(329c) High-Performance Bifunctional Sulfur Carriers for Hydrogen Production from Hydrogen Sulfide with Regeneration Using Carbon Dioxide in Cyclic Redox Scheme

Hydrogen sulfide (H₂S) is a dangerous pollutant, largely formed during the processing of fossil fuels. Claus process and reactive adsorption are the widely adopted decomposition technologies for safe H₂S removal. However, they suffer from various drawbacks including unfavorable thermodynamics, high energy requirement, and poor adsorbent recyclability. Moreover, their fundamental oxidative chemistry converts H-content of H₂S into low-value steam instead of valuable hydrogen (H₂).

In this work, we develop a cyclic redox scheme for continuous decomposition of H₂S into H₂ and elemental sulfur (S) using Ni-based sulfur carrier in two steps—sulfidation and regeneration. In first step, nickel sulfide (Ni₃S₂) decomposes H₂S into H₂ while forming NiS and then captured S is removed by decomposing NiS into Ni₃S₂ by passing carbon dioxide (CO₂). The scheme reduces the number of processing units and energy requirement and eliminates the need of air separation unit by utilizing readily available CO₂. Ni₃S₂ is impregnated on highly stable supports—ZrO₂ and MgAl₂O₄ to tackle its thermal instability and enhance the active material dispersion. Both sulfur carriers exhibit a stable reactivity over 10 redox cycles and show complete regeneration in CO₂ with ~95% selectivity towards the formation of S over SO₂. Furthermore, ZrO₂-supported sulfur carrier yields universally higher reactivity towards H₂S decomposition compared to MgAl₂O₄-supported sulfur carrier with ~100% increase in sulfur uptake during redox cycles. The reaction mechanisms investigated by density functional theory reveal that MgAl₂O₄ acts as an inert support due to its weak interaction towards H₂S, while ZrO₂ serves as a bi-functional catalytic support that increases the surface area and participate in the H₂S decomposition via a non-oxidative route, forming S₂ and H₂. The formed S₂ then actively adsorbs onto the Ni₃S₂ surface further sulfurizing Ni₃S₂, thereby promoting the overall kinetics. These findings demonstrate a novel technology for cost and energy efficient H₂S utilization.