1,721,011 research outputs found
A Theoretical Study of the CO and NO Chemisorption on Cu2O(111)
No full textThe density functional theory coupled to the molecular cluster approach has been used to study the bonding of CO and NO to
the Cu20 ( 111 ) non-polar surface. Two molecular orientations, X- and O-down (X = C, N), at two distinct sites, mono- and threecoordinated,
of the Cu20( 111 ) surface have been considered. Theoretical outcomes indicate that the former coordinative position is
more favoured than the latter and that the X-down bonding is definitely stronger than the O-down one. Both XO~Cu(I) G donation
and Cu(I)~XO ~ backdonation participate in the adsorbate/substrate interaction. In agreement with literature experimental results,
the C-O and N O stretching frequencies are significantly red-shifted upon coordination
Pairing of Hydrogen Atoms on the Si(100)-2×1 Surface: The Role of Interactions Among Dimers
No full textLocal-density-functional calculations of the pairing of H atoms on the (100)-2×1 surface of Si are presented. We find that significant interactions between neighboring dimers along a row are present, favoring configurations where H-unpaired dimers are in the same row and have the H-atoms all on the same side. We discuss how these effects can contribute to the effective H-pairing energies determined experimentally
Coordination Chemistry of CO and NH3 on ZnO(0001): a Molecular Cluster Study of the CO and NH3 Bonding Interaction with a d10 Ion
No full textA detailed investigation of the electronic structure of CO chemisorbed on the ZnO(0001) polar surface has been carried out by using the local-density-functional molecular-cluster approach. A Zn22O22/CO model cluster is used to evaluate internuclear distances, adsorption energies and vibrational frequencies of the surface-CO and C-O vibrational modes. The chaining to the rest of the solid has been simulated with pseudo-atoms carrying a not-integer nuclear charge. A realistic description of the electronic structure of the Lewis acid site is shown to be crucial to obtain a correct chemisorptive interaction. In particular, it appears that removing half an electron from each surface Zn allows an excellent agreement between experiment and theory as a consequence of the absence of occupied dangling bonds on ZnO(0001). Preliminary results relative to NH3 chemisorption over the same surface are also presented and discussed. The bonding mechanism of CO to ZnO(0001) is dominated by a covalent interaction involving a donation from the highest occupied molecular orbital of CO into the empty levels of the coordinatively unsaturated Zn surface ions. The same kind of mechanism is active in NH3 chemisorption, even though the electrostatic interaction between the nh3 dipole moment and the field at ZnO(0001) is at least as important as the covalent one. Slight variations of the electronic structure of the surface greatly affect the bonding of CO to the surface itself as a consequence of a consistent donation into the CO lowest unoccupied molecular orbital from the partially occupied dangling bonds of the Zn unsaturated ions nearest to the Lewis acid site. Transition state calculations seem to indicate that the assignments of the ultraviolet photoelectron spectra of ZnO(0001) after exposure to CO or NH3 need to be slightly modified
A Comparative Study of NH3 Chemisorption on ZnO(10-10) and Cu2O(111) Non Polar Surfaces
No full textThe chemisorption of NH3 on the ZnO(10-10) and Cu2O(111) non-polar surfaces is theoretically investigated to look into the different Lewis acidity of exposed, unsaturated Zn(II) and Cu(I) ions. Both molecular and dissociative processes have been taken into account for NH3 on ZnO(10-10), whereas only the molecular chemisorption of NH3 on Cu2O(111) has been considered. Calculations include the evaluation of surface relaxation before/after chemisorption, optimized adsorbate geometries, chemisorption energies and adsorbate vibrational frequencies. Theoretical data indicate that the chemisorption of NH3 on the defect free ZnO(10-10) surface occurs only molecularly and that the adsorbate is more tightly bound to Cu(I) than to Zn(II) sites. The agreement with available experimental data is satisfactory
A LCAO-LDF study of Bronsted acids chemisorption on ZnO(0001)
No full textThe local density functional theory coupled to the molecular cluster approach has been used to study the chemisorption of Bronsted acids (H2O, H2S, HCN, CH3OH and CH3SH) on the ZnO(0001) polar surface. Geometrical parameters and vibrational frequencies for selected species molecularly and dissociatively chemisorbed have been computed. The agreement with literature experimental data, when available, has been found to be good. The nature of the interaction between the conjugate base of the examined Bronsted acids and the Lewis acid site available on the surface has been elucidated, confirming its leading role in determining the actual relative acidity scale obtained by titration displacement reactions. The strength of this interaction follows the order OH(-)approximate to CN- > CH3O- > SH- > CH3S-
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
SO2 on TiO2(110) and Ti2O3(10-12) Nonpolar Surfaces: A DFT Study
No full textDensity functional molecular cluster calculations have been used to investigate the interaction of SO2 with defect-free TiO2(110) and Ti2O3(101-2) surfaces. Adsorbate geometries and chemisorption enthalpies have been computed and discussed. Several local minima have been found for TiO2(110), but only one seems to be relevant for the catalytic conversion of SO2 to S. In agreement with experiment, the bonding of SO2 to Ti2O3(101-2) is much stronger than that on TiO2(110). Moreover, our results are consistent with the surface oxidation and the formation of strong Ti-O and Ti-S bonds. On both substrates, the bonding is characterized by a two-way electron flow involving a donation from the SO2 HOMO into virtual orbitals of surface Lewis acid sites (Ls
a), assisted by a back-donation from surface states into the SO2 LUMO. However, the localization of surface states and the strength of back-donation are very different on the two surfaces. On TiO2(110), back-donation is weaker, and it involves unsaturated bridging O atoms, while on Ti2O3(101-2), it implies the Lsa-based valence band maximum and significantly weakens the S-O bond
A LCAO-LDF Study of Formate Chemisorption on Cu(100)
No full textThe coordination of formate on the Cu(100) surface has been investigated by using the LCAO-LDF molecular-cluster approach. Two different adsorption geometries, the short bridge and the cross bridge, have been investigated. In the former coordination, the adsorbate/substrate bonding is mainly σ in character, while in the latter π interactions, even though rather weak, seem to play a leading role. In both cases the molecular orbitals responsible of the Cu-O bonding have been identified. Total energy calculations indicate that the short bridge configuration is more stable than the cross bridge one by about 20 kcal/mol. Furthermore, a good agreement with experimental (ARPEFS, SEXAFS, XPD) structural parameters and formate ARUPS data is obtained only for the short bridge local geometry
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