Adsorption precursor complex formation

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

Figure 13.29. Schematic sorption isotherms of a metal ion (Me) on an oxide (XO ) at constant pH (a) adsorption only (H) adsorption and surface precipitation via ideal solid solution (c) adsorption and heterogeneous nucleation in the absence of a free energy nucleation barrier (AG 0) adsorption and heterogeneous nucleation of a metastable precursor (e) same as in (3) but with transformation of the precursor into the stable phase. The arrows show the isotherm evolution for continual addition of dissolved Me. The initial isotherm with the slope of 1 (in the double logarithmic plot) corresponds to a Langmuir isotherm (surface complex formation equilibrium). [Me]s , = solubility concentration of Me for the stable metal oxide [Me]p = solubility concentration of Me for a metastable precursor (e.g., a hydrated Me oxide phase). (From Van Cappellen, 1991.)...

In contrast, toluene and methanol coadsorbed on Rb-X do not form a bimolecular precursor complex and both reactants seem to be independently adsorbed at the surface. It should be noted, however, that after equilibration of the catalyst with equal partial pressures of both reactants, toluene was the main adsorptive. During toluene methylation, sorbed toluene was again the main surface species, the reaction rate, however, was proportional to the surface concentrations of both chemisorbed species (toluene, formaldehyde). The onset of the reaction was observed at much higher temperatures than in the ring alkylation which is at large ascribed to the indispensable conversion of methanol to a formaldehyde (or formate) species. 

Inner- or outer-sphere surface complex formation is a. necessary prerequisite for most surface chemical redox reactions. (ESR may provide important information regarding the nature of the precursor complex.) When electron transfer is fast k2 ArJArOH]), overall rates of reaction are influenced by rates of organic reductant adsorption. When electron transfer is slow kl < ArJArOH]), Eq. [18] can be modeled as a pseudoequilibrium reaction, using the equilibrium constant... 

In contrast, others claim the formation of an inner-shell complex during the adsorption of metal-precursor complexes on carbon [7], sometimes even... 

In this chapter, the adsorption of catalyst precursors on oxidic surfaces will be discussed in terms of a chemical-interaction model, i.e. allowing the formation of inner-sphere complexes, but it is important to realize that other models are being advocated in the literature as well on the one hand, one has the physical adsorption models, allowing only the formation of outer-sphere complexes, and on the other, it is proposed that during adsorption one really has a reaction between the precursor and the support to form a new phase or phases. We will meet the latter situation in our discussion of deposition-precipitation (Section 10.3.3), but we will disregard it when discussing impregnation chemistry (Section 10.3.2). 

In the case of other systems prepared by adsorption of gold acetyl acetonate, it seems that only cationic gold complexes could be active in CO oxidation. In the case of Au/NaY zeolite the Au precursor is reduced to Au under reaction conditions at 298 K with no evidence of formation of Au [164]. The catalyst shows activity although the Au complex (initial catalyst) is an order of magnitude more active than Au. The authors, however, report that the catalyst is less active than... 

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