K.Dj. Popović

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 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.

Formic Acid Oxidation on Pt/Ru Nanoparticles: Temperature Effects

The formic acid oxidation on Pt/Ru nanoparticles in acid solution over the temperature range 298-333 K has been studied by thin-film rotating disk method (RDE). Transmission electron microscopy in combination with scanning tunneling microscopy was used to determine the size (4.3 ± 0.3 nm) and shape (cuboctahedral) of the particles. Kinetic analysis revealed that at elevated temperatures (313 K, 333 K) the reaction rate is much higher than at room temperature (295 K), indicating that formic acid oxidation on supported Pt/Ru catalyst is a highly activated process. Based on experimental kinetic parameters we propose that the HCOOH oxidation on the PtRu alloy most likely follows a dual pathway, but the branching ratio is still very high, i.e. Pt-like. The principal effect of opening the dehydration channel at steady-state (via the presence of Ru in the surface) is to lower the coverage of COads on Pt sites and permit the dehydrogenation path to increase in rate.

Insight into electrocatalytic stability of low loading Pt-Bi/GC and Pt/GC clusters in formic acid oxidation

Formic acid oxidation was examined on platinum-bismuth deposits on glassy carbon substrate prepared by two-step process, i.e., electrochemical deposition of Bi followed by electrochemical deposition of Pt as described in our previous article (J Electrochem Soc 161:H547–H554, 2014). Upon treatment of as-prepared clusters by slow anodic sweep, bimetallic structure consisting of Bi core occluded by Pt and Bi-oxide was obtained and exhibited significant activity and exceptional stability in HCOOH oxidation. In order to explain such electrocatalytic stability, in this work, the electrochemical properties of Pt@Bi/GC catalyst were investigated applying same protocols in supporting electrolyte with or without HCOOH and compared with Pt/GC. The protocols comprised potentiodynamic, quasi-steady-state, and chronoamperometric measurements combined with the surface characterization by COads stripping voltammetry. Application of potential cycling at Pt@Bi/GC electrode in supporting electrolyte containing HCOOH leads to minor change in surface morphology, mildly leaching of Bi from the electrode surface, and negligible decrease in activity. On the other hand, significant Bi dissolution and considerable decrease in activity are the effects of the same treatment without HCOOH. Contrary to Pt@Bi/GC, Pt/GC electrodes subjected to the same protocols exhibit completely opposite properties being more stabile during potential cycling without HCOOH than in the presence of this acid. Exceptional stability in formic acid oxidation of Pt@Bi/GC catalyst is thus most probably the result of the combination of predominant dehydrogenation path of the reaction, suppressed Bi leaching, and compensation of dissolved Bi from the core as its source due to which surface morphology endured minor changes.

Particle Size Effect: Methanol Oxidation on Supported Pt Catalysts

The methanol oxidation was studied at two differently prepared supported Pt electrodes (Pt-C/GC and Pt/GC) in 0.5 M H2SO4 and 0.1 M NaOH. The supported Pt electrodes were characterized by AFM, STM TEM and HRTEM. The higher activity of Pt-C/GC than of Pt/GC catalyst, as well as negligible differences in the activities between the supported Pt catalysts and the corresponding single crystal electrodes oriented as the sites in the catalyst deposits in which Pt particles are dominant, clearly suggest the influence of the particle size effect on the catalyst activity.

Study of Supported Pt and PtRu Catalysts in Methanol and Formic Acid Oxidation

The kinetics of methanol and formic acid oxidation on supported Pt and Pt2Ru3 catalysts are measured in 0.5 M H2SO4 at ambient temperature using thin-film rotating disc electrode (RDE) method. Both catalysts are characterized by HRTEM and STM techniques. Only negligible differences in the kinetics are observed between Pt and Pt2Ru3 catalysts in both reactions studied. The activity of Pt and Pt2Ru3 catalysts is much higher in formic acid than in methanol oxidation at the potentials of electrocatalytic interest.

Structural Effects in Electrocatalysis: Formic Acid Oxidation at Model and Real Pt Catalysts

Formic acid oxidation was studied at low-index Pt single crystals (model systems) as well as at the platinum catalyst supported on high area carbon (real catalyst) in HClO4. The Pt single crystals were characterized by LEED. The LEED patterns obtained after a mild heating of flame-annealed crystals have shown clean, well ordered unreconstructured surfaces. Pt-C supported catalyst was analyzed by AFM and STM in air and by XRD. AFM and STM images revealed the presence of Pt-C agglomerates of several tenth of nm consisting of Pt particles ranged from 2 nm to 6 nm. The electrocatalytic activity of these catalysts in formic acid oxidation increased in a sequence: Pt(100) < Pt(110) < Pt-C/GC < Pt(111).