Oxidation induced ionization and reactions of metal carbide clusters (Nb, Zr, V, Ta)

Following our recent report of the oxidation induced formation of Ti8C12+ (H.T. Deng, K.P. Kerns, and A.W. Castleman, Jr., J. Chem. Phys. 104 (1996) 4862), the oxidation induced ionization of niobium and zirconium carbide clusters are studied using a triple quadrupole mass spectrometer coupled with a laser induced plasma reaction source. It was found that reactions of both of these neutral carbide clusters with dioxygen leads to formation of carbide ions. The ion product distributions show that zirconium carbide clusters mainly take the form of Met-Car cations, but niobium carbide clusters favor a cubic-like crystalline pattern. Furthermore, reactions of mass-selected NbxCy+ with dioxygen result in a sequential loss of C-2 units from NbxCy+, and leads to formation of Nb-x(+) and NbxC+ depending on y being an even or odd number. However, NbxCy+ shows comparably low reactivity towards nitrous oxide through a single oxygen abstraction mechanism. In comparison with the reaction products of VxCy+ with dioxygen, the complementary information obtained in the present study suggests that the C-2 unit is a basic building block for formation of small early transition metal carbide clusters. The oxidation induced ionization mechanisms are also discussed in relation with the stability, ionization potentials, and structures of the clusters. (C) 1997 Elsevier Science B.V.