Adsorption and oxidation of CO on ceria nanoparticles exposing single-atom Pd and Ag: A DFT modelling

Various COx species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCOx moieties are found to be stabilised in the order MCO < MCO2 < MCO3, similar to the trend for COx species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO2 transformation are found for O4Pd site of the Pd/Ce21O42 model featuring a square-planar coordination of the Pd2+ cation. The removal of one O atom and formation of the O3Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO2 reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO2-species are strongly bound in palladium carboxylate complexes, whereas the CO2 molecule easily desorbs from oxide-supported AgCO2 moieties. Opposite to metal-free ceria particle, the formation of neither PdCO3 nor AgCO3 carbonate intermediates before CO2 desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface COx moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO2 and CO32− species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction

Dynamics of metal clusters in rare gas clusters

We investigate the dynamics of Na clusters embedded in Ar matrices. We use a hierarchical approach, accounting microscopically for the cluster's degrees of freedom and more coarsely for the matrix. The dynamical polarizability of the Ar atoms and the strong Pauli-repulsion exerted by the Ar-electrons are taken into account. We discuss the impact of the matrix on the cluster gross properties and on its optical response. We then consider a realistic case of irradiation by a moderately intense laser and discuss the impact of the matrix on the hindrance of the explosion, as well as a possible pump probe scenario for analyzing dynamical responses.Comment: Proceedings of the 30th International Workshop on Condensed Matter Theories, Dresden, June 05 - 10, 2006, World Scientific. 3 figure

Adsorption and Oxidation of CO on Ceria Nanoparticles Exposing Single-Atom Pd and Ag: A DFT Modelling

Various COx species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCOx moieties are found to be stabilised in the order MCO &lt; MCO2 &lt; MCO3, similar to the trend for COx species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO2 transformation are found for O4Pd site of the Pd/Ce21O42 model featuring a square-planar coordination of the Pd2+ cation. The removal of one O atom and formation of the O3Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO2 reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO2-species are strongly bound in palladium carboxylate complexes, whereas the CO2 molecule easily desorbs from oxide-supported AgCO2 moieties. Opposite to metal-free ceria particle, the formation of neither PdCO3 nor AgCO3 carbonate intermediates before CO2 desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface COx moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO2 and CO32− species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction

Layered structure of the near surface region of oxidized chalcopyrite CuFeS2 hard X ray photoelectron spectroscopy, X ray absorption spectroscopy and DFT U studies

Metal-depleted layers with different S species are found, and mechanisms for their formation and metal sulfide ‘passivation’ are proposed.</p

Density Functional Calculation of Dioxygen Adsorption at Complexes of Ceria Nanoparticle with Atoms, Trimers and Tetramers of Silver

В статье рассмотрены продукты молекулярной адсорбции (МА) и диссоциативной адсорбции (ДА) молекулы О2 на комплексах модельной наночастицы Ce21O42 (NP) с атомом и небольшими кластерами серебра. Согласно данным расчетов методом функционала плотности энергии образования координированных с O2 мономеров, тримеров и тетрамеров серебра (относительно невзаимодействующих NP, Agn и О2) составляют 2.0–4.4 эВ. Энергия адсорбции молекулы O2 (Ead(O2)) в наиболее стабильном атомарном AgOO{Ce}-комплексе с молекулой О2 в мостиковом положении между атомами Ce и Ag на поверхности нанограни {111} достигает ~1.3 эВ. AgnOO{Ce}- и OAgnO{Ce}-мостиковые структуры образуются и в наиболее стабильных изомерах МА- и ДА-комплексов Ag3 на {100} и {111} и Ag4 на {100} наногранях. Характеристики химической связи О-О в AgnOO{Ce}-структурах указывают на образование стабильных супероксидных группировок О2 –. Ead(О2) для наиболее стабильных МА и ДА комплексов на кластерах серебра и ограничиваются величинами 0.5–1.1 и 1.4–2.0 эВ соответственно. Барьеры активации диссоциации молекулы O2 в AgnOO{Ce}-комплексах с n = 3 и 4 составляют 1.5–2.1 эВ, что свидетельствует о низких скоростях протекания данных окислительных перегруппировокMolecular adsorption (MA) and dissociative adsorption (DA) of O2 on complexes of a model nanoparticle Ce21O42 (NP) with an atom and small clusters of silver have been addressed. According to results of density functional calculations formation energies of such systems (with respect to the non-interacting NP, Agn and O2) are calculated to be 2.0–4.4 eV. O2 adsorption energy (Ead(O2)) in the lowest-energy atomic complex with О2 in a bridging position between Ce ion and Ag bound on the {111} nanofacet (AgOO{Ce}-complex) is as high as ~1.3 eV. AgnOO{Ce}- and OAgnO{Ce}- bridging structures are formed in MA and DA lowest-energy complexes of Ag3 on {100} and {111} as well as Ag4 on {100} nanofacets. Bonding characteristics of the AgnOO{Ce}-structures match those for stable superoxo groups О2 –. Ead(О2) of the lowest-energy МА and DA complexes of O2 with Ag clusters are in the range of 0.5–1.1 and 1.4–2.0 eV, respectively. Activation energies for O2 dissociation in AgnOO{Ce}- complexes (n=3, 4) calculated to be 1.5–2.1 eV indicate low rates of these oxidation rearrangement

Density functional embedded cluster calculations on Lewis acid centers of the α-Al₂O₃(0001) surface: Adsorption of a CO probe

We have investigated the adsorption of CO on the cation-terminated (0001) surface of -Al2O3 with a generalized-gradient density functional approach. We used cluster models that are consistently embedded in an elastic polarizable environment (EPE), employing a (classical) shell model scheme and bare pseudopotentials (Alpp*) for cations at the cluster boundary. The newly implemented EPE method was found to perform well for describing structure and adsorption properties of Lewis acidic centers of the polar (0001) surface known for its strong relaxation with respect to the bulk terminated geometry. The calculated data are stable with respect to the size of the cluster models. For the embedded stoichiometric clusters [Al4O6]/Al and [Al10O15]/Al of different structure, the adsorption induced shift of the CO stretching vibration and the binding energy (BE) are 30-42 cm-1 and 0.38-0.47 eV, respectively. The results for the frequency shifts are in good agreement with the value of 39 cm-1 measured for CO/-Al2O3(0001) at low coverages. Judged by the frequency shift and the adsorption energy, the interaction of CO molecules with three-coordinated Al3+ cations at the regular surface is very similar to that with Mg2+ corner sites of MgO polycrystallites, =39 cm-1, BE=0.38 eV. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 200

On the electronic and geometric structure of bimetallic clusters : a comparison of the novel cluster Na6Pb to Na6Mg

Density functional studies of the abundant cluster Na6Pb and of its analogue Na6Mg are reported. The structure of Na6Pb has been optimized for a series of symmetry constraints (Oh, D3d, D3h, C5v, C3v and C2v). The resulting binding energies fall within a narrow range of less than 0.1 eV whereas a spread of more than 0.5 eV is calculated for Na6Mg. These findings indicate a high structural flexibility of Na6Pb. The Pb atom exhibits a propensity to occupy a highly coordinated site in contrast to Mg which, in the most stable structures, is attached to the outside of a Na6 moiety. Analysis of the bonding mechanism revealed two major contributions which increase the atomization energy of Na6Pb compared to Na6Mg: an enhanced charge transfer from the Na6 subsystem and a stronger polarization of the Pb atom. A significant contribution to the overall cluster stability comes from the interaction between the alkali atoms