Design of stable catalysts for methane-carbon dioxide reforming

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed CO₂. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles. The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO₂, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH₄ dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni. Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically. Well dispersed Ni supported on ZrO₂ is thus a viable alternative to the noble metal based materials.