Plasmon-Driven Ammonia Decomposition on Pd(111): Hole Transfer’s Role in Changing Rate-Limiting Steps

Ammonia (NH₃) has the potential to be a hydrogen carrier because it can be transported and stored with ease, but only if it also can be decomposed easily when needed. Understanding how to control the frequently rate-limiting N-H bond breaking and N-N bond forming on catalytic surfaces may help design efficient means for NH₃ decomposition. Yuan et al. recently demonstrated photocatalytically selective N-H bond breaking in NH₃ on plasmon-driven aluminum-palladium (Al-Pd) antenna-reactor heterostructures [Yuan et al. ACS Nano 2022, 16 (10), 17365 ]. Using embedded correlated wavefunction (ECW) theory, we predict that the rate-determining step (RDS) for NH₃ decomposition on Pd(111) via thermocatalysis (dissociating the first N-H bond, *NH₃ → *NH₂ + *H, in the ground state, where * means adsorbed) differs from that via photocatalysis (dissociating the second N-H bond, *NH₂ → *NH + *H, in the excited state). This result is consistent with the measured catalytic efficiency and selectivity of NH₃−deuterium (D₂) exchange reactions (an indirect way to measure N-H bond breaking) on Al-Pd heterodimers. We also determine the origin of the observed selectivity of thermocatalysis and photocatalysis on Pd(111) toward doubly deuterated (NHD₂) and monodeuterated (NH₂D) products, respectively, and explore viability of the full NH₃ decomposition path, also via ECW theory. Additionally, we predict that the associative desorption of *N as N₂ from Pd(111) is extremely difficult in thermocatalysis at least at low surface coverages; metal-to-adsorbate hole transfer in photocatalysis stabilizes the transition state for the first N-H bond dissociation, shifting the RDS to the second N-H bond breaking. Furthermore, the redistribution of electrons around *N upon excitation reduces the electron density in the Pd-N bonds, which may lower the barrier for N₂ associative desorption in photocatalysis. Thus, light-induced, plasmon-mediated, excited-state hole transfer may provide an efficient mechanism to accelerate NH₃ decomposition.