Reactive Metals and Oxides

The structure of the passive oxide film formed on iron has been the subject of much controversy dating back to the discovery of the phenomenon in the 1700s because of the difficulty in characterizing the thin film in the aqueous environment, and it is only recently that SXS has been able to resolve some of the issues. M. F. Toney and co-workers [92] used SXS to study the passive oxide films formed on single-crystal Fe(OOl) and Fe(llO) substrates at a high anodic formation potential. Fig. 19 a and b show the measured diffraction patterns from the passive film on Fe(OOl) and Fe(llO) respectively. For growth on Fe(OOl), the oxide (001) planes 

The formation of passive oxide films on the (111) surfaces of Cu and Ni has also been studied in detail by SXS [94, 95]. Measurements of Cu(lll) in 0.1 M NaCl04 (at pH 4.5) showed that the oxide exhibited a crystalline cuprite structure (CU2O) that was epitaxially aligned with the underlying Cu substrate [94]. Although a similar oxide structure was observed for oxidation in air, there were some key differences in the structure of the aqueous oxide. In particular it was found that a preferred reversed orientation of the oxide film was formed, and this indicated that oxide growth occurs at the interface between the oxide and the Cu(lll) surface 

In this case a combined SXS/AFM study revealed that cycling the electrode potential over the Ni(OH)2/NiOOH redox peaks led to an increase in thickness of both the compact NiO layer and the amorphous hydroxide phase [97]. The thickening of the dense NiO phase Hmits further growth of the hydroxide phase, because of either hmited Ni diffusion from the Ni substrate or the reduction in potential gradient across the NiO fihn. 

The small light grey balls represent the hydrogen atoms, conjectured to show conceivable hydrogen bonds. Horizontal direction (010) [(l-lO)buikj Vertical direction (100) [(OOl)buikj for upper panels (top views) (001) [(n0) ikj for lower panels (side views) (taken from Ref [99]). 

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Plutonium is a very reactive metal and oxidizes readily in moist air. In finely divided form, plutonium metal is pyrophoric (Taylor 1973). Plutonium exhibits five oxidation states from plutonium(lll) to plutonium(VII). The four lower oxidation states are stable in solution and may co- exist in the same solution. Complex (coordination) compounds are formed with many of the common inorganic anions, such as plutonium nitrate (Pu(NO 3) 4). 

The lanthanides are rather reactive metals, and a good indication of their ease of oxidation and reduction can be seen by considering their reduction potentials. For comparison, the reduction potentials for magnesium and aluminum are as follows ... 

The high affinity between the two elements results in a ductile, strong and selfrepairing oxide layer, and it is this layer that gives the titanium its unique performance. Titanium is a reactive metal and without the oxide layer it would simply dissolve in water. 

Magnesium is a very reactive metal and makes an excellent fuel under the proper conditions. It is oxidized by moist air to form magnesium hydroxide, Mg(OH) 2, and it readily reaets with all acids, including weak species such as vinegar (5% acetic acid) and boric acid. The reactions of magnesium with water and an acid (HX) are shown below ... 

Theoretically, all metal oxides can be thermally decomposed to give the metal and oxygen gas. In reality, it is usually too difficult to decompose the oxides of reactive metals, and even some moderately reactive metals, such as aluminium, have oxides that require thousands of degrees to make them decompose. 

Self-assembly phenomena on solid substrates are usually studied in ultra-high vacuum (UHV) or at the liquid-solid interface. Surface analytical methods involving electrons require vacuum. But UHV has also the advantage that reactive metal and metal oxide surfaces can be used as substrate since the very low background pressure also guarantees long investigation times on a non-altered sample. 

The oxide layer that forms on aluminum is more complex than with other metal substrates. Aluminum is a very reactive surface, and oxide forms almost instantaneously when a freshly machined aluminum surface is exposed to the atmosphere. Fortunately, the oxide is extremely stable, and it adheres to the base metal with strength higher than could be provided by most adhesives. The oxide is also cohesively strong and electrically nonconductive. These surface characteristics make aluminum a desirable metal for adhesive bonding, and they are the reasons why many adhesive comparisons and studies are done with aluminum substrates. 

During the handling of microgram-sized samples of berkelium metal, it was observed that the rate of oxidation in air at room temperature is not extremely rapid, possibly because of the formation of a protective oxide film on the metal surface (135). Berkelium is a chemically reactive metal, and berkelium hydride (123), some chalco-genides (123, 136, 137) and pnictides (138, 139) have been prepared directly from the reaction of Bk metal with the appropriate nonmetal-lic element. 

Finally, the Aksay s model was used to explain the empirical correlation between reactivity and wettability observed in several metal/ceramic systems. It will be shown, for instance for metal/oxide couples (see Sections 6.4.1 and 6.5.2), that such a correlation can be explained by taking into account only the effect of reactions between liquid metals and oxides on interfacial energies. 

Although a valence-type force field of the type illustrated by Eq. [1] is most suitable for modeling molecular systems, the electronegativity equalization approach to treating polarization can be coupled equally well to other types of potentials. Streitz and Mintmire used an EE-based model in conjunction with an embedded atom method (EAM) potential to treat polarization effects in bulk metals and oxides. The resulting ES + EAM model has been parameterized for aluminum and titanium oxides, and has been used to study both charge-transfer effects and reactivity at interfaces. 

The author believes that students are well aware of the basic reaction pathways such as substitutions, additions, eliminations, aromatic substitutions, aliphatic nucleophilic substitutions and electrophilic substitutions. Students may follow undergraduate books on reaction mechanisms for basic knowledge of reactive intermediates and oxidation and reduction processes. Reaction Mechanisms in Organic Synthesis provides extensive coverage of various carbon-carbon bond forming reactions such as transition metal catalyzed reactions use of stabilized carbanions, ylides and enamines for the carbon-carbon bond forming reactions and advance level use of oxidation and reduction reagents in synthesis. 

The reactivity of oxide supported metals has received considerable attention because of the importance of such systems in heterogeneous catalysis. The morphology (structure and size) of the supported particle and its stability, the interaction of the particle with the support, and the crossover of adsorbed reactants, products and intermediates between the metal and oxide phases are all important in determining the overall activity and selectivity of the system. Because of the relative insensitivity of an optical technique such as IR to pressure above the catalysts, and the flexibility of transmission and diffuse reflection measurement techniques, vibrational spectroscopy has provides a considerable amount of information on high area (powder) oxide supported metal surfaces. Particularly remarkable was the pioneering work of Eichens and Pliskin [84] in which adsorbed CO was characterised by IR spectroscopy on... 

Cadmium is a reactive metal and dissolves in nonoxidizing and oxidizing acids, but unhke Zn, it does not dissolve in aqueous alkali. In moist air, Cd slowly oxidizes, and when heated in air, it forms CdO. When heated, Cd reacts with the halogens and sulfur. 

The most common technique to obtain clean rare earth metal surfaces is the deposition of thin films by evaporation of highly purified rare earth samples in an ultrahigh vacuum system onto a suitable substrate. The high reactivity and thermal stability of the rare earth metals and oxides demand both careful design of the evaporation source and an elaborate procedure. 

It is tempting to take into consideration the results we obtained using pure metals and come to the conclusion that oxygen is more reactive towards tin dioxide associated with metals because of a synergic effect between metal and oxide. In order to eliminate the exclusive action of metal, complementary experiments were conducted using equal metal surface areas in both cases, that is to say for pine metals and also for metal associated with tin dioxide. 

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