We present a joint experimental and density functional theory (DFT) study on the effect of atomic vacancies on the restructuring of platinum—transition metal alloy nanocatalysts and the associated changes in electrocatalytic activity. Atomic vacancies were introduced into slabs composed of pure Pt monolayers, and the structures were relaxed using the Vienna ab initio simulation package code. Effects of i) the concentration and ii) the spatial distribution of atomic vacancies in the slabs on surface and bulk restructuring were investigated. Highly disordered nanostructures featuring large variations of the in‐plane and out‐of‐plane nearest‐neighbour distances around the mean were observed upon relaxation. These findings were confirmed experimentally by using hollow PtNi/C nanoparticles synthesized by a combination of galvanic replacement and the nanoscale Kirkendall effect (a vacancy‐mediated interdiffusion mechanism). The experimental results also show that hollow PtNi/C nanoparticles feature a combination of oxophilic and oxophobic catalytic sites on their surface and are thus highly active both for electrochemical oxidation and reduction reactions.

In the course of (electro)catalytic reactions, reversible and irreversible changes, namely the formation of adsorbed poisons, catalyst degradation, surface roughening, etc., take place at distinct time-scales. Reading the transformations on the catalyst surface from the measurement of the reaction rates is greatly desirable but generally not feasible. Herein, we study the effect of random surface defects on Pt(100) electrodes toward the electro-oxidation of methanol in acidic media. The surface defects are gently generated in situ and their relative magnitudes are reproducibly controlled. The system was characterized under conventional conditions and investigated under an oscillatory regime. Oscillatory patterns were selected according to the presence of surface defects, and a continuous transition from large amplitude/low frequency oscillations (type L) on smooth surfaces to small amplitude/high frequency oscillations (type S) on disordered surfaces was observed. Importantly, self-organized potential oscillations were found to be much more sensitive to the surface structure than conventional electrochemical signatures or even other in situ characterization methods. As a consequence, we proved the possibility of following the surface fine structure in situ and in a non-invasive manner by monitoring the temporal evolution of oscillatory patterns. From a mechanistic point of view, we describe the role played by surface defects and of the adsorbed and partially oxidized, dissolved species on the oscillations of type S and L.

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Non-covalent interactions between hydrated alkali metal cations and adsorbed oxygenated species on platinum might considerably inhibit some electrocatalytic reactions. We report in this communication the effect exerted by electrolyte alkali metal cations on the electro-oxidation of formate ions on platinum. The system was investigated by means of cyclic voltammetry and chronoamperometry in the presence of an electrolyte containing Li⁺, Na⁺, or K⁺. As already observed for other systems, the general activity towards the electro-oxidation of formate ions was found to increase in the sequence Li⁺ < Na⁺ < K⁺. In addition, we observed that the inhibition caused by smaller cations has a peculiar potential dependence because of the multi-peaked current profile of the electro-oxidation of formate on platinum. In this respect, we have also identified a new effect caused by cation inhibition at intermediate potentials, namely a peak splitting towards the use of smaller cations. Results are discussed in connection with mechanistic aspects of this model system

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Recebido em 31/1/05; aceito em 12/8/05; publicado na web em 14/3/06 THE pK a OF ACID-BASE INDICATORS AND THE INFLUENCE OF COLLOIDAL SYSTEMS. An experiment is proposed that can be included in undergraduate courses of chemistry. The subject is the acidity of organic compounds, which are employed as pH indicators, particularly in acid-base titrations. The indicators used are methyl orange, bromophenol blue and bromocresol green in aqueous medium. The influence of colloidal systems on the equilibrium is evaluated by the pK a . The colloids employed are surfactants like sodium dodecyl sulfate, cetyl-trimethylammonium bromide and a polymeric non-ionic F127 (pluronics). The effect of stabilization promoted by the system on the acidic or basic structureof the indicator establishes the action mechanism of the colloid on the pK a values.Keywords: pK a ; acid-base indicator; colloidal systems. INTRODUÇÃONo âmbito universitário, pesquisa científica e ensino são áreas que devem estar em simbiose e, quanto mais próximas, maiores são os benefícios a ambas. Uma das ações nesta linha é apresentar e motivar discussões em salas de aulas, de pesquisas científicas avançadas conectadas aos conteúdos programáticos ministrados nas disciplinas. Em muitos casos, porém, não se tem esta associação devido, principalmente, à complexidade e especificidade da pesquisa, provocando o distanciamento desta em relação ao aluno. Tendo em mente este fato, nosso grupo tem trabalhado no desenvolvimento de práticas laboratoriais para disciplinas experimentais de físico-química, abordando sistemas usados em pesquisas científicas e apresentando-os em experimentos "relativamente" simples, visando correlacionar propriedades físico-químicas com conceitos contidos no programa curricular dos cursos de Química. Dessa forma, de maneira mais motivadora, focada e didática, procura-se clarificar conceitos e propriedades físi-co-químicas desses sistemas apresentando experimentos que possam ser incluídos nas disciplinas dos cursos de graduação. Neste trabalho abordamos temas que despertam o interesse devido à importância científica e no cotidiano/indústria: compostos ácido-base, particularmente, o conceito de pK a e sua determinação; os sistemas coloidais. Mostra-se, aqui, a determinação de pK a e, a partir da comparação dos valores obtidos em água e em sistemas coloidais, avalia-se a modificação micro-local do ambiente e do equilíbrio químico.A classe de compostos investigada, quanto ao pK a , é a de indicadores ácido-base utilizados em determinações de acidez através de curvas de titulação, os quais, em boa parte, propiciaram o desenvolvimento da química analítica quantitativa volumétrica moderna 1 . Estes compostos são moléculas orgânicas cromóforas que absorvem na região do visível (corantes), cuja sensibilidade à concentração de íons H + permite indicar o grau de acidez do ambiente onde estão inseridos. Esta propriedade advém do fato deles próprios serem substâncias ácidas ou básicas fracas e que têm seu sistema cromóforo fortemente influenciado pelo grupamento ...

We present in this work an experimental investigation of the effect of temperature (from 25 to 180 o C) in the electro-oxidation of ethanol on platinum in two different phosphoric acid concentrations. We observed that the onset potential for ethanol electro-oxidation shifts to lower values and the reaction rates increase as temperature is increased for both electrolytes. The results were rationalized in terms of the effect of temperature on the adsorption of reaction intermediates, poisons, and anions. The formation of oxygenated species at high potentials, mainly in the more diluted electrolyte, also contributes to increase the electro-oxidation reaction rate.

Electrodeposition of the first Pd layers onto Pt(100) was investigated using cyclic voltammetry at a low scan rate (0.1 mV·s). Ultrathin films were characterized by cyclic voltammetry in 0.1 M HSO solution and with ex situ AFM (atomic force microscopy). For the first time, we evidenced the underpotential character of the deposition of the first two Pd layers, characterized by a two-step mechanism, each step corresponding to the deposition of a complete Pd atomic layer. For thicker deposits, especially above 10 monolayers as equivalent thickness, the electrochemical characterization displays a strong irreversibility and a broadening of the adsorption/desorption peaks, associated with a reduction of long-range ordered flat areas. Ex situ AFM images are in agreement with this description. They show rough thick deposits and the growth of (100)-oriented rectangular shaped islands with their sides aligned with the two [011] and [0-11] perpendicular directions of the (100) Pt surface.