Platinum complexes anodic oxidation

K.-I. Machida, A. Fukuoka, M. Ichikawa, M. Enyo, Preparation of platinum cluster-derived electrodes from metal carbonyl complexes and their electrocatalytic properties for anodic oxidation of methanol. J. Electrochem. Soc. 1991, 138(7) 1958-1965. 

Studies by cyclic voltammetry of anodic oxidation of organotin compounds at a platinum electrode in acetonitrile show that the primary irreversible process is the outer sphere oxidation of R4Sn to the radical cation, followed by rapid fragmentation into R3Sn+ and R which is then rapidly oxidised further to R+. The rate constants of the reactions correlate with those for the oxidation by Fe(III) complexes. Values for the oxidation potentials and the ionisation energies are given in Table 5-3. 

The electroactive labels most used in genosensing design are ferrocene and its derivates [24-27] (the reversible oxidation process of ferrocene can be detected by means of several electrochemical techniques), osmium complexes [28], platinum complexes [29], gold complexes [30, 31], and metallic [32-36] or semiconductor nanoparticles [37]. Among the last ones, gold nanoparticles are the most used, their detection can be carried out by means of the measurement of resistance or capacitance changes, usually after an amplification procedure with silver, or by means of the anodic stripping voltammetry of Au(lll) obtained after the nanoparticle oxidation Fig. 9.3. 

R. TarozaitS, L. Tamasauskaite TamaSiunaite, V. Jasulaitien4, Platinum-tin complexes as catalysts for the anodic oxidation of borohydride, J. Solid State Electrochem. 13 (2009) 721-731. 

Investigation of the anodic oxidation of organic substances involves certain experimental difficulties due, on the one hand, to the complex multistage nature of the reactions and, on the other, to the special characteristics of the surface state in platinum metals. This makes it necessary to employ a range of experimental methods to determine the mechanism of the anodic processes. 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

The solution should be free from the following, which either interfere or lead to an unsatisfactory deposit silver, mercury, bismuth, selenium, tellurium, arsenic, antimony, tin, molybdenum, gold and the platinum metals, thiocyanate, chloride, oxidising agents such as oxides of nitrogen, or excessive amounts of iron(III), nitrate or nitric acid. Chloride ion is avoided because Cu( I) is stabilised as a chloro-complex and remains in solution to be re-oxidised at the anode unless hydrazinium chloride is added as depolariser. 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

The character of the oxide layers influences the kinetics and mechanism of the electrochemical reactions occurring on the platinum anode surface. The relation between the rate of oxygen evolution and oxide layer thickness is complex. In the region where the a-oxides exist, the reaction rate decreases with increasing oxide layer thickness. In the region where the P-oxides exist, the reaction rate depends little on oxide layer thickness or, according to some data, increases with increasing oxide layer thickness. 

The products of electrochemical oxidation of conjugated dienes are considerably affected by the reaction conditions such as the material of the electrode, the supporting electrolyte and the solvent. The oxidation of butadiene with a graphite or carbon-cloth anode in 0.5 M methanolic solution of NaClCU mainly yields dimerized products along with small amounts of monomeric and trimeric compounds (equation 5)1. The use of platinum or glassy carbon mainly gives monomeric products. Other dienes such as isoprene, 1,3-cyclohexadiene, 2,4-hexadiene, 1,3-pentadiene and 2,3-dimethyl-l,3-butadiene yield complex mixtures of isomers of monomeric, dimeric and trimeric compounds, in which the dimeric products are the main products. 

Butadienes give a complex mixture of methoxylated products by electrochemical oxidation in methanol with sodium perchlorate as supporting electrolyte [44]. Dimethoxybutenes are formed together with dimers from reaction of medioxybu-tenyl radicals. A platinum anode gives the highest yields of monomeric products while graphite anodes yield only dimeric products. This is a distinction from the... 

The hydroxyalkyl- and aminoalkylpyridines are also almost totally unexplored. The oxidation of various hydroxyalkylpyridines was studied by voltammetry at a platinum anode.122 The ring substituents had no effect on the position of the discharge wave. Oxidation of the metal-complexed amine 147 to the imine 148 was quantitative.209... 

An additional interpretation issue involves the presence of oxidation reactions that are not metal dissolution. Figure 28 shows polarization curves generated for platinum and iron in an alkaline sulfide solution (21). The platinum data show the electrochemistry of the solution species sulfide is oxidized above -0.8 V(SCE). Sulfide is also oxidized on the iron surface, its oxidation dominating the anodic current density on iron above a potential of approximately -0.7 V(SCE). Without the data from the platinum polarization scan, the increase in current on the iron could be mistakenly interpreted as increased iron dissolution. The more complex the solution in which the corrosion occurs, the more likely that it contains one or more electroactive species. Polarization scans on platinum can be invaluable in this regard. 

Bryce and co-workers reported that the crown-annulated TTF derivatives 98 and 99 were used for UV-Vis spectroscopic and electrochemical studies of metal complexation <1996J(P2)1587>. Solution electrochemical studies showed that metal complexation to the crown unit leads to a small anodic shift in the first oxidation potential of the TTF system. Langmuir-Blodgett films of amphiphilic 99 have been assembled on solid substrates by Y-type deposition. Compounds 100-104 were used to prepare self-assembled monolayers on gold and platinum surfaces <1998AM395, 2000JOC8269>. The self-assembled monolayers of 104 were the most stable in this series of TTF-crowns. Electrochemical data for the self-assembled monolayers of 100-104 in MeCN showed two reversible one-electron waves, typical of the TTF system. The self-assembled monolayers of 102-104 exhibited an electrochemical response in aqueous electrolyes, which was observed between 50 and 100 cycles. 

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