Platinum Surface Oxides Research Articles

Hydroxylation of Platinum Surface Oxides Induced by Water Vapor.

With its high stability and well-tuned binding strength for adsorbates, platinum is an excellent catalyst for a wide range of reactions. In applications like car exhaust purification, the oxidation of hydrocarbons, and fuel cells, platinum is exposed to highly oxidizing conditions, which often leads to the formation of surface oxides. To reveal the structure of these surface oxides, the oxidation of Pt in O2 has been widely studied. However, in most applications, H2O is also an important or even dominant part of the reaction mixture. Here, we investigate the interaction of H2O with Pt surface oxides using near-ambient-pressure X-ray photoelectron spectroscopy. We find that reversible hydroxylation readily occurs in H2O/O2 mixtures. Using time-resolved measurements, we show that O–OH exchange occurs on a time scale of seconds.

Probing the Dynamics of Platinum Surface Oxides in Fuel Cell Catalyst Layers Using in Situ X-ray Diffraction

The processes leading to degradation of Pt nanoparticles inside commercial hydrogen fuel cell catalyst layers are studied in situ, using time-resolved high-energy powder X-ray diffraction. Advances...

Catalytic Duality of Platinum Surface Oxides in the Oxygen Reduction and Hydrogen Oxidation Reactions

In polymer electrolyte membrane fuel cells (PEMFCs), the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) take place on the surface of platinum nanoparticles (Pt-NPs) residing on carbon support. Polycrystalline platinum (Pt(poly)) serves as a model polyoriented system due to its randomly oriented grains separated by grain boundaries, and research using Pt(poly) creates important background knowledge that is used to identify and understand electrochemical phenomena occurring in fuel cells. In this study, we report new results on the electrochemical behavior of Pt(poly) in 0.50 M H2SO4 aqueous solution saturated with reactive gases, namely O2(g) and H2(g). We analyze the influence of the potential scan rate over a broad range of values (1.00–50.0 mV s−1) on the cyclic voltammetry (CV) behavior of Pt(poly). A comparative analysis of the impact of dissolved O2 and H2 on the electrochemical behavior of Pt(poly) is performed using CV profiles and capacitance transients. Their analysis reveals the existence of new features that are observed in the potential range corresponding to the Pt surface oxide formation and reduction. The results indicate that the Pt surface oxide reveals catalytic duality because it acts both as an inhibitor and a catalyst in both the ORR and HOR. In the case of the ORR, the anodic-going transients reveal that the process becomes inhibited as the Pt surface oxide develops, while in the cathodic-going transients, the reduction of Pt surface oxide significantly (ca. 65%) increases the reaction rate. In the case of the HOR, the anodic-going transients also reveal that the process becomes inhibited as the Pt surface oxide develops, while in the cathodic-going transients, the reduction of Pt surface oxide increases (ca. 15%) the reaction rate. The catalytic effect can be attributed either to changes in the surface electronic structure that accompanies the surface oxide reduction or to short-lived increase in the electrochemically active surface area.

Growth of Stable Surface Oxides on Pt(111) at Near‐Ambient Pressures

AbstractDetailed knowledge of the structure and degree of oxidation of platinum surfaces under operando conditions is essential for understanding catalytic performance. However, experimental investigations of platinum surface oxides have been hampered by technical limitations, preventing in situ investigations at relevant pressures. As a result, the time‐dependent evolution of oxide formation has only received superficial treatment. In addition, the amorphous structures of many surface oxides have hindered realistic theoretical studies. Using near‐ambient pressure X‐ray photoelectron spectroscopy (NAP‐XPS) we show that a time scale of hours (t≥4 h) is required for the formation of platinum surface oxides. These experimental observations are consistent with ReaxFF grand canonical Monte Carlo (ReaxFF‐GCMC) calculations, predicting the structures and coverages of stable, amorphous surface oxides at temperatures between 430–680 K and an O2 partial pressure of 1 mbar.

Growth of Stable Surface Oxides on Pt(111) at Near-Ambient Pressures.

Detailed knowledge of the structure and degree of oxidation of platinum surfaces under operando conditions is essential for understanding catalytic performance. However, experimental investigations of platinum surface oxides have been hampered by technical limitations, preventing in situ investigations at relevant pressures. As a result, the time-dependent evolution of oxide formation has only received superficial treatment. In addition, the amorphous structures of many surface oxides have hindered realistic theoretical studies. Using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) we show that a time scale of hours (t≥4 h) is required for the formation of platinum surface oxides. These experimental observations are consistent with ReaxFF grand canonical Monte Carlo (ReaxFF-GCMC) calculations, predicting the structures and coverages of stable, amorphous surface oxides at temperatures between 430-680 K and an O2 partial pressure of 1 mbar.

Redox titration of gold and platinum surface oxides at porous microelectrodes.

The extension of the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) is demonstrated for porous electrode materials. These materials are often high surface area powders which are very important electrocatalysts for instance in fuel cells or water electrolyzers. The powdered electrocatalyst material is filled into a cavity-microelectrode which is then operated as the sample electrode in SECM. After a surface oxide generation step, the oxides on the porous sample are reduced by [Ru(NH3)6]2+ formed at the microelectrode probe of the SECM while the sample is at open circuit potential. Such porous electrodes pose the difficulty to cope with unavoidable variations in the filling of the cavity and to access the entire surface by the mediator. The electrochemically active surface area is used to compensate the variation in filling. It can also be used for calculating coverages of surface oxides for a better comparison between different electrodes. We found a complete and fast accessibility for all investigated porous electrodes which is based on electron transfer. Therefore, we propose a "vertical feedback" mechanism analogous to SECM feedback experiments on extended flat samples at open circuit potential. Moreover, the current transients indicate that distinctive oxide species with different kinetics are present. Taken together, these measures ensure consistent determination of oxide coverages for nanoporous gold and carbon-supported platinum nanoparticles.

Oxidation of a platinum microwire surface applied in glucose detection

In this paper, we report the results of the surface oxidation of platinum (Pt) microwires in aqueous sulfuric acid (H 2 SO 4) solutions by using a cyclic voltammetry technique. The Pt microwire chips were scanned and applied with voltage potentials ranging from 0 to 1.4 V in the H 2 SO 4 solution with concentrations from 0.0003 to 0.0018 M to find out the optimized concentration of sulfuric acid for the oxidation process. The cyclic voltammetry (CV) measurements show the oxidation peak at a potential range from 1.1 to 1.2 V. This is the peak of the interfacial place exchange of chemisorbed O (O chem ) and surface Pt atoms, resulting in the formation of a quasi-3D surface PtO lattice comprising Pt 2+ and O 2−. The oxidized surface Pt microwires were then functionalized with a 3-aminopropyl triethoxy silane (APTES) and glucose oxidase (GOD) was immobilized onto the functionalized chips for further application in glucose detection. By using this process, Pt microwires have been used for the successful detection of glucose in solution with concentrations in the range of 4–20 mM.

A thermogravimetric analysis/mass spectroscopy study of the thermal and chemical stability of carbon in the Pt/C catalytic system

Vulcan XC72 carbon powder and Pt/Vulcan XC72 catalytic powder were characterized by transmission electron microscopy (TEM) and their reactivity under controlled atmospheres was studied as a function of the temperature. Under air atmosphere, production of water was detected by thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) measurement at m/z 18, which evidenced that hydrogenated surface functions were present on the carbon substrate. Under argon atmosphere, the comparison of TGA-MS measurements performed at m/z 18 and m/z 44 with TEM and XRD results, together with XPS measurements, indicated that platinum surface oxides are rather Pt(OH)2 than PtO or PtO2 species. Such reactive surface species is involved in the degradation mechanism of carbon support under air and inert atmospheres. Under H2(3%)/Ar atmosphere, hydrocarbon production coming from “reforming” reactions of the carbon support started at very low temperatures (below 373K). TEM images of the same catalytic powder region before and after thermal treatment at 423K under reducing atmosphere clearly displayed consumption of the carbon substrate. The reaction products may not only affect the intrinsic properties of the support but also the catalytic properties of platinum particles: reaction products could poison the anodic catalyst.

Electrochemical Durability of Carbon Nanostructures as Catalyst Support for PEMFCs

Electrochemical surface oxidation of platinum supported on carbon black (Vulcan XC-72, ) and carbon nanostructures (Pt/CNS) was studied to understand and compare the durability as catalyst support in polymer electrolyte membrane fuel cells (PEMFCs). The subsequent electrochemical characterization at different treatment time intervals suggests that CNS is more electrochemically stable than carbon black due to its higher degree of graphitization. Furthermore, the reported membrane electrode assembly durability measurements suggest that highly ordered CNS would be a better catalyst support than turbostratic Vulcan XC-72.

Electrochemical behaviour of squaric acid on single-crystal platinum electrodes with basal orientations in aqueous sulphuric acid medium

The electro-oxidation of squaric acid (SQA) on basal platinum single-crystal surfaces in aqueous 0.5 M H 2SO 4 solutions has been studied. Voltammetric results show that two different processes occur in the overall reaction. In the potential range 0.7–0.8 V vs. reversible hydrogen electrode, the oxidation of strongly bonded species formed after adsorption of SQA at less positive potentials takes place. This process is a structure-sensitive reaction and its characteristics allow us to assume that CO-like species are the stable adsorbed residues. At more positive potentials, the direct oxidation of SQA takes place overlapping with the oxidation of the platinum surface; Pt(110) is the most active electrode for this reaction. Its sensitivity to the surface structure indicates that this reaction involves the interaction of SQA molecules with the first platinum surface oxides. Only a slight inhibition effect on the direct oxidation of SQA due to strongly adsorbed CO is observed on Pt(100) and Pt(110) electrodes.

Investigation of xanthate interaction on platinum and chalcopyrite by small amplitude cyclic voltammetry

The surface oxidation of platinum and chalcopyrite in borate solutions of pH 9.2 has been studied by small amplitude cyclic voltammetry. Triangular voltage sweeps of small amplitude and relatively high potential sweep rates were used at different off-set potentials. The apparent polarization resistance, diagonal resistance and Delta I values were calculated from the current-potential hysteresis loop responses. It was found that the formation of dixanthogen which is not revealed by classical high amplitude cyclic voltammetry, was observed by the present technique. The presence of CuEtX resulting from a direct reaction between chalcopyrite and xanthate was substantiated. The present research supports thus the importance of CuEtX on the onset of chalcopyrite flotation.

XPS study of the electrochemical surface oxidation of Platinum in N H 2SO 4 acid electrolyte

The surface of a platinum electrode was anodically polarized at 3 V vs standard hydrogen electrode in 1 N H 2SO 4at 300 K for 16 h so that a thick uniform oxidation layer was formed. Analysis by X-ray photoelectron spectroscopy (XPS) showed that the stoichiometry O-to-Pt of this layer was equal to four, with O1s and Pt4f 7 2 binding energies at 53.1 and 74.4 eV respectively. The oxidation layer is therefore proposed to consist of Pt(OH) 4. Cyclic voltammograms showed a single cathodic oxygen peak in support of a single uniform oxidation compound. The comparison with spectra of thinner layers, grown at potentials between 1.2 and V vs she for only 15min, led to the conclusion that those are also made up predominantly of platinum hydroxide. Surface Pt(OH) 4 thermally decomposes at 405 ± 5 K into a mixture of Pt 4/Pt 2+ hydroxides and oxides and Pt-metal with a measured ratio Pt:O = 1. At about 700 ± 50 K only a monolayer of adsorbed oxygen is left and at 1070 K all oxygen is desorbed.

Study of multilayer surface oxidation of platinum by electrical resistance technique

Multilayer surface oxidation of polycrystalline Pt wire is studied for the first time at moderate oxygen pressures using a high precision electrical resistance technique. The dependence of the oxidation rate on pressure, temperature, and time is investigated. Oxide films of up to 90 nm thickness are grown, and the temperature region for rapid dissociation of the oxide is identified. The dissociation pressure-temperature relationship for the oxide is determined in the region 450–600°C, yielding high precision values for the heat of decomposition and entropy change. A two zone model for the electrical resistance is developed and shown to agree qualitatively with the experimental results. The oxide is identified as the β phase of PtO 2 on the basis of its heat of decomposition, electron diffraction pattern, and acid solubility properties.

The comparative study of CO 2 + H ads reaction on platinum electrode in H 2O and D 2O

The observed isotopic effect in CO 2 + H ads reaction showed that at 0.05 V the rate determining step is ▪ formation whereas at 0.2 V the reorientation of adsorbed intermediate: ▪ is probably the slowest step of reaction. The oxidation of adsorbed product is slower in D 2SO 4 than H 2SO 4 solution like the surface oxidation of platinum. The rate determining step of COOH ads oxidation is a reaction with OH ads.

Voltammetric detection of short half-life intermediates formed during the anodic surface oxidation of platinum in molten potassium bisulphate

Voltammetric detection of short half-life intermediates formed during the anodic surface oxidation of platinum in molten potassium bisulphate

Voltammetric detection of short half-life intermediates formed during the anodic surface oxidation of platinum in molten potassium bisulphate

Voltammetric detection of short half-life intermediates formed during the anodic surface oxidation of platinum in molten potassium bisulphate

Effect of platinum surface oxides on the rate of anodic evolution of oxygen in solutions of sulphuric acid

Effect of platinum surface oxides on the rate of anodic evolution of oxygen in solutions of sulphuric acid

Electrochemical surface oxidation of platinum

The surface oxidation of platinum in 1 N HClO4 has been studied by means of the rigorous pre-treatment and fast “potentiodynamic” sequences previously described. Electrode capacitance was studied as a function of potential and surface coverage with “oxygen”. The capacitance was found to increase to a maximum with increasing anodic potential and to decrease with increasing extent of surface oxidation. A family of constant θo capacitance/potential plots was approximated. Current/time and charge/time plots at constant potential were obtained by means of an anodic step. The results allow speculation on the mechanism of the surface oxidation.