Iron-silica interaction during reduction of precipitated silica-promoted iron oxides using in situ XRD and TEM

The effect of silica-promotion on the reduction of iron oxides in hydrogen was investigated using in situ X-ray diffraction and aberration-corrected transmission electron microscopy to understand the mechanism of reduction and the identity of the iron(II) silicate phase that has historically been designated as the cause of the iron-silica interaction in such materials. In the absence of a silica promoter the reduction of hematite to {\alpha}-Fe proceeds via magnetite. Silica promoted amorphous iron oxide is reduced to {\alpha}-Fe via stable magnetite and w\"ustite phases. During reduction of silica-promoted iron oxide, Fe0 diffuses out of the amorphous silica-promoted iron oxide matrix upon reduction from Fe2+ and coexists with an amorphous Fe-O-Si matrix. Certain portions of w\"ustite remain difficult to reduce to {\alpha}-Fe owing to the formation of a protective silica-containing layer covering the remaining iron oxide regions. Given sufficient energy, this amorphous Fe-O-Si material forms ordered, crystalline fayalite.