Deactivation of bimetallic nickel–copper alloy catalysts in thermocatalytic decomposition of methane

Abstract

Bimetallic alloys are extensively used as catalysts because of their superior activity relative to the single constituent. Unfortunately, bimetallic catalysts suffer from complex deactivation. It is fundamentally important, but always highly challenging, to clearly elucidate the deactivation mechanism because of its complexity. Herein, the authors have endeavored to present a detailed study on the deactivation of Ni–Cu alloy catalysts supported on carbon nanotubes (Ni–Cu/CNT) for the thermocatalytic decomposition of methane. The activity of the catalysts was examined under two temperature-programmed modes, namely, a constant temperature mode and a cyclic heating–cooling temperature mode. The methane conversion, carbon deposition rate and carbon yield of the catalyst were recorded as a function of time. The structures of the resulting filamentous carbons were characterized by transmission electron microscopy, Raman spectroscopy and N₂ adsorption–desorption isotherm analysis. The effects of the reaction temperature and copper content on the methane conversion, lifespan and carbon yield of the catalyst were investigated. The structural properties of the resulting carbons were correlated with the deactivation mechanism. It was found that the deactivation of the Ni–Cu/CNT catalysts is rather complex, which involves three deactivation mechanisms – atomic erosion, fragmentation and copper segregation. Under the constant temperature mode, the deactivation of the Ni–Cu/CNT catalysts is mainly attributed to the fragmentation of Ni–Cu nanoparticles at temperatures over 700 °C while atomic erosion plays a dominant role in the catalyst deactivation when the working temperature is less than 700 °C. Under the cyclic heating–cooling temperature mode, the activity of the deactivated Ni–Cu/CNT catalysts can be partially restored due to the re-establishment of the alloy surface composition after cooling to room temperature. Meanwhile, fragmentation of the Ni–Cu alloy also takes place under the cyclic heating–cooling temperature mode, which is responsible for the activity decay at each cycle and eventual catalyst deactivation.