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

The hydrogen evolution reaction (HER) activity of certain Transition-metal phosphides (TMP) has been attributed to their nearly ideal hydrogen binding energy (HBE), similar to Pt, as deduced by first-principles calculations. The structural complexity of TMP and its impact on the formation of adsorbed H (Had) and catalysis are discussed along with the validity of HBE theory on TMP and whether a single value of HBE is sufficient to describe TMP activity trends. The diversity of adsorption sites introduces challenges in bridging computational and experimental results, because of large discrepancies in calculated TMP HBE values and difficulties in characterizing the surface of a catalyst. Most importantly, the diversity of sites present on TMP surfaces is crucial in understanding mechanisms of more-complex reactions involving hydrogen.