Revealing the active sites of the structured Ni-based catalysts for one-step CO2/CH4 conversion into oxygenates by plasma-catalysis

doi:10.1016/j.jcou.2021.101675

Journal of CO2 Utilization

Volume 52, October 2021, 101675

https://doi.org/10.1016/j.jcou.2021.101675 Get rights and content

Highlights

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One-step conversion of CO₂/CH₄ was achieved over hierarchical foamed catalysts.
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Ni-based catalysts with different valence states showed greatly varied activities.
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NiAl-LDH/NF with abundant −OH groups obviously increased the CH₃OH selectivity.
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4. The −OH groups also suppressed the C-deposition process for CO₂/CH₄ conversion.
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A “−OH reservoir” mechanism was proposed for plasma-enable catalysis.

Abstract

The direct conversion of CO₂/CH₄ into liquid chemicals is an attractive way for using carbon resources, but it is still unclear which types of catalysts are suitable for plasma-involved catalysis. Herein, a serious of nickel foam (NF) supported Ni-based catalysts derived from Layered double hydroxides (LDH) were newly designed by a green hydrothermal strategy. The crystal structures, morphologies, and the redox behaviors of these catalysts (NiAl-LDH/NF, NiO/NF, Ni/NF and NiGa/NF) were systemically characterized. Plasma-enabled direct conversion of CO₂/CH₄ was performed in a co-axial dielectric barrier discharge reactor using a nanosecond repetitive-pulse power, and the results showed that the selectivities of the liquid oxygenates were strongly depended on their microstructures, surface compositions (valence states) and reducibilities. The zero-valent Ni/NF and NiGa/NF showed high total liquid selectivity (> 30 %) and dominant CH₃COOH (> 15 %), while the Ni²⁺ in NiO/NF gave the comparable activities for CH₃OH (8.9 %) and CH₃COOH (9.6 %). Unexpectedly, NiAl-LDH/NF exhibited the highest CH₃OH selectivity (12.3 %) and negligible carbon deposition. According to systemic characterizations, an “−OH reservoir” mechanism was proposed that numerous −OH groups over the ultrathin nanosheets (∼60 nm) of NiAl-LDH may react with the interfacial CH_x* radicals and, on the other hand, possess inferior electron affinity for electronegative C atom, thus endowing NiAl-LDH the prominent CH₃OH selectivity and high resistance to carbon deposition.

Graphical abstract

Introduction

The increase of consumption of fossil energy leads to vast greenhouse gases such as CO₂ and CH₄, which makes global warming become one of the most arduous climate challenges [1]. Among the technologies and methods that have been proposed to reduce the concentration of greenhouse gases, the direct or indirect conversion of CO₂ and CH₄ into high value-added chemical products has attracted widespread attention [2,3]. Dry reforming of methane (DRM) with carbon dioxide can directly convert the CO₂ and CH₄ into syngas (CO and H₂) [4,5]. The syngas can be further subjected to the Fischer-Tropsch synthesis reaction to produce high value-added chemical products such as alcohols, acids and aldehydes [[6], [7], [8]]. Usually, DRM reaction needs to be carried out at high temperatures (> 700 °C) to ensure high conversion rate. However, high temperatures will cause catalysts sintering and carbon deposition, leading to rapid deactivation of the catalysts [[9], [10], [11]]. With the development of plasma technology, non-thermal plasma (NTP) is increasingly used in various fields, such as material modification and chemical synthesis [[12], [13], [14]]. Besides, the combination of NTP and heterogeneous catalyst, which called plasma-catalysis, has attracted more and more attentions [15,16]. NTP can generate massive high-energy electrons (1−10 eV) and dissociate the originally stable gas molecules of CO₂ (5.5 eV) and CH₄ (4.5 eV) by electron impact excitation, forming highly active radicals, excited molecules, atoms, and ions, thus promoting the chemical reactions at low temperatures [17,18]. And up to-date, the most used reactor for DRM reaction was the dielectric barrier discharge (DBD) reactor [19,20].

A great deal of researches have been done to exploring the plasma-catalysis on dry reforming of CH₄, including the influence of DBD characteristics and reaction conditions on the conversion rate of CO₂/CH₄ and on the selectivity of products [21,22]. Meanwhile, different catalysts have also been investigated [23,24]. Tu et al. [25] studied the effect of material filling on plasma-catalysis, and found that partially packing can maintain a strong plasma-catalysis interaction and significantly improve the selectivity of H₂ and CO (32.8 % and 54.9 %). Li et al. [26] filled the DBD reactor with the solid catalysts to directly convert CO₂ and CH₄ into liquid chemicals and synthesis gas under atmospheric condition, and proved the feasibility of directly synthesizing high value-added liquid chemicals and fuels from CO₂ and CH₄. The current studies mainly focus on the effects of the geometric parameters of the DBD reactor, the filling materials, the types of active materials and other reaction conditions on the conversion and the selectivity of the products [27,28]. However, the interactions between plasma and catalysts are complicated and there are few reports on revealing the true active sites of the catalysts for plasma-enabled dry reforming of CH₄. Zeng et al. [29] found the combination of plasma with the NiO/γ-Al₂O₃ and MnO₂/γ-Al₂O₃ significantly improved the CH₄ conversion and exhibited a plasma-catalytic synergy. Sheng et al. [30] used Ni/Al₂O₃ for dry reforming of CH₄ and found that NTP can not only enhance the activation of CO₂ and form surface carbonate species, but also promote the formation of CH_x* species from CH₄. Nevertheless, although both Ni-based catalysts possess high catalytic activities for CO₂/CH₄ conversion, it is still unclear that how the catalysts with varied valence states affect the distribution of the products during plasma catalysis, especially for the attractive liquid oxygenates. Therefore, it is urgently necessary to clarify the relationship between product selectivity and the corresponding active sites.

Layered double hydroxides (LDH) has shown great potentials in the fields of catalysis, energy storage and environmental remediation due to their unique lamellar structure, in which the cations are orderly distributed on the edge-sharing MO₆ octahedral layers. Additionally, nickel foam (NF) has attracted much attention because of its three-dimensional porous structure, high specific surface area as well as heat and mass transfer capacity [31,32]. Our most recent work showed that the LDH/NF based structured catalysts can maintain high CO₂ conversion and build strong synergy effect for electric-assisted catalysis process [33,34]. Herein, we use a one-step hydrothermal strategy to grow NiAl-LDH on NF surface to obtain highly-dispersed catalyst (NiAl-LDH/NF) involving uniform nanosheets structure. In order to investigate the DRM performance over different active sites for plasma-catalysis, we treated NiAl-LDH/NF in air and the mixture of H₂/Ar to obtain NiO/NF and Ni/NF catalysts, respectively. To investigate the metal-support interaction on catalytic activity, NiGaAl/NF with partial replacement of Al by Ga was also prepared. The effects of the catalysts with varied valence states on the conversion of CO₂/CH₄, the selectivity of CO, H₂, C_xH_y, and especially for liquid products in DRM process were carefully tested. To elucidate the structure-activity relationship and the possible mechanism for plasma-induced CO₂/CH₄ conversion, systemic characterizations were carried out and further discussed combined with DFT simulations.

Section snippets

Materials

All reagents including Nickel acetate tetrahydrate [Ni(CH₃COO)₂·4H₂O], Aluminum nitrate nonahydrate [Al(NO₃)₃·9H₂O], Urea [CO(NH₂)₂], Ammonium fluoride (NH₄F), Gallium nitrate [Ga(NO₃)₃·xH₂O] and ethanol, were analytical reagent and were purchased from Macklin. Nickel foam (6.2 cm × 5.0 cm × 1.5 cm) was ultrasonic treated in dilute hydrochloric acid solution (3.0 M) for 30 min, and then further ultrasonic treated in ethanol and water for three times respectively, and then dried for 3 h at 80 °C.

Characterization of fresh catalysts

Fig. 3. depicts the XRD patterns of the prepared samples. The diffraction peaks of NF substrate at 44.5 °, 51.8 ° and 76.3 ° can be indexed to the (111), (200) and (220) planes of metallic Ni (JCPDS-04−0850) [36]. The main characteristic peaks of NiAl-LDH/NF can be ascribed to NiAl-LDH crystalline phase (JCPDS-38−0715). Correspondingly, these peaks at 11.5 °, 23.2 °, 34.8 °, 39.3 °, 46.6 °, 60.7 ° and 62.0 ° correspond to the planes of (003), (006), (012), (015), (018), (110) and (113) of LDH

Conclusion

This study exemplified that the valence states and surface microstructure of the catalysts can efficiently modulate the distributions of liquid oxygenates for plasma-enabled catalysis and thereby achieve potential power-to-chemical energy conversion. It is found that the formation of CH₃COOH is closely related to the reducibilities of the catalysts as well as surface active Ni⁰ content, resulting in the high CH₃COOH selectivity (> 15 %) of Ni/NF and NiGa/NF. Differently, Ni²⁺ in NiO/NF gave the

CRediT authorship contribution statement

Jiangwei Li: Investigation, Validation, Visualization, Writing - original draft. Liguang Dou: Software, Writing - review & editing, Supervision. Yuan Gao: Data curation, Visualization, Conceptualization. Xueting Hei: Investigation, Formal analysis, Visualization. Feng Yu: Methodology, Funding acquisition, Supervision. Tao Shao: Resources, Funding acquisition, Supervision, Project administration.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

This work is financially supported by the National Science Fund for Distinguished Young Scholars [grant no. 51925703], the National Natural Science Foundation of China [grant nos. 21908219, 22068034 and 51807190], Program of Science and Technology Innovation Team in Bingtuan (grant no. 2020CB006). Dr. Dou thanks the financial support from State Key Laboratory of Advanced Electromagnetic Engineering and Technology (grant no. AEET2021KF001). We acknowledge the National Supercomputing Center in

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