1,721,127 research outputs found
Chemisorption of CO on defect sites of MgO
No full textChemisorption of a CO molecule on regular and defect sites of the MgO(100) surface has been investigated by means of cluster model calculations. At all sites studied, CO bonds at the cation with the C atom closest to the surface. The bonding is considerably larger at a three-coordinated corner site than for a regular five-coordinated surface site. A blueshift in the C-O stretching frequency, ωe, of adsorbed CO compared to free CO is found; the shift is much higher for a corner than for a surface site because of the larger local electric field for low-coordinated cations. Both the bond strength and the ω shift are largely due to electrostatic effects and not to the formation of a dative σ-bond with the surface. Surface relaxation effects have also been considered. © 1992
We present experimental and theoretical results for the surface core-level binding-energy shifts of Al(100), representative of an sp metal, and Cu(100), representative of a transition metal. Our analysis of these results leads to a unified interpretation for the different behavior of sp and transition metals. The d-electron contribution to smaller surface core-level binding energies is elucidated.
No full textWe present experimental and theoretical results for the surface core-level binding-energy shifts of Al(100), representative of an sp metal, and Cu(100), representative of a transition metal. Our analysis of these results leads to a unified interpretation for the different behavior of sp and transition metals. The d-electron contribution to smaller surface core-level binding energies is elucidated
FINAL-STATE EFFECTS FOR THE CORE-LEVEL XPS SPECTRA OF NIO
No full textIonization of the Ni 3s core level in NiO has been studied using ab initio wavefunctions for an NiO6 cluster model. Three important final state effects are studied: (I ) ligand to Ni 3d charge transfer; (2) exchange coupling of the ionized core level within the open 3d shell; and (3) atomic correlation effects among the metal 3s, 3p, and 3d shells. Analysis of the cluster wave-functions shows that these mechanisms are strongly coupled and must be treated on an equal footing. The ligand to metal charge transfer is often fractional, i.e. intermediate between 0 and 1
STUDIES OF THE CU-O BOND IN CUPRIC OXIDE BY X-RAY PHOTOELECTRON-SPECTROSCOPY AND ABINITIO ELECTRONIC-STRUCTURE MODELS
No full textX-ray photoemission spectroscopy (XPS) and ab initio molecular orbital cluster model wavefunctions are used in a combined study of the electronic structure of CuO. High-resolution XPS spectra for single-crystal CuO identify and clarify several features of the core and valence level spectra. The molecular orbital cluster wavefunctions show that there is a significant covalent contribution to the ionic Cu-O bond. For the copper core hole final states, the states where the core hole is screened by charge transfer from O2p to Cud lie at lower binding energy than the states where charge transfer does not screen the core hole. The multiplet splitting of the higher binding energy 2p-hole states is large (ca. 3 eV)
Solar-driven chemistry: towards new catalytic solutions for a sustainable world
No full textThe topic of production of useful chemical compounds with the help of solar light has been debated at a recent meeting organized in Rome on October 18 and 19, 2018, by the Accademia Nazionale dei Lincei. Some of the contributions presented at this event are collected in this special issue of the Rendiconti Lincei. Scienze fisiche e Naturali. In this paper, we briefly discuss some recent results concerning the use of solar energy by artificial photochemical reactions for four important applications: (i) conversion of solar energy into fuels or (ii) conversion of sunlight into electrical energy, (iii) use of solar energy to perform organic synthesis that cannot be obtained by conventional chemistry, and (iv) photochemical reactions to reduce pollution
Mechanisms responsible for chemical shifts of core-level binding energies and their relationship to chemical bonding
No full textA comprehensive review of different mechanisms which contribute to the chemical shifts of core-level binding energies, BEs, is made. A principle focus is on showing how the mechanisms can be used to relate the BE shifts to features of the chemical bonding and chemical interactions in the studied system. Several initial state mechanisms are identified; while some are well. known, the importance of others has been only recognized fairly recently. A theoretical framework is presented which places the analysis and interpretation of these BE shifts on a firm foundation. A rigorous definition and distinction of initial and final state effects is presented. This definition is applied to show that initial state effects are often the dominant factors for the chemical BE shifts. It is also shown that, in many cases, theoretical approaches involving the use of constrained variations can permit a clear and definitive separation of the contributions of the different mechanisms. Several representative applications to the analysis and interpretation of core-level BE shifts are described which show how the theoretical methods of analysis can be used to identify the mechanisms important for the BE shifts. Often more than one mechanism makes an important contribution to the shifts and it is common that the contributions will be canceling. When all of the relevant mechanisms are taken into account in the analysis of the BE shifts, these shifts do provide valuable information about the chemical bonding and electronic structure of the materials being studied. The mechanisms presented and the theoretical frameworks described provide a unified view of BE chemical shifts
MECHANISMS RESPONSIBLE FOR THE SHIFTS OF CORE-LEVEL BINDING-ENERGIES BETWEEN SURFACE AND BULK ATOMS OF METALS
No full textInterpretations are given for the core-level binding energies, BE's, of the surface and bulk atoms of metals; in particular, the origin of possible differences, or shifts, between these BE's is considered. The mechanisms which are responsible for the BE shifts are identified and characterized through theoretical analyses of the surface electronic structure. The dominant mechanisms for the BE shifts are those which lead to the initial-state changes in the orbital energies and, hence, the Koopmans' Theorem BE's. This is particularly important because it means that the shifts can be directly related to the chemistry and physics of the systems. The surface electronic structure is described with molecular-orbital wavefunctions for cluster models of the surface; this theoretical approach is particularly well suited for the study of local chemical interactions. The different mechanisms act in a cancelling fashion which leads to rather small net shifts between the BE's of bulk and surface atoms. The mechanisms which explain the surface and bulk core-level BE shifts are rather general and may provide a basis for understanding other cases of core-level BE shifts
Decoding the Role of Adsorbates Entropy in the Reactivity of Single-Atom Catalysts
Full text linkSingle-atom catalysts (SACs) are rapidly gaining attention as a versatile class of materials that combine the advantages of both homogeneous and heterogeneous catalysis. A growing number of studies aim to identify potential new SACs or to describe their structure and reactivity through ab initio quantum chemical simulations. While many computational studies primarily address reactions involving small molecules, such as water splitting or CO2 reduction, the application scope of SACs is rapidly broadening to include the production of fine chemicals and the conversion of biomass-derived platform molecules, processes that involve larger, more complex reactants. Using density-functional theory (DFT) simulations, we demonstrate that, while an approximate treatment of entropy is acceptable for molecules with up to three atoms, it introduces substantial errors in reactions involving more complex molecules. Our results reveal a linear correlation between the entropy of adsorbed molecules and that of the corresponding isolated species, mirroring trends observed on extended catalytic surfaces. For the largest systems investigated in this study, the entropy of the free molecule is reduced by approximately 10-20% upon adsorption; for small molecules, this reduction can range from 50 to 70%. This disparity arises because, on SACs, the translational entropy is effectively zero, the rotational entropy is minimal, and the vibrational entropy increases with the size of the molecule. Moreover, the entropy of adsorbates scales linearly with the number of atoms in the molecule, allowing for the prediction of entropic contributions of adsorbates on SACs without additional computational cost. Using propyne hydrogenation as a test, we demonstrate that the reaction energy profile computed with current approximate approaches for estimating the entropy of adsorbates differs significantly from the profile where entropy is explicitly included. These findings highlight the importance of considering adsorbate entropy for accurately predicting the catalytic activity of SACs, particularly for reactions involving complex molecules
SURFACE ELECTRONIC-STRUCTURE OF HEAVILY-ION-IMPLANTED AND LASER-ANNEALED SI SINGLE-CRYSTALS
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