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Experiments and theoretical calculations for N2O physisorption on ceria(111)
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
Queen Mary, University of London, School of Biological & Chemical Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
##### Abstract [en]

We have investigated N2O physisorption on the CeO2(111) surface using electronic structure calculations for embedded cluster and periodic slab models. Infrared (IR) measurements were performed and give $\Delta \omega_{symm}$ < 0 and $\Delta \omega_{anti-symm}$ > 0, with respect to the gas phase. For the adsorption of single N2O molecules we found two adsorption modes with similar interaction energies, one with the N-end of the molecule pointing to an Ce ion on the surface (E$E_{ads}$ = 0.36 eV at the MP2 level) and one with the O-end pointing to an Ce ion on thesurface ($E_{ads}$ = 0.33 eV at the MP2 level). The calculations give (at the B3LYP level) for the N-down case $\Delta \omega_{symm}$ = +20 $cm^{-1}$ and $\Delta \omega_{anti-symm}$ = +40 $cm^{-1}$ but in the O-down case$\Delta \omega_{symm}$ = -20 $cm^{-1}$ and $\Delta \omega_{anti-symm}$ = +20 $cm^{-1}$ suggesting that only the O-down case occured in experiment. Calculations with higher N2O coverage and electron correlation treatment beyond MP2 will probably stabilize the O-down mode.

##### Keyword [en]
nitrous oxide, ceria, physisorption, vibrational frequencies, B3LYP, MP2, embedded cluster, periodic slab, IR-spectroscopy
##### National Category
Theoretical Chemistry
##### Research subject
Quantum Chemistry
##### Identifiers
OAI: oai:DiVA.org:uu-101210DiVA, id: diva2:212078
Available from: 2009-04-21 Created: 2009-04-21 Last updated: 2010-01-14
##### In thesis
1. Physisorption of CO and N2O on ceria surfaces
Open this publication in new window or tab >>Physisorption of CO and N2O on ceria surfaces
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

Physisorption of CO and N2O on surfaces of ceria (CeO2) was investigated by means of high-level quantum-mechanical embedded cluster calculations. Both systems have high relevance in the field of environmental chemistry and heterogeneous catalysis. The CO/CeO2 system, has been investigated in a couple of both experimental and theoretical studies, but for the N2O/CeO2 system, this is the first study in the literature, experimental or theoretical.

In physisorption, the interaction relies entirely on classical electrostatic interactions and electron dispersion forces. No covalent bond is formed between the molecule and the surface. A proper description of the dispersion requires some of the most accurate quantum-mechanical methods available, such as MP2 or CCSD(T). Moreover, even the most sophisticated methods cannot heal errors anywhere else in the theoretical treatment. Standard periodic models cannot be used with methods such as CCSD(T), but embedded cluster models can, and have been thoroughly explored in this thesis.

In this thesis, embedded cluster models were constructed for the CeO2(110) and (111) surfaces. Using a range of assessment tests, it was verified that the electronic structure of the central region of a large and fully embedded surface cluster agrees well with the corresponding region in a periodic system. CO physisorption was investigated at the CCSD(T) level. Due to the prohibitively large expenses (in computer time) for standard CCSD(T) calculations, the method of increments, previously used in the literature for bulk systems, was extended to adsorption problems. It was found that, electron correlation contributes by 30 - 80% to the molecule-surface interaction and that the contribution depends on the topology of the surface. The calculated CO-ceria interaction energy is 20 kJ/mol for the (111) surface and 27 kJ/mol for the (110) surface.

In low temperature TPD experiments for the N2O/CeO2(111) system, one surface species was found with an adsorption energy of about 29 kJ/mol. IR measurements showed stretching frequencies that are typically assigned to N2O adsorption with the O-end directed towards surface cations. However, theoretical calculations up to the MP2 level predicted two equally favorable adsorption species. Improvements in the structural model (larger clusters, consideration of molecule-induced relaxation) or the computational method (larger basis sets) did not affect this result. Only at the CCSD(T) level was one dominating surface species found, namely N2O adsorbed over a Ce ion, with the O-end of the molecule directed towards the surface. The calculated stretching vibrational frequency shifts (with respect to the gas phase) for this adsorbed species agree well with the measured IR spectra.

##### Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2009. p. 68
##### Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 644
##### Keyword
ceria, carbon monoxide, nitrous oxide, embedded cluster, physisorption, method of increments, CCSD(T), vibrational frequencies
##### National Category
Theoretical Chemistry
##### Research subject
Quantum Chemistry
##### Identifiers
urn:nbn:se:uu:diva-101271 (URN)978-91-554-7527-7 (ISBN)
##### Public defence
2009-06-05, Angström 2001, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
##### Supervisors
Available from: 2009-05-14 Created: 2009-04-22 Last updated: 2009-05-14Bibliographically approved

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Müller, Carsten

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