"METHOD FOR SIMULTANEOUS TREATMENT OF NITROUS OXIDE AND FORMATION OF NITRIC OXIDE"

FIELD OF THE INVENTION

The present invention relates to a method for simultaneous treatment of nitrous oxide and formation of nitric oxide respectively in reduction and oxidation catalysts o f a membrane , being both oxidation and reduction catalysts of the membrane at the same reaction temperature . This is a new N2O abatement method that can be retrofitted to nitric acid plants to treat N2O while advantageously producing NO, a valuable product in nitric acid formation .

BACKGROUND OF THE INVENTION

The electrochemical formation of nitric oxide is a known process and it has been approached in several literature documents , being particularly the focus of the following patent literature .

European patent application EP 0023813 relates to a method and apparatus for forming nitric oxide (NO) from ammonia . An ammonia-containing gas is contacted with an oxidation catalyst deposited on a surface of a solid electrolyte while an oxygen containing gas i s contacted with a second catalyst capable of dissociating oxygen gas to oxygen ion deposited upon a second surface of the solid electrolyte. Oxygen ion is transported through the solid electrolyte to react with ammonia to form nitric oxide under simultaneous production of electric energy. So, NO is formed by the electrochemical oxidation of ammonia (NH3) across an oxide-ion conducting membrane.

International patent application WO 2006041300 refers to a catalytic membrane reactor for the oxidation of NH3 to NO during the manufacture of nitric acid, where the membrane reactor comprises a mixed electronic and ionic conducting membrane which is capable of transporting oxygen, comprising a multi-component oxide having a perovskite or perovskite-related structure represented by the formula AxA'x'BOs-z, wherein A represents a lanthanide element or a mixture thereof, A' represents an alkaline earth metal or a mixture thereof, B represents a transition metal or a mixture thereof, x and x' each represent numbers such that 0 < X<1.1, 0<x'<l.l and 0.9 < (x+x' ) < 1.1, and z represents a number rendering the compound charge neutral. The invention also relates to a method for the oxidation of NH3 to NO. The product and method according to the invention lead to NO selectivities up to 98 % and no N2O formation during high- temperature NH3 oxidation, giving rise to a highly efficient and intensified process for nitric acid manufacture.

One of the drawbacks of current, non-electrochemical, nitric acid manufacturing processes is the emission of N2O. For the last few decades, the concentration of nitrous oxide (N2O) in the atmosphere has been increasing at a rate of 0,2-0, 3% per year, leading to current levels that are 18% higher than those before the industrial revolution. This is a factor of major concern due to the large contribution of N2O to global warming (with an impact 298 times greater than CO2) , with severe negative impacts on ozone depletion and climate change.

Nitric acid manufacture is the largest single source of anthropogenic N2O emissions with 125xl0 ⁶ t C02-equiv produced per annum. For reference, for each tonne of HNO3 produced in a typical nitric acid plant, approximately twice of that number of tonnes of C02-equiv of N2O is produced, underscoring a serious environmental impact.

Typical processes for the abatement of N2O are those of thermal or catalytic decomposition. Nonetheless, both of these methods convert the N2O solely into heat and its reduction products, namely those of nitrogen and oxygen. For typical industrial processes, yields of NO are commonly 95-97%, with the remainder being approximately 1.5-2.5% of N2O and 4-4.5% of N2. It has been estimated that an increase in yield of NO of 1% in a medium nitric acid plant would roughly correspond to an additional profit of ~500 k€ per year. Thus, while solving the environmental concern, typical processes for abatement of the N2O bi-product by its reduction have major negative impacts on profit .

In view of the above, there is still the need to provide a method that provides less nitrous oxide emissions, while still forming the intended reaction products of nitric acid manufacturing process. SUMMARY OF THE INVENTION

Surprisingly, the aforementioned problem was solved by the present invention .

The present invention provides a method for simultaneous treatment of nitrous oxide and formation of nitric oxide characteri zed in that it comprises feeding a N2O gas stream into reduction catalyst of a membrane simultaneously with feeding a NH3 gas stream into oxidation catalyst of said membrane , wherein both oxidation and reduction catalysts of the membrane are at the same reaction temperature .

In one embodiment , the method of the invention further comprises a pre-treatment step of the membrane by f lowing reducing gases into oxidation catalyst . Preferably, the reducing gases are selected from hydrogen and ammonia .

In another embodiment , the method of the invention further comprises a heating step of the membrane up to the reaction temperature before feeding the oxidation and reduction catalysts of the membrane .

In an embodiment , the reaction temperature of the method of the invention ranges between 500- 900 ° C . In a preferred embodiment , the reaction temperature of the method of the invention ranges between 600- 800 ° C and in a more preferred embodiment , the reaction temperature of the method of the invention ranges between 650-750 ° C .

In one embodiment of the invention, the reduction catalyst is made of a material of general formula (Ai-xA' x) a (B) b0 _c , wherein A represents a rare earth element or a mixture thereof, A' represents an alkaline earth metal or a mixture thereof, B represents a transition metal or a mixture thereof, x, a and b represent numbers such that 0 < x<l, 0.6 < a < 1.4, 0.6 < b < 1.4, and c represents a number rendering the compound charge neutral. Preferably, the reduction catalyst is made of perovskite or perovskite-based material.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1A is a schematic representation of one embodiment of the method of the invention in a tubular fuel cell having a pure oxide-ion conducting membrane: (1) represents an anode offering mixed oxide-ion and electron conductivity and catalytic activity for the NH3 oxidation reaction; (2) represents a pure oxide-ion conducting electrolyte; (3) a cathode offering mixed oxide-ion and electron conductivity and catalytic activity for the N2O reduction reaction; and (4) represents an electronically conducting interconnect material.

Figure IB is a schematic representation of several tubular fuel cells of Figure 1A connected by an electronically conducting interconnect material and therefore assuming a tubular stack configuration.

Figure 2 is a schematic representation of one embodiment of the method of the invention in a planar fuel cell configuration having a pure oxide-ion conducting membrane.

Figure 3 is a schematic representation of one embodiment of the method of the invention in a tubular mixed conducting membrane : ( 1 ) represents a material of fering catalytic activity for the NH3 oxidation reaction at the permeate side ; ( 2 ) represents a mixed oxide- ion and electron conducting membrane ; and ( 3 ) represents a material at the feed side of fering mixed oxide-ion and electron conductivity and catalytic activity for the N2O reduction reaction .

Figure 4 represents a comparison of the theoretical cell potential obtained (A) with the method of the invention using a pure oxide-ion conducting electrolyte membrane and (B ) with the method known from prior art (EP 0023813 ) .

Figure 5 shows the conductivity profiles in a preferred embodiment of the invention using perovskite anode compositions La _x Sr 0 .9 _x Feo.4Tio. 6O3-5 .

Figures 6 and 7 show the conductivity profiles in a preferred embodiment of the invention using perovskite cathode electrocatalyst compositions ( LaSr ) ( Fe , Ti ) O3-5.

DETAILED DESCRIPTION OF THE INVENTION

The obj ect of the present invention is a method for simultaneous treatment of nitrous oxide and formation of nitric oxide comprising comprises feeding a N2O gas stream into reduction catalyst of a membrane simultaneously with feeding a NH3 gas stream into oxidation catalyst of said membrane , wherein both oxidation and reduction catalysts of the membrane are at the same reaction temperature . Surprisingly, the method of the invention performs simultaneously both oxidation and reduction reactions respectively involved in the formation of nitric oxide and treatment of nitrous oxide . This is due to the presence of a membrane having oxidation and reduction catalysts being both catalysts at the same reaction temperature thereby promoting the conversion of N2O to N2 electrochemically at the same temperature as NH3 oxidi zes to NO on the reverse side .

In view of the known methods for forming nitric oxide and nitric acid, the skilled person would not expect that the use of a single reaction temperature for both oxidation and reduction catalysts of a membrane would provide a workable method since each oxidation and reduction reaction has speci fic requirements to be met in order to occur .

Further, nitrous oxide which is a pollutant gas formed during the ordinary manufacturing process of nitric acid as an unwanted side product is now retrofitted by the method of the present invention due to the workable reaction temperature of both sides of catalyst membrane . Therefore , this recovery and reuse of nitrous oxide in the method of the invention clearly reduces ( or even eliminates ) its emission to the atmosphere as a harmful gas . Thus , the simultaneous treatment of nitrous oxide and formation of nitric oxide was not expected to occur success fully .

So , the method of the present invention allows the otherwise environmentally detrimental chemical nitrous oxide to be ef ficiently and simultaneously converted to the nitric oxide and nitrogen, the former being beneficial for several chemical processes , such as nitric acid formation, while the latter can be safely released to the atmosphere .

In addition to the above , the method of present invention further removes the need of supplying molecular oxygen to the reduction side of the membrane . The method of the invention produces the required oxide-ions at the respective cathode or feed side of the electrochemical cell by the intrinsic decomposition of N2O directly to oxide-ions at the cathode due to the presence of a suitable catalyst on the reduction side .

Moreover, the method of the invention provides a higher cell potential in comparison with methods known from the prior art ( Figure 4 ) . This increases the driving force for oxide-ion migration, improving ef ficiency thereof . The invention provides a new N2O abatement method that can be retrofitted to nitric acid plants to treat N2O while advantageously producing NO, a valuable product in nitric acid formation .

The method of the invention further comprises an additional step which is a pre-treatment of the membrane by flowing reducing gases into oxidation catalyst , being the reducing gases selected from hydrogen and ammonia, to activate the oxidation catalyst .

In addition, the method can comprise a heating step of the membrane up to the reaction temperature before feeding the gases to the oxidation and reduction catalysts of the membrane , avoiding loss of these reaction precursors .

As already indicated, the reaction temperature is a critical parameter for performing the method of the invention . The reaction temperature ranges between 500-900 °C, preferably 600-800 °C and more preferably 650-750 °C.

The reduction catalyst used in the method of the invention should be made of a material allowing a high catalytic activity for the N2O reduction reaction. Said material is of general formula (Ai-xA' x) a (B) b0 _c , wherein A represents a rare earth element or a mixture thereof, A' represents an alkaline earth metal or a mixture thereof, B represents a transition metal or a mixture thereof, x, a and b represent numbers such that 0 < x<l, 0.6 < a < 1.4, 0.6 < b < 1.4, and c represents a number rendering the compound charge neutral. Preferably, the reduction catalyst is made of perovskite or perovskite-based material, such as (La, Sr) (Fe,Ti)O3-5 based electrocatalysts. Electrocatalysts made of said materials contain high La and Fe contents, thereby offering high catalytic activity and also high oxide-ion and electronic conductivity in the oxidizing conditions of the cathode compartment to benefit the electrochemical reaction. The reduction catalyst used in the method of the same invention is designed to convert N2O to nitrogen electrochemically at the same reaction temperature as the NH3 oxidation catalysts on the reverse side.

In an embodiment, a tubular membrane comprising a mixed oxide-ion and electron conducting anode, such as that of a perovskite or perovskite-based material, Lni- _x A _x M03, where Ln is a rare earth element, A is a alkaline earth metal and M is a transition metal or mixture of transition metals, preferentially containing Fe, is fabricated by extrusion in green state containing a pore former. Figure 5 shows an example of anode compositional selection in a perovskite system containing (La, Sr) (Fe,Ti)O3-5, in particular potential perovskite anode electrocatalyst compositions La _x SrO,9- _x Feo.4Tio.603-5 where it is clearly observed that higher conductivities are obtained in materials that contain higher La contents in the reducing conditions of the anode compartment ( 10~ ¹² Pa) . Alternatively, in an embodiment for better performance, the anode may be formed from a composite mixture of an oxide-ion conducting ceramic oxide or a mixed oxide-ion and electron conducting ceramic oxide, where the second phase of the composite is that of phase offer high electronic conductivity, such as a metal or mixture of metals, preferentially those of Pt, Pd, Ni, Cu, or Co, or a ceramic phase offering high electronic conductivity in reducing atmospheres. On this extruded tube a thin layer of an oxide-ion conducting electrolyte, such as yttria stabilized zirconia is formed, by a suitable process, such as dip coating. With the aim to lower potential ohmic resistance losses the thickness of the electrolyte layer should be minimized, with thicknesses of less than 20 m being preferable for target working temperatures <600°C. A further layer is deposited on the electrolyte to form the cathode, of a single material or composite material, which offers both mixed oxide-ion and electron conductivity and catalytic activity for N2O reduction and which is designed to be catalytically active for the N2O reduction reaction at the same temperature as the NH3 oxidation reaction at the opposing side of the cell. Examples such as perovskite or perovskite-based material, such as Lni- _x A _x M03, or Lnl/hCh where Ln is a rare earth element, A is a alkaline earth metal and M is a transition metal or mixture of transition metals, preferentially containing Fe, Co, Cu or Ni, or materials of the spinel structure AB2O4, where A and B in the spinel structure may be divalent, trivalent, or tetravalent cations , preferably containing high concentrations of transition elements such as Fe , Co , Cu or Ni .

Figure 6 shows an example of cathode compositional selection in a perovskite system containing ( La, Sr ) ( Fe , Ti ) O3-5, where it is clearly observed that higher conductivities are obtained in materials that contain lower La and higher Fe contents in the oxidi zing conditions of the cathode compartment . These phases may be combined with oxide-ion conducting phases , such as yttria stabili zed zirconia, to form the overall requirements of the cathode of mixed oxide-ion and electron conductivity and catalytic activity for N2O reduction . The combination of anode-electrolyte and cathode forms a membrane electrode assembly (MEA) , a repeat unit that can be combined in series to yield higher voltage , and/or in parallel to allow a higher current where each unit is separated by an electronically conducting interconnect material . The formation of the functional MEA can be formed by a single sintering step, involving the complete MEA, or in two steps , firstly where the anode and electrolyte are sintered at an elevated temperature , preferably in the range 1300- 1600 ° C, followed by a second sintering step at lower temperature , preferably in the range 1000- 1300 ° C, after deposition of the cathode . In the first option, it is beneficial to add a pore forming agent to the cathode before its deposition . NH3 is passed in the anode compartment and N2O is passed in the cathode compartment . N2O is reduced at the cathode side producing nitrogen, thus , converting this otherwise polluting gas to the environmentally safe product of nitrogen, whi lst also providing dissociated oxide-ions that are subsequently transported across the membrane to the anode . At the anode these oxide-ions lead to the oxidation of NH3 gas to NO . Electricity and heat are simultaneously provided by this embodiment . Typical working temperatures of this embodiment will be in the range 500- 900 ° C . Moreover, the thermodynamic information provided in Figure 4 highlights that this embodiment provides a method that is spontaneous and that can be used to generate electricity .

In a further embodiment , a planar MEA design is formed comprising of the same anode , electrolyte and cathode materials . In this embodiment , the combination of planar anode , electrolyte and cathode layers forms a planar membrane electrode assembly (MEA) , a repeat unit that can be combined in series to yield higher voltage , and/or in parallel to allow a higher current where each unit is separated by an electronically conducting interconnect material . Gas leakages between each layer are managed by using of ceramic or glass seals . Operation conditions of this planar cell are as those outlined for the previous tubular arrangement . In this embodiment , it is provided the simultaneous conversion of N2O to the useful product NO, a precursor of industrial processes , such as nitric acid synthesis , environmentally harmless , N2 as well as the production of heat and electricity . Moreover, the thermodynamic information provided in Figure 4 highlights that this embodiment provides a method that is spontaneous and that can be used to generate electricity .

In an embodiment , a tubular membrane comprising a mixed oxide-ion and electron conducting material , such as that of a perovskite or perovskite-based material , Lni- _x A _x M03, where Ln is a rare earth element , A is a alkaline earth metal and M is a transition metal or mixture of transition metals , preferentially containing Fe , is fabricated by extrusion in green state . This extruded tube is densi fied by sintering at elevated temperature high enough to remove all open porosity . At the feed side of this tube a suitable electrocatalyst is also provided that is designed to be catalytically active for the N2O reduction reaction at the same temperature as the NH3 oxidation reaction at the opposing side of the cell . Examples of suitable catalyst materials are perovskite or perovskite-based materials , such as Lni-xAxMCh, or Lnl/hCh where Ln is a rare earth element , A is a alkaline earth metal and M is a transition metal or mixture of transition metals , preferentially containing Fe , Co , Cu or Ni , or materials of the spinel structure AB2O4 , where A and B in the spinel structure may be divalent , trivalent , or tetravalent cations , preferably containing high concentrations of transition elements such as Fe , Co , Cu or Ni . On the permeate side catalysts to promote NH3 oxidation may be employed such as perovskite or perovskite-based material , Lni-xA _x M03, where Ln is a rare earth element , A is a alkaline earth metal and M is a transition metal or mixture of transition metals , preferentially containing Fe , or a composite mixture of an oxide-ion conducting ceramic oxide or a mixed oxide-ion and electron conducting ceramic oxide , where the second phase of the compos ite of fers high electronic conductivity, such as a metal or mixture of metals of fering catalytic performance , preferentially those of Pt, Pd, Ag, Ni , Cu, or Co .

Figures 5 , 6 and 7 show examples of compositional selection in a perovskite system containing ( La, Sr ) ( Fe , Ti ) O3-5 tailored to function at the same temperature for simultaneous N2O reduction and NH3 oxidation, such that these reactions provide the treatment of N2O while produce NO which is a valuable product in nitric acid formation. NH3 is passed in the permeate compartment and N2O is passed in the feed compartment . N2O is reduced at the feed side producing nitrogen, thus , converting this otherwise polluting gas to the environmentally safe product o f nitrogen, whilst also providing dissociated oxide-ions that are subsequently transported across the mixed conducting membrane permeate side . At the permeate side these oxide-ions lead to the oxidation of NH3 gas to NO . Heat is simultaneously provided . Typical working temperatures of this embodiment wil l be in the range of 500- 900 ° C . In this embodiment , the method provides the simultaneous conversion of N2O to the useful product , NO, a precursor of industrial processes , such as nitric acid synthesis , environmentally harmless , N2 as well as the production of heat .

Moreover, Figure 4 highlights that said embodiment of fers a method that is spontaneous . Figure 4 reveals that the method of the invention provides complete conversion of N2O to N2 at the cathode and NH3 to NO at the anode when a pure oxide-ion conducting electrolyte membrane is used, thereby generating a much higher driving force for oxide-ion migration . By contrary the prior art methods describe the use of oxygen or an oxygencontaining gas on the cathode ( feed) side (where oxygen is instead present in its molecular form) and where no electrocatalytic decomposition of N2O is simultaneously performed . This figure demonstrates that , in addition to N2O abatement and production of useful NO product , a higher cell potential is provided by the method of the current invention, thereby advantageously increasing the driving force for oxide- ion migration, improving process ef ficiency .

In a further embodiment , a planar electrochemical cell is formed comprising the same component materials . In this embodiment , the combination of these components forms a planar assembly . Gas leakages are managed by using of ceramic or glass seals . Operation conditions of this planar device are as those outlined for the previous tubular arrangement .

In a further embodiment a mixture of gases that contain both NO and the undesired side product N2O are passed over the cathode or feed side of the aforementioned devices . Here the desired reduction method ( that of N2O reduction) is preferentially targeted over that of NO reduction due to control of temperature in the range 500-700 ° C and selection of reduction catalyst . In this embodiment this selectivity for N2O reduction is intrinsically obtained by profiting from the slower kinetics of NO reduction in comparison to that of N2O reduction in order to di f ferentiate the compound preferentially undergoing reduction . In this embodiment the invention, thus , avoids the need of pre-separation of N2O from the NO containing product of NH3 reduction, a highly attractive feature for its implementation by retrofitting into the nitric acid process . This is possible due to an identical reaction temperature which is speci fically selected to preferentially select N2O reduction over that of NO reduction .