Developing an understanding of structure-activity relationships and reaction mechanisms of catalytic processes is critical to the successful design of highly efficient catalysts. As a This is a previous version of the article published in

The oxygen reduction reaction (ORR) at different pH values has been studied at platinum single crystal electrodes using the hanging meniscus rotating disk electrode (HMRDE) configuration. The use of NaF/HClO4 mixtures allows investigating the reaction up to pH = 6 in solutions with enough buffering capacity and in the absence of anion specific adsorption. The analysis of the currents shows that the kinetic current density measured at 0.85 V for the Pt(111) electrode follows a volcano curve with the maximum located around pH = 9. This maximum activity for pH = 9 can be related to the effects of the electrode charge and/or water structure in the ORR. On the other hand, the catalytic activity for the other basal planes shows a monotonic behavior with a small dependence of the activity with pH. For stepped surfaces with (111) terraces, the behavior with pH changes gets closer to that of the Pt(111) surface as the terrace length increases. Additionally, the ORR curves show a dependence of the limiting diffusion current with pH. It is observed that the limiting current density diminishes as the pH increases in a potential region where hydrogen peroxide is readily reduced. These results suggest the existence of a bifurcation point in the mechanism previous to peroxide formation, in which OOH • is proposed as the bifurcation intermediate. The reduction of OOH • requires proton addition and would be more difficult at neutral pH values, justifying the diminution of the limiting currents.

The study of the oxygen reduction reaction (ORR) at high-index Pt­(hkl) single crystal surfaces has received considerable interest due to their well-ordered, typical atomic structures and superior catalytic activities. However, it is difficult to obtain direct spectral evidence of ORR intermediates during reaction processes, especially at high-index Pt­(hkl) surfaces. Herein, in situ Raman spectroscopy has been employed to investigate ORR processes at high-index Pt­(hkl) surfaces containing the [011̅] crystal zonei.e., Pt(211) and Pt(311). Through control and isotope substitution experiments, in situ spectroscopic evidence of OH and OOH intermediates at Pt(211) and Pt(311) surfaces was successfully obtained. After detailed analysis based on the Raman spectra and theoretical simulation, it was deduced that the difference in adsorption of OOH at high-index surfaces has a significant effect on the ORR activity. This research illuminates and deepens the understanding of the ORR mechanism on high-index Pt­(hkl) surfaces and provides theoretical guidance for the rational design of high activity ORR catalysts.

ABSTRACT:The electrochemical production of hydrogen peroxide can be implemented in small-scale plants under "on demand" approach. For that, selective catalysts for the oxygen reduction reaction (ORR) towards the desired species are required. Here, we report about the synthesis, characterization, ORR electrochemical behavior and reaction mechanism of an aza-fused π-conjugated microporous polymer, which presents high selectivity towards hydrogen peroxide. It was synthesized by polycondensation of 1,2,4,5-benzenetetramine tetrahydrochloride and triquinoyl octahydrate. A cobaltmodified version of the material was also prepared by a simple post-synthesis treatment with a Co(II) salt. The characterization of the material is consistent with the formation of a conductive robust porous covalent laminar poly-aza structure. The ORR properties of these catalysts were investigated using rotating disk and rotating disk-ring arrangements. The results indicate that hydrogen peroxide is almost exclusively produced at very low overpotential values on these materials. Density functional theory calculations provide key elements to understand the reaction mechanism. It is found that, at the relevant potential for the reaction, half of the nitrogen atoms of the material would be hydrogenated. This hydrogenation process would destabilize some carbon atoms in the lattice and provides segregated charge. On the destabilized carbon atoms, molecular oxygen would be chemisorbed with the aid of charge transferred from the hydrogenated nitrogen atoms and solvation effects. Due to the low destabilization of the carbon sites, the resulting molecular oxygen chemisorbed state, which has the characteristics of a superoxide species, would be only slightly stable and would promote the formation of hydrogen peroxide.

The hydrogen peroxide reduction reaction (HPRR) on Pt(111) has been studied for the first time in acid to neutral pH values in the absence of anion specific adsorption using the HMRDE configuration. The onset for the reduction is ca. 0.95 V (RHE) for the Pt(111), irrespective of the pH value. At more negative potential values, the reduction is inhibited. It has been found that the diminution of the activity on Pt(111) occurs at the same potential value in the SHE scale for the different pH values. This indicates that this deactivation is not dependent on hydrogen adsorption process, as has been previously suggested. However, it should be related to the interface water reorganization and the potential of zero free charge of the metal surface. In addition, studies in alkaline conditions suggest that the presence of adsorbed OH species promotes the total conversion of hydrogen peroxide to water.

Bromide adsorption on Pt (111) is investigated by means of cyclic voltammetry and CO displacement experiments at different pH values. In acidic pH bromide adsorption is strongly overlapped with hydrogen desorption process. However, as the pH increases, hydrogen adsorption process displaces towards negative potentials while bromide adsorption remains nearly in the same potential region. In consequence, both processes decouple at higher pH values. The structural transition from Pt(111)-(1×1) to Pt(111)(3×3)-4Br is pH independent, in the SHE scale, and not observed for pH > 9.1. Values of pztc are extracted from the combination of voltammetric and CO displacedcharges. An alternative approach to obtain charge curves is based on the coincidence of the curves the structural transition characteristic of the bromide adlayer completion.Pztc values obtained from different approaches with and without bromide are compared and their dependence on pH discussed. A thermodynamic analysis is carried out to obtain hydrogen Gibbs excess and charge number from the Esin Markov analysis.or "commensurate" depending on what type of interaction is stronger, as pointed out by Lucas et al. 15 On the one hand, if the interaction between nearby adatoms is stronger, the structural adlayer will be "incommensurate". On the other hand, if the adatomsubstrate interaction is stronger, then structural adlayer will be "commensurate".Bromide adsorption on Pt(111) was subject of investigation by Hubbard's group [16][17][18] providing precious information about the characterization of the formed adlayers.UHV measurements by LEED and Auger spectroscopy were employed to characterize the adlayer, and these works opened the way for further similar investigations. 3,19-22 The Pt(111)-Br ads adlayer was also characterized by STM 3,23 and surface X-ray scattering. 19 The bromide adlayers can be formed either from bromide solutions or bromide vapors. 3 A structural transition at 0.25 V vs. RHE in 0.1 M HClO 4 was observed. 3,18,23 It was first proposed that this feature corresponds to the change from a Pt(111)-(3×3)-4Br adlattice at high potentials to a Pt(111)-(4×4)-7Br adlattice at low potentials, 18 but Itaya et al. observed an incommensurate Pt(111)-(1×1) structure instead of Pt(111)-(4×4)-7Br. 23 Independently of the technique used for the characterization of the Pt(111)-Br ads adlayer, it was shown that the Pt(111)-(3×3)-4Br structure is dense and the bromine adatoms are arranged in a closed-packed hexagonal layer, where the Br-Br distance is close to van der Waals diameter of bromine. They pointed out also that these adlayers are ordered preferentially along the directions of the substrate densest rows. 3,[16][17][18]24 Lucas et al. 19 proposed similar observations, although in these studies they observed that the (3×3) adlayer structure was incommensurate, whereas Orts et al. 3 proposed it as commensurate.In the present work, the knowledge about bromide adsorption on Pt(111) is extended by performing cyclic voltammetry and CO displacement experiments...

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We report a study using Pt(111) and Pt(100) electrodes of the role of adsorbed formate in both the direct and indirect pathways of the electrocatalytic oxidation of formic acid. Cyclic voltammetry at different concentrations of formic acid and different scan rates and pulsed voltammetry were used to obtain a deeper insight into the effect of formate coverage on the rate of the direct pathway. Pulsed voltammetry also provided information on the effect of the concentration of formic acid on the rate of the formation of adsorbed CO on Pt(100). At low to medium coverage, increasing formate coverage increases the rate of its direct oxidation, suggesting that decreasing the distance between neighboring bidentate-adsorbed formate favors its interconversion to and/or stabilizes monodentate formate (the reactive species). However, increasing the formate coverage beyond approximately 50% results in a decrease of the rate of the direct oxidation, probably because bidentate formate is too closely packed for its conversion to monodentate formate to be possible. Cyclic voltammetry at very high scan rates reveals the presence of an order–disorder phase transition within the bidentate formate adlayer on Pt(111) when the coverage approaches saturation. The dependence of the potential of the maximum rate of dehydration to adsorbed CO, and of the rate at the maximum, on the concentration of formic acid is in good agreement with predictions made for a mechanism, in which adsorbed CO is formed through the adsorption of formate followed by its reduction to adsorbed CO, thus confirming that monodentate-adsorbed formate is the last intermediate common to both the direct and indirect pathways.