Density functional theory study on improved reactivity of alkali-doped Fe2O3 oxygen carriers for chemical looping hydrogen production

The key obstacle preventing the widespread use of chemical looping hydrogen production is the scarcity of high-performance oxygen carrier (OC) materials. Here, for the first time we investigated improved reactivity of all the alkali-doped Fe2O3 OCs by means of density functional theory (DFT) calculations. Firstly, the location of alkali dopants (Li, Na, K, Rb and Cs) in the Fe2O3 crystal structure was studied. Our calculation results showed that all the alkali dopants prefer to be located at the surface of Fe2O3 rather than in the bulk. Then oxygen vacancy formation energies (E-vac) of Fe2O3 and alkali-doped Fe2O3 OCs, which could be used to evaluate the activity of OC surface oxygen, were calculated via DFT. It was found that the E-vac of surface oxygen adjacent to the dopants for all the alkali-doped Fe2O3 are much lower than that for undoped Fe2O3. The smaller the ionic radius is and the stronger the electronegativity is, the lower the E-vac of surface oxygen away from the dopants is. The surface oxygen adjacent to the dopants will show the better activity compared to the one away from the dopants. Finally, we analyzed the effect of alkali dopants on the reactivity of Fe2O3 OC. We concluded that the addition of Li, Na and K dopants are certainly able to enhance the activity of surface oxygen, thereby improving the reactivity of Fe2O3 OC. Compared with Li, Na and K, Rb and Cs will have a worse synergetic effect on the reactivity of Fe2O3 OC. Li, Na and K were identified as the optimal dopants. A quick screening of promising dopants for Fe2O3 OC could be realized by our DFT calculations.