Correlations between hydrogen electrosorption properties and composition of Pd-noble metal alloys

Hydrogen adsorption on and absorption into Pd alloys with other noble metals was studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry. Correlations were found between the potentials of adsorbed/absorbed hydrogen oxidation peaks and surface/bulk compositions of Pd–Rh alloys. The potential of the α–β-phase transition depends linearly on Pd bulk content in Pd–Au, Pd–Rh, Pd–Pt and Pd–Pt–Rh alloys. The obtained relationships can be utilized for the determination of the composition of homogeneous Pd-noble alloys from hydrogen electrosorption experiments.     

Hydrogen electrosorption in Ni–Pd alloys

Hydrogen absorption in Ni–Pd alloys has been investigated. The amount of absorbed hydrogen in alloys containing below 20 at.% of nickel is equal to the amount of hydrogen sorbed in pure palladium. Hydrogen absorption occurs in the range 0–40 at.% of nickel concentration. Cyclic voltammograms recorded at Ni–Pd alloys have characteristic peaks which overlap with the responses due to processes occurring on the surface at Ni and Pd atoms. Also some of the processes characteristic of the pure metals can be distinguished from the recorded voltammograms.     

Influence of surface contaminations on the hydrogen storage behaviour of metal hydride alloys

Hydrogen storage in metal hydrides is a promising alternative to common storage methods. The surface of a metal hydride plays an important part in the absorption of hydrogen, since important partial reaction steps take place here. The development of surface contaminations and their influence on hydrogen absorption is examined by means of absorption experiments and surface analysis, using X-ray photoelectron spectroscopy (XPS), thermal desorption mass spectrometry (TDMS) and secondary neutral mass spectrometry (SNMS), in this work. All investigations were carried out on a modern AB₂ metal hydride alloy, namely Ti_0.96Zr_0.04Mn_1.43V_0.45Fe_0.08. Surface analysis (SNMS, XPS) shows that long-term air storage (several months) leads to oxide layers about 15 nm thick, with complete oxidation of all main alloy components. By means of in situ oxygen exposure at room temperature and XPS analysis, it can be shown that an oxygen dose of about 100 Langmuirs produces an oxide layer comparable to that after air storage. Manganese enrichment (segregation) is also clearly observed and is theoretically described here. This oxide layer hinders hydrogen absorption, so an activation procedure is necessary in order to use the full capacity of the metal hydride. This procedure consists of heating (T = 120° C) in vacuum and hydrogen flushing at pressures like p = 18 bar. During the activation process the alloy is pulverized to particles of ∼20 μm through lattice stretches. It is shown that this pulverization of the metal hydride (creating clean surface) during hydrogen flushing is essential for complete activation of the material. Re-activation of powder contaminated by small doses of air (p ≈ 0.1 bar) does not lead to full absorption capacity. In ultrahigh vacuum, hydrogen is only taken up by the alloy after sputtering of the surface (which is done in order to remove oxide layers from it), thus creating adsorption sites for the hydrogen. This is shown by TDMS measurements with and without sputtering and oxygen exposure. Figure Investigated metal hydride before and after activation     

Study of hydrogen electrosorption in Pd-Ni alloys by the quartz crystal microbalance

The electrochemical quartz crystal microbalance (EQCMB) method has been used to study the processes of hydrogen absorption/desorption in Pd-Ni alloy electrodes. It was found that hydrogen electrosorption is accompanied by an additional frequency shift, attributed to the stresses generated inside the alloy. The influence of stresses on the EQCMB response depends on the amount of absorbed hydrogen and the alloy composition. From the comparison of the EQCMB results with Pd-Ni alloy absorption capabilities data, it was concluded that the decrease of the hydrogen sorption capacity at a Ni content of ca. 25–30 at% is due to an excessive generation of stresses in the alloy lattice. Also, a dependency of the rate of hydrogen absorption in Pd-Ni alloys on potential is reported. Electronic Publication     

Unique activity of Pd monomers: hydrogen evolution at AuPd(111) surface alloys

Well-defined Au/Pd(111) alloy films have been prepared on a Ru(0001) substrate by electrochemical metal deposition and subsequent heating up to 700 degrees C. The electrochemical behaviour of the 20 monolayers thick epitaxially-grown films is in excellent agreement with both equilibrium surface composition and distribution for Au/Pd alloys on Mo(110) as previously reported (D. W. Goodman et al., J. Phys. Chem., 2005, B109, 18535). The electrocatalytic activity of the AuPd(111) surface alloys was studied for the hydrogen evolution in 0.1 M H(2)SO(4) as a function of surface composition. Maximum activities were found for Pd fractions of 0.2 +/- 0.1, where the population of Pd atoms surrounded by Au has its maximum. These Pd monomers are found to be about 20 times more active than Pd atoms in the Pd overlayer.     

Hydrogen electrosorption properties of electrodeposited Pd-Ir alloys

Journal of Solid State Electrochemistry - The study of the hydrogen sorption process in metals/alloys of different forms is crucial for developing the fields of catalysis and energy storage. The...     

The method of limited volume electrodes as a tool for hydrogen electrosorption studies in palladium and its alloys

The review summarizes selected aspects of the electrochemical investigations on hydrogen absorption in palladium and its alloys prepared as thin layers deposited on a conductive substrate. These kinds of electrodes are named ??limited volume electrodes?? (LVEs). We demonstrate that LVE methodology creates new possibilities for electrochemical studies on hydrogen-absorbing materials. In this paper, we describe the procedures of preparation and characterization of Pd-based LVEs and give a survey of the investigations on hydrogen electrosorption performed with the use of LVEs. We discuss the influence of such factors as electrode potential, temperature, electrolyte composition, electrode thickness, and other experimental conditions on the process of hydrogen and deuterium absorption in Pd LVEs. The results concerning Pd alloys are thoroughly reviewed. The studies on thermodynamics, kinetics, and mechanism of hydrogen electrosorption in Pd and its alloys are also summarized. Attention is paid to the examination of the interrelation between hydrogen absorption and other electrochemical processes taking place on LVEs. Finally, possible applications of the LVE methodology in the context of electrochemical power sources are mentioned.     

Catalytic oxidation of methanol on Pd metal and oxide clusters at near-ambient temperatures

Supported Pd clusters catalyze methanol oxidation to methyl formate with high turnover rates and >90% selectivity at near ambient temperatures (313 K). Metal clusters are much more reactive than PdO clusters and rates are inhibited by the reactant O(2). These data suggest that ensembles of Pd metal atoms on surfaces nearly saturated with chemisorbed oxygen are required for kinetically-relevant C-H bond activation in chemisorbed methoxide intermediates. Pd metal surfaces become more reactive with increasing metal particle size. The higher coordination of surface atoms on larger clusters leads to more weakly-bound chemisorbed species and to a larger number of Pd metal ensembles available during steady-state catalysis. Chemisorbed oxygen removes H-atoms formed in C-H bond activation steps and inhibits methoxide decomposition and CO(2) formation, two functions essential for the high turnover rates and methyl formate selectivities reported here.     

The method of limited volume electrodes as a tool for hydrogen electrosorption studies in palladium and its alloys

The review summarizes selected aspects of the electrochemical investigations on hydrogen absorption in palladium and its alloys prepared as thin layers deposited on a conductive substrate. These kinds of electrodes are named “limited volume electrodes” (LVEs). We demonstrate that LVE methodology creates new possibilities for electrochemical studies on hydrogen-absorbing materials. In this paper, we describe the procedures of preparation and characterization of Pd-based LVEs and give a survey of the investigations on hydrogen electrosorption performed with the use of LVEs. We discuss the influence of such factors as electrode potential, temperature, electrolyte composition, electrode thickness, and other experimental conditions on the process of hydrogen and deuterium absorption in Pd LVEs. The results concerning Pd alloys are thoroughly reviewed. The studies on thermodynamics, kinetics, and mechanism of hydrogen electrosorption in Pd and its alloys are also summarized. Attention is paid to the examination of the interrelation between hydrogen absorption and other electrochemical processes taking place on LVEs. Finally, possible applications of the LVE methodology in the context of electrochemical power sources are mentioned.

     

Effect of CO on surface oxidation of uranium metal

The surface reactions of uranium metal with carbon monoxide at 25 and 200 °C have been studied by X-ray photoelectron spectroscopy (XPS); respectively. Adsorption of carbon monoxide on the surface layer of uranium metal leads to partial reduction of surface oxide and results in U4f photoelectron peak shifting to the lower binding energy. The content of oxygen in the surface oxide is decreased and O1s/O4f ratio decreases with increasing the exposure of carbon monoxide. The investigation indicates the surface layer of uranium metal has resistance to further oxidation in the atmosphere of carbon monoxide.     

Sintering of Pd particles on the surface of carbon supports in hydrogen

High resolution electron microscopy methods have been used to study the sintering of Pd particles on the surface of carbon support. It was found that geometric non-uniformities, fractures and edge planes of graphite-like carbon, can function as localization centers of Pd particles.     

Catalytic oxidation of hydrogen in solutions of metal complexes

The kinetics of H₂ oxidation at 25°C in solutions of Pd-histidine complexes has been studied and a reaction mechanism suggested for the explanation of the experimental data. In terms of this mechanism, the correlation between the activities of homogeneous catalysts and reactant-catalyst bond energies are considered. The oxygen-catalyst bond energy proves to be a determining factor similarly as in heterogeneous catalytic oxidation.

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Catalytic activity of transition metal silicides in hydrogen oxidation

The catalytic activity of transition metal silicides in H₂ oxidation is much higher compared to metal-like carbides, which can be due to the lower electronegativity of silicon compared to carbon.

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Effect of Pt and Pd additives on the state of the surface layer and catalytic activity of niobium oxides in the oxidation of hydrogen

Catalysts prepared by the hydrogen reduction of Nb₂O₅ in the presence of Pt or Pd have specific surface much greater than for the starting oxide and their catalytic activity in the oxidation of hydrogen is much greater than the activity of Pt/Al₂O₃ or Pd/Al₂O₃. X-ray phase analysis and X-ray photoelectron spectroscopy were used to establish the existence of Nb₂O_5–x nonstoichiometric oxides in the catalyst, which enhances the catalytic activity of the surface. The kinetic behavior of the oxidation of hydrogen on these catalysts is explained in the framework of the Eley–Riedel mechanism.     

Theory of surface segregation in transition metal alloys and hydrides

The surface segregation of one component in binary transition metal alloys and the surface segregation of one hydrogen isotope in transition metal hydrides containing a mixture of hydrogen isotopes are discussed in the scope of the same thermodynamic model. The binary alloys are assumed to form a disordered substitutional alloy and the random distribution of hydrogen isotopes is assumed in the case of transition metal hydrides.     

Influence of surface morphology on the oxidation of metal electrodes studied by in-situ grazing incidence x-ray diffractometry

The successful application of in-situ grazing incidence x-ray diffractometry (GIXD) for the investigation of oxidation processes at copper electrodes in pH 12 electrolytes is demonstrated. A penetration/escape depth of about 1 μm could be detected for a smooth polycrystalline copper foil and an x-ray incidence angle of 1.7°. Oxide layers generated at overpotentials less than about 0.5 V in respect to the equilibrium formation potentials of Cu₂O or CuO, respectively, showed a dependence of the crystalline oxide formation on the defect density of the copper substrate. Highly disordered ground or polished specimens exhibited an order of magnitude higher GIXD reflexes from crystalline Cu₂O than electrodeposited copper. Beyond overpotentials of 0.5 V, this epitaxial information for the Cu₂O crystal growth became irrelevant. Further, GIXD turned out to be an appropriate tool to monitor atmospheric corrosion processes under thin humidity films with oxygen access. When oxygen diffusion through the polymer window membrane is allowed, oxygen reduction led to the concurrent formation of a crystalline CuO phase coexisting with amorphous Cu(OH)₂ and Cu₂O, though the potential was kept in the region of Cu₂O. Received: 30 July 1997 / Revised: 28 May 1998 / Accepted: 13 July 1998     

First principles study of oxygen adsorption and dissociation on the Pd/Au surface alloys

The formation of Pd/Au surfaces and their catalytic performance toward oxygen dissociation were investigated using periodic density functional methods. We show that Pd can readily incorporate into the second layer of Au(100) and Au(111) substrates with the assistance of Au vacancies. Pd/Au(100) exhibits better catalytic activity toward oxygen dissociation than Pd/Au(111). Specifically, the sub-layer Pd atoms of Pd/Au(100) can promote the oxygen dissociation and stabilize the surface structure after adsorbing oxygen atoms. On the contrary, the sub-layer Pd atoms of Pd/Au(111) slightly hinder the oxygen dissociation.     

Electrochemical absorption and oxidation of hydrogen on palladium alloys with platinum, gold and rhodium

Thin layers of Pd and its alloys with Pt, Au and Rh were prepared by electrodeposition on a Au substrate. Hydrogen electrosorption by the obtained electrodes was studied in 0.5 M H(2)SO(4) solution using cyclic voltammetry and chronoamperometry. The influence of the alloying process on selected thermodynamic (the amount of absorbed hydrogen, the stability of the β-phase, the extent of the absorption/desorption hysteresis) and kinetic aspects (the rate of hydrogen absorption and absorbed hydrogen oxidation) of hydrogen absorption and desorption was examined. It was found that the addition of the non-absorbing elements to Pd results in faster kinetics of the hydrogen electrosorption process and a smaller absorption/desorption hysteresis.     

Influence of atmospheric oxygen on hydrogen detection on Pd using Kelvin probe technique

Influence of oxygen concentration in the measurement atmosphere on detection of hydrogen using Kelvin probe was studied. The studied material was a 100-μm-thick palladium foil, which was mounted in a 3D printed electrochemical flow cell. The used setup enables hydrogen loading with in-situ contact potential measurement of the hydrogen exit side of the Pd electrode. The hydrogen loading and unloading procedure, including insertion of different amounts of hydrogen into the Pd membrane and recording resulting values of contact potential difference, was performed at distinct oxygen concentrations ranging between 1 and 80 vol%. An increasing amount of oxygen in the atmosphere surrounding the hydrogen-loaded Pd electrode resulted in an accelerated removal of hydrogen from the Pd. The kinetics of this reaction was studied based on Kelvin probe measurements, and a reaction mechanism is discussed.