A Computational Study On Molecular Adsorption States Of Nitrogen On A Tungsten Tetramer

Molecular adsorption states of a dinitrogen (N-2) on a free tungsten tetramer (W-4) 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 N-2 and W-4 is weak, and therefore the electronic structure of N-2 is not largely modified from that of a free N-2. 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 N-2 is significantly activated. The electronic structure of the bridge-type state explains a single-peaked X-ray photoelectron spectrum, previously observed experimentally for N-2 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.