Catalytic performance of different carbon materials for hydrogen production in sulfur–iodine thermochemical cycle

•Four carbon catalysts are investigated for HI decomposition.•The AC catalysts present the best catalytic performance.•Structural analysis indicates that AC possesses the largest amount of edge sites.•Quantity of the edge sites relates to disordering and content of aliphatic carbon.•A detailed mechanism of HI decomposition over carbon materials is established.

This study examines four carbon materials through a series of characterization methods and a hydrogen iodide (HI) decomposition test. Applying a traditional structure to carbon materials facilitates the quantitative analysis of the four samples. The X-ray diffraction and Raman spectroscopy results indicate that lesser stacked graphite-like layers and shorter lateral diameter La result in higher disordering structure. However, the quantity of active sites is not determined only by the degree of disordering. The aliphatic carbon in the inter-layer correlations decreases the amount of graphite carbon and occupies its edge, thereby inhibiting the formation of edge sites in carbon materials. A low ratio of amorphous carbon with a high degree of disordering corresponds to a high concentration of surface active sites associated with the edges of graphite-like layers. The catalytic performance combined with characterization results demonstrates that the edges of graphite carbon are active sites in HI decomposition. A computational chemistry study is also conducted to illustrate the dominant role of edge sites in the reaction. The calculation results build a detailed mechanism of the catalytic decomposition of HI over the carbon materials and verify that the adsorbed I on the edge sites facilitate the HI decomposition.

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