Insights into the role of metal dopant on ammonia decomposition over graphene-supported Ni6, Ni5Pt, and Ni5Rh clusters

Abstract

Ammonia (NH₃) is a promising hydrogen (H₂) carrier, owing to its high hydrogen content and ease of storage. However, efficient catalytic decomposition of NH₃ to H₂ remains challenging. This study investigates the catalytic activity of graphene-supported Ni₆, Ni₅Pt, and Ni₅Rh clusters for NH₃ decomposition to N₂ and H₂ using density functional theory (DFT) calculations. The results show that the addition of Pt and Rh dopants weakens the adsorption of intermediates formed during the NH₃ dehydrogenation step and slightly increases the activation energies. Interestingly, however, this drawback becomes a key factor in enhancing the activity of the subsequent N–N association and H–H recombination steps, which are critical for the reaction to cycle. Our electronic structure analysis reveals that the decreased activity in the NH₃ dehydrogenation step but increased activity in the N₂ and H₂ formation steps are due to the ligand effect exerted by Pt and Rh. Specifically, since the metal dopants have higher electronegativities than Ni, the number of electrons at the preferred Ni-edge site decreases and eventually weakens the adsorption. Furthermore, the spatially broad nature of the Pt-5d and Rh-4d orbitals delocalizes the cluster's electron distribution, which weakens the 2N and 2H adsorption and thus eases the N₂ and H₂ formation. This study provides insights into the role of noble metal dopants in optimizing Ni-based catalysts for NH₃ decomposition, paving the way for efficient H₂ utility technologies.