(303i) Hydrogen Production from Acid Gas: Structural Changes for Enhanced Reactivity in FeS-Based Hydrogen Sulfide Decomposition Systems

Hydrogen Sulfide (H₂S) is a highly toxic gas produced during the processing of fossil fuels. Despite the growing use of renewable energy sources, non-renewable hydrocarbon-based fuels are projected to dominate the world’s energy production scenario until 2050. Therefore, efficient, and economic treatment of H₂S during fossil use continues to be an industrial concern. Claus process, the current state of the art for H₂S treatment, faces several drawbacks, such as unfavorable reaction thermodynamics, high capital cost, and the requirement of an air separation unit (ASU). Moreover, it converts the ‘H’ content of H₂S into low-cost H₂O instead of valuable H₂. The valorization of H₂S to produce H₂ can be achieved through a two-step thermochemical process facilitated by a sulfur carrier undergoing ‘sulfidation’ and ‘regeneration’ cycles periodically. In the first step– sulfidation, the H₂S in the acid gas stream reacts with a sulfur lean metal sulfide carrier to produce a sulfur-rich carrier and hydrogen. In the second step– regeneration, the sulfur-rich metal sulfide is regenerated at a higher temperature via sulfur uncoupling, generating sulfur which can be recovered later. For the successful realization of this process, developing a high-performance sulfur carrier that exhibits fast reaction kinetics and stable long-term recyclability is vital. Among various metal candidates, iron sulfide (FeS), a low-cost, environmentally benign, and earth-abundant mineral, can be a suitable candidate for the sulfur carrier material. However, the reactivity of FeS towards H₂S is poor, which hinders its potential for effective utilization. To bridge this gap, we structurally modify FeS to improve its reactivity towards H₂S decomposition.

In this work, we report the design of engineered FeS nanoscale sulfur carrier particles embedded in mesoporous silica SBA-15 (FeS@SBA-15). FeS nanoparticles of size ~3 nm were prepared by the wet impregnation technique. For comparison, micro-sized FeS particles impregnated on SiO₂ support (FeS@SiO₂) were also synthesized. Thermogravimetric analyzer (TGA) experiments showed that FeS@SBA-15 maintained a stable performance over 10 sulfidation-regeneration cycles with nearly ~70% improvement in reaction rate compared to FeS@SiO₂. Several solid characterization techniques like Temperature-programmed sulfidation (TPS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, and transmission electron microscopy (TEM) were used to investigate this improvement in the reaction rate. The analysis revealed that the mesoporous, tubular, and ordered structure of SBA-15 provides the FeS nanoparticles with a large surface area and prevents their agglomeration over extended cycles. Further, fixed-bed studies were carried out to evaluate H₂ yield at a larger scale of operation. To corroborate the experimental results and elucidate the effect of particle size on carrier reactivity, atomistic scale density functional theory calculations were performed. The experimental studies in combination with characterization and atomistic calculations confirm the improved reactivity of the engineered particles. The findings from this study provide vital insights into the design of sulfur carriers for two-step H₂S decomposition.