Highly active and coke resistant Ni/CeZrO2 catalyst prepared by cold plasma decomposition for CO2 reforming of methane
Graphical abstract
Introduction
The CO2 reforming of methane or the dry reforming of methane (DRM) can use methane and carbon dioxide, the two most abundant greenhouse gases, as the feedstock to produce syngas (hydrogen and carbon monoxide) (Eq. (1)). With the increasing concern on the emission control and utilization of greenhouse gases, DRM has received increasing attention worldwide. Moreover, the syngas produced by DRM with a lower H2/CO ratio (∼1) is ideal for further syntheses of chemicals [1]. The supported nickel catalyst shows high activity towards DRM. Despite its potential economic and environmental benefits, DRM over Ni catalyst has not been an industrial process [2]. The Ni catalysts usually suffer from serious carbon deposition, originated from two principal coke formation pathways: the methane dissociation (Eq. (2)) and the Boudouard reaction (Eq. (3)) [3]. During the practical operation with high temperature, methane dissociation mainly causes the carbon formation, eventually leading to the catalyst deactivation.
The development of catalysts with good coke resistance has been the focus of recent studies on DRM. Noble metal catalysts exhibit superior activity and stability, but their high prices and scarcity make them an uneconomic option [[4], [5], [6]]. The design and preparation of Ni catalysts with better low-temperature activity and high carbon resistance are very necessary for future applications.
The previous studies have confirmed that the catalyst structure, particle size, and physicochemical properties, like basicity, reducibility and oxygen storage capacity, of the support are among the principal influencing factors for the performance of Ni catalysts [7]. The support material plays a key role not only in keeping the active metallic phase well-dispersed but also in promoting CO2 activation and coke elimination. MgO, Al2O3, SiO2, TiO2, CeO2, ZrO2, and many other materials have been exploited as the support of Ni catalysts for DRM [8]. Particularly, many studies have revealed that ceria-containing catalysts usually exhibit superior performance due to the excellent redox property and oxygen storage capacity of ceria [9,10]. CeO2 can readily release lattice oxygen species via reduction of Ce4+ to Ce3+, helping the oxidation of intermediate carbon species. Moreover, the oxygen vacancies and Ce3+ cations formed under reducing atmosphere motivate CO bonds to be dissociated and promote the CO2 activation [11]. However, pure CeO2 support is known to be thermally unstable and frequently suffers from rapid deactivation due to severe carbon deposition. Therefore, efforts to improve the performance of CeO2 by the addition of transition, alkaline-earth, or rare-earth metals have been carried out [12,13]. The non-stoichiometric fluorite CexZr1-xO2 solid solutions have been proved to be effective for the promotion of the redox property, oxygen mobility, thermal stability, and catalytic activity of CeO2 [14]. With the improved redox couples (Ce3+/ Ce4+) formation and oxygen storage properties, the ceria-zirconia solid solution exhibits better catalytic behavior in DRM than pure CeO2 [15]. Therefore, the ceria-zirconia solid solution can serve as promising support for nickel-based DRM catalysts.
Except for the supports, the preparation methods strongly influence the physicochemical properties and catalytic performance of nickel catalysts as well. Loading the nickel precursor onto the support via the impregnation, deposition-precipitation, or co-precipitation method, following by thermal decomposition and hydrogen reduction, is a typical preparation method for Ni catalysts [16]. However, the decomposition of the precursor at an elevated temperature usually makes the catalyst in big size and even be sintered. The catalyst in big size could reduce the activity and promote carbon deposition [17]. Recently, catalyst preparation with various gas discharge plasmas draws more and more attention [18]. The cold plasma, as a highly reactive mixture of energetic electrons, ions, molecules, radicals, photons, and excited species at relatively low temperatures, can cause rapid decomposition of nickel precursors at a lower temperature while avoiding thermal effect [18]. It has been reported that the cold plasma decomposition, especially with the dielectric barrier discharge (DBD) plasma, leads to the Ni catalysts with the smaller Ni particle size, stronger Ni-support interaction, more oxygen vacancies, and enhanced coke resistance in comparison to thermal decomposition [[19], [20], [21]]. These are responsible for the significantly improved performance of Ni catalysts prepared by plasma decomposition for CO2 methanation [19], CO methanation [20], and reforming reactions [21]. However, to the best of our knowledge, there is still no reported work in the literature on the use of the plasma decomposition method to prepare the Ni/CeZrO2 catalyst for DRM. In this work, we attempt to prepare Ni/CeZrO2 catalyst using the DBD plasma decomposition method for DRM, in order to extend the plasma decomposition to the catalyst preparation with the solid solution support. Detailed analyses of structural and chemical properties of the plasma decomposed Ni/CeZrO2 catalyst have been performed to explain the improved activity for DRM. The Ni/CeZrO2 catalyst prepared by the conventional thermal decomposition method was tested as well for comparison.
Section snippets
Catalyst preparation
The cerium-zirconium solid solution support was prepared by the co-precipitation method. Calculated quantities of Ce(NO3)3·6H2O and Zr(NO3)4·5H2O (Tianjin Kemiou Chemical Reagent) (molar ratio of Ce/Zr is 4:1) were dissolved in deionized water. Aqueous ammonia (20 % w/w, Tianjin Kemiou Chemical Reagent) was added dropwise to the mixture solution under vigorous stirring to attain a pH of 10. After that, the solution was kept for 2 h under stirring. The obtained slurry was hydrothermally aged at
The activity
The evolution of the CH4 conversion, CO2 conversion, and H2/CO molar ratio as a function of the temperature from 650 to 800 °C with the space velocity of 60,000 mL h−1 gcat−1 are presented in Fig. 1. Both CO2 and CH4 conversions of the catalysts increase with the increasing temperature, determined by the thermodynamics of the DRM reaction [22]. Obviously, the plasma decomposed catalyst shows higher activity. For example, at 650 °C, the conversion of CO2 and CH4 is 66.9 % and 55.7 %,
Conclusion
In this work, a highly active and coke resistant Ni/CeZrO2 catalyst for dry reforming of methane has been prepared via the DBD plasma decomposition of nickel precursor followed by hydrogen reduction. The plasma decomposition, as a rapid process at low temperature, not only keeps Ni/CeZrO2 with a larger specific surface area but also positively controls the Ni NPs size and crystal plane of the catalyst. In addition, the enhanced reducibility is observed, which benefits the formation of Ni0 and Ce
Author contribution statement
Planning and organization by C.J.L. Catalysts and reactions designed by C.J.L. and Y.X.D. Catalysts prepared by Y.X.D., R.Z., T.B. and J.Z. Reactions performed by Y.X.D. Catalyst characterizations by Y.X.D., R.Z., T.B. and J.Z. The manuscript written by Y.X.D. and C.J.L.
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.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. 21536008 and No.21621004).
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