Promotion effect of alkali metal addition on WGS performance of Ni–FeOx structured catalyst

Abstract

This study investigates the role of alkali metals (Li, Na, K, Cs) and alkaline earth metals (Ca, Sr, Ba) in promoting the water–gas shift (WGS) reaction over a nickel-doped iron oxide (Ni–FeO_x) structured catalyst. The introduction of alkali metals markedly enhanced both CO conversion and CO₂ selectivity. Among them, the Cs/Ni–FeO_x catalyst showed the best performance in the water–gas shift reaction, achieving the highest CO conversion under the tested conditions, while the K–Ni–FeO_x catalyst also exhibited similarly high activity and was selected for mechanistic investigation due to its greater practicality. The overall activity trend followed the order of the first ionization energy of the alkali metals. X-ray photoelectron spectroscopy (XPS) confirmed electron transfer from the alkali promoters to the Ni active sites, which contributed to the improved catalytic performance. Fourier-transform infrared (FT-IR) spectroscopy revealed that formate species were formed at 300 °C on both unpromoted and alkali-promoted catalysts. The presence of alkali metals accelerated both the formation and decomposition of the formate intermediate, and the rate of decomposition strongly correlated with the observed WGS activity. Mechanistic investigations using the highly active K/Ni–FeO_x catalyst demonstrated that the reaction proceeds via a formate-mediated pathway, in which formate decomposition is the rate-determining step.