Electric Field and Mobile Oxygen Promote Low-Temperature Oxidative Coupling of Methane over La1–xCaxAlO3−δ Perovskite Catalysts

Oxidative coupling of methane (OCM) over La1–xMxAlO3−δ (M = Ca, Sr, Ba; x = 0, 0.1, 0.2, 0.3) in an electric field at low temperature (423 K) was investigated. Among the tested catalysts, the La0.7Ca0.3AlO3−δ catalyst showed the highest performance in terms of C2H6 + C2H4 yield (11.1%). Surface mobile oxygen species (O22– or O–), which were considered as active oxygen species for the OCM reaction, increased with increasing Ca doping amount, and thereby the La0.7Ca0.3AlO3−δ catalyst showed the best catalytic activity.


■ INTRODUCTION
Oxidative coupling of methane (OCM) is a reaction to produce C 2 hydrocarbons from methane and oxygen directly. It has been attracting lots of attention and has been investigated from the 1980s because of the need for technical development to convert natural gas to added value chemicals. 1−5 Even though the OCM reaction is exothermic thermodynamically, it requires a high reaction temperature over 900 K because of high energy required for dissociation of the C−H bond in methane. 6 At such high temperature, C 2 hydrocarbons tend to be oxidized to CO and CO 2 , resulting in a drastically decreased C 2 selectivity. To prevent the undesirable nonselective reactions, we conducted the OCM reaction in a low-temperature region by applying an electric field. Under constant current to the catalyst-bed, the OCM reaction proceeded at the low reaction temperature of 423 K over Ce 2 (WO 4 ) 3 /CeO 2 catalyst. 7−9 Many researchers have reported that the active oxygen species on the catalyst is an active site of the OCM reaction. 1,10−13 These oxygen species (e.g., O 2− , O − , O 2 2− , and/or O 2 − ) abstract hydrogen atom from methane. 6 Perovskite-type oxide materials are known to generate such oxygen species with high mobility in low-temperature regions relatively. 14 Especially, among many kinds of perovskite-type oxides, LaAlO 3 shows high OCM activity. 6,10,15,16 By doping alkali or alkali-earth metal cations, which have strong basic nature, high C 2 selectivity can be obtained. 17,18 In addition, substitution of La 3+ cation by such cations resulted in increase of oxygen deficiency in the lattice structure and this, in turn, contributed to production of O − and O 2 2− type oxygen species. 19,20 Herein, we investigated catalytic activity of OCM in the electric field at the low temperature of 423 K over alkali-earth metal cations (Ca, Sr, and Ba)-substituted LaAlO 3 catalysts. The reaction mechanism and effects of substituted alkali-earth metal cations on its structure, electronic state, and OCM activity were also investigated.

■ RESULTS AND DISCUSSION
First, we investigated the catalytic OCM activity of La 0.7 M 0.3 AlO 3−δ (M = Ca, Sr, and Ba) at 423 K in the electric field, and the results are presented in Figure 1 and Table S1. The results show that La 0.7 Ca 0.3 AlO 3−δ , La 0.7 Sr 0.3 AlO 3−δ , and La 0.7 Ba 0.3 AlO 3−δ showed OCM activity at the low temperature of 423 K in the electric field (imposed current 3.0 mA). Among the tested catalysts, La 0.7 Ca 0.3 AlO 3−δ showed the highest C 2 H 6 + C 2 H 4 yield and selectivity. It is noteworthy that the La 0.7 Ca 0.3 AlO 3−δ catalyst showed higher C 2 H 6 + C 2 H 4 yield than that of the previously reported catalysts, 7,8 thanks to its redox property (details are discussed in following section). Moreover, the La 0.7 Ca 0.3 AlO 3−δ catalyst showed stable activity for at least 180 min ( Figure S1) without significant structural deformation and coke formation ( Figures S2 and S3). The effect of electric field on the activity of La 0.7 Ca 0.3 AlO 3−δ was investigated by comparing the activity with or without the electric field. Results are shown in Figure 2 and Table S2. As shown in Figure 2, with the aid of electric field, CH 4 conversion, O 2 conversion, and C 2 H 6 + C 2 H 4 yield for the reaction in the electric field were extremely high at low temperature, whereas no OCM activity was found without the electric field. On the other hand, in the reaction without the electric field, C 2 H 6 + C 2 H 4 yield steeply increased with temperature above 800 K. It is well known that perovskite oxide releases its lattice oxygen at high temperature (i.e., above 800 K for La 0.7 Sr 0.3 AlO 3−δ ), and it contributed to hydrocarbon conversion by the Mars van Krevelen mechanism. 21−24 The isotopic oxygen exchange tests revealed that the lattice oxygen was exchanged by gas phase molecular oxygen at 473 K with the electric field and at above 800 K without the electric field (see the Supporting Information, Figures S4 and S5). The temperature of the catalyst was measured by a thermocouple, and the effect of Joule heat by the electric field on the catalytic activity was negligible. From these results, it is considered that applying electric field to La 0.7 Ca 0.3 AlO 3−δ promotes the release of its lattice oxygen even at low temperature below 700 K, and thereby the OCM reaction takes place.
In addition, as shown in Table S1, field intensity of the applied electric field was ca. 150 V mm −1 , which is much lower than that of a plasma-catalyst hybrid system. 25 Moreover, the present electrocatalytic system can proceed the OCM reaction at lower temperature (<700 K) than that of a high-temperature electrochemical reaction using a SOFC (solid oxide fuel cell) system (>873 K), 14,26 although the field intensity was higher than that of the SOFC system (<30 V mm −1 ). Therefore, the present reaction system, catalytic reaction in an electric field, is a mild and efficient low-temperature electrocatalytic OCM.
To clarify effects of Ca doping amount in LaAlO 3 on its structure and OCM activity, La 1−x Ca x AlO 3−δ catalysts with various Ca doping amount (x = 0, 0.1, 0.2, and 0.3) were prepared and evaluated. Details of the structural characterizations are described in the Supporting Information (Figures S6, S7, and Table S3). Figure 3 and Table S4 show results of activity tests over La 1−x Ca x AlO 3−δ catalysts in the electric field. O 2 partial pressure decreased from 15% to 5% for comparison of these activities and selectivity precisely. In the case of LaAlO 3 catalyst, spark discharge was observed due to its low electron, hole, and/or ion conductivity, 27 and no products were formed. As the Ca doping amount increased, CH 4 and O 2 conversions and C 2 H 6 + C 2 H 4 yield increased. Reportedly, the amount of lattice oxygen defect increases by substituting cations with different oxidation numbers in perovskite structures, which promotes redox reaction using lattice oxygen through the Mars van Krevelen mechanism. 21−24 In the present catalytic system, the lattice oxygen contributes to methane activation and C 2 product formation. Therefore, it is considered that Ca doping to LaAlO 3 contributed to increase the amount of lattice oxygen defects, and thereby the conversions and C 2 H 6 + C 2 H 4 yield increased as Ca doping amount increased.
X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the difference in electronic states of lattice or surface oxygen over La 1−x Ca x AlO 3−δ catalysts. Figure S8 shows O 1s spectra of La

ACS Omega
Article Tables 1 and S5 present the result of O 1s XPS  measurements. From Table 1, the O s ratio increases by increasing the amount of Ca doping. Therefore, we deduced the relationship between C 2 H 6 + C 2 H 4 yield and O s ratio as shown in Figure 4. A linear correlation was found between the O s ratio and C 2 H 6 + C 2 H 4 yield (and CH 4 conversion), suggesting that the O s ratio over La 1−x Ca x AlO 3−δ catalyst plays an important role in the OCM reaction. It was reported that electrophilic oxygen species (e.g., O − ), which was formed by reaction of O 2 molecule with metal cations or oxygen vacancies over the surface of metal oxide catalysts, 4,31,32 contribute homolytic C−H bond dissociation to form methyl radicals. 33 To elucidate the reaction pathway on the La 0.7 Ca 0.3 AlO 3−δ catalyst in the electric field, the effect of contact time (W/ F CH 4 ) was investigated. O 2 partial pressure was set to 5% to evaluate the product selectivity at the initial stage. Figure 5 and Table S6 present the influence of W/F CH 4 on CH 4 conversion, O 2 conversion, and selectivity of each product. As shown in Figure 5a, CH 4 conversion and O 2 conversion increase as the contact time increases. As shown in Figure 5b, C 2 H 6 selectivity decreased gradually as the W/F CH 4 increased, and C 2 H 4 selectivity was almost constant in low W/F CH 4 region, and it decreased with further increase in W/F CH 4 . On the other hand, C 2 H 2 selectivity was almost zero under these conditions, suggesting that C 2 H 2 was hardly produced by successive dehydrogenation of C 2 H 4 . CO and CO 2 were formed even at low W/F CH 4 , and CO selectivity increased as the contact time increased, which implies that CO and CO 2 were produced from methane directly (parallel reaction) in low W/F CH 4 region and by oxidation of C 2 products (successive reaction) in high W/F CH 4 region. From these results, the main reaction pathway is suggested as follows: C 2 H 6 was formed by OCM reaction over the La 0.7 Ca 0.3 AlO 3−δ catalyst at first, and then C 2 H 4 was produced by oxidative dehydrogenation of C 2 H 6 , and finally they were oxidized to CO and CO 2 .
In order to investigate the reactivity of C 2 products, the reactant was changed from CH 4 to C 2 H 6 or C 2 H 4 . The results of each catalytic activity test are shown in Table S7. In each case, C 2 hydrocarbons, CH 4 , CO, and CO 2 were produced; however C 3+ hydrocarbons were not detected. From Table S6, C 2 H 6 conversion was lower than CH 4 conversion, and C 2 H 6 was mainly converted to C 2 H 4 by oxidative dehydrogenation. Similar to the CH 4 reaction, C 2 H 2 selectivity in C 2 H 6 or C 2 H 4 reaction was also very low. Although main products from C 2 H 4 were CO and CO 2 , C 2 H 4 conversion was low despite the presence of unreacted O 2 . Considering the obtained results, the C−H bond of C 2 H 6 was selectively dissociated rather than C−C bond dissociation. Also, both dehydrogenation and oxidation of C 2 H 4 hardly proceeded in the present system.

ACS Omega
Article Scheme 1 shows a possible reaction pathway. Therefore, the high C 2 H 6 + C 2 H 4 yield was obtained over the La 0.7 Ca 0.3 AlO 3−δ catalyst with the electric field.

■ CONCLUSIONS
In conclusion, OCM has been conducted over various perovskite oxide catalysts in the electric field at low temperature. Applying the electric field facilitated the OCM reaction under low reaction temperature (423 K). The La 0.7 Ca 0.3 AlO 3−δ catalyst showed the highest C 2 H 6 + C 2 H 4 yield (11.1%) than ever. Surface mobile oxygen species (O s : O 2 2− or O − ) on La 1−x Ca x AlO 3−δ was considered as active oxygen species for the OCM reaction via the Mars van Krevelen mechanism. The O s ratio increased with increasing the amount of Ca doping, and thereby the OCM activity improved. In this catalytic system, oxidative dehydrogenation of C 2 H 6 easily proceeds, whereas successive C 2 H 4 conversion hardly does, resulting in high C 2 H 6 + C 2 H 4 yield. La 0.7 Ca 0.3 AlO 3−δ generated effective oxygen species in the electric field, which brought high C 2 H 6 + C 2 H 4 yield at low reaction temperature of 423 K.

■ EXPERIMENTAL SECTION
Catalyst Preparation. Perovskite oxide catalysts (La 1−x M x AlO 3−δ ; M = Ca, Sr, Ba; x = 0, 0.1, 0.2, 0.3) were prepared by a citric acid complex method according to the reported procedure. 21,22,27 First, metal nitrate precursors were dissolved with distilled water in a Teflon beaker and stirred. Next, ethylene glycol and citric acid were added to this solution. The molar ratio of metal/citric acid/ethylene glycol was 1:3:3. This solution was heated to 353 K for 15 h, and the gel was heated and stirred to dry up the water completely. Precalcination was conducted at 673 K for 2 h. Thereafter, the obtained powder was calcined at 1123 K for 10 h.
Activity Tests. Catalytic activity tests were conducted in a fixed-bed flow type reactor with a quartz tube (6 mm o.d., 4 mm i.d.). 37 Figure S9 shows schematic diagram of the reactor. The catalysts were sieved to 355−500 μm, and 100 mg of it was inserted into the reactor. To impose electric field to the catalyst, two stainless-steel electrodes were inserted to the upper-side and the bottom-side of the catalyst-bed. The constant current of 3.0 mA was imposed to the catalyst bed using a dc power supply. The reactant feed gases were methane, oxygen, and argon in the ratio of CH 4 Characterization. The catalyst structure was characterized by powder X-ray diffraction (SmartLab III; Rigaku Corp.) at 40 kV and 40 mA with Cu Kα radiation and Raman spectroscopy (NRS-4500; Jasco Corp.) with green laser (λ = 538 nm). Transmission electron microscopy (TEM) observations were performed using field emission-transmission electron microscope (JEM2100-F; JEOL Ltd.) operated at 200 kV. The specific surface area of each catalyst was measured by N 2 physisorption at 77 K with the Brunauer−Emmett− Teller method (GeminiVII; Micromeritics Instrument Corp.) after pretreatment at 573 K for 2 h in an Ar atmosphere (Table  S8). XPS (VersaProbe2; ULVAC-PHI Inc.) measurements were conducted with an Al Kα X-ray source. The binding energies were referenced to C 1s peak at 284.8 eV. The 16 O 2 / 18 O 2 isotopic oxygen exchange tests in the temperatureprogrammed reaction or isothermal transient reaction were conducted using the fixed-bed continuous flow-type reactor equipped with a quadrupole mass spectrometer (QGA; Hiden Analytical Ltd.