N 2 on tungsten clusters: Molecular and dissociative adsorption

Abstract

We have studied the size-dependent reactivity of W10–W60 with N2 under single-collision-like conditions by using a laser-vaporization source, a low-pressure reaction cell and a laser-ionization time-of-flight mass spectrometer. The reaction probability with the first and second N2 molecule was measured at two different cluster-source temperatures: room temperature (RT) and liquid-nitrogen temperature (LNT). For the RT clusters, a strong size dependence in the reaction probability was observed in the size range ∼10–26 atoms, with distinct local maxima at W16, W22, and W23. Upon cooling of the cluster source, the reaction probability increased significantly overall, and the relative variations with size decreased, but persisted. To get an indication of the bond strength of N2 on Wn, we heated the cluster products after reaction through irradiation with 4.02 eV photons from a XeCl excimer laser and checked for consequent desorption of adsorbate atoms or molecules. For the LNT clusters, heating with laser light caused a substantial decrease in the abundance of reaction products with nitrogen, whereas no significant change in the abundance of WnN2 was observed for the RT clusters. This indicates that a proportion of the N2 is relatively weakly bound to the LNT clusters, whereas on Wn produced at RT, only the strongly bound state/states of N2 exist. Based on comparisons with the N2-adsorption on W bulk surfaces, we conclude that the weakly and strongly bound states represent molecularly and dissociatively bound N2, respectively.