Vibrationally excited nitrogen molecules formed in the catalytic decomposition of ammonia on platinum

Abstract

Catalytic decomposition of ammonia on polycrystalline platinum was studied with a modulated molecular beam mass spectrometer. Threshold ionization measurements on N2 have established the production of significant quantities of vibrationally excited nitrogen molecules (N*2 ). Although the overwhelming majority of molecules have excitation energies less than 1.3 eV, a few of the molecules have energies as high as 2.4 eV, requiring excitation to at least the v=9 vibrational level of N2. It is suggested that the observed vibrational energy is provided by two reactions involving recombination of chemisorbed NH radicals: NHad+NHad→N*2+H2 and NHad+NHad→N*2+2Had. The ionization data on N*2 indicate that during ionization vibrational energy is being efficiently converted into electronic energy via autoionization processes. Measurements on the appearance potential of NH+2 show no evidence for the evolution of NH2 radicals from NH3 decomposition, confirming recently published results. Pulsed heating experiments show that N*2 molecules have, as expected, shorter lifetimes than N2 molecules. Reaction mechanisms for N*2 production are discussed.