The Role of Hydrogen Adsorption Site Diversity in Catalysis on Transition Metal Phosphide Surfaces

Hydrogen production via clean technologies such as water electrolysis, and its use in the synthesis of value-added chemicals (e.g., ammonia production), play a critical role in the pursuit of decarbonization. This realization requires effective earth-abundant catalysts that facilitate making and breaking H—H and H—X bonds. Transition metal phosphides (TMPs), such as nickel phosphide, cobalt phosphide, and molybdenum phosphide, have been historically used as hydro-processing catalysts in the petroleum industry, enabling critical hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) reactions. Over the past two decades, TMPs have been attracting extensive attention for applications in energy conversion due to their exceptional activity towards the hydrogen evolution reaction (HER). The exceptional HER activity of certain TMPs has been attributed to their nearly ideal H binding energy (HBE), similar to Pt, as deduced by first-principles calculations. In contrast to Pt and other noble metals, where HER and the hydrogen oxidation reaction (HOR) activities are usually correlated, TMPs are usually inactive for HOR. This challenges the applicability of HBE theory to describe activity trends for H2 electrocatalysis on TMPs. In this viewpoint, we discuss the structural complexity of TMPs and its impact on the formation of adsorbed H (Had) and catalysis. In light of this we discuss the validity of HBE theory on TMPs and whether a single value of HBE is sufficient to describe TMP activity trends. Lastly, we propose that the presence of diverse adsorption sites on TMP surfaces can be leveraged to design selective and efficient hydrogenation and electrochemical reduction reactions beyond HER.