Vladislava M. Jovanović

Glassy carbon electrodes. I. Characterization and electrochemical activation

Abstract Electrochemical properties of glassy carbon electrodes of two types were examined, one thermally treated at 1000°C (sample K) and another thermally treated at 2500° (sample G). Mechanically polished or electrochemically polarized electrodes were characterized in NaOH, HClO4 and H2SO4 solutions by cyclic voltammetry (cv) at different sweep rates in the potential range between the hydrogen and oxygen evolution. The activity of the electrodes depended on the properties of the glassy carbon examined, as determined by both the temperature of thermal treatment and the mechanical or electrochemical pretreatment of the sample. It was noticed that both types of electrodes, when polished exhibited an increase in the double layer charge upon increasing the pH value of the solution. The cv charges, for both types of samples, increase upon anodic polarization. The higher the potential of oxidation, the more pronounced is the increase in charge, particularly in acidic solution. The increase in charge amounts from below 1 mC cm−2 for polished glassy carbon up to few hundreds of mC cm−2 for surfaces anodically polarized in acidic solution. Analysis of the dependence of voltammetric charge, as well as morphological changes of the electrode surface, on the time of oxidation suggests the existence of three stages in the electrochemical activation process. The first one occurs only once at the beginning of the activation, while the other two repeat themselves, reflecting a periodical activation and deactivation process. These stages were discussed and ascribed to a surface layer oxidation, graphite oxide layer growth and mechanical destruction of the surface. Independent surface analysis by AES, XPS and STM confirms the results obtained by electrochemical methods.

The electrocatalytic properties of the oxides of noble metals in the electro-oxidation of some organic molecules☆

The electro-oxidation of organic compounds on gold and silver electrodes can occur either in a potential range more negative than that where the metal oxides are formed or within the more positive range where the surface is covered by oxides. Oxidation of methanol on gold electrodes proceeds in the potential range where the electrode is not covered with oxide and only at the slowest scan speed used (0.1 mVs−1). At the same scan rate the Au(110) orientation as well as the Au(111) orientation favor the catalytic activation of the anodic electro-oxidation of methanol in the region of oxide formation. Oxidation of malic acid on a gold electrode proceeds only in the region where the electrode is covered by gold oxide. On a glassy carbon electrode modified by silver, reactivation of the formaldehyde anodic oxidation is observed in the region where the electrode is covered by silver oxide. On the bulk silver electrode and on the glassy carbon electrode modified by silver, alcohols such as methanol, ethylene glycol and glycerol are oxidized only in the region where the electrodes are covered by silver oxide. On such electrodes the observed anodic currents increase in the sequence: methanol < methylene glycol < glycerol, and in the same sequence the corresponding peaks occur at more negative pot

Glassy carbon electrodes: II. Modification by immersion in AgNO3

Abstract The modification of glassy carbon by immersion in AgNO 3 solution was studied by cyclic voltammetry, vacuum techniques (AES and XPS) and STM. The influence of Ag + concentration, time of immersion, the presence of oxygen in AgNO 3 solution and previous thermal treatment of the glassy carbon were examined on ‘as received’ (untreated), polished and electrochemically treated samples. The results show that glassy carbon electrode immersed in AgNO 3 solution is modified by silver, which is deposited in its elemental state on the surface and near surface layers of the material. Cyclic voltammograms of silver-modified glassy carbon electrodes are similar to those of a silver electrode. The quantity of silver deposited on glassy carbon strongly depends on the pretreatment of the material before its immersion. The results obtained suggest that functional groups participate in the reduction of Ag + , and thus in the electrode modification, as active centers. Metal particles are not uniformly deposited on the surface and the deposition is 3-dimensional with deposits of laminar structure. A mechanism for the modification process is proposed.

The roles of the ruthenium concentration profile, the stabilizing component and the substrate on the stability of oxide coatings

Abstract Electrocatalytic oxide coatings with variable concentration profiles of RuO2 as the active component were obtained through a combination of separately applied layers of RuO2, TiO2, IrO2, RuO2 + TiO2 and RuO2 + IrO2 on titanium and glassy carbon substrates. The stability of the samples was examined by accelerated tests performed at high anodic current densities. Electrochemical techniques, cyclic voltammetry for assessing the charge associated with the coating, polarization measurements for assessing the electrocatalytic activity, and Auger electron spectroscopy to register surface composition of the coatings, were applied to follow changes due to the stability experiments. The stability and the charge depended strongly on the sequence of layers in the RuO2−TiO2 coating, with the samples having the RuO2 + TiO2 layer facing the electrolyte exhibiting the highest values for both properties. In contrast to this, the stability of the RuO2−IrO2 coatings, besides being lower than the stability of RuO2−TiO2 coatings, showed no dependence on the sequence of the applied layers. Much lower stability was exhibited by the coatings applied on glassy carbon rather than on titanium. A mechanism of the stability of the coatings based on the interaction of lower than four valency state titanium with higher than four valency ruthenium, proposed for single-crystal surfaces, is corroborated. Finally, during the thermal treatment a diffusion of titanium originating in the titanium substrate through the coating was established.

SYNTHESIS, CHARACTERIZATION, STRUCTURE AND POSSIBLE CATALYTIC PROPERTIES OF CIS-OXALATO(1,4,8,11-TETRAAZACYCLOTETRADECANE)COBALT(III) NITRATE

Abstract A new complex of cobalt(III) with 1,4,8,11-tetraazacyclotetradecane (cyclam) and oxalato ion as a bidentate ligand was prepared and characterized by elemental analysis, IR, electronic and 1H NMR spectroscopy and cyclic voltammetry. X-Ray analysis has shown that this compound crystallizes in the orthorhombic system, space group Pccn, with a = 8.583(1), b = 12.854(2), c = 14.944(1)A, V = 1649.3(4) A3, Z = 4, R = 0.512, Rw = 0.545, and has a crystallographic two-fold rotation axis. The complex was identified as cis-oxalato(1,4,8,11-tetraazacyclotetradecane)cobalt(III) nitrate, [Co(ox)cyclam]NO3 (oxH2 = oxalic acid), and it can be described in terms of a cis octahedral geometry with a folded cyclam configuration around the cobalt atom with the oxalato ion occupying the remaining two sites. Cyclic voltammetric data suggest a large stability for this compound, as well as its possible catalytic effect on electrochemical CO2 reduction.

Anodic oxidation of small organic molecules on silver modified glassy carbon electrodes

Small organic molecules (HCOOH, C,H,(OH),, CH,OH, CH,O, etc.) have become more interesting with the possibility of their application in fuel cells. Therefore, the anodic oxidation of such molecules has been studied on a number of noble metals [1,2]. The present paper is intended to expand these investigations by introducing glassy carbon as an electrode material and/or a support to enhance the catalytic activity of silver deposited on its surface. Glassy carbon (GC) has been widely used in electrochemistry due to its chemical inertness, good conductivity, rather small porosity and large electrochemical window [31 that allow examination and assessment of a number of reactions over a wide range of potentials. If appropriately treated, glassy carbon can be used either as an electrocatalyst [4,5] or as a substrate for other catalysts [6]. According to Jenkins and Kawamura [7], glassy carbon consists of randomly oriented ribbon molecules tangled in an intricate way. Several investigations [&lo] have demonstrated the presence of different oxygen-containing functional groups on the surface of this material. A glassy carbon surface can be easily modified by metals simply by exposing it to solutions with the corresponding metal ions 1111. In prolonged contact with AgNO, solution, GC acquires a large percentage of zero valency silver on the surface and in the near surface region [11,12]. In general, the aim of this work has been to attain a simple and coherent procedure for achieving an active and stable, silver-modified glassy carbon electrodes for the anodic oxidation of small organic molecules. In this note we report the effects of silver modified GC on the oxidation of HCOOH, C,H,(OH),, CH,OH, and CH,O in alkaline solution. As observed by cyclic voltammetry,

Synthesis, Crystal Structure, Magnetic Properties and Cyclic Voltammetry of the Unsymmetric (μt-Oxalato)-[N, N′, N″, N″′-Tetrakis-(2-Pyrii)Ylmethyl)-1, 4, 8, 11-Tetraazacyclotetfw-Decane]Dicobalt(II) Pekchlokate Trihydkate

Abstract A new dinuclear cobalt(II) complex, [Co2(ox)tpmc](ClO4)2 3H2O, (tpmc = N, N′, N″, N″′-tetrakis(2-pyridylmethyl)-1, 4, 8, 11-tetraazacyclotetradecane, ox2 -oxalate ion), has been synthesized and its crystal structure solved by single crystal X-ray diffraction studies. It crystallizes in the monoclinic system, space group P21, with a = 97558(8) A, b = 21.455(2) A, c - 21.241(2) A, β=100.069(8)°, V=4391.0(8) A3, Z =4,R = 0.083, Rw = 0.086. Its structure consists of two dinuclear, crystallographically unique molecules in which the oxalate ion bridges two cobalt(II) ions unsymmetrically. E:ach cobalt is hexa-coordinated with four macrocyclic nitrogens and two oxygens in an octahedral arrangement of an “em” coordination of a macrocyclic ligand. The third oxygen is simultaneously bonded to both cobalt ions and the last one remains uncoordinated, which is the unique way of oxalato ion coordination. Intramolecular antiferromagnetic coupling between the two cobalt(II) ions is observed and yielded J = -9.3 ...

Structural Effect in Electrocatalysis: Formic Acid Oxidation on Pt Electrodeposited on Glassy Carbon Support

The structural effect of Pt nanoparticles on formic acid oxidation was studied using Pt electrochemically deposited on glassy carbon as a model system. The morphology of Pt deposited on glassy carbon is defined by agglomerates whose number, size, and distribution depend on Pt loading and support pretreatment as revealed by atomic force microscopy characterization. A scanning tunneling microscopy analysis of the electrodes showed that an increase of Pt loading leads to an increase of Pt particles size and their coalescences. Electrochemical treatment of the support prior to Pt deposition results in a decrease of the particle size on acidic treated support and in their negligible change on alkaline treated support. The coalescences of the particles detected cause the formation of different defects. The most active are the electrodes with the smallest Pt loading, and with the support treated in acid having the lowest defected surface and the highest contribution of high coordinated (111) facets exposed to the reaction. The activity of the electrode decreases as the number of defects grows with increasing of the loading or with alkaline pretreatment of support, i.e., coalescence of the particles. The results obtained suggest that the ratio between the facets and the defect sites rather than particle size determines the rate of the formic acid oxidation.

Electrochemical Examination of Mixed-Ligand Cobalt(III) Complexes with Tetraazamacrocyclic Ligand and Heterocyclic Dithiocarbamates

Electrochemical stability of eight complexes of the general formula [CoIIIRdtc(1–8)cyclam](ClO4)2, where cyclam=1,4,8,11-tetraazacyclotetradecane and Rdtc− (1–8)=4-morpholine (Morphdtc), 4-thiomorpholine (Timdtc), 4-piperazine (Pzdtc), N-methyl piperazine (N-Mepzdtc), piperidine (Pipdtc), 2-, 3- or 4-methylpiperidine (2-, 3- or 4-Mepipdtc) dithiocarbamates, respectively, were studied. The substances were examined in aqueous NaClO4 solution and nonaqueous LiClO4 in CH3CN solution by cyclic voltammetry. In aqueous solution, macrocyclic ligand cyclam is characterized by the anodic peak at 0.95 V. The Rdtc− ligands have two anodic peaks, one in the region 0.25–0.30 V and the other in the 0.78–0.95 V region. Absence of these anodic peaks in the case of the complexes indicates that coordination to cobalt(III) stabilizes both cyclam and Rdtc− ligands, but reversible peaks in the range −0.68 to −0.78 V support the Co(III) redox reaction. In nonaqueous solutions cyclam has one anodic peak at 1.75 V. The ligands with heteroatom in the ring (Morphdtc, Timdtc, Pzdtc, N-Mepzdtc) have two anodic peaks, while the other four ligands (Pipdtc, 2-, 3- and 4-Mepipdtc) have only one anodic peak. In nonaqueous solution again, coordination to Co(III) ion stabilizes the Rdtc− ligands and contrary to aqueous solution no Co(III) redox reaction occurs, indicating a greater stability of the complexes in this media. Finally, the electrochemical results are compared with spectroscopic data obtained previously.

Determination of low sulphide and cyanide levels in biochemical and environmental control using a deposited-on-wire Ag-Ag2S electrode

A laboratory-made version of a deposited-on-wire Ag-Ag2S electrode was tested in sulphide and cyanide solutions. The electrode was applied to the determination of low sulphide levels in spa-water and dilute thiourea solutions, and also for the determination of free and total cyanide in a gold-plating bath solution. The behaviour of this electrode was compared with that of some commercial sulphide and cyanide electrodes. The results achieved with the laboratory-made Ag-Ag2S electrode appear to be comparable to those obtained with the commercial electrodes.