CATALYSTS FOR USE IN AMMONIA OXIDATION PROCESSES

Catalysts for use in ammonia oxidation processes

This invention relates to a method for preparing supported catalysts suitable for use in an ammonia oxidation process and in particular mixed metal cobalt oxide ammonia oxidation catalysts and nitrous oxide abatement catalysts.

Ammonia oxidation is carried out industrially with air to generate nitric oxide, which is used to make nitric acid (the Ostwald Process) or with air and methane to generate hydrogen cyanide (the Andrussow Process). In both processes, the reactant gases are mixed and passed at elevated temperature and pressure through a reaction vessel in which is placed a pack of platinum/rhodium gauzes that catalyse the oxidation reactions. The gauzes are typically circular and are supported on a frame or basket that holds them perpendicular to the flow of gases through the reactor. The catalyst pack may also comprise one or more palladium-rich gauzes, known as "catchment gauzes" that act to capture volatilised platinum. In recent years there has been a desire to thrift the amount of platinum used in the catalyst packs and catalyst arrangements combining both precious metal gauzes and particulate metal oxide catalysts have been tested. WO 99/64352 describes a process wherein a mixture of ammonia and air at an elevated temperature is fed to a catalyst comprising one or more gauzes of at least one precious metal in elemental filamentary form, and the resultant gas mixture passed through a layer of a particulate oxidic cobalt-containing catalyst placed below the gauzes.

In addition, the evolution of nitrous oxide (N₂0) from ammonia oxidation processes has come under scrutiny as a potent greenhouse gas, and nitrous oxide abatement catalysts have also been included in the ammonia oxidation vessels as layers underneath the precious metal ammonia oxidation catalyst.

Preferred ammonia oxidation catalysts are based on mixed metal cobalt oxides. Such catalysts are described in WO 98/28073. The catalyst preparation methods disclosed include combining and calcining metal oxides at high temperature; co-precipitation, drying and calcining of mixed cobalt oxides; impregnation of an oxidic support with soluble cobalt compounds followed by drying and calcining; or by applying a wash coat comprising a mixed metal cobalt oxide catalyst to a gauze, mesh or pad of steel or ceramic wires, or to a monolithic honeycomb or foam, followed by calcining. The latter preparation method can be complex and expensive, and pelleted catalysts prepared from co-precipitated compositions are currently used. However, ammonia oxidation reactions occur very quickly and we have realised that a short path length, obtainable in coated catalysts, may improve catalyst selectivity and offer an effective catalyst with reduced cobalt loading. We have found an alternative method that overcomes the problems of the previous preparative routes.

Accordingly the invention provides a method for preparing a catalyst composition suitable for use in an ammonia oxidation process, comprising the steps of:

(i) spraying a slurry containing a particulate mixed metal cobalt oxide on to the surface of a shaped support in a pan coater to form a coated support material, and

(ii) drying and optionally calcining the coated support material to form the catalyst having the mixed metal cobalt oxide in a surface layer.

The method provides the mixed cobalt oxide in an outer layer on the shaped support. The invention further provides catalysts obtainable by the above method and the use of such catalysts for performing an ammonia oxidation process or for the destruction of nitrous oxide generated by an ammonia oxidation process.

Suitable mixed metal cobalt oxides in which the cobalt is locked-in to a structure and therefore has improved stability in the ammonia oxidation process are cobalt oxide spinels and cobalt oxide perovskites.

Mixed metal cobalt oxide spinels that may be used may be of formula M M²0₄ wherein M is selected from Co, optionally with Cu, Ni, Mg, Zn and Ca, M² is selected from Al, or Cr, and so includes Cu_xCo₃._x0₄ (where x = 0-1 ), Co_X'Mg-i__X'AI₂0₄ (where x' = 0-1 ), Co₃._x-Fe_x-0₄ or Co₃. χ-ΑΙχ-0₄ (where x" = 0-2). The slurry preferably comprises 0.1-10 mol% Co₃._xM_x0₄, where M is Fe or Al and x=0-2, on a cerium oxide support.

Mixed metal cobalt oxide perovskites that may be used may be of formula AB0₃ wherein A may be selected from La, Nd, Sm and Pr, B may be selected from Co optionally with Ni, Cr, Mn, Cu, Fe and Y. Partial substitution of the A-site (e.g. up to 20mol%) may be performed with divalent or tetravalent cations e.g. Sr²⁺ or Ce⁴⁺. In addition, if desired, partial substitution of one B-site element (e.g. up to 50mol%) with another may be performed. Suitable perovskite materials include LaCo0₃, La-|._xSr_xCo0₃, La-|._xCe_xCo0₃ (where x≤0.2) and LaCu_yCo-|._y0₃ (where y≤0.5).

In a preferred embodiment, the mixed metal cobalt oxide is effective both as a nitrous oxide decomposition catalyst and for the oxidation of ammonia. Hence a particularly preferred mixed metal cobalt oxide is a particulate composition containing oxides of cobalt and other metals, particularly rare earths, for example as described in EP-B-0946290. The preferred catalyst comprises oxides of (a) at least one element Vv selected from cerium and praseodymium and at least one element Vn selected from non-variable valency rare earths and yttrium, and (b) cobalt, said cobalt and elements Vv and Vn being in such proportions that the (element Vv plus element Vn) to cobalt atomic ratio is in the range 0.8 to 1.2, at least some of said oxides being present as a mixed oxide phase with less than 30% of the cobalt (by atoms) being present as free cobalt oxides. Preferably less than 25% (by atoms) of the cobalt is present as free cobalt oxides, and in particular it is preferred that less than 15% (by atoms) of the cobalt is present as the cobalt monoxide, CoO. The proportion of the various phases may be determined by X-ray diffraction (XRD) or by thermogravimetric analysis (TGA) making use, in the latter case, of the weight loss associated with the characteristic thermal decomposition of Co₃0₄ which occurs at approximately 930°C in air. Preferably less than 10%, particularly less than 5%, by weight of the composition is free cobalto-cobaltic oxide and less than 2% by weight is free cobalt monoxide. Thus there may be a perovskite phase, e.g. VnCo0₃ or VvCo0₃, mixed with other phases such as Vv₂0₃, Vn₂0₃, (Vv_xVn-|._x)₂0₃ or VvxVn-|._x0₂. A particularly preferred mixed metal cobalt oxide is a La-|._xCe_xCo0₃ material (where x < 0.2) such as La₀.₈Ce₀.₂CoO₃. Such materials may be prepared according to examples of EP-B-0946290 herein incorporated by reference.

The particulate mixed metal cobalt oxide particles preferably have a d50 average particle size in the range 1-80μΐη, preferably 1-50μηη, more preferably 1 to 30μηη, especially 1 to 10μΐη.

To form the slurry, the particulate mixed metal cobalt oxide is dispersed in a liquid medium, which is desirably aqueous. The solids content of the slurry may suitably be in the range 10 - 60% wt. The slurry may suitably be formed using milling techniques used in the preparation of catalyst wash-coats. Binder materials such as zirconia, titania, alumina or hydrated alumina sols may be included and other conventional wash-coat preparation techniques may be applied, such as milling and mixing of the dispersion to achieve the desired particle size prior to coating the support. In a preferred embodiment, polyethylene graft co-polymers such as Kollicoat™ are included in the slurry. These materials, which have been used in

pharmaceutical tablet coating, lower the surface tension of the Washcoat, thus enhancing the wetting and droplet formation in the spray, and lend a degree of plasticity to the coating enabling it to withstand the attrition that occurs in the coating process itself.

The shaped support is in the form of shaped units such as extrudates, pellets or granules, which are typically prepared from powdered support material and which may also comprise lubricants or binders. Extrudates and pellets are preferred shaped supports. The extrudates, pellets or granules may be commercially available or a re readily prepared from suitable powders using methods known to those skilled in the art.

Shaped units may have a variety of shapes and particle sizes, depending upon the mould or die used in their manufacture. For example the units may be in the form of spheres, cylinders, rings, or multi-holed units, which may be multi-lobed or fluted, e.g. of cloverleaf cross-section. The extrudates or pellets may be spheres or cylindrical, i.e. circular in cross-section, but are preferably lobed or fluted to increase their geometric surface area without increasing the pressure drop through a layer formed from the units. This has the advantage that the pressure drop through a bed of the catalyst is minimised.

The shaped units desirably have a smallest unit dimension preferably in the range 1 mm to 50 mm. The smallest dimension may be width, e.g. diameter or length, e.g. height. The shaped units may have a length from 1 mm to 50 mm, preferably 1.2mm to 25 mm. The cross sectional width or diameter of the shaped units may be from 1 mm to about 25 mm, preferably from 1.2 mm to 10 mm, particularly from 1.2 mm to 5 mm. The aspect ratio, i.e. the largest dimension divided by the smallest dimension e.g. length/cross-section, is preferably less than 10. The use of shaped units with these dimensions is advantageous for catalyst recovery and recycle.

The shaped oxidic support may be a refractory oxide comprising one or more of alumina, silica, alumino-silicate, titania, zirconia, magnesia, ceria or lanthana, including layered structures in which the shaped support comprises two or more support oxides in a layered arrangement. Preferably, the oxidic support comprises a high-temperature-stable oxide such as a metal aluminate cement or alpha-alumina.

If desired, the porosity or other properties of the shaped units such as the BET surface area, attrition resistance or crush strength may be altered by a physical or chemical treatment. For example the shaped units may be calcined to temperatures >900°C or treated with organic or inorganic compounds. In particular we have found that the attrition resistance of the coated catalysts is significantly enhanced when the shaped support is first coated with a thin layer of a particulate zirconia using the same pan coating method prior to applying the slurry of mixed metal cobalt oxide. The zirconia layer is preferably present in the dried catalyst at an amount in the range 0.5 - 15% by weight, preferably 2 - 8% by weight. The particulate mixed metal cobalt oxide is present within a layer on the surface of the support. The layer may be applied to particulate shaped units by spraying a slurry containing the particulate mixed metal cobalt oxide onto heated, tumbling shaped support units in a pan coater, which may be of the type used in the pharmaceutical or foodstuff industry for preparing coated tableted products. Such apparatus is commercially available. Multiple sprays may be applied with drying in-between spraying. The slurry is preferably applied to supports at temperatures in the range 30-60°C, preferably 30-50°C. In this way the support is not over- wetted and the possibility of spray-drying the slurry is avoided. The resulting coated material is then dried. The drying step may be performed at 20-150°C, preferably 20-120°C, more preferably 95-1 10°C, in air or under an inert gas such as nitrogen, or in a vacuum oven for a period as required up to 24 hours. The thickness of the layer containing the mixed metal cobalt oxide in the dried material is preferably in the range 5 to 250 μηη (micrometres), but more preferably is in the range 5-150 micrometres, most preferably 10-100 micrometres. Thinner layers make more efficient use of the applied cobalt. The thickness of the cobalt-containing layer may be determined by methods known to those skilled in the art. For example optical microscopy in the present case is useful for measuring the thickness of the coloured cobalt compounds in the surface of the white oxidic supports. Alternatively electron microprobe analysis may be used for determining the thickness of cobalt-containing layers.

Loadings of the mixed metal cobalt oxide are preferably in the range 1-15% by weight, more preferably 2 to 10% by weight on the dried catalyst.

Preferably, the dried catalyst precursors are calcined, i.e. heated at temperatures above 250°C, for example 250-1000°C for 0.5 to 5 hours to produce a catalyst that will be stable at the operating conditions in the ammonia oxidation process. However in the method of the present invention calcination is not essential to provide active catalysts.

The dried or calcined particulate catalyst may be provided to the ammonia oxidation vessel. This may be as one or more fixed beds placed in the conventional catalyst support through which the reacting gases pass axially. Alternatively the particulate mixed metal cobalt oxide catalyst may be supported beneath the conventional Pt-based precious metal and/or Pd-based catchment gauzes. The thickness of such particulate beds is typically < 500mm, preferably < 300mm, more preferably < 100mm.

Alternatively, the particulate catalyst may be provided to the ammonia oxidation vessel in a radial-flow catalyst support structure, such as that described in our application

WO 2010/046675. In such arrangements, for a low-pressure ammonia oxidation process, the catalyst assembly may comprise 1 or 2 precious metal ammonia oxidation catalyst gauzes followed by one or more gauzes of palladium catchment, followed by a radial flow bed of shaped units of an oxidic cobalt-containing nitrous oxide decomposition catalyst. Likewise in a high-pressure plant, there may be less than 15, e.g. 10, precious metal ammonia oxidation catalyst gauzes followed by the catchment and the bed of the oxidic cobalt-containing nitrous oxide decomposition catalyst. Thus, for example, the catalyst assembly may comprise 10 or fewer gauzes of a platinum or platinum-alloy ammonia oxidation catalyst, one or more gauzes of palladium catchment, a radial-flow bed of shaped units of a mixed metal rare-earth cobalt perovskite catalyst. In a preferred embodiment, the catalyst assembly comprises 10 or fewer platinum or platinum alloy ammonia oxidation catalyst gauzes followed by one or more gauzes of palladium catchment comprising <5% wt Rh, followed by a radial flow bed of shaped units of mixed metal rare-earth cobalt perovskite catalyst, prepared according to the present invention.

In the oxidation of ammonia to nitric oxide for the manufacture of nitric acid, the oxidation process may be operated at temperatures of 750-1000°C, particularly 850-950°C, pressures of 1 (low pressure) to 15 (high pressure) bar abs., with ammonia in air concentrations of 7-13%, often about 10%, by volume. In the Andrussow Process, i.e. the oxidation of ammonia with air in the presence of methane for the manufacture of hydrogen cyanide, the operating conditions are similar.

Under operating conditions described heretofore it has been usual practice to fully oxidise the ammonia passing through precious metal catalyst gauzes and then if desired pass the resultant nitrogen oxides over a bed of nitrous oxide decomposition catalyst. Apart from the reduction in process efficiency, to operate otherwise could expose the operator to the highly undesirable risk of passing ammonia (i.e. "ammonia slip") to the nitric oxide absorber where explosive ammonium nitrate may form. By incorporating a nitrous oxide decomposition catalyst that is also an effective ammonia oxidation catalyst into the catalyst assembly, it is possible to permit a controlled portion of the ammonia fed to the precious metal catalyst to pass through it. This may enable a reduction in the amount of precious metal catalyst required or possibly enable a higher ammonia flowrate to be used. In addition, conventional precious metal gauze catalysts, as have been referred to earlier, lose platinum in use, and eventually this is sufficient to cause a loss of conversion and an increased risk of ammonia slip. The use of combined precious metal and mixed metal cobalt oxide catalysts, under preferred conditions, may allow increased catalyst life or "campaign length" before shutdown to replace precious metal catalyst, because the preferred nitrous oxide decomposition catalyst is effective to catalyse the oxidation of ammonia. Such increased campaign lengths are of great significance to plant operators and are highly desirable.

The process using the catalysts made by the method described herein may provide high conversion of the ammonia with aggregate N₂0 levels below 1600 ppm, preferably below 600 ppm, more preferably below 500 ppm and most preferably below 200 ppm, e.g. 50- 200ppm, when a particulate nitrous oxide abatement catalyst is provided.

The invention will now be further described by reference to the following examples.

Pellet coating was carried out using a Profile Automation Pilot XT bench top side vented pan coater or a Capco Conical Lab Mixer. The coating was applied with a spray gun fed by a peristaltic pump (Watson Marlow 101 U/R) that supplies the washcoat to the pan coater through silcone tubing (8 mm od, 5 mm id).

Slurries were pre-milled using an Eiger Torrence minimotor mill 250 using 1 mm YSZ beads as the milling media. The particle size distribution was measured using a Malvern Mastersizer laser diffraction particle size analyser.

Slurry pH was measure using a Jenway 370 pH meter and the slurry solids content was measured using a Sartorius MA45 solids content balance

Attrition testing was carried out in a 135 mm diameter rotating steel pan fitted with four 20 mm steel baffles. Samples (100 g) were tumbled at 26 rpm for 15 minutes and the weight loss recorded. This gave an indication adhesion based on total weight loss but samples were also assessed by ICP analysis pre- and post attrition to assess actual coating loss as a percentage.

Example 1 : Preparation of Catalyst 1

a) Slurry preparation. A Lao.₈Ce_0.2Co0₃ material (200 g) was dispersed in demineralised water (400 g) at 2600 rpm using a Silverson mixer with a high shear head. The initial pH of 9.5 was reduced to 7.3 with the addition of acetic acid and the slurry milled for 8 minutes at 350 rpm. The particle size was measured and the d(50) was 2.1 microns. The solids content was 24.27%. b) Coating. α-ΑΙ₂0₃ cylinders (262 g, 3 mm by 3 mm) were heated to 55 °C in the pan coater. Kollicoat IR (2.36 g) was dissolved in demineralised water (200 ml) and added to the slurry (324 g) prepared in (a). The coating was applied using the pan coating equipment. During the coating the pellets were maintained at between 40 and 50 °C. The product was dried in an oven at 105 °C and calcined at 500 °C for 2 hours. Catalyst 1 was found by ICP elemental analysis to have a La₀.₈Ceo_.2Co0₃ loading of 3.9 %

Example 2: Preparation of Catalysts 2 and 3

a) Slurry preparation. A La₀.₈Ceo_.2Co0₃ material (700 g) was dispersed in demineralised water (1556 g) at 2600 rpm using a Silverson mixer with a high shear head. The mixture was milled for 1 minute at 350 rpm. The particle size was measured and the d(50) was 4.7 microns. The pH was 9.8 and the solids content was 43.36 %. The dispersible boehmite Disperal 10/7 (61.5 g) (Sasol) was added to the milled slurry and mixed using a Silverson mixer for 10 minutes. b) Coating - catalyst 2. α-ΑΙ₂0₃ cylinders (1500 g, 3 mm by 3 mm) were heated to 50 °C in the Pilot XT pan coater. A 3% wt zirconia base coat was then applied to the pellets using NanoUse Zr40 BL colloidal zirconia (37.5 g) (Nissan Chemical, APS 6-8 nm) immediately before the catalyst coating and dried in the pan coater at 50 °C. Kollicoat IR (7.50 g) was dissolved in demineralised water (58 ml) and added to the washcoat (344 g) prepared in (a). The same coating parameters as those employed in the base coating were used for the catalyst coating. The product was dried in the coater in flowing air at 50 °C and calcined at 500 °C for 2 hours. Catalyst 2 was found by ICP elemental analysis to have a Lao.₈Ce₀.₂Co0₃ loading of 5.3%. c) Coating - catalyst 3. α-ΑΙ₂0₃ cylinders (1500 g, 3 mm by 3 mm) were heated to 50 °C in the Pilot XT pan coater. Kollicoat IR (3.75 g) was dissolved in demineralised water (58 ml) and added to the washcoat (344 g) prepared in (a). The product was dried in the coater in flowing air at 50 °C and calcined at 500 °C for 2 hours. Catalyst 3 was found by ICP elemental analysis to have a La₀.₈Ceo.₂Co0₃ loading of 4.3%.

Example 3: Attrition Testing

Catalysts 2 and 3 were subjected to attrition testing as set out above. The results were as follows;

The zirconia base-coat has significantly improved the attrition resistance of the catalyst.

Example 4: Catalyst Selectivity.

Ammonia oxidation testing was carried out in laboratory apparatus using a 5% NH₃ stream at heating rate of 20 °C min^" and a dwell at 800 °C for 30 minutes. Catalysts 1 and 2 were compared to a commercially available ammonia oxidation catalyst under the same conditions.

The results were as follows;

The results show high conversion to NO for all three catalysts with the N₂0 produced under these conditions for Catalysts 1 and 2 lower than the commercially available catalyst, with a significantly lower Co content.