Understanding the Reduction of Transition-metal Phosphates to Transition-metal Phosphides by Combining Temperature-programmed Reduction and Infrared Spectroscopy.

Temperature-programmed reduction of transition-metal phosphate precursors is the most commonly used method for the preparation of transition-metal phosphides (a new class of versatile metal catalysts), but the reduction processes are still not clear. Here we describe the construction of a temperature-programmed reduction-infrared spectroscopy apparatus (TPR-IR) to analyze the gas flows during the reduction of nickel, molybdenum, and tungsten phosphates. PH 3 and P n+ species already appeared in the gas flows at low temperature (ca. 200 o C), and PH 3 was involved in the formation of phosphides. The emission of PH 3 and P n+ during the reduction of the molybdenum and tungsten phosphates was smaller than that of the nickel phosphate. Ni 2 P drastically accelerated the formation of the PH 3 and P n+ . These results explain why excess phosphorus is needed for the preparation of Ni 2 P, and also demonstrate that the TPR-IR technique is an efficient method to understand the complex processes of catalyst preparation.