Adsorption of oxygen and coadsorption of oxygen and carbon monoxide on polycrystalline iron films at 273 K

Abstract

Iron films of thicknesses of about 10 nm evaporated under ultrahigh vacuum conditions have been used to study the sorption of oxygen and the influence of a preadsorption of oxygen (carbon monoxide) on the subsequent adsorption of carbon monoxide (oxygen) at 273 K. Experimental techniques included measurements of adsorbed amounts, of changes in resistance and in work function, and of thermal desorption. Under the applied pressures (p<10-2Pa) oxygen forms a multilayer, whereas carbon monoxide forms a monolayer. The different states of the adsorbed oxygen can be derived from the change in work function, not, however, from the heat of adsorption or from the change in resistance. Since oxygen is more strongly bound to the iron surface than carbon monoxide, it is able to displace the latter from the surface. When carbon monoxide is coadsorbed with oxygen it is more loosely adsorbed than on a clean iron surface. A quantitative description of the effects is given.