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

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


Aitbekova, A. - Presenter, Stanford University
Bare, S., SLAC National Accelerator Laboratory
Cargnello, M., Stanford University
Wu, L., Brown University
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.