Reverse water gas shift by chemical looping with iron-substituted hexaaluminate catalysts

The Fe-substituted Ba-hexaaluminates (BaFeHAl) are active catalysts for reverse water-gas shift (RWGS) reaction conducted in chemical looping mode. Increasing of the degree of substitution of Al³⁺ for Fe³⁺ ions in co-precipitated BaHAl from 60% (BaFeHAl) to 100% (BaFe-hexaferrite, BaFeHF), growing its surface area from 5 to 30 m²/g, and promotion with potassium increased the CO capacity in isothermal RWGS-CL runs at 350–450^◦C, where the hexaaluminate/hexaferrite structure is stable. Increasing H₂-reduction temperature converts BaFeHAl to a thermally stable BaFeHF modification that contains additional Ba-O-Fe bridges in its structure, reinforcing the connection between alternatively stacked spinel blocks. This material displayed the highest CO capacity of 400 µmol/g at isothermal RWGS-CL run conducted at 550^◦C due to increased concentration of oxygen vacancies reflected by greater surface Fe²⁺/Fe³⁺ ratio detected by XPS. The results demonstrate direct connection between CO capacity measured in RWGS-CL experiments and calculated CO₂ conversion.