Carbidisation of Pd Nanoparticles by Ethene Decomposition with Methane Production

In the presence of oxygenated organic molecules pure Pd, which is widely used in chemicals processing and the pharmaceutical industry, tends to defunctionalise and dehydrogenate such molecules to H2, CO and surface/bulk carbon, in the form of a palladium carbide. We have investigated the formation of this carbide by ethene adsorption using a variety of techniques, including pulsed flow reaction measurements, XAS and DFT calculations of the lattice expansion during carbidisation. These experiments show that two main reactions take place above 500 K, that is, both total dehydrogenation, but also disproportionation to methane and the carbide, after which the activity of the Pd is completely lost. We estimate the value of x in PdCx to be 0.28 (±0.03), and show by computer modelling that this fits the lattice expansion observed by XAFS, and that there is charge transfer to C from Pd of around 0.2–0.4 e. Carbide controversy! Palladium carbide is an elusive substance in that the stoichiometry of the material is disputed. Here we show, by using chemical titration in addition to spectroscopy and structural analysis, that we can determine that stoichiometry to be close to Pd4C. This is formed by reaction with ethene at elevated temperatures between 200 and 300 °C with a silica supported Pd catalyst. The reaction with Pd occurs via two routes – dehydrogenation and a disproportionation reaction that also forms methane.