Elsevier

Applied Catalysis A: General

Volume 575, 5 April 2019, Pages 230-237
Applied Catalysis A: General

Effect of Si-modified zirconia on the properties of MoO3/Si-ZrO2 catalysts for sulfur-resistant CO methanation

https://doi.org/10.1016/j.apcata.2019.01.028Get rights and content

Highlights

  • Doping of Si into ZrO2 achieves more O defects, Brönsted acid sites and larger surface area.

  • More active species are obtained over catalysts with modified zirconia.

  • Si-modified catalysts have improved sulfur-resistant CO methanation catalytic performance.

Abstract

A series of Si-modified ZrO2 supports were prepared by incorporating different content of silicon and loaded 25 wt.% MoO3 for sulfur-resistant CO methanation reaction. The prepared catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), N2 physisorption measurements, infrared spectra of adsorbed pyridine (Py-IR) and hydrogen temperature-programmed reduction (H2-TPR). The results demonstrated that Si-modified ZrO2 achieved more stable ZrO2 phase, more oxygen vacancies, larger specific surface area and more Brønsted acid sites. Catalysts prepared by modified ZrO2 exhibited smaller metal particles and different interaction between metal and support, which possessed higher activity with 63.4% CO conversion and 59% CH4 selectivity than unmodified catalyst at 550 °C under 2.5 MPa with 100 ppm H2S.

Graphical abstract

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Catalysts prepared by Si-modified zirconia possess more active species and more Brönsted acid sites, which can improve CO conversion and CH4 selectivity.

Introduction

In recent decades, energy crisis and environment pollution issues are concerned by public. Hence, the continuous development of fossil fuels has attracted increasing attention, especially the efficient and clean utilization of coal. Synthetic natural gas (SNG) produced by syngas is an extremely reasonable way to convert coal resource and the key step is CO methanation reaction [1]. Generally, Ni-based catalyst is intensely attractive for its excellent catalytic performance and relatively low cost [2,3]. However, it is limited by the rigorous conditions of high H2/CO ratio (more than 3) and low sulfur content (less than 0.1 ppm) [4]. Therefore, the removal of sulfur species and adjusting H2/CO ratio are necessary for Ni-based catalysts to react efficiently. It has been reported that Mo-based catalysts could stand not only the low H2/CO ratio, but also the high sulfur concentration in feed gas [5]. Hence, Mo-based catalysts have been studied widely for their outstanding performance of water-gas-shift and sulfur resistance, which eliminate the need of the adjustment process of syngas [[6], [7], [8]].

Various oxides such as Al2O3 [6,9], CeO2 [10], ZrO2 [11], SiO2 [12] and composite oxides such as Al2O3-CeO2 [7,13], CeO2-ZrO2 [14], Al2O3-ZrO2 [15],SiO2-Al2O3 [12] have been applied to the Mo-based catalysts. Myoung Yeob Kim et al. [12] have reported that ZrO2-loaded Mo catalysts have faster CH4 generation rate than other commercial supports, for instance, γ-Al2O3, SiO2, SiO2-Al2O3, ZrO2, YSZ, CeO2 and TiO2. Meanwhile, ZrO2 possesses superior redox ability, high thermal stability, acid-base properties and strong interaction with metal [16]. Consequently, ZrO2 is a favorable support as sulfur-resistant CO methanation catalysts to load molybdenum.

Although pure ZrO2 exhibits these advantages as a catalyst support, it tends to possess a low specific surface area [17], single acid sites (Lewis acid) [18] and limited oxygen defects [19]. To enhance the specific surface area, the incorporation of Y [20], Cu [21] and Si [[22], [23], [24], [25]] into ZrO2 have been reported and the effect is remarkable. Ca [19,26], Sm [19], K [27], Si [18,23,24,28] and Mg [27,29,30] are also effective modifiers to change the acid-base properties of supports. Additionally, the amount of oxygen vacancies of ZrO2 can be increased by doping Ca [26], Ce [22] and La [22]. Interestingly, the incorporation of Si can not only form Si-O-Zr structure to enhance the specific surface area but also lead to an undercoordination of surface oxygen to induce the Brønsted acid sites [18,24,28]. The large specific surface area is helpful to disperse active species and the Brønsted acid sites promote the cleavage of C–O bonds and hydrogenation [28,31]. Consequently, incorporation of silicon into ZrO2 is a promising treatment to improve the dispersion of active species and promote the hydrogenation to enhance the CH4 selectivity for sulfur-resistant CO methanation catalysts.

Few cases have reported that Si-modified ZrO2 supported molybdenum as catalysts were employed in sulfur-resistant CO methanation. In this paper, the effect of Si incorporation on the physicochemical properties of Si-modified zirconia supports was studied and the catalytic performance of MoO3/Si-ZrO2 for sulfur-resistant CO methanation reaction was tested. All samples were characterized by X-ray diffraction (XRD), N2 physisorption measurements, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), hydrogen temperature-programmed reduction (H2-TPR) and infrared spectra of adsorbed pyridine (Py-IR).

Section snippets

Experimental

Zirconyl chloride octahydrate (ZrOCl2·8H2O, ≥98%), ammonium hydroxide solution (NH4OH, 25–28%), anhydrous ethanol (CH3CH2OH, ≥99.7%) and ammonium molybdate (VI) tetrahydrate ((NH4)6Mo7O24·4H2O, ≥99.0%) were purchased from Titan Scientific Corporation (Shanghai, China). Ethyl silicate (TEOS, (C2H5)4SiO4) was purchased from Lingfeng Chemical Reagent Corporation (Shanghai, China). All reagents were used without purification.

Characterization of supports

To analyze the change of Zr 3d binding energy, the results were summarized in Fig. 1 and Table 1. It can be clearly seen that signals corresponding to ZrO2 (Zr4+) and Zr(OH)x(Zr3+) are observed in the Zr 3d5/2 spectrum at 182.2 and 183.7 eV for the ZrO2 without modified, respectively [34,35]. With the increasing of Si/Zr molar ratio, the binding energy of ZrO2 (Zr4+) and Zr(OH)x (Zr3+) gradually shifts towards higher binding energy, up to 182.8 and 184.1 eV respectively. This can be due to the

Conclusion

A series of investigations on physicochemical properties of xSi-ZrO2 supports and catalytic performance of Mo/xSi-ZrO2 catalysts had been carried out. The CO conversion of modified catalysts Mo/xSi-ZrO2 approached 63.4% when Si/Zr molar ratio was 0.04, and the CH4 selectivity of modified catalysts Mo/xSi-ZrO2 approached 59% when Si/Zr molar ratio was 0.08, while the CO conversion and CH4 selectivity of unmodified catalyst Mo/ZrO2 were 58.9% and 45% respectively.

The characterization results

Acknowledgments

This research was financially supported by the National Natural Science Funds of China (Grant numbers U1203293, 21776091 and 21808062), Fundamental Research Funds for the Central Universities (22A1817025), the program of Shanghai Subject Chief scientist (Grant number 10Xd1401500), China Postdoctoral Science Foundation (2017M611474), and the Program of Shanghai Leading Talents (2013).

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