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| 2. |
- Mattsson, A, et al.
(författare)
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MODEL STUDIES OF THE CHEMISORPTION OF HYDROGEN AND OXYGEN ON CU(100)
- 1987
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 36:14, s. 7389-7401
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Tidskriftsartikel (refereegranskat)abstract
- Atomic chemisorption of hydrogen and oxygen on Cu(100) has been studied using up to 25 copper atoms as a model of the surface. The computational procedure used involves a reduction of the metal atoms to one-electron systems and extensive configuration-interaction calculations of the adsorbate and the cluster-adsorbate bonds. The calculations support the fourfold hollow site as the preferred chemisorption site for oxygen, with a barrier-to-surface migration of 25 kcal/mol. The calculated chemisorption energies for both hydrogen and oxygen, 51 and 90 kcal/mol, respectively, are in good agreement with experimental estimates (56 kcal/mol for hydrogen and 97 kcal/mol for oxygen). The effects of reducing the metal atoms to one-electron systems have been investigated through comparisons with all-electron calculations for Cu5H and Cu5O at the self-consistent-field level and by comparisons to previous calculations on Cu5Cl and Cu9Cl in which the 3d electrons were treated explicitly.
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| 3. |
- Panas, Itai, 1959, et al.
(författare)
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COMPARISON OF THE BINDING OF CARBON, NITROGEN, AND OXYGEN-ATOMS TO SINGLE NICKEL ATOMS AND TO NICKEL SURFACES
- 1988
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Ingår i: Journal of Physical Chemistry. - 0022-3654. ; 92:11, s. 3079-3086
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Tidskriftsartikel (refereegranskat)abstract
- Multireference CCI calculations have been performed for the diatomic molecules NiC, NiN, and NiO, and for the cluster systems NiSC, NiSN, and Ni50. Of the diatomic molecules, NiN has by far the smallest dissociation energy, which we argue is primarily due to the small electron affinity of nitrogen. The bonding to nickel 3d, which is pronounced for the diatomic molecules, disappears almost entirely for the cluster systems. In the full geometry optimization of the NiSX systems, includingcluster relaxation, the final geometries are rather similar for the three adsorbates, all being adsorbed below the surface. The energy gain in penetrating the surface is much larger for carbon and nitrogen than it is for oxygen, however. This is in line with the experimentally determined surface geometries, where carbon and nitrogen adsorb close to the surface and reconstruct the surface, whereas oxygen stays further out and does not reconstruct the surface. For oxygen the restoring forces from neighboring atoms in the surface are larger than the driving force to reconstruct the surface. The origin of the difference between the adsorbates is that the 2p, orbital, which points toward the surface, is almost doubly occupied for oxygen, whereas this orbital is only partially occupied for carbon and nitrogen.
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| 4. |
- Panas, Itai, 1959, et al.
(författare)
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MECHANISM FOR H-2 DISSOCIATION ON TRANSITION-METAL CLUSTERS AND SURFACES
- 1989
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Ingår i: ACS Symposium Series. - 1947-5918 .- 0097-6156. ; 394, s. 125-139
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Tidskriftsartikel (refereegranskat)abstract
- Basic rules are set up for the requirements on a cluster for dissociatively chemisorbing H2. These rules are built on previous experience from model calculations on transition metal complexes and surfaces. If these rules are combined with the knowledge of the atomic spectra of the transition metals, an understanding of both the low reactivity of the Co6-Co9 clusters compared to the corresponding nickel clusters and the similarly large reactivities of Co10 and Ni10 can be obtained. It is found that all the Co6-Co9 clusters have stable closed valence shell configurations. The present model for dissociating H2 does not involve the electron donation from the cluster to H2 that has been suggested previously. The correlation between ionization energies and reactivities of the clusters is shown to be a secondary consequence of the electronic structure of the clusters.
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| 5. |
- Panas, Itai, 1959, et al.
(författare)
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MODEL STUDIES OF THE CHEMISORPTION OF HYDROGEN AND OXYGEN ON NICKEL SURFACES .1. THE DESIGN OF A ONE-ELECTRON EFFECTIVE CORE POTENTIAL WHICH INCLUDES 3D RELAXATION EFFECTS
- 1987
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Ingår i: Chemical Physics. - 0301-0104. ; 112:3, s. 325-337
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Tidskriftsartikel (refereegranskat)abstract
- The present article is the first in a series describing investigations of the chemisorption and dissociation of hydrogen and oxygen on nickel surfaces. This paper deals with methodological questions, mainly concerning the form and properties of the crucial one-electron effective core potential (ECP) on nickel, which is used to describe chemisorption situations with sufficiently large nickel—adsorbate distances. Atomic chemisorption in the fourfold position on the Ni(100) surface is chosen as the model problem for testing and designing the nickel ECP. All-electron calculations on Ni4 and Ni5 with and without adsorbed atoms have been performed to produce reference results. Particularly noteworthy of the all-electron results is the large effect of 3d relaxation. To mimic cluster 3d relaxation effects a diffuse attractive 3d projection operator was added to the original standard form of the ECP.
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| 6. |
- Panas, Itai, 1959, et al.
(författare)
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MODEL STUDIES OF THE CHEMISORPTION OF HYDROGEN AND OXYGEN ON NICKEL SURFACES .2. ATOMIC CHEMISORPTION ON NI(100)
- 1988
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Ingår i: Theoretica Chimica Acta. - 0040-5744. ; 74:3, s. 167-184
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Tidskriftsartikel (refereegranskat)abstract
- Atomic chemisorption of hydrogen and oxygen on the Ni(100) surface has been studied using an Effective Core Potential (ECP) approach described in a previous paper. Clusters of up to 50 nickel atoms have been used to model the surface. The computed chemisorption energies are 62 kcal/mol (exp. 63 kcal/mol) for hydrogen and 106 kcal/mol (exp. 115–130 kcal/mol) for oxygen. Correlating the adsorbate and the cluster-adsorbate bonds is extremely important for obtaining accceptable results, particularly for oxygen. Reasonable convergence of chemisorption energies is obtained with 40–50 cluster atoms for both hydrogen and oxygen. For hydrogen the addition of a third cluster layer stabilizes the results considerably. Both hydrogen and oxygen are adsorbed at (or close to) the four-fold hollow site. The calculated barriers for surface migration are also in good agreement with the experimental estimates. The calculated equilibrium heights above the surface are on the other hand too high compared with experiments. This disagreement is believed to be due to core-valence correlation effects, which are not incorporated in the present ECP. The cluster convergence for the height above the surface is much slower than for the chemisorption energy.
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| 7. |
- Panas, Itai, 1959, et al.
(författare)
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ON THE CLUSTER CONVERGENCE OF CHEMISORPTION ENERGIES
- 1988
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Ingår i: Chemical Physics Letters. - 0009-2614. ; 149:3, s. 265-272
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Tidskriftsartikel (refereegranskat)abstract
- A new rule is suggested for calculating chemisorption energies using the cluster model. This rule is built on the realization that relatively large clusters (50 atoms and more) often need to be prepared for bonding by making an excitation to a proper bonding state (such a state will always be easily accessible in an infinite cluster). For hydrogen chemisorption, this bonding state must have a singly occupied orbital of the same symmetry as the hydrogen 1 s orbital. The new rule is applied to hydrogen chemisorption in the hollow sites of Ni(100) and Ni(111). When a cluster is prepared in a bonding state, even quite small clusters (≈ 10 atoms) give chemisorption energies in reasonable agreement with experimental surface results.
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| 8. |
- Schüle, Josef, et al.
(författare)
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Ab initio investigation of methyl adsorption on Ni(111)
- 1988
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Ingår i: Journal of Molecular Structure: THEOCHEM. - 0166-1280. ; 47, s. 151-153
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Tidskriftsartikel (refereegranskat)abstract
- Single reference CI calculations have been performed to investigate the adsorption of methyl on Ni(111) using clusters as models for the surface. Methyl is adsorbed in the threefold hollow position. The calculated symmetric C-H stretching frequency is in reasonable agreement with experiment. The lowering of the C-H frequency is explained by the large charge transfer from the surface to methyl.
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| 9. |
- Siegbahn, Per E.M., et al.
(författare)
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A COMPARISON OF THE ON-TOP DISSOCIATION OF H-2 ON NI(100) AND CU(100)
- 1989
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Ingår i: Theoretica Chimica Acta. - 0040-5744. ; 75:2, s. 143-159
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Tidskriftsartikel (refereegranskat)abstract
- The on-top dissociations of H2 on Ni(100) and Cu(100) are studied using a cluster approach. Correlation effects are accounted for through the use of CASSCF and CCI methods. The central metal atom is treated with all its electrons whereas the other cluster atoms are described by recently developed one electron ECP's. A molecular chemisorbed H2 state on nickel, similar to that recently observed experimentally, was identified in the cluster calculations and also for the triatomic NiH2. No such state was found on copper. The large differences found for the on top dissociation of H2 on nickel and copper are attributed solely to the difference in 3d orbital occupation. The parallel between the on top dissociation reaction on the cluster and the dissociation on a single atom is also studied. While the neutral triatomic NiH2 represents a qualitatively correct model in the nickel case, the negatively charged CuH 2 – is required as a model in the copper case.
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