O2 activation

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

Catalytic oxidations on the surface of oxidic materials usually proceed according to the Mars-Van Krevelen mechanism [P. Mars and D.W. van Krevelen, Chem. Eng. Sci. 3 (1954) 41], as illustrated in Fig. 9.17 for the case of CO oxidation. Instead of a surface reaction between CO and an adsorbed O atom, CO2 is formed by reaction between adsorbed CO and an O atom from the metal oxide lattice. The vacancy formed is filled in a separate reaction step, involving O2 activation, often on defect sites. 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

A coupled binuclear copper active site is found in a variety of different metalloprotelns Involved in dloxygen reactions. These Include hemocyanln (reversible O- binding), tyrosinase (O2 activation and... 

Fueled by the biological relevance, the initial steps of Cu-based O2 activation have attracted much interest in the last decades. Various, very different Cux/02 species that result from the reaction of dioxygen with Cu complexes have meanwhile been identified (Fig. 2) [70-90], where mononuclear species A and B as well as dinuclear type E and F species (and some tricopper systems as in, for example, laccase) are considered the most relevant in nature. 

Since Fenton s work in the late nineteenth century, the role of transition metals in oxygen chemistry is known, but the formation of oxygen adducts with coordination metal complexes and their importance for O2 activation have been studied much later [1, 97]. The lively interest in ORR catalysis comes from its utmost importance to the development of fuel cells and this justifies that only a few studies have been done with metal complexes in solution most have been devoted to carbon electrodes modified by immobilization of a catalyst. The research for good catalysts that could be efficient substitutes for the expensive platinum naturally moved toward porphyrins. 

In summary, the heme-copper oxidases provide two crucial functions in cell respiration O2 activation/reduction and energy conservation by redox-linked proton translocation. Significant progress has been made in this field since the first edition of this series in the understanding of both structure and function. X-ray structures at atomic resolution have now been solved for several heme-copper oxidases, " and this has provided an important basis for further functional work. A summary of the recent advances that have been made in understanding the architecture of the heme-copper oxidases and their metal-containing active sites and a discussion of insights into the molecular mehanisms of operation are presented here with special emphasis on important issues that still remain to be solved. 

The active site similarities listed above belie a remarkable functional diversity, which includes phosphate ester hydrolysis, dioxygen and NO reduction, reversible O2 binding, and O2 activation, the last of which includes enzymes involved in ribonucleotide reduction, hydrocarbon monooxygenation, and fatty acyl desaturation. At the overall protein level, the purple acid phosphatases (PAPs) seem to be completely unrelated, both structurally and functionally, to any of the others in this class. Similarly, the flavo-diiron enzymes form a structurally and probably functionally distinct family of proteins, catalyzing both dioxygen and NO reduction. These last two examples illustrate that attempts to shoehorn all of these enzymes into a single class can sometimes provide a simplistic and misleading view of their chemistry and biochemistry. 

It is apparent that more than one oxygen activation pathway exists. The excited triplet state of tin porphyrins is known to be quenched in the presence of 2, suggesting a possible direct mechanism of O2 activation by the photosensitizer. We have examined reactions of both singlet O2 and superoxide anion under our reaction conditions. Chemically-produced superoxide (K02/18-crown-6) is not reactive under our experimental conditions. On the other hand, singlet oxygen, produced by irradiation of free base protoporphyrin (H2ProtoP), is reactive in the presence of tertiary amines and gives about the same hexanol to hexanone ratio (2.7, see Table 1) as is observed in the presence of the SnP photosensitizer. 

From the cyclic voltammogram of pure Pt in deaerated 0.1 M H2SO4, it can be seen that 0.68 V is a potential at which snrface oxide (or hydroxide) has jnst started, and the amonnt formed is minuscnle compared to that when the potential is at 1.229 V. In other words, even at satmation concentrations, O2 gas is unable to oxidize a pnre Pt snrface to the extent that the OCP is driven up to 1.229 V. The effect of Co has been smmised to facihtate O2 activation. It is further conjectured that the oxygen atoms formed on the Co sites irreversibly spill over to the Pt sites, to increase the amount of Pt-surface oxide and, consequently, the also the OCP. " The present work suggests that the optimal Pt-Co surface concentration for such synergistic process is that of PtsCo. ... 

In recent years there have been many attempts to isolate PSII proteinaceous components containing Mn and possibly restoring the Oj evolving ability. It has been reported on proteins [162,163] partially able to restore O2 evolution in protein-depleted vesicles the Mn associated to these preparations was however insufficient to account for the Mn content correlated to O2 evolution in the native membranes. By washing inside-out thylakoids with 250 mM NaCl, Akerlund [164] has extracted a 23 kDa protein free of Mn but reconstituting O2 activity. Several proteinaceous components, not all associated with Mn, could be involved in the oxidizing side of PSII. 

Figure 14.6 presents a possible mechanism for PHM and OPH. Substrate and O2 bind to the reduced enzyme, triggering initial O2 activation involving electron transfer from the type 2 Cu atom, to form the Cu-superoxo intermediate. A second electron is then transferred from the other Cu site, followed by-product release and reduction of the two Cu sites by ascorbate. The question of the exact mechanism of long-range electron transfer between the two Cu sites remains to be established. 

---