Chemical-looping combustion (CLC) has developed significant interest over the recent years because of its ability to capture nearly 100% of the carbon dioxide produced from hydrocarbon combustion. CLC uses oxygen from reducible transition metal oxides to react with the hydrocarbons in the absence of air. This allows for the CO2 product to be easily captured without the need of expensive gas phase separation techniques used today. Before being used at the industry level, these metal oxides have to show high reactivity with thermal and mechanical stability. Experimental work has shown these transition metal oxides may follow two different reduction processes. Through the Nucleation and Nuclei Growth Model (NNGM) reduction of the metal first occurs inside the particle bulk before being seen on the surface. The Uniform Shrinking Core Model (USCM) shows reduction occurs at the surface before slowly working its was to towards the bulk. To increase metal oxide reactivity, an understanding of why they tend to reduce through different methods is needed. In this work, DFT + U methodology is used to obtain an understanding of the reduction along the catalysis’ surface through oxygen vacancies and electronic structure. We find that transition metal oxides which reduce through the NNGM process show a favorability for an oxygen vacancy along its subsurface compared to its surface. The opposite is seen for transition metal oxides that reduce through the USCM. The surface electronic structure is then studied through the inclusion of band centers of the oxygen and metal states. This method allows for the prediction of transition metal oxide reduction mechanisms and defines a tunable parameter that can be used to increase reactivity.

Combining experiment and density functional theory to understand transition metal oxide reduction mechanisms in chemical-looping combustion

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.