Metal surfaces, reactive

This j taper describes equipment, procedures and results for investigation of transition metal surface reactivity. Specifically, the surface reactivity of copper single crystals was examined under conditions relevant to the electrochemistry and corrosion of copper. 

This chapter is organized as follows Section 13.2 gives a discussion of a multitude of concepts in this held between physics and chemistry band structure, chemical bond, iff-LDOS, ad-atoms, chemisorption, and metal surface reactivity. The next section presents a brief description of metal NMR theory, with which the values of iff-LDOS can be deduced. In Section 13.4 selected examples from Pt NMR are presented, and in Section 13.5 those for CO. Section 13.6 offers brief conclusions. 

We have proposed an analogy for metal surface reactivity among metal particles with similar surfaces (made of the same metal, with comparable sizes, etc.) those that have the higher local density of states at the Fermi energy on their surface sites will be the more reactive [54,55]. This is a weaker statement than the sometimes-heard proposal that the iff-LDOS is a useful yardstick to compare more widely different systems, e.g., a series of transition metals. 

The sticking probability S of the hydrogen molecule being the simplest measure of metal surface reactivity, its dependence upon crystal face and coverage may provide ideas for understanding other reactions. In the absence of a precursor state and on an energetically homogeneous surface S will depend upon 9 as... 

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

F1 NMR of chemisorbed hydrogen can also be used for the study of alloys. For example, in mixed Pt-Pd nanoparticles in NaY zeolite comparaison of the results of hydrogen chemisorption and F1 NMR with the formation energy of the alloy indicates that the alloy with platinum concentration of 40% has the most stable metal-metal bonds. The highest stability of the particles and a lowest reactivity of the metal surface are due to a strong alloying effect. 

Aluminium is a very reactive metal with a high affinity for oxygen. The metal is nevertheless highly resistant to most atmospheres and to a great variety of chemical agents. This resistance is due to the inert and protective character of the aluminium oxide film which forms on the metal surface (Section 1.5). In most environments, therefore, the rate of corrosion of aluminium decreases rapidly with time. In only a few cases, e.g. in caustic soda, does the corrosion rate approximate to the linear. A corrosion rate increasing with time is rarely encountered with aluminium, except in aqueous solutions at high temperatures and pressures. 

The unequal attack which occurs in tap water, condensate and other mild electrolytes may lead to perforations of thin-gauge sheet and even to deep pitting of castings. In stronger electrolytes the effect is variable. In chloride solutions such as sea-water, attack on the metal usually results in the pitting of some areas only, but where the metal surface has been rendered reactive, as by shot blasting, attack may be so rapid that uniform dissolution over the whole surface may occur. In either case magnesium-base alloys are not usually suitable for use in aqueous liquids since they are not intrinsically resistant to these electrolytes. 

General Titanium is intrinsically very reactive, so that whenever the metal surface is exposed to air, or to any environment containing available oxygen, a thin tenacious surface film of oxide is formed. This oxide, which is present on fabricated titanium surfaces at normal or slightly elevated temperatures, has been identified as rutile, a tetragonal form of titanium dioxide, and it is the presence of this surface film which confers upon titanium excellent corrosion resistance in a wide range of corrosive media. 

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