Phthalocyanins catalytic activity

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Lalande G, Faubert G, Cote R, Guay D, Dodelet JP, Weng LT, Bertrand P. 1996. Catalytic activity and stability of heat-treated iron phthalocyanines for the electroreduction of oxygen in polymer electrolyte fuel cells. J Power Sources 61 227-237. 

The first catalysts reported for the electroreduction of C02 were metallophthalocyanines (M-Pc).126 In aqueous solutions of tetraalkylammonium salts, current-potential curves at a cobalt phthalocyanine (Co-Pc)-coated graphite electrode showed a reduction current peak whose height was proportional to the C02 concentration and to the square root of the potential sweep rate at a given C02 concentration. On electrolysis, oxalic acid and glycolic acid were detected, but formic acid was not. Mn and Pd phthalocyanines were inactive, while Cu and Fe phthalocyanines were slightly active. At the potentials used for C02 reduction, M-Pc catalysts would be in their dinegative state, and the occupied dz2 orbital of the metal ion in the metallophthalocyanine was suggested to play an important role in the catalytic activity. 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

Copper, and occasionally silver, have been used as catalysts for hydroformylation of a-olefins. Phosphite complexes of copper(I) chloride have been claimed as catalysts (126). Phthalocyanine complexes of Group IB metals have been stated to show a low degree of catalytic activity (127). One of the more interesting examples of copper catalysis was disclosed by McClure (128). Copper powder, with a controlled amount of water (0.2-4.0 moles H20/mole Cu), gave a slow conversion of pro-... 

Photo-oxidation reactions, 32 118 Photoreduction, metal oxides, 31 123 Phthalic acid, esterification, 17 340 Phthalocyanines EDA complexes of, 20 328-330 catalytic activity for hydrogen exchange reaction, 20 329,330 electronic configuration of, 20 330 organometallic complexes, 30 276-277 Phyllosilicates, see Layer lattice silicates, catalysts... 

An aluminum electrode modified by a chemically deposited palladium pen-tacyanonitrosylferrate film was reported in [33]. Vitreous carbon electrode modified with cobalt phthalocyanine was used in [34]. Electrocatalytic activity of nanos-tructured polymeric tetraruthenated porphyrin film was studied in [35]. Codeposition of Pt nanoparticles and Fe(III) species on glassy-carbon electrode resulted in significant catalytic activity in nitrite oxidation [36]. It was shown that the pho-tocatalytic oxidation at a Ti02/Ti film electrode can be electrochemically promoted [37]. 

Catalytic activity of the aluminum-Schiff base system is dramatically enhanced by adding a bulky Lewis acid (Table 2). Inoue et al. reported that a combination of 3 with 2c led to over 1000 times acceleration in the polymerization of PO at room temperature compared with the polymerization in the absence of 2c.The resulting polymers have narrow MWDs, molecular weights close to those estimated, assuming that every molecule of 3 forms one polymer chain. The same accelerating effect of 2c is also demonstrated in the polymerization of PO by using aluminum-phthalocyanine and aluminum-tetraazaannulene complexes, 4 and 5, which exhibit very low catalytic activities without 2c. 

Manecke et al.16s synthesized a semiconducting polymeric complex which possessed both bis(ethylene-l,2-dithiolato)Cu(II) and a phthalocyanine-Cu(II)-type structure 54. This Cu complex exhibited high catalytic activity in the oxidative polymerization of XOH, about 50 times higher than that of pyridine-Cu. A synchronous four-electron-transfer mechanism was proposed for the catalysis of 54. The phthalo-cyanine-Cu(II) type structure of 54 is presumed to form a complex with molecular... 

Cobalt complexes of dendritic phthalocyanines (Fig. 6.37) showed a 20% lower catalytic activity (TON 339 min-1 for G2 dendrons) as catalysts for the oxidation of 2-mercaptoethanol than non-dendritic phthalocyanines [56]. By way of compensation, however, the dendritic catalysts proved to be more stable than non-dendritic ones, which is probably attributable to enclosure of the metallo-phthalocyanine core unit by the dendrons. This also prevents molecular aggregation of the phthalocyanines in polar solvents and thin films. 

An interesting effect is the catalytic activity of an iron phthalocyanine film, deposited on iron metal, which was shown to have enhanced activity compared with that of metal complex or metal alone 48). 

Hock and Kropf compared the catalytic activities of different metal complexes of phthalocyanine for the oxidation of cumene, and their results are recorded in Table 3. 

Interesting results have been obtained in studies of the catalytic activity for oxidation by phthalocyanine polymers, containing different metal ions in the same molecule 87-90>. If Fe was mixed with a series of other transition metal ions, differences in activity were found to be dependent on the metal ion, and correlations between the catalytic activity and the thermal activation energy of semiconductivity were found. With copper as the second metal ion, maximum activities were found at a ratio Fe/Cu = 1. Many other chelate polymers have been tested for their oxidation activity, and a dependence of the catalytic activity on the donor properties of the ligand was found 91>92). 

By complexing phthalocyanine or tetraphenylporphyrin molecules with different bivalent metal ions, their oxidation potential may be changed, and this also appeared to change their catalytic activity. In Fig. 3 this is shown graphically. 

Metal phthalocyanine complexes are also frequently used as autoxidation catalysts (see Section II.B.2). They have generally been found to be more active than the corresponding stearates or acetylacetonates. Thus, Uri145 compared the catalytic activity of a series of transition metal stearates with the corresponding metal phthalocyanines in the autoxidation of methyl linoleate. The phthalocyanine complexes afforded faster rates of oxidation. In addition, the phthalocyanine ligand is stable and is not easily destroyed under autoxidizing conditions. Interest in metal phthalocyanine catalysts has also been stimulated by their resemblance to the metal-porphyrin structures contained in many oxidative enzymes (see Sections II.B.2 and V). 

Catalytic Activity. It is perhaps surprising that the differences in activity for a wide range of [M(PC)] studied here are so small (approximately two-fold difference). It must be assumed that the catalytic activity is much more dependent on the phthalocyanine macrocycle than on the particular metal present. 

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