Structure and catalytic activity of ultra-high-entropy rare-earth orthoferrite (UHE REO) towards thermal hydrogen oxidation

Abstract

The application of new catalysts based on high-entropy oxides to replace traditional catalysts has proven to be effective in both economics and energy. The present research focuses on the design and development of novel materials based on ultra-high-entropy rare-earth orthoferrites (UHE REOs, ∑REFeO₃) based on 16 rare-earth elements (RE = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). The solution combustion method was used to synthesize the ∑REFeO₃ nanopowder, which was then applied for catalytic hydrogen oxidation. By varying the ratio of fuel/oxidant (φ) towards values of 1.0 ÷ 3.5, six samples were obtained. It was found that the change in φ can lead to a difference in combustion mode and, therefore, result in the synthesized product's physicochemical properties. In particular, all these samples displayed an orthorhombic crystal structure of perovskite-like compounds with an average crystallite size of ∼31–36 nm and a specific surface area of ∼6.7–13.7 m² g⁻¹. The sample with φ = 2.0 showed its highest reducing property, as compared to those with φ = 1.0 and 3.0 when the total amount of H₂ consumption in the H₂-TPR analysis equals 2.75, 1.77, and 0.83 mL, respectively. Furthermore, the chemical phase purity, elemental composition, element oxidation state, and morphological features of the sample with φ = 2.0 were additionally examined by EDXS, XPS, and EDX-mapping analyses. The catalytic performance of the ∑REFeO₃ samples demonstrated that the sample with φ = 2.0 exhibits the highest activity, with a hydrogen conversion of ∼86% at 500 °C and an observed activation energy of 56.0 kJ mol⁻¹ (for a temperature region of 300–500 °C). Thus, this study highlights the great potential of UHE REOs as a catalytic platform for thermal hydrogen oxidation.