N.M. Marković

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.

Structural effects in electrocatalysis: Oxygen reduction on the Au (100) single crystal electrode

Abstract The kinetics of O2 reduction on the Au (100) single crystal electrode have been examined in alkaline electrolytes using the rotating dise method. A comparison has been made with the Au (110) and Au (111) surfaces. The quality of the surfaces has been determined by Auger electron spectroscopy and LEED. A pronounced effect of the crystallographic orientation on the kinetics and mechanism of O2 reduction has been found. The half-wave potential for the Au (100) face is the most positive. O2 reduction, only on this surface, proceeds with the exchange of four electrons in a “series” pathway in the potential region of AuOH formation. The cathodic and anodic kinetic data for the O2/HO2− and HO2−/OH− couples support the mechanism O2+e−→ O2− (ads) 2 O2− (ads)+H2O HO2−+O2+OH− Further reduction of HO2− is controlled by an initial one-electron transfer: HO2−+e−+H2O→ 2 OH−+OH(ads) OH(ads)+e− OH− The OH species adsorbed on the Au (100) face appears considerably discharged. Such a surface apparently interacts more strongly with O2, O2− and HO2−, and is probably the origin of the high activity of the Au (100) face for O2 and HO2− reduction.

Structural effects in electrocatalysis: Oxygen reduction on the gold single crystal electrodes with (110) and (111) orientations

Abstract The kinetics of O2 reduction on the Au (110) and the Au (111) single crystal electrodes have been examined using the rotating dise method. The quality of the surfaces has been determined by Auger electron spectroscopy and LEED. Two electrons are exchanged in O2 reduction on both surfaces. HO2 appears completely stable on the (111) face. The half-wave potential for the Au (110) surface is ca. 70 mV more positive than that for the Au (111) face. The cathodic and anodic kinetic data for the O2/HO2 and HO2−/OH− couples support the mechanism with the following initial two steps: O2+e−→O2−(ads) 2 O2−(ads)+H2O→HO2−+O2+OH− Slow further reduction of HO2−, which occurs on the Au (110) face, is controlled by a chemical reaction. The higher activity of the Au (110) face is explained by some reduction of HO2− on a partially discharged OH−, whose adsorption on the Au (111) face is negligible.

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.

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.

Hydrogen adsorption on single-crystal platinum electrodes in alkaline solutions

Abstract Hydrogen adsorption on single-crystal platinum low-index and stepped surfaces was studied in 0.1 M and 0.01 M NaOH solutions. A strong dependence on the crystallographic orientation of the electrode surface was found. The multiple states of hydrogen were correlated with the symmetry of surface sites. The data for the stepped surfaces were found to be useful in explaining the nature of processes occurring on the low index surfaces. Several strong indications were found for the nature of the so-called “butterfly” peak, indicating that it is due to adsorption of the hydroxil species rather than to hydrogen adsorption. Indications for hydrogen adsorption on Pt(111) in the H2 evolution region was found by impedance measurements. An effort was made to identify the multiple adsorption states for Pt(100). Hydrogen adsorption on the step sites for surfaces from all three crystallographic zones was identified and found to be proportional to the step density. A comparison with hydrogen adsorption in acid solutions was made for some surfaces.

Oxygen reduction on electrode surfaces modified by underpotential deposited species: Thallium on gold

Abstract The addition of small amounts of Tl(I) to NaOH results in a substantial catalytic effect on the kinetics of O2 reduction on gold. This effect is associated with the formation of an underpotential deposited layer of Tl. Rotating disk-ring electrode measurements indicate that the O2 reduction still proceeds via the series mechanism through peroxide as an intermediate, as is the situation on pure gold in the absence of Tl. The peroxide reduction is strongly catalyzed with the result that four electrons are realized per O2-molecule reaching the disk electrode. Various explanations for the enhanced catalytic activity are proposed.

Structural effects in electrocatalysis: Oxidation of D-glucose on single crystal platinum electrodes in alkaline solution

Abstract Electrochemical oxidation of D-glucose has been studied in 0.1 M NaOH on single crystal platinum electrodes with thirteen different orientations. The reaction rate depends strongly on the crystallographic orientation of the electrode surface. For most of the surfaces investigated, three voltammetric peaks were observed in the potential region of hydrogen adsorption, the double layer region and the region of anodic film formation. The most active surface is Pt(111). The poisoning adsorbates seem to be less strongly adsorbed on this plane than on Pt(100) or Pt(110). The (100) and (111) oriented steps cause a decrease of the activity of Pt(111). A strongly bound species appears to be adsorbed preferentially on the step sites. Its oxidation was found to coincide with the Pt(OH)ads layer formation.

Structural effects in electrocatalysis: Ethylene glycol oxidation on platinum single-crystal surfaces

Abstract The oxidation of ethylene glycol (EG) has been studied on platinum single-crystal surfaces in 0.1 M NaOH. A strong structural dependence of the reaction kinetics was found for all 12 single-crystal orientations investigated. The onset of the reaction occurs in the sequence Pt (110) > Pt (100) ≈ Pt (111). However, the peaks of the voltammetry curves for the low-index planes decrease in the order (111) > (110) > (100). Either the (111) or the (100) oriented steps cause a decrease in the activity of the (111) plane. Surfaces vicinal to the (100) plane exhibit a higher activity than Pt (100). The (111) oriented steps in the Pt (110) plane cause an increase in activity, while the (100) oriented steps decrease it. A parallelism between PtOH layer formation and the onset of the oxidation of EG was found which indicates that the reaction involves interaction of the adsorbed and dehydrogenated EG with Pt(OH)ads species.