxide, said catalyst being obtained by a preparation process comprising at least the following steps: a) a colloidal suspension of palladium oxide or palladium hydroxide is prepared in aqueous phase; b) said suspension is impregnated onto the support comprising titanium dioxide; c) the catalyst precursor obtained in step b) is dried at a temperature below 250°C; d) the catalyst precursor obtained in step c) is calcined at a temperature between 250°C and 900°C.

2. A method according to claim 1, wherein the gaseous feed comprises between 10 ppm and 3000 ppm molar of NOx relative to the total weight of the gaseous feed.

3. A process according to any one of claims 1 or 2, wherein said catalyst further comprises at least one dopant selected from platinum, gold and / or tungsten, taken alone or in mixture.

4. A method according to claim 3, wherein when said catalyst comprises platinum as a dopant, the platinum content is between 0.01 and 10% by weight relative to the total weight of the catalyst.

5. A method according to claim 4, wherein the molar ratio between palladium and platinum is between 0.1 and 10 mol / mol.

6. A method according to claim 3, wherein when said catalyst comprises gold as a dopant, the gold content is between 0.01 and 10% by weight relative to the total weight of the catalyst.

7. A method according to claim 6, wherein the molar ratio between palladium and gold is between 0.1 and 10 mol / mol.

8. A process according to any one of claims 3, 6 or 7, wherein where the dopant is gold, the gold is added before step a) of the preparation process, by dry impregnation, which consists of bringing the support into contact with a solution, containing at least one gold precursor.

9. A method according to claim 3, wherein said catalyst comprises platinum and tungsten as dopants.

10. A process according to claim 9, wherein the tungsten content is between 0.5 and 30% by weight of tungsten element relative to the total weight of the catalyst.

11. A method according to any one of claims 1 to 10, wherein the specific surface area of ​​the support is between 10 m² 2 / g and 300 m 2 / g.

12. A method according to any one of the preceding claims, wherein the catalyst is shaped by deposition as a coating on a honeycomb structure or a plate structure, or is shaped as an extrudate containing up to 100% of said catalyst.

13. Method according to claim 12, wherein the honeycomb structure is formed of parallel channels open at both ends or comprises filtering porous walls for which the adjacent parallel channels are alternately blocked on either side of the channels.

14. Method according to claim 13, wherein the quantity of catalyst deposited on said structure is between 50 and 240 g / L for filter structures and between 80 and 320 g / L for structures with open channels.

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