Bifunctional oxygen catalysts

Hydrotreating. In order to successfully reform a naphtha with the conventional bifunctional reforming catalyst, heteroatoms must be reduced to a level such that they will not deactivate the reforming catalyst. Sulfur and nitrogen levels of well below 5 ppm seem to be required for conventional reforming catalysts while levels below 1 ppm appear necessary for the newer bimetallics. The assumption was made that, if sulfur and nitrogen levels could be reduced to specification, the chemical oxygen levels would follow. 

Although photoelectrochemical systems are able to offer respectable conversion efficiencies, the refinement of other solution-based processes continues. Gratzel has reviewed photoredox processes, paying particular attention to the use of organized assemblies such as micelles and vesicles. He emphasizes the central role of efficient colloidal metal catalysts in these schemes and also describes the recent development of bifunctional redox catalysts that allow the combination of cycles for the generation of hydrogen and oxygen. 

Figure 3 The reaction scheme for the bifunctional redox catalyst. RuOj/Pt on Ti02- The conduction and valence hand energies are compared with the standard hydrogen and oxygen potentials...

Olson TS, Pylypenko S, Fulghum JE, Atanassov P (2010) Bifunctional oxygen reduction reaction mechanism on non-platinum catalysts derived from pyrolyzed porphyrins. J Electrochem Soc 157 B54—B63... 

A good catalyst for the bifunctional hydrogen electrode is platinum, and for the bifunctional oxygen electrode the most promising catalyst material is a mixture of platinum and iridium oxide. The use of thin catalyst layers in the electrode helps to minimize mass transport and ohmic limitations [57]. In addition to the catalyst composition, the ionomer content, the catalyst layer thickness, and the PTFE content are varied and the influence of these variations on performance has been described [58-63]. The highest efficiency can be achieved using a catalyst with a high amount of platinum and a low amount of iridium [58, 60]. 

In alkaline solutions, bifunctional properties are exhibited by catalysts having the pyrochlore structure A2B2O7, where A = Pb, Bi and B = Ru, Ir (Horowitz et al., 1983), and by oxide catalysts having the perovskite structure (e.g., Lao,6Cao,4Co03) (Wu et al., 2003). The properties of bifunctional oxygen electrodes are discussed in greater detail in a paper by Jorissen (2006). 

It should be mentioned here that Sn sites are not considered to be the solitary source for OHad, which could be adsorbed on Pt sites owing to the influence of adjunct Sn atoms [Stamenkovic et al., 2005], The promotional effect of Sn was later confirmed on a PtSn/C nanocatalyst [Arenz et al., 2005], which exhibits similar behavior that was assigned primarily to the formation of reactive OH species at much lower potential than on pure Pt catalysts. Based on these findings, the bifunctional effect was unambiguously confirmed for Pt-Sn surfaces, where Sn sites serve as a source of oxygenated species that boost CO oxidation at low potentials and allow these surfaces to be employed as CO-tolerant catalysts. 

Finally, we want to compare the main mechanistic findings of our study with the classic bifunctional mechanism, which is generally used to explain the improved CO oxidation reactivity of PtRu surfaces and catalyst particles [Watanabe and Motoo, 1975]. According to that mechanism, Ru acts as a promotor for the formation of oxygenated adspecies on bimetallic PtRu surfaces, which can then react with CO... 

As it follows from Table 5, many catalysts contain metallic platinum. We have developed bi-layer porous hydrophobic air electrodes, which do not contain platinum metals, are active and can be cycled [24, 25] (Figures 4-6). These bifunctional catalysts are pyrolized Co - macrocyclic compounds. Said catalyst has high catalytic activity for the oxygen reduction and also features acceptable stability, however its activity for the oxygen evolution is not high enough. 

Takanabe, K. Aika, K.-I. Inazu, K. T. B. Seshan, K. Lefferts, L., Steam reforming of acetic acid as a biomass derived oxygenate Bifunctional pathway for hydrogen formation over Pt/ZrOz catalysts. Journal of catalysis 2006,243(2), 263-269. 

Snapper and Hoveyda reported a catalytic enantioselective Strecker reaction of aldimines using peptide-based chiral titanium complex [Eq. (13.11)]. Rapid and combinatorial tuning of the catalyst structure is possible in their approach. Based on kinetic studies, bifunctional transition state model 24 was proposed, in which titanium acts as a Lewis acid to activate an imine and an amide carbonyl oxygen acts as a Bronsted base to deprotonate HCN. Related catalyst is also effective in an enantioselective epoxide opening by cyanide "... 

Axially chiral phosphoric acid 3 was chosen as a potential catalyst due to its unique characteristics (Fig. 2). (1) The phosphorus atom and its optically active ligand form a seven-membered ring which prevents free rotation around the P-0 bond and therefore fixes the conformation of Brpnsted acid 3. This structural feature cannot be found in analogous carboxylic or sulfonic acids. (2) Phosphate 3 with the appropriate acid ity should activate potential substrates via protonation and hence increase their electrophilicity. Subsequent attack of a nucleophile and related processes could result in the formation of enantioenriched products via steren-chemical communication between the cationic protonated substrate and the chiral phosphate anion. (3) Since the phosphoryl oxygen atom of Brpnsted acid 3 provides an additional Lewis basic site, chiral BINOL phosphate 3 might act as bifunctional catalyst. 

Shortly thereafter, Terada demonstrated that the Mannich reaction between several N-Boc aryl imines and acetoacetone was effectively catalyzed by only 2 mol% of le (Scheme 5.2) [4]. In view of AMyama s work, this study is particularly significant because it suggested that le may act as a bifunctional catalyst [9] not only to form a chiral ion pair with the electrophile but also to activate the nucelo-phile through hydrogen bonding of the a-proton with Lewis basic phosphoryl oxygen. 

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