The role of surface defects in the chemisorption of nitric oxide and sulfur dioxide on variable-sized crystalline .alpha.-iron hydroxide oxide

Abstract

{alpha}-FeOOH crystals of different crystalline sizes were prepared by aging Fe{sub 2}(SO{sub 4}){sub 3} solution at 303 K for various periods up to 10 days. The synthetic {alpha}-FeOOH crystals were examined by x-ray diffraction, BET surface area measurements, Fe K-edge XANES, and dc and microwave conductivity measurements. The precipitates without aging were amorphous solids. Aging for more than 18 h gave highly crystalline {alpha}-FeOOH, whereas {alpha}-FeOOH obtained by aging for less than 18 h was very defective. The XANES examination showed the presence of tetrahedral Fe(III) sites in the insufficiently aged {alpha}-FeOOH. The chemisorption abilities for NO and SO{sub 2} increase with the crystallite size up to 18 h of aging and become almost constant after 18 h of aging. The dc electrical conductivity of {alpha}-FeOOH decreases upon the first dose of NO or SO{sub 2}; the succeeding dose of the gas increases the conductivity. FT-IR examinations showed the formation of a nitrosyl, nitrito, and nitrato species and a sulfito species for chemisorption of NO and SO{sub 2}. The relationship between the increase in the electrical conductivity upon chemisorption of NO or SO{sub 2} and the formation of the pseudo oxygen vacancy due to the chemisorbed species is discussed frommore » the viewpoint of the surface defect chemistry.« less