Method for oxidising ammonia into nitric oxide or for reacting ammonia with oxygen and methane comprising recovery of rhodium

Abstract

The present invention discloses an ammonia oxidation process and a method for recovering Rh from a gas phase comprising volatile precious metals, comprising the steps of: a) generating or providing volatile precious metals, particularly by operating a catalytic system comprising a catalyst, wherein the catalyst contains precious metals, wherein the precious metals at least comprise Rh, wherein the volatile precious metals comprise at least volatile Rh; b) contacting volatile Rh generated in step a) with a recovery system comprising a compound of formula Co3-xMxO4, where M is Fe or Al and x = 0-2; and c) forming a solid composition comprising the compound of formula Co3-xMxO4 and Rh, wherein the compound of formula Co3-xMxO4 is bonded with Rh, particularly as determined by SEM-EDS and TEM analysis. The present invention further relates to the use of the compound of formula Co3-xMxO4, where M is Fe or Al and x = 0-2, or the use of the recovery system, for recovering at least volatile Rh.

Description

[0001] METHOD FOR OXIDISING AMMONIA INTO NITRIC OXIDE OR FOR REACTING AMMONIA WITH OXYGEN AND METHANE

[0002] Field of the disclosure

[0003] The present disclosure relates to field of the oxidation of ammonia into nitric oxide in the production of nitric acid or the reaction of ammonia with oxygen and methane for generating hydrogen cyanide, in particular for the recovery of volatile precious metals, particularly volatile Rh, from a gas phase, generated in these processes.

[0004] Background of the disclosure

[0005] Several reactions require a catalyst to proceed at an acceptable rate and to produce the desired products. Such catalysts are often based upon precious metals, such as platinum group metals (PGMs). The six platinum-group metals are ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt).

[0006] For example, platinum (Pt), palladium (Pd) and / or rhodium (Rh) are commonly used as catalysts among others in the Ostwald process for nitric acid production, for oxidising ammonia into nitric oxide, and in the Andrussow process for hydrogen cyanide production, for reacting ammonia with oxygen and methane. A problem associated with catalysis using platinum group metals, particularly Pt, Pd and Rh is that due the high temperature at which catalysis is performed, for example from 700 to 950 °C or over 1000 °C or over 1100 °C, some of these metals evaporate, in particular when the catalysis is performed in the presence of oxygen.

[0007] The recovery of volatile Pt with, in particular, palladium (Pd)-containing recovery systems is well documented. For example, GB1343637 relates to a process and a related device for recovering platinum metals entrained in a hot gas stream (as in the manufacture of nitric acid) wherein the gas is passed through a gettering device in the form of an inert ceramic honeycomb structure which is coated with a getter containing Pd to absorb the volatile platinum.

[0008] EP63450 generally discloses a getter device and a related process for recovery of a precious metal lost from a precious metal-containing catalyst operating at elevated temperature, wherein the getter comprises an agglomeration or assemblage of unwoven fibres made from a metal selected from the group ruthenium, palladium, iridium, platinum, gold, silver, rhodium, and alloys containing one or more or the said metals. The document primarily focuses on Pd / Au alloys.

[0009] GB668935 relates to a process and related device for platinum recovery of volatilized platinum, originating from a catalyst. In this context, GB668935 claims a process for recovery of platinum, wherein the platinum is trapped on the surface of baffles, disposed at a place where the temperature is at least 700°C and wherein some of the baffles have a coating of silver or of a silver alloy with gold, palladium, or platinum.

[0010] LIS20130149207 relates to an exhaust system arrangement comprising a Pt and Pd catalyst and a downstream SCR catalyst and a component capable of trapping and / or alloying with a gas phase platinum group metal, wherein this component is typically a metal selected from the group consisting of gold, palladium, and silver, preferably a Pd / Au alloy.

[0011] On the side of Rh capture, US4774069A discloses a process for the manufacture of nitric oxide by oxidising ammonia in the presence of a catalyst comprising platinum and from 0 to 20 wt % of rhodium and from 0 to 40 wt % of palladium (based on the weight of alloy), the catalyst being located upstream from a catchment trap for scavenging platinum or rhodium lost from the catalyst. The catchment trap comprises an alloy of Pd with at least one compound selected from the group consisting of the oxides, borides, carbides, silicides, nitrides and silicates of aluminum, zirconium, boron, silicon, magnesium, titanium, yttrium, beryllium, thorium, manganese, lanthanum, scandium, calcium, uranium, chromium, niobium, and hafnium.

[0012] Further, rhodium capture is significantly less documented than platinum capture, which indicates that it is more challenging to achieve. Nonetheless, at the moment, one of the most expensive precious metals is Rh . Thus, it is of great interest to recover as much volatile Rh as possible.

[0013] There is thus a need in the art for compounds and materials for recovering at least volatile Rh and that are stable under ambient air.

[0014] Summary of the disclosure

[0015] The present application addresses one or more of the above indicated needs. The inventor has surprisingly found a method for recovery of at least volatile Rh from a gas phase comprising volatile precious metals. The gas phase comprising volatile precious metals is particularly generated in high temperature processes, particularly in the Ostwald’s process or in the Andrussow process. The method of the disclosure involves recovery of at least volatile Rh with a recovery system comprising a compound of formula Co3-XMXO4, where M is Fe or Al and x =0-2. The recovery system acts as a stable and effective recovery agent for recovering at least volatile Rh. The inventor has further found that the recovery system comprising a compound of formula CO3.XMXO4on a cerium oxide support further acts as a stable and effective recovery agent for recovering Pd. The compound of formula Co3-XMXC>4, where M is Fe or Al and x =0-2 on a cerium oxide support is also known to act as a catalyst for nitrous oxide decomposition. Thus, both nitrous oxide emissions in the air and loss of precious and expensive metals, particularly Rh and / or Pd is mitigated. Further, such a recovery system is stable and active over a long period of time at extreme burner conditions. In the production of nitric acid, this long period of exposure could be 10-12 years.

[0016] In one aspect of the present disclosure, a method for recovering Rh from a gas phase comprising volatile precious metals, particularly comprising at least volatile Rh, is disclosed. The method comprises the steps of: a) catalytically oxidising ammonia into nitric oxide, particularly in the production of nitric acid or catalytically reacting ammonia with oxygen and methane, thereby generating hydrogen cyanide, whereby volatile precious metals are generated, wherein the precious metals comprise at least volatile Rh; b) contacting volatile Rh generated in step a) with a recovery system comprising a compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2; and c) forming a solid composition comprising the compound of formula Co3-xMxC>4 and Rh, wherein the compound of formula Co3-XMXC>4 is bonded with Rh, particularly as determined by SEM-EDS and TEM analysis.

[0017] In one embodiment according to the method of the disclosure, the method further comprises the step of d) recovering Rh from the solid composition of step c).

[0018] Stated differently, in one aspect of the disclosure, a method for oxidising ammonia into nitric oxide in the production of nitric acid or for reacting ammonia with oxygen and methane thereby generating hydrogen cyanide is disclosed. The method comprises the steps of: a) providing or generating a gas phase comprising volatile precious metals, wherein the volatile precious metals comprise at least volatile Rh; b) contacting volatile Rh generated in step a) with a recovery system comprising a compound of formula Co3-XMXC>4, where M is Fe or Al and x =0-2; and c) forming a solid composition comprising the compound of formula Co3-XMXC>4 and Rh, wherein the compound of formula Co3-XMXC>4 is bonded with Rh, particularly as determined by SEM-EDS and TEM analysis.

[0019] In one embodiment according to the method of the disclosure, the method further comprises the step of: operating a catalytic system comprising a catalyst, wherein the catalyst contains precious metals, wherein the precious metals at least comprise Rh, in a system for the catalytic conversion of ammonia and oxygen into nitric oxide or in a system for the catalytic conversion of ammonia, oxygen and methane into hydrogen cyanide, wherein volatile precious metals are evaporated from the catalytic system, thereby producing a gas phase comprising volatile precious metals.

[0020] In one embodiment according to the method of the disclosure, in step c) the solid composition of formula Co3-xRhxC>4 is formed.

[0021] In one embodiment according to the method of the disclosure, in step c) the solid composition of formula Co2RhC>4 is formed when x= 1 or wherein in step c) the solid composition of formula CoRh2C>4 is formed when x= 2.

[0022] Stated differently, in step b) Rh contacts the recovery system comprising a compound of formula Co2AIC>4when M= Al, x = 1 or a compound of formula Co2FeO4when M= Fe, x = 1 to form a solid composition of formula Co2RhC>4 in step c).

[0023] Alternatively, in step b) Rh contacts the recovery system comprising a compound of formula CoAhO4when M = Al, x = 2 or a compound of formula CoFe2C>4 when M = Fe, x = 2 to form a solid composition of formula CoRh2O4 in step c).

[0024] In one embodiment according to the method of the disclosure, in step b), the recovery system comprising the compound of formula Co3-xMxC>4 is on a cerium oxide support.

[0025] In one embodiment according to the method of the disclosure, the recovery system comprises 0.1-10 mol% of the compound of formula Co3-XMXC>4 and 90-99.9 mol% cerium oxide support, and preferably comprises 1-5 mol% of the compound of formula Co3-XMXC>4 and 95-99 mol% cerium oxide support.

[0026] In one embodiment according to the method of the disclosure, the volatile precious metals further comprise volatile Pd, wherein in step b) volatile Pd contacts the cerium oxide support, thereby forming, in step c), a solid composition, particularly with the formula PdxCei-xC>2-x-5, where 0<x<1 and 0<b<1.

[0027] In one embodiment according to the method of the disclosure, the solid composition of formula Co3-xRhxC>4 is dissolved in a mixture of at least one mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C.

[0028] In one embodiment according to the method of the disclosure, the method further comprises a step of recovering Pd from the solid composition of formula PdxCei-xC>2-x-5, wherein the solid composition of formula PdxCei-xC>2-x-5 is dissolved in a mixture of at least one mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C.

[0029] In one embodiment according to the method of the disclosure, the mineral acid is selected from the group consisting of HNO3, HCI, H2SO4, HF, or mixtures thereof.

[0030] In one embodiment according to the method of the disclosure, the mineral acid is a mixture of nitric acid and hydrochloric acid.

[0031] In one embodiment according to the method of the disclosure, step a) is carried out at a temperature in the range from 700 to 1500°C, preferably from 700 to 1400°C, more preferably from 700 to 1100°C, most preferably from 700 to 950°C.

[0032] In one embodiment according to the method of the disclosure, step a) is performed with a catalytic gauze, and wherein step b) is performed with a recovery system that has the shape of a honeycomb, a tablet, a pellet, a sponge, a net, a gauze.

[0033] In another aspect of the present disclosure, use of a compound of formula Co3-XMXC>4, where M is Fe or Al and x =0-2, for recovering volatile Rh from a gas phase comprising volatile precious metals, wherein the precious metals comprise at least volatile Rh, is disclosed.

[0034] In one embodiment according to the use of the disclosure, volatile Rh is generated during the catalytic oxidation of ammonia into nitric oxide, for the generation of nitric acid, or wherein volatile Rh is generated during the catalytic reaction of ammonia with oxygen and methane, thereby generating hydrogen cyanide.

[0035] In one embodiment according to the use of the disclosure, the compound of formula Co3-XMXC>4 is on a cerium oxide support.

[0036] In one embodiment according to the use of the disclosure, the compound of formula Co3-xMxC>4 is CO2AIO4, where M = Al, x = 1 or wherein the compound of formula Co3-xMxC>4 is COAI2O4, where M = Al, x = 2. In one embodiment according to the use of the disclosure, the compound of formula Co3-xMxC>4 is Co2FeC>4, where M = Fe, x = 1 or wherein the compound of formula Co3-xMxC>4 is CoFe2C>4, where M= Fe, x = 2.

[0037] In one embodiment according to the use of the disclosure, the volatile precious metals further comprise volatile Pd that contacts the cerium oxide support, wherein the cerium oxide support recovers volatile Pd.

[0038] In one embodiment according to the use of the disclosure, the compound of formula Co3-XMXC>4 is bonded with Rh to form a solid composition of formula Co3-xRhxC>4. The solid composition of formula Co3-xRhxC>4 is dissolved in a mixture of at least one mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C, thereby recovering Rh.

[0039] In one embodiment according to the use of the disclosure, the cerium oxide support is bonded with Pd to form a solid composition of formula PdxCei-xC>2-x-5, where x = 0-1 , 5 = 0-1. The solid composition of formula PdxCei-xC>2-x-5 is dissolved in a mixture of at least one mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C, thereby recovering Rh.

[0040] In one embodiment according to the use of the disclosure, the mineral acid is selected from the group consisting of HNO3, HCI, H2SO4, HF, or mixtures thereof.

[0041] In one embodiment according to the method of the disclosure, the mineral acid is a mixture of nitric acid and hydrochloric acid.

[0042] Description of the figures

[0043] The following description of the figures of specific embodiments according to the present disclosure is only given by way of example and is not intended to limit the present explanation, its application or use.

[0044] Figure 1 shows the TEM image of sample 2 on the nanoscale confirming that Al3+and Rh3+form crystals with cobalt.

[0045] Figure 2 shows the XPS analysis indicating that Pd is present in the pellet core and prefers to attach itself to the cerium oxide support. Detailed description

[0046] Throughout the description and claims of this specification, the words “comprise,” and variations thereof mean “including but not limited to,” and they are not intended to (and do not) exclude other moieties, additives, components, integers, or steps. Throughout the description and claims of this disclosure, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the disclosure is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0047] Features, integers, characteristics, compounds, chemical moieties, or groups described in conjunction with a particular aspect, embodiment or example of the disclosure are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All the features disclosed in this disclosure (including the description, claims, abstract and drawing), and / or all the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The disclosure is not restricted to the details of any foregoing embodiments. The disclosure extends to any novel one, or any novel combination, of the features disclosed in this disclosure (including the description, claims, abstract and drawing), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0048] The enumeration of numeric values by means of ranges of figures comprises all values and fractions in these ranges, as well as the cited end points. The terms “ranging from ... to ...” or “range from ... to ...” or “up to” as used when referring to a range for a measurable value, such as a parameter, an amount, a time period, and the like, is intended to include the limits associated to the range that is disclosed.

[0049] The present disclosure provides a method for the recovery of one or more volatile platinum group metals, particularly Pd and / or Rh, present in a gas phase.

[0050] As used herein, the terms “recover” / ” recovery” are used interchangeably with the terms “capture” / ”capturing”,” catchment”, “retain” / ”retaining”, “entrapping / trapping”, “gettering” or “scavenging”. These terms are used in the meaning that the element that is recovered or captured by a recovery system according to the present application is incorporated into the crystal lattice of the recovery system. The recovery or capture of a particular element or metal by a recovery system according to the present disclosure can be determined by various techniques such as Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectroscopy analysis (EDS), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) analysis, Transmission Electron Microscopy (TEM) analysis, X-ray Fluorescence (XRF) analysis and X-Ray Photoelectron Spectroscopy (XPS) analysis, as well known to the skilled person.

[0051] The term “on stream” refers to the period for which the recovery system has been installed in the oxidation burner while the reactor is in operation.

[0052] The present disclosure provides the method of recovery of at least volatile Rh from a gas phase comprising volatile precious group metals generated in an ammonia oxidation process.

[0053] As known to the skilled person, the ammonia oxidation process 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 gas phase with a precious metal catalyst often in the form of a gauze prepared from Pt, Pd and / or Rh. In both processes, the gas mixture is passed at an elevated temperature (e. g. from 700 to 1500°C) over a catalyst to effect the oxidation. However, the oxidation of ammonia using Pt, Pd and / or Rh catalysts gives rise to undesirable nitrous oxide (N2O). The content of nitrous oxide is typically reduced by contacting the exhaust gas with a nitrous oxide decomposition catalyst. At the same time, due to high burner temperatures the precious metal catalysts, in particular Pt, Pd and / or Rh get evaporated and are eventually lost in the gas phase.

[0054] The inventor has surprisingly found that in the ammonia oxidation process, a recovery system comprising a compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2, particularly on a cerium oxide support, as defined herein, retains at least volatile Rh and / or Pd thereby allowing recovery of volatile Rh and / or Pd that would otherwise be lost in the gas phase. The compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2 on a cerium oxide support is also known to act as a catalyst for nitrous oxide decomposition as discussed in W00202230 (A1). Thus, both nitrous oxide emissions in the air and loss of precious and expensive metals, particularly Rh and / or Pd is mitigated. Further, such a recovery system is stable over a long period of time at extreme burner conditions. In the production of nitric acid, this long period of exposure could 10-12 years. Accordingly in one aspect of the disclosure, a method for recovering Rh from a gas phase comprising volatile precious metals is disclosed. The method comprises the steps of: b) contacting a gas phase comprising volatile precious metals, wherein the precious metals comprise at least volatile Rh, with a recovery system comprising a compound of formula CO3-XMXC>4, where M is Fe or Al and x =0-2; and c) forming a solid composition comprising the compound of formula Co3-xMxC>4 and Rh, wherein the compound of formula Co3-xMxC>4 is bonded with Rh, particularly as determined by SEM-EDS and TEM analysis.

[0055] More in particular, a method for recovering Rh from a gas phase comprising volatile precious metals is disclosed. The method comprises the steps of: a) catalytically oxidising ammonia into nitric oxide, particularly in the production of nitric acid or catalytically reacting ammonia with oxygen and methane, thereby generating hydrogen cyanide, whereby volatile precious metals are generated, wherein the precious metals comprise at least volatile Rh ; b) contacting volatile Rh generated in step a) with a recovery system comprising a compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2; and c) forming a solid composition comprising the compound of formula Co3-xMxC>4 and Rh, wherein the compound of formula Co3-xMxC>4 is bonded with Rh, particularly as determined by SEM-EDS and TEM analysis.

[0056] In one embodiment according to the method of the disclosure, the method further comprises the step of d) recovering Rh from the solid composition of step c).

[0057] Stated differently, in one aspect of the disclosure, an ammonia oxidation process, in particular a method for oxidising ammonia into nitric oxide in the production of nitric acid or for reacting ammonia with oxygen and methane thereby generating hydrogen cyanide, is disclosed. The method comprises the steps of: a) providing or generating a gas phase comprising volatile precious metals, wherein the volatile precious metals comprise at least volatile Rh; b) contacting volatile Rh generated in step a) with a recovery system comprising a compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2; and c) forming a solid composition comprising the compound of formula Co3-xMxC>4 and Rh, wherein the compound of formula Co3-xMxC>4 is bonded with Rh, particularly as determined by SEM-EDS and TEM analysis. In one embodiment according to the method of the disclosure, the method further comprises the step of: operating a catalytic system comprising a catalyst, the catalyst containing precious metals and the precious metals at least containing Rh, in a system for the catalytic conversion of ammonia into nitric oxide, or in a system for the catalytic conversion of ammonia, oxygen and methane into hydrogen cyanide, wherein volatile precious metals are evaporated from the catalytic system, thereby producing a gas phase comprising volatile precious metals.

[0058] The method of the present disclosure addresses the recovery of at least volatile Rh from a gas phase. Two very well know industrial production processes are the production of nitric acid according to the Ostwald process and the production of hydrogen cyanide according to the Andrussow process. Both these processes involve the presence of at least Rh containing catalyst. Indeed, in the production of nitric acid according to the Ostwald process, the first step involves the reaction of gaseous ammonia with gaseous oxygen provided for example through air onto a Pd / Rh containing catalyst, thereby producing gaseous nitric oxide. The catalyst comprising Pd and Rh usually is part of a so-called ammonia oxidation burner in which the Pd / Rh containing catalyst is located at the top surface of a so-called burner basket and supported by Raschig rings or catalyst particles for example for N2O gas conversion and abatement located inside the burner basket. After contacting the Pd / Rh containing catalyst, the ammonia and oxygen react at suitable temperatures and pressures as known to the skilled person to form gaseous nitric oxide, which passes through the burner basket and is further subject to the subsequent steps of the nitric acid production process. EP3727667A1 and W02004 / 005187A1 describe potential designs for the ammonia oxidation burner basket. Regarding the production of hydrogen cyanide according to the Andrussow process, it also involves the reaction of gaseous ammonia, oxygen and methane that are fed to a reactor and subsequently reacted in the reactor onto a catalyst bed comprising Pd / Rh gauzes at suitable temperatures and pressures as known to the skilled person. It results that the method of the present disclosure is particularly helpful for performing the Ostwald and Andrussow processes generating nitric acid and hydrogen cyanide respectively, particularly for recovering at least volatile Rh generated during the Ostwald and Andrussow processes. In one embodiment according to the method of the disclosure, the Rh containing catalyst is in the form of a catalytic gauze.

[0059] The inventor has surprisingly found that the recovery system comprising the compound of formula CO3-XMXO4 where M is Fe or Al and x = 0-2 particularly recovers Rh, through retaining or capturing Rh, in particular by bonding Rh with said compound to form a solid composition comprising the compound of formula Co3-xMxO4 and volatile Rh. The term “bond” or “bonding” is not limited to a particular type of interaction. In a particular embodiment, this bonding is such that the catalyst metal, particularly Rh, is incorporated into the crystal lattice of the compound of formula Co3-XMXC>4. The bonding of Rh with the compound of formula Co3-XMXC>4 can further be evaluated by analytic techniques such as SEM / EDS analysis, ICP-MS / ICP-OES analysis, TEM analysis, XRF analysis and XPS analysis, particularly by comparing the data obtained by these techniques before and after contacting the recovery system according to the present disclosure with a gas comprising one or more volatile platinum group metals, in particular Rh. The recovery system as defined above is in a crystalline form. Advantageously, the formed solid composition is stable such that upon shutdown of the reactor and exposure to ambient temperature and ambient air, these compositions do not hydrate and do not become brittle and dusty. As known to the skilled person, in ammonia oxidation burner, the recovery system is present downstream of the precious metal catalyst system.

[0060] In one embodiment according to the method of the disclosure, the recovery system has the shape of a honeycomb, a tablet, a pellet, a sponge, a foam, a net, a gauze.

[0061] Honeycomb monoliths and nets offer a large geometric surface area and also very low pressure drop, which is of benefit for the gas to contact the catalytic system or the recovery system. The shape of pellets or tablets can be adjusted and optimised such as to offer a maximised geometric surface area. Further, pellets and tablets are easy to produce, and large volumes can be easily installed and subsequently used in large sized reactors such as circular ammonia oxidation burners. Sponges, foams also offer a large geometric surface area and also exhibit a very low pressure drop, which is of benefit for the gas to contact either the Pt, Pd and / or Rh catalytic metal or the transition metal oxide. Moreover, sponges, foams and ceramics present the advantage of increased mass transfer with respect to other shapes. Gauzes also offer the advantage of low pressure drop, in addition to high mass transfer which favours both the catalytic conversion onto the Pt, Pd and / or Rh metal and the interaction of the volatile Pt, Pd and / or Rh with the transition metal oxide, resulting in the retaining of the volatile Pt, Pd and / or Rh by the transition metal oxide. The desired shape of the recovery system can be produced by extrusion, moulding, slip-casting, pressing, granulating the compound of formula Co3-XMXC>4 and optionally depositing it onto a suitable support. In one embodiment according to the method of the disclosure, in step c) the solid composition of formula Co3-xRhxC>4 is formed.

[0062] In one embodiment according to the method of the disclosure, in step c) the solid composition of formula Co2RhC>4 is formed when x= 1 or wherein in step c) the solid composition of formula CoRh2C>4 is formed when x= 2.

[0063] Stated differently, in step b) Rh contacts the recovery system comprising a compound of formula Co2AIC>4when M= Al, x = 1 or a compound of formula Co2FeO4when M= Fe, x = 1 to form a solid composition of formula Co2RhC>4 in step c).

[0064] Alternatively, in step b) Rh contacts the recovery system comprising a compound of formula CoAhO4when M = Al, x = 2 or a compound of formula CoFe2C>4 when M = Fe, x = 2 to form a solid composition of formula CoRh2O4 in step c).

[0065] In one embodiment according to the method of the disclosure, in step b) the recovery system comprising the compound of formula Co3-xMxC>4 is on a cerium oxide support.

[0066] The inventor has further found that when the compound of formula Co3-XMXC>4 is on a cerium oxide support, volatile Pd present in the gas phase comprising volatile precious metals is retained or captured by the cerium oxide support, in particular by bonding Pd with the cerium oxide to form a solid composition comprising cerium oxide and Pd. The term “bond” or “bonding” is not limited to a particular type of interaction. In a particular embodiment, this bonding is such that the catalyst metal, particularly Pd, is incorporated into the crystal lattice of cerium oxide. In particular, the cerium oxide support is porous in nature. When a pellet of such a recovery system is formed, the compound of formula Co3-xMxO4will be located both in the bulk of the cerium oxide matrix and the surface of the pores inside the pellet and preferably more on the surface of the pores inside the pellet. The volatile Pd has a better ability to reach the core of the pellet. Without being bound by the theory, Pd is expected to have two different paths to reach the core. One is to diffuse in the gas phase into the pellet through the pellet pores, while the other is solid diffusion when forming a solid composition comprising cerium oxide and Pd in the carrier bulk matrix. The bonding of Pd with cerium oxide can further be evaluated by known analytic techniques such as ICP-MS / ICP- OES analysis, TEM analysis, XRF analysis and XPS analysis, particularly by comparing the data obtained by these techniques before and after contacting the recovery system according to the present disclosure with a gas comprising one or more volatile platinum group metals. The present disclosure further provides the use of a porous cerium oxide support to recover volatile Pd from a gas phase comprising volatile precious metals. Furthermore, the present disclosure also provides the method of recovery of Pd with a recovery system comprising a porous cerium oxide support.

[0067] Cerium oxide supported recovery system could be made in several ways using conventional methods being employed in catalyst manufacturing. Cobalt salts, particularly cobalt-aluminium- salts and cobalt-iron-salts can be precipitated on or impregnated into cerium oxide powder and the resulting slurry could be dried and calcined. The catalyst particles can then be formed into useful shape by tableting, compacting, extrusion etc. A high surface area of the cerium oxide will be advantageous and as it will be reduced during calcination. Therefore, the cerium oxide with high initial surface area is used. At operating temperature, the surface area of cerium oxide is larger than 10 m2 / g, preferably larger than 50 m2 / g. Advantageously, the efficiency of Rh capture by the compound of formula Co3-xMxC>4 may increase due to the large surface area of cerium oxide support.

[0068] In one embodiment according to the method of the disclosure, the recovery system comprises 0.1-10 mol% of the compound of formula Co3-XMXO4 and 90-99.9 mol% cerium oxide support, and preferably comprises 1-5 mol% of the compound of formula Co3-XMXC>4 and 95-99 mol% cerium oxide support.

[0069] In one embodiment according to the method of the disclosure, the volatile precious metals further comprise volatile Pd, wherein in step b) volatile Pd contacts the cerium oxide support, thereby forming a solid composition of formula PdxCei-xO2-x-s, where, 0<x<1 and 0< b<1.

[0070] In one embodiment according to the method of the disclosure, in step d) the solid composition of formula Co3-xRhxC>4 is dissolved in a mixture of at least one mineral acid, particularly in a concentrated mineral acid, and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C.

[0071] In one embodiment according to the method of the disclosure, the method further comprises a step of recovering Pd from the solid composition of formula PdxCei-xC>2-x-5, wherein the solid composition of formula PdxCei-xO2-x-s is dissolved in a mixture of at least one mineral acid, particularly in a concentrated mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C. In one embodiment according to the method of the disclosure, the mineral acid, particularly concentrated mineral acid, is selected from the group consisting of HNO3, HCI, H2SO4, HF, or mixtures thereof.

[0072] In one embodiment according to the method of the disclosure, the mineral acid is a mixture of nitric acid and hydrochloric acid, such as aqua regia.

[0073] In one embodiment according to the method of the disclosure, in step a) is carried out at a temperature in the range from 700 to 1500°C, preferably from 700 to 1400°C, more preferably from 700 to 1100°C, most preferably from 700 to 950°C.

[0074] In one embodiment according to the method of the disclosure, step a) is performed with a catalytic gauze, and wherein step b) is performed with a recovery system that has the shape of a honeycomb, a tablet, a pellet, a sponge, a net, a gauze.

[0075] In another aspect of the present disclosure, use of a compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2, for recovering volatile Rh from a gas phase comprising volatile precious metals, wherein the precious metals comprise at least volatile Rh, is disclosed.

[0076] In particular, the use of a recovery system comprising a compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2, for recovering volatile Rh from a gas phase comprising volatile precious metals, wherein the precious metals comprise at least volatile Rh, is disclosed.

[0077] In one embodiment according to the use of the disclosure, volatile Rh is generated during the catalytic oxidation of ammonia into nitric oxide, for the generation of nitric acid, or wherein volatile Rh is generated during the catalytic reaction of ammonia with oxygen and methane, thereby generating hydrogen cyanide. It results that the use of the compound of formula Co3-XMXC>4, where M is Fe or Al and x =0-2, in particular the use of the recovery system of the disclosure, for recovering volatile Rh, is particularly helpful for performing the Ostwald and Andrussow processes generating nitric acid and hydrogen cyanide, respectively.

[0078] In one embodiment according to the use of the disclosure, the compound of formula Co3-XMXO4 is on a cerium oxide support. In one embodiment according to the use of the disclosure, the compound of formula Co3-xMxC>4 is CO2AIO4, where M = Al, x = 1 or wherein the compound of formula Co3-xMxC>4 is COAI2O4, where M = Al, x = 2.

[0079] In one embodiment according to the use of the disclosure, the compound of formula Co3-XMXC>4 is Co2FeC>4, where M = Fe, x = 1 or wherein the compound of formula Co3-XMXC>4 is CoFe2C>4, where M= Fe, x = 2.

[0080] In one embodiment according to the use of the disclosure, the volatile precious metals further comprise volatile Pd that contacts the cerium oxide support, wherein the cerium oxide support recovers volatile Pd.

[0081] In one embodiment according to the use of the disclosure, the compound of formula Co3-XMXC>4 is bonded with Rh to form a solid composition of formula Co3-xRhxC>4. The solid composition of formula Co3-xRhxC>4 is dissolved in a mixture of at least one mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C, thereby recovering Rh.

[0082] In one embodiment according to the use of the disclosure, the cerium oxide support is bonded with Pd to form a solid composition of formula PdxCei-xC>2-x-5, where x = 0-1 , 5 = 0-1. The solid composition of formula PdxCei-xC>2-x-5 is dissolved in a mixture of at least one mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C, thereby recovering Rh.

[0083] In one embodiment according to the use of the disclosure, the mineral acid is selected from the group consisting of HNO3, HCI, H2SO4, HF, or mixtures thereof.

[0084] In one embodiment according to the method of the disclosure, the mineral acid is a mixture of nitric acid and hydrochloric acid, such as aqua regia.

[0085] Under high temperature conditions in the ammonia oxidation burner, the volatile precious metals, particularly Rh, can be captured with a recovery system comprising the compound of formula C03-XMXO4. The capture of Rh by said compound is a continuous process where the replacement of the metal with Rh takes place over a period of time. As the duration of exposure increases, the metal is expected to be partially replaced by Rh such that a matrix comprising Co-M-Rh-0 is formed. As the time passes, the metal is expected to be completely replaced by Rh, thereby forming a solid composition of formula Cos-xRhxC Volatile Pd can be captured by the cerium oxide support, thereby forming a solid composition of formula PdxCei-xO2-x-5.

[0086] In particular embodiments, the compound of formula Co3-xMxC>4 is CO2AIO4 when M=AI, x = 1 on a cerium oxide support. When the pellets of Co2AIO4 are installed in the ammonia oxidation burner, during time installed there is a constant loss of cobalt due to the high temperature inside the ammonia oxidation burner. Thus, the active phase in the recovery system comprising CO2AIO4 is a combination of CO2AIO4 and COAI2O4. After the desired period of exposure of volatile Rh with the recovery system comprising CO2AIO4, Rh gets captured in the crystal lattice of both Co2AIO4 and

[0087] COAI2O4. Rhodium in a gaseous state is typically in the form of Rh3+and will replace Al as shown in below equations:

[0088] CO2AIO4 (s) + 2 Rh3+(g) - — > CO2RI-1O4 (s) + AI2O3 (s) COAI2O4 (s) + 2 Rh3+(g) - — > CoRh2O4(s) + AI2O3 (s)

[0089] Further, Pd gets captured in the crystal lattice of cerium oxide support. The reaction taking place can be shown below:

[0090] CeO2(s) + Pd2+(g) I Pd°--> PdxCei.xO2-x-5(s)

[0091] The method of present disclosure offers several advantages. The recovery system comprising a compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2 as defined herein, retains volatile Rh thereby allowing recovery of volatile Rh that would otherwise be lost in the gas phase. When the recovery system also offers capture of volatile Pd, when the compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2 is on a cerium oxide support. Further, as known to the skilled person, the compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2 on a cerium oxide support is also known to function as a catalyst for nitrous oxide decomposition as discussed in W00202230 (A1). Thus, both nitrous oxide emissions in the air and loss of precious and expensive metals, particularly Rh and / or Pd is mitigated. Further, such a recovery system is stable and active over a long period of time at extreme burner conditions. In the production of nitric acid, this long period of exposure could be 10-12 years.

[0092] EXAMPLES

[0093] Example 1 : use of cobalt aluminium oxide for Rh catchment

[0094] Two samples of cobalt aluminium oxide on a CeC>2 support were pressed into pellets of 8 mm diameter and 8 mm thickness. The pellets were sintered at 900°C for 12 hours. o Sample 1 : Co2AIO4was pressed and sintered (active phase being Co2AIO4 and COAI2O4) o Sample 2: CoAhOtwas pressed and sintered (active phase being COAI2O4)

[0095] The pellets hence obtained were installed downstream of Pd / Rh gauzes in an ammonia oxidation reactor. The reactor was operated at 5 bar pressure at the temperature in the range of 800-900°C for 11 years on stream. The combusted gas contacting the pellets comprised 10% NO, 15% H2O, 6% O2 and nitrogen. In addition to these gases, the gas contained traces of volatile palladium and rhodium. During time installed, for sample 1 there is a rearrangement due to the high temperature inside the oxidation burner. As time goes by, more COAI2O4 is formed due to loss of cobalt from CO2AIO4. Thus, the active phase for sample 1 is a combination of CO2AIO4 and COAI2O4. The vaporization of cobalt (Co3+or Co2+) can be shown as by the equation: 2 CO2AIO4 (s) — > COAI2O4 (s) + 3 Co3+(g).

[0096] After the exposure in the reactor the pellets were recovered from the reactor for analysis. Prior to installation in the reactor, the lower side of the pellets had been marked, so that after exposure the upper side of the pellets that had most direct contact with the incoming flow of combusted gas could be identified. The capture of Rh by the active phases of the pellets and the capture of Pd by the cerium oxide support was evaluated by following methods.

[0097] Al SEM-EDS analysis of the pellets of sample 1 and 2:

[0098] Analysis in a scanning electron microscope (SEM) with energy dispersive X-ray fluorescence analysis (EDS) was carried out at low magnification to obtain an average chemical composition of the near surface region of the upper side. The normalised masses for Co, Al, Rh and Pd are shown in Table 1.

[0099] Table 1. EDS analysis of the upper surface of the pellets of sample 1 and 2:

[0100] These results show that at the surface of the pellets Rh has been captured by CO2AIO4 and / or COAI2O4.

[0101] ICP bulk of the pellets of sample 1 and 2: ICP bulk measurements were initiated to get an estimate of the quantity of Rhodium captured by the pellets. The pellets of sample 1 and 2 were crushed and then dissolved in acid. To completely dissolve the Co-AI particles and the CeC>2 support, a strong inorganic acid solution of equal parts 98% H2SO4, 38% HCI, 68% HNO3, and 49% HF was heated in a microwave to around 200°C in a closed container. Two different types of ICP sensors were used to get the ICP-MS and ICP-OES measurements as provided in Table 2 and 3. As known to the skilled person, ICP-MS measures an atom's mass by mass spectrometry (MS) whereas ICP-OES analysis quantifies the measurement of excited atoms and ions at the wavelength characteristics for the specific elements being measured.

[0102] Table 2. ICP-MS: Amount of Co, Al, Rh detected in pellets of sample 1 and 2.

[0103] Table 3: ICP-OES: Amount of Co, Al, Rh detected in pellets of sample 1 and 2.

[0104] It can be seen that the amounts of Rh detected for ICP-MS and ICP-OES are close with insignificant deviations (~ 1000 ppm Rhodium on average). From Table 2 and 3, an estimate of the quantity of Rhodium captured by the pellets of sample 1 and 2 can be made.

[0105] Cl XRF analysis of pellets of sample 1 and 2:

[0106] A new sample (sample 3) was created by scraping the outer layer of pellet of sample 1. Sample 1-3 were then analyzed by XRF (see Table 4).

[0107] Table 4: Insignificant amount of Palladium is observed in scraped surface material. It is clear from Table 4 that Pd is not found or has extremely low concentration at the outer pellet surface of the pellets. In conclusion, the data of SEM-EDS analysis, ICP bulk analysis and XRF analysis indicates that Rh is found in higher concentration at the outer pellet surface as compared to Pd and Rh prefers to attach itself to the Co-AI component.

[0108] DI TEM analysis of pellets of sample 1 and 2:

[0109] Transmission electron microscopy (TEM) image of sample 2 on the nanoscale confirms that both Al3+and Rh3+form crystals with cobalt (see Fig. 1). It is found that CoRh2O4 is present in some regions of the spinel particle while COAI2O4 is found in separate regions. As known to the person skilled in the art, CoRt^CU is thermodynamically more stable than COAI2O4, implying that Rhodium is replacing Al from COAI2O4 crystal.

[0110] Example 2: use of cerium oxide support from the cobalt aluminium oxide pellets for Pd catchment The capture of volatile Pd by the cerium oxide support was evaluated for five samples discussed herein. o Sample 1 : Pellet comprising approximately 97% cerium oxide carrier (grade 1) and Co-AI compound with active phase being CO2AIO4. The pellet was exposed in the plant for less than a year on stream. o Sample 2: Pellet comprising approximately 97% cerium oxide carrier (grade 1) and Co-AI compound with active phase being CO2AIO4 and COAI2O4. The pellet was exposed in the plant for 3 years on stream. o Sample 3: Pellet comprising approximately 97% cerium oxide carrier (grade 2) and active phase being Co2AIC>4. The pellet was exposed in the plant for less than a year on stream. o Sample 4: Pellet comprising approximately 97% cerium oxide carrier (grade 2) and Co-AI compound with active phase being CO2AIO4 and COAI2O4 . The pellet was exposed in the plant for 1 year on stream. o Sample 5: Pellet comprising approximately 97% cerium oxide carrier (grade 2) and Co-AI compound with active phase being COAI2O4. The pellet was exposed in the plant for 4 years on stream.

[0111] All five samples were pressed into a pellet of 88 mm diameter and 8 mm thickness and sintered at 900 °C for 12 hours. The pellet hence obtained were installed downstream of Pd / Rh gauzes in an ammonia oxidation reactor. The reactor was operated at 5 bar pressure at the temperature in the range of 800-900°C. The combusted gas contacting the pellets comprised 10% NO, 15% H2O, 6% O2 and nitrogen. In addition to these gases, the gas contained traces of volatile palladium and rhodium. The presence of Pd was evaluated by following method:

[0112] XPS analysis of pellets of samples 1-5:

[0113] The pellets after exposure were cut in two and X-ray photoelectron spectroscopy (XPS) measurements were performed on the exposed core material to analyze the presence of elements and their oxidation state. The atomic percentages of the elements present in the analyzed crosssections are provided in Fig. 2. From Fig. 2, it is clear that Pd was found in analyzed pellets and the level of Pd was significantly higher than Rh. Pd is thus found at a higher level in the core of the pellets relative to the levels of Rh detected. Further, aged pellets of samples 2, 4 and 5 showed capture of Palladium as compared to the pellets of sample 1 and 3.

[0114] The cerium oxide support is porous in nature. When the Co-AI compound is on a cerium oxide support, the Co-AI compound will be located both in the bulk of the cerium oxide matrix and the surface of the pores inside the pellet and preferably more on the surface of the pores inside the pellet. From the XRF data in Example 1 , it is clear that no Pd was observed in the vicinity of the Co-AI particles, indicating that the chemical adsorption may have taken place in the cerium oxide support. This indicates that palladium is captured by the CeC>2 matrix and is spread inside the bulk of the pellet. Further, Fig. 2. indicates the higher concentration of Pd in the XPS analysis. This suggests that Pd is captured by the cerium oxide support instead of the Co-AI component.

[0115] From the XRF analysis of Example 1 and the XPS analysis of Example 2, it can be concluded that Pd has a better ability to reach the core of the pellet. Without wishing to be bound by theory, Pd may have two different paths to reach the core. One is to diffuse in the gas phase into the pellet through the pellet pores, while the other is solid diffusion when forming PdxCei-xO2-x-5 crystals (or as Pd°) in the carrier bulk matrix. Without wishing to be bound by theory, it may be likely that Pd uses both paths to reach the core, but to varying degrees.

[0116] While the preferred embodiments of the present disclosure have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.

Claims

Claims:

1. A method for oxidising ammonia into nitric oxide in the production of nitric acid or for reacting ammonia with oxygen and methane thereby generating hydrogen cyanide, the method comprising the steps of: a) providing or generating a gas phase comprising volatile precious metals, wherein the volatile precious metals comprise at least volatile Rh; b) contacting volatile Rh generated in step a) with a recovery system comprising a compound of formula Co3-xMxC>4, where M is Fe or Al and x =0-2; and c) forming a solid composition comprising the compound of formula Co3-xMxC>4 and Rh, wherein the compound of formula Co3-XMXC>4 is bonded with Rh, particularly as determined by SEM-EDS and TEM analysis.

2. The method according to claim 1 , wherein step a) comprises operating a catalytic system comprising a catalyst, wherein the catalyst contains precious metals, wherein the precious metals at least comprise Rh, in a system for the catalytic conversion of ammonia and oxygen into nitric oxide, or in a system for the catalytic conversion of ammonia, oxygen and methane into hydrogen cyanide, wherein volatile precious metals are evaporated from the catalytic system, thereby producing a gas phase comprising volatile precious metals.

3. The method according to claim 1 or 2, further comprises the step of d) recovering Rh from the solid composition of step c).

4. The method according to claim 3, wherein in step d) the solid composition formed in step c) is dissolved in a mixture of at least one mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C.

5. The method according to any one of claim 1 to 4, wherein in step c) the solid composition of formula Co2RhC>4 is formed when x= 1 , or wherein in step c) the solid composition of formula CoRh2C>4 is formed when x= 2.

6. The method according to any one of claims 1 to 5, wherein, in step b), the recovery system comprising the compound of formula Co3-XMXC>4 is on a cerium oxide support.

7. The method according to claims 6, wherein the volatile precious metals further comprise volatile Pd, wherein in step b) volatile Pd contacts the cerium oxide support, thereby forming in step c) a solid composition wherein the cerium oxide is bonded with Pd, particularly thereby forming a solid composition of formula PdxCei-xO2-x-5, with 0<x<1 and 0< b<1.

8. The method according to claim 7, further comprising a step of recovering Pd from the solid composition of step c), particularly the solid composition of formula PdxCei-xO2-x-s, wherein the solid composition of formula PdxCei-xO2-x-s is dissolved in a mixture of at least one mineral acid and heated at a temperature of 180 to 280°C, preferably at a temperature of 200°C.

9. The method according to claim 4 or 8, wherein the mineral acid is selected from the group consisting of HNO3, HCI, H2SO4, HF, or mixtures thereof.

10. The method according to any of the claims 2 to 9, wherein step a) is carried out at a temperature in the range from 700 to 1500°C, preferably from 700 to 1400°C, more preferably from 700 to 1100°C, most preferably from 700 to 950°C.

11. The method according to any one of claims 2 to 10, wherein step a) is performed with a catalytic gauze, and wherein step b) is performed with a recovery system that has the shape of a honeycomb, a tablet, a pellet, a sponge, a net, a gauze.

12. The method according to any one of claims 1 to 11 or the use according to any one of claims 11 to 14, wherein the compound of formula Co3-XMXC>4 in the recovery system is CO2AIO4 or COAI2O4.

13. Use of a compound of formula Co3-XMXC>4, where M is Fe or Al and x =0-2, for recovering volatile Rh from a gas phase comprising volatile precious metals, wherein the volatile precious metals comprise at least volatile Rh.

14. The use according to claim 13, wherein volatile Rh is generated during the catalytic oxidation of ammonia into nitric oxide, for the generation of nitric acid, or wherein volatile Rh is generated during the catalytic reaction of ammonia with oxygen and methane, thereby generating hydrogen cyanide.

15. The use according to claim 13 or 14, wherein the compound of formula Co3-xMxC>4 is on a cerium oxide support.

16. The use according to claim 15 for additionally recovering volatile Pd from the gas phase comprising volatile precious metals, wherein the volatile precious metals further comprise volatilePd that contacts the cerium oxide support, and wherein the cerium oxide support recovers volatile Pd.

17. The use according to any one of claims 13 to 16, wherein the compound of formula Co3-xMxC>4 is CO2AIO4, or wherein the compound of formula Co3-XMXC>4 is COAI2O4.

Images