METHOD AND EXHAUST SYSTEM FOR TREATING NOX IN EXHAUST GAS FROM STATIONARY EMISSION SOURCES

Abstract

A method of selectively catalysing the reduction of oxides of nitrogen (NO.sub.x) including nitrogen monoxide in an exhaust gas of a stationary source of NO.sub.x emissions also containing oxides of sulfur (SO.sub.x) comprising the steps of passively oxidising nitrogen monoxide to nitrogen dioxide (NO.sub.2) over an oxidation catalyst comprising a platinum group metal so that a NO.sub.2/NO.sub.x content is from 40-60%; introducing a nitrogenous reductant into the exhaust gas; and contacting exhaust gas having the 40-60% NO.sub.2/NO.sub.x content and containing the nitrogenous reductant with a selective catalytic reduction (SCR) catalyst comprising an aluminosilicate zeolite promoted with copper.

Claims

1. A method of selectively catalysing the reduction of oxides of nitrogen (NO.sub.x) including nitrogen monoxide in an exhaust gas of a stationary source of NOx emissions, which exhaust gas also containing oxides of sulfur (SO.sub.x), which method comprising the steps of passively oxidising nitrogen monoxide to nitrogen dioxide over an oxidation catalyst comprising a platinum group metal so that a NO.sub.2/NO.sub.x content is from 40-60%; introducing a nitrogenous reductant into the exhaust gas; and contacting exhaust gas having the 40-60% NO.sub.2/NO.sub.xcontent and containing the nitrogenous reductant with a selective catalytic reduction (SCR) catalyst comprising an aluminosilicate zeolite promoted with copper.

2. A method according to claim 1, wherein the SO.sub.x content of the exhaust gas is from 0.5 ppm to 20 ppm.

3. A method according to claim 1, wherein the temperature at which the exhaust gas contacts the SCR catalyst is about 200° C. to about 450° C.

4. A method according to claim 1, wherein the temperature at which the exhaust gas contacts the oxidation catalyst is about 150° C. to about 450° C.

5. A method according to claim 1, wherein the nitrogenous reductant is ammonia (NH.sub.3) and the alpha ratio of ammonia molecules to NO.sub.x molecules is about 0.90 to about 1.10.

6. A method according to claim 1, wherein the copper is ion exchanged in the aluminosilicate zeolite.

7. A method according to claim 1, wherein the aluminosilicate zeolite has a maximum ring size of eight tetrahedral atoms.

8. A method according to claim 7, wherein the aluminosilicate zeolite has a Framework Type Code that is CHA, AEI or AFX.

9. A method according to claim 1, wherein the aluminosilicate zeolite has a Framework Type Code BEA, MOR, MFI or FER.

10. A method according to claim 1, wherein the platinum group metal in the oxidation catalyst is platinum supported on a particulate support and is disposed on a substrate at from about 1 to about 40 g/ft.sup.3, wherein the support for the platinum group metal is silica-doped alumina, titania or optionally doped zirconia.

11. A method according to claim 1, wherein the exhaust gas is a product of a cogeneration plant, preferably a stationary natural gas-fired engine.

12. An exhaust system for selectively catalysing the reduction of oxides of nitrogen (NO.sub.x) including nitrogen monoxide in an exhaust gas of a stationary source of NOx emissions, which exhaust gas also containing oxides of sulfur (SO.sub.x), optionally for use in a method according to any preceding claim, which system comprising an oxidation catalyst comprising a platinum group metal for passively oxidising nitrogen monoxide to nitrogen dioxide so that a NO.sub.2/NO.sub.x content is from 40-60%; an injector for introducing a nitrogenous reductant into the exhaust gas located downstream from the oxidation catalyst; and a selective catalytic reduction (SCR) catalyst comprising an aluminosilicate zeolite promoted with copper located downstream of the injector.

13. An exhaust system according to claim 12, wherein the copper is ion exchanged in the aluminosilicate zeolite.

14. An exhaust system according to claim 12, wherein the aluminosilicate zeolite has a maximum ring size of eight tetrahedral atoms.

15. An exhaust system according to claim 14, wherein the aluminosilicate zeolite has a Framework Type Code that is CHA, AEI or AFX.

16. An exhaust system according to claim 12, wherein the aluminosilicate zeolite has a Framework Type Code BEA, MOR, MFI or FER.

17. An exhaust system according to claim 12, wherein the SCR catalyst is a washcoat coated onto a substrate or is a component of an extruded honeycomb body.

18. An exhaust system according to claim 12, wherein the platinum group metal in the oxidation catalyst is platinum supported on a particulate support and is disposed on a substrate at from about 1 to about 40 g/ft.sup.3, wherein the support for the platinum group metal is silica-doped alumina, titania or optionally doped zirconia.

19. An exhaust system according to claim 12 comprising a heat recovery steam generator (HRSG).

20. A cogeneration plant, preferably a stationary natural gas-fired engine, comprising an exhaust system according to claim 12, wherein the exhaust system comprises a heat recovery steam generator (HRSG).

Description

DETAILED DESCRIPTION

[0019] According to a first aspect, the invention provides a method of selectively catalysing the reduction of oxides of nitrogen (NO.sub.x ) including nitrogen monoxide in an exhaust gas of a stationary source of NO.sub.x emissions, which exhaust gas also containing oxides of sulfur (SO.sub.x ), which method comprising the steps of passively oxidising nitrogen monoxide to nitrogen dioxide over an oxidation catalyst comprising a platinum group metal so that a NO.sub.2/NO.sub.x content is from 40-60%; introducing a nitrogenous reductant into the exhaust gas; and contacting exhaust gas having the 40-60% NO.sub.2/NO.sub.x content and containing the nitrogenous reductant with a selective catalytic reduction (SCR) catalyst comprising an aluminosilicate zeolite promoted with copper.

[0020] The SO.sub.x content of the exhaust gas is dependent on the gas mixture emitted by the stationary source of NO.sub.x , e.g. power plant etc. So, for example, an exhaust gas from a waste incinerator may reach 10 ppm SO.sub.2 but an exhaust gas from a natural gas-fired engine can be as low as 0.50 ppm SO.sub.2. The method according to the invention is capable of treating exhaust gas containing from about 0.50 ppm to about 20 ppm. However, the oxidation catalyst activity can become poisoned by the sulphur through extended use. Therefore, preferably, the oxidation catalyst is adapted to resist sulphur poisoning, as described in greater detail hereinbelow.

[0021] Unlike automotive applications, where the act of driving creates dynamic changes in the temperature of the exhaust gas entering the SCR catalyst, the exhaust gas temperatures in a stationary source application are relatively constant for extended periods of use, tens of thousands of hours. Also the temperatures to which automotive SCR catalysts are exposed can vary widely, through dynamic driving but also system processes such as filter regeneration. The relatively constant temperatures to which stationary source SCR catalysts are exposed can be relatively low by comparison with automotive applications. For example, the temperature at which the exhaust gas contacts the SCR catalyst can be about 250° C. to about 400° C.

[0022] The step of oxidising nitrogen monoxide to nitrogen dioxide using the oxidation catalyst can be an upstream process step from the step of selectively catalysing the reduction NO.sub.x on the SCR catalyst. Accordingly, the temperature at which the exhaust gas contacts the oxidation catalyst is generally higher, e.g. about 275° C. to about 425° C.

[0023] The nitrogenous reductant entering the SCR catalyst is ammonia (NH3), but the ammonia can be injected into the exhaust gas either as ammonia as such or as a precursor of ammonia, such as urea, or ammonia can be evolved from a solid source e.g. by heating ammonium carbamate. It is important that the mixing of ammonia and the exhaust gas is done as completely as possible before the exhaust gas/ammonia enters the SCR catalyst. This can be done using various baffles and static mixers having e.g. herringbone ad skew channel shapes, as necessary provided that they do not significantly contribute to an increase in backpressure. Good mixing of ammonia and exhaust gas can be promoted by a thorough two dimensional coverage of reducing agent injection across a duct carrying the exhaust gas. Such arrangements are known in the art as ammonia injection grids (AIG).

[0024] In order to promote reaction (3) (and reactions (1) and (2) in addition), the alpha ratio of ammonia molecules to NO.sub.x molecules used in the reaction is preferably about 0.90 to about 1.10.

[0025] Although various ways of making copper-promoted aluminosilicate zeolites are known, including impregnation, incipient wetness (or capillary) impregnation and electrostatic adsorption, preferably the copper is ion exchanged in the aluminosilicate zeolite and the quantity of “free”, i.e. un-ion exchanged copper is minimised. This is because we have found that un-ion exchanged copper can promote unselective oxidation of NH.sub.3 (4NH.sub.3.fwdarw.5O.sub.3.fwdarw.4NO+6H.sub.2O), which reduces net NOx reduction.

[0026] In view of the extended period of the desired life cycle of SCR catalysts for use in stationary source NO.sub.x treatment, it is preferable that the catalysts are as active as possible, as durable as possible and as resistant to poisons as possible phosphorus and zinc and alkali metals. More active catalysts can use the nitrogenous reductant more efficiently and/or enable lower SCR catalyst volume to be used.

[0027] Aluminosilicate zeolites that are particularly durable to hydrothermal ageing and hydrocarbon “coking” have a maximum ring size of eight tetrahedral atoms and are referred to in the art as “small pore” zeolites. Zeolites are categorised by the International Zeolite Association by their Framework Types. Preferred aluminosilicate zeolites according to the invention have a Framework Type Code that is CHA, AEI or AFX.

[0028] However, in certain applications where durability over an extended period is not a primary requirement, e.g. in relatively low temperature applications, “medium pore” zeolites, such as those having the Framework Type Codes MFI or FER or “large pore” zeolites, e.g. BEA or MOR, may be equally applicable.

[0029] A silica-to-alumina ratio (SAR) of the zeolite can be any appropriate SAR for promoting the reactions (1), (2) and (3). Generally, this is a balance between thermal stability on the one hand, wherein a relatively high silica content is preferred, and the promoting effect of an ion-exchanged transition metal, wherein a relatively high alumina content is preferred. In practice, the SAR selected may be dependent on the framework type code of the zeolite, but is typically within the range of about 10 to about 256, with SAR of about 15 to about 40 preferred for small pore zeolites such as CHA, AEI and AFX.

[0030] The SCR catalyst can be in the form of a washcoat that is coated onto a substrate, such as an inert ceramic honeycomb monolith, e.g. made from cordierite, or a metal monolith or it can be prepared as an extruded honeycomb body, wherein the catalyst is mixed with a paste of binder (or matrix) components and then extruded into the desired shape having flow channels extending therethrough. Washcoat compositions containing the zeolites for use in the present invention for coating onto the monolith substrate or manufacturing extruded type substrate monoliths can comprise a binder selected from the group consisting of alumina, silica, (non-zeolite) silica-alumina, naturally occurring clays, TiO.sub.2, ZrO.sub.2, and SnO.sub.2.

[0031] The oxidation catalyst for use in the system according to the present invention comprises a platinum group metal, preferably platinum, which is supported on a particulate support. The platinum/support is then disposed on a substrate, such as an inert honeycomb monolith, in order to achieve the desired oxidation of NO to NO.sub.2 thereby to promote the so-called “fast reaction, i.e. reaction (3), at from about 1 to about 40 g/ft.sup.3.

[0032] Since the exhaust gas comprises SO.sub.x , the oxidation catalyst activity can become poisoned as SO.sub.2 chemisorbs onto the platinum active sites at temperatures below 300° C., resulting in an inhibition of the CO and NO oxidation reactions. Above about 300-350° C., the SO.sub.2 is converted to SO.sub.3, which can react with alumina washcoat components forming Al.sub.2(SO.sub.4).sub.3, which can lead to deactivation by pore blockage in the alumina support. See the

[0033] Heck reference mentioned hereinabove at chapter 13. In order to prevent or reduce this effect, preferably the platinum group metal component is supported on acidic particulate support components such as silica-doped alumina, titania or zirconia. The silica-doped alumina can comprise up to 40wt % silica, e.g. 1.5-40 wt % silica.

[0034] The method according to the first aspect of the invention can be used to treat exhaust gas from any stationary source of NO.sub.x emission also containing SO.sub.x. In particular, the exhaust gas can be a product of a power station, an industrial heater, a cogeneration power plant, a combined cycle power generation plant, a wood-fired boiler, a stationary diesel engine, a stationary natural gas-fired engine, a marine propulsion engine, a diesel locomotive engine, an industrial waste incinerator, a municipal waste incinerator, a chemical plant, a glass manufacturing plant, a steel manufacturing plant or a cement manufacturing plant.

[0035] In a preferred application, the exhaust gas is a product of a cogeneration plant, preferably a stationary natural gas-fired engine.

[0036] According to a second aspect, the invention provides an exhaust system for selectively catalysing the reduction of oxides of nitrogen (NO.sub.x) including nitrogen monoxide in an exhaust gas of a stationary source of NO.sub.x emissions, which exhaust gas also containing oxides of sulfur (SO.sub.x), optionally for use in a method according to any preceding claim, which system comprising an oxidation catalyst comprising a platinum group metal for passively oxidising nitrogen monoxide to nitrogen dioxide so that a NO.sub.2/NO.sub.x content is from 40-60%; an injector for introducing a nitrogenous reductant into the exhaust gas located downstream from the oxidation catalyst; and a selective catalytic reduction (SCR) catalyst comprising an aluminosilicate zeolite promoted with copper located downstream of the injector.

[0037] The composition of the SCR catalyst and the oxidation catalyst can be the same as that disclosed hereinabove in connection with the method according to the first aspect of the invention.

[0038] Preferably, the exhaust system comprises a heat recovery steam generator (HRSG).

[0039] According to a third aspect, the invention provides a stationary source of NOx emissions, which is a power station, an industrial heater, a cogeneration power plant, a combined cycle power generation plant, a wood-fired boiler, a stationary diesel engine, a stationary natural gas-fired engine, a marine propulsion engine, a diesel locomotive engine, an industrial waste incinerator, a municipal waste incinerator, a chemical plant, a glass manufacturing plant, a steel manufacturing plant or a cement manufacturing plant comprising an exhaust system according to the second aspect of the invention.

[0040] Preferably, the cogeneration plant, which is preferably a stationary natural gas-fired engine, comprises an exhaust system which is a heat recovery steam generator (HRSG).

[0041] In order that the invention may be more fully understood, the following Examples are provided by way of illustration only.

EXAMPLES

EXAMPLE 1

Cu/CHA SCR Catalyst Activity in Reactions (1), (2) and (3)

[0042] A commercially available aluminosilicate CHA zeolite was ion-exchanged with 3wt % copper according to the methods disclosed in WO 2008/132452, i.e. the CHA was NH.sub.4.sup.+ ion exchanged in a solution of NH.sub.4NO.sub.3, then filtered. The resulting materials were added to an aqueous solution of Cu(NO.sub.3).sub.2 with stirring. The slurry was filtered, then washed and dried. The procedure can be repeated to achieve a desired metal loading. The final product was calcined. The resulting product was washcoated on a 300 cells per square inch (cpsi) (46.5 cells cm.sup.−2) cordierite honeycomb substrate monolith having cell walls of 5 mil (thousandths of an inch (0.127 mm)) and the coated substrate was calcined in air at 500° C. for 2 hours to obtain a sample referred to herein as “fresh”—as opposed to “aged”—catalyst. Prior to testing, the calcined, coated substrate was aged in air/water (steam) at 450° C. for 48 hours. A 1.5 inch×3.5 inch core was removed from the aged substrate. The sensitivity of the catalyst's activity for reactions (1), (2) and (3) was tested using a laboratory reactor and a base gas mixture for all tests of 15% O.sub.2, 8% H.sub.2O, 3% CO.sub.2, 50 ppm CO, and balanced by N.sub.2 at a space velocity of 120,000 hr.sup.−1 and a total inlet NOx concentration of 30 ppm (30 ppm NO only for reaction (1), 30 ppm NO.sub.2 for reaction (2); and 15 ppm NO and 15 ppm NO.sub.2 for reaction (3)). Ammonia was introduced to the base gas mixture for all SCR reactions. The ammonia:NO.sub.x ratio (ANR) for all SCR reactions was 1, i.e. 30 ppm NH.sub.3 was used. Following an initial assessment of SCR activity, the catalyst was aged in sulphur dioxide by switching off the ammonia and adding 20ppm SO.sub.2 to the base gas mixture at a constant 300° C. and at a flow rate of 100 standard litres gas per minute (slpm) for 60 minutes. For reaction (1), the ageing cycle in SO.sub.2 was repeated.

[0043] The results of the testing of reaction (1) are shown in Table 2

TABLE-US-00001 TABLE 2 % NOx SCR catalyst Inlet Conversion/ Temperature (° C.) Temperature 200 250 300 350 Fresh catalyst 55% 87% 93% 95% Sulphated run #1 21% 54% 80% 93% Sulphated run #2 22% 64% 84% 92%

[0044] The results in Table 2 show high fresh NO conversion for reaction (1), significantly reduced NO conversion following sulphation (sulphated run #1), and only slightly increased NO conversion in sulphated run #2 after multiple low sulphur tests demonstrating that the catalyst is readily poisoned by sulphur for reaction (1) and that poisoning is not easily reversed.

[0045] The results of the testing of reaction (2) are shown in Table 3

TABLE-US-00002 TABLE 3 % NOx Conversion/ Temperature (° C.) Temperature 200 250 300 350 Fresh catalyst 21% 43% 80% 92% Sulphated run #1 15%  7% 28% 64%

[0046] These results show that the high fresh NO.sub.2 conversion for reaction (2) is also significantly poisoned by the sulfation treatment.

[0047] Finally, the results of the testing of reaction (3) are shown in Table 4.

TABLE-US-00003 TABLE 4 % NOx Conversion/ Temperature (° C.) Temperature 200 250 300 350 Fresh catalyst 74% 93% 96% 96% Sulphated run #1 65% 92% 96% 97%

[0048] These results show that the high fresh NO+NO.sub.2 conversions for reaction (3) are maintained above 250° C. even after the sulphation treatment. This result demonstrates that reaction (3) is very robust to sulphur exposure and indicates that the Cu zeolite SCR catalyst can be used to remove NOx from an exhaust stream without need for a sulphur management cycle as long as the NOx is removed via reaction C.

[0049] In this invention, it has been realized that since the rates of reactions (1) and (2) over a Cu/zeolite catalyst are significantly reduced by exposure of the catalyst to sulphur but reaction (3) is not deactivated by sulphur, a system configured to provide an approximately 50/50 ratio mix of NO/NO.sub.2 to a Cu/zeolite SCR would take advantage of the high intrinsic activity of the Cu/zeolite SCR catalyst while not requiring the sulphur maintenance cycle typically used in mobile applications.

EXAMPLE 2

Oxidation Catalyst

[0050] The NO.sub.2/NO.sub.x ratio in a lean combustion derived exhaust can be modified by directing that exhaust through the appropriate oxidation catalyst at the appropriate temperature and flow rate so that reaction (8) proceeds to the desired extent.

2NO+O.sub.2.fwdarw.2 NO.sub.2   (8)

[0051] In practice, this can be quite difficult for a mobile diesel application since the exhaust flows and temperatures continuously vary over wide ranges. However in some stationary applications, such as emission control from a combined cycle gas turbine, the system operates over narrow flow and temperature ranges making it practical to match the oxidation catalyst to the reaction conditions to get the desired NO2/NOx ratio in the exhaust gas fed to a downstream SCR. For example, Table 5 shows the NO2:NOx ratio that results from passing a gas mixture over a standard gas turbine CO oxidation catalyst at different space velocities.

[0052] The oxidation catalyst was applied as a washcoat on a 200 cells per square inch metal honeycomb substrate. The washcoat contained an alumina support material doped with 30% silica, which was impregnated with 0.6 wt % platinum. The oxidation catalyst was field aged in a turbine for about six months of constant operation in a HRSG of a gas turbine with temperature exposures of between 302° C. to 354° C. The aged oxidation catalyst substrate was recovered and a 1.5 inch×1.7 inch core was tested in a laboratory reactor using a gas mixture of 50 ppm CO, 20 ppm NO, 15 ppm as Cl propene, i.e. the quantity of propene as a C3 molecule calculated as delivering the equivalent ppm of Cl, 15% O.sub.2, 8% H.sub.2O, 3% CO.sub.2, and balanced by N.sub.2.

[0053] From the results shown in Table 5, it is apparent that the conditions can be adjusted so that the NO.sub.2/NO.sub.x ratio in the outlet gas is approximately 0.5 over the 250 to 400° C. temperature range which is the temperature range of interest for a combined cycle exhaust.

TABLE-US-00004 TABLE 5 % NOx Conversion/ Space Velocity Temperature (° C.) (hr.sup.−1 × 1000) 220 250 300 350 400 60 10% 66% 75% 69% 55% 125 25% 38% 52% 53% 46% 300  6% 11% 21% 27% 28%

[0054] Table 6 and Table 7 below compare the CO oxidation activity of fresh and hydrothermally sulphur aged (HTSA) alumina supported Pt oxidation catalyst (no silica dopant) (Table 6) and an alumina-silica supported Pt catalyst (Table 7) at equivalent Pt loadings of 0.6 wt % washcoated on 1.5 inch×2.0 inch 200 cells per square inch metal cores. The cores were aged at 300° C. in air/10% H.sub.2O/5ppm SO.sub.2 for 336 hours. The catalysts were then evaluated for oxidation activity at 200,000 hr.sup.−1 Gas Hourly Space Velocity (GHSV) in a laboratory test apparatus using a gas composition of 50 ppm CO, 10 ppm NH.sub.3, 30 ppm as Cl propane, 30 ppm as Cl propene, 15% O.sub.2, 8% H.sub.2O, 3% CO.sub.2, and N.sub.2 balance. Chemical analysis by ICP of the Pt/Al.sub.2O.sub.3 sample showed that the HTSA catalyst contained 3.38 wt % sulphur and the Pt/Al.sub.2O.sub.3—SiO.sub.2 catalyst contained 1.94 wt % sulphur.

TABLE-US-00005 TABLE 6 % NOx Conversion/ Temperature (° C.) Temperature 200 225 260 300 350 Pt/Al.sub.2O.sub.3-Fresh  28% 53% 88% 90% 90% Pt/Al.sub.2O.sub.3-HTSA  25% 51% 82% 86% 87% % Conversion −11% −4% −7% −4% −3% reduction from Fresh to HTSA

TABLE-US-00006 TABLE 7 % NOx Conversion/ Temperature (° C.) Temperature 200 225 260 300 350 Pt/Al.sub.2O.sub.3—SiO.sub.2-Fresh 40% 73% 85% 87% 88% Pt/Al.sub.2O.sub.3—SiO.sub.2-HTSA 23% 73% 86% 88% 89% % Conversion reduction −43% 0% +1% +1% +1% from Fresh to HTSA

[0055] From the results shown in Tables 6 and 7 and the ICP analysis it can be seen that the silica-doped Pt support retains oxidation activity because it is less easily sulphated.

[0056] Tables 8 and 9 below compare CO oxidation of fresh and hydrothermal sulphur aged (HTSA) titania and doped zirconia supported 0.6 wt % Pt oxidation catalysts. The zirconia catalyst support was doped with 11% SiO2 and 8% Y.sub.2O.sub.3. Both catalysts were washcoated on 200 cells per square inch 1.5 inch×2.0 inch metal cores and were evaluated at 200,000 hr.sup.−1 GHSV in a laboratory test apparatus. The HTSA conditions were 300° C. in air/10% H.sub.2O/5 ppm SO2 for 336 hrs. The gas composition for testing the oxidation catalyst activity was 50 ppm CO, 10 ppm NH3, 30ppm as Cl propane, 30 ppm as Cl propylene, 15% O.sub.2, 8% H.sub.2O, 3% CO.sub.2, and N.sub.2 balance.

TABLE-US-00007 TABLE 8 % NOx Conversion/ Temperature (° C.) Temperature 200 225 260 300 350 Pt/TiO.sub.2-Fresh 56 70 85 86 87 Pt/TiO.sub.2-HTSA 19 70 85 87 88 % Conversion reduction −66% 0% 0% +1% +1% from Fresh to HTSA

TABLE-US-00008 TABLE 9 % NOx Conversion/ Temperature (° C.) Temperature 200 225 260 300 350 Pt/Zr—Si—Y-Fresh 67 78 87 88 89 Pt/Zr—Si—Y-HTSA 66 78 88 89 90 % Conversion reduction −1% 0% +1% +1% +1% from Fresh to HTSA

[0057] From the results presented in Tables 8 and 9 it can be seen that both oxidation catalysts retain oxidation activity after HTSA. However, the oxidation catalyst containing doped zirconia also retains oxidation activity even at very low temperatures. This is an advantage during start-up of a gas turbine power plant when the exhaust gas composition is transient and the exhaust gas temperature is relatively low.

[0058] Table 10 below compares the CO oxidation activity of alumina- and alumina-silica 0.72 wt % supported Pt oxidation catalysts coated on 1.5 inch×1.7 inch 200 cells per square inch metal cores. The catalysts were field aged in a gas turbine for 6 months. The oxidation catalysts were evaluated in a laboratory test apparatus at 200,000 hr.sup.−1 GHSV with a gas mixture of 50 ppm CO, 15% O.sub.2, 8% H.sub.2O, 3% CO.sub.2 and N.sub.2 balance.

TABLE-US-00009 TABLE 10 % NOx Conversion/ Temperature (° C.) Temperature 150 174 203 224 Pt/Alumina 69% 77% 81% 82% Pt/Silica-Alumina 81% 83% 85% 86%

[0059] These results show that the silica-alumina support provides superior low temperature performance which would be beneficial during start-up of a gas turbine power plant when the exhaust gas composition is transient and the exhaust gas temperature is relatively low.