Mechanistic Insights into Photocatalyzed Hydrogen Desorption from Palladium Surfaces Assisted by Localized Surface Plasmon Resonances

Nanoparticles synthesized from plasmonic metals can absorb low-energy light, producing an oscillation/excitation of their valence electron density that can be utilized in chemical conversions. For example, heterogeneous photocatalysis can be achieved within heterometallic antenna-reactor complexes (HMARCs), by coupling a reactive center at which a chemical reaction occurs to a plasmonic nanoparticle that acts as a light-absorbing antenna. For example, HMARCs composed of aluminum antennae and palladium (Pd) reactive centers have been demonstrated recently to catalyze selective hydrogenation of acetylene to ethylene. Here, we explore within a theoretical framework the rate-limiting step of hydrogen photodesorption from a Pd surface - crucial to achieving partial rather than full hydrogenation of acetylene - to understand the mechanism behind the photodesorption process within the HMARC assembly. To properly describe electronic excited states of the metal-molecule system, we employ embedded complete active space self-consistent field and n-electron valence state perturbation theory to second order within density functional embedding theory. The results of these calculations reveal that the photodesorption mechanism does not create a frequently invoked transient negative ion species but instead enhances population of available excited-state, low-barrier pathways that exhibit negligible charge-transfer character.