The growth of FeOOH microcrystals and chemisorption rate of NO

The adsorption of NO over a period of 1 ms–1 min on α- and γ-FeOOH with different degrees of crystallization has been examined in order to clarify the fast chemisorption phenomenon. The FeOOH crystals were prepared under the conditions of various reaction temperatures and ageing times. The changes in X-ray diffraction patterns, particle size, specific surface area and electrical conductivity were examined. The fast chemisorption rate constant, k_f, for NO was obtained at 30°C from the conductivity change in the initial stage of NO adsorption. The α-FeOOH aged for 2 days and 6 days, the crystallite size of which approximately 100 Å gave the highest k_f and the greatest amount of chemisorption; the high activity is assumed to be caused by a special surface structure accepting NO molecules. The maximum k_f for α-FeOOH was also for a crystallite size of approximately 100 Å. As the change of k_f with crystal growth for γ-FeOOH is similar to that of electrical conductivity, an electronic factor seems to be most important in the fast chemisorption process of NO on γ-FeOOH.