Redox-Responsive Multimetal Phosphides for Reverse Water–Gas Shift: Does High Entropy Improve Catalytic Activity?

Abstract

High-entropy phosphides (HEPs) offer compositional complexity and potential redox flexibility, yet their role in CO2 conversion remains unclear. Here, silica-supported mono-metal, tri-metal, and high-entropy-composition phosphide catalysts were examined for the reverse water-gas shift (RWGS) reaction to clarify how phosphide structure and redox chemistry govern catalytic performance. FeCoNiPx/SiO2 and CrMnFeCoNiPx/SiO2 formed compositionally uniform phosphide nanoparticles and showed stable RWGS activity. Among the tested catalysts, FeCoNiPx/SiO2 showed a more favorable balance of activity under the evaluated conditions, reaching 51% CO2 conversion at 700 oC with CO as the only detectable product and an apparent activation energy of 63.4 kJ mol−1, lower than that of CrMnFeCoNiPx/SiO2 at 75.6 kJ mol−1. X-ray absorption (XAS) analysis revealed that Fe, Co, and Ni species in FeCoNiPx/SiO2 and CrMnFeCoNiPx/SiO2 underwent reversible oxidation under CO2 and reduction under H2, whereas Ni in CrMnFeCoNiPx/SiO2 was less redox-responsive. P K-edge XAS further showed partial phosphide-to-phosphate transformation under CO2 and recovery under H2, indicating that both metal and phosphorus species participate in the redox cycle. These results show that high entropy composition can provide stable and selective RWGS catalysts but does not necessarily enhance activity. Instead, RWGS performance is governed by reversible metal-phosphide redox ensembles, with FeCoNiPx/SiO2 providing a more favorable balance than CrMnFeCoNiPx/SiO2.