MATERIAL FOR N2O DECOMPOSITION

Abstract

The present invention concerns a material with a non-stoichiometric spinel-type crystalline structure based on cobalt oxide doped with alkaline elements, its production process for obtaining it by precipitation with controlled washing, and its particular use as a highly active catalyst in the N.sub.2O decomposition reaction. Therefore, we understand that the present invention is in the area of green industry aimed at reducing N.sub.2O emissions into the atmosphere.

Claims

1-13. (canceled)

14. A material, comprising: a non-stoichiometric spinel-type crystal structure with a general formula Co.sub.3O.sub.4-x/2A.sub.y, wherein: x has a value between 0.02 and 0.3; A is an alkali element; y has a value of between 0.06 and 0.18; with an A/Co ratio between 0.02 and 0.10; a Co.sup.2+/Co.sup.3+ ratio between 0.55 and 0.80; and a primary particle size equivalent to the crystallite size between 5 and 30 nm.

15. The material according to claim 14, exhibiting a specific surface area BET of between 40 m.sup.2/g and 80 m.sup.2/g.

16. The material according to claim 14, exhibiting a pore volume of between 0.2 cm.sup.3/g and 0.4 cm.sup.3/g.

17. The material according to claim 14, wherein the material is mesoporous.

18. The material according to claim 14, wherein the alkali element A is K, x is 0.182, and y is 0.09.

19. The material according to claim 14, wherein the alkali element A is Cs, x is 0.235, and y is 0.15.

20. The material according to claim 14, wherein the material is configured as a catalyst.

21. The material according to claim 14, wherein the material is configured as a catalyst in oxidation/decomposition of gases.

22. The material according to claim 14, wherein the material is configured as a catalyst for decomposition of N.sub.2O.

23. A process for obtaining a material, the process comprising: a) dissolving a cobalt salt in water to obtain a first solution; b) dissolving an alkali metal salt or hydroxide in water to obtain a second solution; c) slowly adding the second solution obtained in step (b) to the first solution prepared in step (a) until a pH of between 8 and 11 is reached, thereby obtaining a solid; d) filtering the solid obtained in step (c) and washing the solid with 5 ml to 75 ml of water for each gram of cobalt salt added in step (a) to thereby obtain a second solid; e) drying the second solid obtained in step (d) at a temperature between 50° C. and 200° C. for a time between 12 h and 20 h to thereby obtain a third solid; and f) calcining the third solid obtained in step (e) at a temperature between 200° C. and 700° C. in an air atmosphere for at least 30 min.

24. The process according to claim 23, wherein the cobalt salt in step (a) is selected from at least one of cobalt nitrate hexahydrate, cobalt sulphate, cobalt chloride, or cobalt acetate.

25. The process according to claim 23, wherein the alkali metal salt or hydroxide in step (b) is selected from at least one of alkali metal carbonate, alkali metal nitrate, alkali metal hydroxide, or alkali metal acetate.

26. The process according to claim 25, wherein if the cobalt salt is cobalt nitrate hexahydrate and the alkali metal salt is alkali metal carbonate, the washing of step (d) is with an amount of water between 16 ml/g and 21 ml/g of cobalt nitrate hexahydrate.

27. The process according to claim 23 wherein the material includes: a non-stoichiometric spinel-type crystal structure with a general formula Co.sub.3O.sub.4-x/2A.sub.y, wherein: x has a value between 0.02 and 0.3; A is an alkali element; y has a value of between 0.06 and 0.18; with an A/Co ratio between 0.02 and 0.10; a Co.sup.2+/Co.sup.3+ ratio between 0.55 and 0.80; and a primary particle size equivalent to the crystallite size between 5 and 30 nm.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0038] FIG. 1 X-ray diffractograms of the materials of the invention.

[0039] FIG. 2 Scanning electron microscopy images of the materials of the invention.

[0040] FIG. 3 Pore size distribution of the materials of the invention obtained by adsorption-desorption isotherms of N.sub.2.

[0041] FIG. 4 XPS diagrams of the materials of the invention.

[0042] FIG. 5 Programmed temperature reduction with H.sub.2 of the materials of the invention.

[0043] FIG. 6 Desorption of O.sub.2 at programmed temperature of the materials of the invention.

[0044] FIG. 7 Conversion of N.sub.2O as a function of space time in the presence of O.sub.2 and H.sub.2O vapour of the materials of the invention and of materials reported in the state of the art.

EXAMPLES

[0045] The invention is illustrated below by means of the results of tests carried out by the inventors which demonstrate the effectiveness of the product of the invention.

Example 1

[0046] 14.84 grams of cobalt nitrate (Co(NO.sub.3).sub.2.6H.sub.2O) were dissolved in 100 ml of water and the solution was kept under stirring. 100 ml of a solution of potassium carbonate (K.sub.2CO.sub.3) 15% w/w was prepared, placed in a burette and slowly added to the cobalt nitrate solution. The addition of carbonate was continued until the pH reached was 9. The solid was filtered off and washed with 250 ml of water at 15° C. It was dried at 100° C. for 16 h and calcined at 400° C. for 2 h, to obtain a catalyst with the formula Co.sub.3O.sub.3.88K.sub.0.08.

Example 2

[0047] When a sample of the material obtained according to example 1 was introduced into a tubular reactor and a gas stream of Ar with an N.sub.2O concentration of 1400 ppm, was fed at a ratio (Gas flow rate:catalyst volume) GHSV=50,300 h.sup.−1, gradual heating of the gases inside the reactor resulted in a progressive decrease of the N.sub.2O concentration at the reactor outlet equivalent to N.sub.2O conversion values of 73% at 260° C., 95% at 280° C. and 98% at temperatures above 310° C.

[0048] When a sample of this material was introduced into a tubular reactor and a gas stream of Ar with an N.sub.2O concentration equal to 1400 ppm and O.sub.2=3% v/v was fed at a ratio (total gas flow rate:catalyst volume) GHSV=50,300 h.sup.−1, gradual heating of the gases inside the reactor resulted in a progressive decrease of the N.sub.2O concentration at the reactor outlet, equivalent to N.sub.2O conversion values of 17% at 250° C., 88% at 300° C. and 96% at temperatures above 350° C.

[0049] When a sample of this material was introduced into a tubular reactor and a gas stream of Ar with an N.sub.2O concentration equal to 1400 ppm and [H.sub.2O]=0.5% v/v was fed at a ratio (Total gas flow rate:catalyst volume) GHSV=50,300 h.sup.−1, gradual heating of the gases inside the reactor resulted in a progressive decrease of the N.sub.2O concentration at the reactor outlet, equivalent to N.sub.2O conversion values of 25% at 250° C., 75% at 280° C. and 98% at temperatures above 340° C.

[0050] When a sample of the material obtained according to example 1 was introduced into a tubular reactor and a gas stream of Ar with a concentration of N.sub.2O equal to 1400 ppm, [O.sub.2]=3% v/v and H.sub.2O=0.5% v/v was fed at a ratio (Total gas flow:catalyst volume) GHSV=50,300 h.sup.−1, gradual heating of the gases inside the reactor resulted in a progressive decrease of the N.sub.2O concentration at the reactor outlet, equivalent to N.sub.2O conversion values of 17% at 260° C., 42% at 275° C., 73% at 290° C., 94% at 315° C. and 97% at temperatures above 350° C.

[0051] When a sample of the material obtained according to example 1 was introduced into a tubular reactor and a stream of Ar gas was fed, with a concentration of N.sub.2O equal to 1400 ppm, [O.sub.2]=3% v/v and H.sub.2O=0.5% v/v, at a ratio (Total gas flow:catalyst volume) GHSV=50,300 h.sup.−1, keeping the reaction temperature equal to 360° C., an initial N.sub.2O conversion of 93% was obtained, experiencing a slight increase in conversion to 96% over 65 hours in reaction.

Example 3

[0052] 14.84 grams of cobalt nitrate (Co(NO.sub.3).sub.2.6H.sub.2O) were dissolved in 100 ml of water and the solution was kept under stirring. 100 ml of a solution of caesium carbonate (Cs.sub.2CO.sub.3) at 30% w/w was prepared. This solution was poured into a burette. The carbonate solution was slowly added and maintained until the pH reached was 9. The total volume of carbonate added was 57.5 ml. The solid was filtered off and washed with 220 ml of water at 15° C. It was dried at 100° C. for 16 h and calcined at 400° C. for 2 h to obtain a material with the formula Co.sub.3O.sub.3.88Cs.sub.0.15.

Example 4

[0053] When a sample of the material obtained according to example 3 was introduced into a tubular reactor and a gas stream of Ar with a concentration of N.sub.2O equivalent to 1400 ppm, [O.sub.2]=3% v/v and H.sub.2O=0.5% v/v was fed at a ratio (Total gas flow:catalyst volume) GHSV=50,300 h.sup.−1, gradual heating of the gases inside the reactor resulted in a progressive decrease of the N.sub.2O concentration at the reactor outlet equivalent to N.sub.2O conversion values of 50% at 280° C., 80% at 300° C., 93% at 320° C. and 97% at temperatures above 340° C.

Example 5

[0054] 59.36 grams of cobalt nitrate (Co(NO.sub.3).sub.2.6H.sub.2O) were dissolved in 400 ml of water and the solution was kept under stirring. 400 ml of a solution of caesium carbonate (Cs.sub.2CO.sub.3) at 30% w/w was prepared. This solution was poured into a burette. The carbonate solution was slowly added and maintained until the pH reached was 9. The total volume of carbonate added was 207 ml. The solid was filtered off and washed with 1160 ml of water at 15° C. It was dried at 100° C. for 16 h and calcined at 400° C. for 2 h to obtain a material with the formula Co.sub.3O.sub.3.88Cs.sub.0.06.

Example 6

[0055] When a sample of the material obtained according to example 5 was introduced into a tubular reactor and a gas stream of Ar with a concentration of N.sub.2O equivalent to 1400 ppm, [O.sub.2]=3% v/v and H.sub.2O=0.5% v/v was fed at a ratio (Total gas flow:catalyst volume) GHSV=24,000 h.sup.−1, gradual heating of the gases inside the reactor resulted in a progressive decrease of the N.sub.2O concentration at the reactor outlet equivalent to N.sub.2O conversion values of 47% at 250° C., 88% at 280° C., 95% at 300° C. and 99% at temperatures above 320° C.

Example 7

[0056] 14.84 grams of cobalt nitrate (Co(NO.sub.3).sub.2.6H.sub.2O) were dissolved in 100 ml of water and the solution was kept under stirring. 100 ml of a solution of potassium carbonate (K.sub.2CO.sub.3) at 15% w/w was prepared, placed in a burette and the potassium carbonate solution was slowly added to the cobalt nitrate solution and the addition of carbonate was continued until the pH reached was 9. The total volume of carbonate added was 54 ml. The solid was filtered off and washed with 400 ml of water at 15° C. It was dried at 100° C. for 16 h and calcined at 400° C. for 2 h, obtaining a K-free material with the formula Co.sub.3O.sub.4.

Example 8

[0057] When a sample of the material obtained according to example 7 was introduced into a tubular reactor and an Ar gas stream with an N.sub.2O concentration equivalent to 1400 ppm was fed at a ratio (Gas flow rate:catalyst volume) GHSV=50,300 h.sup.−1, gradual heating of the gases inside the reactor resulted in a progressive decrease of the N.sub.2O concentration at the reactor outlet, equivalent to N.sub.2O conversion values of 9% at 280° C., 20% at 320° C., 34% at 360° C. and 44% at 380° C. A decrease in N.sub.2O conversion due to the absence of the alkaline element was observed throughout the range of temperature tested.

Example 9

[0058] X-ray diffractograms (XRD) of the materials described in examples 1 and 3 show that these materials have a cubic structure close to that of the Co.sub.3O.sub.4 spinel described in example 7 (JCPDS 00-042-1467) (FIG. 1). The average crystallite size was calculated according to the Scherrer equation respectively obtaining values around 18 nm and 10 nm.

[0059] Scanning Electron Microscopy micrographs of these materials (FIG. 2) showed an average primary particle size between 10 and 20 nm for example 1 and 8-15 nm for example 3.

[0060] These microphotographs show the general surface appearance of the materials, formed by agglomerates of primary particles with sizes similar to those obtained for crystallite sizes by XRD, and with pores whose diameter falls within the range of mesoporous materials (2-50 nm), as determined by N.sub.2 adsorption-desorption isotherms (FIG. 3).

Example 10

[0061] The samples of the materials described in examples 1 and 7 were analysed by X-ray photoelectron spectroscopy (XPS) showing a shift of the Co 2p level in the sample Co.sub.3O.sub.3.88K.sub.0.08 (example 1) towards lower binding energies with respect to the sample without K (example 7) which is explained by the increase in the proportion of Co(II) (CoO) species that are stabilised by the electron donation effect promoted by the presence of K.sup.+ ions (FIG. 4).

[0062] According to these results, the material obtained following the procedure described in the invention is clearly different, as regards its redox properties, with respect to those previously described.

Example 11

[0063] Programmed temperature reduction experiments were performed on the materials described in examples 1 and 3, detecting a shift of the first reduction peak towards lower temperatures (246 and 262° C. respectively for Co.sub.3O.sub.3.88K.sub.0.08 and Co.sub.3O.sub.3.88Cs.sub.0.15) compared to those reported in the state of the art for conventional spinel (FIG. 5). From these results, the Co.sup.2+/Co.sup.3+ ratio was calculated, obtaining a value higher than the stoichiometric value (0.7 versus 0.5). This increase in the ratio of Co(II) with respect to Co(III) in the spinel lattice leads to the appearance of a certain proportion of oxygen vacancies, which give the surface of the material special properties for adsorbing and activating the N.sub.2O molecule. This changes the stoichiometry of the spinel, making it oxygen deficient.

[0064] According to these results, the material obtained following the procedure described in the invention is clearly different, as regards its redox properties, with respect to those previously described.

Example 12

[0065] O.sub.2 temperature-programmed desorption (O.sub.2.TPD) experiments were performed with the materials described in examples 1, 3 and 7. In the materials whose washing was controlled (examples 1 and 3), the peak relative to surface oxygen (P.sub.O2-I) appeared at temperatures around 100° C. (FIG. 6), compared to 190° C. for the material without K (example 7).

[0066] Furthermore, the peak relative to the lattice oxygen (P.sub.O2-II) shifted to lower temperatures (180-350° C.) whereas in the K-free spinel this peak appeared from 300° C. onwards.

[0067] Based on these results, it can be concluded that the materials covered by this patent are clearly different in terms of their O.sub.2 adsorption/desorption capacity.

[0068] The most accepted mechanism for the N.sub.2O decomposition reaction is through adsorption of N.sub.2O on an active centre [A], releasing N.sub.2 and leaving an O atom adsorbed on that centre. A second N.sub.2O molecule is adsorbed on this centre, giving rise to another N.sub.2 molecule. The two adsorbed O atoms must recombine to form molecular O.sub.2, this being the limiting stage of the reaction:

N.sub.2O.sub.(g)+[A].fwdarw.N.sub.2 (g)+[O.sup.−-A] [O.sup.−-A]+N.sub.2O.sub.(g).fwdarw.N.sub.2 (g)+O.sub.2 (g)+[A]

[0069] The O.sub.2 DTP results obtained show that the samples described in examples 1 and 3 have the ability to perform the O.sub.2 desorption processes from the lattice oxygen (P.sub.O2-II) at a temperature (200-300° C.) significantly lower than that required by the sample with conventional spinel structure (>300° C.). This can be considered to be related to the lowering of the temperature required by these new catalysts to carry out the N.sub.2O decomposition process under the indicated conditions.

Example 13

[0070] A comparison is made of the data obtained from the catalyst obtained according to example 1 of the present invention against the data described in the prior art documents, “D1” [Li Xue, Changbin Zhang, Hong He, Yasutake Teraoka, Applied Catalysis B: Environmental, Volume 75, Issues 3-4, 2007, Pages 167-174, FIG. 8], where, for instance, at 350° C., it is possible to observe that the conversion obtained with this catalyst, under conditions of humidity and the presence of O.sub.2, is close to 77%, compared to the 96% conversion obtained with the Co.sub.3O.sub.3.88K.sub.0.08 material. To compare these results, the data have been analysed as a function of the space velocity (GHSV) at which the different experiments have been carried out. FIG. 7 shows the results of N.sub.2O conversion under conditions of humidity and in the presence of O.sub.2 for the mentioned catalysts as a function of space time or contact time, i.e. considering the volume of the catalyst and the flow rate used in each experiment (T=V.sub.cat/F=1/GHSV). This comparison highlights the great improvement in the catalytic activity of this material compared to the state of the art since a shorter contact time (lower volume of catalyst) is needed to achieve similar or higher conversions.

[0071] The following table shows the values of the reaction constant at 350° C., K.sub.350° C., calculated considering that the reaction runs according to a first-order kinetics, which would be the best quantitative expression of the catalytic activity.

TABLE-US-00001 TABLE 1 Values of the reaction constant of the catalysts. 1st order eq. Catalyst K.sub.350° C. (s.sup.−1) Co.sub.3O.sub.3.88K.sub.0.08 45.0 Cat Co.sub.3O.sub.4 (D1) 2.9

[0072] According to these data, the increase in reaction rate produced by the catalysts covered by the patent is more than an order of magnitude greater than the best described in the article mentioned in this example.

[0073] On the other hand, the catalyst described in this article, D1, as Co.sub.3O.sub.4, showed a binding energy of the Co 2p 3/2 component of 780.1 eV similar to the K-free spinel described in example 7 (FIG. 3), and the desorption of the lattice oxygen obtained by O.sub.2 DTP takes place, as with the K-free spinel described in example 7, at 300° C. and above (FIG. 5).

Example 14

[0074] The material obtained according to example 1 and according to example 3 of the present invention and the data shown in the prior art document, “D2” [Paweł Stelmachowski, Gabriela Maniak, Andrzej Kotarba, Zbigniew Sojka, Catalysis Communications, Volume 10, Issue 7, 2009, Pages 1062-1065, FIG. 7] are compared. It can be seen from FIG. 7 of “D2” that under humid conditions (option b) the Cs-doped catalyst achieves a conversion value close to 90% at 350° C. The K-doped catalyst is significantly less active, since at this temperature the estimated conversion in this curve is close to 50%, while the conversion of the materials in examples 1 and 5 is respectively 96 and 98%, using a much smaller volume of catalyst (shorter contact time).

[0075] Furthermore, the following table shows the values of the reaction constant at 350° C., K.sub.350° C., calculated considering that the reaction runs according to a first-order kinetics, which would be the best quantitative expression of the catalytic activity.

TABLE-US-00002 TABLE 2 Values of the reaction constant of the catalysts. 1st order eq. Catalyst K.sub.350° C. (s.sup.−1) Co.sub.3O.sub.3.88K.sub.0.08 45.0 Co.sub.3O.sub.3.88Cs.sub.0.15 49.0 Cat Cs (D2) 4.3

[0076] According to these data, the increase in reaction rate produced by the materials covered by the patent is more than an order of magnitude greater than the best described in the article mentioned in this example.