Metal surfaces electronic consequences

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. Hydrogen bonds are formed mainly within the adsorbate layer but also between the adsorbate and the second layer. Fig. 3 already shows quite clearly that the requirements of hydrogen bond maximization and minimization of interfacial dipoles lead to preferentially planar orientations. On the metal surface, this behavior is modified because of the anisotropy of the water/metal interactions which favors adsorption with the oxygen end towards the metal phase. 

The change in the electronic structure of a bulk metal catalyst, in consequence of its transformation into the hydride, influences respectively the metal surface atoms (ions) or, strictly speaking, their d orbitals. Recent achievements and the present knowledge of the subject only permit us so far to formulate such general conclusions. 

Equation (38) still includes the electronic term. On the other hand, SB(dip) ir may differ from gB(dip) at the metal surface as a consequence of different interactions with the environment. Therefore the interpretation of adsorption potential shifts is always subject to a number of assumptions that cannot be easily checked. 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

The fact that evaporated potassium arrives at the surface as a neutral atom, whereas in real life it is applied as KOH, is not a real drawback, because atomically dispersed potassium is almost a K+ ion. The reason is that alkali metals have a low ionization potential (see Table A.3). Consequently, they tend to charge positively on many metal surfaces, as explained in the Appendix. A density-of-state calculation of a potassium atom adsorbed on the model metal jellium (see Appendix) reveals that the 4s orbital of adsorbed K, occupied with one electron in the free atom, falls largely above the Fermi level of the metal, such that it is about 80% empty. Thus adsorbed potassium is present as K, with 8close to one [35]. Calculations with a more realistic substrate such as nickel show a similar result. The K 4s orbital shifts largely above the Fermi level of the substrate and potassium becomes positive [36], Table 9.2 shows the charge of K on several metals. 

Although chemisorption is not essential, when it does occur there may be further enhancement of the Raman signal, since the formation of new chemical bonds and the consequent perturbation of adsorbate electronic energy levels can lead to a surface-induced RR effect. The combination of surface and resonance enhancement (SERRS) can occur when adsorbates have intense electronic absorption bands in the same spectral region as the metal surface plasmon resonance, yielding an overall enhancement as large as 10lo-1012. 

Due to its importance in many industrial processes, the prototypical reaction of CO binding to metal surfaces has received much attention. Using Hiickel molecular orbital theory, Blyholder showed that CO bonding at top sites consists of the donation of electrons from the filled CO 5a HOMO to the metal d 2 orbitals with a back-donation of electrons from the metal dxz caddy orbitals to the CO 2n LUMO. Consequently,... 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

The modification of an x-wave sample state due to the existence of the tip is similar to the case of the hydrogen molecule ion. For nearly free-electron metals, the surface electron density can be considered as the superposition of the x-wave electron densities of individual atoms. In the presence of an exotic atom, the tip, the electron density of each atom is multiplied by a numerical constant, 4/e 1.472. Therefore, the total density of the valence electron of the metal surface in the gap is multiplied by the same constant, 1.472. Consequently, the corrugation amplitude remains unchanged. 

The SMSI effect in Mn-promoted Ru/Ti02 catalysts was studied in more detail making use of the SSIMS technique, as well as with TEM, and selective chemisorption experiments. The SSIMS technique revealed the presence of TiO c forming two new surface sites, TiO -Ru and TiO-Mn. These species were found to be located at the immediate vicinity of the Ru nanoparticles. These new surface sites were considered to alter the electronic properties of the Ru metal surface and, as a consequence, the product selectivity. 

The presence of adsorbates modifies the potential barrier at the metal surface and this may lead to the adsorption of positive or negative ions. In the former case, as seen in Fig. 4a, the highest occupied level of the adsorbate of ionization potential I lies above the Fermi level of the metal so that an electron may transfer from the adsorbate to the metal. When, for example, a Th atom is located near a W surface (Fig. 4b), the neutral atom is unstable as the ionization potential of Th (4.0 v.) is less than the work function of W (4.5 v.). Consequently, the outermost electron of the Th atom transfers to the metal, and a positively charged layer of Th ions is... 

Through the jellium model of the metal we have explained the effect of the metal electrons on the interfacial properties. We also know that the spillover of electrons creates a separation of charges at the metal edge, and consequently, a surface potential. However, what is the magnitude of this surface potential How important is its contribution to the total potential drop in the interfacial region ... 

Catalytic reactions at a metal surface involve a subtle and delicate balance of adsorption forces. Too weak an adsorption and the catalyst will have low activity, too strong and the surface becomes poisoned by adsorbed reactants or products. Consequently, quite small changes in the nature of a metal surface may result in significant variations in catalytic properties. Structure sensitivity is known to exist. There is good evidence that the selectivity and activity of a metal catalyst are affected by changes in structure and/or electronic properties. 

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