Elsevier

Catalysis Today

Volumes 293–294, 15 September 2017, Pages 97-104
Catalysis Today

Enhancement of methanation of carbon dioxide using dielectric barrier discharge on a ruthenium catalyst at atmospheric conditions

https://doi.org/10.1016/j.cattod.2017.01.022Get rights and content

Highlights

  • Enhancement of CO2 methanation using dielectric barrier discharge (DBD) is described.

  • Ru catalyst activates the methanation in the DBD plasma at atmospheric conditions.

  • Optical emission spectroscopy is used for the plasma diagnostics in the CO2 methanation.

  • Excited and ionized hydrogen atoms are confirmed as the primary accelerators.

Abstract

This paper describes the enhancement of CO2 methanation using dielectric barrier discharge (DBD) on Ru catalyst under atmospheric conditions. The DBD plasma leads to increased deoxygenation of CO2, which is decomposed into CO. Subsequently, the Ru catalyst activates the methanation in the DBD plasma at atmospheric conditions. The effect of the discharge frequency of the DBD plasma and the H2/CO2 mixture ratio on the CO2 conversion and CH4 selectivity is investigated. The CO2 conversion and CH4 selectivity rapidly increase at a discharge frequency above 2.5 kHz, and reach 23.20% and 95.02%, respectively, when the H2/CO2 molar ratio is 7. CO2 methanation and deoxygenation are simultaneously enhanced by adding Ar. Optical emission spectroscopy is used for the plasma diagnostics in the CO2 methanation process. The optical emission spectra of the γ-Al2O3 and Ru/γ-Al2O3 catalysts are measured to investigate the effect of the presence of Ru during plasma generation. Finally, the excited and ionized hydrogen atoms are confirmed as the primary accelerators in the plasma-assisted CO2 methanation process.

Introduction

Recently, methanation of CO2 has become an important challenge. CO2 methanation is important for two reasons: the first is CO2-related environmental issues and the second is the potential of the “power to gas (P2G)” process [1], [2]. Since the industrial revolution, atmospheric concentration of CO2 has continuously increased; the increased CO2 concentration causes climate change and global warming [3], [4]. The major source of CO2 emissions is from the combustion of fossil fuels, such as in an internal combustion engine. However, human beings cannot stop using fossil fuels to prevent CO2 emissions because the modern human lifestyle is highly dependent on the fossil fuel-based economy. For this reason, reasonable and alternative solutions to reduce CO2 emissions involve recycling or utilizing the emitted CO2.

Either recycling or utilizing CO2 can be achieved by the Sabatier reaction. The Sabatier reaction is a well-known process to convert CO2 into useful products, such as CH4 and H2O, according to the process shown in Eq. (1),CO2 + 4H2  CH4 + 2H2O (ΔH = −165.0 kJ/mol)

The CO2 methanation process has a great potential for environmental, industrial, and economical applications because fuels or fuel sources can be regenerated from the waste CO2 gas. Based on the renewable energy growth, CO2 methanation can be used for the P2G process that converts electrical power to a gas fuel. It means that the excess power can be utilized and stored in a storable gas state, such as CH4.

For this reason, the CO2 methanation process has been widely studied. Typically, CO2 methanation has been achieved for group VIII–based metal catalysts, such as Ni and Ru [5], [6], [7], [8], [9]. However, the catalytic methanation process using the Sabatier reaction is usually activated above 300 °C and 20 bar [10], [11], [12]. The temperature and pressure requirements make the process complex because a heater, pressurization device, and pressurized reactor are required. It means that the system has a limitation for miniaturization and simplification, which limits its applicability to various industrial applications. Therefore, an alternative approach to lower the temperature and pressure to activate the CO2 methanation process is required.

Recently, non-thermal plasma-assisted processes have been proposed for various reactions, including treatment, decomposition, conversion, and reforming processes. Many studies have reported that the non-thermal plasma-assisted process provides a synergistic effect between the plasma and catalyst. The surfaces of the catalysts were pretreated by the non-thermal plasma to enhance their catalytic performance; this surface-modifying treatment also improved the durability of the catalysts [13], [14], [15], [16], [17]. The enhancement of the catalytic performance using the non-thermal plasma was also reported for pollutant removal and reforming species, such as methanol and methane [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28].

The non-thermal plasma has many advantages in various chemical processes. A few groups have reported the study of CO and CO2 hydrogenation using a non-thermal plasma [29], [30], [31], [32]. Therefore, the non-thermal plasma-assisted CO2 methanation process was investigated in this study. Among the non-thermal plasma techniques, dielectric barrier discharge (DBD) was used to generate the plasma on the catalyst. The activation and enhancement of the CO2 methanation in the plasma at atmospheric conditions were verified, and the mechanism between the plasma and catalyst was investigated in this study.

Section snippets

Catalyst preparation

Ru is a popular catalyst in the chemical industry. For example, Ru-promoted cobalt catalysts have been commonly used for energy conversion processes, such as Fischer-Tropsch synthesis and the CO2 methanation process [33], [34], [35]. Therefore, Ru was selected as the catalyst for the DBD plasma-assisted CO2 methanation process in this study. The Ru chloride powder (RuCl3∙3H2O, >99% purity) and alumina spheres (γ-Al2O3, 1/16 inches) that are used as the catalyst support were purchased from Kojima

Effect of the DBD plasma on the CO2 conversion and CH4 selectivity

DBD plasma-assisted CO2 methanation was performed at atmospheric temperature and pressure. The discharge voltage and frequency were 9 kV and 3 kHz, respectively. The CO2 conversion and CH4 selectivity were measured when the plasma was applied in the empty reactor, when the catalyst was used without the plasma, and when both the plasma and catalyst were used simultaneously (Fig. 5 and Table 1). The CO2 conversion and CH4 and CO selectivity were essentially zero in the absence of the plasma. The

Conclusions

The DBD plasma was used to activate the Ru/γ-Al2O3 catalyst for CO2 methanation at atmospheric conditions, and the effect of the interaction between the plasma and catalyst on the conversion and selectivity was investigated. The CO2 conversion increased with the DBD plasma regardless of the presence of the catalyst at atmospheric conditions. However, when the DBD plasma was used alone, most of the CO2 conversion was induced by CO2 deoxygenation because CO2 molecules were exposed to electron

Acknowledgements

Funding: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning [grant number 2014R1A2A1A11054686].

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