Thermal and photon induced reactions of vinyl chloride adsorbed on Ag(111).

摘要

Vinyl Chloride physisorbs on liquid nitrogen cooled Ag(111). Thermal Desorption Spectroscopy (TDS) indicates that most of the monolayer desorbs molecularly at 119 K in a first order process with desorption activation energy of 6.7 kcal/mol. The multilayer desorbs molecularly at 92 K in a first order process with desorption activation energy of 4.3 kcal/mol. High Resolution Electron Energy Loss Spectroscopy (HREELS) indicates that the monolayer vinyl chloride is adsorbed with the molecular plane approximately parallel to the Ag(111) surface plane. TDS and HREELS indicate that as the Ag(111) surface is heated to temperatures greater than 126 K, small quantities of the physisorbed vinyl chloride monolayer dissociate into adsorbed vinyl and Cl. Further heating to temperatures greater than 180 K results in vinyl and Cl recombination with subsequent molecular desorption in a second order process with a desorption activation energy of 10.7 kcal/mol.; Pulsed ultraviolet laser irradiation of the monolayer results in desorption and dissociation through a substrate mediated, electron transfer mechanism. The calculated photon excited, substrate electron energy required for vertical attachment is 4.03 eV above the Fermi level while the experimental photon energy threshold is 3.49 eV. The vinyl chloride photodissociation products are vinyl and Cl. Upon heating the substrate, two adsorbed vinyl species combine on the Cl-Ag(111) surface at 249 K to produce 1,3 butadiene in a first order process with a desorption activation energy of 10.4 kcal/mol. The proposed bi-molecular reaction mechanism indicates adsorbed vinyl surface diffusion determines the rate of 1,3-butadiene formation.; Pre-adsorbed Cl on the Ag(111) surface causes physisorbed vinyl chloride to desorb at 150 K in TDS. In addition, pre-adsorbed Cl-Ag(111) increases substrate electron energy required for vertical attachment and hence decreases the photochemical dissociation cross-section. HREELS indicates that physisorbed vinyl chloride on the Cl-Ag(111) surface has its molecular plane even more parallel to the surface plane than for the case of no pre-adsorbed Cl and that the vinyl chloride "hydrogen bonds" to the Cl-Ag(111).