Catalytic Conversion of H₂S to H₂: Challenges and Catalyst Limitations

H2S, a highly toxic chemical, is produced in massive quantities worldwide as a byproduct. Environmental regulations require >99% sulfur recovery, which is currently met using sulfur recovery units based on the Claus process, where H2S is converted to sulfur and water. Ideally, hydrogen in H2S is recovered as H2. Despite much effort to achieve this objective, especially in thermal catalysis, an industrial application remains distant. A fundamental factor is the lack of an effective catalyst. In this work, we employ density functional theory to illustrate the main limitations in existing catalysts. We use pure metals to explain this by studying the full elementary steps in H2S decomposition. We find that many catalysts, though capable of decomposing H2S, are limited due to sulfur poisoning. We conclude by outlining the ideal properties of a catalyst for this process

Formation of Reversible Clusters with Controlled Degree of Aggregation

We develop a reversible colloidal system of silica nanoparticles whose state of aggregation is controlled reproducibly from a state of fully dispersed nanoparticles to that of a colloidal gel and back. The surface of silica nanoparticles is coated with various amino silanes to identify a silane capable of forming a monolayer on the surface of the particles without causing irreversible aggregation. Of the three silanes used in this study, <i>N</i>-[3-(trimethoxysilyl)­propyl]­ethylenediamine was found to be capable of producing monolayers up to full surface coverage without inducing irreversible aggregation of the nanoparticles. At near full surface coverage the electrokinetic behavior of the functionalized silica is completely determined by that of the aminosilane. At acidic pH the ionization of the amino groups provides electrosteric stabilization and the system is fully dispersed. At basic pH, the dispersion state is dominated by the hydrophobic interaction between the uncharged aminosilane chains in the aqueous environment and the system forms a colloidal gel. At intermediate pH values the dispersion state is dominated by the balance between electrostatic and hydrophobic interactions, and the system exists in clusters whose size is determined solely by the pH. The transformation between states of aggregation is reversible and a reproducible function of pH. The rate of gelation can be controlled to be as fast as minutes while deaggregation is much slower and takes several hours to complete