Investigation of the effects of alkali and alkaline earth metals on performance of oxygen carrier for chemical looping hydrogen production

Chemical looping hydrogen production is considered as an efficient way for hydrogen generation with carbon capture simultaneously. The development oxygen carrier with low cost and high activity is the foundation of chemical looping technology, which is considered as an efficient way for hydrogen generation. The promotion of reaction rate has always been the main challenge that limits the development of chemical looping technology. To address this issue, pyrite cinder, which has been investigated with ideal property, was utilized as a precursor for OCs preparation and alkali metals were doped for the enhancement of reactivity. Oxygen carriers modified with sodium (Na), magnesium (Mg), calcium (Ca), and potassium (K) were prepared and the influence of alkali metals were experimentally investigated. Alkali metal doping significantly enhanced the hydrogen yield and transformed the hydrogen production stage into three distinct plateaus with different reaction rates. K-doped oxygen carrier exhibited the highest hydrogen production. Density functional theory (DFT) analysis revealed that potassium doping drastically reduced the oxygen vacancy formation energy (from 4.31 eV to 0.49 eV). FeO bond is weakened by doping alkali metals and it leads to the deep reduction of oxygen carrier. It provides effective charge compensation for positively charged oxygen vacancies. Furthermore, the K-doped oxygen carrier demonstrated robust cyclic stability and superior resistance to sintering during long-term redox cycling. This study identifies the oxygen carrier doped with 15% potassium as optimal for hydrogen production performance. And it also provides better ideas for the development of oxygen carriers.