燃料化学学报（中英文）

Sample	BET surface area S^a/(m²·g⁻¹)	Total pore volume v^b/(cm³·g⁻¹)
^a: Calculated with the BET equation; ^b: Referring to the BJH desorption pore volume.
La	3	0.02
LaMg_0.05	12	0.06
LaBa_0.05	11	0.04
LaMg_0.04Ba_0.01	13	0.05
LaMg_0.025Ba_0.025	10	0.04
LaMg_0.01Ba_0.04	13	0.06

Sample	BET surface area S^a/(m²·g⁻¹)	Total pore volume v^b/(cm³·g⁻¹)
^a: Calculated with the BET equation; ^b: Referring to the BJH desorption pore volume.
La	3	0.02
LaMg_0.05	12	0.06
LaBa_0.05	11	0.04
LaMg_0.04Ba_0.01	13	0.05
LaMg_0.025Ba_0.025	10	0.04
LaMg_0.01Ba_0.04	13	0.06

Catalyst	O 1s binding energy/eV (percentage fraction/%)	${\mathrm{O_2^-/O^{2-}}} $
LaMg_0.05	532.2 (13.4)	531.6 (68.2)	528.9 (18.3)	0.7
LaBa_0.05	533.2 (14.2)	531.4 (63.2)	528.7 (22.6)	0.6
LaMg_0.04Ba_0.01	532.1 (16.7)	530.7 (68.8)	528.1 (14.3)	1.2
LaMg_0.025Ba_0.025	532.2 (34.6)	531.1 (41.1)	528.6 (24.1)	1.4
LaMg_0.01Ba_0.04	532.2 (31.8)	530.9 (37.6)	528.5 (30.4)	1.0

Catalyst	O 1s binding energy/eV (percentage fraction/%)	${\mathrm{O_2^-/O^{2-}}} $
LaMg_0.05	532.2 (13.4)	531.6 (68.2)	528.9 (18.3)	0.7
LaBa_0.05	533.2 (14.2)	531.4 (63.2)	528.7 (22.6)	0.6
LaMg_0.04Ba_0.01	532.1 (16.7)	530.7 (68.8)	528.1 (14.3)	1.2
LaMg_0.025Ba_0.025	532.2 (34.6)	531.1 (41.1)	528.6 (24.1)	1.4
LaMg_0.01Ba_0.04	532.2 (31.8)	530.9 (37.6)	528.5 (30.4)	1.0

Mg和Ba共掺杂调控氧化镧表面氧物种以增强其在催化甲烷氧化偶联反应中的性能

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王珂 ¹^,² , 张祺 ¹ , 牛鹏宇 ¹^,^* , 林明桂 ¹^,^* , 贾丽涛 ¹ , 李德宝 ¹ , 章日光 ²

燃料化学学报（中英文） | 2026,54(7): 20250374-1273-20250374-1285

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燃料化学学报（中英文） 2026 , 54 (7) : 20250374-1273 -20250374-1285

Mg和Ba共掺杂调控氧化镧表面氧物种以增强其在催化甲烷氧化偶联反应中的性能

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王珂¹^,², 张祺¹, 牛鹏宇¹^,^*, 林明桂¹^,^*, 贾丽涛¹, 李德宝¹, 章日光²

作者信息

¹中国科学院山西煤炭化学研究所煤炭高效低碳利用全国重点实验室山西太原 030001

²太原理工大学化学与化工学院煤基能源清洁高效利用国家重点实验室山西太原 030024

通讯作者:

牛鹏宇, 林明桂

Tuning surface oxygen species via Mg-Ba co-doping on La₂O₃ to enhance oxidation coupling of methane performance

Ke WANG¹^,², Qi ZHANG¹, Pengyu NIU¹^,^*, Minggui LIN¹^,^*, Litao JIA¹, Debao LI¹, Riguang ZHANG²

Affiliations

¹State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

²State Key Laboratory of Clean and Efficient Coal Utilization, College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

Corresponding Author:

Tel: 0351-4040428, E-mail: niupy@sxicc.ac.cn

linmg@sxicc.an.cn

出版时间: 2026-07-10 doi: 10.1016/S1872-5813(26)60677-9

文章导航

摘要

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采用水热法合成了Mg、Ca、Sr和Ba单掺杂改性及Mg、Ba共掺杂改性La₂O₃催化剂，并评价了其在甲烷氧化偶联（OCM）反应中的催化性能。实验结果表明，Mg改性La₂O₃催化剂表现出较高的O₂和CH₄活化能力，但C₂₊选择性较低。而Ba改性La₂O₃催化剂则表现出较高的C₂₊选择性，但反应活性较低。Mg与Ba共掺杂改性La₂O₃催化剂，能够在维持较高C₂₊选择性的同时提升催化活性。当Mg/Ba物质的量比为1∶1时，该催化剂在700 ℃条件下实现29.5%的CH₄转化率、54.5%的C₂₊选择性及16.1%的C₂₊产物收率。通过O₂/H₂-TPR、原位FT-IR和O₂脉冲等方法对催化剂活化O₂和CH₄进行了研究。表征结果表明，Mg和Ba共掺杂改性La₂O₃催化剂可在催化剂表面产生更多${{\mathrm{O}}_2^- }$物种，从而显著提高La₂O₃催化剂的活性和选择性。

关键词

甲烷氧化偶联 / La₂O₃ / 镁和钡共掺杂 / ${\mathrm{O}}_2^- $物种

Abstract

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Mg-, Ca-, Sr- and Ba-single-doped La₂O₃ as well as Mg-Ba co-doped La₂O₃ catalysts were synthesized via a hydrothermal method and evaluated for the oxidative coupling of methane (OCM). The experimental results revealed that the Mg-modified La₂O₃ catalyst activates O₂ and CH₄ effectively, yet achieves only moderate C₂₊ selectivity. Conversely, the Ba-modified analogue affords high C₂₊ selectivity, albeit at the expense of lower reaction activity. Notably, the Mg-Ba co-doped La₂O₃ catalyst strikes an effective balance between activity and selectivity, enhancing catalytic performance while maintaining a high C₂₊ selectivity. Specifically, at a Mg/Ba molar ratio of 1∶1 and 700 °C, it achieved a CH₄ conversion of 29.5%, a C₂₊ selectivity of 54.5% and a corresponding C₂₊ yield of 16.1%.The characterization results indicate that Mg and Ba co-doped La₂O₃ catalysts promotes the formation of more ${\mathrm{O}}_2^- $ species on the catalyst surface, which in turn significantly enhances both the activity and selectivity of La₂O₃ catalysts.

Key words

oxidation coupling of methane / La₂O₃ / Mg and Ba co-doped / ${\mathrm{O}}_2^- $ species

引用本文

王珂, 张祺, 牛鹏宇, 林明桂, 贾丽涛, 李德宝, 章日光. Mg和Ba共掺杂调控氧化镧表面氧物种以增强其在催化甲烷氧化偶联反应中的性能. 燃料化学学报（中英文）, 2026 , 54 (7) : 20250374-1273 -20250374-1285 . DOI: 10.1016/S1872-5813(26)60677-9

Ke WANG, Qi ZHANG, Pengyu NIU, Minggui LIN, Litao JIA, Debao LI, Riguang ZHANG. Tuning surface oxygen species via Mg-Ba co-doping on La₂O₃ to enhance oxidation coupling of methane performance[J]. Journal of Fuel Chemistry and Technology, 2026 , 54 (7) : 20250374-1273 -20250374-1285 . DOI: 10.1016/S1872-5813(26)60677-9

正文

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The increasing exploitation of unconventional natural gas resources, such as shale gas and coalbed methane, has investigated the demand for efficient, high-value utilization of methane (CH₄)^[1]. In general, CH₄ conversion strategies are categorized into indirect and direct pathways. The indirect route typically involves the initial reforming of CH₄ into synthesis gas (a mixture of CO and H₂), which is then transformed into various high-value chemicals^[2−4]. However, this multi-step process is often hampered by its complex infrastructure, high energy consumption, and suboptimal carbon atom economy. In contrast, the direct catalytic conversion of CH₄ into target chemicals offers a more streamlined process with potentially higher efficiency and carbon utilization, presenting a promising frontier in catalysis^[5]. Within this field, the oxidative coupling of methane (OCM) to C₂₊ hydrocarbons (C₂₊) has garnered significant research attention due to its conceptual simplicity and potential for direct production of valuable olefins and paraffins in a single step^[6−8].

Since the seminal discovery of the OCM reaction in 1982, extensive research has been devoted to developing efficient catalytic systems^[9−11]. These systems are broadly categorized into four categories: alkali metal/alkaline earth metal oxides (e.g., Li/MgO)^[12], multi-component transition metal oxides (e.g., MnO_x-Na₂WO₄/SiO₂)^[13−16], composite oxides (e.g., A₂B₂O₇)^[17−18], and rare earth oxides (e.g., La₂O₃)^[19]. While Li/MgO catalysts offer the merits of compositional simplicity and high initial activity, they suffer from the volatility and leaching of lithium species, leading to rapid deactivation^[20−21]. The benchmark MnO_x-Na₂WO₄/SiO₂ system delivers exceptional C₂₊ selectivity, but this performance typically requires reaction temperatures exceeding 800 °C. Under such harsh conditions, the sublimation of the Na₂WO₄ active component remains a critical challenge, severely limiting long-term stability^[22−23]. Composite oxide catalysts exhibit superior structural and thermal robustness, resisting phase transitions at high temperatures. However, they are often hindered by lower C₂₊ selectivity due to a higher propensity for the complete oxidation of methane and desired C₂ products^[24−25]. In this landscape, rare earth oxides (particularly La₂O₃) have emerged as promising candidates. They present a balanced profile of high thermal stability and effective catalytic performance under comparatively milder conditions^[26−29]. Nevertheless, achieving commercially viable C₂₊ yields over La₂O₃-based catalysts remains an ongoing research challenge, motivating further efforts in material design and optimization^[30−32].

The OCM over La₂O₃ primarily involves two key steps: initial activation of CH₄ on surface active sites to generate methane radicals (CH₃·), followed by their gas-phase coupling to form C₂₊ hydrocarbon products. Studies have shown that surface oxygen species (such as O⁻,

${\mathrm{O}}_2^- $

${\mathrm{O}}_2^{2-} $

and O^2–) play a decisive role in directing activation of CH₄ to form C₂₊ hydrocarbon products or CO_x^[32−33]. Superoxide species (

${\mathrm{O}}_2^- $

) are recognized as crucial for selective C−H bond cleavage, directly abstracting a hydrogen atom from CH₄ to yield methyl radicals^[34−36]. This role of

${\mathrm{O}}_2^- $

has been corroborated in various catalytic systems, including barium halide-modified La₂O₃/CaO^[37−38], La₂Ce₂O₇^[35] and Mn₂O₃-Na₂WO₄ supported on Ce_xZr_1−xO₂^[34]. When peroxide species (

${\mathrm{O}}_2^{2-} $

) interact with CH₄, one of its oxygen atoms inserts directly into a C−H bond of CH₄, forming a methanol intermediate^[39]. However, under typical OCM reaction temperatures, methanol is very easily overoxidized to CO₂, leading to complete oxidation via this pathway. Theoretical calculations by Wang et al.^[39] suggest that the presence of peroxide species (

${\mathrm{O}}_2^{2-} $

) limits the C₂₊ selectivity on La₂O₃ to only 67%. Although not directly detected experimentally, atomic oxygen anions (O⁻) are theorized to form at defect sites on La₂O₃, though their precise role in CH₄ oxidation remains ambiguous. Lattice oxygen (O^2–) species on La₂O₃ generally exist in two forms: fully coordinated oxygen bound to lanthanum ions, and coordinatively unsaturated lattice oxygen. The interaction of CH₄ with surface lattice oxygen (O^2–) of the catalyst predominantly leads to the deep oxidation. Therefore, the regulation of oxygen species has become a focus in OCM. Doping L₂O₃ with foreign metals is an effective strategy to modulate oxygen species and enhance catalytic performance^[40−41]. In particular, alkaline earth metal-doped lanthanum-based catalysts can significantly enhance their OCM reaction performance. Kiatsaengthong et al.^[42] reported that Mg, Ca, Sr and Ba-activated La₂O₃ catalysts can boost the C₂ selectivity by increasing the number of surface superoxide species (

${\mathrm{O}}_2^{2-} $

). Xiong et al.^[43] studied the effects of Li, Mg, Zn and Ce-doped La₂O₃ nanorods on the OCM and found that Mg²⁺ doping generates oxygen vacancies, thereby promoting the formation of more surface oxygen species and enhancing C−C bond coupling.

Co-doping lanthanum oxides with multiple metals has recently emerged as a promising strategy to enhance catalytic performance for the oxidative coupling of methane (OCM) by harnessing synergistic effects between dopants. For instance, Ptrolini et al.^[44] demonstrated in Mg- and Ce-modified La₂(Ce_1−xMg_x)₂O₇-δ catalysts that Mg incorporation modulates reducibility, stabilizes Ce³⁺ species, and creates basic sites, collectivelyleading to superior C₂₊ selectivity at lower temperatures. Sollier et al.^[45] reported that introducing Sr into La_xCe_yO_z solid solutions could promote the formation of structural defects, thereby enhancing the generation of superoxide species (

${\mathrm{O}}_2^- $

).This ultimately achieved a C₂₊ yield of 21.7% at 600 °C. Wu et al.^[46] reported that co-doping of Ca and Sr in La₂Ce₂O₇ could increase surface oxygen vacancies, which elevated the density of surface active oxygen species (

${\mathrm{O}}_2^- $

), resulting in a C₂ product yield of 21.3% at 650 °C. However, the influence of alkaline-earth metal co-doping on OCM performance over lanthanum-based catalysts, as well as the underlying mechanism regulating the formation of superoxide species (

${\mathrm{O}}_2^- $

), remains inadequately understood.

In this work, a series of La₂O₃-based catalysts modified by single (Mg, Ca, Sr or Ba) and co-doping (Mg and Ba) were prepared by a hydrothermal method. The co-doping strategy, particularly the incorporation of Mg and Ba, was found to significantly enhance both the catalytic activity and C₂₊ selectivity for OCM. To gain mechanistic insights into this performance improvement, the structural and chemical properties of the catalysts were interrogated using a combination of characterization techniques. Specifically, O₂/H₂-TPR, in situ FT-IR and O₂ pulse experiments were employed to systematically probe the dynamic activation of CH₄ and O₂ as well as the nature and evolution of surface oxygen species on catalysts under relevant conditions. This study clarifies the enhanced oxygen storage and activation behavior and elucidates the synergistic effect of Mg-Ba co-doping in La₂O₃, offering fundamental insights for the rational design of highly active and selective OCM catalysts.

1　Experimental

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1.1　Catalyst preparation

The La₂O₃ catalysts co-doped with Mg and Ba were synthesized via a hydrothermal-calcination method. Specifically, 18 g of NH₄HCO₃ was dissolved in 480 mL of ethylene glycol under vigorous stirring for 60 min. Separately, La(CH₃COO)₃, Ba(CH₃COO)₂ and Mg(CH₃COO)₂ (the molar ratio of La(CH₃COO)₃, Ba(CH₃COO)₂ and Mg(CH₃COO)₂ was 20∶0.5∶0.5, with a total amount of 0.04 mol) were dissolved in 480 mL of deionized water. This aqueous solution was then slowly added dropwise to the NH₄HCO₃/ethylene glycol solution. After stirring at 25 °C for 30 min, the mixture was transferred into a 200 mL Teflon-lined autoclave and maintained at 180 °C for 24 h. The resulting precipitate was collected by centrifugation, washed thoroughly with deionized water and absolute ethanol, and dried at 60 °C overnight. Finally, the dried precursor was calcined in air at 800 °C for 4 h to obtain the catalyst. After cooling to room temperature, the catalyst labeled as LaMg_0.025Ba_0.025 was obtained (subscripts represent the molar ratio of elements). In addition, catalysts with alternative La∶Mg∶Ba ratios of 20∶0.2∶0.8 and 20∶0.8∶0.2 were also synthesized, denoted as LaMg_0.04Ba_0.01 and LaMg_0.01Ba_0.04. A set of single M-doped La₂O₃ samples (M = Mg, Ca, Sr, Ba) were prepared with an M∶La molar ratio of 0.05∶1 and are labeled LaM_0.05. Pure La₂O₃ (La) was obtained using the same procedure with only lanthanum acetate (0.04 mol).

1.2　Catalyst characterization

The morphology of the catalysts was characterized by scanning electron microscopy (SEM) on a JSM-7001F scanning electron microscope with a voltage of 5 kV.

The phase of the catalysts was determined by a PANalytical X'pert3 X-ray diffraction analyzer using a Cu target, Kα radiation (wavelength λ=0.154 nm), tube current of 50 mA, tube voltage of 50 kV, and a scanning rate of 10(°)/min.

Nitrogen adsorption-desorption isotherms of the catalysts were obtained on a Micromeritics ASAP 2460 physical adsorption analyzer.

XPS testing was performed using an AIX ULTRAADLD X-ray photoelectron spectrometer. The spectra were charge-corrected using the C 1s signal located at 284.5 eV. The O 1s spectra of all samples were fitted using XPSPEAK41 software, and the peaks were resolved using an 80% Gaussian-20% Lorentzian model function. The full width at half maximum (FWHM) of all O 1s spectra was approximately 2 eV.

O₂/H₂-TPR testing was conducted using an AMI300 fully automatic chemisorption analyzer. The sample (0.05 g) was first pretreated in a 10% H₂/Ar (volume fraction) at 800 °C for 30 min, then exposed to a 10% O₂/Ar (volume fraction) at 550, 650 or 750 °C for 30 min, subsequently cooled to 50 °C, switched to Ar purging, and finally heated to 850 °C in a 10% H₂/Ar (volume fraction).

In situ DRIFT was performed on a Nicolet iS10 infrared spectrometer, collecting 128 scans at a resolution of 4 cm⁻¹. The sample was pretreated in H₂ at 680 °C for 10 min, then cooled to 550 °C in Ar. After 10 min, a background spectrum was collected in Ar. Then, a 5% O₂/Ar (volume fraction) was introduced into the reaction cell, and infrared spectral signals were collected at 550 °C (infrared spectra were labeled as 0, 1, 2, 3, 4, 5 min according to sampling time). Subsequently, the sample was pretreated in H₂ at 680 °C for 10 min, then cooled to 650 °C in Ar. After running in Ar for 10 min, a background spectrum was collected. A 5% O₂/Ar gas mixture was introduced into the reaction cell, and spectra were collected at 650 °C(infrared spectra were labeled as 0, 1, 2, 3, 4, 5 min according to sampling time).

O₂-TPD experiments were performed on AutoChem II chemisorption analyzer. The catalyst (0.1 g) was treated with 10% O₂/Ar (volume fraction, 30 mL/min) at 800 °C for 30 min, and cooled to 50 °C in 10% O₂/Ar. At 50 °C, the gas was switched to Ar and purged for 60 min to flatten the baseline. Finally, the temperature was raised to 800 °C under Ar. Results were recorded by a mass spectrometer (MS, IPI GAM200).

O₂ pulse experiments were performed on AutoChem II chemisorption analyzer. The catalyst (0.1 g) was treated with H₂ (30 mL/min) at 700 °C for 30 min to remove oxygen species, then switched to Ar for 30 min purging. Subsequently, 5% CH₄ (volume fraction, 30 mL/min) was turned on, and 10 pulses of 5% O₂ (volume fraction) were performed at 5 min intervals. Results were recorded by a mass spectrometer (MS, IPI GAM200).

1.3　Catalyst evaluation

The performance of the catalysts was evaluated in a fixed-bed quartz reactor (inner diameter 8 mm) with a feed gas of CH₄∶O₂=3∶1 (CH₄ flow rate: 100 mL/min, O₂/N₂ flow rate: 66 mL/min) at a space velocity of 80000 mL/(g·h). 0.1 g of catalyst (40−60 mesh) and quartz sand were mixed uniformly to a volume of 0.5 mL and placed in the constant temperature zone of the reactor. Prior to the reaction, the catalyst was pretreated in N₂ at 800 °C for 2 h, then cooled to 550 °C. Catalytic activity was measured while increasing the temperature from 550 to 800 °C. A chromatograph (Agilent 7890B) equipped with TCD and FID detectors was used for online analysis of feed gas and products.

CH₄ conversion (

$x_{{\mathrm{CH_4}}} $

), O₂ conversion (

$x_{{\mathrm{O_2}}} $

), C₂₊ selectivity (

$s_{{\mathrm{C}}_{2+}} $

), and C₂₊ yield (

$Y_{{\mathrm{C}}_{2+}} $

) were calculated with the following equations：

$ {{x}}_{{{\mathrm{CH}}_{\text{4}}}}=\frac{{\text{CH}}_{\text{4, in}}-{\text{CH}}_{\text{4,out}}}{{\text{CH}}_{\text{4,in}}}{\times 100\%} $

(1)

$ {{x}}_{{{\text{O}}_{\text{2}}}}=\frac{{\text{O}}_{\text{2,in}}-{\text{O}}_{\text{2,out}}}{{\text{O}}_{\text{2,in}}}{\times 100\%} $

(2)

${{s}}_{{{\text{C}}_{\text{2+}}}}\text=\frac{\rm{2 \times }\left({\rm{C}}_{\rm{2}}{\rm{H}}_{\rm{4}}\rm+{\rm{C}}_{\rm{2}}{\rm{H}}_{\rm{6}}\right)\rm{+3 \times }\left({\rm{C}}_{\rm{3}}{\rm{H}}_{\rm{6}}\rm+{\rm{C}}_{\rm{3}}{\rm{H}}_{\rm{8}}\right)\rm{+4 \times }\left({\rm{C}}_{\rm{4}}{\rm{H}}_{\rm{8}}\rm+{\rm{C}}_{\rm{4}}{\rm{H}}_{\rm{10}}\right)}{\rm{2 \times }\left({\rm{C}}_{\rm{2}}{\rm{H}}_{\rm{4}}\rm+{\rm{C}}_{\rm{2}}{\rm{H}}_{\rm{6}}\right)\rm{+3 \times }\left({\rm{C}}_{\rm{3}}{\rm{H}}_{\rm{6}}\rm+{\rm{C}}_{\rm{3}}{\rm{H}}_{\rm{8}}\right)\rm{+4 \times }\left({\rm{C}}_{\rm{4}}{\rm{H}}_{\rm{8}}\rm+{\rm{C}}_{\rm{4}}{\rm{H}}_{\rm{10}}\right)\rm{+CO+}{\rm{CO}}_{\rm{2}}} \times 100\% $

(3)

$ {{Y}}_{{{\text{C}}_{\text{2+}}}}={{x}}_{{{\text{CH}}_{\text{4}}}}{\times}{{s}}_{{{\text{C}}_{\text{2+}}}}\times 100\% $

(4)

2　Results and discussion

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2.1　Reaction performance

Figure 1 presents the OCM reaction performance of Mg, Ca, Sr and Ba doped La₂O₃ catalysts within the temperature range of 550−800 °C. The undoped La₂O₃ catalyst reaches its optimal performance at 750 °C, with CH₄ and O₂ conversions of 30.4% and 95.1%, respectively and a C₂₊ selectivity of 48.3%. For alkaline earth metal Mg, Ca, Sr and Ba doped La₂O₃ catalysts, a clear trend was observed: the light-off temperature gradually increased with the ionic radius of the dopant. At 600 °C, the order of activity for both O₂ and CH₄ conversion was LaMg_0.05 > LaCa_0.05 > LaSr_0.05 > LaBa_0.05. This sequence indicates that Mg doping significantly enhances the O₂ activation capability of La₂O₃, leading to improved CH₄ conversion. Conversely, the C₂₊ selectivity showed an opposite trend, increasing with the size of the dopant cation. LaBa_0.05 exhibited the highest C₂₊ selectivity, which was 6.7% higher than that of undoped La₂O₃ catalyst at 750 °C. However, this gain in selectivity came at the cost of lower activity at 700 °C, LaBa_0.05 showed conversions of only 20.2% for CH₄ and 59.0% for O₂. These results indicate that while Ba doping promotes C₂₊ formation, it concurrently weakens the catalyst's oxygen activation capacity, necessitating higher operating temperatures to achieve a competitive C₂₊ yield. Motivated by the complementary properties of Mg-doped La₂O₃ (strong O₂ activation) and Ba-doped La₂O₃ (high C₂₊ selectivity), this study investigates the synergistic integration of both dopants into La₂O₃, aiming to concurrently achieve high CH₄ conversion and C₂₊ selectivity under relatively mild reaction conditions. As illustrated in Figure 2, the catalytic properties showed a clear dependence on the Mg/Ba molar ratio. Specifically, with an increasing Mg/Ba molar ratio, the C₂₊ selectivity progressively increased, while the activation capability for both O₂ and CH₄ concurrently decreased. The catalyst with an equimolar Mg/Ba ratio (LaMg_0.025Ba_0.025) demonstrated balanced and superior performance at 700 °C, achieving a CH₄ conversion of 29.5% and a C₂₊ selectivity of 54.5%. This represents a 6% enhancement in C₂₊ selectivity compared to the solely Mg-doped counterpart (LaMg_0.05) and an 11% improvement in CH₄ conversion relative to the solely Ba-doped catalyst (LaBa_0.05) at the same temperature. Therefore, the synergistic co-doping of Mg and Ba into La₂O₃ successfully reconciled the trade-off between activity and selectivity at 700 °C, yielding a substantially higher C₂₊ yield. In addition, the long-term stability of the optimal LaMg_0.025Ba_0.025 catalyst was evaluated under industrially relevant, high-severity conditions (GHSV = 80000 mL/(g·h)). As shown in Figure 3(a), the catalyst maintained exceptionally stable CH₄ conversion and C₂₊ selectivity over the 24 h test, showing its robustness, which is crucial for the OCM reaction.

The composition distribution of C₂₊ products over the LaMg_0.025Ba_0.025 catalyst was analyzed as a function of temperature (Figure 3(b)). Below 700 °C, the products were mainly C₂ hydrocarbons (C₂H₄ and C₂H₆), with no C₃₊ products detected. As the temperature increased, longer-chain C₃₊ products began to form, and their selectivity gradually increased. The ratio of C₂H₄/C₂H₆ in C₂ hydrocarbon products (Figure 3(c), (d)) continued to increase with temperature, indicating that higher temperatures promote the dehydrogenation of C₂H₆, thereby favoring the formation of the higher-value product, C₂H₄.

2.2　Catalyst structure

Figure 4 shows the XRD patterns of alkaline earth metal doped La₂O₃ catalysts. All diffraction patterns for both single-doped (Mg, Ca, Sr, Ba) and co-doped (Mg-Ba) catalysts correspond exclusively to the hexagonal phase of La₂O₃ (PDF#05-0602)^[43]. No diffraction peaks corresponding to oxides or carbonates of the doped elements were observed, indicating high dispersion of the alkaline-earth metals within the La₂O₃. A comparison of the XRD patterns revealed that the diffraction peak of La₂O₃ (101) crystal plane corresponding to Mg and Ca single-doped La₂O₃ catalysts shifted toward higher angles. This shift results from the smaller ionic radii of Mg²⁺ (0.072 nm) and Ca²⁺ (0.100 nm) compared with La³⁺ (0.103 nm), which induce lattice contraction and a reduction in the interplanar spacing. In contrast, the (101) peak shifts to lower angles for the Sr- and Ba-doped samples due to the larger ionic radii of Sr²⁺ (0.118 nm) and Ba²⁺ (0.135 nm), leading to lattice expansion. The above analysis confirms that the successful incorporation of the dopant ions into the La₂O₃ lattice. Furthermore, the magnitude of the shift increased with the atomic radius of the dopant. A similar trend was observed in the co-doped samples, with LaMg_0.025Ba_0.025 exhibiting the most pronounced shift toward lower angles, suggesting the formation of a greater number of structural defects.

Figure 5 shows SEM images of La, LaMg_0.05, LaBa_0.05, LaMg_0.04Ba_0.01, LaMg_0.01Ba_0.04 and LaMg_0.025Ba_0.025 catalysts. The undoped La₂O₃ catalyst exhibited a well-defined triangular flake morphology. Upon doping with alkaline earth metals, all modified catalysts LaMg_0.05, LaBa_0.05, LaMg_0.04Ba_0.01, LaMg_0.01Ba_0.04 and LaMg_0.025Ba_0.025 retained the characteristic triangular lamellar morphology, indicating that alkaline earth metal doping treatment does not affect the fundamental morphology of the catalysts. However, the doped catalyst exhibited a more open, loosely packed lamellar structure with a developed microporous texture, which is expected to facilitate reactant diffusion and enhance mass transport during the reaction.

The specific surface area and pore volume of the catalysts were determined, and the results are shown in Table 1. Compared to pure La₂O₃, the alkaline-earth-metal-doped catalysts showed increased specific surface area and pore volume. A larger specific surface area can expose more surface active sites^[46], which is beneficial for catalytic reactions.

2.3　Characterization of surface oxygen species

The reducibility of adsorbed oxygen species on Mg- and Ba-doped La₂O₃ catalysts was analyzed by O₂/H₂-TPR. All catalysts were first reduced in H₂ to remove oxygen species adsorbed, followed by exposure to O₂ at different temperatures (550, 650 and 750 °C) to (re)adsorb oxygen. Afterward, H₂ temperature-programmed reduction was performed, and the amount of reducible oxygen species was quantified based on the H₂ consumption peaks. The characterization results are shown in Figure 6. All catalysts showed three distinct H₂ consumption peaks, corresponding to the reduction of different oxygen species: a low-temperature H₂ consumption peak (200–300 °C) assigned to the reduction of oxygen species adsorbed on the catalyst surface; a medium-temperature H₂ consumption peak (300–500 °C) attributed to the reduction of oxygen species oxygen migrating from the near-surface region; and a high-temperature H₂ consumption peak above 500 °C associated with the reduction of bulk-phase oxygen species diffusing to the surface. With increasing O₂ pretreatment temperature, the areas of the low- and medium-temperature H₂ consumption peaks gradually increased, indicating that the number of reducible oxygen species on its surface and near-surface layer increased with O₂ pretreatment temperature.

This trend indicates an enhanced ability of catalysts to adsorb and activate O₂ at elevated temperatures. Moreover, this trend correlates well with the observed increase in O₂ conversion with temperature, suggesting that the abundance of active oxygen species is a key factor governing O₂ conversion, which in turn drives CH₄ conversion. Furthermore, as the O₂ treatment temperature increased, the high-temperature H₂ consumption peak of the LaMg_0.05 catalyst moved toward lower temperatures, suggesting that bulk phase oxygen migrated to the sub-surface layer and merged with near-surface oxygen species, which reflects its strong capacity for O₂ dissociation and explains its highest O₂ conversion at 600 °C. In contrast, the LaBa_0.05 catalyst had a relatively weak ability to dissociate O₂, thus resulting in the lowest O₂ conversion at 600 °C. The relatively weak O₂ dissociation capacity of the LaBa_0.05 catalyst may be associated with enhanced C₂₊ selectivity, as dissociated oxygen species frequently lead to the complete oxidation of CH₄^[44]. In Mg and Ba co-doped La₂O₃ catalysts, as the O₂ pretreatment temperature increased, the phenomenon of oxygen species in the bulk phase migrating to the sub-surface layer was also observed. Especially under the O₂ pretreatment condition of 750 °C, the high-temperature H₂ consumption peaks of LaMg_0.04Ba_0.01 and LaMg_0.025Ba_0.025 catalysts merged with the medium-temperature H₂ consumption peaks. This feature indicates that LaMg_0.04Ba_0.01 and LaMg_0.025Ba_0.025 catalysts possess excellent capabilities in activating O₂ and generating active oxygen species, thus enabling high O₂ conversion and CH₄ conversion at a reaction temperature of 700 °C.

The effects of single-doping (with Mg or Ba) and co-doping (with Mg and Ba) modifications on the surface oxygen species of La₂O₃ catalysts were investigated by O₂-TPD, as shown in Figure 7. All catalysts displayed distinct O₂ desorption peaks in the range of 200–300 °C, which are ascribed to chemisorbed oxygen species, such as superoxide (

${\mathrm{O}}_2^{-} $

) and peroxide (

${\mathrm{O}}_2^{2-} $

) ions^[18,38,46]. Comparing the O₂ desorption profiles of Mg and Ba single-doped and co-doped La₂O₃ catalysts, the O₂ desorption peak of Mg-modified La₂O₃ was broad, suggesting the presence of multiple types of chemisorbed oxygen species on its surface. The Ba-modified sample presented a sharp desorption peak, indicating the formation of a more uniform type of chemisorbed oxygen species on its surface. When Mg and Ba were co-doped at a 1∶1 molar ratio (LaMg_0.025Ba_0.025), the desorption peak became broader and its intensity increased. Correlating with catalyst performance, the Mg and Ba co-doped LaMg_0.025Ba_0.025 catalyst achieved 96.3% O₂ conversion at 700 °C, whereas LaMg_0.05 and LaBa_0.05 required 750 °C to exceed 95% O₂ conversion. This indicates that the Mg and Ba co-doping enhanced the activation capacity for O₂, significantly increasing both the type and quantity of surface chemisorbed oxygen species. It should be noted that the desorption peaks in this temperature regime may encompass both superoxide (

${\mathrm{O}}_2^{-} $

) and peroxide (

${\mathrm{O}}_2^{2-} $

) species, whose similar thermal stabilities make their precise deconvolution challenging^[47].

The surface oxygen species distributions of La₂O₃ catalysts, modified by single (Mg or Ba) and co-doping (Mg-Ba), were analyzed via XPS, with results presented in Figure 8 and Table 2. The O 1s spectra can be deconvoluted into three characteristic peaks, corresponding to

${\mathrm{O}}_2^{-} $

(532 eV),

${\mathrm{CO}}_3^{2-} $

(531 eV), and O²⁻ (529 eV) respectively^[18,26,35]. Moreover, the quantitative results of surface oxygen species indicate that the concentration of

${\mathrm{O}}_2^{-} $

is significantly enhanced in Mg and Ba co-doped La₂O₃ catalysts, with the LaMg_0.025Ba_0.025 catalyst exhibiting the highest

${\mathrm{O}}_2^{-} $

concentration. However, XPS analysis cannot reflect the true surface state of the catalyst under high-temperature reaction conditions. Moreover, La-based catalysts easily adsorb CO₂ and H₂O to generate carbonates and hydroxyl groups, which seriously interfere with the O 1s signal and compromise the accuracy of active-oxygen quantification. Therefore, to investigate reactive oxygen species under reaction conditions, in situ DRIFT experiments were conducted.

In situ DRIFT was employed to further investigate the types of surface active oxygen species on La-based catalysts, and the results are shown in Figure 9. Both Mg and Ba single-doped and co-doped La₂O₃ catalysts exhibit an infrared band at approximately 1100 cm⁻¹, which is assigned to surface superoxide species (

${\mathrm{O}}_2^- $

)^[43]. In contrast, undoped La₂O₃ exhibits no detectable signal for surface superoxide species (

${\mathrm{O}}_2^- $

). As the temperature of the O₂ treatment increased, the intensity of the band at 1100 cm⁻¹ enhanced, indicating that the amount of superoxide species (

${\mathrm{O}}_2^- $

) generated on the catalyst increased with rising temperature. This trend correlates with the temperature-dependent increase in C₂₊ selectivity observed for these catalysts, further confirming that superoxide species (

${\mathrm{O}}_2^- $

) serve as the active oxygen species in the reaction. Specifically, for the LaMg_0.025Ba_0.025 catalyst, both the intensity of the

${\mathrm{O}}_2^- $

signal and the C₂₊ selectivity increased with temperature, with the latter rising from 3% to 52%. Doping with either Mg or Ba effectively promotes the formation of superoxide species (

${\mathrm{O}}_2^- $

) on the La₂O₃ surface. Notably, the signal for superoxide species (

${\mathrm{O}}_2^- $

) of LaBa_0.05 was significantly stronger than that of LaMg_0.05. This may be attributed to the strong charge-induced dipole interaction between Ba with a larger ionic radius and superoxide species (

${\mathrm{O}}_2^- $

), which can stabilize superoxide species (

${\mathrm{O}}_2^- $

)^[48]. Importantly, compared to single-doped catalysts, the signal for superoxide species (

${\mathrm{O}}_2^- $

) was markedly enhanced in Mg and Ba co-doped La₂O₃ catalyst, demonstrating a synergistic effect of Mg and Ba in promoting the formation of superoxide species (

${\mathrm{O}}_2^- $

As evidenced by the O₂/H₂-TPR results, the co-doping of Mg and Ba significantly enhances the activation and dissociation of O₂ on the La₂O₃ catalyst. Consequently, this co-doping strategy not only increases oxygen vacancies on the La₂O₃ catalyst, thereby promoting oxygen activation, but also significantly elevates the concentration of superoxide species (

${\mathrm{O}}_2^- $

) present on the catalyst surface. Combined with catalytic performance, the LaMg_0.025Ba_0.025 catalyst exhibited the highest C₂₊ selectivity of 52.4%, while the C₂₊ selectivity of other catalysts was lower than 50% at 700 °C. This indicates that the presence of superoxide species (

${\mathrm{O}}_2^- $

) is closely related to the C₂₊ selectivity.

2.4　Effect of surface oxygen species on OCM reaction

CH₄/O₂ pulse experiments were conducted to investigate the formation of surface oxygen species and their efficacy in activating CH₄ to form C₂H₄. The variation in CH₄ consumption under pulsed O₂ at 700 °C is shown in Figure 10. Prior to O₂ pulse, the catalyst was pretreated with H₂ to remove adsorbed oxygen species. During the subsequent O₂ pulses, oxygen species were progressively adsorbed, gradually establishing an equilibrium distribution between the surface and the bulk phase. All samples exhibited a similar two-stage response in CH₄ signal intensity: as the number of O₂ pulses increased, the CH₄ signal intensity first gradually increased, and then tended to stabilize. The initial increase in CH₄ signal intensity corresponds to the accumulation of surface active oxygen species on the catalyst, whereas the subsequent plateau of CH₄ signal intensity indicates that adsorption equilibrium has been reached. LaBa_0.05 required five O₂ pulses to reach a stable CH₄ signal, while La, LaMg_0.05 and LaMg_0.025Ba_0.025 require three O₂ pulses to achieve equilibrium CH₄ signal intensity. The Mg and Ba co-doped catalyst LaMg_0.025Ba_0.025 achieved 86% of the equilibrium intensity in the CH₄ signal during the first oxygen pulse, indicating that this catalyst promotes O₂ activation and rapidly establishes adsorption equilibrium of oxygen species on both the catalyst surface and bulk phase.

The C₂H₄ generation profiles of the catalysts following O₂ pulses at 700 °C is shown in Figure 11. The signal intensity of C₂H₄ also presented a trend of initial increase followed by stabilization as the number of O₂ pulses increased. A detailed comparison revealed that the C₂H₄ signal for LaBa_0.05 stabilized at the 4th O₂ pulse, whereas that for LaMg_0.05 required until the 5th pulse. Combined with in situ DRIFT (Figure 9) results, which showed a higher concentration of

${\mathrm{O}}_2^- $

species on LaBa_0.05 than on LaMg_0.05, this further supports the governing role of superoxide species (

${\mathrm{O}}_2^- $

) in the formation of C₂₊ products. Notably, no C₂H₄ was detected for LaBa_0.05 and LaMg_0.05 catalysts in the first O₂ pulse. This is attributed to the limited amount of

${\mathrm{O}}_2^- $

available during the first pulse, which favors the complete oxidation of CH₄ to CO₂. The resulting CO₂ is strongly adsorbed by the basic catalyst surface, explaining its absence in the gas-phase detection. It is worth noting that for the LaMg_0.025Ba_0.025 catalyst, the C₂H₄ signal intensity reached 81% of the equilibrium intensity during the first O₂ pulse, which was significantly superior to other catalysts. This indicates that LaMg_0.025Ba_0.025 facilitates the generation of superoxide species (

${\mathrm{O}}_2^- $

) more readily, further verifying the in situ FT-IR results.

2.5　Activation of O₂ and CH₄ on La₂O₃ catalyst

The surface activation of O₂ proceeds through reactive intermediates such as superoxide (

${\mathrm{O}}_2^- $

) and dissociated oxygen species (O⁻ or O²⁻)^[46]. Mg doped La₂O₃ promotes the activation of O₂ on the catalyst surface. However, excessive O₂ activation capacity led to the formation of a large quantity of dissociated oxygen species (O⁻), which promote the over-oxidation of CH₄ to CO_x and consequently lower the C₂₊ selectivity. The Ba doped La₂O₃ catalyst possesses a weaker O₂ activation ability, yet the dopant interacts favorably with superoxide species (

${\mathrm{O}}_2^- $

), stabilizing them on the surface. This results in a higher surface concentration of

${\mathrm{O}}_2^- $

, which selectively promotes the formation of C₂₊ products. When Mg and Ba are co-doped, the activation of O₂ toward superoxide species (

${\mathrm{O}}_2^- $

) is further enhanced, increasing the surface concentration of this selective oxygen species (Figure 12). Consequently, the Mg-Ba co-doped La₂O₃ catalyst achieves superior C₂₊ selectivity (54.5%) at a relatively mild temperature of 700 °C, outperforming both singly doped La₂O₃ catalysts.

3　Conclusion

收起

In this study, La₂O₃ catalysts single-doped with alkaline earth metals (Mg, Ca, Sr, Ba) and co-doped with Mg and Ba were prepared via a hydrothermal method, and their structure, catalytic performance, and oxygen species were systematically investigated. In situ DRIFTS revealed the presence of superoxide species (

${\mathrm{O}}_2^- $

) on the surface of both Mg- and Ba-doped catalysts, with the co-doped system exhibiting a significantly more intense signal for the superoxide species

${\mathrm{O}}_2^- $

. This indicates a synergistic effect between Mg and Ba in promoting the formation and stabilization of these key reactive intermediates. O₂/H₂-TPR studies revealed that Mg and Ba co-doped La₂O₃ catalysts exhibit superior O₂ activation capabilities compared to those doped with Mg or Ba alone. Furthermore, CH₄/O₂ pulse experiments revealed that the co-doped catalysts facilitate faster establishment of oxygen adsorption equilibrium, thereby enhancing CH₄ activation and the subsequent formation of C₂ products. This work establishes that co-doping La₂O₃ with Mg and Ba represents an effective strategy for improving catalytic performance in the oxidative coupling of methane (OCM), primarily by modulating the generation and stabilization of key active oxygen species.

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2026年第54卷第7期

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doi: 10.1016/S1872-5813(26)60677-9

接收时间：2025-11-20
首发时间：2026-09-23
出版时间：2026-07-10

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收稿日期：2025-11-20
修回日期：2025-12-23
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¹中国科学院山西煤炭化学研究所煤炭高效低碳利用全国重点实验室山西太原 030001

²太原理工大学化学与化工学院煤基能源清洁高效利用国家重点实验室山西太原 030024

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2种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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