Catalytic behavior of nickel nanoparticles: gasborne vs. supported state

To study the pure catalytic activity of metallic nanoparticles, the formation of methane on gasborne Ni nanoparticles, so called aerosol catalysis experiments, were performed. Beside effects typical for the methanation such as poisoning of the particle surface at temperatures above 385°C, the maximum of the catalytic activity was observed for Ni particles of about 14 nm, i.e. in a size range, which is quite uncommon for typical nanoeffects of metallic particles. To clarify, which catalytic phenomena are related to the aerosol state, the same reaction was performed on supported Ni nanoparticles, which were also generated and conditioned in the gas phase and deposited on a SiO₂ surface by thermophoresis. For these supported particles, the same reaction conditions were established as before for the gasborne Ni nanoparticles. However, differences in the mass transport characteristics of educt and product molecules to the particles were encountered and led to lower overall reaction rates. While qualitatively poisoning kinetics and activation energies agreed for both cases, significant differences were observed for the size dependence of the catalytic activity and for the sintering kinetics. The observed shift of the optimum size for the methanation from 14 nm (aerosol) to 25 nm (on support) can be explained by different adsorption enthalpies of the educt gases on aerosol and supported Ni nanoparticles, respectively.