Chemisorption and surface corrosion in the tungsten + carbon monoxide system, as studied by field emission and field ion microscopy

Abstract

A new interpretation of the occurrence of different adsorption states of carbon monoxide on tungsten has been achieved by combining field emission and field ion microscopy techniques. It is shown that the adsorption of CO on a field-evaporated tungsten surface causes a re-arrangement of the surface atoms. The activation energy required decreases with increasing coverage and is smallest for the less densely packed crystal faces. At 78°K re-arrangement takes place to a very little extent only. Two adsorption layers can be distinguished: a chemi-adsorption layer and a physisorbed layer on top. At 300°K re-arrangement leads to complete surface corrosion, creating vacancies in the layers beneath the corrosive chemisorption layer. On top of the latter layer, additional CO can be weakly adsorbed.

Hydrogen does not cause any re-arrangement of tungsten. However, CO adsorption on tungsten precovered with hydrogen, again gives rise to re-arrangements with removal of hydrogen. The results, together with the evidence obtained with other systems, justify the conclusion that surface re-arrangement can be an important factor in adsorption, even at low temperatures and on metals with a high cohesive energy.