Different metal surfaces reactivity

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

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. 

In this section, we move from the elucidation of molecular and atomic adsorption to the fundamental features that control smface reactivity. We start by initially describing dissociative adsorption processes. We focus on elucidating surface chemistry as well as the understanding of how the metal substrate influences the intrinsic surface reactivity. We will also pay attention to geometric ensemble-size related requirements. The Brpnsted-Evans Polanyi relationship between transition-state energy and reaction energy discussed in Chapter 2 is particularly useful in understanding differences in reactivity between different metal surfaces. 

Much of what we know about the reactive intermediates for a particular reaction has been established from either in situ or ex situ spectroscopic analyses of the reaction surface. What is usually measured, however, is the most stable species on the surface and not necessarily the most reactive sp>ecies. There are a growing number of examples which have shown, through either experiment or theory, that the reaction may be controlled by species that are very reactive on the surface. They have very short hfetimes, thus making it difficult to catch them in action . Some of the notable examples include the TT-bound ethylene species on Pt and Pd which are more weakly boimd than their di-cr-bound intermediate but also tend to be the more predominant reaction channels. Similarly, transient O2 surface intermediates on Cu and La203 and also 0 on different metal surfaces have been identified. 

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... 

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-... 

In this review, we will specifically discuss the similarities and the differences between the chemistry on surfaces and molecular chemistry. In Sect. 2, we will first describe how to generate well-dispersed monoatomic transition metal systems on oxide supports and understand their reactivity. Then, the chemistry of metal surfaces, their modification and the impact on their reactivity will be discussed in Sect. 3. Finally, in Sect. 4, molecular chemistry and surface organometallic chemistry will be compared. 

Different metal species vary in their biological reactivity.98 99 For example, the free ionic form of a metal may act by substituting a cofactor for a vital enzyme. Hydroxylated metal ions have been suggested to bind to the cell surface and alter the net charge of the cell to reduce its viability.101 Because different species may have different effects on biological processes, some species may be more toxic than others. There is a paucity of information in the literature regarding the relative toxicity of different metal species. 

During the catalytic cycle, surface intermediates include both the starting compounds and the surface metal atoms. This working site is a kind of supramolecule that has organometallic character, and, one hopes, the rules of the organometallic chemistry can be valid for this supramolecule. The synthesis of molecular models of these supramolecules makes it possible to study the elementary steps of the heterogeneous catalysis at a molecular level. Besides similarities there are, of course, also differences between the reactivity of a molecular species in solution and an immobilized species. For example, bimo-lecular pathways on surfaces are usually prohibited. 

In fact, the orientation of water at the potential of zero charge is expected to depend approximately linearly on the electronegativity of the metal.9 This orientation (see below) may be deduced from analysis of the variation of the potential drop across the interface with surface charge for different metals and electrolytes. Such analysis leads to the establishment of a hydrophilicity scale of the metals ( solvophilicity for nonaqueous solvents) which expresses the relative strengths of metal-solvent interaction, as well as the relative reactivities of the different metals to oxygen.23... 

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. 

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