Unveiling the temperature-driven microkinetic processes of NH3 decomposition on the Co(111) surface

Abstract

The limited understanding of the reaction mechanism, particularly the unresolved debate on surface dynamics, hinders the advancement of catalytic ammonia (NH₃) decomposition as a viable pathway for carbon-free hydrogen production. This study integrates spin-polarized DFT + U calculations with microkinetic simulations to investigate the microkinetic of NH₃ decomposition on the promising transition metal Co(111) surface. The results show that NH₃ decomposition on Co(111) exhibits temperature-dependent properties. The N∗−N∗ coupling (N∗ + N∗ → N₂∗) is identified as the rate-limiting step across all temperatures, despite being highly favorable both thermodynamically (ΔH = −0.85 eV) and kinetically (Ea = 0.23 eV). Below 750 K, NH₃ decomposition is limited by the second dehydrogenation step (NH₂∗ + H∗ → NH∗ + 2H∗), due to its high activation energy (Ea = 2.27 eV) and strong NH₃ adsorption (Eads = 1.54 eV). As the temperature rises (750–1000 K), the reverse shift in the equilibrium of the third dehydrogenation step (NH∗ + 2H∗ → N∗ + 3H∗, ΔH = +0.59 eV) and H∗−H∗ coupling (H∗ + H∗ → N₂∗, ΔH = +1.02/+1.05 eV) causes NH₂∗ and NH∗ accumulation, reducing H₂ and N₂ production rates. This study aims to clarify the long-standing debate on rate-determining steps in NH₃ decomposition and provides theoretical insights for developing efficient Co-based catalysts.