Chemisorption on metal

The structure of the adsorbate layer formed at high surface coverage is more relevant to catalytic reactions at high pressures. Besenbacher and coworkers [29-33] found for CO on Pt(l 1 0) and Pt(l 11) and NO on Pd(l 11) that the structure ofhigh 

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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. 

The above studies show that the chemisorptions on metals could often alter the composition and structure of metal surfaces. To bridge the pressure gap, in situ STM has played a critical role in observing the dynamic behavior of catalytic surfaces from UHV to atmospheric pressures. 

Copper clusters, as reported by the Rice group(lc), do not react with hydrogen. Hydrogen chemisorption on copper surfaces is also an activated process. Surface beam scattering experiments place this barrier between 4-7 kcal/mole(33). This large value is consistent with the activated nature oT hydrogen chemisorption on metal clusters, and the trend toward bulk behavior for relatively small clusters (>25 atoms in size). 

A series of papers by Shustorovi ch(63) and/or Baetzo1d(64) summarized in a recent article(65) have addressed the problem of chemisorption on metal surfaces in terms of electron accepting and donating interactions. Saillard and Hoffmann (66) developed qualitatively identical pictures of these interactions but starting from fragment orbital type analysis. These papers are only a few of the theoretical discussions that consider hydrogen activation, however we will use their approach because it address the problem in a fashion that can interpolate between the organometallic cluster and the bulk. 

The heat of chemisorption is, of course, the energy difference between the chemical bonds formed and those broken. One of the strongest bonds to be broken in dissociative chemisorption on metals is the N-N bond of N2. This chemisorption is known to be rate limiting in ammonia synthesis. Brill et al. reported in 1967 field emission results indicating that N2 adsorption on Fe is strongest on the (111) face." Then-suspicion that this might be the initial step in ammonia synthesis over Fe catalysts... 

It is hardly possible, and probably not very useful, to find the exact form for Instead, as frequently happens, much progress can be made by adopting a model Hamiltonian. The work of Blandin et al, Bloss and Hone and the earlier study of sputtering by Sroubek shows that a suitable one is the TDAN Hamiltonian, which is a generalization of the time-independent one originally introduced to discuss impurities in metals and later applied to hydrogen chemisorption on metals ... 

The explanation for the apparent correlation between catalytic activity and electron affinity of metals cannot be as simple as that which has been advanced for the homogeneous catalysts. This is because chemisorption on metals (unlike the splitting of hydrogen by metal ions in solution ) is an exothermic process and, hence, as shown earlier, catalytic activity depends not only on a low activation energy of adsorption but also on a low heat of adsorption. The interpretation applied earlier to homogeneous catalysts can account for an inverse dependence of Ea on the work function, but does not suggest any obvious reason why Q should show a similar dependence. 

Another quite different area where ECP s have proven to be very useful for the development of transition metal cluster models. By using a very simplified description of the metal atoms, where all electrons including the d-electrons are considered as core, certain properties of the solid material such as chemisorption on metal surfaces or the reactivity of metal clusters has been studied theoretically with considerable success. 

Compared to the corresponding carbides the heats of oxygen chemisorption on metals are higher. For example, on metallic tungsten the heat of adsorption is 812 kJ/mole 02, while on metallic chromium it is 730 kJ/ mole 02n. These values are significantly higher that those of the carbides of the same metals (Table 16.2). Thus, carbon atoms, when implanted in the metal lattice, reduce the adsorption affinity of the metal atoms towards oxygen. 

Reduction with hydrogen is a preliminary step in practically all of the experiments dealing with chemisorption on metals. In most cases the sample is cooled to 35° C. in an atmosphere of hydrogen. Usually an attempt is made to observe a band due to chemisorbed hydrogen even though this may not be related to the major objective of the experiment. Despite many such attempts with a wide variety of samples, no band has been observed which could be attributed to chemisorbed hydrogen. 

Heats of Adsorption and Desorption and Activation Energies Connected with Chemisorption on Metals... 

Since we also learned that the mutual repulsion of dipoles does not materially contribute to a decrease of the heat of adsorption and since it can easily be proved that, also in the case of chemisorption on metals,... 

The energy transfer processes during the chemisorption on metal surfaces have been generally explained in terms of electron transfer from metal to incoming species (Cox et al., 1983 Gesell et al., 1970 Prince et al., 1981). 

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