Investigations on the adsorption and electrocatalytic oxidation of carbon monoxide on gold + palladium alloys

A bifunctional mechanism of electrocatalysis was found for the CO-oxidation at Au + Pd alloys. The synergetic effect works best at the 20% Au alloy. The different behaviour of An + Pd and Ag + Pd alloys was assigned to the formation of CO-induced ensembles of Pd-atoms at Au + Pd alloys with high gold content, which is possible due to a high mobility of Pd-atoms at the surface. As CO could be shown to react preferably with the Au-oxide species, the course of the energy of activation for the gas phase reaction might be understood on the-basis of geometric effects.     

Electrochemical absorption and oxidation of hydrogen on palladium alloys with platinum, gold and rhodium

Thin layers of Pd and its alloys with Pt, Au and Rh were prepared by electrodeposition on a Au substrate. Hydrogen electrosorption by the obtained electrodes was studied in 0.5 M H(2)SO(4) solution using cyclic voltammetry and chronoamperometry. The influence of the alloying process on selected thermodynamic (the amount of absorbed hydrogen, the stability of the β-phase, the extent of the absorption/desorption hysteresis) and kinetic aspects (the rate of hydrogen absorption and absorbed hydrogen oxidation) of hydrogen absorption and desorption was examined. It was found that the addition of the non-absorbing elements to Pd results in faster kinetics of the hydrogen electrosorption process and a smaller absorption/desorption hysteresis.     

The oxidation of formic acid at noble metal electrodes Part 4. Platinum + palladium alloys

Formic acid oxidation has been studied on Pt + Pd alloys of known surface composition using cyclic voltammetry. The results are interpreted in terms of a strongly bound (poisoning) intermediate which adsorbs only on the Pt atoms and a direct oxidation path which involves an intermediate occupying a single site and a second intermediate occupying two sites. Steady state oxidation currents on the alloys at potentials in the range of about 0.4 to 0.9 V are larger than expected from a linear combination of the currents on the pure metals. This effect may be due to the modification of metal properties at the junction between two types of metallic atoms.     

Chemisorption and oxidation of carbon monoxide on palladium alloys

Kinetic and adsorption data (programmed thermal desorption in situ) for the oxidation of carbon monoxide on palladium alloys with silver and gold have been discussed. It has been shown that the local reaction plays a determining role and that active cluster sites are best for catalysis which are monoatomic with respect to palladium, weakly binding the chemisorbed CO molecule. The role of silver is to activate the second component: oxygen. The cluster model makes it possible to predict the optimum composition of Pd-Ag catalysts for the oxidation of CO and to explain the dependence of Pd-Au activity on the composition.Kiev University. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 5, pg. 574–578 September–October, 1991. Original article submitted July 12, 1991.     

The determination of gold by anodic stripping voltammetry

A method is described for the routine determination of gold as its chloride or cyanide complex by anodic stripping voltammetry at a glassy carbon electrode coupled to a microprocessor-controlled voltammeter. The preferred supporting electrolyte is 0.1 M HCl/0.32 M HNO₃, with plating at ?200 mV or ?1200 mV (vs. Ag/AgCl). The stripping peak potentials range from 830 to 1150 mV (vs. Ag/AgCl) depending on concentration and plating time. Precision (percent relative standard deviation) is better than 5 % for a range of concentrations between 5 μg l^?1 and 1000 μg l^?1. The detection limit is about 5 μg l^?1 for a 5-min plating period. Interferences from Cu, Hg, Ag and other electroactive species are overcome by preliminary extraction with diethyl ether.     

Organocatalyzed anodic oxidation of aldehydes

A method for the catalytic formation of electroauxiliaries and subsequent anodic oxidation has been developed. The process interfaces N-heterocyclic carbene-based organocatalysis with electro-organic synthesis to achieve direct oxidation of catalytically generated electroactive intermediates. We demonstrate the applicability of this method as a one-pot conversion of aldehydes to esters for a broad range of aldehyde and alcohol substrates. Furthermore, the anodic oxidation reactions are very clean, producing only H(2) gas as a result of cathodic reduction.     

Dithizonates of gold and palladium

Contrary to some previous reports, it has been found that gold reacts with dithizone in the ratio 1:1 forming a red-brown complex, and in the ratio 2:1 forming a yellow complex, both extractable into chloroform. The proportion of these two complexes in the organic extract depends critically on several factors. Therefore, the use of dithizone as a spectrophotometric reagent and especially as an extractive titrant for gold cannot be recommended. Previous reports on the composition and colour of the dithizonates of palladium are confirmed, except that PdDz is soluble in carbon tetrachloride and in chloroform.     

The anodic oxidation of 2,4,5-triarylimidazoles in aprotic medium

The anodic oxidation of 2-(p-dimethylaminophenyl)-4,5-diphenylimidazole and 2,4,5-tris-p-methoxyphenylimidazole in benzonitrile solutions has been studied, using rotating disc electrode, rotating double ring electrode, and coulometry as electrochemical techniques. ESR and UV-vis. spectroscopy were used for product analysis. The triarylimidazoles exhibit two or more oxidation waves. The first one is interpreted as a non-conventional e.c.e. mechanism where the parent molecule is involved in the chemical step. Further anodic waves are due to the oxidation of protonated triarylimidazole, produced in the e.c.e. mechanism.     

Efficient chemoselective alcohol oxidation using oxygen as oxidant. Superior performance of gold over palladium catalysts

Gold nanoparticles supported on nanocrystalline ceria (Au/CeO₂) is a general, air- and moisture-stable, commercial catalyst for the atmospheric pressure, solventless oxidation of aromatic, primary and secondary alcohols to the corresponding benzaldehyde or ketone compound. Aliphatic primary alcohols are oxidized to the corresponding alkyl ester and aliphatic secondary alcohols are oxidized to ketones. Conversions and product yields are in most of the cases excellent. The oxidizing reagent and the experimental conditions are almost ideal from the environmental point of view. Comparison with analogous ceria supported and hydroxyapatite-supported palladium catalysts, Au/CeO₂ clearly shows the superior performance of Au/CeO₂ in terms of higher chemoselectivity. In contrast to palladium catalysts that promote CC double isomerization, Au/CeO₂ oxidizes selectively allylic alcohols to conjugated ketones.     

The anodic behavior of gold in phosphoric acid at elevated temperatures

Summary The anodic dissolution of gold in concentrated phosphoric acid was investigated at 145–200°C. Gold dissolves in a narrow potential region (150 mV) about 300 mV positive to the oxygen potential in the same electrolyte. At higher potentials it passivates. At very high overvoltages dissolution, which increased rapidly with temperature, was also observed in the transpassive region. The product of the dissolution is finely divided gold, probably formed by immediate decomposition of the oxydation product. At 200°C, at which temperature pyrophosphoric acid is formed, the gold dissolution also yields a clear yellow solution of gold pyrophosphate

---

Zusammenfassung Die anodische Auflösung von Gold in konzentrierter Phosphorsäure wurde im Temperaturbereich von 145–200° C untersucht. Gold geht in Lösung in einem engen Potentialintervall (150 mV) etwa 300 mV positiver als das Sauerstoffpotential im gleichen Elektrolyten. Bei höheren Potentialen wird es passiv. Bei sehr hohen Überspannungen wurde auch im transpassiven Bereich Auflösung beobachtet, deren Geschwindigkeit mit der Temperatur stark ansteigt. Bei der Auflösung entsteht fein verteiltes Gold, vermutlich durch unmittelbar folgende Zersetzung des Oxydationsproduktes. Bei 200° C, wo Pyrophosphorsäure gebildet wird, ergibt die Goldauflösung auch eine klare gelbe Lösung von Goldpyrophosphat.

---

---

Dedicated to Prof. Dr. M. v. Stackelberg on his 70th birthday.     

Rotating-disc electrode studies of the anodic oxidation of iodide in concentrated iodine + iodide solutions

The anodic oxidation of iodide on platinum in concentrated iodine + iodide solutions has been investigated using a rotating disc electrode. The conventional limiting diffusion current, which is produced by the diffusion of iodide ions towards the electrode, was not observed due to the formation of an iodine film on the electrode. On the other hand, the steady-state anodic current after a current/time transient is the genuine limiting diffusion current in the anodic oxidation due to diffusion of iodine species from the electrode surface towards the bulk solution. Thus, the dissolution-diffusion control mechanism of the iodine film is confirmed. This is interesting as a typical example of an anodic process in a redox system governed by diffusion of the anodic product species from the electrode surface towards the bulk solution. When an iodine film is formed on the electrode, the maximum driving force of the iodine species is Δm_I2,max, which is defined as the extent of unsaturation of the iodine, and the limiting current of the anodic oxidation of iodide is always directly proportional to Δm_I2,max, regardless of the forms of iodine species in the solution, which may be I₂, I⁻₃, i₅⁻, etc. δm_I2,max is clearly determined by the solution composition and temperature, and it is different in definition and value from the usual degree of unsaturation of iodine.     

The negative adsorption of chromium atoms on alloy-oxide film boundaries during oxidation in air and anodic passivation of Fe-Cr and Ni-Cr alloys

Equilibrium of Cr atoms between the surface layer and bulk of a binary alloy was analyzed. The Gibbs adsorption equation was used to obtain the dependence of the adsorption activity of atoms in the surface layer on their activity in the bulk. An approximate thermodynamic method was used to calculate the adsorption of Fe (Ni) and Cr atoms in the surface layers of Fe-Cr and Ni-Cr alloys. According to calculations, there was negative adsorption, X _Cr ≪ 1, in the surface layer of the alloys caused by a large difference between the Gibbs surface energies of Cr and Fe (or Ni). The negative adsorption of Cr shifted chemical reaction equilibria on the alloy-oxide film boundary both in oxidation in air and in anodic passivation, 3FeO (NiO) + 2Cr = Cr₂O₃ + 3Fe(Ni), toward oxide film enrichment in the FeO (or NiO) oxide. A unified method for calculating the composition of oxide films on alloys was used for both processes. The method was based on the use of the initial data on the Gibbs surface energy of metals constituting alloys. The calculated oxide film compositions were close to the experimental X-ray photoelectron spectroscopy data.     

The role of elastic scattering analysis in studies of anodic oxidation

Résumé L'analyse par diffusion élastique peut détecter de microquantités à condition qu'elles soient concentrées dans des films minces. En outre, la méthode peut directement fournir des indications sur l'emplacement des éléments. Ces caractéristiques ont été mises à profit pour apporter un instrument utile à l'étude des processus d'oxydation.