Adsorption of Carbon Monoxide on Iron Surfaces

The most common method for the estimation of the surface area of a metallic phase in a supported catalyst is by measuring the extent of the hydrogen adsorption. With iron surfaces, however, the adsorption of hydrogen often does not proceed consistently. The free iron surface area is therefore usually calculated from the extent of adsorption of carbon monoxide, measured at 90 K. In contrast to adsorption of hydrogen, which has a low boiling point, physical adsorption of carbon monoxide by oxidic surfaces present in the catalysts is appreciable at 90 K. The extent of the adsorption is therefore measured twice after the first measurement, the catalyst is evacuated at 195 K and the extent of adsorption is determined again. The amount of carbon monoxide chemisorbed by the iron surface is assumed to be the difference between the values obtained from the two adsorption isotherms.  

The amount of carbon monoxide that can be desorbed at 195 K is dependent upon the period of time of the evacuation and the rate of evacuation of the sample. Consequently, Scholten used the value obtained from the adsorption isotherm of physically adsorbed nitrogen for subtraction from the isotherm representing the total adsorption of carbon monoxide. On different inert adsorbants, he determined the ratio of adsorption isotherms of physically adsorbed carbon monoxide and nitrogen to be 1.05. 

The number of iron atoms at the surface suggests a level of chemisorption of about one carbon monoxide molecule per two iron surface atoms. Taking into account the size of an adsorbed carbon monoxide molecule, which exhibits a diameter of about 0.3 nm, a coverage of about 1.0 x 10 molecules per cm is reasonable. 

Experimental results on single crystals show that at temperatures below about 250 K, dissociative adsorption of carbon monoxide on the surface does not proceed. Molecular adsorption of carbon monoxide occurs on different sites, each showing different heats of adsorption. 

Experiments on vapor-deposited iron films also point to an extent of adsorption of carbon monoxide of about 1.0 x 10 molecules per cm. For example, the effect of carbon monoxide on the electrical conductance of an iron film is represented in Fig. 5.6. When the level of adsorption reaches 0.7 to 1.0 x 10 molecules per cm, the conductance is unaffected by further addition of carbon monoxide. It appears that the smaller ratios measured on iron catalysts after more severe reduction treatments arise from segregation of oxides to the iron surface. The oxides are present initially within the iron and migrate to the iron surface at high temperatures. 

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