WO2004096703A2

WO2004096703A2 - Ammonia oxidation process

Info

Publication number: WO2004096703A2
Application number: PCT/GB2004/001785
Authority: WO
Inventors: Sean Alexander Axon; Duncan Roy Coupland; James Richard Foy; John Ridland; Ian Carmichael Wishart
Original assignee: Johnson Matthey Plc
Priority date: 2003-04-29
Filing date: 2004-04-27
Publication date: 2004-11-11
Also published as: CN1780789A; EP1633677A2; NO20054967L; KR20060017503A; WO2004096703A3; TW200502162A; TWI346087B; GB0315643D0; ZA200508788B; CO5660283A2; JP2006525216A; TW200513435A; BRPI0409944A; NO20054967D0; IL171472A; ZA200508783B; WO2004096703A8; CN100363252C

Abstract

A process for the oxidation of ammonia, including the Andrussow process is described, wherein a mixture comprising air and ammonia is contacted with a precious metal ammonia oxidation catalyst and the resultant gas mixture contacted with a catalyst effective for the decomposition of nitrous oxide, characterised in that prior to contacting the gas mixture with the catalyst effective for the decomposition of nitrous oxide, the gas mixture is contacted with a guard material. The guard material protects the nitrous oxide decomposition catalyst from deposition of precious metal and/or catalyst poisoning. In a preferred embodiment the catalyst effective for the decomposition of nitrous oxide also functions as an ammonia oxidation catalyst.

Description

Ammonia oxidation process

This invention relates to an ammonia oxidation process and in particular an ammonia oxidation process employing a precious metal catalyst.

Ammonia oxidation is widely employed in the manufacture of nitric acid (the Ostwald process) and hydrogen cyanide (the Andrussow process). In the manufacture of nitric acid, ammonia is oxidised with air to nitric oxide, while in the manufacture of hydrogen cyanide a mixture of ammonia and methane (often as natural gas) is oxidised with air. Both are typically performed by contacting the gasses with a precious metal catalyst often in the form of a gauze prepared from platinum or a platinum-alloy. In both processes, the gas mixture is passed at an elevated temperature (e.g. 800 to 1000°C) over a catalyst to effect the oxidation.

However the oxidation of ammonia using platinum or platinum-alloy catalysts gives rise to nitrous oxide (N₂0) and nitrogen, as well as nitric oxide. The formation of nitrogen and in particular nitrous oxide represents undesirable side reactions. Whereas discharge of any nitrogen produced into the atmosphere is acceptable, discharge of nitrous oxide is becoming environmentally unacceptable due to its potency as a so-called 'greenhouse gas'. Thus it increasingly necessary to reduce the nitrous oxide present in the exhaust gas produced after absorption of the nitric oxide in an aqueous medium to give nitric acid, before discharge to the atmosphere. This may be achieved by contacting the exhaust gas with a nitrous oxide decomposition catalyst. However such exhaust gas treatment requires costly ancillary equipment. Preferably, a catalyst combination is provided in the reactor at the ammonia oxidation stage to decompose the nitrous oxide by conversion of the nitrous oxide to either (a) nitrogen by catalytic reduction or (b) nitric oxide by catalytic oxidation according to the following equations;

(a) N₂0 → 2 N₂ + /₂ 0₂

(b) 2 N₂0 + 0₂ → 4 NO.

It has been proposed in WO 99/07638 to oxidise ammonia by combusting ammonia with air in the presence of a platinum gauze catalyst and passing the resultant gasses over a bed of nitrous oxide decomposition catalyst comprising a ceramic doped with specific metals or metal oxides in an ammonia oxidation reactor. Similarly, WO 00/13789 describes a process whereby a metal oxide selected from those of La, Cr, Mn, Fe, Co, Ni and Cu was used as a nitrous oxide decomposition catalyst immediately after a platinum gauze in an ammonia oxidation reactor. 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 bed of a particulate oxidic cobalt-containing catalyst. In the examples it appears that while the bulk of the ammonia oxidation was effected by the precious metal (platinum) gauze, the cobalt- containing catalyst also functioned as an ammonia oxidation catalyst to provide, in combination with the precious metal catalyst a lower overall nitrous oxide level than that obtained by the use of precious metal gauze alone.

However, the operation of these processes has not been examined over extended periods of time and therefore the long-term stability of the nitrous oxide decomposition catalysts has not been established. The long term stability of the nitrous oxide decomposition catalyst is important because unlike exhaust gas treatments it will be necessary, when the nitrous oxide decomposition catalyst is located in the ammonia oxidation reactor, to shut down the whole ammonia oxidation process in order to replenish the nitrous oxide decomposition catalyst. Certain metal oxide catalysts effective for nitrous oxide decomposition e.g. the mixed metal oxides described in WO 99/64352, appear to be more sensitive to poisoning by sulphur compounds present in the gasses, e.g. the air, fed to the ammonia oxidation process than the platinum or platinum-alloy catalysts. As a result of the poisoning, the metal oxide catalysts can lose their effectiveness and therefore require replacement. Consequently, there is a need to provide combined ammonia oxidation / nitrous oxide decomposition catalyst systems which are resistant to poisoning. Preferably the lifetime of the nitrous oxide decomposition catalyst should be at least the same as the platinum or platinum-alloy ammonia oxidation catalyst. Replacement of platinum or platinum-alloy catalysts is necessary because the platinum or platinum-alloy is slowly vaporised by the reacting gasses. The vaporised metal then deposits downstream in the process. Accordingly in a combined catalyst system wherein the nitrous oxide decomposition catalyst is downstream of the platinum or platinum alloy catalyst, the nitrous oxide decomposition catalyst will slowly be coated in a layer of platinum and alloy metals. This coating is undesirable as it may reduce the effectiveness of the nitrous oxide decomposition catalyst and represents a problem in precious metal recovery.

We propose that the effectiveness of the nitrous oxide decomposition catalyst in a combined catalyst system used in an ammonia oxidation process may be maintained over extended periods by providing a guard material with the precious metal catalyst and the nitrous oxide decomposition catalyst.

Accordingly the invention provides a process for the oxidation of ammonia, including the Andrussow process, wherein a mixture comprising air and ammonia is contacted with a precious metal ammonia oxidation catalyst and the resultant gas mixture contacted with a catalyst effective for the decomposition of nitrous oxide characterised in that prior to contacting the gas mixture with the catalyst effective for the decomposition of nitrous oxide, the gas mixture is contacted with a guard material. We also provide a catalyst assembly suitable for ammonia oxidation, including the Andrussow process, comprising a precious metal ammonia oxidation catalyst, a guard material and a nitrous oxide decomposition catalyst and the use of such catalyst assemblies in an extended campaign length ammonia oxidation process, whereby aggregate nitrous oxide production is reduced.

By the term "air" we include air and other oxygen-containing gas mixtures, including oxygen- enriched air having oxygen concentrations above 21 % by volume.

The precious metal ammonia oxidation catalysts may be in the form of gauzes or may be supported on a suitable inert refractory or metal support. The support may take the form of a bead, foam, honeycomb, or fibre. Preferably, the precious metal catalyst comprises one or more gauzes of at least one precious metal in elemental filamentary form.

Precious metal gauzes may be formed by weaving or knitting or otherwise forming precious metal filaments into a gauze-like structure. Such catalyst gauzes are well established and may consist of platinum or platinum alloy filaments of thickness from 0.02 to 0.15 mm woven to provide rectangular interstices, knitted to provide a regular looped structure or simply agglomerated to provide a non-woven irregular structure. Herein the term 'filament ' is meant to include wires that have a substantially circular cross-section and also wires that are flattened or otherwise shaped and thereby have a non-circular cross section. Woven gauzes are well established and typically comprise 0.076 mm diameter wire, woven to provide 1024 apertures per square centimetre and prepared to a specific weight per unit area dependant upon the wire composition. Knitted gauzes offer a number of advantages in terms of catalyst physical properties, catalyst activity and lifetime. Knitted gauzes comprise a regular looped structure and may be formed using wire with diameters in the same range as woven materials, in a variety of shapes and thicknesses using variety of stitches such as tricot, jacquard, satin stitch (smooth sunk loops) and raschel. EP-B-0364153, page 3, line 5 to line 56 describes knitted gauzes of particular use in the present invention. Non-woven gauzes are described for example in GB 2064975 and GB 2096484.

The precious metal ammonia oxidation catalyst is preferably platinum (Pt) or a platinum alloy, such as an alloy of platinum with rhodium (Rh) and/or palladium (Pd) containing ≥85% preferably ≥90% Pt by weight. Alloys used in ammonia oxidation in the production of nitric acid or hydrogen cyanide include 10% Rh 90% Pt, 8% Rh 92% Pt, 5% Pd 5% Rh 90% Pt and 5% Rh 95% Pt. Alloys containing upto about 5% of iridium (Ir) may also be used in the present invention. The precious metal catalyst may desirably be formulated to reduce nitrous oxide by-product formation, and may thus have an increased rhodium (Rh) content, or may contain other components such as cobalt (Co). Generally, where gauzes are employed, they are circular but other shapes e.g. hexagonal, oblong or square may also be used. In a conventional nitric acid plant, the number of gauzes employed depends on the pressure at which the process is operated. For example in a plant operating at low pressure, e.g. up to about 5 bar abs., typically <10, often 3 to 6 gauzes may be employed, while at higher pressures, e.g. up to 20 bar abs., a greater number of gauzes, typically >20, often 35-45, may be employed. The gauzes may be incorporated individually into the reactor or may be pre-formed into a pad comprising a number of gauzes that may be hammer welded at their periphery. The pad may comprise a combination of woven and knitted or possibly non-woven gauzes whose elemental composition may be the same or different. If adjacent woven gauzes are present, to facilitate replacement, they are preferably arranged so that their warps or wefts are at an angle of 45° to each other. Angular displacement, suitably at 90°, may also be used between adjacent woven gauzes to reduce opportunities for gas channelling.

The guard material used in the present invention functions by guarding the nitrous oxide decomposition catalyst and ensuring the overall activity and in particular selectivity of the precious metal and nitrous oxide decomposition catalyst combination is substantially maintained for at least the lifetime of the precious metal catalyst. For example, deposition of precious metal onto the nitrous oxide decomposition catalyst may not have a significant effect of the overall conversion of ammonia to nitrogen oxides (activity) but may ultimately cause an increase in the amount of nitrous oxide in the exhaust gasses towards that generated by the precious metal catalyst alone. Guarding against such deposition is an object of the present invention. Secondly, the guard material may also reduce or prevent the passage of catalyst poisons to the nitrous oxide decomposition catalyst. By trapping catalyst poisons, the guard material should allow the activity as well as selectivity of the catalyst combination to be substantially maintained.

Hence guard materials may comprise catchment gauzes and/or beds comprising catchment materials that reduce or prevent the deposition of precious metal on the nitrous oxide decomposition catalyst. Preferably the guard material comprises one or more catchment gauzes. Catchment gauzes based on palladium are used in ammonia oxidation plants to act as so-called "getters" or collectors of 'vaporised' platinum lost by chemical action, evaporation or mechanical losses from the precious metal catalyst. Such catchment gauzes may be in the form of woven or knitted gauzes or agglomerated non-woven gauzes akin to those described above for the precious metal catalysts. Any palladium present in a gauze will be able to catch vapourised platinum passing over it, hence the palladium content of the catchment gauze may be from 10 to > 95% wt , preferably >40%, more preferably >70%. One or more palladium based catchment gauzes may be used. The catchment gauzes may be provided underneath the precious metal catalyst gauzes individually or form a lower or final gauze as part of a precious metal catalyst pad. The catchment gauzes may be knitted, e.g. according to the aforesaid EP-B-0364153 and may form a layer or layers in a precious metal catalyst knitted structure, e.g. a layer in a knitted pad. Alternatively the palladium-based guard material is woven or knitted into a precious metal ammonia oxidation catalyst gauze by using it as a filament in the weaving or knitting process. Palladium-based guard materials suitable for weaving or knitting into gauze structures are palladium or palladium alloys with nickel (Ni), cobalt (Co) or gold (Au). For example a catchment gauze may be fabricated from a 95:5% wt Pd:Ni alloy, in addition the palladium-based guard material may desirably be formulated to reduce nitrous oxide by-product formation, and may thus preferably contain a small amount, e.g. <5% rhodium (Rh). In particular, palladium gauzes containing amounts of platinum and rhodium may be used. Such gauzes may comprise 8-25% wt, preferably 10-20% wt platinum. Examples of suitable catchment gauze materials include >92% wt palladium, 2-4% wt rhodium and the remainder platinum, or alternatively 82-83% wt palladium, 2.5-3.5% wt rhodium and the remainder platinum.

Ceramic fibres comprising an inert refractory material, such as alumina, zirconia or the like, may also be woven or knitted into catchment gauzes in addition to the palladium-based guard materials.

Catchment beds may be used, alone or in combination with a catchment gauze. Catchment beds may comprise shaped units of inert refractory materials, for example, in the form of pellets, spheres, rings, cylinders, multi-holed pellets and the like. Alternatively the inert refractory material may take the form of a honeycomb or foam. Suitable inert refractory materials are alumina, mullite, cordierite, yttria-stabilised zirconia or magnesia.

Alternatively or additionally the guard material may comprise a poison trap, i.e. the guard material traps catalyst poisons by absorption of, and/or reaction with, the catalyst poison. The catalyst poison may be any that deactivates the nitrous oxide decomposition catalyst. Such poison trap guard materials may of course also function to catch precious metals. Typical catalyst poisons include metals, such as iron, antimony, lead and arsenic, compounds of sulphur and phosphorus and halides. In particular sulphur compounds present in the gasses, particularly the air, fed to the ammonia oxidation process are poisons for many metal oxide nitrous oxide decomposition catalysts, for example Co-based catalysts. It is understood that the sulphur compounds react with metal oxide catalysts by forming sulphates. We have recognised that the ability of the guard material to form a stable sulphate is important in achieving satisfactory protection of the nitrous oxide decomposition catalyst. It is an object of the present invention to provide a poison trap that is effective for sulphur compounds by using in any combination of precious metal catalyst and nitrous oxide decomposition catalyst, a guard material that is capable of forming a stable sulphate, as stable or preferably more stable than any of the components of the nitrous oxide decomposition catalyst under the reaction conditions in the ammonia oxidation reactor. Particularly suitable guard materials are oxides and mixed oxides of at least one of lanthanum (La), gadolinium (Gd), cerium (Ce), yttrium (Y) and ytterbium (Yb). A particularly preferred poison trap guard material is lanthana (La₂0₃).

The poison trap guard material may be provided as a bed of shaped units, for example, pellets, spheres, rings, cylinders, multi-holed extrudates and the like that have maximum and minimum dimensions in the range 1.5 to 20 mm, particularly 3 to 10 mm. The aspect ratio of the shaped units, i.e. the ratio of the maximum to minimum dimensions, is preferably less than 2. The use of a shaped unit poison trap guard material disposed in a bed provides a potentially a useful high surface area for absorption and/or reaction with catalyst poisons. To overcome potential difficulties arising from the resistance to flow created by the shaped units and the consequential pressure drop, they may be disposed in beds that provide a short path for the gas stream. Such beds preferably have thicknesses of 10 to 100 mm, more preferably 10 to 50 mm.

The shaped units may be prepared from or incorporate particles of the poison trap guard material. Preferably such particles have an average (by weight) particle size below 100 μm. Particularly the poison trap guard material particles have an average (by weight) particle size below 50 μm and preferably substantially all the particles have a size below 120 μm. The shaped units may be made by various techniques known to those skilled in the art, such as a "dry" technique wherein a powder composition is compacted to the desired shape, in e.g. a pelleting machine, or a "wet" method wherein a powder composition is mixed with a suitable liquid to form a paste which is then extruded to the desired cross section and the extrudate is cut or broken into units of the requisite length. A granulation method may alternatively be employed wherein a powder composition is mixed with a small amount of liquid, often water, insufficient to give a paste, and the resulting damp mixture granulated or pelletised by means of a pellet mill.

Alternatively, the poison trap guard material may take the form of a honeycomb or foam. The use of a poison trap guard material in the form of a honeycomb or foam may be advantageous compared to a bed of shaped units where such beds cause an undesirable resistance to flow of the gas stream.

The poison trap guard material may also be provided as a coating applied to a suitable inert fibrous, monolithic or particulate support. Suitable supports include ceramic fibres, ceramic or metal honeycombs or foams, and particulate oxides of aluminium (Al), alkaline earth metals e.g. magnesium (Mg) or calcium (Ca), titanium (Ti) and zirconium (Zr). The particulate supports are preferably formed into shaped units as described above. The support may be coated or impregnated with a solution for example an aqueous solution of a suitable metal compound and dried. Alternatively, the support may be coated or impregnated with an aqueous solution of a precursor to the poison trap guard material, e.g. a metal salt such as a nitrate or an acetate, and then transformed, if necessary, to the poison trap guard material by appropriate treatment, for example treatment with a solution of an alkali metal hydroxide or carbonate and/or heating. Alternatively, where the desired poison trap guard material is insoluble in water, the support may be impregnated with a slurry of the poison trap guard material. Alternatively, a supported poison trap guard material may be provided by precipitating the poison trap guard material in the presence of support particles or by co- precipitating poison trap guard material and support, or support precursor, compounds followed by heating as necessary and forming the precipitated compounds into shaped units before or after such a heating step. Preferably the coating comprises between 0.5 and 50%, more preferably between 1 and 25% by weight of the supported poison trap guard material.

Where the poison trap guard material is supported on ceramic fibres, such fibres may be knitted or woven into a gauze and accordingly may form part of a precious metal catalyst pad, which may also incorporate a palladium-based catchment gauze as described above.

Alternatively, the poison trap guard material may be provided as a coating e.g. a 'wash-coat', on the precious metal catalyst. Coatings may be applied to the precious metal gauze or supported precious metal catalyst by precipitation of the poison trap guard material, or a precursor to it, from a suitable solvent, or by treating the precious metal catalyst with a slurry of the poison trap guard material, or a precursor to it. Where a precursor is employed a transformation step, e.g. a heating step, will be required to generate the poison trap guard material. This transformation step may be performed before or after the precious metal catalyst is installed into an ammonia oxidation reactor.

In an alternative embodiment, the poison trap guard material may be provided as a coating on a honeycomb, foam, fibrous or particulate nitrous oxide decomposition catalyst using the methods described above.

The nitrous oxide decomposition catalyst may be a supported metal, a pure or mixed metal oxide or a zeolitic system (for example those described on pages 30-32 of Kapteijn et al, Applied Catalysis B: Environmental, 9 (1996) pages 25-64 and the references provided therein).

Supported metal catalysts that may be used in the present invention include one or more of rhodium, ruthenium, palladium, chromium, cobalt, nickel, iron and copper on particles or shaped units of oxides of alkaline earth metals e.g. magnesium (Mg) or calcium (Ca), alumina, silica, titania or zirconia. Particulate supports are preferably formed into shaped units as described above. Other suitable supports include ceramic fibres, ceramic or metal honeycombs or foams. In one embodiment the metals may be supported within a zeolite framework. For example zeolite systems, typically zeolite Y or ZSM-5 impregnated with transition metals selected from Fe, Co, Cu, Ni, Mn, Rh, Ru, Pd or Pt may be effective nitrous oxide decomposition catalysts. The metal loading in the supported metal nitrous oxide decomposition catalysts will depend upon the activity of the metal and the nature of the support used. The loading may be 1% by weight or less but may be greater than 20% by weight. The supported metal catalyst may form oxide phases on the support under the reaction conditions.

Suitable pure oxide nitrous oxide decomposition catalysts include oxides of rhodium (Rh), iridium (Ir), cobalt (Co), iron (Fe), nickel (Ni), copper Cu(ll), lanthanum (La), calcium (Ca), strontium (Sr), vanadium V(lll), hafnium (Hf), manganese Mn(lll), cerium (Ce), thorium (Th), tin (Sn), chromium (Cr), magnesium (Mg), zinc (Zn) and cadmium (Cd), preferably Rh, Ir, Co, Fe and Ni. The pure oxide nitrous oxide decomposition catalyst may be provided as particles or shaped units or a honeycomb or foam as described above.

Alternatively the pure metal oxide may be supported. Supported pure metal oxides that may be used in the present invention include any of the above pure oxides, particularly oxides of Fe, Cr(lll), Mn(lll), Rh, Cu and Co supported on oxides of alkaline earth metals e.g. magnesium or calcium, alumina, silica titania, zirconia or ceria. The pure metal oxide may be supported on particles or shaped units. Alternative suitable supports include ceramic fibres, ceramic or metal honeycombs or foams. Particulate supports are preferably formed into shaped units as described above and preferably the supported oxide comprises between 0.5 and 50% by weight of the pure metal oxide catalyst.

Alternatively, the pure metal oxides may be provided as a coating e.g. a 'wash-coat', on the precious metal catalyst using the methods described above for a poison trap guard material.

Mixed metal oxides herein includes doped-oxides or solid solutions, spinels, pyrochlores and perovskites. Other useful mixed oxide catalysts that may be used in the process of the present invention include transition metal-modified hydrotalcite structures containing Co, Ni, Cu, La, Mg, Pd, Rh and Ru and solid solutions comprising Co(ll) oxide and Mn(lll) oxide in magnesia or alumina. However preferred mixed oxide nitrous oxide decomposition catalysts are spinels and perovskites.

Spinel catalysts that may be used in the present invention may be of formula M¹M²0₄ wherein M¹ is selected from Co, Cu, Ni, Mg, Zn and Ca, M² is selected from Al, Cr, or Co (and thus also includes Co₃0₄), Cu_xCo_3-x0₄ (where x = 0-1 ), Co_x>Mg _-xAI₂0₄ (where x' = 0-1 ), Co_3-x.>Fe_x»0₄ or Co₃._x»Al_x»0₄ (where x" = 0-2). A suitable non-Co containing spinel catalyst is CuAI₂0₄.

Perovskite nitrous oxide decomposition catalysts may be represented by the general formula AB0₃ wherein A may be selected from La, Nd, Sm and Pr, B may be selected from Co, 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⁺ to provide further useful nitrous oxide decomposition catalysts. In addition, if desired, partial substitution of one B-site element (e.g. up to 50mol%) with another may be performed to provide further useful nitrous oxide decomposition catalysts. Suitable perovskite catalysts include LaCo0₃, La_1-xSr_xCo0₃, La_1-xCe_xCo0₃ (where x<0.2) and LaCu_yCo_1-y0₃ (where y≤0.5).

The mixed metal oxide nitrous oxide decomposition catalyst may be provided as particles or shaped units or a honeycomb or foam as described above for the pure metal oxides. If supported, the spinel and perovskite mixed metal oxide catalysts may be supported on particles or shaped units as described above for the pure oxide catalysts. Alternatively, the mixed metal oxides may be provided as a coating e.g. a 'wash-coat', on the precious metal catalyst using the methods described above for a poison trap guard material. Where shaped units of the nitrous oxide decomposition catalyst are used they may be provided as a bed of shaped units, for example, pellets, spheres, rings, cylinders, multi-holed extrudates and the like that have maximum and minimum dimensions in the range 1.5 to 20 mm, particularly 3 to 10 mm. The aspect ratio of the shaped units, i.e. the ratio of the maximum to minimum dimensions, is preferably less than 2. The shaped units may be disposed as a thin bed, i.e. a bed whose thickness is less than the diameter of the reactor. Typically the bed thickness of shaped unit nitrous acid decomposition catalyst will be 10-500 mm, preferably 25-250 mm, more preferably 25-100 mm in depth to reduce the pressure drop through the bed.

If desired, shaped units of nitrous oxide decomposition catalyst, whether they comprise supported metal or unsupported or supported pure or mixed metal oxides, may be combined as a layer, or mixed intimately in a bed with particles or shaped units of the guard material, e.g. a poison trap guard material. Where a combined bed is employed, it is preferable that the particles or shaped units of nitrous oxide decomposition catalyst are beneath a layer of guard material.

Preferred nitrous oxide decomposition catalysts are supported Rh catalysts and supported or unsupported pure and mixed metal oxides of one or more of Co, Mn, Fe, Cu, Cr and Ni, preferably Co in a spinel or perovskite structure. In a preferred embodiment, the nitrous oxide decomposition catalyst is also an effective ammonia oxidation catalyst. Accordingly, we have realised that in order to accommodate the guard material within the catalyst assembly, use of a catalyst that acts both as an ammonia oxidation catalyst and as a nitrous oxide decomposition catalyst offers practical advantages in catalyst assembly design and construction. . We have realised that a bed of guard material may be incorporated successfully by altering the quantities of both the precious metal catalyst and the nitrous oxide decomposition catalyst where the latter also functions as an effective ammonia oxidation catalyst.

Hence a particularly preferred nitrous oxide decomposition catalyst is a particulate composition containing oxides of cobalt and other metals, particularly rare earths, for example as described in EP-B-0946290. These cobalt-containing catalysts have the further advantage in that they are highly active ammonia oxidation catalysts in their own right. 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_xVnι_-x)₂0₃ or VvxVn-i__x0₂. A particularly preferred catalyst is a La_1-xCe_xCo0₃ material. Such catalysts may be prepared according to examples 2 and 3 of EP-B-0946290 herein incorporated by reference.

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 oxidation of ammonia with air in the presence of methane for the manufacture of hydrogen cyanide, the Andrussow Process, 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 present invention, 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 of the present invention may provide aggregate N₂0 levels below 1600 ppm, preferably below 600 ppm and more preferably below 500 ppm. Furthermore using the catalyst assemblies of the present invention the campaign length of ammonia oxidation processes may be increased, e.g. by ≥10%, preferably >20%.

In circumstances where the process is operated such that less than 100% of the ammonia is oxidised by the precious metal catalyst it is possible to consider the relative activity of the precious metal catalyst and the nitrous oxide decomposition catalyst for the oxidation of ammonia. WO 99/64352 suggests that 75% or more of the ammonia oxidation reaction may be performed by the precious metal catalyst in combination with the particulate composition containing oxides of cobalt and other metals as described in EP-B-0946290. In the present invention this figure may be varied by using alternative catalyst combinations to accommodate for the guard material. Hence in a process wherein a precious metal ammonia oxidation catalyst is combined with a nitrous oxide decomposition catalyst that functions also as an ammonia oxidation catalyst, the relative activity of the catalysts with respect to the molar percentage of ammonia oxidised will be in the range 99:1 to 1 :99, preferably 75:25 to 25:75 (precious metal catalyst: nitrous oxide decomposition catalyst). In particular, where the nitrous oxide decomposition catalyst is a bed of particulate lanthanum-cerium cobaltate catalyst as described in EP-B-0946290, we have found that it is possible to replace >20%, preferably >30% and most preferably >40% by weight of platinum alloy gauzes used before inclusion of the cobaltate catalyst in the catalyst system without loss in NO efficiency whilst reducing nitrous oxide levels to below 50% of that obtained before inclusion. In the process of the present invention a mixture comprising ammonia and air are fed at an elevated temperature to a catalyst assembly wherein the gasses first contact a precious metal ammonia oxidation catalyst which may be provided as one or more gauzes or may be supported on a suitable metal or inert refractory support. The resultant gasses then contact a guard material which may be in the form of a coating on the precious metal catalyst, fibres which may be incorporated into a gauze, a honeycomb or foam, or a bed of particles or shaped units that catches precious metal and/or catalyst poisons. The gasses then contact a catalyst effective for the decomposition of nitrous oxide which may be in the form of a coating on the precious metal catalyst, fibres which may be incorporated into a gauze, a honeycomb or foam, or a bed of particles or shaped units.

Accordingly, in one embodiment the catalyst assembly may comprise one or more gauzes of a precious metal ammonia oxidation catalyst, one or more palladium-based catchment gauzes and a bed comprising shaped units of a poison trap guard material and a nitrous oxide decomposition catalyst. If desired, either palladium-based catchment gauze or the poison trap guard material may be omitted. Preferably a palladium-based catchment gauze is present. Alternatively the catalyst assembly may comprise a knitted pad comprising filaments of a precious metal ammonia oxidation catalyst and filaments of a palladium-based guard material over a honeycomb, foam or bed of shaped units of a nitrous oxide decomposition catalyst. If desired, a poison trap guard material may be provided in the form of a coating on the precious metal oxidation catalyst or as shaped units held in a layer between the precious metal catalyst and the nitrous oxide decomposition catalyst.

In an alternative embodiment, the catalyst assembly may comprise one or more gauzes of a precious metal ammonia oxidation catalyst and a honeycomb, foam or bed of shaped units of a nitrous oxide decomposition catalyst having on it a wash coat of a poison trap guard material.

Furthermore, if desired, one or more additional palladium-based catchment gauzes may be provided after the nitrous oxide decomposition catalyst.

Hence for a low-pressure ammonia oxidation process, the catalyst assembly of the present invention may comprise 1 or 2 precious metal ammonia oxidation catalyst gauzes followed by guard materials, for example one or more gauzes of palladium catchment and optionally a bed of shaped units of a poison trap guard material such as Ianthana of thickness between 10 and 50 mm, followed by a bed of shaped units of an oxidic cobalt-containing nitrous oxide decomposition catalyst of thickness between 25 and 100 mm. Likewise in a high-pressure plant, there may be less than 15, e.g. 10 precious metal ammonia oxidation catalyst gauzes followed by the guard materials 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, optionally a bed of shaped units of a Ianthana, and a bed of shaped units of a mixed metal rare-earth cobalt perovskite catalyst. In a further embodiment the catalyst assembly has a platinum or platinum alloy gauze supported on the oxidic cobalt-containing bed in the form of a thin layer disposed between two metal gauzes which comprise palladium-based catchment gauzes.

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 bed of shaped units of mixed metal rare-earth cobalt perovskite catalyst, preferably as described in EP-B-0946290, of thickness between 25 and 100 mm.

The invention is further illustrated by the following examples.

Example 1 - Comparative Example

Ammonia was oxidised with normal un-enriched air in a tubular laboratory reactor of 40-mm internal diameter. The reaction conditions were as follows; Pressure 4.5 ± 0.1 bara, Flowrate 7.7 ± 0.1 m³/hr of an air-ammonia mixture

Composition 10.4 ± 0.5% NH₃,

Preheat Temp. 201+ 1°C

Exit Temp. 830 ± 30°C

The catalyst combination comprised three 1 mm plies (2.026 g) of platinum-rhodium gauze (Prolok 95:5 available from Johnson Matthey PLC) above 25 mm (83.0 g) of a Co-perovskite catalyst in the form of 3 mm pellets, prepared according to EP-B-0946290 having the composition Lao.₈Ce₀.₂Co0₃ and <25%(by atoms) of the cobalt present as free cobalt oxides. The process was monitored over a 31 -day period for overall oxidation efficiency and the amount of N₂0 produced.

On days 11 and 12, additional N₂0 (100 ppm) was blended in to the feed gasses. Between days 15 and 26, S0₂ (50 ppb) was blended in to the feed gasses. The results, averaged for each day, are set out in the following table.

Infra-red and UV-vis spectrometry were used to measure [NO], [N0₂], and [N₂0] concentration in the dried product gas. Infra-red spectrometry was used to measure [NH₃] concentration in the gaseous feed. The efficiency of the catalyst was calculated on the basis of the nitrogen balance using the following formula: Efficiency to NOx = {[NO] + [N0₂]} / [NH₃] with a correction for molar volume changes in the reaction.

Under the same conditions as days 1 to 10, using only 6 plies of the platinum rhodium gauze, the average amount of nitrous oxide produced was ca. 1600 ppm with a comparable NOx efficiency.

For days 11 and 12, although 100 ppm of N₂0 was added, an increase of only ca 50 ppm was observed in the exhaust gasses demonstrating the high capacity for the Co-perovskite catalyst to decompose nitrous oxide. However, when 50ppb of S0₂ was added to the feed the concentration of N₂0 leaving the catalyst bed rose from ca. 190ppm to 700-800ppm over a period of 10 days. This is consistent with a model of sulphur poisoning in which active surface area is lost. Thus in spite of the presence of the platinum ammonia oxidation catalyst, the cobalt perovskite N₂0 abatement catalyst in the absence of a guard material under these conditions loses its effectiveness if sulphur compounds are present in the feed gas.

Example 2

Using the same equipment and under the same conditions as Example 1 , ammonia was oxidised with normal un-enriched air using two different catalyst assemblies. a) The first catalyst assembly comprised four plies (3.252 g) of platinum-rhodium gauze of 60 micron wire (Prolok 95:5 available from Johnson Matthey PLC) above a set of catchment gauzes, comprising two (2.839 g) 95% palladium-5% nickel gauzes sandwiched between three stainless steel support gauzes, in turn above 50 mm (133.7 g) of a Co-perovskite catalyst in the form of 3 mm pellets, prepared according to EP-B-0946290 having the composition Lao.sCeo.₂Co0₃.

b) For comparison a second catalyst assembly comprised four plies (3.417 g) of platinum- rhodium gauze of 60 micron wire (Prolok 95:5 available from Johnson Matthey PLC) above 50 mm (134.0 g) of a Co-perovskite catalyst in the form of 3 mm pellets, prepared according to EP-B-0946290 having the composition La₀.₈Ce₀.₂CoO₃, in turn above a set of catchment gauzes, comprising two (2.040g) 95% palladium-5% nickel gauzes sandwiched between three stainless steel support gauzes.

The process was monitored over a 31 -day period for overall oxidation efficiency and the amount of N₂0 produced. The results, averaged for each day, are set out in the following table.

The results show extremely low levels of N₂0 for the present invention compared with 6 plies of platinum alone and also surprisingly demonstrate the effectiveness of the claimed assembly over one in which the catchment gauze is beneath the cobalt perovskite catalyst.

Example 3 Ammonia was oxidised over different catalyst arrangements using a mixture of 10.5% oxygen: 1.0% argon: 88.5% helium in a tubular laboratory reactor of 25 mm internal diameter. The reaction conditions were as follows: Pressure ambient, with a back pressure of 0.05barg

Flowrate (total) 36.84 litres min^"1 (normalised to NTP)

Composition 5.0% NH₃

Preheat Temp. 415°C Exit Temp. 850°C

The compositions of the product gases (NO/N0₂, N₂0 and N₂), were determined by a mass spectrometer with Ar used as an internal standard in the helium carrier gas. The selectivity of the catalysts towards each of the detected products was calculated on the basis of the nitrogen balance using the following formulae:

Selectivity to NO = [NO] / {2 * ([N₂] + [N₂0]) + [NO]}

Selectivity to N₂0 = 2 * [N₂0] / {(2 * [N₂] + [N₂0]) + [NO]}

Selectivity to N₂ = 2 * [N₂] / {(2 * [N₂] + [N₂0]) + [NO]} where [NO], [N₂] and [N₂0] represent the concentrations of nitric oxide, nitrogen and nitrous oxide respectively in the product gas.

The catalyst arrangements placed in the reactor comprised either: a) a single 0.5 mm ply (0.35 g) of 60 μm platinum-rhodium gauze (ProLok 95:5, available from Johnson Matthey pic) above 2 plies (0.75 g) of 95:5 palladium-nickel gauze above 20 mm (20.42 g) of a Co-perovskite catalyst in the form of 3 mm cylindrical pellets, prepared according to EP-B-0946290 having the composition La₀.aCe₀.₂CoO₃ and <25% (by atoms) of the cobalt present as free cobalt oxides, or b) a single 0.5 mm ply (0.35 g) of 60 μm platinum-rhodium gauze (ProLok 95:5, available from Johnson Matthey pic) above 2 plies (0.75 g) of 95:5 palladium-nickel gauze above 20 mm (13.61 g) of copper aluminate (CuAI₂0 ) in the form of 3 mm cylindrical pellets pre-fired at 900°C for 6h, or c) a single 0.5 mm ply (0.35 g) of 60 μm platinum-rhodium gauze (ProLok 95:5, available from Johnson Matthey pic) above 15 mm (15.04g) of a Ianthana (La₂0₃) guard bed in the form of 3 mm cylindrical pellets above 20 mm (21.12 g) of a Co-perovskite catalyst in the form of 3 mm cylindrical pellets, prepared according to EP-B-0946290 having the composition Lao.₈Ce₀.₂Co0₃ and <25% (by atoms) of the cobalt present as free cobalt oxides.

The process was monitored over 5 hours for 3a and 3b and 8.5 hours for 3c. The results obtained under steady state conditions are set out in the following table:

3-plies platinum-rhodium gauze in the same equipment under similar conditions gave a N₂0 selectivity of 3.9% (with a NO selectivity of 90%). Furthermore the combination of 1 -ply platinum-rhodium gauze on top of 20 mm cobalt perovskite in the absence of any guard layer gave a N₂0 selectivity of 1.95-2.3% (with a NO selectivity of 92-94%). These results show the benefit of the combination of guard material and nitrous oxide abatement catalyst of the present invention over catalyst assemblies where they are omitted.

Example 4

Ammonia was oxidised with normal un-enriched air in a tubular laboratory reactor of 28-mm internal diameter. The reaction conditions were as follows;

Flowrate 3.3 ± 0.1 m³/hr of an air-ammonia mixture

Composition 10.5 ± 0.3% NH₃,

The pressure, preheat and exit temperatures varied and are given in the table below.

A mass spectrometer was used to analyze the composition of NO, N0₂ and N₂0 in the product gas, with Ar used as an internal standard. Infra red spectrometry was used to measure [NH₃] concentration in the gaseous feed. The efficiency of the catalyst was calculated on the basis of the nitrogen balance using the following formula: Efficiency to NOx = {[NO] + [N0₂]} / [NH₃] with a correction for molar volume changes in the reaction using Ar as an internal standard.

The catalyst arrangements placed in the reactor comprised either: a) a single ply (0.54 g) of 76 μm platinum-rhodium gauze (NitroLok 95:5, available from Johnson Matthey pic) above 2 plies (0.95 g) of 95:5 palladium-nickel gauze above 24 mm (14.9 g) of 0.5% Rhodium on alumina in the form of 3 mm cylindrical pellets, or b) a repeat of Example 4a using 3-plies (1.53 g) of platinum-rhodium gauze, or c) a single ply (0.46 g) of 76 μm platinum-rhodium gauze (NitroLok 95:5, available from Johnson Matthey pic) above 3 plies (1.21 g) of a 76 μm 81 :3:16 Pt:Rh:Pd catchment gauze above 50 mm (67.8 g) of a Co-perovskite catalyst in the form of 3 mm pellets, prepared according to EP-B-0946290 having the composition La₀.₈Ce₀.₂CoO₃ and <25% (by atoms) of the cobalt present as free cobalt oxides, or d) a single ply (0.56 g) of 76 μm platinum-rhodium gauze (NitroloLok 95:5, available from Johnson Matthey pic) above 3 plies (1.18 g) of a 76 μm 55:1 :44 Pt:Rh:Pd catchment gauze above 50 mm (74.4 g) of a Co-perovskite catalyst in the form of 3 mm pellets, prepared according to EP-B-0946290 having the composition La₀.₈Ce₀.₂CoO₃ and <25% (by atoms) of the cobalt present as free cobalt oxides.

The tests were performed over 7-22 hours. The results obtained under steady state conditions are set out in the following table:

In comparison, in the same equipment under similar conditions, 4-plies of platinum-rhodium gauze make 2345 ppm N₂0 (at an efficiency of 90.0%). Furthermore, 50 mm of cobalt perovskite under 2 plies of platinum rhodium gauze in the absence of a Pd catchment made 232 ppm N₂0 (at an efficiency of 83.9%). Hence it can be seem in Examples 4c and 4d that the rhodium-containing Pd-catchment in combination with a platinum-rhodium gauze and cobalt perovskite nitrous oxide decomposition catalyst provide a particularly effective combination both in terms of higher efficiency and low N₂0 production.

Claims

Claims.

1. A process for the oxidation of ammonia, including the Andrussow process, wherein a mixture comprising air and ammonia is contacted with a precious metal ammonia oxidation catalyst and the resultant gas mixture contacted with a catalyst effective for the decomposition of nitrous oxide, characterised in that prior to contacting the gas mixture with the catalyst effective for the decomposition of nitrous oxide, the gas mixture is contacted with a guard material.

2. A process according to claim 1 wherein the precious metal ammonia oxidation catalyst comprises one or more gauzes of platinum or a platinum alloy in elemental filamentary form.

3. A process according to claim 1 or claim 2 wherein the guard material comprises a palladium-based material.

4. A process according to claim 3 wherein the palladium material comprises <5% by weight rhodium (Rh).

5. A process according to any one of claims 1 to 4 wherein the guard material comprises an inert refractory material.

6. A process according to any one of claims 1 to 4 wherein the guard material comprises an oxide or mixed oxide of one or more of lanthanum (La), gadolinium (Gd), cerium (Ce), yttrium (Y) and ytterbium (Yb).

7. A process according to any one of claims 1 to 6 wherein the catalyst effective for the decomposition of nitrous oxide is a supported Rh catalyst or a supported or unsupported pure or mixed metal oxide of one or more of Co, Mn, Fe, Cu, Cr and Ni.

8. A process according to any one of claims 1 to 7 wherein the catalyst effective for the decomposition of nitrous oxide is also an effective ammonia oxidation catalyst.

9. A process according to claim 8 wherein the catalyst effective for the decomposition of nitrous oxide 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.

10. A catalyst assembly suitable for ammonia oxidation, including the Andrussow process, comprising a precious metal ammonia oxidation catalyst, a guard material and a nitrous oxide decomposition catalyst.

11. A catalyst assembly according to claim 10 comprising a precious metal ammonia oxidation catalyst in the form of one or more gauzes or supported on a metal or inert refractory support, a guard material in the form of a coating on the precious metal catalyst, fibres, a honeycomb, a foam, or a bed of particles or shaped units, and a catalyst effective for the decomposition of nitrous oxide in the form of a coating on the precious metal catalyst, fibres, a honeycomb, a foam, or a bed of particles or shaped units.

12. A catalyst assembly according to claim 11 wherein the catalyst assembly comprises one or more gauzes of a precious metal ammonia oxidation catalyst, one or more palladium- based catchment gauzes, optionally a bed comprising shaped units of a poison trap guard material, and a nitrous oxide decomposition catalyst.

13. A catalyst assembly according to claim 11 wherein the catalyst assembly comprises a knitted pad comprising filaments of a precious metal ammonia oxidation catalyst and filaments of a palladium-based guard material over a honeycomb, foam or bed of shaped units of a nitrous oxide decomposition catalyst.

14. A catalyst assembly according to claim 11 wherein the guard material comprises a coating on the precious metal ammonia oxidation catalyst or shaped units held in a layer between a precious metal catalyst and the a nitrous oxide decomposition catalyst.

15. A catalyst assembly according to claim 11 wherein the catalyst assembly comprises one or more gauzes of a precious metal ammonia oxidation catalyst and a honeycomb, foam or bed of shaped units of a nitrous oxide decomposition catalyst having supported on it a wash coat of a poison trap guard material.

16. A catalyst assembly according to any one of claims 11 to 15 wherein one or more additional palladium-based catchment gauzes are provided after the nitrous oxide decomposition catalyst.

17. The use of a catalyst assembly according to any one of claims 10 to 16 in an extended campaign length ammonia oxidation process, whereby aggregate nitrous oxide production is reduced.

18. The use according to claim 17 wherein the nitrous oxide production is below 1600 ppm.

19. The use according to claim 17 or claim 18 wherein the campaign length is extended by 10% or greater.