The importance of Pd carbide formation for reactions with ethene and other organic molecules

Abstract

The reactions of a range of unsaturated hydrocarbons and oxygenated organic molecules with pure Pd surfaces can result in the carbidisation of the bulk of the metal, even though Pd is not usually recognised as a carbide-forming material. This can have important implications for catalysis by Pd. In this personal review it is shown that for the case of ethene, such bulk carbidisation is very fast above about 400 K. Adsorption and reaction with both pure single crystal Pd and supported nanoparticulate Pd is reported. For the former every ethene molecule has a 70 % probability per collision with the surface of dehydrogenating and depositing C into the metal. At low temperature (400 K) this accumulates near the surface, while at higher temperatures the C dissolves in the bulk and is reduced in the surface region. It can be retrieved by treatment in oxygen. For the nanoparticulate metal, the bulk is saturated very quickly with C, ultimately making a Pd4C-like material. During this time a disproportionation reaction occurs in which methane only evolves into the gas phase and C is deposited in the Pd. Once the bulk is saturated with C, then reaction with ethene stops. It is shown that similar carbidisation also takes place with acetaldehyde.