The adsorption, desorption, and exchange reactions of oxygen, hydrogen, and water on platinum surfaces. Pt. 4

Field electron emission microscopy has been used to study the adsorption of water in platinum and its production by the reaction of hydrogen with adsorbed oxygen. Techniques are described whereby clean reproducible Pt field emitters can be produced without irreversible contamination or excessive tip blunting. Direct water adsorption always led a emitter failure, consequently adsorbed water was produced by the reaction of hydrogen with adsorbed oxygen. This reaction occurred from 100 to 175 K with a decrease in work function from Δphi = +0.6 eV, for oxygen adsorption, to a minimum work function, Δphi = -0.55 eV, at 175 K. From 175 to 300 K the work function increases and reattains a positive shift, Δphi approx. +0.2 eV, and a field emission pattern characteristic of adsorbed oxygen which are uninfluenced by further H2 interaction up to 600 K. There is little change in emission anisotropy, but enhanced emission from Pt(110) planes suggests that the reaction is particularly facile on this plane. The work function shifts indicate the formation of adsorbed water up to 175 K, which therefore forms an electropositive adsorbed state, followed by desorption of water in the interval 175 to 300 K from an adlayer stabilised by the presence of unreacted oxygen. These observations are in agreement with thermal desorption results, although the observation that part of the oxygen adlayer could not be reduced was surprising since thermal desorption studies suggested that this efficient reaction proceeded to completion. Rationalisation requires that the unreactive oxygen is undetected in the thermal desorption experiments and supporting evidence is given from oxygen adsorption, desorption and exchange, field emission and Auger electron spectroscopy experiments. (orig.)