WO2012114288A1

WO2012114288A1 - Composite catalyst for the low temperature decomposition of nitrous oxide, and method of manufacture thereof

Info

Publication number: WO2012114288A1
Application number: PCT/IB2012/050810
Authority: WO
Inventors: Marcin Wilk; Marek Inger; Magdalena SARAMOK; Paweł KOWALIK; Zbigniew Sojka; Andrzej Kotarba; Paweł STELMACHOWSKI; Witold Piskorz; Filip Zasada; Andrzej Adamski; Gabriela MANIAK; Ewelina Franczyk
Original assignee: Instytut Nawozów Sztucznych; Uniwersytet Jagielloński
Priority date: 2011-02-22
Filing date: 2012-02-22
Publication date: 2012-08-30
Also published as: EP2678104A1

Abstract

A composite catalyst according to the present invention contains cobalt oxide at 25 - 88.6 % by weight, zinc oxide at 10.99 - 60 % by weight as the substantive components, wherein the zinc oxide is introduced in two stages, during precipitation as well as prior to the forming of the catalyst. The ratio of zinc oxide introduced during the precipitation to the zinc oxide added prior to the forming of the catalyst is 1 :0.26-13.3and it also contains components which improve activity, in the form of alkali metals at 0.01 -5 % by weight calculated in terms of oxides of these metals, and graphite at 0.4-10 %. Preferably, the alkali metals present in the catalyst composition are sodium and/or potassium.

Description

Composite catalyst for the low temperature decomposition of nitrous oxide, and method of manufacture thereof

The subject of the present Invention Is a composite catalyst for the low-temperature decomposition of nitrous oxide in tail gases from nitric acid installations as well as a method of producing It.

Under the provisions of Kyoto Protocol of 1997, nitrous oxide has been defined as one of the six greenhouse gases, whose potential for causing this effect is some 296 times that of carbon dioxide. One o1 the largest sources of of nitrous oxide emission is the chemical Industry, and In particular, nitric acid Installations, whose participation In the global emission of this gas Is 20 %.

Nitrous oxide is formed as a by-product of the catalytic oxidation of ammonia, During the technological process of nitric acid manufacturing, the nitrous oxide does not undergo any reactions and Is emitted Into the atmosphere In Its entirety along with the other tail gases.

The removal of nitrous oxide in nitric acid installations can occur by way of decomposition or reduction In the presence of an appropriate catalyst, wherein this process can be performed at both high and low temperatures. At high temperatures, 800 - 940^eC, the decomposition of nitrous oxide occurs in the stream of nitroeo gases rich in nitric oxide NO, formed due to ammonia oxidation, During the low-temperature process, at 200 - 450^eC, the removal of nitrous oxide may be performed by way of the catalytic reduction or catalytic decomposition in the tall, gas stream, which Is directed Into the atmosphere upon NO_x reduction. Economically more preferable solution is the catalytic decomposition of nitrous oxide, compared to reduction, due to the lack of need for the use of additional reducing substances. The main limitation of the direct use of decomposition is that the low temperature of the tail gases that prevents effective catalyst functioning.

The low-temperature process may be realised through catalytic reduction or decomposition, In the case of reduction, the most frequently used reductant Is ammonia or hydrocarbons. For economic reasons, direct decomposition Is preferable. The main limitation of the direct use of decomposition is that the low temperature of the tail gasses that prevents effective catalyst functioning.

Patent description WO2001/051181 discloses a two-stage removal process for NO* and nitrous oxide from the tail gases of nitric acid installations at low temperature area. According to said description, the removal of nitrous oxide may occur through the catalytic decomposition on a zeolite catalyst at a temperature of 425 - 520"C or through catalytic reduction with a hydrocarbon at a temperature of 300 - 520^aC, Patent description No. PCT/NO02/00439 discloses a method of obtaining and activating a zeolite catalyst, its composition and use in nitrous oxide removal.

The problem of the catalytic decomposition of nitrous oxide is also the subject of many scientific publications. Research has been performed on many different systems: metallic, oxides with various structures as well as mixed systems. A noteworthy place among the catalytic systems is held by oxide systems with a spinel structure. The appropriate modification of cobalt oxide Co₃0₄ with heteroatoms allows to obtain a catalyst active in tail gases at temperature occurring in nitric acid installations. The hetero-atomic modification of cobalt oxide can be of two types: structural and surface modification.

In the case of structural modification, a portion of the cobalt ions is replaced with ions of other metals, usually nickel, magnesium or zinc, not affecting the structure of the spinel. In the case of surface modifications, group I metal ions are introduced onto the surface of the catalyst.

The publication by Y. Liang et al. Catalysis Communications 4 (2003) 505- 509; Y. Liang et al. Applied Catalysis B: Environmental 45 (2003) 85-90 reports the positive effect of structural modifications on catalyst activity. The positive effect of surface modifications with such additives as potassium, sodium, barium and cerium, is known from F. Zasada et al. Catalysis Letters 127 (2008) 126-131 , K. Karaskova et al. Chemical Engineering Journal 160 (2010) 480^187, L. Obalova et al. Applied Catalysis B: Environmental 90 (2009 132-140. The concentrations of the above mentioned ions on a surface has a significant effect on catalytic activity.

Polish patent description No. 386 890, 2008 discloses a catalyst for the low-temperature decomposition of nitrous oxide, based on cobalt oxide which contains, in addition to the main component, nickel oxide, zinc oxide as well as alkali earth metal oxides, such as Ca and/or Mg and alkali metal oxides, such as Na and/or K.

The publications P. Stelmachowski et al. Catalysis Letters 130 (2009) 637 - 641 ; M. Inger et al. Przemysl Chemiczny 12 (2009) 1307 - 1313 disclose a synergistic effect of structural and surface modification of a catalyst and the effect of these modifications on its activity.

The goal of the present invention is to deliver a composite catalyst for removing nitrous oxide from the tail gasses of nitric acid installation at a temperature below 450 °C in a form that facilitates ts use in an industrial reactor, as well as to present a method of producing it. The active components are ions of cobalt, zinc and potassium, possibly sodium, and the catalyst composition additionally comprises zinc oxide and graphite not incorporated in the spinel structure, which enhance the durability of the catalyst tablets.

The composite catalyst according to the present invention contains cobalt oxide at 25 - 88.6 % by weight, zinc oxide at 10.99 - 60 % by weight as the substantive components, wherein the zinc oxide is introduced in two stages, during precipitation as well as prior to the forming of the catalyst. The ratio of zinc oxide introduced during the precipitation to the zinc oxide added prior to the formation is 1 :0.26-13.3 and it also contains components which improve activity in the form of alkali metals at 0.01 -5 % by weight calculated in terms of the oxides of these metals, and graphite at 0.4-10 %. Preferably, the alkali metals present in catalyst composition are sodium and/or potassium.

The essence of the present invention relating to a method of producing a composite catalyst is based on the fact that a solution with a total concentration of cobalt and zinc cations in the range of 0.8-2.5 mol/dm³ and the ratio of cobalt to zinc in the range of 1 :0.12-0.17 is mixed, preferably in a circulatory system, and potassium carbonate or potassium hydroxide at a concentration of 1 .0-1 .5 mol/dm³ is added in an amount at least 1 :0.4 to a solution of cobalt nitrate and zinc nitrate to reach a pH of 9-10. The precipitate is rinsed to reach a potassium concentration of 0.4 - 1 % by weight, and then it is dried and calcined at a temperature of 450 °C and possibly impregnated with a solution of potassium carbonate at a concentration of 0.03 - 0.1 mol/dm³. Next, zinc oxide calcined at a temperature of 250 - 600 °C is added at 1 - 50% by weight and graphite at 0.4 - 10 % by weight. The mixture is thoroughly mixed and formed in molds, which results in the formation of a product, active in the decomposition reaction of nitrous oxide at a temperature below 450 °C.

During research performed using various catalytic systems it unexpectedly turned out that a cobalt catalyst with an addition of zinc, which at the same time contains zinc oxide as a separate phase induces an increase of the relative reaction rate of the decomposition of nitrous oxide, calculated per mass of the active phase.

An additional advantage of such use of zinc oxide is a better dispersion of the active phase nanoparticles which facilitates access of reagents and removal of reaction products of the decomposition of nitrous oxide. Furthermore, using the selection of the calcination temperature of zinc oxide as well as the amount of added graphite, one can control the size of zinc oxide grains as well as the porosity of the entire system of diphase catalyst.

Unexpectedly, it turned out that the method of introducing zinc at the precipitation stage and then at the formation stage after calcination induces an increase of the relative reaction rate of the decomposition of nitrous oxide (calculated in terms of active phase mass). Furthermore the use of this solution facilitates a cost reduction of catalyst production.

Physicochemical characteristics determined using XRD, BET, SEM and TEM microscopy shows that the introduction of zinc oxide as well as graphite during the formation stage does not disrupt the spinel structure, nor the grain shape, and the obtained catalyst exhibits a high composition and texture homogeneity. The use of this solution facilitates a cost reduction during catalyst production.

Composite catalysts of various compositions according to the present invention were tested in a quartz reactor, through which passed a mixture of nitrous oxide and helium or tail gases from a pilot nitric acid installation. The composition of the tail gases was identical to that occurring in industrial nitric acid installation. The research on the 5 % of nitrous oxide in helium mixture was performed in a flow-through quartz reactor with sinter, in the range of temperatures from ambient to 450 °C, at a GHSV load of 7000 h^" . The composition of the post-reaction mixture was determined using a mass spectrometer, measuring the partial pressures of nitrous oxide as well as its decomposition products: oxygen and nitrogen. It was determined that at the temperature of 350 °C the degree of decomposition of nitrous oxide is 50-100 %, depending on ZnO content. The only decomposition products were nitrogen and molecular oxygen. Research performed on tail gases from nitric acid installation confirms the catalytic activity despite the presence of NO_x, oxygen and water vapour in tail gas stream.

The present invention is illustrated by the following examples:

Example 1

In a vessel of 10000 cm³ volume it was dissolved 1554.0 g of cobalt(ll) nitrate hexahydrate and 244.5 g of zinc (II) nitrate hexahydrate in 3100 cm³ of distilled water such a way that the total cation concentration in the solution was 2M, and the ratio of cobalt ions to zinc ions was 1 :0.153. The resulting solution mixture was mixed using a mechanical mixer, at the same time adding the precipitant in the form of a solution of potassium carbonate at a concentration of 1 .2 mol/dm3. This was performed until a pH of solution was reached at the range of 9.0-9.5, which resulted in precipitation. The precipitate was maintained in the parent solution for 15 h at ambient temperature, after which the precipitate was drained off and rinsed, until reaching a pH of filtrate equal 7.0-7.5. The resulting precipitate was dried at a temperature of 120 °C for 15 hours, and then the filtrate was impregnated with a 0.06 molar solution of potassium carbonate, dried at a temperature of 120 °C and calcined at a temperature of 400-450 °C for4 hours, gradually increasing the temperature from 120 to 400 °C. The mixer was loaded with 450 gof the initially produced catalyst, 150 g of zinc oxide calcined at a temperature of 450 °C, which constitutes 24.88 % by weight, as well as 3 g of graphite which constitutes 0.5 % by weight and mixed until homogeneity was obtained. Next, the resulting mass was formed into tablets with the following dimensions d = 5 mm, h = 5 mm. It was obtained a composite catalyst containing 63.16 % by weight Co₃0₄, 35.71 % by weight ZnO, 0.63 % by weight K₂0 as well as 0.50 % by weight C, calculated in terms of oxides. A phase analysis using powder diffraction demonstrated the presence of only spinel and zinc oxide phases.

The specific surface area determined via N₂-BET method was 39 m²/g. The activity test tail gases from a nitric acid installation with the following composition: 800 ppm NOx, 1450 ppm N₂0, 0,40 % vol. H₂0, 1 .50 % vol. 0₂, N₂ - remainder, was used at the temperature of 380 °Cand an a catalyst (GHSV) load of 10 000 h^" obtained a degree of decomposition of N₂0 of 93 %.

Example 2 In a vessel of 10000 cm³ volume we dissolved 1554.0 g of cobalt(ll) nitrate hexahydrate and 244.5 g of zinc (II) nitrate hexahydrate in 3100 cm³ of distilled water such a way that the total cation concentration in the solution was 2M, and the ratio of cobalt ions to zinc ions was 1 :0.153. The resulting solution mixture was mixed using a mechanical mixer, at the same time adding the precipitant in the form of a solution of potassium hydroxide at a concentration of 1 .5 mol/dm³. This was performed until a pH of solution was reached at the range of 9.0-9.5, which resulted in precipitation. The precipitate was maintained in the parent solution for 15 h at ambient temperature, after which the precipitate was drained off and rinsed with volumes of water, controlling the potassium content in the precipitate. When the potassium concentration in the precipitate became 1 .4 % by weight, the rinse was terminated, and the resulting precipitate was dried at a temperature of 120 °C for 15 hours and calcined at a temperatue of 400-450 °C for 4 hours, gradually increasing the temperature from 120 to 400 °C. The mixer was loaded with 300 g of the produced catalyst precursor, 300 g of zinc oxide calcined at a temperature of 450°C, which constitutes 49.75 % by weight, as well as 3 g of graphite which constitutes 0.5 % by weight and mixed until homogeneity was obtained.

Next, the resulting mass was formed into tablets with the following dimensions: d=5 mm, h=5 mm.

It was obtained a composite catalyst containing 42.07 % by weight Co₃0₄, 57.01 % by weight ZnO, 0.42 % by weight K₂0 as well as 0.50 % by weight C. A phase analysis using powder diffraction demonstrated the presence of only spinel and zinc oxide phases. The specific surface area determined via N₂-BET method was 28 m²/g. The activity test tail gasses from a nitric acid installation with the following composition: 1500 ppm NO_x, 560 ppm N₂0, 0.90 % vol. H₂0, 1 .55 % vol. 0₂, N₂ - remainder, was used at the temperature of 400 °C and a catalyst (GHSV) load of 10000 h^" obtained a degree of decomposition of N₂0 of 92 %.

Example 3 In a vessel of 10000 cm³ volume we dissolved 1554.0 g of cobalt(ll) nitrate hexahydrate and 244.5 g of zinc (II) nitrate hexahydrate in 3100 cm³ of distilled water. The resulting solution mixture was mixed using a mechanical mixer, at the same time adding the precipitant in the form of a solution of potassium carbonate at a concentration of 1 .2 mol/dm³. This was performed until a solution pH was reached at the range of 9.0-9.5. The precipitate was maintained in the parent solution for 15 h at ambient temperature, after which the precipitate was drained off and rinsed. The resulting precipitate was dried and then the precipitate was impregnated with a 0.1 molar solution of potassium carbonate, dried and calcined. 450 g of the obtained mass was mixed until homogeneity was obtained with 150 g of zinc oxide calcined at the temperature of 450 °C, which constitutes 49.75 % by weight, as well as 3 g of graphite .

It was obtained a composite catalyst containing 63.16 % by weight Co₃0₄, 35.71 % by weight ZnO, 0.63 % by weight K₂0 as well as 0.50 % by weight C, calculated in terms of oxides.

Example 4 Catalyst was prepared using the method of Example 3, with the difference that the impregnation was made with the use of a 0.8 mol/dm³ solution of sodium carbonate in place of the potassium carbonate solution, and then the resulting 450 g of dry mass was supplemented with 150 g of zinc oxide as well as 5 g of graphite and thoroughly mixed.

We obtained a composite catalyst containing 62.95 % by weight Co₃0₄, 35.63 % by weight ZnO, 0.02 % by weight K₂0 as well as 0.60 % Na₂0 and 0.80 % by weight C.

Claims

1 . A composite catalyst for the low-temperature decomposition of nitrous oxide in tail gases from nitric acid installation on the basis of cobalt oxide characterised in that it contains cobalt oxide at 25 - 88.6 % by weight and zinc oxide at 10.99 - 60 % by weight as the substantive components, wherein the zinc oxide is introduced in two stages, during precipitation as well as prior to the forming of the catalyst, and the ratio of zinc oxide introduced during the precipitation to zinc oxide added prior to the forming of the catalyst is 1 :0.26-13.3 as well as containing components which improve activity, in the form of alkali metals at 0.01 -5 % by weight calculated in terms of oxides of these metals, and graphite at 0.4-10 %.

2. Catalyst according to Claim 1 characterised in that it contains Na and/or K as the alkaline metals.

3. A method of producing a composite catalyst for the low-temperature decomposition of nitrous oxide in tail gases from nitric acid installations through the precipitation from a solution of cobalt nitrate and zinc nitrate using a precipitant, which precipitate is filtered, rinsed, dried, and calcined, characterised in that a solution with a total concentration of cobalt and zinc cations in the range of 0.8-2.5 mol/dm³ and a ratio of cobalt to zinc in the range of 1 :0.12-0.17 is mixed, preferably in a circulatory system, and a solution of potassium carbonate or potassium hydroxide at a concentration of 1 .0-1 .5 mol/dm³ is added in an amount at least 1 :0.4 to a solution of cobalt nitrate and zinc nitrate to obtain a pH of 9-10, after which the precipitate is rinsed to obtain a potassium concentration of 0.4 - 1 % by weight, dried and calcined at a temperature of 450 °C and possibly impregnated with a solution of potassium carbonate at a concentration of 0.03 - 0.1 mol/dm³, and then calcined zinc oxide is added at 1 - 50 % by weight and graphite at 0.4 - 10 % by weight in relation to the to precipitate, and the mixture is thoroughly mixed and formed in molds.

4. A method according to Claim 3 characterised in that it the zinc oxide is calcined at a temperature of 250 - 600 °C.