H2/D2 isotopic exchange: A tool to characterize complex hydrogen interaction with carbon-supported ruthenium catalysts

• The H2/D2 isotopic exchange is very fast at 0 °C over clean Ru surfaces.
• If chemisorbed H species are still present, the rate of exchange is slowed down and H2/D2 mixtures cannot reach equilibrium.
• Reverse H spillover from the active carbon to the metal can be observed during exchange.
• The presence of Na inhibits this spillover phenomenon. H2/D2 exchange is a pure homolytic process on Ru-Na/AC catalysts.

Hydrogen interaction with ruthenium surfaces is a complex phenomenon that can play an important role in catalytic reactions such as ammonia synthesis. In this study, H2/D2 isotopic homomolecular exchange has been used to characterize four different catalytic surfaces, namely Ru/AC0, Ru/AC1, Ru-Na/AC0 and Ru-Na/AC1. They are composed of 2%Ru or 2%Ru-4.5%Na supported on an active carbon with (AC0) and without surface groups (AC1). The AC1 carbon (1162 m2 g−1) is obtained by treatment of AC0 (960 m2 g−1) in N2 at 900 °C. Ru/AC0 and Ru/AC1 catalysts are prepared by impregnation of the supports with aqueous solutions of Ru(NO)(NO3)3. Na-promoted catalysts are then prepared by impregnation of Ru catalysts with NaOH solutions. An overall picture of the surface mobility phenomena on the four different catalytic surfaces has been described. It has been demonstrated that the presence of Na promoter in Ru-Na/AC0 and Ru-Na/AC1 catalysts inhibits the spillover of H atoms from the Ru particles to the AC surface. However, for non Na promoted catalysts, the extension of the H spillover phenomenon depends on the amount of oxygen groups present on the AC surface. Likewise, the formation of a ruthenium hydride during the reduction treatment is suggested for catalysts promoted or not by Na. Finally, this work shows how the kinetics of the H2/D2 isotopic exchange reaction may be related to the surface electron density of the catalyst, which allows us to better understand the effect that both promoter and support have on the metal particles.

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