Oxygen Vacancy Creation Energy in Mn-Containing Perovskites: An Effective Indicator for Chemical Looping with Oxygen Uncoupling

Abstract

Chemical looping with oxygen uncoupling (CLOU) is a novel process for carbon dioxide capture from coal combustion. Designing a metal oxide oxygen carrier with suitable oxygen release and uptake (redox) properties represents one of the most critical aspects for CLOU. The current work aims to correlate oxygen vacancy creation energy of metal oxide oxygen carriers with their redox properties. Oxygen vacancy creation energies of CaMnO 3−δ , Ca 0.75 Sr 0.25 MnO 3−δ , CaMn 0.75 Fe 0.25 O 3−δ , and BaMnO 3−δ were determined through density functional theory (DFT) calculations. The effect of the Hubbard U correction on the ground state magnetic configurations and vacancy creation energies was investigated, along with the effect of lattice oxygen coordination environment. It was determined that Hubbard U only slightly changes the relative differences in vacancy creation energies between the Mn-containing perovskites investigated. Therefore, ranking of oxygen vacancy creation energies among the various oxides can be determined using a simplified method without using Hubbard U. Comparisons with experimental data confirmed that vacancy creation energy is an effective indicator for oxygen release properties of the perovskites investigated: oxygen carrier materials with lower vacancy creation energies can release their lattice oxygen more readily. Thermogravimetric analysis indicated increased oxygen release with decreasing oxygen vacancy creation energy at temperatures below 700 °C. Higher activities for coal char combustion were also observed. The simplified DFT strategy also satisfactorily predicted the effects of iron and strontium doping on lattice distortions as well as the crystal volume changes upon oxygen vacancy creation. These findings indicate that oxygen vacancy creation energies in Mncontaining perovskites can potentially be used as an effective design parameter for oxygen carrier development and optimizations.

A‐ and B‐site Codoped SrFeO3 Oxygen Sorbents for Enhanced Chemical Looping Air Separation

“…Ca‐ and Co‐codoped SrFeO 3 perovskites were prepared by a sol–gel method . In a typical synthesis of Sr 0.8 Ca 0.2 Fe 0.4 Co 0.6 O 3− δ perovskite, strontium nitrate (8 mmol; Sigma–Aldrich, Sr(NO 3 ) 2 , 99.9+%,), calcium nitrate tetrahydrate (2 mmol; Sigma–Aldrich, Ca(NO 3 ) 2 ⋅4 H 2 O, 99.0+%), iron nitrate nonahydrate (4 mmol; Sigma–Aldrich, Fe(NO 3 ) 3 ⋅9 H 2 O, 98+%), and cobalt nitrate hexahydrate (6 mmol; Sigma–Aldrich, Co(NO 3 ) 2 ⋅6 H 2 O, 98 %,) were dissolved in deionized water (15 mL).…”

Chemical looping beyond combustion – a perspective

“…perovskites, can accommodate significant lattice defects prior to phase transition. 129,130 As such, the redox thermodynamics of these redox catalysts are unique and cannot be categorized as redox pairs. That is, the equilibrium oxygen partial pressure of the oxide is a continuous function of the oxide's (oxygen) vacancy concentration without incurring a distinct phase change.…”

Structural Insight into La0.5Ca0.5Mn0.5Co0.5O3 Decomposition in the Methane Combustion Process

“…These effects can be associated, in particular, with the influence of low oxygen partial pressure in the medium. An increase in the cell parameters and a decrease in the crystallite size to 150 Å can be caused by lattice oxygen loss and the formation of defects in the structure, as was observed in [ 38 ].…”