B. Kastening

Light-induced generation of hydrogen at CdS-monograin membranes

Abstract A new kind of membrane consisting of small particles of single-crystalline CdS embedded in a polymer film is described. In various photoelectrolysis experiments the properties of the membrane which separates a cell in two compartments were investigated. Relatively large quantities of H 2 were produced in the presence of an electron donor such as S 2− ions or EDTA and small quantities by direct photoelectrolysis of H 2 O. This technique gives information on the role of catalysts deposited on one or both faces of the membrane.

Elektrochemische bildung, reaktivität und eigenschaften des nitrobenzol-radikalanions☆

A special electrolysis vessel was constructed, the cathode of which rotates rapidly and possesses a large area, thus exhibiting a half-time for potentiostatic electrolysis below one minute. Primary products of electrolysis with relatively short life times can be produced in this vessel with concentrations allowing the study of their properties (e.g. light absorption) and reactivity. The anion radical C6H5—2−, formed by the primary one-electron reduction of nitrobenzene, was so produced and studied. This radical, exhibiting considerable stability in some non-aqueous solvents, dismutates in aqueous solutions by two reactions of first- and second-order kind respectively, producing nitrobenzene and phenylhydroxylamine. The reaction rates have been determined and a mechanism is proposed. The life time of the anion radical is greatest in alkaline solutions. The solutions of the anion radical are yellow, the absorption curve showing five peaks in the range 390–450 mμ. The results are supplied by polarographic studies. The rapid dismutation of the anion radical in solutions of pH < 10 and the consecutive rebuilding of nitrobenzene increases the height of the polarographic one-electron wave of nitrobenzene. The height of this wave is also increased if certain oxidizing agents are present, by which the anion radical is reoxidized; thus nitrobenzene acts as a catalyst for the reduction of these compounds. The reaction rates between them and the anion radical have been determined.

The double layer of activated carbon electrodes: Part 1. The contribution of ions in the pores

Abstract The changes produced in the electrolyte concentration upon introducing well-defined activated carbon particles to the solution and upon polarizing the carbon material have been studied in order to obtain quantitative information about the double layer. Immobilization of a certain amount of water, corresponding to a monomolecular layer on the pore surface, results in an increase in the external electrolyte concentration in the case of KF and K 2 SO 4 , while the adsorption of anions (Cl − , Br − , NO 3 − , I − , ClO 4 − , SCN − ) or cations (H + , tetra-alkyl ammonium ions) produces a substantial accumulation of electrolyte in the pores. Owing to the narrowness of the pores, there is a membrane potential at the pore openings instead of a Gouy-Chapman layer near the phase boundary. In the presence of adsorbed anions, the charge on the polarized carbon material is balanced by a variation of the amount adsorbed, with the cation concentration in the pores remaining almost constant. In the variation of which is balanced by the SO 4 2− ion. This behaviour suggests the formation of COH groups on adsorption of H + at CO surface groups.

FUNDAMENTAL PROBLEMS OF WATER SPLITTING AT CADMIUM SULFIDE

Abstract Oxygen is directly involved in the anodic photocorrosion of CdS leading to sulfate formation according to the overall reaction CdS + 4h + + 2H 2 O + O 2 → Cd 2+ + SO 2− 4 + 4H + . Sulfate formation leads to cleaning of the CdS surface from elemental sulfur and therefore improves the electrode behaviour dramatically. Removal of sulfur from the surface shifts the flatband potential of CdS to about −1.7 V(SCE) and the valence-band edge to −0.6 V(SCE). Accordingly holes lose their thermodynamic capability to oxidise water.

Electronic properties and double layer of activated carbon

Abstract According to the ion exchange, between the electrolyte in the pores and the external electrolyte, as well as to the conductivity of the solid material, the large charge capacity of the activated carbon under investigation does not arise from redox properties of surface groups but from mobile charge carriers in the solid. The dependence of the conductivity (minimum value 1000 S m −1 ) and of the thermopower on the electrode potential and on the temperature shows that the mechanism of the electronic conduction resembles neither that of metals nor of semiconductors. It is characterized as “hopping” between electronically coherent domains and resembles the process in conducting polymers. According to e.s.r. measurements, the spin density is low and does not change markedly with the potential. The prevalence of paired charges is proposed as a hypothetic explanation.

Die bedeutung der adsorption in der polarographie

Zusammenfassung In der vorliegenden Arbeit werden die wichtigsten Phanomene diskutiert, die als Folge der Adsorption an der Phasengrenze Metall/Elektrolytlosung bei polarographischen Untersuchungen in Erscheinung treten.

Protonen- und Elektronen-transfer an inhibitorbedeckten Elektroden

Zusammenfassung Chemische Oberflachenreaktionen, wie sie vor allem in der heterogenen Katalyse massgeblich sind, spielen neben Ladungsubergangen auch im Ablauf von Elektrodenprozessen eine wichtige Rolle; besondere Bedeutung kommt dem heterogenen Protonentransfer zu. Untersuchungen mit aromatischen Carbonyl- und Nitroverbindungen sowie mit Sauerstoff zeigen, dass ein Protonentransfer ebenso wie der Ladungsaustausch mit der Elektrode auch dann noch stattfinden kann, wenn die Elektrode nahezu vollstandig mit Inhibitormolekulen belegt ist. Fur die Geschwindigkeit des Protonen- ebenso wie des Elektronenubergangs ist dabei die neben dem Inhibitor in der Adsorptionsschicht noch vorhandene Grenzflachenkonzentration des reagierenden Stoffes massgeblich. Diese Grenzflachenkonzentration steht uber eine fruher abgeleitete Beziehung mit der Losungskonzentration des Inhibitors im Zusammenhang, wenn die Vorgange an homogenen Oberflachen (Quecksilber) ablaufen. Polarographische Untersuchungen mit Nitroverbindungen zeigen, dass das Produkt der primaren Elektrodenreaktion (das Radikalanion) zwei verschiedene Orientierungen in der Adsorptionsschicht einnehmen kann, wobei die eine (mit dem Ring parallel zur Grenzflache) den Elektronenubergang von der Elektrode, die andere (senkrecht zur Grenzflache, mit der Gruppe −NO 2 − zur Losung) den Protonentransfer aus der Losung begunstigt.

Electrochemical polarization of activated carbon and graphite powder suspensions: Part II. Exchange of ions between electrolyte and pores

Abstract Potentials of zero charge have been determined for activated carbon suspensions in dilute aqueous solutions of acids (H2SO4, HCl, HClO4) from the distinct minima on the capacity vs. potential curves at sufficiently low electrolyte concentrations; these are about +0.1 V vs. SHE. Upon polarizing the suspension from the potential of zero charge towards negative or positive potentials, the corresponding charge of the solid is counterbalanced by protons and anions which leave the free electrolyte and enter the electrolyte in the pores, or vice versa. The amounts of protons and anions, respectively, have been determined as a function of the charge. At negative charges, a limited amount of anions leaves the pores, constituting the entire content of anions of at least the greater part (0.8 cm3/g) of the whole pore volume accessible to ions. A corresponding, though less distinct, limit applies to protons at positive charges in the case of H2SO4. When the limit is approached, all the charge is balanced by the counterion. Unlike the case of H2SO4, where a normal electrolytic double layer is predominant and surface redox compounds contribute to the ion exchange only at higher positive charges, the behaviour observed with HCl indicates the participation of such compounds over the whole range of positive charges, which also seem to be the case with HClO4.

The double layer of activated carbon electrodes part 2. Charge carriers in the solid material

Abstract Considerable differences in the electronic conductivity of activated carbon when conditions are varied can be accounted for formally by treating the material as an intrinsic semiconductor with a narrow bandgap. According to the temperature dependence of the resistance, the bandgap is 0.034 eV. The minimum conductivity of the material investigated was 103 S m−1. Upon exposing the formerly dry material to a humid atmosphere or dipping it in water or an electrolyte solution, the conductivity increases by 15%–20%. This effect can be attributed to a H2O monolayer at the pore surface, with the system H2O + CO surface group acting as an acceptor and creating holes. Upon shifting the electrode potential from the immersion potential (potential of zero charge), the conductivity changes considerably because, owing to the thinness of the solid walls between the pores, the space-charge layer extends throughout the solid material. Unusual effects are observed in the presence of H+ or OH− ions; they are attributed to interactions of the adsorbed ions with the CO surface groups. The large capacity of the material is due to the mobile charge carriers but not to the surface groups.

Design of a slurry electrode reactor system

A ’slurry electrode‘ is a suspension of particles with large double-layer capacity, such as activated carbon, in an electrolyte solution. These particles transfer charge from an electrochemical cell to an external reactor, where a substance is oxidized or reduced, and are recharged in the cell. The circulated particles must be separated from the solution of the reaction product. Various aspects of a system consisting of an electrochemical cell, a reactor, and a sedimentation vessel for separation are discussed.For the reactor the following aspects are considered: rate of influx, residence time in the reactor, size of the reactor, concentration of particles in the suspension, rate and degree of conversion, flow rate of particles and single or cascade reactors.The transport of charge by the particles determines the current flowing in the electrochemical cell as well as the rate of conversion, or, for a given degree of conversion, the maximum initial concentration. Specific, and yet unsolved, problems of the cell are examined. These result from the contact between the particles and the feeder electrode, of which either the duration is too short or the resistance is too great.Possible application of a slurry electrode working in steady state conditions is considered.