The key elements in the mechanism of the formic acid oxidation reaction on platinum have been completely elucidated, not only for the direct path through an active intermediate, but also for the CO formation route.

In order to improve catalytic processes, elucidation of reaction mechanisms is essential. Here, supported by a combination of experimental and computational results, the oxidation mechanism of formic acid on Pt(111) electrodes modified by the incorporation of bismuth adatoms is revealed. In the proposed model, formic acid is first physisorbed on bismuth and then deprotonated and chemisorbed in formate form, also on bismuth, from which configuration the C−H bond is cleaved, on a neighbor Pt site, yielding CO 2 . It was found computationally that the activation energy for the C−H bond cleavage step is negligible, which was also verified experimentally.

The engineered search for new catalysts requires a deep knowledge about reaction mechanisms. Here, supported by a combination of computational and experimental results, the oxidation mechanism of formic acid on Pt(111) electrodes modified by adatoms of the pblock is elucidated for the first time. DFT calculations reveal that some adatoms, such as Bi or Pb, have positive partial charge when adsorbed on the bare surface whereas others, Se or S, remain virtually neutral. When the partial charge is correlated with previously reported experimental results for the formic acid oxidation reaction, it is found that the partial positive charge is directly related to the increase in catalytic activity of the modified surface. Further, it is obtained that such a positive partial charge is directly proportional to the electronegativity difference between the adatom and Pt. Thus, the electronegativity difference can be used as an effective descriptor for the expected electrocatalytic activity. Furthermore, this partial positive

A numerical treatment suitable for the computational investigation of physisorption of molecular hydrogen on carbon nanostructures has not been sufficiently discussed. In this paper it is shown that results used as a reference are actually a product of poorly solved interactions and contaminated estimates with errors which would be of the order of 60%. Moreover, using ab initio molecular orbital theory, under the rigid monomer supermolecular approach, the physisorption energy of molecular hydrogen on graphene was reinvestigated. The graphene surface was modeled as a coronenelike (C(24)H(12)) graphene sheet. The basis set superposition error was corrected by means of the counterpoise method. The H(2)-H(2) and H(2)-benzene interactions were examined, under systematic combinations of basis sets and correlation methods, including the aug-cc-pVQZ basis set and the coupled cluster correlation method with single, double, and noniterative triple excitations, searching for a numerical treatment with a reasonable trade-off between efficiency and accuracy. Asymmetrical modeling strategies, using diffusion augmented basis sets with preference for the adsorbate, were found to be effective. Also local modeling strategies, using more complete basis sets for the nearest atoms to the adsorbate than for the rest of the substrate, were considered. The aug-cc-pVTZ basis set for the adsorbate and for the nearest atoms to the adsorbate and the cc-pVTZ basis set for the rest of the cluster-modeled graphene, at the second-order Moller-Plesset perturbation theory correlation level, was selected as reference treatment. It was found that the physisorption energy of molecular hydrogen on graphene would be of the order of 0.06 eV, which would be 25% less than what has been previously published, though it would be sufficient to permit the storage of hydrogen physisorbed on carbon. To our knowledge this would be the most realistic theoretical estimate of the mentioned energy to date.

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.

Controversial experimental results have suggested that the presence of alkali metals in graphitic adsorbents would intensify hydrogen adsorption. Mainly by use of density functional theory methods, several authors have confirmed this intensification, which has been explained by a significant charge transfer from dopand, located on top of the center of a ring (the hollow configuration), to substrate. However, in this work, it was found that, for each distance of the hollow configuration of the Li−graphene interaction, depending on the initial guess, not only one but two qualitatively different self-consistent field solutions would be possible. One of them would concentrate charge in the region between lithium and graphene. The other one would be just the opposite. At fully correlated second-order Möller−Plesset level, for each distance, the latter was found to be significantly more stable than the former. So, it seems that the charge transfer from lithium to graphene described by several authors would not take place. The set of results reported here would provide evidence that solutions for the interaction of alkali metals with single-layer all-carbon materials would be extremely sensitive to initial guesses and correlation treatments. It is concluded that, in the case of alkali metal doping being capable of significantly intensifying physisorption of molecular hydrogen on single-layer all-carbon materials, the postulated charge transfer from dopand (located on top of the center of a ring) to substrate would not be the mechanism.

Untangling the mechanism of the methanol electro-oxidation on platinum in water as a model reaction is essential to optimize fuel cells using alcohols as fuel. Recent experiments unexpectedly suggested that this electro-oxidation process would take place only in the presence of adsorbed OH. The here reported results, carefully obtained under low methanol concentrations on the three basal planes of platinum at different scan rates to discriminate between oxidation and adsorption processes, confirm such an unexpected preliminary observation. It is found that adsorbed CO from methanol is only formed when adsorbed OH is already present on the surface. This observation is a clear indication that adsorbed OH is involved in the mechanism beyond providing the oxygen group required to oxidize adsorbed CO, which has never been considered before. Supported by density functional theory calculations, the role played by adsorbed OH in the methanol electro-oxidation to CO on platinum in water and the reason why this reaction is not observed in the absence of adsorbed OH are also here both elucidated. A combination of kinetic and thermodynamic factors, such as the presence of multiple water molecules per methanol molecule, the high adsorbed OH mobility on the surface, the favorable coadsorption of methanol in the presence of adsorbed OH, and the favorable and virtually barrier-less hydrogen transference from the hydroxy group of methanol to adsorbed OH to yield water result in the immediate activation of methanol (as soon as the molecule approaches the surface) through the favorable substitution of adsorbed OH by adsorbed methoxy. This contribution represents a change of paradigm in the understanding of how alcohols are electro-oxidized in reference systems and have crucial implications in the search for better electrocatalysts.

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