A novel approach to the synthesis of unsupported nickel phosphide catalysts using nickel thiophosphate as precursor

Nickel thiophosphate, NiPS3 synthesized at room temperature using a soft chemistry approach (a-NiPS3) has been successfully used as precursor for low-temperature preparation of unsupported nickel phosphide. Using Raman spectroscopy and EXAFS, comparison with a well-crystallized NiPS3 reference (c-NiPS3) first confirmed that nickel thiophosphate can be effectively obtained at low temperatures of preparation. In situ XRD diffraction and EXAFS showed that Ni2P was obtained by the reduction of a-NiPS3 at a temperature as low as 573 K. For c-NiPS3, slower kinetics of reduction first led to the formation of an intermediate richer phosphorus-containing nickel phosphide phase, Ni5P4 at a temperature of reduction of 623 K while Ni2P started being formed at 773 K. Under hydrodesulfurization (HDS) thiophene catalytic conditions (613 K, 2.8 mol% thiophene/H2), a-NiPS3 was decomposed into a mixture of Ni2P and Ni5P4 while c-NiPS3 was converted into Ni5P4. Both nickel phosphide catalysts exhibit much higher activities compared to a MoS2 reference. Ni5P4 exhibited a specific activity (per gram of catalyst) seven times as high as for MoS2.