Bonding to Transition Metal Surfaces

The valence electron distribution of a transition metal surface is sketched in Fig. 4.1. A narrow d valence electron band is overlapped by a broad s-p valence 

Generally, the bonding of adatoms other than hydrogen to a metal surface is highly coordination-dependent, whereas molecular adsorption tends to be much less discriminative. For the different metals the bond strength of an adatom also tends to vary much more than the chemisorption energy of a molecule. Atoms bind more strongly to surfaces than molecules do. Here we will discuss the quantum chemical basis of chemisorption to the transition metal surfaces. We will illustrate molecular chemisorption by an analysis of the chemisorption bond of CO [3] in comparison with the atomic chemisorption of a C atom. 

The valence electron orbitals and energies of CO and C are sketched in Fig. 4.2. In a molecule, atomic orbitals form bonding and antibonding orbitals, separated by an energy gap. In CO the highest occupied molecular orbital, the 5a orbital is a symmetric with respect to the molecule s axis. It is separated by approximately 7 eV from the lowest two unoccupied degenerate 2re orbitals, of re symmetry with respect to the molecule s axis. The unoccupied 2k orbitals are antibonding and result from the interaction between the CPt and 0Px and CPy and 0Pj( orbitals. In the atom, atomic p orbitals are partially occupied, separated by approximately 20 eV from the doubly occupied 2s atomic orbital. CO adsorbs perpendicular to the transition metal surface, attached via its carbon atom. When adsorbed atop the surface valence s-electrons will interact with the 5a orbital, but their interaction is symmetry forbidden with the p-symmetric 2re orbitals. 

Interaction of re-type CO orbitals with the s valence atomic orbitals is only possible in high coordination sites (in the organometallic nomenclature these are called bridging sites, denoted i, p.3, and p4 for sites involving 2, 3 and 4 metal atoms, respectively). As illustrated in Fig. 4.3 they can then interact with asymmetric group orbitals that are linear combinations of atomic s orbitals of the surface atoms. 

When the d valence electron band is nearly completely filled, interaction with the doubly occupied CO 5o orbital, leading to a significant fraction of occupied antibonding orbital fragments a between adsorbate and surface atoms, will be repulsive. This Pauli repulsion is proportional to the number of surface atom neighbours and hence is a minimum in atop coordination. This counteracts the 

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In the following, we will discuss a number of different adsorption systems that have been studied in particular using X-ray emission spectroscopy and valence band photoelectron spectroscopy coupled with DFT calculations. The systems are presented with a goal to obtain an overview of different interactions of adsorbates on surfaces. The main focus will be on bonding to transition metal surfaces, which is of relevance in many different applications in catalysis and electrochemistry. We have classified the interactions into five different groups with decreasing adsorption bond strength (1) radical chemisorption with a broken electron pair that is directly accessible for bond formation (2) interactions with unsaturated it electrons in diatomic molecules (3) interactions with unsaturated it electrons in hydrocarbons ... 

From the calculated adsorbate-substrate bond length a substantial covalent character of alkali bonding to transition metal surfaces is deduced in the low coverage limit. A spin-polarized calculation shows that the unpaired spin of the alkali atom is almost completely quenched upon chemisorption. 

The O- species on the surface will be further stabilized by coulombic interactions with the lattice cations. For the sake of completeness, the review also includes a brief section on oxygen ions which are closely bonded to transition metal ions (M=0) and which may play an important part in selective oxidation [Weiss et al. (4) The coverage of the mononuclear oxygen species is restricted to those papers where there is direct evidence on the nature of the oxygen species concerned. 

This introductory section intends to present the basic bonding concepts, necessary to understand chemical bonding to transition metal complexes, clusters and transition metal surfaces. 

The bond strength to transition metal surfaces decreases with increasing d-valence electron occupation. This is because more anti-bonding adsorbate-surface orbital interactions occur. It tends to decrease when moving downward along a column of the periodic table. 

Bonding to a metal surface can also be described in terms of frontier orbital interactions similar as in the cluster case. Symmetry considerations now apply to the coefficients of the metalsurface molecular orbitals at the Fermi elevels that interact with the HOMOs and LUMOs of the adsorbing molecule or atom. Such a theory helps to predict bonding topology of adsorbates, as will be illustrated for bonding of CO to the transition metals. 

It is evident from the earlier discussion that scaling among adsorption energies should not be limited to transition metal surfaces. In fact, even for metal-terminated surfaces of more complex systems like transition metal compounds (oxides, nitrides, sulfides, and carbides), where there is mixed covalent, ionic bonding between the... 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

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