Electrodeposition of submonolayer amounts of Os onto polycrystalline Pt (original) (raw)
Related papers
Langmuir, 2006
Catalytic activity of the Pt(111)/Os surface toward methanol electrooxidation was optimized by exploring a wide range of Os coverage. Various methods of surface analyses were used, including electroanalytical, STM, and XPS methods. The Pt(111) surface was decorated with nanosized Os islands by spontaneous deposition, and the Os coverage was controlled by changing the exposure time to the Os-containing electrolyte. The structure of Os deposits on Pt(111) was characterized and quantified by in situ STM and stripping voltammetry. We found that the optimal Os surface coverage of Pt(111) for methanol electrooxidation was 0.7 ( 0.1 ML, close to 1.0 ( 0.1 Os packing density. Apparently, the high osmium coverage Pt(111)/Os surface provides more of the necessary oxygen-containing species (e.g., Os-OH) for effective methanol electrooxidation than the Pt(111)/Os surfaces with lower Os coverage (vs e.g., Ru-OH). Supporting evidence for this conjecture comes from the CO electrooxidation data, which show that the onset potential for CO stripping is lowered from 0.53 to 0.45 V when the Os coverage is increased from 0.2 to 0.7 ML. However, the activity of Pt(111)/Os for methanol electrooxidation decreases when the Os coverage is higher than 0.7 ( 0.1 ML, indicating that Pt sites uncovered by Os are necessary for sustaining significant methanol oxidation rates. Furthermore, osmium is inactive for methanol electrooxidation when the platinum substrate is absent: Os deposits on Au(111), a bulk Os ingot, and thick films of electrodeposited Os on Pt , all compare poorly to Pt(111)/Os. We conclude that a bifunctional mechanism applies to the methanol electrooxidation similarly to Pt(111)/Ru, although with fewer available Pt sites. Finally, the potential window for methanol electrooxidation on Pt(111)/Os was observed to shift positively versus Pt(111)/Ru. Because of the difference in the Os and Ru oxophilicity under electrochemical conditions, the Os deposit provides fewer oxygen-containing species, at least below 0.5 V vs RHE. Both higher coverage of Os than Ru and the higher potentials are required to provide a sufficient number of active oxygen-containing species for the effective removal of the site-blocking CO from the catalyst surface when the methanol electrooxidation process occurs.
Osmium nanoislands spontaneously deposited on a Pt(111) electrode: an XPS, STM and GIF-XAS study
Journal of Electroanalytical Chemistry, 2003
Scanning tunneling microscopy (STM) characterized adlayers of spontaneously deposited osmium on a Pt(111) electrode were investigated using ex-situ X-ray photoemission spectroscopy (XPS) and in-situ grazing incidence fluorescence X-ray absorption spectroscopy (GIF-XAS). After a single spontaneous deposition, monoatomic (or nearly monoatomic) nanoislands of osmium are formed. The island diameter varies from 2 to 5 nm depending on the Os coverage, which in turn is adjusted by varying the concentration of the Os precursor salt (OsCl 3 ) in the deposition bath and/or by the deposition time. XPS reveals three oxidation states: a metallic Os (the 4f 7 /2 core level binding energy of 50.8 eV), Os(IV) (51.5 eV) and Os(VIII) (52.4 eV). The metallic osmium exists at potentials below 500 mV (vs. RHE) while above 500 mV osmium is oxidized to Os(IV). Electrodissolution of osmium begins above 900 mV and occurs simultaneously with platinum oxidation. At ca. 1200 mV V versus the RHE reference, the oxidation state of some small amounts of osmium that survive dissolution is the Os(VIII). We demonstrate, for the first time, that mixed or odd valencies of osmium exist on the platinum surface at potentials higher that 800 mV. In-situ GIF-XAS measurements of an Os L III edge also reveal the presence of three Os oxidation states. Namely, below the electrode potential of 400 mV, the X-ray fluorescent energy at maximum absorption is 10.8765 keV, and is characteristic of the metallic Os. In the potential range between 500 and 1000 mV this energy is gradually shifted to higher values, assignable to higher valencies of osmium, like Os(IV). This tendency continues to higher potentials consistent with the third, highly oxidized osmium form present, most likely Os(VIII). The variation of the ''raw edge jump height'' of Os with the electrode potential, which is equivalent to a drop in osmium surface concentration, demonstrates that the electrochemical stripping of Os begins below 1.0 V versus RHE, as expected from voltammetry. Also, the observed intensity of the white line of Os in the 100 Á/400 mV region is larger than the value reported for metallic bulk Os. This discrepancy may result from the difference in the electronic properties of the metallic Os layers on Pt(111) and the metallic bulk Os: in the potential region between 100 and 400 mV, the 5d electrons in Os and Pt form a mixed electronic band, and the density of electronic states near the Fermi level, the main factor determining the white line intensity, may not be the same as in metallic bulk. The presented results on osmium adlayers are much more comprehensive than those available in our previous work due to the combined STM, GIF-XAS and XPS investigations. A nearly perfect convergence of the in situ and ex situ data is one of the main research outcomes of this project. Finally, platinum XPS spectra taken in the context of Os electrooxidation from the electrode surface are also presented and conclusions are made, that up to 900 mV platinum remain metallic, irrespective of a significant osmium oxidation on its surface. #
Hydrogen adsorption and oxide formation on platinum single crystal electrodes
Journal of Electroanalytical Chemistry, 1979
The adsorption of hydrogen on Pt(100), (110) and (111) electrodes from 0.5 M HfSO 4 has been investigated by measuring potentiodynamic adsorption and desorption spectra. Distinct differences in the adsorption behaviour of H on the various faces of single crystalline Pt are found and the results are compared with those previously reported in the literature. The quality of the single crystal surface was checked by RHEED. In addition, the influence of the substrate crystallographic orientation on the surface oxide formation was stud!ed, where again differences on the three low index faces of Pt are observed.
Osmium Nanoislands Spontaneously Deposited on a Pt(111) Electrode: the XPS, STM and GIF-XAS Study
Adlayers of spontaneously deposited osmium on a Pt(111) electrode were investigated in detail using insitu scanning tunneling microscopy (STM), ex-situ x-ray photoemission spectroscopy (XPS) and in-situ grazing incidence fluorescence x-ray absorption spectroscopy (GIF-XAS). After a single deposition, monoatomic (or nearly monoatomic) height adislands of osmium are formed. Depending on the osmium coverage, the size of the islands spans from 2 to 5 nm. The XPS measurements demonstrate the dependence of the oxidation state of Os on the electrode potential. Apparently, there are three oxidation states: metallic Os (4f 7/2 core level binding energy of 50.8 eV), Os(IV) (51.5 eV) and Os(VIII) (52.4 eV). Predominantly metallic layer of Os exists in the electrode potential region below 500 mV (vs. RHE) but osmium is oxidized to Os(IV) above 500 mV. Electrodissolution of osmium does not begin until the potential reaches 900 mV, but then the dissolution occurs simultaneously with platinum oxide formation. Notably, above 1000 mV, the oxidation state of the residual Os increases to Os(VIII). In-situ GIF-XAS measurements of Os L III edge also reveal the presence of three oxidation states of Os on Pt(111) depending on the electrode potential. The x-ray energy at maximum absorption appeared at 10.8765 keV below 400 mV, was gradually shifted to higher values in the potential range between 500 mV and 1000 mV, reaching the limiting value of 10.8793 keV. In addition, the variation of the raw edge jump height of Os, equivalent to surface concentration, demonstrates that the electrochemical stripping of Os begins at ca. 1000 mV, as expected from voltammetry. We demonstrate, for the first time, that a mixed valency or some uncommon valencies of osmium exists on the platinum surface at potentials higher that 800 mV. Both qualitatively and quantitatively the results from ex-situ XPS and in-situ GIF-XAS appear uniquely consistent. The results on osmium adlayers obtained by spontaneously deposition are much more comprehensive than in previous work due to the combined in situ STM and GIF-XAS measurements. The convergence of the in situ and ex situ data is one of the main research outcomes of this project.
Potential Shift for OH(ads) Formation on the Pt Skin on Pt[sub 3]Co(111) Electrodes in Acid
Journal of The Electrochemical Society, 2005
A study combining theoretical predictions and experimental measurements was made to gain an understanding of the beneficial effect of alloying cobalt into platinum for electroreduction of oxygen. Carbon-supported Pt 3 Co catalyst particles were characterized by X-ray diffraction spectroscopy and X-ray absorption near-edge structure, which gave evidence for a surface layer composed of Pt, called the Pt skin. Electrochemical measurements were made in 1 M trifluoromethane sulfonic acid with a rotating ring disk setup. Cyclic voltammetry showed significantly less oxide formation in the Ͼ0.8 V range over the skin on the alloy compared to nonalloyed Pt. Tafel plots showed a 50-70 mV reduction in overpotential for O 2 reduction over the Pt skin. The Vienna Ab Initio Simulation Program was used for calculating H 2 O and OH adsorption bond strengths on the Pt skin on Pt 3 Co͑111͒ for comparison with prior work with the Pt͑111͒ surface. The bond strength variations were used to estimate the shift in reversible potential for OH ads formation from H 2 O ads oxidation. A shift of 80 mV was found, which indicates that an increase in the reversible potential for OH ads formation correlates with the decrease in overpotential for O 2 reduction over the Pt skin on Pt 3 Co nanoparticles.
Surface-oxide growth at platinum electrodes in aqueous H2SO4
Electrochimica Acta, 2004
The mechanism of platinum surface electro-oxidation is examined by combined cyclic-voltammetry (CV), in situ electrochemical quartzcrystal nanobalance (EQCN) and ex situ Auger electron spectroscopy (AES) measurements. The CV, EQCN and AES data show that the charge density, interfacial mass variation and intensity of the O-to-Pt AES signal ratio increase in a continuous, almost linear manner as the potential is raised from 0.85 to 1.40 V. In addition, the charge density, mass variation and O-to-Pt signal ratio profiles follow each other, thus indicating that the surface oxidation proceeds by a progressive coordination of O-containing species to the Pt substrate. The coupled CV and EQCN measurements lead to in situ determination of the molecular weight of the interfacial species; these were identified as chemisorbed O (O chem ) at 0.85 ≤ E ≤ 1.10 V and as O 2− in the form of a surface PtO at 1.20 ≤ E ≤ 1.40 V. The AES results reveal that the first halfmonolayer of O chem is formed through discharge of H 2 O molecules and such formed O chem resides on the Pt surface. Subsequent discharge of H 2 O molecules leads to formation of the second half-monolayer of O chem that is accompanied by the interfacial place exchange of O chem and surface Pt atoms; this process results in the development of a quasi-3D surface PtO lattice comprising Pt 2+ and O 2− . AES data demonstrate that the place-exchange process occurs in the 1.10-1.20 V potential range. The experimentally determined molecular weight of the species added to the surface is 15.8 g mol −1 , which points to O and to anhydrous PtO as the surface oxide formed.
Electrochimica Acta, 2004
The mechanism of platinum surface electro-oxidation is examined by combined cyclic-voltammetry (CV), in situ electrochemical quartzcrystal nanobalance (EQCN) and ex situ Auger electron spectroscopy (AES) measurements. The CV, EQCN and AES data show that the charge density, interfacial mass variation and intensity of the O-to-Pt AES signal ratio increase in a continuous, almost linear manner as the potential is raised from 0.85 to 1.40 V. In addition, the charge density, mass variation and O-to-Pt signal ratio profiles follow each other, thus indicating that the surface oxidation proceeds by a progressive coordination of O-containing species to the Pt substrate. The coupled CV and EQCN measurements lead to in situ determination of the molecular weight of the interfacial species; these were identified as chemisorbed O (O chem ) at 0.85 ≤ E ≤ 1.10 V and as O 2− in the form of a surface PtO at 1.20 ≤ E ≤ 1.40 V. The AES results reveal that the first halfmonolayer of O chem is formed through discharge of H 2 O molecules and such formed O chem resides on the Pt surface. Subsequent discharge of H 2 O molecules leads to formation of the second half-monolayer of O chem that is accompanied by the interfacial place exchange of O chem and surface Pt atoms; this process results in the development of a quasi-3D surface PtO lattice comprising Pt 2+ and O 2− . AES data demonstrate that the place-exchange process occurs in the 1.10-1.20 V potential range. The experimentally determined molecular weight of the species added to the surface is 15.8 g mol −1 , which points to O and to anhydrous PtO as the surface oxide formed.
Journal of the Electrochemical Society, 2005
A study combining theoretical predictions and experimental measurements was made to gain an understanding of the beneficial effect of alloying cobalt into platinum for electroreduction of oxygen. Carbon-supported Pt 3 Co catalyst particles were characterized by X-ray diffraction spectroscopy and X-ray absorption near-edge structure, which gave evidence for a surface layer composed of Pt, called the Pt skin. Electrochemical measurements were made in 1 M trifluoromethane sulfonic acid with a rotating ring disk setup. Cyclic voltammetry showed significantly less oxide formation in the Ͼ0.8 V range over the skin on the alloy compared to nonalloyed Pt. Tafel plots showed a 50-70 mV reduction in overpotential for O 2 reduction over the Pt skin. The Vienna Ab Initio Simulation Program was used for calculating H 2 O and OH adsorption bond strengths on the Pt skin on Pt 3 Co͑111͒ for comparison with prior work with the Pt͑111͒ surface. The bond strength variations were used to estimate the shift in reversible potential for OH ads formation from H 2 O ads oxidation. A shift of 80 mV was found, which indicates that an increase in the reversible potential for OH ads formation correlates with the decrease in overpotential for O 2 reduction over the Pt skin on Pt 3 Co nanoparticles.
Electrochimica Acta, 2010
The current study is concerned with the preparation and characterization of tantalum oxide-loaded Pt (TaO x /Pt) electrodes for hydrogen spillover application. XPS, SEM, EDX and XRD techniques are used to characterize the TaO x /Pt surfaces. TaO x /Pt electrodes were prepared by galvanostatic electrodeposition of Ta on Pt from LiF-NaF (60:40 mol%) molten salts containing K 2 TaF 7 (20 wt%) at 800 • C and then by annealing in air at various temperatures (200, 400 and 600 • C). The thus-fabricated TaO x /Pt electrodes were compared with the non-annealed Ta/Pt and the unmodified Pt electrodes for the hydrogen adsorption/desorption (H ads /H des ) reaction. The oxidation of Ta to the stoichiometric oxide (Ta 2 O 5 ) increases with increasing the annealing temperature as revealed from XPS and X-ray diffraction (XRD) measurements. The higher the annealing temperature the larger is the enhancement in the H ads /H des reaction at TaO x /Pt electrode. The extraordinary increase in the hydrogen adsorption/desorption at the electrode annealed at 600 • C is explained on the basis of a hydrogen spillover-reverse spillover mechanism. The hydrogen adsorption at the TaO x /Pt electrode is a diffusion-controlled process.
Stability of atomic oxygen chemisorption on Pt-alloy surfaces
Surface and Interface Analysis, 2016
A density functional theory calculation is used to investigate the atomic oxygen (O) stability over platinum (Pt) and Pt-based alloy surfaces. Here, the stability is connected with the preferential adsorption sites for O chemisorptions and the adsorption energy. Thus, the interaction mechanism between atomic O and metal surfaces is studied by using charge transfer analysis. In this present paper, atomic structure and binding energy of oxygen adsorption on the Pt(111) are in a very good agreement with experiment and previous density functional theory calculations. Furthermore, we obtained that the addition of ruthenium (Ru) and molybdenum (Mo) on the pure Pt surface enhances the adsorption energy. Our charge transfer analysis shows that the largest charge transfer contributing to the metal-O bonding formation is observed in the case of O/PtRuMo surface followed by O/PtRu surface. This is in consistency with metal d-orbital characteristic, where Mo has much more empty d-orbital than Ru in correspondence to accept electrons from atomic oxygen.