The reduction of Fe-bearing copper slag for its use as a catalyst in carbon oxide hydrogenation to methane. A contribution to sustainable catalysis.

Reduction of Fe-phases in a slag from the copper smelting process is studied for its use as a catalyst in methanation of carbon oxide (CO). This material contains 36.4 wt% Fe and the main Fe-phases in its fresh and reduced forms were identified and quantified. Chemical analysis and X-ray diffraction (XRD) for crystalline phase detection and determination of Fe dispersion were carried out. Reducibility of Fe-oxides was studied by thermal programmed reduction (TPR) under H2 at 650 and 800 °C using 0.5 and 2 h soak time. In the fresh slag, iron was found to be in the form of Fe3O4 (17.4 wt%) and fayalite, Fe2SiO4 (43.4 wt%). The composition was experimentally determined and verified by stoichiometric balances and thermogravimetric analysis (TGA). Upon reduction at 800 °C and 2 h soak time, 87 % of the Fe-phases were reduced, leaving an activated catalyst with a 35.2 % Fe0, which is the active phase for CO hydrogenation to methane. An expression was derived to determine the Fe0 concentration in the reduced slag based on the composition of the fresh slag and its reduction degree. The catalytic activity of the reduced slag during CO hydrogenation was evaluated in a fixed bed differential reactor. The selectivity to methane, at 300 °C, was 87 %, thus confirming its catalytic activity for the selected reaction.