CATALYST AND CATALYTIC REDUCTION

Abstract: A catalyst for the selective reduction of nitric oxide NO in flue gas to nitrogen using ammonia NH3 comprises oxidatively heat treated manganese oxide loaded on a porous substrate of gamma-alumina or titania- or titania-/WO3-modified silica.

Description: CATALYST AND CATALYTIC REDUCTION This invention relates to a catalyst comprising a metal on a porous support, and to a method of making such a catalyst. The invention also relates to a method of reduction using the catalyst. The natural gas industry has long sought a way in the presence of excess oxygen of removing oxides of nitrogen from flue gas, which may typically be at 375-425K and may contain 13-14 volume 1 water vapour, 5-6X oxygen, 6-7.. carbon dioxide and lOOppm nitric oxide. Oxides of nitrogen can also arise in flue -gas in other industries where combustion occurs and in petrol motor car exhausts. It is desirable to convert these compounds into harmless compounds ' before they are released into the atmosphere. EP-B-256590 discloses a titanium/vanadium catalyst on a silica carrier for reducing nitrogen oxides in the presence of ammonia to nitrogen and water. This is stated in the EP-B to be only 64% effective at 423 even at a gas hourly space velocity (explained later) of 10000, which is too low a throughput for some large-scale industries. Further catalysts for this reaction are disclosed in Catalysis Today 7 (1990) 157-165, (copper salts on carbon supports), Ind Eng Chem Prod Res Dev 20 (1981) 301-304 (molybdenum on titania/zirconia/alumina/si lica, International Chemical Engineering 15 (July 1975) 546-549 (Pt, CuO-Cr 2 0 , CuO, V 2 0 5 , NiO, Fe 2 0 3 , Mn0-Cr 2 O 3 , Mo0 3 , Co 3 0 4 , h0 2 , Cr 2 0 3 and W0 3 ), and Bulletin of Chemical Society of Oapan 54 (1981) 3347-3350 (Mn2U3 and Mn 2 θ3-Cθ3θ4 in 1mm particles from calcined carbonate- precipitated nitrate solution), the last-named suffering from slow partially irreversible poisoning by sulphur dioxide. Wakker and Gerritsen, Proc American Chemical Society, Division of Fuel Chemistry 35(1) (1990) 170 discloses a catalyst of MnO supported on gamma-alumina, the MnO being obtained by impregnating the alumina in manganese acetate, sulphate, nitrate or oxalate solution, drying, optionally calcining, and reducing with hydrogen at 600°C. This catalyst can remove K 2 S from hot reactor gases. However, it cannot reduce oxides of nitrogen. A similar catalyst (manganese acetate or nitrate solution Impregnated into steam-activated carbonised peat and calcined In 2% oxygen) was disclosed for reducing NO with ammonia at the International Symposium on Carbon, Tsukuba, 1990, page 590 of the Extended Abstracts. This catalyst can successfully reduce NO below 425K but Its activity decreases irreversibly above that temperature. This is a serious problem since it cannot be guaranteed that the temperature of a flue 1n an industrial process (such as petrochemical cracking) will not accidentally rise above 425K. According to the present Invention, a process of preparing a catalyst for the reduction of nitrogen oxide comprises applying a manganese compound in solution to a porous carrier, removing the solvent, and subjecting the carrier (Including the manganese compound) to a heat treatment at from 550K to 850 characterised in that said heat treatment tends to oxidise Mn(II) and in that the porous carrier is inorganic. The heat treatment is preferably in the presence of at least 0.01 or 0.03 atmospheres oxygen, more preferably at least 0.05 atmospheres, for example at least 0.1 atmospheres. The oxygen presence is most conveniently the air, or may be Y- atmosphere or more, such as pure oxygen. The heat treatment may be performed at from 720K, more preferably from 750K, and preferably at up to 830K, more preferably up to 800K. The preferred duration of the heat treatment depends on the temperature, as follows:- 550-625K: 12 to 48 hours, preferably 18 to 36 hours 625-720K: 4 to 18 hours, preferably 6 to 12 hours 720-830-1. Vf- to 6 hours, preferably 2 to 4 hours. The upper time- limits may be exceeded but there is no advantage. Beforehand, if proceeding above 625K, it preferably takes at least 1 hour from passing 525K to passing 625K. The porous Inorganic carrier, which preferably comprises silica or alumina or both, may be loaded with titanium dioxide or tungsten trioxide or both, before or at the same time as or after applying the manganese compound, and the tungsten trioxide may be applied in the form of a tungstate, preferably in solution, preferably of ammonium. If T10 2 is used as the support, tungsten and manganese compounds may be added in either order or simultaneously. The preferred use of the catalyst made in this way is to reduce nitrogen oxide in the presence of oxygen, using a compound containing N and H such as ammonia, preferably at up to 475K. For applications where ammonia is not practical, such as motor cars, a solid compound may be used, preferably containing amine groups, e.g. urea The invention will now be described by way of example. A catalyst was synthesised for the selective catalytic reduction of nitric oxide NO in stack or flue gases at low temperatures (100-200°C). The reduction is effected with ammonia NH 3 and proceeds thus: 4N0 + 4NH 3 + 0 2 -> 4N 2 + 6H 0. Also nitrogen dioxide NO2 is in principle able to be reduced to environmentally innocuous nitrogen and water by a corresponding reaction, but consuming less elemental oxygen. The catalyst was based on manganese oxide on a porous inorganic particulate support, and was made as follows: The support used in series A of the examples was γ-Al 2 03, in series B = Si0 2 , C = Ti0 2 , and D = Siθ2-Al 2 0 3 . The γ-Al 2 0 3 had a BET surface area of 200m 2 /g, a V p of 0.5cm 3 /g and a d p of 210-250μm. The support could be modified by loading or covering its internal surface (i.e. its pores) and its external surface with a highly dispersed modifier in as near as possible monolayer form, or quarter-mono!ayer, or up to 10% by weight. The modifier is designated in the examples thus: 0 = no modifier 1 = Ti0 2 2 = WO3 Examples A0 thus refer to an unmodified γ-alumina support, and Examples Bl to a silica support loaded with Ti0 2 , in that case 10% by weight of T10 2 . The WO3 was applied by decomposing ammonium tungstate in situ and ammounted to 6 weight %. Both 05 could be present together. Heavier and lighter loadings of modifier may also be used. The manganese oxide was applied by stirring the support (which had been dried for 24 hours at 393 ) in a near-saturated (or, if desired, less concentrated) precursor solution (in 10 demineralised water or any other solvent not leaving undesirable residues) of a manganese salt or complex. This could be .inorganic, e.g.. (i) manganous nitrate Mn(N0 3 ) 2 or organic, e.g. (ii) manganous acetate - Q^COO^, T5 (iii) manganous oxal te, (iv) manganese-EDTA complex or (v) for the Mn-W combination catalyst, a mixture of (ii) + ammonium tungstate ( NH4 gH2Wj2θ4o> used in that order (see below) 0 Examples AOi thus refer to an unmodified γ-alumina support on which manganese oxide was deposited derived from manganous nitrate precursor. The volume of the precursor solution was equal to the volume of the pores in the support. The precursor solution was allowed 5 to fill the pore volume of the support by the so-called "pore volume impregnation" or "incipient wetness" method. Solvent was removed (in the case of water) by allowing to dry at room temperature overnight (24 hours was used in case (v) above), then raising to 50°C for 2 hours, then 60°C for 2 hours, then 70°C for 0 2 hours, then 85°C (about 360 ) overnight. The procedure was- repeated as often as necessary to achieve the desired manganese loading of the support. A less concentrated solution of precursor could be sufficient if a less heavy manganese loading is desired; for the same loading, 5 correspondingly more repetitions of the impregnation-plus- solvent-removal woufd be needed. Alternatively, a dry procedure performed directly at 85-100°C could be sufficient. The sample was then calcined by heating it in air or oxygen (no differences found) at 4K/min to 575K (300 β C), held for 1 hour to allow the salts/complexes to decompose to oxide, heated further at 4K/min to 775K and held for 3 hours. Instead of 775 , 725K could be used at the cost of incomplete calcination (which would lower the catalytic activity) and 830K could be used but at the cost of a diminished catalytic activity caused by vapour-phase loss of manganese or thermal transformation of catalytically active phase into a less active one. The 3 hours could be extended to e.g. 24 hours but it made no difference. After slow cooling to room temperature, the sample was ready for use. This procedure, according to the number of impregnations with the precursor, yielded loadings of 3.3% Mn (calculated as the metal) by weight, 8.3% and 15.5%. These are designated in the examples as A0i3.3 etc. For case (v), these operations are followed by impregnating with the ammonium tungstate solution, with again drying and calcining as described above. In more general terms, where a modifier is present, it could either be calcined before the addition of manganese precursor or could be merely dried, or vice versa, i.e. add manganese precursor first, then dry and optionally calcine, then add modifier, dry and calcine. The kind of active phase present on the catalyst due to the oxidative thermal treatment of the catalyst can be sensitively detected using thermal programmed reduction, which yields distinctive "signatures", representing the several reduction steps of the metal oxide on the support. The catalysts were then tested on a gas stream flowing at a rate of 60 litres (STP equivalent ) of gas per gram of catalyst-including-support per hour, in other words a volume hourly space velocity of 54000h~^ , otherwise equivalent to 54000 changes per hour, the gas stream being of composition 500 ppm NO, 550 ppm NH 3 , 2% 0 2 and balance helium. Catalyst AOii achieved over 95% conversion of the NO to N 2 at 425K with minimal N O byproduct over a 200-hour test. The latter finding is valuable since the catalyst will not decompose N 2 0 with ammonia. Above 475K, the catalyst started to oxidise the ammonia, with undesirable production of NO ( counterproductive) and N 2 0 (nuisance). Above 300°C the catalyst 1s not recommended because of the possible decreasing selectivity towards 2 . The catalyst was selective towards NO reduction; in particular, it did not catalyse the reduction of other flue gases such as C0 2 . The N 2 , and the H-0 which is sometimes observed, have been proved to arise from the reaction ^0 + N 2 H 3 + oxygen • *• H - H- or N^N 2 0 + water, i.e. not solely from the oxidation of ammonia but jointly from the NO. The formation of N 2 0 can be almost eliminated by -K-3-loading of a gamma-aluminia catalyst support (Examples A2xn), and can be lessened (with an accompanying gain in percentage removal of NO) by 10% T10 2 -loading of a silica catalyst support (Examples Blxn). It can also be lessened by diminishing the amount of manganese catalyst used, but this may be undesirable especially in cleaning damp flue gas. Certain combinations of support and precursor yielded especially active and selective catalysts, such as (B/C/) (i), and (A/D) (ϋ/iii/iv). Especially preferred were Examples A.H8.4, B(l+2)ii8.4 (at higher temperatures, e.g. 540-560K) and B0i8.4. Exposure to sulphur dioxide, in particular 2% S0 2 at 525 , tends to deactivate the catalysts, but Example series A survived best. Exposure to water inhibits the catalyst but this effect can be compensated by increasing the amount of manganese or using a lower volume hourly space velocity (which might however be industrially undesirable). Example Bl118.4 removed 80% of NO at 400K and 100% in the range 440-475K, and still removed over 95% at 563K. Example B(1+2)118. achieved 100% removal in the range 525-575 . Example A0H8.4 has- achieved 95% conversion of NO continuously for 200 hours, and can do so over the wide temperature range 425-525 , which simplifies process control. Example A0i8.4 can achieve. 97% or better around 475-525 but falls away more sharply outside this range. Varying the manganese loading while keeping all other conditions constant had little effect up to 500K, but at 575 the 3.3 weight % catalyst was still achieving 97% NO conversion (same as 500K) while the 8.4% and 15.5% catalysts had fallen to 90% conversion. Examples D0x8.4 were superior at all temperatures to Examples D2x8.4 A survey of Examples AOv is given 1n Table 1. The catalysts are designated by the number of metal atoms per nm 2 of initial support area. For the sake of clarity the weight content of the metal oxide is also included in Table 1. Table 1 Experiment (i) The standard activity measurements on NO conversion were carried out at atmospheric pressure, using 50 mg of catalyst and between 385 K and 575 K at a volume hourly space velocity of 58.000 h -1 . The reactor feed contained 500 ppm NO, 550 ppm NH 3 , 2 vol% O2 and He as balance. The volume hourly space velocity was doubled for (c) and lessened for (a) so that VHSV/Mn was constant. Experiment (ii) To investigate the effect of H 2 0 on the NO conversion some experiments have been carried out with addition of H2O at 435 K at a volume hourly space velocity of 31.000 h -1 . In this case the reaction mixture had the following composition: 525 ppm NO, 550 ppm NH 3 , 2.5 vol% 0 2 , with either 0, 4.6 or 8.7 vol% H2O and Ar as balance. Sample (c) was used for this experiment. Experiment (iii) A long run experiment of 600 h at 425 K under standard conditions has been carried out. In some of the experiments 15 NH 3 was employed to trace the origin of both N atoms in the reaction products. Experiment (iv) Sulfatation of the samples has been performed in a thermobalance by using an air flow, containing 2000 ppm S0 2 . The sulfatation procedure starts with drying the sample at 425 K in air, after which the temperature is decreased to 325 K. Then the sulfatation of the sample starts, while the temperature is increased at a rate of 5 K.min" 1 to 475K. At this final temperature sulfatation is continued for 15 minutes, after which the catalysts have been tested at 425 under the standard experimental conditions. The results of Experiment (i) on the catalysts from Table 1 show that sample (e) hardly exhibits any NO reduction activity, having a comparable activity as the γ-Al 2 03 support, (d) being slightly inferior. Differences in NO conversions between the other catalysts are only observable below 475 K. In the higher temperature region no differences can be discerned, all the NO conversions being 95-100% but decreasing above 525K. . Besides N 2 formation from the NO, less desirable N 2 0 formation has been observed above 425 for all the catalysts of Table 1, but by far the least N 2 0 arose from (d), and the most from the most Mn-rich catalysts. Experiment (ii) yielded the following results: NO conversion - 98% (no H 2 0); = 67% (4.6 vol% H 2 0); = 47% (8.7 vol% H2O . H2O reversibly inhibits the reaction, since upon removing the H 2 0 the NO conversion returns within a few minutes to the original higher value. In Experiment (iv) on sample (b), the NO conversion decreases from 95% to 85% after the sulfatation procedure. So it can be concluded that sample (b) still retains a high (85%) activity- even after being subjected to 2000 ppm S0 2 in air. Experiment (iii) on sample ( b ) demonstrates that this catalyst exhibits a highly stable activity (about 94% NO conversion) after approximately 150 h. Sample (d) already operates at 325 . Ammonia was adsorbed first, catalyst was flushed to remove gaseous residual ammonia, and a mixture of NO and oxygen was passed over the catalyst. This resulted in the instantaneous formation of N 2 due to the reaction between NO and ammonia at 50C. The presence of oxygen, is needed, otherwise the temperature has to be raised to above lOOC. Generally speaking, the reaction rate is zero order in ammonia, and 0.4 order in NO and also in oxygen. Nitrous oxide (N 2 0) was not observed in this type of experiment. This opens up applications in systems whereby the catalyst is first loaded with ammonia, and then exposed to the NO-containing atmosphere. This separately loading and reacting may avoid the escape of ammonia to the environment, an important issue in this type of catalysis. Experiments with catalysts loaded with 1 and 2 wt% Mn on alumina confirmed that these are as active per unit of Mn and selective as the 3 wt% Mn catalyst.

Claims: CLAIMS 1. Process of preparing a catalyst for the reduction of nitrogen oxide, comprising applying a manganese compound in solution to a porous carrier, removing the solvent, and subjecting the carrier (Including the manganese compound) to a heat treatment at from 550K to 850 characterised in that said heat treatment tends to oxidise Mn(II) and in that the porous carrier 1s inorganic. 2. Process according to Claim 1, wherein the oxygen 1s present in air. 3. Process according to Claim 1, wherein the heat treatment is in the presence of at least 0.03 atmosphere oxygen. .4. Process according to any preceding claim, wherein the heat treatment is at 720-830K. 5. Process according to Claim 4, wherein the heat treatment is at 750-800 . 6. Process according to any preceding claim, wherein the duration of the heat treatment is related to the temperature thereof as follows: 550-625K: at least 12 hours 625-720K: at least 4 hours 720-830K: at least 1% hours 7. Process according to Claim 6, wherein the duration of the heat treatment is related to the temperature thereof as follows: 550-625K: 18-36 hours 625-720K: 6-12 hours 720-830K: 2-4 hours 8. Process according to any preceding claim, wherein the heat treatment is at above 625 and takes at least 1 hour from passing 525. to passing 625K. 9. Process according to any preceding claim, wherein the porous carrier is gamma-alumina, silica, titania, or alumina-silica. 10. Process according to Claim 9, wherein the carrier is loaded with titanium dioxide or with tungsten trioxide or with both, before or at the same time as applying the manganese compound. 11. Process according to Claim 10, wherein the tungsten trioxide is applied in the form of a tungstate which is decomposed in situ. 12. Process according to Claim 11, wherein the tungstate is an ammonium tungstate solution. 13. Process according to any preceding claim, wherein the manganese compound is a nitrate, acetate, oxalate or EDTA complex. 14. Use of a catalyst made according to any preceding claim 15. Use of a catalyst according to Claim 14, to reduce nitrogen oxide in the presence of oxygen, using a compound containing nitrogen and hydrogen, such as urea or ammonia.

Inventors: KAPTEIJN FREEK, SINGOREDJO LYDIA, KAPTEIJN, FREEK, SINGOREDJO, LYDIA
Applicants: BRITISH TECH GROUP, BRITISH TECHNOLOGY GROUP LTD.
Application year: 1992
Main classification: B01J23
IPC codes: B01D53, B01J23, B01J37