Rare earth metal promoters (La, Ce, Nd, Sm) on nickel-supported Al2O3 catalysts for ammonia decomposition

Abstract

Ammonia is the 2^nd most widely produced chemical, storing 17.6 wt% of hydrogen, but the economic generation of hydrogen from it needs a more affordable solution. Hence, replacing precious metals, such as ruthenium (Ru), with inexpensive nickel (Ni) is desirable. In this study, a series of rare-earth metals (La, Ce, Nd and Sm) promoted nickel nanoparticles supported on alumina (Al₂O₃) have been investigated for ammonia decomposition. Here, 3, 5 and 10 % promoters loaded on 50 wt% Ni on Al₂O₃ have been prepared and characterized by XRD, SEM, TEM, BET, H₂-TPR and XPS. HRTEM and elemental mapping reveal a homogeneous distribution of La-promoters on the surface of Ni nanoparticles with an average size within a narrow range of 31 nm. Catalyst 5%La/Ni/Al₂O₃ demonstrates 90 % ammonia decomposition activity at 500 ℃, outperforming the 5%Ce/Ni/Al₂O₃ under the optimized gas hourly speed velocity (GHSV) of 20,400 mL/g_cat/h. respectively. The impact of promoters on 50%Ni/Al₂O₃ can be established as 5%La > 5%Ce > 5%Sm > 5%Nd catalyst. Optimizing 5%La loaded catalyst showed better catalytic activity than 10%La in terms of ammonia decomposition. The 5%La/Ni/Al₂O₃ and 5%Ce/Ni/Al₂O₃ catalysts retained their stability for an extended period of time (65 h). The experimental findings are substantiated by first-principles density functional theory (DFT) calculations, which provide insights into the catalytic reaction pathway. The results demonstrate that the incorporation of La into the Ni(111) surface significantly reduces the activation energy for NH₃ dissociation, thereby promoting enhanced catalytic efficiency for ammonia cracking.