HZSM-5 catalyst with enhanced stability and activity

Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 02004 Elsevier B.V. All rights reserved.

475

The catalytic combustion of CH4 o v e r Pd-Zr/HZSM-5 catalyst with enhanced stability and activity Chun-Kai Shi, Le-Fu Yang, and Jun-Xiu Cai* Department of Chemistry and State Key Laboratory for Physical Chemistry of Solid Surface, Xiamen University, Xiamen 361005, China 1. INTRODUCTION Methane (CH4) is a more effective greenhouse gas than C O 2 and hence CH4 emission from natural gas engines, power plants, petroleum and petrochemical industries should be completely converted to CO2 and H20 by catalytic combustion [1-2]. Recently the Pd-supported zeolite catalysts are paid considerable attention due to their better low-temperature activity and high conversion efficiency for CH4 combustion than conventional Pd/A1203 catalysts [3-5]. These Pd-supported zeolite catalysts are typically prepared by an ion-exchanged method and show a poor thermal/hydrothermal stability. It is widely accepted that palladium oxide species play an important role as active sites during the reaction and their activities are dependent on the kinds of supports and additives used. In this work, we report the preparation of a series of Pd catalysts with different supports using the impregnation method, and the effects of these supports on the methane oxidation activity of Pd catalysts. In addition, thermal and hydrothermal stability of Pd-Zr/HZSM-5 and Pd/HZSM-5 are also evaluated at 370 ~ from a practical point of view.

2. EXPERIMENTAL 2.1. Catalyst preparation Firstly, Zr/HZSM-5 (Si/A12 = 100) support was prepared by impregnating the parent HZSM-5 powders for 12 h with aqueous solution of Zr(NO3)4, followed by drying at 120 ~ for 12 h and calcining at 500 ~ for 3 h. Then, the powders obtained were impregnated with Pd(NO3)2 aqueous solution. The simple and ZrO2 modified Pd/HZSM-5 catalysts were obtained by repeating the above drying and calcining procedures. Moreover, Pd/7-A1203 and Pd/SiO2 samples were also similarly prepared for comparison. The desired loading of all the metals in their metallic states was 1 wt.% of the catalysts.

476

2.2. Catalyst characterizations The specific surface area and micropore volume (Table 1) of the samples were measured by nitrogen-adsorption at 77 K with a Carlo 1900 Sorptomatic instrument. Oxygen temperature-programmed desorption (O2-TPD) tests of the catalysts were conducted in a conventional fixed-bed flow system. The samples were firstly pre-treated in air-flow (20 ml/min.) at 500 ~ for 30 min, let self-cool down to room temperature (RT), and then were flushed with a high purity He at a flowing rate of 15 ml/min for 60 rain to remove the physically adsorbed oxygen. Finally, the sample bed temperature was linearly increased from RT to 900 ~ at a heating rate of 20 ~ The change of oxygen concentration from the catalysts was detected with a quadrupole mass spectrometer (Balzers QMS 200 Omnistar). The details for methane temperature-programmed reduction (CH4-TPR) tests of the catalysts were the same as for O:-TPD except that a mixture of CH4/He (4%: 96% in volume) was used as the flushing and reducing gas. Moreover, CO: was monitored as a product of the redox reaction between CH4 and the PdO species on the catalysts.

2.3. Activity evaluation The catalytic activity and lifetime tests of the catalysts were carried out using a tubular quartz microreactor of the down-flow fixed-bed type at atmospheric pressure. A powder sample of 100 mg (45-60 mesh) was placed in the reaction bed, and the reaction mixture of CH4, O: and N2 (2: 8:90 vol.%) was fed into the reactor at a space velocity of 48000 h -~. For hydrothermal stability test, the water vapor of the feed gas was provided by a glass bubbler. CH4 conversion was analysed by an on-line gas chromatograph (Shimadzu GC-14B) equipped with an automatic sampling device and a TCD detector using a 5A sieve column at 55 ~ 3. RESULTS AND DISCUSSION

3.1. Activity and stability evaluation Table 1 Catalytic properties of Pd-supported catalysts for the complete combustion of methane Surface Micropore Catalyst area volume T l o % a / T s 0 % a / Tloo%~/ /m 2 g-1 /cm 3 g-1 oC oC ~ Pd/HZSM-5 488.7 b 0.1674 b 315 355 425 Pd-Zr/HZSM-5 482.5 b 0.1630 b 292 337 380 pd/7-A1203 172.7 c 0.0927 c 352 443 490 Pd/SiO2 197.5 c 0.1049 c 410 526 --aTemperatures required for 10, 50 and 100% conversion of CH4, respectively. bCalculated from the Dubinin plot; CCalculated from the BET plot.

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Fig. 1 compares the catalytic activities for methane combustion over various Pd catalysts as a function of temperature. Pd catalysts with different supports display a considerable discrepancy in terms of activity. In comparison to Pd/HZSM-5, Pd-Zr/HZSM-5 shows a remarkably higher methane ignition activity and a conversion efficiency of 100%, whereas Pd/y-AI203 and Pd/SiO2 exhibit more or less lower activities for ignition and total conversion. In addition, Pd/HZSM-5 shows a higher catalytic activity than Pd/y-A1203 and Pd/SiO2, indicating that the HZSM-5 support has a better enhancing effect on the activity of loaded PdO species than the y-A1203 or SiO2 supports, and ZrO2 further improves Pd/HZSM-5's activity. For further comparison, the methane conversion temperatures of all the catalysts are listed in Table 1. T~0%, T50% and T~00% represent the temperatures for 10, 50 and 100% methane conversions, respectively. One can clearly see that Pd-Zr/HZSM-5 exhibits the best activity among all the catalysts in terms of the three conversion levels. The T~0o/o,Ts0o/oand T~00% of Pd-Zr/HZSM-5 are 23, 18 and 45 ~ lower than those of Pd/HZSM-5, respectively. The thermal/hydrothermal stability of Pd-Zr/HZSM-5 was compared with that of Pd/HZSM-5 through monitoring the time dependence of CH4 conversion over them at 370 ~ (Fig. 2). In the presence or absence of 4% water vapor, Pd-Zr/HZSM-5 is found to show a remarkably lower tendency to deactivate compared to Pd/HZSM-5. Moreover, it is also worth to mention that the CH4 conversion over Pd-Zr/HZSM-5 could easily be recovered to its initial value by increasing the reaction temperature to 390 ~ or removing the H20 in the feed gas. It is suggested that ZrO2 can improve the thermal stability of the support, prevent the sintering of the supported palladium, and thus stabilize the dispersed state of palladium. As a result, with the addition of ZrO2, Pd/HZSM-5 possesses remarkably enhanced thermal/hydrothermal stability.

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Fig. 2 Time dependence of methane conversion over Pd-Zr/HZSM-5 (a, b) and Pd/HZSM-5 (c, d) at 370 ~ in the presence (b, d) and absence (a, c) of 4% water vapor

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Fig. 3. O2-TPD spectra of the samples. 3.2. O2-TPD tests The chemical states of PdO on the catalysts were studied by performing O2-TPD measurements (Fig. 3). One can see that the initial O2 evolution peak appears at ~661 ~ for Pd/HZSM-5, while at ~651, 671 and 674 ~ respectively, for Pd-Zr/HZSM-5, Pd/y-AI203 and Pd/SiO2. In addition, the Pd/~,-A1203 and Pd/SiO2 catalysts display obvious broad-peaks at the high-temperature region. According to Klingstedti et al. [6], the O2 evolution peaks at lower temperature (651-674 ~ could be attributed to the decomposition of crystalline PdO, while the broad-peaks appearing at high temperatures (722-790~ are mainly associated with the decomposition of amorphous PdO. Apparently, amorphous PdO possesses stronger Pd-O bonds than crystalline Pd-O. And ZrO2 additive on Pd/HZSM-5 obviously affects the oxygen desorption properties of surface PdO species. In combination with the activity data listed in Table 1, one could come to the conclusion that the PdO species desorbing oxygen at lower temperature possess higher CH4 activities, and v i c e v e r s a .

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3.3. CH4-TPR tests

In order to examine directly the CH4 reducibility of PdO species on the catalysts, CH4-TPR tests were carried out (Fig. 4). One can see that the CH4 reduction properties of these PdO species on Pd catalysts are greatly influenced by the kinds of support and additive. All the TPR curves show two major peaks. Only the low temperature peak exhibits the reduction property of the PdO species [7]. Though the low-temperature peaks can be further split into three or more components corresponding to different PdO species reacting with methane, the initial reduction temperatures of the samples are in the order: Pd/SiO: > Pd/y-A1203 P > Pd/HZSM-5 > Pd-Zr/HZSM-5, with Pd/SiO2 being 337 ~ and Pd-Zr/HZSM-5 being 267 ~ Through CH4-TPR testing, Carstens et al. [2] also found that the crystalline form of PdO could be reduced more rapidly than the amorphous one. Therefore, we are tempted to ascribe the initial reduction peak to the reduction of the crystalline PdO, and the following ones to that of its amorphous state. Crystalline PdO was shown to be more catalytically active than amorphous PdO [2,8-9]. ZrO2 could act as an oxygen reservoir and is also a well-known oxygen supplier, exhibiting large oxygen mobility [2]. Therefore, ZrO2 could offer oxygen atoms a relatively free pathway for approaching the Pd atoms. So the oxygen mobility of ZrO2 might be a contributing factor for improving the crystallizability of PdO on Pd/HZSM-5. The crystalline PdO possesses the weak Pd-O bonds which can increase its stability and surface density of oxygen vacancies, contributing to the high turnover rate of CH4 oxidation [ 10]. 4. CONCLUSION In a summary, a series of Pd catalysts with different supports were prepared by the impregnation method. It is found that Pd/HZSM-5 shows considerably higher CH4 combustion activity than the conventional Pd/7-A1203 and Pd/SiO2. ZrO2 additive on Pd/HZSM-5 can greatly improve its catalytic activity. In addition, the

480 introduction of ZrO2 also endows Pd-Zr/HZSM-5 with higher thermal and hydrothermal stability than those of Pd/HZSM-5. Therefore, Pd-Zr/HZSM-5 could be considered as a promising catalyst for practical low-temperature CH4 combustion. ACKNOWLEDGEMENT The authors would like to thank for the financial support by the Ministry of Science and Technology of China (G1999022400).

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