INFRARED SPECTROSCOPIC STUDY OF THE ADSORPTION AND DISSOCIATION OF CH2CL2 ON PD/SIO2 - GENERATION OF CH2 SPECIES

The adsorption and dissociation of CH2Cl2 have been investigated by means of infrared spectroscopy combined with mass spectrometry. CH2Cl2 adsorbs molecularly on Pd/SiO2 at low temperature (193-213 K) and interacts strongly with the OH groups of silica to form H-bridge-bonded methylene halide. This is exhibited by a significant attenuation of the 3744 cm(-1) band exhibited by silica and by the development of a broad spectral feature at 3662 cm(-1). The dissociation of CH2Cl2 was observed at 233-243 K. The primary product of dissociation, CH2, was characterized by absorption bands at 2984 and 2907 cm(-1). The CH2 species dimerized into C2H4 above 263 K. A fraction of ethylene desorbed, while another fraction was transformed into ethylidyne. The latter compound is characterized by absorption bands at 2922 and 2857 cm(-1). By means of mass spectrometry C2H4 and HCl were identified in the desorbing gases at 250-330 K. Decomposition of CH2Cl2 occurred above 350 K yielding C2H4, CH4, HCl, and H2O. Some exploratory measurements were also performed with CH2I2. (C) 1995 Academic Press, Inc.