The Experimental And Computational Chemistry Study On The Catalytic Decomposition Of Hydrogen Iodide Over Carbon Materials

With the depletion and pollution of fossil energy, renewable energy gradually attracted our attention. Hydrogen is considered an alternative source of energy because of its environmentally friendly characteristics. An efficient hydrogen production system is necessary in developing a hydrogen energy system. The thermochemical water-splitting cycle, which produces hydrogen and oxygen from water through a series of correlative chemical reactions, has attracted much attention in current research. Among various potential thermochemical cycles, the sulfur-iodine (SI) thermochemical cycle presents significant advantages. The SI cycle involves three reactions:Bunsen reaction:I2+SO2+2H2O=2HI+H2SO4Sulfuric acid decomposition:H2SO4=SO2+H2O+I/2O2Hydrogen iodide decomposition:2HI=H2+12The decomposition of hydrogen iodide (HI) serves as the step toward hydrogen evolution in several thermochemical water-splitting steps. However, the homogeneous gas-phase conversion during the decomposition is rather low, occurring below833K. The use of catalysts is therefore desirable to accelerate the reaction rate. Research on HI decomposition catalysts has mainly focused on three systems, namely, carbon materials, supported precious metals, and supported transition metals. Though investigators have done numerous of work in the study activated carbon, including preparation and modification methods, the mechanism of the HI catalytic decomposition remains unknown. In this study, both the experimental and computational chemistry researches were associated to study the mechanism of the HI decomposition over carbon materials.This study examines four carbon materials through a series of characterization methods and a 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.In the quantum chemistry study, we investigated three elemental reactions among HI homogeneous decomposition at first, calculation results were close to the datas from NIST. The mechanism of the HI catalytic decomposition was established based on the density functional theory (DFT). The DFT results verifies the significant function of edge sites in the reaction, which serve as adsorption sites initially and then take part in the breakage and formation of the bonds. The reaction pathway shows that the adsorbed I on the edge sites have a dominant role in the decomposition. Besides, the reactions between the adsorbed I and HI molecules are demonstrated to be dominant during the HI catalytic decomposition process. The dissociation of the H2has the highest energy barrier, leading to the introduction of some transition metals to support the carbon materials and facilitating the formation of H2.