Electrochemical preferential oxidation

Zhang, J., Datta, R. (2005). Electrochemical preferential oxidation of CO in reformate. 

The electrochemical behavior of single-crystal (100) lead telluride, PbTe, has been studied in acetate buffer pH 4.9 or HCIO4 (pH 1.1) and KOH (pH 12.9) solutions by potentiodynamic techniques with an RRDE setup and compared to the properties of pure Pb and Te [203]. Preferential oxidation, reduction, growth, and dissolution processes were investigated. The composition of surface products was examined by XPS analysis. It was concluded that the use of electrochemical processes on PbTe for forming well-passivating or insulating surface layers is rather limited. 

Fig. 15. A chemically and electrochemically controllable molecular shuttle 104+. When the dumbbell is in its unperturbed state, the cyclophane exists in an 84 16 ratio (CD3CN, -44 °C) encircling preferentially the comparatively more 7t-electron-rich benzidine site. The cyclophane can be enticed to translate to the biphenol site exclusively either chemically -through protonation of the benzidine nitrogen atoms - or electrochemically - through oxidation of the benzidine unit to its radical cation. Both of these processes are completely reversible, through addition of base, or electrochemical reduction, respectively...

The counterion should also be stable both chemically and electrochemically otherwise, breakdown products may interfere in the polymerization process. If it is electroactive at potentials lower than the monomer oxidation potential, it can be incorporated using potentiostatic methods but not with constant-current techniques because the counterion will be preferentially oxidized. 

The reactions at the cathode will be similar to those when a sacrificial anode is employed. If the electrode is a noble metal or is electrochemically inert, oxidation of the environment will occur preferentially. For an aqueous environment the reaction could be ... 

One possibility is the electrocatalytic preferential oxidation (cPrOx) (Zhang Datta, 2005), which is simply the electrochemical version of PrOx. By exploiting the strong selective CO adsorption on the anode Pt catalyst (Eqn (3.22)), coupled with the fact that H2O can be activated by the anode catalyst at certain electrode potentials to produce surface hydroxyl radicals, especially on Ru cocatalyst, the cPiOx process involves the following chemistry ... 

Kahlich, M.J., Gasteiger, H.A., and Behm, R.J. (1998) Preferential oxidation of CO over Pt/p-Al203 and Au/a-Fe203 reactor design calculations and experimental results. J. New Mater. Electrochem. Syst., 1, 39—46. 

Arenz M, Stamenkovic V, Ross PN, Markovic NM. Preferential oxidation of carbon monoxide adsorbed on Pd submonolayer films deposited on Pt(l 0 0). Electrochem Commun 2003 5 809-13. 

CO methanation is an effective means of removing trace CO in a hydrogen steam. The process is technically simple because it does not require the introduction of any gases, such as air, as is required in CO preferential oxidation. The process is normally operated at temperatures from 180 to 280°C [47]. However, CO methanation is not suitable for an onboard application as compared to preferential oxidation of CO, which requires temperatures of only 80 to 180°C. In the CO preferential oxidation process, since air must be mixed with the hydrogen stream, its onboard application raises concerns with hydrogen safety issues. The unique feature of the CO electrochemical WGS reaction is that it can operate under... 

AlCu alloys containing a very reactive element and a seminoble metal of extremely different electrochemical properties, anodic oxidation causes the preferential oxidation of Al, which forms passivating aluminum oxide films whereas Cu remains at the metal surface. Although... 

Electrochemical oxidation of 4-aryl-substituted thiane in aqueous organic solvents containing various halide salts as electrolytes gave selectively the trans-sulfoxide (lOe). Under acidic conditions a preferential formation of the cis-sulfoxide was attained328. The stereoselective potential of this method for the oxidation of cyclic sulfides139,329 is apparent (equation 123). 

Solla-Gullon J, Solla-Gullon J, Vidal-Iglesias FJ, Herrero E, Feliu JM, Aldaz A. 2006. CO monolayer oxidation on semi-spherical and preferentially oriented (100) and (111) platinum nanoparticles. Electrochem Commun 8 189-194. 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

From this point of view, a brief comparison of acyloxylation of cis- or irany-stilbenes in electrochemical and chemical conditions is also relevant. Oxidation of cis- or irany-stilbene at the platinum anode in the presence of acetic or benzoic acid gives predominantly meyo-diacylates of hydroxy-benzoin or, if some water is present, t/treo-monoacylate. None of the stereoisomeric erythro-mono-acylate and rac-diacylate were obtained in either case. There was no evidence of isomerization of cis- to trany-stilbene nnder the electrolytic conditions employed (Mango and Bonner 1964, Koyama et al. 1969). The sequence of reaction steps in Scheme 2.27 was proposed. Adsorption-controlled one-electron oxidation of the snbstrate takes place. Then the cis-stilbene cation-radical interact with acetate to form an oxonium ion. The phenyl groups in the oxoninm adopt the trans mntnal disposition which is thermodynamically preferential. The trany-benzoxoninm ion is the common intermediate for conversions of both cis- and trany-stilbenes and, of conrse, for all the final prodncts (Scheme... 

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