Bismuth phosphate nanoparticle catalyst for direct oxidation of methane into formaldehyde

Abstract

The direct oxidation of methane (CH₄) to formaldehyde (HCHO) with molecular oxygen (O₂) as the sole oxidant was studied over various bismuth-based catalysts (BiPO₄, α-Bi₂O₃, β-Bi₂O₃) using a fixed-bed flow reactor. The catalytic activity of monoclinic BiPO₄ nanoparticles (BiPO₄-DEG) synthesized in a mixed solvent of diethylene glycol (DEG) and water for the direct oxidation of CH₄ was the highest among the catalysts tested. In high temperature region, BiPO₄-DEG was more selective for HCHO formation than FePO₄ nanoparticles. Based on mechanistic studies including the catalyst effect, kinetics, pulse-reaction experiments, infrared (IR) spectroscopy, operando near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), and density functional theory (DFT) calculations, surface active oxygen species generated on BiPO₄ possibly react with CH₄ to give HCHO as the primary product. In contrast, the phosphate units of FePO₄ nanoparticles react with CH₄ followed by rapid reoxidation to FePO₄.