Molybdenum phosphide (MOP) catalyst has been widely applied in hydrogenation reactions,while the preparation of unsupported MoP catalysts with ultra-small size and large specific surface area (SBET) is still challenging.Herein,we have provided a facile method for preparing a series of MoP-x (x =P/Mo ratios ranging from 1 to 5) catalysts by pyrolyzing phytic acid (PA)-derived Mo complexes in a H2 atmosphere.The physicochemical properties and the catalytic activity of MoP catalysts were investigated.The results showed that the obtained MOP-5 catalyst had the largest SBET and exhibited ultra-small nanoparticle diameter of 3.6 nm,which ascribed to the chelation of PA and the confinement of deposited products.As the content of PA increased,the synthetic mechanism of MoP was also affected,which led to the difference in the valence of surface Mo species.The characterization results further confirmed that Moδ+ sites in MoP catalysts are active sites for methanation reaction and its content on the surface of MoP-x catalysts determines the catalytic activity.