Surface kinetics using line of sight techniques: the reaction of chloroform with Cu(111)

Abstract

The adsorption of chloroform (CHCl₃) on Cu(111) in the temperature range 100–480 K has been studied using line of sight sticking probability (LOSSP) measurements, line of sight temperature programmed desorption (LOSTPD), low energy electron diffraction (LEED), He I ultra-violet photoelectron spectroscopy (UPS) and work function measurements. Chloroform adsorbs molecularly at 100 K with a sticking probability of 0.98±0.02, the monolayer reacting on heating to 170 K to form chemisorbed chlorine and adsorbed ethyne. The adsorbed ethyne desorbs at just above room temperature with first order kinetics, an activation energy of 77±6 kJ mol^-1 and a pre-exponential factor of 10^11±1 s^-1. The sticking probability of chloroform on clean Cu(111) at 320 K is 0.23±0.04, which corresponds to activated adsorption at zero coverage with an activation energy of 3.5±0.7 kJ mol^-1. The initial sticking probability is found to increase slightly for temperatures above room temperature, and also for temperatures below room temperature, while the sticking probability at finite coverage is greatly increased by the presence of the dissociation product, ethyne, on the surface. These observations are explained in terms of activated adsorption at zero coverage which becomes non-activated at finite coverage due to attractive intermolecular interactions between adsorbed chloroform molecules, and adsorbed chloroform and ethyne molecules.