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Effect of water on the performance of Pd-ZSM-5 catalysts for the combustion of methane
Journal of Natural Gas Chemistry 17(2008)87–92
Effect of water on the performance of Pd-ZSM-5 catalysts for the combustion of methane Bo Zhang, Xingyi Wang∗ , Ogtour M’Ramadj,
Dao Li, Hua Zhang,
Guanzhong Lu
Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China [ Manuscript received July 10, 2007; revised September 27, 2007 ]
Abstract: Palladium-based catalysts were prepared using impregnation (I) and ion-exchange method (E) with ZSM-5 as support. Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts presented the high activity for the combustion of methane. The order of activity was consistent with Brønsted acidity of the catalysts: Pd-ZSM-5(I)>Pd-ZSM-5(E). It was shown by FT-IR that methane was adsorbed on the acidic bridging hydroxyl groups of ZSM-5-supported Pd catalysts. The effect of water on the activity of Pd-ZSM-5 was investigated. The inhibition effect of water on the conversion of methane was observed. However, water promoted the stability of Pd-ZSM-5 obviously during extended time periods. XPS measurement showed that Pd/Si ratio near the surface of Pd-ZSM-5(E) decreased more pronouncedly with time in dry stream than that of Pd-ZSM-5(I), this is attributed to the dispersion of Pd into the micropores. The addition of water, however, retarded Pd dispersion. And high partial pressure of methane reduced this effect of water vapor. The decrease in activity during the stability test can be explained on the basis of the reduction of Pd/Si ratio. Key words: methane combustion; Brønsted acid; palladium; ZSM-5; water
1. Introduction Catalytic combustion of methane is an important technology employed to decrease the emission of NOx in the application of natural gas. Supported palladium catalysts possess an excellent activity for the oxidation of methane and therefore have been extensively used for catalytic combustion purposes [1], but the thermal stability of Pd catalysts remains to be a problem. The state and dispersion of Pd supported on Al2 O3 [2], SiO2 [3] or zeolites [4,5] were less stable at lower temperature than the decomposition temperature of PdO during the catalytic combustion of methane. Even though the initial activity is high at this temperature, the catalysts are not able to maintain the high conversion level during extended time periods, that is, the activity is not stable [6,7]. Previous studies showed that the addition of other metal elements into Pd catalysts can improve their stability to some extent [4,8,9]. There were also studies on the effects of water and carbon dioxide on the activity of Pd catalysts for the combustion of methane [10]. The effect of water was generally considered as an inhibition effect, which depended on the reaction temperature. Burch observed that the inhibition effects of water were ∗
additive which could be completely reversible or permanent in deactivation of Pd catalysts, depending on the time period of exposure [11]. Recently, Ribeiro tested the kinetic rate of methane oxidation in the presence of water on Pd catalysts and found the magnitude of inhibition effect of water on the oxidation of methane as a function of temperature. At 553 K, the reaction order of water was about −1; above 723 K, the water reaction order was close to 0 [12]. Currently, carbenium ion chemistry is considered to be the most probable route for the initiation of alkane via hydride transfer on a solid acid [13,14]. Truitt and co-workers [15] discovered that trace amounts of Lewis acid sites facilitated the abstraction of hydride from alkane and generate reactive carbenium ions. Liu and Flytzani-Stephanopoulos [16] proved that the strength and geometry of acidic sites on the catalyst surface played an important role in the breakage of C−H bond and the formation of intermediate species. However, several recent publications still proposed the direct protonation of alkanes. It was suggested by Sommer and coworkers that strong solid acids could protolyse the C−H bond and thus the activation of short chain alkanes was achieved at low temperature [17]. According to the recent result [15],
Corresponding author. Tel: +86-21-64253372; E-mail: [email protected] This work was supported by the National Natural Science Foundation of China (No. 20377012).
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alkane did not simply undergo physical adsorption on the zeolite but rather formed a specific adsorption complex with the proton on Brønsted acid sites. The formation of adsorption complex was the route of activating the C−H bonds. However, the mechanismic steps catalyzed by solid acids and the role of acid sites for the combustion of methane are ambiguous. The proof related to the C−H activation is inadequate [13,14]. Therefore, an explicit understanding is needed to identify the role of the surface acidity of catalyst in the activation of methane during the catalytic combustion of methane. In this study, palladium-based catalysts supported on ZSM-5 were prepared. The effect of water vapor on the activity and stability of Pd-ZSM-5 catalysts for the oxidation of CH4 was studied. 2. Experimental 2.1. Preparation of catalyst Palladium-based catalysts were prepared using impregnation (I) and ion-exchange method (E). H-ZSM-5 zeolite (40−60 mesh, Si/Al = 80) was used as the support. In impregnation method, Pd-ZSM-5(I) catalyst was prepared by immersing support into an aqueous solution of palladium chloride at room temperature for 24 h. The impregnated solid was dried at 110 ◦ C for 12 h and then calcined in air at 550 ◦ C for 4 h. In ion-exchange method, the support was added into the aqueous solution of palladium chloride and the pH value of the suspension was adjusted to 7.5 by the addition of NH4 OH solution. Stirred at 90 ◦ C for 24 h, Pd-ZSM-5(E) catalyst was filtered, dried at 110 ◦ C for 12 h, and then calcined at 550 ◦ C for 4 h. The loading of Pd determined by inductively coupled plasma-atomic emission spectroscopy (ICP) was 0.55 wt%.
2.3. Characterization of catalysts BET surface area of catalyst was measured by nitrogen adsorption at 77 K on a Micromeritics ASAP 2000 surface analyzer. XPS spectrum of catalyst was recorded on a PerkinElmer PHI-5400 spectrometer, using a monochromatic MgKα radiation (1253.6 eV). The binding energy of adventitious C1s (284.6 eV with an accuracy of ± 0.2 eV) was used as a reference. The FT-IR spectra were recorded using a NICOLET NEXUS 470 spectrophotometer with a resolution of 4 cm−1 . A total of 15 mg sample was ground finely, pressed into a self-supporting wafer, and mounted into a quartz IR cell with CaF2 windows. Before adsorbing pyridine at room temperature, the wafer was heated at 300 ◦ C for 4 h under 10−3 torr (1 torr = 133 Pa). The FT-IR spectra of pyridine adsorbed on the catalyst were recorded after subsequent evacuation with the increase of temperature from room temperature to 300 ◦ C. The pre-adsorption of methane was also carried out before adsorption of pyridine. The background created by zeolite was subtracted. 3. Results and discussion 3.1. Catalytic activity The conversion of methane on Pd-ZSM-5(I) and PdZSM-5(E) as the function of temperature is shown in Figure 1 and the data with regard to the activity and properties of catalysts are summarized in Table 1, in which T10% , T50% , and T90% represent the temperatures for 10%, 50%, and 90% conversion of CH4 , respectively. It is obvious that both PdZSM-5 catalysts were active, leading to almost total conversion of CH4 at the temperatures of 200 to 460 ◦ C, of which
2.2. Activity test of catalysts for the combustion of methane The catalytic activity for the combustion of methane was evaluated at atmospheric pressure in a conventional fixed bed quartz reactor. Temperature was monitored by a thermocouple at the bottom of the catalyst bed and controlled by a temperature controller. The catalyst of 1.0 g was placed on the reactor bed and a gas mixture (0.2%CH4, 2%O2 and N2 balance) was fed at a rate of 420 ml·min−1 into the reactor (space velocity 36000 h−1 ). For the measurement of thermal stability, the catalyst bed was fed with reaction stream for 100 h. For the activity test on wet stream, the water vapour of the feed gas was provided by a glass bubbler. Conversion measurements were conducted in an ascending temperature mode. After the reaction was kept for 30 min in the steady state, the composition in outlet of reactor was analyzed on stream by a gas chromatograph (G-120, Shanghai).
Figure 1. Light-off curves for the combustion of methane over Pd-ZSM-5(I) and Pd-ZSM-5 (E). • and in the reactant feed with 0.2%CH4 , 2%O2 and N2 balance; ◦ and in the reactant feed with 0.2%CH4 , 0.4%O2 and N2 balance; space velocity 36000 h−1
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Table 1. The properties and activity of Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts BET
Relative amount of Brønsted acid sites a
Activity b
Catalysts
(m2 /g)
100 ◦ C
200 ◦ C
300 ◦ C
T10% / ◦ C
T50% / ◦ C
T90% / ◦ C
Pd-ZSM-5(I)
346
2.081
1.898
1.590
277//380//308
335//431//341
374//456//380
Pd-ZSM-5(E)
344
1.045
0.88
0.651
337//329//342
393//422//412
−1
a
435//456//471 b
The relative amount of Brønsted acid sites estimated from the integrated areas of the band at 1540 cm at different temperatures; Reaction in dry stream of O2 /CH4 = 10//in dry stream of O2 /CH4 = 2//in wet stream of O2 /CH4 = 10; T10% , T50% , and T90% are the temperatures required for 10%, 50%, and 90% conversion of CH4 , respectively.
Pd-ZSM-5(I) was more active than Pd-ZSM-5(E). When O2 /CH4 = 10 in dry stream, the former presented T10% of 277 ◦ C, and T90% of 374 ◦ C, whereas for the latter, the conversion of 10% and 90% was obtained at 337 and 435 ◦ C, respectively. The reaction products were mainly CO2 , and no other by-product was observed. When the partial pressure of oxygen was changed from O2 /CH4 = 10 to stoichiometric ratio (O2 /CH4 = 2) and the concentration of methane was maintained at 0.2%, 10% conversion of methane over Pd-ZSM-5(I) occurred at higher temperature than that over Pd-ZSM-5(E), and the temperature required for 90% conversion of methane on both Pd catalysts tended to be the same value. On the other hand, for the fresh Pd-ZSM-5 catalyst, its activity on dry stream was higher than that on wet stream, which indicated the inhibition effect of water. However, this effect decreased at high temperature over Pd-ZSM-5(I) and the conversion of methane reached more than 90% when temperature was raised to 380 ◦ C. For Pd-ZSM-5(E) catalyst, the inhibition effect of water retarded the 90% conversion to occur at 471 ◦ C, in line with the result reported by Ribeiro et al. [12]. The thermal stability of Pd-ZSM-5 catalysts was tested in dry or wet reactant stream for 100 h (shown in Figure 2). To avoid the inhibition effect of water, the reaction temperature was controlled at 430 and 480 ◦ C for Pd-ZSM-5(I) and Pd-ZSM-5(E), respectively. In Pd-ZSM-5(I) first, the conversion of methane in the dry stream (without water) was rapidly decreased from 100% to 82%, then increase up to 90%. On the other hand, the conversion in wet stream over the same catalyst gradually decreased from 100% to about 80% from the start to 60 h, thereafter the catalytic activity kept constant. When the concentration of CH4 was increased from 0.2% to 0.5%, the decrease in the conversion occurred earlier. In the case of Pd-ZSM-5(E), the conversion of methane was changed from 90% to about 55% within 60 h in the dry stream containing 0.2% methane. However, the catalyst was able to maintain the high conversion level during extended time period of 100 h, when water vapor was added into the stream. After the concentration of methane in wet stream was raised to 0.5%, the conversion of methane decreased gradually from 98% to 80% within the time range of 60−90 h, indicating that the interaction between methane and active sites could result in the deactivation of catalyst to some extent. From the way that the
Figure 2. The activity of methane oxidation over Pd-ZSM-5 (I) (a) at 430 ◦ C and Pd-ZSM-5 (E) (b) at 480 ◦ C in the reactant feed with different methane concentration, 2%O2 and N2 balance with or without water vapor and space velocity 36000 h−1
conversion gradually reached a constant, it can be seen that there were two types of active sites, of which one was stable and the other unstable. Obviously, water vapor retarded this deactivation and enhanced the resistance to thermal environment. 3.2. Pyridine FT-IR Py-FT-IR spectra over Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts are shown in Figure 3. For comparison, the relative amount of Brønsted acid sites estimated from the integrated areas of the band at 1540 cm−1 at different temperatures are given in Table 1. It can be seen from Fig-
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ure 3 that the intensity of bands at 1540 cm−1 is stronger on Pd-ZSM-5(I) catalyst than that on Pd-ZSM-5(E). The intensity difference between 100 ◦ C and 300 ◦ C is similar in both samples, but the intensity of this band for Pd-ZSM-5(I) at 300 ◦ C is much higher than that for Pd-ZSM-5(E), indicating that larger amount of strong acid sites exist on Pd-ZSM-5(I) (shown in Table 1). By ion-exchange method, Pd2+ ions were firstly substituted for H+ ions with strong acidity on ZSM-5, whereas Pd species are anchored without choice on ZSM-5 by using impregnation method. Therefore, the difference in acidity between Pd-ZSM-5(I) and Pd-ZSM-5(E) reflects the different amount of strong Brønsted acid sites. The sequence of catalytic activities is consistent with that of the Brønsted acidities (Pd-ZSM-5(I)>Pd-ZSM-5(E)), and the increase in catalytic activities is partly attributed to the existence of significant amount of Brønsted acid sites on the catalysts.
that the Brønsted acid sites accessible to pyridine decrease after the adsorption of methane on Pd-ZSM-5 catalysts, and this is attributed to the transfer of protons from strong Brønsted acid sites to methane.
Figure 4. FT-IR spectra of hydroxyl group on Pd-ZSM-5 (I) and Pd-ZSM5 (E) before and after the adsorption of methane at room temperature for different time
Figure 3. Py-FT-IR profiles of Pd-ZSM-5 (I) and Pd-ZSM-5 (E) at 150 ◦ C
The acidic bridging hydroxyl group in the ZSM-5 structure is a typical Brønsted acid site, whose vibration in FT-IR spectrum appears at 3610 cm−1 . Before and after the adsorption of methane, the acidic bridging hydroxyl group on PdZSM-5(E) and Pd-ZSM-5(I) was investigated by FT-IR (see Figure 4). And the results show that the band at 3610 cm−1 appears on both Pd-ZSM-5(I) and Pd-ZSM-5(E), and the intensity of former is much stronger than that of the latter, in line with the acidity shown in Table 1. The appearance of negative band after the adsorption of methane indicates that the adsorption of methane occurs on the acidic hydroxyl groups of both Pd-ZSM-5(E) and Pd-ZSM-5(I). Therefore, it is reasonable to assume that adsorption of methane is more preferred on the acidic sites of Pd-ZSM-5(I). The equilibrium of methane adsorption takes a period of time as the inverse band becomes large with the adsorption of methane until the intensity of inverse bands becomes almost equal to the positive bands after 30 min. In addition, the acidities after the adsorption of methane were tested by FT-IR. The experimental results show
According to the result of activity test of methane combustion (Figure 1), the sequence of activity over Pd catalysts supported on ZSM-5 with both Brønsted and Lewis acid sites is consistent with Brønsted acidities (Pd-ZSM-5(I)>Pd-ZSM-5(E)). On the basis of this consideration, it is plausible that the acidity of the support could affect the catalytic performances of zeolite supported palladium catalysts toward the combustion of methane. In the presence of water, the acidic sites could be occupied to some extent, which would result in the inhibition of the methane adsorption on the Brønsted acidic sites. As there were less acidic sites on the Pd-ZSM-5(E) than Pd-ZSM-5(I), the effect of water vapor on catalytic activity of Pd-ZSM-5(E) was higher. When the temperature was raised, the action of water molecules on the acidic sites became weak. So, the magnitude of inhibition effect on the oxidation of methane could be presented as a function of temperature. On the other hand, Pd2+ hydroxyl complexes formed because of the ionisation of the water molecules are induced by the strong electronic field of Pd cations. This may inhibit methane oxidation to some extent. 3.3. XPS Figure 5 shows the bonding energy of Pd on catalysts treated under different conditions measured by XPS, and the Pd/Si ratio near surface estimated from the peak intensity of XPS is summarized in Table 2. It can be seen from Table 2 that both fresh and aged Pd catalysts presented Pd3d 5/2 peaks at 337.0 eV, which indicated that Pd mainly exists in the form of PdO (337.0 eV) [18]. When treated with
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the reactant stream with or without water vapor at 480 ◦ C, Pd3d5/2 bonding energy of Pd-ZSM-5(E) shifts with time to a higher value, 337.6 ± 0.2 eV, corresponding to isolated Pd2+ (337.8 eV) [18], which suggests that the structure of Pd was changed during the oxidation of methane. On the other hand, Pd-ZSM-5(I) treated with the reactant stream with or without water vapor at 430 ◦ C maintained the value of 337.0 eV.
Figure 5. XPS spectra of Pd-ZSM-5 (E) catalyst treated in dry or wet stream. 0.2% CH4 , 2% O2 and N2 balance for 100 h at 480 ◦ C, space velocity 36000 h−1
For Pd-ZSM-5(E), Pd/Si ratio near surface of the fresh catalyst was 0.015 much higher than the value expected by the uniform model which gives the Pd/Si ratio of 0.003 for Pd-ZSM-5 with 0.55 wt% of Pd [6]. Therefore, Pd may be richer on the external surface of the catalyst than in the bulk. During the reaction in dry stream (0.2% CH4 ), the ratio dramatically decreased with time from 0.015 to 0.008 for first
24 h. After that, the ratio decreased at a slow rate. In wet stream with 0.2%CH4 , first, the Pd/Si ratio decreased at 24 h or 48 h from 0.015 to 0.012, but did not change significantly after that. Consequently, the peak intensity of Pd 3d of PdZSM-5 catalyst treated for 100 h with wet stream was much stronger than that with dry stream (shown in Figure 5). However, the amount of Pd on Pd-ZSM-5(E) catalyst remained constant, 0.55 wt%, measured by ICP, before and after the reaction for 100 h with dry or wet stream, which indicates that the decrease of Pd/Si ratio near surface did not result from the loss of Pd. Misono et al. [5] found a change in the Pd/Si ratio of Pd-ZSM-5 in the NO-CH4 -O2 stream because of the gradual dispersion of Pd species into the micropores as isolated Pd2+ . Pecchi found a slight increase in the C/Pd ratios in Pd/SiO2 catalysts used [3]. Demoulin et al. considered the decrease of Pd/Al ratios after the reaction as the results from sintering of Pd species [18]. In this study, C/Si ratios maintained about 0.8 and did not increase obviously with reaction time in dry stream. So, it is reasonable to assume that the decrease of Pd/Si ratio near surface is not resulted from the carbonaceous compounds covering on Pd species. To find the cause for the decrease of Pd/Si ratio, TEM analysis was carried out on Pd catalysts before and after the aging test. The experimental results showed that the particles of Pd on Pd-ZSM-5(E) were too small to see and almost not observed after the treatments. And BET area was maintained at approximately 340 m2 /g. It is suffice to say that most of the PdO species over Pd-ZSM5(E) were transformed into isolated Pd2+ , indicating that the decrease of Pd/Si ratio near surface can be partly ascribed to the dispersion of Pd2+ into the micropores of HZSM-5.
Table 2. The data of XPS spectra of Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts used in oxidation of methane Catalyst Pd-ZSM-5(I) d Pd-ZSM-5(I) e Pd-ZSM-5(E) e Pd-ZSM-5(E)d Pd-ZSM-5(E) f
Pd content (wt%) a Fresh 72 h c 0.54 0.56 0.54 0.55 0.55 0.59 0.55 0.54 0.55 –
Fresh 0.014/377.0 0.014/377.0 0.015/337.0 0.015/337.0 0.015/337.0
24 h 0.012/337.1 0.013/337.1 – 0.008/337.7 0.010/337.0
Pd/Si ratio b /Pd3d5/2 (eV)c 48 h 72 h 0.010/337.2 0.009/337.1 0.012/337.2 0.012/337.1 0.012/337.6 0.011/337.9 0.008/337.6 0.006/337.5 0.008/337.0 –
100 h 0.009/337.2 0.012/337.1 0.012/337.6 0.007/337.6 0.007/337.0
a Measured by ICP; b the relative ratio of Pd/Si near the surface estimated by I /I , the peak intensity ratio of Pd 3d(3d c Pd Si 5/2 +3d 3/2 ) and Si 2p; the reaction time at 430 ◦ C for Pd-ZSM-5(I) and at 480 ◦ C for Pd-ZSM-5(E); d dry-stream containing 0.2% CH4 ; e wet-stream containing 0.2% CH4 ; f wet-stream containing 0.5% CH4
Pd2+ ions in Pd-ZSM-5 have been proposed to anchor at oxygen anions of the zeolite structure in the crystallographic position of the framework [18−20] and can be dispersed gradually into the micropores with reaction time when exposed to stream [5]. Pd2+ hydroxyl complexes formed because of the ionisation of the water molecules are induced by the strong electronic field of the Pd cations. Aylor et al. [21] suggested that palladium may be associated with the zeolite either as charge-compensating species, viz. Z-Pdn+ (OH− )(n−1) or
as neutral PdO species attached to the zeolite where two Brønsted acid groups proximate to each other serve to stabilise it. In this study, water on the surface of Pd-ZSM-5 was further ionized by the strong electronic field of Pd cations, and the interaction between water and [AlO]-Pd2+ could occur, which could prevent the removal of Pd species. However, when the concentration of methane was increased from 0.2% to 0.5% in wet stream, it was observed that the drop of Pd/Si
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ratio with the reaction time was enhanced. This implies that the decrease of Pd/Si ratio is related to the interaction between methane and [AlO]-Pd2+. This interaction reduced the interaction between Pd2+ and ZSM-5 framework and promoted the dispersion of Pd into micropores. For Pd-ZSM-5(I), Pd/Si ratio near the surface of the fresh catalyst was 0.014, which is lower than the value of Pd/Si ratio over Pd-ZSM-5(E), and this can be attributed to the lower Pd dispersion of Pd-ZSM-5(I). Pd/Si ratio decreased with time in dry stream to greater extent than in wet stream. Pd species on Pd-ZSM-5(I) existed more in the form of PdO and less in the form of Pd2+ . Moreover, there were more Bronsted acid groups to retard the dispersion of Pd2+ into the micropores, so that Pd/Si ratio decreased during the reaction to a smaller extent. The Pd loading will affect the activity of Pd catalysts for combustion of methane. The decrease in activity during the stability test can be explained on the basis of the reduction of Pd/Si ratio near surface. The poor stability of Pd-ZSM5(E) in dry stream resulted from the reduction of Pd/Si ratio. The addition of water retarded the dispersion of Pd, but high partial pressure of methane reduced the effect of water vapor. Pd-ZSM-5(I) presented the high resistance to the thermal deactivation because of more stable Pd/Si ratio near surface of Pd-ZSM-5(I) during the reaction. From the results in Figure 2(a), it can be seen that the activity for Pd-ZSM-5(I) dropped to a significant extent during the reaction in wet stream than in dry stream, although Pd/Si ratio dropped more significantly during the reaction in dry stream than in wet stream. Persson et al. reported that the presence of water promoted the degradation process of Pd/Al2 O3 catalyst with respect to the production of surface hydroxyls [22]. As mentioned above, the covering of water on acid sites and the production of Pd-OH species led to the decrease of activity to some extent. However, the presence of water decreased the reduction of Pd/Si ratio near surface, which is very important in terms of the promotion and stability of activity for the oxidation of methane. 4. Conclusions Pd/ZSM-5 catalysts prepared by impregnation and ionexchange method were effective for the combustion of methane. The order of activity is consistent with the Brønsted acidity of catalysts: Pd-ZSM-5(I)>Pd-ZSM-5(E). It is shown by FT-IR that methane is adsorbed on the acidic bridging hydroxyl groups of HZSM-5-supported Pd catalysts. With the addition of water into the feed, the inhibition effect of water on the conversion of methane was observed. However, water markedly promoted the stability of Pd-ZSM-5, especially for Pd-ZSM-5(E) prepared by ion-exchange method. XPS measurement showed that Pd/Si ratio near the surface of Pd-ZSM-
5 markedly decreased with time in dry stream because of the dispersion of Pd into micropores. The addition of water retarded the dispersion of Pd, whereas high partial pressure of methane reduced the effect of water vapor. The decrease in activity during the stability test can be related to the reduction of Pd/Si ratio. Acknowledgements We would like to acknowledge the National Basic Research Program of China (No. 2004CB719500) and the National Natural Science Foundation of China (NNSFC) (No. 20377012) for their financial support.
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Effect of water on the performance of Pd-ZSM-5 catalysts for the combustion of methane Bo Zhang, Xingyi Wang∗ , Ogtour M’Ramadj,
Dao Li, Hua Zhang,
Guanzhong Lu
Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China [ Manuscript received July 10, 2007; revised September 27, 2007 ]
Abstract: Palladium-based catalysts were prepared using impregnation (I) and ion-exchange method (E) with ZSM-5 as support. Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts presented the high activity for the combustion of methane. The order of activity was consistent with Brønsted acidity of the catalysts: Pd-ZSM-5(I)>Pd-ZSM-5(E). It was shown by FT-IR that methane was adsorbed on the acidic bridging hydroxyl groups of ZSM-5-supported Pd catalysts. The effect of water on the activity of Pd-ZSM-5 was investigated. The inhibition effect of water on the conversion of methane was observed. However, water promoted the stability of Pd-ZSM-5 obviously during extended time periods. XPS measurement showed that Pd/Si ratio near the surface of Pd-ZSM-5(E) decreased more pronouncedly with time in dry stream than that of Pd-ZSM-5(I), this is attributed to the dispersion of Pd into the micropores. The addition of water, however, retarded Pd dispersion. And high partial pressure of methane reduced this effect of water vapor. The decrease in activity during the stability test can be explained on the basis of the reduction of Pd/Si ratio. Key words: methane combustion; Brønsted acid; palladium; ZSM-5; water
1. Introduction Catalytic combustion of methane is an important technology employed to decrease the emission of NOx in the application of natural gas. Supported palladium catalysts possess an excellent activity for the oxidation of methane and therefore have been extensively used for catalytic combustion purposes [1], but the thermal stability of Pd catalysts remains to be a problem. The state and dispersion of Pd supported on Al2 O3 [2], SiO2 [3] or zeolites [4,5] were less stable at lower temperature than the decomposition temperature of PdO during the catalytic combustion of methane. Even though the initial activity is high at this temperature, the catalysts are not able to maintain the high conversion level during extended time periods, that is, the activity is not stable [6,7]. Previous studies showed that the addition of other metal elements into Pd catalysts can improve their stability to some extent [4,8,9]. There were also studies on the effects of water and carbon dioxide on the activity of Pd catalysts for the combustion of methane [10]. The effect of water was generally considered as an inhibition effect, which depended on the reaction temperature. Burch observed that the inhibition effects of water were ∗
additive which could be completely reversible or permanent in deactivation of Pd catalysts, depending on the time period of exposure [11]. Recently, Ribeiro tested the kinetic rate of methane oxidation in the presence of water on Pd catalysts and found the magnitude of inhibition effect of water on the oxidation of methane as a function of temperature. At 553 K, the reaction order of water was about −1; above 723 K, the water reaction order was close to 0 [12]. Currently, carbenium ion chemistry is considered to be the most probable route for the initiation of alkane via hydride transfer on a solid acid [13,14]. Truitt and co-workers [15] discovered that trace amounts of Lewis acid sites facilitated the abstraction of hydride from alkane and generate reactive carbenium ions. Liu and Flytzani-Stephanopoulos [16] proved that the strength and geometry of acidic sites on the catalyst surface played an important role in the breakage of C−H bond and the formation of intermediate species. However, several recent publications still proposed the direct protonation of alkanes. It was suggested by Sommer and coworkers that strong solid acids could protolyse the C−H bond and thus the activation of short chain alkanes was achieved at low temperature [17]. According to the recent result [15],
Corresponding author. Tel: +86-21-64253372; E-mail: [email protected] This work was supported by the National Natural Science Foundation of China (No. 20377012).
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alkane did not simply undergo physical adsorption on the zeolite but rather formed a specific adsorption complex with the proton on Brønsted acid sites. The formation of adsorption complex was the route of activating the C−H bonds. However, the mechanismic steps catalyzed by solid acids and the role of acid sites for the combustion of methane are ambiguous. The proof related to the C−H activation is inadequate [13,14]. Therefore, an explicit understanding is needed to identify the role of the surface acidity of catalyst in the activation of methane during the catalytic combustion of methane. In this study, palladium-based catalysts supported on ZSM-5 were prepared. The effect of water vapor on the activity and stability of Pd-ZSM-5 catalysts for the oxidation of CH4 was studied. 2. Experimental 2.1. Preparation of catalyst Palladium-based catalysts were prepared using impregnation (I) and ion-exchange method (E). H-ZSM-5 zeolite (40−60 mesh, Si/Al = 80) was used as the support. In impregnation method, Pd-ZSM-5(I) catalyst was prepared by immersing support into an aqueous solution of palladium chloride at room temperature for 24 h. The impregnated solid was dried at 110 ◦ C for 12 h and then calcined in air at 550 ◦ C for 4 h. In ion-exchange method, the support was added into the aqueous solution of palladium chloride and the pH value of the suspension was adjusted to 7.5 by the addition of NH4 OH solution. Stirred at 90 ◦ C for 24 h, Pd-ZSM-5(E) catalyst was filtered, dried at 110 ◦ C for 12 h, and then calcined at 550 ◦ C for 4 h. The loading of Pd determined by inductively coupled plasma-atomic emission spectroscopy (ICP) was 0.55 wt%.
2.3. Characterization of catalysts BET surface area of catalyst was measured by nitrogen adsorption at 77 K on a Micromeritics ASAP 2000 surface analyzer. XPS spectrum of catalyst was recorded on a PerkinElmer PHI-5400 spectrometer, using a monochromatic MgKα radiation (1253.6 eV). The binding energy of adventitious C1s (284.6 eV with an accuracy of ± 0.2 eV) was used as a reference. The FT-IR spectra were recorded using a NICOLET NEXUS 470 spectrophotometer with a resolution of 4 cm−1 . A total of 15 mg sample was ground finely, pressed into a self-supporting wafer, and mounted into a quartz IR cell with CaF2 windows. Before adsorbing pyridine at room temperature, the wafer was heated at 300 ◦ C for 4 h under 10−3 torr (1 torr = 133 Pa). The FT-IR spectra of pyridine adsorbed on the catalyst were recorded after subsequent evacuation with the increase of temperature from room temperature to 300 ◦ C. The pre-adsorption of methane was also carried out before adsorption of pyridine. The background created by zeolite was subtracted. 3. Results and discussion 3.1. Catalytic activity The conversion of methane on Pd-ZSM-5(I) and PdZSM-5(E) as the function of temperature is shown in Figure 1 and the data with regard to the activity and properties of catalysts are summarized in Table 1, in which T10% , T50% , and T90% represent the temperatures for 10%, 50%, and 90% conversion of CH4 , respectively. It is obvious that both PdZSM-5 catalysts were active, leading to almost total conversion of CH4 at the temperatures of 200 to 460 ◦ C, of which
2.2. Activity test of catalysts for the combustion of methane The catalytic activity for the combustion of methane was evaluated at atmospheric pressure in a conventional fixed bed quartz reactor. Temperature was monitored by a thermocouple at the bottom of the catalyst bed and controlled by a temperature controller. The catalyst of 1.0 g was placed on the reactor bed and a gas mixture (0.2%CH4, 2%O2 and N2 balance) was fed at a rate of 420 ml·min−1 into the reactor (space velocity 36000 h−1 ). For the measurement of thermal stability, the catalyst bed was fed with reaction stream for 100 h. For the activity test on wet stream, the water vapour of the feed gas was provided by a glass bubbler. Conversion measurements were conducted in an ascending temperature mode. After the reaction was kept for 30 min in the steady state, the composition in outlet of reactor was analyzed on stream by a gas chromatograph (G-120, Shanghai).
Figure 1. Light-off curves for the combustion of methane over Pd-ZSM-5(I) and Pd-ZSM-5 (E). • and in the reactant feed with 0.2%CH4 , 2%O2 and N2 balance; ◦ and in the reactant feed with 0.2%CH4 , 0.4%O2 and N2 balance; space velocity 36000 h−1
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Table 1. The properties and activity of Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts BET
Relative amount of Brønsted acid sites a
Activity b
Catalysts
(m2 /g)
100 ◦ C
200 ◦ C
300 ◦ C
T10% / ◦ C
T50% / ◦ C
T90% / ◦ C
Pd-ZSM-5(I)
346
2.081
1.898
1.590
277//380//308
335//431//341
374//456//380
Pd-ZSM-5(E)
344
1.045
0.88
0.651
337//329//342
393//422//412
−1
a
435//456//471 b
The relative amount of Brønsted acid sites estimated from the integrated areas of the band at 1540 cm at different temperatures; Reaction in dry stream of O2 /CH4 = 10//in dry stream of O2 /CH4 = 2//in wet stream of O2 /CH4 = 10; T10% , T50% , and T90% are the temperatures required for 10%, 50%, and 90% conversion of CH4 , respectively.
Pd-ZSM-5(I) was more active than Pd-ZSM-5(E). When O2 /CH4 = 10 in dry stream, the former presented T10% of 277 ◦ C, and T90% of 374 ◦ C, whereas for the latter, the conversion of 10% and 90% was obtained at 337 and 435 ◦ C, respectively. The reaction products were mainly CO2 , and no other by-product was observed. When the partial pressure of oxygen was changed from O2 /CH4 = 10 to stoichiometric ratio (O2 /CH4 = 2) and the concentration of methane was maintained at 0.2%, 10% conversion of methane over Pd-ZSM-5(I) occurred at higher temperature than that over Pd-ZSM-5(E), and the temperature required for 90% conversion of methane on both Pd catalysts tended to be the same value. On the other hand, for the fresh Pd-ZSM-5 catalyst, its activity on dry stream was higher than that on wet stream, which indicated the inhibition effect of water. However, this effect decreased at high temperature over Pd-ZSM-5(I) and the conversion of methane reached more than 90% when temperature was raised to 380 ◦ C. For Pd-ZSM-5(E) catalyst, the inhibition effect of water retarded the 90% conversion to occur at 471 ◦ C, in line with the result reported by Ribeiro et al. [12]. The thermal stability of Pd-ZSM-5 catalysts was tested in dry or wet reactant stream for 100 h (shown in Figure 2). To avoid the inhibition effect of water, the reaction temperature was controlled at 430 and 480 ◦ C for Pd-ZSM-5(I) and Pd-ZSM-5(E), respectively. In Pd-ZSM-5(I) first, the conversion of methane in the dry stream (without water) was rapidly decreased from 100% to 82%, then increase up to 90%. On the other hand, the conversion in wet stream over the same catalyst gradually decreased from 100% to about 80% from the start to 60 h, thereafter the catalytic activity kept constant. When the concentration of CH4 was increased from 0.2% to 0.5%, the decrease in the conversion occurred earlier. In the case of Pd-ZSM-5(E), the conversion of methane was changed from 90% to about 55% within 60 h in the dry stream containing 0.2% methane. However, the catalyst was able to maintain the high conversion level during extended time period of 100 h, when water vapor was added into the stream. After the concentration of methane in wet stream was raised to 0.5%, the conversion of methane decreased gradually from 98% to 80% within the time range of 60−90 h, indicating that the interaction between methane and active sites could result in the deactivation of catalyst to some extent. From the way that the
Figure 2. The activity of methane oxidation over Pd-ZSM-5 (I) (a) at 430 ◦ C and Pd-ZSM-5 (E) (b) at 480 ◦ C in the reactant feed with different methane concentration, 2%O2 and N2 balance with or without water vapor and space velocity 36000 h−1
conversion gradually reached a constant, it can be seen that there were two types of active sites, of which one was stable and the other unstable. Obviously, water vapor retarded this deactivation and enhanced the resistance to thermal environment. 3.2. Pyridine FT-IR Py-FT-IR spectra over Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts are shown in Figure 3. For comparison, the relative amount of Brønsted acid sites estimated from the integrated areas of the band at 1540 cm−1 at different temperatures are given in Table 1. It can be seen from Fig-
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ure 3 that the intensity of bands at 1540 cm−1 is stronger on Pd-ZSM-5(I) catalyst than that on Pd-ZSM-5(E). The intensity difference between 100 ◦ C and 300 ◦ C is similar in both samples, but the intensity of this band for Pd-ZSM-5(I) at 300 ◦ C is much higher than that for Pd-ZSM-5(E), indicating that larger amount of strong acid sites exist on Pd-ZSM-5(I) (shown in Table 1). By ion-exchange method, Pd2+ ions were firstly substituted for H+ ions with strong acidity on ZSM-5, whereas Pd species are anchored without choice on ZSM-5 by using impregnation method. Therefore, the difference in acidity between Pd-ZSM-5(I) and Pd-ZSM-5(E) reflects the different amount of strong Brønsted acid sites. The sequence of catalytic activities is consistent with that of the Brønsted acidities (Pd-ZSM-5(I)>Pd-ZSM-5(E)), and the increase in catalytic activities is partly attributed to the existence of significant amount of Brønsted acid sites on the catalysts.
that the Brønsted acid sites accessible to pyridine decrease after the adsorption of methane on Pd-ZSM-5 catalysts, and this is attributed to the transfer of protons from strong Brønsted acid sites to methane.
Figure 4. FT-IR spectra of hydroxyl group on Pd-ZSM-5 (I) and Pd-ZSM5 (E) before and after the adsorption of methane at room temperature for different time
Figure 3. Py-FT-IR profiles of Pd-ZSM-5 (I) and Pd-ZSM-5 (E) at 150 ◦ C
The acidic bridging hydroxyl group in the ZSM-5 structure is a typical Brønsted acid site, whose vibration in FT-IR spectrum appears at 3610 cm−1 . Before and after the adsorption of methane, the acidic bridging hydroxyl group on PdZSM-5(E) and Pd-ZSM-5(I) was investigated by FT-IR (see Figure 4). And the results show that the band at 3610 cm−1 appears on both Pd-ZSM-5(I) and Pd-ZSM-5(E), and the intensity of former is much stronger than that of the latter, in line with the acidity shown in Table 1. The appearance of negative band after the adsorption of methane indicates that the adsorption of methane occurs on the acidic hydroxyl groups of both Pd-ZSM-5(E) and Pd-ZSM-5(I). Therefore, it is reasonable to assume that adsorption of methane is more preferred on the acidic sites of Pd-ZSM-5(I). The equilibrium of methane adsorption takes a period of time as the inverse band becomes large with the adsorption of methane until the intensity of inverse bands becomes almost equal to the positive bands after 30 min. In addition, the acidities after the adsorption of methane were tested by FT-IR. The experimental results show
According to the result of activity test of methane combustion (Figure 1), the sequence of activity over Pd catalysts supported on ZSM-5 with both Brønsted and Lewis acid sites is consistent with Brønsted acidities (Pd-ZSM-5(I)>Pd-ZSM-5(E)). On the basis of this consideration, it is plausible that the acidity of the support could affect the catalytic performances of zeolite supported palladium catalysts toward the combustion of methane. In the presence of water, the acidic sites could be occupied to some extent, which would result in the inhibition of the methane adsorption on the Brønsted acidic sites. As there were less acidic sites on the Pd-ZSM-5(E) than Pd-ZSM-5(I), the effect of water vapor on catalytic activity of Pd-ZSM-5(E) was higher. When the temperature was raised, the action of water molecules on the acidic sites became weak. So, the magnitude of inhibition effect on the oxidation of methane could be presented as a function of temperature. On the other hand, Pd2+ hydroxyl complexes formed because of the ionisation of the water molecules are induced by the strong electronic field of Pd cations. This may inhibit methane oxidation to some extent. 3.3. XPS Figure 5 shows the bonding energy of Pd on catalysts treated under different conditions measured by XPS, and the Pd/Si ratio near surface estimated from the peak intensity of XPS is summarized in Table 2. It can be seen from Table 2 that both fresh and aged Pd catalysts presented Pd3d 5/2 peaks at 337.0 eV, which indicated that Pd mainly exists in the form of PdO (337.0 eV) [18]. When treated with
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the reactant stream with or without water vapor at 480 ◦ C, Pd3d5/2 bonding energy of Pd-ZSM-5(E) shifts with time to a higher value, 337.6 ± 0.2 eV, corresponding to isolated Pd2+ (337.8 eV) [18], which suggests that the structure of Pd was changed during the oxidation of methane. On the other hand, Pd-ZSM-5(I) treated with the reactant stream with or without water vapor at 430 ◦ C maintained the value of 337.0 eV.
Figure 5. XPS spectra of Pd-ZSM-5 (E) catalyst treated in dry or wet stream. 0.2% CH4 , 2% O2 and N2 balance for 100 h at 480 ◦ C, space velocity 36000 h−1
For Pd-ZSM-5(E), Pd/Si ratio near surface of the fresh catalyst was 0.015 much higher than the value expected by the uniform model which gives the Pd/Si ratio of 0.003 for Pd-ZSM-5 with 0.55 wt% of Pd [6]. Therefore, Pd may be richer on the external surface of the catalyst than in the bulk. During the reaction in dry stream (0.2% CH4 ), the ratio dramatically decreased with time from 0.015 to 0.008 for first
24 h. After that, the ratio decreased at a slow rate. In wet stream with 0.2%CH4 , first, the Pd/Si ratio decreased at 24 h or 48 h from 0.015 to 0.012, but did not change significantly after that. Consequently, the peak intensity of Pd 3d of PdZSM-5 catalyst treated for 100 h with wet stream was much stronger than that with dry stream (shown in Figure 5). However, the amount of Pd on Pd-ZSM-5(E) catalyst remained constant, 0.55 wt%, measured by ICP, before and after the reaction for 100 h with dry or wet stream, which indicates that the decrease of Pd/Si ratio near surface did not result from the loss of Pd. Misono et al. [5] found a change in the Pd/Si ratio of Pd-ZSM-5 in the NO-CH4 -O2 stream because of the gradual dispersion of Pd species into the micropores as isolated Pd2+ . Pecchi found a slight increase in the C/Pd ratios in Pd/SiO2 catalysts used [3]. Demoulin et al. considered the decrease of Pd/Al ratios after the reaction as the results from sintering of Pd species [18]. In this study, C/Si ratios maintained about 0.8 and did not increase obviously with reaction time in dry stream. So, it is reasonable to assume that the decrease of Pd/Si ratio near surface is not resulted from the carbonaceous compounds covering on Pd species. To find the cause for the decrease of Pd/Si ratio, TEM analysis was carried out on Pd catalysts before and after the aging test. The experimental results showed that the particles of Pd on Pd-ZSM-5(E) were too small to see and almost not observed after the treatments. And BET area was maintained at approximately 340 m2 /g. It is suffice to say that most of the PdO species over Pd-ZSM5(E) were transformed into isolated Pd2+ , indicating that the decrease of Pd/Si ratio near surface can be partly ascribed to the dispersion of Pd2+ into the micropores of HZSM-5.
Table 2. The data of XPS spectra of Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts used in oxidation of methane Catalyst Pd-ZSM-5(I) d Pd-ZSM-5(I) e Pd-ZSM-5(E) e Pd-ZSM-5(E)d Pd-ZSM-5(E) f
Pd content (wt%) a Fresh 72 h c 0.54 0.56 0.54 0.55 0.55 0.59 0.55 0.54 0.55 –
Fresh 0.014/377.0 0.014/377.0 0.015/337.0 0.015/337.0 0.015/337.0
24 h 0.012/337.1 0.013/337.1 – 0.008/337.7 0.010/337.0
Pd/Si ratio b /Pd3d5/2 (eV)c 48 h 72 h 0.010/337.2 0.009/337.1 0.012/337.2 0.012/337.1 0.012/337.6 0.011/337.9 0.008/337.6 0.006/337.5 0.008/337.0 –
100 h 0.009/337.2 0.012/337.1 0.012/337.6 0.007/337.6 0.007/337.0
a Measured by ICP; b the relative ratio of Pd/Si near the surface estimated by I /I , the peak intensity ratio of Pd 3d(3d c Pd Si 5/2 +3d 3/2 ) and Si 2p; the reaction time at 430 ◦ C for Pd-ZSM-5(I) and at 480 ◦ C for Pd-ZSM-5(E); d dry-stream containing 0.2% CH4 ; e wet-stream containing 0.2% CH4 ; f wet-stream containing 0.5% CH4
Pd2+ ions in Pd-ZSM-5 have been proposed to anchor at oxygen anions of the zeolite structure in the crystallographic position of the framework [18−20] and can be dispersed gradually into the micropores with reaction time when exposed to stream [5]. Pd2+ hydroxyl complexes formed because of the ionisation of the water molecules are induced by the strong electronic field of the Pd cations. Aylor et al. [21] suggested that palladium may be associated with the zeolite either as charge-compensating species, viz. Z-Pdn+ (OH− )(n−1) or
as neutral PdO species attached to the zeolite where two Brønsted acid groups proximate to each other serve to stabilise it. In this study, water on the surface of Pd-ZSM-5 was further ionized by the strong electronic field of Pd cations, and the interaction between water and [AlO]-Pd2+ could occur, which could prevent the removal of Pd species. However, when the concentration of methane was increased from 0.2% to 0.5% in wet stream, it was observed that the drop of Pd/Si
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ratio with the reaction time was enhanced. This implies that the decrease of Pd/Si ratio is related to the interaction between methane and [AlO]-Pd2+. This interaction reduced the interaction between Pd2+ and ZSM-5 framework and promoted the dispersion of Pd into micropores. For Pd-ZSM-5(I), Pd/Si ratio near the surface of the fresh catalyst was 0.014, which is lower than the value of Pd/Si ratio over Pd-ZSM-5(E), and this can be attributed to the lower Pd dispersion of Pd-ZSM-5(I). Pd/Si ratio decreased with time in dry stream to greater extent than in wet stream. Pd species on Pd-ZSM-5(I) existed more in the form of PdO and less in the form of Pd2+ . Moreover, there were more Bronsted acid groups to retard the dispersion of Pd2+ into the micropores, so that Pd/Si ratio decreased during the reaction to a smaller extent. The Pd loading will affect the activity of Pd catalysts for combustion of methane. The decrease in activity during the stability test can be explained on the basis of the reduction of Pd/Si ratio near surface. The poor stability of Pd-ZSM5(E) in dry stream resulted from the reduction of Pd/Si ratio. The addition of water retarded the dispersion of Pd, but high partial pressure of methane reduced the effect of water vapor. Pd-ZSM-5(I) presented the high resistance to the thermal deactivation because of more stable Pd/Si ratio near surface of Pd-ZSM-5(I) during the reaction. From the results in Figure 2(a), it can be seen that the activity for Pd-ZSM-5(I) dropped to a significant extent during the reaction in wet stream than in dry stream, although Pd/Si ratio dropped more significantly during the reaction in dry stream than in wet stream. Persson et al. reported that the presence of water promoted the degradation process of Pd/Al2 O3 catalyst with respect to the production of surface hydroxyls [22]. As mentioned above, the covering of water on acid sites and the production of Pd-OH species led to the decrease of activity to some extent. However, the presence of water decreased the reduction of Pd/Si ratio near surface, which is very important in terms of the promotion and stability of activity for the oxidation of methane. 4. Conclusions Pd/ZSM-5 catalysts prepared by impregnation and ionexchange method were effective for the combustion of methane. The order of activity is consistent with the Brønsted acidity of catalysts: Pd-ZSM-5(I)>Pd-ZSM-5(E). It is shown by FT-IR that methane is adsorbed on the acidic bridging hydroxyl groups of HZSM-5-supported Pd catalysts. With the addition of water into the feed, the inhibition effect of water on the conversion of methane was observed. However, water markedly promoted the stability of Pd-ZSM-5, especially for Pd-ZSM-5(E) prepared by ion-exchange method. XPS measurement showed that Pd/Si ratio near the surface of Pd-ZSM-
5 markedly decreased with time in dry stream because of the dispersion of Pd into micropores. The addition of water retarded the dispersion of Pd, whereas high partial pressure of methane reduced the effect of water vapor. The decrease in activity during the stability test can be related to the reduction of Pd/Si ratio. Acknowledgements We would like to acknowledge the National Basic Research Program of China (No. 2004CB719500) and the National Natural Science Foundation of China (NNSFC) (No. 20377012) for their financial support.
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