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

Abstract

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.