The Role of Surface Sites on the Oscillatory Oxidation of Methanol on Stepped Pt[n(111) × (110)] Electrodes

Reaction rates and mechanisms of most electrocatalytic reactions are known to critically depend on the structure of the electrode surface. Examples of structure sensitive electrocatalytic systems include the reduction of oxygen and the oxidation of small organic molecules on platinum, for example. Even more intricate is the effect of the interfacial structure on the oscillatory dynamics commonly observed in some electrocatalytic systems. This is somewhat expected because several adsorption and reaction steps are simultaneously active during self-organized potential or current oscillations. Herein we present results of the effect of surface structure on the oscillatory electro-oxidation of methanol in acidic media on Pt(111), Pt(110), and stepped surfaces Pt(776), Pt(554), Pt(775), and Pt(332). The system was investigated at two methanol concentrations and under voltammetric and galvanostatic regimes. The voltammetric activity toward the electro-oxidation of methanol on stepped surfaces followed this sequence: Pt(776) < Pt(554) < Pt(775) < Pt(332), at high methanol concentration. The reaction rates increase with the density of (110) sites, but small (111) terraces were also found to contribute to the overall process. In terms of potential oscillations, we found specificities that were unambiguously assigned to the surface structure. In particular, the following features were found according to the specific surface studied: period-adding sequences of mixed-mode oscillations; a new type of mixed-mode oscillation; and a particular separation between two types of sequential oscillations. The understanding of the relationship between the surface structure and the underlying dynamics of the surface chemistry during oscillations is a key challenge and our findings in this direction are discussed.G.T.F. and H.V. (Grant 2013/16930-7) acknowledge São Paulo Research Foundation (FAPESP) for financial support. H.V. (Grant 306060/2017-5) and GTF (Grant 305273/2017-5) acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support. This study was partially financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001

The ethanol electrooxidation at Pt layers deposited on polycrystalline Au

The ethanol electro-oxidation reaction was evaluated using a polycrystalline Au substrate modified with two different amounts of Pt using the galvanic exchange methodology. FTIR results suggest that Pt deposits have a greater ability to break the C-C bond present in the ethanol molecule. However, under potentiostatic conditions both modified Au surfaces undergo faster deactivation in comparison with polycrystalline platinum as indicated by the chronoamperometric results. XPS results indicate the presence of two phases depending on the Pt content. These are: (i) Pt-Au alloy and (ii) segregated Pt. The structural and electronic properties of these phases were related to the differences observed in the catalytic activity.Conselho Nacional de Pesquisa (CNPq) [Proc. 142507/2007-5]Conselho Nacional de Pesquisa (CNPq)Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP

Corrosion resistance evaluation of AISI 420 steel deposited by various thermal spray process

Entre as técnicas utilizadas no melhoramento de desempenho de materiais, a deposição na superfície de componentes pode ser eficaz para recuperação de elementos desgastados. Os processos de aspersão térmica, devido a sua versatilidade, quanto à aplicação, tipos de camadas e espessuras, constituem-se numa forma bastante adequada na obtenção de camadas de elevada dureza para proteção ou reparo do componente base, sendo possível revestir substratos metálicos com polímeros, metais ou cerâmicas. Entre esses processos, destacam-se: HVOF (High Velocity Oxygen Fuel), Arc-spray (arco elétrico) e Flame-spray (chama-pó). A seleção de um aço inoxidável, para aplicação, envolve algumas considerações como resistência à corrosão da liga, propriedades mecânicas, fabricabilidade e custo. Nesse trabalho foram usadas amostras do aço AISI 1045, revestidas, com aço inoxidável AISI 420, por meio das técnicas de Arc-Spray, HVOF e Flame-Spray, para o estudo comparativo de suas resistências à corrosão em água do mar, visando à produção de peças de menor custo, em comparação com peças maciças desse aço. Os melhores desempenhos em termos de dureza, porosidade e resistência à corrosão ocorreram na seguinte seqüência crescente: Flame-Spray, Arc-Spray, e HVOF.Among the techniques used to improve materials performance, deposition on the surface of components is a proper way of recovering worn elements. Thermal spraying processes were developed during the last few years and they are a very suitable method to obtain layers with high hardness for protecting or repairing the base component. Employing these processes, it is possible to overlay metallic substrates with polymers, metals and ceramics. Among these processes are: HVOF, Arc-Spray and Flame-Spray. The selection of a particular type of stainless steel for an application involves some considerations, as the corrosion resistance of the alloy, mechanical properties, manufacture feasibility and cost. In this work, used were samples of AISI 1045 steel, coated with stainless steel AISI 420, using the techniques of Arc-Spray, HVOF and Flame-Spray for the comparative study of their corrosion resistance in sea water, aimed at producing low-cost alternative pieces, compared with massive pieces of steel. The best performances in terms of hardness, porosity levels and corrosion resistance of the layers occurred in the following sequence growing: Flame-Spray, Arc-Spray, and HVOF.Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)CNP

Eletrocatálise da oxidação de monóxido de carbono

This work discusses some important aspects related to the carbon monoxide electrooxidation reaction on Pt single crystal electrodes in acidic media. The mechanistic aspects are discussed in terms of the formation of compact structures developed when CO is adsorbed. The main ideas presented here are focused on the mechanistic aspects that take into account the existence of such structures. The classical kinetic mechanisms of Lagmuir-Hinshelwood and Eley-Rideal are discussed considering the superficial mobility of CO or nucleation-growing of islands formed by oxygen-containing adsorbates.Este trabalho discute alguns aspectos importantes relacionados à reação de eletrooxidação do monóxido de carbono sobre monocristais de platina, em meio ácido. Aspectos mecanísticos são discutidos em termos da formação das estruturas compactas que o CO forma quando este é adsorvido. As principais idéias aqui apresentadas, levam em consideração as existências dessas estruturas. Os clássicos mecanismos LagmuirHinshelwood e Eley-Rideal são aqui discutidos, especialmente o primeiro considerando a mobilidade do CO e também a nucleação e crescimento de ilhas formadas por espécies adsorvidas contendo oxigênio

Corrosion resistance evaluation of precipitation hardening stainless steels with nb

Os aços inoxidáveis endurecíveis por precipitação foram desenvolvidos a partir de 1945, em decorrência das necessidades da indústria aeroespacial em termos da disponibilidade de aços com resistência mecânica e à corrosão, em temperaturas mais elevadas, superiores às dos aços inoxidáveis tradicionais, aliadas à facilidade de soldagem. Esses aços apresentam microestruturas martensíticas do tipo substitucional, endurecidas posteriormente por precipitados. Nesse trabalho, foram produzidos dois aços com composições alternativas usando o Nb como formador de precipitados, bem como um aço PH13-8Mo para efeito de comparação, em termos de resistências mecânica e à corrosão. O Aço 1 apresentou resistência à corrosão semelhante à do aço PH13-8Mo e o Aço 2 apresentou resistência mecânica próxima à do aço PH13-8Mo.Precipitate-hardened stainless steel was developed in 1945 as a consequence of the aerospace industry&rsquo;s need for a high-strength steel that would be resistant to corrosion at high temperatures, and easily welded exceeding the properties of conventional stainless steel. This steel possesses substitutional martensitic microstructures that can afterwards be hardened by precipitation. For this research, two types of steel with alternative compositions were produced by using Nb as a precipitate producer, and PH13-8Mo steel for comparison purposes in terms of mechanical and corrosive resistance. Steel 1 showed corrosive resistance similar to PH13-8Mo and Steel 2 presented mechanical resistance close to that of PH13-8Mo.Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)CNP

Characterization of plasma nitrocarburized layers produced on AISI 316L austenitic stainless steel

A dureza e, conseqüentemente, a resistência ao desgaste dos aços inoxidáveis austeníticos podem ser acentuadamente aumentadas, sem perdas na resistência à corrosão, com a produção de camadas superficiais pelo tratamento de nitrocementação por plasma. Nesse trabalho, foram nitrocementadas por plasma, na temperatura de 450°C, amostras do aço AISI 316L. As camadas obtidas foram caracterizadas por meio de ensaios micrográficos, de microdureza e de microdesgaste. A camada produzida constituiu-se de duas regiões, sendo a mais externa constituída de nitrocarbonetos de cromo e, abaixo dela, verificou-se a presença de austenita expandida, rica em nitrogênio, com dureza próxima a 850HV. A espessura média total da camada foi de, aproximadamente, 40µm. Sua resistência ao desgaste foi muito superior à do substrato.Hardness and consequently wear resistance of austenitic stainless steel can be highly increased, without losing corrosion resistance, by plasma nitrocarburizing surface treatment. In this re-search, an AISI 316L stainless steel was plasma nitrocarburized at 450°C, and the obtained layers were characterized by optical microscopy, microhardness and micro-wear tests. It was verified that the layer is composed of chromium nitrides precipitates and beneath them, a nitrogen rich expanded austenite, with hardness around 850HV. The total average thickness of the layer was about 40µm and presented good uniformity. The wear resistance of the layer was much higher than that of the substrate.Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)CNP

Eletrocatálise da oxidação de monóxido de carbono

Este trabalho discute alguns aspectos importantes relacionados à reação de eletrooxidação do monóxido de carbono sobre monocristais de platina, em meio ácido. Aspectos mecanísticos são discutidos em termos da formação das estruturas compactas que o CO forma quando este é adsorvido. As principais idéias aqui apresentadas, levam em consideração as existências dessas estruturas. Os clássicos mecanismos LagmuirHinshelwood e Eley-Rideal são aqui discutidos, especialmente o primeiro considerando a mobilidade do CO e também a nucleação e crescimento de ilhas formadas por espécies adsorvidas contendo oxigênio

The effect of Pt surface orientation on the oscillatory electro-oxidation of glycerol

In the present paper, we have studied the influence of (bi)sulfate anion (0.1 and 0.5 M) on the electro-oxidation of glycerol on basal Pt(hkl) and stepped surfaces belonging to the series of Pt(S)[n(1 1 1) × (1 1 1)]. Cyclic voltammograms and derivative voltammetry pointed out that the catalytic activity decreases for Pt(1 1 1) and Pt(1 1 0) and, to a minor extent, for stepped surfaces in 0.5 mol/L H2SO4. Chronoamperometric curves demonstrated that above 0.60 V (vs RHE), for both concentrations (0.1 and 0.5 mol/L H2SO4), stepped surfaces and Pt(1 1 0) showed greater ability to catalyze the glycerol electro-oxidation in comparison with Pt(1 1 1). Potential oscillations were mapped along with slow galvanodynamic sweeps and studied at constant current. For Pt(1 1 1), no oscillations were found in the galvanodynamic regime, however, under the galvanostatic regime, period 1 oscillations were observed after a long induction period. The oscillations showed a very similar profile for stepped surfaces, even for the Pt(3 3 2) surface, which has a high density of (1 1 0) steps. Pattern changes were observed only for Pt(1 1 0) compared to other surfaces. Therefore, we conclude that (1 1 0) step sites influence the oscillatory behavior, thus the insertion of the steps favors the path of formation of inactive species, which compete for the same catalytic sites in a given potential region. The extinction of the mechanism oscillatory occurs differently due to the intrinsic characteristics of each surface electrode for the formation of (hydro)oxides.The authors acknowledge FAPESP (Grants No. 2013/16930-7 and 2019/22183-6), FAPEAL (process 60030-001076/2016), CAPES - Brasil (CAPES, Grant No. 0001, and for the scholarship, GBM, 88887.341974/2019-00). HV (Grant No. 306060/2017-5) and GTF (Grant No. 313455/2021-0) acknowledge CNPq for financial support. We gratefully acknowledge the support of the RCGI – Research Centre for Gas Innovation, hosted by the University of São Paulo (USP) and sponsored by FAPESP (2014/50279-4 and 2020/15230-5) and Shell Brazil, and the strategic importance of the support given by ANP (Brazil’s National Oil, Natural Gas, and Biofuels Agency) through the R&D levy regulation

Study of pitting corrosion mechanism of supermartensitic stainless steels microalloyed with Nb and ti in sea water

Os aços inoxidáveis supermartensíticos (SMSS) são usados em ambientes agressivos devido à sua boa soldabilidade, boas propriedades mecânicas em temperaturas elevadas e superior resistência à corrosão sob tensão. Aplicações na exploração de petróleo demandam superior combinação de propriedades e os aços inoxidáveis duplex e superduplex têm sido aplicados nessa área, a despeito de seus custos elevados. Os SMSS consistem numa alternativa técnica e econômica ao uso daqueles aços. Nesse trabalho, adições de Nb e Ti foram realizadas com o intuito de minimizar o efeito de sensitização, promover o refino de grãos e foram estudados os aspectos microestruturais e a resistência à corrosão por pites em água do mar. A formação e a evolução dos pites foram acompanhadas por ensaios de corrosão, microscopia óptica e eletrônica, focalizando suas morfologias. O aço com Ti apresentou o melhor desempenho quanto à corrosão, com o maior potencial de corrosão e menor potencial de pite entre os aços em estudo. O aço com Nb, apesar de apresentar potencial de corrosão superior ao do aço sem adição, teve um potencial de pite inferior ao do mesmo.Supermartensitic stainless steel (SMSS) is increasingly used in harsh environments due to its good weldability and mechanical properties at higher temperatures and high resistance to corrosion under stress. Applications in oil exploration demand a superior combination of properties and duplex and superduplex stainless steels have been widely applied in this area, despite their high costs. SMSS provides a technical and economical alternative for these steels. In this research, additions of Nb and Ti were made in order to minimize the sensitization effect and to promote grain refinement, studying the microstructural aspects and the pitting corrosion resistance in seawater. Pitting formation and evolution were accompanied by corrosion testing, optical and electronic microscopy. The Ti alloyed steel showed the best corrosion performance, with the greatest corrosion potential and the lowest pitting potential. The steel with Nb addition presented a higher corrosion potential than that of the steel without additions but had a lower pitting potential.Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)CNP

Effect of palladium on gold in core-shell catalyst for electrooxidation of ethanol in alkaline medium

In this paper the effect of small amounts of palladium deposited on gold nanoparticles supported on Vulcan XC-72 carbon (core-shell structure denoted Au@Pd/C) is studied. Different nominal atomic compositional ratios of Au@Pdx maintaining fixed gold nuclei and varying the amount of palladium (x = 0.10; 0.80 and 1.60) were synthesized via seed growth method for the ethanol oxidation reaction in alkaline medium. UV–Vis spectrometric, X-ray powder diffraction, X-ray energy dispersive spectroscopy, transmission electron microscopy and electrochemical measurements were performed for the characterization of these catalysts. Electrocatalytic activity toward ethanol oxidation on Au@Pd/C catalysts were investigated by cyclic voltammetry and chronoamperometry showed that [email protected]/C electrocatalyst has the highest current density and low onset potential for ethanol oxidation reaction in alkaline medium. In-situ Fourier transform infrared spectroscopy measurements demonstrated that acetate is the main product of ethanol oxidation and CO2 can be slightest observed, the latter could be visualized in greater quantity on catalyst [email protected]/C catalyst