PNNL

Abstract

Cobalt phosphide is one of the most promising earth-abundant replacements for noble metal catalysts and electrocatalysts for energy-related and other reactions. Many of these reactions, starting with the simplest, the hydrogen evolution reaction (HER), involve the catalyst binding hydrogen atoms (protons and electrons). Theoretical studies have computed the preferred sites and energetics of bound hydrogen on phosphide surfaces. However, direct experimental studies of hydrogen on transition metal phosphides surfaces are scarce, especially at the solid/solution interfaces. Herein, we describe measurements of hydrogen stoichiometry and thermochemistry for hydrogen bound to cobalt phosphide. We studied both mesoscale cobalt phosphide particles, exhibiting phosphide surfaces after an acidic pretreatment, and colloidal cobalt phosphide nanoparticles. Treatment with H2 introduced large amounts of reactive hydrogen to cobalt phosphide, on the order of one H per Co or P surface atom. This stoichiometry was determined by titration of the bound hydrogen using alkyne hydrogenation and H-atom transfer reactions with phenoxy radicals. There are also substantial amounts of reactive hydrogen-atoms present on the as-prepared materials, before hydrogen treatment. Cobalt phosphide was found to be reactive with hydrogen atom transfer reagents, accepting hydrogen from molecules with low X–H bond dissociation free energies (BDFEs) and donating hydrogen to good H-atom acceptor reagents. Based on this reactivity we estimate that there is a distribution of possible binding energies for hydrogen on cobalt phosphide, between ~53 kcal/mol and ~63 kcal/mol. Operando X-ray absorption spectroscopy gave first indications about the structure of hydrogenated CoP, showing a slight lattice expansion under H2-flow and indicating that the addition of hydrogen does not significantly change the effective nuclear charge of the cobalt. These results provide a new picture of catalytically active cobalt phosphide, with a substantial amount of reactive hydrogen atoms. This revised view of cobalt phosphide is of fundamental relevance for its catalytic and electrocatalytic properties, and the approach developed here provides a road map to examine hydrogen on other materials.