Colloidally Prepared Pt Nanoparticles for Heterogeneous Gas-Phase Catalysis: Influence of Ligand Shell and Catalyst Loading on CO Oxidation Activity

In contrast to conventional methods, colloidally prepared heterogeneous supported metal catalysts are excellent systems to study the catalytic properties as a function of metal loading, monodispersity, particle shape, or the type of support without changing the other parameters, as will be demonstrated herein. Colloidal, ligand-capped Pt nanoparticles deposited on oxide supports are investigated for CO adsorption and oxidation. Dodecylamine and different alkanethiols are used as ligands. IR spectroscopic experiments reveal that small molecules, such as CO, can pass through the ligand shell and can adsorb on the particle surface, even if the ligand shell is not removed by a special pretreatment. The ability to penetrate the shell was found to depend on the type of ligand used which renders ligand-capped nanoparticles potentially interesting for reaction and selectivity control. In the case of CO oxidation, high activity is detected only at temperatures at which a partial loss of ligands has already occurred, resulting in a rather similar catalytic behavior independent on the type of ligand. However, there are no indications for poisoning of the catalysts by decomposition of the ligand shell. Simple purification procedures of the Pt nanoparticles are sufficient to avoid further poisoning effects. Depositing nanoparticles with the same size in different amounts on a support enabled a detailed study of the influence of metal loading on the activity. The activity per gram metal increases with the metal loading. Local auto-thermal heating is responsible for this effect, which is also detected for a reference system consisting of Pt nanoparticles prepared without a ligand shell.