Investigating interfacial parameters with platinum single crystal electrodes

The concepts of total and free charge of platinum single crystal electrodes are revised in this paper, together with the associated concepts of potential of zero total and free charge. Total charges can be measured from CO displacement method. Results on solution of different pH are described. A novel buffer composition is used to attain pH values close to neutrality while avoiding interferences from anion adsorption processes. Stress is made on the fact that free charges are not accessible through electrochemical measurement for systems at equilibrium since adsorption processes (hydrogen and hydroxyl) interfere with free charge determination. Still, a model is described that allows, under some assumptions, extract free charge values and the corresponding potential of zero free charge for Pt(111) electrodes. On the other hand, fast measurement outside equilibrium can separate free charges from adsorption processes based on their different time constant. In this way, the laser induced temperature jump experiment allows determination of the potential of maximum entropy, a magnitude that is intimately related with the potential of zero free charge. Values of the potential of maximum entropy as a function of pH are given for the different basal planes of platinum.     

Electro-oxidation of o-phenylenediamine at platinum electrodes in acetonitrile solutions

The study of o-phenylenediamine electro-oxidation at platinum electrodes in acetonitrile solutions under different experimental conditions is presented. Cyclic voltammograms show 4 oxidation peaks, which are assigned to o-phenylenediamine, o,o′-diamineazobenzene and protonated o-phenylenediamine anodic oxidation. An additional prepeak system is apparent at potentials less anodic than the first peak in successive scans. This prepeak system evidences the presence of two redox systems diphenylamine and H⁺, derived from the initial oxidation product o-phenylenediamine cation radical. The effect of base and acid addition is also studied. o,o′-Diamineazobenzene was identified as one of the principal soluble products in preparative scale electrolysis and a general mechanism for o-phenylenediamine oxidation is proposed.     

Electrochemical behavior of acidic acetonitrile solutions at platinum and gold electrodes

Hydrogen evolution reactions from acidic MeCN solutions were studied at Pt and Au electrodes. The acids studied were HCl and HPic in a wide range of concentrations. LiClO₄ and Et₄NClO₄ (0.4 M) were used as supporting electrolytes. Quasi steady state polarization curves show that the reaction is irreversible with a Tafel slope of 2 RT/F after diffusion polarization and pseudo-ohmic drop have been corrected. Non-stationary techniques reveal that the electrochemical reaction is preceded by a chemical reaction attributed to slow dissociation of ACN·2 HCl and ACN·3 HPic species. Equilibria and chemical rate constants for both species were evaluated by chronopotentiometry. The values obtained explain satisfactorily the results accomplished under potentiodynamic conditions. Data at the RDE confirm the existence of a previous chemical reaction. MeCN electroadsorption in the double layer potential region at Pt accounts for the solvent levelling effect towards H adsorption between Pt and Au. Furthermore, an inhibitory effect over hydrogen evolution and hydrogen oxidation is reported at more negative potentials at both Pt and Au electrodes.     

Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. −0.44 V (vs. Ag|AgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed.     

New insights on hydrogen evolution at Au single crystal electrodes

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Thirty years of platinum single crystal electrochemistry

The electrochemistry of platinum single crystals is historically reviewed. After a brief revision of historical results dating before the publication of the landmark experiment by J. Clavilier of the flame annealing in 1980, the controversy introduced by this experiment into the surface electrochemistry community is described. Questions about the structure and composition of the platinum surface after the flame annealing and their implications on the characteristic voltammetry of platinum single crystal electrodes were slowly answered in the years that followed the first introduction of this methodology. One of the last questions to be solved was that about the nature of the chemical species responsible for the charge transfer process that leads to the so-called unusual features in the voltammogram. This was solved with the charge displacement experiment. Nowadays, a great deal of knowledge has been gathered about the structure of the interphase between platinum electrodes and electrolytic solutions and also about the electrocatalytic behaviour of platinum surfaces. State-of-the-art information about platinum electrochemistry is provided, with emphasis on results from our group, especially those obtained with a thermodynamic analysis, involving either constant or variable temperatures and with the laser-induced temperature jump method.     

Cu UPD at Pt(100) and stepped faces Pt(610), Pt(410) of platinum single crystal electrodes

The processes of adsorption/desorption of copper adatoms on the basal Pt(100) face and stepped Pt(610), Pt(410) surfaces have been studied in perchloric acid solution by cyclic voltammetry. It has been shown that the positions of the Cu stripping peaks are determined by perfection of the adlayer. The “island” model is suggested to describe electrochemical behavior of the Pt(hkl)+Cu_ad system. Obtained results are important for target modification of shape-controlled nanoparticles that are used in electrocatalysis.     

Hydrogen redox reactions in 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide on platinum single crystal electrodes

Hydrogen oxidation and the subsequent proton reduction are studied on platinum single crystal electrodes in purified 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid. The hydrogen redox reaction shows some dependence of the surface orientation. The highest reversibility is observed with Pt(111) whereas the reaction in electrodes with {100} sites is less reversible and with a slow kinetics. Adsorption states are observed in the presence of hydrogen along with the main oxidation reaction. Also, it is possible to detect protons after oxidation of water and H₂O₂.     

Borohydride electro-oxidation on Pt single crystal electrodes

The borohydride oxidation reaction on platinum single-crystal electrodes has been studied in sodium hydroxide solution using static and rotating conditions. The results show that borohydride electro-oxidation is a structure sensitive process on Pt surfaces. Significant changes in the measured currents are observed at low potentials. In this region, the Pt(111) electrode exhibits the lowest activity, whereas the highest currents are measured for the Pt(110) electrode. The behavior of the different electrodes is discussed taking as reference the observed behavior on the blank electrolyte and the possible formation of weakly adsorbed intermediates.     

Electrochemical behaviour of isopropanol at platinum electrodes

The accessible potential range of isopropanol was found to be from +1.1 to -1.2 V vs. Ag/AgCl in non-aqueous 0.01 M LiCl, with acetone being formed at the anode and hydrogen at the cathode. Water was formed in a side-reaction, probably by ketal formation, the rate being proportional to the electrode area. Another side-reaction at the cathode produced an insoluble product after prolonged electrolysis. The coulometric current efficiency was nevertheless very high (99.6-99.97%) and almost independent of current density. The results indicate that the cathode reaction proceeded with somewhat less than theoretical current efficiency.     

Oxidation of glucose at electrodeposited platinum electrodes

The oxidation of glucose at low concentrations was studied at electrodeposited platinum electrodes in 0.1 M HClO₄ using an electrochemical quartz crystal microbalance (EQCM). Experiments were performed over the whole potential range and then over selected regions of potential to investigate the processes giving rise to specific features in the cyclic voltammetry and mass response. In the region of potential where adsorbed hydrogen is present, EQCM experiments reveal the presence of adsorbate derived from glucose through the removal of features associated with the presence of adsorbed anions in the background electrolyte. Cycling over this potential region leads to a decrease in the mass of the electrode as the coverage of poisoning species increases. In the double-layer region of potential an increase in mass is seen as the adsorbate is oxidized and removed from the electrode surface, and is suggested to be a result of the replacement of adsorbed residues (formed from progressive oxidation of glucose) by both anions and fresh reactant. Restriction of the electrode potential to values above 0.2 V (SCE) prevents poison formation and mass responses indicate the presence of adsorbed glucose between 0.2 V and 0.6 V. Mass responses accompanying voltammetry and injection experiments also indicate that glucose or species derived from it can adsorb on an oxidized platinum surface.     

Reaction mechanism of the benzoquinone/hydroquinone couple at platinum electrodes in aqueous solutions: Retardation and enhancement of electrode kinetics by single chemisorbed layers

The electrochemical kinetics of the benzoquinone (Q)/hydroquinone (H₂Q) redox couple at platinum electrodes in aqueous solutions has been found to be extremely sensitive to the nature of species adsorbed on the electrode surface at monolayer coverages. Experimental measurements were based on thin-layer cyclic voltammetry; the use of thin-layer electrodes was dictated by the need to minimize surface contamination. Bulky neutral or anionic aromatic adsorbates led to the familiar U-shaped rate-vs.-pH curves; the rate minimum occurred near pH 4. Kinetic effects due to oriental changes of chemisorbed species were noted only when the rate was low. Adsorbed 1 atoms led to comparatively rapid reactivity (rate constant k° > 10^?3 cm s^?1) and virtual independence of pH. Profound retardation resulted from pretreatment ofthe surface with CN^? and SCN^?; total irreversibility (k° < 10^?6 cm s^?1) was observed at pH 4, with a further decrease in rate at pH 7. In contrast, when the surface contained n layer of chemisorbed phenyltriethylammonium cations, the electrode rate increased with increasing pH. The results indicate that different reaction pathways predominate when different absorbates are present.     

Selective electrocatalysis of acetaldehyde oxime reduction on (111) sites of platinum single crystal electrodes and nanoparticles surfaces

The reduction of acetaldehyde oxime (AO) in acid medium on platinum surfaces is a structure sensitive reaction that takes place almost exclusively on (111) sites of Pt electrodes, and it is strongly inhibited on Pt(100) and Pt(110) surfaces. A study using stepped electrodes with (111) terraces and monoatomic steps either with (100) and (110) orientation shows that the activity of the electrode is also dependent on the terrace width, i.e., the wider the terrace is, the higher current density is recorded and the more positive the peak potential for AO reduction appears. Moreover, in the electrodes with (100) step sites, the reduction process appears at more negative potential than the electrodes with (111) step sites. Nanoparticles with some preferential orientations were also tested for the AO reduction reaction to check the presence of (111) ordered domains on the nanoparticles surface. Dedicated to Teresa Iwasita on the occasion of her retirement and for her contributions to Electrochemistry.     

Surface-enhanced Raman scattering of pyridine on platinum and nickel electrodes in nonaqueous solutions

Surface-enhanced Raman scattering (SERS) spectra of pyridine adsorbed onto bare platinum and nickel electrodes in nonaqueous solutions are reported in this Letter. There are similarities and differences between the SERS from aqueous and nonaqueous solutions. The surface enhancement factor for platinum in acetonitrile solution has been calculated to decrease by a factor of ca. 10 compared with that in the aqueous media. The double-band character for the ring breathing mode is observed at 1009 and 1019 cm⁻¹. Two adsorption modes of pyridine on the platinum surface were assumed. Part of the pyridine molecules may be chemisorbed onto the surface, with the ring plane oriented vertical to the surface; other pyridine molecules may co-adsorb with lithium cations onto the surface.     

Peculiarities of hydrogen-oxygen mixture ionization at platinum electrodes

Polarization curves at platinum electrodes are measured in acid and alkaline solutions saturated with hydrogen, oxygen, and their mixture, under the condition of the controlled solution agitation. It is shown that the current of the gaseous mixture ionization differs markedly of that expected according to the principle of the currents’ additivity. The non-fulfillment of the currents’ additivity is observed even in the region of the limiting currents of hydrogen electrooxidation and oxygen electroreduction measured in the gases’ individual solutions. Possible reasons for the described behavior of the oxygen-hydrogen mixture at platinum electrode are analyzed. It was concluded that the oxygen reduction is hindered when hydrogen is present in the solution.     

The effect of platinum oxide films on reaction kinetics at platinum electrodes

It has been known for some time that the pre-treatment of platinum electrodes often effects subsequent electrochemical reactions. Part of the effect of pre-treatment is due to the fact that anodized or chemically oxidized platinum electrodes become coated to some degree with a film of platinum oxide. This work was concerned with quantitative measurement of kinetic parameters as a function of the extent of oxide film formation. Whenever possible, variations in reaction mechanisms are proposed.

Most of the experimental evidence has been taken from current-potential curves but the techniques of chronopotentiometry and microscopy were also used.

The reduction of vanadium^V chromium^VI, arsenic^V, iodate and oxygen were investigated as well as the oxidation of vanadium^IV, arsenic^III, oxalic acid, and formic acid. The presence of the platinum oxide film effected the reactions studied in a variety of ways but in every case some variation in the kinetic parameters of the reactions studied was recorded. For a number of cases, a modified oxygen bridge theory was found useful.