Adsorbate different metal surfaces reactivity

F. Comparative Reactivities of Hydrocarbon Species Adsorbed on Different Metal Surfaces... 

Both the cluster, as well as periodic approaches, will likely play invaluable roles in the future toward the quantitative prediction of transition metal surface chemistry. Herein, we discuss some of the recent developments on the application of DFT-cluster calculations to chemisoiption and reactivity of adsorbates on metal surfaces. We demonstrate how these results can subsequently be used to begin to model overall catalytic cycles and interpret different selective oxidation chemistries. 

Given its ability to adsorb onto metal surfaces in different binding modes, CO can be used as a probe molecule to apprehend the surface state of NPs using IR and NMR spectroscopies, for example [82]. This provides indirect information on location and mobifity of ligands and even on their electron promotion at metal surface and can help to understand the surface properties and further the catalytic reactivity of NPs. 

Extensive studies are still needed on hydrogen-metal surface interactions, leading to various forms of adsorbed hydrogen of different specific reactivity with the metal catalyst surface. Nevertheless, one can conclude on the basis of the experimental evidence presented that certain facts al-... 

However, when the reductions were carried out with lithium and a catalytic amount of naphthalene as an electron carrier, far different results were obtained(36-39, 43-48). Using this approach a highly reactive form of finely divided nickel resulted. It should be pointed out that with the electron carrier approach the reductions can be conveniently monitored, for when the reductions are complete the solutions turn green from the buildup of lithium naphthalide. It was determined that 2.2 to 2.3 equivalents of lithium were required to reach complete reduction of Ni(+2) salts. It is also significant to point out that ESCA studies on the nickel powders produced from reductions using 2.0 equivalents of potassium showed considerable amounts of Ni(+2) on the metal surface. In contrast, little Ni(+2) was observed on the surface of the nickel powders generated by reductions using 2.3 equivalents of lithium. While it is only speculation, our interpretation of these results is that the absorption of the Ni(+2) ions on the nickel surface in effect raised the work function of the nickel and rendered it ineffective towards oxidative addition reactions. An alternative explanation is that the Ni(+2) ions were simply adsorbed on the active sites of the nickel surface. 

The differences in antiwear properties of disulfides are related to their ability to be physisorbed about 100 to 1000 times faster than monosulfide on metal surfaces. The differences can be explained in terms of the lower energy needed for the formation of the same number of RS" ions from disulfides (Kajdas,1994). The exposed metal surface is extremely reactive to lubricant components, especially antiwear and extreme-pressure additives resulting the formation of a film on the contact surface. The reaction of emitted electrons of low energy (1 to 4 eV) with molecules of oil additives adsorbed on the friction surface may lead to formation of negative ions and negative ion radicals. The investigator (Kajdas, 1994 and 1985) pointed out the indispensability of the metal oxide film on the rubbing surface from the viewpoint of the theory of sulfide film formation. 

Impurities are a concern in ionic liquids electrochemistry. Whereas even considerable amounts of impurities, like different metal ions, water or organic impurities, might not disturb a technical process (e.g. extractive distillation, organic synthesis) the wide electrochemical windows of an ionic liquid ( 3 V vs. NHE) allow the electrodeposition of even reactive metals like lithium and potassium, as well as the oxidation of halides to the respective gases. In the best case this codeposition only leads to a low level of impurities, in the worst case fundamental physicochemical studies are made impossible as the impurities are adsorbed onto the electrode surface and subsequently reduced. Furthermore, passivation or activation effects at the counter electrode have to be expected. 

The very language I have used here conceals a trap. It puts the burden of motion and reactive power on the chemisorbed molecules, and not on the surface, which might be thought of as passive, untouched. Of course, this can t be so. We know that exposed surfaces reconstruct, i.e., make adjustments in structure driven by their unsaturation.20 They do so first by themselves, without any adsorbate. And they do it again, in a different way, in the presence of adsorbed molecules. The extent of reconstruction is great in semiconductors and extended molecules, and generally small in molecular crystals and metals. It can also vary from crystal face to face. The calculations I will discuss deal with metal surfaces. One is then reasonably safe (we hope) to assume minimal reconstruction. It will turn out, however, that the signs of eventual reconstruction are to be seen even in these calculations. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

---