La2O3 Catalyst

CO2 Reforming of Methane over Ni0/La2O3 Catalyst Without Reduction Step: Effect of Calcination Atmosphere Eun-hyeok Yang & Dong Ju Moon

Topics in Catalysis ISSN 1022-5528 Top Catal DOI 10.1007/s11244-017-0779-z

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Author's personal copy Top Catal DOI 10.1007/s11244-017-0779-z

ORIGINAL PAPER

CO2 Reforming of Methane over Ni0/La2O3 Catalyst Without Reduction Step: Effect of Calcination Atmosphere Eun‑hyeok Yang1,2 · Dong Ju Moon1,2

© Springer Science+Business Media New York 2017

Abstract La-Ni precursor prepared by EDTA-cellulose method was calcined under different atmosphere (Air or Ar), and the catalysts were characterized by various techniques. In this study, the possibility of reduction free catalyst for dry reforming of methane was investigated as well. It was observed that LaNiO3 perovskite structure was formed under the calcination of Air atmosphere, while N i0/ La2O3-C structure was obtained under the calcination of Ar atmosphere due to the reducing and the oxidizing agents generated by the decomposition of organic species under inert atmosphere. It was found that even if LaNiO3-Ar had much larger size of nickel particle than LaNiO3-Air, the remained carbon species derived positive effect: the interfacial area among carbon, L a2O3 and N i0 could lead to synergetic sites such as basic sites, which enhanced resistance to carbon deposition. Furthermore, the higher CH4 activation energy and basicity of LaNiO3-Ar catalyst might ascribe to equilibrium between C H4 decomposition and C O2 gasification rates. Thus, it is suggested that remained carbon species in Ar calcined catalyst did not negatively affect the catalytic activity, but it affected stability positively.

Electronic supplementary material The online version of this article (doi:10.1007/s11244-017-0779-z) contains supplementary material, which is available to authorized users. * Dong Ju Moon [email protected] 1

Clean Energy Research Center, Korea Institute of Science and Technology, 10, Hwarang‑ro 14‑gil, Seongbuk‑gu, Seoul, South Korea

2

Clean Energy & Chemical Engineering, University of Science and Technology, 217, Gajeong‑ro, Yuseong‑gu, Daejeon, South Korea

Keywords Ar calcination · EDTA · Metallic nickel · Dry reforming · Reduction free · Carbon

1 Introduction Dry reforming of methane has attracted considerable scientific interest in the past years, since it has the possibility of simultaneous removal of two inexpensive and abundant carbon containing greenhouse gases. In addition, it could decrease operating cost amongst the commercially operating methane reforming processes such as steam reforming, partial oxidation and auto-thermal reforming because it does not require water supply or air separation unit for oxygen [1]. Dry reforming produces low synthesis gas ratio around 1 by Eq. (1), which can be used as a feedstock for producing synthetic fuels or chemicals such as dimethyl ether, GTL (gas to liquid), higher alcohols and light olefins.

CH4 + CO2 ↔ 2H2 + 2CO, ΔH◦298 = 247 kJ mol−1

(1)

However, the carbon deposition during the reaction is a main drawback. Thus, it is of main importance to develop catalysts for dry reforming of methane with high tolerance against carbon deposition. Although noble metals (Rh, Ru, Pd, Pt and Ir) are promising active metals for dry reforming in terms of catalytic activity and resistance to carbon formation, noble metals are not suitable for industrial applications owing to its high cost and limited availability [2]. Thus, nickel is mainly used as an active metal for reforming reactions due to its low cost, high activity, and relative opulence. However, nickel induces carbon formation easily, leading to catalytic deactivation during reactions [3]. To solve this problem, enhanced metal dispersion on the supports can be derived

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by reduction of perovskite type oxides [4–10]. Moreover it is possible to substitute A or B site in perovskite by small amount of promoters since perovskite oxides have various valency, stoichiometry and vacancy properties [11]. However, all researches mentioned above included reduction step to vitalize active sites. Therefore, some studies about obtaining reduced metals during calcination step were carried out. Y. Jiang et al. reported that metal Co, Ni, Cu, Ag, Bi, and Co–Ni alloy were obtained by auto-combustion method with citric acid under inert gas calcination [12]. It is also reported that reduced Cu0/ZnO and C o0/SiO2 catalysts were obtained by sol-gel auto-combustion method under the calcination of Ar atmosphere, and C u0/ZnO was used for methanol syn0 thesis and Co /SiO2 was used for Fischer-Tropsch synthesis [13–15]. The author insisted that the prepared catalysts represented good catalytic performance without reduction step because active metals were already existed as reduced form. In the previous studies, we prepared catalysts using EDTA [9, 16], and EDTA generates metal-EDTA complex like citric acid. Thus, reduced nickel could be obtained by calcination of EDTA-metal complex under Ar atmosphere. In reforming process, the catalyst should be reduced with pure or diluted hydrogen prior to reaction step. However, it will be more efficient if reduction step can be omitted in reaction process. Thus, in this study, the feasibility of reduction free catalyst is investigated. Subsquently, LaNiO3 precursor was prepared by EDTA-cellulose method, and the precursor was calcined under Air or Ar atmosphere, respectively. The calcined catalysts were characterized by various analytical techniques to explore any differences in physicochemical properties, and dry reforming of methane for catalytic activity test without reduction step was performed as well.

2 Experimental 2.1 Catalysts Preparation Aqueous solution of lanthanum nitrate hexahydrate (La(NO3)3·6H2O) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) were mixed in equal molar stoichiometric ratio (1:1). After the nitrate precursors were fully dissolved in deionized water, cellulose was added to the solution, and the solution was stirred for 1 h. Subsequently, ethylenediaminetetraacetic acid (EDTA)-ammonia solution was added to the solution, and the pH of the mixture was adjusted to 5 by adding ammonia solution (30 wt% in H 2O). The resulting mixture was aged for 3 h and dried at 110 °C for 12 h. Finally, the catalyst precursor was pre-calcined at 300 °C for 3 h and calcined at 800 °C for 5 h with the heating rate of 5 °C min− 1 under Air or Ar atmosphere, respectively.

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The prepared catalysts were designated as L aNiO3-Air or LaNiO3-Ar. 2.2 Characterization The physical properties of the catalysts were analyzed using a physisorption analyzer (Moonsorp-I, KIST, Korea) under nitrogen adsorption at −196 °C. The BET method was employed to measure the surface area of the catalysts. X-ray diffraction (XRD) measurements were performed on fresh catalysts, with 2θ values between 20°–80° using a XRD-6000 diffractometer (Shimadzu Co., Japan) with graphite-filtered CuKα radiation (λ = 1.5406 Å). Temperature programmed reduction (TPR) experiments were carried out using a temperature programmed analyzer (Autochem 2910, Micromeritics Co., USA). For TPR studies, approximately 30 mg of calcined sample was placed between quartz wool in a U-shape quartz reactor. The samples were heated with the rate of 10 °C min− 1 from room temperature to 900 °C under 10% H2/Ar stream with the flow rate of 50 ml min− 1. The amount of hydrogen consumed was measured by a thermal conductivity detector (TCD). Temperature programmed oxidation (TPO) experiments were carried out using the same procedure of TPR. The spent catalysts were used and 10% O2/Ar gas were used for TPO analysis. Temperature programmed surface reaction (TPSR) experiments were carried out using the same equipment with TPR. Before the measurement, LaNiO3-Air was reduced, while L aNiO3-Ar was used for the experiment without reduction. About 20 mg of the catalyst was charged in a U-shaped quartz tube, and the samples were pretreated in He flow at 300 °C for 1 h, and then were cooled to 60 °C. At the first step, after the pretreatment, CH4 gas was introduced while the temperature increased from 60 °C to 900 °C with the ramping rate of 10 °C min− 1. Subsequently, the gas flow was changed to He, and the temperature decreased to 60 °C. 10% CO2/He gas was used for the second step. And then, the temperature increased from 60 to 900 °C with the ramping rate of 10 °C min− 1. Each temperature increase step was recorded by a thermal conductivity detector. CO2-temperature programmed desorption (TPD) experiments were carried out using the same equipment with TPR. About 30 mg of the reduced catalyst was charged in a U-shape quartz tube, pretreated in He flow at 300 °C for 1 h, and then cooled to ambient temperature. After the pretreatment, 10% CO2/He gas was used as an adsorption gas while the temperature was retained at 50 °C for 2 h and the sample was flushed with He flow at 50 °C for another 30 min. CO2-TPD was carried out in He flow, while increasing the temperature from ambient to 900 °C with the

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ramping rate of 10 °C min− 1. The amount of adsorbed C O2 was measured by a thermal conductivity detector. Transmission electron microscope (TEM) images of the catalysts were characterized using an FEI microscope (Talos F200X, USA) in STEM mode. Each particle was identified by EDX mapping equipped in TEM. Thermogravimetric analysis (TGA) was carried out using SDT Q600 (TA instruments Co, USA) equipment. Approximately 20 mg of each sample was placed in an alumina cup and the temperature was raised from room temperature to 1000 °C under air atmosphere with the heating rate of 5 °C min− 1. 2.3 Catalytic Reactions Catalytic activity tests were carried out under the atmospheric pressure in a fixed-bed down flow reactor. Since this study focus on activity test without reduction step, the reactions were conducted without reduction. The flow rates of the reactants, C H4 and CO2, were controlled by mass flow controller (MFC). Gaseous products were analyzed by online gas chromatograph unit (Agilent HP 7890A, USA) equipped with a carbosphere 60/80 packed column and a thermal conductivity detector. All reactions were carried out under the temperature of 700 °C, pressure of 1 bar, weight hourly space velocity (WHSV) of 20 h− 1 and feed molar ratio (CH4:CO2) of 1:1. Nitrogen was used as an internal standard gas.

catalyst was significantly low because perovskite structure was formed during the calcination step. It was reported that low surface area is derived when the temperature of calcination is high (>800 °C), leading to severe sintering of the solid and large grain size [17]. It was also observed in the TEM images that large grains of perovskite phase were generated after the calcination under Air atmosphere. However, relatively high surface area was obtained for the Ar calcined catalyst. It is considered that, during the decomposition of organic species, the released gases generated porous carbon residue, thus a lot of textural pores were created. Therefore, the Ar calcined catalyst had higher surface area and pore volume. On the contrary, the LaNiO3-Air catalyst indicated high surface area after the reaction due to severe carbon deposition, which led to a lot of textural pores like mentioned previously. For the L aNiO3-Ar catalyst, however, showed almost the same surface area, even if the parameters of pore volume and pore diameter were somewhat changed. It might imply that the structure of LaNiO3-Ar catalyst was changed by slight elimination of carbon or by slight sintering of materials. 3.2 Temperature Programmed Reduction Profiles The reduction behaviors of prepared catalysts are depicted in Fig. 1. It is clearly seen that the L aNiO3-Air catalyst had two reduction peaks at 380 and 520 °C, respectively. There are some literatures about reduction behavior of LaNiO3 perovskite [18–20], that the first peak at 380 °C is reduction of N i3+ 2+ 2+ 0 to Ni , and the second peak is reduction of Ni to Ni , following the reduction steps below:

3 Results and Discussion

2LaNiO3 + H2 → La2 Ni2 O5 + H2 O

3.1 Physical Properties of Catalysts

La2 Ni2 O5 + 2H2 → 2Ni + La2 O3 + 2H2 O

Table 1 shows the physical properties of the fresh and the spent catalysts. The surface area (S. A.) of the Air calcined

(2)

(3) However, the LaNiO3-Ar catalyst displays totally different reduction behavior; it shows one small reduction peak

Table 1 Physical properties and crystalline size of the catalysts LaNiO3

Calcination

Reduction

Tempera- Atmosphere ture (°C) Before reaction After reaction

800 800 − −

Air Ar Air Ar

− − × ×

Surface area (m2 g−1)*

Total pore vol- Mean pore ume (cm3 g− 1)* diameter (nm)*

2.6 66.7 101.5 69.6

0.017 0.077 0.284 0.139

26.0 4.6 11.2 7.9

Crystalline size (nm)**

CO2 adsorption (mmol g− 1)

Perovskitea

Nib

1st peak

2nd peak

14.4 -

30.6 17.4 29.8

29.03c 34.85

14.64c 46.72

*Measured by N2 physisorption analyzer (Moonsorp-I)

**Measured by Scherrer equation with the peak a

32.8°

b c

44.5°

Measured after reduction

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Fig. 1 Temperature programmed reduction profiles of fresh catalysts

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well formed. In contrast, the Ar calcined catalyst presented metallic nickel (JCPDS No. 870712) and L a2O3 (JCPDS No. 831344). After the Ar calcination, carbon species was remained, however, the carbon (JCPDS No. 75-1621) related peak at 26.2° is overlapped with the peak related to La2O3. However, the carbon peak at 26.2° is normally originated from whiskers carbon, and the remained carbon is considered as amorphous or polymeric carbon species. Thus, the peak at 26.2° for fresh LaNiO3-Ar might be ascribed to La2O3 phase. After the dry reforming reaction of LaNiO3-Air, LaNiO3 phase was decomposed and metallic nickel and La2O3 phases were newly formed and the strong carbon related peak was observed at 26.2°. It means that C H4 was decomposed on the surface defect, vacancies and low-coordination sites [21], releasing solid carbon and H 2 gas. Thus, the emissive hydrogen gas reduced L aNiO3 into Ni and La2O3; however generated carbon from the CH4 decomposition was rapidly accumulated on the surface of the catalyst as carbon nanotubes (CNTs), resulting in catalyst deactivation. It is also considered that deposited carbon on the catalyst covered La2O3 surface which could react with CO2 to form L a2O2CO3 (JCPDS No. 84-1963), therefore La2O2CO3 phase was not detected in LaNiO3-Air after the reaction. However, LaNiO3-Ar catalyst showed different phase compared with L aNiO3-Air after the reaction. The main phases were La2O2CO3 and metallic Ni because La2O3 reacted with CO2 to form L a2O2CO3 as follows.

La2 O3 + CO2 → La2 O2 CO3

(4)

(5) La2O2CO3 could act as carbon eliminator by Eq. (5) [9, 22], and this is one of the reasons that carbon was not detected in XRD patterns, meaning that additional carbon source such as filamentous or graphite carbons were not deposited. Carbon deposition in both catalysts was also confirmed by TEM analysis.

La2 O2 CO3 + C → La2 O3 + 2CO

Fig. 2 X-ray diffraction patterns of fresh and spent catalysts (filled diamond LaNiO3open diamond La2O3inverted open triangle Ni filled star carbon filled circle La2O2CO3)

at 660 °C. This reduction peak was identified by the mass analysis (inset image in Fig. 1) that C H4 was detected, thus it can be said that remained carbon is eliminated partially by hydrogenation. Besides, H 2O peak was not observed because nickel was already existed as reduced species. 3.3 X‑ray Diffraction Patterns XRD patterns after the calcination and the reaction are shown in Fig. 2. Each peak was identified by JCPDS cards. In the case of Air calcined catalyst, perovskite structure with rhombohedral symmetry (JCPDS No. 34-1181) was

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3.4 Dry Reforming of Methane without Reduction Dry reforming of methane over the prepared catalysts was conducted without reduction step, and the results are shown in Fig. 3 and Table 2. In the dry reforming of methane reaction, CO2 conversion is higher than C H4 conversion due to reverse water gas shift reaction (H2 + CO2 → CO + H2O) [23], and it was observed that generated water was caught by the water trap. The LaNiO3-Air catalyst exhibited catalytic activity without reduction step; however C O2 conversion gradually decreased as time went by, while C H4 conversion was relatively constant. It is considered that CH4 was decomposed on the surface defects of the catalyst, and


LaNiO3-Air could be evidence that active sites (basic site) for CO2 were blocked by carbon deposition. In the case of CH4 conversion, the activity of C H4 lasted until the reaction was stopped. Because CNTs generated by diffusion of C through Ni-crystal and filamentous growth with Ni on top of CNTs do not cause deactivation of Ni active sites, whereas it breakdown catalyst and increases ΔP [24, 25]. The rapidly deposited carbon blocked the reactor, leading to increase in the reactor pressure up to 20 bar. Therefore, the reaction was stopped before it reached 50 h of reaction time. On the other hand, the L aNiO3-Ar showed quite stable activity and stability without reduction step during the reaction period because reduced Ni was already formed during the Ar calcination. We will discuss the reasons of the stable activity of the L aNiO3-Ar later. However, the conversions of CH4 and CO2 are lower than the Air calcined catalyst due to the larger nickel particle size compared with the LaNiO3-Air. To verify the stability of the L aNiO3-Ar catalyst, longterm activity test was carried out (Fig. 3b). It is considered that active sites for CH4 and CO2 were not deactivated because quite stable activity and stability were observed during the 120 h of reaction time. Thus, it can be said that LaNiO3-Ar catalyst could be an option for dry reforming of methane without reduction step. 3.5 Thermogravimetric Analysis Figure 4 depicts TGA profiles of the calcined and the used catalysts. In the case of fresh L aNiO3-Air catalyst, there was no weight change since every organic residue was eliminated during the calcination step under Air atmosphere. However, the fresh L aNiO3-Ar catalyst showed approximately 44% of the weight loss, which was derived from oxidation of carbon species, and this coincided with the previous results (Fig. 1S). As mentioned previous sections, the Air calcined catalyst represented severe carbon formation in the TGA profile. Approximately 83% of the weight change was detected due to the oxidation of deposited CNTs which were mainly formed by C H4 decomposition. However, the weight change in the LaNiO3-Ar after the reaction (50 and 120 h) indicated almost the same weight change as

Fig. 3 Dry reforming of methane over L aNiO3 at 700 °C, 1 bar, WHSV = 20 h− 1, CH4:CO2 = 1:1. a 50 h, b 120 h

hydrogen and solid carbon were produced as mentioned previously. The produced hydrogen reduced L aNiO3 into Ni and La2O3; however carbon species from CH4 decomposition, which grew in CNTs, was continuously deposited on the surface of the catalyst. Thus, the deposited carbon covered active sites for CO2 activation, leading to decrease in CO2 conversion. Non-detection of La2O2CO3 phase for the Table 2 Results of dry reforming of methane over LaNiO3 catalysts at 700 °C, 1 bar, WHSV = 20 h−1, CH4:CO2 = 1:1

Catalyst

LaNiO3-Air LaNiO3-Ar

Reaction time (h)

Conversion (%)

Outgas faction (%)

H2/CO ratio

CH4

CO2

H2

CO

CH4

CO2

50 50 120

57.6 39.8 40.8

65.6 52.3 53.4

33.6 23.8 24.7

40.4 36.2 36.5

14.3 22.3 21.7

11.7 17.7 17.1

0.83 0.66 0.68

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3.6 Transmission Electron Microscope Analysis

Fig. 4 Thermogravimetric analysis of fresh and spent catalysts

the catalyst before the reaction, but both of the catalysts after the reaction showed two steps of weight changes at 360–540 °C and 540–760 °C. The first step was due to the oxidation of carbon which was remained during the calcination, while the second one was ascribed to oxy-carbonate or strongly adsorbed C O2 [26, 27], and it was confirmed by the TG and mass analysis of the used LaNiO3-Ar catalyst (Fig. 2S).

Transmission electron microscope with EDS mapping analysis was introduced to investigate the morphologies of the fresh and the spent catalysts (Fig. 5). It is observed that approximately hundreds nm of bulk phases were stacked each other for the fresh L aNiO3-Air catalyst (Fig. 5a). The crystalline size of L aNiO3, calculated by Scherrer equation was 14.4 nm (Table 1) because particles of L aNiO3 consisted of several crystallites [28]. The EDX mapping images of La, Ni and O are exactly the same with one another, thus it can be said that LaNiO3 type perovskite was well established under the calcination of Air atmosphere. On the contrary, the Ar calcined catalyst showed totally different morphology (Fig. 5b). La and O have almost the same mapping forms, considered as L a2O3 phase, while Ni particles were isolated, referred to metallic Ni, which was existed as quite large particles (up to 200 nm), and it looks like remained carbon species was widely dispersed through the sample. For the LaNiO3-Air catalysts after the reaction without the reduction, the TEM and EDX mapping images are represented in Fig. 5c. Even if the reduction step was not conducted, LaNiO3 was decomposed and became metallic Ni and La2O3 because of the hydrogen produced by CH4 decomposition as mentioned in XRD section. However, large amount of the CNTs were generated, and some of metallic nickel particles were lifted up on top of the CNTs (Fig. 3S). The carbon atoms from hydrocarbons would

Fig. 5 Transmission electron microscope with EDS mapping images of fresh and spent catalysts

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migrate through the metal particles, and the carbon atoms finally precipitate as tubular structures at the rear of nickel particles [29]. In the case of LaNiO3-Ar after the reaction without reduction (Fig. 5d), the morphology was almost similar to the catalysts before the reaction, and the mapping images of La and O could refer to L a2O2CO3 since La2O2CO3 phase was mainly detected in XRD patterns. The nickel particles size was still quite large (up to 200 nm) like the particles before the reaction, and no filamentous type carbon was observed in TEM images. 3.7 Discussions In the dry reforming reaction without reduction step, Ar calcined catalyst showed quite stable activity even if the conversions were somewhat lower than the LaNiO3-Air catalyst. The large Ni particles obtained under the calcination of Ar atmosphere might be the reason of lower CH4 and CO2 conversions for the dry reforming. It is considered that the remained carbon species during the calcination in Ar atmosphere could decrease the interaction between La2O3 and metallic Ni, thus the Ni particles were sintered and became larger than normal size. Generally it is reported that large nickel particle favors carbon formation than smaller size of nickel [30–32]. In this case, however, the LaNiO3-Ar catalyst showed good resistance to carbon formation under the tested conditions in spite of the large nickel particles. To make sure the fact that the LaNiO3-Ar had good resistance to carbon formation, additional dry reforming of methane was carried out with LaNiO3-Air with reduction step (Figs. 5S, 6S). It was observed that LaNiO3-Air showed better catalytic activity and lower rate of carbon formation (Table 2S) compared with the reaction without reduction step over L aNiO3-Air, and represented better conversions than L aNiO3-Ar. However, it still indicated gradual deactivation of conversions and severe carbon deposition after the reaction. Thus, it is considered that carbon residue from Ar calcination has some roles to inhibit carbon formation during dry reforming. It is reported that C O2 is not only activated by basic catalytic sites but also activated by the interfacial sites of the active metals and supports [33–35]. Therefore, it is surmised that finely dispersed carbon residue through the catalyst may fabricate interfacial active sites between Ni, La2O3 and C for C O2 activation. Thus, the C O2-TPD was carried out to investigate basic site of each catalyst (Fig. 6; Table 1). The L aNiO3-Air after reduction showed basic sites at 330 and 710 °C, respectively. Therefore, the catalyst after the reduction showed better stability than the catalyst before the reduction, but it still showed large amount of carbon formation after 50 h reaction time under the same reaction conditions. On the contrary, the LaNiO3-Ar showed basic sites at 190 and 520 °C, respectively. The difference in

Fig. 6 CO2-Temperature programmed desorption of the catalysts

peak shapes between L aNiO3-Ar and LaNiO3-Air could be the evidence that active site for CO2 were newly generated after the calcination under Ar atmosphere. Furthermore, the total amount of CO2 adsorption for the LaNiO3-Ar was larger than that of the LaNiO3-Air, implying that the LaNiO3-Ar have more basic sites for C O2 adsorption. It is reported that the basic sites could enhance the ability of CO2 adsorption, supplying the surface oxygen to prevent carbon formation [36, 37]. For further studies, TPSR experiments were carried out (Fig. 7) to investigate ability of C H4 decomposition and carbon elimination by carbon gasification by CO2. During the CH4-TPSR, CH4 was decomposed as the temperature increased, thus carbon was deposited on the surface of the catalysts and hydrogen was produced simultaneously. Even if the initiation temperature of C H4 decomposition was 430 °C for both catalysts, the peak intensity started increasing at 450 °C for LaNiO3-Air catalyst, while the peak intensity began to increase at 600 °C for LaNiO3-Ar catalyst. This phenomenon could be related to distinction of activation energies of C H4. Thus, the activation energies of CH4 and CO2 were determined by the Arrhenius equation based on power law method (Fig. 7S and Table 3S). It is reported that activation energy of the catalysts increases with increasing particle size [38, 39]. It is shown that C H4 activation energy of L aNiO3-Ar is higher than the activation energy of LaNiO3-Air. Thus, it is speculated that CH4 decomposition for L aNiO3-Ar at higher temperature was mainly caused by higher C H4 activation energy of the LaNiO3-Ar catalyst. Thus, the lower C H4 conversion over LaNiO3-Ar could be attributed to the fact that the large particle size of active metal could decrease reaction rate because the surface area of activated nickel was decreased

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LaNiO3-Ar catalyst. In TPO results (Fig. 8S), it is also shown that L aNiO3-Ar catalysts had more ability of C O2 adsorption.

4 Conclusions

Fig. 7 Temperature programmed surface reactions of the catalysts. a CH4, b CO2

[40, 41]. After C H4-TPSR, CO2 was introduced to eliminate deposited carbon by gasification (Fig. 7b). It is obvious that carbon was removed from around 530 °C for both catalysts. However, the amount of C O2 consumption for LaNiO3-Air LaNiO3-Ar catalyst was larger than that of because of remained carbon residue during the calcination step in LaNiO3-Ar; however it is clearly seen that CO2 was activated at almost the same point of L aNiO3-Air catalyst. Thus, it can be said that the rate of carbon gasification by C O2 was almost the same in both L aNiO3-Air and LaNiO3-Ar, while the rate of CH4 decomposition over LaNiO3-Air was faster than the rate of CH4 decomposition over L aNiO3-Ar. Therefore, carbon was deposited over LaNiO3-Air during the reaction period; on the contrary carbon was not formed over L aNiO3-Ar because the rates of carbon deposition and gasification were equilibrated over

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La-Ni precursor prepared by EDTA-cellulose method was calcined under different atmosphere (Air or Ar), and the catalysts were characterized by various techniques. Subsequently, the possibility of reduction free dry reforming of methane was investigated under the tested conditions. During the calcination of La-Ni precursor under different atmosphere (Ar or Air), it was observed that LaNiO3 perovskite structure was formed under Air atmosphere, while Ni0/La2O3-C structure was generated under Ar atmosphere because C H4 and C O2 from the decomposition of organic species acted as reducing and oxidizing agents under Ar atmosphere, thus Ni was reduced and La was oxidized. Therefore, it showed stable activity without reduction step for dry reforming of methane. However, the calcination under Ar atmosphere derived large nickel particles and remained carbon species on the surface of the catalyst. Even if the L aNiO3-Ar catalyst had much larger nickel particle size than the LaNiO3-Air catalyst, the remained carbon species caused a positive effect in terms of catalytic stability. It was found that interfacial area among carbon, La2O3 and Ni0 could lead to synergetic sites such as basic sites, which enhanced the resistance to carbon deposition. Therefore, it is concluded that the lower reaction rate of CH4 caused by large nickel particles and better basicity derived by interfacial sites among carbon, La2O3 and Ni0 led to long term stability of the L aNiO3-Ar catalyst for dry reforming of methane. Acknowledgements This work was supported and funded by Korea Institute of Science and Technology (Project No. 2E26570) and supported and funded by Ministry of Trade, Industry and Energy Republic of Korea (Project No. 20142010102790).

References 1. Ross JRH (2005) Natural gas reforming and CO2 mitigation. Catal Today 100:151–158 2. Usman M, Wan DWMA, Abbas HF (2015) Dry reforming of methane: Influence of process parameters-A review. Renew Sust Energy Rev 45:710–744 3. Lee HC, Siew KW, Khan MR, Chin SY, Gimbun J, Cheng CK (2014) Catalytic performance of cement clinker supported nickel catalyst in glycerol dry reforming. J Energy Chem 23:645–656 4. Gallego GS, Marín JG, Batiot-Dupeyrat C, Barrault J, Mondragón F (2009) Influence of Pr and Ce in dry methane reforming catalysts produced from L a1–xAxNiO3–δ perovskites. Appl Catal A Gen 369:97–103

Author's personal copy Top Catal 5. Wang N, Yu X, Wang Y, Chu W, Liu M (2013) A comparison study on methane dry reforming with carbon dioxide over LaNiO3 perovskite catalysts supported on mesoporous SBA-15, MCM-41 and silica carrier. Catal Today 212:98–107 6. Zhu Q, Cheng H, Zou X, Lu X, Xu Q, Zhou Z (2015) Synthesis, characterization, and catalytic performance of La0.6Sr0.4NixCo1–xO3 perovskite catalysts in dry reforming of coke oven gas. Chin J Catal 36:915–924 7. Touahra F, Rabahi A, Chebout R, Boudjemaa A, Lerari D, Sehailia M, Halliche D, Bachari K (2016) Enhanced catalytic behaviour of surface dispersed nickel on L aCuO3 perovskite in the production of syngas: an expedient approach to carbon resistance during CO2 reforming of methane. Int J Hydrog Energy 41:2477–2486 8. Zheng XG, Tan SY, Dong LC, Li SB, Chen HM, Wei SA (2015) Experimental and kinetic investigation of the plasma catalytic dry reforming of methane over perovskite LaNiO3 nanoparticles. Fuel Process Technol 137:250–258 9. Yang EH, Noh YS, Ramesh S, Lim SS, Moon DJ (2015) The effect of promoters in L a0.9M0.1Ni0.5Fe0.5O3 (M = Sr, Ca) perovskite catalysts on dry reforming of methane. Fuel Process Technol 134:404–413 10. Moradi GR, Rahmanzadeh M, Khosravian F (2014) The effects of partial substitution of Ni by Zn in LaNiO3 perovskite catalyst for methane dry reforming. J CO2 Util 6:7–11 11. Tanaka H, Misono M (2001) Advances in designing perovskite catalysts. Curr Opin Soild Solid State Mater 5:381–387. 12. Jiang Y, Yang S, Hua Z, Huang H (2009) Sol-gel autocombustion synthesis of metals and metal alloys. Angew Chem Int Ed 48:8529–8531 13. Shi L, Tao K, Yang R, Meng F, Xing C, Tsubaki N (2011) Study on the preparation of Cu/ZnO catalyst by sol-gel auto-combustion method and its application for low-temperature methanol synthesis. Appl Catal A Gen 401:46–55 14. Shi L, Yang RQ, Tao K, Yoneyama Y, Tan YS, Tsubaki N (2012) Surface impregnation combustion method to prepare nanostructured metallic catalysts without further reduction: as-burnt CuZnO/SiO2 catalyst for low-temperature methanol synthesis. Catal Today 185:54–60 15. Shi L, Tao K, Kawabata T, Jun ZX, Shimamura T (2011) Impregnation combustion method to prepare nanostructured metallic catalysts without further reduction: As-burnt Co/SiO2 catalysts for fischer–tropsch synthesis. ASC Catal 1:1225–1233 16. Yang EH, Kim NY, Noh YS, Lim SS, Jung JS, Lee JS, Hong GH, Moon DJ (2015) Steam C O2 reforming of methane over La1–xCexNiO3 perovskite catalysts. Int J Hydrogen Energy 40:11831–11839 17. Valderrama G, Goldwasser MR, De Navarro CU, Tatibouët JM, Barrault J, Batiot-Dupeyrat C, Martínez F (2005) Dry reforming of methane over Ni perovskite type oxides. Catal Today 107–108:785–791 18. Lima SM, Assaf JM, Peña MA, Fierro JLG (2006) Structural features of La1–xCexNiO3 mixed oxides and performance for the dry reforming of methane. Appl Catal A Gen 311:94–104 19. Kuras M, Roucou R, Petit C (2007) Studies of L aNiO3 used as a precursor for catalytic carbon nanotubes growth. J Mol Catal A Chem 265:209–217 20. Valderrama G, Kiennemann A, Goldwasser MR (2008) Dry reforming of CH4 over solid solutions of LaNi1–xCoxO3. Catal Today 133–135:142–148 21. Muradov N, Smith F, T-Raissi A (2005) Catalytic activity of carbons for methane decomposition reaction. Catal Today 102–103:225–233 22. Sutthiumporn K, Maneerung T, Kathiraser Y, Kawi S (2012) CO2 dry-reforming of methane over La0.8Sr 0.2Ni0.8M0.2O3 perovskite (M = Bi, Co, Cr, Cu, Fe): Roles of lattice oxygen on

C-H activation and carbon suppression. Int J Hydrog Energy 37:11195–11207 23. Xu X, Jiang E, Wang M, Xu Y (2015) Dry and steam reforming of biomass pyrolysis gas for rich hydrogen gas. Biomass Bioenergy 78:6–16 24. De Llobet S, Pinilla JL, Lazaro MJ, Moliner R, Suelves I (2013) CH4 and CO2 partial pressures influence and deactivation study on the catalytic decomposition of biogas over a Ni catalyst. Fuel 111:778–783 25. Bartholomew CH (2001) Mechanisms of catalyst deactivation. Appl Catal A Gen 212:17–60 26. Singha RK, Yadav A, Agrawal A, Shukla A, Adak S, Sasaki T, Bal R (2016) Synthesis of highly coke resistant Ni nanoparticles supported MgO/ZnO catalyst for reforming of methane with carbon dioxide. Appl Catal B 191:165–178 27. Barros BS, Melo DMA, Libs S, Kiennemann A (2010) CO2 reforming of methane over La2NiO4/α-Al2O3 prepared by microwave assisted self-combustion method. Appl Catal A Gen 378:69–75 28. Wang H, Zhu Y, Liu P, Yao W (2003) Preparation of nanosized perovskite LaNiO3 powder via amorphous heteronuclear complex precursor. J Mater Sci 38:1939–1943 29. Shaijumon MM, Bejoy N, Ramaprabhu S (2005) Catalytic growth of carbon nanotubes over Ni/Cr hydrotalcite-type anionic clay and their hydrogen storage properties. Appl Surf Sci 242:192–198 30. Jiang Z, Liao X, Zhao Y (2013) Comparative study of the dry reforming of methane on fluidised aerogel and xerogel Ni/Al2O3 catalysts. Appl Petrochem Res 3:91–99 31. Rostrup-Nielsen J, Bak Hansen J-H (1993) CO2-reforming of methane over transition metals. J Catal 144:38–49 32. Kim J-H, Suh DJ, Park T-J, Kim K-L (2000) Effect of metal particle size on coking during CO2 reforming of C H4 over Ni–alumina aerogel catalysts. Appl Catal A Gen 197:191–200 33. Bitter JH, Seshan K, Lercher JA (2000) On the contribution of X-ray absorption spectroscopy to explore structure and activity relations of Pt / Z rO2 catalysts for C O2/CH4 reforming. Top Catal 10:295–305 34. Bitter JH, Seshan K, Lercher JA (1998) Mono and bifunctional pathways of CO2/CH4 reforming over Pt and Rh based catalysts. J Catal 176:93–101 35. Guo J, Lou H, Mo L, Zheng X (2009) The reactivity of surface active carbonaceous species with C O2 and its role on hydrocarbon conversion reactions. J Mol Catal A Chem 316:1–7 36. Koo KY, Roh HS, Seo YT, Seo DJ, Yoon WL, Park SB (2008) Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process. Appl Catal A Gen 340:183–190 37. Li X, Wu M, Lai Z, He F (2005) Studies on nickel-based catalysts for carbon dioxide reforming of methane. Appl Catal A Gen 290:81–86 38. Fu Q-S, Xue Y-Q, Cui Z-X, Wang M-F (2014) J Nanomater 2014:1–8 39. Zhou L, Rai A, Piekiel N, Ma XF, Zachariah MR (2008) Study on the Size-Dependent Oxidation Reaction Kinetics of Nanosized Zinc Sulfide. AIChE Annual Meeting-Nano Energetic materials session 40. Da Silva ALM, Den Breejen JP, Mattos LV, Bitter JH, De Jong KP, Noronha FB (2014) Cobalt particle size effects on catalytic performance for ethanol steam reforming—smaller is better. J Catal 318:67–74 41. Bhavani AG, Kim WY, Lee JW, Lee JS (2015) Influence of metal particle size on oxidative C O2 reforming of methane over supported nickel catalysts: effects of second-metal addition. ChemCatChem 7:1445–1452

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