Mixed metal oxide catalyst and production of nitric oxide by oxidation of ammonia

Abstract

The present invention provides a catalyst for production of nitric oxide from ammonia and oxygen. The catalyst has the composition A.sub.3-xB.sub.xO.sub.9-y, wherein A and B are selected from the group Mn, Co, Cr, Fe and Al, x is between 0 and 3 and y is between 0 and 6. The catalyst has a high selectivity towards nitric oxide and a low ignition temperature in the reactor. Further the present invention relates to a method for the production of gas comprising nitric oxide by the catalyst of the present invention. The produced gas has a low content of nitrous oxide.

Claims

1. A method for the production of a gas comprising nitric oxide, the method comprising: converting a gas blend comprising ammonia and oxygen in a reactor at a reactor temperature T.sub.c of 800 to 950° C. in the presence of a catalyst comprising the composition A.sub.3-xB.sub.xO.sub.4, wherein a combination of A and B are selected from: A=Mn or Cr and B═Co, and x is 0<x≦1.5.

2. The method according to claim 1, further comprising the steps of: (a) continuously feeding said gas blend to the reactor comprising said catalyst, whereby a temperature of the feed T.sub.a1 is increased until ignition of a reaction at ignition temperature T.sub.b, and (b) thereafter adjusting a temperature of the feed T.sub.a2 to achieve the reactor temperature T.sub.c.

3. The method according to claim 2, wherein the ignition temperature T.sub.b of said gas blend is 240° C. to 465° C.

4. The method according to claim 3, wherein the ignition temperature T.sub.b, of said gas blend is 240° C. to 380° C.

5. The method according to claim 4, wherein the ignition temperature T.sub.b of said gas blend is 240° C. to 300° C.

6. The method according to claim 5, wherein the ignition temperature T.sub.b of said gas blend is 240° C. to 270° C.

7. The method according to claim 1, wherein the selectivity of the conversion towards nitrous oxide and nitrogen dioxide is 90% to 96%.

8. The method according to claim 7, wherein the selectivity of the conversion towards nitrous oxide and nitrogen dioxide is 92% to 96%.

9. The method according to claim 8 wherein the selectivity of the conversion towards nitrous oxide and nitrogen dioxide is 94% to 96%.

10. The method according to claim 9, wherein the selectivity of the conversion towards nitrous oxide and nitrogen dioxide is 95% to 96%.

11. The method according to claim 1, wherein the produced gas has a concentration of nitrous oxide lower than 500 ppm.

12. The method according to claim 11, wherein the produced gas has a concentration of nitrous oxide lower than 400 ppm.

13. The method according to claim 12, wherein the produced gas has a concentration of nitrous oxide lower than 300 ppm.

14. The method of claim 1 wherein the reactor temperature T.sub.c is from 800 to 900° C.

15. The method of claim 1 wherein the reactor temperature T.sub.c is from 850 to 950° C.

16. The method of claim 15 wherein the reactor temperature T.sub.c is from 900 to 950° C.

17. A method for converting a gas blend comprising ammonia and oxygen to nitric oxide, comprising contacting a catalyst selected from the group consisting of Mn.sub.1.5Co.sub.1.5O.sub.4, Cr.sub.1.5Co.sub.1.5O.sub.4, Cr.sub.0.5Co.sub.2.5O.sub.4 and mixtures thereof with the blend in a reactor at a reactor temperature T.sub.c of 800 to 950° C.

18. The method of claim 17 wherein the catalyst is Mn.sub.1.5Co.sub.1.5O.sub.4.

19. A method for the production of a gas comprising nitric oxide, the method comprising: converting a gas blend comprising ammonia and oxygen to nitric oxide in a reactor at a reactor temperature T.sub.c of 800 to 950° C. in the presence of a catalyst selected from Cr.sub.0.5Co.sub.2.5O.sub.4, Cr.sub.1.5Co.sub.1.5O.sub.4 or mixtures thereof.

20. The method of claim 19 wherein the catalyst is Cr.sub.1.5Co.sub.1.5O.sub.4.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the experimental properties of Co.sub.1.5Cr.sub.1.5O.sub.4 which is one embodiment of the catalyst of the present invention.

(2) FIG. 2 shows the selectivity calculated from FIG. 1.

(3) FIG. 3 shows the experimental properties of Mn.sub.1.5Co.sub.1.5O.sub.4 which is one embodiment of the catalyst of the present invention.

(4) FIG. 4 shows the selectivity calculated from FIG. 3.

(5) The following non-limiting examples illustrate certain embodiments of the invention.

EXAMPLES

Example 1

(6) The catalysts were tested in an atmospheric pressure reactor, with an internal diameter of 8 mm. The catalyst bed (0.15 cm.sup.3), consisting of catalyst granules in the size range of 0.2 to 0.5 mm, was supported on a quartz frit. The gas feedstock, consisting of 10 volume % ammonia in air or 20% oxygen/80% argon was passed through the catalyst bed, at a rate of 3 N l/min, and the product gas was analysed using infrared spectroscopy and mass spectrometry.

(7) The experimental procedure involved increasing the temperature of the gas feedstock at a rate of 5° C./min until the catalyst initiated combustion, defined as an increase in temperature in the catalyst bed (measured with a thermocouple placed in the catalyst bed) at a rate exceeding 20° C./min second. After ignition, the temperature of the gas feedstock was adjusted, sequentially, to give catalyst bed temperatures of 800, 850 and 900° C. The nominal duration of each of these temperatures was 30 minutes.

(8) The ignition temperatures towards formation of NO and NO.sub.2, of the catalysts described in Table 1, are shown in Table 2.

(9) TABLE-US-00002 TABLE 2 Ignition temperatures of mixed oxide catalysts in air + 10% NH.sub.3 Values of x Base System 0 0.5 1.0 1.5 2.0 2.5 Mn.sub.3−xCo.sub.xO.sub.9−y 322 305 309 272 317 Mn.sub.3−xAl.sub.xO.sub.9−y 331 305 314 291 202 Mn.sub.3−xCr.sub.xO.sub.9−y 336 310 292 276 278 Cr.sub.3−xAl.sub.xO.sub.9−y 281 269 276 Cr.sub.3−xCo.sub.xO.sub.9.−y 281 263 303 294 317 374 Fe.sub.3−xCo.sub.xO.sub.9−y 453 461 >480 Fe.sub.3−xAl.sub.xO.sub.9−y

(10) The selectivity towards formation of NO and NO.sub.2 of the catalysts described in Table 1, are shown in Table 3.

(11) TABLE-US-00003 TABLE 3 Selectivity of mixed-oxide catalysts towards NO + NO.sub.2 in 10% NH.sub.3 + 18% O.sub.2 + 72% Argon Values of x Base System 0 0.5 1.0 1.5 2.0 2.5 Mn.sub.3−xCo.sub.xO.sub.9−y 94 95 96 95 93 Mn.sub.3−xAl.sub.xO.sub.9−y 82 Mn.sub.3−xCr.sub.xO.sub.9−y 93 89 84 83 90 Cr.sub.3−xAl.sub.xO.sub.9−y 90 92 84 70 75 74 Cr.sub.3−xCo.sub.xO.sub.9.−y 90 92 80 93 94 Fe.sub.3−xCo.sub.xO.sub.9−y Fe.sub.3−xAl.sub.xO.sub.9−y

Example 2

(12) An oxide with a composition Cr.sub.1.5Co.sub.1.5O.sub.4 was prepared by co-precipitation. A mixed metal nitrate solution containing 1 mole/litre Co(NO.sub.3).sub.2.6H.sub.2O and 1 mole/litre of Cr(NO.sub.3).sub.3.9H.sub.2O was pumped together with a solution of ammonium carbonate (1 molar) heated to 60° C. After the precipitation reactor, the slurry passed into a holding tank, again heated to 60° C. The pH of the slurry was measured in the line between the precipitation reactor and the holding tank. The flow rates of the nitrate and the base were adjusted. When the precipitation was completed, the slurry in the holding tank was stirred overnight.

(13) The resulting aged slurry was filtered with a vacuum filter and the filter cake dried at 90° C. overnight. The dried filter-cake was ground and heated in an air muffle oven at 400° C. for 12 hours. The calcined oxide was then reground and heated to 900° C. in air for 12 hours. XRD analysis confirmed that this material was a single phase, cubic mixed-oxide with the composition Cr.sub.1.5Co.sub.1.5O.sub.4 (x=1.5 and y=5). The calcined catalyst was pressed into pellets, which were crushed to produce a sieve fraction between 0.2 and 0.5 mm. These granules (0.2776 g) were loaded into the ammonia oxidation reactor, and were subjected to a test procedure described above. The temperatures in the catalyst bed, concentrations of N.sub.2O and the selectivity towards NO+NO.sub.2 are shown in FIG. 1.

(14) From FIG. 1, we observe that ignition of the ammonia occurs after 20 minutes, at a temperature of 294° C. The reduction of the ammonia concentration is accompanied by a rapid increase in the catalyst temperature (to circa 870° C.). Prior to ignition it is observed that the N.sub.2O level increases, at temperatures above 230° C., prior to the ignition, and spikes during the ignition process, before decreasing again. At time=80 minutes the temperature of the inlet gas is adjusted, so the catalyst bed temperature rises to 900° C. During the isothermal period, between 20 and 80 minutes, the N.sub.2O level increases slowly. As the catalyst temperature is raised to 900° C., the N.sub.2O level decreases rapidly. The nitrogen concentration also spikes during ignition and then falls to a constant level.

(15) FIG. 2 shows the temperature and selectivity towards (NO+NO.sub.2) for the Cr.sub.1.5Co.sub.1.5O.sub.4 catalyst. After ignition at time=20 minutes, when the catalyst temperature is circa 870° C., the selectivity rises to 90 to 91%. When the temperature in the reactor is adjusted to 900° C., the selectivity over the Cr.sub.1.5Co.sub.1.5O.sub.4 catalyst increases to 92.5%.

Example 3

(16) An oxide with the composition Mn.sub.1.5Co.sub.1.5O.sub.4 was prepared by complexation. 1 molar Mn(NO.sub.3).sub.2.6H.sub.2O and Co(NO.sub.3).sub.2.6H.sub.2O were mixed in a 1:1 ratio. To 100 ml of the metal nitrate solution was added 25 ml of 64% HNO.sub.3, 100 ml of ethylene glycol and 211 g of citric acid. Stirring at 120° C. resulted in the formation of a viscous liquid/gel. The gel was heated at 400° C., in an air muffle oven, for 12 hours. The resulting powder was ground and reheated in the air muffle oven at 900° C. for 12 hours.

(17) X-ray diffraction confirmed that this material was a single phase, cubic mixed-oxide with the composition Mn.sub.1.5Co.sub.1.5O.sub.4 (x=1.5 and y=5).

(18) The calcined catalyst was pressed into pellets, which were crushed to produce a sieve fraction between 0.2 and 0.5 mm. These granules (0.2813 g) were loaded into the ammonia oxidation reactor, and were subjected to a test procedure described above. The temperatures in the catalyst bed, concentrations of N.sub.2O and the selectivity towards NO+NO.sub.2 are shown in FIG. 3.

(19) From FIG. 3, we observe that ignition of the ammonia occurs after 18 minutes, at a temperature of 309° C. The reduction of the ammonia concentration is accompanied by a rapid increase in the catalyst temperature (to circa 800° C.). Prior to ignition it is observed that the N.sub.2O level increases, at temperatures above 230° C., prior to the ignition, and spikes during the ignition process, before decreasing again. At time=50 minutes the temperature of the inlet gas is adjusted, so the catalyst bed temperature rises to 850° C. During the isothermal period, between 20 and 50 minutes, the N.sub.2O level increases slowly. As the catalyst temperature is raised to 850° C., the N.sub.2O level decreases rapidly. At time=80 minutes, the temperature of the catalyst bed is increased to 900° C. Again, when the temperature of the catalyst bed is raised, the N.sub.2O level decreases. The nitrogen concentration also spikes during ignition and then falls to a constant level. There is a slight decrease in nitrogen concentration, when the catalyst temperature is raised from 800 to 850° C.

(20) FIG. 4 shows the temperature and selectivity towards (NO+NO.sub.2) for the Mn.sub.1.5Co.sub.1.5O.sub.4 catalyst. It is observed, that after ignition, when the temperature of the Mn.sub.1.5Co.sub.1.5O.sub.4 catalyst is 800° C., selectivity reaches 95%. As the temperature in the reactor is increased to 850 and 900° C., the selectivity increases to circa 96%.