WO2009142520A1

WO2009142520A1 - Catalyst for low-temperature decomposition of dinitrogen oxide and a process for the preparation thereof

Info

Publication number: WO2009142520A1
Application number: PCT/PL2009/000050
Authority: WO
Inventors: Marcin Wilk; Marek Inger; Urszula Prokop; Ewelina Franczyk; Zbigniew Sojka; Andrzej Kotarba; Andrzej Adamski; Pawel Stelmachowski; Witold Piskorz; Filip Zasada
Original assignee: Instytut Nawozow Sztucznych; Uniwersytet Jagiellonski
Current assignee: Instytut Nawozow Sztucznych; Uniwersytet Jagiellonski
Priority date: 2008-05-21
Filing date: 2009-05-19
Publication date: 2009-11-26
Anticipated expiration: 2010-11-21

Abstract

A catalyst for low-temperature decomposition of dmitrogen oxide in tail gases from a nitric acid plant based on cobalt oxide comprises, calculated as simple oxides: cobalt oxide at the level of 45.00-99.97 weight %, nickel oxide at the level of 0.01-30.00 weight %, zinc oxide at the level of 0.01-20.00 weight % as principal structural components and activity promoters in the form of alkali metals, such as Na and/or K, at the level of 0.01-5.00 weight % and shape-facilitating alkaline earth metal oxides, such as Ca and/or Mg, at the level of 0.01-5.00 weight %. The invention provides also a process for the preparation of a catalyst.

Description

Catalyst for low-temperature decomposition of dinitrogen oxide and a process for the preparation thereof

The invention provides a catalyst for low-temperature decomposition of dinitrogen oxide in tail gases from a nitric acid plant and a process for the preparation thereof

Dinitrogen oxide (N₂O) is formed as a by-product during catalytic oxidation of ammonia in nitric acid manufacture plants, is not absorbed in water and is discharged with tail gases into the atmosphere. Considering global nitric acid production on the level of 55 mln t HNO₃ and average emission rate of 7-9 kg N₂CVt HNO₃, the emission of dinitrogen oxide from a nitric acid plant has significant impact on the greenhouse effect, due to global warming potential for dinitrogen oxide being 310 times higher of that for carbon dioxide. Moreover, dinitrogen oxide can add up to destroying the ozone layer in the stratosphere. The effective removal thereof from tail gases of both stationary and mobile sources is therefore necessary.

Catalytic decomposition of dinitrogen oxide in nitric acid production plants can be carried out at both high and low temperatures. At higher temperatures (800 - 940⁰C) a catalyst is placed in ammonia oxidizer directly below catalytic gauzes for ammonia oxidation. At low temperatures (200 - 450⁰C) a catalyst is placed in a special reactor, in the tail gas stream which is heated in heat exchangers of the nitric acid plant.

From a mechanistic point of view, dinitrogen oxide removal can be carried out by both selective catalytic reduction and by direct decomposition of N₂O into oxygen and nitrogen. Low-temperature catalytic decomposition of dinitrogen oxide is definitely more beneficial as compared with reduction, due to the lack of the necessity of using additional reducing agents and a relatively low operating temperature. Additionally, in contrast to catalysts operating in a high temperature zone, use of a catalyst in the tail gas stream allows to minimize losses connected to possible depletion of the gas blend in NO_x, being a starting material for the nitric acid production. On the other hand, use of a catalyst in a low temperature zone is connected with the requirement of high activity, resistance to other components present in the tail gas stream, i.e. oxygen and water, as well as other nitrogen oxides. Catalytic decomposition of dinitrogen oxide was a subject of numerous studies concerning metalic, oxide and mixed catalysts (F. Kapteijn et al./Appl. Catal. B 9 (1996) 25-64, F. Kapteijn et al./Appl. Catal. B 44 (2003) 117-151). Besides simple oxides, composite oxides of diverse structures, i.a. perovskites, spinells, zeolites, hydrotalcite derivatives were studied. Model systems such as Co₃O₄, MgO, CaO were also studied (A. Scagnalli et al./Surf. Sci. 600 (2006) 386-394; A. Satsuma et al./J. MoI. Catal. A 155 (2000) 81-88). It can be stated based on reports in the literature that depending on a system, active components of catalysts could be cobalt, iron, nickel, zinc, copper, manganese, chromium, magnesium, calcium ions (N. Russo et al./Cat. Today 119 (2007) 228-232; T. Goto et al./React. Kinet. Catal. Lett. 69 2 (2000) 375- 378)). Oxide catalysts on the zirconium oxide support were also studied (A. Snis, H. Miettinen/J. Phys. Chem. B 102(1998), 2555-2561). Promoting effects in model systems were confirmed as yet for dopants such as potassium, sodium, barium, cerium (L. Xue et al./Appl. Catal. B 75 (2007) 167-174; Ch. Ohnishi et al./Catal. Today 120 (2007) 145- 150; L. Xue et al./Catal. Today 126 (2007) 449-455). Activity of a developed catalyst is also influenced by a process of the preparation thereof (K. W. Yao et al./Appl. Catal. B: Env. 16 (1998) 291-301, M. Rauscher/Appl. Catal. A 184 (1999) 246-256).

Problem of catalytic decomposition of N₂O is also a topic of numerous theoretical works. However, the calculations carried out recently are based on highly simplified models of metal and oxide surfaces (B. -Z. Sun et al. / Applied Surface Science 253 (2007) 7501-7505; A. Spis, H. Miettinen/ J. Phys. Chem. B, 102 (1998) 2555; A. Delabie, K. Pierloot/J. Phys. Chem. A, 106 (2002) 5679-5685). Due to limited calculation power of computers only selected energetic pathways OfN₂O decomposition could be studied, while neglecting numerous essential parameters (T, p) of the real process, which are requisite for developing an industrially active catalyst.

A disadvantage of the numerous previous solutions was experimenting with decomposition of N₂O on the said catalysts on a laboratory scale, which most probably was the reason for not developing a catalyst for dinitrogen oxide decomposition satisfactory active at low temperatures.

The invention was aimed at providing a catalyst for removal of dinitrogen oxide from tail gases from a nitric acid plant at the temperature below 45O⁰C, having a high activity and resistance to tail gas components.

The catalyst of the invention is characterized in that it comprises (calculated as simple oxides): cobalt oxide at the level of 45.00-99.97 weight %, nickel oxide at the level of 0.01-30.00 weight %, zinc oxide at the level of 0.01-20.00 weight % as principal structural components and activity promoters in the form of alkali metals, such as Na and/or K, at the level of 0.01-5.00 weight % and formation-enhancing alkaline earth metal oxides, such as Ca and/or Mg at the level of 0.01-5.00 weight %.

Preferably, cobalt, nickel and zinc ions introduced at the synthesis step form a solid oxide solution of a spinel (Co₃O₄) structure.

Preferably structural promoters, nickel and zinc ions, are incorporated into the spinel Co₃O₄ structure, while alkaline promoters remain at the surface of the catalyst.

A process for the preparation of a catalyst of the invention is characterized in that a solution of cobalt(II) nitrate at a concentration of from 50 to 120 g/dm³, nickel(II) nitrate at a concentration from 0.01 to 35.00 g/dm³, zinc nitrate at a concentration from 0.01 to 30.00 g/dm³ is stirred vigorously, most preferably in a circulation system, and a precipitating agent is added simultaneously, at the temperature from ambient to 100°C. A ratio of the precipitating agent to the solution of cobalt, nickel and zinc nitrates should be at least 1.0:0.4 for pH of the solution to range of 9.0-9.5, resulting in the forming of a precipitate. The precipitate is then left in the mother solution at the ambient temperature for at least 15 hrs, and filtered off and washed with water until obtaining the filtrate at pH 7.0-7.5. The prepared precursor is dried at 120°C for at least 15 hrs and calcined at 400-450°C for at least 4 hrs, with gradual heating of the precipitate from 120 to 400°C, to obtain a final product, which is optionally comminuted, and then formed into desired shapes and calcined at 450°C for at least 4 hrs.

Preferably, the cobalt(II) nitrate solution is prepared by dissolving cobalt(II) nitrate hexahydrate in water, at the room temperature, the nickel(II) nitrate solution is prepared by dissolving nickel(II) nitrate hexahydrate in water, at the room temperature, and the zinc nitrate solution is prepared by dissolving zinc nitrate in water, at the room temperature.

As the precipitating agent, an aqueous solution of potassium carbonate and/or sodium carbonate or ammonia at a concentration of 150 g of the reagent/1 is used, which is added to the solution of nitrates at the rate of 5 cmVmin.

Preferably, promoters in the form of alkali metals, Na and/or K, and alkaline earth metals, Mg and/or Ca, are added at the precipitation step and/or during the precursor impregnation after the calcination.

According to the invention, for the catalyst, addition of promoters in the form of alkali metals, such as sodium and potassium, enhances the activity of the catalyst, while addition of alkaline earth metals, such as magnesium and calcium, promotes the catalyst forming and provides its appropriate structure and strength. The catalyst according to the invention, intended to be used in a low-temperature zone, is characterized by high activity and resistance to other components present in a tail gas stream such as oxygen, water vapour and other nitrogen oxides, ubiquitous in such circumstances.

The catalysts according to the invention with different compositions were tested in a quartz reactor, through which a mixture of dinitrogen oxide and helium or tail gases from a nitric acid pilot plant were passed. Composition of the tail gas was identical with the one of a commercial nitric acid plant. Studies on a mixture of 5 % of dinitrogen oxide in helium were carried out in a flow quartz reactor with a frit, at the temperature range from the room temperature to 450°C. Composition of a post-reaction mixture was measured on a mass spectrometer by measuring partial pressures of dinitrogen oxide and decomposition products thereof: oxygen and nitrogen. A decomposition degree of dinitrogen oxide at 300°C was measured to be 95 %, while the sole products of decomposition were dioxygen and dinitrogen. For tail gases comprising: 917 ppm of NO_x, 874 ppm of N₂O, 0.40 vol. % of H₂O, 0.68 vol. % O₂, with N₂ as a remainder, studies were carried out at 400⁰C and at catalyst loading (GHSV) of about 29000 h^"1. Composition of gases before and after a catalyst bed was measured on a FT-IR analyzer. When the above-mentioned parameters were used, the decomposition degree of N₂O was found to be 99 %.

The invention is illustrated by the following examples. Example 1

In a 800 cm³ flask, 12.73 g cobalt(II) nitrate hexahydrate, 3.63 g nickel(II) nitrate hexahydrate, 1.24 g zinc(II) nitrate hexahydrate and 0.49 g calcium(II) nitrate tetrahydrate were dissolved in 30 cm³ of deionized water, to the total concentration of cations in the solution about 2M. The obtained solution was stirred with a magnetic stirrer, with simultaneous addition of a 15 % solution of potassium carbonate as the precipitating agent. The operation was continued until pH of the solution was adjusted to pH in the range of 9.0-9.5, resulting in the forming of a precipitate. The precipitate was left in the mother solution for 15 h at the ambient temperature, followed by filtering and washing the precipitate until pH of the filtrate was 7.0-7.5. The precursor prepared in the process was dried at 120 ⁰C for 15 hrs, and calcined at 400-450⁰C for 4 hrs, with gradual heating of the precipitate from 120 to 400⁰C. The ready catalyst was shaped into pellets and comminuted to obtain granulate of the size of 0.6 -1.0 mm which was calcined at 45O⁰C for 4 hrs. The obtained catalyst contained 70.0 wt. % Of Co₃O₄, 21.0 wt. % OfNi₂O₃, 7.0 wt. % of ZnO, 1.9 wt. % of CaO and 0.1 wt. % of K₂O, calculated as oxides. Phase analysis of a sample by an X-ray powder diffractometry revealed the presence of a spinel phase only. The surface area was measured to be 72 m²/g by the N₂-BET method. Example 2

In a 800 cm³ flask, 12.66 g of cobalt(II) nitrate hexahydrate, 2.41 g of nickel(II) nitrate hexaliydrate, 2.46 g of zinc(II) nitrate hexahydrate and 0.49 g of calcium(II) nitrate tetrahydrate was dissolved in 30 cm³ of deionized water, to the total concentration of cations in the solution about 2M. The obtained solution was stirred with a magnetic stirrer bar, with simultaneous addition of a 15 % solution Of K₂CO₃ as the precipitating agent. The operation was continued until pH of the solution was adjusted to pH in the range of 9.0-9.5, resulting in the forming of a precipitate. The precipitate was left in the mother solution for 15 h at the ambient temperature, followed by filtering and washing the precipitate until pH of the filtrate was 7.0-7.5. The precursor prepared in the process was dried at 120 ⁰C for 15 hrs, and calcined at 400- 45O⁰C for 4 hrs, with gradual heating of the precipitate from 120 to 400⁰C. The ready catalyst was shaped into pellets and comminuted to obtain granulate of the size of 0,6- 1 ,0 mm which was calcined at 45O⁰C for 4 hrs.

The obtained catalyst contained 71.0 wt. % Of Co₃O₄, 14.0 wt. % Of Ni₂O₃, 13.0 wt. % of ZnO, 1.9 wt. % of CaO and 0.1 wt. % of K₂O. Phase analysis of a sample by an X-ray powder diffractometry revealed the presence of a spinel phase only. The surface area was measured to be 69 m²/g by the N₂-BET method.

Claims

1. A catalyst for low-temperature decomposition of dinitrogen oxide in tail gases from a nitric acid plant, based on cobalt oxide, characterized in that the catalyst comprises the following, calculated as simple oxides: cobalt oxide at the level of 45.00-99.97 weight %, nickel oxide at the level of 0.01-30.00 weight %, zinc oxide at the level of: 0.01- 20.00 weight % as principal structural components, and activity promoters in the form of alkali metals, such as Na and/or K, at the level of 0.01-5.00 weight %, and catalyst shape-facilitating alkaline earth metal oxides, such as Ca and/or Mg, at the level of 0.01-5.00 weight %.

2. A catalyst according to claim 1, characterized in that as cobalt oxide, the catalyst contains Co₃O₄ in the form of spinel structure.

3. A catalyst according to claim 1 characterized in that nickel and zinc ions are incorporated into the spinel Co₃O₄ structure, while alkaline promoters remain at the surface of the catalyst.

4. A process for the preparation of a catalyst for low-temperature decomposition of dinitrogen oxide in tail gases from a nitric acid plant by controlled precipitation of a precursor from aqueous solution of cobalt, nickel and calcium nitrates, leaving the precipitate in the mother solution, filtering and washing, and drying, forming and calcination, characterized in that a solution of cobalt(II) nitrate at a concentration of from 50 to 120 g/dm³, nickel(II) nitrate at a concentration of from 0.01 to 35.00 g/dm³, zinc nitrate at a concentration of from 0.01 to 30.00 g/dm³ is stirred vigorously, preferably in a circulation system, and simultaneously a precipitating agent is added, at the temperature from ambient to 100 °C, wherein a ratio of the precipitating agent to the solution of cobalt, nickel and zinc nitrates is at least 1.0:0.4, for pH of the solution to be in the range of 9.0-9.5, resulting in the forming of a precipitate which is then left in the mother solution at the ambient temperature for at least 15 hrs, and filtered off and washed with water until obtaining the filtrate at pH 7.0-7.5, wherein the obtained precursor is dried at 120°C for at least 15 hrs and calcined at 400-450 °C for at least 4 hrs with gradual heating of the precipitate from 120 to 400 °C to prepare a final product, which is optionally comminuted, and then formed into desired shape and calcined at 450°C for at least 4 hrs.

5. A process according to claim 4, characterized in that solution cobalt(II) nitrate is prepared by dissolving cobalt(II) nitrate hexahydrate in water, at the room temperature.

6. A process according to claim 4, characterized in that solution nickel(II) nitrate is prepared by dissolving nickel(II) nitrate hexahydrate in water, at the room temperature.

7. A process according to claim 4, characterized in that solution zinc nitrate is prepared by dissolving zinc nitrate in water, at the ro.om temperature.

8. A process according to claim 4, characteristic in that as the precipitating agent an aqueous solution of potassium carbonate and/or sodium carbonate or of ammonia is used at the concentration of 150 g of reagent/1, which is added to the solution of nitrates at the rate of 2-5 cm³/min.

9. A process according to claim 4 characteristic in that alkali metal promoters, Na and/or K, and alkaline earth promoters, Mg and/or Ca, are added at the precipitation stage and/or during impregnation of the precursor after calcination.