A new preparative procedure for ultrafine spinel ferrites

Ferrites have been used as industrial catalysts of butene-1 to butadiene. Recently, great attention has been focused on ultrafine ferrites in our laboratory. It is well established that preparatory procedures have a major influence on the performance of catalysts. Thus, a series of ultrafine ferrites were prepared by different procedures. In an attempt to find suitable procedures, the explosion method of excessive urea was first developed by ourselves. The process was as follows. First, excessive urea was added to the solution of nitrates. On controlled heating, a white slurry was formed. The slurry became hardened at room temperature. Eventually, the slurry was placed in a muffle furnace at a calcination temperature, whereupon a severe explosion occurred and the corresponding ultrafine powder was obtained. If the slurry producing step was well controlled, the single spinel phase would be obtained. It was found that the samples obtained in this way, denoted as E-type samples, exhibited the best activities and selectivities in oxydehydrogenation of ethylbenzene [1]. The M6ssbauer spectra of ultrafine magnesium ferrites taken at 298 K are shown in Fig. 1. The solid lines through the data points are the result of computer simulations of the spectra. The parameters derived from the least-square fits are listed in Table I. The C-type magnesium ferrite shown in Fig. 1 was prepared by hydrolysis in citric acid in the conventional way. The two kinds of magnesium ferrites both have spinel structures as illustrated by X-ray powder diffractometry (XRD). Observations by transmission electron microscopy (TEM) suggested that the particle sizes of the two samples were all below 20 nm. However, the E-type magnesium ferrite exhibited completely superparamagnetic spectrum only with doublets, whereas the C-type magnesium ferrite mainly exhibited normal Zeeman-split sextets, one due to the Fe 3÷ ions at the tetrahedral (A) site and (a)