Chemisorption-induced metal atom

Chemisorption-induced metal atom reorganization at low temperatures had been discovered in the 1960s by field ion microscopy in chemisorption studies of nitrogen and carbon monoxide on a tungsten single-crystal tip (55-58). Likewise, surfece reconstruction was concluded from research of chemisorption on metal and bimetal films (59-61). This surface reconstruction was observed particularly on open crystal faces the more closely packed faces are often corroded from their edges with the former faces. Surface reconstruction is of particular relevance to alloy surfaces, where it leads to chemisorption-induced segregation of one alloy component to the... 

The chemisorption of hydrogen atoms on the external surface of SWNTs can be used to tailor both the physical and chemical properties of CNTs. Computational studies showed that attachment of hydrogen atoms induces change in the electronic and molecular structures of SWNTs [30, 36, 43]. For instance, hydrogenation of zigzag SWNT with half coverage on the external surface makes them metallic with very high density of states at the Fermi level. 

On metallic catalysts, sulfur is strongly adsorbed, and even if only minute amounts are found in the feedstock, accumulation can occur on a significant part of the metallic surface area. In the adsorbed state, the poison molecule will deactivate the surface on which it is adsorbed then the toxicity will depend on the number of geometrically blocked metal atoms. On the other hand, the chemisorption bond between the poison and the metal can modify the properties of the neighboring metallic atoms responsible for the adsorption of reactants. If the interaction between the poison and the metal is weak, the structure of the metal will remain unchanged, but it can induce a perturbation all around the adsorption site, which will be able to modify the catalytic properties of this surface. Yet if the interaction between the metal and the adsorbate is strong, it can go as far as to modify the metal-metal bond. The mobility of the surface atoms can be increased and a new superficial structure can appear. 

An extreme case of chemisorption-induced restructuring of metal surfaces is coirosive chemisorption as observed by SFG. In this circumstance, metal atoms break away from step or kink surface sites and form bonds with several adsorbate molecules. Carbon monoxide can form several carbonyl ligand bonds with platinum atoms leading to the creation of metal-carbonyl species. Thus, metal-metal bonds are broken in favor of forming metal-carbonyl clusters that are more stable at high CO pressures. The SFG vibrational spectra detect the reversible formation of new adsorb carbon monoxide species above 1(X) Torr on Pt(l 11), that appear to be platinum-carbonyl clusters Pt (CO) , with (m/n) > 1 and a CO commensurate overlayer. 

The chemisorption of an atom or a molecule often induces rearrangement of the substrate atoms around the adsorption site. For example, the chemisorption of carbon atoms on the nickel (100) surface occurs at the fourfold site. The nearest-neighbor nickel atoms are displaced away from the carbon, permitting it to move more into the metal surface and bond to the metal atom in the second layer [5, 6]. A small in-plane rotation of the surface nickel atoms around the carbon, shown in Figure... 

Ethylidyne restructures the Rh(l 11) crystal face [30], sulfur restructures the Fe(l 10) face [7], and carbon restructures the Ni(lOO) face [6, 46]. The surface metal atoms move into new equilibrium p>ositions upon chemisorption in different ways, and there is evidence of restructuring even in the second substrate layer under the surface. Review the available data and point out the important electronic and structural parameters that influence the nature and magnitude of chemisorption-induced surface restructuring. 

Interpretation of the first three terms in Eq.(2.229a) is straightforward. Term (1) results from the assumption that in the adsorbate no levels higher than Ef were occupied before adsorption. If such terms were occupied a correction to account for electron transfer from the adsorbate to the Fermi-level has to be introduced. Term (1) reflects electron transfer front the occupied undisturbed adsorbate levels to the Fermi level, term (2) electron transfer from a surface electronic state present before chemisorption to the Fermi level and term (3) electron transfer from the Fermi level to a surface molecule- or chemisorption- induced surface state level. In the surface molecule limit the contribution to the chemisorption energy is easy to calculate for s-type orbitals. If an orbital has Z" metal atom neighbors, the solutions cf become the solution of Ek. (2.215) ... 

More recently, Lang and Kohn (1973) gave a detailed discussion of this problem, including the question of chemisorption, e.g., of alkali metal atoms on transition metal surfaces such as those of W, Ta, Re. The problem is approached first by considering the profiles of the charge induced by a uniform external electric field for metals of different bulk electron densities. At electrodes, it is to be noted, an additional factor is the potential-dependent surface electron density, q, given, in the case of liquid metals of surface tension y> by q = -(8y/9E),... 

Recently a novel experimental approach using Schottky diodes with ultra-thin metal films (see Fig. 11) makes direct measurement of reaction-induced hot electrons and holes possible. See for example Refs. 64 and 65. The chemical reaction creates hot charge carriers which travel ballistically from the metal film towards the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and N02 molecules and of certain hydrocarbons on Ag. Similar results have been found with metal-insulator-metal (MIM) devices, which also show chemi-currents for many exothermic surface reactions.64-68... 

Considerable attention has been given for many years to the nature of the forces active in chemisorption and catalysis and in particular to the electronic nature of these forces these investigations have in the main treated the surface as a rigid structure, often considered as energetically uniform, with no mobility of its constituent atoms at the temperature of catalytic reactions. This assumption of rigidity is usually valid up to around room temperature and for metals is often true at quite high temperatures. There are, of course, some well-known cases where the gas dissolves in the metal, e.g., H2 into Pd 02 into Zr N2 into a-Fe, etc., and also instances where the adsorbate induces mobility in the upper layer of the adsorbent, e.g., H2 on K [see de Boer (I)], but these are exceptional. 

Recent experiments determining the so-called chemicurrent [111] have provided some information on the importance of electron-hole pair excitation in adsorption processes. Using thin films deposited on n-type Si(l 1 1) as a Schottky diode device, the nonadiabatically generated electron-hole pairs upon both atomic and molecular chemisorption create the chemicurrent which can be measured [111, 112]. It has been estimated that for example in the NO adsorption on Ag one quarter of the adsorption energy is dissipated to electron-hole pairs. Adsorption-induced electron-hole pair creation has also been found for other metal substrates, such as Au, Pt, Pd, Cu, Ni and Fe, and even for semiconductors such as GaAs and Ge [112, 113]. 

A way to stretch or compress metal surface atoms in a controlled way is to deposit them on top of a substrate with similar crystal symmetry, yet with different atomic diameter and lattice constant. Such a single monolayer of a metal supported on another is called an overlayer. Metal overlayers strive to approach the lattice constant of their substrate without fully attaining it hence, they are strained compared to their own bulk state [24, 25]. The choice of suitable metal substrates enables tuning of the strain in the overlayer and of the chemisorption energy of adsorbates. A Pt monolayer on a Cu substrate, for instance, was shown to bind adsorbates much weaker than bulk platinum due to compressive strain induced by the lattice mismatch between Pt and Cu, with Cu being smaller [26]. 

The dependence of the angular distribution of the adsorbate - induced photoelectron current on the initial state wave function has been recently calculated by Gadzuk (47—49). He has considered adsorption of atoms on transition metals, assuming the interaction of the adatom with the 7-type orbitals of the substrate to be responsible for the chemisorption. To avoid the difficulties associated with Penn s antiresonance effect he assumed the adsorbate level to be non-degenerate with the metal 7-band. Neglecting the variation of the optical matrix element over the width of the adsorbate level the angular distribution is in this case given by... 

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