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 N(2)O formation, and therefore, the bridge-type state is a possible candidate for a reaction precursor to the N(2)O formation under a mild condition.