Revealing the effects of Ni on sorption-enhanced water-gas shift reaction of CaO for H2 production by density functional theory

Abstract

The sorption-enhanced water-gas shift (SE-WGS) reaction promoted by Ni-doped CaO (Ni-CaO) was sufficiently investigated by experiments that only displayed macroscopic results instead of microscopic reaction mechanisms. In this work, density functional theory (DFT) was employed to clarify the mechanisms of SE-WGS reaction which was catalyzed by Ni in the presence of CaO. The SE-WGS reaction only promoted by CaO was used as a comparison to clarify the catalysis of Ni. The analysis of electron differential densities, the partial density of states, and formation energy indicate that Ni causes the higher stability and reactivity of the Ni-CaO model than the CaO model. The Ni promotes the release of H2* by rising the adsorption energy of H2*(−0.05 eV) and retains the efficient CO2 capture with the adsorption energy of −1.56 eV. The transient state calculations indicate the SE-WGS reaction prefers to proceed along with the redox mechanism compared with the carboxyl and formate mechanisms on the Ni-CaO surface. The energy barriers for the CO2* generation, H2* generation and desorption are respectively 0.74, 1.77 and 0.10 eV on the Ni-CaO surface, which are lower than those on the CaO surface. Therefore, Ni helps to improve CO conversion and H2 productivity which is consistent with the previous experimental results.