High-energy resolution X-ray absorption and emission spectroscopy reveals insight into unique selectivity of La-based nanoparticles for CO2

The lanthanum-based materials, due to their layered structure and f-electron configuration, are relevant for electrochemical application. Particularly, La₂O₂CO₃ shows a prominent chemoresistive response to CO₂. However, surprisingly less is known about its atomic and electronic structure and electrochemically significant sites and therefore, its structure–functions relationships have yet to be established. Here we determine the position of the different constituents within the unit cell of monoclinic La₂O₂CO₃ and use this information to interpret in situ high-energy resolution fluorescence-detected (HERFD) X-ray absorption near-edge structure (XANES) and valence-to-core X-ray emission spectroscopy (vtc XES). Compared with La(OH)₃ or previously known hexagonal La₂O₂CO₃ structures, La in the monoclinic unit cell has a much lower number of neighboring oxygen atoms, which is manifested in the whiteline broadening in XANES spectra. Such a superior sensitivity to subtle changes is given by HERFD method, which is essential for in situ studying of the interaction with CO₂. Here, we study La₂O₂CO₃-based sensors in real operando conditions at 250 °C in the presence of oxygen and water vapors. We identify that the distribution of unoccupied La d-states and occupied O p- and La d-states changes during CO₂ chemoresistive sensing of La₂O₂CO₃. The correlation between these spectroscopic findings with electrical resistance measurements leads to a more comprehensive understanding of the selective adsorption at La site and may enable the design of new materials for CO₂ electrochemical applications.CO₂ has become a challenge for our society and we have to develop new materials for its photo/electrocatalysis, chemoresistive sensing, and storage (1–8). Particularly, for the variety of electrochemical applications the selective interaction of CO₂ and charge transfer with solids is in the foreground. At the same time, the interaction of CO₂ with solids in the electrochemical cell or sensing device is rather complex, thus it remains challenging to experimentally identify the key elements determining their selectivity and efficiency. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) provide complementary information on the electronic structure of materials (9, 10) and on the orbitals participating in the interaction with absorbing molecules (11). High-energy resolution fluorescence-detected (HERFD) XAS probes unoccupied states with a spectral resolution higher than regular XAS. Furthermore, with the same experimental setup XES can be measured, which allows one to probe the occupied states within the valence band (12). In situ HERFD XAS or XES experiments have been previously carried out to study the catalytic reaction at the surface of noble metals (11, 13–16), zeolites (17), and metal organic frameworks (18). Thus far, no such in situ experiments have been performed to directly track the changes of the electronic structure of a solid and its electrochemical activity toward CO₂. The rare-earth–based materials like perovskites and oxycarbonates, owing to their unique f-electron configuration of Ln (Ln = rare earth) and layered crystal structure, emerge as the most interesting for future photo- and electrochemical applications (3–8). Among rare-earth oxycarbonates (19, 20), particularly lanthanum strongly responds to CO₂ and shows up to 16-fold conductivity changes, not seen before for any metal oxides (21). This is very surprising because a direct injection of an electron into CO₂ molecule requires the activation energy of nearly 2 eV (22). To assess the origins of the unique CO₂ sensitivity of rare-earth oxycarbonate, it is essential to study in situ the interplay between the changes of the electronic structure of La-based nanoparticles upon CO₂ adsorption and changes of the macroscopic conductivity of a device.Here, to elucidate the underlying mechanism we first determine the structure and atomic positions of the lanthanum oxycarbonate. Using HERFD XAS and valence-to-core (vtc) XES results, we gain information about the electronic structure and band gap. Moreover, we combine in situ HERFD XAS and XES measurements with sensing performance tests to obtain the structure–function relationship. Finally, with all of the obtained information we discuss a mechanism of CO₂ adsorption on the La₂O₂CO₃ surface.