Magnetite surface area titration using nitric oxide

The adsorption of nitric oxide on unsupported Fe₃O₄ has been investigated. Consistent with previous studies, the rate of adsorption follows Elovich kinetics, and the adsorption isotherm obeys the Freundlich equation over the range from 0.133 to 27 kPa at 273 K. However, the time of evacuation of the sample at 650 K prior to NO adsorption is shown to be critical for obtaining reproducible results. Specifically, if the evacuation is carried out for 1 hr, then consistent values of the NO uptake are subsequently obtained. Furthermore, this NO uptake normalized by the nitrogen BET monolayer capacity gives an estimate of the surface cation density that agrees with model calculations based on various low index faces of magnetite. On the other hand, long (ca. 25 hr) evacuation of the sample leads to a reduction of the surface oxidation state resulting in additional NO uptake and reduction of NO to primarily N₂O.