GB2361653A

GB2361653A - Improvements in catalytic reduction of NOx

Info

Publication number: GB2361653A
Application number: GB0010318A
Authority: GB
Inventors: Philip Gerald Blakeman
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2000-04-28
Filing date: 2000-04-28
Publication date: 2001-10-31
Also published as: GB0010318D0

Abstract

A low temperature catalyst composition for NO<SB>x</SB> reduction in combustion gases, especially from diesel engines, the catalyst composition comprising an oxide of manganese alone or in combination with an oxide of cerium on a wash-coat of titania which contains an oxide of tungsten, the titania being of a relatively high surface area. The catalyst composition is used with a reductant such as ammonia, urea or ammonium carbamate.

Description

2361653 IMPROVEMENTS IN CATALYTIC REDUCTION OF NOk 5 The present invention

relates to improvements in selective catalytic reduction of NOX in waste gas streams such as exhausts from engines powered by diesel or gasoline.

The technique of selective catalytic reduction (hereafter "SCW') is well established for industrial plant combustion gases and for vehicular exhaust gases, and may be broadly described as passing a hot exhaust gas over a catalyst in the presence of a nitrogenous reductant, especially ammonia or urea, which process converts nitrogen oxides (hereafter "NO,, ") into harmless components such as nitrogen and water. This reaction can be represented as follows:

4NO + 4NH3 + 02 4N2 + 6H20 6NO2 + 8NH3 7N2 + 12H20 This method effectively reduces the NO,, content of the exhaust gases by about 20-25% at elevated temperatures using a catalyst comprising a platinum group metal. Platinum based catalysts tend to oxidise N113 to NOx during high temperature operations of this type. Other catalysts such as those containing vanadium pentoxide, manganese oxides and the like combined with a titania, zirconia, silica, alumina or zeolite support have been used for this purpose. Furthermore, such supports have been wash- coated with (rare-earth - remove) metal oxides such as e.g. ceria -to improve their toughness. In such cases, vanadium pentoxide exhibits optimum catalytic activity only at temperatures of about 300-500'C. These relatively high temperature processes impose a severe limitation because particulate matter present in exhaust emissions tend to reduce the life of the catalyst used. In view of this, the catalyst is sited after the particulates removal section of a process train for treatment of exhaust gases. If the catalyst is sited downstream at a point where the temperature of the exhaust gases has declined to below the temperature at which the catalyst exhibits optimum activity, then it is necessary to reheat the exhaust gases to the desired temperature. This is wasteful of energy and adds to the process cost.

Prior published US-A-5380692 describes a catalyst for the catalytic reduction of nitrogen oxides using a hydrocarbon and/or oxygen containing organic compound as reducing agent rather than ammonia or urea as described above. A whole host of catalytic compositions are disclosed including inter alia a composition of the formula 3XB21_.XC2 3 r.

A yC 1-y"3 2 wherein A3 represents La or Ce; B 2 represents Ba, Sr, Ca, Mg, PB, Zii or Ag; C2 represents Mn or Co; C3 represents Fe, Ni, Cr, Cu, V, Mo, W, Ti, Zr, Nb, Pd, Rh, Ru or Pt; x and y each is a number from 0 to 1; the catalyst being supported on "a solid acid 5 carrier such as zeolite, metallosilicate, SAPO, ALPO, MAPO, alumina, titania, zirconia, silica- alumina, etc.". The acronyms are defined in column 4 of the '692 patent. This catalyst again is used to convert NQ, at a temperature of 300-600'C (col. 21, lines 1-2, see also Table 4 on cols. 49 and 50). Thus, it would appear that these catalysts in all their broad scope operate only at relatively high temperatures.

It has now been found that by a suitable choice of a catalyst composition which can function at relatively low temperatures, some of the problems encountered hitherto can be mitigated.

Accordingly, the present invention is a catalyst composition comprising an oxide of manganese alone or in combination with an oxide of cerium on a washcoat of titania which contains an oxide of tungsten, the titania being of a relatively high surface area.

According to a further embodiment, the present invention is a process for reducing the content of nitrogen oxides in waste or exhaust gases said process comprising admixing a reductant which is a source of ammonia with said gases and passing said admixture over a catalyst composition comprising an oxide of manganese alone or in combination with an oxide of cerium supported on a washcoat of titania which contains an oxide of tungsten, the titania being of a relatively high surface area.

The reductant which is a source of ammonia is suitably a compound from which ammonia can be generated in situ under the reaction conditions. Such reductant is suitably selected from ammonia, urea and ammonium carbamate but is preferably ammonia or urea.

The washcoat of titania is of a type which also contains (ie in the washcoat as a layer over the titania surface) tungsten oxide. Such a washcoat containing tungsten oxide can be obtained from Millennium Inorganic Chemicals for instance under grade DT52. The tungsten oxide stabilises the titania phase, anatase (high surface area), and stops it becoming the rutile (low surface area) phase until higher temperatures are reached. Tungsten also supplements the S tolerance of titania.

v 3 The catalyst composition is suitably prepared by wet impregnation of the tungsten/titania washcoat (previously calcined at 500'C for I hr) with an aqueous solution of a salt of manganese or a mixture of cerium and manganese salts, the concentration of the metals in the solution being adequate to give the desired metal loading. The wet catalyst is suitably dried and calcined for I hr at 500'C.

The catalyst compositions of the present invention may contain manganese loadings over a wide range. Where the composition has manganese alone, the loading thereof is suitably in the range from I - 25 Kg/M3, preferably from 1.5 - 20 Kg/M3.

Within this range, the lower loadings tend to relatively shift the optimum activity to slightly higher temperatures. The catalyst suitably contains from 0.002-0. 010 g/cc of Mn and 0.005-0.050 g/cc of Ce, e.g. 0.00706g/ec (200 g/ft-3) of Mn and 0. 01413g/cc of Ce (400 g/ft-3).

The catalyst composition of the present invention is suitably coated onto a support which is a porous monolith. By "porous" is meant that the monolith is capable of allowing gases to pass therethrough, especially the exhaust gases from an automotive vehicle powered by an internal combustion engine. Theoretically, the metal impregnation can be suitably coated onto any 'porous' material (titania is a slurry so it would not go into small pores), preferably a 'honeycomb' (flow-through) monolith which is an array of small, long channels (cells) running the whole length of the catalyst unit. The cell densities may vary over a moderately wide range limited only by the strength of the final unit (for low densities) and manufacturing limitations (high densities). The cell densities are -suitably in the range from 7-186 cells/cm 2 (50 - 1200 cells/in 2). The movement of gaseous reactants from the centre of the cell to the catalyst-coated wall as the gas flows through is dictated by mass transfer considerations. The monolith is suitably made from e.g. a ceramic oxide such as cordierite, a wound metallic foil or a packed bed of coated beads. Where coated beads are used, the gas would flow around the beads and thus come into contact with the catalyst coating The monolith used is suitably selected from commercially available materials such as eg'Honeyceram' (from NGK) and 'Celcor' brand (from Coming).

Where a porous monolith is used, such as e.g. a ceramic flow through monolith, the catalyst composition is suitably bound to the porous monolith with a binder which may be derived from a silica sol such as e.g. LudoxO AS-40 ( ex DuPont) or a titania sol S5-30013 (ex Millennium Inorganic Chemicals) in order to facilitate and maximise the adhesion of the titania wash-coat onto the monolith support. This binder is suitably 4 added in 10 - 15% (by dry weight) to the washcoat slurry, before being coated onto the support. The washcoated support is then suitably calcined at 50WC before the wet impregnation step is carried out.

The catalyst compositions of the present invention can be made to any shape or size. For instance, they may be in the form of pellets, granules, agglomerates of irregular shape or extrudates.

The catalyst and the process of the present invention is particularly applicable to exhaust emissions from vehicles powered by diesel or gasoline although it can also be employed to remove NQ, gases present in any other waste gases generated in the industry or power stations which use fuels capable of generating the same.

A feature of the catalyst composition and the NO, reduction process of the present invention is that it can be operated at relatively low temperatures. The catalyst compositions of the present invention are suitably used at temperatures below 400'C since the activity thereof tends to decline at higher temperatures. For optimum activity the catalyst composition is suitably used at temperatures between 130 and 40CC over which range they are active, and are preferably used at temperatures between 200 and 35WC. Tests with these catalysts show that the activity of the catalyst composition is typically 40% NQ, conversion at 150'C in an exhaust gas mix with NO as the only NQ, component and 85% NQ, conversion at 15WC in an exhaust gas which contains a mixture of NO and N02 in a 1: 1 ppm ratio. The gas mixture tested was found to contain in addition about 200 ppm carbon monoxide, 100 ppm C3 hydrocarbons, 12% oxygen, 4.5% each of water and carbon dioxide, 200 ppm each of ammonia and NO, and an space velocity ('SV') of 30,000 hr -1.

The catalyst compositions of the present invention perform best with emissions which are relatively low in sulphur dioxide content since there is a risk of the low temperature activity of the catalyst composition being adversely affected. The presence of titania and tungsten in the catalyst composition improves, as mentioned previously, the sulphur tolerance thereof.

In the low temperature process of NO, reduction, the catalyst compositions of the present invention may be suitably used in conjunction with (i) an oxidation catalyst upstream of the ammonia injection point to remove other pollutants such as CO, HC and/or (ii) an oxidising slip catalyst downstream of the SCR catalysts to oxidise any ammonia slipping through from the previous stages of the reaction. If necessary, the catalyst compositions of the present invention can also be used in conjunction with a conventional high temperature, ainmonia-SCR catalyst which is active e.g. in range 300- 600'C provided that the present catalysts are located downstream of such a high temperature SCR catalyst. The high temperature catalyst has to go upstream otherwise when the low temperature catalyst stops working, it turns all of the NH3 to NOx and the high temperature catalyst does not see any NI-13. If the high temperature catalyst is in front it does not cause any problems since it is easily designed to be effectively transparent to NH3 at lower temperatures. Thus, the catalyst composition of the present invention does not have to be the only catalyst or even the only SCR catalyst used in a system. By using the catalyst compositions of the present invention with - other SCR catalysts the active temperature window of a system can be widened, or, if an oxidation catalyst is included, the system can remove CO and I-IC as well as NOx.

The catalyst compositions of the present invention are particularly suitable for use in vehicles with relatively small engines due to the ability of the catalyst to catalyse the reduction reaction at low temperatures.

The catalyst compositions of the present invention are not only costefficient because they operate at relatively low temperatures but also because they do not contain any of the platinum group metals.

The present invention is further illustrated with reference to the following Examples:

For a catalyst supported on a flow through ceramic monolith having a manganese concentration of 0.00706 g/cc (200 g/ft3) the following recipe was used:

5kg Tungsten-stabilised titania (DT52, from Millenium. Inorganic Chemicals) was suspended in deionised water (5 litres) and Ti sol binder added (600 g dry weight) to form a slurry. More water was added to the slurry to adjust its solids content to 30%. A ceramic monolith was passed through a circulating curtain of this slurry to give 0.146g/ec (2.4 g washcoat per cubic inch) of monolith when dry. The washcoated monolith was then calcined at 50WC for 1 hour.

Manganese nitrate was dissolved in deionised water to give 45 g Mn/litre (ie the appropriate concentration was determined from water adsorption of washcoated monolith 6 to give Mn loading of 0.00706g/cc (200 g/ft3). The washcoated monolith was dipped into this solution to pick up a pre-determined amount of solution, dried on hot (90'C) air flow and then calcined for 1 hr at 50WC. . 5 The tests were carried out in the laboratory at between 1 WC and 600'C with the following two gas mixes.

GAS MIX A GAS MIX B Space Velocity: 30,000 hr-1 Space velocity: 30,000 hr-1 CO: 200 ppm CO: 200 ppm NO: 200 ppm NO: 100 ppm N02: 0 ppm N02: 100 ppm N113: 200 ppm N113: 200 ppm HC (C3): 100 ppm HC (C3): 100 ppm 02: 12% 02: 12% H20: 4.5% H20: 4.5% C02: 4.5% C02: 4.5% N2: balance N2: balance NOx conversions for a catalyst with a manganese concentration of 0. 00706g/ce (200 g/ft3) were as follows:

Temp Mix A Mix B 35% 88% 55% 98% 77% 100% 225 86% 100% 250 91% 99% 275 94% 97% 300 94% 96% 325 94% 96% 350 93% 94% 375 82% 86% 400 63% 73%

Claims

7 CLAIMS

1. A catalyst composition comprising an oxide of manganese alone or in combination with an oxide of cerium on a washcoat of titania which contains an oxide of 5 tungsten, the titania being of a relatively high surface area.

2. A catalyst composition according to Claim 1 wherein the reducatant which is a source of ammonia is selected from ammonia, urea and ammonium carbamate.

3. A catalyst composition according to Claim 1 or 2 wherein the washcoat of titania is of a type which contains tungsten oxide as a layer over the titania surface.

4. A catalyst composition according to any one of the preceding Claims wherein the catalyst composition is prepared by wet impregnation of the tungsten/titania washcoat which has previously calcined at 50WC with an aqueous solution of a salt of manganese or a mixture of cerium and manganese salts.

5. A catalyst composition according to any one of the preceding Claims wherein the loading of manganese when used alone is in the range from 1 25 Kg/M3.

6. A catalyst composition according to any one of the preceding Claims 14 wherein the loading of a combination of manganese and cerium in said composition is in the range from 0.002-0.010 g/cc of Mn and 0.005-0.050 g/cc of Ce 7. A catalyst composition according to Claim 6 wherein the loading of a combination of manganese and cerium in said composition is in the ratio of 0.00706g/ce (200 g/ft-3) of Mn to 0.01413g/cc of Ce (400 g/ft-3).

8. A catalyst composition according to any one of the preceding Claims wherein the catalyst composition is coated onto a support.

9. A catalyst composition according to Claim 8 wherein the support is a porous monolith capable of allowing gases to pass therethrough.

10. A catalyst composition according to Claim 9 wherein the porous monolith is a honeycomb (flow-through) monolith which is an array of small, long channels (cells) running the whole length of the catalyst unit.

8 11. A catalyst composition according to Claim 10 wherein the cell densities on the monolith are in the range from 7-186 cells/cm 2 (200 1200 cells/in 2).

12. A catalyst composition according to Claim 11 wherein the monolith is made from a ceramic oxide, a wound metallic foil or a packed bed of coated beads.

13. A catalyst composition according to any one of the preceding Claims 912 wherein said catalyst composition is bound in a binder derived from a silica sol or a titania sol before being applied on to a porous monolith support.

14. A catalyst composition according to Claim 13 wherein the binder is added in 10 15% (by dry weight) to the washcoat slurry, before being applied onto the support and. the washcoated support is then calcined at 500T.

15. A process for reducing the content of nitrogen oxides in waste or exhaust gases said process comprising admixing a reductant which is a source of ammonia with said gases and passing said admixture over a catalyst composition comprising an oxide of manganese alone or in combination with an oxide of cerium supported on a washcoat of titania which contains an oxide of tungsten, the titania being of a relatively high surface area as claimed in any one of the preceding Claims 1-14.

16. A process according to Claim 15 whenever applied to reduce the nitrogen oxides content of exhaust emissions from vehicles powered by diesel or gasoline.

17. A process according to Claim 15 or 16 wherein said process is operated at catalyst temperatures from 130-400'C.

18. A process according to any one of the preceding Claims 15-17 wherein the catalyst composition is used in conjunction with (i) an oxidation catalyst upstream of the admixing point of the reductant with the gases and/or (ii) an oxidising slip catalyst downstream of any SCR catalysts employed in the process.

19. A process according to any one of the preceding Claims 15-17 wherein the catalyst compositions of the present invention is employed in conjunction with a high temperature ammonia-SCR catalyst active in range 300-600'C such that the catalyst composition is located downstream of said high temperature SCR catalyst.