(618a) X-Ray Absorption Spectroscopy Reveals the Dynamic Nature of Catalysts

The rising levels of greenhouse gas emissions necessitate reduction in the amounts of these harmful gases in the atmosphere. The transformative capabilities of catalysts to facilitate these chemical transformations depend on our ability to identify the specific sites on a catalyst surface that form desirable products – given that the catalysts change under reaction conditions – and engineer materials with such properties. My work focuses on synthesizing catalysts with controlled properties and studying their dynamic nature using X-ray absorption spectroscopy (XAS). Here I demonstrate the application of this approach to identify two unique changes in the catalyst’s structure that occur during CO₂ reduction: i) redispersion of ruthenium (Ru) nanoparticles (NPs) into single atoms; ii) reduction of iron oxide (Figure 1). In-situ and operando XAS has allowed me to observe catalyst restructuring that would have otherwise gone unseen.

First, when oxidized at temperatures < 200 °C, Ru NPs on ceria disintegrate into single atoms, as evidenced by the disappearance of the Ru-Ru scattering in the oxidized catalyst (Figure 1,I). This change impacts the catalyst’s CO₂ hydrogenation performance: while the NPs convert CO₂ and H₂ into methane, the single atoms exclusively make carbon monoxide. Second, by applying linear combination fitting of in-situ Fe K- edge XANES, I have identified the stepwise reduction of iron oxide to metallic iron (Figure 1,II). This information enables tuning of the catalyst’s ability to make C₂₊ products from CO₂ and H₂. In-situ characterization tools were instrumental in this discovery, given iron’s susceptibility to oxidize in ambient air. Overall, well-defined nanomaterials and XAS has enabled me to track the state of the catalysts as they perform CO₂ reduction reactions and design materials with desired properties for sustainable energy applications.