Alcohol oxidation with high efficiency and selectivity by nickel phosphide phases

Abstract

Small aliphatic alcohols are excellent fuels that have a much higher energy density than liquid H₂. It is advantageous to avoid their complete oxidation to CO₂ and produce chemicals with higher commercial value along with moderate energy extraction. We explored three different phases of nickel phosphide (Ni₁₂P₅, Ni₂P, and Ni₅P₄) for the electro-oxidation of methanol, ethanol, isopropanol, ethylene glycol, and glycerol. Ni₁₂P₅ exhibits excellent activity (210 mA cm⁻² at 1.72 V vs. RHE), durability, and mass activity (∼4.2 A mg⁻¹), outperforming the state-of-the-art catalysts. We compared the reaction mechanisms and concluded that except ethylene glycol, the various alcohols are oxidized following a similar reaction mechanism, both on Ni₁₂P₅ and on Ni₂P. The high selectivity of the reaction and the large suppression of further oxidation to CO₂ are a marker of a preferred bidentate adsorption configuration that conveys a specific O–H activation reaction path. The catalysts show excellent activity towards alcohol oxidation and durability, likely thanks to the mild conditions required for the process, which allow the formation of a regenerating thin active layer of oxidized nickel.