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

TL;DR

This study investigates how silica promotes the reduction of iron oxides using in situ XRD and TEM, revealing the formation of iron-silicate phases and the reduction pathways influenced by silica.

Contribution

It provides new insights into the reduction mechanism of silica-promoted iron oxides and identifies the formation of crystalline fayalite during reduction.

Findings

Silica promotes reduction via stable magnetite and wüstite phases.

Fe0 diffuses out of silica matrix during reduction.

Crystalline fayalite forms with sufficient energy.

Abstract

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.