First-principles-based microkinetic rate equation theory for oxygen carrier reduction in chemical looping

Abstract

The reduction kinetics of oxygen carriers with reacting gases are typically modeled using shrinking-core-based models, in which, the elemental surface reaction mechanisms are simplified as only one global reaction, and the kinetic parameters are obtained by fitting with experimental data. In this study, a theoretical scheme is proposed to couple the elemental surface reaction mechanisms with the bulk diffusion of lattice oxygen, and a first-principle-based microkinetic rate equation theory is developed to model the reduction kinetics of oxygen carrier in chemical looping. The elemental surface reaction mechanisms are computed using quantum chemistry and are integrated into the mean-field microkinetic model, in which, oxygen bulk diffusion is also included. The elemental steps of gas adsorption, surface reaction, and surface species desorption are considered in the theory. The kinetic parameters used in the model are directly calculated by using transition state theory and density functional theory instead of fitting with experimental data. The developed theory is applied to predict the reduction kinetics of a CaMn0.775Ti0.125Mg0.1O2.9-δ perovskite oxygen carrier and validated to accurately predict the reduction kinetics by H2 and CO under a wide range of temperatures and gas concentrations. The effect of surface reaction and bulk diffusion on the overall reaction rate is analyzed and the rate-controlling step is discussed. When the bulk diffusion is considerably rapid, the interior oxygen concentration in the grain remains uniform throughout the reduction process and can be considered as a function of time only. Lumped parameter analysis is used to simplify the bulk diffusion of lattice oxygen without sacrificing accuracy. The developed microkinetic rate equation theory couples the surface reaction mechanism with the bulk diffusion, bridges the gap between atom-scale first-principles quantum chemistry calculations with particle-scale heterogeneous kinetics, and provides a theoretical framework for oxygen carrier reduction/oxidation kinetics, gas conversion mechanisms, and product selectivity in chemical looping.