Charging phenomena of supported metal clusters and effects on chemical reactitity: a d.f.t. study

In the 80’s Masatake Haruta pointed out the striking catalytic activity of supported gold clusters toward carbon monoxide oxidation at the temperature of 200 K (Haruta’s catalysts). Several experimental and theoretical efforts have been dedicated to clarify the properties that actually enhance the chemical activity of gold. In particular, the charge state of the particles is still now at the center of a debate all over the scientific community. Several studies have shown that negatively charged clusters are chemically more active than neutral or positively charged clusters: the presence of an extra negative charge results in the activation of adsorbed molecules and thus in the enhanced catalytic activity. Charge transfer phenomena are due to the interaction between clusters and the support. A simple yet viable way to stabilize and chemically activate metals nanoclusters on MgO supports consists in functionalizing the MgO surface with creation of trapped electrons by exposure to atomic hydrogen, molecular hydrogen and UV light or deposition of small amounts of alkali metals on partially hydroxylated surfaces. The calculations show that electron traps act as nucleation and charging sites for clusters. A catalytic cycle for CO oxidation has been proposed in order to verify the catalytic activity of gold clusters. Charging of supported metal atoms can also be obtained with the use of ultra-thin oxide films grown on a metal substrate. It has been found that electrons can actually flow by direct tunneling from the metal substrate through the oxide thin film to adsorbed molecules and clusters. CO is largely used as a probe molecule: according to the Blyholder model, the vibrational frequency of the C-O stretching mode is very sensitive to changes in the electron density of the atoms where the molecule is bound. In this scenario, CO adsorption can represent a powerful tool to distinguish between neutral and anionic clusters formed on MgO/Ag.