Metal dissolution dissociative adsorption

What are the molecular pathways for metal or ligand adsorption (4) How can we infer molecular information from dissolution experiments (5) Do the macroscopic properties of a surface in water, such as Bronsted acidity, change with size of the molecule (6) How do the microscopic properties of a molecule, such as the rates of bridge dissociation, relate to calculable bond properties, such as electronic charge densities (7) Over what time scales are different types of bond dissociations complete (8) Can geochemists predict rates of multi-step reactions ... 

The aqueous solution layer that forms at the metal interface can ultimately provide a medium for the dissolution of Pd ions or oxidized Pd clusters into the supported liquid layer where they can then act as homogeneous catalysts. As was discussed earlier, the acetoxylation of ethylene can be carried out over various Pda,OAcj, clusters where alkali metal acetates are typically used as promoters. DFT calculations were carried out on both the Pd2(OAc)2 and Pd3(OAc)e clusters in order to examine the paths that control the solution-phase chemistry. The Pd3(OAc)e cluster is the most stable structure but is known experimentally to react to form the Pd2(OAc)2 dimer and monomer complexes in the presence of alkali metal acetates. The reaction proceeds by the dissociative adsorption of acetic acid to form acetate ligands. Elthylene subsequently inserts into a Pd-acetate bond. The cation is then reduced by the reaction to form the neutral Pd°. The reaction is analogous to the Wacker reaction in which ethylene is oxidized over Pd + to form acetaldehyde. Pd° is subsequently reoxidized by oxygen to form pd2+[35,36,44]... 

Sorption reactions, which introduce potentially reactive ligands to a metal at the mineral surface, or modify a preexisting ligand, are important to the dissolution rate (Westall 1988). The adsorption reactions, however, are very rapid whereas dissolution is slow. The discrepancy in reaction time suggests that the adsorbates indirectly influence but do not control dissolution rates. To understand the influence of solution composition on dissolution rates, it is useful to examine the simplest possible analog for mineral dissolution dissociation of a dissolved metal dimer. 

In this respect the solution chemistry of common anions is very different. For example with phosphate (Fig. 4.4), HP04 and H2P04 are the dominating solution species over the typically studied pH range, and fully dissociated anions occur only at extremely high pH values, which are of limited interest in adsorption studies, e.g. many common adsorbents are unstable at such a high pH (dissolution). On the other hand, the final products of hydrolysis on anions, i.e. fully protonated acid molecules are usually water soluble, thus, the applicability of surface complexation model is not limited by surface precipitation as it was discussed above for metal cations. 

Hydrogen interacts with metals in three principal ways (i) by dissociative chemisorption at the surface (ii) by physical adsorption as molecules at very low temperatures and (iii) by dissolution or occlusion. As we shall see, to these three extreme forms have been added numerous intermediate states of various lifetimes and stabilities, some of which may have importance in catalysis. There is for example clear evidence for a molecular state formed at about 100 K on stepped surfaces saturated with atomic hydrogen (on Ni(510), Pd(510) and (210) ) this is distinct from a molecular precursor stzlt such as that seen with deuterium on Ni( 111) at 100 K. The role of such states will be discussed further below (Sections 3.2.2 and 3.3.3). The small size and electronic simplicity of the hydrogen atom formed by dissociation enable it to bond to metal surfaces in different ways, and simple-minded notions about its forming only a single covalent bond to another atom have to be abandoned. 

Metallic membranes, (Pd-Ag) alloys, are typically used for separation of H2, either as an unsupported foil or a supported thin film. In these membranes, the hydrogen transport is by adsorption and atomic dissociation on one side of the membrane, dissolution in the membrane, followed by diffusion, and finally desorption (on the other side). Due to the H2 dissociation step, H2 separation is driven by a transmembrane difference of the square roots of the hydrogen partial pressures. The preparation technologies of both unsupported and supported Pd-Ag membranes are well developed and such membranes are commercially available. Since the membrane reformer performance is limited by separation capability, optimization of membrane permeability is one of the important issues. 

Metals and metalloids on the surface of silicate minerals are also connected by oxide or hydroxide ion bridges, which undergo acid-base reactions with the adjacent aqueous solution. The results of these acid-base reactions are measurable as surface charge, which varies in concentration with solution pH. (The situation is a little more complicated than this, especially for minerals that have a structural charge due to uncompensated cation substitutions. The reader is directed to, for example, Schindler and Stumm 1988.) The enhancement of dissolution rates via adsorption of hydrogen ions is referred to as the proton-promoted pathway for dissolution (Furrer and Stumm 1986) and is analogous to the proton-promoted pathway for the dimer dissociation discussed above. 

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