Proton exchange membrane fuel cell (PEM FC) is considered as one promising clean and highly efficient power generation technology in the 21st 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.