NH3 adsorption around Nin, (n ≤ 4) clusters

The attachment of NH3 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 NH3 gas. The experimental mass spectra show that in Ni-n-(NH3)(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 NH3 on the electronic and magnetic behavior of Ni-n clusters. It is shown that the binding energy (BE) of NH3 to Ni-n clusters changes non-monotonically with size and that the observed peaks in the mass spectrum of Ni-n-(NH3)(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. (C) 2002 Elsevier Science B.V. All rights reserved.