NH 3 adsorption around Ni n ( n ≤ 4) clusters

Abstract

The attachment of NH 3 to Ni n clusters was studied both theoretically and experimentally. Experiments were conducted by generating the clusters in a flow tube reactor and reacting them with NH 3 gas. The experimental mass spectra show that in Ni n –(NH 3) m , the dominant products for cations occur at 1:2, 1:3, 3:3, 4:4, 5:5 and 6:6. This paper also reports the results of theoretical ab initio density functional calculations, which were carried out to examine the nature of binding, stability and the effect of NH 3 on the electronic and magnetic behavior of Ni n clusters. It is shown that the binding energy (BE) of NH 3 to Ni n clusters changes non-monotonically with size and that the observed peaks in the mass spectrum of Ni n –(NH 3) m clusters can be understood from the theoretical energetics. Although the magnitude of the bond energies is greater for ammonia bound to Ni + than to the neutral atom, based on the energetics, the trend leading to a truncation in the cluster distribution is expected to be the same in accord with the experimental findings. Theoretical attention was also directed to a study of the trends in the magnetic properties of the system. We show that the absorption leads to a gradual quenching of the Ni n moments. In some cases, however, the moments can reappear at higher coverage. This reappearance of magnetism is shown to be a consequence of the variation of Ni–N distance with coverage of the adsorbent.