Chemical Sensitivity of Valence-to-Core X-ray Emission Spectroscopy Due to the Ligand and the Oxidation State: A Computational Study on Cu-SSZ-13 with Multiple H2O and NH3 Adsorption

5 years ago

Chemical Sensitivity of Valence-to-Core X-ray Emission Spectroscopy Due to the Ligand and the Oxidation State: A Computational Study on Cu-SSZ-13 with Multiple H2O and NH3 Adsorption

Hui Li, Jean-Sabin McEwen, Renqin Zhang

Valence-to-core X-ray emission spectroscopy (vtc-XES) is a powerful experimental tool that can overcome the sensitivity limitations of X-ray absorption near edge structure (XANES) measurements with regard to ligand identification. To further elucidate the sensitivity of this experimental technique, the corresponding emission spectrum of a Cu cation when exchanged within a chabazite structure (Cu-SSZ-13) was calculated from first principles. By comparing vtc-XES spectra of H₂O and NH₃ adsorbed on Cu⁺ (or Cu²⁺) cations, we find a blue shift for the kβ″ lines from a Cu–O to a Cu–N ligation. In addition, the adsorption of NH₃ results in a stronger kβ_2,5 line intensity than the corresponding one for H₂O. Therefore, one can discriminate the adsorption of H₂O or NH₃ by the different vtc-XES emission lines of Cu–O and Cu–N. By scanning the vtc-XES of multiple H₂O and NH₃ adsorbed Cu-SSZ-13 structures, we find a shift in the energy of the kβ″ line between H₂O and NH₃ adsorbed conformations, which increases by increasing the population of Cu–ligand bonds for both the Cu⁺ and Cu²⁺ cations. By analyzing the partial density of states (PDOS) for these structures, the kβ″ emission line results from a N 2s to Cu 1s transition, while the kβ_2,5 emission line is generated from the transition going from a mixed N 2p and Cu 3d and 4p state to a Cu 1s core hole, where the Cu 4p state plays a key role in this transition. Further PDOS analysis shows the chemical sensitivity of vtc-XES, since the ligand environment is intrinsically determined by the different potential binding energies of ligand 2s states. Finally, we compare our computed XES results to the measured XES of several reference compounds, which seem to suggest a different assignment than what was suggested in the literature. As such, the vtc-XES spectrum is a complementary tool to XANES and is well-suited for uncovering the state of the active site and the nearest neighbor environment of Cu ions in Cu-SSZ-13 during the selective catalytic reduction of NO or, more generally, of metal ions in a working catalyst.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b04309

DOI: 10.1021/acs.jpcc.7b04309