Results 1 - 4 of 4
Results 1 - 4 of 4. Search took: 0.012 seconds
|Sort by: date | relevance|
[en] Systems containing self-assembled monolayers (SAMs) of thiols on metallic and nonmetallic surfaces are attractive as they provide structurally well-defined surfaces with a controllable chemical functionality, in addition to their stability and stiffness. These special features have inspired several studies addressed to electron transport ability through the SAMs and to the influence of the chemical functionality of thiols. We studied the electrocatalytic activity for the oxidation of L-cysteine of gold electrodes modified with self-assembled monolayers of 4-mercaptopyridine (4MPy) and 4-aminothiophenol (4ATP) on Au(111) surfaces chemically functionalized with substituted and unsubstituted Fe(II)-phthalocyanines (FePc). Attention was focused on the study of the effect of the thiol-end-FePc on the electron transfer rate from L-cysteine to the gold–SAM–FePc assembly, as well as on the effect of Pc-ring substituents on the ET kinetics. It is found that the effect of substituents on the Pc-ring (using Hammett parameters) on the Fe(III)/(II) redox potential is weak when FePc molecules are confined directly on Au and with the 4MPy SAMs/FePc, but their more pronounced when FePc molecules are located on the outermost position of the SAM assembly with a 4ATP molecules
[en] CoN4 complexes like Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the oxygen reduction reaction (ORR) but they only promote the 2-electron reduction of O2 to give peroxide. In contrast, vitamin B12 a Co macrocyclic naturally occurring molecule has attracted the attention of the scientific community because instead of catalysing the 2-electron reduction of O2 to H2O2 like the other Co macrocycles it promotes the 4-electron reduction to H2O. Vitamin B12 possesses an axial back ligand and this seems to be the reason for its higher activity and selectivity for the 4-electron reduction of O2. To test this hypothesis, we synthetized a CoPc axially coordinated to pyridine anchored to carbon nano-tubes (CoPc-Py-CNT). The Co centre is therefore coordinated to 5 N as in vitamin B12. The modified CoPc containing catalytic material was characterized by EPR and XPS spectroscopy. Ab initio calculations, Koutecky– Levich extrapolation and Tafel plots well describe the similarities between the 2 complexes and reveal insights into the mechanism of action of Co penta-coordinated complexes. According to our results the pyridine back ligand increases the Co-O2 binding energy and making it more similar to that of Vitamin B12, favouring the splitting of the O-O bond. The back ligand then plays a crucial role in modifying Co-O2 binding energy which is a well know reactivity descriptor.
[en] Graphite electrodes modified with four different cobalt N4 macrocyclics, namely Co tetrapentapyridinophthalocyanine, (CoTPenPyrPc), Co tetrapyridinoporphyrazine (CoTPyPz), Co octa(hydroxyethylthio)phthalocyanine (CoOEHTPc) and Co tetranitrophthalocyanine (CoTNPc) exhibit catalytic activity for the oxidation of glucose in alkaline media. The purpose of this work is to establish correlations between the catalytic activity of these complexes and their redox potential. The activity of the different modified electrodes was tested by linear voltammetry under hydrodynamic conditions using the rotating disk technique. Tafel plots constructed from mass-transport corrected currents give slopes ranging from 0.080 to 0.160 V/decade for the different catalysts which suggests that a first one-electron step is rate controlling with the symmetry of the energy barrier depending on the nature of the ligand of the Co complex. A plot of log I versus the Co(II)/(I) formal potential gives a volcano curve that also includes catalysts studied previously. This illustrates the concept that the formal potential of the catalyst needs to be tuned to a certain value for achieving maximum activity. A theoretical interpretation of these results is given in terms of Langmuir isotherms for the adsorption of glucose on the Co sites of the surface-confined metal complexes