High-entropy oxide synthesis in concentrated alkaline solutions for plasma-catalytic formaldehyde oxidation

Abstract

High-entropy oxides (HEOs) have emerged as promising candidates for catalytic oxidation reactions, but present a considerable challenge in achieving high surface area and small particle size. In this work, a high-concentration sodium hydroxide co-precipitation method was developed to synthesize high-surface-area HEO catalysts. This method allows for the rapid precipitation of various metal precursor salts to form a uniform high-entropy compound, reducing nanoparticle size and increasing surface area. Mn, Cu, Co, Fe, and Ni are selected as model elements. The HEO materials synthesized via 10% sodium hydroxide co-precipitation and subsequent calcination at 400 °C exhibited a specific surface area of 173 m² g⁻¹. Plasma-catalytic tests for formaldehyde degradation demonstrate that HEO-10%-400 achieved a 59.7% improvement in removal efficiency, a 93.1% reduction in O₃ production and a 91.8% reduction in NO_x production, compared to plasma-only treatments. The synergistic effect is attributed to the high specific surface area, increased number of active sites and reactive oxygen species in the HEO catalysts.