Gas phase contributions to the catalytic formation of HCN from CH4 and NH3 over Pt: An in situ study by molecular beam mass spectrometry with threshold ionization

Abstract

Molecular beam mass spectrometry has been used for an in situ study of the Pt-catalyzed formation of hydrocyanic acid from methane and ammonia. The goal was to identify transient gas phase intermediates which would indicate homogeneous contributions to the reaction mechanism. A catalytic wall reactor operated at 1300 °C, 1013 mbar, and 74% HCN yield was connected via a molecular beam interface with a quadrupole mass spectrometer, which allowed the measurement of ionization- and appearance potentials by electron impact. Shape and width of the electron energy spread function were determined by analyzing the ionization efficiency curve of helium; the experimental uncertainty of the measured threshold values was found to be 0.6 eV. By use of the threshold ionization technique it could be shown that methylamine (CH₃NH₂) and methylenimine (CH₂NH) are present in the gas phase under reaction conditions. The measured threshold potentials at m/z = 30 u (9.9 ± 0.6 eV) and m/z = 29 u (10.6 ± 0.6 eV) were unambiguously assigned to the appearance potential of CNH₄⁺/CH₃NH₂ and the ionization potential of CNH₃⁺/CH₂NH, respectively. Both molecules dehydrogenate rapidly at reaction temperature to HCN so that they can be considered as true gas phase intermediates.