Role of A (A = Ca, Mg, Sr) over Hexaaluminates La0.8A0.2NiAl11O19

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Role of A (A = Ca, Mg, Sr) over Hexaaluminates La0.8A0.2NiAl11O19 for Carbon Dioxide Reforming of Methane Jing Li, Dong Wang, Guangdong Zhou,* Yingxue Xue, Chao Li, and Tiexin Cheng* College of Chemistry, Jilin University, Changchun 130023, People’s Republic of China

bS Supporting Information ABSTRACT: A study on hexaaluminates La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) as catalysts for CO2 reforming of CH4 was carried out by means of the XRD, TPR, TGA, XPS, and TEM techniques. It was found that the active Ni dispersion and Ni particle size depend strongly on the ionic radius of the alkaline earth metal (Ca, Mg, Sr). The ionic radius of Ca is less than that of La, and the difference between the ionic radii of Ca and La is lowest. The La0.8Ca0.2NiAl11O19 catalyst exhibited the highest activity compared to those of the other two catalysts, and it also showed a remarkably low carbon deposition. The presence of two types of carbon (deactivating encapsulating carbon and nondeactivating filamentous carbon) was observed by TEM. The filamentous carbon formed more easily over the La0.8Ca0.2NiAl11O19 catalyst.

1. INTRODUCTION Syngas is a crucial intermediary in the chemical industry, as it is used in highly selective syntheses of different types to obtain a wide range of chemical compounds, such as formaldehyde, acetic acid, dimethyl ether, methyl tert-butyl ether, olefins, and liquid hydrocarbons.1,2 Syngas can be produced from almost any source of carbon, such as natural gas and biogas, with natural gas being the predominant raw material.3 Increasing concerns about the world dependence on petroleum oil have generated interest in the more efficient use of natural gas.4 The catalytic process of carbon dioxide reforming of methane into synthesis gas (CH4 + CO2 f 2H2 + 2CO, ΔH298 K = 247 kJ/mol) has attracted many researchers for the chemical utilization of the undesirable greenhouse gases methane (the main component of natural gas) and carbon dioxide. This reaction produces synthesis gas with a H2/CO ratio of about 1, which is adequate for hydroformylation and carbonylation reactions, as well as for both methanol and Fischer Tropsch syntheses.5,6 Noble metals (Rh, Ru, Pt, Pd, etc.) have been employed successfully as highly active catalysts for CO2 reforming of CH4 to synthesis gas by many researchers.7 13 Although these noble metals tend to resist severe carbon deposition, they have not been developed commercially for economic reasons. Serious efforts have been made to improve and develop non-noble metal catalysts that can prevent coke formation. In particular, the investigation of supported catalysts consisting of Ni metal and promoters can lead to catalysts that have high catalytic activity but do not suffer from coke deactivation. For this reason, many research efforts have been focused on the improvement of their stability toward sintering and resistance to the formation of carbonaceous deposits.14 20 Suitable supports have to be resistant to the high temperatures applied and to maintain the metal dispersion of the catalyst during operation. At the same time, a series of Ni-based hexaaluminate oxides have been studied for the CO2 reforming of CH4. Xu reported that ANiAl11O19 δ (A = Ca, Sr, Ba, and La) hexaaluminates catalysts exhibited significant catalytic activity and stability at 780 °C.21 Liu and Zhen reported r 2011 American Chemical Society

that hexaaluminate La0.8Pr0.2NiAl11O19 showed long-term stability (300 h) and good resistance to carbon deposition.22 Then, Zhang et al. reported that the addition of A = Pr or Ce promoted the activities of LaxA1 xNiAl11O19 catalysts, and they found that the interaction of Ni particles and hexaaluminate crystalline structure can stabilize small Ni crystallites and increase catalyst lifetime by decreasing carbon deposition.23 In the present work, the Ni-based hexaaluminate La0.8A0.2NiAl11O19 catalysts (A = Ca, Mg, Sr) were studied, with the aim of evaluating the effects of the substitution of the A ions (A = Ca, Mg, Sr) for some of the La ions in La0.8A0.2NiAl11O19 on the structure and catalytic properties, especially their resistance to coke formation. The effects of these modifiers on hexaaluminate catalysts were also characterized by XRD, TPR, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) to evaluate several important factors for the catalytic properties of these catalysts.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. A series of hexaaluminate catalysts, La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr), were prepared as follows: Lanthanum nitrates; calcium, magnesium, and strontium nitrates; Ni(NO3)2 3 6H2O; and Al(NO3)3 3 9H2O were dissolved in distilled water at a molar ratio of 0.8:0.2:1:11. Then, the aqueous solution was slowly poured into a hot polyethylene glycol isopropyl alcohol solution under magnetic stirring. The mixture was evaporated to dryness at 80 °C for 12 h in the water bath and then stored in an oven to remove the polyethylene glycol and decompose the nitrates at 150 °C for 5 h. After being ground into a fine powder, the sample was sequentially calcined Received: December 27, 2010 Accepted: August 29, 2011 Revised: August 4, 2011 Published: August 29, 2011 10955

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Figure 1. Catalytic activity and stability of the hexaaluminates La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) at 750 °C: (a) CH4 conversion, (b) CO2 conversion. CH4/CO2 = 1:1, flow rate = 30 mL min 1, catalyst amount = 0.3 g, GHSV = 9000 h 1.

at 400 °C for 2 h and then calcined at 1250 °C for 5 h in the muffle-type furnace. 2.2. Characterization. The crystal structures of the catalysts before and after reaction were determined by X-ray diffraction (XRD) (Rigaku D/max-2500). XRD experiments were performed using Cu Kα radiation at 200 mA and 50 kV. Diffraction peaks recorded in the 2θ range between 10° and 80° were used to identify the structure of the samples. The reducibility of the catalysts was characterized by temperature-programmed reduction (TPR), for which 0.1 g of catalyst was embedded in a quartz tube with an inner diameter of 4 mm. Before reduction, the sample was purged, and then the reactor was heated from room temperature to 1200 °C at a heating rate of 7 °C min 1 in a 5% H2/Ar gas flow at a rate of 30 mL min 1. The effluent gases were analyzed using a gas chromatograph (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD). Suitable amounts of CuO were reduced under the same experimental conditions to calibrate the method and quantify the hydrogen consumed.24,25 The binding energy and chemical composition of surface elements of the catalysts were measured by X-ray photoelectron spectroscopy (XPS) (V.G. ESCA Mark II) using Al Kα radiation. The measurements were performed at a pass energy of 50 eV and a step size of 0.05 eV. The amount of carbon deposited on the catalysts was determined by oxidation conducted using a thermogravimetric analyzer (Perkin-Elmer TGA7). Nickel dispersion on the reduced catalysts was measured by oxygen chemisorption and the titration of absorbed oxygen with hydrogen (HOT) at 500 °C and pulse of 0.2 mL (5% H2/Ar).26,27 Transmission electron microscopy (TEM) micrographs of used catalysts were obtained on an FEI Tecnai F20 electron microscope operated at 200 kV. Samples were dispersed in aqueous ethanol by ultrasonication and deposited onto a carboncoated copper grid. 2.3. Catalytic Performance Assessment. Catalytic activities were tested in a fixed-bed quartz reactor under atmospheric pressure at 750 °C. A thermocouple was placed in the center of the catalyst bed to monitor the reaction temperature. The reactant mixture consisted of CH4 and CO2 at a molar ratio of 1:1 and a flow rate of 30 mL min 1. About 0.3 g of catalyst was embedded in the reactor with an inner diameter of 8 mm. Before reaction, the catalyst was reduced at 900 °C in a flow of 10% H2/Ar mixture gas for 40 min. The exit gases (reactant/product mixtures) were analyzed with a gas chromatograph (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD) and Porapack-Q and 5A molecular sieve columns.

3. RESULTS AND DISCUSSION 3.1. Catalytic Activity Measurement. The catalytic reaction was carried out over reduced La0.8A0.2NiAl11O19 catalysts at 750 °C and a space velocity of 9000 h 1. The stability of the hexaaluminates La0.8A0.2NiAl11O19 over 10 h is plotted in Figure 1. Both the catalytic activity and the long-term stability increased in the following order: Sr < Mg < Ca. At the same time, La0.8Ca0.2NiAl11O19 was the most active for CH4 and CO2 conversion, and the activity of La0.8Sr0.2NiAl11O19 was quickly lost. For the reduced La0.8Ca0.2NiAl11O19 catalyst, the conversions of CH4 and CO2 remained over 93.5% and 96.6%, respectively. For A = Ca, Mg, and Sr, the amounts of carbon deposition (in 10 h) were 3.6, 5.2, and 7.5 wt %, respectively. Consequently, the La0.8Ca0.2NiAl11O19 catalyst showed the best catalytic activity and the lowest amount of carbon deposition. Therefore, it is concluded that the activity and stability of La0.8A0.2NiAl11O19 are sensitively affected by modifiers Ca, Mg, and Sr and that La0.8Ca0.2NiAl11O19 is the best in this series of catalysts for CO2 reforming of CH4. 3.2. Crystal Structure and Stability of La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr). Figure 2a shows the XRD analyses of the hexaaluminates La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) before reaction. It was found that the crystalline structures of the series of samples were all similar to the hexaaluminate crystalline structure: the characteristic diffraction peaks of La0.8A0.2NiAl11O19, A = Ca, Mg, Sr, were at 35.9°, 33.8°, and 31.9°, respectively. The crystalline structure of the hexaaluminate samples obtained by decomposition of the nitrates and calcination at high temperature was consistent with that of the hexaaluminates synthesized by the metal alkoxide hydrolysis method and the aerogel-derived approach reported previously.28,29 After the TPR experiments at high temperature, the peak positions (2θ) and intensities of the catalysts were still the same as those measured before reaction (Figure 2b), indicating that the crystalline structure of hexaaluminates La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) is extremely stable. A peak of metallic Ni0(111) at 44.5° can be clearly seen in the patterns of the samples after TPR experiment (see Figure 2b), indicating that a large fraction of metallic Ni0 is separated from the hexaaluminate phase to form an individual metallic phase. It is believed that metallic Ni0 is the active component for the reaction. 3.3. Reducibility of the Catalysts. The reducibility of the La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) catalysts was studied by TPR under 5% H2/Ar flow, presented in Figure 3. It can be seen that the three hexaaluminates La0.8A0.2NiAl11O19, A = Ca, Mg, Sr, exhibit almost the same reduction profiles in terms of shape and need extremely high reduction temperatures. At the same time, these La0.8A0.2NiAl11O19 materials display only one H2-consumption peak. 10956

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Figure 2. XRD patterns of hexaaluminates La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr): (a) before reaction, (b) after TPR experiment.

Figure 3. TPR profiles of hexaaluminates of La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr).

This illustrates that the Ni ions inlaid in the hexaaluminate lattices are extremely stable and that there is only oxidation state, namely, Ni2+, in the hexaaluminate lattices.30 As shown in Table 1, the reduction temperature increased in the order Ca < Mg < Sr, and the H2 consumptions followed the sequence La0.8Ca0.2NiAl11O19 > La0.8Mg0.2NiAl11O19 > La0.8Sr0.2NiAl11O19. As a result, the hexaaluminate La0.8Ca0.2NiAl11O19 had the lowest reduction temperature, but its H2 consumption was the highest. Therefore, it can be concluded that the Ni ions in the lattice of both hexaaluminates La0.8Mg0.2NiAl11O19 and La0.8Sr0.2NiAl11O19 are more difficult to reduce than those in the lattice of hexaaluminate La0.8Ca0.2NiAl11O19. The characterization results indicate that there are more Ni2+ ions that can be reduced to metallic Ni0 as the active components in the hexaaluminate La0.8Ca0.2NiAl11O19. 3.4. Oxidation State and Chemical Composition of Surface Elements. Table 2 gives the binding energies of the surface elements of hexaaluminates La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) before reduction by XPS. It was found that the binding energies of surface elements La, Al, and O were not affected by different A ions, that is, their valence states were basically unchanged. Therefore, it can be concluded that their oxidation states are La3+, Al3+, and O2+. Based on the binding energy data, it can also be assured that the oxidation states of the modifier A in the samples are Ca2+, Mg2+, and Sr2+. Figure 4 shows the XPS spectra of Ni on the surface, in which one can see that the position of the Ni 2p3/2 peak remains unchanged. At the same time, all of the binding energies of Ni in the samples can be fitted to one component at about 856 eV, indicating that the valence states of the Ni ions on the surface are the same and are not changed by the modifiers. According to the attribution of Ni 2p3/2 at 856.0 eV, the oxidation state of the Ni ions on the hexaaluminate lattices is attributed to Ni2+,23,31 part of which can be completely reduced to Ni0 under the reduction conditions.

Table 3 reports the chemical compositions of surface elements of the hexaaluminate catalysts La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) before reduction. It is clear that the Ni content is much lower at the surface than in the bulk. This phenomenon provides a clue to the difference between the Ni concentration of the surface and bulk phases of the samples, which, on one hand, can be explained from the viewpoint of surface segregation.30 At the same time, the modification by A in the mirror plane shows different effects on the Ni content in the surface phase. Among the La0.8A0.2NiAl11O19 catalysts, the Ni content increase in the following order Sr < Mg < Ca, a nd the La0.8Ca0.2NiAl11O19 catalyst has the highest Ni content in the surface phase. TPR and XPS are complementary techniques when used to identify the oxidation states of reducible metal oxides. Based on both the XPS and TPR results, it can be confirmed that the oxidation state of the Ni ions in the samples is Ni2+, part of which can be completely reduced to Ni0 under the reduction conditions. In the meantime, it can also be concluded that the modifier Ca can increase the reducibility of Ni and the Ni content in the surface phase. 3.5. Metal Dispersion and Particle Size. Figure 5 shows typical TEM profiles and corresponding histograms of the Ni particle size distribution of the hexaaluminate catalysts La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) after H2 reduction. It is obvious that La0.8Ca0.2NiAl11O19 had the most metal Ni particles, about 60% of which are between 10 and 20 nm (Figure 5a). Highly dispersed active Ni0 species were also detected in La0.8Ca0.2NiAl11O19 catalysts. Figure 5b shows that some slight aggregation and sintering of nickel crystallites occurred in the La0.8Mg0.2NiAl11O19 catalyst; at the same time, a lower density of Ni particles on the support was detected, and the portion of the larger particles increased. It can be clearly seen in Figure 5c that severe aggregation and sintering of Ni crystallites occurred in the La0.8Sr0.2NiAl11O19 catalyst, which had the highest content of large Ni particles. Moreover, the Ni dispersions measured by HOT for reduced La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) were 28.8%, 19.7%, and 12.2%, respectively, and exhibited the following order: La0.8Ca0.2NiAl11O19 > La0.8Mg0.2NiAl11O19 > La0.8Sr0.2NiAl11O19. The Ni dispersion data validate the TEM results for reduced samples. Among the samples, the Ni dispersion is related to the ionic radius of the alkaline earth metal (Ca, Mg, Sr). Table 4 lists the ionic radii of the elements (La, Ca, Mg, and Sr). Every hexaaluminate compound crystallizes in either the β-Al2O3 or magnetoplumbite structure. Both of these structures consist of alternative stacking of a spinel block and a mirror plane. In addition, the structure type of hexaaluminates depends on the charge and radius of the large modification cations in the mirror plane layer,32 and 10957

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Table 1. Reduction Temperatures and H2 Consumptions in TPR Runs for Reduced La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) Catalysts reduction

H2 consumption

catalyst

temperature (°C)

(mmol of H2 gcat 1)

La0.8Ca0.2NiAl11O19

1116

0.62

La0.8Mg0.2NiAl11O19

1122

0.48

La0.8Sr0.2NiAl11O19

1130

0.41

Table 2. XPS Results for Hexaaluminates La0.8A0.2NiAl11O19 before Reaction (eV) A

La 3d5/2

Ca

834.5

Mg

834.8

Sr

834.7

A 2p3/2(3d5/2) 348 50.3 133

Ni 2p3/2

Al 2p3/2

Figure 4. XPS spectra of Ni on the surface of hexaaluminates La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) before reduction.

O 1s

856.0

73.7

530.9

855.9

73.8

531.0

855.7

74.0

530.8

the Ni ions as active components are inlaid in the hexaaluminate lattices to substitute some of the Al ions. The large cation plays an important role in the catalytic activity and thermal stability. In our work, the A ions (A = Ca, Mg, Sr) were found to be substituted for some of the La ions in the La0.8A0.2NiAl11O19 samples. As shown in Table 4, the ionic radius of Sr is much larger than that of La, whereas the ionic radii of Ca and Mg are less than that of La, and the difference between the ionic radii of Ca and La is lowest. That is, for the hexaaluminate La0.8Ca0.2NiAl11O19, the minimum difference in ionic radii between the modifier Ca and La can lead to the highest dispersion of active Ni0 species and smaller metal particles. Although the ionic radius of Mg is less than that of La, the difference between them is greater, and greater differences cause a slight sintering and aggregation of Ni crystallites and an increase in larger metal particles in the La0.8Mg0.2NiAl11O19. Because the ionic radius of Sr is larger than that of La, serious sintering and aggregation of Ni crystallites occur in the hexaaluminate La0.8Sr0.2NiAl11O19, and it contains the highest portion of large Ni particles throughout the sample. According to Jing et al.,33 the aggregation and growth of particles at high temperature lead to a rapid decrease of the dispersion of the active phase, which is one of the main reasons that the supported Ni-based catalyst is easily deactivated. This explains why the activity of hexaaluminate La0.8Ca0.2 NiAl11O19 is better than those of the other two catalysts from another point of view. The number of active centers or sites is usually less than the total number of surface metallic atoms of the catalyst. More small particles mean more edges and more metal metal interactions and, as a result, more active sites formed, and small metal particles are crucial to the suppression of carbon deposition.34,35 That is, the more small metal particles, the more active sites, and the higher the catalytic activity. This result is consonant with the regular pattern of the catalytic activity, as shown in Figure 1. Therefore, it is believed that the ionic radius of the alkaline earth metal (Ca, Mg, Sr) influences the Ni dispersion and the size of the Ni particles in hexaaluminate La0.8A0.2NiAl11O19 catalysts. The Ni dispersion and the size of the Ni particles are two key factors that can affect the catalytic activity and stability of the hexaaluminate La0.8A0.2NiAl11O19 catalysts for this reaction at high temperature. As a result, the addition of Ca can improve the Ni dispersion and reduce the particle size of the metallic Ni of the

Table 3. Chemical Composition of Surface Elements of Hexaaluminates La0.8A0.2NiAl11O19 A

A (%)

La (%)

Ni (%)

Al (%)

O (%)

Ca

0.91

1.31

0.93

43.26

53.58

Mg

0.95

1.05

0.81

44.33

52.85

Sr

0.86

1.22

0.69

43.83

53.40

catalyst. Thus, the modifier Ca is the most suitable modifier for hexaaluminate La0.8A0.2NiAl11O19 catalysts. 3.6. Carbon Accumulation. TEM was used to characterize the surface morphology of the reacted catalysts. Figure 6 shows TEM images of the La0.8Ca0.2NiAl11O19 catalyst (Figure 6a,b) and La0.8A0.2NiAl11O19 (A = Mg, Sr) catalysts (Figure 6c,d) after being subjected to the reforming reaction for 10 h at 750 °C. Chen et al. reported that, in the reforming reaction, two different types of carbonaceous species, one deactivating (encapsulating carbon) and the other nondeactivating (filamentous carbon), are generally found and that the types of carbon deposition depend on the metal particle size and the components of the catalyst.36,37 Filamentous carbon could be readily eliminated by gasification with CO2, whereas encapsulating carbon cannot easily be eliminated by gasification with CO2, and this type of carbon-encapsulated Ni particles would lead to the deactivation of the Ni-based catalyst.22,38,39 Figure 6 shows the carbon deposits formed on the three spent La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) catalysts during the CO2 reforming of CH4. It clearly demonstrates the different types of carbonaceous deposits. Two different types of carbonaceous species, the encapsulating carbon and the filamentous carbon, are observed on the La0.8Ca0.2NiAl11O19 catalyst by TEM, and the majority of the carbon deposition is filamentous carbon. Interestingly, only the La0.8Ca0.2NiAl11O19 showed growth of filamentous carbon with a hollow inner channel (Figure 6a,b). The outside diameter of the nanotubes on the La0.8Ca0.2NiAl11O19 was about 20 30 nm, and the inner diameter was about 10 nm. It has been reported that there is a hollow inner channel inside a lot of thin filamentous carbon material and that the reactive filamentous carbon species can readily be eliminated; hence, catalyst deactivation by blocking of the catalytic active sites by deposited carbon is greatly hindered.37,40 Furthermore, the growth of filamentous carbon was not found on the La0.8A0.2NiAl11O19 (A = Mg, Sr) catalysts (Figure 6c,d), and only the encapsulating carbon was observed on these two samples. The encapsulating carbon forms a shell above the active Ni sites and can directly result in deactivation because the active sites are covered.41 10958

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Figure 5. TEM images of reduced La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) catalysts and Ni particle size distributions: (a) La0.8Ca0.2NiAl11O19, (b) La0.8Mg0.2NiAl11O19, (c) La0.8Sr0.2NiAl11O19.

Table 4. Ionic Radii of Investigated Elements (La, Ca, Mg, and Sr) element

ionic radius (pm)

La

106.1

Ca

99

Mg

65

Sr

113

As a result, the type of carbon species is another key factor that can affect the catalytic activity and stability of the hexaaluminate

La0.8A0.2NiAl11O19 catalysts for the continuous CO2 reforming reaction of CH4. The La0.8Ca0.2NiAl11O19 catalyst showed more stable activity than the other two samples because of the dominant formation of reactive filamentous carbon during the reaction process. Moreover, the particle sizes of the nickel crystallites in the three samples did not change even after reaction for 10 h (see Figure 6). However, some very slight agglomeration of Ni particles can be seen in the La0.8Ca0.2NiAl11O19 catalyst in Figure 6a,b, as well as much more severe agglomeration of Ni particles in the La0.8Mg0.2NiAl11O19 and La0.8Sr0.2NiAl11O19 catalysts (see Figure 6c,d). The La0.8Ca0.2NiAl11O19 catalyst 10959

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Figure 6. TEM images of catalysts La0.8A0.2NiAl11O19 after reaction for 10 h at 750 °C: (a,b) La0.8Ca0.2NiAl11O19, (c) La0.8Mg0.2NiAl11O19, (d) La0.8Sr0.2NiAl11O19.

exhibited a better capability for suppressing Ni agglomeration during the reaction process than the other two samples. Thus, it is suggested that the agglomeration of Ni particles during the reaction process also affects the catalytic activity of La0.8A0.2NiAl11O19 catalysts.

4. CONCLUSIONS A series of La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) catalysts with the same hexaaluminate crystalline structure were prepared and evaluated regarding their catalytic performance for carbon dioxide reforming of methane to synthesis gas. Ni ions as the active component were found to be inlaid in the hexaaluminate lattices to substitute some of the Al ions. The ionic radius of the alkaline earth metal (Ca, Mg, Sr) was found to influence the active Ni dispersion and the Ni particle size of the hexaaluminate La0.8A0.2NiAl11O19 catalysts. The ionic radius of Ca is less than that of La, and the difference between the ionic radii of Ca and La is lowest. Therefore, the addition of Ca can reduce the particle size of the metallic Ni and improve the active Ni dispersion after reduction. The La0.8Ca0.2NiAl11O19 catalyst was found to have the highest

activity among the three catalysts. In the meantime, the catalyst stability was also strongly influenced by deactivation due to carbon deposition and the agglomeration of Ni on the catalyst surface during the reaction process. Two species were identified: encapsulating carbon and filamentous carbon. The formation of encapsulating carbon can directly result in deactivation because the active sites are covered, whereas the filamentous carbon species can be removed by carbon dioxide treatment, regenerating the catalyst. Filamentous carbon tended to occur over the La0.8Ca0.2NiAl11O19 catalyst. At the same time, the La0.8Ca0.2NiAl11O19 catalyst exhibited the best capability for suppressing Ni particle agglomeration during the reaction process. Therefore, Ca is a more valuable modifier than Mg and Sr in hexaaluminate La0.8A0.2NiAl11O19 catalysts for the CO2 reforming reaction of CH4.

’ ASSOCIATED CONTENT

bS

Supporting Information. Specific surface areas of the catalysts La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr). This information is available free of charge via the Internet at http://pubs.acs.org/.

10960

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: +86 431 88499356. E-mail: [email protected] (G.Z.), [email protected] (T.C.).

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dx.doi.org/10.1021/ie201044z |Ind. Eng. Chem. Res. 2011, 50, 10955–10961