Local Chemical Interactions Research Articles

ChemInform Abstract: Quantum‐Chemical Cluster Models of Acid‐Base Sites of Oxide Catalysts

Publisher Summary This chapter focuses on the quantum-chemical cluster models of acid-base sites of oxide catalysts. It is desirable, on the one hand, that the size of such a cluster should be as small as possible to permit a sufficiently rigorous quantum-chemical description of its electron properties and the interaction with adsorbed molecules. On the other hand, the cluster should be large enough to produce both the real geometry of the whole crystal lattice of a catalyst and its electron structure, which are important for chemisorption. The numerous quantum-chemical computations carried out for such clusters have demonstrated that chemisorptions interactions are indeed sufficiently localized, even in the case of metals where maximal cooperative effects should be anticipated. Thus quantum chemistry quite unambiguously advocates the use of the local approach in describing the catalytic phenomena. The cluster approach opens the way for a direct application of quantumchemical methods to the problems of chemisorption and catalysis. Quantum chemistry has a rich experience in describing the local chemical interactions in molecular systems. Its methods are continuously refined and improved, and the computational techniques have become more and more accurate and predictive.

Disordering effect of Al clusters on GaAs(110) measured with medium-energy ion scattering

The effect of room-temperature adsorption of Al on the surface structure of GaAs(110) has been investigated with medium-energy ion scattering using shadowing and blocking. For the coverage range studied (0 to 2.8 × 10 15 Al atoms cm -2) we have observed the formation of three-dimensional islands. In between clusters the surface relaxation is identical to that of the atomically clean surface. Under the clusters the number of substrate atoms visible to the incident ion beam has increased by an amount equivalent to one monolayer. This indicates a local chemical interaction which leaves the substrate in a disordered state.

The coadsorption of nitrogen with carbon monoxide and oxygen on the Ru(001) surface: Local chemical interactions in mixed overlayers

High resolution electron energy loss spectroscopy and thermal desorption mass spectrometry have been employed to investigate the molecular chemisorption of N2 on both disordered and ordered overlayers of atomic oxygen on the Ru(001) surface, as well as the chemisorption of CO on overlayers of N2 on Ru(001). Pertinent results obtained for the adsorption of N2 on the clean Ru(001) surface are also presented for comparison. Disordered oxygen poisons a fraction of the surface to the subsequent adsorption of N2 whereas the N2 that does adsorb is indistinguishable from N2 on clean Ru(001). The fraction of the surface that is poisoned to the adsorption of N2 is approximately twice the fractional surface coverage of disordered oxygen. The p(2×2) overlayer of ordered oxygen adatoms, which is formed at a fractional surface coverage of 0.25, stabilizes the chemisorption of N2 into a new binding state with a heat of adsorption that is approximately 1.5 kcal/mol greater than any one observed for the adsorption of N2 on the clean surface. Coverage measurements indicate that this state results from the stoichiometric addition of one N2 molecule to each unit cell of the p(2×2)–O overlayer. Electron energy loss spectroscopic results suggest that this N2 binding state results from stabilization of the dominant σ donor contribution to the Ru–N2 bond, due to the presence of the electronegative oxygen adatoms of the p(2×2) overlayer. Measurements of the adsorption of CO on saturated overlayers of N2 show that N2 is displaced from the surface by increasing coverages of subsequently adsorbed CO. For low coverages of CO in the presence of N2, the observed value of ν(CO) is lower than observed under any conditions for the adsorption of CO alone on the Ru(001) surface. The N2 admolecules enhance the ability of the surface ruthenium atoms to backdonate electron density into the 2π orbital of coadsorbed CO under these conditions. At coverages of CO in excess of 0.10 monolayer, the results are consistent with CO island formation and segregation of N2 and CO admolecules into different local regions on the surface.

Local electronic structure of amorphous metal alloys using cluster models. Evidence for specific metalloid-metal interactions

Although considerable effort has been expended over the past ten years on constructing structural models of amorphous metal alloys, mostly through the application of dense random packing of hard spheres ideas and extensions thereof, almost no theoretical effort has been devoted to understanding the local electronic structure and chemical interactions among the various components in such alloys. The present work is part of a substantial effort which is being made to rectify this situation. Detailed quantum-mechanical calculations are carried out on clusters of atoms which simulate possible local environments in amorphous metal alloys. The clusters used are based on simple Bernal polyhedra and distortions of these polyhedra. The materials being studied are the (Fe, Ni, B, P) alloys. In this paper, the simplest Bernal polyhedron, the tetrahedron, is considered as a first model for the local environment in ${\mathrm{Fe}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{B}}_{20}$ and ${\mathrm{Fe}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{20}$ alloys. The tetrahedron contains two Fe atoms and two Ni atoms at the vertices with either a B atom or a P atom at the center. Spin-polarized self-consistent-field---$X\ensuremath{\alpha}$---scattered-wave calculations have been employed to investigate the electronic structure. It is found that there are considerable bonding interactions between the metalloid atoms (B or P) and the metal atoms of the clusters. Furthermore a detailed analysis of the charge densities of these models leads to the conclusion that the B atom should have a stronger interaction with the Ni atoms than with the Fe atoms, whereas the P atom should have a stronger interaction with the Fe atoms than with the Ni atoms. A brief discussion of recent experimental data which supports this conclusion is presented.