Jiří Pancíř

Topological study of the chemisorption behavior of carbon monoxide on the Ni(112) and Cu(112) surfaces

Abstract A semi-empirical topological method was applied to a study of Ni(112) and Cu(112) surfaces as well as associative and dissociative chemisorption on these surfaces. A basic structural unit was used which consisted of 27 metal atoms. The terminal atoms were coiled into a torus in order to achieve a conservation of the translation symmetry corresponding to the infinite crystal. The high reactivity of the Ni surface can be ascribed to the rather unusual charge alternation as well as to the degeneracy of frontier orbitals. Calculations revealed that a charge alternation near a Cu surface is much smaller than that near a Ni surface which is considered to be in accord with the smaller reactivity of Cu. It has been shown that CO adsorption can take place on all metal surface sites, the most effective being in the valley of the step. In all the cases studied, an attachment of carbon to the surface is energetically more favorable than an attachment of oxygen. On the other hand, dissociation of CO can energetically compete with the simple adsorption only at the ledges of the Ni surface. The dissociative chemisorption on the Cu surface is endoergic on all adsorption centers investigated. At a higher coverage (and correspondingly higher CO pressure) a set of energetically favorable reactions may lead to the replacement of chemisorbed O by additional CO molecules on the Ni surface. The adsorbed C atoms surrounded by adsorbed CO molecules remain at the ledges of the (112) surface. This conclusion seems to be in accord with the methanation reaction of CO and H 2 in which CH 4 is formed on Ni catalyst.

Semiempirical calculations of singlet-triplet and triplet-triplet transitions

Semiempirical calculations of the PPP-type with optimum values of parameters (γμμ=8.2 eV, βc=−2.5 eV) reproduce fairly well the singlet-triplet (S-T) and triplet-triplet (T-T) excitation energies and oscillator strengths with conjugated hydrocarbons of various structural types. The usefulness of this parametrization is shown with anthracene. Moreover, the applicability of the Hückels N→V1 energies for estimates of the S-T transitions is mentioned.

Topological study of the chemisorption behavior of carbon monoxide on the Pt(112) surface

Abstract The chemisorption of CO on the Pt(112) surface has been investigated using a quantum-chemical topological method. A basic structure unit was used which consisted of 27 Pt atoms. Terminal atoms were coiled into a torus in order to assure a conservation of the translation symmetry of an infinite crystal. The calculations indicate that CO adsorbs linearly at sites on terraces and perpendicular to them. An attachment of carbon to the surface is energetically more favorable than an attachement of oxygen in all the cases studied. There was no indication that CO dissociates on the stepped surface (112) of platinum between multiple n -fold hollow positions. On the other hand, dissociation of CO between atop positions can take place even if it is energetically less favorable than the simple adsorption at the top positions.

Cis-trans isomerization of glyoxal

The CNDO/2 method with the original parameter set predicts the conformation with the mutually perpendicular CH=O groups to be the most stable isomer of glyoxal; the cis form is favoured over the trans form. The order of stabilities is not changed upon the full optimization of coordinates of the isomers. Calculations fail to reproduce the observed order of isomer stabilities unless allowance is made for limited configuration interaction with the lowest ππ* doubly excited state and for geometry optimization. Doubly excited configurations of the ππ* and σσ* types have negligible effect on the energies of isomers.

Topological study of the chemisorption behavior of carbon monoxide on the (112) surface of NiCu alloys

Abstract Chemisorption properties of bimetallic NiCu systems are studied. Ni atoms in a Cu lattice are strongly negatively charged as compared with a pure Ni crystal. The insertion of Cu atoms into a Ni lattice causes the opposite effect. The chemisorption properties are practically not changed by insertion of other metal atoms into the bulk. However, the substitution of metal atoms on the surface leads to a substantial change in chemisorption properties which cannot be explained on the basis of the behavior of pure monometallic species.