Suchergebnisse

The interaction of small molecules with acid–base and redox centers in small Ce21O42 nanoparticles has been theoretically investigated using the DFT + U approach with the PW91 functional and U = 0.2 and 4 eV, in order to determine the influence of the U value on the trends observed in selected properties describing such interactions. CO adsorption at low coordinated Ce4 + Lewis acid centers, water adsorption and dissociation at acid–base pairs, formation of oxygen vacancy defects by removal of an oxygen atom from the system, and interaction of molecular O2 with such defects have been considered. The largest effect of the value of U is found for the description of the reduced Ce21O41 nanoparticle. In all other cases involving stoichiometric and oxidized Ce21O42 and Ce21O43 systems, the trends in the calculated adsorption and reaction energies, optimized geometries, charge distribution, and vibrational frequencies are quite similar at the three levels considered. ; Financial support from the Spanish Science and Innovation Ministry (Consolider Ingenio 2010-MULTICAT CSD2009-00050 and Subprograma de apoyo a Centros y Universidades de Excelencia Severo Ochoa SEV 2012 0267) is acknowledged. The European Union is also acknowledged by ERC-AdG-2014-671093 — SynCatMatch. Red Española de Supercomputación (RES) and Centre de Càlcul de la Universitat de València are gratefully acknowledged for computational facilities and technical assistance. T. L.-A. thanks ITQ for a contract. ; Peer Reviewed

A nanoparticulate CeO2 catalyst is presented that is able to oxidize cyclohexane to K/A-oil (a mixture of cyclohexanone and cyclohexanol) using hydroperoxides as oxidizing species. The improvement in selectivity with decreasing particle size suggests the existence of a structure-activity relationship in this process, and points to a preferential activation of cyclohexane at defective corner or edge sites. A detailed theoretical study of the reaction mechanism over three different CeO2 catalyst models shows that cyclohexane is preferentially activated by bicoordinated oxygen atoms present at the edges of small particles, following a Mars van Krevelen mechanism which has been confirmed by in situ IR spectroscopy and 18O/16O isotopic exchange experiments. The process is fully heterogeneous, and the catalyst can be reused without loss of activity up to four cycles. ; The authors thank Solvay, MINECO (Consolider Ingenio 2010-MULTICAT, CSD2009-00050 and Severo Ochoa program, SEV-2012-0267), Generalitat Valenciana (PROMETEOII/2013/011 Project) and the European Union (ERC-AdG-2014-671093 — SynCatMatch) for financial support. Red Española de Supercomputación (RES) and Centre de Càlcul de la Universitat de València are gratefully acknowledged for computational facilities and technical assistance. The technical support of the Electronic Microscopy Service of UPV is kindly appreciated. T.L.-A. thanks ITQ for a contract. ; Peer Reviewed