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

Abstract

The effect of the Fe:Ni ratio in metallic nanoalloys and the nature of metal oxide overlayer supports (MO_x@Al₂O₃) on catalytic activity, selectivity, and stability in the reverse water–gas shift reaction (RWGS) was investigated. To obtain the Fe_yNi alloy phase, oxidic (Ni_yFe_1–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 MO_x overlayers (M = Cr or Ga). The surface of the prepared materials is related to the chemical and electronic properties of the respective MO_x, 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 Fe_yNi 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 MO_x@Al₂O₃ overlayers, specifically when supported on CrO_x@Al₂O₃, form a tandem system, supporting high activity and stability. The catalytic performance is dependent on both the alloy composition and the MO_x support, with the surprising observation of a reversal of the trend in activity with iron content between CrO_x@Al₂O₃ and GaO_x@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.