EFFECTS OF POTASSIUM ON THE ADSORPTION AND DISSOCIATION OF CH3CL ON PD(100)

The effects of potassium on the adsorption and dissociation of CH3Cl on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES) electron energy loss spectroscopy (in the electronic range EELS), temperature-programmed desorption (TPD) and work function change. In contrast to the clean surface, the adsorption of CH3Cl caused a significant work function increase, 0.9-1.4 eV, of potassium-dosed Pd. Preadsorbed K enhanced the binding energy of CH3Cl to the surface and induced the dissociation of adsorbed molecules. The extent of the dissociation increased almost linearly with the potassium content. The appearance of a new emission in the UPS spectrum at 9.2 eV, attributed to adsorbed CH3 species, and the low-temperature formation of ethane suggest that a fraction of adsorbed CH3Cl dissociates even at 115-125 K on potassium-dosed Pd(100). At the same time, a significant part of adsorbed CH3 radical is stabilized, the reaction of which occurs only at 250-300 K. By means of TPD measurements, H2, CH4, C2H6, C2H4, KCl and K were detected in the desorbing gases. The results are interpreted by assuming a through-metal electronic interaction at low potassium coverage and by a direct interaction of the Cl in the adsorbed CH3Cl with potassium at high potassium coverage. The latter proposal is supported by the electron excited Auger fine structure of the Cl signal and by the formation of KCl in the desorbing gases.