Desorption of carbonium ions from zeolite surfaces; A comparison of reactive surface ionization with field ionization

In comparison with field ionization on platinum the thermal desorption of surface carbonium ions from zeolite surfaces has been investigated mass spectrometrically. CaY-zeolites — forming carbonium ions — were supported by Pt filaments or Pt emitter tips where hydrocarbons are chemisorbed in a non-ionic form. Thermal ion desorption at very low external fields has been observed with stable carbonium ions of triphenylmethyl compounds Ph₃ C-X. This ion desorption was achieved on CaY zeolite surfaces and occurred in the order X  Br > X  Cl > X  OH; however, no ions could be obtained with X  H and X  COOH up to 825°K. On Pt surfaces high fields were required for desorbing molecular ions.