Direct Observations of Oxygen-induced Platinum Nanoparticle Ripening Studied by In Situ TEM

This study addresses the sintering mechanism of Pt nanoparticles dispersed on a planar, amorphous Al2O3 support as a model system for a catalyst for automotive exhaust abatement. By means of in situ transmission electron microscopy (TEM), the model catalyst was monitored during the exposure to 10 mbar air at 650 degrees C. Time-resolved image series unequivocally reveal that the sintering of Pt nanoparticles was mediated by an Ostwald ripening process. A statistical analysis of an ensemble of Pt nanoparticles shows that the particle size distributions change shape from an initial Gaussian distribution via a log-normal distribution to a Lifshitz-Slyozov-Wagner (LSW) distribution. Furthermore, the time-dependency of the ensemble-averaged particle size and particle density is determined. A mean field kinetic description captures the main trends in the observed behavior. However, at the individual nanoparticle level, deviations from the model are observed suggesting in part that the local environment influences the atom exchange process

Supported nanoclusters: Preadsorbates tuning catalytic activity

Using a density-functional approach, we investigate the reactivity of supported metal nanoclusters. We focus on the sequential adsorption of N-2 molecules on a Fe nanocluster supported by an MgO substrate. For an increasing number of N atoms preadsorbed on the nanocluster, we found that the binding energy of the N-2 molecule increases, and can become higher than that of its dissociation products, in marked contrast with the behavior at the respective metallic surface. We identify the electrostatic interaction as a primary factor determining this behavior, and discuss the observed trends in terms of interactions with highest occupied and lowest unoccupied molecular orbitals

Nitrogen adsorption on a supported iron nanocluster

Sequential adsorption of N atoms on a MgO(100) supported Fe-7 cluster is studied using a density functional approach. For the number of adsorbates varying between one and six, the most favorable adsorption geometries are determined and the corresponding potential energy diagram is given. Up to five N atoms are found to bind strongly to the oxide supported cluster, with binding energies ranging from 0.98 to 1.11 eV per N atom. When a sixth N atom is added to the Fe-7/MgO(100) cluster with five preadsorbed N atoms, the potential energy sharply increases due to the strong repulsive interaction between N adsorbates at short distances. The MgO(100) support plays an important role in increasing the binding energy of the adsorbed species. (C) 2003 Published by Elsevier Ltd

Effect of Carbon Adsorption on the Isomer Stability of Ir4 Clusters

The atomic structure and electronic properties of gas-phase and MgO(100)-supported iridium tetramers are studied using density functional theory. At variance with experimental data, the most stable Ir4 isomer on MgO(100) is the square one, as in the gas phase, and the metastable tetrahedral isomer is highly distorted by interactions with the substrate. In the presence of a single carbon adatom, the most stable structure of Ir4 is tetrahedral for both environments and the structural distortion of the adsorbed cluster is reduced. On MgO(100), the binding energy of a C adatom to tetrahedral Ir4 is 1.6 eV larger than that to the square isomer, due to strong interactions between C-2p orbitals and a low-energy unoccupied molecular orbital of tetrahedral Ir4

Electronic properties of an epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface: A first-principles investigation

Using a density functional scheme, the authors investigate the electronic properties of an epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface, as recently realized experimentally. Simulated scanning-tunneling-microscopy images of filled and empty states agree well with the experiment, lending support to the proposed atomic structure. In accord with the experiment, the local density of states indicates that the electronic band gap in the thin silicate layer at the surface is close to that of bulk SiO2. The authors show that this effect results from the surface of the epitaxial adlayer acting as a high-barrier potential for the SiC states induced in the oxide band gap

Nitrogen fixation at passivated Fe nanoclusters supported by an oxide surface: Identification of viable reaction routes using density functional calculations

Using density-functional calculations, we investigate the possibility of ammonia synthesis at supported Fe nanoclusters along catalytic routes closely resembling those in biological nitrogen fixation. To achieve similar catalytic conditions as at the active site of the enzyme nitrogenase, the clusters are passivated with either S or N atoms. From calculated potential-energy profiles for the N-2 hydrogenation, we find that low-temperature synthesis of ammonia is viable at the clusters passivated by N atoms due to the strong binding energy of the N-2 molecule in the initial adsorption step

Transition from Mn4+ to Mn3+ induced by surface reconstruction at lambda-MnO2(001)

Structural and electronic properties of the lambda-MnO2(001) surface are investigated applying density functional theory approach. The calculations show that all Mn ions at unreconstructed smooth surface preserve the +4 oxidation state observed in the bulk. Upon the lambda-MnO2(001) reconstruction, one fourth of Mn ions at the surface undergo a change of the oxidation state from +4 to +3, although the reconstruction does not change the Mn coordination number with oxygen. This is accompanied with the filling of initially empty 3d(z2) states localized on cations with one electron denoted by two neighboring O atoms. Although the reconstruction leads to an energy gain of 0.04 eV per surface unit cell, it is not a spontaneous process since it proceeds with an activation energy of 0.12 eV. (C) 2010 American Institute of Physics. [doi:10.1063/1.3509401

Chiral recognition of organic molecules by atomic kinks on surfaces

Two distinct non mirror symmetric conformations of D and L cysteine were found after adsorption on Au 17 11 9 S. This demonstrates chiral heterorecognition, i.e. enantioselectivity of S kinks on vicinal Au 111 . The structures as determined by angle scanned X ray photoelectron diffraction XPD agree well with those from density functional theory DFT calculations. The calculations predict adsorption energies of 2 eV where D cysteine binds 140 meV stronger than L cysteine. The classical three point contact model for molecular recognition fails to explain these finding