Enhancement in oxygen transfer rate of CuMn2O4 oxygen carrier via selective dopants: Role of dopant effects on O migration for chemical looping combustion

Abstract

Chemical looping combustion technology has been widely studied because this platform can convert fossil fuels efficiently into sustainable energy with an in situ carbon capture capability. The redox property of the oxygen carrier (usually a transition metal oxide) plays a critical role in determining the kinetics of the combustion process. This study investigated computationally and experimentally methane oxidation on spinel structure of Mn–Cu bimetallic oxygen carriers due to its facile oxygen uncoupling (Mn and Cu) with high durability in the reduction cycle. Our simulation predicted that the mechanism of complete methane oxidation on CuMn2O4 involved two competitive rate-limiting steps: 1) thermodynamic controlled step of CH2O* formation (CH3O* + O → CH2O* + OH) and 2) kinetic controlled step of CO formation (CHO* + O → CO* + OH). Multiple dopants (Co, Cr, Fe, Ni, Ti, V, and Zn) to the activation barriers of the rate-limiting steps were tested to enhance the performance of oxygen carriers by improving the oxygen transfer rate. Ni-doped CuMn2O4 and V-doped CuMn2O4 showed promising results computationally (significant decrease in ∆E of rate limiting steps). The enhanced oxygen transfer rate of Ni-doped CuMn2O4 was confirmed experimentally. On the other hand, experimentally, V-doped CuMn2O4 provided a lower oxygen transfer rate than the other CuMn2O4 materials, which is inconsistent with the computational predictions. Computation studies showed that the lower oxygen transfer rate stems from the hindrance of O migration by surface V dopants; surface V atom energetically hampers O migration from the subsurface to the top surface. This behavior adversely affects the oxygen transfer rate, resulting in a lower net oxygen transfer rate. These results suggest that the effects of dopants on O migration should be carefully considered when designing enhanced oxygen carriers using a dopant strategy.