Crystallite size effect in the selective oxidation of butene to butadiene on iron oxide. 1. Mössbauer, X-ray, and magnetization characterization of the catalysts

A series of silica-supported and unsupported iron oxide catalysts were characterized by X-ray diffraction, room-temperature Mössbauer spectroscopy, and low-temperature magnetization measurements. The average crystallite sizes were determined by the X-ray line-broadening technique. They ranged from 2.5 to 9.5 nm for the supported samples and from 14.5 to 61 nm for the unsupported samples. X-ray diffraction detected only the presence of α-Fe₂O₃. Fourier line-shape analyses showed that the samples were rather strain free, and the width of the crystallite size distribution increased with increasing average crystallite size. The Mössbauer patterns showed the presence of a six-line magnetic component and a superparamagnetic component. The Mössbauer parameters of both components are consistent with the assignment of α-Fe₂O₃. In particular, no component with zero quadrupole splitting assignable to γ-Fe₂O₃ was observed. Magnetization measurements showed that the small crystallite samples possess magnetic moments higher than that of bulk α-Fe₂O₃ but much lower than that of γ-Fe₂O₃. The data was explained by the absence of Morin transition and incomplete cancellation of spins in very small crystallites of α-Fe₂O₃.