Mechanistic insight into the controlled synthesis of metal phosphide catalysts from annealing of metal oxides with sodium hypophosphite

Understanding and manipulating synthetic progress for precisely controlling the components and defects of nanomaterials is an important and challenging task in materials synthesis and nanocatalysis. Metal phosphides (MPs) have been explored as cheap advanced materials in various catalytic fields. MP materials are usually synthesized through gas-solid phosphorization reaction in a trial-to-error manner, but their formation mechanism and the origin of controlled synthesis remain unclear. Here, we combine in situ thermogravimetric analysis-mass spectrometry (TG-MS) and quasi-in situ X-ray powder diffraction (XRD) analysis to probe the transformation mechanism from metal oxides (MOs) to MPs during the phosphorization process mediated by hypophosphite. Temperature, time, and the amount of hypophosphite are revealed as the driven forces while oxophilicity and crystallinity as the impeded forces, simultaneously control the component and defect level of a series of MP (M = Ni, Co, W, Mo, and Nb). The as-obtained WO_2.9/WP is proved to be an efficient Z-scheme photocatalyst for oxidative coupling of methane with the total C₂₊ production and C₂H₄ selectivity in C₂₊ products reaching 10.75 μmol·g⁻¹ and 98.25%. Our work provides a fundamental understanding of the phosphorization treatment and thereby guides a viable synthetic route to the controlled synthesis of MO_x−δ, MP, MO_x−δ/MP, and MP/M heterostructured materials.