Redox-mediated reverse water gas shift integrated with ammonia cracking over Ni/La0.75Sr0.25Cr0.5Mn0.5O3−δ

Abstract

NH₃ and CO₂ can react to produce syngas (H₂ + CO), which serves as a feedstock for the production of chemicals or synthetic fuels. Combining NH₃ cracking (NH₃ → 0.5N₂ + 1.5H₂) and the redox-mediated reverse water gas shift reaction (RWGS, CO₂ + H₂ → CO + H₂O), we propose the “NH₃-RWGS” process in a two-reactor system that can produce N₂-free syngas without requiring a downstream gas separation step. We investigate the role of La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) in the redox-mediated RWGS, combined with a stabilized Ni catalyst to impart NH₃ cracking functionality. The mixture of LSCM and Ni catalyst at a weight ratio of 10:1 increases the NH₃ cracking activity fivefold compared to using only LSCM. Because the reduction of LSCM proceeds through a two-step mechanism that requires the prior cracking of NH₃, it also substantially increases the redox reactivity of LSCM. The Ni catalyst exhibits undesirable nitrogen uptake at ∼500 °C, and the redox capacity of LSCM with NH₃ and CO₂ decreases with temperature. Therefore, the process is best implemented at ∼600 °C. Under these conditions, the application of the “NH₃-RWGS” process with mixtures of LSCM and Ni catalyst is promising to produce high-quality, N₂-free syngas directly from NH₃ and CO₂.