METHANE COMBUSTION CATALYST, METHOD FOR PRODUCING THE SAME AND METHOD FOR PURIFYING COMBUSTION EXHAUST GAS

Abstract

The present invention relates to a methane combustion catalyst including platinum and iridium supported on a tin oxide carrier for combusting methane in a combustion exhaust gas containing sulfur oxide. In the methane combustion catalyst, a ratio R.sub.TO of platinum oxides to metal platinum is 8.00 or more, wherein the ratio R.sub.TO is based on existence percentages of the metal platinum (Pt) and the platinum oxides (PtO and PtO.sub.2) obtained from a platinum 4f spectrum analyzed and measured by X-ray photoelectron spectroscopy (XPS) and calculated in accordance with the following expression. In the following expression, R.sub.Pt is an existence percentage of the metal platinum (Pt), R.sub.Pto is an existence percentage of PtO, and R.sub.Pto2 is an existence percentage of PtO.sub.2.

R.sub.TO=(R.sub.PtO+R.sub.PtO2)/R.sub.Pt  [Expression 1]

Claims

1. A methane combustion catalyst comprising platinum and iridium supported on a tin oxide carrier, and for combusting methane in a combustion exhaust gas containing sulfur oxide, wherein a ratio R.sub.TO of platinum oxides to metal platinum is 8.00 or more, the ratio R.sub.TO being based on existence percentages of the metal platinum (Pt) and the platinum oxides (PtO and PtO.sub.2) obtained from a platinum 4f spectrum of the methane combustion catalyst measured by X-ray photoelectron spectroscopy (XPS) and calculated in accordance with the following expression:
R.sub.TO=(R.sub.PtO+R.sub.PtO2)/R.sub.Pt  [Expression 1] wherein R.sub.Pt is an existence percentage of the metal platinum (Pt), R.sub.Pto is an existence percentage of PtO, and R.sub.Pto2 is an existence percentage of PtO.sub.2.

2. The methane combustion catalyst according to claim 1, wherein a supported amount of platinum on a mass basis with respect to the entire catalyst is 2.0% by mass or more and 15.0% by mass or less.

3. The methane combustion catalyst according to claim 1, wherein a supported amount of iridium on a mass basis with respect to the entire catalyst is 0.1% by mass or more and 5.0% by mass or less.

4. The methane combustion catalyst according to claim 1, wherein the methane combustion catalyst is in a shape of any one of a grain, a granule, a pellet, and a tablet.

5. The methane combustion catalyst according to claim 1, wherein the methane combustion catalyst is supported on a support in a shape of any one of a plate, a cylinder, a sphere, and a honeycomb.

6. A method for producing the methane combustion catalyst defined in claim 1, comprising: a first supporting step of impregnating a carrier of tin oxide with a platinum salt solution, and a first burning step of burning the carrier after the first supporting step; and a second supporting step of impregnating the carrier after the first burning step with an iridium salt solution, and a second burning step of burning the carrier after the second supporting step, wherein in the first supporting step, the carrier is impregnated, a plurality of times, with a platinum salt solution having a smaller platinum content than a platinum salt solution containing a target supported amount of platinum, and the carrier after impregnation is dried at a temperature of 60° C. or more and 150° C. or less after every time of the impregnation performed a plurality of times to provide a step of impregnating the target supported amount of platinum, and then a heating temperature in the first burning step is set to 350° C. or more and 500° C. or less.

7. The method for producing the methane combustion catalyst according to claim 6, wherein the method impregnates a platinum salt solution having a uniform platinum content a plurality of times in the first supporting step.

8. The method for producing the methane combustion catalyst according to claim 6, wherein the method sets a heating temperature in the second burning step to 350° C. or more and 500° C. or less.

9. A method for purifying combustion exhaust gas comprising oxidatively removing methane from a combustion exhaust gas containing sulfur oxide, wherein the method brings the combustion exhaust gas into contact with the methane combustion catalyst defined in claim 1 at a reaction temperature of 340° C. or more and 500° C. or less.

10. The methane combustion catalyst according to claim 2, wherein a supported amount of iridium on a mass basis with respect to the entire catalyst is 0.1% by mass or more and 5.0% by mass or less.

11. The methane combustion catalyst according to claim 2, wherein the methane combustion catalyst is in a shape of any one of a grain, a granule, a pellet, and a tablet.

12. The methane combustion catalyst according to claim 3, wherein the methane combustion catalyst is in a shape of any one of a grain, a granule, a pellet, and a tablet.

13. The methane combustion catalyst according to claim 2, wherein the methane combustion catalyst is supported on a support in a shape of any one of a plate, a cylinder, a sphere, and a honeycomb.

14. The methane combustion catalyst according to claim 3, wherein the methane combustion catalyst is supported on a support in a shape of any one of a plate, a cylinder, a sphere, and a honeycomb.

15. A method for producing the methane combustion catalyst defined in claim 2, comprising: a first supporting step of impregnating a carrier of tin oxide with a platinum salt solution, and a first burning step of burning the carrier after the first supporting step; and a second supporting step of impregnating the carrier after the first burning step with an iridium salt solution, and a second burning step of burning the carrier after the second supporting step, wherein in the first supporting step, the carrier is impregnated, a plurality of times, with a platinum salt solution having a smaller platinum content than a platinum salt solution containing a target supported amount of platinum, and the carrier after impregnation is dried at a temperature of 60° C. or more and 150° C. or less after every time of the impregnation performed a plurality of times to provide a step of impregnating the target supported amount of platinum, and then a heating temperature in the first burning step is set to 350° C. or more and 500° C. or less.

16. A method for producing the methane combustion catalyst defined in claim 3 comprising: a first supporting step of impregnating a carrier of tin oxide with a platinum salt solution, and a first burning step of burning the carrier after the first supporting step; and a second supporting step of impregnating the carrier after the first burning step with an iridium salt solution, and a second burning step of burning the carrier after the second supporting step, wherein in the first supporting step, the carrier is impregnated, a plurality of times, with a platinum salt solution having a smaller platinum content than a platinum salt solution containing a target supported amount of platinum, and the carrier after impregnation is dried at a temperature of 60° C. or more and 150° C. or less after every time of the impregnation performed a plurality of times to provide a step of impregnating the target supported amount of platinum, and then a heating temperature in the first burning step is set to 350° C. or more and 500° C. or less.

17. A method for producing the methane combustion catalyst defined in claim 4, comprising: a first supporting step of impregnating a carrier of tin oxide with a platinum salt solution, and a first burning step of burning the carrier after the first supporting step; and a second supporting step of impregnating the carrier after the first burning step with an iridium salt solution, and a second burning step of burning the carrier after the second supporting step, wherein in the first supporting step, the carrier is impregnated, a plurality of times, with a platinum salt solution having a smaller platinum content than a platinum salt solution containing a target supported amount of platinum, and the carrier after impregnation is dried at a temperature of 60° C. or more and 150° C. or less after every time of the impregnation performed a plurality of times to provide a step of impregnating the target supported amount of platinum, and then a heating temperature in the first burning step is set to 350° C. or more and 500° C. or less.

18. A method for producing the methane combustion catalyst defined in claim 5, comprising: a first supporting step of impregnating a carrier of tin oxide with a platinum salt solution, and a first burning step of burning the carrier after the first supporting step; and a second supporting step of impregnating the carrier after the first burning step with an iridium salt solution, and a second burning step of burning the carrier after the second supporting step, wherein in the first supporting step, the carrier is impregnated, a plurality of times, with a platinum salt solution having a smaller platinum content than a platinum salt solution containing a target supported amount of platinum, and the carrier after impregnation is dried at a temperature of 60° C. or more and 150° C. or less after every time of the impregnation performed a plurality of times to provide a step of impregnating the target supported amount of platinum, and then a heating temperature in the first burning step is set to 350° C. or more and 500° C. or less.

19. The method for producing the methane combustion catalyst according to claim 7, wherein the method sets a heating temperature in the second burning step to 350° C. or more and 500° C. or less.

20. A method for purifying combustion exhaust gas comprising oxidatively removing methane from a combustion exhaust gas containing sulfur oxide, wherein the method brings the combustion exhaust gas into contact with the methane combustion catalyst defined in claim 2 at a reaction temperature of 340° C. or more and 500° C. or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 is a schematic diagram illustrating a structure of a test device for a methane combustion test conducted in the present embodiment; and

[0063] FIG. 2 is a diagram illustrating a Pt4f spectrum and an Ir4f spectrum measured by XPS analysis of a methane combustion catalyst of Example 2 in First Embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] First Embodiment: An embodiment of the present invention will now be described. In the present embodiment, a Pt—Ir/SnO.sub.2 catalyst was produced following production process (Basic Production Process) according to a method for producing a methane combustion catalyst of the present invention. Also, Pt—Ir/SnO.sub.2 catalysts were produced through production processes (Comparative Production Processes 1 and 2) referring to a conventional technique (Patent Document 2). Each catalyst was subjected to XPS analysis to measure a ratio of platinum oxides (R.sub.TO), and a combustion test of a gas containing methane and sulfur oxide was performed to measure/evaluate a methane conversion.

[0065] [Basic Production Process]

[0066] The production process of the methane combustion catalyst of the present embodiment was as follows. A commercially available tin oxide powder was burnt at 600° C. A commercially available SnO.sub.2 sol was mixed/ground, as a binder, with the burnt tin oxide powder with a ball mill to produce a tin oxide slurry. The tin oxide slurry was applied by air blowing on a commercially available cordierite honeycomb (manufactured by NGK Insulators, Ltd.: ϕ25.4 mm×50 mmL) to obtain a tin oxide carrier (specific surface area of tin oxide: 12.09 m.sup.2/g). At this point, the number of times of performing air blowing was adjusted to adjust the mass of the tin oxide carrier.

[0067] On the tin oxide carrier (honeycomb support), platinum was dividedly supported. As a platinum salt solution to be impregnated in the divided supporting, a dinitrodiamine platinum-ammonia aqueous solution was used. As the dinitrodiamine platinum-ammonia aqueous solution, one obtained by dissolving dinitrodiamine platinum in ammonia water, and adjusting the resultant to pH 12 was used. In the present embodiment, the number of times of the divided supporting was set to 4 times, and a platinum concentration in a platinum salt solution used in every time was adjusted with respect to a target supported amount. Through the impregnation with the platinum salt solution of the respective times, the platinum salt solution was air blown onto the tin oxide carrier. After the impregnation with the platinum salt solution, a drying step was performed. In the drying step, the tin oxide carrier after the impregnation was put in a drier machine kept at 110° C., and held therein for 30 minutes as a drying treatment. The impregnation with the platinum solution and the drying step were performed 4 times.

[0068] Then, after the impregnation with the platinum salt solution and the drying, the resultant tin oxide carrier was burnt. The temperature was increased from the drying temperature (110° C.) in the final drying step in the supporting step described above at a temperature increase rate of 1° C./min to 275° C., and was kept at this temperature for 3 hours. A heating step described so far was performed in consideration of decomposition of platinum salt. Then, the temperature was increased from 275° C. at a temperature increase rate of 1° C./min to a set burning temperature, and was kept at this temperature for 3 hours for a burning treatment. In this manner, a tin oxide carrier supporting platinum oxides was produced.

[0069] On the tin oxide carrier (platinum supported), iridium was supported, and the resultant was burnt. Here, as an iridium salt solution, a hexachloroiridate aqueous solution was used, and a target supported amount was adjusted in accordance with an iridium concentration therein. A method for impregnating the iridium salt solution was performed similarly to that with the platinum salt solution, and the total amount of the solution was impregnated at one time. After the impregnation of the iridium salt, drying was performed at 110° C. for 0.5 hours, and the temperature was increased from this temperature at a temperature increase rate of 5° C./min to a burning temperature the same as that for platinum, and was kept at this temperature for 3 hours for burning treatment. Through these steps, the methane combustion catalyst of the present embodiment was produced.

[0070] In the present embodiment, the catalyst was produced in accordance with the above-described Basic Process, with the supported amount of platinum on a mass basis with respect to the entire catalyst set to 8.0% by mass, and with an iridium supported amount set to 0.8% by mass. At this time, the number of times of the divided supporting in the platinum supporting step was set to 4 times, and a platinum salt solution corresponding to 2% by mass was supported/dried every time, and the burning treatment was performed after the platinum supporting. In the present embodiment, five types of methane combustion catalysts were produced with a burning temperature employed after the platinum supporting respectively set to 350° C. (Example 1), 400° C. (Example 2), 450° C. (Example 3), 500° C. (Example 4), and 650° C. (Reference Example 1). It is noted that a burning temperature employed after the iridium supporting was the same temperature as the burning temperature employed after the platinum supporting.

[0071] [Comparative Production Process 1]

[0072] As a comparative example (Comparative Example 1) for the production process of a methane combustion catalyst of the present embodiment, referring to the conventional technique (Patent Document 2), a catalyst was produced by performing the drying step and the burning treatment respectively at higher temperatures than in Basic Production Process. As compared with Basic Production Process of the present embodiment, the divided supporting (4 times) was performed, and as a drying treatment after every time of the impregnation, heating was performed at 175° C. for 3 hours, and thereafter, heating was performed at 275° C. for 3 hours. In the burning step after the final drying treatment, heating was performed at 550° C. for 3 hours. Except for these drying treatment and burning treatment, the platinum supporting step was the same as that in Basic Production Process of the present embodiment. The iridium supporting step and the burning step were performed in the same manner as in Basic Production Process. The supported amounts of platinum and iridium were the same as those of Example 1 and the like.

[0073] [Comparative Production Process 2]

[0074] As another comparative example (Comparative Example 2) to the present embodiment, a catalyst supporting platinum without employing the divided supporting was produced. The supported amounts of platinum and iridium were the same as those of Example 1 and the like. In this Comparative Production Process 2, as compared with Basic Production Process of the present embodiment, the entire amount of a platinum salt solution containing a target supported amount of platinum was impregnated in a tin oxide carrier through one operation without employing the divided supporting. After the impregnation with the platinum salt solution, drying and burning were performed. In the drying treatment, heating was performed at a temperature of 110° C. for 1 hour, and in the burning treatment, heating was performed at 450° C. for 3 hours. A temperature increase rate in each treatment was the same as that employed in Basic Production Process. Thereafter, the iridium supporting step and the burning step were performed in the same manner as in Basic Production Process.

[0075] Measurement of Ratio of Platinum Oxides by XPS

[0076] Each of the methane combustion catalysts of the respective examples and comparative examples produced by the above-described various methods was subject to XPS analysis to obtain a ratio (R.sub.TO) of platinum oxides. In the XPS analysis, each catalyst was crushed with an agate mortar to prepare samples, and the XPS analysis was performed under the following conditions. In the XPS analysis, survey scan and narrow scan were performed to obtain a Pt4f spectrum and an Ir4f spectrum. [0077] Analyzer: K-Alpha+ manufactured by Thermo Fisher Scientific [0078] Irradiation X-ray: Al Kα ray for single crystal spectroscopy [0079] X-ray spot system: 400 μm [0080] Neutralizing electron gun: used [0081] Normalization of binding energy: normalized assuming that C—C and C—H have 2884.6 eV

[0082] FIG. 2 illustrates a Pt4f spectrum and an Ir4f spectrum measured by narrow scan of Example 2 (platinum supported amount: 8.0% by mass, iridium supported amount: 0.8% by mass, burning temperature: 400° C.). In the XPS spectrum of platinum, a peak top was obtained in a range of binding energy of 71 to 75 eV. In this range, as platinum oxides, a peak in a range of 72.8 to 73.2 eV was identified as PtO, and a peak in a range of 74.6 to 75 eV corresponded to PtO.sub.2. Referring to the spectrum of iridium of FIG. 2, iridium was in the form of Ir.sup.3+ or Ir.sup.4+, and a peak of metal iridium was not found. Based on this, it is understood that substantially all iridium was present in the form of iridium oxide.

[0083] For calculating the existence percentage of platinum oxides, the XPS profile thus obtained was subjected to waveform separation processing to measure a peak area representing each of the states of PtO.sub.2, PtO, metal Pt, and iridium (oxide). Also, peak areas of respective elements of oxygen (O), tin (Sn) and carbon (C) were simultaneously measured. Then, the peak areas of the respective components of Pt, Ir, O, Sn and C were corrected with respective relative sensitivity factors (RSF), and with a total peak area of the respective components used as a reference (100), the existence percentages of PtO.sub.2, PtO and metal Pt were calculated. Furthermore, based on the existence percentages of PtO.sub.2, PtO and metal Pt thus calculated, a ratio (R.sub.TO) of platinum oxides to metal platinum was calculated. It is noted that analysis software (Advantage-Thermo) was used in the above analysis.

[0084] Evaluation Test of Methane Combustion Performance

[0085] Next, each of the thus produced methane combustion catalysts was used for performing a test for performance evaluation. In this evaluation test, each catalyst was set in a test device of FIG. 1 simulating a fixed bed reactor, and a test gas was allowed to pass therethrough to measure a methane conversion. Test conditions were as follows: [0086] Reaction temperature (catalyst temperature): 400° C. [0087] Test gas composition: [0088] CH.sub.4: 2000 ppm [0089] CO.sub.2: 5% [0090] O.sub.2: 10% [0091] H.sub.2O: 10% [0092] SO.sub.2: 1 ppm [0093] N.sub.2: balance [0094] Space velocity (GHSV): 80,000 h.sup.−1

[0095] The test gas was caused to pass through the catalyst under the above-described conditions, and the composition of an exhaust gas was analyzed at 5 hours after starting the test to measure a methane conversion. For the measurement of the methane conversion, the exhaust gas was analyzed with an FID THC analyzer, a nondispersive infrared analyzer, and a magnetic oxygen analyzer to obtain CH.sub.4, CO.sub.2 and O.sub.2 concentrations. Then, the methane conversion was calculated from the measured values in accordance with the following expression:

[00001]$\begin{array}{cc}{\mathrm{CH}}_4\mathrm{conversion}\left(\%\right)=\frac{\begin{array}{c}(\left({\mathrm{CH}}_4\mathrm{concentration}\mathrm{before}\mathrm{reaction}\right)-\\ {\mathrm{CH}}_4\mathrm{concentration}\mathrm{after}\mathrm{reaction}))\end{array}}{\left({\mathrm{CH}}_4\mathrm{concentration}\mathrm{before}\mathrm{reaction}\right)}\times 100& \left[\mathrm{Expression}2\right]\end{array}$

[0096] Results of the evaluation test of the methane combustion performance performed in the present embodiment, and results of the ratio (R.sub.TO) of platinum oxides obtained by the XPS analysis of the respective methane combustion catalysts are shown in Table 1.

TABLE-US-00001 TABLE 1 Burning Methane Production Pt Ir SnO2 Temperature Estimated Existence Pt Oxide Conversion Process [wt %] [wt %] [wt %] [° C.] Assignment percentage Ratio (R.sub.TO) [%] Example 1 Basic Process 8.0 0.8 91.2 350 Metal Pt 0.5 15.60  97.8 PtO 1.7 PtO2 6.1 Ir3+, Ir4+ 0.8 — Example 2 400 Metal Pt 0.6 12.00  94.7 PtO 1.8 PtO2 5.4 Ir3+, Ir4+ 0.8 Example 3 450 Metal Pt 0.5 8.60 92.0 PtO 1.2 PtO2 3.1 Ir3+, Ir4+ 0.5 — Example 4 500 Metal Pt 0.8 8.25 84.7 PtO 1.7 PtO2 4.9 Ir3+, Ir4+ 0.7 — Reference 650 Metal Pt 0.9 3.22 24.3 Example 1 PtO 0.8 PtO2 2.1 Ir3+, Ir4+ 0.3 Comparative Comparative 550 Metal Pt 0.5 7.40 57.9 Example 1 Process 1 PtO 0.6 PtO2 3.1 Ir3+, Ir4+ 0.5 — Comparative Comparative 450 Metal Pt 1.2 3.83 75.9 Example 2 Process 2 PtO 2.1 PtO2 2.5 Ir3+, Ir4+ 1 —

[0097] Referring to Table 1, the methane combustion catalysts of Examples 1 to 4 had a methane conversion beyond 80%. As compared with the methane combustion catalysts of Comparative Examples 1 and 2, it is deemed that these catalysts have extremely good activity. Regarding the ratio (R.sub.TO) of platinum oxides measured by XPS, the methane combustion catalysts of Examples 1 to 4 all had a ratio of 8.00 or more, and it is thus understood that platinum oxides were generated at a high ratio. This can be also confirmed from the Pt4f spectrum of FIG. 2. On the other hand, in the catalyst of Reference Example 1 obtained with a high burning temperature (650° C.), the ratio (R.sub.TO) of platinum oxides measured by XPS is as low as less than 8.00. The methane conversion thereof was lower than those of the respective examples.

[0098] Although platinum oxides were sufficiently generated also in the methane combustion catalysts of Comparative Examples 1 and 2, the ratios were obviously lower than those of Examples 1 to 4. It is presumed that catalytic activity is basically in proportion to the supported amount of platinum, but the supported platinum amount was the same in the examples and the comparative examples, and hence, it is presumed that the activity was improved owing to the increase of the ratio of platinum oxides.

[0099] It can be understood, from a difference in the ratio of platinum oxides, that the drying treatment performed after the impregnation with the platinum salt solution in Basic Process of the present embodiment is significant, and that temperature control in the burning step is significant. In Comparative Production Process 1 employed for producing the catalyst of Comparative Example 1, the temperature in the drying treatment was high, and the burning temperature was also high. It is presumed that platinum salt was decomposed through the drying treatment performed at a high temperature, and hence platinum oxides were insufficiently generated in the subsequent burning, and in addition, that platinum oxides were decomposed also due to the high burning temperature. In Comparative Production Process 2 employed for producing the catalyst of Comparative Example 2, the divided supporting was not employed, but the platinum salt solution containing the target supported amount (8.0% by mass) of platinum was supported at one time. Therefore, it is presumed that dispersion of platinum was inferior, and that platinum oxides were insufficiently generated.

[0100] Second Embodiment: In the present embodiment, methane combustion catalysts different in the platinum supported amount and the iridium supported amount were produced to evaluate performances. Here, through the same steps as those of Basic Production Process of First Embodiment, a plurality of methane combustion catalysts were produced with a platinum concentration in a platinum salt solution and an iridium concentration in an iridium salt solution changed for adjusting the supported amounts of the noble metals. A methane combustion catalyst for comparison was also produced based on Comparative Production Process 1. Then, a performance evaluation test similar to that of First Embodiment was performed. In the performance evaluation test, two reaction temperatures (340° C. and 400° C.) were employed. Results thus obtained are shown in Table 2.

TABLE-US-00002 TABLE 2 Reaction Methane Production Pt Ir SnO2 Temperature Conversion Process [wt %] [wt %] [wt %] [° C.] [%] Example 5 Basic 2.1 0.2 97.6 340 3.0 Example 6 Process 4.2 0.4 95.4 340 7.4 Example 7 7.9 1.6 90.5 340 12.7 Example 8 8.0 0.4 91.6 340 11.4 Example 9 8.0 0.8 91.2 340 13.4 Example 10 11.5 1.1 87.4 340 12.2 Example 11 14.7 1.5 83.8 340 21.7 Comparative 2.8 0.3 96.9 340 0 Example 3 Comparative 8.0 0.8 91.2 340 0 Example 4 Example 5 Basic 2.1 0.2 97.6 400 56.8 Example 6 Process 4.2 0.4 95.4 400 76.9 Example 7 7.9 1.6 90.5 400 88.8 Example 8 8.0 0.4 91.6 400 90.0 Example 9 8.0 0.8 91.2 400 87.8 Example 10 11.5 1.1 87.4 400 89.0 Example 11 14.7 1.5 83.8 400 89.5 Comparative 2.8 0.3 96.9 400 15.9 Example 3 Comparative 8.0 0.8 91.2 400 57.9 Example 4

[0101] Referring to Table 2, the methane combustion catalysts of the comparative examples were extremely poor in the methane combustion activity when the reaction temperature was set to 340° C. On the other hand, the methane combustion catalysts of the examples exhibited the methane combustion activity even when it was 340° C. When the catalysts having the same noble metal supported amounts (Example 9 and Comparative Example 4) were compared, the methane combustion catalyst of the example obviously had higher activity.

[0102] When the reaction temperature was increased to 400° C., the methane combustion catalysts of the examples exhibited definite methane combustion activity. In particular, when the platinum supported amount was 4% by mass or more, high activity of 70% or more was exhibited. Even the catalyst having a platinum supported amount of 2.1% by mass (Example 5) exhibited activity equivalent to that of Comparative Example 4 having a platinum supported amount of 8.0% by mass, and hence this catalyst is deemed to be a useful catalyst depending on required performances in consideration of catalyst cost.

[0103] Third Embodiment: In the present embodiment, a proper range of a treatment temperature in the burning step performed after the impregnation with the platinum salt solution and drying was checked. Catalysts were produced with the burning temperature after the platinum supporting step set to 350° C. (Example 12), 450° C. (Example 13), 500° C. (Example 14), 550° C. (Reference Example 2), and 600° C. (Reference Example 3) in Basic Production Process of First Embodiment. The temperature in the burning step for iridium was set to the same as that in the burning step for platinum. A methane combustion catalyst for comparison was also produced based on Comparative Production Process 1 (drying temperature: 175° C., burning temperature: 550° C.) (Comparative Example 5). Then, each of the catalysts was subjected to a performance evaluation test in the same manner as that of First Embodiment (reaction temperature of 400° C.). Results thus obtained are shown in Table 3.

TABLE-US-00003 TABLE 3 Burning Methane Production Pt Ir SnO2 Temperature Conversion Process [wt %] [wt %] [wt %] [° C.] [%] Example 12 Basic 8.69 0.87 90.44 350 97.8 Example 13 Process 8.15 0.82 91.04 450 92.0 Example 14 7.89 0.79 91.32 500 91.7 Reference 8.19 0.82 91.00 550 73.5 Example 2 Reference 7.83 0.78 91.38 600 59.9 Example 3 Comparative Comparative 7.92 0.80 91.28 550 60.0 Example 5 Process 1

[0104] Referring to Table 3, the methane combustion catalysts obtained with the burning temperature set to 350° C. (Example 12), 450° C. (Example 13), and 500° C. (Example 14) exhibited a high methane conversion of 90% or more. On the other hand, in the methane combustion catalysts obtained by burning with the burning temperature over 500° C. (550° C.: Reference Example 2, and 600° C.: Reference Example 3), the methane conversion was likely to decrease. Accordingly, it was confirmed, also from the results of the present embodiment, that the treatment temperature in the burning step for platinum is preferably 500° C. or less. However, the methane combustion catalyst of Reference Example 2 obtained by burning at 550° C. had higher activity than the methane combustion catalyst of Comparative Example 5 produced with the same burning temperature. In the production process for the catalyst of Comparative Example 5 (Comparative Production Process 1), the drying temperature after the impregnation with the platinum salt solution was high (175° C.), and hence, it is presumed that the activity of Comparative Example 5 was inferior because a difference was caused in generation of platinum oxides by the difference in the drying temperature.

[0105] Furthermore, in the present embodiment, methane combustion catalysts of Example 13, Reference Example 2, Reference Example 3, Reference Example 4, and Comparative Example 5 were measured/evaluated for degree of metal dispersion by a CO gas adsorption method. In analysis by the CO gas adsorption method, a sample mass was set to 50 mg, a pre-treatment with He gas flow was performed at 50° C. for 2 hours, and then CO gas adsorption measurement was performed at 50° C. In this evaluation test, a catalyst (Reference Example 4) obtained by applying Basic Production Process with the burning temperature in the burning treatment set to a high temperature (650° C.), and a catalyst (Comparative Example 6) obtained by a production method of Comparative Production Process 2 in which platinum was supported through one impregnation operation without employing the divided supporting were produced, and were similarly evaluated. Measurement results thus obtained are shown in Table 4.

TABLE-US-00004 TABLE 4 Burning Methane CO Production Pt Ir SnO2 Temperature Conversion Adsorption Dispersion Process [wt %] [wt %] [wt %] [° C.] [%] (cm.sup.3/g) (%) Example 13 Basic 8.15 0.82 91.04 450 92.0 0.33 6.55 Reference Process 8.19 0.82 91.00 550 73.5 0.20 3.99 Example 2 Reference 7.83 0.78 91.38 600 59.9 0.14 2.82 Example 3 Reference 8.07 0.40 91.53 650 24.3 0.09 1.67 Example 4 Comparative Comparative 7.92 0.80 91.28 550 60.0 0.11 2.18 Example 5 Process 1 Comparative Comparative 9.00 0.90 90.10 450 75.9 0.17 2.92 Example 6 Process 2

[0106] When Example 13 (burning temperature: 450° C.), Reference Example 2 (burning temperature: 550° C.), Reference Example 3 (burning temperature: 600° C.), and Reference Example 4 (burning temperature: 650° C.) of Table 4 were compared, it was understood that the degree of metal dispersion is deteriorated in accordance with the increase of the burning temperature. In the methane combustion catalyst produced by burning at 550° C. to 650° C., it is presumed that the grain size was coarsened due to sintering of a platinum particle resulting from decomposition and metalation of platinum oxides, and hence the degree of metal dispersion was deteriorated.

[0107] The catalyst of Comparative Example 5 was inferior in degree of metal dispersion to the catalyst of Example 13 because the drying temperature and the burning temperature were higher. The catalyst of Comparative Example 6 was obtained with the same burning temperature (450° C.) as Example 13, but was produced by supporting the platinum salt solution at one time without employing the divided supporting. It is presumed that since the metal salt solution was not dividedly supported, dispersion of platinum was inferior, and that platinum oxides were insufficiently generated.

INDUSTRIAL APPLICABILITY

[0108] A Pt/SnO.sub.2-based methane combustion catalyst of the present invention is excellent in methane combustion activity as compared with that of conventional techniques. This is because platinum oxides working as an activity source are efficiently generated in production process. According to the present invention, owing to improvement of initial activity, a combustion exhaust gas can be treated for a long period of time with catalyst poisoning with sulfur oxide suppressed. The methane combustion catalyst of the present invention can be suitably applied to purification of various exhaust gases generated from engines, boilers, and power generation systems using hydrocarbon fuels such as natural gas and city gas. In addition, the present invention is useful for a power generation system such as a cogeneration system and a gas heat pump (GHP).