α-Al2O3 catalysts: The effect of La2O3 contents on the kinetic performance

Applied Catalysis A: General 331 (2007) 60–69 www.elsevier.com/locate/apcata

The CO2 reforming of CH4 over Ni/La2O3/a-Al2O3 catalysts: The effect of La2O3 contents on the kinetic performance Yuehua Cui, Huidong Zhang, Hengyong Xu *, Wenzhao Li Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China Received 16 March 2007; received in revised form 22 May 2007; accepted 27 June 2007 Available online 25 July 2007

Abstract The aim of this work is to quantitatively investigate the effect of La2O3 contents on the intrinsic activity of Ni active sites for the CO2 reforming of CH4 on Ni/La2O3 (1, 2, 4, and 6 wt%)/a-Al2O3 catalysts using steady and pulse kinetic methods. The Ni particle sizes were decreased with increasing La2O3 contents. And the 4-La catalyst had the most Ni–H species. The Ni active sites on the 4-La catalyst showed the highest reforming activity and the lowest activation energy. Among CH4, CO2, CO, and H2, only CH4 reaction orders were not zero and they were affected by the La2O3 contents. In 550–600 8C, the CH4 dissociation is a rate-determining step for the reforming reaction on the Ni/La2O3/a-Al2O3 catalysts. The 4-La catalyst gave the highest reforming activity because the CH4 dissociation was a structure sensitive reaction and 4-La catalyst has the most suitable Ni particle size. In 650–750 8C, the CH4 dissociation and the formation of Ni–H reached equilibration and the reaction between CHx species with La2O2CO3 became the rate-determining step. The 4-La catalyst with the most Ni–H could keep the highest x value of CHx intermediates, leading to the highest reforming activity in 650–750 8C. Therefore, the La2O3 contents on the catalysts tuned the Ni particle sizes and Ni–H amounts on the catalysts, which lead to higher reforming activity on the 4-La catalyst. # 2007 Elsevier B.V. All rights reserved. Keywords: Kinetics; Reforming; CH4; CO2; Ni/La2O3/a-Al2O3

1. Introduction The production of CO and H2 through CO2 reforming of CH4 over Ni-based catalysts has received much attention. The reactants CH4 and CO2 are considered to generate the greenhouse effect [1]. The reforming products CO and H2 with equal molar ratio are more suitable for the subsequent Fischer-Tropsh synthesis compared to the products of the steam reforming or partial oxidation of methane [2]. And Ni-based catalysts are highly active for the catalytic dry reforming and lower cost and plentifully available compared to the noble metal catalysts [3]. Generally, some promoters are added to the Ni-based catalysts to improve the catalytic activity and the catalyst stability, and to depress the carbon deposition on the catalysts.

* Corresponding author. Tel.: +86 411 84581234; fax: +86 411 84581234. E-mail addresses: [email protected] (Y. Cui), [email protected] (H. Zhang), [email protected] (H. Xu), [email protected] (W. Li). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.06.039

The promoters include the alkaline oxide, MgO, CaO, or K2O [4,5], metal K or Cr [6,7], and lanthanon oxide La2O3 [4,8]. Among these promoters, La2O3 is most often used in the reforming reaction due to their typical physical and chemical properties [9,10]. The addition of La2O3 to Ni/g-Al2O3 can improve the dispersion of Ni particles on the supports and depress the assembling of Ni particles during the reforming reaction [11–13]. The La2O3 can also adsorb and react with CO2 to form La2O2CO3 species on the catalyst surface [14–17], which can accelerate the conversion of surface CHx species (x = 0–3) [18,19] and improve the reforming reaction. It was reported that the catalytic stability of the Ni/La2O3/SiO2 catalysts was remarkably promoted when the La2O3 content was higher than 5 wt% in the CH4/O2/CO2 reaction [20]. The previous research results qualitatively describe the effects of La2O3 on the reforming reaction. However, the investigation of the quantitative influence of La2O3 on the reforming reaction over the Ni-based catalysts, especially on the intrinsic activity of the Ni active sites, has been less reported [21]. We have recently studied the effect of La2O3 on the dry reforming over the Ni/a-Al2O3 catalyst. The addition of La2O3

Y. Cui et al. / Applied Catalysis A: General 331 (2007) 60–69

to the Ni/a-Al2O3 catalyst can increase Ni–H amount on the catalysts [22,23], which further improved the reaction between CHx intermediates and CO2 and increased the reforming reaction rate. We have proven that CH4 dissociation on Ni/aAl2O3 catalyst is a structure sensitive reaction [24] and the addition of La2O3 as a structure promoter to the catalyst can alter the CH4 dissociation rate [25], which may further affect the reforming performance if CH4 dissociation becomes a ratedetermining step. Additionally, the addition of La2O3 to Ni/aAl2O3 catalyst can also alter the rates of CH4 dissociation and CO2 conversion under the kinetic conditions [26], avoiding the deposition of carbon species in the reforming reaction of CH4/CO2 (1:1, molar ratio) in 550–750 8C under the kinetic conditions. Based on our previous preliminary results, the aim of this work is to investigate the quantitative effects of La2O3 contents on the CH4/CO2 reforming reaction on the Ni active sites of the Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents using steady and pulse kinetic methods.

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2. Experimental

lysts (40 mg) in a flow of deoxidated highly pure Ar were increased from RT to 500 8C, and then kept for 0.5 h. Then the sample was cooled down to RT in Ar. The TPR experiments were performed under a flow of 5% H2/Ar (30 ml/min) from RT to 900 8C at a constant heating rate of 10 8C/min. The consumption of hydrogen was monitored with a thermal conductivity detector (TCD). The selection of experimental conditions for TPR was in agreement with the criterion developed by Malet and Caballero [27], and the effect of intraparticle mass transfer resistance of reactant in terms of the Weisz–Prater criterion [28] can be negligible. The unreduced Ni/La2O3/a-Al2O3 catalysts (40 mg) were reduced by H2 at 850 8C for 30 min and then cooled to RT for H2-TPD-MS measurements in an Ar flow (30 ml/min) at a constant heating rate of 10 8C/min [22,23]. The effluents from the reactor were analyzed via an on-line GC-MS (GC Model 6890, carbon sieve as column, TCD as detector; MS 5973, three channels for the detection of H2, H2O, and O2.). The amount of H2 desorbed above 400 8C is calculated according to the integral area of the desorption curves.

2.1. Preparation of catalysts

2.3. Steady-state kinetic methods

All the catalysts were prepared by impregnating a-Al2O3 (2.1 m2 g1, 100–160 mesh) with aqueous solution of La(NO3)36H2O (1, 2, 4, and 6 wt%) for 12 h at room temperature (RT). Then the catalysts were dried at 120 8C for 3 h and calcined in air at 800 8C for 6 h. Then the La2O3/aAl2O3 samples were impregnated with aqueous solution of Ni(NO3)26H2O (8 wt%) for 12 h at RT. Then the catalysts were dried at 120 8C for 3 h and calcined in air at 800 8C for 6 h to give the unreduced Ni/La2O3/a-Al2O3 catalysts. These catalysts were reduced by H2 at 850 8C for 30 min before used for the dry reforming. The obtained Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents, 0, 1, 2, 4, and 6 wt%, were denoted as 0-La, 1-La, 2-La, 4-La, and 6-La, respectively. CH4 (99.995% purity), CO2 (99.98% purity), N2 (99.95% purity), He (99.99% purity), Ar (99.99% purity), and H2 (99.99% purity) were used in the studies.

The amount of Ni/La2O3/a-Al2O3 catalysts employed in the dry reforming was 5–45 mg. The partial pressure of N2 was used to tune the total flow rate at 360 ml/min. The conversions of the reforming reaction were kept below 20%, which was far away from the corresponding thermodynamic equilibrium values, and the reaction was controlled by kinetics. The reverse reforming reaction can be basically excluded under the kinetic conditions. The observed reaction rates were almost equal to the forward reaction rates (rf). Due to the presence of reverse water–gas shift reaction (RWGS, CO2 + H2 = CO + H2O), the forward rates of the dry reforming were calculated based on the conversions of CH4 (Eq. (1)):

2.2. Characterization of catalysts X-ray powder diffraction (XRD) patterns of the unreduced and reduced Ni/La2O3/a-Al2O3 catalysts were measured on a Philips CM-1 (Cu Ka, l = 0.1543 nm, 208 < 2u < 808) powder X-ray diffractometer. The Ni particle sizes were calculated based on the Scherrer Equation. The Ni/La2O3/a-Al2O3 catalysts were reduced by H2 at 850 8C for 0.5 h, decreased to 100 8C under Ar and kept for 20 min, and was then decreased to RT for the in situ H2 chemisorption measurements. The exposed Ni atoms (Ni active sites) on the catalysts were estimated based on the H/Ni atomic ratio of 1 according to a spherical model for the metallic particles. Temperature-programmed reductions (TPR) were carried out in a conventional setup equipped with a programmable temperature furnace. The unreduced Ni/La2O3/a-Al2O3 cata-

r f ¼ ConCH4 %  V CH4 =22; 400  m

(1)

where ConCH4 % is the CH4 conversion and V CH4 is the volume flow rate of CH4 in the reforming reaction (ml/s), and m is the catalyst mass (g). The forward rates of the dry reforming (CH4 + CO2 = 2CO + 2H2) were also expressed as Eq. (2): r f ¼ k0 PaCH4 PbCO2 PcH2 PdCO

(2)

where k0 is the rate constant of the dry reforming and a, b, c, and d are the reaction orders of CH4, CO2, H2, and CO, respectively. The experiments were performed at atmospheric pressure in the temperature range of 550–750 8C. According to the initial rate methods [19,29], one of the partial pressures of CH4, CO2, H2, or CO was varied and the other three partial pressures were kept constant. These basically constant partial pressures and the rate constant k0 can be combined as k, and Eq. (2) was written as Eq. (3): r f ¼ kPn

or lnr f ¼ ln k þ n ln P

(3)

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The reaction orders of CH4, CO2, H2, and CO can be obtained by plotting ln rf against ln P at different temperatures. The TOF of the reforming reaction per Ni active site of the Ni/La2O3/a-Al2O3 catalysts were calculated according to Eq. (4): TOF ¼ ConCH4 %  V CH4  6:02  1023 =22; 400  Ni0  m (4) where ConCH4 % is the conversion of CH4; V CH4 the volume flow rate of CH4 (ml/s); Ni0 the amount of Ni active sites per gram catalyst (sites/g catalyst), determined by H2 chemisorption; and m is the catalyst mass (g). The activity energy (Ea) of the reforming reaction was expressed as Eq. (5) and it can be calculated by plotting ln TOF against 1/T.   Ea TOF ¼ Aexp  RT

or

ln TOF ¼ ln A 

Ea RT

(5)

where A is the pre-exponential factor of the reforming reaction and R is gas constant. 2.4. Transient kinetic methods The carrier gas Ar (36–60 ml/min) carrying CH4 or CO2 from a quantitative tube (0.3058 ml) was pulsed through the catalysts bed containing 2–10 mg of Ni/La2O3/a-Al2O3 catalyst in a quartz tube reactor (6 mm o.d.  4 mm i.d.  40 cm length). A thermocouple was introduced in the catalyst bed to monitor the reaction temperature. The effluents were analyzed via an on-line GC (Model 103, TCD, carbon sieve as column). The contact times were tuned to keep the pulse conversions of CH4 or CO2 (ConP%) far below the corresponding equilibrium conversions for the dry reforming. The ConP% of CH4 or CO2 was calculated according to the following equation: Conp % ¼

molbefore pulse  molafter pulse  100 molbefore pulse

3. Results 3.1. Characterization of the catalysts with different La2O3 contents The XRD patterns of the unreduced and reduced Ni/La2O3/ a-Al2O3 catalysts are respectively shown in Fig. 1A and B. Fig. 1A shows the presence of the NiO species and the diffraction peaks became wider with increasing the La2O3 contents, meaning that the NiO particles became smaller with increasing La2O3 contents. The patterns corresponding to the NiO species (37.28, 63.08) are disappeared in Fig. 1B, showing that all the NiO species were reduced. The diffraction peaks of Ni particles become wider with the increase in the content of La2O3, suggesting the decrease in the Ni particle sizes with the increase in the La2O3 contents. The addition of La2O3 can improve the dispersion of Ni metal on the support, leading to the exposure of more Ni active sites. The XRD patterns of the reduced Ni/La2O3/a-Al2O3 catalysts (Fig. 1B) show the presence of the NiAl2O4 species on all of the Ni-based catalysts. La2NiO4 can be clearly detected out on the 4-La and 6-La catalysts. These structures were also observed by Slagtern et al. [2]. Table 1 summarizes that when the La2O3 contents on Ni/La2O3/a-Al2O3 catalysts were increased from 0 to 6 wt%, the Ni particle sizes were decreased from 39.7 to 21.9 nm, and the exposed Ni active sites per gram catalyst were increased

(6)

The TOF values of the pulse of CH4 or CO2 were calculated according to the following equation: TOFP ¼ ConP %  V  6:02  1023 =22; 400  t  m  Ni0 (7) Here p means pulse; V is volume of quantitative tube (0.3058 ml); t = V/SV (s); Sv the flow rate of carrier gas (ml/s); m the catalyst mass (g); Ni0 the amount of Ni active sites per gram catalyst (atoms/g catalyst); TOFP;CH4 and TOFP;CO2 are respectively TOF of the pulse of CH4 and CO2 through the fresh catalyst (s1); and TOFP;CH4 =CO2 is the TOF of CO2 for the first pulse of CH4 and then CO2 through the same catalysts (s1).

Fig. 1. XRD patterns of the unreduced catalysts (A) and reduced catalysts (B) of Ni/La2O3/a-Al2O3 catalysts.

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Table 1 The Ni particle sizes and active sites on the Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents Ni/La2O3/a-Al2O3 a

Ni particle size (nm) Ni active site (Ni0, 1019 g1)b H atom amount in Ni–H (1019 g1)c a b c

0-La

1-La

2-La

4-La

6-La

39.7 15.0 41.2

36.7 34.6 47.9

32.3 36.7 51.6

26.4 42.0 70.2

21.9 44.2 61.2

Estimated based on the half-width of peaks in the XRD patterns. Determined by H2-chemisorption based on Ni/H of 1. Determined by integral area of desorption curve of H2-TPD (Fig. 3).

from 15.0  1019 to 44.2  1019 g1. Therefore, the increase in the La2O3 contents leads to the wider diffraction peaks of NiO species (smaller NiO particles) and smaller Ni particles after the reduction of the catalysts. Temperature-programmed hydrogen reduction (H2-TPR) of the Ni/La2O3/a-Al2O3 catalysts can give the information about the influence of La2O3 on the interactions between Ni and aAl2O3. The H2-TPR spectrum of the catalyst Ni/a-Al2O3 (0-La, Fig. 2) shows three reduction peaks by deconvolving the overlapped peaks using Gaussian-type functions. The H2-TPR spectra of Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents show not only the three peaks at low temperature but also a new peak in high temperature range of 620–760 8C. The presence of the new high-temperature peak can be considered as the existence of the Ni species with the strongest interaction with support after the addition of La2O3 on the catalysts. Therefore, the new peaks can directly disclose the effect of the La2O3 contents on the interaction of the Ni active sites and the support. The area ratios of the new high-temperature peaks in the patterns of the five catalysts with La2O3 contents of 0, 1, 2, 4, and 6 wt% were estimated as 0:8:10:31:13. The Ni oxide species that have the strongest interaction with support and promoter became the most on the unreduced Ni/La2O3 (4 wt%)/ a-Al2O3 catalyst. Fig. 3 gives the H2-TPD spectra of Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents. It was found that hydrogen gas was desorbed from these catalysts at high

Fig. 2. The H2-TPR spectra of the unreduced Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents.

Fig. 3. The H2-TPD spectra of the Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents.

temperature, which was confirmed by MS. As we have recently reported [22,23], the hydrogen gas was originated from the Ni– H species formed in the reduced Ni particles on the catalysts. The addition of La2O3 on the catalyst can obviously increase the Ni–H amount on the catalysts. The Ni–H amount on the Ni/ La2O3/a-Al2O3 catalysts was also affected by the La2O3 contents. Table 1 summarizes that the amount of H atom in Ni–H species formed above 400 8C per gram catalyst is 41.2  1019 (0-La), 47.9  1019(1-La), 51.6  1019 (2-La), 70.2  1019(4-La), and 61.2  1019(6-La). When the La2O3 content on the Ni (8 wt%)/La2O3/a-Al2O3 catalyst was tuned to 4 wt%, the same amount of Ni metal on the 4-La catalyst can produce the most Ni–H species. These results mean that the amount of Ni–H formed on the catalysts was also affected by the amount of La2O3 on the catalysts. 3.2. TOF of the reforming reaction The reforming reaction was performed over Ni/La2O3/aAl2O3 catalysts with different La2O3 contents. In order to consider the effect of possible carbon deposition on the kinetic tests, we did the stability test for 10 h at the lowest and highest temperature before the kinetic study. The catalysts show very stable catalytic activities in 10 h. So these catalysts can give stable kinetic performance during the kinetic tests. Due to the presence of the reaction of CO2 with H2 in the concomitant RWGS, the TOF of the reforming reaction per Ni active site was calculated based on the conversion of CH4 in the reforming reaction. Fig. 4 shows the variations of TOF with the contents of La2O3 on the catalysts at different temperatures. TOF values were increased with increasing the temperature for the reforming reaction on every catalyst. The TOF values were also initially increased and then decreased with increasing the La2O3 contents on the catalysts, giving a ‘‘mountain’’ shape with the highest TOF value of the reforming reaction on the Ni active sites of the 4-La catalyst. It indicates that the La2O3 contents on the catalyst directly affected the catalytic activity of

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Fig. 4. The variation of TOF of the reforming reaction with the La2O3 contents at different temperatures.

the Ni active sites. And when the La2O3 content was tuned to 4 wt%, the Ni active sites can exhibit the highest catalytic reforming activity. 3.3. The reaction orders of the reforming The effect of CH4 partial pressure on the rates of the reforming reaction (based on CH4 conversions) was studied on the Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents at different temperatures. The reforming rates were increased as the orders of 1-La < 2-La < 6-La < 4-La under the same conditions, which was similar to the increase tendency of TOF of the reforming reaction with La2O3 contents shown in Fig. 4. The reforming rates were also increased with increasing the CH4 partial pressure. According to Eq. (3), the reaction orders of CH4 in the reforming reaction can be calculated based on the line slopes of the plots of ln rf against ln PCH4 . It was found that the CH4 orders were varied with the La2O3 contents on the catalysts even at the identical temperature (Fig. 5). The CH4 orders were in the range of 0.8–1.05 for 1-La, 2-La, and 6-La

Fig. 5. The variation of the reaction orders of CH4 with the La2O3 contents in the reforming reaction on the Ni/La2O3/a-Al2O3 catalysts at different temperatures.

catalysts and in the range of 0.6–0.8 for 4-La catalyst. The relationship between the CH4 orders and the La2O3 contents shows a converse ‘‘mountain’’ shape at every temperature and the 4-La catalyst gives the lowest CH4 reaction orders. The La2O3 contents on the catalysts show an obvious effect on the CH4 reaction orders in the reforming reaction. The effect of CO2 partial pressure on the reforming reaction rates on the Ni/La2O3/a-Al2O3 catalysts with different La2O3 contents was also studied at different temperatures. It was found that the reforming rates were nearly not affected by the partial pressure of CO2 at any temperature. The obtained reaction orders of CO2 were almost zero on every Ni/La2O3/aAl2O3 catalyst (Fig. 6). It has been reported that CO2 partial pressure obviously affected the reforming rates on the Ni/aAl2O3 catalyst [30]. However, the reaction orders of CO2 became zero in the reforming reaction on the La2O3-modified catalysts because of the effect of La2O3 on the reforming reaction. The variations of the reforming rates (rf) with CO partial pressure on the different Ni/La2O3/a-Al2O3 catalysts at different temperatures were also studied. The reforming rates were nearly not affected by CO partial pressure at the identical temperature, and the reaction orders of CO were almost zero on every La2O3-modified catalyst (Fig. 7). These results show that the formation and desorption of CO from the Ni catalysts do not affect the reforming reaction rate on the Ni/La2O3/a-Al2O3 catalysts. The effects of partial pressure of H2 on the reforming reaction rates were also studied on every catalyst at different temperatures. The variation of H2 partial pressure did not nearly affect the reforming rates on these catalysts. The obtained H2 reaction orders were almost zero in the reforming reaction on every catalyst (Fig. 8). 3.4. The activation energy of the reforming The activation energies (Ea) of the reforming reaction per Ni active site of Ni/La2O3/a-Al2O3 catalysts were calculated from the line slopes by plotting ln TOF of the reforming reaction

Fig. 6. The variation of the reaction orders of CO2 with the La2O3 contents in the reforming reaction on the Ni/La2O3/a-Al2O3 catalysts at different temperatures.

Y. Cui et al. / Applied Catalysis A: General 331 (2007) 60–69

Fig. 7. The variation of the reaction orders of CO with the La2O3 contents in the reforming reaction on the Ni/La2O3/a-Al2O3 catalysts at different temperatures.

against 1/T according to Eq. (5) and these plots are shown in Fig. 9. It was obviously found that there were inflexions at about 625 8C for these plots. The plots below and above 625 8C can be well fitted as lines. The Ea values of the reforming reaction per Ni active site of every catalyst were obtained in two separated temperature regions: 550–600 8C and 650–750 8C (Fig. 10). The Ea values obtained in the low temperature region were higher than those obtained in the high temperature region on every catalyst, indicating that the reaction mechanism was changed in the two temperature regions. The Ea values were also varied with the La2O3 contents on the catalysts in both temperature regions. When the La2O3 content on the catalyst was tuned to 4 wt%, the Ni active sites on the 4-La catalyst showed the lowest Ea of the reforming reaction in either temperature region, leading to the highest reforming activity (shown in Fig. 4), despite the lowest CH4 order on the 4-La catalyst at every the reaction temperature. These results further confirm that La2O3 contents on the catalysts remarkably affect the reforming activity of the Ni active sites.

Fig. 8. The variation of the reaction orders of H2 with the La2O3 contents in the reforming reaction on the Ni/La2O3/a-Al2O3 catalysts at different temperatures.

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Fig. 9. The plots of ln TOF against 1/T for the reforming reaction on the Ni/ La2O3/a-Al2O3 catalysts with different La2O3 contents.

3.5. The pulse TOF of CH4 and CO2 It was generally accepted that CH4 was adsorbed and dissociated into CHx intermediates (0  x  3) on Ni active sites [22,31,32]. Fig. 11 shows the TOF of CH4 conversion (TOFP;CH4 ) per Ni active site in the single pulse of CH4 through the catalysts with different La2O3 contents in 550–750 8C. The TOFP;CH4 values were varied with La2O3 contents on the catalysts, leading to a ‘‘mountain’’ shape. The Ni active sites on the 4-La catalyst showed the highest TOFP;CH4 at every temperature. The results agree to the increase tendency of TOF of the reforming reaction per Ni active site with the La2O3 contents (Fig. 4), which suggests the direct relation between the CH4 dissociation rate and the reforming reaction rate. TOFP;CH4 were also obviously varied with the pulse temperature (Fig. 11). TOFP;CH4 were increased in 550– 600 8C and reached the maximum at 600 8C; and then TOFP;CH4 remained constant in 650–750 8C. The different variation trends of TOFP;CH4 with temperature below and above 625 8C

Fig. 10. The variation of the activation energies (Ea) with the La2O3 contents for the reforming reaction on the Ni/La2O3/a-Al2O3 catalysts at low and high temperatures.

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Fig. 11. The TOF of CH4 for the single pulse of CH4 through the different Ni/ La2O3/a-Al2O3 catalysts at different temperatures.

show that the reaction mechanism should be changed at about 625 8C, similar to the results shown in Fig. 9. The pulse conversions of CO2 for the single pulse of CO2 through every catalyst are shown in Fig. 12. The CO2 conversions on every catalyst were increased with increasing temperature in 550–750 8C. The CO2 conversions were also increased with the La2O3 contents as the orders of 1-La < 2La < 4-La < 6-La at the same temperature. The increase orders of CO2 conversion with La2O3 contents is different from those of the CH4 reforming of CO2, which shows that CO2 can easily react with La2O3 to form La2O2CO3 species during the pulse of CO2 through the fresh Ni/La2O3/a-Al2O3 catalysts [16,17,18]. And the increase orders of the CO2 conversion are also different from those of TOF of the reforming reaction shown in Fig. 4, indicating that CO2 activation should not be the ratedetermining step for the CH4 reforming of CO2. It is accepted that CH4 is firstly dissociated into CHx species on the Ni active sites and then CO2 (or its other intermediates) reacts with these CHx species on the Ni sites during the

Fig. 12. The conversions of CO2 for the single pulse of CO2 through the different Ni/La2O3/a-Al2O3 catalysts at different temperatures.

Fig. 13. The TOF of CO2 for the sequential pulses of CH4 and CO2 through the different Ni/La2O3/a-Al2O3 catalysts at different temperatures.

reforming reaction [18,19,31,33]. Therefore, first CH4 and then CO2 were pulsed through the same Ni/La2O3/a-Al2O3 catalyst in order to study the interaction between the CHx species and CO2 on the La2O3-modified catalysts. The apparent TOF of CO2 conversion for the sequential pulses of CH4 and then CO2 (TOFP;CH4 =CO2 ) were obviously affected by the La2O3 contents (Fig. 13). And the Ni active sites on the 4-La catalyst gave the highest apparent TOF values. Such a variation trend was similar to that of TOF of CH4 pulse conversion (Fig. 11). The TOFP;CH4 =CO2 values were also varied with the pulse temperature. The TOF values were increased with temperature in 550–600 8C and reached maximum at 600 8C and then remained almost constant in 650–750 8C. Such a variation trend was completely same as that of TOFP;CH4 shown in Fig. 11. 4. Discussion 4.1. The effect of La2O3 on the catalyst properties The increase in the La2O3 contents improved the dispersion of Ni metal on the support and decreased the sizes of the Ni particles on the catalysts (Fig. 1). The presence of La2NiO4 and NiAl2O4 (spinel) is helpful for the explanation for the better dispersion of Ni with increasing the La2O3 contents [2]. The La2O3 contents on the catalysts also affected the interaction between Ni and the support [34]. The TPR results in Fig. 2 show several reduction peaks corresponding to the different Ni oxides. The amount of Ni metal that has the strongest interaction with support was the most on the 4-La catalyst among all the catalysts according to the H2-TPR spectra (Fig. 2). The degree of reduction might be different for these catalysts, because the NiAl2O4 was observed on all the reduced catalysts but La2NiO4 was only clearly detected out on the 4-La and 6-La catalysts (Fig. 1B). Without La2O3 on the catalyst, the reduction peak should correspond to the reduction of NiO; with increasing the La2O3 content on the catalyst, the reduction peaks in 620–720 8C possibly correspond to the reduction of NiAl2O4 or La2NiO4. The addition of La2O3 on the Ni catalysts

Y. Cui et al. / Applied Catalysis A: General 331 (2007) 60–69

can also change the chemical environment [2] and electronic state of Ni [34]; and the 4-La catalyst produced the most Ni–H species after the hydrogen reduction in their H2-TPD patterns (70.2  1019 compared to 41.2–61.2  1019 H atoms per gram catalyst) (Fig. 3). Here, the hydrogen determined in the H2-TPD corresponds to the total amount of hydrogen desorbed above 400 8C, relating to the internal Ni atoms in Ni particles. While the hydrogen determined by H2-chemisorption was operated at room temperature and it only contains the surface chemisorption hydrogen, corresponding to the amount of the surface Ni active sites. 4.2. The effect of La2O3 on the reaction orders Figs. 6–8 show that the reaction orders of CO2, CO, and H2 were almost zero in the reforming reaction and only the CH4 reaction orders were not zero and they were affected by the La2O3 contents on the catalysts. Therefore, only the CH4 partial pressure affected the reforming reaction rates but CO2, CO, and H2 did not affect the reforming reaction rates on the Ni/La2O3/ a-Al2O3 catalysts. The CO2 reaction orders were almost zero on the Ni/La2O3/ a-Al2O3 catalysts, which is different from the reforming rates on the Ni/a-Al2O3 catalyst [30]. The reason could be due to the equilibrium state of the formation of La2O2CO3 species through the reaction of La2O3 with CO2 [17,35]. Then the La2O2CO3 species were further reacted in the reforming reaction [18,19]. Due to the constant content of La2O3 on a catalyst and the rapid reaction of CO2 with La2O3 on the catalyst surface to form a constant concentration of La2O2CO3 species over each catalyst, CO2 reaction orders became almost zero in the reforming reaction. Additionally, the formation of La2O2CO3 on the catalysts can further explain the results in Fig. 12 that CO2 conversions were increased with the La2O3 contents on the catalysts for the single pulse of CO2 through the fresh catalysts. The similar result was also reported by Mo et al. [20] that the desorbed CO2 amount was increased with increasing the La2O3 contents on the Ni/La2O3/SiO2 catalysts in the CO2-TPD spectra of these catalysts. The reaction orders of CO were zero for the reforming reaction on the Ni/La2O3/a-Al2O3 catalysts, indicating that CO partial pressure did not affect the reforming rates. While CO partial pressure restrained the reforming reaction rates below 650 8C on the Ni/a-Al2O3 catalyst [30]. The reason can be attributed that the addition of La2O3 on the catalysts altered the electronic state of Ni metal [34], and CO desorption from the Ni active sites of the La2O3-modified catalysts became much easier than that from the Ni/a-Al2O3 catalyst [36,37]. Therefore, the reaction orders of CO became almost zero on these catalysts. It has been reported that hydrogen can be adsorbed on the Ni catalysts to form the Ni–H species on the catalyst during the reforming reaction, and H2 and Ni–H species can reach the rapid equilibrium in the reforming reaction [22]. We also found the Ni–H species were formed on the La2O3-modified catalysts (Fig. 3). Therefore, the hydrogen partial pressure did not affect the reforming reaction rates on the Ni/La2O3/a-Al2O3 catalysts and the H2 orders became almost zero.

67

The reaction orders of CH4, CO2, CO, and H2 suggest that the CH4 dissociation or the reaction between CHx and La2O2CO3 should be a rate-determining step in the reforming reaction on the Ni/La2O3/a-Al2O3 catalysts. 4.3. The effect of La2O3 on the reforming reaction rates The variation of the structure and properties of the Ni active sites with La2O3 contents directly affected the reforming reaction on the catalysts. Martı´nez et al. [21] reported that Ni/ La2O3/a-Al2O3 catalyst with low La2O3 content (up to 8 wt%), which was prepared by variable pH co-precipitation, showed higher stability in the CH4/CO2 reforming reaction than other catalysts with the La2O3 contents of 0%, 4%, and 12%. Liu et al. [34] also found that Ni/La2O3/theta-Al2O3 catalysts with an optimum amount of La2O3 loading with a La/Al ratio of 0.05 could give the highest activity and stability during the CO2 reforming of CH4. Mo et al. [20] also noticed that the catalytic activity and the stability of Ni/La2O3/SiO2 catalyst with 5 wt% La2O3 was slightly higher than that with 10 wt% La2O3 in the reaction of CH4/O2/CO2 performed in a fluidized-bed reactor. The results above have suggested that La2O3 contents have some effect on the activity and stability of the catalysts in the reforming reaction. In order to further explore the effect of the La2O3 contents on the intrinsic reforming activity per Ni active site on the catalysts, TOF of CO2 reforming of CH4 per Ni active site of the catalysts was obtained under the kinetic conditions. It was found that the TOF values were obviously varied with the La2O3 contents on the catalysts and the Ni active sites on the 4-La catalyst exhibited the highest TOF values (Fig. 4). Fig. 10 shows that the Ea values of the reforming reaction per Ni active site were obviously affected by the La2O3 contents on the catalysts with the lowest Ea on the 4-La catalyst. The lowest Ea of the reforming reaction on the 4-La catalyst provided the direct explanations for the highest TOF of the reforming reaction on the Ni active sites of the 4-La catalyst. The addition of 4 wt% La2O3 on the catalysts promotes the CH4/CO2 reforming not by increasing the reaction orders of CH4 but by increasing the pre-exponential factor and by decreasing the reaction activation energy. 4.4. The mechanism of the reforming reaction The obvious inflexions on the lines of the plots of ln TOF of the reforming reaction against 1/T at about 625 8C in Fig. 9, as well as the constant TOFP;CH4 and TOFP;CH4 =CO2 values on very catalyst above 650 8C shown in Figs. 11 and 12, indicate that the mechanism of the reforming reaction was varied at about 625 8C and the reforming mechanism can be divided into two temperature regions: 550–600 8C and 650–750 8C. The variation trends of TOFP;CH4 =CO2 with La2O3 contents (Fig. 13) were completely same as those of TOFP;CH4 with La2O3 contents (Fig. 11). Only CH4 reaction orders were not zero but other orders were all zero in the reforming reaction (Figs. 5–8). Therefore, the reforming reaction rates were directly determined by the rate of CH4 dissociation or the rates

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of the reaction between the CHx intermediate and La2O2CO3 on the Ni/La2O3/a-Al2O3 catalysts. Therefore, the effect of the La2O3 contents on the reforming rates could be explained by the studying the effect of La2O3 contents on the CH4 dissociation rates or on the CHx formation. Throughout all the temperature regions, the TOF of the pulse of CH4 showed a ‘‘mountain’’ shape with the La2O3 contents (Fig. 11). In 550–600 8C, the TOFP;CH4 was increased, indicating that the CH4 dissociation and the formation of Ni–H have not reached equilibrium. The TOFP;CH4 =CO2 had the same change tendency as TOFP;CH4 . These results support that the dissociation of CH4 is rate-determining step for the CH4 reforming of CO2 in 550–600 8C. Therefore, the mechanism of CH4 reforming of CO2 in 550–600 8C can be proposed as Eqs. (8)–(12): 4x CH4 þ 6x 2 NiH ! CHx  þ 2 H2 ;

RDS

CO2 þ La2 O3 Ð La2 O2 CO3

(8) (9)

x CHx  þLa2 O2 CO3 þ 2þx 2 NiH Ð 2CO  þ2H2  þLa2 O3

(10) 2CO  Ð 2CO þ 2NiH

(11)

2H2  Ð 2H2 þ 2NiH

(12)

We have proved that the dissociation of CH4 on the Ni active sites of the Ni-based catalysts was a structure sensitive reaction [24] and the sizes of the Ni particles on the catalysts directly affected the CH4 dissociation rates. Table 1 has shown that the Ni particle sizes were decreased from 36.7 to 21.9 nm with increasing La2O3 contents from 1 to 6 wt%. When the La2O3 content was tuned to 4 wt%, the average size of Ni particles on the 4-La catalyst was 26.4 nm, which resulted in the highest TOF for the CH4 dissociation, as well as the highest reforming rate. This should be one reason for the highest reforming activity per Ni active site on the 4-La catalyst in 550–600 8C. In 650–750 8C, TOFP;CH4 and TOFP;CH4 =CO2 were decreased and then kept unchanged, indicating that the CH4 reversible dissociation [38,39] and the formation of Ni–H reached equilibrium at higher temperature (650–750 8C). The only possible explanation for the effect of the CH4 partial pressure on the reforming rate can be due to that the reaction between the adsorbed CHx and the adsorbed CO2 (or La2O2CO3) is the RDS for the reforming reaction at higher temperature. The mechanism of CH4 reforming of CO2 on the Ni–La2O3-based catalysts in 650–750 8C can be proposed as Eqs. (13)–(17): 4x CH4 þ 6x 2 NiH Ð CHx  þ 2 H2 

(13)

CO2 þ La2 O3 Ð La2 O2 CO3

(14)

CHx  þLa2 O2 CO3 þ  þLa2 O3 ;

2þx 2 NiH ! 2CO

RDS



þ2xH2 (15)

2CO  Ð 2CO þ 2NiH

(16)

2H2  Ð 2H2 þ 2NiH

(17)

We have found that the amount of the Ni–H species formed on the catalysts was increased with increasing temperature. The formed Ni–H can inhibit the CH4 deep dissociation and the Ni– H species formed on the catalysts can increase the x values of CHx species, further increase the reforming reaction rate [22]. In this work, the H2-TPD spectra of Ni/La2O3/a-Al2O3 catalysts (Fig. 3) have shown that the amount of Ni–H species formed on the catalysts was varied with the La2O3 contents on the catalysts and the 4-La catalyst has the most Ni–H species. Therefore, the 4-La catalyst has the highest x value of CHx, which has the highest reaction activity with La2O2CO3. Different amounts of Ni–H on the catalysts originating from the varied La2O3 contents affected the reforming reaction rates in range of 650–750 8C. So, the CH4 reforming of CO2 on the Ni–La-based catalysts can be considered as the LangmiurHinshelwood model [2,16,19,40]. 5. Conclusion Among CH4, CO2, H2, and CO, only CH4 reaction orders were not zero in the CH4/CO2 reforming on the Ni/La2O3/aAl2O3 catalysts. And the CH4 orders were also affected by the La2O3 contents on the catalysts. The Ni active sites on the 4-La catalyst showed the highest reforming activity and the lowest reforming activation energy, as well as the lowest CH4 reaction orders. The results of TOFP;CH4 and TOFP;CH4 =CO2 on the catalysts show that the CH4 dissociation can be proposed as rate-determining step in the range of 500–600 8C. The Ni particles on the 4-La catalyst have the optimized size, leading to the highest CH4 dissociation rate due to the structure sensitive dissociation of CH4. Therefore, the 4-La catalyst shows the highest reforming activity in 500–600 8C. With the increase in temperature, the CH4 dissociation and the formation of Ni–H reached equilibrium, and the reaction between the intermediate CHx and La2O2CO3 became RDS in 650–750 8C. The 4-La catalyst has the most of Ni–H species, which leads to the highest x value in the CHx species, further improving the reaction rate between the CHx and La2O2CO3 in 650–750 8C. Therefore, the La2O3 contents on the catalysts affected the Ni particle sizes and the Ni–H amount on the catalysts, which further affected the CH4 dissociation rates and the x values in the CHx species, which leading to the highest reforming activity per Ni active site on the 4-La catalyst. Acknowledgements All the authors thank Yanxin Chen, Qingjie Ge, Yuzhong Wang, and Shoufu Hou for their significant help and discussion. Reference [1] H.Y. Wang, C.T. Au, Catal. Lett. 38 (1996) 77. [2] A. Slagtern, Y. Schuurman, C. Leclercq, X.E. Verykios, C. Mirodatos, J. Catal. 172 (1997) 118. [3] Z. Zhang, X.E. Verykios, S.M. MacDonald, S.J. Affrossman, J. Phys. Chem. 100 (1996) 744. [4] R.G. Ding, Z.F. Yan, Catal. Today 68 (2001) 135. [5] T. Osaki, T. Mori, J. Catal. 204 (2001) 89.

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