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

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 degreesC, 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 (CH2=NH) 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.