Structure and reactivity of ruthenium nanoparticles

We present a method for obtaining detailed structural information of ruthenium nanoparticles in at least the diameter range from 1.5 to 5 nm. The method is based on an ensemble approach where a large number of low-energy structures are collected in an ensemble, from which average properties can be extracted using Boltzmann averaging. The method is used to obtain the number of catalytic active step sites present on the surface of the ruthenium particles. We find that the presence of highly catalytic active step sites does not depend significantly on the temperature within a relevant temperature range; the presence of step sites is mainly a function of the lowest energy shape of the cluster, i.e., a function of the number of atoms. By combining the structural information with estimations of the single site activities in the ammonia synthesis, we find that the optimal particle diameter is approximately 3 nm. The single site activities are estimated by using density functional theory to calculate the barrier of the rate limiting step, the dissociation of a nitrogen molecule.