Mechanistic investigation of ammonia adsorption and reforming over Ni, Ru loaded catalysts toward green H2 production of marine engines: DFT calculation and experimental evaluation

Abstract

Highly active catalysts suitable for ammonia reforming reactions have been comprehensively investigated by density functional theory (DFT) calculations and experimental tests. Non-precious metal Ni, precious metal Ru and Ru-Ni composite metal are selected as catalytic activity surfaces, and a series of three-dimensional metal-packed catalyst models are constructed parallelly. Then, to elucidate the rate-determining step of reforming reaction, in-depth analysis of microscopic reaction mechanisms for amino groups (including H and NHx, x=0–3) adsorption, dissociation, recombination and desorption are carried out by simulation parameters and experimental data. The results indicate that NH3 exhibits highly stable adsorption configurations (Bridge site) on both Ni, Ru-based catalyst surfaces, with the adsorption position having a relatively minor effect on adsorption energy. It is worth noting that Ni-based catalyst exhibits better adsorption behaviors towards NH3 compared to Ru. Whereas, during stepwise dehydrogenation of NH3 and subsequent N2 and H2 recombination and desorption processes, the reaction energy barriers confirm experimental observations of higher activity displayed by Ru-based catalyst in low-temperature range of 400–500 °C. Interestingly, N2 synthesis behavior of innovative Ru-Ni bimetallic tandem catalyst surface possesses the lowest reaction energy barrier than other models, which is attributed to the adsorption capability of Ni layers and the catalytic activity of Ru atoms. The experimental results also demonstrate that under typical engine exhaust conditions, the H2 production efficiency of 1 %Ru-3 %Ni/Al2O3 surpasses that of 1 %Ru/Al2O3 by 6.2 %, showcasing exceptional stability.