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