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

Abstract

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. PH3 and Pn+ species already appeared in the gas flows at low temperature (ca. 200 °C), and PH3 was involved in the formation of phosphides. The emission of PH3 and Pn+ during the reduction of the molybdenum and tungsten phosphates was smaller than that of the nickel phosphate. Ni2 P drastically accelerated the formation of the PH3 and Pn+ . These results explain why excess phosphorus is needed for the preparation of Ni2 P, and also demonstrate that the TPR-IR technique is an efficient method to understand the complex processes of catalyst preparation.