Internal carbon dioxide reforming by methane over Ni-YSZ-CeO2 catalyst electrode in electrochemical cell

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Abstract

The carbon dioxide reforming by methane over Ni-YSZ-CeO2 catalyst electrode in an electrochemical cell (CO2, CH4, Ni-YSZ-CeO2 | YSZ | La0.79Sr0.16Mn0.8Co0.2O3, air) under open- and short-circuit conditions was studied at 800 °C and atmospheric pressure. The microstructure of the catalyst electrode was characterized by SEM. The current–voltage test showed that the electric power generation performance of the electrochemical cell for methane and carbon dioxide was close to that for hydrogen. Under open-circuit condition, the catalyst electrode was deactivated by coke deposition. On the other hand, the catalyst electrode was stable and the selectivity of carbon monoxide was increased under short-circuit condition. The electrochemically pumped oxygen ion was reacted with the surface carbon which formed in the dry reforming on the three boundary phase (Ni/YSZ/gas) to yield carbon monoxide.

Introduction

In recent years, global warming has become a serious world-wide environmental problem. Since CO2 is a greenhouse effect gas and contributes much to global warming, the elimination of CO2 has been attracting interest from an environmental perspective. The CO2 reforming by CH4 (so-called dry reforming) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] is one of the CO2 elimination methods.CH4+CO2→2H2+2CO,ΔH2980=247kJ/mol

This reaction has an advantage of the production of synthesis gas as well as the elimination of greenhouse gas. And the CO2 reforming by CH4 is of special interest from an industrial perspective since it produces synthesis gas with low hydrogen to carbon monoxide ratio, which can be preferentially used for Fischer–Tropsch synthesis. Furthermore, both CH4 and CO2 are the cheapest reactants and most abundant carbon-containing materials. Therefore, this reaction is an important area of current catalytic research.

But the CO2 reforming by CH4 has two serious problems. The dry reforming is an intensively endothermic reaction (ΔH2980=247 kJ/mol), which consumes much energy. And the catalysts used in the dry reforming are inclined to deactivate due to coke deposition on the catalysts surface. So, these problems must be overcome to apply the method for commercial processes.

During the past decades, much effort has been focused on the development of catalysts which show high activity and have resistance against coking for long-term operation. Numerous metal catalysts were studied for this reaction. Among them, nickel-based catalysts [12], [13], [14], [15], [16], [17], [18] and noble metal catalysts (Rh, Ru, Ir, Pd, and Pt) [19], [20], [21], [22] have been showing the promising catalytic performance in terms of activity and selectivity to synthesis gas. Noble metal catalysts have been found to have resistance against coking. But it does not seem to be practical because of the high cost of the noble metals.

In this work, electrocatalytic dry reforming over Ni-YSZ-CeO2 catalyst electrode in the electrochemical cell using solid electrolyte (CO2, CH4, Ni-YSZ-CeO2 | YSZ | La0.79Sr0.16Mn0.8Co0.2O3, air) was studied in order to overcome the two main problems in the catalytic dry reforming. Electrocatalytic dry reforming in electrochemical cell has some advantages over the catalytic dry reforming. The electrochemical cell used in this study is similar to the solid oxide fuel cell that produces electric energy. And catalyst deactivation by coking is suppressed by oxygen ions which are being supplied electrochemically to catalyst electrode through solid electrolyte.

This paper presents the effects of electrochemically pumped oxygen ion on the CO2, CH4 reaction rates and the stability of the catalyst electrode against coking. Also, the power generation performance of the electrochemical cell is discussed.

Section snippets

Experimental

The structure of electrochemical cell and reactor are shown in Fig. 1. A disk of 8 mol% Y2O3 doped ZrO2 (8-YSZ, TOSOH Ceramic Division) was used as the electrolyte. The thickness and diameter of the YSZ disk was 1.2 and 24 mm, respectively. The catalyst electrode and counter electrode were prepared by coating onto each side of the disk. The catalyst electrode material was a mixture of NiO and YSZ (NiO:YSZ=45:55 v/v). And CeO2 powder (4 wt.%) was added to this mixture. The binder solution for slurry

Characterization of electrode microstructure

The microstructure of Ni-YSZ-CeO2 catalyst electrode was characterized by SEM, and it was highly porous, as shown in Fig. 2. The porosity of the electrode is very important, because it is related to the electrochemical charge transfer reaction sites (three-phase site, interface among Ni, YSZ and gases). The sizes of Ni particles and YSZ particles of catalyst electrode were ca. 2–5 and 0.3 μm, respectively. The Ni particle was surrounded by YSZ particles which prevent Ni particles from

Conclusions

The CO2 reforming by CH4 was investigated over Ni-YSZ-CeO2 catalyst electrode in the electrochemical cell. The electric power generation performance of the electrochemical cell for CO2 and CH4 was close to that of H2 from current–voltage characteristics. Under open-circuit condition, catalyst electrode was deactivated by coke deposition. On the other hand, catalyst electrode was stable and synthesis gas was produced with high yield, in addition to the generation of electric power under

Acknowledgements

We acknowledge financial support from the R&D Management Center for Energy and Resources, The Korea Energy Management Corporation.

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