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

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

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-

Crystallographic and electronic evolution of lanthanum strontium ferrite (La0.6Sr0.4FeO3-δ) thin film and bulk model systems during iron exsolution.

We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La0.6Sr0.4FeO3-δ). By using thin films of orthorhombic LSF, grown epitaxially on NaCl(001) and rhombohedral LSF powder, the materials gap is bridged. The orthorhombic material transforms into a fluorite structure after the exsolution has begun, which further hinders this process. For the powder material, by a combination of in situ core level spectroscopy and ex situ neutron diffraction, we could directly highlight differences in the Fe chemical nature between surface and bulk: whereas the bulk contains Fe(iv) in the fully oxidized state, the surface spectra can be described perfectly by the sole presence of Fe(iii). We also present corresponding magnetic and oxygen vacancy concentration data of reduced rhombohedral LSF that did not undergo a phase transformation to the cubic perovskite system based on neutron diffraction data