Creating The Next High Efficiency Power Generation Technology Through Selective Methanation Of CO

Proton exchange membrane fuel cell (PEM FC) is considered as one promising clean and highly efficient power generation technology in the 21^st century. PEM FC is of great recent interest in our society today due to their high efficiency and potential for low emissions. Also, technologies for its fabrication are well-developed due to integration with water electrolyzers. It reduces costs and extends market.

Conventional fuel for PEM FC is hydrogen (H₂). Currently, it is being produced at big-scale facilities from natural gas and stored in high-pressure tanks. The latter is not so efficient due to low capacity, high weight and safety reasons. The alternative way is to produce hydrogen from conventional transport fuels (gasoline, diesel, natural gas, NGL, bio-ethanol, etc.) directly at the place of use, even on board of the vehicle. Such H₂-rich gas production could be realized in a multistep catalytic converter device. Let’s imagine, one can use good old gasoline, but twice more effective and without any vibrations and noise. One of the main problems in this field is final-step deep carbon monoxide (CO), – strong poison for PEM FC, – removal from H₂-rich mixture, which contains about 1 % CO, 20 % CO₂ and 10 % H₂O in addition to hydrogen.

One of the ideas how to do it is the catalytic reaction of selective (preferential) methanation of CO (CO-SMET), i.e. one needs to convert all CO in the mixture to methane (CH₄) without touching on the excess of CO₂. So, it is a challenging problem from both fundamental and practical points of view, considering that all industrial methanation catalysts are very active in the hydrogenation of both CO and CO₂. Therefore, the main question was how to make catalyst selective towards CO.

A team from Novosibirsk State University and Boreskov Institute of Catalysis paid special attention to relatively novel very active nickel-cerium oxide (Ni/CeO₂) catalytic system. The first studies showed that Ni/CeO₂ is active in both CO and CO₂ methanation reaction. However, CO methanation is predominantly driven by Ni surface, while CO₂ methanation – by CeO₂ surface.

The obvious idea was to selectively poison CeO₂ surface keeping Ni surface active. F, Cl, Br additives, being “classical” catalytic poisons, were chosen as promising candidates to do it. Halogen-doped Ni(F)/CeO₂, Ni(Cl)/CeO₂ and Ni(Br)/CeO₂ catalysts were prepared by treating halogen-free Ni/CeO₂ with aqueous solutions of NH₄F, NH₄Cl, and NH₄Br, respectively. And the reached effect was very clear: fluorine did not change catalytic activity, chlorine inhibited CO₂ methanation activity providing high selectivity towards CO methanation, while bromine totally inhibited both CO and CO₂ methanation activity.

The deeper understanding of chlorine doping exclusive selectivity boosting effect was associated with the formation of surface cerium oxychlorine (CeOCl) species and, specifically, with the blockage of surface Ce³⁺-coupled oxygen vacancy sites by CeOCl species that inhibited ceria-assisted CO₂ activation and hydrogenation. These findings are featured in an upcoming article in Applied Catalysis B: Environmental.

Finally, very efficient (active and selective) Cl-doped Ni/CeO₂ catalyst was designed. Authors believe it would be soon applied in PEM FC-based portable devices, such as range extenders for electric cars.

This study, Preferential Methanation of CO using Halogen-Doped (F, Cl, Br) Ceria-Supported Nickel Catalysts was recently published by M.V. Konishcheva, D.I. Potemkin, P.V. Snytnikov, V.P. Pakharukova and V.A. Sobyanin in the journal Energy Technology.