Liu - Characterization Zsm On Plasma

Applied Catalysis B: Environmental 47 (2004) 95–100

Characterization of plasma treated Pd/HZSM-5 catalyst for methane combustion Chang-jun Liu a,b,∗ , Kailu Yu a,b , Yue-ping Zhang b , Xinli Zhu a , Fei He a , Baldur Eliasson c a

b

School of Chemical Engineering and Technology, Tianjin 300072, PR China ABB Plasma Greenhouse Gas Chemistry Laboratory, Tianjin University, P.O. Box 796666, Tianjin 300072, PR China c ABB Switzerland Ltd., Corporate Research, Baden CH5405, Switzerland Received 14 February 2003; received in revised form 30 June 2003; accepted 29 July 2003

Abstract In this work, a novel glow discharge plasma treatment of Pd/HZSM-5 catalyst, followed by calcination thermally, has been conducted. Such prepared catalyst presents a higher catalytic activity and an enhanced stability over the catalyst prepared without plasma treatment. The methane conversion over the plasma treated catalyst is close to 100% at 450 ◦ C, but it is only ca. 50% at the same temperature over the catalyst without plasma treatment. The XPS, H2 chemisorption and XRD characterizations confirm an enhanced dispersion has been achieved with the plasma reduction (during treatment) followed by oxidation (during calcination). Upon FT-IR analyses, the plasma treatment, followed by calcination thermally, also leads to enhanced Brönsted and Lewis acidities. The amount of Brönsted acid sites of the plasma treated Pd/HZSM-5 catalyst is 1.13 times larger than that of the catalyst without plasma treatment, while the amount of Lewis acid sites of the plasma treated Pd/HZSM-5 catalyst is 1.21 times higher. The enhanced acidities are definitely helpful to increase the dispersion of PdO over the support and improve the interaction between PdO and HZSM-5 support, which leads to a remarkable improvement in the catalyst stability. © 2003 Elsevier B.V. All rights reserved. Keywords: Plasma treatment; Glow discharge; Pd/HZSM-5; Methane combustion

1. Introduction Catalytic combustion of hydrocarbons has recently drawn more and more attentions worldwide [1–3]. Among all the hydrocarbon fuels, methane is the most difficult to be oxidized. The supported palladium catalyst has been found to be the most active catalyst for methane combustion [3–7]. Alumina has been extensively studied to be the support for methane combustion [8–10]. Zeolites were also employed in the preparation of the supported palladium catalysts, including modenite [11–14], ZSM-5 [4,6,13,14] and SAPO-5 [15]. PdO has been identified as the active species for methane combustion. The preparation method, dispersion of active species and the structure of the support were found to be the influencing factors to the catalytic activity and stability. The use of zeolites as the support can lead to a better low ∗ Corresponding author. Tel.: +86-22-27890078; fax: +86-22-27890078. E-mail address: [email protected] (C.-j. Liu).

0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.07.005

temperature activity, compared to the supporting materials like alumina and ZrO2 . However, the poor stability of the zeolite-supported catalysts limited their further application. Low Brönsted acidity of the support has a negative effect on the dispersion of PdO [4,6], while one can regulate the degree of dispersion through the acid–base interaction between acid sites and basic metal oxides [6]. In this work, we attempt to use a glow discharge plasma treatment, followed by calcinations thermally, to modify Pd/HZSM-5 catalyst and get higher catalytic activity and improved stability for methane combustion.

2. Experimental 2.1. Catalyst preparation The preparation of Pd/HZSM-5 catalyst includes the following steps: impregnation, drying, glow discharge plasma treatment and calcination thermally. For the purpose of

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charge treatment, the pressure of the discharge tube was in the range between 100 and 200 Pa. The applied voltage to the electrode was ca. 900 V. The discharge treatment time was ranged from 30 to 60 min. Fig. 2 presents an image of the glow discharge with the powder being treated. The further details with the measurement of electron temperature were previously described [18]. After plasma treatment, the catalyst was calcined for 4 h at 500 ◦ C. The amount of palladium loaded for all catalysts was 2 wt.%. 2.2. Reaction studies

Fig. 1. The apparatus for the plasma catalyst treatment using glow discharge.

comparison, a catalyst prepared without the step of plasma treatment has also been made. The incipient wetness impregnation of the zeolite with an aqueous solution of PdCl2 was conducted. Before the impregnation, hydrochloric acid (36–38%) was added into the solution of PdCl2 (the molar ratio of HCl/PdCl2 is 3) to promote the dissolution of PdCl2 in water. The zeolite, HZSM-5 (made in Zeolite Plant of Nankai University, Tianjin, China), was obtained commercially and was used as received. After impregnation, the catalysts were dried at 50 ◦ C for 4 h. The obtained sample was then held in a container and placed in a discharge tube for the plasma treatment. Fig. 1 shows the apparatus for the plasma treatment of Pd/HZSM-5 powder (40–60 mesh). The discharge tube (i.d. 40 mm) was made of glass and the distance between the two electrodes was set at 100 mm. A dc high-voltage generator was used to generate glow discharge plasma. The properties and characterization of glow discharge plasma have been addressed in the literature [16,17]. The catalyst powder was placed in the “positive column” of glow discharge that characterizes itself with highly energetic electrons at low gas temperature. The glow discharge was initiated at room temperature in this work. Argon (>99.99% in purity) was applied as the plasma-forming gas. During the glow dis-

The catalytic activity for methane combustion was tested using a quartz tube reactor (i.d. 8 mm) at atmospheric pressure. The loading amount of catalyst was 220 mg. A gaseous mixture containing 1 vol.% methane and 4 vol.% oxygen in nitrogen (GHSV = 15,000 h−1 ) was fed into the reactor. Before reaction, pure nitrogen (20 ml/min) flowed through the catalyst bed at a temperature-increasing rate of 5 ◦ C/min until the temperature reached 300 ◦ C. The methane conversion was measured as a function of temperature between 300 and 600 ◦ C with successive heating steps of 50 ◦ C, maintaining a 150 min duration at each temperature. The effluent gas from the reactor went through a condenser firstly to remove the condensable water. Then the effluent gas containing CH4 , O2 , CO2 , N2 and trace H2 O was analyzed by a gas chromatograph (AGILENT 4890D) using a TCD detector with a Paropak Q column. 2.3. Catalyst characterization XRD characterization was conducted using a Rigaku D/max-2500 diffractometer with Cu K␣ radiation (λ = 1.5418 Å). The scan rate was 8◦ /min. XPS analysis was performed using a PHI-1600 system operated at 1.2 × 10−8 under Mg K␣ radiation (1253.6 eV). The contaminative C1s peak at 284.6 eV was used for calibration. The acidity analysis was conducted using IR-pyridine adsorption with a Bruker Vector22 IR spectrometer. And, the specific surface area was measured by nitrogen adsorption in helium at 77 K using a CHEMBET-3000 instrument.

Fig. 2. Image of the glow during the plasma treatment of Pd/HZSM-5 catalyst.

C.-j. Liu et al. / Applied Catalysis B: Environmental 47 (2004) 95–100

The H2 -chemisorption measurement was executed in a Micromeritics 2910 analyzer by using a dynamic pulse technique using flowing argon at 20 ml/min with pulses of a 5% hydrogen/argon mixture.

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Table 1 Particle size of PdO obtained by Scherrer formula

Pd/HZSM-5(C) Pd/HZSM-5(PC)

Full-width at half-maximum (rad)

Particle size (nm)

0.00691 0.00904

21.0 16.0

3. Results and discussion In the following discussion, the catalyst prepared without plasma treatment is referred to as Pd/HZSM-5(C), while the sample just after glow discharge plasma treatment (before the calcination) is referred to as Pd/HZSM-5(PT) and the calcined catalyst of plasma treated sample is referred as Pd/HZSM-5(PC). 3.1. XRD characterization The specific surface area of Pd/HZSM-5(C) is 214.2 m2 /g and it is 245.4 m2 /g over Pd/HZSM-5(PC). The specific surface area of Pd/HZSM-5(C) is slightly less than Pd/HZSM5(PC). This suggests that the plasma treatment using glow discharge does not affect the zeolite structure. The XRD characterization presents further evidence with it. Fig. 3 shows the XRD patterns of Pd/HZSM-5(C), Pd/HZSM-5(PT) and Pd/HZSM-5(PC). It can be seen clearly that both Pd/HZSM-5(C) and Pd/HZSM-5(PC) own a sharp peak at near 33.8◦ that is attributed to the tetragonal PdO. However, this characteristic peak of Pd/HZSM-5(PC) is much broader than that of Pd/HZSM-5(C), which suggests that the particle size of PdO over Pd/HZSM-5(PC) is smaller compared to that without plasma treatment. The full-width at half-maximum (FWHM) and the particle size calculated by Scherrer formula are shown in Table 1. It can be seen clearly that the PdO size is 16.0 nm for Pd/HZSM-5(PC), but it is 21.0 nm for Pd/HZSM-5(C), which confirms that the particle size of PdO over the plasma treated sample is smaller compared to that without plasma treatment. An enhanced dispersion has been achieved with the plasma

Table 2 Results of H2 -chemisorption measurement

Pd/HZSM-5(C) Pd/HZSM-5(PC)

Metal dispersion (%)

Active particle diameter (nm)

4.57 5.92

24.5 18.9

treatment followed by calcination thermally. The results of H2 -chemisorption shown in Table 2 also indicates that the palladium dispersion of Pd/HZSM-5(PC) is higher than that of Pd/HZSM-5(C), which is consistent with the calculated results from XRD patterns with Scherrer formula. On the other hand, Pd/HZSM-5(PT) exhibits a sharp peak at 40◦ that can be attributed to the metallic palladium, which indicates that the palladium species has been reduced during glow discharge plasma treatment. During the plasma treatment, the color change of Pd/HZSM-5 catalysts from brown to black can be observed. This also indicates the remarkable change in the palladium species over the HZSM-5 support. Once the plasma treated sample, Pd/HZSM-5(PT), is oxidized during calcination, the active palladium species, PdO, is formed and dispersed into the pore of zeolite. Therefore, the combination of plasma reduction and oxidation is an effective way to enhance the dispersion of PdO. Okumura and Niwa reported a thermally reduction-oxidation way to increase the dispersion of PdO over the ZSM-5 supports [6] via controlling temperatures carefully. They also mentioned the dispersion of PdO is related to the amount of acid sites of zeolite catalysts. As we will discuss below, the plasma treatment will lead to an enhanced acidities. To increase the dispersion of PdO, the plasma treatment is an easy and excellent way. 3.2. XPS analysis Table 3 presents the atomic composition of the surface of Pd/HZSM-5(C) and Pd/HZSM-5(PC). In this table, the bindTable 3 The surface atomic composition of Pd/HZSM-5(C) and Pd/HZSM-5(PC) Sample

Fig. 3. XRD patterns of Pd/HZSM-5(C), Pd/HZSM-5(PT) and Pd/HZSM5(PC) 䉬:HZSM-5; 䊏: Pd; 夹: PdO.

Fresh Pd/HZSM-5(C) Used Pd/HZSM-5(C) Fresh Pd/HZSM-5(PC) Used Pd/HZSM-5(PC)

Surface atomic concentration (%)

Intensity ratio

O

Al

Pd

Si

Pd/Si

Pd/Al

56.8 58.1 60.4 60.2

0.9 1.6 1.3 1.3

0.2 0.1 0.5 0.4

24.8 23.4 24.9 25.6

0.81 0.43 2.01 1.56

22.22 6.25 38.46 30.77

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Fig. 4. The XPS profiles of Pd/HZSM-C(a), Pd/HZSM-PT(b) and Pd/HZSM-PC(c).

ing energy of Pd3d5 of both Pd/HZSM-5(C) and Pd/HZSM5(PC) are 337.5 eV (as shown in Fig. 4), which indicates the existence of PdO. These confirm the results of XRD characterization. Upon Table 3, the palladium composition on the surface of Pd/HZSM-5(PC) is ca. 2.5 times higher than that of Pd/HZSM-5(C). And, the intensity ratios of Pd/Si and Pd/Al of Pd/HZSM-5(PC) are also higher than those of Pd/HZSM-5(C), which indicates an enrichment of palladium species on the surface and a higher dispersion of palladium species on Pd/HZSM-5(PC), compared to Pd/HZSM-5(C). We have also observed such surface enrichment of palladium species with our previous investigation on Pd/Al2 O3 catalyst for methane combustion [19]. But the surface enrichment over Pd/Al2 O3 catalyst is much better than Pd/HZSM-5(PC). More palladium species has dispersed into the pore of the zeolite. After reaction, the surface palladium composition decreases from 0.2% to 0.1% over Pd/HZSM-5(C), but it is just from 0.5% to 0.4% over Pd/HZSM-5(PC). Especially, the intensity ratios of Pd/Si and Pd/Al decrease significantly with Pd/HZSM-5(C), which suggests a significant decrease in the dispersion of PdO or an aggregation of palladium species during reaction. The aggregated palladium species would block the pore of the zeolite and induce a deactivation of the catalyst. However, the ratios of Pd/Al and Pd/Si with Pd/HZSM-5(PC) just reduce slightly. Therefore, the glow discharge plasma treatment followed by calcination thermally is very helpful to hold or stabilize the dispersed palladium species over the zeolite support, which guarantees the better catalytic performance of Pd/HZSM-5(PC). Fig. 4 also exhibits the XPS spectrum of Pd/HZSM-5(PT). Obviously, two kinds of palladium species exist in this plasma treated sample. Upon the results of the curve-fitting of the spectrum, two peaks at 335.4 eV and 337.3 eV are obtained. The first peak (the relative amount is 67%) can be attributed to the metallic palladium and the latter can

be considered to be palladium ion (+2) species. The result is very different from what we obtained with Pd/Al2 O3 catalyst [19]. Almost all the palladium ions are reduced to metallic phase during the plasma treatment with Pd/Al2 O3 catalyst. It is thought that the reduction is related with the interaction between catalyst support and palladium ions. The zeolite can hold the palladium ions better and prevent the migration of palladium ions to the external surface. 3.3. The effect of plasma treatment on the acidity of the zeolites In general, PdO is a kind of basic oxides. Upon Okumura and Niwa [6,20], PdO in ZSM-5 can be anchored at the Brönsted acid sites [6]. And, Okumura and Niwa indicated too that the degree of dispersion of PdO is determined simply through the acid amount of ZSM-5. They concluded one could regulate the degree of dispersion through the amount of acid sites and through the acid–base interaction between acid sites of zeolite and basic PdO. In this work, we used IR–pyridine adsorption to analyze the effect of plasma treatment on the Brönsted and Lewis acidities for further understanding the plasma enhanced catalytic performance. Fig. 5 shows a comparative result of acidities obtained from FT-IR analyses. It can be seen that both Brönsted acid and Lewis acid exist in the two catalysts, based upon the appearance of bands at 1540 and 1450 cm−1 , respectively (corresponding to PyrH+ and PyrL). It is obvious that the peak area with Pd/HZSM-5(PC) is larger than that of Pd/HZSM-5(C), which would suggests a higher concentration of Brönsted acid and Lewis acid sites. The concentration of Brönsted acid and Lewis acid sites can be calculated from the integrated intensities of the PyrH+ and PyrL bands, and the values of the molar absorption coefficients of these bands (1.67 cm/␮mol and 2.22 cm/␮mol,

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Table 4 The concentration of Brönsted acids and Lewis acids based upon IR analyses with pyridine adsorption

sites/mmol g−1

Lewis acid Brönsted acid sites/mmol g−1

Pd/HZSM-5(C)

Pd/HZSM-5(PC)

1.025 1.222

1.239 1.375

respectively) have been determined by Emeis [21]. The calculated results are shown in Table 4. The concentrations of Brönsted and Lewis acidic sites over Pd/HZSM-5(PC) are 1.375 and 1.239 mmol/g, respectively, which are higher than that of Pd/HZSM-5(C) (1.13 times and 1.21 times higher, respectively). Therefore the plasma treatment followed by calcination thermally can lead to enhanced Brönsted and Lewis acidities. In addition, the enhanced Lewis acid sites will enhance their neighbor Brönsted acid sites furthermore [22]. The enhanced Brönsted acidity are definitely helpful to increase the dispersion of PdO over the support and improve the interaction between PdO and HZSM-5 support, which leads to a remarkable improvement in the catalyst stability. 3.4. Catalytic activity Fig. 6 presents a comparison of catalytic activity and stability between Pd/HZSM-5(C) and Pd/HZSM-5(PC) for methane combustion. The main products in the effluent gas were carbon dioxide and water. No carbon monoxide was detected. It can be seen clearly that the catalytic activity of Pd/HZSM-5(C) catalyst is lower. The methane conversion is only ca. 25% at 400 ◦ C and close to 100% at 600 ◦ C. Moreover, the catalyst Pd/HZSM-5(C) is very easy to be deactivated. At each temperature, the methane conversion reduces quickly with time. The reduced amount in methane conversion is ca. 27.7, 26.1, 22.4 and 14.9% at 400, 450, 500 and 550 ◦ C, respectively, with the reaction time at each temperature.

Fig. 6. The activity and stability of the plasma-prepared Pd/HZSM-5 catalyst (A) and the conventional one (B).

By contrast, Pd/HZSM-5(PC) shows a higher catalytic activity for methane combustion. The methane conversion is ca. 40% at 350 ◦ C and 90% at 400 ◦ C, respectively. When the temperature reaches 450 ◦ C, no methane can be detected in the effluent. In addition, the stability of this catalyst is also greatly improved. The methane conversion is very stable at each temperature, especially, at the temperature over 450 ◦ C with 100% methane conversion.

1540

1450

4. Conclusion Pd/HZSM-5(PC)

Pd/HZSM-5(C)

1300

1400

1500

1600

1700

wave number, cm-1 Fig. 5. FT-IR spectra of pyridine adsorbed on Brönsted and/or lewis acidic sites.

In this work, we report a novel plasma catalyst preparation method that has led to a preparation of Pd/HZSM-5 catalyst with higher activity and enhanced stability for methane combustion. This novel preparation includes the glow discharge plasma catalyst treatment followed by calcination thermally. Upon the catalyst characterizations, the plasma treatment will induce a reduction of palladium species. The combination of such plasma reduction and oxidation leads to the enhanced acidities. The increased amount of acid sites is very helpful to increase the dispersion of PdO species and to stabilize the active species, which makes the catalyst exhibit higher catalytic activity and much better thermal

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stability for methane combustion. The methane conversion over the plasma treated catalyst is close to 100% at 450 ◦ C, but it is only ca. 50% at the same temperature over the catalyst without plasma treatment. The present investigation will lead to a significant improvement in the catalyst preparation for methane combustion.

Acknowledgements This work was supported partly by the National Natural Science Foundation of China (under the contract of 20225618) and by ABB Switzerland Ltd.

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