R.R. Adžić

Structural effects in electrocatalysis: oxygen reduction on platinum low index single-crystal surfaces in perchloric acid solutions

Oxygen reduction has been studied in 0.1 M HClO4 on the platinum low index surfaces, employing a hanging meniscus rotating-disk technique. A special cleaning procedure has been developed to remove impurities from the surface prior to the oxygen reduction measurements. Oxygen reduction was found to be sensitive to the crystallographic orientation of the platinum electrode surface. The activity for oxygen reduction discerned from the half-wave potential decreases in the sequence (110) & >; (111) & >; (100). The reaction proceeds on all well-ordered low index planes with exchange of four electrons per O2 molecule. Tafel slopes of −120 mV per decade at high current densities and −60 mV / per decade at low current densities were found for all three planes at room temperature. Oxygen-containing species chemisorbed on platinum appear to be the cause of the change in the Tafel slope.

Determination of the kinetic parameters of the oxygen reduction reaction using the rotating ring-disk electrode: Part II. Applications☆

A general scheme of oxygen reduction is given and analysis of the disk—ring measurements is performed. Expressions for the diagnostic criteria for such a scheme have been derived and their implications are discussed. The existence of superoxide and oxygen ions and the interaction of the series and direct paths have been assumed in postulating the scheme. Also, all the electrochemical reactions involved only one-electron exchange. The possibilities and limitations of the disk-ring measurements in the mechanistic study of oxygen reduction are discussed together with a re-evaluation of some well-accepted criteria which are shown to be misleading. New definitions and terminology for different pathways of O2 reduction are proposed.

Electrosorption of hydrogen and sulphuric acid anions on single-crystal platinum stepped surfaces: Part I. The [110] zone

Abstract The adsorption of hydrogen and sulphuric acid anions has been studied on single-crystal platinum stepped surfaces from the [110] zone. Both processes depend on the orientation of the electrode surface. The multiple states of hydrogen adsorption can be correlated with the nature and density of the adsorption sites. There is a negligible effect of adsorbate-adsorbate interaction. The voltammetric profiles are in a wide potential range determined by bisulphate and sulphate adsorption/desorption. A sharp peak found in voltammetry is associated with hydrogen adsorption in the step sites, composed of the (100)-(111) step-terrace combination, concurrent with sulphate desorption. Bisulphates and sulphates are strongly adsorbed in the trigonal sites in the step, which can be formed from the steps and terraces of different orientations from this zone. The exact position of the adsorbed hydrogen atoms and anions is discussed. A comparison with the adsorption in HClO4 is given for some surfaces.

Structural effects in electrocatalysis: Oxidation of formic acid and oxygen reduction on single-crystal electrodes and the effects of foreign metal adatoms

Abstract Structural effects in electrocatalytic reactions have been demonstrated on examples of the oxidation of HCOOH on Pt and the O 2 reduction on Au. Catalytic effects of foreign metal adatoms on these reactions have been studied on single-crystal surfaces. Hypothetical superlattice structures of foreign adatoms have been considered in explaining the coverage effect on the kinetics of the HCOOH oxidation. The OH chemisorption on Au in alkaline solution is strongly dependent on the crystallographic orientation of the electrode surface which is the origin of the structural dependence of the O 2 reduction. Thallium adsorbates at small overpotentials inhibit the O 2 reduction on the Au(100) plane, but catalyse it at the Au(111) and the Au(110) faces. At higher overpotentials a four-electron reduction is observed with all three planes covered with thallium adatoms.

The influence of pH on reaction pathways for O2 reduction on the Au(100) face

Oxygen reduction on the Au(100) face was studied by the rotating disk-ring electrode technique in solutions of anions which adsorb strongly on gold (HSO4−SO42− and/or OH−) over the entire pH range. The specific adsorption of OH− anions, which is a pH dependent process, is found to play the key role in determining the reaction pathways. In the absence of OH− adsorption, for pHs below 6, the reduction of O2 begins as a 2e-process. Due to the increase in local pH during O2 reduction, the reaction pathway turns into a 4e-reduction at a certain potential depending on the pH of the solution. For pHs higher than 6, O2 reduction begins as a 4e-process in the potential region where specifically adsorbed OH− anions are present.

Electrocatalysis of oxygen on single crystal gold electrodes

Abstract Oxide formation and oxygen reduction on gold have been investigated with single crystal electrode surfaces of various orientations. Both reactions show sensitivity to the surface orientation, presence of steps, their density and orientation. The initial stages of oxide formation can be correlated with calculated relative surface energy. Anions exert a pronounced effect on oxide formation. The activity for oxygen reduction can be correlated with the same surface property except for Au(100). Structural effect from this particular plane outweighs the electronic effects. No correlation for alkaline solutions was found. Rotating ring-disc measurements show no hydrogen peroxide generation Au(100) in the presence of almost neutral AuOH −(1 − λ) on the surface. This makes it the most active electrocatalyst for oxygen reduction in alkaline solutions. At more negative potentials (no AuOH on the surface) a series mechanism up to the peroxide stage is operative. The Au(311) plane also shows a high activity dependent on AuOH −(1 − λ) . Data for both reactions clearly demonstrate that each crystallographic orientation gives an electrode surface with distinct electrochemical properties.

Oxygen reduction on electrode surfaces modified by foreign metal ad-atoms: Lead ad-atoms on gold

Abstract Oxygen reduction on gold is considerably catalysed by foreign metal ad-atoms. The catalytic effects of lead have been studied in more detail as most illustrative. The two-electron reduction of O2 to HO2− on Au changes into a four-electron process on Au modified by lead. In the potential region where AuOH constitutes the surface, the interaction of Pb ions with AuOH causes catalytic effects. At more negative potentials, on bare Au surface, the underpotential deposition of Pb ad-atoms gives rise to the catalytic effects. At AuOH surface modified by Pb ions the O2 reduction involves a “series” mechanism, with only minute quantities of HO2− leaving the electrode surface. The reduction of HO2− is considerably catalysed. The mechanism of this reaction is changed from the rate-determining chemical step into the charge-transfer rate-determining step. The rate-determining step for O2 reduction involves the first charge transfer: O2+e→O2−(ads) The mechanism of HO2− formation is uncertain, while its reduction most probably involves a direct process. There are indications that on Au surface with Pb ad-atoms a “parallel” mechanism may be operative. The catalytic effect originates in the interaction of Pb2+ with AuOH surface, which considerably reduces a partial negative charge on OH. Such a surfaces, as well as that of Au covered by Pb ad-atoms, are more suitable for adsorption of O2, O2− and HO2− which considerably alters the free energy of adsorption of these species.

Oxygen reduction on a ruthenium electrode in acid electrolytes

The kinetics and mechanism of O2 reduction on an Ru electrode in acid electrolytes have been measured by using a disc—ring method. The reaction proceeds through a “parallel” mechanism with the exchange of approximately four electrons. It shows a pronounced dependence on the oxidation state of the Ru surface. A catalytic decomposition of H2O2 does not occur on the surface obtained at potentials E > 0.0 V. Tafel slopes, some rate constants and reaction orders for O2 and H2O2 reduction have been determined. Calculated rate constant for a direct reduction of O2 (k1) is higher than k2, the reduction with the H2O2 intermediate. The experimental value of k3, the H2O2 reduction, is lower than k1 and k2 at E > 0.05 V. The Tafel slope for O2 reduction with the value higher than 120 mV has been obtained.

Structural effects in electrocatalysis: oxidation of formaldehyde on gold and platinum single crystal electrodes in alkaline solution

The oxidation of formaldehyde in sodium hydroxide solution has been studied on platinum and gold single crystal electrodes with the (111), (110) and (100) orientations. There is apparently no structural sensitivity of this reaction, since minor differences have been found between the three low index faces. This is valid for both platinum and gold electrodes. The hydrogen adsorption on platinum and the AuOH formation on gold electrodes exhibit structural sensitivity in the same solution. Similar activity of platinum and gold electrodes is noteworthy. A weak adsorption of gem-diol, formed in the interaction of formaldehyde with H2O or OH− appears as the origin of the structural insensitivity of this reaction.

Oxygen reduction on a ruthenium electrode in alkaline electrolytes

The kinetics of oxygen reduction have been studied on a ruthenium electrode in alkaline solutions using the rotating disc and rotating disc-ring methods. The reaction kinetics and mechanism were found to depend on the oxidation state of the ruthenium surface. For a surface having up to two monolayers of oxide a “parallel” mechanism of O2 reduction has been found, which goes predominantly through a four-electron direct reduction to OH−. All the rate constants of the O2 and HO−2 reactions, including those of the adsorption equilibrium of HO−2 on the Ru surface, have been determined. No catalytic decomposition of HO−2 was found. Although the Tafel slope is higher than −120 mV/dec it appears that an initial electron exchange is the rate-determining step. The kinetics are slower on a Ru surface with a thicker oxide layer. The mechanism of the reaction is also different, showing a potential-dependent ratio of the rate constants for a direct and series reduction of O2 and a catalytic decomposition of HO−2.