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. 2026 Feb 18;6(3):1757–1766. doi: 10.1021/jacsau.5c01660

Probing Methane Coupling on Liquid Metal Indium: In Situ Elucidation of Active Sites

Lizhuo Wang †, Ang Li ‡,*, Yasuhiro Sakamoto †, Wenjie Yang †, Wei Li ‡, Haoyue Sun †, Yue Jiang ⊥, Yusheng Li ⊥, Xiaozhou Liao §, Kourosh Kalantar-Zadeh †,*, Xiaodong Han ‡,∥,*, Jun Huang †,*
PMCID: PMC13014191  PMID: 41889734

Abstract

The nonoxidative coupling of methane (NOCM) offers a promising route to convert methane into value-added chemicals. Liquid metals have emerged as potential catalysts for NOCM, due to their propensity against coke formation and the flexible atomic arrangement that facilitates methane activation, with liquid state indium (In) gaining attention. However, the reaction pathways and catalytic mechanisms of In during NOCM have yet to be fully understood. Here, we report the discovery of locally generated In liquid metal active sites on In2O3 for the NOCM reaction, supported by silicon dioxide substrates, in the vicinity of an in situ formed In silicon oxide (In2Si2O7) interfacial layer. By the implementation of combined in situ transmission electron microscopy and electron energy loss spectroscopy, we directly observed the formation of liquid metal “In active sites”, near the interfacial layer, at >600 °C. The spectroscopy analysis reveals that In2Si2O7 is a reservoir in methane conversion, storing reactive H* and CH x * intermediate spillover from “In” for driving the NOCM reaction, avoiding the overcracking of CH4 over metallic In. This finding provides a practical approach for the rational design of efficient and noncorrosive liquid metal-based catalysts.

Keywords: Liquid metal, Interface, Methane coupling, Indium oxide, Silicon oxide

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Introduction

Methane (CH4) conversion is a pivotal reaction in the field of catalysis. Owing to the substantial reserves of natural gas and recent advancements in the extraction of unconventional petroleum resources, CH4 has emerged as an abundant and low-cost carbon resource for the synthesis of value-added chemicals and fuels. − However, the nonpolar symmetric structure of CH4 makes it more inert, and its C–H bonding energy is 440 kJ/mol, presenting a formidable challenge for activation under mild and energy-efficient conditions. In addition, considering that CH4 is a potent greenhouse gas, its low-energy conversion is a critical issue in many settings. Thus, developing effective catalytic systems capable of activating CH4 is a critical scientific pursuit.

During the past few years, liquid metal-based catalysts, such as indium, − gallium, − or multicomponent liquid metal alloys, − have emerged as promising candidates for different catalytic processes. − This includes nonoxidative coupling of methane (NOCM) processes, taking the production of ethane and ethylene as examples, and also cracking and pyrolysis.

Due to the density differences and lack of polarity on the surface of metal in the liquid state, the coke formed during the reaction can be intrinsically separated, thereby providing durability and stability to liquid metal catalysts in the CH4 conversion reaction. Meanwhile, the fluidity of liquid metal enables dynamic surface atomic rearrangement on liquid metal nanoparticles, giving rise to distinctive active site structures that facilitate reactant activation. ,, Finally, compared with conventional solid catalysts, − CH4 activation on liquid metal surfaces can exhibit a significant reduction (∼150 kJ/mol), highlighting the critical role of liquid metal in the heterogeneous catalysis process. Nevertheless, liquid metal catalysts are accompanied by the drawbacks of corrosion to metal, which impedes their industrial implementation. Conversely, the oxides of liquid metals, such as In2O3 and Ga2O3, although regarded as not active to NOCM, are noncorrosive and have been employed as catalysts for hydrocarbon conversion reactions. − For example, using theoretical calculations, it has been demonstrated that In2O3 and Ga2O3 are potential candidates for concerted C–C coupling between CO2 and CH4. In another experiment, it has also been reported that In2O3–Al2O3 could carry out propane dehydrogenation in the presence of N2O. Another report showed In2O3-Ga2O3-Al2O3 mixed oxide in propane nonoxidative dehydrogenation, and the experimental results hinted at the role of the post-transition metal oxides in the dehydrogenation activity. Despite such research, the detailed reaction mechanisms of hydrocarbon conversion over the oxides and the potential role of liquid metal have remained unclear. The structure of the interface between the oxide of liquid metal and the supporting material has been ignored, leaving a black box in this research area.

Some pioneering computational studies have begun to emerge focusing on the structures of active sites in liquid metals and their oxides. For instance, density functional theory (DFT) has been used for building the structure–performance relationship between different surface oxygen vacancy sites in In2O3 for the CO2 hydrogenation reaction. Also, C–H activation has been investigated using DFT and X-ray adsorption spectroscopy, , with attention to the role of indium liquid metal. However, the direct observation of the active sites in experiments is critical to validate the presence of liquid metal as an active site, which is missing in such investigations. The implementation of in situ transmission electron microscopy (TEM) provides the opportunity to perform detailed localized analysis for oxides of liquid metals. For example, the recently demonstrated liquid phase formation for CO2 reduction gives basic evidence for the redox cycle of lattice oxygen during reactions. Adding to this argument, the integration of electron energy loss spectroscopy (EELS) enables the investigation of changes in the chemical composition of the catalyst during the reaction, allowing research to simultaneously observe both structural and compositional transformations. , However, despite their critical importance, such observations have not been applied to NOCM, particularly in the presence of a liquid metal.

In our work, we implement in situ TEM coupled with EELS to investigate the liquid metal active site evolution on an In2O3/SiO2 model catalyst under NOCM reaction, exploring such sites as catalysts and determining the synergetic interaction between In2O3, SiO2, and liquid metal. Our research determines that indium liquid metal active sites are formed during NOCM at elevated temperatures, while an interfacial In2Si2O7 layer emerges. We provide evidence that this interfacial layer functions as a reservoir for reactive fragments such as H* and CH x * generated from CH4 activation, thereby effectively suppressing the overcracking of CH4, leading to an efficient NOCM reaction.

Results and Discussion

Synthesis of In2O3/SiO2 Base

The In2O3/SiO2 catalyst base was synthesized using a wet-impregnation method. In a typical experiment, In­(NO3)3·nH2O was added to DI water and mixed with fumed silica (details can be found in the Supporting Information, Experimental Method). The residue solid was then dried and calcined at 500 °C. Here, In2O3 was chosen with the aim of accessing a noncorrosive metal oxide that supports liquid metal indium active sites during the NOCM reaction.

The X-ray diffractometry (XRD) pattern of the as-synthesized In2O3/SiO2 sample, presented in Figure a, displays sharp diffraction peaks located at 21.5°, 30.5°, 35.4°, 50.9°, and 60.6°, corresponding to the (211), (222), (400), (440), and (622) planes of In2O3 (ICSD#14387). The broad bump between 16 and 29° is attributed to the amorphous SiO2 substrate. The In 3d X-ray photoelectron spectroscopy (XPS) spectrum displays In 3d5/2 and In 3d3/2 peaks at 445.9 and 453.4 eV, corresponding to good peak symmetry. No In0 In 3d5/2 was observed at 444.4 eV, indicating that In3+ is the only In species. The O 1s XPS spectrum (Figure S1a) was deconvoluted into two peaks located at 533.2 and 531.7 eV, corresponding to oxygen in SiO2 and In2O3, respectively. Meanwhile, the Si 2p spectrum (Figure S1b) exhibits a peak at around 104 eV, attributed to SiO2. All of these results suggest that the indium species that exists on SiO2 is in the form of full stoichiometry In2O3. As for the surface morphology, compared to the bare silica support (Figure S2), some bright particles were observed in the scanning electron microcopy (SEM) images of In2O3/SiO2 (Figure c) acquired with a backscattered electron detector (BSD). As indium (N = 49) is much heavier than Si (N = 14), these bright particles were assigned to In2O3. Meanwhile, these bright particles show an approximately cubic form, which is consistent with the possibility of the body-centered cubic phase of the In2O3 crystal, as also suggested by XRD (Ia3̅, space group 206).

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Analysis of the as-synthesized In2O3/SiO2. XRD pattern of as-synthesized In2O3/SiO2 (a); In 3d XPS spectrum of as-synthesized In2O3/SiO2 (b); SEM image of as-synthesized In2O3/SiO2 (c); ex situ TEM image of as-synthesized In2O3/SiO2 (d); the FFT of the red square area in panel d (e); diffraction pattern analysis of the FFT (f); and crystal arrangement illustration of the as-synthesized In2O3/SiO2 (g).

Interfacial Incorporation between In2O3 and SiO2

The TEM image was utilized to further ascertain the structural properties of the catalyst (Figure d). The TEM image indicates that the crystals are placed on the amorphous support. The fast Fourier transform (FFT) (Figure e) of the selected area (red square in Figure d) further evidences the presence of In2O3. The FFT pattern indexing (Figure f) and measurement determine a distance of d = 0.29 nm for the (222) plane and d = 0.18 nm for the (4̅4̅0) plane, which corresponds to the In2O3 observed from the [1̅10] direction. All of these results indicate that the structures of as-synthesized In2O3/SiO2 are In2O3 nanoparticles supported by amorphous silica, as illustrated in Figure g.

In situ TEM was first applied to investigate the thermal-induced dynamic on In2O3/SiO2. When In2O3/SiO2 in a N2 environment at room temperature was investigated (Figure a1), the boundaries between the crystallized In2O3 and the amorphous SiO2 could be clearly observed. Lattice fringes could be measured on the crystal particle, and the lattice distances were 0.20 and 0.32 nm, which are attributed to the (413) and (1̅1̅3) planes of In2O3, respectively. This crystal structure was also verified by the corresponding FFT pattern. As shown in Figure S3, no structure change of In2O3/SiO2 was detected when the temperature was increased to 200 °C. The high-resolution TEM (HRTEM) images (Figure S3a,b) and the corresponding FFT images, shown in Figure S3c,d, confirm that In2O3 was the only crystalline phase.

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In situ analysis of In2O3/SiO2 in N2. In situ TEM image and selected area (red square) FFT analysis of In2O3/SiO2 in N2 (a1); in situ TEM image and selected area (red square) FFT analysis of In2O3/SiO2 in N2 at 600 °C (a2); in situ TEM image and selected area (red square) FFT analysis of sample at 700 °C (a3); in situ EELS Si L2,3 spectra of sample at 25 (black), 200 (purple), 400 (cyan), 600 (pink), and 700 °C (orange) (b); and illustration of the generated In2Si2O7 interface between In2O3 and SiO2 (c) and the incorporation of oxygen (d).

The support structural change was noted subsequent to heating at 400 °C in N2. As shown in Figure S4, the diffraction pattern corresponding to In2O3 with the incident beam along the [223̅] zone axis was observed and confirmed through simulated diffraction (Figure S4d). The inverse FFT image calculated from the dominant diffraction spots aligns well with the In2O3 structural model, providing further evidence that the lattice fringes observed in the TEM image primarily originated from In2O3. Noticeably, two weak diffraction spots with a reciprocal distance of 3.4 nm–1 were detected in the FFT, corresponding to the (201̅) and (2̅01) planes of In2Si2O7, indicating the presence of a very minor interfacial phase as the binary oxide phase of indium and silicon. Additionally, the corresponding in situ EELS spectra of the Si L2,3 edge (Figure b) displayed an increase of peak A/B ratio. Peak A of Si L2,3 corresponds to dipole-allowed transitions from the Si 2p core level to s-like states in the conduction band, while peak B originates from the electron transfer from the p level to s- and d-like states. This increased A/B peak ratio suggests that indium species have been introduced into the support. These introduced indium species modify the local electronic environment around silicon by reducing the relative contribution of d-like states and enhancing s-like states, leading to the increased A/B ratio.

Upon heating the sample to 600 °C in N2, as displayed in Figure a2, fringes emerged on the silica support adjacent to In2O3. The measured lattice spacing of 0.30 nm on the support differs from the 0.58 nm spacing corresponding to the (111) plane of In2O3 and is attributed to the (201̅) plane of In2Si2O7. FFT was implemented to further confirm the formation of the minor layer lattice structure on the support. The FFT of the selected area in Figure a2 revealed that the diffraction spot originating from the support (indicated by a blue circle) has a shorter distance to (000) in reciprocal space compared to that of In2O3. This observation suggests the formation of a new species on the support. The A/B peak ratio at 600 °C remains comparable to that observed at 400 °C, indicating a similar chemical environment of silicon at both temperatures.

We investigated the temperature modulation to observe the morphological and crystallographic changes at different temperatures. At 700 °C (Figure a3), observations were consistent with those at 600 °C. The lattice distance measured on the TEM image were 0.58 and 0.24 nm, corresponding to the (111) plane from In2O3 and the (2̅21) plane from In2Si2O7, respectively, which was also confirmed by FFT analysis. The TEM image accompanied by FFT analysis suggests the copresence of In2O3 and In2Si2O7 in samples at elevated temperature.

A more in-depth look at the Si L2,3 EELS spectra acquired from room temperature (RT) to 700 °C (Figure b) and the A/B peak ratio trend in Figure S5 indicate the A/B ratio rises from approximately 0.6 to 0.7 once the temperature exceeds 400 °C, confirming that the generation of In2Si2O7 is thermally induced at the interface and occurs at over 400 °C. We propose that this interfacial In2Si2O7 stems from the intrinsic properties of In2O3. As illustrated in Figure c,d, since In2O3 inherently contains oxygen vacancies, the incorporation of Si from SiO2 into In2O3 would reduce these vacancies and enhance the overall stability of the oxide structure. Consequently, the interfacial area between SiO2 and In2O3 spontaneously transforms, establishing a layer of In2Si2O7.

In Situ Liquid Metal Active Site Generation

To investigate whether the presence of the reactant gas influences the formation of the interfacial layer at elevated temperature, in situ TEM was used to monitor the In2O3/SiO2 heated under a CH4 atmosphere. At room temperature, the in situ TEM images (Figure a1) of In2O3/SiO2 in a CH4 and N2 atmosphere exhibit no significant differences. No crystalline fringe was observed on the silica support, and the FFT analysis determined the (411), (341̅), (420), and (514) planes in reciprocal space, confirming the same In2O3-loaded silica structure as that revealed in Figure a1.

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In situ analysis of samples in CH4. In situ TEM image and selected area (red square) FFT analysis of a sample with the initial formulation of In2O3/SiO2 in a CH4 environment at room temperature (a1), 600 (a2), and 700 °C (a3); Si L2,3 EELS spectra of this sample in a CH4 atmosphere at room temperature (black), 200 (purple), 400 (cyan), 600 (pink), and 700 °C (orange), as well as the spectrum acquired in a N2 atmosphere at 600 °C (gray), which was used for comparison (b); and the intensity of H2 detected by MS in the outlet product from environmental TEM presented as a function of temperature when In2O3/SiO2 was exposed to a CH4 environment (c).

Upon heating to 400 °C in CH4 (Figure S6), two new lattice fringes with spacings of 0.21 and 0.24 nm appeared. These were assigned to the (221) and (201) planes of In2Si2O7, respectively. The corresponding Si L2,3 EELS spectra presented in Figure b exhibited an increased A/B peak ratio of approximately 0.7 (Figure S8), confirming the formation of interfacial In2Si2O7 from SiO2 and In2O3 at this temperature. At 600 °C, lattice fringes with a spacing of 0.43 nm were observed (Figure a2), corresponding to the (020) plane of In2Si2O7. The FFT analysis also indicates the presence of In2Si2O7 under this condition. The Si L2,3 EELS spectra (Figure b) showed a further increase in the A/B peak ratio compared to that at 400 °C in CH4. This phenomenon may originate from the influence of introducing CH4. The in situ TEM images of In2O3/SiO2 indicate In2O3 edge retraction when exposed in CH4 at 600 °C (Figure S7), suggesting a transformation in the In2O3 phase (such as reduction) under this condition.

When the temperature increased to 700 °C, as shown in Figure a3, the TEM image reveals well-resolved lattice fringes with lattice distances of 0.25 and 0.22 nm. Coupled with the corresponding FFT analysis, these two planes were assigned to the (400) lattice plane from In2O3 and the (131) plane from In2Si2O7, respectively. The corresponding EELS spectrum obtained in a CH4 environment at 700 °C closely resembles those observed at 600 °C, exhibiting a comparable A/B peak ratio. In fact, this Si L2,3 A/B peak ratio trend (Figure S8) can be categorized into three stages: In stage 1 (below 200 °C), the A/B peak ratio is around 0.6, which corresponds to noninteracted In2O3 and SiO2. When the temperature reaches 400 °C (stage 2), the formation of interfacial In2Si2O7 makes the A/B peak ratio rise to 0.7. For stage 3 (above 600 °C), the influence of CH4 starts to be observed, and the A/B ratio further increases to around 0.9, indicating the higher electron density in Si. Moreover, the mass spectrometry (MS) data presented in Figure c revealed the generation of a small amount of H2 in the CH4 environment starting at 600 °C, indicating the onset of CH4 conversion under these conditions. This coincides with the third stage of A/B ratio development.

We further increased the temperature in our in situ TEM test to investigate the conversion of methane on this catalyst. The in situ TEM image of the sample in a CH4 atmosphere at 800 °C is shown on Figure a1. An amorphous region, highlighted by a red circle, is observed in the TEM image, and the corresponding inverse FFT image (Figure c) confirms the formation of this amorphous site. This amorphous site could be attributed to the reduced state for the oxide of indium facilitated by the reductive gas in the environment. This corresponds to the dynamic wetting–dewetting of nano indium liquid metal at its original site (Video S1). Besides, both In2O3 and In2Si2O7 were identified in TEM images and FFT analysis, confirming that only a portion of the indium oxide underwent reduction. A noticeable disparity is detected in the O K EELS spectra: as shown on Figure g, the new peak at 541.2 eV emerges at high temperature (>800 °C), which is attributed to the electronic transition from the O 1s orbital to a σ* bond. This discovery indicates that the function of In2Si2O7 is to store the activated species: the methane coupling process involving the methane activation and coupling. The CH4 activation over indium liquid metal leads the CH4 splitting, forming hydrocarbon fragments (CH x *) and activated hydrogen (H*). The observed σ* bond determined the activated CH x * and H* were transferred to the In2Si2O7 surface and adsorbed on it with a σ bond, forming O–H or O–CH x as illustrated on Figure h. This observation is consistent with the Si L2,3 A/B peak ratio shown in Figure S8. As aforementioned, the electron density of Si in In2O3/SiO2 was further increased at 600 °C in the CH4 environment. This change is plausibly associated with the transportation of reductive CH x * and H* species to the In2Si2O7 interface layer. In contrast, intrinsic SiO2 does not display a similar trend. Furthermore, H2 temperature-programmed reduction (H2-TPR) measurements (Figure S9) exhibit 3 peaks located at 332, 555, and 800 °C. The reduction peaks located at around 332 and 800 °C were attributed to the reduction of surface and bulk In2O3 species, respectively. , We propose that the intermediate peak located at 555 °C originates from the hydrogen spillover from the indium surface to the In2Si2O7 interface, which likely increases the H2 consumption.

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In situ analysis of converted samples in CH4. In situ TEM images of samples with the formulation of In/In2O3/ In2Si2O7/SiO2 in a CH4 environment at 800 (a1), 850 (a2), and 900 °C (a3); FFT analysis of panel a1 (b1) and panel a2 (b2); inverse FFT image of sample in a CH4 environment at 800 °C (c); FFT filtered image that distinguishes In2O3 (green) and In2Si2O7 (red) phases in a CH4 environment at 900 °C (d); Intensity map of in situ TEM images acquired on the sample in a CH4 environment at 900 °C for 12 s (e1) and 64 s (e2); Si L2,3 EELS spectra of the sample in a CH4 environment at 800 °C, 850 °C, and 900 °C (f); O K EELS spectra of the sample in a CH4 environment at room temperature, 600, 800, and 900 °C (g); and illustration of the formation of σ bond on In2Si2O7 during methane activation (h).

At 850 °C, the in situ TEM image (Figure a2) shows an In2O3 nanoparticle overlapping with the support, and the corresponding FFT image (Figure b2) confirmed the coexistence of In2O3 and In2Si2O7. It is noticeable that the A/B peak ratio in Si L2,3 EELS increased compared to that at 800 °C. This evidences that Si-containing species evolve in the CH4 activation process. Given that the silica support is considered as highly inert, , it is highly probable that In2Si2O7 serves as the storage sites for activated fragments, and the stored H* and CH x * fragments on In2Si2O7 alter the chemical environment of Si in In2Si2O7. This claim is strengthened by further experiments, which will be presented later in this paper.

Further increasing the temperature to 900 °C, the TEM image presented in Figure a3 and its corresponding FFT filtered TEM image (Figure d) reveal the In2O3 (the green section) is supported by In2Si2O7 (the red section). The dynamic change in the support relationship between In2O3 and In2Si2O7 was observed in the intensity map. As illustrated in Figure e1,e2, the In2O3 boundary migrates, and as the reaction progresses, In2O3 decreases in size, which can be attributed to its partial reduction. Additionally, voids and defects were observed on the catalysts. As shown in Figure e2, the red circles on the support exhibit a lower contrast. The formation of these defects may be associated with the influence of H* and CH3* on the In2Si2O7 support, which causes the In2Si2O7 rearrangement. The Si L2,3 EELS spectrum at this temperature shows the highest A/B peak ratio among all tested samples, revealing that the In2Si2O7 is most significantly affected by methane under this condition. The O K EELS spectrum shows an obvious peak at 541.2 eV, further validating that activated hydrocarbon fragments are stored on In2Si2O7.

Methane Conversion by In, In2O3, and In2Si2O7 Synergy

In addition to the aforementioned in situ TEM investigation, the methane activation and NOCM performance over the sample were also evaluated. As seen in the bulk reaction results shown in Figure a, H2 generation commenced at 600 °C. The hydrogen signal intensity, measured by online MS, increased steadily with rising temperature. Notably, the hydrogen productivity exhibited a significant increase when the temperature exceeded 800 °C.

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The sample conversion mechanisms. Online MS signal intensities of product gases: H2 (m/z = 2) (a), H2O (m/z = 18) (b), and CO2 (m/z = 44) (c); a comparison of H2O and H2 signals at 600 °C in a CH4 environment (d); and the atomic illustration of two different pathways for reducing In2O3 as discussed in the paper (e).

Importantly, at 800 °C, C2 species such as ethylene and ethane were also detected, as indicated by the MS data (Figure S10). Water production (Figure b) was observed at temperatures exceeding 600 °C, although its signal intensity was significantly lower than that of H2. Given that the ionization energy of water (∼12.56 eV) is lower than the ionization energy of hydrogen (∼16 eV), this weak water signal measured by MS indicates only trace amounts of water were generated. Meanwhile, only a trace amount of CO2 was observed in the product (Figure c).

The MS spectra of CO2, H2, and H2O were further compared to investigate the dynamics of methane activation over the catalyst. As shown in Figure d, the H2O signal appeared approximately 30 s after the detection of the CO2 and H2 signals. As no oxygen-containing reactant was introduced into the reactor, it is likely that the CO2 and water originated from the reduction of the In2O3/SiO2 catalyst. Considering the exceptionally high reduction energy of SiO2 (4.95 eV), this reduction should occur over In2O3, which explains the retraction of the In2O3 edge at high temperature in the CH4 environment. The delayed appearance of H2O suggests that CH4 initially reduces a portion of In2O3 to form metallic indium and simultaneously generates both CO2 and H2. Once H2 is produced, it can further reduce In2O3 to form water; thus, part of In2O3 can be converted into metallic indium active sites. As illustrated in Figure e, through this procedure, the catalyst consisting of In/In2O3/In2Si2O7/SiO2 is formed, further driving the NOCM reaction.

The XRD pattern of the post-used catalyst given in Figure S11 again confirms the formation of In2Si2O7 and reduction of In2O3 during the reaction. Weak but distinguishable peaks were observed at 2θ = 28.5° and 30.1°, corresponding to the (021) and (201̅) planes of In2Si2O7 (ICSD#74897). Additional XRD diffraction peaks were detected at 2θ = 32.9°, 36.4°, 39.2°, 54.5°, and 66.9°, which correspond to the (101), (002), (110), (112), and (211) planes of metallic indium (ICSD#41094). The persistence of In2O3 diffraction indicates that only a portion of the In2O3 was reduced. Meanwhile, no graphitic carbon was observed in the post-used catalyst.

The methane conversion rates at different temperatures are presented in Figure S12, showing an increase from 0.21 mmol/h at 800 °C to 1.04 mmol/h at 900 °C. To identify the true active sites responsible for this reaction, we calculated the apparent activation energy of CH4 conversion based on these activity results. The corresponding Arrhenius plot is depicted in Figure S13. The calculated activation energy is 48.1 ± 2.7 kcal/mol, which aligns well with the theoretical predictions for CH4 activation over liquid indium, reported to be in the range of 45–55 kcal/mol. This correlation strongly suggests that the in situ generated liquid indium is the key active site for CH4 conversion. The stability test, shown in Figure S14, presents the good stability of the catalyst when continuously operated for 20 h, determining the durability of the catalyst.

Based on these observations, the reaction mechanism of this process can be proposed. As shown in Scheme , at temperatures above 400 °C, an interfacial layer of In2Si2O7 forms between In2O3 and SiO2. When this temperature is further increased (i.e., 600 °C), In2O3 undergoes partial reduction by CH4, resulting in the formation of liquid indium active sites. These active sites are responsible for the sustained CH4 conversion. During CH4 activation, reactive intermediates such as H* and CH x * are generated on indium liquid metal. These species are subsequently transferred and adsorbed onto the In2Si2O7 layer, where further coupling reactions may occur. This migration of hydrocarbon fragments away from the indium surface mitigates carbon deposition, a common challenge in CH4 activation over liquid metal, , by preventing overcracking and carbon buildup on the active sites.

1. Formation of In/In2O3/​In2Si2O7/SiO2 Structure and the Consequent CH4 Conversion Process .

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a The red arrow represents the pathway for hydrocarbon fragment migration, while the green arrow represents the pathway of H2 and C2 production.

Conclusion

In summary, this study utilized spherical aberration-corrected in situ high-resolution TEM, combined with EELS and MS, to investigate the dynamic structural and chemical transformations of silica-supported indium oxide during CH4 conversion. The in situ environmental TEM and EELS analyses revealed that, regardless of the reaction gas environment, a new In2Si2O7 layer forms at the interface between In2O3 and SiO2 upon heating to 400 °C.

It is important to emphasize the observation that in the CH4 environment, the In2O3 undergoes partial reduction above 600 °C, eventually leading to the formation of a liquid state of indium metal, which serves as the active sites for CH4 conversion. Further in situ EELS analysis confirms that the newly formed In2Si2O7 layer participates in the CH4 conversion process by temporarily storing activated H* and CH x * species prior to their coupling. This mechanism effectively suppresses the overcracking of CH4 on liquid metal. This research provides fundamental insights into the active structure of In2O3/SiO2 that transforms into In/In2O3/​​In2Si2O7/SiO2 and participates in the NOCM reaction. Also, this work demonstrates a strategy for the in situ generation of liquid metal active sites under reaction conditions, offering a promising direction for the future application of liquid metal catalysts.

Supplementary Material

au5c01660_si_001.pdf (945.1KB, pdf)
Download video file (83.1MB, avi)

Acknowledgments

This work was supported by the Australia Research Council Discovery Project (DP22002851) and Future Fellowship (FT220100601). The authors acknowledge the financial support from the Sydney Nano Grand Challenge, International SDG Collaboration Program (4891165261), Partnership Collaboration Awards (1372369152) at the University of Sydney. This work was also supported by grants from the National Natural Science Foundation of China (Nos. 52371001 and 52301004). The authors also acknowledge the scientific and technical support from the Microscopy Australia node at the University of Sydney (Sydney Microscopy & Microanalysis).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.5c01660.

  • Experimental details of sample preparation, characterization, catalytic performance test, and in situ TEM test; XPS spectra of In2O3/SiO2; SEM image of SiO2; in situ TEM images of In2O3/SiO2 in N2 or CH4 at various temperatures, as well as the FFT and IFFT analysis; online MS results; H2-TPR profile of In2O3/SiO2; XRD pattern of post-used In2O3/SiO2; methane conversion rate over In2O3/SiO2; and stability test results (PDF)

  • Video S1: Dynamic wetting–dewetting of nano indium liquid metal at its original site (AVI)

The authors declare no competing financial interest.

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