Chemisorption modeling

Cobalt, nickel and copper naked metal clusters and olefin chemisorption models. G. A. Ozin, Coord. Chem. Rev., 1979, 28, 117-146 (45). 

Fig. 1.1. Chemisorption model, where aa(Pa) is adatom site (bond) energy, as(a) surface (chain) atom site energy and (3 chain bond energy.

Models of CO adsorption show that top site binding is governed by the CO HOMO (5cr orbital) donating electrons into the metal unoccupied states, with simultaneous back-donation of electrons from the metal s occupied dxz and dyz states into the CO LUMO 2tt orbital). Therefore, it follows that the standard chemisorption model, which considers shifts in the total d-band center, can be inaccurate for systems in which individual molecular orbitals, involved in bonding with the adsorbate, shift differently due to external interactions. In particular, we have shown that the formation of hybrid orbitals with the support material can lead both to downward shifts in the metal d-band center, which do not affect the adsorption of molecules to the metal surface, and to upward shifts that are vitally important. 

Fig. 10. Fourier transform of Pt Lj-edge SEXAFS data for 0.8 ML Pt/Si(l 11)7x7 (dots) and of the simulated EXAFS for a Pt atom in the sixfold interstitial site within the top Si(lll) double layer (solid line). The derived chemisorption model is represented in the insets. The first neighbour distance is Pt—Si = 2.48 + 0.03 A, and the Pt coordination number is 6 + 1 Si neighbours. The distances beyond the first neighbour peak are fitted up to the fifth Pt—Si distance, and indicate the deformation of the Si cage, as indicated in the modot, due to the presence of the host Pt atom...

In view of the linearity of the curves presented in Figure 8 and the high activation energies obtained by using Equation 1 at constant values of there seems to be considerable justification for the chemisorption model of volatile matter release. The fact that the initial rates of H2 release show a zero-order dependency may be attributed to the fact that for raw anthracite the fraction of surface coverage is practically unity. Under these conditions, where almost all of the sites available for H2 adsorption are filled, Equation 1 leads to the result that the rate of change of coverage with respect to time is initially constant. 

We adopt the same chemisorption model as in our previous work [3], which within the unrestricted Hartree-Fock approximation involves a self-consistent calculation of the electronic charge on the adatom. The basis elements needed for the calculation are the... 

Comparison of HRELS" for C2H4 on Ni(lll) with Ni C2H4) Chemisorption Models 121)... 

The above chemisorption model can formally be extended to desorption kinetics. In fact, assuming the same rate equation as in Eq. (58)... 

The LCGTO-MCP-DF method has been already applied for a wide range of systems in the last few years, inluding transition metal atoms, dimers and clusters, organometallic molecules and chemisorption models [6-9]. The investigations showed that this computational scheme is a very powerfull and efficient tool and that there is certainly a wide class of problems where it is the methodology of choice because of its ability to treat some types of electronic correlation at a reasonable level and because it can offer computational economies. 

It is important to explicitly account for the possibility that the metal surface atoms coordinated to the adsorbate can also be metal surface nearest-neighbor atoms. It appears to be essential to incorporate this feature in the chemisorption model, and we will discuss it for the particular case of the threefold coordination. If the adsorbate orbital o has a s-symmetry, it will coordinate with a surface fragment orbital that is a symmetric combination of surface s-atomic orbitals 1, 2 and 3, located on the coordinating surface atoms. If the adsorbate orbital has p-symmetry, it will coordinate with an antisymmetric fragment orbital. The surface orbital fragment... 

Because gold displays very weak chemisorbing properties, the activated chemisorption model of electrocatalysis is assumed to be inapplicable in the case of this metal in aqueous media. The alternative, which is well established in the chemically modified electrode [48] and redox sensor [49] area, is the interfacial cyclic redox mediator model which, in the case of gold in aqueous media, is sometimes referred to as the incipient hydrous oxide/adatom mediator (IHOAM) [18,33] model. In the case of the Group 11 metals the mediator systems are unusual in that their redox transitions involve couples with nonequilibrium (or metastable) reduced and oxidized states. 

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