Support effect of zeolite on the methane combustion activity of palladium

Applied Catalysis B: Environmental 40 (2003) 151–159

Support effect of zeolite on the methane combustion activity of palladium Kazu Okumura∗ , Sachi Matsumoto, Noriko Nishiaki, Miki Niwa Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho Minami, Tottori 680-8552, Japan Received 19 February 2002; received in revised form 17 June 2002; accepted 18 June 2002

Abstract Catalytic combustion of methane was carried out over Pd loaded on MFI and MOR zeolites. The activity and durability of the catalysts in the methane combustion was dependent on the Al concentration, the kind of cation and the structure of zeolite supports. Especially, Pd loaded on MOR was more active than that on MFI. The catalytic performance of Pd was correlated with the measurement of reaction order, activation energy and local structure of Pd determined by Pd K-edge EXAFS. A significant difference in the state of Pd was observed between MFI and MOR, where the Pd metal and PdO existed during the reaction over MOR and MFI, respectively. The high activity of Pd/MOR for methane combustion was ascribed to the generation of a mixture of Pd metal and PdO during the reaction. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Palladium; MOR; MFI; Methane combustion; EXAFS; Support effect

1. Introduction Recently, catalytic combustion of hydrocarbon has widely been studied from the viewpoint of environmental protection [1]. High catalytic activity is required for the operation at low temperature with low emission of NOx gas. Among the catalysts consisting of active metal and support oxide, Pd loaded on Al2 O3 has been studied most extensively, in order to identify the active phase, the reaction mechanism [2,3], and the structure sensitivity [4,5]. These studies have revealed that the activity of Pd was correlated with the particle size, the crystal structure and the reaction conditions. Although previous attention has primarily been directed to the investigation of the active sites, ∗ Corresponding author. Tel.: +81-857-31-5257; fax: +81-857-31-5684. E-mail address: [email protected] (K. Okumura).

the role of support is also an important factor, because the nature of support surface possibly affected the catalysis of Pd. We have reported that the of Pd for the methane and toluene combustion activity was very much dependent on the acid–base character of supports in the case of Pd catalysts loaded on simple metal oxides and zeolites [6,7]. The fact means that the acid and base property of support is one of the important factors, and determines the properties of supported Pd, such as dispersion and oxidation state. In the present study, in order to elucidate how the acid property of supports affected the structure and oxidation state of Pd, zeolites with different acid amount or structure were employed for supports of Pd. The experiments were carried out using zeolites with MFI and MOR structures. Zeolite is an especially suitable material for the investigation of the support effect, because it is easy to control the amount and the strength of acid sites through changing the Al concentration

0926-3373/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 1 4 9 - 2

152

K. Okumura et al. / Applied Catalysis B: Environmental 40 (2003) 151–159

and structure. In addition, Pd loaded zeolite catalyst is practically important because of its high activity and durability for the methane combustion [8,9]. In the present study, EXAFS technique was applied for the characterization of Pd structure. The data were correlated with the catalytic performance of Pd to investigate the active phase of Pd as well as the role of zeolite support. 2. Experimental 2.1. Catalysts preparation Zeolites with MFI and MOR structure were used for the support of Pd. MFI with different Al content were synthesized (Si/Al2 = 55) or supplied from Tosoh Co. (Si/Al2 = 24), Japan Gasoline Co. (Si/Al2 = 38), Mobil Catal. Co. Japan (Si/Al2 = 75), and reference catalysts, Catalysis Society of Japan (Si/Al2 = 90; JRC-Z5-90H). As for MOR structure, JRC-Z-HM15 and JRC-Z-HM20 were supplied from the Catalysis Society of Japan. In order to prepare MOR samples with low Al concentration, JRC-Z-HM20 was dealuminated with HCl solution with different concentrations (0.02–8 mol dm−3 ) at 353 K for 24 h. After the dealumination, it was washed with deionized water until a drop of silver nitrate solution detected no Cl− . Then the samples were calcined in a N2 flow at 773 K. The H-form of zeolite was prepared by an ion exchange of zeolite with NH4 NO3 solution, followed by calcination under N2 at 773 K for 4 h. The catalysts were subsequently filtered, washed, and dried at 373 K, followed by calcination in a N2 flow (30 ml min−1 ) at 773 K for 4 h. The chemical composition of the Pd loaded catalyst was measured by ICP method after digestion of the sample by HF solution. The loading of Pd was calculated to be 0.2–0.3 wt.%. 2.2. Catalytic reaction Catalytic activity of Pd in the combustion of methane was measured by the continuous-flow method. In the typical reaction conditions, CH4 :O2 :N2 (1:2:7) gas mixture (flow rate: 100 ml min−1 ) was fed into a Pyrex glass reactor under the atmospheric pressure, where 5 mg of catalyst diluted with 1 g of glass bead was placed. The reaction was conducted at 723 K.

The products were analyzed by gas chromatography equipped with TCD detector. Molecular Sieves 13X and Porapak Q columns were used for the analysis of products. 2.3. EXAFS measurement and analysis Pd K-edge XAFS was measured at BL01B1 station of Japan Synchrotron Radiation Research Institute (SPring-8). The storage ring was operated at 8 GeV with a ring current of 44–100 mA. Si(3 1 1) single crystal was used to obtain monochromatic X-ray beam. Two ion chambers filled with Ar and Kr were used as detectors for I0 and I, respectively. The measurements were conducted at room temperature. The sample was transferred to aluminum cells with two Kapton windows connected to a flow reaction system without contacting air. The thickness of the cell was chosen to be 30 mm to give edge jump of 0.4. In spite of the low concentration of Pd, fairly good spectra were obtained due to the high brilliancy of the X-ray source. For EXAFS analysis, oscillation was first extracted from the EXAFS data by a spline smoothing method [10]. The oscillation was normalized by the edge height around 50 eV above the threshold. The Fourier transformation of the k3 -weighted EXAFS oscillation from k space to r space was performed over the range 30–155 nm−1 to obtain a radial distribution function. The inversely Fourier filtered data were analyzed by a usual curve fitting method based on Eq. (1) χ (k)=




 kj =

Nj Fj (k) exp(−2σj2 kj2 )

k 2 − 2mE0j h2

1/2

sin(2krj + φj (k)) kr2j

,

(1)

where Nj , rj , σ j and E0j represent the coordination number, the bond distance, the Debye–Waller factor, and the difference of the threshold energy between reference and sample, respectively. The degree of error bars in the present curve fitting analysis for E0j and σ j is estimated to be 4 eV and 0.002 nm, respectively. Fj (k) and fj (k) represent amplitude and phase shift functions, respectively. For the curve fitting analysis, the empirical phase shift and amplitude functions for Pd–O and Pd–Pd were extracted from the data for PdO and Pd foil, respectively. Errors in the

K. Okumura et al. / Applied Catalysis B: Environmental 40 (2003) 151–159

153

analysis were estimated by R factor (Rf ) determined by Eq. (2).  3 obs |k χ (k) − k 3 χ calc (k)|2 dk  Rf = (2) |k 3 χ obs (k)|2 dk The analysis of EXAFS data was performed using the “REX2000” program (RIGAKU). 3. Results 3.1. Catalytic performance 3.1.1. Catalysis of Pd loaded on various zeolites The initial rates of methane combustion over Pd/H-MFI and Pd/H-MOR were summarized in Fig. 1. As can be seen in the figure, the structure and composition of zeolites affected the catalytic performance of Pd. The maximum conversions of methane attained on H-MOR and H-MFI when Si/Al2 ratios reached at 30 and 200, respectively. The initial activity of Pd/H-MOR was higher than that of Pd/H-MFI independent of the Al concentration, where the maximum conversion of Pd/H-MOR was higher than that of Pd/H-MFI by eight times. In contrast, the activity of Pd loaded on Al2 O3 (Al2 O3 : JRC-ALO4, reference catalyst supplied from the Catalysis Society of Japan;

Fig. 1. Dependence of initial methane combustion activity on the Al concentration; the numbers indicated the Si/Al2 ratios: (䊉) Pd/H-MOR, (䉭) Pd/H-MFI; Pd loading, 0.2–0.3 wt.%; reaction temperature, 723 K; total flow rate, 100 ml min−1 ; CH4 :O2 :N2 = 10:20:70.

Fig. 2. Methane combustion activity of Pd loaded on H- or Na-form MFI zeolite plotted as a function of time: (䊉) Pd/H-MFI, (䊊) Pd/Na-MFI; reaction temperature, 723 K; total flow rate, 100 ml min−1 ; CH4 :O2 :N2 = 10:20:70.

Pd: 0.2 wt.%) was as low as 0.5 mmol min−1 g−1 under the same reaction conditions. Apparently, it was possible to mention that the activity of Pd/H-MOR was outstanding. Figs. 2 and 3 show the methane combustion activity of Pd loaded on H-MFI and H-MOR as a function of time-on-stream, respectively. Pd/H-MOR exhibited better durability compared with Pd/H-MFI. For

Fig. 3. Methane combustion activity of Pd loaded on H-MOR zeolite plotted as a function of time on stream: reaction temperature, 723 K; total flow rate, 100 ml min−1 ; CH4 :O2 :N2 = 10:20:70.

154

K. Okumura et al. / Applied Catalysis B: Environmental 40 (2003) 151–159

instance, the conversion of Pd/H-MFI decreased on the average to 91% after 2 h from the beginning of the reaction, while Pd/H-MOR kept 97% of the initial activity. Particularly, the decrease in the activity of Pd was fast on Na-MFI in which the activity dropped by 46% after 2 h. Thus, it could be noted that the degree of deactivation was dependent on the structure and the cation of zeolite. 3.1.2. Activation energy and reaction order against oxygen Fig. 4 shows the Arrhenius plots of methane combustion based on the activity measured in the temperature range between 588 and 663 K. From these plots, the apparent activation energy of Pd loaded on H-MOR and H-MFI were calculated to be 131–169 and 59–63 kJ mol−1 , respectively. The values observed on Pd/H-MOR and Pd/H-MFI coincided with the previously reported data of Pd metal and PdO, respectively [11]. In order to obtain information about the oxidation state of Pd surface, the reaction order with respect to the oxygen pressure was measured. In the measurement, the flow rate of oxygen and N2 balance was varied while holding the methane pressure at 10.1 kPa. Fig. 5 shows the dependence of methane combustion activity on the oxygen pressure. From these plots, the reaction order against oxygen pressure was calculated to be 0.1 over Pd/H-MFI. Probably, the fact meant the surface of Pd on H-MFI was covered with adsorbed

Fig. 5. Dependence of the initial methane combustion activity on the partial pressure of oxygen: (䊉, 䉱) Pd/H-MOR; (䊊, 䉭) Pd/H-MFI.

oxygen completely, since the data indicated that oxygen in the gas phase did not directly participate in the reaction. On the other hand, the reaction order was calculated to be 1.0–1.2 over Pd/H-MOR, suggesting the presence of the Pd metal sites that could adsorb oxygen. Therefore, the formation of PdO and Pd metal on H-MFI and H-MOR was inferred from the measurement of reaction order against oxygen, respectively. These conclusions were consistent with the measurement of the apparent activation energy in both Pd loaded on H-MOR and H-MFI catalysts. 3.2. Characterization of Pd by EXAFS

Fig. 4. Dependence of the initial methane combustion activity on the temperature: (䊉, 䉱) Pd/H-MOR; (䊊, 䉭) Pd/H-MFI.

3.2.1. Pd/zeolites oxidized with O2 Pd K-edge EXAFS spectra were measured after the oxidation of Pd/MFI in an 1% O2 flow diluted with He at 773 K for 3 h. Fig. 6 shows the Fourier transforms of Pd/MFI measured after the treatment. The spectrum of PdO (standard sample) was attached in the figure in which three major peaks appeared at 0.16, 0.26 and 0.31 nm (phase shift uncorrected). These were attributed to the O and two kinds of Pd atoms, respectively. The Pd–Pd peaks appeared in the spectrum of Pd/MFI with low Al concentration (Si/Al2 = 90). However, the intensity of the Pd–Pd peaks gradually decreased accompanied by increase in Al content, and disappeared in H-MFI with the

K. Okumura et al. / Applied Catalysis B: Environmental 40 (2003) 151–159

Fig. 6. k3 -Weighted Pd K-edge EXAFS Fourier transforms for PdO, and Pd loaded on H-MFI measured after oxidation with 1% oxygen at 773 K.

highest Al concentration (Si/Al2 = 24). The fact meant that the size of PdO was affected by the acid amount in H-MFI, since the size of PdO reflected on the intensity of Pd–Pd shells. The coordination number (CN = 4) and the bond distance (0.202 nm) of Pd–O bond were quite similar between standard PdO sample and highly dispersed PdO on MFI. Thus, the

155

Fig. 7. k3 -Weighted Pd K-edge EXAFS Fourier transforms for Pd foil, and Pd loaded on H-MOR measured after oxidation with 1% oxygen at 773 K.

valence of the dispersed PdO was the same as that of bulk PdO, which was confirmed from the coincidence in the XANES region as reported elsewhere [12]. Fig. 7 shows the Fourier transforms of Pd/H-MOR treated with oxygen. The conditions for oxidation treatment were identical to that for Pd/H-MFI. The

Fig. 8. k3 -Weighted Pd K-edge EXAFS oscillations and their Fourier transforms for Pd/H-MFI (Si/Al2 = 24, 90) measured before and after methane combustion at 663 K.

156

K. Okumura et al. / Applied Catalysis B: Environmental 40 (2003) 151–159

spectra of Pd foil and PdO were attached in the figure. Pd kept highly dispersed form in the Si/Al2 ratio between 15 and 40 on H-MOR, which was evident from the disappearance of Pd–Pd peaks in these spectra. The growth of Pd–Pd bonds of PdO was found in the Si/Al2 ratio above 55. However, in the case of Pd/H-MFI, the Pd–Pd bonds (PdO) appeared at Si/Al2 ≥ 38. Therefore, it could be noted that Pd loaded on H-MOR showed better dispersion in comparison with Pd/H-MFI at a given Al concentration. Another characteristic of Pd/H-MOR was found in H-MOR with low Al concentration. In this region, Pd metal phase appeared in Pd/H-MOR (Si/Al2 = 65), and it became clearer in Pd/H-MOR (Si/Al2 = 260). Particularly, a characteristic Pd–Pd bond due to Pd metal was obvious in the spectrum of Pd/H-MOR (Si/Al2 = 260) as seen from the comparison with the spectrum of Pd foil. In contrast, palladium oxide was stabilized on H-MFI independent of the Al concentration. Thus, it was noted that the Pd metal phase was stable on H-MOR with low Al concentration.

3.2.2. EXAFS spectra of Pd/zeolites measured after methane combustion In order to obtain information about the structure of Pd during the methane combustion, EXAFS spectra were measured after the methane combustion at 663 K for 1 h. The samples were quenched after switching the gas to N2 and cooling down the temperature immediately. Then the sample was transferred to the EXAFS cell without contact with air. Figs. 8 and 9 show the Pd K-edge EXAFS spectra of Pd/H-MFI and Pd/H-MOR measured after the reaction, respectively. The dispersed PdO and agglomerated PdO was found on Pd/H-MFI samples with Si/Al2 = 24 and 90, respectively. From the comparison with the Fig. 6, it was seen that the structures were similar to those measured before the reaction, which was confirmed from the Fourier transforms and the curve fitting data listed in Table 1, implying the reaction atmosphere did not affect the structure of Pd on H-MFI. In contrast, a significant change was found on Pd/H-MOR. The fact was clearly seen from the appearance of novel peaks

Fig. 9. k3 -Weighted Pd K-edge EXAFS oscillations and their Fourier transforms for Pd foil, PdO and Pd/H-MOR (Si/Al2 = 15, 30, 65) measured before and after methane combustion at 663 K.

K. Okumura et al. / Applied Catalysis B: Environmental 40 (2003) 151–159

157

Table 1 Curve fitting analysis of Pd K-edge EXAFS data for Pd/H-MFI measured after oxidation and methane combustion Treatment

Si/Al2

Scatter atom

CNa

r (nm)b

E0 (eV)c

σ (nm)d

Rf (%)e

Oxidized

24

O Al

4.3 ± 0.9 1.4 ± 0.7

0.202 ± 0.001 0.305 ± 0.004

6 −3

0.0069 0.0063

3.6

Reacted

24

O Pd Pd Pd

4.4 1.5 0.3 1.0

± ± ± ±

0.001 0.002 0.010 0.006

8 2 −8 5

0.0071 0.0060 0.0054 0.0059

5.1

Oxidized

90

O Pd Pd

4.2 ± 0.8 3.9 ± 0.5 6.6 ± 0.9

0.202 ± 0.001 0.304 ± 0.001 0.344 ± 0.001

1 −5 −3

0.0063 0.0059 0.0059

1.4

Reacted

90

O Pd Pd

3.9 ± 0.4 3.8 ± 0.5 6.6 ± 0.8

0.202 ± 0.001 0.304 ± 0.001 0.343 ± 0.001

2 −3 −3

0.0059 0.0053 0.0054

0.6

Pd foilf

Pd

12

0.274

PdOf

O Pd Pd

4 4 8

0.202 0.304 0.342

± ± ± ±

1.1 0.9 0.2 0.8

0.203 0.274 0.303 0.346

a

Coordination number. Bond distance. c Difference in the origin of photoelectron energy between the reference and the sample. d Debye–Waller factor. e Residual factor. f Data from X-ray crystallography. b

Table 2 Curve fitting analysis of Pd K-edge EXAFS data for Pd/H-MOR measured after oxidation and methane combustion Treatment

Si/Al2

Scatter atom

CN

Oxidized

15

O

4.4 ± 1.0

Reacted

15

O Pd Pd Pd

2.3 4.6 0.8 2.0

Oxidized

30

O Al

4.3 ± 0.9 1.4 ± 0.7

Reacted

30

O Pd Pd Pd

2.7 2.8 1.7 3.2

± ± ± ±

0.3 0.4 0.7 1.0

Oxidized

65

O Pd Pd Pd

3.5 2.1 0.6 2.5

± ± ± ±

Reacted

65

O Pd Pd Pd

2.7 3.7 2.6 4.8

± ± ± ±

Notation as in Table 1.

E0 (eV)

r (nm)

± ± ± ±

0.203 ± 0.001

σ (nm)

Rf (%)

6

0.0074

1.6

0.002 0.001 0.004 0.002

−1 −4 −7 −4

0.0065 0.0060 0.0062 0.0060

1.2

0.202 ± 0.001 0.305 ± 0.004

6 −3

0.0069 0.0063

3.6

0.201 0.275 0.305 0.345

± ± ± ±

0.001 0.001 0.002 0.002

−2 0 −2 −1

0.0065 0.0061 0.0062 0.0062

1.2

0.5 0.9 0.3 1.0

0.201 0.277 0.304 0.343

± ± ± ±

0.001 0.002 0.002 0.002

4 10 3 3

0.0066 0.0064 0.0050 0.0068

2.7

0.4 0.5 0.6 0.9

0.202 0.275 0.306 0.344

± ± ± ±

0.001 0.001 0.001 0.001

−1 0 0 −1

0.0062 0.0060 0.0060 0.0059

0.7

0.4 0.5 0.6 0.8

0.203 0.278 0.304 0.341

± ± ± ±

158

K. Okumura et al. / Applied Catalysis B: Environmental 40 (2003) 151–159

at 0.25 and 0.31 nm (phase shift uncorrected). These peaks were primarily assigned to Pd–Pd bonds of the Pd metal and PdO, respectively, suggesting the generation of the mixture of Pd metal and agglomerated PdO. At the same time, the intensity of the Pd–O bond decreased after the reaction. Thus, the spectra measured after the reaction indicated that the partial reduction as well as the growth of Pd particle had occurred during reaction over the Pd/H-MOR. Table 2 gives the data obtained by the curve fitting analysis of Pd/H-MOR measured after oxidation and reaction conditions. The generation of Pd–Pd bonds characteristic of both Pd metal and bulk PdO was confirmed from the curve fitting analysis. The phenomena observed here were in contrast with those in Pd/MFI, where reaction atmosphere did not influence on the structure of Pd. Pd K-edge EXAFS spectra measured after methane combustion was dependent on the Al concentration of H-MOR as shown in Fig. 9. The Pd metal was predominantly observed on Pd/H-MOR with the highest Al concentration (Si/Al2 = 15), the sample exhibited low activity (1.9 mmol min−1 g−1 ) in the reaction. On the other hand, the generation of Pd–Pd bonds ascribed to PdO and Pd metal was found in Pd/H-MOR (Si/Al2 = 30) that exhibited the highest activity in the reaction. Though a similar spectrum was obtained on Pd/H-MOR (Si/Al2 = 65), the intensity of Pd–Pd bonds due to Pd metal and PdO was larger than that on Pd/H-MOR (Si/Al2 = 30). The fact implied that the particle size of Pd/PdO on Pd/H-MOR (Si/Al2 = 65) was larger than that on Pd/H-MOR (Si/Al2 = 30). 4. Discussion The methane combustion activities of Pd catalysts were sensitive to the structure and the composition of zeolite supports. In the case of H-MFI zeolite, the activity of Pd increased with decrease in the Al concentration, where the maximum activity was obtained at Si/Al2 = 200. In these samples, Pd kept the oxidized form before and after the methane combustion as indicated by EXAFS. These data were consistent with the data on the activation energy and the reaction order against oxygen characteristic of PdO phase. Therefore, it was considered that the change in the methane combustion activity over Pd/H-MFI was simply brought about the size effect of PdO, since the

growth of the Pd–Pd peaks characteristic of the agglomerated PdO was observed accompanied by the decrease in the Al concentration. The tendency observed here agreed well with the size effect of PdO reported in the literature [13,14]. For example, Hicks et al. [13] described that methane combustion activity of dispersed PdO was 10–100 times lower than that of the small palladium crystallites. Probably the size of PdO was determined through the acid–base interaction between acid sites of H-MFI and basic PdO [15]. Furthermore, it seemed that this acid–base interaction played an important role not only in the determination of the initial activity of PdO but also in the durability in the reaction. Because it was expected that the interaction between acid sites of zeolite and PdO prevented PdO from sintering, and the appropriate size of PdO was kept for combustion of methane. In fact, Pd supported on H-form MFI showed better durability than Pd on Na-form as seen in Fig. 2. The kinetic and structural characteristics of Pd/HMOR were considerably different from those of Pd/H-MFI in many aspects. The methane combustion activity over Pd/H-MOR was higher than that of Pd/H-MFI structure when comparison was made at a fixed Si/Al2 ratio. At the same time, in the case of oxidized samples, the acid amount required to keep the dispersed PdO on MOR was lower than on MFI. Probably, the difference in the acid strength of zeolite caused the different behavior between MOR and MFI. It was expected that the higher acid strength of H-MOR played a role in keeping the higher dispersion of PdO on H-MOR than on H-MFI, because the acid–base interaction was considered to be the driving force for anchoring of PdO. Indeed, the acid strength of MOR and MFI structure has been reported to be 145 and 130 kJ mol−1 , respectively [16]. The structures of Pd on H-MOR zeolite measured after methane combustion was also significantly different from those of the oxidized ones, while the reactant gas mixture did not affect the structure of Pd loaded on H-MFI. These facts were directly proved from EXAFS spectra as well as the kinetic measurements. The EXAFS spectra revealed the generation of a mixture of PdO and Pd metal over H-form MOR after the reaction. Thus, it was supposed that the easiness of the reduction of Pd/H-MOR led the generation of active Pd species, which was composed of the mixture of Pd metal and PdO. The fact was consistent

K. Okumura et al. / Applied Catalysis B: Environmental 40 (2003) 151–159

with the previous study reported in the literature. For instance, Carstens et al. [2] noted that the methane combustion activity of PdO was enhanced by producing a small amount of metallic Pd on the surface of PdO. They proposed that the products of the dissociatively adsorbed CH4 diffuse into the Pd/PdO interface in which the reaction occurred. In addition, Pfefferle and co-workers [11,17,18] reported the sharp increase in the conversion of methane when the Pd ⇔ PdO transition occurred at 993 K. The methane combustion activity of Pd/H-MOR was considerably dependent on the Al concentration of H-MOR. The optimum composition for the reaction was observed at Si/Al2 = 30. At this Al concentration, the generation of the mixture of Pd metal and PdO was observed as described above. However, the activity of Pd loaded on H-MOR (Si/Al2 = 15) was lower than Pd/H-MOR (Si/Al2 = 30). In this sample, the Pd metal phase was predominantly found after reaction. This was clearly seen from Fig. 9 and Table 2. Therefore, the reason for the low activity on Pd/H-MOR (Si/Al2 = 15) could be ascribed to the predominant formation of Pd metal. The observation agreed well with the literature in that the single Pd metal component was essentially inactive in the reaction [19]. On the other hand, Pd loaded on H-MOR with high Al concentration (e.g. Si/Al2 = 65) was also relatively inactive. The EXAFS spectra revealed that the intensity of Pd–Pd bonds of Pd/H-MOR (Si/Al2 = 65) was larger than that of H-MOR (Si/Al2 = 30). Therefore, it was considered that the growth of the Pd particle led the decrease in the number of the active sites, which existed in the boundary between Pd metal and PdO. Consequently, the influence of Al concentration of H-MOR on the activity of Pd was explained through the change in the composition and the size effect of Pd particle stabilized on H-MOR.

5. Conclusions The support effect of zeolite on the methane combustion activity and the structure of Pd were studied. The activity of Pd was considerably dependent on the kind and acid amount of zeolite. The activity of Pd loaded on H-form MFI reached a maximum at Si/Al2 = 90. The reason was explained through the

159

size effect of PdO that was regulated though the degree of interaction between PdO and acid sites of H-MFI. Pd supported on H-form MFI exhibited better durability to the methane combustion reaction than Pd on Na-form MFI zeolite. The acid–base interaction between acid sites of zeolite and PdO was considered to be responsible for the retardation of the sintering of PdO. Pd loaded on H-MOR exhibited much higher activity than that of H-MFI. The mixture of the Pd metal and PdO presented in the reaction was active phase in Pd loaded on H-MOR. This was characterized by the kinetic data and the Pd K-edge EXAFS measurement. From the experiments using a series of zeolites with different Al concentrations and structure, the order of methane combustion activity was summarized as follows: PdO dispersed < PdO agglomerated mixture of Pd metal and PdO. References [1] Y.H. Chin, D.E. Resasco, Catalysis, Vol. 14, Royal Society of Chemistry, London, 1998. [2] J.N. Carstens, S.C. Su, A.T. Bell, J. Catal. 176 (1998) 136. [3] S.C. Su, J.N. Carstens, A.T. Bell, J. Catal. 176 (1998) 125. [4] F.H. Ribeiro, M. Chow, R.A. Dalla Betta, J. Catal. 146 (1994) 537. [5] R.F. Hicks, H. Qi, M.L. Young, R.G. Lee, J. Catal. 122 (1990) 295. [6] K. Muto, N. Katada, M. Niwa, Appl. Catal. A 134 (1996) 203. [7] K. Okumura, H. Tanaka, M. Niwa, Catal. Lett. 58 (1999) 43. [8] Y. Li, J.N. Armor, Appl. Catal. B 3 (1994) 275. [9] H. Maeda, Y. Kinoshita, K.R. Reddy, K. Muto, S. Komai, N. Katada, M. Niwa, Appl. Catal. A 163 (1997) 59. [10] K. Moller, D.C. Koningsberger, T. Bein, J. Phys. Chem. 93 (1989) 6116. [11] M. Lyubovsky, L. Pfefferle, Appl. Catal. A 173 (1998) 107. [12] K. Okumura, J. Amano, N. Yasunobu, M. Niwa, J. Phys. Chem. B 104 (2000) 1050. [13] R.F. Hicks, H. Qi, M.L. Young, R.G. Lee, J. Catal. 122 (1990) 280. [14] C.A. Muller, M. Maciejewski, R.A. Koeppel, A. Baiker, J. Catal. 166 (1997) 36. [15] K. Okumura, M. Niwa, J. Phys. Chem. B 104 (2000) 9670. [16] N. Katada, H. Igi, J.-H. Kim, M. Niwa, J. Phys. Chem. 101 (1997) 5969. [17] M. Lyubovsky, L. Pfefferle, Catal. Today 47 (1999) 29. [18] A.K. Datye, J. Bravo, T.R. Nelson, P. Atanasova, M. Lyubovsky, L. Pfefferle, Appl. Catal. A 198 (2000) 179. [19] K. Sekizawa, M. Machida, K. Eguchi, H. Arai, J. Catal. 142 (1993) 655.