The kinetics of chemisorption of hydrogen and carbon monoxide on evaporated iron films

Abstract

The kinetics of chemisorption of hydrogen and carbon monoxide on evaporated iron films have been investigated at liquid-air temperatures. With hydrogen, the rate varies as the square root of the hydrogen pressure, and this dependence is attributed to the fact that an activated migration of adatoms which results in the production of additional surface sites is the rate-determining process. The rate process is kinetically monotonic, indicating a single-surface process in contradistinction to Beeck’s concept of an instantaneous surface adsorption followed by a slower absorption into the interior. The rate falls exponentially with the amount adsorbed, and the activation energy (3 to 6 kcal/mole) of the process increases with coverage. The mechanism of activated migration over a non-uniform surface having a constant distribution function is consistent with these, and other, experimental results. The results with carbon monoxide are similar to those obtained with hydrogen except that the rate increases linearly with the gas pressure, indicating that the activated migration of adsorbed carbon monoxide molecules is probably the rate-controlling process.