Preparation of LaAlO3 perovskite catalysts by simple solid-state method for oxidative coupling of methane

Journal Pre-proof Preparation of LaAlO3 perovskite catalysts by simple solid-state method for oxidative coupling of methane Yujin Sim, Inchan Yang, Dahye Kwon, Jeong-Myeong Ha, Ji Chul Jung

PII:

S0920-5861(19)30603-0

DOI:

https://doi.org/10.1016/j.cattod.2019.10.038

Reference:

CATTOD 12543

To appear in:

Catalysis Today

Received Date:

24 June 2019

Revised Date:

22 October 2019

Accepted Date:

30 October 2019

Please cite this article as: Sim Y, Yang I, Kwon D, Ha J-Myeong, Jung JC, Preparation of LaAlO3 perovskite catalysts by simple solid-state method for oxidative coupling of methane, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.10.038

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Catalysis Today (SI: 17th KOR-JPN Catalysis)

Preparation of LaAlO3 perovskite catalysts by simple solid-state method for oxidative coupling of methane

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea

These authors contributed equally.

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Department of Chemical Engineering, Myongji University, Yongin 17058, Republic of Korea

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Yujin Sima,1, Inchan Yanga,1, Dahye Kwona, Jeong-Myeong Hab, Ji Chul Junga,*

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*Corresponding author E-mail address: [email protected]

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Graphical abstract

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Highlights

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► We attempt to obtain high performance LaAlO3 catalyst using a solid-state method. ► LaAlO3 catalyst was prepared by controlling calcination conditions.

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► Time and temperature affect the crystallinity and homogeneity of the catalyst.

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► Oxygen fraction controls the oxygen species on the catalyst surface.

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Abstract The preparation of LaAlO3 perovskite catalysts for the oxidative coupling of methane (OCM) using a very simple solid-state method with good reproducibility is described. To obtain high performance LaAlO3 catalysts, preparation conditions such as time, temperature, and oxygen content, which have a strong effect on the catalytic activity, are controlled during the calcination process. As the calcination time and temperature increase, the catalytic activity of LaAlO3 improves due to the good homogeneity and high crystallinity of LaAlO3 perovskite. Especially, the LaAlO3 catalyst

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calcined at 1350 °C for 24 h shows the highest selectivity and yield of C2 hydrocarbons. LaAlO3 catalysts calcined at various oxygen fractions show a volcano-shaped curve with respect to oxygen content in calcination gas, and the LaAlO3 catalyst prepared with an oxygen content of 5% exhibits

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the best catalytic performance in this reaction. The appropriate oxygen fraction provides the LaAlO 3

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catalyst with a large amount of electrophilic lattice oxygen species and a sufficient amount of adsorption oxygen species, which are known to play key roles in the OCM reaction. This study clearly

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reveals this simple, highly reproducible, and mass-productive solid-state method as a promising

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preparation method for high performance LaAlO3 perovskite catalysts for OCM.

Keywords: oxidative coupling of methane; LaAlO3 perovskite; solid-state method; crystallinity;

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homogeneity; oxygen species

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1. Introduction Shale gas is drawing increasing attention as the upcoming alternative energy source. Consequently, many research efforts are currently directed toward the efficient conversion of methane (CH4), the main component of shale gas, into high-value compounds such as methanol (CH3OH), ethane (C2H6), ethylene (C2H4), and aromatics [1-5]. The methods for the conversion of CH4 can be divided into direct and indirect conversion methods. In the direct conversion methods, CH4 is directly converted into high-value compounds, whereas CH4 conversion proceeds via synthesis gas (carbon

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monoxide and hydrogen gas) in the indirect conversion methods. The reason for using synthesis gas is that CH4 is thermodynamically stable; however, the manufacturing process of synthesis gas by reforming requires high temperature and a large amount of energy, which renders the indirect

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conversion methods not cost effective [6-9]. Therefore, further progress in the direct conversion

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methods is required, particularly for the development of good catalysts for the direct conversion of CH4 [10-12].

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Among the direct CH4 conversion methods, the oxidative coupling of methane (OCM) to form C2 hydrocarbons such as C2H6 and C2H4 has recently attracted the attention of many researchers [13-

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16] particular, OCM has become one of the hottest issues in heterogeneous catalysis. However, the selective conversion of CH4 into C2 hydrocarbons is a challenge, due not only to the chemical stability

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of CH4 but also to the thermodynamically favorable formation of carbon monoxide (CO) and carbon dioxide (CO2) from CH4 [17-19]. Unfortunately, despite the intense research efforts devoted to the

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development of OCM catalysts with highly efficient CH4 conversion and high C2 hydrocarbon selectivity, there is no commercially available catalyst yet [20-25]. Na2WO4/Mn/SiO2 is currently considered the best catalyst for the OCM reaction [20,24,25]. Nevertheless, there remains a great deal of ambiguity concerning the nature and properties of the active 4

sites owing to its complex composition and structure [26-28]. A number of studies on OCM catalysts have focused on improving the catalytic activity of Na2WO4/Mn/SiO2 using various additives; however, to systematically improve the catalytic activities of the OCM catalysts, more fundamental studies using catalysts with simpler compositions and structures are needed to unveil their essential properties and intrinsic characteristics. In this context, perovskites have received attention as promising catalysts for fundamental studies on the OCM reaction [29,30]. Particularly, we previously found that a LaAlO3 perovskite catalyst showed considerable catalytic activity in the OCM reaction [31]. Therefore, the

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LaAlO3 perovskite catalyst, having a simple structure and high thermal stability, can be considered as a good model catalyst to study the active sites in OCM.

Perovskite can be prepared by various methods such as citrate sol-gel, co-precipitation, and solid-

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state methods [32-34]. Among them, the citrate sol-gel method allows preparing highly homogeneous

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and crystalline perovskite at relatively low calcination temperature [34-37]. However, there are an excessive number of parameters that must be carefully controlled during various preparation steps that

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affect the catalytic activity of the perovskite [38-39]. In addition, the complicated processes of the citrate sol-gel method limit its reproducibility and mass productivity. Meanwhile, perovskites prepared

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by solid-state method are usually not well crystallized, and byproducts are easily generated due to low homogeneity [35,40]. Thus, in our previous study, the LaAlO3 perovskite catalyst prepared by solid-

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state method showed lower OCM catalytic activity than the citrate sol-gel catalyst [41]. In spite of these drawbacks, the solid-state method is normally the method of choice for preparing perovskite

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catalysts because it is very simple, highly reproducible, and suitable for mass production for commercial application [42,43]. Therefore, we were interested in investigating the parameters involved in the solid-state method to gain a better understanding of this process and to enhance the low activity of LaAlO3 perovskite as OCM catalyst.

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In this work, a series of LaAlO3 perovskite catalysts were prepared by controlling preparation conditions such as time, temperature, and oxygen content during the calcination process. The catalysts were applied to the OCM reaction using a fixed-bed reaction and were characterized by using X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and X-ray photoelectron spectroscopy (XPS) analyses. Through this work, the correlation between catalytic activity and various characteristics such

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as crystallinity, homogeneity, and surface oxygen species were also discussed.

2. Materials and methods 2.1. Catalyst preparation

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Lanthanum nitrate (La(NO3)3·6H2O, Sigma-Aldrich), aluminum nitrate (Al(NO3)3·9H2O, SigmaAldrich), and citric acid (C6H8O7, Sigma-Aldrich) were employed as starting materials for the

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preparation of LaAlO3 catalysts. All chemicals were directly used without further purification after

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purchase. In this study, the LaAlO3 catalysts were prepared via two different methods, i.e., solid-state method and citrate sol-gel method. The LaAlO3 catalysts prepared by the solid-state method were

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named LAOS, and those prepared by the citrate sol-gel method were named LAOC. The LAOS catalysts were prepared by controlling time, temperature, and oxygen content during the calcination

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process.

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2.1.1. LAOS catalyst preparation

0.01 mol of lanthanum nitrate and 0.01 mol of aluminum nitrate were thoroughly mixed and

ground in a mortar. Then, the resulting powder was directly calcined under the following three different calcination conditions: (1) The mixed powder was calcined at 950 °C at different calcination times (X) 6

in a muffle furnace. The obtained catalysts were named LAOS_X h (X = 5, 24, and 48); (2) the mixed powder was calcined at different calcination temperatures (Y) for 24 h in a muffle furnace. The obtained catalysts were named LAOS_Y °C (Y = 850, 950, 1050, 1150, 1250, and 1350); (3) the mixed powder was directly calcined at 950 °C for 24 h using a tube furnace under various nitrogen and oxygen fractions. The total flow rate of nitrogen and oxygen gases was fixed at 200 mL min−1. After then, the

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obtained catalysts were named LAOS_Z%, where Z is the oxygen fraction (Z = 0, 5, 21, and 100).

2.1.2. LAOC catalyst preparation

0.01 mol of lanthanum nitrate and 0.04 mol of citric acid were dissolved in deionized water to

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prepare a 75 mL solution (A). Separately, 0.01 mol of aluminum nitrate was dissolved in deionized water to prepare a 25 mL solution (B). The solution B was added dropwise into the solution A under

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vigorous stirring. After stirring the resulting solution at room temperature for 1 h, the gel was obtained by evaporating the water at 80 °C. The obtained gel was dried in a convection oven at 200 °C for one

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day, and the dried gel was ground. Finally, the ground gel was calcined at 950 °C for 5 h in a muffle

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furnace. The obtained catalyst was named LAOC.

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2.2. Catalyst characterization

XRD measurements were performed on an X'pert-Pro PAN analytical diffractometer with Cu Kα

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radiation (λ = 1.54056 Å) for confirmation of the crystal structure of the prepared catalysts and calculation of the crystallite size. The XRD pattern was recorded within a 2θ range of 10°–90°. To confirm the specific surface area of the prepared catalysts, nitrogen adsorption−desorption isotherms were determined using a constant-volume adsorption apparatus (TriStar II 3020, Micromeritics) at 7

−196 °C. The specific surface area was determined using the BET equation [44]. XPS analysis was also performed using a K-alpha instrument (Thermo Fisher) equipped with a monochromatic Al Kα X-ray source to determine the O 1s binding energies of the catalysts, which were calibrated using the C 1s peak at 284.5 eV.

2.3. Catalytic reaction

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The OCM reaction was conducted in a continuous flow fixed-bed reactor that was made of quartz with an inner diameter of 6 mm. The reaction temperature was raised to 775 °C under a nitrogen gas flow of 20 mL min−1. Then, the reactor feed was changed to CH4:O2:N2 = 3:1:1 (v/v/v). The total flow

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rate of the reactor feed was fixed at 20 mL min−1 with a gas hourly space velocity of 10000 h−1. After 10 min for stabilization, the catalytic activity was analyzed every 50 min. The water vapor in the

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gaseous products was removed using a cooling trap. The activity was measured using an on-line gas chromatograph system (YL-6500, Younglin) equipped with a flame ionization detector and a thermal

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conductivity detector. A carboxen-1000 column was used to separate the reaction products. The catalytic activity was calculated using the following equation. The yield of C 2 hydrocarbon was

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obtained by multiplying the conversion of CH4 and the selectivity of C2 hydrocarbons.

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Methane conversion (%) =

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C2 Selectivity (%) =

moles of CH4 consumed × 100 moles of CH4 in the feed

2 × moles of C2 hydrocarbons × 100 moles of CH4 consumed

COX Selectivity (%) =

moles of COX × 100 moles of CH4 consumed

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3. Results and Discussion 3.1. LAOS_X h catalysts in the OCM reaction For the preparation of high-performance LaAlO3 catalysts for the OCM reaction, we selected the solid-state method with controlled preparation conditions such as calcination time, temperature, and atmosphere because it is a simple and highly reproducible method suitable for mass production for commercial application [42,43]. First, we varied the calcination time (X) to obtain a series of LaAlO3

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catalysts (LAOS_X h; X = 5, 24, and 48). Moreover, the LaAlO3 catalyst LAOC was prepared by a citrate sol-gel method for comparison. The prepared LAOS_X h and LAOC catalysts were applied to the OCM reaction, and the obtained results are shown in Fig. 1. As expected, calcination time strongly affected the LAOS_X h catalysts activity, which underwent a gradual increase with calcination time.

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In particular, CH4 conversion slightly decreased with calcination time, while C2 selectivity gradually

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increased. After 24 h of calcination, the C2 yield was found to be similar due to the trade-off effect of two factors. However, it was found that the catalytic activity of LAOS_24 h was much lower than that

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of the LAOC catalyst.

From the XRD analysis, we confirmed that the lower catalytic activity of LAOS_X h compared

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with that of LAOC was due to the low homogeneity and crystallinity of the former catalysts. As shown in Fig. 2, we found various characteristic peaks corresponding to La-containing phases such as La2O3

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and La(OH)3 in the XRD patterns of the LAOS_X h catalysts. In addition, the relative crystallinity of LAOS_X h was found to be lower than that of LAOC. On the other hand, the LAOC catalyst prepared by citrate sol-gel method showed well-dseveloped LaAlO3 perovskite peaks without any byproducts. According to previous literatures, the homogeneity and crystallinity of catalysts has a great influence on the catalytic activities in the OCM reaction [41]. We therefore considered the low homogeneity and 9

crystallinity of LAOS_X h the reason for the low C2 yield, and accordingly, we assumed that the catalytic activity of LaAlO3 prepared by solid-state method could be improved by controlling other reaction parameters. On the basis of the results extracted from Fig. 1, we fixed the calcination time at 24 h for further study.

3.2. LAOS_Y °C catalysts in the OCM reaction

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Next, we evaluated the effect of the calcination temperature (Y) on the catalytic activity of the corresponding LAOS catalysts (LAOS_Y °C catalysts; Y = 850, 950, 1050, 1150, 1250, and 1350). The calcination temperature was controlled from 850 °C to 1350 °C in intervals of 100 °C. Fig. 3

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shows the catalytic activities of the LAOS_Y °C catalysts in the OCM reaction. Noticeably, C2

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selectivity increased with calcination temperature, whereas CH4 conversion remained relatively constant. Accordingly, the yield of C2 hydrocarbons increased with the calcination temperature, with

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LAOS_1350 °C affording the highest C2 hydrocarbon yield. Surprisingly, the catalytic activity of LAOS_1350 °C reached that of LAOC in terms of C2 hydrocarbon yield. In our previous work [41],

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we found that crystallinity could be one of the key factors to determine the activity of LaAlO 3 perovskite catalyst in the OCM reaction. Therefore, we assumed that high calcination temperature

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could allow the LAOS_1350 °C to retain the enhanced homogeneity and crystallinity, leading to the

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high C2 selectivity in this reaction.

Fig. 4 shows the XRD patterns of the prepared LAOS_Y °C catalysts. As expected, the characteristic peak corresponding to LaAlO3 perovskite grew sharper and higher as the calcination 10

temperature increased. Moreover, the LAOS_ Y °C catalysts prepared at relatively lower calcination temperatures (LAOS_850 °C, 950 °C, and 1050 °C) retained La-containing phases such as La2O3 and La(OH)3 as byproducts. Interestingly, the peak intensity of La-containing phases gradually decreased as the calcination temperature increased, with the LAOS_1350 °C catalyst affording only intense and sharp LaAlO3 perovskite peaks. Therefore, it can be concluded that highly crystalline and homogeneous catalysts with high activity in the OCM reaction can be prepared by controlling the

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calcination temperature of the solid-state method.

For the evaluation of other factors that could affect the catalytic activity of LAOS in the OCM reaction, we calculated the crystallite size and specific surface area of the catalysts, which are generally

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considered important factors in heterogeneous catalysis reactions (Table 1). The crystallite size of the

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LAOS_Y °C catalysts was calculated using the following Scherrer equation: 0.9λ βcosΘ

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𝐷=

where D is the crystallite size, λ is the X-ray wavelength, Θ is the Bragg angle, and β is the pure

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full width of the diffraction line at half its maximum intensity. As expected, it was found that the crystallite size gradually increased with the calcination temperature. The specific surface area of the

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LAOS_Y °C catalysts was also obtained by determining the nitrogen adsorption–desorption isotherms. As shown in Table 1, the surface area of the LAOS_Y °C catalysts decreased as the calcination

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temperature increased, which is due to the formation of larger crystallites at high calcination temperatures. In other words, the trend of specific surface area is in good agreement with that of the crystallite size calculated using the Scherrer equation. However, the specific surface area and crystallite size were not significantly related to the catalytic activity of the LAOS catalysts. Considering that the OCM reaction proceeds at very high temperature, we believe that the formation of selective active 11

sites for C2 production by controlling the characteristics of the active sites is more determining than exposing many catalytic sites by increasing the specific surface area. To sum up, we prepared LAOS_Y °C catalysts by a solid-state method using different calcination temperatures (Y), and applied them to the OCM reaction. The catalytic activity of LAOS_Y °C increased with increasing the calcination temperature. Accordingly, LAOS_1350 °C calcined at the highest temperature showed the highest catalytic activity, which is similar C2 hydrocarbon yield obtained with LAOC catalyst. The XRD analysis clearly revealed that the enhanced catalytic activity

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of LAOS_1350 °C was attributed to its high crystallinity and good homogeneity. Therefore, we assumed that LaAlO3 perovskite catalysts with considerable OCM activity can be readily prepared via

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3.3. LAOS_Z% catalysts in the OCM reaction

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a solid-state method by simply controlling the calcination conditions.

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The oxygen species present on the surface of metal oxide catalysts are one of the crucial factors that determine the catalytic activity in the OCM reaction [45-47]. Many researchers believe that OCM

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consists of two processes: a heterogeneous reaction and a homogeneous reaction. In the heterogeneous reaction, surface oxygen species in the catalyst play a key role in generating a methyl radical by

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abstracting hydrogen from methane. Once the methyl radicals are formed, they can be dimerized to C2 hydrocarbons in a homogenous gas-phase reaction [49-51]. We assumed that the surface oxygen

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properties of the LaAlO3 perovskite catalysts could be easily tuned by varying the oxygen content in the calcination flow gas. Accordingly, we prepared a series of LAOS_Z% catalysts using a simple solid-state method with different oxygen content (Z = 0, 5, 21, and 100) in the calcination gas balanced with nitrogen gas. The total flow rate of calcination gas was fixed at 200 mL min −1. Fig. 5 represents the obtained catalytic activities of the LAOS_Z% catalysts in the OCM reaction, in which an upward 12

convex parabolic curve can be seen for the C2 hydrocarbon yield. Particularly, LAOS_5% showed the highest C2 hydrocarbon selectivity and yield, which were comparable to those obtained with LAOC. Interestingly, the LAOS_100% catalyst showed the lowest C2 yield among the catalysts prepared using the present solid-state method. This implies that excessive oxygen content in the calcination atmosphere was not favorable to generate selective oxygen species for C2 production on the surface of

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the LaAlO3 perovskite catalyst.

Fig. 6 shows the XRD patterns of the LAOS_Z% catalysts, in which LaAlO3 perovskite peaks with similar peak intensities can be observed. In addition, La-containing phases such as La(OH)3 and La2O3 were present as byproducts in all the catalysts. It should be noted that the LAOS_Z% catalysts

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exhibited different catalytic activities in spite of retaining similar homogeneity and crystallinity, which

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was found to have an effect on the catalytic activity in the OCM reaction. This indicates that there is

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formed on the catalyst surface.

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another factor affecting the catalytic activity, which might be related to the nature of the oxygen species

To confirm the nature of oxygen species formed on the surface of the LAOS_Z% catalysts, they

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were subjected to XPS analysis. The obtained O 1s XPS spectra are presented in Fig. 7, which shows that three different types of oxygen species were generated on the surface of the LAOS_Z% catalysts.

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According to the previous studies on OCM catalysts, lattice oxygen (Olat) and surface adsorption oxygen (Oads) have a great influence on the catalytic activity in the OCM reaction [48-55]. The Olat of OCM catalysts is known to be the active site for the conversion of CH 4 into C2 hydrocarbons by forming methyl radicals [48-51]. Moreover, Olat can be divided into electrophilic lattice oxygen (Olat(e)) and nucleophilic lattice oxygen (Olat(n)). The former is considered to be more active and selective in 13

the formation of methyl radicals, and the latter can easily convert CH4 into COx through the complete oxidation of CH4 [31,55-59]. Meanwhile, Oads is formed by adsorption of oxygen gas on the oxygen vacancies of the catalyst surface. Oads also plays a key role, filling the lattice oxygen vacancies generated by the reaction of CH4 and Olat [53,54]. Therefore, the amount of Oads is believed to be important for maintaining the CH4 conversion rate in the OCM reaction. In other words, an efficient OCM catalyst is expected to have a large amount of Olat(e) with sufficient amount of Oads on its surface [52-55]. Accordingly, all the LAOS Z% catalysts would show significant CH4 conversion due to their

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sufficient Oads content. However, LAOS_Z% exhibited different C2 selectivity depending on the amount of Olat(e) species, i.e., Olat(e) on the catalysts surface are selective for producing C2 hydrocarbons, while Olat(n) could promote the oxidation of CH4 into carbon dioxide. Thus, LAOS_5%, which showed

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the highest C2 hydrocarbon selectivity, retained the largest amount of Olat(e) and the lowest amount of Olat(n). As a result, we finally obtained the highest C2 yield using LAOS_5%. On the other hand,

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LAOS_100% showed the lowest C2 yield owing to its small Olat(e) content. These results are in good

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agreement with our previous studies on the OCM reaction, which further confirm that it is possible to

4. Conclusion

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prepare a high performance LaAlO3 catalyst for OCM by simple solid-state method.

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In this study, calcination parameters including calcination time, calcination temperature, and

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oxygen content in the calcination gas were controlled to obtain a high performance LaAlO3 perovskite catalyst using a simple solid-state method for the oxidative coupling of methane (OCM). As a result, LaAlO3 catalysts (LAOS) prepared by the solid-state method with considerable catalytic activity were obtained and compared with a LaAlO3 catalyst (LAOC) prepared by the commonly used citrate solgel method. The calcination time and temperature strongly affected the crystallinity and homogeneity 14

of the LaAlO3 catalysts, whose activities gradually increased with increasing their homogeneity and crystallinity. Particularly, long calcination times and high calcination temperatures were required to obtain good homogeneity and high crystallinity, with the corresponding LaAlO3 catalyst calcined at 1350 °C for 24 h showing the best catalytic performance. Moreover, the oxygen content in the calcination gas greatly affected the amount of electrophilic lattice species and adsorption oxygen species of the LaAlO3 catalysts. In particular, an oxygen fraction of 5% produced the sufficient amount of adsorption oxygen species and the highest amount of electrophilic oxygen species that are known

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to be advantageous for C2 hydrocarbons production on the catalyst surface. Therefore, the LaAlO3 catalyst prepared with 5% oxygen content showed the highest C2 hydrocarbon selectivity and yield. In summary, a high performance LaAlO3 perovskite catalyst for OCM can be successfully prepared using

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a simple solid-state method with high reproducibility, which is suitable for mass production for

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commercial application.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors declare the following financial interests/personal relationships which may be

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considered as potential competing interests:

Acknowledgements

This research was supported by C1 Gas Refinery Program through the National Research

Foundation of Korea(NRF) funded by the Ministry of Science, ICT & Future Planning (2015M3D3A1A01064908). 15

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Fig. 1. Catalytic activities of LAOS_X h (X = 5, 24, and 48) and LAOC catalysts.

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Fig. 2. XRD patterns of LAOS_X h (X = 5, 24, and 48) and LAOC catalysts.

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Fig. 3. Catalytic activities of LAOS_Y °C (Y = 850, 950, 1050, 1150, 1250, and 1350) and LAOC

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catalysts.

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Fig. 4. XRD patterns of LAOS_Y °C catalysts (Y = 850, 950, 1050, 1150, 1250, and 1350).

Fig. 5. Catalytic activities of LAOS_Z% catalysts (Z = 0, 5, 21, and 100) and LAOC catalysts.

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Fig. 6. XRD patterns of LAOS_Z% catalysts (Z = 0, 5, 21, and 100).

Fig. 7. XPS spectra of O 1s of LAOS_Z% catalysts (Z = 0, 5, 21, or 100).

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Table 1. Crystallite size and specific surface area of LAOS_Y °C catalysts (Y = 850, 950, 1050, 1150, 1250, and 1350). SBET (m2 g−1)a

LAOS_850 °C

25.5

12.4

LAOS_950 °C

33.2

12.3

LAOS_1050 °C

37.1

10.4

LAOS_1150 °C

44.9

11.0

LAOS_1250 °C

49.5

8.0

LAOS_1350 °C

51.3

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Crystallite size (nm)

5.2

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SBET was determined via the Brunauer−Emmett−Teller plot.

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Catalyst

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