Experimental and theoretical studies of reactions of neutral vanadium and tantalum oxide clusters with NO and NH3

Reactions of neutral vanadium and tantalum oxide clusters with NO, NH3, and an NO/NH3 mixture in a fast flow reactor are investigated by time of flight mass spectrometry and density functional theory (DFT) calculations. Single photon ionization through a 46.9 nm (26.5 eV) extreme ultraviolet (EUV) laser is employed to detect both neutral cluster distributions and reaction products. Association products VO3NO and V2O5NO are detected for VmOn clusters reacting with pure NO, and reaction products, TaO3,4(NO)(1,2), Ta2O5NO, Ta2O6(NO)(1-3), and Ta3O8(NO)(1,2) are generated for TamOn clusters reacting with NO. In both instances, oxygen-rich clusters are the active metal oxide species for the reaction MmOn + NO -> MmOn(NO)(x). Both VmOn and TamOn cluster systems are very active with NH3. The main products of the reactions with NH3 result from the adsorption of one or two NH3 molecules on the respective clusters. A gas mixture of NO:NH3 (9:1) is also added into the fast flow reactor: the VmOn cluster system forms stable, observable clusters with only NH3 and no VmOn(NO)(x)(NH3)(y) species are detected; the TamOn cluster system forms stable, observable mixed clusters, TamOn(NO)(x)(NH3)(y), as well as TamOn(NO)(x) and TamOn(NH3)(y) individual clusters, under similar conditions. The mechanisms for the reactions of neutral VmOn and TamOn clusters with NO/NH3 are explored via DFT calculations. TamOn clusters form stable complexes based on the coadsorption of NO and NH3. VmOn clusters form weakly bound complexes following the reaction pathway toward end products N-2 + H2O without barrier. The calculations give an interpretation of the experimental data that is consistent with the condensed phase reactivity of VmOn catalyst and suggest the formation of intermediates in the catalytic chemistry. (C) 2010 American Institute of Physics. [doi:10.1063/1.3497652]