Bismuth phosphate nanoparticle catalyst for direct oxidation of methane into formaldehydeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy00590a

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 BiPO4nanoparticles (BiPO4-DEG) synthesized in a mixed solvent of diethylene glycol (DEG) and water for the direct oxidation of CH4was the highest among the catalysts tested. In high temperature region, BiPO4-DEG was more selective for HCHO formation than FePO4nanoparticles. Based on mechanistic studies including the catalyst effect, kinetics, pulse-reaction experiments, infrared (IR) spectroscopy, operandonear ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), and density functional theory (DFT) calculations, surface active oxygen species generated on BiPO4possibly react with CH4to give HCHO as the primary product. In contrast, the phosphate units of FePO4nanoparticles react with CH4followed by rapid reoxidation to FePO4.