Electronic and structural study of Pt-modified Au vicinal surfaces: a model system for Pt–Au catalysts

FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOTwo single crystalline surfaces of Au vicinal to the (111) plane were modified with Pt and studied using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in ultra-high vacuum environment. The vicinal surfaces studied are Au(332) and Au(887) and different Pt coverage (θPt) were deposited on each surface. From STM images we determine that Pt deposits on both surfaces as nanoislands with heights ranging from 1 ML to 3 ML depending on θPt. On both surfaces the early growth of Pt ad-islands occurs at the lower part of the step edge, with Pt ad-atoms being incorporated into the steps in some cases. XPS results indicate that partial alloying of Pt occurs at the interface at room temperature and at all coverage, as suggested by the negative chemical shift of Pt 4f core line, indicating an upward shift of the d-band center of the alloyed Pt. Also, the existence of a segregated Pt phase especially at higher coverage is detected by XPS. Sample annealing indicates that the temperature rise promotes a further incorporation of Pt atoms into the Au substrate as supported by STM and XPS results. Additionally, the catalytic activity of different PtAu systems reported in the literature for some electrochemical reactions is discussed considering our findings. © 2014 The Owner Societies.Two single crystalline surfaces of Au vicinal to the (111) plane were modified with Pt and studied using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in ultra-high vacuum environment. The vicinal surfaces studied are Au(332) and Au(887) and different Pt coverage (yPt) were deposited on each surface. From STM images we determine that Pt deposits on both surfaces as nanoislands with heights ranging from 1 ML to 3 ML depending on yPt. On both surfaces the early growth of Pt ad-islands occurs at the lower part of the step edge, with Pt ad-atoms being incorporated into the steps in some cases. XPS results indicate that partial alloying of Pt occurs at the interface at room temperature and at all coverage, as suggested by the negative chemical shift of Pt 4f core line, indicating an upward shift of the d-band center of the alloyed Pt. Also, the existence of a segregated Pt phase especially at higher coverage is detected by XPS. Sample annealing indicates that the temperature rise promotes a further incorporation of Pt atoms into the Au substrate as supported by STM and XPS results. Additionally, the catalytic activity of different PtAu systems reported in the literature for some electrochemical reactions is discussed considering our findings.Two single crystalline surfaces of Au vicinal to the (111) plane were modified with Pt and studied using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in ultra-high vacuum environment. The vicinal surfaces studied are Au(332) and Au(887) and different Pt coverage (yPt) were deposited on each surface. From STM images we determine that Pt deposits on both surfaces as nanoislands with heights ranging from 1 ML to 3 ML depending on yPt. On both surfaces the early growth of Pt ad-islands occurs at the lower part of the step edge, with Pt ad-atoms being incorporated into the steps in some cases. XPS results indicate that partial alloying of Pt occurs at the interface at room temperature and at all coverage, as suggested by the negative chemical shift of Pt 4f core line, indicating an upward shift of the d-band center of the alloyed Pt. Also, the existence of a segregated Pt phase especially at higher coverage is detected by XPS. Sample annealing indicates that the temperature rise promotes a further incorporation of Pt atoms into the Au substrate as supported by STM and XPS results. Additionally, the catalytic activity of different PtAu systems reported in the literature for some electrochemical reactions is discussed considering our findings.16261332913339FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO07/54829-5; 2011/12.566-3; 2012/16860-6160172/2011-0Greeley, J., Stephens, I.E.L., Bondarenko, A.S., Johansson, T.P., Hansen, H.A., Jaramillo, T.F., Rossmeisl, J., Nørskov, J.K., (2009) Nat. Chem., 1, pp. 552-556Wu, G., More, K.L., Johnston, C.M., Zelenay, P., (2011) Science, 332, pp. 443-447Chu, Y.H., Shul, Y.G., (2010) Int. J. Hydrogen Energy, 35, pp. 11261-11270Kowal, A., Li, M., Shao, M., Sasaki, K., Vukmirovic, M.B., Zhang, J., Marinkovic, N.S., Adzic, R.R., (2009) Nat. 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Chem., 607, pp. 47-53Kobiela, T., Moors, M., Linhart, W., Cebula, I., Krupski, A., Becker, C., Wandelt, K., (2010) Thin Solid Films, 518, pp. 3650-3657Petkov, V., Wanjala, B.N., Loukrakpam, R., Luo, J., Yang, L., Zhong, C.-J., Shastri, S., (2012) Nano Lett., 12, pp. 4289-4299Authors thank Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP-07/54829-5) and Conselho Nacional de Pesquisa (CNPq) for financial support. Specially thank to L. H. Lima for experimental assistance with STM measurements and Prof. G. Tremiliosi-Filho for lending the Au(332) crystal. MJP, SF and EAC thank FAPESP and CNPq for the fellowships granted (Procs. FAPESP 2011/12.566-3 and 2012/16860-6; Proc. CNPq 160172/2011-0)

Surface Electron-Hole Rich Species Active in the Electrocatalytic Water Oxidation.

Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrOx centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally IrV species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ1-O and μ1-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species

Cationic Copper Species Stabilized by Zinc during the Electrocatalytic Reduction of CO2 Revealed by In Situ X‐Ray Spectroscopy

Advanced in situ X-ray absorption spectroscopy characterization of electrochemically co-electrodeposited bi-element copper alloy electrodes shows that zinc yields the formation of a stable cationic Cu species during the electroreduction of CO2 at high cathodic polarization. In contrast, the formation/stabilization of cationic Cu species in copper oxides, or doping Cu with another element, like Ni, is not possible. It is found that the pure and mixed Cu:Zn electrodes behave similarly in term of electrocatalytic selectivity to multi-carbon products. At higher Zn concentrations the electrode behaves like the pure Zn catalyst, which indicates that the Cu cationic species do not have a significant influence on the selectivity to multi-carbon products. It is found that in the non-monotonically distribution of products is dominated in term of surface energy in which copper prefers the surface. Otherwise, this work highlights the importance of in situ characterization to uncover the mechanisms mediating the catalytic reactions in contrast to ex situ or post mortem analysis, which can be a source of misinterpretation

Assessment of the Degradation Mechanisms of Cu Electrodes during the CO

Catalyst degradation and product selectivity changes are two of the key challenges in the electrochemical reduction of CO on copper electrodes. Yet, these aspects are often overlooked. Here, we combine X-ray spectroscopy, electron microscopy, and characterization techniques to follow the long-term evolution of the catalyst morphology, electronic structure, surface composition, activity, and product selectivity of Cu nanosized crystals during the CO reduction reaction. We found no changes in the electronic structure of the electrode under cathodic potentiostatic control over time, nor was there any build-up of contaminants. In contrast, the electrode morphology is modified by prolonged CO electroreduction, which transforms the initially faceted Cu particles into a rough/rounded structure. In conjunction with these morphological changes, the current increases and the selectivity changes from value-added hydrocarbons to less valuable side reaction products, , hydrogen and CO. Hence, our results suggest that the stabilization of a faceted Cu morphology is pivotal for ensuring optimal long-term performance in the selective reduction of CO into hydrocarbons and oxygenated products

Synthesis and characterization of carbon suported M-MxSy-type nanoparticles (M = Pt, Rh) for electrocatalysis of fuel cell reactions

As Células a combustível são conversores de energia química em energia elétrica. As Células do tipo PEM que funcionam com metanol como combustível tem uma ampla variedade de aplicações. Os materiais utilizados como eletrocatalisadores nas células são responsáveis por uma grande parte do custo das mesmas. Outros problemas, que provocam diminuição da eficiência da célula, são a cinética lenta da reação de redução de oxigênio (RRO) e o potencialmisto gerado devido ao cruzamento de metanol através da membrana. Neste trabalho apresenta-se um estudo de catalisadores do tipo M-MxSy (M = Pt, Rh) para a RRO em meio ácido, com diferentes relações M:MxSy. Os materiais preparados a partir da modificação do método do ácido fórmico (MAF) com tiouréia (TU) foram caracterizados mediante XRD, XPS e XAS. Foi determinado mediante estas técnicas que os catalisadores consistem numa mistura de fases: Pt ou PtRh, PtS, RhxSy e PtS2. O efeito de um tratamento térmico em H2/Ar foi reduzir completamente o PtS2 e parcialmente o PtS. A fase de RhxSy mostrou ser mais estável nas condições do tratamento. Todos os materiais mostraram ter atividade para a RRO e alta seletividade na presença de metanol. Foi determinado que para que a RRO ocorra via 4 elétrons, deve haver sítios metálicos na superfície das nanopartículas. Determinou-se que os materiais contendo maior quantidade de fase MxSy podem ser ativados mediante tratamento térmico ou eletroquímico, melhorando a atividade catalítica frente a RRO e conservando a seletividade na presença de metanol.Fuel cells are dispositives that convert chemical energy into electricity. The PEM fuel cell types that function with methanol as fuel have a wide variety of applications. The materials used as electrocatalysts in the cells are responsible for the major part of their cost. Other problems, that cause decrease in efficiency of the cell, are the slow kinetics of oxygen reduction reaction (ORR) and the mixed potential generated due to methanol crossover through the membrane. This thesis presents a study of M-MxSy-type catalysts (M = Pt, Rh) for the ORR in acid medium, with different M:MxSy ratios. The materials prepared from the modification of the formic acid method (FAM) with thiourea (TU) were characterized by XRD, XPS and XAS. It was determined by these techniques that the catalysts consist of a mixture of phases: Pt or PtRh, PtS, RhxSy and PtS2. The effect of heat treatment in H2/Ar atmosphere was to reduce PtS2 completely and PtS partially. The RhxSy phase proved to be more stable under the treatment conditions. All materials showed to have activity for the ORR and high selectivity in the presence of methanol. It was determined that for the ORR to occur via four electrons, there must be metallic sites at the surface of the nanoparticles. It was determined that the materials containing higher amount of MxSy phase can be activated by thermal or electrochemical treatment, improving the ORR catalytic activity and retaining the selectivity in the presence of methanol

Temperature effects on the oscillatory electro-oxidation of methanol on platinum

We report in this paper the effect of temperature on the oscillatory electro-oxidation of methanol on polycrystalline platinum in aqueous sulfuric acid media. Potential oscillations were studied under galvanostatic control and at four temperatures ranging from 5 to 35 degrees C. For a given temperature, the departure from thermodynamic equilibrium does not affect the oscillation period and results in a slight increase of the oscillation amplitude. Apparent activation energies were also evaluated in voltammetric and chronoamperometric experiments and were compared to those obtained under oscillatory conditions. In any case, the apparent activation energies values fell into the region between 50 and 70 kJ mol(-1). Specifically under oscillatory conditions an apparent activation energy of 60 +/- 3 kJ mol(-1) and a temperature coefficient q(10) of about 2.3 were observed. The present findings extend our recently published report (J. Phys. Chem. A, 2008, 112, 4617) on the temperature effect on the oscillatory electro-oxidation of formic acid. We found that, despite the fact that both studies were carried out under similar conditions, unlike the case of formic acid, only conventional, Arrhenius, dynamics was observed for methanol.Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)[142095/2007-9]Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)[302698/2007-8]Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)[06/04636-3]Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)[04/04528-0

Structural and electronic characterization of Co nanostructures on Au(332)

FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCo nanoislands were grown on (332) vicinal surface of Au in UHV using the e-beam evaporation technique. Scanning tunneling microscopy results reveal that Co deposition occurs following an islanding mode for θCo ranging from 0.17 to 0.64 ML. At low coverage nanoislands show a monolayer height, while at higher Co loadings, islands have a maximum bilayer height. XPS measurements rule out the possibility of alloy formation provided that binding energy of Co2p core lines remains unchanged as cobalt loading increases. Also, XPS data reveals that, when subjected to thermal annealing, Co atoms diffuse into Au crystal retaining its chemical nature as before the annealing. Finally, NO adsorption experiments show that Co nanostructures are partially oxidized upon adsorption, as evidenced by changes in core photoemission lineshapes of the Co2p lines. Also, NO adsorption seems to inhibit Co atom diffusion into Au crystal during moderate thermal treatment.Co nanoislands were grown on (332) vicinal surface of Au in UHV using the e-beam evaporation technique. Scanning tunneling microscopy results reveal that Co deposition occurs following an islanding mode for θCo ranging from 0.17 to 0.64 ML. At low coverage nanoislands show a monolayer height, while at higher Co loadings, islands have a maximum bilayer height. XPS measurements rule out the possibility of alloy formation provided that binding energy of Co2p core lines remains unchanged as cobalt loading increases. Also, XPS data reveals that, when subjected to thermal annealing, Co atoms diffuse into Au crystal retaining its chemical nature as before the annealing. Finally, NO adsorption experiments show that Co nanostructures are partially oxidized upon adsorption, as evidenced by changes in core photoemission lineshapes of the Co2p lines. Also, NO adsorption seems to inhibit Co atom diffusion into Au crystal during moderate thermal treatment.6178793FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP [Proc. 2011/12.566-3]CNPq [Proc. 160172/2011-0]2011/12.566-3160172/2011-

Oxygen reduction on methanol-tolerant carbon-supported PtxSy nanoparticles prepared by a single-step low-temperature method

FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOIn direct methanol fuel cells (DMFCs), the methanol crossover from the anode to the cathode is a major cause of power density loss because of the overpotential arising due to the parasitic reaction of methanol oxidation at the cathode. Catalysts modified with S have shown better methanol tolerance; however, the preparation routes often require high temperatures or pressures and very long times, making these expensive and unlikely to be used in large scale. Here, we report on a single-step low-temperature method used to prepare a carbon-supported PtxSy catalyst. Moreover, we show that the catalyst shows lower depolarization in the presence of methanol and study the effect of reductive thermal treatment and electrochemical potential cycling.105516523FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOSem informação142095/2007-

From 1D to 3D Ru Nanostructures on a Pt Stepped Surface as Model Systems in Electrocatalysis: UHV-STM and XPS Study

A Ru-decorated Pt single crystal vicinal to the (111) plane was studied using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in ultrahigh-vacuum environment. Pt(332) vicinal surface was used, and different coverages of Ru (θ_Ru) were deposited. STM images show that Ru initial growth is highly influenced by the stepped nature of the surface. For instance, 1D and 2D nanostructures grow decorating the steps at low coverage (θ = 0.34), while at higher coverage a second layer is stabilized and bilayer (3D) growth sets in. The size and shape of the Ru nanostructures differ from those reported for Ru on Pt(111) due to the high density of steps that induces anisotropic growth and delays the monolayer to bilayer growth transition. Sample annealing at 623 K promotes further anisotropy and an increase in the amount of 3D structures. XPS suggests that no bulk alloying (bulk diffusion) of Ru occurs at this temperature. Additionally, our findings are used to discuss the different electrocatalytic behavior reported for some Pt–Ru systems and to explain tendencies observed for Ru-decorated Pt nanoparticles toward the CH₃OH and CO electrooxidation reactions