Dynamic investigation of the mechanism of reaction between sulphur and oxygen on a molybdenum surface by means of Auger electron spectroscopy

Abstract

The reaction of gaseous oxygen with segregated sulphur on a polycrystalline molybdenum surface was studied by Auger electron spectroscopy between 700 and 1300°C. The “working state” of the surface was examined directly and the concentration changes of all surface species which participate in the reaction were determined during the course of the reaction.

At temperatures around 800°C, reaction is predominantly between ambient oxygen and surface sulphur to form sulphur dioxide molecules, followed by the adsorption of oxygen at the vacant sites; this reacts with the surface sulphur very slowly at this temperature. At temperatures around 1200°C, surface diffusion becomes rapid because of its large activation energy and the dominant process becomes the reaction between the sulphur and chemisorbed oxygen on the surface.

Characteristic phenomena in the reaction, such as an induction period for the oxygen chemisorption on the sulphur-segregated surface and production of clean surface by removal of the segregated sulphur with oxygen at higher temperatures are discussed in connection with this mechanism.