A.V. Tripković

Methanol electrooxidation on supported Pt and PtRu catalysts in acid and alkaline solutions

The kinetics of methanol oxidation on supported 47.5 wt.% Pt and 54 wt.% PtRu (with nominal Pt:Ru ratios of 2:3) catalysts are measured in 0.5 M H2SO4 and 0.1 NaOH at 295 and 333 K using thin-film rotating disk electrode (RDE) method. It was found that the activity of Pt and PtRu for methanol oxidation is a strong function of pH of solution and temperature. The kinetics are much higher in alkaline than in acid solution; at 333 K, a factor of 30 for Pt and a factor of 20 for Pt2Ru3 at 0.5 V. The pH effect is attributed to the pH competitive adsorption of oxygenated species with anions from supporting electrolytes. The activity of Pt and Pt2Ru3 catalysts at 333 K is higher (a factor of 5) than at 295 K. Irrespective of pH, only negligible differences in the kinetics are observed between Pt and on high Ru content Pt alloys, presumably owing to a slow rate of methanol dehydrogenation on the Ru-rich surface and insufficient number of Pt sites required for dissociative chemisorption of methanol.

The influence of the oxygen-containing species on the electrooxidation of the C1–C4 alcohols at some platinum single crystal surfaces in alkaline solution

The electrochemical oxidation of methanol, ethanol, n-propanol and n-butanol has been carried out at the Pt(111) and the stepped Pt(755) and Pt(332) surfaces in sodium hydroxide solution. Special attention was paid to the study of the oxygen-containing species generated and adsorbed during surface oxidation. The existence of reversible OHad, irreversible OHad and PtO species was suggested in the potential region relevant for the alcohol oxidation. The role of these species in the reaction was determined and a dual path mechanism was proposed for the alcohols studied.

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.

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.

Structural effects in electrocatalysis: oxidation of formic acid and hydrogen adsorption on platinum single-crystal stepped surfaces

Abstract The oxidation of formic acid and hydrogen adsorption have been measured on platinum single-crystal stepped surfaces with five orientations. A pronounced structural sensitivity of both reactions has been found. They show a sensitivity to the terrace orientation, the density of steps and to the step orientation The surfaces which are blocked for reaction by intermediates formed in the oxidation of formic acid, as the Pt(100) and Pt(110) surfaces are, become more active upon introduction of steps. The surface which are not blocked, i.e. the Pt(111) surface, become less active upon introduction of steps because more active steps increase their activity for the “wrong” reaction — the formation of blocking intermediates. Preliminary data on hydrogen adsorption show the possibility of using this reaction as a check of the degree of order and orientation of sigle-crystal Pt surfaces.

Optical and electrochemical study of electrocatalysis by foreign metal adatoms: Oxidation of formic acid on rhodium

Abstract Specular reflectance spectroscopy, in conjunction with linear potential sweep voltammetry, has been used in studying catalytic effects of Pb and Cd adatoms on the oxidation of HCOOH on Rh. The intermediates formed in that reaction exhibit a similar behaviour as those formed on Pt, thus indicating the same mechanism of reaction on the two metals. The catalytic effect of Pb and Cd adatoms has a dual origin. The adatoms prevent the formation and the adsorption of strongly bound intermediates, as found for Pt electrode. Specular reflectance spectroscopy was used to follow the coverage of foreign metal adatoms in the absence and the presence of the electrode reaction. Different catalytic effects of Pb and Cd adatoms have been attributed to their different adsorbabilities. The adsorption of Pb atoms was unaffected by the presence of HCOOH and its intermediates. However, their presence caused a considerable desorption of Cd adatoms. Certain differences in the voltammograms of Rh electrode in H 2 SO 4 and HClO 4 electrolytes were also demonstrated.

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.

Effect of temperature on the methanol oxidation at supported Pt and PtRu catalysts in alkaline solution

Abstract The effect of temperature on the kinetics of methanol oxidation on supported 47.5 wt% Pt, 54 and 33.5 wt% Pt/Ru catalysts was studied in 0.1 M NaOH at 295, 313 and 333 K using thin-film rotating disk electrode (RDE) method. The catalysts were characterized by ex situ STM prior to the voltammetric studies. It was found that the activity of those catalysts for methanol oxidation was determined by a delicate balance between the surface coverage by CO ad and by OH ad species. Significantly faster kinetics at higher temperatures clearly indicates that methanol oxidation at the catalysts studied is highly activated process. The highest effect of temperature is obtained at the least active Pt catalyst which has the highest activation energy. On the contrary, the smallest temperature effect is detected on the most active Pt 3 Ru 2 catalyst having the lowest activation energy. These phenomena have been explained by the important role of the adsorption energies of both reactive intermediates (CO ad and OH ad ).

Oxidation of methanol on platinum single crystal stepped electrodes from [110] zone in acid solution

Abstract The methanol oxidation has been studied on high index (755), (211) and (311) and low index (111) platinum surfaces in perchloric and sulfuric acid. The most active of the stepped surfaces is (755) in both acids. (755), (211) and (111) surfaces are more active in perchloric than in sulfuric acid. The Pt(311) plane shows the same activity in both acids. Methanol is preferentially adsorbed competing with sulfuric acid anions. The oxidation of methanol proceeds without any important effect caused by specific adsorption of sulfuric acid anions at lower anodic potentials. The presence of a reactive (OH) ads species on the surface seems to be the major cause for the larger activities of the electrodes at higher anodic potentials. Two different reaction mechanisms are proposed for the Pt(755) plane. The poisoning species formation dominates at the potentials lower than E = 0.25 V (sce). The oxidative removal of poisoning species followed by the bulk methanol oxidation at the surface partially covered by (OH) ads species take place at the more positive potentials. The increase of (OH) ads coverage, and the formation of inactive oxygenated species cause the inhibition of the methanol oxidation at potentials more positive than the peak potentials.