Elsevier

Catalysis Today

Volume 356, 1 October 2020, Pages 589-596
Catalysis Today

Carbon dioxide reforming of methane over MgO-promoted Ni/SiO2 catalysts with tunable Ni particle size

https://doi.org/10.1016/j.cattod.2020.01.006Get rights and content

Highlights

  • The additive MgO to Ni/SiO2 catalyst could enhance CO2 activation and dissociation.

  • Nien-0.4 %MgO catalyst possessed high activity and strong coke resistance.

  • The providing electron capacity of additive MgO became stronger as the Ni particle size was smaller.

  • The smaller Ni particle size and higher Ni electron density promoted CO2 activation and dissociation.

Abstract

Design and preparation of coke resistant nickel-based (Ni-based) catalyst is the key point for the industrial application of carbon dioxide reforming of methane. In this work, MgO-promoted Ni/SiO2 catalysts with tunable particle size were prepared by the multiple-impregnation method and were employed to catalyze carbon dioxide reforming of methane. Results showed that the additive MgO was an electron donor, and its providing electron capacity to Ni became stronger as the Ni particle size was smaller. Catalytic activity evaluation presented that the Nien-0.4 %MgO catalyst possessed superior catalytic activity (CH4 and CO2 conversions maintained at 83.6 % and 89.9 %) at 750 °C. Its carbon deposition amount was only 0.16 wt.% after 100 h runs. The higher activity and the stronger coke resistance of the Nien-0.4 %MgO catalyst were due to the smaller Ni particle size (8.4 nm) and higher Ni electron density, which could promote CO2 activation and dissociation, and finally balance the formation rates of C species and O species over the MgO-promoted Ni/SiO2 catalysts.

Introduction

CO2 reduction and utilization has attracted more and more global attention due to the increasing pressure from environmental pollution and energy crisis. Great efforts have been concentrated to reduce CO2 emission and utilize CO2 to produce energy chemicals so as to solve the global warming and energy problems [1,2]. CO2 reforming of methane (or CO2 reforming of CH4, CRM) presents a powerful way to simultaneously convert two major greenhouse gases (CO2 + CH4) into syngas (CO + H2) with a low or adjustable H2/CO ratio, which is suitable for Fischer-Tropsch synthesis and carbonyl synthesis [[3], [4], [5], [6], [7]]. However, due to the high bond energies of CO2 and CH4 molecules, CRM reaction had to be proceeded only at very high temperature (> 700 °C) and inevitably resulted in catalyst deactivation. Thus, how to design and prepare a kind of catalyst with high activity and stability is a hot topic for CRM reaction.

Compared with the noble metals-based catalysts, the nickel-based (Ni-based) catalysts have been widely investigated due to their higher activity and lower cost [8,9]. Both theoretical prediction and experimental results have proved that Ni possessed strong C–H bond breaking ability, meanwhile, Ni-based catalyst sintering and carbon deposition occurred during long term run. The catalyst sintering enlarged Ni particle size and decreased active sites, while the carbon deposition encapsulated catalysts and hindered the contact between catalysts and reactants.

To counteract the above-mentioned challenge, promising strategies have been proposed to improve the sintering and coke resistance capacity of Ni-based catalysts, such as controlling metal particle size, metal-support interaction, properties of support and active metal component etc. [[10], [11], [12], [13], [14], [15], [16]], of which the particle size effect was demonstrated to be the most important factor. It is agreed that the formation rate of carbon deposition dramatically slowed down on the smaller Ni particles where the driving force of carbon diffusion in Ni particles would be weakened [10,17,18]. Therefore, the small Ni particle size must be guaranteed in the design of catalysts for CRM reaction. Moreover, from the viewpoint of reaction mechanism, it is popular belief that carbon deposition was ascribed to the unbalance between the formation rates of C species and O species [19]. It is known that C species and O species came from the activation and dissociation of CH4 and CO2, respectively; and CO2 is more difficult to be activated than CH4 [20,21]. Thus, in order to make the balance between C species and O species, it is essential to enhance the activation and dissociation of CO2 on the Ni-based catalysts.

Many reports indicated that the Ni/MgO solid solution catalysts had high activity and stability, because the MgO could promote the dissociation and activation of CO2, and the strong interaction between Ni and MgO in the solid solution could control the Ni particle size. On the one hand, adding MgO into the Ni-based catalysts has been verified a promising approach to improve the activation and dissociation of CO2 [[22], [23], [24], [25]]. Wang et al. indicated that the MgO-coated Ni/SBA-15 catalysts showed high catalytic activity and stability, which derived from an improved Ni dispersion and larger medium basic sites in the presence of MgO [22]. Zhao et al. reported that MgO-doped Ni/MAS-24 exhibited high catalytic activity and stability due to the smaller Ni particle size and the MgO improved the activation and dissociation capacity of CO2 [24]. On the other hand, it has been proved that the activation energy barrier of CO2 would be greatly decreased if the linear structure of CO2 converts into a bent structure, which involves the electron transfer from metal atom to CO2 molecules [10,[26], [27], [28], [29]]. Rostrup-Nielsen et al. believed that CO2 was dissociated on the surface of the transition metal [10]. Solymosi et al. proposed that the activation and dissociation of CO2 was dominated by electron transfer from transition metal to the antibonding orbital of CO2 forming the CO2 precursor [27]. Wang et al. pointed out that the electron transfer from Ni to the antibonding orbital of CO2 was able to promote the CO2 activation and dissociation, and the increased Ni electron density could enhance the electron transfer [29]. In summary, the higher electron density of Ni could enhance the activation and dissociation of CO2, which was beneficial to the balance between C species and O species.

In recent years, Yoon et al. reported that there was electron transfer between oxygen vacancies of MgO and Au, which could increase the electrons density of Au [30]. Lin et al. found that Au particle size had a direct influence on the providing electron capacity of MgO, and the electrons density of Au was increased with decrease of Au particle size [31]. In our previous DFT investigations about Ni/MgO catalysts [[32], [33], [34]], it is found that more electron would transfer to Ni from MgO as the Ni particle size became smaller. Thus, it is reasonable to suppose that the Ni particle size might also have a direct influence on the providing electron capacity of MgO, which could influence the electron density of Ni and finally affect the dissociation and activation of CO2.

In the present work, a series of MgO-promoted Ni/SiO2 catalysts with tunable Ni particle sizes were prepared to investigate how the Ni particle size affect the providing electron capacity of MgO in CRM reaction. It should be noticed that the MgO-promoted Ni/SiO2 catalysts were different from the conventional Ni/MgO solid solution catalysts. Herein, the MgO was employed as an additive and its content was only 0.4 wt.%, which was chosen based on our previous work [35]. Moreover, in order to exclude other influence factors for the activation and dissociation of CO2, the Ni particle size was adjusted by only changing the Ni salt precursors [36]. The coke-resistant mechanism of the Ni-MgO/SiO2 catalysts was also proposed.

Section snippets

Materials and reagents

The SiO2 (SBET = 227 m2 g−1) powder purchased from Aldrich was used as support. The Ni precursor reagents of analytical grade, including nickel nitrate (Ni(NO3)2·6H2O), magnesium nitrate (Mg(NO3)2·6H2O), nickel acetate (Ni(CH3COO)2·4H2O), and ethylenediamine (C2H8N2), were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. The [Ni(en)3]2+ (en, ethylenediamine) solution was obtained by mixed Ni(NO3)2·6H2O and C2H8N2 with a molar ratio of 1–3. Ultrapure water with a resistivity of

Crystallographic structure

XRD patterns of the reduced MgO-promoted Ni/SiO2 catalysts were shown in Fig. 1. It is clear that all the as-prepared MgO-promoted Ni/SiO2 catalysts presented diffraction peaks at 2θ = 44.5° and 51.8°, which is corresponding to Ni(111) and Ni(200) (JCPDS no. 65-2865). The diffraction peaks at 2θ = 22° belongs to that of SiO2. The absence of typical diffraction peaks of MgO (2θ = 42.9° and 62.3°, JCPDS no. 45-0946) and NiO-MgO solid solution (2θ = 75° and 79°) might be ascribed to the low

Conclusions

MgO-promoted Ni/SiO2 catalysts with tunable particle size were prepared by the multiple-impregnation method via only changing Ni salt precursors. The additive MgO was found to be an electron donor, and electrons in MgO could be transferred to Ni particles, resulting in a higher Ni electron density. Moreover, the providing electrons capacity of MgO became stronger as the Ni particle size was smaller. Catalytic activity evaluation showed that the Nien-0.4 %MgO catalyst (Ni particle size was

CRediT authorship contribution statement

Jie-ying Jing: Conceptualization, Resources, Writing - original draft, Writing - review & editing, Validation, Methodology, Supervision. Ze-hua Wei: Investigation, Methodology. Yu-bin Zhang: Investigation, Data curation. Hong-cun Bai: Writing - review & editing. Wen-ying Li: Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank the financial support from National Natural Science Foundation of China (21978190), Natural Science Foundation of Shanxi Province (201701D221237), and Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2017-K27). Dr. Jing would like to thank “The International Clean Energy Talent Program” hosted by the Future Energy Profile at Malardalen University in cooperation with China Scholarship Council and Applied Energy Journal.

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