Carbon-supported Ni nanoparticles in CO₂ methanation: role of a superficial NiO shell observed by in situ TEM†

Katherine E. MacArthur‡ *^a, Liliana P. L. Gonçalves‡^bcd, Juliana P. S. Sousa^b, O. Salomé G. P. Soares^cd, Hans Kungl^e, Eva Jodat^e, André Karl^e, Marc Heggen^a, Rafal E. Dunin-Borkowski^a, Shibabrata Basak*^e, Rüdiger-A. Eichel^efg, Yury V. Kolen'ko^b and M. Fernando R. Pereira*^cd
aErnst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, 52425 Jülich, Germany. E-mail: k.macarthur@fz-juelich.de
^bInternational Iberian Nanotechnology Laboratory (INL), Braga 4715-330, Portugal
^cLSRE-LCM – Laboratory of Separation and Reaction Engineering – Laboratory of Catalysis and Materials, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal. E-mail: fpereira@fe.up.pt
^dALiCE – Associate Laboratory in Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
^eInstitute of Energy Technologies – Fundamental Electrochemistry (IET-1), Forschungszentrum Jülich GmbH, Jülich, Germany. E-mail: s.basak@f-juelich.de
^fInstitute of Physical Chemistry, RWTH Aachen University, Aachen, Germany
^gFaculty of Mechanical Engineering, RWTH Aachen University, Aachen, Germany

First published on 16th July 2025

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1 Introduction

The CO₂ methanation reaction has been the subject of intense studies, since it offers an interesting pathway to produce a carbon-neutral methane fuel (e-CH₄), thus combating the CO₂ emissions to the atmosphere.^1–5 Mostly Ni-based catalysts supported on a variety of supporting materials, such as metal oxides,^6–8 zeolites,⁹ metal organic frameworks,¹⁰ and carbons,^3,11–18 are used for CO₂ methanation. Interestingly, the supporting material plays a major role in the catalyst’s performance, and it has been demonstrated that the same active phase, such as Ni nanoparticles, can present different activity, selectivity, and stability, depending upon the supporting material being used.^3,5,11,19 Besides the fact that CO₂ methanation involves reactant diffusion, adsorption, surface reaction, product desorption, and diffusion, the metal–support interactions can also play a role in the catalytic mode of action, which is consequently reflected in the performance of the catalyst.^3,20

Notably, the mechanism of CO₂ methanation is still under debate. The proposed mechanisms are largely governed by the catalytic system and the reaction conditions, and the methanation could proceed through (i) the associative pathway, in which CO₂ is adsorbed associatively with a former adsorbed H atom, forming formate as an intermediate, and (ii) the dissociative pathway, where CO₂ is dissociated in CO, which is the main intermediate of the reaction.⁴ The mechanism behind nickel-catalyzed CO₂ methanation involves the dissociation of H₂ over the Ni nanoparticles, while CO₂ adsorption usually occurs on the supporting material or nickel–support interface with dissociation and subsequent adsorption of the resultant carbonyl on Ni nanoparticles.²¹

A comprehensive understanding of the behavior of the supported Ni catalyst under reaction conditions, coupled with a thorough elucidation of the associated reaction mechanism, is imperative for facilitating the rational design of high-performance catalyst materials. Hence, in situ techniques are particularly important and have been extensively used to elucidate the mechanism of CO₂ methanation over various supported catalytic systems. For instance, in situ infrared (IR) spectroscopy has been used for underpinning the reaction intermediates,^20,22–25 while X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) have been employed to elucidate the chemical nature of the real catalysts under methanation conditions.^26,27 At the same time, the fine microstructural modifications, that the supported Ni catalysts undergo during CO₂ methanation, have been studied to a less extent. For example, deactivation of the Ni catalysts is often attributed to the sintering of the Ni nanoparticles and/or coke building up on the active phase, which has been investigated by ex situ transmission electron microscopy (TEM).^4,28 Importantly, in situ observation of fine microstructural modifications should assist in understanding and further developing active, selective and stable methanation catalysts. Hence, in this study, in situ TEM is applied to elucidate the behavior of the Ni catalyst supported on the reduced activated carbon during CO₂ methanation, affording insight into the presently lacking understanding of the catalyst’s real microstructure and its dynamics.

The understanding and atomic level information provided by aberration-corrected TEM is unparalleled, particularly for the study of catalyst nanoparticles. The development of a specialized microelectromechanical system (MEMS)-based sample holder allows us to contain gas near the sample at more than 1 bar pressure with precise control over temperature, as well as gas flow and gas compositions.

In this study, we have utilized a MEMS-based nanoreactor in which two Si/SiNx chips are positioned on top of each other and sealed with an O-ring, allowing for a maximum gas pressure of up to 2 bar. A 5 μm spacer on the bottom chip guides the gas mixture from the gas supply system towards the microheater. The system can be heated up to 1000 °C with small power consumption (in the range of mW), thus offering a low drift, while the 30 nm thin electron transparent SiN_x windows, that are present in these chips, allow transmission of electron beams, thus affording structural characterization of the samples under realistic reaction conditions.

2 Results and discussion

Our earlier study³ demonstrated excellent CO₂ methanation performance of a Ni catalyst supported on reduced activated carbon (Ni/ACR). Interestingly, after 90 h of CO₂ methanation at 450 °C and 0.1 MPa, with 10% H₂, 40% CO₂ and 50% He, a distinct crystalline shell of Ni oxide was formed on the surface of the Ni nanoparticles supported on ACR observed via ex situ TEM. Fig. 1 shows TEM images of the Ni nanoparticles pre- and post-reaction, with the oxide shell of thickness 2–3 nm highlighted in Fig. 1b. The observation of a Ni oxide shell on the Ni nanoparticles after prolonged CO₂ methanation was intriguing, as it contradicted the prevailing expectation that metallic Ni was the active phase of the reaction. However, these samples were transferred in air, after cooling to room temperature, for the post-reaction analysis; therefore, it was unclear whether the oxide shell was formed during the air transfer or as a consequence of the reaction conditions. Understanding this detail, along with the possible deactivation mechanism of the supported Ni catalyst, will not only shed light on the debatable methanation mechanism but also pave the pathway for engineering improved catalysts.

To exactly replicate the reaction conditions, the drop-cast Ni/ACR catalyst was first reduced under a H₂ environment (4% H₂ with N₂ as the carrier gas at 1 bar pressure) at 400 °C, and then imaged under the reactive H₂ and CO₂ gases providing methanation conditions. It is found that once a temperature of 450 °C is reached under the reactive gases, the Ni nanoparticles began to dynamically change shape whilst still retaining crystallinity. Fig. 2(a–d) shows the dynamics of such a nanoparticle within the thick ACR supporting material. The arrows highlight the dynamic reduction-growth of the nanoparticle that simulates a ‘breathing’ behaviour. We believe that this ‘breathing’ behavior is the macroscopic manifestation of a dynamic equilibrium at the nanoscale. It likely involves the continuous, competitive oxidation of the nickel surface by CO₂ to form NiO, and the simultaneous reduction of this oxide shell by H₂. The observed shape changes reflect the local fluctuations in the rates of these opposing reactions. The corresponding breathing process can be better portrayed in Movie S1 in the ESI.† It should be pointed out that the reduction performed at 400 °C during the in situ experiments followed from our previous work on the same Ni on the ACR sample, where during the temperature programmed reduction (TPR) experiment, it was verified that even the Ni species that interact more strongly with the carbon were reduced at ca. 380 °C.³

To determine whether the dynamic behavior is impacted by the H₂ pre-treatment, Fig. 3 compares in situ results where the sample was initially reduced in an H₂ atmosphere to another set of experiments where the H₂ pre-treatment was not used. Overall, like the nanoparticle shown in Fig. 2, Fig. 3 shows the dynamic behaviour of Ni nanoparticles towards the edge of the ACR supporting material under the dynamic conditions. The red arrows highlight a few of these changes in two particles, as examples. Interestingly, the presence of an oxide shell is clearly visible in both cases. The green dashed lines in Fig. 3a and j highlight the oxide shell. The high-resolution image in Fig. S1† shows the polycrystalline nature of the oxide layer outside the nanoparticles. The apparent absence of such an oxide layer for the particle shown in Fig. 2 is most likely due to the difficulty to resolve such a layer in the presence of the thick ACR supporting material. Notably, it is clear that the particles towards the edge of the ACR support changed more dynamically and, in some cases, appear almost molten, rolling around within an oxide shell. This is likely due to improved mass transport, allowing for more efficient diffusion of the CO₂ and H₂ reactant gases to the nanoparticle surface compared to particles embedded deeper within the porous support. The red arrows in Fig. 3a–i indicate one such nanoparticle. This behaviour was seen regardless of whether or not the reducing H₂ pre-treatment was applied, as observed in Fig. 3a–i (the sample without H₂ reduction) vs. Fig. 3j–n (similar sample, under similar conditions, but with H₂ reduction pre-treatment). Primarily, we noticed that the formation of the oxide shell was not a mechanism of the deactivation of the particles. In fact, the most dynamic behaviour was seen for particles exhibiting an oxide shell, thereby suggesting a permeable structure in which the reactive gases can penetrate to reach metallic Ni active sites. Thus, we demonstrated that the observed NiO shell after CO₂ methanation for 90 h (Fig. 1) is not a result of the non-reduced catalyst but rather formed under reaction conditions, as observed during the in situ TEM studies.

Using in situ XPS, Heine and co-workers²⁹ reported the formation of NiO even at room temperature in CO₂ gas alone, which requires the transfer of two electrons from metallic Ni to the CO₂ molecule, forming Ni²⁺ due to the oxidation of Ni⁰ and CO due to the reduction of CO₂. However, after the introduction of H₂ into the system mimicking methanation conditions, NiO species were found to be reduced again to metallic Ni⁰. In another interesting study that uses in situ XPS and in situ NEXAFS spectroscopies, Giorgianni and co-workers²⁶ reported a similar behavior, wherein the metallic Ni surface was found to oxidize rapidly upon exposure to CO₂. Under methanation conditions (i.e., CO₂ + H₂), while in situ XPS revealed that Ni is predominantly in a reduced state, in situ NEXAFS indicated that the shell of Ni nanoparticles is prevalently in a Ni²⁺ state due to incomplete reduction. Interestingly, Mutz and co-workers³⁰ showed that, in the presence of CO₂, Ni starts to oxidize after the removal of H₂ from the gas flow. When H₂ is again introduced into the stream, the reduced fraction of metallic Ni was found to increase to 94%, yet having a 6% fraction of oxidized Ni. In our study, we would not expect the Ni nanoparticles to oxidize, since they are under an atmosphere rich in H₂; however, CO₂ should be responsible for the oxidation of the surface of the Ni nanoparticles, thus forming the NiO shell according to the following reaction: Ni + CO₂ = NiO + CO. The Ni⁰ species can be oxidized by CO₂ to form Ni²⁺ species, activating the CO₂ as well, to form CO.³¹ Importantly, an experiment where only CO₂ gas was used (Fig. S2†) shows that the presence of both H₂ and CO₂ is required for the oxide-shell formation (i.e., only surface oxidation), since in the presence of CO₂ alone, no oxide shell can be seen. Therefore, the synergetic relationship between CO₂ and H₂ should be responsible for the formation of the Ni@NiO core–shell structure during CO₂ methanation, as observed herein.

The formation of this Ni@NiO core@shell structure under reaction conditions might be one of the reasons for the good performance of this catalyst. On the one hand, the NiO formation in close contact with the supporting carbon material, where CO₂ is adsorbed, leads to the activation of this molecule,^25,31 while the Ni⁰ in the core is responsible for H₂ dissociation, since the dynamic nature of the oxide shell allows for the diffusion of the H₂ to the metallic Ni⁰ core of the nanoparticles. This way, both reactants can be in close contact, which facilitates the reaction. On the other hand, NiO might present oxygen vacancies, which are known to induce the dissociation and activation of CO₂ on the surface of the catalyst.³² It has been reported before that the rate-determining step for the CO₂ methanation reaction should be related to the availability of adjacent adsorbed H to hydrogenate adsorbed CO;³³ thus, this proximity of CO₂ activation and CO formation on the NiO shell and H₂ dissociation in the Ni core might explain the good performance on the catalyst.³

After examining the dynamic behaviour of the particles under a reactive methanation environment, we then explored the mechanisms of deactivation of the catalyst. The main deactivation mechanism was observed to be related to a particle size increase. This can be observed in Movie S2,† obtained at 450 °C in the presence of both H₂ and CO₂. The larger particles demonstrated no dynamic changes, while the smaller particles demonstrated to be very active. Upon cooling, the particle structures stabilised to fill the oxide shell again (this can be seen in Fig. S3†), yielding a structure that matches the original ex situ characterisation (Fig. 1b). Finally, post in situ energy dispersive X-ray spectroscopy analysis in scanning TEM mode (STEM-EDX) was performed to confirm the presence of a 2–3 nm thick NiO shell (Fig. 4(a–f)). The heating and gas profile during these in situ experiments can be found in Fig. S4.†

Particle size distribution was not thoroughly studied in this work; however, since methanation is a known structure-sensitive reaction, the catalyst particle size is an important aspect. Thus, further experiments should be carried out to address this topic.

Our experimental in situ TEM findings have demonstrated that the nickel oxide shell observed on the catalytic Ni nanoparticles supported on reduced activated carbon in our previous study³ is in fact formed under reaction conditions of CO₂ methanation, and it is expected to be an integral part of the reaction process. Fig. 5 presents a summary of the findings of this work. We believe that the Ni oxide shell structure, in situ formed during methanation, exhibits a permeable nature, allowing for the penetration of the reaction CO₂ and H₂ gases to the active metallic Ni⁰ core, otherwise dynamic changes under reaction conditions would not continue to be observed in the nanoparticles demonstrating a shell. We hypothesize that the Ni oxide shell acts as an enhancer of the CO₂ adsorption/activation, while the Ni⁰ in the core is responsible for H₂ dissociation, which leads to both reactants being in close proximity, facilitating the CH₄ formation.^35,36 Notably, we observed in situ that the main deactivation mechanism was related to a particle size.

3 Conclusions

In summary, we have presented an in situ TEM investigation of our previously reported high-performance nickel nanoparticle catalyst supported on reduced activated carbon.³ The unprecedented in situ formation of a NiO shell around the metallic Ni⁰ core was directly imaged and studied under CO₂ methanation conditions. An important proposed feature of real Ni@NiO core@shell catalyst nanoparticles is coupling of the enhanced CO₂ adsorption/activation over the NiO shell with H₂ dissociation over the Ni⁰ core, bringing the reactants and intermediates together to accomplish CO₂ hydrogenation into CH₄. This fundamental understanding of CO₂ methanation over the Ni catalyst provides a framework for the judicious design and realization of new high-performance and stable core@shell catalysts for this reaction, in particular, and other interesting CO₂ hydrogenation reactions, in general.

Data availability

Data for this article are available in the ESI.†

Author contributions

Katherine E. MacArthur: conceptualization, data curation, formal analysis, investigation, methodology, and writing – original draft; Liliana P. L. Gonçalves: conceptualization, formal analysis, investigation, methodology, and writing – original draft; Juliana P. S. Sousa: supervision and writing – review & editing; O. Salomé G. P. Soares: conceptualization, supervision, and writing – review & editing; Hans Kungl: writing – review & editing; Eva Jodat: writing – review & editing; André Karl: writing – review & editing; Marc Heggen: writing – review & editing; Rafal E. Dunin-Borkowski: funding acquisition, project administration, and writing – review & editing; Shibabrata Basak: conceptualization, data curation, investigation, methodology, writing – review & editing, and supervision; Rüdiger-A. Eichel: writing – review & editing; Yury V. Kolen'ko: conceptualization, funding acquisition, project administration, resources, supervision, and writing – review & editing; M. Fernando R. Pereira: conceptualization, funding acquisition, project administration, resources, supervision, and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the support of the German Research Foundation (DFG) for supporting this work under grant number HE 7192/1-2. S. B. acknowledges support for the project ‘Electroscopy’ (Grant No. 892916) from the Marie Skłodowska-Curie action. L. P. L. G. thanks the Portuguese Foundation for Science and Technology (FCT) for the PhD grant (SFRH/BD/128986/2017). This work was partially supported by national funds through Fundação para a Ciência e a Tecnologia, I.P./MCTES: LSRE-LCM, UID/50020; ALiCE, LA/P/0045/2020 (DOI: https://doi.org/10.54499/LA/P/0045/2020). O. S. G. P. S. acknowledges FCT funding under the Scientific Employment Stimulus – Institutional Call (CEECINST/00049/2018).

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Footnotes

† Electronic supplementary information (ESI) available: Detailed experimental procedures, together with the additional TEM data and a video. See DOI: https://doi.org/10.1039/d5im00033e

‡ These authors contributed equally.

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