Nickel, covalent surface bonding

Beeck at Shell Laboratories in Emeryville, USA, had in 1940 studied chemisorption and catalysis at polycrystalline and gas-induced (110) oriented porous nickel films with ethene hydrogenation found to be 10 times more active than at polycrystalline surfaces. It was one of the first experiments to establish the existence of structural specificity of metal surfaces in catalysis. Eley suggested that good agreement with experiment could be obtained for heats of chemisorption on metals by assuming that the bonds are covalent and that Pauling s equation is applicable to the process 2M + H2 -> 2M-H. 

Contact-potential measurements usually indicate only a small surface dipole associated with chemisorbed hydrogen, and in the case of metals such as tungsten and nickel, the negative end of the dipole appears to be on the outside. While there is some uncertainty about the detailed interpretation of these measurements, they are more readily reconciled with covalent than with ionic bonding. 

From various sources Dowden (27) has accumulated data referring to the density of electron levels in the transition metals and finds an increase from chromium to iron. The density is approximately the same from a-iron to /3-cobalt there is a sharp rise between the solid solution iron-nickel (15 85) and nickel, and a rapid fall between nickel-copper (40 60) and nickel-copper (20 80). From Equation (2), the rates of reaction can be expected to follow these trends of electron densities if positive ion formation controls the rates. On the other hand, both trends will be inversely related if the rates are controlled by negative ion formation. Where the rate is controlled by covalent bond formation, singly occupied atomic orbitals are deemed necessary at the surface to form strong bonds. In the transition metals where atomic orbitals are available, the activity dependence will be similar to that given for positive ion formation. In copper-rich alloys of the transition elements the activity will be greatly reduced, since there are no unpaired atomic d-orbitals, and for covalent bond formation only a fraction of the metallic bonding orbitals are available. 

Similar covalent bonds may be formed between metal surfaces and many other atoms, including atoms forming part of molecules or radicals. In many cases the dipoles point with their negative ends away from the metal surface. In other cases, however, as with C2H2 and C2H4 on nickel, they form dipoles pointing with their positive poles away from the surface (60). 

The necessity of covalent bonding between anions and cations of the reconstructed layer allows May and Carroll ) to discard the possibility of finding protons in such layers. Consequently the presence of hydrogen, as observed in the form of — SH by certain authors on polycrystaUine films of iron ), nickel and tungsten ), may be an indication that the state of the surface is not that of a proper reconstructed layer. 

R. Suhrmann Hanover) We have found by the method to be described on page 223 that the benzene molecules of the first layer give off hydrogen when they are adsorbed on nickel, iron, and platinum films at room temperature. There seems to be no decomposition on copper and gold. From simultaneous measurements of photoelectric sensitivity and resistance, it is concluded that the bond between phenyl radicals and the metal surface is covalent. 

The results presented in Fig. 4 show the evolution of the number of adsorbed benzene in the different system as a function of the simulation time. The ReaxFF force field allows the creation and breaking of covalent bonds between the different atoms of the system during the molecular dynamics simulation. In fliis work, we considered that a benzene molecule was adsorbed, if it formed at least one bond with the Ni (100), Ni (111), or Raney-Nickel surface, respectively. In the first 5 ps, the benzene adsorption is comparable for all three systems evaluated, i.e. both clean Ni surfaces and Raney Ni model. After the first 5 ps, about 6-7 % of benzene molecules have been adsorbed. In the very beginning of the simulation time, the adsorption process is even faster on Ni (100) and Ni (111) surface (blue and green line in Fig. 4) compared to the catalyst (purple line). After the first few ps, the adsorption on Ni (100) surface (blue line in Fig. 4) remains rather constant and does not increase much. After 25 ps, only about 12 % of benzene have been adsorbed. In contrast to this finding, the benzene adsorption on the Ni (111) surface and the Raney Ni model system (green and purple line in Fig. 4) increases more... 

After being physisorbed molecules may break open and their fragments can chemisorb. Chemisorption means strong reaction with the surface e.g., hydrogen or nitrogen gas with metallic surfaces (Figure 6.1). The atoms that were covalently bound to each other in molecules break off and form separate metallic bonds with the atoms of a metallic substrate surface such as platinum, nickel, or iron. There also can be a covalent bond between a surface atom and a nonmetal atom of an adsorbate or a er-bound organic moiety. Ionic bonds... 

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