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

[Display omitted] •In situ X-ray diffraction and aberration-corrected TEM analysis of silica-promoted iron oxides catalyst precursors.•Silica promoted precursor are reduced to α-Fe via stable magnetite and wüstite phases.•Fe0 diffuses from the silica-promoted iron oxide upon reduction from Fe2+ and coexists with an amorphous Fe-O-Si matrix.•Some wüstite remain irreducible to α-Fe owing to the formation of a protective silica-containing layer.•Given enough energy, the amorphous Fe-O-Si material forms crystalline fayalite. 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 α-Fe proceeds via magnetite. Silica promoted amorphous iron oxide is reduced to α-Fe via stable magnetite and wüstite 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 wüstite remain difficult to reduce to α-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.