Verständnis der katalytischen Eigenschaften verschiedener Seltenerdoxide: Auf dem Weg zum wissensbasierten Design von Katalysatoren mithilfe quantenchemischer Rechnungen
Festkörper- und Oberflächenchemie, Materialsynthese
Physikalische Chemie von Festkörpern und Oberflächen, Materialcharakterisierung
Zusammenfassung der Projektergebnisse
Rare earth oxides (REO) are attracting increasing interest in catalysis and electrocatalysis but theoretical studies of catalytic processes on the surfaces of REO are challenging due to the localized nature of 4f electrons; thus, standard methods of density functional theory (GGA and LDA functionals) fail for these systems. The DFT+U (where U is a Hubbard-like term describing the on-site Coulomb interactions) and hybrid density functionals have been suggested to give at least qualitatively correct description of 4f electrons in lanthanide compounds. This project has been devoted to a systematic assessment of the PBE+U (Perdew-Burke-Ernzerhof) and hybrid HSE06 (Heyd-Scuseria-Ernzerhof) functionals with the aim of suggesting a consistent set of U values to be used with PBE+U for various lanthanide elements. The PBE+U approach is less expensive compared with the usage of hybrid functionals and has been previously applied for modeling ceria and lanthana but not to other REOs. Our study has revealed that the assessment of the two approaches and finding an optimal range of U is even more challenging than anticipated, due to scarcity of reliable experimental data and to unsatisfactory performance of the hybrid functional or PBE+U for certain cases studied. Nevertheless, based on our critical analysis, we were able to make the following recommendations. The U value around 3 eV gives the best description of the lattice parameters of bulk oxides, except for Eu2O3, Gd2O3 and Er2O3 for which U=8 eV gives the best results. U=2-3 eV is found to be the optimal range of U for the reaction energies of bulk La2O3, Ce2O3, Nd2O3, Er2O3 and Ho2O3. U=1 eV gives the best results for Pr2O3, Pm2O3, Eu2O3, Tm2O3 and Lu2O3, whereas Gd2O3 could not be accurately calculated by the PBE+U method. The U values around 3 eV, found optimal for reaction energies of most bulk REO oxides, also work well in the calculations of adsorption of small molecules on Nd2O3(0001) and CeO2(111), although larger U values are required to obtain full localization of 4f electrons. Also, as a part of this project, we have applied the PBE+U approach and ab initio molecular dynamics to study inverse catalytic systems represented by ceria nanoparticles supported on flat and stepped Au surfaces, Au(111) and Au(321). We successfully modelled the mechanism of CO oxidation on these inverse systems, which generated new insights into reactivity of such systems and demonstrated that the mechanism involves redox steps, in which CO reacts with the lattice O of the ceria nanoparticle. That facilitates the activation of an O2 molecule adsorbed at a perimeter interface site between Au and ceria in the subsequent step. On the stepped Au(321) surface functionalized with ceria, our simulations revealed the formation of short-living gold carbonyl AuCO complexes that were very reactive. Our results suggest possible prominent role of such complexes in the exceptional activity of Au-based nanostructured catalysts. Finally, to assess the accuracy of high-level multireference ab initio wavefunction methods for modelling lanthanides we performed a benchmark study of adsorption and emission spectra of a luminescent coordination compound of Tb(III) at the CASSCF and SAS-NEVPT2 level including spin-orbit treatment. The spectral transitions were predicted in excellent agreement with experiment.
Projektbezogene Publikationen (Auswahl)
- Combined Relativistic Ab Initio Multireference and Experimental Study of the Electronic Structure of Terbium Luminescent Compound. J. Phys. Chem. A. 2020, 124, 82/89
C.-X. Wang, Y. Li, Z.-F. Li, Z.-J. Liu, E. F. Valeev, L. V. Moskaleva
(Siehe online unter https://doi.org/10.1021/acs.jpca.9b11089) - What Changes on the Inverse Catalyst? Insight From CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics. ACS Catal. 2020
Y. Li, S. Li, M. Bäumer, E. A. Ivanova-Shor, L. V. Moskaleva
(Siehe online unter https://doi.org/10.1021/acscatal.9b05175)
