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Partial oxidation of methane to syngas over the catalyst derived from double perovskite (La0.5Sr0.5)2FeNiO6−δ
Applied Catalysis A: General 371 (2009) 153–160
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Partial oxidation of methane to syngas over the catalyst derived from double perovskite (La0.5Sr0.5)2FeNiO6d Xiong Yin, Liang Hong * Department of Chemical and Biomolecular Engineering, National University of Singapore, BLK E5 02-02, 4 Engineering Drive 4, Singapore 117576, Singapore
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 July 2009 Received in revised form 25 September 2009 Accepted 29 September 2009 Available online 7 October 2009
Double perovskite-type oxide (La0.5Sr0.5)2FeNiO6d (LSFN) was invented as the precursor of a catalyst for the partial oxidation of methane (POM). The catalyst derived from LSFN is the K2NiF4-supported Ni(0) system, where K2NiF4 denotes the oxide (La0.5Sr0.5)2Ni1xFeO4+d, a chemically stable structure with mixed ionic and electronic conducting properties. Among the four catalysts derived from LSFN, the best catalyst showed a high CH4 conversion (XCH4 > 99%), high syngas selectivity (>98%) and, most importantly, nil coke formation at 900 8C. Detailed structural characterizations revealed that the presence of a small amount of SrCO3, left initially by incomplete formation of LSFN, and of nano-Ni(0) domains (or clusters <5 nm on average) on the K2NiF4 support is vital to this extraordinary catalytic performance. Furthermore, the decrease of H2/CO molar ratio with the increase in methane conversion happening in the course of activation was simulated using a group of the proposed key reaction steps of POM. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Methane Catalytic partial oxidation Syngas Double perovskite Nickel catalyst
1. Introduction Syngas, which is a mixture of H2 and CO, is a superior fuel over methane for solid oxide fuel cells (SOFC) and is a sustainable feedstock for the production of H2, ultra-clean liquid fuels without sulfur, and other fine organic chemicals. Conversion of methane, the most destructive greenhouse gas, into syngas has received extensive attention over the past decades [1–3]. Steam reforming of methane (SRM) to syngas is currently the principal industrial syngas production process 0 (CH4 + H2O ! CO + 3H2; DH298 K ¼ þ206 kJ=mol) despite a highly endothermic and capitally expensive process [4–7]. In comparison with SRM, partial oxidation of methane (POM) to form syngas is a far more cost-effective process due to its mild 0 exothermic nature (CH4 + 1/2O2 ! CO + 2H2; DH298 K ¼ 36 kJ=mol). Nevertheless, a lack of POM catalysts with both acceptable cost to industry and stable performance (i.e., activity, selectivity, and coke resistance) is the major barrier to the commercialization of POM process. Transition metals (e.g., Rh, Ru, Pt, Pd, Ir, Co, Ni) are the most widely used catalysts for methane reforming. Among them, the noble metals [8–10], though they possess high catalytic activity, high selectivity to syngas and long-run performance stability, are prohibitively expensive. As a result, the nickel-based POM catalysts are still attractive due to the much lower cost of Ni relative to noble
* Corresponding author. Tel.: +65 6516 5029; fax: +65 6779 1936. E-mail address: [email protected] (L. Hong). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.044
metals. Since Prettre et al. [11] reported the first refractory supported nickel catalyst for POM more than 60 years ago, solving the problem of heavy coke (carbon filament) formation on Ni(0) catalytic sites has been an active area of research. Formation of coke led to deactivation of catalyst unless a large amount of steam is cosupplied with methane. However, supplying steam to this catalytic reaction requires energy and is therefore undesirable. With respect to the supported Ni catalytic system, a series of promoters, such as alkali elements [12], alkaline earth elements [13], rare earth elements [14–16], boron [17], and even noble metals [18,19], have been developed to improve the support with the intention of lessening coking extent. Doping the Ni surface with other metals to form an alloy was also a tactic to inhibit coking [20– 22]. NiO–MgO solid solution [23] and the use of perovskite BaTiO3 as support [24] have also improved the stability of Ni catalyst. A valuable conclusion drawn from the studies to date is that coke formation is sensitive to the sizes of Ni metal crystallites [25,26]. The smaller the Ni particles, the stronger the resistance to coke formation the catalyst will possess. Thinking along this line, the use of nickel-containing perovskite-type oxides as precursor of the POM catalyst has manifested an attractive progress on enhancing the resistance to coke deposition, because very tiny nickel clusters could be generated from these precursors. Besides this advantage, the support could function as a mixed ionic and electronic (O2/e) conductor (MIEC) in the catalytic system [27–31] since POM benefits from its mixed conductivity. La1ySryFe1xNixO3d oxides that have perovskite structure with low doping degrees at A-site or B-site [32– 36] have been reported to function as the precursor of POM catalyst. Provendier et al. [36] found that the B-site doped oxide,
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LaFe1xNixO3d, formed solid-solutions for all x values. Hence, this structure allows partial substitution for La3+ in LaFe1xNixO3d with Sr2+ in order to promote oxygen vacancy concentration and hence the diffusion coefficient of O2 anions at the expense of thermodynamic stability. A further concern for this catalyst is that tiny nickel clusters, generated in situ while reducing a perovskite oxide precursor, tend to merge together on the surface of support at the POM reaction temperature. In consequence, the issue of retaining long-term catalytic reactivity of Ni clusters remains unresolved. It is thus a great challenge to the realization of a commercial Ni POM catalyst. Inspired by the latest development in the perovskite oxidesupported Ni catalyst system, we focused on double perovskitetype oxide (DPO) with the formula of (AxA0 1x)2BB0 O6d, where A or A0 is a rare earth or alkaline earth ion, and B and B0 are different transition metal ions. The DPO with the composition of (La0.5Sr0.5)2FeNiO6d (LSFN) was specifically designed as the precursor of POM catalyst in this work. Such design is based on the fact that the LSFN phase is unstable when calcined in a reducing atmosphere (e.g., in the feed stream of POM), and it will degrade to the layered K2NiF4 type oxide by releasing a small amount of NiO. The NiO phase, in the form of miniature domains dispersed in the K2NiF4 oxide, can be reduced subsequently to Ni(0) clusters. Compared to perovskite oxide, the layered K2NiF4 matrix deserves an investigation regarding its effect on preventing Ni(0) clusters from aggregating. In addition, the K2NiF4 type oxide is chemically stable and possesses oxygen vacancies due to the presence of a perovskite layer of LaxSr1xFe1yNiyO3d. It is, therefore, expected that a synergy can be achieved by combining the Ni(0) clusters and the K2NiF4 support for overcoming the coking problem. 2. Experimental 2.1. Preparation of catalysts (La0.5Sr0.5)2FeNiO6d (LSFN) was prepared by modified Pechini method. A stoichiometric amount of metal (La, Sr, Fe and Ni) nitrates was dissolved in deionized (DI) water and followed by addition of citric acid and glycine. The mole ratio of La:Sr:Fe:Ni:citric acid:glycine was 1:1:1:1:2:2. The solution was thickened and dried at 80 8C with stirring until a gel was formed. The gel was precalcined at 400 8C for 2 h to generate an oxide powder, which was then subjected to calcination at one of the following temperatures: 600 8C, 800 8C, 1000 8C and 1200 8C, under ambient atmosphere. The short forms: LSFN600, LSFN800, LSFN1000 and LSFN1200, are used to denote the respective resulting oxides in the following sections. 2.2. Partial oxidation of methane POM was carried out in a fixed-bed micro-reactor at atmospheric pressure. The catalyst (100 mg) was packed in a tubular quartz reactor (ID = 8 mm). For the safety concerns and the purpose of running POM at isothermal conditions, a dilute methane stream composed of CH4 (5 vol.%) and He was used as feed, and air was supplied instead of pure oxygen to the reactor. The volume ratio of the inlet gas mixture (CH4:O2:N2:He) was 2:1:4:38. The POM reaction was carried out under gas hourly space velocity (GHSV) of 45,780 cm3/(g h). The composition of the product was analyzed by on-line gas chromatography (Perkin Elmer ARNEL, Clarus 500). 2.3. Characterization of catalysts The crystalline structure of catalyst was identified by X-ray diffraction (XRD, SHIMADZU XRD-6000, Cu Ka radiation). The
micro-morphologies of testing samples were scrutinized on a field emission scanning electron microscope (FESEM, JEOL JSM-6700F). The nano-scaled images of the testing samples were obtained from a transmission electron microscope (TEM, JEOL JEM-2100F). The adsorption/desorption isotherm of N2, temperature-programmed reduction with H2 (H2-TPR), and O2-temperatureprogrammed desorption (O2-TPD) were performed on a physicochemical adsorber (Quantachrome Autosorb-1 instrument). The specific surface area of the sample was determined by applying the multi-point BET method. The pore volume and average pore diameter of catalyst were obtained from the N2 desorption isotherm using the BJH method. H2-TPR was performed using a sample of 50 mg and a gas (5% H2 in N2) flow rate of 80 mL min1. O2-TPD was performed using a sample of 200 mg and a He gas flow rate of 80 mL min1. For both H2-TPR and O2-TPD, the heating rates of 15 8C/min below 850 8C and 1 8C/min above 850 8C were set. 3. Results and discussion 3.1. Preparation of K2NiF4-supported Ni catalyst by using double perovskite LSFN as precursor When the mixture consisting of the four individual metal oxides with the stoichiometry of LSFN was subjected to calcination at a temperature in the range from 600 8C to 1200 8C, the solid state reaction of the four oxides took place, which led to formation of DPO solid solution, i.e., LSFN. It was noted that the product of this reaction displayed different particle morphologies with the change in calcination temperature, from irregular grains to short bars and then to flakes (Fig. 1). Correspondingly, behind the variation of particle morphology of the four oxides generated, phase transformations took place, from the formation of LSFN (a to b) to the conversion of it to tetragonal K2NiF4 (c to d); these had taken place according to their XRD diagrams (Fig. 2). The pure DPO phase present in LSFN600 and LSFN800 is characterized by the XRD 2u peaks at 32.68, 46.78, and 58.08. It should also be noted that the formation of DPO had not yet been completed in LSFN600 for it still contained some amounts of SrCO3 as shown by its XRD pattern. On the contrary, the XRD diagram of LSFN1000 represents a mixture of DPO (i.e., the cubic LSFN) and K2NiF4. The cubic LSFN was finally degraded to tetragonal K2NiF4 structure as presented on the XRD diagram of LSFN1200 (Fig. 2). Furthermore, regarding the porous characteristics of these four powders, they do not have high specific surface areas (Table 1) and, in particular, LSFN1200 reveals a smaller BET surface area (2 m2/g) and pore volume (0.01 cm3/g) than the other three powders. This suggests that the K2NiF4 structure is highly compact. It was reported by Tsipis et al. [37] that the composition domain of perovskite-type solid solution series, La1ySryFe1xNixO3d, is rather narrow (y < 0.3 and x < 0.4) in order to uphold the cubic structure at 1100 8C. On this basis, we attempted a higher doping level (x = y = 0.5) in this research work and found that, providing the calcination temperature was below 1000 8C, the composition led to the double perovskite structure (La0.5Sr0.5)2FeNiO6d (i.e., LSFN600 and LSFN800) (Fig. 2a and b). In this double perovskite structure, Ni3+ and Fe3+ ions occupy alternatively the perovskite Bsites, resulting in alternating NiO6 and FeO6 octahedra. Such a double perovskite structure is normally thermally vulnerable. Hence, the structure not only decays to K2NiF4 but also releases NiO phase upon calcination at temperatures above 1100 8C at ambient atmosphere. The generated La1ySryFe1xNixO3d layer in K2NiF4 was stable, as described by Tsipis et al. [37]. In light of the NiO crystallites generated, the XRD pattern of LSFN1000 shows a trace of NiO phase identified by the characteristic diffraction peaks at about 2u = 378 and 438 [38] besides the strong peaks of the K2NiF4 phase. The formation of NiO crystallites became apparent in
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Fig. 1. The particulate morphologies of the LSFN precursors obtained from the different calcination temperatures for 4 h: (a) LSFN600 at 600 8C, (b) LSFN800 at 800 8C, (c) LSFN1000 at 1000 8C, and (d) LSFN1200 at 1200 8C.
LSFN1200 (Fig. 2d). Fig. 3 portrays the changes in microstructures with the rising of calcination temperature. In principle, the structural change from DPO to K2NiF4, driven by thermal degradation, must have accompanied a change in the oxygen vacancy concentration. The O2-TPD diagrams of the four composite oxides reveal such a correlation (Fig. 4). The first three diagrams consist of two O2 desorption peaks, in which the low temperature peak is due to desorption of O2 from the catalyst
Table 1 Surface area, pore volume, and pore diameter of the fabricated LSFN catalysts.
Fig. 2. XRD diagrams of the LSFN precursor series: (a) LSFN600, (b) LSFN800, (c) LSFN1000, and (d) LSFN1200.
Fig. 3. Schematic illustration of the calcination temperature-related microstructures and the formation of catalytic reactive Ni(0) clusters in the POM activation stage.
Multi-point BET surface area (m2/g) BJH desorption pore volume (cm3/g) BJH desorption average pore diameter (nm)
LSFN600
LSFN800
LSFN1000
19
19
18
0.09 18
0.09 20
0.09 21
LSFN1200 2 0.01 25
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Fig. 4. O2-TPD curves of the LSFN precursor series: (a) LSFN600, (b) LSFN800, (c) LSFN1000, and (d) LSFN1200.
surface while the high temperature peak emerging at about 800 8C can be attributed to oxygen that escaped from the subsurface layer [39]. The increase in calcination temperature brought about reductions in both the O2-TPD peak areas; in particular, the tangible reductions were found in curves (b)–(d). This is indicative of the increase in oxygen vacancy concentration due to the retreat of LSFN phase from the oxide composites with the increase in calcination temperature. The observed O2 desorption trend is in line with the phase composition of the four powders as confirmed by their XRD patterns. The TPR profiles of the four composite oxides have been investigated with the aim of understanding the reducibility of Ni2+ ion in the four samples (Fig. 5). This message is crucial to the assessment of the performance of the supported Ni(0) clusters in POM. As it is impossible to reduce the La3+ and Sr2+ ions at A-site under H2-TPR condition, the four H2 consumption peaks displayed on the TPR diagrams of the oxides come from the reduction of B-
site metals. Peaks I and II are due to the reduction of M3+ to M2+ (M = Fe, Ni) [36,40]. Peak III corresponds to the reduction of Ni2+ to Ni0 [36,40]. Peak IV is assigned to the reduction of Fe2+ to Fe0, which does not happen in the reaction system of POM [41]. Regarding the first two oxides, in contrast to LSFN600 (curve a), LSFN800 (curve b) is predominant by the double perovskite phase, and this results in a shift of peaks I, II and III in (b) to slightly higher temperatures. It is worthy to note that (a) displays a much stronger peak III than peak II, while (b) has a much weaker III than peak II. Compared to LSFN800, a greater amount of NiO in LSFN600 was reducible. As to the cause of this difference, the impurity phases existed in LSFN600 (Fig. 2a), owing to incomplete formation of the designated DPO, played the key role. The SrCO3 phase labeled by the XRD peaks at 2u = 258 and 268, being relatively stagnant and therefore acting to prevent sintering of perovskite phase formed [42], is deemed to retard the reducibility of the DPO (i.e., small peak II) in the present case. While for LSFN800, since the DPO with the designated composition had been formed prior to TPR, it would be therefore quickly degraded to K2NiF4 and the NiO released would become inaccessible. As a result, a small peak III was observed. Moving on to LSFN1000 (curve c), according to its XRD diagram it is an oxide composite due to calcination-driven degradation; it comprises a DPO phase, a K2NiF4 phase and a trace amount of NiO before TPR. Since the K2NiF4 phase was derived from the DPO phase, these two major phases were interpenetrating; in them, NiO was embedded. A broad Ni2+ reduction peak extending to high temperature side was observed. From the area of this peak, it can be estimated that a larger amount of NiO than that in LSFN800 was reducible. Such a broad reduction temperature span can be attributed to a distribution of NiO crystallites in different chemical microenvironments. Finally, regarding LSFN1200 (curve d), it contained a predominant K2NiF4 phase and small amounts of DPO and NiO, as shown in Fig. 2d. Compared to LSFN1000, a noticeable increase in the area of peak III and a sliding down of its temperature range all indicate that a larger number of NiO crystallites were produced and they were readily reducible. However, it is not clear why a small amount of NiO displayed by the XRD of LSFN1200 could give rise to an overriding TPR peak. Presumably, the reducing atmosphere of TPR would facilitate the further release of NiO from the K2NiF4 phase due to the reduction of it to Ni(0) clusters. The TPR investigation exhibits that a larger amount of NiO crystallites was reducible in either LSFN600 or LSFN1200 than in the other two precursors, and this means that more NiO could be reduced in situ of POM in the former two oxides. To clarify the distribution and dimensions of Ni(0) sites formed in LSFN600 and LSFN1200, these two oxides were subjected to reduction in a H2 (5%)–N2 stream of 80 sccm at 600 8C for 1 h and then scrutinized by TEM (Fig. 6). After this treatment, oxide LSFN600 showed metallic Ni(0) clusters with sizes smaller than 5 nm, which were not typical grains in shape and were randomly distributed over the support, whereas oxide LSFN1200 showed embedded Ni(0) nodules having both large (10–20 nm) and small (5 nm) particle sizes. 3.2. Assessment of partial oxidation of methane over the catalysts
Fig. 5. H2-TPR curves of the LSFN precursor series: (a) LSFN600, (b) LSFN800, (c) LSFN1000, and (d) LSFN1200.
The preceding structural characterizations have confirmed that the temperature, at which the oxide mixtures from the Pechini synthesis (Section 2.1) were calcined, affected the structure of the precursors of POM catalyst (i.e., LSFN series), and in turn the reducibility of NiO to Ni(0). For simplicity reasons, the resulting catalysts are still labeled by the same LSFN series name. The precursors were activated at the reaction temperature for 25 h in the purging stream consisting of CH4, O2, N2, and He (with molar ratio 2:1:4:38) and the GHSV of 45,780 cm3/(g h) was adopted. A decreasing trend of catalytic activity was found with the increase in calcination temperature of the precursors, namely, a monotonic
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Table 2 Catalytic partial oxidation of methane using LSFN catalysts. Reaction temperature
Catalyst
XCH4 (%)
SCO (%)
H2/CO
850 8C
LSFN600 LSFN800 LSFN1000 LSFN1200
96.5 70.0 61.4 30.2
98.4 68.0 86.0 38.6
2.0 3.0 3.0 3.8
800 8C
LSFN600 LSFN800 LSFN1000 LSFN1200
88.8 66.0 42.6 30.2
95.2 59.0 74.2 36.4
2.3 3.4 3.6 4.2
Fig. 7. Evaluation of the temperature-dependent POM performance data in the system catalyzed by LSFN600.
Fig. 6. TEM images of the H2-reduced samples: (a) LSFN600 and (b) LSFN1200.
decrease in the methane conversion XCH4 but a rise of the H2/CO molar ratio (Table 2). Additionally, with respect to a single catalyst (LSFN600 catalyst), Fig. 7 shows how the XCH4 , CO selectivity SCO and H2/CO ratio vary with reaction temperature. The methane conversion and CO selectivity increases drastically with raising the reaction temperature range from 600 8C to 850 8C, while the H2/CO ratio reduced with the increase in methane conversion. This phenomenon can be attributed to the reforming of carbon deposited on Ni(0) clusters by steam and CO2; with the increase in reaction temperature, more deposited carbon filaments were reformed to produce CO. The carbon deposition is due to the catalytic thermal cracking of CH4 on Ni(0) clusters, which produces H2 as well. This interpretation will be further elaborated in the last section. With the use of the highest POM reaction temperature, i.e., 900 8C, the H2/CO molar ratio slid to about 2 and correspondingly the methane conversion approached to 100%. Furthermore, the stability of catalytic performance represented by XCH4 , SCO and H2/CO ratio has also been examined at 900 8C over
a 100-h reaction period of time. Fig. 8 shows the outcome of the LSFN600 catalyst: a very high methane conversion (>99%), high CO selectivity (>98%) and the desired H2/CO molar ratio (2.0) were maintained through the rest of the evaluation course (75 h) after the activation of catalyst in the first 25 h. In other words, the catalyst did not show any sign of the loss of activity in the end of this arbitrarily selected reaction duration. The catalyst was then examined using FESEM after it was cooled down to room temperature under the flow of feed stream. Non-carbon filaments on its surface could be identified (Fig. 9a). Similarly, for LSFN1200,
Fig. 8. The POM performance profiles of LSFN600 at 900 8C, which consists of two stages: the activation stage (<25 h) and the steady operation stage.
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Fig. 10. The XRD diagrams of the catalysts: (a) LSFN600 and (b) LSFN1200 after POM reaction for 100 h at 900 8C.
course, carbon filaments had already extensively grown on the catalyst before it was discharged from the reactor (Fig. 9c). It is presumed that a combination of the ionic conducting K2NiF4 support and the nano-scaled Ni(0) clusters generate an effective catalytic surface, which allows faster reforming of the deposited coke than that on the benchmarking catalyst under the POM condition. 3.3. Structural attributes of different catalytic behaviors
Fig. 9. The FESEM images of the catalysts: (a) LSFN600, (b) LSFN1200, and (c) HiFUEL after POM reaction for 100 h at 900 8C.
despite having much lower catalytic activity than LSFN600, it also left behind a carbon-free surface morphology after the same testing process (Fig. 9b). In comparison to these two catalysts, a commercial Ni-supported catalyst (HiFUELTM R110 from Alfa Aesar) was used. Although this commercial catalyst displayed no apparent loss of catalytic activity in the end of the same testing
After the LSFN600 catalyst was used to drive POM for more than 70 h at 900 8C, its XRD diagram (Fig. 10) revealed the predominant K2NiF4 phase, indicating that K2NiF4 phase produced from the activation of LSFN, as illustrated in Fig. 3, remained stable throughout the entire reaction process. For LSFN1200, the K2NiF4 phase remained unchanged. Besides the K2NiF4 phase, the NiO phase was also found on the two XRD diagrams (Fig. 10), which came from the oxidation of catalytic Ni(0) clusters after they were exposed to atmosphere. As far as the significantly different catalytic activity between these two catalysts is concerned, which represented the boundary cases of the four catalysts in question, the presence of accessible nano-Ni(0) clusters as well as a small amount of highly dispersed SrCO3 phase in close proximity to Ni(0) catalytic sites, happening only in the LSFN600 catalyst (Figs. 2a and 10a), are deemed to be the key factors to sustain its high activity. The role of SrCO3 is presumed to trim the Lewis basicity of Ni(0) catalytic sites, by referencing the previous work [43]. On the other hand, from the micro-morphology of K2NiF4 (Fig. 1c), we conclude that its BET parameters (Table 1) and the embedded Ni(0) granules (Fig. 6b), the lowest catalytic performance of LSFN1200 might be due to covering of the NiO phase by the compact K2NiF4 matrix. It has been indicated in the above discussion in Fig. 7 that the H2/CO molar ratio decreased with the increase in conversion of methane, XCH4 , caused by raising the reaction temperature. To understand this reliance from the perspective of reaction mechanism, we used the catalysts, LSFN600 and LSFN800, as the model systems (Fig. 11); in these, the H2/CO ratio decreased with the increase in XCH4 through the initial 25 h (e.g., Fig. 8). We found from the following four key reaction steps (Eqs. (1)–(4) a simple formula that describes the experimental H2 =COXCH4 dependency can be concluded, although the detailed POM mechanism over the Ni-based catalyst involves a series of chemical reactions [7,36,44– 48]. CH4 þ 2O2 ! CO2 þ 2H2 O
(1)
CH4 ! Cs þ 2H2
(2)
Cs þ H2 O ! CO þ H2
(3)
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Combining Eqs. (5) and (7) results in a simple relation between H2/ CO ratio and XCH4 (Eq. (8)). This relation allows testing if it reflects the experimental trend, viz. XCH4 increases with the increasing of the available Ni(0) sites, while H2/CO ratio decreases with the increase in XCH4 . H2 ð8=3Þb 1 þ XCH4 ¼ CO 1þð4=3Þb XCH4
Fig. 11. The simulation of the calculated XCH4 H2 =CO relationship (Eq. (8)) to the experimental counterpart.
Cs þ CO2 ! 2CO
(4)
The methane combustion (Eq. (1)) takes place primarily on the surface of K2NiF4. The thermal cracking of methane (Eq. (2)) over tiny nickel clusters on the surface is widely accepted as the major carbon formation process in the high reaction temperature region (T > 800 8C). H2 gas is produced from the thermal cracking of methane (Eq. (2)) and the reaction of surface carbon (Cs) with the steam (Eq. (3)) generated from methane combustion. Therefore, on the basis of the stoichiometry of these two reactions (2 + k)-mole H2 gas can be generated from consuming 1 mole Cs, where the value of k is the molar fraction of Cs reformed by steam in Eq. (3) over the total Cs consumed in both Eqs. (3) and (4). From reactions (3) and (4) (2 k) mole CO is to be formed from 1 mole Cs provided that the Cs generation rate equals the Cs removal rate, which has been explicitly supported by the FESEM image of the LSFN600 after POM reaction (Fig. 9a). As a result, the H2/CO molar ratio is then expressed as H2 2 þ k ¼ CO 2 k
(5)
As the activation process is a non-steady state process, its ending point can thus be set when XCH4 becomes steady, for example, Fig. 8 shows that the steady conversion is close to 100% after activation. According to the stoichiometry relationship, Cs reacted in reaction (3) is double of that reacted in reaction (4) because the molar ratio of CO2 to H2O is 1/2 in Eq. (1). Therefore, taking XCH4 ¼ 100% as the boundary condition, then the k value equals 2/3. This k value depends upon the production of Cs and H2O in reactions (1) and (2) besides the rate of reaction (3), and alternatively these two reactions on the whole represent the conversion XCH4 . In light of the activation process, such a scenario must exist: reaction (1) is almost independent of whether the numbers of K2NiF4 sites are sufficient since they largely exceed the Ni(0) sites most of the time. Therefore the contributions of reaction (2) to XCH4 , with the increase of Ni(0) clusters, and to the mounting of reaction (4), which depresses the k value, follow an approximately linear relation as proposed by Eq. (6), because both steps are directly linked with the concentration of Cs. Therefore the rates of these two elementary steps must be proportional: XCH4 ¼ a þ bk
(6)
In Eq. (6) parameter a can be determined using the boundary condition of XCH4 ¼ 1 and k = 2/3. Thus, Eq. (6) can be rewritten as k¼
2 1 XCH4 3 b
(7)
(8)
Fig. 11 shows that the calculated H2 =COXCH4 curve (b = 0.96) fits well the experimental data for the two catalytic systems. For LSFN800 catalyst, only at 950 8C, its XCH4 H2 =CO could fit the model because its conversion could reach almost a quantitative value in the end of activation. The satisfactory fitting supports the above assumption that the rates of reactions (2) and (4) increase with the number and activity (i.e., temperature effect for an individual catalyst) of Ni(0) sites by following a near linear manner. In other words, for the supported Ni(0) sites (clusters or nodules), owing to either a low reaction temperature (Fig. 7) or steric blockage because of their embedment in the K2NiF4 support (Table 2), the last two reaction steps especially the reforming of the deposited carbon by CO2 must slow down, causing a higher H2/CO molar ratio according to the stoichiometry relations of Eqs. (1)–(4). 4. Conclusions Double perovskite-type (La0.5Sr0.5)2FeNiO6d (LSFN) solid solution was designed as the precursor of the K2NiF4-supported Ni(0), which catalyzes the POM to produce syngas. To synthesize LSFN, a mixture of four metal oxides with the required stoichiometry was prepared by the wet chemistry method and then calcined at a fixed temperature in the range of 600–1200 8C. The phase composition of the precursor obtained from this process is affected by the calcination temperature used. Double perovskite-type LSFN can be completely formed and retained only in the temperature range roughly from 800 8C to 1000 8C since the thermal degradation of LSFN to K2NiF4 becomes progressively more severe with the increasing of temperature from the upper bound, and a small amount of NiO is released in the same course. Four precursors were prepared by the use of four different calcination temperatures, and subsequently they were converted to the corresponding catalysts in the POM reactor, where LSFN underwent reductive degradation to K2NiF4 and a small amount of B-site Ni3+ ions was reduced to Ni(0). Of the four catalysts, the best catalyst came from the precursor bound to the lowest calcination temperature. It demonstrated a high methane conversion (XCH4 > 99%), high syngas selectivity (SCO > 98%) and nil coke formation over a100h test. The main structural attribute for this catalyst lies in two aspects: firstly, the minor NiO and SrCO3 phases that were left in the formation of LSFN and in close proximity to each other, the former being transformed to nano-Ni(0) clusters after the activation; secondly, the O2 conducting K2NiF4 support. For the other three catalysts, although all were based on K2NiF4, the lower POM catalytic activity can be attributed to a low concentration Ni(0) clusters or heavy coverage of them by the support. Both TEM and catalytic activity assessment suggest that the NiO phase is severely embedded if it is produced from the thermal degradation of LSFN to K2NiF4, and similarly Ni(0) clusters generated from the reductive degradation of LSFN to K2NiF4 in the course of the activation were not prevalent according to the TPR investigation. Furthermore, to understand how the Ni(0) clusters collaborate with K2NiF4 to catalyse POM, the four elementary steps were selected from the known methane reforming reactions and were used to simulate the decreasing trend of H2/CO molar ratio, as happened in the activation process. The satisfactory simulation result suggests that CO2 and steam formed from the combustion of
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CH4 on K2NiF4 are consumed to reform the carbon deposit from the thermal cracking of CH4 on the Ni(0) sites. The important inference is therefore that the reforming steps show control over the combustion step. Acknowledgement The work has been performed with the financial support by NRF/CRP ‘‘Molecular engineering of membrane research and technology for energy development: hydrogen, natural gas and syngas’’ (R279-000-261-281). References
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Partial oxidation of methane to syngas over the catalyst derived from double perovskite (La0.5Sr0.5)2FeNiO6d Xiong Yin, Liang Hong * Department of Chemical and Biomolecular Engineering, National University of Singapore, BLK E5 02-02, 4 Engineering Drive 4, Singapore 117576, Singapore
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 July 2009 Received in revised form 25 September 2009 Accepted 29 September 2009 Available online 7 October 2009
Double perovskite-type oxide (La0.5Sr0.5)2FeNiO6d (LSFN) was invented as the precursor of a catalyst for the partial oxidation of methane (POM). The catalyst derived from LSFN is the K2NiF4-supported Ni(0) system, where K2NiF4 denotes the oxide (La0.5Sr0.5)2Ni1xFeO4+d, a chemically stable structure with mixed ionic and electronic conducting properties. Among the four catalysts derived from LSFN, the best catalyst showed a high CH4 conversion (XCH4 > 99%), high syngas selectivity (>98%) and, most importantly, nil coke formation at 900 8C. Detailed structural characterizations revealed that the presence of a small amount of SrCO3, left initially by incomplete formation of LSFN, and of nano-Ni(0) domains (or clusters <5 nm on average) on the K2NiF4 support is vital to this extraordinary catalytic performance. Furthermore, the decrease of H2/CO molar ratio with the increase in methane conversion happening in the course of activation was simulated using a group of the proposed key reaction steps of POM. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Methane Catalytic partial oxidation Syngas Double perovskite Nickel catalyst
1. Introduction Syngas, which is a mixture of H2 and CO, is a superior fuel over methane for solid oxide fuel cells (SOFC) and is a sustainable feedstock for the production of H2, ultra-clean liquid fuels without sulfur, and other fine organic chemicals. Conversion of methane, the most destructive greenhouse gas, into syngas has received extensive attention over the past decades [1–3]. Steam reforming of methane (SRM) to syngas is currently the principal industrial syngas production process 0 (CH4 + H2O ! CO + 3H2; DH298 K ¼ þ206 kJ=mol) despite a highly endothermic and capitally expensive process [4–7]. In comparison with SRM, partial oxidation of methane (POM) to form syngas is a far more cost-effective process due to its mild 0 exothermic nature (CH4 + 1/2O2 ! CO + 2H2; DH298 K ¼ 36 kJ=mol). Nevertheless, a lack of POM catalysts with both acceptable cost to industry and stable performance (i.e., activity, selectivity, and coke resistance) is the major barrier to the commercialization of POM process. Transition metals (e.g., Rh, Ru, Pt, Pd, Ir, Co, Ni) are the most widely used catalysts for methane reforming. Among them, the noble metals [8–10], though they possess high catalytic activity, high selectivity to syngas and long-run performance stability, are prohibitively expensive. As a result, the nickel-based POM catalysts are still attractive due to the much lower cost of Ni relative to noble
* Corresponding author. Tel.: +65 6516 5029; fax: +65 6779 1936. E-mail address: [email protected] (L. Hong). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.044
metals. Since Prettre et al. [11] reported the first refractory supported nickel catalyst for POM more than 60 years ago, solving the problem of heavy coke (carbon filament) formation on Ni(0) catalytic sites has been an active area of research. Formation of coke led to deactivation of catalyst unless a large amount of steam is cosupplied with methane. However, supplying steam to this catalytic reaction requires energy and is therefore undesirable. With respect to the supported Ni catalytic system, a series of promoters, such as alkali elements [12], alkaline earth elements [13], rare earth elements [14–16], boron [17], and even noble metals [18,19], have been developed to improve the support with the intention of lessening coking extent. Doping the Ni surface with other metals to form an alloy was also a tactic to inhibit coking [20– 22]. NiO–MgO solid solution [23] and the use of perovskite BaTiO3 as support [24] have also improved the stability of Ni catalyst. A valuable conclusion drawn from the studies to date is that coke formation is sensitive to the sizes of Ni metal crystallites [25,26]. The smaller the Ni particles, the stronger the resistance to coke formation the catalyst will possess. Thinking along this line, the use of nickel-containing perovskite-type oxides as precursor of the POM catalyst has manifested an attractive progress on enhancing the resistance to coke deposition, because very tiny nickel clusters could be generated from these precursors. Besides this advantage, the support could function as a mixed ionic and electronic (O2/e) conductor (MIEC) in the catalytic system [27–31] since POM benefits from its mixed conductivity. La1ySryFe1xNixO3d oxides that have perovskite structure with low doping degrees at A-site or B-site [32– 36] have been reported to function as the precursor of POM catalyst. Provendier et al. [36] found that the B-site doped oxide,
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LaFe1xNixO3d, formed solid-solutions for all x values. Hence, this structure allows partial substitution for La3+ in LaFe1xNixO3d with Sr2+ in order to promote oxygen vacancy concentration and hence the diffusion coefficient of O2 anions at the expense of thermodynamic stability. A further concern for this catalyst is that tiny nickel clusters, generated in situ while reducing a perovskite oxide precursor, tend to merge together on the surface of support at the POM reaction temperature. In consequence, the issue of retaining long-term catalytic reactivity of Ni clusters remains unresolved. It is thus a great challenge to the realization of a commercial Ni POM catalyst. Inspired by the latest development in the perovskite oxidesupported Ni catalyst system, we focused on double perovskitetype oxide (DPO) with the formula of (AxA0 1x)2BB0 O6d, where A or A0 is a rare earth or alkaline earth ion, and B and B0 are different transition metal ions. The DPO with the composition of (La0.5Sr0.5)2FeNiO6d (LSFN) was specifically designed as the precursor of POM catalyst in this work. Such design is based on the fact that the LSFN phase is unstable when calcined in a reducing atmosphere (e.g., in the feed stream of POM), and it will degrade to the layered K2NiF4 type oxide by releasing a small amount of NiO. The NiO phase, in the form of miniature domains dispersed in the K2NiF4 oxide, can be reduced subsequently to Ni(0) clusters. Compared to perovskite oxide, the layered K2NiF4 matrix deserves an investigation regarding its effect on preventing Ni(0) clusters from aggregating. In addition, the K2NiF4 type oxide is chemically stable and possesses oxygen vacancies due to the presence of a perovskite layer of LaxSr1xFe1yNiyO3d. It is, therefore, expected that a synergy can be achieved by combining the Ni(0) clusters and the K2NiF4 support for overcoming the coking problem. 2. Experimental 2.1. Preparation of catalysts (La0.5Sr0.5)2FeNiO6d (LSFN) was prepared by modified Pechini method. A stoichiometric amount of metal (La, Sr, Fe and Ni) nitrates was dissolved in deionized (DI) water and followed by addition of citric acid and glycine. The mole ratio of La:Sr:Fe:Ni:citric acid:glycine was 1:1:1:1:2:2. The solution was thickened and dried at 80 8C with stirring until a gel was formed. The gel was precalcined at 400 8C for 2 h to generate an oxide powder, which was then subjected to calcination at one of the following temperatures: 600 8C, 800 8C, 1000 8C and 1200 8C, under ambient atmosphere. The short forms: LSFN600, LSFN800, LSFN1000 and LSFN1200, are used to denote the respective resulting oxides in the following sections. 2.2. Partial oxidation of methane POM was carried out in a fixed-bed micro-reactor at atmospheric pressure. The catalyst (100 mg) was packed in a tubular quartz reactor (ID = 8 mm). For the safety concerns and the purpose of running POM at isothermal conditions, a dilute methane stream composed of CH4 (5 vol.%) and He was used as feed, and air was supplied instead of pure oxygen to the reactor. The volume ratio of the inlet gas mixture (CH4:O2:N2:He) was 2:1:4:38. The POM reaction was carried out under gas hourly space velocity (GHSV) of 45,780 cm3/(g h). The composition of the product was analyzed by on-line gas chromatography (Perkin Elmer ARNEL, Clarus 500). 2.3. Characterization of catalysts The crystalline structure of catalyst was identified by X-ray diffraction (XRD, SHIMADZU XRD-6000, Cu Ka radiation). The
micro-morphologies of testing samples were scrutinized on a field emission scanning electron microscope (FESEM, JEOL JSM-6700F). The nano-scaled images of the testing samples were obtained from a transmission electron microscope (TEM, JEOL JEM-2100F). The adsorption/desorption isotherm of N2, temperature-programmed reduction with H2 (H2-TPR), and O2-temperatureprogrammed desorption (O2-TPD) were performed on a physicochemical adsorber (Quantachrome Autosorb-1 instrument). The specific surface area of the sample was determined by applying the multi-point BET method. The pore volume and average pore diameter of catalyst were obtained from the N2 desorption isotherm using the BJH method. H2-TPR was performed using a sample of 50 mg and a gas (5% H2 in N2) flow rate of 80 mL min1. O2-TPD was performed using a sample of 200 mg and a He gas flow rate of 80 mL min1. For both H2-TPR and O2-TPD, the heating rates of 15 8C/min below 850 8C and 1 8C/min above 850 8C were set. 3. Results and discussion 3.1. Preparation of K2NiF4-supported Ni catalyst by using double perovskite LSFN as precursor When the mixture consisting of the four individual metal oxides with the stoichiometry of LSFN was subjected to calcination at a temperature in the range from 600 8C to 1200 8C, the solid state reaction of the four oxides took place, which led to formation of DPO solid solution, i.e., LSFN. It was noted that the product of this reaction displayed different particle morphologies with the change in calcination temperature, from irregular grains to short bars and then to flakes (Fig. 1). Correspondingly, behind the variation of particle morphology of the four oxides generated, phase transformations took place, from the formation of LSFN (a to b) to the conversion of it to tetragonal K2NiF4 (c to d); these had taken place according to their XRD diagrams (Fig. 2). The pure DPO phase present in LSFN600 and LSFN800 is characterized by the XRD 2u peaks at 32.68, 46.78, and 58.08. It should also be noted that the formation of DPO had not yet been completed in LSFN600 for it still contained some amounts of SrCO3 as shown by its XRD pattern. On the contrary, the XRD diagram of LSFN1000 represents a mixture of DPO (i.e., the cubic LSFN) and K2NiF4. The cubic LSFN was finally degraded to tetragonal K2NiF4 structure as presented on the XRD diagram of LSFN1200 (Fig. 2). Furthermore, regarding the porous characteristics of these four powders, they do not have high specific surface areas (Table 1) and, in particular, LSFN1200 reveals a smaller BET surface area (2 m2/g) and pore volume (0.01 cm3/g) than the other three powders. This suggests that the K2NiF4 structure is highly compact. It was reported by Tsipis et al. [37] that the composition domain of perovskite-type solid solution series, La1ySryFe1xNixO3d, is rather narrow (y < 0.3 and x < 0.4) in order to uphold the cubic structure at 1100 8C. On this basis, we attempted a higher doping level (x = y = 0.5) in this research work and found that, providing the calcination temperature was below 1000 8C, the composition led to the double perovskite structure (La0.5Sr0.5)2FeNiO6d (i.e., LSFN600 and LSFN800) (Fig. 2a and b). In this double perovskite structure, Ni3+ and Fe3+ ions occupy alternatively the perovskite Bsites, resulting in alternating NiO6 and FeO6 octahedra. Such a double perovskite structure is normally thermally vulnerable. Hence, the structure not only decays to K2NiF4 but also releases NiO phase upon calcination at temperatures above 1100 8C at ambient atmosphere. The generated La1ySryFe1xNixO3d layer in K2NiF4 was stable, as described by Tsipis et al. [37]. In light of the NiO crystallites generated, the XRD pattern of LSFN1000 shows a trace of NiO phase identified by the characteristic diffraction peaks at about 2u = 378 and 438 [38] besides the strong peaks of the K2NiF4 phase. The formation of NiO crystallites became apparent in
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Fig. 1. The particulate morphologies of the LSFN precursors obtained from the different calcination temperatures for 4 h: (a) LSFN600 at 600 8C, (b) LSFN800 at 800 8C, (c) LSFN1000 at 1000 8C, and (d) LSFN1200 at 1200 8C.
LSFN1200 (Fig. 2d). Fig. 3 portrays the changes in microstructures with the rising of calcination temperature. In principle, the structural change from DPO to K2NiF4, driven by thermal degradation, must have accompanied a change in the oxygen vacancy concentration. The O2-TPD diagrams of the four composite oxides reveal such a correlation (Fig. 4). The first three diagrams consist of two O2 desorption peaks, in which the low temperature peak is due to desorption of O2 from the catalyst
Table 1 Surface area, pore volume, and pore diameter of the fabricated LSFN catalysts.
Fig. 2. XRD diagrams of the LSFN precursor series: (a) LSFN600, (b) LSFN800, (c) LSFN1000, and (d) LSFN1200.
Fig. 3. Schematic illustration of the calcination temperature-related microstructures and the formation of catalytic reactive Ni(0) clusters in the POM activation stage.
Multi-point BET surface area (m2/g) BJH desorption pore volume (cm3/g) BJH desorption average pore diameter (nm)
LSFN600
LSFN800
LSFN1000
19
19
18
0.09 18
0.09 20
0.09 21
LSFN1200 2 0.01 25
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Fig. 4. O2-TPD curves of the LSFN precursor series: (a) LSFN600, (b) LSFN800, (c) LSFN1000, and (d) LSFN1200.
surface while the high temperature peak emerging at about 800 8C can be attributed to oxygen that escaped from the subsurface layer [39]. The increase in calcination temperature brought about reductions in both the O2-TPD peak areas; in particular, the tangible reductions were found in curves (b)–(d). This is indicative of the increase in oxygen vacancy concentration due to the retreat of LSFN phase from the oxide composites with the increase in calcination temperature. The observed O2 desorption trend is in line with the phase composition of the four powders as confirmed by their XRD patterns. The TPR profiles of the four composite oxides have been investigated with the aim of understanding the reducibility of Ni2+ ion in the four samples (Fig. 5). This message is crucial to the assessment of the performance of the supported Ni(0) clusters in POM. As it is impossible to reduce the La3+ and Sr2+ ions at A-site under H2-TPR condition, the four H2 consumption peaks displayed on the TPR diagrams of the oxides come from the reduction of B-
site metals. Peaks I and II are due to the reduction of M3+ to M2+ (M = Fe, Ni) [36,40]. Peak III corresponds to the reduction of Ni2+ to Ni0 [36,40]. Peak IV is assigned to the reduction of Fe2+ to Fe0, which does not happen in the reaction system of POM [41]. Regarding the first two oxides, in contrast to LSFN600 (curve a), LSFN800 (curve b) is predominant by the double perovskite phase, and this results in a shift of peaks I, II and III in (b) to slightly higher temperatures. It is worthy to note that (a) displays a much stronger peak III than peak II, while (b) has a much weaker III than peak II. Compared to LSFN800, a greater amount of NiO in LSFN600 was reducible. As to the cause of this difference, the impurity phases existed in LSFN600 (Fig. 2a), owing to incomplete formation of the designated DPO, played the key role. The SrCO3 phase labeled by the XRD peaks at 2u = 258 and 268, being relatively stagnant and therefore acting to prevent sintering of perovskite phase formed [42], is deemed to retard the reducibility of the DPO (i.e., small peak II) in the present case. While for LSFN800, since the DPO with the designated composition had been formed prior to TPR, it would be therefore quickly degraded to K2NiF4 and the NiO released would become inaccessible. As a result, a small peak III was observed. Moving on to LSFN1000 (curve c), according to its XRD diagram it is an oxide composite due to calcination-driven degradation; it comprises a DPO phase, a K2NiF4 phase and a trace amount of NiO before TPR. Since the K2NiF4 phase was derived from the DPO phase, these two major phases were interpenetrating; in them, NiO was embedded. A broad Ni2+ reduction peak extending to high temperature side was observed. From the area of this peak, it can be estimated that a larger amount of NiO than that in LSFN800 was reducible. Such a broad reduction temperature span can be attributed to a distribution of NiO crystallites in different chemical microenvironments. Finally, regarding LSFN1200 (curve d), it contained a predominant K2NiF4 phase and small amounts of DPO and NiO, as shown in Fig. 2d. Compared to LSFN1000, a noticeable increase in the area of peak III and a sliding down of its temperature range all indicate that a larger number of NiO crystallites were produced and they were readily reducible. However, it is not clear why a small amount of NiO displayed by the XRD of LSFN1200 could give rise to an overriding TPR peak. Presumably, the reducing atmosphere of TPR would facilitate the further release of NiO from the K2NiF4 phase due to the reduction of it to Ni(0) clusters. The TPR investigation exhibits that a larger amount of NiO crystallites was reducible in either LSFN600 or LSFN1200 than in the other two precursors, and this means that more NiO could be reduced in situ of POM in the former two oxides. To clarify the distribution and dimensions of Ni(0) sites formed in LSFN600 and LSFN1200, these two oxides were subjected to reduction in a H2 (5%)–N2 stream of 80 sccm at 600 8C for 1 h and then scrutinized by TEM (Fig. 6). After this treatment, oxide LSFN600 showed metallic Ni(0) clusters with sizes smaller than 5 nm, which were not typical grains in shape and were randomly distributed over the support, whereas oxide LSFN1200 showed embedded Ni(0) nodules having both large (10–20 nm) and small (5 nm) particle sizes. 3.2. Assessment of partial oxidation of methane over the catalysts
Fig. 5. H2-TPR curves of the LSFN precursor series: (a) LSFN600, (b) LSFN800, (c) LSFN1000, and (d) LSFN1200.
The preceding structural characterizations have confirmed that the temperature, at which the oxide mixtures from the Pechini synthesis (Section 2.1) were calcined, affected the structure of the precursors of POM catalyst (i.e., LSFN series), and in turn the reducibility of NiO to Ni(0). For simplicity reasons, the resulting catalysts are still labeled by the same LSFN series name. The precursors were activated at the reaction temperature for 25 h in the purging stream consisting of CH4, O2, N2, and He (with molar ratio 2:1:4:38) and the GHSV of 45,780 cm3/(g h) was adopted. A decreasing trend of catalytic activity was found with the increase in calcination temperature of the precursors, namely, a monotonic
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Table 2 Catalytic partial oxidation of methane using LSFN catalysts. Reaction temperature
Catalyst
XCH4 (%)
SCO (%)
H2/CO
850 8C
LSFN600 LSFN800 LSFN1000 LSFN1200
96.5 70.0 61.4 30.2
98.4 68.0 86.0 38.6
2.0 3.0 3.0 3.8
800 8C
LSFN600 LSFN800 LSFN1000 LSFN1200
88.8 66.0 42.6 30.2
95.2 59.0 74.2 36.4
2.3 3.4 3.6 4.2
Fig. 7. Evaluation of the temperature-dependent POM performance data in the system catalyzed by LSFN600.
Fig. 6. TEM images of the H2-reduced samples: (a) LSFN600 and (b) LSFN1200.
decrease in the methane conversion XCH4 but a rise of the H2/CO molar ratio (Table 2). Additionally, with respect to a single catalyst (LSFN600 catalyst), Fig. 7 shows how the XCH4 , CO selectivity SCO and H2/CO ratio vary with reaction temperature. The methane conversion and CO selectivity increases drastically with raising the reaction temperature range from 600 8C to 850 8C, while the H2/CO ratio reduced with the increase in methane conversion. This phenomenon can be attributed to the reforming of carbon deposited on Ni(0) clusters by steam and CO2; with the increase in reaction temperature, more deposited carbon filaments were reformed to produce CO. The carbon deposition is due to the catalytic thermal cracking of CH4 on Ni(0) clusters, which produces H2 as well. This interpretation will be further elaborated in the last section. With the use of the highest POM reaction temperature, i.e., 900 8C, the H2/CO molar ratio slid to about 2 and correspondingly the methane conversion approached to 100%. Furthermore, the stability of catalytic performance represented by XCH4 , SCO and H2/CO ratio has also been examined at 900 8C over
a 100-h reaction period of time. Fig. 8 shows the outcome of the LSFN600 catalyst: a very high methane conversion (>99%), high CO selectivity (>98%) and the desired H2/CO molar ratio (2.0) were maintained through the rest of the evaluation course (75 h) after the activation of catalyst in the first 25 h. In other words, the catalyst did not show any sign of the loss of activity in the end of this arbitrarily selected reaction duration. The catalyst was then examined using FESEM after it was cooled down to room temperature under the flow of feed stream. Non-carbon filaments on its surface could be identified (Fig. 9a). Similarly, for LSFN1200,
Fig. 8. The POM performance profiles of LSFN600 at 900 8C, which consists of two stages: the activation stage (<25 h) and the steady operation stage.
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Fig. 10. The XRD diagrams of the catalysts: (a) LSFN600 and (b) LSFN1200 after POM reaction for 100 h at 900 8C.
course, carbon filaments had already extensively grown on the catalyst before it was discharged from the reactor (Fig. 9c). It is presumed that a combination of the ionic conducting K2NiF4 support and the nano-scaled Ni(0) clusters generate an effective catalytic surface, which allows faster reforming of the deposited coke than that on the benchmarking catalyst under the POM condition. 3.3. Structural attributes of different catalytic behaviors
Fig. 9. The FESEM images of the catalysts: (a) LSFN600, (b) LSFN1200, and (c) HiFUEL after POM reaction for 100 h at 900 8C.
despite having much lower catalytic activity than LSFN600, it also left behind a carbon-free surface morphology after the same testing process (Fig. 9b). In comparison to these two catalysts, a commercial Ni-supported catalyst (HiFUELTM R110 from Alfa Aesar) was used. Although this commercial catalyst displayed no apparent loss of catalytic activity in the end of the same testing
After the LSFN600 catalyst was used to drive POM for more than 70 h at 900 8C, its XRD diagram (Fig. 10) revealed the predominant K2NiF4 phase, indicating that K2NiF4 phase produced from the activation of LSFN, as illustrated in Fig. 3, remained stable throughout the entire reaction process. For LSFN1200, the K2NiF4 phase remained unchanged. Besides the K2NiF4 phase, the NiO phase was also found on the two XRD diagrams (Fig. 10), which came from the oxidation of catalytic Ni(0) clusters after they were exposed to atmosphere. As far as the significantly different catalytic activity between these two catalysts is concerned, which represented the boundary cases of the four catalysts in question, the presence of accessible nano-Ni(0) clusters as well as a small amount of highly dispersed SrCO3 phase in close proximity to Ni(0) catalytic sites, happening only in the LSFN600 catalyst (Figs. 2a and 10a), are deemed to be the key factors to sustain its high activity. The role of SrCO3 is presumed to trim the Lewis basicity of Ni(0) catalytic sites, by referencing the previous work [43]. On the other hand, from the micro-morphology of K2NiF4 (Fig. 1c), we conclude that its BET parameters (Table 1) and the embedded Ni(0) granules (Fig. 6b), the lowest catalytic performance of LSFN1200 might be due to covering of the NiO phase by the compact K2NiF4 matrix. It has been indicated in the above discussion in Fig. 7 that the H2/CO molar ratio decreased with the increase in conversion of methane, XCH4 , caused by raising the reaction temperature. To understand this reliance from the perspective of reaction mechanism, we used the catalysts, LSFN600 and LSFN800, as the model systems (Fig. 11); in these, the H2/CO ratio decreased with the increase in XCH4 through the initial 25 h (e.g., Fig. 8). We found from the following four key reaction steps (Eqs. (1)–(4) a simple formula that describes the experimental H2 =COXCH4 dependency can be concluded, although the detailed POM mechanism over the Ni-based catalyst involves a series of chemical reactions [7,36,44– 48]. CH4 þ 2O2 ! CO2 þ 2H2 O
(1)
CH4 ! Cs þ 2H2
(2)
Cs þ H2 O ! CO þ H2
(3)
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159
Combining Eqs. (5) and (7) results in a simple relation between H2/ CO ratio and XCH4 (Eq. (8)). This relation allows testing if it reflects the experimental trend, viz. XCH4 increases with the increasing of the available Ni(0) sites, while H2/CO ratio decreases with the increase in XCH4 . H2 ð8=3Þb 1 þ XCH4 ¼ CO 1þð4=3Þb XCH4
Fig. 11. The simulation of the calculated XCH4 H2 =CO relationship (Eq. (8)) to the experimental counterpart.
Cs þ CO2 ! 2CO
(4)
The methane combustion (Eq. (1)) takes place primarily on the surface of K2NiF4. The thermal cracking of methane (Eq. (2)) over tiny nickel clusters on the surface is widely accepted as the major carbon formation process in the high reaction temperature region (T > 800 8C). H2 gas is produced from the thermal cracking of methane (Eq. (2)) and the reaction of surface carbon (Cs) with the steam (Eq. (3)) generated from methane combustion. Therefore, on the basis of the stoichiometry of these two reactions (2 + k)-mole H2 gas can be generated from consuming 1 mole Cs, where the value of k is the molar fraction of Cs reformed by steam in Eq. (3) over the total Cs consumed in both Eqs. (3) and (4). From reactions (3) and (4) (2 k) mole CO is to be formed from 1 mole Cs provided that the Cs generation rate equals the Cs removal rate, which has been explicitly supported by the FESEM image of the LSFN600 after POM reaction (Fig. 9a). As a result, the H2/CO molar ratio is then expressed as H2 2 þ k ¼ CO 2 k
(5)
As the activation process is a non-steady state process, its ending point can thus be set when XCH4 becomes steady, for example, Fig. 8 shows that the steady conversion is close to 100% after activation. According to the stoichiometry relationship, Cs reacted in reaction (3) is double of that reacted in reaction (4) because the molar ratio of CO2 to H2O is 1/2 in Eq. (1). Therefore, taking XCH4 ¼ 100% as the boundary condition, then the k value equals 2/3. This k value depends upon the production of Cs and H2O in reactions (1) and (2) besides the rate of reaction (3), and alternatively these two reactions on the whole represent the conversion XCH4 . In light of the activation process, such a scenario must exist: reaction (1) is almost independent of whether the numbers of K2NiF4 sites are sufficient since they largely exceed the Ni(0) sites most of the time. Therefore the contributions of reaction (2) to XCH4 , with the increase of Ni(0) clusters, and to the mounting of reaction (4), which depresses the k value, follow an approximately linear relation as proposed by Eq. (6), because both steps are directly linked with the concentration of Cs. Therefore the rates of these two elementary steps must be proportional: XCH4 ¼ a þ bk
(6)
In Eq. (6) parameter a can be determined using the boundary condition of XCH4 ¼ 1 and k = 2/3. Thus, Eq. (6) can be rewritten as k¼
2 1 XCH4 3 b
(7)
(8)
Fig. 11 shows that the calculated H2 =COXCH4 curve (b = 0.96) fits well the experimental data for the two catalytic systems. For LSFN800 catalyst, only at 950 8C, its XCH4 H2 =CO could fit the model because its conversion could reach almost a quantitative value in the end of activation. The satisfactory fitting supports the above assumption that the rates of reactions (2) and (4) increase with the number and activity (i.e., temperature effect for an individual catalyst) of Ni(0) sites by following a near linear manner. In other words, for the supported Ni(0) sites (clusters or nodules), owing to either a low reaction temperature (Fig. 7) or steric blockage because of their embedment in the K2NiF4 support (Table 2), the last two reaction steps especially the reforming of the deposited carbon by CO2 must slow down, causing a higher H2/CO molar ratio according to the stoichiometry relations of Eqs. (1)–(4). 4. Conclusions Double perovskite-type (La0.5Sr0.5)2FeNiO6d (LSFN) solid solution was designed as the precursor of the K2NiF4-supported Ni(0), which catalyzes the POM to produce syngas. To synthesize LSFN, a mixture of four metal oxides with the required stoichiometry was prepared by the wet chemistry method and then calcined at a fixed temperature in the range of 600–1200 8C. The phase composition of the precursor obtained from this process is affected by the calcination temperature used. Double perovskite-type LSFN can be completely formed and retained only in the temperature range roughly from 800 8C to 1000 8C since the thermal degradation of LSFN to K2NiF4 becomes progressively more severe with the increasing of temperature from the upper bound, and a small amount of NiO is released in the same course. Four precursors were prepared by the use of four different calcination temperatures, and subsequently they were converted to the corresponding catalysts in the POM reactor, where LSFN underwent reductive degradation to K2NiF4 and a small amount of B-site Ni3+ ions was reduced to Ni(0). Of the four catalysts, the best catalyst came from the precursor bound to the lowest calcination temperature. It demonstrated a high methane conversion (XCH4 > 99%), high syngas selectivity (SCO > 98%) and nil coke formation over a100h test. The main structural attribute for this catalyst lies in two aspects: firstly, the minor NiO and SrCO3 phases that were left in the formation of LSFN and in close proximity to each other, the former being transformed to nano-Ni(0) clusters after the activation; secondly, the O2 conducting K2NiF4 support. For the other three catalysts, although all were based on K2NiF4, the lower POM catalytic activity can be attributed to a low concentration Ni(0) clusters or heavy coverage of them by the support. Both TEM and catalytic activity assessment suggest that the NiO phase is severely embedded if it is produced from the thermal degradation of LSFN to K2NiF4, and similarly Ni(0) clusters generated from the reductive degradation of LSFN to K2NiF4 in the course of the activation were not prevalent according to the TPR investigation. Furthermore, to understand how the Ni(0) clusters collaborate with K2NiF4 to catalyse POM, the four elementary steps were selected from the known methane reforming reactions and were used to simulate the decreasing trend of H2/CO molar ratio, as happened in the activation process. The satisfactory simulation result suggests that CO2 and steam formed from the combustion of
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