Methane Dissociation on α-Fe2O3(0001) and Fe3O4(111) Surfaces: First-Principles Insights into Chemical Looping Combustion

Chemical looping combustion (CLC) has drawn much attention in recent years for its near 100% efficiency in generating CO2. Unlike conventional combustion processes, CLC uses transition-metal oxides (TMOs) to transform hydrocarbons into carbon dioxide in the absence of air. Instead of an air atmosphere acting as a source of gaseous oxygen, the TMO surface acts as a solid reservoir of oxygen. This decreases the cost of CO2 production because the CLC process creates a CO2 product that does not need to be separated from O-2, N-2, and other gases found in the atmosphere. Although CLC can lead to clean, efficient gas production, there are still a few key needs to further optimize the process. The most pressing need is to understand the chemical changes that occur by fully characterizing reaction products and surface reconstructions. Here, we use DFT + U methodology to obtain an atomistic picture of the surface transformations and chemical reactions that take place during the initial dissociation of methane into CH3 and H on hematite alpha-Fe2O3(0001) and magnetite Fe3O4(111) surfaces at the beginning of the CLC process. We find that a homolytic adsorption pathway is energetically preferred over a heterolytic pathway and that it is necessary to include Hubbard U corrections to both Fe and O to accurately describe surface processes, such as adsorption and transformations, at the atomistic level. After a comparison of the two surfaces, we go on to show that they may exhibit competitive adsorption and that oxygen-deficient hematite surfaces may result in enhanced methane dissociation, an intermediate that may be a key step to optimizing the CLC process.