Enhanced CO capture properties of Li2MnO3via inducing layered to spinel transition by cation doping with Fe, Co, Ni and Cu

Abstract

Lithium manganate (Li₂MnO₃) was the first alkaline ceramic to show selective CO chemisorption in non-oxidative atmospheres, which is of interest for gas separation processes, such as high purity H₂ production. Thus, in the present study, the partial substitution of 10 mol% of the Mn ions in Li₂MnO₃ with Fe, Co, Ni or Cu ions to modify the CO oxidation–chemisorption properties was examined. All solid solutions were synthesized by a solid-state reaction and then characterized by XRD, Raman, EPR, XPS, SEM and EDS-mapping. Hence, it was evidenced that the addition of the different transition metal ions to the Li₂MnO₃ crystal structure induces several structural modifications, such as the formation of spinel Li_1+xMn_2−xO₄ (0 < x < 0.33) and oxygen deficiencies at the particle surface. Also, in the copper case, partial integration of the Cu²⁺ ions due to CuO formation was identified. Afterwards, the CO oxidation–chemisorption processes were evaluated dynamically and isothermally through thermogravimetry. The kinetics were determined and fitted to the modified Jander–Zhang model for CO capture. These results showed that in all the solid solutions the CO oxidation–chemisorption surface and bulk processes were improved due to the easy lithium and oxygen diffusion. However, the lack of surface oxygens due to the different oxidation states of each transition metal limited the total CO capture in the Co-, Ni- and Cu-containing solid solutions. In fact, Fe–Li₂MnO₃ displayed the fastest kinetics due to the surface oxygen release as the oxygen deficient structure (Li₂MnO_3−δ) leads to faster lithium diffusion, increasing the CO capture efficiency in comparison to the pristine Li₂MnO₃. Thus, the Fe–Li₂MnO₃ solid solution may be of interest for further studies in gas separation applications.