| Summary: | The emerging energy and environmental concerns nowadays are highlighting the need to turn to clean fuels, such as hydrogen. In this regard, hydrogen sulfide (H<sub>2</sub>S), an abundant chemical compound found in several natural sources and industrial streams, can be considered a potential carbon-free H<sub>2</sub> source through its decomposition. In the present work, the H<sub>2</sub>S decomposition performance of Co<sub>3</sub>O<sub>4</sub>/CeO<sub>2</sub> mixed oxide catalysts toward hydrogen production is investigated under excess H<sub>2</sub>O conditions (1 <i>v/v</i>% H<sub>2</sub>S, 90 <i>v/v</i>% H<sub>2</sub>O, Ar as diluent), simulating the concentrated H<sub>2</sub>S-H<sub>2</sub>O inflow by the Black Sea deep waters. The effect of key operational parameters such as feed composition, temperature (550–850 °C), and cobalt loading (0–100 wt.%) on the catalytic performance of Co<sub>3</sub>O<sub>4</sub>/CeO<sub>2</sub> catalysts was systematically explored. In order to gain insight into potential structure-performance relationships, various characterization studies involving BET, XRD, SEM/EDX, and sulfur elemental analysis were performed over the fresh and spent samples. The experimental results showed that the 30 wt.% Co/CeO<sub>2</sub> catalyst demonstrated the optimum catalytic performance over the entire temperature range with a H<sub>2</sub> production rate of ca. 2.1 μmol H<sub>2</sub>∙g<sup>−1</sup>·s<sup>−1</sup> at 850 °C and a stable behavior after 10 h on stream, ascribed mainly to the in-situ formation of highly active and stable cobalt sulfided phases.
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