| AVS 62nd International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS-TuP |
| Session: | Surface Science Poster Session |
| Presenter: | Alexei Nefedov, Karlsruhe Institute of Technology, Germany |
| Authors: | A. Nefedov, Karlsruhe Institute of Technology, Germany C. Yang, Karlsruhe Institute of Technology, Germany F. Bebensee, Karlsruhe Institute of Technology, Germany C. Wöll, Karlsruhe Institute of Technology, Germany |
| Correspondent: | Click to Email |
Ceria, one of the most reducible metal oxides, has proven to be a highly active catalyst for NOx reduction to N2. Here, we use synchrotron based photoemission spectroscopy (PES) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) to monitor the conversion of N2O to N2 over reduced ceria surfaces (N2O + CeO2-x → N2 + CeO2) in a time-resolved fashion. This enables us to determine the kinetics of this process.
The NEXAFS and PES measurements were carried out using the HE-SGM beamline at the synchrotron facility BESSY II operated by the Helmholtz-Zentrum Berlin. The clean and stoichiometric CeO2(111) single crystal was annealed at 800 K for 15 min in vacuum to create surface oxygen vacancies prior to exposure to N2O, whereas ceria powders were annealed at 1000 K for 30 min. Exposure to 50 Langmuir N2O at sample temperatures typically below 120 K was achieved by backfilling the analysis chamber up to 10-9 mbar before NEXAFS spectra acquisition. The NEXAFS spectra were recorded in the partial electron yield mode for the π* resonance region of N K absorption edge. The Ce oxidation state was judged by PES before and after N2O exposure.
In the NEXAFS spectra, two intense resonances (401.2 eV and 404.8 eV) are observed. The resonances are assigned to the transition from the 1s orbital into the lowest unoccupied molecular orbital (3π*) of the terminal and central nitrogen atom of N2O, respectively. In agreement with previous experimental spectra for N2O on CeO2(111) thin films, both resonances exhibit equal intensity. Theoretical calculations for thin films indicate that N2O is adsorbed with the oxygen-end towards a cation on the surface also on the CeO2 single crystal. Moreover, these two resonances decrease in parallel over time and one may speculate at this point that the decrease is caused by conversion of N2O to N2 over reduced CeO2(111). The difference in valence band photoemission spectra measured before and after the introduction of 50 Langmuir N2O clearly demonstrate that the initial reduced CeO2(111) surface is re-oxidized to some extent and thus confirms the speculation above that N2O can interact with oxygen vacancies resulting in vacancy healing.
Annealing can easily reduce ceria, while N2O can heal the oxygen vacancies on the reduced ceria surface, giving rise to a complete catalytic cycle. The first set of data on N2O adsorption and reaction over ceria surface suggest the feasibility of the study of the conversion of N2O to N2 by using PES and NEXAFS, which gives us a chance to determine the kinetics of this reaction.