Mechanism insights into sorption enhanced methane steam reforming using Ni-doped CaO for H2 production by DFT study

Abstract

Sorption Enhanced Steam Methane Reforming (SESMR) provides a promising method to produce high purity hydrogen by in-situ CO2 capture. Ni-doped CaO (Ni-CaO) with catalytic and adsorption active sites can effectively improve the hydrogen production in the SESMR process. It is difficult to determine the enhancement mechanism of Ni-CaO in the SESMR simply by the research experiment. In this study, the reaction mechanisms of SESMR promoted by Ni in the presence of CaO were investigated by the density functional theory (DFT) calculations. The reaction pathway was determined by analyzing the activation barriers along the possible reaction pathways in the SESMR reaction. The SESMR reaction promoted by CaO was also studied as a comparison to clarify the catalysis of Ni. The reaction mechanism of SESMR on Ni and Ni-CaO surfaces was also compared. The results show that the SESMR reaction is more prone to follow path CH4 → CH3 → CH2 → CH → CHO → HCOO → CO2. The OH-assisted dissociation assists in the breaking of first C–H bond in CH4. Process CH3 → CH2 → CH is accomplished by direct dissociation in two steps. Next, CHO is spontaneously formed from CH and O. CHO is oxidized to generate HCOO. Finally, the CO2 is formed by the HCOO dehydrogenation path. The presence of Ni atoms causes a change in the reaction rate limiting step of the SESMR reaction from CH dissociation (on CaO surface) to CH3 dissociation (on Ni-CaO surface). Compared with the reaction energy barrier of CH dissociation (3.215 eV), the SESMR reaction of the Ni-CaO surface is easier to occur with lower reaction energy barrier of 2.030 eV. Besides, Ni reduces the adsorption energy of CH4 (-0.106 eV) and H2O (-1.32 eV) as well as increases the adsorption energy of H2 (-0.047 eV). The SMR reaction is easier to occur on the Ni surface than Ni-CaO surface. The DFT calculations determine the possible mechanism of Ni-CaO in SESMR for H2 production.