A computational study on molecular adsorption states of nitrogen on a tungsten tetramer

Abstract

Molecular adsorption states of a dinitrogen (N₂) on a free tungsten tetramer (W₄) were investigated in detail by using a density functional theory method. It was found that adsorption states with end-on type geometries are the most energetically favorable for molecular adsorption. In the states, interaction between N₂ and W₄ is weak, and therefore the electronic structure of N₂ is not largely modified from that of a free N₂. There exist, however, another type of adsorption state with a W–N–N–W bridge-type binding configuration. In the adsorption state of this type, local N (s, p)–W (d) interactions are fairly stronger than in the end-on-type state, and, as a result of this, the adsorbed N₂ is significantly activated. The electronic structure of the bridge-type state explains a single-peaked X-ray photoelectron spectrum, previously observed experimentally for N₂ adsorbed on supported tungsten nanoclusters. The N 1s spectrum was found in our previous study of low-temperature N₂O formation, and therefore, the bridge-type state is a possible candidate for a reaction precursor to the N₂O formation under a mild condition.