Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production

Abstract

Quantum chemistry calculations using a reasonably simplified char model were performed to clarify the mechanisms of hydrogen iodide (HI) decomposition over carbon materials. The density functional theory at the B3LYP/3-21G** level was used to optimize the geometries of the reactants, products, stable intermediates, and transition states in possible reaction pathways. The main elementary reactions of the homogeneous decomposition of HI were simulated to verify the applicability of the chosen calculation method and basis set. The adsorptions of HI on two prototypical model chars were all irreversible chemisorption reactions. The results revealed that HI chemisorption preferentially occurred in the zigzag model compared with the energy barriers. Based on the chemisorption results, the decomposition process of HI in the zigzag model was calculated at the same level. The process of HI decomposition on carbon materials took place not directly but by a series of chemisorption and desorption reactions, which demonstrated that the desorption of I2 and H2 controls overall catalysis reaction. The initial carbon structure disappeared and turned into the iodine absorbed structure, which played a real catalytic role in the overall process. The two structures were compared by analyzing their electrostatic potentials (ESPs), which demonstrated that the iodine absorbed structure presented a higher activity than the initial carbon structure. A detailed mechanism of the catalytic decomposition of HI over the carbon materials was proposed based on the pathways that we obtained from the calculation results.