CO₂ Reduction Over Iron–Nickel Alloy Catalysts─Tandem Effect of Support and Alloy Composition

The effect of the Fe:Ni ratio in metallic nanoalloys and the nature of metal oxide overlayer supports (MOx@Al2O3) on catalytic activity, selectivity, and stability in the reverse water–gas shift reaction (RWGS) was investigated. To obtain the FeyNi alloy phase, oxidic (NiyFe1–y)Fe2O4 precursor nanoparticles of varying composition were synthesized (Fe:Ni = 3, 4, and 6, as well as pure iron oxide) with a narrow size distribution and without the use of surfactants. The effect of the varying physical properties of the respective bulk oxides on catalyst performance was circumvented via the preparation of bespoke support materials by impregnating a γ-Al2O3 carrier with MOx overlayers (M = Cr or Ga). The surface of the prepared materials is related to the chemical and electronic properties of the respective MOx, but the pore geometry of γ-Al2O3 is maintained. An inert high-surface-area SiO2 support material was also tested to isolate the performance of the FeyNi phases. The reduced catalysts contain a mixture of the bcc and fcc alloy phases irrespective of the support material. The relative concentrations of each phase are a function of iron content, with an increase in iron content increasing the concentration of the bcc alloy phase. The bcc phase has a high affinity toward reoxidation via CO2 activation, while the fcc phase was only found to be partially reoxidized at elevated temperatures (above 600 °C). When exposed to RWGS conditions, all samples tested show >99.5% CO selectivity. The SiO2-supported samples deactivate rapidly, while the alloys supported on the MOx@Al2O3 overlayers, specifically when supported on CrOx@Al2O3, form a tandem system, supporting high activity and stability. The catalytic performance is dependent on both the alloy composition and the MOx support, with the surprising observation of a reversal of the trend in activity with iron content between CrOx@Al2O3 and GaOx@Al2O3. Spent catalyst characterization showed that the rapid deactivation seen on SiO2 cannot be explained by sintering, oxidation, or carbon deposition. The deactivation is instead attributed to the consumption of the bcc phase under reaction conditions. The results show that there is a beneficial interaction between the fcc phase, an exsoluted amorphous Fe-oxide formed from the bcc phase, and the active support, which enhances the catalytic performance in the RWGS.