Article
ZrO2-modified Ni/LaAl11O18 catalyst for CO methanation: Effects of catalyst structure on catalytic performance

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Abstract

We report Ni/LaHA@ZrO2 catalysts prepared by a facile modified successive adsorption and reaction method for CO methanation. N2 adsorption, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, thermogravimetric analysis, H2 temperature-programmed reduction, H2 temperature-programmed desorption, X-ray photoelectron spectroscopy, thermogravimetric analysis, and inductively coupled plasma atomic emission spectrometry were used to characterize the samples. The results indicated that the ZrO2 nanoparticles were distributed over the surface of the Ni/LaHA@ZrO2 catalyst and even partially covered some Ni particles, resulting in the coating exerting a confinement effect. The excess ZrO2 had an adverse effect on the enhancement of CO conversion because of the coverage of the surface Ni particles; however, the Ni/LaHA@ZrO2 catalyst displayed much higher CH4 selectivity than Ni/LaHA because of the activation of the byproduct CO2 molecules by ZrO2 species. Therefore, even though 20Ni/LaHA@ZrO2-5 exhibited similar CO conversion as 20Ni/LaHA, the use of the former resulted in a higher CH4 yield than the use of the latter. A 107-h-lifetime test revealed that the Ni/LaHA@ZrO2 catalyst was highly stable with superior anti-sintering and anti-coking properties because of its coating structure and the promoter effect of ZrO2.

Graphical Abstract

The ZrO2-modified Ni/LaHA catalyst shows slight increment of CH4 yield and obvious enhancement stability for CO methanation reaction compared with Ni/LaHA catalyst owing to its coating structure as well as the promoter effect of ZrO2.

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Introduction

The CO methanation reaction from syngas has attracted intensive attention from both academia and industry, especially in coal-rich regions and countries such as China 1, 2, 3, 4. This reaction is strongly exothermic and thermodynamically feasible (CO + 3H2 → CH4 + H2O, ΔH298 K =−206.1 kJ mol−1), leading to high demand for methanation catalysts. Ideal catalysts should be highly active at low temperatures (∼300 °C) and highly stable at high temperatures (∼600 °C). Some transition metals such as Ru, Rh, Pt, Ni, Fe, and Co have been used in the methanation reaction 5, 6, 7, 8, 9. Among them, Ru-based catalysts are the most active; however, the limited resources and high cost of Ru restrict the large-scale industrial application of these catalysts [10]. In addition, both the activity and CH4 selectivity of Fe- and Co-based catalysts are still relatively low [4]. Hence, Ni is the most favorable choice for CO methanation because of its relatively high activity and low cost [11]. Moreover, many supports such as Al2O3 [11], SiO2 [12], SiC [13], TiO2 [14], and ZrO2 [15] have been investigated for Ni catalysts; however, these catalysts often suffer from Ni sintering and coke formation on the surface of the Ni particles during the methanation process. Therefore, considering that the methanation process is generally performed at high temperatures (≥ 400 °C) 1, 3, 16, together with its strongly exothermic nature, the control of the thermally induced sintering of Ni particles and supports as well as coke formation is critical for maintaining the catalyst activity.

Hexaaluminate (HA) materials are highly stable at high temperatures because of their unique layered structure of alternately stacked spinel blocks separated by mirror planes 5, 17 and have been used as catalysts or catalyst supports for high-temperature reactions such as methane catalytic combustion 18, 19, methane reforming reaction [20], and CO methanation 5, 17. In addition, the rare-earth oxide La2O3 has been widely used as an excellent promoter for various catalysts because of its unique properties, such as neutralization of acid sites [21], stabilization of the support 22, 23, 24, enhancement of the catalytic performance [25], and suppression of carbon deposition by activating adsorbed H2O and CO2 26, 27. Lanthanum hexaaluminate (LaAl11O18)-supported Ni catalysts are thus expected to be promising CO methanation catalysts.

To prevent both the sintering of Ni particles and coke formation, several strategies have been employed in the literature, such as the addition of an inorganic oxide via atomic layer deposition 28, 29, encapsulation using the precipitation– deposition method [11], or the use of core–shell 30, 31, core–sheath [32], or ordered mesoporous structures 33, 34. Although partial success has been achieved, these approaches still have drawbacks, such as poor control of the shell thickness [30], the need for special equipment [28], costly raw materials [33], and complicated and harsh preparation processes [30], which limit their wide and large-scale applications. Recently, we prepared ZrO2-modified Ni/α-Al2O3 catalysts using a facile modified impregnation method for CO methanation with the construction of a ZrO2-on-metallic Ni surface coating structure, yielding significant enhancement of the catalytic activity and anti-sintering of Ni particles [35]. However, the anti-coking property was poor and thus required further improvement.

Following our previous works on catalysis 6, 35, 36, 37, 38, 39, 40, in this work, ZrO2-modified Ni/LaAl11O18 catalysts were prepared using a modified successive adsorption and reaction method to overcome the technical barriers of sintering and coking. To the best of our knowledge, no reports on this type of CO methanation reaction route are available in the literature. To better understand the effect of the catalyst structure on the catalytic activity and stability, a series of tests and characterizations were performed. The results revealed that the coating of ZrO2 nanoparticles over the Ni/LaAl11O18 catalyst could suppress both Ni sintering and coke formation.

Section snippets

Preparation of LaHA support

The LaHA support was prepared using a coprecipitation method with carbon black as the hard template, similar to a method previously described in the literature [17]. First, 0.165 mol Al(NO3)3·9H2O and 0.015 mol La(NO3)2·6H2O were dissolved in 300 mL of deionized water at 60 °C; then, 10 g carbon black was added, and the mixture was stirred for 5 h to obtain a precursor slurry. Next, a (NH4)2CO3 aqueous solution (3 mol L−1) was heated to 60 °C and added to the above precursor slurry while

Catalyst characterization

Fig. 2(a) presents the N2 adsorption isotherms of the as-synthetized samples, which are Type-IV isotherms with H4 hysteresis loops. The hysteresis loops of the samples were very small because of the small pore volumes. However, the onsets of the relative pressure of the hysteresis loops of all the samples appeared at relatively high positions, indicating that macropores over 50 nm in size dominated the samples. Moreover, the PSD curves provide further evidence of the pore diameters. The PSD

Conclusions

We conducted a systematic investigation of Ni/LaHA@ZrO2 catalysts synthesized using a modified successive adsorption and reaction method to produce synthetic natural gas via the CO methanation reaction. After the addition of ZrO2, the ZrO2 nanoparticles were distributed over the surface of the Ni/LaHA catalyst and even partially covered some Ni particles, resulting in the coating exerting a confinement effect. The Ni/LaHA@ZrO2 catalyst with the addition of an appropriate amount of ZrO2 even

Acknowledgments

The authors sincerely appreciate Prof. Fabing Su from Institute of Process Engineering (CAS) for his suggestion on the experiment.

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    Published 5 February 2018

    This work was supported by the National Natural Science Foundation of China (21606146), Natural Science Foundation of Shandong Province (ZR2016BB17, 2016ZRB01037), Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (2016RCJJ005, 2016RCJJ006), Government Sponsored Visiting Scholar Foundation of Shandong University of Science and Technology (2016), Qingdao Postdoctoral Applied Research Project (2015202), and China National Coal Association Science and Technology Research Program (MTKJ2016-266).

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